ISSN:0301-5696    

DOI:10.1039/FS97308FX001

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Pulse Calorimetry and Transient Measurement of Thermal Properties at High Temperatures * BY AREDCEZAIRLIYAN National Bureau of Standards Washington D.C. 20234 U.S.A. Received 29th August 1973 A system is described for the transient (subsecond) measurement of selected thermal and related properties of electrically-conducting substances in the temperature range 1500K to the melting point of the specimen. The method is based on resistive self-heating of the specimen from room temperature to any desired high temperature in less than 1s by the passage of an electrical current pulse through it ; and on measuring and recording the experimental quantities every 0.4 ms with a full-scale signal resolution of one part in 8OOO. The system has been used to measure heat capacity electrical resistivity hemispherical total emittance normal spectral emittance and the melting point of selected refractory elements and alloys.The results of preliminary experiments have shown the potential application of the system to measurements of temperatures and energies of solid-solid phase trans- formations and heat of fusion at high temperatures. Most measurements of thermal and related properties at high temperatures employ steady-state or quasi-steady-state techniques. In all these techniques the specimen is exposed to high temperatures for relatively long periods of time (minutes- to-hours). When these techniques are extended to measurements above 2000 K many problems are created as the result of increased heat transfer chemical reactions evaporation diffusion loss of mechanical strength etc.Because of these limitations of the conventional techniques it has become necessary to develop transient techniques which permit the heating of the specimen and the measurement of the pertinent quantities in a very short time. The objective of this paper is to describe a system developed at the National Bureau of Standards for the transient measurement of selected thermal and related properties of electrically conducting substances in the temperature range from 1500 K to the melting point of the specimen. The system was initially developed for the purpose of extending the limits of accurate calorimetry to high temperatures. The results of extensive work have demonstrated that the technique can also be applied to investigations in various other areas of thermophysics.In this paper the emphasis is placed on the description of the measurement system primarily for calorimetric studies. However brief discussions of the extension of the technique to other measurements are also included to demonstrate the potentials of the transient measure- ment technique. The system has been successfully used to measure heat capacity electrical resis- tivity hemispherical total emittance normal spectral emittance and melting point of several refractory metals and alloys. The results of preliminary experiments have shown the feasibility of measuring heat of fusion temperatures and energies of solid-solid phase transformations and radiance temperature of electrical con- ductors at their melting points.* This work was supported in part by the U.S. Air Force Office of Scientific Research. 7 PULSE CALORIMETRY EXPERIMENTAL METHOD AND MEASUREMENT SYSTEM The method is based on resistive self-heating of the specimen from room temperature to any high temperature (in the range 1500 K to its melting point) in less than 1 s by the passage of an electrical current pulse through it ; and on measuring with millisecond resolution such experimental quantities as current through the specimen potential drop across the speci- men and specimen temperature. Formulation of the relations for properties in terms of the experimental quantities are given elsewhere. The measurement system consists of an electric power pulsing circuit and associated measuring and control circuits.A functional diagram of the complete system is presented in fig. 1. The details of the measurement system and its operational characteristics are given in earlier publications.'. In this section some of the important features are summarized. FIG.1.-Functional diagramof the system for the transient measurement of thermal properties. PULSE CIRCUIT The pulse circuit includes the specimen in series with a battery bank a standard resistance and a switching system. The battery bank consists of 14 series-connected2 V batteries each having approximately 1100A h capacity. A standard resistance (0.001 Q) is used to measure the pulse current through the specimen. An adjustable resistance (total resistance 30 ma) enables control of the heating rate of the specimen and the shape of the current pulse.The switching system consists of two series-connected fast-acting switches. The second switch is used as a back-up in the event the first one fails to open at the end of the heating period. SPECIMEN AND EXPERIMENT CHAMBER The specimen is a tube of the following nominal dimensions length 102 mm ; outside diameter 6.3 mm; and wall thickness 0.5 mm. A small rectangular hole (1 x 0.5 mm) is fabricated in the wall at the middle of the specimen to approximate blackbody conditions for pyrometric temperature measurements. For the above geometry blackbody quality of the sighting hole is estimated to be approximately 0.99. The specimen is mounted vertically 6 mm off-centre with respect to the axis of the experi- ment chamber to reduce the effect of internal reflections.The chamber wall as well as the specimen clamps are water cooled. Thermocouples are connected (electrically insulated) FIG.2.-Oscilloscope trace photograph of radiance of rapidly heating specimen. Dots forming the long horizontal lines correspond to radiances from the reference source. Equivalence of each major division is time 20 ms ; radiance arbitrary unit. To face page 91 A. CEZAIRLIYAN to the two end clamps to measure the specimen temperature before each pulse experiment. An expansion joint allows the expansion of the specimen in the downward direction. The potential probes are knife-edges made of the specimen material and are placed at a distance approximately 13 mm from the end clamps.The knife-edges define a portion of the specimen which should be free of axial temperature gradients for the duration of the experi- ment. The chamber is designed for conducting experiments with the specimen either in vacuum or in a controlled atmosphere. HIGH-SPEED PYROMETER Temperature of the specimen is measured with a high-speed photoelectric pyrometer which permits 1200 evaluations of the specimen temperature per second. The pyrometer alternatively passes precisely-timed samples of radiance from the specimen and a reference source (gas-filled tungsten filament lamp) through an interference filter (wavelength 650 nm bandwidth 10 nm) to a photomultiplier. During each exposure the photomultiplier output is integrated and is recorded.For measurements at temperatures above 2500 K calibrated optical attenuators are placed in the path of the radiation from the specimen. The pyro- meter target is a circular area 0.2mm in diameter. Typical results of the radiance from the sighting hole in the specimen as well as that of the reference lamp as seen by the pyro- meter as a function of time during a pulse experiment are shown in fig. 2. The details regarding the construction and operation of the pyrometer are given in the literat~re.~ DATA RECORDING SYSTEM Data corresponding to temperature current and voltage are recorded with a high-speed digita1 data acquisition sys tem which consists of a mu1 t iplexer ,analog- t 0-digi tal converter and a core memory together with control and interfacing equipment.All signals are brought to the multiplexer through differential amplifiers in order to avoid inaccuracies arising from common ground points. The multiplexed signals go to the analog-to-digital converter which has a full-scale reading of & 10V and a full-scale resolution of one part in 8192 (8192 = 213). Digital output from the converter consists of 13 binary bits plus a sign bit. This output is stored in a core memory having a capacity of 2048 words of sixteen bits each. The data acquisition system is capable of recording a set of signals corresponding to temperature voltage and current approximately every 0.4 ms. At the end of the pulse experiment information stored in the memory is retrieved in the form of numeric printing and punched tape using a teletypewriter.Since the laboratory has access to a time-shared computer it is possible to unload the memory directly to the computer bypassing the inter- mediate stage of punching paper tape. Oscilloscopes are used only to monitor the general pattern of the experimental results and to detect any anomalies. TYPICAL OPERATIONAL CHARACTERISTICS The operational characteristics of the system depend primarily on the electrical circuit parameters properties of the specimen and specimen maximum temperature. The system characteristics for typical experiments are as follows current 1300-2200A power 7000-1 5000 W heating rate 3000-8000K s-' heating duration 0.3-0.7s heating rate 10-100 cooling rate The above quoted figures are the results of typical experiments and do not represent the full capabilities of the measurement system.MEASUREMENT OF THERMAL PROPERTIES The system described in this paper has been used to measure heat capacity electrical resistivity hemispherical total emittance and normal spectral emittance of the refractory molybdenum,l tantal~rn,~ metals ni~bium,~ tungsten ; and the alloys tantalum-10 (wt. %) PULSE CALORIMETRY tungsten,' niobium-1 (wt. %) zirconium in the temperature range from 1500 K to near their respective me1 ting temperatures. Similar measurements were also performed on graphite in the range1500-3000K.'. lo In addition the melting points of niobium,l1 molybdenum,12 and tungsten l3 were also measured.HEAT CAPACITY Heat capacity is determined from the measurements of voltage current and temperature during the rapid heating period. A correction for heat Ioss from the specimen due to thermal radiation is made based on hemispherical total emittance determined during the same experiment. ELECTRICAL RESISTIVITY Electrical resistivity is obtained as a by-product from measurements of voltage current and temperature during the heating period. Before and after the pulse experiments specimen resistance at room temperature is measured using a Kelvin bridge. HEMISPHERICAL TOTAL EMITTANCE Hemispherical total emittance is obtained from data taken during the specimen heating period in conjunction with temperature measured during the radiative cooling period that follows the heating period.Only the data during the initial cooling period are considered to eliminate the effect of axial heat conduction on the results. NORMAL SPECTRAL EMITTANCE Normal spectral emittance is measured at the effective wavelength of the pyrometer interference filter (wavelength 650 nm bandwidth 10 rim). For this determination two rapid heating experiments are performed in which the pyrometer is first aimed at the blackbody radiation hole and then at the surface of the tubular specimen. The ratio of radiances (surface-to-hole) corresponding to the same specimen temperature is the normal spectral emittance. The same temperature in the two experiments is found by matching the measured specimen resistances. MELTING POINT During rapid heating of the specimen if the current is not interrupted the specimen temperature reaches its melting point and then collapses under gravitational forces.Melting 2750t 2750.6 K --**....**.........* **.*..-.* I 0 10 20 30 40 50 time arbitrary unit FIG. 3.-Variation of the temperature of a niobium specimen near and at the melting point. One time unit is 0.833 ms. A. CEZAIRLIYAN is manifested by a plateau in the temperature versus time function. Temperatures deter- mined from the pyrometer output for a niobium specimen are shown in fig. 3. By averaging the temperature points on the plateau the melting point is obtained. OTHER INVESTIGATIONS AND PRELIMINARY MEASUREMENTS In this section description and preliminary results of experiments are given for various thermal and related investigations at high temperatures.The same measurement system was used with some modifications. RADIANCE TEMPERATURE AT THE MELTING POINT Utilizing a specimen in the form of a strip it is possible to measure radiance temperature of a substance during rapid heating. The value of this quantity at the melting point is of particular interest. Fig. 4 shows the results of a typical experiment on a niobium specimen having the following dimensions length 102mm ; width 6.3 mm ;and thickness 0.25 mm. l1 The plateau indicates the melting of the specimen. The drop at the beginning of the plateau corresponds to the change in normal spectral emittance of the specimen as its melts. An initial high radiance temperature (peak of spike) indicates that normal spectral emittance of the solid surface is higher than that of the liquid surface.This is likely to be the case in general since solid surfaces regardless of the degree of polish depart from the conditions of ideal smoothness. The results of experiments on twelve specimens l4 performed under different conditions have demonstrated the constancy and reproducibility of the radiance temperature of niobium at its melting point. This may find applications in performing secondary calibrations on instruments and in conducting overall on-thespot checks on complicated measurement systems at high temperatures. The measurement of the radiance temperature at the melting point in addition to a separate measurement of the melting point yields the normal spectral emittance at the melting point.I--'i I I I 1 I f 2424K T . T J . 2410 1 I I I I I I FIG.4.-Variation of the radiance temperature of a niobium specimen as a function of time near and at the melting point. One time unit is 2.5 ms. TEMPERATURE OF SOLID-SOLID PHASE TRANSFORMATION Most solid-solid phase transformations can be detected by monitoring specimen tem- perature as a function of time as the specimen undergoes heating or cooling. For phase PULSE CALORIMETRY transformations that are constant or nearly-constant temperature processes the temperature versus time function exhibits a plateau during the phase transformation. The feasibility of the present system to measure temperatures of solid-solid phase transformations is demon- strated by preliminary measurements on iron.s For simplicity radiance temperature (instead of true temperature) of an iron rod during heating was measured as a function of time (fig. 5). The lower plateau indicates the solid-solid phase transformation (y -+ a) '700r--9 I650 -I600 -1550-1500-I I I I I 0 50 too 150 200 250 time/ms FIG.5.-Variation of the radiance temperature of an iron specimen as a function of time during its heating through the y +-6 phase transformation up to the melting point. and the upper plateau indicates melting of the specimen. From the measurements of voltage and current as a function of time it appears feasible to determine the electrical resistivity of the specimen and the energy imparted to the specimen during the phase transformation.After making a correction for the energy losses (mainly due to thermal radiation at high temperatures) energy of the phase transformations may be obtained. In the final measure-ments tubular specimens will be used. HEAT OF FUSION A preliminary experiment conducted on niobium l6 has demonstrated the feasibility of extending the capabilities of the present system to the measurement of heat of fusion of metals at high temperatures. The specimen was a composite made of three rectangular strips each having the following nominal dimensions length 75 mm width 6.3 mm and thickness 0.25mm. The middle strip was niobium and the two outer strips were tantalum. The specimen was pulse-heated from room temperature to approximately 200 K above the melting point of niobium.During the heating period current through the composite specimen potential drop across the specimen and radiance temperature (fig. 6) of the outer strip were measured. The plateau in radiance temperature corresponds to melting of the inner strip. Since the melting point of tantalum is approximately 500 K higher than that of niobium the outer strips did not melt during the entire experiment and acted as a container for niobium. The heat of fusion was obtained by integrating the power absorbed by the niobium strip over the duration of the plateau. Heat loss due to thermal radiation and energy absorbed by the outer strips were taken into consideration. The value obtained for the heat of fusion of niobium is 6,430 cal mol-1 (26,900J mol-l) which corresponds to an entropy of fusion of 2.34 cal mol-' K-l.A. CEZAIRLIYAN 1 1 I I I 8 9 3 2450' 2400 1 I J I RESULTS ESTIMATE OF ERRORS A summary of imprecision * and inaccuracy t of measured quantities and pro- perties is given in table 1. Tabulated values for imprecision represent averages of the results of several measurements on the four refractory metals (Nb Mo Ta W) corresponding to 2000K. Imprecision and inaccuracy of most quantities do not increase significantly at temperatures above 2000 K. At 3000 K inaccuracies of temperature and heat capacity are estimated to be 8 K and 3 % respectively. Details regarding the estimates of errors and their combination are given in an earlier publication.TABLE AND INACCURACY OF MEASURED QUANT~IESAND PROPERTTES * 1.-IMPRECISION (the values correspond to measurements at 2000 K) quantity imprecision inaccuracy temperature 0.5 K 4K voltage 0.02 % 0.1 % current 0.03 % 0.1 % heat capacity 0.6 % 2% electrical resistivity 0.1 % 0.5 % hemispherical total emittance 0.8 % 3% normal spectral emittance 0.3 % 3% (at 650nm) melting point 1K 6K * see text for definition of terms. *Imprecision refers to the standard deviation of an individual point as computed from the difference between the measured value and that from the smooth function obtained by the least squares method. Inaccuracy refers to the estimated total error (random and systematic).PULSE CALORIMETRY The estimates of errors in the measurement of other properties discussed in the previous section are not finalized yet. Preliminary estimates indicate that inaccuracy in the temperature of solid-solid phase transformation may be approximately the same as that for the melting point (6 K at 2000 K). Inaccuracy in heat of fusion may be 5 %. SOME RESULTS OF HEAT CAPACITY MEASUREMENTS AT HIGH TEMPERATURES As an example results for heat capacity of tungsten obtained using the present system in addition to those obtained by other investigators are shown in fig. 7. The results represent measurements using three different techniques namely "drop " "modulation " and "pulse ". There is no evidence for any bias with respect to a particular technique.The heat capacity results (smooth) for the elements niobium molybdenum tantalum and tungsten obtained using the present system are shown in graphical form in fig. 8 and are given in tabular form in table 2. It may be noted that the heat capacity results at high temperatures are considerably higher than the Dulong and 55 I I I I ----WORTHING (1918) 50 JAEGER (1930) HOCH (1961)..-..-KlRlLLlN (1963) -. -KRAFTMAKHER(1963) .-._ LOWENTHAL(1963) HElN (1968) LEIBOWITZ (1969) WEST PRESENT WORK f -5% f 25 I I I I I 1000 1500 2000 2500 3000 3500 temperature/K FIG.7.-Heat capacity of tungsten reported in the literature. For brevity the names of only the first authors are given in the figure.References to investigations presented in the figure may be found in the publication by Cezairliyan and McClure.6 The techniques that were used are as follows. Pulse Worthing Cezairliyan ; modulation Kraftmakher Lowenthal ; drop remainder. A. CEZAIRLIYAN Petit value of 3R. Some of the departure is due to cp-cv and the electronic terms. However they do not account for the entire departure. Heat capacity at high temperatures may be expressed by c,'= A -BIT2+ CT+ Ac (1) where the constant term A is 3R (24.943 J mol-l K-I) the term in is the hrst I I I I I I I 50 -U c 30 1 I I 1 I I 1 1I 1I - 1500 2000 2500 3000 3500 temperature/K FIG.8.-Hea t capacity (smooth) of niobium molybdenum tantalum and tungsten measured using the pulse calorimeter.TABLE &-HEAT CAPACITY (J m01-l K-') OF SOME REFRACTORY METALS AND GRAPHITE AT HIGH TEMPERATURES temp./K niobium molybdenum tantalum tungsten graphite 1500 29.48 24.14 1600 30.11 24.38 1700 30.73 24.61 1800 31.35 24.83 1900 32.02 33.97 30.66 25.03 2000 32.75 34.95 31.19 31.65 25.22 2100 33.57 36.07 31.71 32.49 25.41 2200 34.53 37.35 32.25 33.29 25.57 2300 35.64 38.77 32.83 34.08 25.73 2400 36.94 40.34 33.46 34.89 25.88 2500 38.46 42.06 34.17 35.72 26.01 2600 40.23 43.92 34.97 36.61 26.13 2700 42.28 45.94 35.89 37.57 26.24 2800 48.10 36.95 38.63 26.34 2900 38.16 39.81 26.43 3000 39.55 41.14 26.50 3100 41.39 42.62 3200 44.48 44.29 3300 46.17 3400 48.27 3500 50.63 3600 53.25 PULSE CALORIMETRY term in the expansion of the Debye function the term in T represents cp-c and first- order anharmonic and electronic contributions and the quantity Ac represents excess in measured heat capacity at high temperatures which is not accounted for by the first three terms.The coefficients B and C can be obtained from heat-capacity data at low and moderate temperatures (below 1000 K). 2000 2500 3000 3500 temperature/K FIG. 9.-Excess heat capacity Ac as defined by eqn (1) for niobium molybdenum tantalum and tungsten. Using eqn (1) and the measured heat capacity values the quantity Ac was com- puted for temperatures above 2000 K. The results are shown in graphical form in fig.9 and in tabular form in table 3. The estimated uncertainty in the computed Ac may be as high as 1 J mol-1 K-l. This is obtained from the combined uncertainties in the coefficients in eqn (1) and the measured heat capacities. TABLE 3.THE QUANTITY AC (IN J 11101-1 K-') IN EQN (1) FOR SOME REFRACTORY METALS IN THE RANGE 2000K TO NEAR THEIR RESPECTIVE MELTING POINTS temp./K niobium molybdenum tantalum tungsten 2000 2.03 3.70 1.08 2.07 2200 3.22 5.42 1.62 3.24 2400 5.06 7.68 2.31 4.37 2600 7.77 10.58 3.30 5.62 2800 14.23 4.76 7.18 3000 6.84 9.21 3200 9.70 11.90 3400 15.41 3600 19.93 Although vacancy generation becomes appreciable at high temperatures it is not likely that the high values are due entirely to vacancies. Higher-order terms in elec- tronic heat capacity and in lattice anharmonicity probably account for most of the excess heat capacity at high temperatures.More accurate theoretical work may produce a quantitative explanation of this phenomena. A. CEZAIRLIYAN DISCUSSION Delay at least until a few years ago in the development of accurate systems for the measurement of thermal properties may be attributed to a large extent to the lack of sophisticated instrumentation. Recent advances in electronics particularly in the area of high-speed digital data acquisition techniques have contributed im- mensely to the development of the system described in this paper. The system has been used successfully for the measurement of several thermal and related properties in the range 1500 K to the melting point of the specimen which for tungsten is 3695 K.The results of preliminary experiments have demonstrated the potential of the technique to extend the measurements to several other properties. In the temperature range 1500-2500 K the accuracy of measured properties by the present method and some of the accurate conventional techniques is approximately the same. However above 2500K,it becomes very difficult if not impossible to conduct accurate steady-state experiments ; thus the transient technique becomes a unique and indispensable tool for the measurement of thermal properties at high temperatures. Additional advantages of this technique are (i) the possibility of measuring simultaneously or consecutively several related properties and (ii) the possibility of conducting experiments to study time-dependent phenomena.The potential of the transient technique has not been completely explored. It shows promise in extending the measurements to (i) other thermal and related properties at high temperatures (ii) other electrically conducting high-temperature substances such as certain carbides borides nitrides and oxides and (iii) liquid metals and other electrically conducting liquids at hi& temperatures. A. Cezairliyan M. S. Morse H. A. Berman and C. W. Beckett J. Res. Nat. Bur. Stand. 1970 74A 65. A. Cezairliyan J. Res. Nat. Bur. Stand. 1971 75C 7. G. M. Foley Rev. Sci. Instr. 1970 41 827. A. Cezairliyan J. Res. Nat. Bur. Stand. 1971,75A 565. A. Cezairliyan J.L. McClure and C. W. Beckett J. Res. Nut. Bur. Stand. 1971,75A 1. A. Cezairliyan and J. L. McClure J. Res. Nut. Bur. Stand. 1971,75A 283. A. Cezairliyan High Temperature-High Pressures 1972,4,541. a A. Cezairliyan,J. Res. Nat. Bur. Stand. 1973,77A 45. A. Cezairliyan Proc. 6th Symp. Thermphysical Properties P. E. Liley ed. (Amer. SOC. Mech. Eng. New York 1973) 279. A Cezairliyan Electrical Resistivity and Thermal Radiation Properties of Graphite in the Range 1500 to 3000 K by a Pulse Heating Method in preparation. It A Cezairliyan High Temperatures-High Pressures 1972,4,453. l2A. Cezairliyan M. S. Morse and C. W. Beckett Rev. In?. Hautes Temp&. Rkfiact. 1970,7,382. l3 A. Cezairliyan High Temperature Sci. 1972,4,248. l4 A. Cezairliyan J.Res. Nat. Bur. Stand. 1973,77A 333. A. Cezairliyan and J. L. McClure A Pulse Heating Technique for the Study of Solid-Solid Phase Transformations at High Temperatures Application to Iron in preparation. I6 A. Cezairliyan A Pulse Heating Technique for the Measurement of Heat of Fusion of Metals at High Temperatures in preparation.

ISSN:0301-5696    

DOI:10.1039/FS9730800007

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

High Temperature Microcalorimetric Studies of the Thermal Decomposition and Iodination of Polynuclear Carbonyls of Fe Co Ru Rh Re 0s and Ir BY J. A. CONNOR,*H. A. SKINNER AND Y. VIRMANI Chemistry Dept. University of Manchester Manchester M13 9PL Received 29th August 1973 From measurements of the enthalpies of thermal decomposition (and also in some cases of the enthalpies of reaction with iodine vapour) at elevated temperatures the standard enthalpies of forma- tion AH; at 298 K of the following crystalline metal carbonyls have been obtained (in kJ mol-l) Fe2(CO)9= -1410+ 12 ; Fe3(CO)12 = -1849k16; COZ(CO)* = -1209+8; cOq(c0)12 = -1845+ 16; RU3(C0)12 = -1920+20 ; Rh4(C0)12 = -(1820+ 12) ; Rha(C0)lG = -(2029) ; Rez(CO)lo = -1653 & 20 ; OS3(CO)12 = -(1749+ 20) ; Ir4(CO)12= -11820k 16 Estimates of the unknown values AHsub(298K) for the crystalline compounds have been used to derive the values AH;"() ; the latter were converted into AH values for the total enthalpies of dis- ruption of the gaseous carbonyls into metal atoms and CO gaseous molecules.These AH values have in turn been reduced to individual enthalpy contributions from the various metal-carbonyl and metal-metal bonds in the molecule. Terminal metal-carbonyl bond enthalpies increase both with atomic number and with enthalpy of atomization of the metal. The enthalpy contribution from a metal-carbon bond in a bridging M-CO-M is of the order one half of the corresponding terminal M-CO linkage. The bond enthalpy contribution from M-M in the polynuclear carbonyls of Fe and Co is found to be approximately 2/3 of that from the M-CO terminal bonds.The enthalpies of formation of several mononuclear metallic carbonyls have been rneas~red,l-~ but similar data are available only for two polynuclear metal carbonyls (of Mn and Co 6 ; nothing is known of the enthalpies of formation of the poly- nuclear carbonyls of Fe Ru Rh Re 0s and Ir. The conventional thermochemical approach (energies of combustion by bomb calorimetry) can be troublesome when applied to metal carbonyls due to incomplete oxidation of the metal. A different approach was used in the present work through the enthalpies of thermal decomposi- tion. The measurements were made by the "drop "calorimetric technique using the high-temperature Calvet twin microcalorimeter for this purpose.Two types of decomposition reaction were investigated calorimetrically (i) direct thermal de- composition in argon gas and (ii) reaction with iodine vapour to form the iodide(s) of the metal and liberate carbon monoxide. EXPERIMENTAL PREPARATION OF COMPOUNDS Commercial samples (Strem Chemicals Inc.) of RU~(CO)~~ and Re,(CO), were used assupplied. The remaining carbonyl samples were prepared and purified as described in the literature references listed below 18 J. A. CONNOR H. A. SKINNER AND Y. VIRMANI Fez(CO)9 from Braye and Hubel '; Fe3(CO)12 from McFarlane and Wilkinson ; CO~(CO)~, from Wender Sternberg Metlin and Orchin ; Co4(CO)12 from Chini Albano from and Martinengo lo; lU4(C0)12 and R~I~(CO)~~ Chini and Martinengo l1; OS~(CO)~~, from Johnson Lewis and Kilty l2 ; and 1r4(C0)12 from Chaston and Stone.13 CALORIMETRIC TECHNIQUE The Calvet twin-cell microcalorimeter (Setaram Lyon) was calibrated electrically over the temperature range 423-773 K and its performance as a '' drop " calorimeter evaluated from measurements of the enthalpy of sublimation of crystalline iodine.Iodine samples (10-15mg) contained in thin capillary tubes were dropped from outside the calorimeter via the inlet chute into the argon-filled " live " cell of the calorimeter at the same time as identical but empty glass capillary tubes were let fall into the twin " reference " cell. The resulting thermogram area was measured and related to the known l4 enthalpy change for the live cell process I2(c G)+ 12k TZ) where T2= fixed calorimeter tempetature and Tl = arrival temperature of the I2 sample on reaching the reaction zone in the calorimeter.Thermal decomposition measurements were made similarly by dropping weighed carbonyl samples (a few mg) into the argon- filled live cell ; iodination measurements were made by dropping carbonyl samples in the live cell charged with excess I2 vapour. The extent of iodination of the metal was deter- mined from analysis of unused iodine and from analysis of the iodine content of the solid product (metal iodidelmetal) formed. AUXILIARY QUANTITIES Enthalpy changes (Ah,AH) and enthalpies of formation (AH:) are given in thermo- chemical calories defined by 1 cal = 4.1840 J.The following standard enthalpies of forma- tion ls were accepted (values in kcal mol-l) ; AH;(CO 9)= -26.416 ; AH"f12,g) = 14.92; AH"fFe12,c) = -27.0 ; AH:(CoI, C) = -21.2 ; AH?(Fe g) = 99.5 ; AHi(C0,g) = 101.5; AHOf(Ru,g)= 153.6; AH:(Rh,g) = 133.1 ; AHi(Re g) = 184.0; AH:(Os g) = 189.0 and AH?(Ir g) = 159.0. (HT-HZg8) values for the metals Fe Coy Ru Re. 0s and Ir were taken from the compilation by Hultgren Orr Anderson and Kelley.16 RESULTS DI -IRO N ENNEACARBoNY L Fe2(C0)9 Thermal decomposition measurements were made by dropping carbonyl samples (2-2.5 mg) into the calorimetric reaction vessel maintained at a fixed temperature T2 within the range 553-573 K. Decomposition took place rapidly within the reaction vessel with deposition of a bright metallic film on the walls of the Pyrex reaction vessel.Typical results are summarized in table la. The column Ah gives the measured enthalpy change in each experiment determined from the recorded thermo- gram area. The values AHubsrefer to the process Fe2(CO)9[c T11 -+ 2Fe[c T21 +9c0b T21 (1) where Tl is the temperature of the carbonyl sample on entering the reaction zone of the calorimeter. The values AH298 refer to the same process but carried out iso- thermally at 298 K; the reduction of AH,, to 298 K made use of tabulated (HT-H298) data 14* l6 for Fe(c) and CO(g). The reaction of Fe2(CO)9 with excess I2 vapour was studied in the range T2 = 524-573 K. The main product was a brown-red powder (FeI,) and there was also HIGH TEMPERATURE MICROCALORIMETRIC STUDIES some metal film deposit mainly within the capillary tube containers.Results are summarized in table lb. The values AHobsrefer to the calorimeter reaction F~,(CO),[C GI + n~,rg,GI + n~e~,[c, T,I+ (2-n)wc T,I+ 9corg GI. (2) The values AH* were obtained from AHobsby removing the exothermic contribution due to the iodination reaction n Fe[c T2] +n 12[g,T2] n FeI,[c T2]. (3) The adjustment made i.e. AH = AHobs+ 441.9 -0.0045(T2-298)] kcalmol-1 (3) was based on recommended values for the enthalpy of formation of FeIz(c) and for ACp of reaction 14(3). [AHi[Fe12 c] = -27.0 kcal mol-l; ACD= 4.5 cal K-l mol-1 ; AH",[12 g] = 14.92 kcal mol-l]. The values AH* provide an indirect measurement of the enthalpy of the thermal decomposition process (I) and were corrected to AHzs8 as described earlier.The thermal decomposition and iodination studies gave virtually identical results (mean AH298 = 99 kcalmol-') corresponding to AH;[Fe,(CO), c] = -337 kcal mol-l. The overall uncertainty in the latter is estimated at &3 kcal mol-l. TABLETHERMAL DECOMPOSITION OF Fez(CO)9 AH,,./_ AHz9al expt . Fez(CO)glmg Ahlcal Tz/K kcalmol 1 kcal mol-1 1 2.025 0.659 553 118 99 2 2.450 0.796 553 118 99 3 2.035 0.669 553 120 100 4 1.975 0.661 573 122 101 5 1.835 0.602 573 119 98 mean AHzss = 99 kcal mol-I. TABLE OF Fez(CO)9 1~.-IODINATION apt. Fez(CO19I mg 12/w M/d TZ/K n AHobsl AH*l kcal mol-1 kcal mol-1 AH2981 kcalmol-1 1 4.380 15.005 0.522 524 1.so 43 117 100 2 6.415 14.315 0.817 544 1.74 46 117 99 3 3.985 14.765 0.459 553 1.84 42 117 98 4 3.965 14.440 0.448 553 1.90 41 119 100 5 4.510 12.770 0.517 573 1.86 42 118 97 mean AHzss = 99kcal mol-l.TRI-IRON DODECARBONYL Fe3(CO)12 Thermal decomposition and iodination measurements on Fe,(CO) were made over the range T2 = 494-544 K. The thermal decomposition results are summarized in table 2a AHobshere relating to the decomposition process Fe,(CO),,[c T11 + 3Fe[c T2l+ 12corg GI. (5) The iodination studies (table 2b) followed the pattern of similar studies on Fe2(CO)9 ; the values AH& refer to (3 -F)J?e[e T,]+ 12 CO[g T2] J. A. CONNOR H. A. SKINNER AND Y. VIRMANI and the values AH* (obtained from AH* = AHobs+3n -c41.9 -O.O045(T -298)] kcaf mol- (7) 2 by removing the exothermic contribution to AHobs from the formation of (3n/2)Fe12) refer to the thermal decomposition process (5).The agreement between the values AHzg8obtained directly (table 2a) and indirectly from iodination (table 2b) is fair ; we accept AH298 = 125f4 kcal mol-' as the final result corresponding to A\HOf[Fe3(C0)12,c] = -442+4 kcal mol-l. TABLEk-THERMAL DECOMPOSITION OF Fe,(CO) 2 Fe3(CO)i21 AHobsl-AH2981 expt. mg Ahlcal T21K kcal mol 1 kcal mol-1 1 2.920 0.814 494 140 120 2 3.215 0.973 544 152 127 3 2.780 0.825 544 149 124 4 3.060 0.881 544 145 120 5 3.095 0.922 544 150 125 mean AHzg8= 123 kcal mol-l. TABLE 26.-IODINATION OF Fe3(CO)12 1 4.415 13.760 0.319 494 1.72 36 142 122 2 3.230 13.950 0.233 518 1.81 36 147 124 3 3.595 12.670 0.242 518 1.88 34 149 126 4 3.945 13.680 0.307 518 1.81 39 153 131 5 3.945 13.910 0.258 518 1.94 33 152 129 6 5.040 14.670 0.399 544 1.75 40 147 122 mean AHzgs= 126 kcal mol-I.COBALT CARBONYLS Thermal decomposition studies were made on Co,(CO) and on CO,(CO)~ over the temperature range T2 = 453-514K. Freshly prepared octacarbonyl was used for each measurement and both carbonyls were handled in an atmosphere of pure N2 at all times. The dicobalt compound is less stable to heat than Co,(CO),, decomposing visibly at temperatures as low as 350 K. Both carbonyls were rapidly decomposed at the temperatures used in these studies yielding a bright clean filmof metal on the walls of the reaction vessel; metal was also formed within the capillary tube containers.DICOBALT OCTACARBONYL CO~(CO)~ Thermal decomposition results with Co,(CO) are summarized in table 3; the values AHobs refer to the process. and were converted to AH298 values using tabulated (HT-H298) data for Co(c)from c] Hultgren et aZ.l6 The mean AH298 leads to AH"~[CO,(CO)~ = -298 kcal mol-l. The error limits in this value are estimated at f2 kcal mol-l. HIGH TEMPERATURE MICROCALORIMETRIC STUDIES TABLE3.-THERMAL DECOMPOSITION OF cO,(co) AHod AH2981 expt. Coz(CO)a/mg Ahlcal T2IK kcal mol-1 kcal mol-1 1 3.000 0.870 470 99 88 2 2.995 0.850 470 98 87 3 3.235 0.891 475 98 86 4 3.355 0.965 475 98 86 mean AH298 = 87 kcal mol-l. TETRA COB A LT D OD E CA CA RBO NY L CO4(CO)12 Both thermal decomposition and iodination studies were made on CO~(CO),~.The thermal decomposition results are summarized in table 4a in which AHobsrefers to the process c04(co)12[c TI1 4c0[c T2] + l2c0[g T2]. (9) TABLETHERMAL DECOMPOSITION OF cOa(c0) 12 AHod AH2981 expt . Co4(CO)12Img Ah/cal TZIK kcal mol-1 kcal mol-1 1 3.505 0.851 458 139 122 2 3.050 0.803 514 151 127 3 3.020 0.793 514 150 126 4 2.805 0.702 514 143 120 mean AH298 = 124 kcal mol-'. TABLE46.-IODINATION OF CO,(CO), em. Co4(CO)lz/mg Idmg Ah/d T2/K n Noadkcal mol-1 AH*l kcal mol-1 AH29el kcal xn01-~ 1 3.480 10.170 0.565 456 0.64 93.0 138 121 2 3.440 11.960 0.445 458 1.02 74.0 146 129 3 2.950 13.700 0.340 514 1.18 65.4 148 125 4 3.355 13.235 0.407 514 1.06 69.4 144 120 5 3.025 10.430 0.328 518 1.19 62.0 146 122 mean AHzg8 = 124 kcal mol-'.In table 4b AHobs refers to the iodination reaction n COq(CO)12[C 2-11 +Z'z[g 2-21 -+ 4COI,[C GI + 12 cots TZI (10) where CoI = (n/2)C012 +(1 -(n/2))Co. The values AH*(obtained from AH,,,by removing the contribution from the iodination ofthe cobalt metal) refer to the thermal decomposition (9). The adjustment AH*= AHobs+ 2n[36.1- o.0045(T2 -29811 kcal mol-' (1 1) was based on the recommended value AhH"fCo12,c) = -21.2 kcal mol-' and used an assumed AC = 4.5 cal mol-l K-' for the reaction Co(c)+12(g)3 C012(c). The mean AH298= 124 kcal mol-l from both thermal decomposition and iodination studies (estimated uncertainty f4 kcal mol-l) leads to the value A.H"fCo,(CO),2 c]= -441 + 4 kcal mol-l .J. A. CONNOR H. A. SKINNER AND Y. VIRMANI TRI-RUTHEN I uM DO D E cA cA RB oN Y L Ru,(CO) Thermal decomposition and iodination studies were made on RU,(CO)~~. The carbonyl decomposes readily at temperatures >550 K ; the results of the thermal decomposition measurements are summarized in table 5. The iodination reaction studied over the range 550-570 K led to a product which was orange-red in colour and also to a metallic deposit. The red powder has not been identified but was thought to be the polymeric carbonyl iodide,17 [Ru(CO)~I~],. Further studies on this reaction are planned. TABLE 5.-THERMAL DECOMPOSITION OF RUs(C0) 12 AHod AHzgsl expt. RuACO)1zlmg Ahlcal TzIK kcal mol-1 kcal mol-1 1 4.010 1.030 553 164 138 2 3.855 1.027 553 170 144 3 3.420 0.921 573 173 145 4 3.815 1.053 573 177 149 5 3.665 0.931 573 162 135 mean AHzsa= 142 kcal mol-I ; AH~[RU,(CO)~~, c] = -459k 5 kcal mol-'.RHODIUM CARBONYLS Rh4(C0)12 AND Rh6(C0)16 Thermal decomposition studies on Rh4(C0)12 were made over the range 518- 574 K. Freshly prepared material was used as this compound may deteriorate on standing. Preliminary results (six measurements) gave AHzg8 = 1 19 3 kcal mol-I corresponding to AHf(Rh4(C0)12 c] = -436 kcal mol-I. Thermal decomposition studies on Rh6(C0)16 were made over the range 458-574K. Preliminary results only are available ; from four measurements AHzss = 62 3 kcal mol-' corres-ponding to AH"f[Rh6(C0)16 c] = -485 kcal mol-'. Further work is needed to complete these studies and both the AH; values given above are tentative.DI-RHENIUM DECACARBONYL Re2(CO)lo Both thermal decomposition and iodination studies were made on Re2(CO),o. The thermal decomposition results obtained at 593 IS,are summarized in table 6a. The iodination reactions were studied at 558 and at 593 K. The product of iodination was a black powder (presumed to be ReI,) and traces of metal deposit were also formed on the walls of the reaction vessel and inside the capillary-tube containers. The overall product ReI, analysed with n = 2.5-2.9. The AHobsvalues (table 6b) relating to the calorimeter reaction Rez(CO)lo[c TI1+nI2[g T2l + 2ReIn[c T21+ locolg,T219 (1 1) were converted to AH* values by removing the exothermic contribution from the for- mation of ReI (2Re1 = (2n/3)Re13+(2-(2n/3))Re).In this case it was necessary to use an estimated value for AH"fReI, c]. Literature values exist for AH;[ReCl, c] and AHf"[ReBr, c] (-63 and -40 kcal mol-1 respectively) ; the increments in AH; for transition metal halides are normally numerically larger in passing from bromides + iodides than in passing from chlorides -+bromides and on this basis a value AHT[ReI, c] = -I0 (f5) kcal mol-I was adopted for present purposes. The adjustments made (table 6b) to AHobs to obtain AH* used AH -32 kcal mol-' for the iodination reaction Re[c T,]++I,[g T2]+ Re13[c T2]. HIGH TEMPERATURE MICROCALORIMETRIC STUDIES The agreement between the AH298values from thermal decomposition and from iodination is satisfactory but may be fortuitous bearing in mind the estimated value for AHi(Rel[, c).Accepting AH298-131 kcal mol-' AH"fRe2(CO)lo,c] is calcu- lated = -395 kcal rnol-l to which we attach an uncertainty of +5 kcal mol-l. TABLE&.-THERMAL DECOMPOSITION OF Rez(CO)lo AT 593 K expt. RedCO) 1olmg Ahlcal AHObs/kcal m01-l AHzg8lkcal mol-1 1 3.325 0.793 156 131 2 3.370 0.793 154 129 3 2.91 5 0.681 153 128 4 2.915 0.676 151 127 5 3.030 0.716 154 130 mean AHzg8= 129 kcal mol-l. TABLE 6b.-IODINATION OF Re2(CO)10 expt. Rez(C0)lolmg Ahlcal T2IK n AHotuI kcal mol-1 AH*/kcal mol-1 AH2981 kcal mol-1 1 3.930 12.080 0.550 558 2.85 91 152 131 2 4.000 13.350 0.526 558 2.85 86 147 126 3 3.735 13.500 0.533 558 2.9 93 155 134 4 3.965 12.385 0.568 558 2.75 94 152 131 5 5.390 14.300 0.875 593 2.6 106 161 137 6 4.220 13.100 0.669 593 2.5 103 157 133 mean AHzga= 132 kcal mol-l.TRI-OSMIUM DODECARBONYL OS~(CO)~~ Os,(CO), was studied by thermal decomposition at 593 K and by iodination at 573 and 593 K. Thermal decomposition was noticeable at 543 K but too slow for microcalorimetric study. At 593 K decomposition appeared to be rapid but was not totally confined to the reaction vessel; there was some escape of vapour from the reaction zone and the metallic deposit extended some way into the inlet tube. Iodination occurred readily at 573 K forming a bright yellow powder. This was thought to be the polymeric carbonyl iodide [Os(CO),I,] ; on strong heating in vacuo it decomposed to liberate iodine vapour and deposit a clean film of metal.Further studies of this reaction are planned. The thermal decomposition results at 593 K are summarized in table 7. TABLE 7.-THERMAL DECOMPOSMTON OF OSj(CO)12 AT 593 K expt. Os3(CO)lz/mg Ahlcal Affobs/kcal mol-1 AH29slkcal mol-I 1 3.565 0.520 132 102 2 3.290 0.479 132 102 3 2.380 0.346 131 101 4 3.365 0.482 130 100 5 4.650 0.659 129 99 mean AHzg8 = 101 kcal mol-I. In view of the deposition of some metal film outside the reaction vessel the mean AH,, = 101 kcal mol-l is regarded as a lower limit corresponding to AH",Os,(CO),, c] < -418 kcal rnol-l J. A. CONNOR H. A. SKINNER AND Y. VIRMANI TETRA -I R I D I UM D OD E cA cA R B oN Y L Ir4(CO)l Thermal decomposition studies on Ir4(CO)12 were made at T2 = 573 and 594 K.The decomposition produced a metallic mirror on the walls of the reaction vessel and a fine black powder within the capillary tube containers. Results are summarized in table 8. TABLE&-THERMAL DECOMPOSITION OF Ir4(CO)1 AHobsI AH2981 expt. 1r I 2/mo Ah/cal T2/K kcal mol-1 kcal mol-I 1 5.765 0.779 594 149 117 2 4.765 0.638 594 148 116 3 5.380 0.760 594 156 124 4 6.680 0.866 573 143 113 mean AHzg8= 118 kcal mo1-'. The mean AH298 corresponds to AH"fIr,(C0)12 c] = -435 kcal mol-' with uncertainty f4 kcal mol-l. DISCUSSION Available knowledge on the enthalpies of formation of metal carbonyls is collected together in table 9. Most of the carbonyls listed are solids at 298 K. The enthalpies of sublimation AHsub of only a few of these compounds have been measured experi- mentally.The bracketted values in table 9 are estimates which we consider acceptable within the broad limits (& 5 kcal mol-l) attached. The values AH:(g) carry both the experimental uncertainty in AH",c) and the uncertainty (experimental or estimated) in AHsub. TABLE~.-ENTHALPIES OF FORMATION OF METAL CARBONYLS AH;/kcal mol-1 AHssut,/kcal mol-1 AH;(g)/kcal mol-1 ref. MONONUCLEAR CARBONYLS -234f 2 17f0.5 -217f 2.5 --183.5f2 -183f 2 9.6+ 0.2* -173f 2 --134f 3 -l50f 1 6.6+0.3* -143.5f 1 -236.5rfI0.5 17.7f 0.3 -218.8f0.6 -229.0+ 0.8 18.1 +0.3 -210.9f 0.9 -(-165) POLYNUCLEAR CARBONYLS -401fl 15+ 1 -386f 1.5 (51 (3) -337f 3 (18+ 5) -319+ 6 this research -442k4 (23f5) -419f 7 this research -298f2 lS+ 1.5 -280f 3 (6); this research -441f4 (23f5) -418f7 this research -459+ 5 (24+ 5) -435f7 this research (-435+3) Wf5) (-4 1 1 +6) this research (485) (28*5) (-457 this research -395f 5 (20k5) -375+ 7 this research (-418+ 5) (25f5) (-393f7) this research (-435+4 (25+ 5) -41Of 7 this researGh * AHvap.HIGH TEMPERATURE MICROCALORIMETRIC STUDIES Table 10 gives the calculated enthalpies of disruption AHdisrupt, of gaseous metal carbonyls into gaseous metal atoms and CO molecules. For a mononuclear carbonyl M(CO), AHdisrupt refers to the process M(CO),[g 2981 -+ M[g 2981 +n CO[g 2981 calculated from AHdisrupt = AH",M SI +n AH",co gl-AH",M(CO)n 91. The metal-ligand bonds in M(CO) are all of the terminal type M-C=O (symbolized in table 10 by T).In a polynuclear carbonyl M,(CO), AHdlsrupt refers to M,(CO),[g 2981 + rnM[g 2981+n CO[g 2981 and includes the contribution from disruption of the metal-metal bonds (symbolized by M)as well as from the metal-to-carbon bonds in the molecule. The latter may include bridging carbonyl linkages M-C-M as well as terminal carbonyl bonds II 0 (each bridging metal-carbon bond M-CO is symbolized by B). The structures I of the polynuclear carbonyls have been determined in the crystalline state 23-31 and the bond descriptions given in table 10 accept that the crystal molecular structures are retained in the gaseous state. The values Tgive the average contribution of a terminal metal-carbonyl bond in a mononuclear carbonyl to the total enthalpy of disruption T = AHdistuptfn.The values given for Band Min thepolynuclear carbonyls of iron were calculated assuming TABLE ~~.-ENTHALPIES OF DISRUPTION OF METAL CARBONYLS carbonyl AHdismpt/kcalmol-1 bonds bond enthalpy/kcal mol-1 MONONUCLEAR 154 22.5 ll8.&2 140.4+2 6T5TST ?" = 25.7k0.4 = 23.7k0.4 T=28.1k0.4 130 +3 4T = 32.5k0.8 l4o.jrfr1 (217) 217.6k 1 255.4+ 1 4T 5T 6T 6'ii F= 35.1k0.3T=36.310.2 T=42.6k0.2 T=(43.4) POLYNUCLEAR 256+ 2 lOT+ M 280+ 6 6T+6fi+R B = 15.4 400+7 lOT+4B+3m = 19.2 272rfr 3 6T+4B+m = 13.8 5071 7 9T+ 6B+ 6m M= 22 579+ 7 (626+6)(833 479+ 7 12T+3M 9T+ 68+ 6M lOT+ El (643_+ 7)729+ 7 12T+ 3m 12"+6m J. A. CONNOR H. A. SKINNER AND Y.VIRMANI that Tin Fe(CO) is transferable unchanged to Fe2(CO)9 and Fe,(CO),, and that the bond enthalpies B and M are likewise transferable from Fe,(CQ) to Fe3(CO)12. Simi- lar assumptions were made in calculating B and M in the cobalt carbonyls CO,(CO)~ and CO~(CO),~. From the T,M and B values so calculated we may note that in the Fe and Co carbonyls M -0.68 T,and that B N +T. If we now assume that these approximate relationships between T M and B apply to other polynuclear carbonyls it is possible to extract the Tand M values for other metals than Co and Fe despite the limited AHdisrupt data available. The values obtained are listed below. The T values for the transition metals of the first row range from 24-35 kcal mol-' for the second row from 36-41 kcal mol-' and for the third row from 43-46 kcal mol-' ; the values are plotted against AH",M g) in fig.1. 46 44 42 7-40 2 38 36 -z 8 34 4 32 R 30 28 26 24 70 90 I00 120 140 160 180 200 dH;(M g)/kcal mol-' FIG.1. Rh6(CO)16 has not been included in the analysis of T M values as thermo- chemical studies are incomplete and also because the unusual structural features 29 raise difficulties of description of the bonding in this compound. Suffice to remark that transference of Tfrom Rh4(CO), to FUI~(CO),~ would appear to lead to signi- ficantly weaker M values in the hexarhodium compound that found in Rh4(CO),2. HIGH TEMPERATURE MICROCALORIMETRIC STUDIES H. A. Skinner Adv. Organometal. Chem.1964,2,49 (Academic Press New York). F. A. Cotton A. K. Fischer and G. Wilkinson J. Amer. Chem. SOC. 1956,78 5168. J. A. Connor H. A. Skinner and Y.Virmani J.C.S. Faraday I 1972 68 1754. D. S. Barnes G. Pilcher D. A. Pittam H. A. Skinner D. Todd and Y. Virmani Int. ConJ Chem. & Uses of Molybdenum (University of Reading Sept. 1973). W. D. Good D. M. Fairbrother and G. Waddington J. Phys. Chem. 1958 62 853. A. Cartner B. Robinson and P. J. Gardner J.C.S. Chem. Comm. 1973 317. E. H. Braye and W. Hubel Inorg. Syntheses 1966 8 178. * W. McFarlane and G. Wilkinson Inorg. Syntheses 1966 8 81. I. Wender H. W.Sternberg S. Metlin and M. Orchin Inorg. Syntheses 1957 5 190. lo P. Chini V. Albano and S. Martinengo J. Organometal. Chem. 1969,16,471.P. Chini and S. Martinengo Inorg. Chim. Acta 1969 3 315. l2 B. F. Johnson J. Lewis and P. A. Kilty J. Chem. SOC.A 1968 2859. l3 S. H. H. Chaston and F. G. A. Stone J. Chem. SOC.A 1969 500. l4 D. R. Stull and H. Prophet JANAF Thermochemical Tables NSRDS-NBS 37 June 1971. l5 D. D. Wagman W. H. Evans V. B. Parker I. Halow S. M. Bailey and R. H. Schumm,N.B.S. Techn. Notes 270,l-4 (1969) Washington D.C. l6 R. Hultgren R. L. Orr P. D. Anderson and K. K. Kelley Selected Values of Thermodynamic Properties of Metals and Alloys (Wiley New York 1963). l7 R. D. Johnston Ph.D. Thesis (Manchester University 1968). 18 D. R. Bidinosti and N. S. McIntyre Canad. J. Chem. 1967 45 641. l9 J. D. Cox and G. Pilcher Thermochernistryof Organic and Organometallic Compounds (Academic Press London 1970).2o D. R. Bidinosti and N. S. McIntyre Chem. Comm.,1967 1. 21 D. A. Pittam and G. Pilcher (to be published). 22 H. J. Svec and G. A. Junk J. Chem. SOC.A 1970,2102. 23 L. F. Dahl and R. E. Rundle Acta Cryst. 1963,16,419. 24 G. G. Sumner H. P. Klug and L. E. Alexander Acta Cryst. 1964 17,732. 25 H. M. Powell and R. V. G. Ewens J. Chem. Soc. 1939,286. 26C. H. Weiand L. F. Dahl J. Amer. Chem. SOC.,1966 88 1821; 1969 91 1351. 27 E. R. Corey and L. F. Dahl Inorg. Chem. 1962 1 521. 28 C. H. Wei G. R. Wilkes and L. F. Dahl J. Amer. Chem. SOC., 1967 89 4792. 29 E. R. Corey L. F. Dahl and W. Beck J. Amer. Chem. SOC.,1963 85 1202. 30 R. Mason and A. I. M. Rae J. Chem. SOC.,1968 778. 31 L. F. Dahl E. Ishishi and R. E. Rundle J.Chem. Phys. 1957 26 1750.

ISSN:0301-5696    

DOI:10.1039/FS9730800018

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

GENERAL DISCUSSION Dr. H. A. Skinner (University of Manchester) said The unexpectedly large in- creases in heat capacity of refractory metals at temperatures approaching their melting points as revealed by levitation “ drop” calorimetry and referred to by Margrave are also indicated by the pulse-heating studies of Cezairliyan at Washing- ton and of Lebedev’ at Moscow. These sharp increases may be due primarily to correspondingly sharp increases in the number of lattice vacancies in the metal structure; this possibility would require a concomitant expansion in volume of the specimen. The levitation technique might have some advantage for density/tempera- ture measurements in that it allows ample time for the heated specimen to achieve the equilibrium density at the observation temperature.The rapid pulse-heating method of Lebedev may be too fast to accommodate fully the volume expansion within the recording time ( s). Are density/temperature measurements con- templated-or feasible-at temperatures approaching the melting-points and beyond of refractory metals? Lebedev has communicated results of recent work at the Institute of High Temper- ature Studies in Moscow which he had intended to present as a paper to this discussion.* The Moscow studies resemble those reported by Cezairliya but the time-scale of the pulse heating used (10-5-10-6 s) was very short compared with the 1 s pulse-heating applied in experiments at the National Bureau of Standards. Refractory metals and alloys in the form of thin wires (diam.0.05-0.15 mm ; length 10-60 mm) were pulse-heated in air and in various insulating liquids by passage of a single near- square current pulse of density -5 x lo6 A cm-2. The wire resistance and the voltage drop across the wire were monitored during the time of melting from en- larged photographs of oscillograms taken from the oscilloscope screen. The start and end of melting was observable with many metals from discontinuities in the oscillograms ; coincidently the start and finish of melting were registered photo- electrically from the surface radiance of the wires. The high-speed pulse technique is understandably less accurate than the slow pulse method but it has proved very useful for the study of melting and for investigation of the liquid state.Because of the extremely short duration the wire specimens are not subject to deformation over the observation period even at temperatures above the melting point. Chemical reaction of the specimen with the ambient medium evaporation of the specimen and thermal losses are negligible over the lo-’ s pulse period. Measurements of the electrical resistivity of solid (ps)and liquid (pJ phases and of the latent heat of fusion (Afus) of the metals W Ta Mo Ir,Nb Rh Pt Fe Ni Au and of a number of alloys have been made by the high-speed pulse heating method. Some results are summarized on p. 30. For several metals (W Ta Fe and alloys W/Re Ni/Cr) the resistivity in the liquid state remained constant almost to the moment of explosion. A slow increase of pl with temperature was found for Mo Ir Nb Pt and Ni; for other liquid metals (Rh Cu Au Al) the increase in p1 with temperature was more rapid.Isthere satisfactory agreement in general between the results obtained by the rapid- ’I. Ya. Dikhter and s.V. Lebedev Teplofizika Vysokokh Temperatur 1970,8,155; 1971,9,929. * Pulse Heating of Metals AHand electrical conductivity of Nb and Rh fusion by S. V. Lebedev. 29 GENERAL DISCUSSION pulse technique with those from the slow-pulse measurements? There appears to be reasonable accord as regards ;Ifusof niobium. Is there more scope for comparison in respect of electrical resistivity? &"a/ metal J g-1 PllPs Au 70 13.1 29.1 2.22 Ni 318 60.8 82 1.35 Fe 244 130 134 1.03 W 299 117.5 127 1.08 Ta 202 114 126 1.10 Mo 430 79.2 93.5 1.18 Ir 200 70.3 92 1.31 Nb Rh 297 257 95.2 61 108.5 85.5 1.14 1.# Pt 128 63.5 94 1.48 Dr.A. Cezairliyan (Nat. Bureau Stand. Washington D.C.) said The transient method described by Lebedev et al. for the measurement of heat of fusion and elec- trical resistivity of metals and alloys during melting is interesting and shows great promise. It is commendable that they could obtain meaningful results in experi- ments of such short duration (10 p). Their results are in reasonably good agreement with those of other investigators. Our recent preliminary work (pulse-millisecond resolution) has yielded a value of 290 J g-l for the heat of fusion of niobium. This value is approximately 2.4 % lower than the value (297 J g-I) reported by Lebedev et al.; however this difference is well within the estimated uncertainties of the re- ported values.Dr. S. V. Lebedev (Inst. High Temperatures Moscow) (communicated) The measured values ps p1 refer to the sample size at room temperature. The estimated limits of error are -2 % for ps p1 and -5 % for &usion. The values of ps p1 and Afusion reported for Au Ni Fe and the values psfor W Ta and Mo are in fair agree- ment with literature values from other types of measurement. The reason for the difference of 5 % in psfor Nb from that obtained by Cezairliyan is not yet clear ; the value obtained for Afusion(Nb) from the same measurement agrees quite well with results reported from use of the magnetic levitation technique.Further details are given in TeploJizika Vysokokh Temperatur 1971,9 635 ; 1973 11 1182 ; and Zhur. Tekh. Fiz. 1972 29 1752. Dr. J. A. Connor (University of Manchester) said Further work has been carried out on the thermal decomposition and iodination of Rh4(C0)12 and of Rh6(C0)16. Thermal decomposition studies on a freshly prepared sample of Rh4(C0)12 were made over the range 485-574 K giving the results summarized in table 1. Measurements on the iodination of Rh4(C0)12 at 516K gave a mean value AH = 30.3 kcal mol-' ,for a reaction approximated by Rh,(CO),,(c 298)+$12(g 516) -+ $Rh13(c 516)+$Rh(c 516)+ 12CO(g 516). No experimental value is available for the enthalpy of formation of RhI,(c); we have estimated a value AH -45 kcal mol-l for the reaction Rh(c)+$I,(g) + Rh13(c) on the basis of comparison with known values for other transition metal halides.Using this the measured reaction heat corresponds to AH -150.3kcal mol-' GENERAL DISCUSSION for thermal decomposition at 516 K,and to AH298 = 126.3 kcal mol-'-in fair agreement with the mean AH298 of table 1. The directly measured AH298 = 124.6(+4) kcal mol-l replaces the preliminary value given in the paper and leads to the revised values AH;[Rh4(CO)12 c] = -442 kcal mol-l AHj![Rh4(CO)12,g] --418 kcal m01-l~ and AHdisrupt633 kcal mol-l. Further studies on Rh6(C0)16 -were made using the original sample of material. Because of the difficulties of accurate analysis of the purity of this compound we continue to regard the results as "preliminary " pending studies on different samples of prepared material.TABLE DECOMPOSITION OF Rh4(C0)12 1.-THERMAL expt Rh.i(COhlmg Ah/cal T2IK i\Hobs/kcal mol-1 AH29dkcal mol-1 1 4.040 0.823 574 152.3 122.0 2 3.805 0.749 574 147.2 116.9 3 3.690 0.756 518 153.2 129.2 4 2.795 0.553 485 148.7 128.3 5 2.690 0.524 524 145.8 120.4 6 3.320 0.682 533 153.6 127.9 7 2.140 0.437 528 152.9 127.3 mean AH2s8= 124.6kcal mol-I The thermal decomposition results on Rh6(C0)16 are summarized in table 2. The mean AH298 = 65.0(&4) kcal mo1-l corresponds to AH;[Rh6(CO)16 c] = -488 kcal mo1-' AHj![Rh6(CO)16tg] -460 kcal mol-l and to AHdlsrupt-836 kcal mol-'. Iodination studies indicated that AHzg8 is -70 5 kcal mol-l some-what higher than measured by direct thermal decomposition (this is in respect of the assumed value for the enthalpy of formation of Rh13).TABLE2.-THERMAL DECOMPOSITION OF Rh,(CO) 1 6 exPt RhdCO)is/mg Ahlcal Tz/K AHobs/kcal mol-1 A H29a/kcal mol-1 1 6.025 0.559 529 98.8 64.1 2 5.800 0.528 518 97.0 64.0 3 7.265 0.704 521 103.3 69.9 4 6.370 0.544 518 92.7 59.7 5 5.280 0.478 513 96.5 64.3 6 5.790 0.500 458 92.0 68.2 mean AH29a= 65.0 kcal mol-1 The enthalpy of disruption of Rh4(C0)12 measures the sum (9T+6B+6M) in the notation used earlier. In this instance in solution at least there is rapid scrambl- ing even at -65°C between bridging and terminal carbonyl groups,l so that the assumption that B -0.5 T receives additional support. If we accept that M -0.68T then T = 39.4 kcal mol-' and M = 26.8 kcal mol-l.The bond analysis of the disruption enthalpy is more difficult for Rh6(C0)16 because the solid-state structure contains four triply bridging carbonyl ligands and also because the molecule is formally an 86-electron system. If the assumptions are made that all bridging carbonyl ligands are bound simultaneously to only two rhodium atoms and that the Rh6 cage obeys the effective atomic number rule then we may equate the total enthalpy of disruption to the sum (12T+8B+ 11M). A J. Evans B. F. G. Johnson J. Lewis J. R. Norton and F.A. Cotton J.C.S. Chern. Cornrn. 1973 807. GBNBPAL DISCUSSION simultaneous solution with the equation relating to Rh4(C0)12 leads to a negative value of M so that one concludes that either M or T changes on going from one molecule to another in contrast to our earlier basic assumption regarding transfer- ability of these quantities.If M is assumed constant in Rh4(CO)1 and Rh6(C0)'6 then T = 34 kcal mol-l in Rh6(C0)1 6 ; if the opposite assumption is made then M = 20 kcal mol-' in R~~(CO)~~. We have shown that there is an apparent linear relation between T and the enthalpy of formation of the gaseous metal atom in that T N (0.28+O.O4)[bHj(M,g)]. Further consideration of the results suggests that M -2[AH;(M,g)]/n where n is the coordination number of M in the bulk metal. Finally the results suggest that M is approximateIy 60-70 % of T. While we do not suggest that there is any direct and necessary relation between M T and AH;(M,g) we believe that these empirical relationships provide a useful index of the bond strengths in polynuclear systems.Employing these relationships it is then possible to make an estimate of the value of T in the binary carbonyls of Pd Pt and Cu ; the values (which are expected to be minima) are approximately 24 35 and 21 kcal mol-l respectively. Carbonyl com- pounds of these elements have been detected spectrometrically in matrices at very low temperatures.' The binary compounds are unstable with respect to [M(c)+ CO(g)]at room temperature. Dr. D. S. Barnes (University of Manchester) said Measurements are being made using a " hot-zone " calorimeter of the enthalpies of iodination of metallic carbonyls to confirm by macro-scale studies some of the results obtained using the micro- calorimeter as described by Connor.It has been confirmed that the product formed by reaction of Cr(CO)6 with excess iodine is essentially the tri-iodide analyzing for CrIn,n > 2.95 < 3.05. With W(CO)6 a product analyzing as W12.,4 was obtained. Dr. H. A. Skinner (Universityof Manchester) said The assumption that the metal carbonyl and metal-metal bond enthalpies are transferable unchanged from Fe(CO) to Fe,(C0)9 and to Fe,(CO), was made of necessity in order to extract values for three unknowns (T and M) from three items of information. The M value so obtained is the average Fe-Fe bond contribution from four Fe-Fe bonds of different lengths (2.46A in Fe,(CO), 2.56A and 2.67A (twice) in Fe,(CO),,). A more sophisticated approach might attempt to discriminate between metal-metal bonds in respect of their lengths as a given bond is expected to be stronger the shorter it is.The problem is complicated however by the association of bridging carbonyls with these metal-metal bonds; thus e.g. the Fe-Fe (2.46A) in Fe,(CO) has three bridging CO molecules assisting it the short Fe-Fe (2.56A) in Fe3(C0)12 has two associated bridging CO molecules but the longer Fe-Fe bonds (2.67A) are acting alone. The latter are longer than the Fe-Fe " bonds " in a-Fe (b.c.c. Fe-Fe = 2.48A) ; in Ru3(C0)12 and OS~(CO)~~ all the metal-metal bonds are " unassisted " and are significantly longer than in the metals themselves (Ru-Ru = 2.85A in RU,(CO)~, against 2.65-2.71A in ruthenium metal ; 0s-0s = 2.88A against 2.67-2.73A in osmium metal).The dissociation energy in the diatomic gaseous molecule Fe, is reported as 25 kcal mol-l but the bond length is not known. The Co-Co bond lengths in CO,(CO)~ (2.52A) and CO~(CO)~~ (2.49A) are similar to the bond length (2.51 A) in cobalt metal-and there would appear to be E. P. Kiindig D. McIntosh M. Moskvits and G. A. Ozin J. Amer. Chem. Suc. 1973 95 7234 (and references therein). S. S. Lin and A. Kant J. Plzys. Chem. 1969 73 2450. GENERAL DISCUSSION little scope for variability of M in these compounds. In Rh4(C0)12 the Rh-Rh bond lengths vary over the range 2.7-2.8A (average 2.73A) and are marginally shorter than in Rh6(C0)16 (Rh-Rh = 2.78 A). The metal has Rh-Rh = 2.69 A. Cocke and Gingerich have recently reported a value 66+6 kcal mol-l for the dissociation energy of Rh,(g)-substantially larger than the value (26.5 kcal mol-l) adopted for M in the present paper.It is probable that the bond length in Rh is very short (the estimated value = 2.28A) compared with Rh-Rh bond lengths in the carbonyls. Analogously the bond length in RhC(g) of 1.61 A is very short com- pared with the bridging Rh-C- bond length in Rh6(C0)16 (2.17 A). II 0 In view of the limited amount of data yet available on dimetal molecules M2 the only general starting point for estimation of M-M bond contributions is from the heats of atomization of the pure metals. For example Rh metal has AHsub-133 kcal mol-1 at 298 K and is f.c.c. with C.N = 12 in the solid state at room tempera-ture supposing that the major contribution to the cohesive energy of the metal TABLE1 -metal "metallic " bond M Mn 11 16 Re 30.7 30.5 Fe 16.6 19.2 Ru 25.6 28 0s 31.5 31.1 co 16.9 22 Rh 22.2 26.5 Ir 26.5 31 originates in the metallic bonding between adjacent Rh atoms in the lattice then each Rh-Rh bond (2.69 A) contributes -133/6 N 22 kcal mol-I.The "metallic bond " is electron deficient with respect to a normal covalent bond M-M which might therefore be expected to be both shorter and stronger than its metallic counter- part. " Metallic '' bond-strengths (values in kcal mol-I) calculated in this way compare with the M values given in the paper are given in table 1. D. L.Cocke and K. L. Gingerich J. Chem. Phys. 1972,57,3654.

ISSN:0301-5696    

DOI:10.1039/FS9730800029

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Thermodynamics of PbO+P205 Melts BY J. H. E. JEFFES" AND A. E. M. WARNER Dept. of Metallurgy Imperial College of Science and Technology London SW7 2BP Received 10th September 1973 The heats of solution of PbO have been measured in a range of PbO+Pz05 melts. The heats of solution of PzOs in the same melts have been determined indirectly. The integral heats of mixing of such melts are compared with the integral Gibbs energies of mixing determined previously by e.m.f. measurements. In a previous communication the activities of PbO in a variety of PbO +P205 melts were reported over a range of temperatures. For PbO +SiO and PbO +B203 melts there exist data on activities and also on heats of mixing of these melts so that it is possible to deduce partial and integral thermodynamical quantities for the com- ponents of such melts by means of "third law " calculations as well as those deter- mined by " second law" calculations based on the temperature coefficients of the activities of their components.The data on PbO+SiO and PbO+B,O melts are given in ref. (2-5). The PbO+Pz05 system is a particularly interesting one because it is the only polyanionic melt system in which the basic component is present as an ionized species and in which the activity of the base has been determined and also the chain length distribution and the glass transition temperature of the cooled melt. This enables useful conclusions to be drawn about the relationship of the polymeric species present in such melts and the activity of its components and has been the subject of of a recent analysis by Cripps-Clark et aL6 Because of the importance of this melt system it was decided to carry out calorimetric measurements on it which would enable its thermodynamical mixing pfoperties to be calculated.(Throughout this paper 1 cal = 4.184 J.) The heat effects accompanying the dissolution of PbO in a variety of PbO +P205 melts were measured from the composition xpbo = I to xpbo = 0.5 in a high- temperature twin-differential calorimeter of the type used by Kleppa and co-workers. *' The equipment used in the present series of experiments was that described by Warner Roye and Jeffes.8 At compo3itions richer in P205 than PbO . Pz05 the activity of P205becomes so great that it is lost from the melt at appreciable rates and this sets a limit to the acidity of melts which can be used in such a calorimeter.For the same reason it is not possible to measure the heat of solution of P205 in these melts directly as can be done for the systems PbO+SiO and PbO+B203. It was neces- sary therefore to determine the heat of formation of PbO .P205(Z) separately. This was done by determining the heat of the reaction PbO +ZnO .PzOs(gZ) = ZnO +PbO .P205(gZ). Since the heat of formation of ZnO .P205 glass has been measured at room tempera- ture it is then possible to calculate the heat of formation of PbO . P2O5(gZ) and this 34 J. H. E. JEFFES AND A. E. M. WARNER has been done in the appendix to this paper. The enthalpies of PbO and PbO .P205 liquids at 1273 K have also been determined relative to 298 K.From these measure- ments AH; for PbO .P20,(Z) at 1273 K was calculated. It is unfortunate that such an indirect method had to be used to calculate the heats of solution of P205in these melts since the uncertainties of the derived values are correspondingly large. EXPERIMENTAL Pb0.-Yellow A.R. PbO powder was compressed isostatically into spherical pellets of about 0.5 g weight using the procedure described in ref. (8). The pellets were annealed before use. PbO+ P205 GLASSES.-These were prepared by heating intimate mixtures of PbO and (NH4)H2P04in suitable proportions ;the mixture was first heated very slowly in platinum to drive off the ammonia and then heated until a clear melt was obtained. This was poured on to an aluminium plate to produce a glassy material which was stored in a desiccator until required.ZnO+PzO5 GLASSES.-These were produced in a manner analogous to that used for PbO+ P205 glasses. The samples used were analyzed by the methods described in ref. (6). In order to be able to measure the heats of solution of PbO in liquid melts across the composition range PbO x = 1.0-0.5 it was necessary to operate the calorimeter at a tempera-ture of 1273 K. Because the operating wells in the calorimeter used had a small diameter the platinum crucibles used in them had an internal diameter of 9 mm. This was found rather small to carry out isothermal solution experiments and so samples of PbO were dropped into the melt from room temperature; since the enthalpy of PbO had been previously determined on the same apparatus,8 it was possible to calculate the isothermal heat of solution of the PbO in the melt.The weights of the PbO samples dropped into the calorimeter were not negligibly small compared with the weight of the solution in the calorimeter as is necessary for the accurate measurement of the partial heat of solution. In these experiments the change in the composition of the melt during an experiment varied from 0.02 to 0.05 in the mol fraction of PbO. It was assumed that the heat effect observed corresponded to the final composition of the melt. All calorimetric runs were immediately preceded or followed by a calibration by dropping in a suitable platinum sphere from room temperature and comparing the areas under the two (e.m.f.time) plots. RESULTS The results of the calorimetric experiments are given in fig. 1 in which the partial molar heats of solution of PbO are plotted as a function of melt composition. Also shown in fig. 1 are the partial molar heats of solution of PbO deduced from the temperature coefficients of the measured activities of PbO in such me1ts.l DISCUSSION Using the value of -33.OfI2.1 kcal for the heat of the reaction PbO(l)+$P401&) = PbO .P205(1) given in the appendix the values of can be calculated by Gibbs-Duhem integration from the values of AHpboin fig. 1. These values are given in fig. 2. From the data in fig. 1 and 2 the integral heats of mixing PbO and +P,O,,(g) can be calculated and these are shown in fig.3. Also shown in fig. 3 are the values THERMODYNAMICS OF PbO+P20 MELTS 1.0 0.9 0.a 0.7 0.6 0.5 mol fraction PbO FIG. 1.-Partial molar heats of solution of PbO in PbO+P205 melts. 0,Calorimetric measure- ments ; x ,deduced from e.m.f. measurements. FIG.2.-Partial molar heats of solution of +P401&) in PbO+P205 melts. of the integral Gibbs energies of mixing these species determined by e.m.f. measure- ments. The latter are only approximate because the activity of P205 in the PbO+ P205melt was estimated from the fact that at Xpbo = 0.55 the reaction 5Pb +(PZO,) = 5(Pb0) +P2(g) J. H.E. JEFFES AND A. E. M. WARNER resulted in the production of phosphorus vapour at about 1 atm pressure.l The error limits shown for AHMrepresent experimental scatter ; the uncertainty due to the value of AHMfor PbO .P205 is indicated on the right-hand side of the diagram. The negative values for ASM are presumably due to the fact that one of the reacting species is a gas. The values for these thermodynamical quantities have been calculated to a hypothetical standard state for +P4Ol0(s) using the heat of sublimation data in the JANAF Tables assuming that the heat and entropy of sublimation at 1273 K are the same as at the sublimation point. These are shown in fig. 4. 1.0 0.9 0.8 0.7 0.6 0.5 mol fraction PbO FIG.3.-Integral heats and Gibbs energies of mixing of PbO and +P4Ol0 at 1273 K relative to PbO(Z) and +P4Ol0(g) as standard states. 0,AHM ; x ,AGM. The values of the entropies of mixing of PbO and P205(s) thus calculated are also shown in fig.5 together with the values for the systems PbO + SO2 and PbO + B203. The large positive values of ASMfor the PbO+P205 system at xPb0 = 0.7 and 0.6 are questionable and attempts will be made to measure the activity of P205 in PbO+Pz05 melts more accurately so that more reliable values of AGM can be calculated. Chain distribution analysis of acidic PbO +P,05 glasses indicate that the ionic species are present to an extent predicted by random reorganisation except for the lowest members ofthe polyanionic species? The partial heat of solution of PbO in PbO + Pz05 melts is small from Xpbo = 1.O to 0.85. Sridhar and Jeffes pointed out that PbO+P205 melts showed strong negative deviations from the ideal Temkin model assuming that Pb2+ 02-,and POI- were the ionic species present and that the phosphate species POS- would better agree with the observed activities.The orthophosphate composition if the latter ion predominates is at xpb0= 0.83. THERMODYNAMICS OF PbO+P,O MELTS -4 -0 r( -12 k -16 CI cd 3 s-20 u a E -24 % a -28 -32 1.0 0.9 0.8 0.7 0.6 0.5 mol fraction PbO FIG. 4.-Integral heats and Gibbs energies of mixing of PbO and 3P4Ol0 at 1273 K relative to PbO(2) and +P4Ol0(s) as standard states. .8 .6 .4 .2 mol fraction PbO FIG.5.-Integral entropies of mixing of PbO(2) with SOl (O) Bz03(A) and P205 ( x). APPENDIX DETERMINATION OF AH; OF LEAD METAPHOSPHATE The heat of the reaction PbO+ ZnO .P20,(gZ) = ZnO+ PbO . P20,(gZ) at 298 K was determined by dropping samples of the reactants and of the products into a melt of composition 9PbO+3CdO+4Bz03 as recommended by Kleppa and referred to as " Kleppa solvent " maintained at 1073K in a platinum crucible in the calorimeter. The samples dropped into the melt were chosen so as to produce a similar composition of melt when either the reactants or the products were dissolved. Because the materials were dropped into the melt at 298 K the difference between the corresponding heat effects re-presented the heat of the above exchange reaction at 298 K. The results obtained in these experiments are summarised in table 1. J. H. E. JEFFES AND A. E. M. WARNER From these results the heat of the above reaction was calculated to be -9.23& 2.39 kcal.Meadowcroft and Richardson give the heat of formation of ZnO . P2OS glass at 308 K as -45.40+2.0 kcal mole-l. Assuming that the heat of formation of this glass at 298 K is the same as at 308 K this combined with the data in table 1 give for the reaction PbO(s)+P20,(s) = PbO. P205(gZ) AH,", = -54.65k3.11 kcal. Since the heats of solution of PbO in PbO+Pz05 melts were measured at 1273 K it is necessary to convert the above value to this temperature. This was done using our own TABLEI material no. of expt. heat of solution/(kcal mol-I) PbO 2 13.73k0.02 ZnO 2 1 1.18+ 0.17 PbO . P2Oj ZnO . P205 2 4 -7.07+ 0.40 -18.86k2.35 values for the enthalpy of PbO (18.30k0.9 kcal) and for that of PbO .P205glass (52.91k 2.0 kcal). The enthalpy of phosphorus pentoxide was taken from the JANAF TabZes (46.0+ 1.0 kcal). This gave the heat of the reaction PbO(Z)+~P4010(g)= PbO . P205(Z) AH,",,,K = -66.04k4.17 kcal. The authors are indebted to the Science Research Council and to the North Atlantic Treaty Organisation. R. Sridhar and J. H. E. Jeffes Trans. Inst. Min. Met. C 1969,78 14. G. Papst and H. Schmalzried 2.phys. Chern. N. F.,1972,82,206. T. Ostvold and 0.J. Kleppa Inorg. Chem. 1969 8,78. R. Sridhar and J. H. E. Jeffes Truns. Inst. Min. Met. C 1967 76 44. 'J. L. Holm and 0.J. Kleppa Inorg. Chern. 1967 6 645. C. J. Cripps-Clark R. Sridhar J. H. E. Jeffes and F. D. Richardson (The Richardson Conference 1973) Trans. Inst. Min. Met. 1974 233. '0.J. Kleppa J. Phys. Chem. 1960,64,1937. A. E. M. Warner M. P. Roye and J. H. E. Jeffes Trans. Insf. Min. Met. 1973 (hpress). T. R. Meadowcroft and F. D. Richardson Trans.Furaduy Soc. 1963,59 1564.

ISSN:0301-5696    

DOI:10.1039/FS9730800034

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Thermodynamic Properties and Phase Diagrams of the Fe-Co and Ni-Pt Systems BY K. C. MILLS,"M. J. RICHARDSON AND P. J. SPENCER National Physical Laboratory Teddington Middlesex Received 22izd October 1973 Heat capacities for Fe-Co and Ni-Pt alloys of various compositions have been measured between 300 and 1130 K. The enthalpies of the orderdisorder transformation occurring in these alloys were determined from these measurements. These enthalpy of transformation values were combined with extant thermodynamic data for the disordered phase to calculate thermodynamic properties for the ordered phase which were then used to derive more information on the equilibrium diagrams for the Fe-Co and Ni-Pt systems. Iron-cobalt and nickel-platinum alloys undergo order-disorder transformations over certain composition ranges in the solid state.The boundaries of the ordered and disordered phases in these systems are not known with certainty despite the com- mercial significance of the alloys. The aim of the present study was to obtain thermodynamic information which could be combined with extant data to calculate reliable order-disorder phase boundaries for the Fe-Co and Ni-Pt systems. For lower temperatures calculated boundaries are often more reliable than those obtained from metallographic examination for the latter frequently correspond to non-equilibriumconditions arising fromdiffusional delays. In contrast the thermodynamic values necessary for calculation of the same boundaries can be measured at higher temperatures where diffusion is sufficiently rapid to achieve equilibrium conditions then extrapolated to lower temperatures.Elliott has reported a phase diagram for the Fe-Co system which contains the three solid phases ' FeCo ' ' Fe,Co ' and ' FeCo '; however Hultgren has proposed that only the 'FeCo ' phase exists as the evidence for the 'Fe,Co ' and ' FeCo,' phases is tenuous. Asano et aL3 employed a variety of techniques to investigate the structural changes in Fe-Co alloys but were unable to obtain any positive evidence for the existence of ' Fe,Co '. The ' FeCo ' phase undergoes an order-disorder transition around 1000 K when the ordered (a') b.c.c. structure is transformed into a disordered (a) b.c.c. structure. Heat capacity measurements have been reported 3-10 which show that the Cp(T) curve undergoes a rapid increase of Cp during the order-disorder transformation culminating in a peak in the Cp(T)curve at the critical temperature T,.However the Cpmeasurements also revealed the elcistence of a small peak in the Cp(T)curve around 820 K ; this is known as the '820 K anomaly '. Gormankov et all1 reported neutron diffraction studies which indicated that the degree of long-range order de- creases in the temperature range 770-820 K. Yokoyama et aL6 have reported that the temperature at which the anomaly occurs increases with increase in heating rate and have proposed that this effect is caused by the disordering process requiring adequate time for equilibrium to be attained. Thermodynamic data for the disordered (a) phase have been determined by Satow 40 K.C. MILLS M. J. RICHARDSON AND P. J. SPENCER et aZ.,12 who measured the Gibbs energy change AG for the reaction Fe(in alloy)+ H,O(g) = FeO(s)+H2(g). However these data were considered unreliable by Hultgren et aL2in their assessment of the thermodynamic properties of the Fe-Co system. Thus the thermodynamic data for the disordered (a) phase employed in this study are those obtained by Kaufman and Nesor l3 from combination of the calorimetric enthalpy of formation values due to Muller and Hayes l4 with known characteristics of the phase diagram. Thermodynamic data for disordered (a) phase alloys were combined with enthalpies and entropies for the order-disorder transformation (AHtrans and AS,,,,) determined in the present work to yield thermo- dynamic data for ordered (a’) phase alloys and by calculating AG(x) curves for both the ordered and disordered phase at a series of temperatures the equilibrium order- disorder phase boundaries were obtained by the common-tangent method.Hansen and Anderko l5 have reported that the Ni-Pt phase diagram contains two ordered phases the cubic ‘Ni,Pt ’phase and the tetragonal ‘NiPt ’phase both of which transform into a disordered f.c.c. structure at higher temperatures. However the phase boundaries of neither ordered phase are known with certainty. Gibbs energies for the disordered phase have been measured by galvanic cell studies l6 but the thermodynamic data listed by Hultgren et aL2have been employed in the present calculations; these data are based on AH; values and nickel vapour pressure measure- ments due to Walker and Darby.17 The data have been combined with the present measurements to yield more information on the equilibrium diagram of Ni-Pt.EXPERIMENTAL MATERIALS The alloys 1-9 listed in table 1 were the samples used in the calorimetric studies made by Miiller and Hayes l4 and the preparation of these alloys has been fully described.14 Alloys 10-12 were kindly donated by Dr. H. B. Bell of Strathclyde University and were prepared by high-frequency melting of weighed mixtures of iron and cobalt powders in an atmosphere of hydrogen. TABLETH THE COMPOSITION OF THE Fe-Co ALLOYS alloy 1 2 3 4 5 6 7 8 9 10 11 12 nominal XF~ 0.3 0.4 0.48 0.5 0.5 0.5 0.52 0.6 0.7 0.25 0.5 0.75 nominalxc, 0.7 0.6 0.52 0.5 0.5 0.5 0.48 0.4 0.3 0.75 0.5 0.25 wt.%C 0.006 0.005 0.012 0.004 0.005 0.006 0.006 0.006 0.005 0.006 0.007 0.008 TABLE2.-THE COMPOSITION OF THE Ni-Pt ALLOYS alloy 1 2 3 4 5 6 7 XPt 0.2234 0.2542 0.2848 0.3972 0.4430 0.4886 0.5192 XNi 0.7766 0.7458 0.7152 0.6028 0.5570 0.5114 0.4808 All alloys were annealed for 4 weeks at 730 K in evacuated silica capsules and slowly cooled to room temperature. The alloys were chemically analysed for carbon the results being given in table 1. The Ni-Pt alloys were provided by Johnson Matthey Co. and were prepared by melting a mixture of high-purity elements in an atmosphere of argon using an arc-furnace with a non-consumable electrode. The samples were turned over and remelted several times to ensure that homogeneity of the samples had been fully attained.The chemi- cal analyses of these alloys were carried out by Johnson Matthey Co. the results being given in table 2. The alloys were annealed for 6 weeks at 700K in evacuated silica capsules and slowly cooled to room temperature. HEAT CAPACITIES FOR Fe-Co AND Ni-Pt ALLOYS THERMAL MEASUREMENTS The measurements were made using a Perkin-Elmer differential scanning calorimeter (DSC) model 2. The controls of the DSC were adjusted to extend the upper limit of the temperature range to 1130 K. The output signal from the DSC was measured with a digital voltmeter and recorded on paper tape. The treatment of the data to yield Cpand enthalpy values was identical with that described previous1y.l8 To check the method the heat capacity of silver was measured between 300 and 800 K,and the experimental Cpvalues obtained were in excellent agreement with the assessed CJT)relationship reported by Hultgren et aLL9 The scatter of individual experimental points from the reported CJT)curve had a standard deviation of 0.5 %.Temperatures are based on the IPTS-68 scale. RESULTS In this study " one mole " refers to the species Fe,Co,- and Ni,Pt,-,. R was taken as 8.3143 J K-l mol-l and the molar masses were taken from the IUPAC Tables (1970). The heat capacities of the Fe-Co alloys are shown in fig. 1-4 and thermodynamic properties for these alloys are listed in table 3. In this investiga- tion with a heating rate of 0.3333 K s-l the ' 820 K anomaly ' was displaced to 869K for Feo,5Coo.5.The heat capacities of the Ni-Pt alloys are shown in fig. 5 and 6. D lOOr I 300 500 700 900 1100 TlK FIG.1.-The heat capacity of Feo.sCoo.s. Values of AHtrans and AS,,,, for the order-disorder transformation were calculated from the CJT)curves. However the value calculated for AHtrans is dependent upon the value used for the heat capacity of the ordered phase C,(ord) in the transformation region. The Cp(T)curve for Feo,sCoo.5 shown in fig. 1 exhibits the following charac- teristics (a) a smooth CJT) relationship for the ordered phase (region AB) (b) a sudden increase in C culminating in a peak (at C) (c) a further increase in Cp culminating in a peak (at D) (d)a smooth Cp(T)relationshipfor the disordered phase (EFG).In this study measurements were restricted to temperatures up to 1130 K 0 300 500 700 900 1100 TIK FIG.2.-The heat capacity of Fe0.3C00.7 Feo.4cOi).6 and Feo.4sCoo.s2 ; --Feo.sCoo.,; --- Feo.4Coo.6; -y Feo.48Coo.52. t 101 1 1 I I I I I I 300 500 700 900 1100 TIK FIG. 3.The heat capacity of Feo.szCoo.48 Feo.6Coo.4 and Feo.7Coo.3 ; - Feo.5zCoo.4a ; --- Feo.&o0.~; --,Feo.7Coo.3. TABLE 3.-THERMODYNAhfIC PROPERTIES OF FeCO ALLOYS T/K 400 2 624 7.56 2 546 7.34 2 541 7.33 2 556 7.37 2 568 7.40 500 5 341 13.62 5 188 13.23 5 130 13.10 5 214 13.29 5 243 13.36 600 8 197 18.82 7 991 18.33 7 858 18.07 8 038 18.44 8 101 18.57 700 11 222 23.48 10 963 22.91 10 746 22.52 11 037 23.06 11 157 23.28 800 14 851 28.32 14 286 27.34 13 900 26.72 14 216 27.30 14 416 27.62 900 18 659 32.81 18 392 32.16 18 100 31.63 18 424 32.23 18 561 32.49 loo0 22 413 36.76 23 429 37.48 24 332 38.13 24 730 38.75 22 758 36.92 1100 26 324 40.49 27 684 41.53 29 542 43.11 28 600 42.00 26 854 40.82 804k 5 9423- 5 10073-5 9763- 5 870&5 K.C. MILLS M. J. RICHARDSON AND P. J. SPENCER and it was not possible either to establish or predict with accuracy the C,(T)relation-ship for the disordered phase (curve FO in fig. 1) for the various alloys. Thus it was not possible to determine C,(ord) in the transformation range (curve BJF) with cer-tainty. For this reason the C,(ord) in the transformation interval is assumed to be c I 500 700 900 110C' T/K ; ; FIG.4.-The heat capacity of Feo.t5C00.75 and Feo.75Coo.25 -- Fe0.25C00.75 -- Fee.7 5C00. 5. given by the line BHE in fig 1. This assumption could readily be applied to all the alloys studied and is denoted as method 1. Values of AHtrans and AStranswere calcu- lated respectively by evaluation of Tat J (Cp-Cp(ord)) dT and Tat B Tat B thus AHtrans is obtained from the area enclosed by BCDKEHB in fig. 1.t Pepperhoff 20' 300 500 700 900 1100 TIK FIG.5.-The heat capacity Of Nio.78Pt0.22 and Ni0.7zPt0.28 ; --,Ni0.78Pf0.22; Ni0.72Pf0.28. -9 f It was not possible to assign a reliable value to AHtrans from the plot of (HT-H~~~) against temperature. HEAT CAPACITIES FOR Fe-Co AND Ni-Pt ALLOYS and Ettwig have reported heat capacities for the temperature range 600-1250 K which are in good agreement with the present measurements ; the CJT) relationship for the range (1 100-1200 K) reported by these workers can be combined with the CJT)relationship obtained in the present study to yield a smoooth relationship for C,(ord) against T in the transformation range.Pepperhoff and Ettwig obtained AHt,,, from the area enclosed by BCDKJB in fig. 1 ; this is referred to as method 11. Values of AHtranscan only be obtained by method I1 for alloys of composition Feo.5Coo.5 and these are 100 J mol-' greater than those obtained by method I. The plot of AH,,,, against composition for the Fe-Co system is given in fig. 7. The value of AHtransobtained from the present work for Feo.4C00.6 was lower than that predicted from the smoothed AH,,,,,(x) curve and the values of AHtransused in the calculation of the equilibrium phase boundaries were taken from the smoothed AHt,,,&) curve.The values of AH:,,, and AS,,,, for the order-disorder transforma- tions in Ni-Pt alloys were calculated in an identical manner to those for the Fe-Co system. To calculate the order-disorder phase boundaries in the Fe-Co and Ni-Pt systems the following procedure was adopted. The experimental values of AH,,,, and AS,,,, for Fe-Co and Ni-Pt alloys were combined with AH; and AS; values for the disordered phase 139 at the appropriate alloy compositions to yield AH; and AS; values for the ordered phase. AG; values were then calculated for the two phases at K.C. MILLS M. J. RICHARDSON AND P. J. SPENCER m 4-4 1 -8 +-.y -$ % a 2-d 0 . I I 8 selected temperatures using the equation AG; = AH; -TAS; and AG(x) curves were hence derived for each temperature. Equilibrium boundaries were obtained by draw- ing common tangents to the AG(x) curves.2o*21 The slightly larger AHtransand AS,,,, values obtained by method I1 would have little affect on the calculated phase I100 900 700 500 0 0.2 0.4 0.6 0.8 xco FIG.8.-Calculated order-disorder phase boundaries for the Fe-Co system. HEAT CAPACITIES FOR Fe-Co AND Ni-Pt ALLOYS boundaries as the increased AH,,,, and AS,,,, terms in the equation AG = AH-TAS tend to cancel each other.The relevant portions of the equilibrium diagrams for the Fe-Co and Ni-Pt systems are given in fig. 8 and 9 respectively. 9 oc 0 00 700 6 OC 5 00 400 0.4 0.6 0.8 xpt FIG. 9.-Calculated equilibrium diagram for the Ni-Pt system; - caIcuIated boundaries ; --,tentative boundaries for the aft' and a" phases. DISCUSSION Fe-Co SYSTEM The heat capacities of the alloys for the temperature range 300-400 K are within 43% of the values calculated by Kopp's rule.? The experimental values of Cp recorded in this study for Feo,5Coo.5 agree with the values obtained by Pepperhoff and Ettwig * to within about 1 % and are in good agreement with preliminary Cp measurements made by Normanton et al.l0 However the Cpvalues recorded here are about 7 % lower than values reported by Japanese worker^,^-^ 10 % lower than the values recorded by Castanet and Ferrier,' and 5 % higher than the preliminary Cpvalues determined by Sale.g The CJT)curve for Feo.2,Co,., (fig.4) shows two small peaks at 760 and 980 K corresponding to the a' -+ a transformation and the onset of the a 4y transformation (see phase diagram 2 respectively.The peak in the Cp(T)curve for Feo.,5Coo.25 at 760 K is presumably associated with the a' -+a transformation but no ready explanation can be given for the small Cppeak observed at 880 K. The scatter of Cp values obtained with these two alloys (+2 %) was greater than that usually observed. t At higher temperatures both iron and cobalt undergo transformations at 1033 and 740 K respectively; at temperatures where the C' values of the elements are influenced by these transfor- mations agreement between Kopp's rule and the experimental values of C of the alloy would not be expected.K. C. MILLS M. J. RICHARDSON AND P. J. SPENCER The values of AHtrans for the order-disorder transformation were calculated by assuming that the ' 820 K anomaly ' is an integral part of the total enthalpy required for the a' a transformation (cf. ref. (7)). The values of AHtrans obtained depend upon the base line used i.e. C,(ord) ; however values obtained for Feo.5Coo.5 by methods I and I1 of 4.1 1 k0.2 and 4.21 k0.2kJ mol-1 respectively are in good agree- ment with the value AH,,,, = 0.5RTc = 4.17 kJ mol-' calculated by the Bragg- Williams theory.22 Values of AHtrans reported by other investigations are shown in fig.7 and are in reasonable agreement with those obtained in this study. Most order-disorder transformations in metallic systems are considered to be "first-order transitions ",2 e.g. the Ni-Pt tran~formation.~~ Although the trans- formation in Fe-Co alloys has been reported as a " second-order transition " the " order " of the transition cannot be considered to be firmly e~tablished.~~ A diffi-culty encountered with the application of DSC to the study of order-disorder trans- formations is that the dynamic nature of the method inevitably produces a broadening of the Cppeaks. Consequently it is difficult to distinguish some "first-order transi- tions " from " second-order transitions ".The equilibrium boundaries between the ordered and disordered phases have been calculated on the assumption that the trans- formation in Fe-Co alloys is "first-order ". Thus the ordered and disordered phases are separated by a two-phase region.23 However the phase boundaries shown in fig. 8 could still be considered to define the transformation limits even if a " second-order transition "pertained. Fig. 8 indicates that the ' FeCo ' phase occurs between Fe,.,Co,. and Fe0.7C00.3. No evidence was found for the existence of ' Fe3Co ' and ' FeCo ' phases; in fact from fig. 8 the compositions Fe0.,SCo0.25 and Feo.25Coo.,5 lie within calculated two-phase regions. It is for this reason that AHtrans values for these compositions were not shown in fig.7 as the measured AH,,,, values probably do not correspond to a complete order-disorder transformation. At temperatures below 800 K the boundary of the orderedphasefor iron-rich compositions was calculated to turn inwards markedly towards the equimolar composition. This behaviour is improbable. The extant thermodynamic data for the disordered phase are not sufficiently accurate to merit drawing the boundary in this manner and hence the boundary has been constructed by a smooth extrapolation of the high-temperature boundary. It is illustrated by a dotted line in fig. 8 to indicate the uncertainty associated with its position. Yokoyama et aL6 noted that the position of the anomalous peak around 820 K observed in dynamic measurements of heat capacity was dependent upon the heating rate used.They attributed the ' 820 K anomaly ' to the slowness of the disordering reaction and applied the Bragg-Williams theory 22 to account for their observations. If the sluggishness of the disordering process is such that an alloy at temperature T results in a dynamic degree of order corresponding to an equilibrium degree of order for a lower temperature 8 then it can be shown from theory 22* that eqn (1) is applicable where E is the activation energy k the Boltzmann constant and A a con- stant with a computed value of In [-(8-T)]= In A +In (dO/dt) +E/RT. (1) In this study peak temperatures of 852f5 869f5 880+5 and 887f5 K were observed for Feo,5Coo,5 with heating rates of 0.167 0.333 0.667 and 1.333 K s-' respectively.The best fit of these data and data from the literature could be obtained with E = 199 kJ mol-l and A = 1.7 x 10-lo in reasonable agreement with the values obtained by Yokoyama et aL6 HEAT CAPACITIES FOR Fe-Co AND Ni-Pt ALLOYS Ni-Pt SYSTEM The C,(T) curves for Ni-Pt alloys in the ‘ Ni3Pt ’ phase field (fig. 5) show two Cp peaks the first corresponding to the Curie temperature and the second to the trans- formation of the ordered cubic (a’) phase into a disordered f.c.c. (a) structure. The Cpvalues calculated by Kopp’s rule are within 2 % of the experimental value for the ‘NiPt ’phase. The C,(T)curves (fig. 6) for alloys of composition Ni0.51Pt0.49 and Ni0.48Pt0.52 in the ‘NiPt ’ phase field show two C peaks at high temperatures.It is proposed here that the first peak is associated with the transformation of the ordered tetragonal (a”) phase into an (a’”) superstructure and the second peak with the transformation of this (a”’) phase into the disordered f.c.c. (a) structure. The CJr) curves for both the other two alloys in the ‘ NiPt ’ phase field show only one Cppeak. However at temperatures slightly above the peak temperature where Cp is decreasing rapidly with temperature there is a small arrest. This arrest could correspond to the formation of the a”’ phase but might also be the result of experi- mental uncertainties associated with individual C values. The evidence for the a’‘’ phase is discussed below. The equilibrium diagram for the Ni-Pt system in fig. 9 is not complete as no structural observations of the a”’ phase field have yet been made.The boundaries of the ‘ Ni3Pt ’ phase were calculated from the experimental AHtrans and AS,,,, values for the three appropriate alloy compositions in an analogous manner to that deccribed for Fe-Co alloys. Unfortunately the position of the maximum in the AH,,,,,(x) plot could not be accurately determined from these three measurements alone with the result that the AG(x) curve for the ordered phase cannot be defined with certainty. Thus the calculated phase boundaries of the ‘Ni,Pt’ or a’ phase are estimated to be accurate to only +3 mol %. The only direct evidence for the existence of the a”‘ phase is the observation of the two maxima in the CJT) curve. However the ordered (a”) phase for ‘NiPt ’ is iso- typic with the ordered f.c.tetragonal phase (a;) for ‘ AuCu ’,and the phase diagram proposed for the Au-Cu system 15*24 shows that with increasing temperature the f.c. tetragonal structure is transformed first into an f.c. orthorhombic superstructure and that at higher temperatures a; transforms into the disordered f.c.c. phase (a). Hirabayashi 25 reported a C,(T) curve for ‘AuCu’ which showed two C peaks corresponding to the temperature for the two transformations. The behaviour of the ‘ AuCu ’ phase alloys appears to be similar to that observed here for the ‘NiPt ’ phase alloys. It was not possible to resolve the twin-peak area of the C,(T)curve (fig. 6) with anv certainty into two AH,,,, values corresponding to the a”-+ a”‘ and the a’” -+a transformations.Thus it was not possible to calculate phase boundaries for a” and a”’ in the usual way. Therefore we have proposed tentative phase boundaries for these phases in fig. 9 based on the observed temperatures of the Cpmaxima and reference to the phase diagram 23 of the Au-Cu system. We thank Johnson Matthey and Co. Ltd for providing the Ni-Pt alloys and Dr. H. B. Bell (Strathclyde University) for providing some Fe-Co alloys. We are grateful to D. G. Nunn and E. B. Lees for rendering assistance and to Dr. E. J. McLaughlan for the chemical analyses of the Fe-Co alloys. We also thank Dr. R. Castanet (Marseille) Dr. F. Sale (Manchester University) Dr. A. S. Normanton Dr. R. Buckley and Prof. B. B. Argent (Sheffield University) and Dr.J. F. Counsel1(NPL) for helpful discussions. K. C. MILLS M. J. RICHARDSON AND P. J. SPENCER R. P. Elliott Constitutionof Binary Alloys 1st suppl. (McGraw-Hill 1965). R. Hultgren R. L. Orr P. D. Anderson and K. K. Kelley Selected Values of Thermodynamic Properties of Metals and Alloys (John Wiley New York 1963) suppl. sheet May 1971. H. Asano Y. Bando N. Nakanishi and S. Kachi Trans. Jap. Inst. Metal. 1967 8 180. S. Kaya and H. Sato Proc. Phys. Math. Soc. Japan 1943,25,261. H. Masumoto H. Saito and M. Shinozaki Sci. Rept. Res. Inst. Tohoku Univ. 1954 6 523. T. Yokoyama T. Takezawa and Y. Higashida Trans. Jap. Inst. Metal. 1971 12,30. R. Castanet and A. Ferrier Coll. int. C.N.R.S. no. 201 Thermochemie (Marseille 19721 collected papers p.345 ; see also Compt. Rend. Ser. C 1971 272 15. W. Pepperhoff and H. H. Ettwig reported by G. Inden and W. Pitsch Chemical Metallurgy of Iron and Steel Proc. Int. Symp. Metallurgical Chemistry-Applications in Ferrous Metallurgy Sheffield 1971 (Iron and Steel Inst. London 1973) p. 314-316. F. R. Sale Chemical Metallurgy of Iron and Steel Proc. of Int. Symp. on Metallurgical Chemistry -Applications in Ferrous Metallurgy Sheffield 1971 (Iron and Steel Inst. London 1973) p. 330. lo A. S. Normanton P. Bloomfield and B. B. Argent (University of Sheffield Sept. 1973) private communication. l1 V.I. Gormankov,D. F. Litvin A. A. Loshmanov G. G. Lyashenko and I. M. Puzei Sou.Phys. Cryst. 1962 7 637. T. Satow S. Kachi and K. Iwase Sci. Rept. Res. Inst. Tohoku Univ.1956,8,502. l3 L. Kaufman and H. Nesor 2. Metallkunde 1973 64,249. l4 F. Miiller and F. H. Hayes J. Chem. Thermodynamics 1971,3 599. M. Hansen and K. Anderko Constitutionof Binary Alloys (McGraw-Hill 1958). l6 K. Schwerdtfeger and A. Muan Acta. Met. 1965 13 509. l7 R. A. Walker and J. B. Darby Jr. Acta. Met. 1970 18 1261. l8 K. C. Mills and M. J. Richardson Thermochim. Acta 1973 6,427. l9 R. Hultgren R. L. Orr P. D. Anderson and K. K. Kelley Selected Values of Thermodynamic Properties of Metals and Alloys (John Wiley New York 1963). 'O F. Muller and 0. Kubaschewski High Temp.-High Pressure 1969 1 543. P. J. Spencer and F. H. Putland J. Iron Steel Inst. 1973 211 293. 22 W. L. Bragg and E. J. Williams Proc. Roy. SOC. A 1935 151 540. 23 R. S.Irani J. Contempory Phys. 1972 13 559. 24 0. Kubaschewski The Carter Memorial Lecture March 1970 ; Metal. J. Uniu. Strathclyde 1971. 2s M. Hirabayashi S. Nagasaki and H. Maniwa Nippon Kinzoku Gakkai-si 1950,14B 1

ISSN:0301-5696    

DOI:10.1039/FS9730800040

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Calorimetric Study of the y-loop Region of the Iron-Chromium System BY A. S. NORMANTON* Dept. of Metallurgy University of SheffieId Received 16th January 1974 A high-temperature adiabatic calorimetric study of the y-loop region of the iron-chromium system is briefly described. Heats of the a +-y and y -+high-temperature-a transformations are given for appropriate mean transformation temperatures. As part of a programme of research on the thermodynamic properties of iron- base alloys heat capacities and heats of transformation have been measured for iron- chromium alloys containing up to 16 at. %Cr. There are only limited similar calorimetric data for this region of the Fe-Cr system 1* and until recently 3* the phase boundaries of the closed y-loop were not known with any certainty.The results of some recent measurements are brieff y described. EXPERIMENTAL The measurements were made in a spherical adiabatic calorimeter capable of operating in the range 600-1700K. The calorimeter and its operation together with data obtained for high-purity iron have been In the current study alloys were prepared by vacuum-melting appropriate quantities of 99.9 % iron and chromium. All alloys con- tained < 0.004wt %C and < 120p.p.m. O2and < 10p.p.m. N2. Specimens weighing approx. 60 g were machined from a hot rolled bar and were subsequently homogenized. Heat-capacity measurements were made discontinuously with the heat input to the specimen of NN 0.35 J s-I for any one measurement of lo3s duration and heating rates were > 10-14 K h-l.Heats of transformation for a -+y and y -+high-temperature-a were measured using heating rates of M 7-15 K h-l. The heat capacities were easily calcu- lated from the temperature rise in the specimens produced by a measured power input together with a knowledge of the specimen weight and " thermal leak " constant of the calorimeter. The heats of transformation were calculated from the total power input during the time of the transformation but because the transformation occurs over the tempera- ture range of the (a+y) two-phase field this total power includes a term to account for the change in temperature and heat capacity as well as the actual transformation heat and this must be taken into consideration. RESULTS A typical set of heat-capacity data are shown in fig.1 for an Fe-3.01 at. %Cr alloy. These show the increase in heat capacity as the Curie temperature is ap- proached and the decrease afterwards as well as the discontinuities at the phase transformations. A typical heating curve for an a -+y transformation is shown in fig. 2 the start and finish of the transformation being respectively taken as the points of departure and * now at British Steel Corporation Advanced Process Laboratory Hoyle Street Sheffield S3 7EY. 52 A. S. NORMANTON 53 I . I I I I 1 1 ] 1 I ; ; ! I ! 1 A ' - 7O- r(I CI2 & 53-- b 0" - 3ot,1 ; 1 I d--z -_ -%-I t 1 sI I 18 ::'!-I ! i --1 - _--I1 f if& ! ; time/min FIG.2.-Typical heating plot of temperature (T/K) against time/& for the a +y transformation of an Fe-7.83 at.%Cr alloy. The dots (0)indicate experimental points some of which are omitted for clarity. return to linearity on this temperature-time graph. The heats of transformation calculated at appropriate mean transformation temperatures are shown in fig. 3. The phase boundaries of the y-loop as determined from the calorimetric measure- ments agree well with the recent measurements of Kirchner et aL3 and are shown in fig.4 CALORIMETRIC STUDY OF Fe-Cr -I I 0 I I ... at. %chromium FIG. 3.-Plot of heat of transformation (kJmol-') against at. %Cr at the mean transformation temperature -x - cc -+y transformation ; - - o -,y-high-temperature-a transformations.The symbols x and 0 on the abscissa represent the values of the o! -+ y and y -+6 transformation for pure Fe. I700 1500-1300. I too. +I I I t I f i; 2 4 6 0 10 12 14 at. %chromium FIG.4.-PIot of temperature (T/K) against at. %Cr showing the phase boundaries of the y-loop according to Kir~hner,~ - and those determined in the current study a. A. S. NORMANTON DISCUSSION The thermodynamic data obtained from a calorimetric study of alloys in the y-loop region of the Fe-Cr system have been briefly described. The heat capacity data are currently being analyzed to yield information on the magnetic transformations. The heats of the a-y transformations show little variation with composition until a marked decrease begins at x 7 at.%Cr. Any comparison of these values with that for pure iron is not easy because of the difference in transformation temperatures and the difference in the degree of magnetic order remaining in the alloys at the start of the transformation. The heats of the y -+ high-temperature-a transformation appear to decrease initially on adding Cr as compared to the heat of the y -+6 transformation in pure iron. This is followed by a rapid rise and then a decrease for Cr contents > 8 at. %. A viable procedure for the correction of the a -+ y heats is currently under consideration. Also it is hoped to determine y + high-temperature-a heats of transformation for a 1 at. %Cr alloy to verify the initial fall in the heats as in fig.3. Other iron binary systems exhibiting closed y-loops have also been studied and it is hoped that data for Fe-V and Fe-Si systems will help in the interpretation of the current results. It is intended to examine the results in terms of thermodynamic models of solid solutions including the applicability of the " two-y-state " approach. The author thanks the Science Research Council for financial support and Professor B. B. Argent for his help and encouragement. W. B. Kendall and R. Hultgren Trans. Amer. Suc. Metals 1961 53 207-14. K. Schroder Phys. Rev. 1962,125,1209. G. Kirchner T. Nishizawa and B. Uhrenius Met. Trans. 1973 4 167. P. Poyot G. Guiraldenq and J. Hochmann Mem. Sci. Rev. Met. 1972 69 no. 77 p. 775. F. R. Sale J. Phys. E 1970 3 646. G. R. Smith and F. R. Sale J. Phys. E 1971,4,366. F. R. Sale and A. S. Normanton Metallurgical Chemistry Symp. Ed. 0. Kubaschewski N.P.L.,1971 (London H.M.S.O. 1972) p. 19.

ISSN:0301-5696    

DOI:10.1039/FS9730800052

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

Thermodynamic Properties of the Cu-A1 System Correlation with Bonding Mechanisms BY JOHN HAIRAND D. €3. DOWNIE" Dept. of Metallurgy University of Strathclyde Glasgow G1 1XN Received 11th October 1973 A wide-ranging study of the thermodynamic properties of the Cu-A1 system at 773 K has been carried out. An e.m.f. technique using solid CaF as electrolyte was used to obtain the partial free energies of Al in a series of selected alloys. Enthalpies of formation (A&) were measured using a liquid-tin solution calorimeter and combined with integral free energies (AGf) to evaluate the excess entropies(ASP). Volume changes on alloying (A Vf) were calculated from existing room temperature lattice parameters. Plots of AHf AS? and A Vf against composition show parallel relationships particularly between ASfs and A Vf.Due to lack of heat capacity data for the alloys it is not possible to separate vibrational and configurational components of entropy but reasons are given for assuming that both may be related to volume changes. Assuming volume contraction is associated with orbital overlap it is concluded that covalency is present in all the intermediate phases to varying extents. There is electronic spectroscopic evidence to support the view that there is an ionic contribution to bonding in the a-phase alloys which diminishes with increasing Al-content beyond this range. From other electron spectroscopic data it is deduced that the covalent bonding in the yz-phase is largely due to orbital overlap while that of the t2-and 0-phases arises mainly from charge localisation.In recent years some attention has been given to the thermodynamic behaviour of intermediate phases in binary metallic systems in relation to the bonding between the specie^.^'^ The Cu-A1 system is one which is characterised by the formation of a large number of intermediate phases some of which are stable only within fairly narrow temperature ranges. However at temperatures around 773 R,at which thermodynamic parameters can conveniently be measured the presence of five inter- mediate and two terminal phases is reported by Hansen and Anderko.6 Since hitherto the Cu-A1 system had not been examined in detail it was decided to make a comprehensive thermodynamic study of the system involving free energy and enthalpy of formation measurements to provide a basis for intercomparison of phases and their possible bonding mechanisms.EXPERIMENTAL ALLOY PREPARATION Alloys of selected compositions were preared by melting under argon in a high-frequency furnace the appropriate weights of the component metals contained in a silicon carbide crucible. The melts were solidified and remelted twice prior to removal from the furnace to ensure a reasonable degree of homogenization. The solid alloys were then wrapped in copper foil sealed in evacuated silica capsules and annealed at 773 K for periods of up to three weeks to effect a greater degree of homogenization and also to ensure the presence of the equilibrium phases. FREE ENERGY MEASUREMENTS The partial free energies of mixing of the Al-component in the alloys were obtained using the solid-electrolyte galvanic cell Al AIF,/CaF2/AIF, AI(Cu) 56 J.HAIR AND D. B. DOWNIE and the relationship Ac~l=-LYE where AGA is the parameter required 2is the state of ionisation (assumed equal to three) F is the Faraday constant (96 487 C/g-atom) and E is the measured e.m.f. Since electrode materials for the galvanic cell were required in an extremely fine particle size the alloys were pulverised in a percussion mortar and the resulting powder intermixed with pre-dried aluminium fluoride using an agate mortar and pestle. The electrodes were formed by pelletizing the powder in a steel die to form discs of about 8 mm dim. and 3 mm thickness and subsequently sintering under argon at 773 K.The reference electrodes were similarly produced from high purity X-ray grade A1 powder. After assembly argon gas was allowed to pass around the cell for one hour before the furnace was switched on. For each alloy sufficient time (ranging from 24 to 48 h) was allowed for the reaction to come to equilibrium before readings were taken. High initial temperatures were chosen in order to encourage equilibrium conditions. In each case the cell was cycled within a 100-150"range in 50" intervals until results of acceptable constancy at each temperature were achieved. ENTHALPY MEASUREMENTS The isoperibol calorimeter used to measure the enthalpies of formation of the alloys has been employed extensively in the Department of Metallurgy at Strathclyde University to make similar measurements on a number of other binary alloy Essentially it is liquid tin solution calorimeter based on the design of Orr Goldberg and H~ltgren.~ Standard procedures for operation were followed.In order to maximise solution rates the temperature of the liquid tin bath was set as high as possible i.e. in the region of 780 K. However with stoichiometric compounds in the composition range 38-50 at. % A1 solution rates were still rather low. Since the problem appeared to arise from a combination of low density and compound stability a number of pelletising techniques O designed to increase both the density and the reacting surfaces of the samples were tried. These attempts resulted in some slight increase in solution rates but to effect further improvement the calorimeter was modified to aIIow operation at higher bath temperatures.This was achieved by re- designing the copper jacket furnace and incorporating a solid state controller used in con- junction with a platinum resistance thermometer.10 This allowed bath temperatures of up to 950 K to be used resulting in greatly increased solution rates. The heats of solution of the pure metals in liquid tin were obtained first of all using lump samples of approximately 0.10g weight. In a similar fashion the heats of solution of the alloys were measured for a preheat temperature of 773 K. Using these results and existing heat capacity and enthalpy of fusion data,ll the enthaipies of formation of the selected alloys at the preheat temperature were obtained.RESULTS FREE ENERGIES Over the relatively narrow range of temperature involved in the measurements (viz. 700-850 K) it was assumed that a linear relationship holds between the measured e.m.f. and the temperature i.e. E = Eo+(dE/dT)T. For each alloy the values of Eo and dE/dT were calculated from the experimental data by the method of least squares. These values are reported in table 1 along with the derived values of AGAlfor a temperature of 773 K. The estimated average error in these values is approximately 6 %. The only published results of free energy measurements on alloys from the Cu-A1 system are those of Ali Samokhval Geiderikh and Vecher.12*l3 These results are plotted along with the present ones against composition in fig.1. Ali et al. used e.m,f. cells of two types a solid electrolyte cell in the temperature range 933-1033 K and a liquid electrolyte in the range 735-825 K. Their results in the a-phase region 58 THERMODYNAMIC PROPERTIES OF Cu-A1 SYSTEM TABLE PROPERTIES OF Cu-A1 ALLOYS AT 773 K 1.-THERMODYNAMIC -(WdTI/ AGAi/ Act/ AH,/ AScI AS I phase xA1 EObV mV K-1 kJ/g-atom kJ/g-atom kJ/g-atom kJ K-'/g-atom J K-t;g-atom composition 0.016 324.3 0.084 -112.67 -1.97 --0.070 237.0 0.075 -85.40 -7.20 -6.19 1.30 -0.79 0.140 229.2 0.039 -75.10 -12.67 -9.15 4.55 + 1.21 0.220 267.2 -0.074 -60.76 -13.92 -0.273 250.8 -0.049 -61.60 -18.43 -0.310 270.5 -0.081 -60.14 -22.34 -20.29 2.65 -2.50 0.340 245.0 -0.117 -44.75 -23.68 -21.69 2.57 -2.76 0.363 121.8 -0.039 -26.56 -24.10 -21.19 3.76 -1.68 0.380 99.8 -0.015 -25.51 --0.395 --20.79 * -24.14 -20.67 4.48 -1.08 -0.425 94.1 -0.050 -16.06 --0.445 --12.79 * -23.39 -20.40 3.87 -1.84 0.468 56.9 -0.03 1 -9.53 --0.490 --8.68 * -22.09 -19.92 2.81 -2.95 -0.520 40.4 -0.021 -6.98 -17.05 -0.615 42.5 -0.026 -6.52 --0.670 4.3 0.008 -3.01 -16.63 -13.05 4.66 -0.61 0.722 3.0 O.OO0 -0.88 -12.33 -0.840 2.7 O.OO0 -0.79 -6.53 -0.950 2.9 O.Oo0 -0.84 -1.62 -0.996 --0.75 -0.17 0.75 +0.53 * values obtained by interpolation.have been extrapolated from the higher temperature range to 773 K and referred to solid A1 as standard state using published data l1 to calculate the free energy of solidification of A1 at 773 K. The phase field limits shown in fig.1 are taken from Hansen and Anderko.6 -20 Ot -1201 1 I I I 0 0.2 0.4 0.6 0.8 1.0 XAl FIG.1.-Partial free energies of mixing of Al in CU(AGA) as a function of composition ; 0,present results ; x,Ali et af. J. HAIR AND D. B. DOWNIE Fig. 1 shows that there is good agreement between the two sets of results in the yz-and (rz+@-phase fields but elsewhere agreement is only fair to poor. In view of their reported lack of reproducibility in e.m.f. values and the uncertainty in the degree of ionisation when using the fused chloride cell in the lower temperature region and also of the large extrapolation of their high temperature measurements the present results are preferred throughout. Fig. 1 also shows that neither set of results includes alloys in the 6- c2-and q2-phase fields.In the case of the present authors the results from alloys within these phase fields were very erratic and considered unreliable. The reason for this is not clear but it may be associated with the difficulty in establishing the equilibrium phase when it is confined to very narrow compositional limits. As a result the AGA values for the compositions from these phase fields shown in table 1 have been ob- tained by interpolation. The integral free energies of formation ((AG,) which are also shown in table 1 were obtained from the partial values using the Cibbs-Duhem relationship. A I 0 0.2 0.4 0.6 0. 0 a XAl FIG.2.-Enthalpies of formation of Cu-Al alloys as a function of composition 0,present results (773 K) ; v,Oelesen and Middel (298 K) ; A,Sinvhal and Khangoankar (303 K) ; .,Kubas-chewski and Heymer (598 K).ENTHALPIES The enthalpy of formation values (AHf)shown in table I are the averages of several determinations at a single preheat temperature. The average error is estimated at 2 %. For purposes of comparison the enthalpy values from table 1 have been plotted along with the published results of other workers as a function of composition in fig.2. The values of the present workers are joined by straight lines to show the trend. From the plot it is clear that overall there is good agreement amongst workers. Agree-ment is particularly good between the present values and those of Oelsen and Middel l4 which were obtained by mixing the liquid metals and evaluating the heats of formation at 298 K.This agreement and the general. accord with the values of the other workers THERMODYNAMIC PROPERTIES OF Cu-A1 SYSTEM for temperatures of 303 K (Sinvhal and Khangaonkar 15) and 598 K (Kubaschewski and Heynier 16) indicate that ACp values are very small. Unpublished results of Wittig l7 measured at 745 K but not included in fig. 1 are considerably less exo- thermic in the composition range 20-45 at. % Al but are in good agreement with the other results outwith this range. ENTROPIES AND VOLUME CHANGES The integral excess entropies of mixing have been derived from the appropriate free energy and enthalpy values obtained in this investigation. The results are recorded in table 1 and plotted against composition in fig.3. Volume changes on alloying have been calculated from the room temperature lattice parameter and crystal structure data of Pearson18 and of Westrnan.lg They are expressed as percentage change in volume over the "ideal " value i.e. the weighted sum of the volumes of the pure metals. The values obtained are plotted in fig. 3 for comparison with the excess entropies of formation of the alloys. XAI FIG.3.-Excess entropies of formation (AS?) and volume changes on alloying (A Vf)as functions of composition 0, ASfs ; X A Vf. DISCUSSION THERMODYNAMIC PARAMETERS At a temperature of 773 K the stable phases in the Cu-A1 system according to Hansen and Anderko,6 are a 0-19.6 at. % Al terminal solid. solution y2 30.9-37.5 at.% Al based on Cu,Al, 6 38.8-39.9 at. % Al based on Cu3A12, r2 43.9-44.8 at. % Al based on Cu,Al, q2 48.7-50.0 at. % Al based on CuAl 6 67.0-67.6 at. % Al based on CuAl, K 98.4-100.0at. % Al terminal solid solution. J. HAIR AND D. B. DOWNIE It has been suggested that the y2-and b-phases may be a single one with varying lattice constants ; the results of this investigation neither support nor contradict this suggestion. If the AHf values of the phases are taken as a measure of their relative stabilities the plot in fig. 2 suggests that the y2-phase around a composition of 34 at. % is the most stable closely followed by the 6- c2-and q,-phases with the &phase showing a much less negative value. This trend is followed approximately by the AV values in fig.3 indicating some correlation between bond strength and volume con-traction. An even more marked parallelism in compositional trends is shown by AV and AS,""values (fig. 3) and the question arises as to whether the relationship is due to varying configurational or vibrational entropy factors. Since all of the intermediate phases have positive entropies of formation (see AS values in table 1) then either the phases are not completely ordered at 773 K or there are positive contributions from vibrational sources. In the absence of the relevant heat capacity data for the alloys it is not possible to evaluate the vibrational contributions to entropy but it seems unlikely that the negative volume changes obtained on alloying would be associated with increases in lattice vibrations.The dilemma could be resolved by postulating sufficiently large positive contributions from electronic sources. In this case the varying vibrational contributions alone would be responsible for the parallelism with the AV values. However since there is no evidence for this unusual electronic behaviour it is more likely that the intermediate phases are not completely ordered and that the parallelism though still mainly associated with changes in vibrational properties may also be related to varying degrees of order. BONDING MECHANISMS Negative heats of formations in compounds are usually associated with ionic (i.e. charge transfer) or covalent (i.e. charge sharing) bonding. Thus in the Cu-A1 system in which negative heats of formation obtain throughout one should look for electronic behaviour which is consistent with these types of bonds.Fuggle,20 using X-ray photoelectric spectroscopy (XPS) has found that a-alloys in the CuAl system are ionic in character this resulting from charge transfer from A1 p-states to available states in the Cu valence-band. These are probably d-states made available by hybridization of the Cu atoms as proposed by Engel,21 to give average outer shells of 3d8.54s1p1-5instead of 3d104s1. This hybridization of Cu is not in accord with the results of Baer et al. in their electron-spectroscopy studies of transition metals 22 in which they found the Cu d-levels to be completely filled. However if Engel's proposal is accepted it is possible to explain the relatively wide compositional stability range of the CI solid solution compared with the low solubility of Cu in Al as shown by the K terminal solution in terms of availability of d-states which will be much less in the Al-rich alloys due to the greater separation of the Cu-atoms preventing hybridization.Also the asymmetric shape of the (AH, composition) curve (fig. 2) would result from the increased ionic contribution to bonding in the Cu-rich alloys. Covalentbonding can arise from two different types of valence electron behaviour (i) orbital overlap and (ii) localization of charges between the atoms. The former is accompanied by volume contraction and the latter results in reduction of valence band width. The fact that all the alloys in the Cu-A1 system form with a reduction in volume would suggest that covalency due to orbital overlap is present in all phases.THERMODYNAMIC PROPERTIES OF Cu-A1 SYSTEM At present investigation of electronic structures has been carried out only on two inter- mediate Cu-A1 alloys viz. Cu9A14 &-phase) and CuA1 (&phase). Using XPS Fuggle et aZ.23have determined that although Cu9A14 shows no reduction in valence band width CuA1 shows considerable reduction. It might be expected therefore that the increased bonding effect due to the charge localization would result in CuAl having the more exothermic heat of formation whereas the reverse is true (fig. 2). However the volume contraction of Cu9A14 is considerably greater than that of CuA1 (fig.3) signifying a greater degree of orbital overlap and as postulated in the preceding paragraph CugA14 being richer in Cu is also likely to have a greater amount of electron transfer resulting in the more exothermic heat of formation. In the absence of information on the electronic behaviour of the other phases the contribution of localization of charges to bonding cannot be assessed. However the AV' values suggest that the orbital overlap of the 8-phase is similar to that of yz while c2 and q2 have much less. The anonialously high (i.e. small contraction) value of AVf for the [,-phase compared to its AHf value (fig. 2) may be due to the presence of considerable charge-localisation as in the @phase. The foregoing discussion has shown that whereas some correlation between thermodynamic and electronic behaviour of alloys in the Cu-A1 system is possible complete analysis requires further experimental data.Information on the electronic behaviour of the 8- c2-,q2-and Ic-phases is required while on the thermodynamic side the low temperature heat capacities of all the phases would allow more thorough examination of the entropies of formation. Investigations to elucidate the electronic behaviour of the neglected phases are presently being pursued by XPS methods in the Department of Metallurgy in the University of Strathclyde and it is hoped to commence work on the cryogenic calorimetric aspects in the same place in the near future. The authors acknowledge the informa tion and helpful discussion provided by their colleagues Dr.J. C. Fuggle and Dr. L. M. Watson particularly on the electronic aspects of the investigation. One of the authors (J. H). also thanks the Science Research Council for provision of a maintenance grant. 0. Kubaschewski and W. Slough A Tentative Analysis of the Bond Types in Binary Metallic Systems (N.P.L. London March 1972). V. A. Geiberikh Russ. J. Chem. 1971,45 1083. W. H. Skelton N. J. Magnani and J. F. Smith Met. Trans. 1971 2,473. A W. Bryant W. G. Bugden and J. N. Pratt Acta Met. 1970 18 101. G M. Lukashenka Russ.J. Phys. Chem. 1968,42,405. M. Hansen and K. Anderko Constitution of Binary Alloys (McGraw-Hill New York,2nd ed. 'G. 1958). R. Blair and D. B. Downie Metal Sci. J. 1970 4 1. * R. A. Come11 and D.B. Downie Metal Sci. J. 1973 7 12. R. L. Orr A. Goldberg and R. Hultgren Rev. Sci. Instr. 1957 28,767. lo John Hair Ph.D. Thesis (Univ. Strathclyde) to be submitted. R. Hultgren R. L. Orr and K. K. Kelley Supplement to Selected Values of Thermodynamic Properties of Metals and Alloys (Lawrence Radiation Laboratory Univ. California Berkley U.S.A.). l2 S. A. Ali V. V. Samakhval V. A. Geiderikh and A. A. Vecher Russ. J. Phys. Chem. 1972,46 139. l3 S. A. Ali V. V. Samakhval V. A. Geiderikh and A A. Vecher Rum. J. Phys. Chem. 1973,47 22. l4 W. Oelsen and W. Middel Mitt. KWI Eisenforsch. Diisseldorf 1937 19 1. l5 R. C. Sinvhal and P. R. Khangaonkar Trans. Indian Znst. Metals 1967 (June) T.P.421 107. l6 0. Kubaschewski and G. Heymer Trans.Faraday SOC., 1960 56,473. J. HAIR AND D. B. DOWNIE l7 F. E. Wittig Univ. Munich Germany (private communication). W. B. Pearson A Handbook of Lattice Spacings and Structures of Metals and Alloys (Pergamon, London 1958). l9 S. Westman Acta Chem. Scand. 1965 19 2371. '* J. C. Fuggle Dept. of Metallurgy Univ. Strathclyde (private communication). 21 N. Engel Acta Met. 1966 15 557. 22 Y.Baer P. F. Heden J. Hedman M. Klasson and K. Siegbahn Physica Scripta 1970,1,55. 23 J. C. Fuggle L. M. Watson D. J. Fabian and P. R. Norris Solid State Comm. 1973 13 507.

ISSN:0301-5696    

DOI:10.1039/FS9730800056

出版商:RSC    

年代:1973

数据来源: RSC

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GENERAL DISCUSSION Mr. D. G. Fraser (University of Oxford) said With reference to the thermo- dynamics of phosphate melts the subject of the paper by Jeffes et al. it might be of interest to outline a theoretical model which Prof. Anderson and I have been working 0n.l The basis of current theories concerning the reaction of basic oxides with oxy-acid melts is the Lux2-Flood expression (I) O”+O” +20’ ; K = (0’)2/(0’’)(00) (1) where 0” = free oxide 0” = 0x0-bridge and 0’ = unshared oxygen. In this the equilibrium between unreacted oxide 0x0-bridges and unshared oxygen is considered and is represented here using the notation of Toop and Sami~.~ One of the assumptioiis of this model which was developed more fully by the latter authors and has been expressed more recently by Masson and his co-workers 5-6 in terms of polymer theory is that the equilibrium constant be independent of bulk composition.This assumption is equivalent to that of the equal reactivity of func-tional groups in co-condensing organic polymers proposed by Flory.’ However in oxy-acid polymers the reactivity of 0x0-bridges may well be dependent on their structural environment and this is especially true of group 5 oxy-acids owing to the differing possibilities for the resonance stabilization of the X-0 double bond in different structural positions.* To account for this we have developed a new model in which reactions are written between oxide ion and the fundamental structural units or structons of which the poly-anions are composed. Using the notation ‘jX where i = no.of unshared oxygens j = no. of oxo- bridges and X = central atom equilibrium in oxy-acid melts may be described by the set of structon equilibria (2) where Slj = structon fraction = nij/Znij+ng. We have included reaction (2.5) in the light of the results of Sridhar and Jeffes.” The overall equilibrium is the resultant of the set of simultaneous structon equilibria and the oxide activity in a binary oxy-acid melt is determined by the set of equilibrium constants and the mass and charge-balance conditions. The chemistry of oxy-acid melts is thus seen to be similar to that of polyprotic acids in aqueous solution com- pleting Lux’s original analogy. The use of the structon model to account for the observed variation in oxide activity with bulk composition for a number of binary oxy-acid melts will be published elsewhere.D. G. Fraser and J. S. Anderson to be published. H. Lux,2.Elektrochem. 1939,45 303. H. Flood and T. Forland Acta Chem. Scand. 1947 1 592. G. W. Toop and C. S. Samis Trans. A.I.M.E. Met. Soc. 1962 224 878. C. R. Masson Proc. Roy.SOC.A 1965 287 201. C. R.Masson I. B. Smith and S. G. Whiteway Can. J. Chem. 1970 48 33 1456. ’P. J. Flory Principles ofpolymer Chemistry (Cornell University Press,Ithaca New York 1953). J. R. Van Wazer Phosphorus and its Compounds vol. 1 (Interscience New York 1958). M. L. Huggins J. Phys. Chem. 1954,58 1141. R. Sridhar and J. H. E. Jeffes Trans. Znst. Min. Met. C 1969 78 14. 64 GENERAL DISCUSSION Dr. J. H.E. Jeffes (Imperial College) (communicated) Fraser’s approach to the problem of polyanionic equilibria in terms of structons appears to be an interesting one and the authors look forward to the elaboration of this treatment. The problem of deviations from ideality which favour formation of oxygen ions and ring-chain equilibria will apparently still create problems in this interpretation of the system. Dr. H. A. Skinner (University of Manchester) said Jeffes refers to “ a calibration by dropping in a suitable platinum sphere from room temperature ”. Would he elaborate on this method and its advantages? Presumably a sphere gains less heat during free fall into the hot zone than would the same mass in the form of a rod or wire; what percentage of (HT-H298)for a Pt sphere was gained during free fall? Dr.J. H. E. Jeffes (Imperial College) (communicated) The heat gained by the platinum spheres during their fall into the calorimeter was determined by the method of Kleppa namely by dropping in spheres of varying mass and extrapolating the resultant effects to an infinitely large mass. This correction was about 2 %. Dr. R. H. Moore (University of Shefield) said In the paper of Mills et al. it is implied that the combined area under the “ 820 K anomaly ” and the a’ -+ a dis-ordering peak was constant for the different heating rates employed. Could the authors state how good the agreement is and compare this agreement with the estimated uncertainty in choosing their base line BHE on fig. 1 ? Dr. K. C. Mills (Nat. Physical Lab.Teddington) said We have measured only the temperature of the CPpeak of the “ 820 K anomaly” as a function of heating rate; all the heat capacity measurements were carried out with a heating rate of 0.333 K s-l. The total enthalpy of the a’ -+a transformation will be unaffected by the heating rate as this represents the total enthalpy required to transform the ordered phase at equilibrium into the disordered phase at equilibrium. Dr. R. H. Moore (University of SheBeld) said I notice that Mills et al. use the data in Kaufmann and Nesor’s paper to calculate the shape of the ordered region in the Fe-Co system. Kaufmann and Nesor’s compilation is a self-consistent set which uses Belton and Fruehan’s liquid data as part of its data-base. The paper by Argent et aL3 suggests that the f.c.c.phase is nearly ideal and that the deviations for the liquid phase are not as strong as suggested by Belton and Fruehaa2 Do Mills et al. think that this discrepancy is sufficiently important to affect significantly the values that they derive for the a’phase? Dr. P. J. Spencer (Nat. Physical Lab. Teddington) said In making calculations of the shape of the ordered region in the Fe-Co system we first attempted to make use of all the available experimental thermodynamic data for Fe-Co alloys. Un-fortunately in contrast to calorimetric studies of the system which in general provide consistent sets of AH data for the liquid f.c.c. and b.c.c. phases there is poor agree- ment between the available experimental Gibbs energy values.The latter include data obtained by Lyubimov et aL4 and Bell in addition to the values given in the L. Kaufmann and H. Nesor 2.Metallkunde 1973 64,249. G. R. Belton and R. J. Fruehan J. Phys. Chem. 1967,71 1403. B. B. Argent P. E. Bloomfield R. H. Moore and D. Robinson this Symposium. A. P. Lyubimov V. Ya. Zobens and V. I. Rakhovski Zhur. Fiz. Khim. 1958 32 1804. H. B. Bell (University of Strathclyde) private communication. SS-3 GENERAL DISCUSSION paper by Argent et al.' Consequently the thermodynamic analysis of the Fe-Co system carried out by Kaufman and Nesor,2 which is consistent both with the pub- lished calorimetric and phase-diagram information was used for the present calcula- tions. In this way the calculated order/disorder boundaries will also be consistent with the data given by Kaufman and Nesor for the different phases of the Fe-Co system.The experimental data of Argent et al. at 1500 K and the analytical expression given by Kaufman and Nesor in fact provide values of AG; for f.c.c. Fe-Co alloys which are in agreement to well within the 15 % accuracy limit quoted by Argent et al. for their experimental activity values. Dr. A. S. Normanton * and P. E. Bloomfield t (University of Shefield) said In principle the Cpvalues obtained using the adiabatic calorimeter at Sheffield Univer- sity agree well with those reported by Mills. The preliminary values reported by Sale were part of the overall set of data obtained on the Fe-Co system and are also consistent with the present data and not 5 % lower as stated in the paper.Heating rates in the order-disorder region were between 7-10 K h-I in an attempt to ensure equilibrium conditions and avoid blurring of the transformation due to fast heating rates. It is thought that further consideration should be given to the interpretation of the order of the order-disorder transformation. Mills et al. state that the phase boundaries that they have determined would still be applicable even if the trans- formation was second order. It is difficult to see how this can be so as second-order transformations are characterized by a single phase for a particular temperature i.e. phase separation does not occur. The use of free energy-temperature curves to define the transformation limits necessitates recognition of the co-existence of ordered and disordered phases.How therefore can the phase boundaries so determined define transformation limits if the transformation is second order ? First-order transformations are accompanied by the evolution of latent heat and with the Fe-Co system using the adiabatic calorimeter we have found no evidence which points conclusively to a latent heat contribution. In our work on the a -+y transformation of pure iron using similar heating rates a latent heat of 930 J mol-' was evolved in 3000 s this time being the length of the plateau on a temperature-time heating curve. Thus if a latent heat of -50 J mole-' had been evolved during the disordering transformation it would have resulted in a plateau of -3 min which would easily have been noticed if present.The behaviour of the Cpmeasurements when passing through the critical temperature was analogous to that occurring when passing through the ferromagnetic to paramagnetic change in dilute Fe-Co alloys. Hence we assume that the latent heat evolved was very small or that the trans- formation occurred entirely by a second-order reaction. Dr. I(. C. Mills (Nat. Physical Lab. Teddington) said In the paper of Mills et al. we have pointed out that the order-disorder transformation in Fe-Co alloys cannot be assigned unequivocally to a " first-order " or " second-order " transition on the basis of extant experimental information. If this transformation should eventually be proved to be " second-order " then the two phase boundaries shown in fig.8 would be more conventionally replaced by a single line lying within these two boun- daries this line representing the critical temperature as a function of composition. B. B. Argent P. E. Bloomfield R. H. Moore and D. Robinson this Symposium. L. Kaufman and H. Nesor 2.Metallkunde 1973 64 249. * now at British Steel Corporation Advanced Process Laboratory Hoyle Street Sheffield. t now at Myers Grove Comprehensive School Wood Lane Sheffield 6. GENERAL DISCUSSION The evidence presented by Normanton and Bloomfield gives some support that the order-disorder transformation in Fe-Co is “ second-order ”. However the evidence is not conclusive and we believe that ‘‘order ” can only be conclusively determined by means of metallographic analyses of the samples annealed for long times at temperatures below the critical temperature so as to attain equilibrium conditions.We are carrying out such experiments but we have no results available at the present time. Dr. Irani has pointed out that heat-capacity measurements are unsuitable for determining the “ order ” of an order-disorder transformation unless they can be carried out at such a slow heating rate that true equilibrium is attained at all times. Irani has also pointed out that CJT) curves obtained for known “first order ” order-disorder transformations do not have the form of classical “first-order ” transformations ; the order-disorder transformation gives rise to a gradual increase in Cpover a large temperature range whereas the “ classical transformation ” gives rise to an infinite increase in Cpat the transformation temperature. R. S. Irani ref. (23) and private communication to National Physical Laboratory Dec. 1973.

ISSN:0301-5696    

DOI:10.1039/FS9730800064

出版商:RSC    

年代:1973

数据来源: RSC

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Atoms and Small Molecules as Useful Chemical Reagents BY P. L. TIMMS School of Chemistry University of Bristol Bristol BS8 1TS Received 31st August 1973 The paper surveys some recent work on reactions of high temperature species with ordinary compounds on a liquid-nitrogen-cooled surface. Useful syntheses with metal vapours metal salt vapours and low-valency boron and silicon compounds are described. Some reactions which failed are also reported and the reasons for unpredictability in the low-temperature reactions are discussed. Over the last few years an increasing number of atomic and molecular species produced at high temperatures have been used successfully in chemical synthesis. 1-3 Potentially useful species include atoms of the metals and metalloids vapours of metal salts and sub-halides and sub-oxides of group 3 and 4 elements.These species may be reactive because they contain elements which are in unstable low oxidation states or which are co-ordinately unsaturated. The reactions are carried out in two main ways. On a preparative scale (> 100mg of the species) the species is formed under a high vacuum and condensed immediately on a surface cooled by liquid nitrogen at the same time as the vapour of another compound. The cold surface acts as a cryogenic pump for the vapours. As there are few collisions in the gas phase at the low pressure thermal decomposition of the added compound on the furnace or by hot vapour is avoided. The products are normally isolated on warming the co-condensate or if they are very unstable they may be reacted with other compounds added at a low temperature.On a very small scale high-temperature species and compounds are co-condensed in the presence of a huge excess of a noble gas at 4-20 K. The species are initially isolated in the inert gas matrix but slight warming permits diffusion and if reactions then occur they are followed spectroscopically. In this paper recent results obtained at Bristol are used to show the potentialities and limitations of the preparative scale method EXPERIMENTAL FORMATION OF HIGH TEMPERATURE SPECIES Methods of vaporizing metals salts and related compounds are well known because of the interest in making thin films of materials by vacuum evaporation. We prepare high temperature species by vaporization of condensed phases by gas-solid reactions and by decomposition of gases at high temperatures.(a) VAPORIZATION TECHNIQUES For species which are formed below 18Oo0C,we prefer to contain the evaporant in an inert crucible (usually alumina) heated resistivity by 18 s.w.g. Mo or W wire. The gauge of wire requires less than 50 A to reach the evaporation temperature so that heavy electrical leads are unnecessary. The crucibles can be efficiently thermally insulated so that radiative heat losses to the cold walls of the vacuum chamber are small. A crucible with a capacity of 1 ml will vaporize 50-100 mmol of most metals. 68 P. L. TIMMS 69 For the more refractory 2nd and 3rd row transition metals we have used electron bom- bardment vaporization but we find that secondary electrons often cause degradation of the compounds condensed with the metal vapours.(b) GAS-PHASE REACTIONS The preparation of the boron monohalides or the silicon dihalides by gas+solid reactions e.g.9 1300°,1 Torr Si(s)+ SiCI4(g) + 2SiCI2 is conveniently conducted in an inductively-heated graphite tube. The sub-halide emerges from the tube into a high vacuum and is immediately condensed at -196°C. Boron monochloride is prepared by flash thermolysis of diboron tetrachloride lOOO" 3-5 Torr B2CUQ) + BCI+BCIB. A short residence time for the vapour in a narrow quartz tube furnace has proved the best condition to avoid the thermodynamically favoured nucleation of solid boron. THE REACTION VESSEL Fig.1 shows a reaction vessel set up for studying co-condensation reactions of metal vapours. The glass vacuum chamber is pumped by a mercury diffusion pump with a speed at the chamber of about 15 1. sec-l. This pumping speed is more than adequate during Reectant inlets \ Water and eI ec tr i c i ty" Solution ATOMS AND SMALL MOLECULES co-depositionof a metal vapour and a compound which has a low vapour pressure at -196"C as the liquid-nitrogen-cooled walls of the vessel act as a cryogenic pump. If traces of per-manent gas are evolved during the deposition or if the compound added has an appreciable vapour pressure at -196"C a higher pumping speed is required to maintain the pressure below Torr ; above this pressure we find that thermal decomposition of the added compound usually becomes appreciable and this may liberate more permanent gas.We use stainless steel apparatuses with much higher pumping speeds when handling very volatile compounds. The apparatus of fig. 1 can be used to vaporize as much as 50 g of suitable metals Iike copper and to condense the vapours with compounds added through the central tube. This tube can be heated to vaporize slightly volatile compounds. The reaction products are recovered either by warming the vessel to room temperature with continuous vacuum pumping into an attached liquid-nitrogen-cooled trap or by adding solvents to the apparatus and sucking out the solution for work-up by standard techniques. The recovery of products is made simpler if a much smaller apparatus is used the vacuum vessel being a one-litre flask.After the condensation the flask and its contents can be handled like a piece of conventional apparatus. This form of metal evaporation system is the basis of an undergraduate teaching experiment at Bristol.* Furnaces which make high temperature species by gas+solid reactions or by flash thermolysis are mounted in vacuum systems similar to those used for metal evaporation. The identification of the products of the reactions has generally depended on a com- bination of infra-red n.m.r. and mass-spectrometric data with microanalysis. RESULTS SOME REACTIONS OF METAL VAPOURS The majority of our work has been with the following metals Cr Mn Fe Co Ni Cu Mo Pd Ag Sn Au (a) WITH BENZENE DERIVATIVES Chromium vapour reacts readily with benzene and its derivatives to give arene- chromium complexes.This route avoids the complication of solvents and the presence of Lewis acids inherent in conventional methods of making these complexes. Thus cumene and other alkylbenzenes give pure di-arene chromium complexes with chromium vapour with no isomerization of the arene~.~ We have prepared the first complexes in which hexafluorobenzene behaves as a six-electron donor; (C6F6)Cr(PF,) and (C$&r(C&) were made in low yield by condensing chromium vapour with the respective ligand mixtures. No (C6F6)2Cr could be isolated the co- condensate exploded. Skell et aL6 have reported the synthesis ofchromium complexes of chlorobenzene fluorobenzene and difluorobenzene; we find that the yield of (C6HSC1)@ is higher than witb any other arene 60-80 % even in the hands of under- graduates.We have also isolated small yields of (.n-pyridine)Cr(PF& and (n-hexamethyl- borazine)Cr(PF,) from chromium vapour and the mixed ligands ; reactions of pyridine with chromium by conventional routes yield only complexes with pyridine co-ordinated through the nitrogen atom. No "20-electron " diarene-iron complexes have yet been isolated from the reac- tions of arenes with iron vapour. Benzene and iron form an explosive product perhaps (C6H6)2Fe but mesitylene and iron give (C9Hl2)Fe(CgH,,) in moderate yield. Manganese vapour gives a low yield ofthe known compound (C6H6)Mn(C5H,) when condensed with benzene and cyclopentadiene.P. L. TIMMS (b) WITH OTHER UNSATURATED ORGANIC COMPOUNDS The reactions of metal atoms with cycloheptatriene are varied. Chromium gives good yields of (C7H8)2Cr ti or in the presence of trifluorophosphine (C,H,)cr(PF,),. Manganese cobalt and nickel vapours gave no simple product with cycloheptatriene but iron gave heptafulvalene (C7H6)2 and no organo-iron complex. Admixture of trifluorophosphine with cycloheptatriene still gave no products with manganese and nickel iron formed heptafulvalene and cobalt gave a good yield of (C7H7)Co(PF3) ; HCo(PF,) was also formed. This hydrogen abstraction in the presence of tri- fluorophosphine is also observed in the reaction of cobalt vapour propene and trifluorophosphine which gives (n-aUyl)Co(PF,),.The reaction of nickel vapour with tetra-ally1 tin causes a simple transfer of groups and bis(n-allyl) nickel is formed in good yield. (C) WITH PHOSPHINES The usefulness of trifluorophosphine as a ligand in metal vapour reactions is clear from the compounds described above. Although its reactions with most transition metals are simple iron causes defluorination and the products include Fe2(PF2)2(PF3)6 and Fe3(PF)2(PF3)9 in ad-dition to Fe(PF3)5. Trimethylphosphine complexes of iron cobalt and nickel are formed on con- densation but phosphine PH3 has proved more difficult. No Ni(PH3) was iso- lated from nickel vapour and phosphine. Much hydrogen was evolved on warming from -196°C; in the presence of trifluorophosphine Ni(PF3)3PH3 and Ni(PF3)2(PH3)2 were obtained.(d) WITH NO N2 AND CO Nitric oxide is a difficult ligand to use in co-condensation reactions because it has a vapour pressure of 0.1 Torr at -196°C and it is an oxidizing agent. On several occasions metal-nitric oxide co-condensates exploded on warming fi om -196°C. We have made the series of compounds Co(NO)(PF,), Fe(NO),(PF,), and Mn(NO),PF, hut we could not obtain Cr(N0)4,5 a compound which has been made photochemically. Nitrogen and carbon monoxide are effectively non-condensible at -196°C. Appreciable gettering of nitrogen was observed when nickel was evaporated under a pressure of 1 Torr of N2. However the condensate did not react with cyclo- octadiene suggesting that no discrete nickel-dinitrogen complex was present.Evaporation of palladium under a pressure of 1 Ton of carbon monoxide followed by addition of trifluorophosphine to the cold surface gave Pd(PF3) ; this indicates that a palladium carbonyl perhaps Pd(CO) as reported by Ogden,I was present at -196°C. (e) REACTION WITH HALOGEN COMPOUNDS We have studied reactions of copper silver and gold atoms with a variety of inorganic and organic halogen compounds. The atoms show selectivity and stereo- specificity. Thus copper atoms efficiently couple the boron atoms in boron trichloride or dichloroalkylboranes making this an excellent route to diboron compounds.* Phosphorus-phosphorus bonds can also be formed from phosphorus chlorine com- pounds and copper vapour but silicon chlorine compounds do not react.Con-densation of copper or silver vapour with R(-)sec-butyl chloride gave s,$(-)3,4- ATOMS AND SMALL MOLECULES dimethylhexane with preservation of 70 % of the optical activity and apparent inversion of configuration. Under the same co-condensation conditions sodium vapour gave an optically inactive 3,4-dimethylhexane. Gold vapour causes coupling of the alkyl groups from alkyl bromides but it does not react with alkyl chlorides. Tin vapour has proved to be rather unreactive towards alkyl halides despite the known stability of the alkyl tin halides. No reaction occurs at -196°C with alkyl chlorides and bromides although alkyl iodides give polymeric (RSnI),. Other research groups are actively studying metal atom dehalogenations and reports have recently appeared on lithi~m,~ zinc,l0 calcium,l and palladium l2 vapour reactions.The work of Lagow on Lithium stands apart for his reactions occur in the gas phase; thus when the flame formed by burning CC14 vapour in a stream of lithium atoms is quenched at -196"C the products behave chemically like Li,C the lithium atoms being readily displaced by other groups. METAL SALT VAPOURS AS REACTANTS Our results in this area are preliminary but it is clear that metal salt vapours are not ideal reactants at low temperatures. We condensed NiC1 and CoC1 vapours with a range of alkenes and dienes and observed polymerization of the organic compounds to high polymers reactions which we thought were catalyzed by the transition metal ions. However the same results were obtained with CaC1 vapour and the polymerization catalyst was found to be the proton liberated in the reaction co-condense HCl(g)+MC12(g) -+ H +MC1,.-196' The hydrogen chloride was formed in trace amounts on heating the " anhydrous " dihalides. Potassium cyanide vapour reacts on condensation with nickel or iron carbonyls to give cyano-carbonyls e.g. co-condense Fe(CO) +KCN(g) -+ K+[Fe(CO),CN]-. -196°C Stannous chloride vapour condensed with alkyl halides does not form alkyl tin halides. A slow reaction is known to occur between stannous halides and alkyl halides above 100°C. SOME REACTIONS OF BORON AND SILICON SPECIES The low-temperature reactions of the boron and silicon species have been studied in more detail than reactions of most of the metal atoms.The compounds provide unique routes to the higher boron and silicon fluorides silicon-boron halides and 1,4-dibora- and 1,4-disila-cyclohexanes and cyclohexadienes. We find the 1,4-diboracyclohexadienesto be powerful ligands to transition metals e.g. \/ \/ co-condense Ni(co)4 BF(g)+CH3C=CCH3 -+ FBoBF -+ FB 1 BF -196'C 0 /-\ /ll\ Ni /\ co co There are surprising differences in the low-temperature behaviour of related species e.g. SiF, SiC12 and SiO. On a cold surface SiFz appears to react mainly P. L. TIMMS in the form of a diradical SiF2SiF2 whereas SiC12 reacts as a monomer. Yields of organo-silicon products from reactions of SiClz are generally higher than those from reactions of SiF2.probably because the reaction mechanism is simpler for SiC12. Silicon monoxide 1s unique in being able to insert into C-H bonds as well as add across C-C multiple bonds. We have studied the chemistry of BC1 only since being able to make its precursor B2C14 in gram quantities from copper vapour and boron trichloride. In its reactions with organic compounds BCl closely resembles Sic],. We have found gaseous boron monoxide O=B-B-0 to react with boron trichloride or with sulphur tetrafluoride at -196°C to give high yields of B2C14 and B2F4 respectively. DISCUSSION Two main points emerge from the work at Bristol and from the work of other groups in this field. First the low-temperature co-condensation method is very effective and enables many compounds which were previously unknown or inaccess- ible to be made in useful quantities.There seems an assured future among synthetic chemists for this interface between " high-temperature chemistry " and " conventional chemistry". Secondly there is much to be understood about the interaction of high-temperature species and compounds on the cold surfzce. Much of the unpredictability of the low-temperature reactions stems from the rapidity with which the) must occur. The rate of recombination of metal atoms to give massive metal at -196°C is probably diffusion controlled. Reaction of the atoms with another compound is only competitive if the activation energy for the reaction is very low and the compound is present in excess. An activation energy of 1 kJ mol-1 can make a reaction quite slow at -196°C.Yields of products based on the metal do improve with an increasing ratio of compound metal. When atoms and molecules react sufficiently quickly the choice between reaction pathways can be governed by subtle kinetic factors rather than thermodynamic ones. Thus it is found that chromium and chlorobenzene give very high yields of (C6H5C1),Cr and not the thermodynanGcally more favoured dechlorination to diphenyl and chromous chloride ; nickel vapour causes efficient dechlorination. There is at present no reliable physical method for following the reactions that occur on co-condensation at -196°C. Studies with clean surfaces have little relevance to a process in which the rate of increase of film thickness may exceed 1000&.Matrix isola.tion spectroscopy can give information about the primary interactions between high-temperature species and compounds but its findings are in a sense artificial. For example Ozin j3 has reported the formation of Ni(N2)4 in an inert gas matrix below 30 K from nickel atoms and N2. We failed to make this compound by condensing nickel vapour in the presence of nitrogen gas on a surface at -196°C. The difference between these results may depend not as much on the temperature in the two experiments as on the isolated state of Ni(N2)4 molecules in the matrix. Decomposition of these molecules in the matrix can only give isolated metal atoms and N2 molecules; Ni(N2)4 will be more stable than the isolated components by approximately [4B(Ni-N,] kJ mol-l.On a surface at -196"C Ni(N2)4 can decompose to solid nickel and N2 ; it will be more stable than these components by approximately [4D(Ni-N,) -AH&,(Ni)] kJ mol-'. As the Ni-N2 bond energy is low Ni(N2)4 decomposes when not matrix isolated. The decomposition will be catalyzed by finely divided nickel on the surface; this catalytic effect probably ATOMS AND SMALL MOLECULES accounts for our failure to make the known compounds Ni(PH& and Cr(NO) which have some kinetic stability in the absence of catalysts. The research at Bristol has been generously supported by grants from the Science Research Council. P. L. Timms Adv. Inorg. Chem. Radiochem. 1972 14 121. J. J. Havel M. J. McGlinchey and P. S. SkeI1 Accts Chem.Res. 1973 6 97. P. L. Timms Accts Chem. Res. 1973 6 118. P. L. Timms J. Chem. Educ. 1972 49,782. R. Middleton J. R.Hull S. R. Simpson C. H. Tomlinson and P. L. Timms J.C.S. Dalton Trans. 1973 120. P. S. Skell D. L. Williams-Smith and M. J. McGlinchey J. Amer. Chem. Soc. 1973 95,3337. ’J. H. Darling and J. S. Ogden J.C.S. Dalton 1973 1079. * P. L. Timms J.C.S. DaZton Trans. 1972 830. R. J. Lagow J.C.S. Chem. Comm. 1972 1078. lo K. J. Klabunde M. Scott Key and J. Y. F. Low J. Amer. Chem. SOC.,1972,94,999. l1 K. J. Klabunde J. Y. F. Low and M. Scott Key J. Fluorine Chem. 1972 2,207. l2 K. J. Klabunde and J. Y. F. Low J. Organomet. Chem. 1973,51 C33. l3 H. Huber E. P. Kundy M. Moskovits and G. A. Ozin J. Amer. Chem. SOC.,1973,95,332.

ISSN:0301-5696    

DOI:10.1039/FS9730800068

出版商:RSC    

年代:1973

数据来源: RSC