ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15156.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

A thermogravimetric investigation of the vaporization of cold‐pressed Pb(Zr0.65Ti0.35)O3in vacuum from 690° to 1130°C shows that the vaporization occurs in two steps. An initial loss of 1 to 7 wt% proceeds by a mechanism with a logarithmic time dependence and represents the vaporization of the unreacted PbO that remains after the initial calcining. The second step proceeds by a slower, diffusion‐controlled mechanism with a parabolic time dependence. It is shown that this rate‐determining step is bulk diffusion across a thickening lead‐depleted layer in the cold‐pressed pellet, despite pore volumes of 24 to 42%. The rate constant, g/cm2sec½, for the parabolic weight loss process is expressed by: logK= (3.37 ± 0.14) ‐ (8.46 ± 0.16) (103/TK). The activation energy is 38.7

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15157.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

Magnesium oxide single crystals containing 0.3% nickel oxide were tested in tension at high temperatures. The stress‐strain relationship and deformation modes were similar to those identified for pure magnesium oxide. Alloy specimens had lower yield and ultimate strengths than pure magnesia at all temperatures above 1200° C and showed a greater tendency for kinking, polygonization, and recrystallization. It is proposed that the reducing atmosphere in the furnace causes the alloy crystals to decompose by the removal of oxygen and internal precipitation of nickel. The excess vacancies so produced enhance the nonconservative mobility of dislocatio

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15158.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

A device was built to provide the capability for out‐reactor simulation of the microstructures seen in irradiated fuel. Out‐reactor simulation of nuclear heating makes possible the precise measurement of temperature gradients in the fuel and provides data for correlating microstructural alterations with temperature. The simulation apparatus was designed to produce controlled thermal gradients sufficient to produce both equiaxed and columnar grain growth in the specimens. Provision was made for calorimetric measurements from which thermal conductance could be determined. Micro‐structural alterations in UO2and ThO2‐UO2materials were achieved, and thermal conductivity data were recorded. Microstructural alterations in the ThO2‐10 wt% UO2specimens were similar to those that other investigators have observed in UO2at lower tem

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15159.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

The effect of stress relaxation by creep on the thermal stress fracture of brittle ceramics at high temperature under conditions of quasi‐static heat flow is discussed. It is shown that, to a good approximation, thermal stress relaxation rates can be calculated on the basis of creep rates which correspond to the minimum temperature of the ceramic workpiece. For materials exhibiting linear stress‐creep rate dependence, expressions for the relaxation time and maximum temperature difference or heat flux to which ceramic bodies can be subjected are derived in terms of the material variables affecting thermal stress fracture and stress relaxation by creep. A numerical example shows that high‐temperature creep can materially affect the thermal stress behavior of brittle ceramics. Appropriate thermal stress parameters are proposed to form the basis of proper material selection for high‐temperature environments involving thermal stress and stress relaxation by creep. Conditions for which thermal stress calculations should be based on an elastic or viscoelastic analysis are o

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15160.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

Creep of selected brands of periclase refractories was studied using rate process theory to provide information on activation energy and on the dominant method of deformation. A brand studied over the range 2200° to 2600°F at 25 psi gave an activation energy of 62 kcal/mole. All brands were studied at selected stresses and 2500°F to determine the flow mechanism. Plastic flow was indicated for all brands test

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15161.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

Beryllium nitride (Be3N2) vaporizes congruently in the range 1640° to 1960°K by the reaction Be, N2(c) = 3Be(g) + N2(g). The equilibrium nitrogen partial pressure, in atmospheres, at the composition for congruent sublimation is given by the expression logPN2= [(–1.952 ± 0.038) × 104]T−1+ (6.509 ± 0.207). The measured enthalpy of decomposition (370 ± 5 kcal at 298° K) yields an enthalpy of formation for Be3N2(c) of –136 ± 6 kcal/mole. The upper limit to the evaporation coefficient at 1600° to 2000°K can be set as 10–4by comparison of equilibrium data to Langmuir data obtained with a sample of 18% porosity. The apparent enthalpy of activation for the reaction is 409 ± 7 kcal/mole at 1800°K for the porous Langmuir specimen. An expression is developed to predict the temperature dependence of the reduced apparent pressures in Knudsen studies of substances with low evaporation coefficients in terms of the enthalpy of activation. The variation in temperature dependence of the Langmuir measurements and Knudsen measurements with three different‐sized orifices is consistent with predictions

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15162.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

The densification produced in sodium borate glasses by pressures to 40 kbars at 25° and 250°C was measured. At a constant temperature and pressure, the densification decreased with increasing alkali content. For specimens pressed at 40 kbars and 250°C, the densification ranged from 14.2% for B2O3to 6.3% for the 33.3 mole % Na2O composition. Under the same conditions, the refractive index increase ranged from 4.8 to 1.1%, and the decrease of the molar refraction of the oxygen ions ranged from 2.5 to 1.5%. The heats of solution of the pressed glasses were more negative than those of the corresponding unpressed glasses. The heats of solution of both the pressed and unpressed glasses pass through a minimum near 20 mole % Na2O. A decrease in density was observed at room temperature for all pressed specimens. Electron micrographs were made of two of the pressed specimens. The results could be explained on the basis of a repacking of structural uni

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15163.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

The Burgers vectors of several nonbasal dislocations in sintered α‐Al2O3were determined from diffraction contrast experiments in the electron microscope. They are consistently of the type ⅓. This is also likely to be the Burgers vector for prismatic slip in α‐Al2O3. It has been generally accepted that is the Burgers vector for prismatic slip, but the evidence for this is not con

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15164.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY

摘要:

The X‐ray thermal expansion of TaCxwas measured over the composition range from TaC0.789to TaC0.997at temperatures to 1000°C. The average thermal expansion coefficient, α, increased with increasing formula weight; α from 25° to 1000°C is 6.04, 6.37, and 6.96 × 10−6/°C for TaC0.789, TaC0.912, and TaC0.997, re

ISSN:0002-7820    

DOI:10.1111/j.1151-2916.1967.tb15165.x

出版商:Blackwell Publishing Ltd    

年代:1967

数据来源: WILEY