J. CHEM. SOC. FARADAY TRANS., 1994, 90(18), 2753-2755 Solutions of Aluminium in Liquid Lithium :Electrical Resistivity of Liquid Alloys Richard J. Pulham," Peter Hubberstey and Petra Hemptenmacher Chemistry Department, Univeristy of Nottingham, Nottingham, UK NG7 2RD The electrical resistivity of solutions of aluminium (up to 15.76 mol% Al) in liquid lithium has been measured by a four-point capillary method from the melting point of lithium, 180.5"C,to 440"C.Aluminium increases linearly the resistivity of liquid lithium by a relatively large amount; 5.9 x lo-' Q m (mol% Al)-' at 320°C to 5.7 x lo-' Q m (mol% AI)-' at 440°C.These relatively large values make the resistivity technique potentially useful for monitoring the concentration of aluminium during chemical reactions in these metallic solutions.The use of electrical resistivity measurements has played a part for a long time in following the chemical reactions of salts and metals when dissolved in the liquid alkali metals. This can be seen in the reactions of the salts Li2C2 and Li3N in liquid lithium to form Li2NCN in the crystalline state,' and the reaction of solutions of silicon in liquid lithium with gaseous nitrogen2 to form Li,SiN, . Moreover, the absolute value of the resistivity provides some help in identifying the nature of the solute species in solution. Correlations between resistivity and solute solvation energy3 and solute core potential/size4 have been developed to rationalise trends in the resistivity increases caused by a variety of solutes in liquid lithium.More recently there has been a role reversal in that it has been the more reactive lithium that has been titrated5 out of liquid Pb-Li (17 mol%) by nitrogen, hydrogen, oxygen and water vapour using an electrical resistivity monitor6 specifi- cally developed for continuous measurement of the lithium concentration in this dense eutectic alloy. This liquid is a potential coolant/tritium-breeder for some nuclear fusion reactor designs. Similarly, should the need arise to plate steel with aluminium compounds at high temperature to reduce tritium diffusion in such systems, then this might be done in situ from liquid lithium selutions using resistance monitoring. This paper measures the increase in resistivity of lithium on adding aluminium to study further the reactions of this Group 13 element in this unusual solvent.Experimental The basic apparatus and procedure for the determination of the resistivities of metallic solutions have been described in detail elsewhere.' It was from this design and accompanying procedure that all the many other versions, including the industrial monitor, are derived. The present work, however, incorporated two specific additional features : a secondary liquid-metal pump to ensure homogeneity of the alloys (aluminium is about 5.5-fold more dense than lithium and could take up to 40 min to dissolve) and a multisample inser- tion device both to hasten the procedure and avoid the chance of contamination from repeated loadings in a glove box.The apparatus is shown in Fig. 1. Briefly, the bulk of the liquid alloy (40 g, ca. 80 cm3) was contained in a cylindrical steel (AISI 321) reservoir, V (diam. 25 mm, height 100 mm) to which was attached a thermocouple pocket, TC, and two vertical (to prevent gas pockets) electromagnetic pumps, P1 and P2, for mixing the alloy by circulation and for contin- uous sampling of the alloy into a capillary (0.d. 3 mm, i.d. 1 mm) for resistance measurement, respectively. Both pumps drew liquid from the bottom of the reservoir and returned it to the top. The capillary loop was short-circuited by two discs, S, so that the resistance of the intervening section, R, (length 160 mm) could be measured by means of silver electri- cal leads, two on each disc, through which a constant current (ca.3 A) was passed. The resistance of the capillary and alloy were determined by measurement of the potential difference across the filled capillary and across a standard resistance (0.01 Q) connected in series. These resistances were converted into resistivities, p, for the alloy from a knowledge of the resistance of the empty capillary and its known dimensions. All necessary calculations and calibrations are described else- where.7 After calibration and filling with lithium (Koch-Light, 99.98%;ca. 30 g) in a glove box under argon, the top of the reservoir was sealed to a glass system to allow both evac- uation of gas through tap T1 and addition of consecutive, known weights of aluminium (Goodfellow, 99.999%; 1 mm thick foil, ca.1 g) through a greaseless tap, T2, from a rotat- able glass dispenser, D. The metal section of the apparatus was enclosed in a thermostatically controlled, fan-assisted, air oven. Two independent sets of experiments were performed. In the first set, successive weights of aluminium were added to lithium to give compositions of 0.98, 2.86, 4.76, 6.63, 8.46, 10.31, 12.19, 14.26 and 15.67 mol% Al. In the second set, the compositions were 1.15, 2.04, 2.97, 3.74, 4.56, 5.39, 6.34 and D T1 P1 7 P2 Fig. 1 Resistivity apparatus 2754 7.12 mol% Al. For each composition the solution was cooled and the resistivity was determined at specific temperatures.Results Experiment 1 The effect of decreasing temperature on the resistivity of the various compositions is shown in Fig. 2 for experiment 1. The resistivities are equilibrium values because on reducing the temperature, measurements were recorded only when they became constant. Each resistivity-temperature graph is of the same basic shape. For example, the homogeneous liquid 6.63 mol% A1 solution has a resistivity which decreases smoothly from 71 x lo-' R m at 460°C to 68 x lo-' R m at 324 "C; below this temperature the resistivity would no longer be linear but would drop sharply due to precipitation of Li,Al, and consequent depletion of aluminium from the solution. Thus the precipitation of solids generally sets the lower temperature limit.The set of near horizontal lines are thus the resistivities of the various homogeneous solutions. The data are summarised in Table 1 as equations portraying the temperature dependence of the resistivity. Experiment 2 The resistivities of these homogeneous solutions are again nearly parallel straight lines, and the data, in the form of 140 I -15.76120 -tct--c 14.26 --+-12.19 I ---.+-. 10.31 ---8.46p 80 MY-6.63 4.76 40 20 150 200 250 300 350 400 450 500 T/T Fig. 2 Resistivity 0s. temperature (experiment 1) for solutions of aluminium (concentrations in mol% Al) in liquid lithium Table 1 Parameters in the resistivity-temperature equations ~/10-~R m = a + bT + cT2 mol% A1 a b c R2/ofor p T/T 0.98 1.15 20.48 23.41 0.056 0.048 -3.99 -3.29 0.997/0.15 0.99910.05 189-441 213-434 2.04 2.86 2.97 3.74 4.56 4.76 5.39 28.20 29.96 36.47 40.87 46.1 1 48.15 51.67 0.051 0.064 0.029 0.030 0.029 0.027 0.026 -3.71 -4.80 1.00/0.04 0.992/0.28 0.996/0.13 0.989/0.18 0.994/0.10 0.994/0.20 0.99710.06 246-451 260-437 265-460 278-435 297-434 310-387 308-435 6.34 6.63 7.12 8.46 57.02 58.36 62.1 1 71.18 0.026 0.029 0.025 0.023 0.995/0.09 0.959/0.14 0.993/0.10 0.9 5910.1 4 318-435 326-441 325-434 340-423 10.31 12.19 80.16 92.53 0.024 0.022 0.8 39/0.2 5 0.53410.27 363-432 376-416 14.26 100.25 0.029 0.76510.21 402-432 15.75 111.12 0.026 0.74710.1 6 4 10-438 J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 resistivity-temperature equations, are combined with those of experiment 1 in Table 1.Discussion Temperature Dependence of the Resistivity Over the relatively small concentration range covered in these experiments, the curvature of the resistivity-temperature lines (experiment 1) in Fig. 2 is similar (experiment 2 also) but not identical to that of pure lithium' (lowest solid line, no points) for which the resistivity is given by p/R m = 16.476 x lo-' + 4.303 x 10-"T -2.297 x 10-13T2 This can be shown mathematically at 4OO"C, for example, where 6p/6T values R m "C-') derived from the equations at the various concentrations (mol% Al) are 0.024/ 0.98, 0.022/1.15, 0.021/2.04, 0.026/2.86, 0.032/2.97, 0.029/3.74, 0.026/4.56, 0.027/5.39, 0.024/6.34, 0.024/6.63, 0.024/7.12, 0.033/8.46, 0.026/10.31, 0.022/12.19 and 0.029/14.26.The value for lithium is 0.025 x lo-' SZ m "C-' at 400°C and falls within the range of values for the solutions. There is no discernible trend in coefficient with increasing concentration. Values at other temperatures can be calculated from the data in Table 1. At a given concentration, the temperature coefficient, 6p/6T, decreases with increasing temperature (Fig. 2 and equations in Table 1). A quantitative measure can be derived from the data in Table 1. Thus, for 0.98 mol% Al, for example, 6p/6T values decrease from 0.040 x lo-' SZ m"C-' at 20°C to 0.021 x lo-' SZ m0C-l at 440°C. Values at other compositions are available from the equations (Table 1). Composition Dependence of the Resistivity The more useful parameter is the concentration dependence of the resistivity, 6p/6x, which is given by the resistivity of the various solution concentrations at a given temperature.For this purpose the results from the two experiments are com- bined in Table 2. Over the relatively small concentrations studied, there is a linear increase in resistivity with increasing concentration of aluminium, the slope of which gives the composition dependence, 6p/6x. The coefficients from 320 "C to 440°C only are shown. The values range from 5.9 x lo-' SZ m (mol% Al)-' to 5.7 x lo-' R m (mol% Al)-'. There are fewer measurements available at 260 "C, 280 "C and 300 "C, and too few at 220 "C and 240 "C to derive any reliable coeffi- cient. The linearity implies that each aluminium atom added to the solution exhibits the same extra electron scattering as its predecessor, but no trend in coefficient with increasing concentration is evident.This is not expected to hold over the entire composition range, of course, because the resistivity of alloys, at a temperature high enough to maintain homoge- neous solutions, should describe a parabola rising to a maximum between lithium and aluminium and then falling to the value for pure liquid aluminium. The slope should vary accordingly. The composition dependence is the feature that is most useful in following chemical reactions in liquid lithium. It provides a simple measure of the concentration of aluminium during, say, a titration of aluminium from the solution by nitrogen to form the insoluble compounds AlN or Li,AlN,.Similarly, a potential use of the technique might be in follow- ing the extraction of aluminium at high temperatures from the solution by steels or nickel alloys as they become plated J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2755 Table 2 Resistivities/lO-' i2 m and resistivity-composition coeff-potential and different size (volume) has on the periodic cients, (Sp/6x)/lO-' i2 m (mol% Al)-' of solutions of aluminium in potential of the lithium solvent matri~.~ The group of solutes liquid lithium Al, Ga, In and T1 are intermediate in the Periodic Table and T/OC it will be informative to determine in the future which of the ~~ two theories best fits this group. mol% A1 320 340 360 380 400 420 440 The determination of the solvation enthalpy of aluminium in liquid lithium from a thermochemical cycle3 requires a0 27.89 28.45 28.99 29.51 30.00 30.49 30.96 0.98 34.3 1 34.9 1 35.47 36.00 36.50 36.96 37.40 knowledge of both the enthalpy of solution, Aso,H, of alu- 1.15 35.40 35.93 36.43 36.90 37.35 37.77 38.16 minium and enthalpy of formation, A,H", of the phase that 2.04 40.72 41.25 41.75 42.22 42.66 43.08 43.46 precipitates, Li9A14.The gradient of the solubility plotI2 of 2.86 45.52 46.17 46.78 47.35 47.88 48.37 48.83 In xAlus. T-(xAl = mol fraction of Al) gives a value of 39.0 2.97 45.75 46.33 46.9 1 47.49 48.07 48.65 49.23 kJ mol- for AsolH, according to the ideal solution equation 3.74 50.47 51.07 51.67 52.27 52.87 53.47 54.07 4.56 55.39 55.97 56.55 57.13 57.71 58.29 58.87 In xAl = -AsolH/RT + AsolS/R4.76 56.79 57.33 57.87 58.41 58.95 59.59 60.03 5.39 59.99 60.5 1 61.03 61.55 62.07 62.59 63.1 1 but the solvation enthalpy cannot be derived until a reliable 6.34 65.34 65.86 66.38 66.90 67.42 67.94 68.46 experimental value of Af H" for Li9A14 becomes available.6.63 -68.22 68.80 69.38 69.96 70.44 71.12 Similarly, it appears that a value of 6p/6x is needed for 7.12 -70.61 71.1 1 71.61 72.1 1 72.61 73.1 1 gallium in lithium before trends in the aluminium group can 8.46 78.54 79.00 79.46 79.92 80.38 10.3 1 ---89.28 89.76 90.24 90.72 be compared with the behaviour of Group 2 elements.12.19 100.9 101.3 14.26 ----11 1.9 112.4 113.0 15.76 -----122.0 122.6 ~ ~~~~ Conclusions 6p/6~= 5.90 5.89 5.88 5.80 5.74 5.73 5.74 k 0.04 0.03 0.03 0.03 0.04 0.03 0.03 The electrical resistivity, p, of liquid lithium is substantially increased, e.g. 6p/6x = 5.7 x lo-* R m (mol% Al)-' at 400"C, on dissolving aluminium (up to 15.76 mol% and 460 "C),but the temperature coefficient of resistivity, 6p/6T, ofwith Ni-Al, Fe-A1 or Ni-Fe-A1 alloy intermediates in corro- the solutions is not much different from that of pure lithium. sion protection. The composition coefficient, 6p/6x, is near the middle of the The resistivity of lithium is increased by adding any solute known range of coefficients for other solutes, e.g. 2.1 for so the technique is not specific. Each solute on its own, oxygen, 11.2 for germanium, in liquid lithium.The present however, increases the resistivity by a characteristic amount, results can be used to follow chemical reactions of aluminium and there is now a substantial bank of values (Table 3) on in metallic lithium solvent. which to draw. For electronegative solutes, such as oxygen, hydrogen, We thank Kim Harper and Susan Smith for practical assist- nitrogen, silicon, germanium, tin and lead, the coefficient ance. 6p/6x has been correlated with the solvation enthalpy (anionic charge/radius) of the solute in liquid lithi~rn.~ For electropositive solutes such as magnesium, calcium, strontium and barium in liquid lithium 6p/6x has been corre- References lated with the effect that a solute atom of different ion core 1 R.J. Pulham, P. Hubberstey, M. G. Down and Anne E. Thunder, J. Nucl. Muter., 1979,854,299. 2 A. T. Dadd and P. Hubberstey, J. Chem. SOC., Dalton Trans., Table 3 Resistivity-composition coeffcients, (6p/6x)/lO-' R m 1982,2175.(molo/oX)-',for solutes in liquid lithium 3 P. Hubberstey and A. T. Dadd, J. Less-Common Met., 1982, 86, 55.solute X 6PI6X T/T 4 P. Hubberstey and P. G. Roberts, Physica B, 1994,198,307. 5 P. Hubberstey, T. Sample and M. G. Barker, J. Nucl. Muter., sodium" 0.3 310 1991,179-181,886.magnesiumb 1.6 402 6 P. Hubberstey, T. Sample and M. G. Barker, Fusion Eng. Des., calciumb 0.3 402 1991, 14, 227.strontiumb 0.7 402 7 C. C. Addison, G. K. Crefield, P. Hubberstey and R. J. Pulham,bariumb 4.8 402 J. Chem. Soc. A, 1971, 1393. aluminium 5.7 400 8 G. K. Creffeld, M. G. Down and R. J. Pulham, J. Chem. SOC.,indium' ca. 6 650 Dalton Trans., 1974, 2325. thalliumd :a. 11 800 9 M. G. Down, P. Hubberstey and R. J. Pulham, J. Chem. SOC., silicon' 10.4 400 Faraday Trans. I, 1975,71, 1387. germaniume 11.2 400 10 C. van der Mare1 and W. van der Lugt, J. Phys. (Paris), 1980,tin' 11.3 400 C8, 516.lead' 9.0 400 11 V. T. Nguyen and J. E. Enderby, Phiios. Mag., 1977,35,1013.hydrogen' 4.9 400 12 R. J. Pulham, P. Hubberstey and Petra Hemptenmacher, J. oxygene 2.1 300 Phase Equilib., in the press. nitrogene 7.0 400 Ref. 9.' Ref. 4. Ref. 10. Ref. 11. Ref. 3. Paper 4/027 151;Received 9th May, 1994