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Solubilities, and solution and solvation enthalpies, for nitrogen and hydrogen in liquid lithium
Journalof 0 Elsevier
the Less-Common
Sequoia
Metals, 42 (1975)
S.A., Lausanne
- Printed
325
- 334
325
in the Netherlands
SOLUBILITIES, AND SOLUTION AND SOLVATION ENTHALPIES, FOR NITROGEN AND HYDROGEN IN LIQUID LITHIUM
PAUL F. ADAMS, PULHAM Department
MICHAEL
of Inorganic
G. DOWN, PETER
Chemistry,
University
HUBBERSTEY
of Nottingham,
and RICHARD Nottingham
NG7
J. 2RD
(Gt. Britain)
(Received
April
28, 1975)
Summary The solubilities of nitrogen and of hydrogen in liquid lithium have been determined up to 2.77 mol% N and 5.68 mol% H by electrical resistance methods, and they can be represented, in part, by the equations 2036 1ogloxN = 1.168 - T
473 < T < 708 K
2308 = 1.523 - -T
523 < T < 775 K
log,,xu
where x is the solute mole fraction. The results show that the nitrogen and hydrogen content of lithium can be reduced to 0.08 and 0.03 mol%, respectively, by filtration at 200 “C. Solubilities provide values of the partial molar enthalpies, Hcsoinj, and entropies, Soolnj (with respect to the precipitating phase), of solution of 39.23 kJ mol-l and 22.35 J K-l mol-l (for LiaN), and 44.18 kJ mol-’ and 29.15 J K-l mol-’ (for LiH). The values of Hcsolnj are used to derive solvation enthalpies of -3473 and -427 kJ mol-’ for nitride and hydride ions, respectively, in the metal. The hydrogen solubilities augment the liquidus of the Li-LiH phase diagram.
Introduction The solubility results constitute part of a study of the solvent properties of liquid metals and of the chemistry of the solute species. From a practical viewpoint, the extent to which non-metals dissolve in lithium can govern the choice of purification technique (e.g., filtration, cold trapping, gettering) for the metal. This aspect is particularly relevant to the possible use of lithium as a combined coolant and tritium breeder in the thermonuclear reactor. Our review of the solubilities of the non-metals in lithium showed that data were not available for dilute solutions of hydrogen (below 625 ‘C), and
326
that nitrogen data were inconsis~nt [l ] . In the present work, have been determined from changes in the electrical resistivity which accompany solution or precipitation. This technique is for dilute solutions, especially for those systems which exhibit eutectic liquidus [ 21. This is frequently difficult to determine native thermal and analytical methods.
solubilities of the metal very effective a steep hyperby the alter-
Experimental Apparatus
The apparatus is shown in Fig. 1 and was ~onst~cted from stainless steel (AISI 321). A cylindrical reservoir, A, (0.10 m high, 0.05 m diam.) which contained the bulk of the lithium (cu. 40 g) was attached to a neck, B, (0.18 m high, 0.025 m diam.) provided with a glass-to-metal seal, M, and glass joint. The liquid metal was circulated independently through two loops using small electromagnetic pumps, Pi and Pa, described previously [3], The main pump, Pi, was used for rapid mixing and, because the orifice was above the surface of the bulk metal, the admitted gas was provided with a continuously regenerated, clean metal surface to ensure rapid reaction. The second pump, P 2, fed the solution into a capillary, R, (0.0015 m i.d., 0.35 m long) for continuous resistance measurement. Getter&g
~a~i~it~e$
The apparatus was equipped with a winding device, Fig. 1 inset, for the addition and removal of gettering metals. Titanium sponge (Koch-Light, 99.9%) and yttrium sponge (Koch-Light, 99.9%) were used to remove nitrogen and hydrogen impurity. The getter, G, was threaded on to a stainless steel rod and supported by a steel chain, C, which was wound round a glass spindle, S.. This was rotated through a greased glass joint enabling the getter to be immersed or withdrawn from the liquid. When gas was admitted to the liquid, titanium was protected by lowering it further into a well, W, in the base of the vessel. The getter was sealed from the bulk of the liquid by the steel plug, D, which fitted closely into the coned neck of the well. Titanium was used in the form of a rod of lightly-compressed sponge which fitted snugly into the well (0.015 m,.diam., 0.045 m depth) and expelled all liquid. Yttrium, however, although sponge, was too hard to fabricate into rods, and randomly-shaped pieces were employed. These allowed unacceptably large volumes of liquid to be withdrawn from the bulk into the well, and this was therefore omitted from the apparatus when using yttrium. During the addition of gas, yttrium was removed completely from the apparatus. Procedure
The vessel was cleaned and dried as before [4]. It was mounted in an air-oven, the temperature of which could be controlled to kO.5 at 400 *C, and attached through a vent to a vacuum frame. The vessel was evacuated, the temperature raised, and the resistance across the empty capillary measured
321
Fig. 1. Apparatus.
by means of a Kelvin-Wheat&one Bridge at selected constant temperatures between 20 and 550 “C. Temperatures were measured to +0.05 o C by means of thermocouples situated at T1 (bulk temperature) and T,, and Ts, (capillary temperatures) as shown in Fig. 1. The apparatus was cooled and charged with partially-purified lithium under argon in a glove box. Lithium was supplied in ingots (Koch-Light, 99.98%) which were cut into lustrous pieces and loaded into the vessel which was reattached to the vacuum frame. The metal was raised to 400 ’ C and the getter immersed. The gettering process was followed by the fall in resistance, and, when complete, the getter was isolated and the resistance remeasured at selected temperatures. Resistivities were calculated from the resistance and capillary dimensions as before, using the formula for parallel conductors [ 41. Nitrogen [5] and hydrogen [6] (Air Products, 99.99 and 99.98%, respectively), purified as reported previously, were added to the metal in small volumes (ca. lo4 mm3 at NTP) at constant temperature. Reaction occurred rapidly to form the salts Li3N and LiH, respectively, which dissolved in the metal. Basis of the method
The resistivity method of determining the solubility of salts in liquid lithium relies on the fact that resistivity increases progressively with increasing solute concentration. Although the solubilities of the salts Li3N and LiH are relatively low, small, accurately-known volumes of gas are easily added, and resistivity changes of the order of 5 X 10-l’ Rm can be detected. More-
Compwtm
(mol
% N)
Fig. 2. Increase in resistivity, Ap, over that of lithium for lithium-nitrogen solutions as a function of composition (temperatures ‘C!, given agitinst the curves) and the solubifity of nitrogen in liquid lithium f 0 - 3 mol%N).
over, the increase in the resistivity of lithi~ caused both by nitrogen and hydrogen is comparatively large, and for these reasons the method is eminently suitable for these solutes. The change in resistivity on saturation is unmis~k~ble, as shown in the upper part of Fig. 2, where the increase in resistivity, Ap, of the metal, caused by adding nitrogen, is shown at 300,375 and 400 “C. At 400 ‘C, for example, the resistivity rises linearly with increasing nitrogen concentration from A, the value for the pure metal, to B. The maximum value of Ap is 10.2 X lo-’ Qm, which corresponds to an increase of 33% in the resistivity of lithium at this temperature. This is caused by 1.45 mol% N giving Ap fc = 7.0 X 10m8 Slm mof% NPrI The corresponding value for hydrogen is 4.8 X lo-* !&?mmol% H-l. At B, the resistivity becomes constant abruptly as more nitrogen is added. This is the saturation point. The gas still reacts to form nitride, but no more dissolves and the resistivity follows BC. The excess salt formed at the gas-metal interface adheres to the vessel walls and is not carried into the capillary. Shauld this occur, the resistivity would not remain constant but would increase sharply, since the salt has a much higher resistivity than the solution. At 375 and 300 ‘C, the resistivity break occurs earlier (at B’ and B”, respectively) since proportionately less nitride dissolves. Each break provides a point on the hypereutectic liquidus of the Li-LisN phase diagram, part of which is shown in Fig. 2. Not all points on the liquidus, however, were obtained in this way. Whereas a mixture at 300 “C, for example, contained more than enough solute at C to saturate the solution, this subsequently dissolved on heating
329
Fig. 3. Increase in resistivity, Ap, over that of lithium for lithium-hydrogen solutions as a function of temperature (compositions, mol%H, given against the curves), and the solubility of hydrogen in liquid lithium (0 - 6 mol% H).
to 400 o C giving an unsaturated solution. This aspect was exploited by stepwise-lowering of the temperature and measuring the equilibrium resistivities to determine the new precipitation temperature. The process is shown in the upper part of Fig. 3 where the difference in resistivity between the solution and lithium, Ap, is plotted against temperature for solutions containing 1.20, 0.89 and 0.46 mol% H, respectively. The solution at A contains 1.20 mol% H but is unsaturated at this temperature, 440 “C. On cooling, the resistivity falls ata characteristic rate for this composition, but at B the solution becomes saturated and precipitation of LiH occurs. From B to C, the solution contains a diminishing quantity of hydride and the resistivity therefore falls much more steeply. The more dilute solutions, 0.89 and 0.46 mol% H, have lower resistivities and precipitate at B’ and B”, respectively. Some precipitation temperatures are shown in the lower part of Fig. 3. Although all solubilities can be obtained in this way, the hydrogen curve was constructed from a combination of results from Ap-T and Ap-C methods. Results and discussion Solubilities are presented in Table 1 for 13 nitrogen solutions (200 450 o C) and 16 hydrogen solutions (212 - 551 “C). The concentration of salt is expressed as mol% non-metal to allow comparison with other solutes. Solubility increases smoothly with increasing temperature as shown in Figs. 2 and 3 for concentrations up to 3 mol%. Over this region, the liquidus for both nitrogen and hydrogen is very similar and nitrogen dissolves onlv marginally more than hydrogen. Subsequently, the similarity ends, since the nitrogen liquidus is a smooth curve up to LisN, but the lithium-lithium
330 TABLE 1 Solubility of nitrogen and hydrogen in liquid lithium Temp. (” C)
MO% N
Temp. (” C)
MO% H
200 225 250 275 300 330 340 352 375 400 400 425 435 450
0.086 0.088 0.207 0.320 0.416 0.580 0.700 0.850* 1.025 1.450 1.450+ 1.940 2.300 2.770
212 227 257 275 296 326 344 361 376 395 397 411 441 499 525 551
0.037 0.063* 0.140 0.212* 0.301 0.462* 0.596 0.753 o&92* 1.148 1.195* 1.420 1.930 3.480 4.540 5.680
* Ap-T method; all other data obtained by Ap-C method. + Yttrium-gettered; all other nitrogen data on titanium-gettered lithium. All hydrogen data on yttrium-gettered lithium.
hydride phase diagram develops into a complex liquid immiscibility region. In neither case was a eutectic composition detected directly in our experiments and any eutectic point must lie adjacent to the axis.
Comparison with previous work Three determinations have been made for nitrogen, and the results are compared with the present work in Fig. 4 (a). Our values lie between those of Fedorov et al. [7] which extend across the entire composition range, and those of Hoffman [8] and Ivanovskii et al. [9]. The high values of Fedorov et al. were obtained using 97.85% pure lithium and thermal analysis. Being a non-equilibrium method, the solutions are prone to supercooling, which can be up to 30 ‘C [9], unless slow cooling is em.ployed. The low results are in agreement with each other, and prior to the present work, the recommended low-temperature solubility was given by the dashed line. Hoffman chemically analysed the filtrate from equilibrated mixtures of LisN with Li; since each point is the mean of four determinations, the scatter probably reflects the analytical difficulties. Ivanovskii et al. used changes in resistance to detect the solution temperature of mixtures of metal with nitride heated at 300 ’ C h-l. We believe that equilibrium methods are necessary to obtain true solubilities. There are no previously reported values for hydrogen over this composition range. The solubility has been determined, however, in sodium and more recently in potassium [lo] , and a comparison is made in all three
I
450
550
( ‘C)
650
12
14
16 _ T ~mh418
t
20
Fig. 4. (a) Soiubility of nitrogen in liquid lithium: l, present results; 0, Fedorovet al. (7 1; 0, Hoffman [ 81; X, Ivanovskii etaL 191. (b) Solubility of hydrogen in liquid lithium, potassium, and sodium.
solvents in Fig. 4 (b) where log& (mol% H) is presented against reciprocal temperature. Data for lithium and potassium extend across the widest and narrowest temperature ranges, respectively. Although these solutions have different temperature coefficients, the solubi~ties are close. There have been numerous de~r~nations of hydrogen solubility in sodium, several of which are represented by the line in the Figure ill ] . It is clear that hydrogen is much less soluble in sodium than in lithium or potassium. This difference is relevant to the purification of these metals. Sodium is often purified by filtration, which is obviously less efficient for the removal of hydride from the other metals due to the higher solubilities. There are no corresponding data for rubidium or caesium. Lithium purification
Due to the chemical reactivity of the metal towards the atmosphere, nitrogen, hydrogen, and oxygen comprise the commonest non-metal impurities in the metal. The present results show that filtration at 200 “C, for example, should reduce nitrogen and hydrogen contents to 0.08 and 0.03 mol%, respectively. Filtration appears to be more effective for oxygen removal since the oxide is less soluble [l] and reduction to 10V3 mol% 0 should be possible. Gettering or, possibly, distillation is necessary to achieve lower levels. Thermodynamic
functions
For dilute solutions, solubilities generally obey the equation
where x is solute mole fraction, H+,lnj and Stsolnj are the partial molar enth-
332 TABLE 2 Solvation
enthalpies
Hydrogen Oxygen Nitrogen
(kJ mol-‘)
D
E
436** 498** 946**
-
for hydrogen,
72** 702** 2142***
oxygen and nitrogen in liquid lithium
- 91.2t -595.8ff -198.6ttt
%A,,)
us
ux
Kc*
44.2 52.5 39.2
- 874 -2856 -4817
- 427 -1960 -3473
- 412 -1991 -3460
* Ref. 14. ** Ref. 13. *** E. C. Vaughn, Trans. Faraday Sot., 55 (1959) 2025. f I. R. Ihle and C. I-I. Wu, J. Inorg. Nuel. Chem., 36 (1974) 2167. ?r F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine and I. Yaffe, Selected values of chemical thermodynamic properties, Nat. Bur. Stand. (U.S.), Circ. 500,1952, 431. wf C. E. Wicks and F. E. Block, U. S., Bur. Mines Bull. 606, 1963, 68.
alpy and entropy, respectively, of solution of one mole of solute element from the precipitating phase, and R is the gas constant. For nitrogen and hydrogen, the respective equations are: 2036 log,, XN = 1.168 - T
473
(21
2308 log,, xu = 1.523 - T
523-C T-C 775 K.
(3)
Equation (2) supercedes that given recently [I 1, which was based on a combination of published results. Above 1.96 mol% N and 3.50 mol% H, dilute solution behaviour begins to break down. Below these limits, the equations hold, and provide values of Htsolnj and SCsoh) of 39.23 kJ mol-’ and 22.35 J K-l mol-r (for LisN), and 44.18 kJ mol-’ and 29.15 J K-l mole1 (for LiH). Although three previous nitrogen solubility determinations have been made [7 - 91, these were not accompanied by estimates of HCsofnjand Sfsoh). A value of Htsoinf of 65 kJ mol-‘” derived from hydrogen solubilities [12] extrapolated from 12 mol% appears too large in view of the present results. Soluation in liquid ~~t~iu~ The values of Hfsolnj though positive are relatively small, and, therefore, the overall enthalpy change (AI?+ + H (solnj) for the solution process is negative, and both nitrogen and hydrogen dissolve. The relationship of IZtsotn) with other terms in the solution process is shown by the cycle below 3Li(s) %N+ (g) - AH* ;NM* ~~~g~ +~~~~g~
3Li’~o~
+ N3-(som)
3Li’( g) + N3-(g) where
AlI;r-: S, I, D,
V,,
and Us are the formation,
sublimation
(161 kJ mol-‘)
333
Li
Fig. 5.The
1”
20 Composttfon
lithium--lithium
hydride
30 (rnOl % Ii)
40
phase diagram.
[13], ionization (520 kJ mol-’ ) [ 131, dissociation, lattice and solution enthalpies, and E is the electron affinity. The solution is considered to consist of nitride ions solvated by cations in a matrix of cations and free electrons. Using this ionic concept, the solvation enthalpy, US, for the components of lithium nitride, is analogous to the lattice enthalpy, and Htsolnj is generally a small positive difference between the two, large negative terms, U, and US. Derivation of solvation enthalpies, however, relies on Htsolnj, and values of US calculated from the cycle for LisN, LiH, and L&O, are given in Table 2. As expected, Us becomes more negative with increasing charge on the solvated anion. According to Thompson [14], the individual solvation enthalpy of the anion, U,, can be extracted from Us by the approximation, U, = Us + I + S - @, where @ is the work function (233.5 kJ mol-‘) [15] of lithium. Values of U, so derived for the three non-metals are included in Table 2. These differ only slightly from the previously calculated values of Thompson [14] (Table 2) who used the solubility data of Hoffman [S] , Messer [ 121, and Konovalov et al. [16], for nitrogen, hydrogen, and oxygen, respectively, to estimate from an ionic model the charge and solvation shell coordination of non-metal solutes in liquid alkali metals. The Li- LiH phase diagram Our results fill in the dilute solution region of the phase diagram and render this virtually complete, as shown in Fig. 5. The high-temperature section of the diagram is composed from five sets of data collected in our recent review [l].The diagram indicates liquid immiscibility over a considerable proportion of the composition range (19.0 - 49.5 mol% H at 685 “C) and is reminiscent of alkali metal-alkali metal halide systems. The consolute temperature and composition are not defined precisely but lie close to 980 ’ C and 40 mol% H, respectively. The present data tally closely with
334
existing results to form a hypereutectic liquidus extending from the lithium axis to the monotectic reaction (685 “C and 19.0 mol% H). We find the eutectic horizontal occurs within 0.5 ‘C of the melting point of lithium, and measurements of freezing-point depression of lithium by hydrogen (to be published) indicate that the eutectic point occurs below 0.02 mol% H. Acknowledgements The authors thank the S.R.C. and U.K.A.E.A. nance grants (M.G.D. and P.F.A., respectively).
(Culham) for mainte-
References 1 P. F. Adams, P. Hubberstey and R. J. Pulham, J. Less-Common Met., 42 (1975) 1. 2 P. Hubberstey and R; J. Pulham, J. Chem. Sot., Dalton Trans., (1972) 819. 3 C. C. Addison, G. K. Creffield, P. Hubberstey and R. J. Pulham, J. Chem. Sot. A, (1971) 1393. 4 C. C. Addison, G. K. Creffield, P. Hubberstey and R. J. Pulham, J. Chem. Sot. A, (1969) 1482. 5 C. C. Addison, R. J. Pulham and E. A. Trevillion, J. Chem. Sot., Dalton Trans., (1975) in the press, 6 R. J. Pull-ram, J. Chem. Soc.A, (1971) 1389. 7 K. A. Bolshakov, P. I. Fedorov and L. Stepina, Izv. Vyssh. Uchebn. Zaved., Tsvetn. Metafl., (1959) 52. 8 E. E. Hoffman, USAEC Rep, ORNL - 2894,196O. 9 M. N. ArnoPdov, M. N. Ivanovskii and B. A. Shmatko, Teplofiz. Vys. Temp., 5 (1967) 380. 10 M. N. Arnol’dov, M. N. Ivanovskii, V. A. Morozov, S. S. Pletenets and V. V. Sitnikov, Proc. Akad. Sci., U.S.S.R., Metals, 1 (1973) 74. 11 A. C. Whittingham, C.E.G.B. Rep. RD/B/N2550,1974. 12 C. E. Messer, E. B, Damon, P. C. Maybury, J. Mellor and R. A. Seales, J. Phys. Chem,, 62 (1958) 220. 13 W. E. Dasent, Inorganic Energetics, Penguin, London, 1970. 14 R. Thompson, J, Inorg. Nucl. Chem., 34 (1972) 2513. 15 Handbook of Chemistry and Physics, Chemical Rubber Co., Cleveland, Ohio, 53rd Edn., 1972, E69. 16 E. E. Konovalov, N. I. Seliverstov and V. P. Emel’yanov, Izv. Akad. Nauk SSSR Met., 3 (1968) 77.
the Less-Common
Sequoia
Metals, 42 (1975)
S.A., Lausanne
- Printed
325
- 334
325
in the Netherlands
SOLUBILITIES, AND SOLUTION AND SOLVATION ENTHALPIES, FOR NITROGEN AND HYDROGEN IN LIQUID LITHIUM
PAUL F. ADAMS, PULHAM Department
MICHAEL
of Inorganic
G. DOWN, PETER
Chemistry,
University
HUBBERSTEY
of Nottingham,
and RICHARD Nottingham
NG7
J. 2RD
(Gt. Britain)
(Received
April
28, 1975)
Summary The solubilities of nitrogen and of hydrogen in liquid lithium have been determined up to 2.77 mol% N and 5.68 mol% H by electrical resistance methods, and they can be represented, in part, by the equations 2036 1ogloxN = 1.168 - T
473 < T < 708 K
2308 = 1.523 - -T
523 < T < 775 K
log,,xu
where x is the solute mole fraction. The results show that the nitrogen and hydrogen content of lithium can be reduced to 0.08 and 0.03 mol%, respectively, by filtration at 200 “C. Solubilities provide values of the partial molar enthalpies, Hcsoinj, and entropies, Soolnj (with respect to the precipitating phase), of solution of 39.23 kJ mol-l and 22.35 J K-l mol-l (for LiaN), and 44.18 kJ mol-’ and 29.15 J K-l mol-’ (for LiH). The values of Hcsolnj are used to derive solvation enthalpies of -3473 and -427 kJ mol-’ for nitride and hydride ions, respectively, in the metal. The hydrogen solubilities augment the liquidus of the Li-LiH phase diagram.
Introduction The solubility results constitute part of a study of the solvent properties of liquid metals and of the chemistry of the solute species. From a practical viewpoint, the extent to which non-metals dissolve in lithium can govern the choice of purification technique (e.g., filtration, cold trapping, gettering) for the metal. This aspect is particularly relevant to the possible use of lithium as a combined coolant and tritium breeder in the thermonuclear reactor. Our review of the solubilities of the non-metals in lithium showed that data were not available for dilute solutions of hydrogen (below 625 ‘C), and
326
that nitrogen data were inconsis~nt [l ] . In the present work, have been determined from changes in the electrical resistivity which accompany solution or precipitation. This technique is for dilute solutions, especially for those systems which exhibit eutectic liquidus [ 21. This is frequently difficult to determine native thermal and analytical methods.
solubilities of the metal very effective a steep hyperby the alter-
Experimental Apparatus
The apparatus is shown in Fig. 1 and was ~onst~cted from stainless steel (AISI 321). A cylindrical reservoir, A, (0.10 m high, 0.05 m diam.) which contained the bulk of the lithium (cu. 40 g) was attached to a neck, B, (0.18 m high, 0.025 m diam.) provided with a glass-to-metal seal, M, and glass joint. The liquid metal was circulated independently through two loops using small electromagnetic pumps, Pi and Pa, described previously [3], The main pump, Pi, was used for rapid mixing and, because the orifice was above the surface of the bulk metal, the admitted gas was provided with a continuously regenerated, clean metal surface to ensure rapid reaction. The second pump, P 2, fed the solution into a capillary, R, (0.0015 m i.d., 0.35 m long) for continuous resistance measurement. Getter&g
~a~i~it~e$
The apparatus was equipped with a winding device, Fig. 1 inset, for the addition and removal of gettering metals. Titanium sponge (Koch-Light, 99.9%) and yttrium sponge (Koch-Light, 99.9%) were used to remove nitrogen and hydrogen impurity. The getter, G, was threaded on to a stainless steel rod and supported by a steel chain, C, which was wound round a glass spindle, S.. This was rotated through a greased glass joint enabling the getter to be immersed or withdrawn from the liquid. When gas was admitted to the liquid, titanium was protected by lowering it further into a well, W, in the base of the vessel. The getter was sealed from the bulk of the liquid by the steel plug, D, which fitted closely into the coned neck of the well. Titanium was used in the form of a rod of lightly-compressed sponge which fitted snugly into the well (0.015 m,.diam., 0.045 m depth) and expelled all liquid. Yttrium, however, although sponge, was too hard to fabricate into rods, and randomly-shaped pieces were employed. These allowed unacceptably large volumes of liquid to be withdrawn from the bulk into the well, and this was therefore omitted from the apparatus when using yttrium. During the addition of gas, yttrium was removed completely from the apparatus. Procedure
The vessel was cleaned and dried as before [4]. It was mounted in an air-oven, the temperature of which could be controlled to kO.5 at 400 *C, and attached through a vent to a vacuum frame. The vessel was evacuated, the temperature raised, and the resistance across the empty capillary measured
321
Fig. 1. Apparatus.
by means of a Kelvin-Wheat&one Bridge at selected constant temperatures between 20 and 550 “C. Temperatures were measured to +0.05 o C by means of thermocouples situated at T1 (bulk temperature) and T,, and Ts, (capillary temperatures) as shown in Fig. 1. The apparatus was cooled and charged with partially-purified lithium under argon in a glove box. Lithium was supplied in ingots (Koch-Light, 99.98%) which were cut into lustrous pieces and loaded into the vessel which was reattached to the vacuum frame. The metal was raised to 400 ’ C and the getter immersed. The gettering process was followed by the fall in resistance, and, when complete, the getter was isolated and the resistance remeasured at selected temperatures. Resistivities were calculated from the resistance and capillary dimensions as before, using the formula for parallel conductors [ 41. Nitrogen [5] and hydrogen [6] (Air Products, 99.99 and 99.98%, respectively), purified as reported previously, were added to the metal in small volumes (ca. lo4 mm3 at NTP) at constant temperature. Reaction occurred rapidly to form the salts Li3N and LiH, respectively, which dissolved in the metal. Basis of the method
The resistivity method of determining the solubility of salts in liquid lithium relies on the fact that resistivity increases progressively with increasing solute concentration. Although the solubilities of the salts Li3N and LiH are relatively low, small, accurately-known volumes of gas are easily added, and resistivity changes of the order of 5 X 10-l’ Rm can be detected. More-
Compwtm
(mol
% N)
Fig. 2. Increase in resistivity, Ap, over that of lithium for lithium-nitrogen solutions as a function of composition (temperatures ‘C!, given agitinst the curves) and the solubifity of nitrogen in liquid lithium f 0 - 3 mol%N).
over, the increase in the resistivity of lithi~ caused both by nitrogen and hydrogen is comparatively large, and for these reasons the method is eminently suitable for these solutes. The change in resistivity on saturation is unmis~k~ble, as shown in the upper part of Fig. 2, where the increase in resistivity, Ap, of the metal, caused by adding nitrogen, is shown at 300,375 and 400 “C. At 400 ‘C, for example, the resistivity rises linearly with increasing nitrogen concentration from A, the value for the pure metal, to B. The maximum value of Ap is 10.2 X lo-’ Qm, which corresponds to an increase of 33% in the resistivity of lithium at this temperature. This is caused by 1.45 mol% N giving Ap fc = 7.0 X 10m8 Slm mof% NPrI The corresponding value for hydrogen is 4.8 X lo-* !&?mmol% H-l. At B, the resistivity becomes constant abruptly as more nitrogen is added. This is the saturation point. The gas still reacts to form nitride, but no more dissolves and the resistivity follows BC. The excess salt formed at the gas-metal interface adheres to the vessel walls and is not carried into the capillary. Shauld this occur, the resistivity would not remain constant but would increase sharply, since the salt has a much higher resistivity than the solution. At 375 and 300 ‘C, the resistivity break occurs earlier (at B’ and B”, respectively) since proportionately less nitride dissolves. Each break provides a point on the hypereutectic liquidus of the Li-LisN phase diagram, part of which is shown in Fig. 2. Not all points on the liquidus, however, were obtained in this way. Whereas a mixture at 300 “C, for example, contained more than enough solute at C to saturate the solution, this subsequently dissolved on heating
329
Fig. 3. Increase in resistivity, Ap, over that of lithium for lithium-hydrogen solutions as a function of temperature (compositions, mol%H, given against the curves), and the solubility of hydrogen in liquid lithium (0 - 6 mol% H).
to 400 o C giving an unsaturated solution. This aspect was exploited by stepwise-lowering of the temperature and measuring the equilibrium resistivities to determine the new precipitation temperature. The process is shown in the upper part of Fig. 3 where the difference in resistivity between the solution and lithium, Ap, is plotted against temperature for solutions containing 1.20, 0.89 and 0.46 mol% H, respectively. The solution at A contains 1.20 mol% H but is unsaturated at this temperature, 440 “C. On cooling, the resistivity falls ata characteristic rate for this composition, but at B the solution becomes saturated and precipitation of LiH occurs. From B to C, the solution contains a diminishing quantity of hydride and the resistivity therefore falls much more steeply. The more dilute solutions, 0.89 and 0.46 mol% H, have lower resistivities and precipitate at B’ and B”, respectively. Some precipitation temperatures are shown in the lower part of Fig. 3. Although all solubilities can be obtained in this way, the hydrogen curve was constructed from a combination of results from Ap-T and Ap-C methods. Results and discussion Solubilities are presented in Table 1 for 13 nitrogen solutions (200 450 o C) and 16 hydrogen solutions (212 - 551 “C). The concentration of salt is expressed as mol% non-metal to allow comparison with other solutes. Solubility increases smoothly with increasing temperature as shown in Figs. 2 and 3 for concentrations up to 3 mol%. Over this region, the liquidus for both nitrogen and hydrogen is very similar and nitrogen dissolves onlv marginally more than hydrogen. Subsequently, the similarity ends, since the nitrogen liquidus is a smooth curve up to LisN, but the lithium-lithium
330 TABLE 1 Solubility of nitrogen and hydrogen in liquid lithium Temp. (” C)
MO% N
Temp. (” C)
MO% H
200 225 250 275 300 330 340 352 375 400 400 425 435 450
0.086 0.088 0.207 0.320 0.416 0.580 0.700 0.850* 1.025 1.450 1.450+ 1.940 2.300 2.770
212 227 257 275 296 326 344 361 376 395 397 411 441 499 525 551
0.037 0.063* 0.140 0.212* 0.301 0.462* 0.596 0.753 o&92* 1.148 1.195* 1.420 1.930 3.480 4.540 5.680
* Ap-T method; all other data obtained by Ap-C method. + Yttrium-gettered; all other nitrogen data on titanium-gettered lithium. All hydrogen data on yttrium-gettered lithium.
hydride phase diagram develops into a complex liquid immiscibility region. In neither case was a eutectic composition detected directly in our experiments and any eutectic point must lie adjacent to the axis.
Comparison with previous work Three determinations have been made for nitrogen, and the results are compared with the present work in Fig. 4 (a). Our values lie between those of Fedorov et al. [7] which extend across the entire composition range, and those of Hoffman [8] and Ivanovskii et al. [9]. The high values of Fedorov et al. were obtained using 97.85% pure lithium and thermal analysis. Being a non-equilibrium method, the solutions are prone to supercooling, which can be up to 30 ‘C [9], unless slow cooling is em.ployed. The low results are in agreement with each other, and prior to the present work, the recommended low-temperature solubility was given by the dashed line. Hoffman chemically analysed the filtrate from equilibrated mixtures of LisN with Li; since each point is the mean of four determinations, the scatter probably reflects the analytical difficulties. Ivanovskii et al. used changes in resistance to detect the solution temperature of mixtures of metal with nitride heated at 300 ’ C h-l. We believe that equilibrium methods are necessary to obtain true solubilities. There are no previously reported values for hydrogen over this composition range. The solubility has been determined, however, in sodium and more recently in potassium [lo] , and a comparison is made in all three
I
450
550
( ‘C)
650
12
14
16 _ T ~mh418
t
20
Fig. 4. (a) Soiubility of nitrogen in liquid lithium: l, present results; 0, Fedorovet al. (7 1; 0, Hoffman [ 81; X, Ivanovskii etaL 191. (b) Solubility of hydrogen in liquid lithium, potassium, and sodium.
solvents in Fig. 4 (b) where log& (mol% H) is presented against reciprocal temperature. Data for lithium and potassium extend across the widest and narrowest temperature ranges, respectively. Although these solutions have different temperature coefficients, the solubi~ties are close. There have been numerous de~r~nations of hydrogen solubility in sodium, several of which are represented by the line in the Figure ill ] . It is clear that hydrogen is much less soluble in sodium than in lithium or potassium. This difference is relevant to the purification of these metals. Sodium is often purified by filtration, which is obviously less efficient for the removal of hydride from the other metals due to the higher solubilities. There are no corresponding data for rubidium or caesium. Lithium purification
Due to the chemical reactivity of the metal towards the atmosphere, nitrogen, hydrogen, and oxygen comprise the commonest non-metal impurities in the metal. The present results show that filtration at 200 “C, for example, should reduce nitrogen and hydrogen contents to 0.08 and 0.03 mol%, respectively. Filtration appears to be more effective for oxygen removal since the oxide is less soluble [l] and reduction to 10V3 mol% 0 should be possible. Gettering or, possibly, distillation is necessary to achieve lower levels. Thermodynamic
functions
For dilute solutions, solubilities generally obey the equation
where x is solute mole fraction, H+,lnj and Stsolnj are the partial molar enth-
332 TABLE 2 Solvation
enthalpies
Hydrogen Oxygen Nitrogen
(kJ mol-‘)
D
E
436** 498** 946**
-
for hydrogen,
72** 702** 2142***
oxygen and nitrogen in liquid lithium
- 91.2t -595.8ff -198.6ttt
%A,,)
us
ux
Kc*
44.2 52.5 39.2
- 874 -2856 -4817
- 427 -1960 -3473
- 412 -1991 -3460
* Ref. 14. ** Ref. 13. *** E. C. Vaughn, Trans. Faraday Sot., 55 (1959) 2025. f I. R. Ihle and C. I-I. Wu, J. Inorg. Nuel. Chem., 36 (1974) 2167. ?r F. D. Rossini, D. D. Wagman, W. H. Evans, S. Levine and I. Yaffe, Selected values of chemical thermodynamic properties, Nat. Bur. Stand. (U.S.), Circ. 500,1952, 431. wf C. E. Wicks and F. E. Block, U. S., Bur. Mines Bull. 606, 1963, 68.
alpy and entropy, respectively, of solution of one mole of solute element from the precipitating phase, and R is the gas constant. For nitrogen and hydrogen, the respective equations are: 2036 log,, XN = 1.168 - T
473
(21
2308 log,, xu = 1.523 - T
523-C T-C 775 K.
(3)
Equation (2) supercedes that given recently [I 1, which was based on a combination of published results. Above 1.96 mol% N and 3.50 mol% H, dilute solution behaviour begins to break down. Below these limits, the equations hold, and provide values of Htsolnj and SCsoh) of 39.23 kJ mol-’ and 22.35 J K-l mol-r (for LisN), and 44.18 kJ mol-’ and 29.15 J K-l mole1 (for LiH). Although three previous nitrogen solubility determinations have been made [7 - 91, these were not accompanied by estimates of HCsofnjand Sfsoh). A value of Htsoinf of 65 kJ mol-‘” derived from hydrogen solubilities [12] extrapolated from 12 mol% appears too large in view of the present results. Soluation in liquid ~~t~iu~ The values of Hfsolnj though positive are relatively small, and, therefore, the overall enthalpy change (AI?+ + H (solnj) for the solution process is negative, and both nitrogen and hydrogen dissolve. The relationship of IZtsotn) with other terms in the solution process is shown by the cycle below 3Li(s) %N+ (g) - AH* ;NM* ~~~g~ +~~~~g~
3Li’~o~
+ N3-(som)
3Li’( g) + N3-(g) where
AlI;r-: S, I, D,
V,,
and Us are the formation,
sublimation
(161 kJ mol-‘)
333
Li
Fig. 5.The
1”
20 Composttfon
lithium--lithium
hydride
30 (rnOl % Ii)
40
phase diagram.
[13], ionization (520 kJ mol-’ ) [ 131, dissociation, lattice and solution enthalpies, and E is the electron affinity. The solution is considered to consist of nitride ions solvated by cations in a matrix of cations and free electrons. Using this ionic concept, the solvation enthalpy, US, for the components of lithium nitride, is analogous to the lattice enthalpy, and Htsolnj is generally a small positive difference between the two, large negative terms, U, and US. Derivation of solvation enthalpies, however, relies on Htsolnj, and values of US calculated from the cycle for LisN, LiH, and L&O, are given in Table 2. As expected, Us becomes more negative with increasing charge on the solvated anion. According to Thompson [14], the individual solvation enthalpy of the anion, U,, can be extracted from Us by the approximation, U, = Us + I + S - @, where @ is the work function (233.5 kJ mol-‘) [15] of lithium. Values of U, so derived for the three non-metals are included in Table 2. These differ only slightly from the previously calculated values of Thompson [14] (Table 2) who used the solubility data of Hoffman [S] , Messer [ 121, and Konovalov et al. [16], for nitrogen, hydrogen, and oxygen, respectively, to estimate from an ionic model the charge and solvation shell coordination of non-metal solutes in liquid alkali metals. The Li- LiH phase diagram Our results fill in the dilute solution region of the phase diagram and render this virtually complete, as shown in Fig. 5. The high-temperature section of the diagram is composed from five sets of data collected in our recent review [l].The diagram indicates liquid immiscibility over a considerable proportion of the composition range (19.0 - 49.5 mol% H at 685 “C) and is reminiscent of alkali metal-alkali metal halide systems. The consolute temperature and composition are not defined precisely but lie close to 980 ’ C and 40 mol% H, respectively. The present data tally closely with
334
existing results to form a hypereutectic liquidus extending from the lithium axis to the monotectic reaction (685 “C and 19.0 mol% H). We find the eutectic horizontal occurs within 0.5 ‘C of the melting point of lithium, and measurements of freezing-point depression of lithium by hydrogen (to be published) indicate that the eutectic point occurs below 0.02 mol% H. Acknowledgements The authors thank the S.R.C. and U.K.A.E.A. nance grants (M.G.D. and P.F.A., respectively).
(Culham) for mainte-
References 1 P. F. Adams, P. Hubberstey and R. J. Pulham, J. Less-Common Met., 42 (1975) 1. 2 P. Hubberstey and R; J. Pulham, J. Chem. Sot., Dalton Trans., (1972) 819. 3 C. C. Addison, G. K. Creffield, P. Hubberstey and R. J. Pulham, J. Chem. Sot. A, (1971) 1393. 4 C. C. Addison, G. K. Creffield, P. Hubberstey and R. J. Pulham, J. Chem. Sot. A, (1969) 1482. 5 C. C. Addison, R. J. Pulham and E. A. Trevillion, J. Chem. Sot., Dalton Trans., (1975) in the press, 6 R. J. Pull-ram, J. Chem. Soc.A, (1971) 1389. 7 K. A. Bolshakov, P. I. Fedorov and L. Stepina, Izv. Vyssh. Uchebn. Zaved., Tsvetn. Metafl., (1959) 52. 8 E. E. Hoffman, USAEC Rep, ORNL - 2894,196O. 9 M. N. ArnoPdov, M. N. Ivanovskii and B. A. Shmatko, Teplofiz. Vys. Temp., 5 (1967) 380. 10 M. N. Arnol’dov, M. N. Ivanovskii, V. A. Morozov, S. S. Pletenets and V. V. Sitnikov, Proc. Akad. Sci., U.S.S.R., Metals, 1 (1973) 74. 11 A. C. Whittingham, C.E.G.B. Rep. RD/B/N2550,1974. 12 C. E. Messer, E. B, Damon, P. C. Maybury, J. Mellor and R. A. Seales, J. Phys. Chem,, 62 (1958) 220. 13 W. E. Dasent, Inorganic Energetics, Penguin, London, 1970. 14 R. Thompson, J, Inorg. Nucl. Chem., 34 (1972) 2513. 15 Handbook of Chemistry and Physics, Chemical Rubber Co., Cleveland, Ohio, 53rd Edn., 1972, E69. 16 E. E. Konovalov, N. I. Seliverstov and V. P. Emel’yanov, Izv. Akad. Nauk SSSR Met., 3 (1968) 77.