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Solubility of lithium deuteride in liquid lithium
Journal of the Less-Common Metals, 55 (1977) 85 - 92 0 Elsevier Sequoia S.A., Lausanne - Printed in the Netherlands
85
SOLUBILITY OF LITHIUM DEUTERIDE IN LIQUID LITHIUM
E. VELECKIS,
R. M. YONCO and V. A. MARONI
Chemical Engineering Division, Argonne National Laboratory, Argonne, Ill. 60439 (U.S.A.)
9700 South Cass Avenue,
(Received October 29, 1976; in revised form January 11, 1977)
Summary The solubility of LiD in liquid lithium between the eutectic and monotectic temperatures was measured using a direct sampling method. Solubilities were found to range from 0.0154 mol.% LiD at 199 “c to 3.32 mol.% LiD at 498 “C. The data were used in the derivation of an expression for the activity coefficient of LiD as a function of temperature and composition and an equation relating deuteride solubility and temperature, thus defining the liquidus curve. Similar equations were also derived for the Li-LiH system using the existing solubility data. Extrapolation of the liquidus curves yielded the eutectic concentrations (0.040 mol.% LiH and 0.035 mol.% LiD) and the freezing point depressions (0.23 “C for Li-LiH and 0.20 “C for Li-LiD) at the eutectic point. The results are compared with the literature data for hydrogen and deuterium. The implications of the relatively high solubility of hydrogen isotopes in lithium just above the melting point are discussed with respect to the cold trapping of tritium in fusion reactor blankets.
Introduction Knowledge of the solubility of hydrogen isotopes in liquid lithium at temperatures near its melting point is important in assessingthe feasibility of cold trapping tritium in fusion reactor blankets. Solubility information is also needed for the completion of the low temperature portions of the LiLiH, Li-LiD and Li-LiT phase diagrams. The liquidus curves for the Li-LiH and Li-LiD systems are only partially known. Above the monotectic temperatures (694 “C for Li-LiH and 690 “C for Li-LiD) these curves had previously been determined in our laboratory [l, 21. Below the monotectic, however, virtually no data were available until recently, when solubility and thermal analysis studies were reported by Adams et al. [3,4] and Hubberstey et al. [ 51. In their solubility studies, successive volumes of hydrogen [ 3] or deuterium [4] were added to molten lithium and the equilibrium resistivity of the melt was measured after each addition. The resistivity increased linearly with concentration up to the
86
liquidus curve, where an abrupt change in the resistivity was observed. Their thermal studies were directed towards the freezing point measurements of hypoeu~ctic solutions of hydrogen [ 51 or deuterium t4) in liquid lithium. Concurrently, we have carried out studies to measure the solubility of LiD in liquid lithium between 199 and 498 “C using a direct sampling method. This method has been successfully employed in our work on the solubility of lithium nitride in liquid lithium [ 6 3 . Experimental Materials Lithium (purity >99.9%) was purchased from the Lithium Corporation of America in 1 lb ingots. Before use the metal was purified in an inert atmosphere box by heating freshly cut slices in a tantalum funnel to a temperature just above the melting point of lithium and allowing the melt to drip through a pinhole in the bottom of the funnel onto a stainless steel chill plate. This technique removed the surface scum and produced shiny spherules about 1 cm in diameter, containing 220 wppm nitrogen and 600 wppm oxygen. The deuterium was obtained from Air Products and Chemicals Inc. and had a stated purity of better than 99.99%. Procedure Solubility measurements were made using the same apparatus and experimental approach as that employed for the Li-LisN system [ 61. The procedure consisted of three steps: (1) preparation of an equilibrated LiLiD melt, (2) ~thdraw~ of filtered melt samples into small metal tubes by pressurizing the melt with helium and (3) analysis of the samples for deuterium. Approximately 250 g of lithium spherules was placed in an Armco iron vessel which was connected to a helium atmosphere glove box. The vessel was covered with a flange that had feed-throughs for a stirrer, a sampling shaft, a thermocouple and a manifold leading to deuterium or helium cylinders and vacuum pumps. Connected to the sampling shaft was a small nickel tube (outer diameter 0.635 cm, length 7.62 cm) the bottom end of which was closed with a press fitted, 10 pm porous nickel frit (0.48 cm thick) for filtering the melt. A Li-5.5 mol.% LiD mixture was prepared in situ by reacting the molten lithium (degassed in uacuo for 30 min) with a measured volume (-22.5 standard litres) of pure deuterium. The gas was added in approximately 21 portions over a period of several days. In order to speed up the reaction during the period of gas additions, the temperature of the melt was gradually increased from 300 to 550 “C. After the last deuterium addition the melt was cooled down, any remaining unreacted gas was pumped out and the iron vessel was refilled with helium. The melt was brought to the desired sampling temperature and samples of the melt were taken by reducing the helium pressure in the iron vessel to about 50 Ton, immersing the fritted end of a preheated sample tube into
87
the melt, restoring the pressure to 1 atm (helium) and finally withdrawing the sample tube from the melt into the cooler portion of the vessel. The filled sample tube was prepared for analysis by washing its outer surfaces with water and alcohol, returning the tube to the glove box, filing down the end of the frit to oxide-free strata and cutting the tube into several sections. The melt was stirred continuously during the sampling operations and its temperature was maintained constant to within tl “C. Sampling temperatures were chosen in a random manner. Deuterium losses from the melt during the helium pump-outs were considered to be negligible because of the low de~ompo~tion pressure of LiD f
88 TABLE
1
Mole fraction concentrations and activity liquidus line of the Li-LiD system Temp.
coefficients
of LID along the
(“C)
198.9 221.1 246.3 271.5 303.6 323.3 351.0 375.1 397.2 397.2 451.4 498.0 690a 705 756 805 840 871
0.000514 0.000768 0.00122 0.00181 0.00322 0.00427 0.00633 0.00866 0.0114 0.0117 0.0208 0.0332 0.214 0.228 0.279 0.333 0.374 0.413
aMonotectic temperature. bAt or above the monotectic the values of Nia from ref. 2. CBelow the monotectic each r& was calculated ding NW using eqn. (1).
270 200 141 107 69.8 57.7 44.1 35.9 30.0 29.3 20.7 15.6 4.60 4.33 3.54 2.94 2.58 2.31
and r’m
from the correspon-
1000
0.8l.O-
were taken
A
Li-LiD
SYSTEM (This Study)
Li-LiD
SYSTEM (ADAMS etaI.
BOO 600
1.2-
500
1.6 -
kiH 0’ hi0 Fig. 1. Phase diagrams for the Li-LiH log(composition) plane.
and Li-LiD
system
projected
on the 1000 K/T us.
89
Figure 1 shows a plot of 1000 K/T uersus log solubility. This particular type of plot construction was used in order to present the data as a conventional phase diagram, but with an expanded scale towards lower temperatures where the solubility data are more important for fusion reactor applications. Owing to the large deviations from ideality of the Li-LiD solutions, the solubility data in Fig. 1 do not fall on a straight line. A smooth liquidus curve best representing the data points was generated in the following manner. Consider a solution having a composition that corresponds to the liquidus curve at the monotectic temperature (21.4 mol.% LiD). At this point the activity of LiD is near unity and therefore the monotectic temperature may be looked upon as the freezing point of pure LiD (solvent). As more Li (solute) is added the freezing point of the solution will be lowered, as shown by the descending branch of the solubility curve in Fig. 1. The activity a’&,, of the saturated solvent may be related to the freezing point lowering [7] by the equation
0)
where the primed symbols refer to saturated solutions, -y;o and N)Ln are the activity coefficient and the mole fraction of LID, respectively, at the liquidus line, AH, the latent heat of fusion of LiD*, T, the monotectic temperature, 0 the freezing point lowering (T, - T), AC,, = Aa + A bT the difference in the heat capacities between liquid and solid LiD* and &in = 0.984 the activity of LiD at the monotectic temperature [ 21, serving here as the standard state for liquid LiD. Equation 1 was used to calculate the activity coefficients from the N&n values for points below the monotectic temperature. At or above this temperature the -y’&nvalues were taken from our previous work [ 21 (Table 1). It is customary [9] to express the activity coefficients of a binary solution in terms of power series in N. Applied to the Li-LiD solutions and with the series truncated at the cubic term, one obtains In rLiD = aNk + flN&. Assuming OLand /I to be linear functions of l/T, this analytical form was used to fit all the activity coefficient data in Table 1, giving (In Yko)/NEi = (1.124 - 2.776 Nu) + (l/T) (3285 + 143.8 NLi)
(2)
which represents the Y’&,-,data to within *2%. Omission of primes from the symbols indicates that eqn. (2) is applicable (in the range 199 - 871 “C) *For the melting of LiH, the values of AH, = 5.307 kcal mole1 and AC, = 20.61 0.192 2’ cal deg-’ mol-’ have been recently reported [ 81. For the melting of LID, the same values were assumed to be valid.
90
throughout the entire homogeneity range of the Li-rich solution region (0 < N,n < A& ). The coefficients in eqn. (2) should be considered superior to those derived in our earlier work [2], where a much narrower temperature range (705 - 871 “C) was used. Equations (1) and (2) were solved simultaneously to yield an expression relating solubility and temperature. Thus for the Li-LiD system In NLin - 10.372 In T + 4.8314 X low3 T + (67.917
+ (6.0800 - 7001.4T-1)N;m + (2.7760
- (7.2040
- 143.80T-1)Nhn’3
= 0
+ 591.35T-l)+
- 3716.4T-l)&n2
+ (3)
Numerical solution of eqn. (3) has generated the liquidus curve in Fig. 1 (solid line) which fits the observed deuterium solubility points (circles) to within ?3%. Deuterium solubilities determined by Adams et al. [4] using the electrical resistance method are also shown in Fig. 1 (triangles). At the high temperature end their data are in fair agreement with ours but towards the lower temperatures they tend to fall to the left of our derived liquidus curve, the extent of disagreement being somewhat exaggerated by the use of a logarithmic abscissa. The reasons for the discrepancy between the two sets of data are not clear but one may speculate that the presence of some residual deuterium (or hydrogen) in lithium before the resistivity experiments could have caused premature indications of saturation, thus yielding low apparent solubilities. This speculation is supported by the observation that for temperatures from 275 to 400 “C, where the relative differences between the resistivity data and our liquidus curve range from 45% to 8%, the actual differences remain essentially constant at NUD = 0.0009. We have found [ 1, 21 that a hydride or deuteride contamination of this magnitude in lithium at the start of a particular experiment is quite possible from earlier exposures of the metal to hydrogen or deuterium. Another reason for the discrepancy could be the possible failure of the filters in our sampling tubes to stop all solid deuteride particles. However, judging by our previous experience [6] with these filters, this possibility appears to be remote. The same analytical treatment was applied to the solubility data of the Li-LiH system, determined by Adams et al. [ 33 using the electrical resistance method. The treatment produced equations* for the activity coefficient of LiH as a function of temperature and composition (analogous to
*For the Li-LiH system the activity coefficient -yf,a of LiH in the Li-rich solution region can be given by (In rm)/N& = 1.135 - 2.690 NQ + (l/T) (3118 + 211.4 N&9 which is valid for temperatures 454 < T(K) < 1176 and concentrations 0 < NLt < Nit. The hydride solubility uers’sI(s temperature relationship is given by In N’EH - 10.372 In T + + 4.8314 x 1O-3 T + (68.024 + 487.77 !I’-l) + (5.8000 - 6870.2 !f’-l) NIW- - (6.9350 3752.2 7-l) NM2 + (2.6900211.40 rl) N”iH3 = 0, which is valid for temperatures 454 < T(K) < 967. In these equations Nit and N;&r are the mole fractions of Li and LiH at the liquidus line and NG is the mole fraction of Li in the unsaturated Li-LiH solutions.
91
eqn, 2) and for hydrogen solubility as a function of temperature (analogous to eqn. (3)). The latter equation is represented in Fig. 1 as the liquidus curve for the Li-LiH system (broken line). (The three lowest temperature points of Adams et al. were omitted from these calculations because of the possible contamination mentioned above for deuterium.) The two liquidus curves run almost parallel to each other. For a given temperature, the ratio NW/N, varies from 1.04 near the monotectic to 1.13 at the eutectic temperature. Extrapolation of the liquidus curves in Fig. 1 to the eutectic temperatures can yield estimates of the corresponding eutectic compositions. For purposes of this extrapolation, the eutectic temperatures were approximated by the melting point of lithium (180.49 “C) [ 41 without introducing any detectable errors. From the eutectic compositions (NMH = 4.0 X 10e4, N,in = 3.5 X 10m4) and from the latent heat of fusion of lithium (717 cal (g atom)-‘) [lo] we calculated freezing point depressions of 0.23 “C for LiLiH and 0.20 “C for Li-LiD at the corresponding eutectic points. These values are higher (-2.5X ) than the freezing point depressions reported from the thermal analysis data [4, 51.
Conclusions The results presented in this paper serve to define the liquidus boundary of the Li-LiD system and clarify the nature of the liquidus boundary for the Li-LiH system. Deuteride solubilities determined in the course of this study are in good agreement with the hydride ~lub~ity meas~emen~ previously reported in the electrical resistance studies [ 31, the hydride being somewhat more soluble in lithium than deuteride, as would be expected from similar trends observed in these systems above the monotectic temperatures [l, 21. A substantial disagreement between our results and the resistivity measurements in the Li-LiD system [ 4] is believed to be caused by the possible presence of small amounts of hydrogen in the starting lithium. Such hydrogen contamination would tend to affect the resistivity data more than it would affect the direct sampling data. Our results show that, because of the large deviations from ideality occurring in solutions of hydrogen isotopes in liquid lithium, curvilinear expressions (such as eqn. 3) should be used to describe the liquidus curves. The D/H isotope effects along the liquidus curves were shown to be small and it is reasonable to expect that the T/H and T/D isotope effects would likewise be quite small. The eutectic compositions for the Li-LiH and Li-LiD systems occur at about 400 appm H or D. Application of these results to solutions of tritium in lithium indicates that the lowest tritium concentration achievable by direct cold trapping of lithium would be well in excess of 100 wppm - a level that would be intolerably high for a fusion reactor blanket system from safety considerations alone [ll] .
92
Acknowledgments We wish to thank H. M. Feder for valuable discussions and A. Engelkemeir for mass spectrometric gas analyses. This work was supported by the Division of Physical Research of the U.S. Energy Research and Development Administration. References 1 E. Veleckis, E. H. Van Deventer and M. Blander, The lithium-lithium hydride system, J. Phys. Chem., 78 (1974) 1933. 2 E. Veleckis, Thermodynamics of the lithium-lithium deuteride system, J. Phys. Chem., 81(1977) 526. 3 P. F. Adams, M. G. Down, P. Hubberstey and R. J. Pulham, Solubilities, and solution and solvation enthalpies for nitrogen and hydrogen in pure lithium, J. Less-Common Met., 42 (1975) 325. 4 P. F. Adams, P. Hubberstey, R. J. Pulham and A. E: Thunder, Depression of the freezing point of lithium by deuterium, and the solubility of deuterium in liquid lithium: a comparison with hydrogen, J. Less-Common Met., 46 (1976) 285. 5 P. Hubberstey, R. J. Pulham and A. E. Thunder, Depression of the freezing point of lithium by nitrogen and by hydrogen, J. Chem. Sot. Faraday Trans., 72 (1976) 431. 6 R. M. Yonco, E. Veleckis and V. A. Maroni, Solubility of nitrogen in liquid lithium and thermal decomposition of solid LiaN, J. Nucl. Mater., 57 (1975) 317. 7 G. N. Lewis and M. Randall, Thermodynamics, revised by K. S. Pitzer and L. Brewer, 2nd edn., McGraw-Hill, New York, 1961, p. 404. 8 E. E. Shpilrayn, K. A. Yakimovich, D. N. Kagan and V. G. Shvalb, Fluid Mech.-Sov. Res., 3, 4 (1974) 3. 9 J. H. Hildebrand and R. L. Scott, The Solubility of Nonelectrolytes, 3rd edn., Reinhold, New York, 1950, p. 34. 10 R. Hultgren, R. L. Orr and K. K. Kelly, Selected values of the thermodynamic properties of metals and alloys, January 1970 supplement, Inorganic Materials Research Division, Lawrence Radiation Laboratory, Berkeley, University of California. 11 V. A. Maroni, R. D. Wolson and G. E. Staahl, Some preliminary considerations of a molten-salt extraction process to remove tritium from liquid lithium fusion reactor blankets, Nucl. Technol., 25 (1975) 83.
85
SOLUBILITY OF LITHIUM DEUTERIDE IN LIQUID LITHIUM
E. VELECKIS,
R. M. YONCO and V. A. MARONI
Chemical Engineering Division, Argonne National Laboratory, Argonne, Ill. 60439 (U.S.A.)
9700 South Cass Avenue,
(Received October 29, 1976; in revised form January 11, 1977)
Summary The solubility of LiD in liquid lithium between the eutectic and monotectic temperatures was measured using a direct sampling method. Solubilities were found to range from 0.0154 mol.% LiD at 199 “c to 3.32 mol.% LiD at 498 “C. The data were used in the derivation of an expression for the activity coefficient of LiD as a function of temperature and composition and an equation relating deuteride solubility and temperature, thus defining the liquidus curve. Similar equations were also derived for the Li-LiH system using the existing solubility data. Extrapolation of the liquidus curves yielded the eutectic concentrations (0.040 mol.% LiH and 0.035 mol.% LiD) and the freezing point depressions (0.23 “C for Li-LiH and 0.20 “C for Li-LiD) at the eutectic point. The results are compared with the literature data for hydrogen and deuterium. The implications of the relatively high solubility of hydrogen isotopes in lithium just above the melting point are discussed with respect to the cold trapping of tritium in fusion reactor blankets.
Introduction Knowledge of the solubility of hydrogen isotopes in liquid lithium at temperatures near its melting point is important in assessingthe feasibility of cold trapping tritium in fusion reactor blankets. Solubility information is also needed for the completion of the low temperature portions of the LiLiH, Li-LiD and Li-LiT phase diagrams. The liquidus curves for the Li-LiH and Li-LiD systems are only partially known. Above the monotectic temperatures (694 “C for Li-LiH and 690 “C for Li-LiD) these curves had previously been determined in our laboratory [l, 21. Below the monotectic, however, virtually no data were available until recently, when solubility and thermal analysis studies were reported by Adams et al. [3,4] and Hubberstey et al. [ 51. In their solubility studies, successive volumes of hydrogen [ 3] or deuterium [4] were added to molten lithium and the equilibrium resistivity of the melt was measured after each addition. The resistivity increased linearly with concentration up to the
86
liquidus curve, where an abrupt change in the resistivity was observed. Their thermal studies were directed towards the freezing point measurements of hypoeu~ctic solutions of hydrogen [ 51 or deuterium t4) in liquid lithium. Concurrently, we have carried out studies to measure the solubility of LiD in liquid lithium between 199 and 498 “C using a direct sampling method. This method has been successfully employed in our work on the solubility of lithium nitride in liquid lithium [ 6 3 . Experimental Materials Lithium (purity >99.9%) was purchased from the Lithium Corporation of America in 1 lb ingots. Before use the metal was purified in an inert atmosphere box by heating freshly cut slices in a tantalum funnel to a temperature just above the melting point of lithium and allowing the melt to drip through a pinhole in the bottom of the funnel onto a stainless steel chill plate. This technique removed the surface scum and produced shiny spherules about 1 cm in diameter, containing 220 wppm nitrogen and 600 wppm oxygen. The deuterium was obtained from Air Products and Chemicals Inc. and had a stated purity of better than 99.99%. Procedure Solubility measurements were made using the same apparatus and experimental approach as that employed for the Li-LisN system [ 61. The procedure consisted of three steps: (1) preparation of an equilibrated LiLiD melt, (2) ~thdraw~ of filtered melt samples into small metal tubes by pressurizing the melt with helium and (3) analysis of the samples for deuterium. Approximately 250 g of lithium spherules was placed in an Armco iron vessel which was connected to a helium atmosphere glove box. The vessel was covered with a flange that had feed-throughs for a stirrer, a sampling shaft, a thermocouple and a manifold leading to deuterium or helium cylinders and vacuum pumps. Connected to the sampling shaft was a small nickel tube (outer diameter 0.635 cm, length 7.62 cm) the bottom end of which was closed with a press fitted, 10 pm porous nickel frit (0.48 cm thick) for filtering the melt. A Li-5.5 mol.% LiD mixture was prepared in situ by reacting the molten lithium (degassed in uacuo for 30 min) with a measured volume (-22.5 standard litres) of pure deuterium. The gas was added in approximately 21 portions over a period of several days. In order to speed up the reaction during the period of gas additions, the temperature of the melt was gradually increased from 300 to 550 “C. After the last deuterium addition the melt was cooled down, any remaining unreacted gas was pumped out and the iron vessel was refilled with helium. The melt was brought to the desired sampling temperature and samples of the melt were taken by reducing the helium pressure in the iron vessel to about 50 Ton, immersing the fritted end of a preheated sample tube into
87
the melt, restoring the pressure to 1 atm (helium) and finally withdrawing the sample tube from the melt into the cooler portion of the vessel. The filled sample tube was prepared for analysis by washing its outer surfaces with water and alcohol, returning the tube to the glove box, filing down the end of the frit to oxide-free strata and cutting the tube into several sections. The melt was stirred continuously during the sampling operations and its temperature was maintained constant to within tl “C. Sampling temperatures were chosen in a random manner. Deuterium losses from the melt during the helium pump-outs were considered to be negligible because of the low de~ompo~tion pressure of LiD f
88 TABLE
1
Mole fraction concentrations and activity liquidus line of the Li-LiD system Temp.
coefficients
of LID along the
(“C)
198.9 221.1 246.3 271.5 303.6 323.3 351.0 375.1 397.2 397.2 451.4 498.0 690a 705 756 805 840 871
0.000514 0.000768 0.00122 0.00181 0.00322 0.00427 0.00633 0.00866 0.0114 0.0117 0.0208 0.0332 0.214 0.228 0.279 0.333 0.374 0.413
aMonotectic temperature. bAt or above the monotectic the values of Nia from ref. 2. CBelow the monotectic each r& was calculated ding NW using eqn. (1).
270 200 141 107 69.8 57.7 44.1 35.9 30.0 29.3 20.7 15.6 4.60 4.33 3.54 2.94 2.58 2.31
and r’m
from the correspon-
1000
0.8l.O-
were taken
A
Li-LiD
SYSTEM (This Study)
Li-LiD
SYSTEM (ADAMS etaI.
BOO 600
1.2-
500
1.6 -
kiH 0’ hi0 Fig. 1. Phase diagrams for the Li-LiH log(composition) plane.
and Li-LiD
system
projected
on the 1000 K/T us.
89
Figure 1 shows a plot of 1000 K/T uersus log solubility. This particular type of plot construction was used in order to present the data as a conventional phase diagram, but with an expanded scale towards lower temperatures where the solubility data are more important for fusion reactor applications. Owing to the large deviations from ideality of the Li-LiD solutions, the solubility data in Fig. 1 do not fall on a straight line. A smooth liquidus curve best representing the data points was generated in the following manner. Consider a solution having a composition that corresponds to the liquidus curve at the monotectic temperature (21.4 mol.% LiD). At this point the activity of LiD is near unity and therefore the monotectic temperature may be looked upon as the freezing point of pure LiD (solvent). As more Li (solute) is added the freezing point of the solution will be lowered, as shown by the descending branch of the solubility curve in Fig. 1. The activity a’&,, of the saturated solvent may be related to the freezing point lowering [7] by the equation
0)
where the primed symbols refer to saturated solutions, -y;o and N)Ln are the activity coefficient and the mole fraction of LID, respectively, at the liquidus line, AH, the latent heat of fusion of LiD*, T, the monotectic temperature, 0 the freezing point lowering (T, - T), AC,, = Aa + A bT the difference in the heat capacities between liquid and solid LiD* and &in = 0.984 the activity of LiD at the monotectic temperature [ 21, serving here as the standard state for liquid LiD. Equation 1 was used to calculate the activity coefficients from the N&n values for points below the monotectic temperature. At or above this temperature the -y’&nvalues were taken from our previous work [ 21 (Table 1). It is customary [9] to express the activity coefficients of a binary solution in terms of power series in N. Applied to the Li-LiD solutions and with the series truncated at the cubic term, one obtains In rLiD = aNk + flN&. Assuming OLand /I to be linear functions of l/T, this analytical form was used to fit all the activity coefficient data in Table 1, giving (In Yko)/NEi = (1.124 - 2.776 Nu) + (l/T) (3285 + 143.8 NLi)
(2)
which represents the Y’&,-,data to within *2%. Omission of primes from the symbols indicates that eqn. (2) is applicable (in the range 199 - 871 “C) *For the melting of LiH, the values of AH, = 5.307 kcal mole1 and AC, = 20.61 0.192 2’ cal deg-’ mol-’ have been recently reported [ 81. For the melting of LID, the same values were assumed to be valid.
90
throughout the entire homogeneity range of the Li-rich solution region (0 < N,n < A& ). The coefficients in eqn. (2) should be considered superior to those derived in our earlier work [2], where a much narrower temperature range (705 - 871 “C) was used. Equations (1) and (2) were solved simultaneously to yield an expression relating solubility and temperature. Thus for the Li-LiD system In NLin - 10.372 In T + 4.8314 X low3 T + (67.917
+ (6.0800 - 7001.4T-1)N;m + (2.7760
- (7.2040
- 143.80T-1)Nhn’3
= 0
+ 591.35T-l)+
- 3716.4T-l)&n2
+ (3)
Numerical solution of eqn. (3) has generated the liquidus curve in Fig. 1 (solid line) which fits the observed deuterium solubility points (circles) to within ?3%. Deuterium solubilities determined by Adams et al. [4] using the electrical resistance method are also shown in Fig. 1 (triangles). At the high temperature end their data are in fair agreement with ours but towards the lower temperatures they tend to fall to the left of our derived liquidus curve, the extent of disagreement being somewhat exaggerated by the use of a logarithmic abscissa. The reasons for the discrepancy between the two sets of data are not clear but one may speculate that the presence of some residual deuterium (or hydrogen) in lithium before the resistivity experiments could have caused premature indications of saturation, thus yielding low apparent solubilities. This speculation is supported by the observation that for temperatures from 275 to 400 “C, where the relative differences between the resistivity data and our liquidus curve range from 45% to 8%, the actual differences remain essentially constant at NUD = 0.0009. We have found [ 1, 21 that a hydride or deuteride contamination of this magnitude in lithium at the start of a particular experiment is quite possible from earlier exposures of the metal to hydrogen or deuterium. Another reason for the discrepancy could be the possible failure of the filters in our sampling tubes to stop all solid deuteride particles. However, judging by our previous experience [6] with these filters, this possibility appears to be remote. The same analytical treatment was applied to the solubility data of the Li-LiH system, determined by Adams et al. [ 33 using the electrical resistance method. The treatment produced equations* for the activity coefficient of LiH as a function of temperature and composition (analogous to
*For the Li-LiH system the activity coefficient -yf,a of LiH in the Li-rich solution region can be given by (In rm)/N& = 1.135 - 2.690 NQ + (l/T) (3118 + 211.4 N&9 which is valid for temperatures 454 < T(K) < 1176 and concentrations 0 < NLt < Nit. The hydride solubility uers’sI(s temperature relationship is given by In N’EH - 10.372 In T + + 4.8314 x 1O-3 T + (68.024 + 487.77 !I’-l) + (5.8000 - 6870.2 !f’-l) NIW- - (6.9350 3752.2 7-l) NM2 + (2.6900211.40 rl) N”iH3 = 0, which is valid for temperatures 454 < T(K) < 967. In these equations Nit and N;&r are the mole fractions of Li and LiH at the liquidus line and NG is the mole fraction of Li in the unsaturated Li-LiH solutions.
91
eqn, 2) and for hydrogen solubility as a function of temperature (analogous to eqn. (3)). The latter equation is represented in Fig. 1 as the liquidus curve for the Li-LiH system (broken line). (The three lowest temperature points of Adams et al. were omitted from these calculations because of the possible contamination mentioned above for deuterium.) The two liquidus curves run almost parallel to each other. For a given temperature, the ratio NW/N, varies from 1.04 near the monotectic to 1.13 at the eutectic temperature. Extrapolation of the liquidus curves in Fig. 1 to the eutectic temperatures can yield estimates of the corresponding eutectic compositions. For purposes of this extrapolation, the eutectic temperatures were approximated by the melting point of lithium (180.49 “C) [ 41 without introducing any detectable errors. From the eutectic compositions (NMH = 4.0 X 10e4, N,in = 3.5 X 10m4) and from the latent heat of fusion of lithium (717 cal (g atom)-‘) [lo] we calculated freezing point depressions of 0.23 “C for LiLiH and 0.20 “C for Li-LiD at the corresponding eutectic points. These values are higher (-2.5X ) than the freezing point depressions reported from the thermal analysis data [4, 51.
Conclusions The results presented in this paper serve to define the liquidus boundary of the Li-LiD system and clarify the nature of the liquidus boundary for the Li-LiH system. Deuteride solubilities determined in the course of this study are in good agreement with the hydride ~lub~ity meas~emen~ previously reported in the electrical resistance studies [ 31, the hydride being somewhat more soluble in lithium than deuteride, as would be expected from similar trends observed in these systems above the monotectic temperatures [l, 21. A substantial disagreement between our results and the resistivity measurements in the Li-LiD system [ 4] is believed to be caused by the possible presence of small amounts of hydrogen in the starting lithium. Such hydrogen contamination would tend to affect the resistivity data more than it would affect the direct sampling data. Our results show that, because of the large deviations from ideality occurring in solutions of hydrogen isotopes in liquid lithium, curvilinear expressions (such as eqn. 3) should be used to describe the liquidus curves. The D/H isotope effects along the liquidus curves were shown to be small and it is reasonable to expect that the T/H and T/D isotope effects would likewise be quite small. The eutectic compositions for the Li-LiH and Li-LiD systems occur at about 400 appm H or D. Application of these results to solutions of tritium in lithium indicates that the lowest tritium concentration achievable by direct cold trapping of lithium would be well in excess of 100 wppm - a level that would be intolerably high for a fusion reactor blanket system from safety considerations alone [ll] .
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Acknowledgments We wish to thank H. M. Feder for valuable discussions and A. Engelkemeir for mass spectrometric gas analyses. This work was supported by the Division of Physical Research of the U.S. Energy Research and Development Administration. References 1 E. Veleckis, E. H. Van Deventer and M. Blander, The lithium-lithium hydride system, J. Phys. Chem., 78 (1974) 1933. 2 E. Veleckis, Thermodynamics of the lithium-lithium deuteride system, J. Phys. Chem., 81(1977) 526. 3 P. F. Adams, M. G. Down, P. Hubberstey and R. J. Pulham, Solubilities, and solution and solvation enthalpies for nitrogen and hydrogen in pure lithium, J. Less-Common Met., 42 (1975) 325. 4 P. F. Adams, P. Hubberstey, R. J. Pulham and A. E: Thunder, Depression of the freezing point of lithium by deuterium, and the solubility of deuterium in liquid lithium: a comparison with hydrogen, J. Less-Common Met., 46 (1976) 285. 5 P. Hubberstey, R. J. Pulham and A. E. Thunder, Depression of the freezing point of lithium by nitrogen and by hydrogen, J. Chem. Sot. Faraday Trans., 72 (1976) 431. 6 R. M. Yonco, E. Veleckis and V. A. Maroni, Solubility of nitrogen in liquid lithium and thermal decomposition of solid LiaN, J. Nucl. Mater., 57 (1975) 317. 7 G. N. Lewis and M. Randall, Thermodynamics, revised by K. S. Pitzer and L. Brewer, 2nd edn., McGraw-Hill, New York, 1961, p. 404. 8 E. E. Shpilrayn, K. A. Yakimovich, D. N. Kagan and V. G. Shvalb, Fluid Mech.-Sov. Res., 3, 4 (1974) 3. 9 J. H. Hildebrand and R. L. Scott, The Solubility of Nonelectrolytes, 3rd edn., Reinhold, New York, 1950, p. 34. 10 R. Hultgren, R. L. Orr and K. K. Kelly, Selected values of the thermodynamic properties of metals and alloys, January 1970 supplement, Inorganic Materials Research Division, Lawrence Radiation Laboratory, Berkeley, University of California. 11 V. A. Maroni, R. D. Wolson and G. E. Staahl, Some preliminary considerations of a molten-salt extraction process to remove tritium from liquid lithium fusion reactor blankets, Nucl. Technol., 25 (1975) 83.