Journal of the Less-Common
Metals, 77 (1981)
205 - 213
PHASE RELATIONS IN THE THORIUM-C~MIUM
Chemical Engineering Division, Argonne, IL 60439 (U.S.A.) (Received
B. S. TAN1 and M. KRUMPELT
9700 South Cass Avenue,
May 8, 1980)
Summary A partial phase diagram for the Th-Cd system was established using chemical analysis, electron probe microanalysis (EPM), differential thermal analysis and X-ray diffraction (XRD) techniques. The solubility of thorium in cadmium can be represented by the equations log (wt.%Th) = 7.349 - 5458T-’ in the range 599 - 841 K and log(wt.%Th)
= 7.705 - 10 142T-1 + 3.688 X 106!F2
in the range 841- 1021 K. The most cadmium-rich phase, ThCdr,, decomposes pe~~ctic~ly at 568 “C into ThCd, plus saturated liquid. Phases previously suggested to be ThCd,,, ThCd, and ThCda were confirmed by EPM and a new phase, ThCds, was identified by EPM and XRD (hexagonal, a = 6.775 A and c = 4.928 A).
1. Introduction Binary phase diagrams of cadmium with uranium [ 11, neptunium [ 21 and plutonium [3, 41 have been reported in the literature, but with thorium the existence of only three intermetallic compounds has been established. ThCd,r has been identified by an X-ray diffraction pattern similar to that of Bang,, [ 51. ThCd,  and ThCdz [ 7 ] have been identified from powder patterns that could be indexed on the basis of the ErZn, and AIBs structures respectively. No chemical analyses to verify the stoichiomet~ have been reported for any of the three compounds. In this study Cd-Th mixtures ranging in composition (atom ratios) from 1.3/l to 13/l were equilibrated at various temperatures and were analyzed by electron probe microanalysis (EPM), X-ray diffraction (XRD) and differential thermal analysis (DTA). A partial liquidus line was determined from chemical analysis of liquid samples taken over the temperature range 326 - 748 “C. @ Elsevier Sequoia/Printed
in The Netherlands
2. Experimental The liquidus line was determined using the apparatus shown in Fig. 1. The liquid alloy was contained in a tantalum crucible which was placed inside a stainless steel furnace tube. The furnace tube was bolted to the bottom of a glove box containing an argon atmosphere. The atmospheres of the glove box and furnace tube were maintained separately so that the furnace tube could be evacuated and purged with research grade argon (99.9995%). The melt temperature was monitored to within +1 “C with a calibrated chromel-alumel thermocouple which was placed inside a tantalum thermowell. The melt was agitated using a tantalum paddle driven by a variable speed motor and filtered samples were taken through a tantalum frit (35 pm porosity) which was press fitted into the bottom of a tantalum tube by pressurizing the melt. The sampling procedure has been described previously [ 81. The thorium metal was obtained from Ames Laboratory, Iowa State University, as a massive ingot and was cut into small pieces. A dense oxide layer was removed mechanically or by dissolving the layer in aqueous HsSiFs-HNOs. Spectroscopic analysis indicated the major impurities in the metal to be phosphors (600 ppm), beryllium (300 ppm), nickel (120 ppm) and iron (100 ppm). A gravimetric analysis showed the sample to contain
Fig. 1. A diagram of the experimental apparatus: a, stirrer motor; b, stirrer housing; c, tantalum thermowell; d, compression fitting; e, flange welded to glove box floor; f, cooling water; g, to argon, vacuum lines; h, radiation shields; i, sampling-addition port; j, ball valve; k, furnace tube cover; 1, furnace tube flange; m, furnace tube; n, furnace tube liner; o, to sampling-addition tube; p, furnace; q, insulating brick.
99.55 wt.% Th, indicating that additional impurities like oxygen and carbon were present. The cadmium was reagent grade metal (melting point 321 "C) witha purity of 99.97%. Three separate solubility experiments were conducted covering the temperature ranges 326 -569,573 -658 and 661- 748 "C.In the experiment performed in the highest temperature range the alloy was covered with a layer of CaCl,-15at.%CaFP (melting point approximately 650 “C) to suppress cadmium vaporization. No distortion of the measured thorium concentration in the cadmium as a result of the salt layer was expected because the thorium is partitioned strongly in the alloy [ 91. The mixtures were initially stirred at the highest temperature of a given range. Liquid samples were then taken at selected lower temperatures after stirring for at least 60 min at each temperature. Sampling temperatures were approached alternately by either heating or cooling. The filtered samples were dissolved in HNOs and were analyzed for thorium by mass spectrometric isotope dilution with =‘Th . The estimated accuracy of the method is approximately 0.2%. Samples for EPM, XRD and DTA analyses were prepared by heating pieces of thorium and cadmium in sealed tantalum tubes 8 mm in diameter and 6 cm in length. The tubes were welded shut in a glove box containing an inert atmosphere and were then held at elevated temperatures. Either a rocking furnace or a stationary furnace was used. The atom ratios and annealing conditions are shown in Table 1. The resulting alloys were sectioned; one-half was used for EPM and the other half was used for DTA and XRD. Because of their sensitivity to air and moisture, especially at the higher Th/Cd ratios, samples were powdered and were sealed into X-ray capillaries under argon or helium atmospheres. SpeciTABLE 1 Reaction conditions of the solid samples Sample number
Th/Cd atom ratio
1 2 3 4 5 6 7 8 9 10 11 12 13
l/12.7 l/5.7 115.7 115.7 l/4.0 114.0 113.5 l/3.5 l/3.5 113.0 113.0 l/2.0 l/l.35
17 days at 545 “C 6.7 days at 850 “C (rocking furnace) 6.7 days at 1000 “C, 21 days at 800 “C 6.8 days at 750 “C (rocking furnace) 6.7 days at 900 “C (rocking furnace) 6.6 days at 800 “C (rocking furnace) 6.7 days at 1000 ‘C, 7.9 days at 900 “C 6.7 days at 1000 “C, 21 days at 800 “C 14 days at 650 “C, 7 days at 800 “C (rocking furnace) 6.7 days at 1000 ‘C, 7.9 days at 900 “C 21 days at 800 “C, 6.7 days at 1000 “C 6.7 days at 1000 ‘C, 7.9 days at 900 “C, 21 days at 950 “C 6.7 days at 1000 ‘C, 7.9 days at 900 “C, 21 days at 950 “C
mens mounted in epoxy molds for EPM were polished in air under oil, were washed with toluene and were stored in an evacuated or helium-filled desiccator. EPM examinations were conducted at an acceleration voltage of 20 kV and a beam current of 0.2 PA using Th Ma and Cd L& lines. The instrument was calibrated against the ThCd, compound, which had been prepared by this technique and identified by X-ray diffraction (see Section 3). The X-ray diff~ctio~ data were collected on a powder camera 114.6 mm in diameter using filtered copper and iron radiations. Transition temperatures were determined with a Rigaku thermal analyzer on samples the compositions of which were known from EPM and XRD analyses. The inst~ment was calibrated against standards from the National Bureau of Standards and thermal arrests were measured on heating curves only since the tantalum sample cup was not sealed and some cadmium could escape at the highest temperatures.
3. Results and discussion The solubility of thorium in liquid cadmium, as determined in the three separate experiments, is given in Table 2. A plot of the logarithm of the TABLE 2 The solubiiity of thorium in cadmium Experimental
326 371 389 423 447 497 524 551 569
0.0178 0.0746 0.126 0.317 0.565 1.973 2.990 5.487 7.301
of Th in Cd (at.%) 0.009
0.036 0.061 0.154 0.275 0.966 1.471 2.736 3.676
573 598 615 637 644 658
7.49 8.03 9.43 10.04 10.93 11.74
3.77 4.06 4.80 5.13 5.61 6.05
661 675 705 72Ti 748
11.89 12.94 15.19 18.24 20.20
6.14 6.72 7.98 9.75 10.92
Fig. 2. The solubility
in liquid cadmium:
solubility against the reciprocal of the absolute temperature is presented in Fig. 2. Evidence that equilibrium was reached is given by the fact that data obtained both on heating and on cooling fall on substantially the same lines. The low temperature data (in the range 599 - 841 K) may be represented by the equation log (wt.%Th) = 7.349 - 5458T-’
with a standard deviation of 0.047 while the high temperature results (in the range 841- 1021 K) can be fitted to the equation log(wt.%Th)
= 7.705 - 10 142T-’
+ 3.688 X 106T-’
The intersection of the two lines, occurring at 565.8 ‘C, corresponds to the temperature at which the most cadmium-rich intermetallic compound peritectically decomposes into a second inter-metallic phase and a saturated liquid phase. Earlier solubility measurements below 568 “C performed by Johnson and Anderson [ 51 were represented by a curved (quadratic) relation between log (wt.%‘Th) and l/T whereas this work supports a linear relation. A recent communication [lo] indicates that the lowest temperature data reported earlier were probably in error since the porous (graphite) frits were subsequently found to be ineffective in filtering out very fine particles. A leastsquares analysis of the earlier measurements excluding the set of points
below 380 “C yielded a straight line with a slope of -5467 K-l. The slope of the present least-squares-fitted line is -5458 K-l and thus both studies agree very well at the higher temperatures. Analysis of the solid samples was complicated by incomplete reaction between the piece of solid thorium contained in each capsule and the liquid cadmium. All the samples contained two or more phases. Nevertheless, the previously reported compounds ThCd ri, ThCda and ThCdz were identified in the samples shown in Table 1 without difficulty from the X-ray powder patterns. In addition, a new phase was identified in samples 5, 9, 10 and 11. The unit cell of this phase is hexagonal with lattice parameters a = 6.775 f 0.002 a and c = 4.928 + 0.002 A. The structure is of the NisSn type. The same structure has been reported for GdCds suggesting that the compound is ThCds. EPM was used to determine the Cd/Th ratios in the solid samples and the results confirm the chemical compositions of the four compounds and also suggest the existence of three additional intermetallics. More than 150 EPMs were conducted. The Cd/Th ratios of all the analyses were clustered in groups and for each group the statistical mean and the standard deviation were calculated, as shown in Table 3. As mentioned earlier ThCd, was used as a calibration standard for the instrument and therefore its atom ratio is 5.00 by definition. The observed atom ratios show that the two compounds which were previously indexed by their similarity to the BaHg,, and AIBz X-ray patterns but were not analyzed chemically are indeed ThCd,, and ThCd,. In addition, the existence of a ThCds compound, suggested by the XRD results, is also indicated by EPM. The EPM results support the possible existence of three more compounds corresponding to ThCds.5, ThCd,., and ThCd. These were not positively identified by XRD. DTAs were done on most, but not all, of the samples listed in Table 1. Each sample when heated to 1000 “C showed characteristic thermal arrests which are listed in Table 4 together with the corresponding XRD and EPM findings. As can be seen, only samples containing ThCdll exhibited a therTABLE 3 The compositions of phases identified using EPM Cd/Th ra tie* (statistical mean)
Number of data points
11.06 5.00 3.50 2.91 2.11 1.64 1.10
0.129 0.156 0.183 0.134 0.098 0.110 0.0979
16 32 25 15 28 15 21
(1.17%) (2.97%) (4.98%) (4.40%) (4.42%) (6.41%) (8.53%)
aThe compositions are referenced to a ThCd, standard.
ThCds.E,, ThCd,.,, ThCdI.,,
in all samples.
in the Th-Cd
mal arrest at 565 “C. Similarly, a minimum arrest temperature of 780 “C (if we disregard cadmium melting at 320 “C) occurs only if appreciable amounts of ThCds are present as indicated by the XRD analysis. Other samples exhibited minimum arrest temperatures at 829,880 and 909 “C. The arrest at 565 “C is attributed to peritectic decomposition of ThCd,, since it conforms to the discontinuity observed in the solub~ity experiments. Examination of Table 4 also suggests that the thermal arrest at 780 “C!is caused by the peritectic decomposition of ThCd,. The other thermal arrests at 829, 880, 909, 946 and 965 “C may correspond to transitions involving ThCds.s+ ThCds, ThCda, ThCdI.5 and ThCd respectively. The enthalpy of the ThCd,, decomposition was measured using the differential scanning calorimetry component of the Rigaku thermal analyzer. A sample of ThCd,, cont~n~g 5 wt.% free cadmium decomposed according to eon. (3) with an enthalpy change measured to be -9.5 ?r:1.5 kcal moT1. ThCd,,(s)
+ GCd(I, saturated)
For comparison, similar enthalpies of decompositions of PuCd,, and NpCd,, were calculated from literature data. These enthalpies were obtained from the free energies of formation of the actinide (An) compounds PuCd,r , PuCds , NpCd,, [ll] and NpCd, [ 111 by taking the difference between AH(AnCdll) and A~(AnCds). The enthalpies of reaction for the. peritectic decompositions of PuCdII and NpCdll are -6.6 kcal mole1 and -14.5 kcal mol-’ respectively. 3.1. Phase diagram The solubility data, DTA and to draw a partial phase diagram for are used above 780 “C to show the The phase diagram resembles other
combined EPM and XRD data were used the Th-Cd system (Fig. 3). Broken lines preliminary nature of the ~si~ments. rare earth-Cd diagrams [ 121.
0” Y +
I I I I 21
I_ I I rI c I I_ I I ::I et--
El ;I i
20 ATOM %
Fig. 3. A partial phase diagram of the Th-Cd system.
Acknowledgments The interest and comments of Dr. M. Steindler and Dr. I. Johnson are appreciated, and the technical help of the Analytical Laboratory is acknowledged. This work was performed under the auspices of the U.S. Department of Energy.
References 1 A. E. Martin, I. Johnson and H. M. Feder, Trans. Metall. Sot. AZME, 221 (1961) 789. 2 M. Krumpelt, I. Johnson and J. J. Heiberger, J. Less-Common Met., 18 (1969) 35. 3 I. Johnson, M. G. Chasanov and R. M. Yonco, Trans. Metall. Sot. AZME, 223 (1965) 1408. 4 D. E. Etter, D. B. Martin, D. L. Roesch, C. R. Hudgens and P. A. Tucker, Trans. Metall. Sot. AZME, 233 (1965) 2011. 5 I. Johnson and K. D. Anderson, U.S. Atomic Energy Commission Rep. ANL-6145, 1960, p. 70. 6 M. L. Fornasini, J. Less-Common Met., 25 (1971) 329. 7 A. Brown, Acta Crystallogr., 14 (1961) 860. 8 M. Krumpelt, J. Fischer and I. Johnson, J. Phys. Chem., 72 (1968) 506. 9 B. Amecke, U.S. Department of Energy Rep. ANL-TRANS 1591, 1979. 10 I. Johnson, personal communication, 1979. 11 M. Krumpelt, I. Johnson and J. J. Heiberger, Metall. Trans., 5 (1974) 65. 12 G. Bruzzone, M. L. Fornasini and F. Merlo, J. Less-Common Met., 25 (1971) 295.