June, 1958
P H A S E EQUILIBRIA IN S Y S T E M S
NAF-ZRF4, U F ~ Z R F . A, ,N D NAF-ZRF~-UF.,
665
PHASE EQUILIBRIA IN THE SYSTEMS NsF-ZrF4,UF4-ZrF4AND NaF-ZrFs-UF4 BY C. J. BARTON, W. R. GRIMES,H. INSLEY, R. E. MOOREAND R. E. THOMA Oak Ridge National Laboratory, Operated by Union Carbide Nuclear Company for the Atomic Energy Commission, Oak Ridge, Tennessee Received December 85,1967
Equilibrium diagrams for the binary systems UF4-ZrF4 and NaF-ZrF4 and the ternary system NaF-ZrF4-UF4 described in this paper are based on data obtained by thermal analysis, differential thermal analysis, equilibration of small samples followed by quenching, visual observations of phase changes and filtration experiments. Phase boundaries were established for the temperature interval between liquidus temperatures and about 300’. UF4 and ZrF4 form a complete series of solid solutions with the minimum melting temperature at 765” for the solution containing 77 mole % ZrF4. The NaF-ZrF4 system contains the compounds 3NaF.ZrF4 and 7NaF.6ZrF4which melt congruently, the compounds 5NaF.2ZrF4, 2NaFsZrF1 and 3NaF.4ZrF4 which melt incongruently, and the subsolidus com ound 3NaF.2ZrFd. A metastable phase has been identified as NaF.ZrF4. The 3NaF.ZrF4-3NaF.UFd and the 7NaF.6Zr$-7NaF.6UF4 joins in the ternary system are complete solid solution series. Liquidus temperatures decrease with increasing UF4 concentration in the former and with decreasing UFe concentration in the latter series. All of the stable binary compounds except the subsolidus phases 3NaF.2ZrF4 and NaF.2UF4 have primary phase areas in the ternary diagram. The system contains neither ternary compounds nor ternary eutectics.
Introduction Molten fluoride mixtures in which uranium tetrafluoride is a component have been shown to be of general value as fuels for high temperature nuclear reactors.’J A recently d e ~ c r i b e d , ~experimental -~ version of such a machine pumped a molten mixture of NaF,ZrFc and UF4, which served both as nuclear fuel and primary heat exchange fluid, through the reactor core and heat exchanger circuit. Molten fluoride mixtures have also been demonstrated to be of value as reaction media for high temperature chemical processes for recovery of uranium from solid fuel elements from more conventional nuclear reactor^.^-^ In these processes the fuel element is immersed in the molten bath and dissolved by application of anhydrous hydrofluoric acid; the uranium (or uranium compound) dissolves as UF4. Subsequent recovery of the uranium is achieved by application of a fluorinating agent capable of converting UF4 t o the volatile compound UFs. Mixtures of NaF and ZrFl have been shown t o provide suitable and convenient reaction media for such a process. A detailed knowledge of the phase equilibrium relations in the NaF-ZrF4-UF4 system is needed for both these practical applications. Phase relationships in the NaF-UF4 binary system along with a discussion of the available literature on alkali fluoride-quadrivalent fluoride systems and a description of most of the techniques used in this investigation have been shown in a previous publication8 from this Laboratory. A (1) A. M. Weinberg qnd R. C. Briant, Nuclear Sci. and Eng., 2, 797 (1957). (2) E. S. Bettis, W. B. Cottrell, E. R. Mann, J. L. Meem and G. D. Whitman, ibid., 2, 841 (1957). W. Schroeder, G. A. Cristy, H. W. Savage, R. G. (3) E. S. Bettis, Affel and L.F. Hemphill, ibid., 2, 804 (1957). (4) W. K. Ergen, A. D. Callthan, C. B . Mills and Dunlap Scott, ibid., 2, 82G (1957). (5) G. I. Cathers and R. E. Leuze, “A Volatilization Process for Uranium Recovery.” .Presented as Paper 278 at Nuclear Engineering and Science Congress, Cleveland, Ohio, December 12-16, 1955. Not
R.
published. (8) G . I. Cathers (Oak Ridge National Laboratory, Oak Ridge, Tennessee), “Uranium Recovery for Spent Fuel by Dissolution in Fused Salt and Fluorination,” presented at Meeting of the hmerican Nuclear Society, Washington, D. C., December 10-12, 1956. (7) R. C. Vogel. “Progress in Nuclear Energy, Vol. 111, Process Chemistry,” Edited by Bruce, Fletcher, Hyman and Katz, McGrawHill Book Co., Inc., New York, N. Y., 1956. (8)C. J. Barton, H. A. Friedman, W. R. Grimes, H. Insley, R . E. Moore and R. E. Thoma, J . Am. Ceram. Soc., 41,2, 63 (1958).
number of complex compounds of NaF and ZrFr prepared by crystallization from aqueous solutions have been described9 and a partial diagram for the NaF-ZrF4 system based on information derived from vapor pressure studies has been presented.l0 However, no detaiIed phase equilibrium study of the NaF-ZrFe, the UFrZrF4, or the NaF-ZrF4UF4 system has appeared in the literature. Accordingly, the phase equilibrium relationships in this ternary system and its associated binary systems have been investigated. The complex phase equilibria in these systems have been examined by the techniques of: (1) thermal analysis, (2) observation by petrographic microscopy and by X-ray diffraction of quenched samples, (3) differential thermal analysis, (4) visual observation of fusion and solidification, and (5) chemical analysis after phase separation by filtration a t high temperature. Experimental Materials.-The sodium fluoride used in this investigation was of Reagent grade from any of several sources. The uranium tetrafluoride was obtained from the Mallinckrodt Chemical Company. Zirconium tetrafluoride was prepared by sublimation at low pressure in nickel equipment of the crude ZrF4 obtained by treatment a t 400’ of commercial ZrCll with anhydrous H F vapor. It was clearly necessary to avoid hydrolytic reactions which would result in the presence of oxygen-containing species in the mixtures under test. Accordingly, prepared mixtures were charged with anhydrous H F vapor a t temperatures above the liquidus or were mixed with ammonium bifluoride which was subsequently removed by volatilization during the heating cycle. The tests were conducted in sealed systems containing an atmosphere of dry helium or argon or were carefully blanketed with such an inert atmoaphere. All necessary grinding or handling of the purified mixtures, as in loading the quenching capsules, was performed in helium-filled dry boxes which used P20bas the desiccant. Apparatus and Methods.-The apparatus and experimental techniques used in this study for thermal analysis, differentialthermal analysis and visual observation of fusion and solidification behavior were identical with those previously described.s Most of the quenches reported in this paper were obtained by use of the gradient quenching furnaces; some of the data, especially in the NaF-ZrF, system, was obtained by very similar techniques but with (9) J. C. G. De Marignac, Ann. chim. phye., 60, 257 (1800): J. W. Mellor, “A Comprehensive Treatise on Inorganic and Theoretical Chemistry,” Voi. VII, Longmans, Green and Co., New York, N. Y., 1927. ( I O ) IC. A. Sense, C. A. Alexander, R . E. Bowman and R. B. Filbert, Jr., THISJOURNAL,61, 337 (1957).
666
C. J. BARTON, W. R. GRIMES,H. INSLEY, R. E. MOOREAND R. E. THOMA
VOl. 62
TABLE I INVARIANT EQUILIBRIA IN THE SYSTEM NaF-ZrF4“ Mole % ZrFtin liquid
Invariant temp. (“C.)
Type of invariant
Phase reaction on cooling
+
747 Eutectic L -+ NaF 3NaF.ZrF4 850 Congruent m.p. L + 3NaF.ZrF4 523 Inversion a5NaFa2ZrFd 485NaF.2ZrF4 500 Eutectoid ~5NaF-2ZrF4(ss, 30 mole % ZrF4) @5NaF.2ZrF4 r2NaF.ZrF4 34 640 Peritectic L 3NaF.ZrFs (ss, ,ca. 27.5 mole % ZrF4) + a5NaF.2ZrF4 L 4- a5NaFa2ZrF4 (ss, 30 mole % ZrF4) 4 a2NaF.ZrF4 39.5 544 Peritectic 40 533 Inversion a2NaF~ZrFd4p2NaF.ZrF4 487 Decomposition r2NaF.ZrF1 7NaF.6ZrF4 ss 43NaF.2ZrFd 40.5 505 Inversion 82NaF.ZrF4 4 r2NaF.ZrF4 40.5 500 Eutectic L r2NaF.ZrFd 7NaF.6ZrF4 ss 46.2 525 Congruent m.p. L 47NaF.6ZrF4 49.5 512 Eutectic L + 7NaF.6ZrF4 3NaF.4ZrF4 56.5 537 Peritectic L ZrFd BNaF.4ZrFd The symbol “L” refers to liquid; the symbol %s” following a formula indicates solid solution of the compound. 20 25
+
-+
+
+
+ +
-+
+
“Fn
,rnC
c
-+
the equilibration performed in a quenching furnace mnintained a t a single temperature. The technique of high temperature filtration was found to be of value in cert8aininstances for determining solid solution boundaries and liquidus surfaces where other methods do not give reliable data. About 20 g. of the desired mixture, previously treated with anhydrous HF above the liquidus, was charged into a nickel cylinder 1.5 inches in diameter
Fig. 1 .-The
.
e 9.1.
system NaF-ZrFc
having a sintered nickel disc 1.37 inches in diameter welded in the flared end of 3/8 inch tubing extending through the top of the cylinder and adjusted in length so that the filter was 0.030 inch above the welded bottom of the cylinder. A nickel thermocouple well reaching from the top of the cylinder to a point just above the filter facilitated measurement of the temperature of the mixture. The loading tube was closed by means of a fitting, the sealed assembly was removed from the helium-filled dry box in which the loading operation was pcrformed, connected by means of flexible tubing to a manifold through which helium could be applied as desired, and mounted in a furnace which could be rocked a t four cycles per minute through a 30’ arc to provide agitation to the contents of the assembly. After the assembly had been maintained with agitation for one to three hours a t the desired temperature the sample was forced through the sintered nickel filter medium11 by applying a vacuum to the filter stick and about 20 p.s.i.a. of helium through the loading tube of the assembly. After the assembly had cooled to room temperature the apparatus was sacrificed and the filtrate and “wet” residue were removed for chemical analysis and examination by petrographic microscopy and X-ray diffraction.
Results and Discussion The System NaF-ZrF4.-The invariant equi(11) The sintcred nickel filter media were obtained from the Micro Metallic Corporation, Glen Cove, New York.
libria for this system are listed in Table I while the equilibrium diagram is shown as Fig. 1. The data from thermal analysis, shown in Table 11,12and from differential thermal analysis, shown in Table III,l2 served to establish the liquidus curve a t high NaF and a t high ZrF4concentrations. The complex relationships a t concentrations between 25 and 60 mole % ZrF4 were established from data obtained by quenching, shown in Table IV, with support from the thermal methods. Thermal effects other than liquidus indications were difficult to interpret, because phases which would not be expected if equilibrium were established were often obtained under the normal cooling conditions. The melting point of ZrF4, as determined by differential thermal analysis was 912 f 0.50.1a An earlier determination at ORNL13using uncalibrated thermocouples gave a value of 910 f 10”. Since the boiling point of ZrF4is very close to the melting point, it was necessary to carry out the determination in welded nickel or inconel containers of the type previously described.* Sense, et aE.,1° have reported a value of 918”, obtained by thermal analysis. The formula of the. congruently melting compound 3NaF.ZrF4 (25 mole % ZrF4) was established by the location of a maximum in the liquidus curve obtained by thermal analysis and by the fact that single-phase material was found by microscopic examination of slowly cooled preparations containing 25 mole % ZrF4. The formulas of 5NaF. 2ZrF4, 2NaFZrF4, 7NaF.6ZrF4 and 3NaF.4ZrF4 ’ ZrF4, respectively) (28.0,33.3,46.2 and 57.1 mole % were established by the observation that almost pure single-phase material was found by microscopic and X-ray diffraction examinations to be present in quenched samples very close to the formula compositions which previously were equilibrated below the solidus temperature for sufficient ’time to achieve equilibrium. Chemical analysis of single-phase (12) Tables 11, 111, V, I X and X have been deposited as Document number 5515 with the AD1 Auxiliary Publications Project, Photoduplication Service, Library of Congress, Washington 25, D. C. A copy may be secured b y citing the Document number and by remitting $2.50for photoprints or $1.75for 35 mm. microfilm, in advance by check or money order payable to: Chief, Photoduplication Service, Library of Congress. (13) D. G. Hill, Duke University, Consultant t o Oak Ridge National Laboratory, Oak Ridge, Tennessee, personal communication.
.
PHASE EQUILIBRIA IN SYSTEMS NAF-ZRF~,UF~-ZRF~, AND NAF-ZRF~-UF~
June, 1958
887
TABLE IV DATAOBTAINED BY QUENCHING NaF-ZrF4 MIXTURES= Comp. (mole % ZrFd
Phase change temp.
("C.)
27
>548
28.6 28.6 29 29 29
642 f 3' 522 f 3 >668 637 f 2 523 f 3
29 30 30 30 30 30
500 f 5
>670 641 f 2 571 f 3 548 f 4 527 f 3
32 32
>GOO
33.3 33.3 33.3
>GOO 545 f 3 531 f 2
33.3
506 f 2
536 f 5
35 35 35
>569 545 f 2 535 f 3
35
504 f 2
39 39
>513 500 i 4
40 40
507 f 3 502 f 3
40
487 f 5
42 42 43 43 43
508 ZIZ2 500 f 2 517 f 3 502 f 2 487 f 3
45 46.5 47
520 487 520 522
50
515 f 5
52
517 f 4
57 57
546 f 4 536 f 3
60 60
GOO f 5 538 f 1
f3
f4 f2 =k 2
Phases found just above phase change
SNaF.ZrF4, Ld a5NaFdZrF4 SNaF.ZrF4, L a5NaF .2ZrF4 a5NaF4?ZrF4 p5NaF.2ZrF4
Phases found just below phase change
SNaF.ZrF4,b a5NaF4ZrF4 a5NaF.2ZrF4 p5NaF.2ZrF4 SNaF.ZrF4, L a5NaF@ZrF4 a5NaF.2ZrF4, p5NaF.2ZrF4 @5NaF.2ZrF4
Interpretation
Solidus Incongruent m.p. of a5NaF.2ZrF4 Inversion temp. Liquidus Incongruent m.p. of a5NaF.2ZrF4 Inversion temp. Lowered inversion temp.
SNaF.ZrF4, L Liquidus Incongruent melting temp. of a5NaF.2ZrF4 05NaF.2ZrF4, L a5NaF.2ZrF4 ss Limit of solid soln. of a5NaF.2ZrF1 a t 571' Incongruent m.p. of a2NaF.ZrF4 a5NaF.2ZrF4 ss, a2NaF.ZrF4 p5NaF.WZrF4, a5NaF.2ZrFpss, Inversion temp. a2NaF.ZrF4 a2NaF.ZrF4 a5NaF.2ZrF4 ss, L Incongruent m.p. of a5NaF.2ZrF4 a5NaF.2ZrF4 ss, a5NaF.2ZrF4 ss, L Solidus a2NaF.ZrF4 aSNaF.2ZrF4 ss, L Incongruent m.p. of a5NaF.2ZrF4 a.5NaF4?ZrF4 ss, L a2NaF.ZrF4, L Incongruent m.p. of a2NaF.ZrF4 p2NaF.ZrF4, aRNaF.ZrF4, L Inversion temp. ( a2NaF.ZrF4) y2NaF.ZrF4 p2NaF.ZrF4, Inversion temp. (a2NaF.ZrF4) (a2NaF.ZrF4) Liquidus a5NaF.2ZrF4 ss, L a5NaFe2ZrF4 ss, L a2NaF.ZrF4, L Incongruent m.p. of a2NaFeZrF4 a2NaF.ZrF4, L p2NaF.ZrF4, Inversion temp. a2NaF.ZrF4, L r2NaF .ZrF4, p2NaF.ZrF4, Inversion temp. and solidus 7NaF.GZrF4 a2NaF.ZrF4, L 72NaF.ZrF4, L r2NaF.ZrF4, r2NaF.ZrF4, L Solidus 7NaF.6ZrF4 ss L r2NaF.ZrF4, L Liquidus 72NaF.ZrF4, L r2NaF.ZrF4, Solidus 7NaF.6ZrF4 ss r2NaF.ZrF4, SNaF.2ZrF4, Dec. temp. of 3NaF.2ZrFd 7NaFeGZrF4 ss ( r2NaF.ZrF4) L 7NaF.6ZrF4 ss, L Liquidus 7NaF4ZrF4 ss, L 7NaF.6ZrF4 ss, y2NaF.ZrF4 Solidus L 7NaF4ZrF4, L Liquidus 7NaF.GZrF4,L 7NaF.6ZrF4 ss, ~ 2 N a F . z r - F ~Solidus y2NaF.ZrF4, 3NaF.2ZrF4, Dec. temp. of 3NaF.2ZrF4 7NaF.GZrF4 ss 7NaF.6ZrF4 ss L 7NaF.6ZrF4, L Liquidus .. 7NaF.GZrF4, 3NaF.2ZrF4 Dec. temp. of 3NaF.2ZrF4 L 7NaF.6ZrF4, L Liquidus. Very near the comp. of 7NaF.6ZrF4 L 7NaF.6ZrF4, (L) Near the m.p. of 7NaF.6ZrF4. The value 522 f 2 was derived from an RV. of 3 gradient quenching series agreeing within l o L 7NaFa6ZrF4, Liquidus and solidus. This comp. must be 3NaF.4ZrFd near the eutectic 3NaF.4ZrF4, L 3NaF.4ZrF4, Solidus 7NaF.6z1-S L ZrF4, L Liquidus ZrF4, L 3NaFe4ZrF4, (L) Incongruent melting temp. of 3NaF~4ZrF4. This composition isvery nearthat of thecompd. L ZrF4, L Liquidus ZrF4, L 3NaF.4ZrF4, ZrF4 Solidus SNaF.ZrF4, L . a5NaF.dZrF4 S S , ~L a5NaF.2ZrF4 ss, (L)'
C. J. BARTON, W. R. GRIMES,H. INSLEY, R. E. MOOREAND R. E. THOMA
668
Vol. 62
a The data for compositions containing 27, 28.6, 29, 30, 32, 33.3, 35 and 47 mole $& ZrFd and part of the data for the composition containing 60 mole yo ZrFd were derived from gradient quenching series; the remainder were obtained from single Most of the phases were identified by optical microscopy. The phases in italics were also identified by X-ray quenches. diffraction. The uncertainty in temperatures shown in column 2 indicates the temperature differences between the quenched samples from which the values were obtained. d The symbol "L" refers to liquid (observed as glass or quench growth). E The symbol %s" following a formula refers to solid solution of the compound as indicated by a variation of the refractive index or the interplanar spacings from those of the pure compound. Phases given in parentheses were found to be present in very small quantities.
"F.
Fig. 2.-The 4.47
4 48
Irnl,%,
system NaF-UF4.
REFRACTIVE INDEX. 4.49
4.50
40
-g -
42
01
e
c44 N
46
0.004
0.005
Fig. 3.-Optical
0.007 0.008 BIREFRINGENCE.
0.006
0.009
0.0040
roperties for the solid solutions between 318aF.2ZrF4 and 7NaF.6ZrF4.
samples mechanically separated from slowly cooled preparations aided in establishing the formulas of the last two compounds. Compounds corresponding to 3NaF.ZrF4, 2NaF.ZrF4 and 7NaF.6ZrF4 have been found in the NaF-UF4 system (Fig. 2).8 Several factors introduce difficulties in determining phase relationships in the narrow region between 25 and 33.3 mole yo ZrF4. I n the first place the three compounds, 3NaF.ZrF4, 5NaF.2ZrF4 and 2NaF-ZrF4,are so near each other in composition that small unintended deviations in composition of quenched samples can produce large variations in relative quantities of phases observed. Other factors complicating the region include the existence of limited regions of solid solution of 3NaF.ZrF4 and 5NaF.2ZrF4and a number of allotropic modifications of 5NaF.2ZrF4and 2NaF.ZrF4. Observations of variations in the refractive index of 3NaF.ZrF4 obtained from slowly cooled and quenched samples gave indication of a limited region of solid solution above 25 mole % ZrF4, but the exact limits were not determined. The incongruent melting temperature (640") of a5NaF. 2ZrF4 was determined by quenching compositions containing 28.6, 29 and 30 mole yo ZrF4. There is a series of thermal effects corresponding to this incongruent melting temperature for compositions
between 27 and 31.5 mole % ZrF4. The temperature of the inversion cr t o p5NaF-2ZrF4(523') was determined from examinations of quenched samples of compositions containing 28.6 and 29 mole % ZrF4. Quenched samples of the latter composition contained both a5NaF.2ZrF4 solid solution and P5NaF.2ZrF4 in the temperature interval 500-525" thus indicating the lowering of the inversion temperature by solid solution of cr5NaF.2ZrF4 which extends to about 30 mole % ZrF4. There is no evidence for the'existence of a range of solid solution of P5NaF.2ZrF4.. A sample of a crystalline material prepared by precipitation from an aqueous solution containing NaF and ZrF4 was obtained from General Chemical Division, Allied Chemical and Dye Corporation. This substance had a chemical composition corresponding t o 5NaF.2ZrF4 and was found by X-ray diffraction examination t o be the phase p5NaF.2ZrF4. The incongruent melting point of cr2NaF.ZrF4, the high temperature modification of the compound, was established a t 544" from an average of data of quench series containing 30, 32, 33.3 and 35 mole % ZrF4. Pure a2NaF.ZrF4 free from quench growth was not observed below the incongruent melting temperature in the quenched samples containing 33.3 mole % ZrF4 possibly because of small variations in composition. The incongruent melting point was confirmed by visual observation. l 4 Temperatures of the inversion CY & P2NaF.ZrF4 (533") and the inversion p & y2NaF.ZrF4 (505") were determined from quench series containing 33.3 and 35 mole % ZrF4. A persistence of a2NaFZrF4 in these quench series at temperatures below 533" shows that the inversion of this phase t o other modifications of the compound is slow. Optical examinations indicate that P2NaF.ZrF4 is hygroscopic. The exact stability limits of another modification, b2NaF.ZrF4, have not been determined, but it has been observed by optical and X-ray diffraction examinations in compositions which had been equilibrated below about 460". The liquidus and solidus relationships from 35 t o 47 mole % ZrF4 were established with a fair degree of precision by quenching, thermal analysis and diff erential thermal analysis. The compound 3NaF.2ZrF4 (40 mole % ZrF4) decomposes a t temperatures above 487" into 2NaF.ZrF4and 7NaF.6ZrF4 solid solution. The formula and decomposition temperature of 3NaF.2ZrF4 were established by optical examinations of previously equilibrated quenched samples over the composition range 39 to 45 mole yoZrF4. When melts in this range are cooled, 3NaF.2ZrF4or solid solution of the compound with 7NaF.6ZrF4 is produced a few degrees belowthe solidus temperature presumably by a solid phase reaction. A melt containing 41 mole Yo ZrF4 cooled in (14) R. J. Sheil, Oak Ridge National Laboratory, Oak Ridge, Tennessee, personal communication.
#
b
8
June, 1958
PHASE EQUILIBRIA IN SYSTEMS NAF-ZRF~,UF~-ZRF~, AND NAF-ZRF~-UF~
the visual observation apparatus appeared t o freeze completely just above 500". When the sample cooled a few degrees below this point, efflorescence took place with a marked volume increase. Microscopic examination showed the preparation to be composed of a single phase consisting of minute prismatic crystals with a very low birefringence and a mean refractive index of about 1.475. There appears to exist at' temperatures below 487" a series of solid solutions between 3NaF.2ZrF4 and 7NaF.6ZrF4 with a miscibility gap between about 43 and 45 mole % ZrF4. This was shown by optical and X-ray diffraction examinations of slowly cooled melts and optical examinations of previously equilibrated quenched samples. Slowly cooled melts at intervals of about 1% from 40 to 46.2 mole yo ZrF4 show the following relationships. At 4oY0 ZrF4 the sample is composed of a single phase consisting of minute elongated prisms with very low birefringence (less than 0.004), and a mean refractive index of 1.470. With increasing content of ZrF4 the mean refractive index increases regularly as does the birefringence to a limit of about &yoZrF4. The existence of a miscibility gap in the solid solution series is evidenced by the observation of two optically similar but distinguishable phases in the preparation containing 44 mole % ZrF4. One phase has a mean refractive index of about 1.485 and a markedly elongated prismatic. habit, and the other has a mean refractive index of about 1.495 with a shorter prismatic habit and a somewhat higher birefringence. At 45y0 and up to 46.2y0there is again a single phase with regularly changing refractive index and birefringence. The changes in optical properties with changing composition are shown in Fig. 3. The X-ray powder diffraction data for this series of preparations are given in Table V.12 The pattern of the compound a t 40 mole % ZrF4(3NaF2ZrF4) is relatively simple, consisting cf a few intense lines. The pattern of the compound a t 46.2% ZrF4(7NaF.6ZrF4)has the same intense lines in the same positions as does the 40% compound but with many additional weaker lines (typical X-ray pattern for the rhombohedral structure). The change in composition from 40 to 46.2% is manifested by a regular increase in intensity of the weaker lines. It is obvious from the absence of shifts in the X-ray lines that the solid solutions are not of the usual substitutional type. The X-ray data in this case do not distinguish between mixtures of phases and solid solution, but the data do show that the crystal structures of the end members have common major planes. A phase having the formula NaF.ZrF4 is often produced when samples containing 46 to 57 mole yoZrF4 are quenched from above the solidus temperature. It is usually obtained when large samples are used and is absent when very small samples are quenched rapidly. Mixtures in this composition range which are cooled slowly always contain 7NaF.6ZrF4 and 3NaF.4ZrF4. The formula NaF.ZrF4 was established by the fact that samples of approximately 3 g. of a composition containing 50 mole yo ZrFl produced almost singlephase material when cooled rapidly. Quenching
669
studies covering the temperature range from 376' t o the liquidus showed that NaF-ZrF4 is not stable within this temperature interval. It is, therefore, either metastable or has a stable existence only a t a lower temperature. The compound 3NaF.4ZrF4exhibits the unusual property of having a refractive index (and therefore probably density) lower than neighboring members of the NaF-ZrF4 system. It is strongly hygroscopic in nature; consequently the compositions containing this phase must be carefully protected from moist air. Liquidus temperatures for four mixtures in NaF-ZrF4 system from 53 t o 74.5 mole % ZrF4 derived from vapor pressure data were re. ~ values ~ are much ported by Sense, e l ~ 2 These higher than those shown by the liquidus curve in Fig. 1, which represents a composite of data obtained by the more conventional methods used in this study. I n addition, liquidus values derived from vapor pressure data obtained a t the Oak Ridge National Laboratory with mixtures containing 66.4 and 74.5 mole yo ZrF4 agreed with the It seems liquidus curve in Fig. 1 to within likely that the high liquidus values reported by Sense are due to a systematic error, po,ssibly in the slope of his vapor pressure curve for ZrF4 which is not in agreement with other data.16r17 1050
1000 A
9 950 v
5
900
Y
6 850 8 g 800 750 700 UF4 10 20
30 40 50 60 7 0 . 8 0 ZrFa (mole %). Fig. 4.-The system UK-ZrF,.
90ZrF4
The optical and X-ray diffraction data for the crystalline phases in the NaF-ZrF4 system are given below. 3NaF.ZrF4 Uniaxial w = 1.386 e = 1.381 X-ray lines: 4.75, 3.06, 1.87
-
aSNaF.2ZrF4 Uniaxial w = 1.396 E = 1.400 X-ray lines: 5.15, 3.09, 1.890
+
@5NaF.2ZrF4 Biaxial 2V = 40" LY = 1.393 y = 1.402 X-ray lines: 4.97, 3.11, 3.05 (15) S. Cantor and R. E. Moore (Oak Ridge National Laboratory, Oak Ridge, Tennessee), persrnal communication. (16) 5. Cantor, R. F. Newton, W. R. Grimes and F. F. Blankenship, THts JOURNAL, 62, 96 (1968). (17) W. Fischer, personal communication, S. Lauter, Dissertation Teohnische Hochachule, Hanoover, 1948.
670
C. J. BARTON,W. R. GRIMES,H. JNSLEY,R. E. MOOREAND R. E. THOMA
-
Biaxial (Y = 1.412 X-ray lines:
TABLE VI QUENCHING DATAFOR UF4-ZrF4 MIXTURES'
2 v = 75" y = 1.419 5.47, 3.11, 1.912
Cornp. (mole % ZrFd
+ -
-
+
+
The System UF4-ZrF4.-The system UF4-ZrF4 consists of a continuous series of solid solutions with a minimum melting temperature of 765" a t 77 mole % ' ZrF4. The diagram shown in Fig. 4 is based on quenching data presented in Table VI. Thermal analysis studies of this system performed in sealed nickel capsules yielded inconsistent vaIues for both liquidus and solidus temperatures and these data are not reported here. Microscopic examinations of samples which were equilibrated and then quenched from below the solidus temperatures showed the presence of only one crystalline phase which varied continuously from dark green to almost colorless with a change in composition from 20 to 95 mole % ZrF4. The possibility of a small immiscibility region in the series near the minimum was not completely ruled out on the basis of optical observations of quenched samples because the optical properties of UF4 and ZrF4 are very nearly alike with the exception of color. X-Ray diffraction patterns of samples of the composition of the minimum in the liquidus curve which had been equilibrated below the solidus temperature show the presence of only one monoclinic crystalline phase. It can be inferred from these data that there is no immiscibility region in the system. This would be expected when one considers the nearness of the unit cell parameters of the pure compounds. (ref. 18)
uo = 12.79 A., bo = 10.72 A.,
Zrl*'a: a. = 11.71 A., bo
=
9.89
cg
= 8.39 A., B = 126'10'
A,, co =
7.66
A.,
Liquidus temp. ("C.)
Solidus temp. ("C.)
20 927 862 25 906 850 30 897 840 40 875 819 50 825 789 60 787 .. 65 792 777 70 784 769 75 779 765 80 783 783 85 >834 817 90 872 853 95 884 .. Samples quenched from above the liquidus temperature were found by microscopic examination to contain only glass or quench growth. Those samples quenched from temperatures between the liquidus and solidus contained crystals of a solid solution of UF4 and ZrF4 as well aR glass or quench growth, while those quenched from below the solididus contained only the well crystallized solid solution.
+
Uniaxial w = 1.376 E = 1.386 X-ray lines: 4.55, 2.893, 1.894 y2NaF.ZrFa 2 v = 75" Biaxial CY = 1.408 y = 1.412 X-ray lines: 5.12, 3.83, 3.25 d2NaFaZrF~ 2V >70° Biaxial a = 1.420 y = 1.429 Polysynthetic twinning common X-ray lines: 4.98, 4.19, 3.07 3NaF.2ZrF4 Very low birefringence Mean refractive index E 1.470 X-ray lines: , 3.13, 1.91, 1.63 7NaF.6ZrFd Uniaxial w = 1.508 E = 1.500 X-ray lines: 3.13, 1.91, 1.63 NaF.ZrF4 Uniaxial w = 1.417 E = 1.446 X-ray lines: 3.37, 3.86, 2.09 3NaF~4ZrFd Biaxial 2V = 30" CY = 1.420 y = 1.432 X-ray lines: 4.15, 3.36, 2.074
UF4:
Vol. 62
(ref. 19) R = 126" 9'
(18) J. J. Kats and E. Rabinowitch, "The Chemistry of Uranium," NNES VIII-5," McGraw-Hill Book Co., Inc., New York, N.Y., 1951. (19) R. W. G. Wyckoff, "Crystal Structures," Bec. I, V, Interscience Pnblishers, New York, N. Y., 1948, p. 20.
The System NaF-ZrF4-UF4.-The phase diagram for the NaF-ZrF4-UF4 system is given in Fig. 5. I n Fig. 6 the primary phase areas are labeled and the invariant points listed in Table VI1 are indicated. The diagram is based on results of .quenching experiments (Table VIII), thermal analysis data listed in Table IX12 and filtration data shown in Table X.12 The quenching results were used for precise definitions of primary phase boundaries, invariant points and solid solution relationships. Thermal data provided information principally for construction of liquidus surfaces. Solidus temperatures are not given in Table IX because these generally cannot be obtained from thermal data in cases such as this where solid solutions exist. Filtration experiments were used to establish solidus temperatures in one solid solution series and to supplement thermal analysis in a region where thermal analysis failed to give reliable liquidus temperatures. The principal feature in this system is the existence of continuous series of solid solutions: that between the compounds UF4 and ZrFl with a minimum a t point 1 (Fig. s), that between the congruently melting compounds 7NaF.6UF4 and 7NaF. 6ZrF4 and that between the congruently melting compounds 3NaF.UF4 and 3NaF.ZrF4. The primary phase fields of these solid solutions are the three largest fields in the diagram. Each of the sections 3NaF.UF4-3NaF.ZrF4 and 7NaF.6UF47NaF.6ZrF4 (Figs. 7 and 8) constitutes a quasibinary section of the system. As far as the equilibria involving the liquidus are concerned, therefore the system may be divided into three independent sub-systems, NaF-3NaF.UF4-3NaF.ZrF4,3NaF.UF4-7NaF.6UF4-7NaF.6ZrF4-3NaF.ZrF4, and
7NaF.6UF4-UF4-ZrF4-7NaF.6ZrF4.
The fact that 3NaF.UF4-3NaF.ZrF4 and 7NaF. 6UF4-7NaF.GZrF4 form continuous solid solution series without maxima or minima was established by thermal analysis, filtration and quenching data. The phase relationships below the solidus temperature at the low zirconium end of the 3NaF.UFr
I
6
June, 1958
PHASE
67 1
EQUILIBRIA I N SYSTEMS NAF-ZRF4, UF~-ZRF~, AND NAF-ZRF~UF~ UF4
1035
ALL TEMPERATURES ARE IN’C
ZrFd
No F 990
9 12
Fig. 5.-The
system NsF-ZrFa-UF4.
TABLE VI1 INVARIANT A N D SINGULAR POINTSIN THE SYSTEM NaF-ZrFd-UF4“ Designation
Comp. (mole 70) Temp. NaF ZrF4 UFd (“C.)
Type of invariant
Solid phases present
I 69.5 4.0 26.5 646 Max. temp. of boundary curve 3NaF.( U,Zr)F4,2NaFeUF4 I1 68.5 5 . 5 26.0 640 Peritectic 3NaF.(U,Zr)F4, 2NaF.UF4, 5NaF.3UF4 tss I11 65.5 12.0 22.5 613 Peritectic or dec. of 5NaF.3UF4 tss 3NaF.(U,Zr)F4, 5NaF.3UF4 tss, 7NaF.G(U,Zr)Fd IV 64.0 27.0 9.0 592 Peritectic 3NaF.(U,Zr)Fd, 5NaFL?ZrF4 ss, 7NaF.G(U,Zr)F( V 61.5 34.5 4 . 0 540 Peritectic 5NaF.ZZrF4 ss, 2NaF.ZrF4, 7NaF.G(U,Zr)F& VI 50.5 47 2 . 5 513 Peritectic 7NaF.G(U,Zr)Fa, (U,Zr)F4,3NaF.4ZrF4 Parentheses indicate solid solution of isomorphous compounds; the symbol “ss” following a binary formula represents solid solution of the compound with one of its binary components; the symbol L‘tss”represents solid solution of the binary compound with a sniall amount of the third component. @
3NaF.ZrF4 series have not been completely tioii series are in the opposite direction while the established, but apparently one or two mole iiitervening compounds do not form solid solutions yo of 3NaF.ZrF4 stabilizes a3NaF.UF4 and results in rather unusual and eomplex phase relaprevents its inversion to /33NaF.UF4 and subse- tionships in this part of the system. The primary queiit decomposition into NaF and 2NaF.UF4. AI- phase fields of the three solid solution series occupy though the continuous solid solution series between about 90% of the area of the diagram which probthe isomorphous compounds 7NaF4UF4 and 7Na- ably accounts for the absence of a ternary eutectic. F.OZrF4 occurs without maximum or minimum, Also, no ternary compounds are believed to exist in the solidus in the high zirconium elid is almost the system. There is a low temperature trough horizontal. There is ‘no detectable solid solution beginning a t the minimum in the UF4-ZrF4 series between 2NaF.UF4 and 2NaF.ZrF4. The fact which penetrates the system t o about 40 mole % ’ that the temperature trends in the two solid solu- NaF.
.
672
C. J. BARTON,W. R. GRIMES,H. INSLEY, R. E. MOOREAND R. E. THOMA
Vol. 62
H
Fig. 6.-Primary
.
phase areas and invariant points in the system NaF-ZrF4-UF4.
BOO
L I .OD
I
,
I
1
1Nd
"9
I !mol
Fig. 7.-The
m o a~z , ~ ,
10
20
3D
IO ,Nor
IO
1MT *rF,
Fig. 8.-The
50
evr,
,rnDI,
60
10
110
90
7M11 W T .
XI
system 7NaF.6ZrF4-7NaF.6UF4.
I.'
system 3NaF,UF4-3NaF.ZrF4.
Because it was found difficult to determine liquidus temperatures in the VNaF primary phase field by conventional thermal analysis techniques, filtration experiments were performed with a misture containing 85 mole % N a F and 7.5 mole yo ZrF4. The data indicated that when NaF precipitates sufficiently to reduce its concentration to 81 mole % a second phase rich in ZrR, presumably
3NaF.ZrF4-3NaF.UF4 solid solution, begins to precipitate and the composition of the liquid moves in the direction of the NaF.UF4 binary system. The compounds in the binary systems NaFZrF4 and NaF-UF4 other than those found in the solid solutioiis discussed above aff ect the ternary system to only a minor extent. The primary phase fields of the compounds 3NaF.4ZrF4, 5NaF. 2ZrF4, 2NaF.ZrF4, 2NaF.UF4 and 5NaF-3UF4in the ternary system are small. The subsolidus
June, 1958
PHASEEQUILIBRIA IN SYSTEMS NAF-ZRF~, UF~--ZRF~, AND NAF-ZRF~-UF~
673
TABLE VI11 DATAOBTAINED BY Camp. (mole %) NaF ZrFd UF4
Phase change temp. ("C.)
QUENCHI~JQNaF-ZrFd-UF4
Phases found just above phase change
Phases found just below phase change
MIXTURES' Interpretation
Liquidus. This series and the following 7 series aided in determining the liquidus and solidus relationships on the join 3NaF.ZrF4-a3NaF.UR Solidus Liquidus Solidus Liquidus Solidus Liquidus Solidus Secondary temp. and secondary phase. Results from this series and the following 7 series aided in establishing phase boundaries and peritectic points involving the phases 3NaF(U,Zr)F4, 7NaF.6(U,Zr)F4, a5NaF.2ZrF4 and a2NaF.ZrF4 Liquidus Secondary temp. and secondary phase
22
669 f 3'
L"
3NaF.(U,Zr)dF4,L
3 6 6 9 9 12.5 12.5 33
22 19 19 16 16 12.5 12.5 5
636 f 3 714 f 2 641 f 3 731 f 3 661 f 4 748 f 3 688 f 3 535 f 2
3NaF.(U,Zr)F4, L L 3NaF.(U,Zr)F4, L
3NaF.(U,Zr)F4 3NaF.(U,Zr)F4, L 3NaF.(U,Zr)F4 3NaF.(U,Zr)Fo, L 3NaF.( U,Zr)F4 3NaF.(U,Zr)Fd, L 3NaF.( U,Zr)F4 7NaF.6(UIZr)F*, a5NaF.2ZrF4, L
63.5 G3.5
31.5 31.5
5 5
575 f 3 565 f 2
L a5NaF.2ZrF4, L
59.5 59.5
33.5 33.5
7 7
578 f 3 540 f 2
L 7NaF*6(U,Zr)F4,L
65 65
25 25
10 10
615 -f 5 598 f 2
L 3NaF.(U,Zr)F4, L
62.5 66.67
19.5 18 10.33 23
634 f 2 639 f 2
L L
66.67
634 f 2
3NaF.(U,Zr)F4, L
3NaF(U,Zr)F4, 5NaFe3UF4, L 593 f 2 3NaF-(U,Zr)F4, 3NaF.( U,Zr)F4, Establishes direction of 3NaF.(U,Zr)F410.33 23 5NaF.3UF4, L 7NaF.6(UJZr)F4 5NaF.3UF4 boundary path 3NaF.(U,Zr)Fd, L 640 f 2 L Liquidus 6 25 3NaF.( U,Zr)F4, 637 f 2 3NaF.(U,Zr)F4, L Secondary temp. and secondary phase 6 25 2NaF.UF4, L 3NaF.( U,Zr)r, 634 f 2 3NaF.(U,Zr)F4, Solidus 6 25 2NaF.UF4, L 2NaF.UF4 3NaF.( U,Zr)F4, 621 f 2 5NaF.3UF4, L Helps to establish the peritectic point 10 25 7NaF..6(U,Zr)F4, L 5NaF.3UF4, L 640 f 2 L Liquidus 10 25 597 i 2 3NaF.(U,Zr)F4, 3NaF .(U,Zr)F4 Solidus 10 25 7NaFsG(U,Zr)F4, L 7NaF.6( U,Zr)F4 649 f 1 L Liquidus 3NaF.(U,Zr)F4, L 5 26 640 f 2 3NaF.(U,Zr)F4, L 3NaF.(U,Zr)F4, Secondary temp. and secondary phase 5 26 2NaF.UF4, L 644 f 2 L 5NaF.3UF4, L Liquidus 7 . 5 26 636 f 2 5NaF.3UF4, L 7 . 5 26 5NaF.3UF4, Secondary temp. and secondary phase 3NaF.(U,Zr)F4, L 613 f 2 5NaF.3UF4, 7 . 5 26 7NaF.6(U,Zr)F4, Peritectic temperature 3NaF.(U,Zr)Fd, L 3NaF.(U,Zr)F4, L 594 f 2 7NaF.6( U,Zr)F4, Solidus 7 . 5 26 7NaF.6(U,Zr)F4, 3NaF.(U,Zr)F4, L 3NaF.( U,Zr)F4 4 26.5 639 i 3 L Location and temp. on boundary psth 2NaF.UF4, 3NaF.(U,Zr)F4, L 4 26.5 629 i 1 2NaF .UF,, 2NaF.UF4, Solidus 3NaF.(U,Zr)F4, L 3NaF.(U,Zr)Fs
75
3
75 75 75 75 75 75 75 62
66.67 69 69 69 65 65 65 69 69 66.5 66.5 66.5 66.5 69.5 69.5
10.33 23
L 3NaF.(U,Zr)F4, L 3NaF(U,Zr)F4, L 7NaF.G(U,Zr)F4, L
a5NaF.2ZrF4, L a5NaF.2ZrF4, 7NaF.6(U,Zr)F4,L 7NaF.6(U,Zr)F4, L 7NaF.6( U,Zr)F4, a2NaF.ZrF4, L 3NaF.(U,Zr)F4, L 3NaF.(UJZr)F4, 7NaF.6(U,Zr)F4, L 7NaF-6(U,Zr)F4,L 3NaF.(UJZr)F4, L
Liquidus Secondary temp. and secondary phase Liquidus Secondary temp. and secondary phase Liquidus Liquidus. Results from this series and the following 24 series aided in establishing the boundary curves and peritectic points involving the phases 7NaF*B(U,Zr)Fd, 3NaF.(U,Zr)F4, 2NaF.UF4 and 5NaF.3UFr Secondary temp. and secondary phase
C. J. BARTON, W. R. GRIMES,H. INSLEY, R. E. MOORE AND R. E. THOMA
674
Vol. 62
TABLE VI11 (Continued) Comp. (mole %) NaF ZrF4 UF4
62.5
11
Phase change temp. (“C.)
26.5 630 f 2
Phases found just above phase change
Phases found ust below phase change
7NaF.6(U,Zr)F4, L
Interpretation
7NaF.6(U,Zr)F4, Secondary temp. and secondary phase 5NaF.3UF4, L 62.5 11 26.5 616 f 5 7NaF.6(U,Zr)F4, 7NaF.6(U,Zr)F4, Peritectic temperature 5NaF.3UF4, L 3NaF (U,Zr)F4 69 3 . 5 27.5 640 f 2 L 2NaF.UF4, 3NaF.I U.Zr)Fd. L Location and temp. on boundary path 64 6.5 29.5 650 f 3 L 5NaF:3UF4; L Liquidus 64 6 . 5 29.5 611 f 3 5NaF.3UF4, L Helps to establish temp. and comp. ( I f 3NaF.( U,Zr)F4, peritectic point. 7NaFeG(U,Zr)Fd,L 61.5 6.5 32 664 f 2 L 7NaF.6( U,Zr)F4, L Liquidus 61.5 6 . 5 32 621 f 3 7NaF.6(U,Zr)F4, L 7NaF.6( U,Zr)F4, Secondary temp. and secondary phase 5NaF.3UF4, L 61.5 6.5 32 605 f 5 7NaF*G(U,Zr)Fd, 7NaFeG(U,Zr)F4, Peritectic temp. 5NaFs3UF4, L 3NaF.(U,Zr)F4 53.8 45.2 1 7NaF.G(U,Zr)Fr, L Liauidus. This series and the following 24 517 f 3 L series aided in establishing the liquidus and solidus relationships on the join 7NaF.6ZrF4-7NaF.6UF4 53.8 44.2 2 7NaF.6(U,Zr)F4, L Liquidus 524 f 3 L Solidus 53.8 44.2 2 517 f 3 7NaF.6(U,Zr)F4, L 7NaF.6(U,Zr)F4 53.8 43.2 3 7NaF.6(U,Zr)F4, L Liquidus 529 f 3 L 53.8 43.2 3 Solidus 521 f 3 7NaF.6(U,Zr)F4, L 7NaF.6( U,Zr)F4 53.8 42.2 4 7NaF.6(U,Zr)F4, L Liquidus 535 f 3 L Solidus 53.8 42.2 4 520 f 3 7NaF.6(U,Zr)F4, L 7NaF.6( U,Zr)F4 53.8 41.2 5 7NaF.6(U,Zr)F4, L Liquidus 532 f 3 L Solidus 53.8 41.2 5 517 f 3 7NaF.6(U,Zr)F4,L 7NaF.6(U,Zr)F4 7NaF.6(U,Zr)F4,L Liquidus 53.8 40.2 6 541 f 3 Solidus 53.8 40.2 6 516 f 3 7NaF.6(U,Zr)F4, L 7NaF.6( U,Zr)F4 7NaF.6(U,Zr)F4, L Liquidus 53.8 39.2 7 551 f 3 L Solidus 53.8 39.2 7 518 f 3 7NaF,G(U,Zr)F4, L 7NaF.6(U,Zr)F4 53.8 37.2 9 7NaF.6(U,Zr)F4, L Liquidus 553 f 4 L Solidus 53.8 37.2 9 523 A 3 7NaF.G(U,Zr)F4, L 7NaF.6(U,Zr)F4 7NaF.6(U,Zr)F4, L Liquidus 53.8 34.2 12 570 i 4 Solidus 53.8 34.2 12 535 f 4 7NaF,B(U,Zr)F4, L 7NaF.6(U,Zr)F4 7NaF.6( U,Zr)F4, L Liquidus 53.8 31.2 15 600 f 3 L 7NaF.6(U,Zr)F4, L Liquidus 53.8 26.2 20 635 f 5 L Solidus 53.8 26.2 20 587 f 2 7NaF.6( U,Zr)F4, L 7NaF.G(U,Zr)F4 7NaF.6( U,Zr)F4, L Liquidus 53.8 23.1 23.1 638 1 3 L Solidus 53.8 23.1 23.1 598 f 4 7NaF.6(U,Zr)F4, L 7NaF.6( U,Zr)F4 7NaF.6(U,Zr)F4,L Liquidus 53.8 667 f 3 L 16.2 30 Solidus 53.8 16.2 30 631 f 3 7NaF.6(U,Zr)F4,L 7NaF.6( U,Zr)F4 Solidus 53.8 11.2 35 641 f 3 7NaF.G(U,Zr)Fd, L 7NaF.6( U,Zr)F4 3NaF.4ZrF4, L Liquidus. This series and the following 12 45 527 f 2 L 53 2 series aided in establishing the primary phase field of 3NaF.4ZrF4 and the peritectic point joining the primary phase fields of 3NaF. 4ZrF4, (U,Zr)Fd and 7NaF.6(U,Zr)F4 3NaF.4ZrF4, Solidus 45 53 2 510 f 4 3NaF.4ZrF4, L 7NaF.B(U,Zr)F4 3NaF.4ZrF4, L Liquidus 41 55 4 528 f 3 L (U,Zr)F4, L Liquidus 8’ 562 f 8 L 42 50 Secondary temp. and secondary phasct 8 519 f 4 (U,Zr)F4, L 42 50 (U,Zr)S, 7NaF.6(U,Zr)F4, L (U,Zr)F4, L Liquidus 41 41 18 695 f 2 L (U,Zr)F4, L Liquidus 48 44 8 563 f 4 L 48 44 8 520 f 3 (U,Zr)F4, L 7NaF.G(U,Zr)F4, (U,Zr)F4 Solidus 3NaF.4ZrF4, L Liquidus 4 516 f 2 L 53 43 44 44 (U,Zr)Fd, L Liquidus 12 652 A 2 L 3NaF.4ZrF4, L Liquidus 41.6 54.4 4 525 f 3 L 7NaF.6(U,Zr)F4, L Liquidus 52 46 2 526 f 2 L (U,Zr)Fd, L Liquidus 6 603 f 2 L 37 57 a The phases listed were identified by optical microscopy confirmed in some cases by X-ray diffraction examinations. * The uncertainty in tem eratures shown in column 4 indicates the temperature differences between quenched samples from which the values were ogtained. c The symbol “L” refers to liquid (observed as glass or quench growth). Parentheses indicate solid solution of isomorphous compounds.
-
June, 1958
PHASEEQUILIBRIA IN SYSTEMS NAF-ZRFI, UF4--Z~F4, AND NAF-ZRF~-UF~
compounds 3NaF.2ZrF4 and NaF.2UF4 do not have primary phase fields in the ternary system although the liquidus surface near the primary phase field of 3NaF.4ZrF4is lower than the upper stability limit of NaFs2UF4. Quenches were made a t the compositions 41NaF41ZrF4-18UF4, 42NaF-50ZrF4-8UF4 and 44NaF-44ZrF4-12UF4 (mole %) through a temperature range which should have shown the presence of NaF.2UF4 if it had a primary phase region in the system. No NaF.2UF4 was found in these quenched samples. The partial solid solution 7NaF.6ZrF4-3NaF-2ZrFc dissolves some uranium in the ternary system as evidenced by optical observations of a pale green color of the solid solutions obtained from ternary compositions. The miscibility limits were not determined. The temperature of the boundary curve f-a (Fig. 6) falls from f to a with precipitation of NaF and 3NaF.(U,Zr)F4. The phases 2NaF.UF4 and 3NaF.(U,Zr)F4 precipitate on cooling along the boundary curve b-11. At the maximum I 2NaF.UF4and 3NaF.(U,Zr)F4containing 10.0 mole % UF4 are in equilibrium with liquid. The composition of this solid solution is represented by the intersection with the join 3NaF.UF4-3NaF.ZrF4 of the extension of a straight line through 2NaF.UF4 and I. Upon cooling compositions on bI 2NaF.UF4 and 3NaF. (U,Zr)F4 containing more than 10.0 mole yo UF4 precipitate and the liquid Composition moves toward b as the composition of the precipitating solid solution moves toward the compound 3NaF. UF4. When compositions on 1-11 are cooled the precipitating solid solution contains less than 10.0 mole yoUF4 and its composition moves in the direction of 3NaF.ZrFc as the liquid composition moves toward the peritectic point 11. The reaction on the curve 11-111 is the precipitation of 5NaF.3UF4 and 3NaF.(U,Zr)F4 on cooling. On the curve d-I11 7NaF.6(U,Zr)F4reacts with liquid to produce 5NaF.3UF4, and on the curve C-I1 5NaF.3UF4 reacts with liquid t o give 2NaF.UF4. The phases 3NaF.(U,Zr)F4 and 7NaF.6(U,Zr)F4 precipitate from liquid on the curve 111-IV. On the curve g-IV 3NaF-(U, Zr)F4 reacts with liquid t o produce 5NaF.2ZrF4 binary solid solution (see Fig. 1). The liquid composition moves toward IV on cooling. When liquids on IV-V are cooled the phases 5NaF.2ZrF4 binary solid solution and 7NaF.6(U,Zr)F4 precipitate and the liquid composition moves toward V. The phase 5NaF-2ZrF4 on h-V binary solid solution reacts with liquid to form 2NaFZrF4. The liquid composition moves in the direction from h to V. On the curve V-i 2NaF.ZrF4 and 7NaF.6(U,Zr)F4precipitate on cooling and the composition of the liquid moves toward the binary eutectic i. Cooling of liquids on e-VI produces (U,Zr)F4 and 7NaF.6(U,Zr)F4. The latter with 3NaF.4ZrF4 precipitate along VI-j with the liquid composition moving toward the binary eutectic j. The reaction on the curve k-VI under’ UF4-55 goes a change a t a point near 5 mole % mole 7 0 ZrR. From the binary peritectic k to this point (U,Zr)F4 reacts with liquid to produce 3NaF.4ZrF4. Beyond this point the liquid produces the two solids (U,Zr)F4 and 3NaF.ZrF4 on
sow4 50 ma
NOF
Fig. 9.-The
675
section NaF-150UFa-50ZrF,1 (mole (Schematic).
%)I
cooling . as . _the _ _ liquid composition moves toward the peritectic VI. The incongruentlv melting comDound 5NaF.3UF4 decomposes& cool”ing at 630” irk0 the pure solids 2NaF.UF4 and 7NaF.6UF4 in the binary system NaF-UF4 (Fig. 2). Its primary phase field in the ternary system NaF-ZrF4-UF4 extends down t o the temperature of I11 (613’). This can only be possible if 5NaF.3UF4 takes ZrF4 into a solid solution which decomposes at a lower temperature than the pure binary phase. There could be found, however, no difference in refractive index or X-ray lines of samples from the ternary system as compared with those from the binary system. The region of solid solution must be too small to detect by the optical and X-ray techniques used. The point I11 may represent either the lowest temperature of existence of the solid solution in the ternary system, or simply the lowest temperature for its equilibrium with the ternary liquid (peritectic point) while the lowest temperature for its existence may be still lower. The data do not distinguish between the two possibilities. A complete discussion of the phase equilibria which may occur in these alternate situations is presented elsewhere.20 The vertical section of the system from NaF to a point midway between UF4 and ZrF4 (Fig. 9) is a section passing through the three adjacent but independent sub-systems. As indicated in this exaggerated form the 3NaF.UF4-3NaF.ZrF4 and 7NaF.6UF4-7NaF.6ZrF4 solid solutions are actually ternary in composition on the NaF-ZrF4 side. They occupy small areas of ternary composition near points 3NaF.ZrF4 and 7NaF.6ZrF4, respectively, not lying simply on the straight lines 3NaF.UF4-3NaF.ZrF4 and 7NaF.6UF4-7NaF.6ZrF4. This is so because the compounds 3NaF. ZrF4 and 7NaF.6ZrF4 form solid solutions in the NaF-ZrF4 system itself beside forming continuous solid solution with the analogous compounds of the NaF-UF4 system. There is theoretically a one phase ternary solid solution band reaching very slightly into the section from both the 3NaF.UF43NaF.ZrF4 line and from the 7NaF.6UF4-7NaF. 6ZrF4line; the dimensions are exaggerated in order (20) J. E. Ricci, New York University, Consultant t o Oak Ridge National Laboratory, Oak Ridge, Tennessee, “Guide to the Phase Diagrpms of the Fluoride Systems,” ORNL-2896.
B. E. CONWAY, R. G. BARFLADAS AND T. ZAWIDZKI
676
Vol. 62
to show the schematic relations. The region 3NaF.(U,Zr)F4 L 7NaF.6(U,Zr)F4 is a cut through space generated by the moving threephase triangles of curve 111-IV.
the liquidus surface, (3) the other binary compounds do not form detectable solid solutions in the ternary system and their primary phase areas are small. The system NaF-ZrF4 provides low melting fused salt solvents for the dissolution or irradiated Conclusions uranium alloy fuel elements. The system NaFA study was made of phase equilibrium relation- ZrF4-UF4 contains a wide range of compositions ships in the ternary system NaF-ZrF4-UF4 and the from which potential fuels may be chosen for use in related binary systems NaF-ZrF, and UF4-ZrF4. fluid fuel nuclear reactors. The binary system NaF-ZrF4 contains the conAcknowledgments.-Experimental data pertigruently melting compounds 3NaF.ZrF4and 7NaF- nent t o this publication were derived by a large 6ZrF4, the incongruently melting compounds number of people a t the Oak Ridge National 5NaF.2ZrF4, 2NaF.ZrF4 and 3NaF.4ZrF4, a sub- Laboratory over a period of seven years. It is a solidus compound 3NaF.2ZrF4, and a compound pleasure t o acknowledge the assistance of a few of with the formula NaF.ZrF4 which is either meta- these people who performed many of the detailed stable or has a stable existence at a temperature laboratory investigations: R. K. Bagweh, J. P. much below the solidus. There are eutectics con- Blakely, R. A. Bolomey, L. M. Bratcher, H. A. taining 20, 40.5 and 49.5 mole % ZrF4 which melt Friedman, R. E. Metcalf, G. J. Nessle and R. J. at 747, 500 and 512", respectively. The binary Sheil. Our thanks are due P. A. Agron who was system UF4-ZrF4 is a continuous series of solid responsible for a portion of the X-ray diffraction solutions with a minimum melting temperature data. The help of T. N. McVay, G. D. White of 765" at 77 mole % ZrF4. The ternary system and B. S. Landau, who were responsible for much has the following characteristics: (1) it contains no of the phase identification by optical microscopy, ternarv eutectics or ternarv comDounds. (2) the was invaluable to this study. I n addition we are primaiy phase fields of the"contirkous solid solu- especially grateful t o R. F. Newton, F. F. Blankentions UF4-ZrF4,! 7NaF.6UF4-7NaF.6ZrF4 and 3- ship and J. E. Ricci for helpful advice concerning NaFmUF4-3NaF.ZrF4occupy approximately 90% of many phases of the investigation.
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DIRECT DETERMINATION OF ADSORPTION OF ORGANIC MOLECULES AT SOLID ELECTRODES BYB. E. CONWAY, R. G. BARRADAS AND T. ZAWIDZKI~ Department of Chemistry, University of Ottawa, Ottawa, Canada Ricsived December 98, lQb7
A method is reported for the direct determination of the adsorption of certain organic ions and molecules a t solid electrodes under thermodynamically reversible conditions. Adsorption isotherms for pyridine, uinoline and acridine in aqueous solutions and polyvinyl yridine in methanol solution, have been obtained for copper, nit& and silver electrodes at 25'. The effects of ultra-pur$cation of solutions have been examined.
The determination of adsorption of ions at e.g., it is unlikely to be valid for more mercury and other liquid electrodes by the elec- loosely held organic compounds. Similarly, trocapillary method is thermodynamically rigorous. methods hitherto described for determination of However, no method has yet been reported for the adsorption from double-layer capacity measure~ ~ electrodes involve non-thermodydetermination of adsorption of organic ions or m e n t ~a t~solid molecules a t solid electrodes in which either (i) the namic assumptions and are limited owing to the requirement of thermodynamic reversibility has less satisfactory state of development of the theory been met, i.e., the adsorbent and adsorbate are of the double-layer a t solid electrodes6s6compared kept in equilibrium during the adsorption determi- with that for mercury, particularly for the case of nation; or in which (ii) non-thermodynamic as- large specifically adsorbed organic molecules or ~,, is equal to sumptions are not made in the calculation of the ions. Thus, - ( d - y / d p ~ ) ~ , ~ ,which amount of substance adsorbed. Use of radio- r2,the surface excess of adsorbate component 2, a t actively labeled compounds by determination of constant electrode potential, C$ and constant solactivity remaining on the electrode after withdrawal vent activity, all cannot be obtained rigorously from the solution followed by washing or wiping from differential capacity measurements which unless these are made over a with some absorbent is unsatisfactory since the give - (d2y/d~2)),,~,,,, thermodynamic equilibrium set up during the im- range of solute activities and electrode potentials mersion of the electrode will be changed upon its and independent knowledge of other parameters, removal from the solution, particularly if the elec(2) N. Hackerman and S. J. Stephens, THISJOURNAL, 68,904 (1954). (3) R. S. Hansen and B. H. Clampitt. i b i d . , 68, 908 (1954). trode was initially polarized by an applied external (4) R. 8. Hansen,R. E. Minturn and D. A. Hickson, ibid., 60, 1186 e.m.f. Whilst this method may be satisfactory (1956). for adsorbates which are irreversibly chemisorbed, ( 5 ) J. O'M. Bockris, B. E. Conway, W . Mehl and L. Young, J . (1) Part of this work was carried out b y T. Zawidzki for the Bachelor's Thesirr at the University of Ottawa.
Chem. Phws., 26, 776 (1956). (6) D. C. Grahame, Ann. Rev. Phgs. Chem., 6 , 337 (1955).
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