The confirmation of phase equilibria in the system ZrOCeO2 below 1400°C

Solid State lonics 3/4 (1981) 477-481 North-Holland Publishing Company

THE C O N F I R M A T I O N OF PHASE EQUILIBRIA IN THE SYSTEM ZrOz--CeO2 B E L O W 1400°C Masahiro Y O S H I M U R A , Eiji T A N I and Shigeyuki S O M I Y A Laboratory for Hydrothermal Syntheses, Research Laboratory of Engineering Materials, and Department of Materials Science and Engineering, Tokyo Institute of Technology, Nagatsuta, Yokohama 22Z Japan

Critical review of the phase diagrams proposed previously suggested incomplete establishment of the phase equilibria in the system ZrOz-CeO2 below 1400°C. Application of hydrothermal techniques enabled us to attain the equilibria in this system. The tetragonal SS (solid solution) was no longer stable below 1000°C, where it decomposed into the stable monoclinic SS and cubic SS. The solubility limit of the tetragonal or cubic SS was 23 wt%(18 mol%) Ce02 or 79 wt%(73 mol%) CeO2, respectively at 1400°C. That of the monoclinic or cubic SS was 5 wt%(3.5 mol%) Ce02 or 96 wt%(95 mol%) CeO~, respectively at 800°C. Thermochemical calculation appears to make it possible to predict true equilibria in this system.

1. Introduction The solid solutions in the system ZrO2-CeO2 are important refractory materials showing mixed conduction [1], which may be considered as electrode materials in M H D [2] and solid electrolyte [3] technology. The stability field and phase relationship of these solid solutions; cubic CeO2 SS, tetragonal ZrO2 SS and monoclinic Z r O 2 S S , have been studied by several investigators. Duwez and Odell [4] proposed a diagram as curve A in fig. 1, which was constructed from the phase analysis of 11 samples with every ten m o l e % composition, fired at 2000°C for 2 h, and annealed at 1375°C for 336 h or at ll00°C for 1000 h. The boundary between the tetragonal and the monoclinic SS was determined by m e a s u r e m e n t of thermal expansion. Curve B shows the results of Longo and Podda [5] who analyzed samples fired at 1700 ° or 1600°C for 8 h, at 1500°C for 150 h, at 1400°C for 1000 h, at 1300°C for 1200 h, or at 1200 or 1100°C for 1500 h, after prefiring at 1400°C for 250-1000 h. They determined the boundary between tetragonal and monoclinic SS by measuring electrical conductivity. They added the c o m p o u n d Ce2Zr3Ol0 as a stable phase below 870°C [6] in this system. Later, curve C was

proposed by Negas et al. [7]. They noticed partial reduction of Ce 4÷ to Ce 3+ at elevated temperatures. The C e 3+ content reached 66% at the liquidus temperatures (>2300°C) of the system ZrOz--CeO2 x in air [8], but seemed to be low below 1600°C [5]. Thus, the ternary system (w tOlo) 0 2800

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M. Y o s h i m u r a et al. / Phase equilibria in Z r O , - ( ~ ' O ~ below 1400°( `

ZrO=,-CeO~Ce203 can be regarded as a binary system ZrO2-CeO2 below 1600°C in air. A m o n g these three results reported, the stability field of tetragonal SS was in good agreement at around 25 wt% (19 mol%) Ce02, and that of cubic SS was in agreement at temperatures between 1700 and 1400°C, but in disagreement at lower temperatures. In these experiments, samples were annealed for a long period after firing at higher temperatures as described above. However long annealing or heating cannot ensure the attainment of the phase equilibria, particularly at low temperatures. The phase assemblage in the samples fired below 1000°C was not necessarily identical with that annealed at the same temperature [9]. A thermochemical assessment [9] suggested that the stability field of tetragonal and cubic SS should become narrower than the experimental one at temperatures lower than 1400°C as illustrated by curve D in fig. 1, which was calculated on the basis of a regular solution model [10] using estimated thermochemical data. One of the possible explanations for such discrepancies is the difficulty in attaining equilibria by solid-state reactions in this system at low temperatures. Thus high-temperature states might exist in metastable form at lower temperatures even after long annealing. It should be noted that cation diffusion is very slow in contrast to anion diffusion in fluorite-type structures [1 I]. In order to accelerate the reactions at low temperatures, this system has been studied under hydrothermal conditions. The present study deals with the effects of temperature, heating duration and starting materials on the phase relations in this system. A phase diagram is proposed on the basis of the experimental results and thermochemical calculations.

appropriate mixture of ZnC14* and cerium nitrate** solutions. They were washed and dried in air at 120°C for 7 days. For runs above 1000°C, they were calcined at 800°C for 1 h to convert into the oxide. The second was a mixture of ZrO2t and CEO25, and the last was prepared by firing the mixed oxide at 1600°C or 1400°C for 4 h in air. These materials were sealed in a platinum capsule, 2.7 mm i.d. and 35 mm long, with water or aqueous solutions of 7-30 wt% LiC1, as a mineralizer. Other solutions were also used as mineralizers. The capsules were heated at the desired temperatures for extended durations; 200-700°C up to for 7 days, 800°C for 1 day, 1200°C for 12 h or 1400°C for 1 h, under a pressure of 100 MPa (1020 kg/cm 2) either in a testtube-type pressure vessel below 800°C or in a high-gas-pressure apparatus [12] above 800°C. After completion of the heating run, the testtube-type vessel was quenched into cold water, while the high-gas-pressure apparatus was cooled down to room temperature by ~20°C/min, The products were decapsuled, washed, dried, then examined by X-ray powder diffraction and observations using a scanning electron microscope and a petrographic microscope.

2. Experimental

* Hf-free ZnCI4, Nikko Engineering Co. Ltd., Tokyo, Japan. ** Ce(NO3)3.6H20, 9 9 . 9 ° , Nissan Kigenso Co. Ltd., Osaka. Japan. t ZrOe, 99.9%, Soekawa Rikagaku Co. Ltd., Tokyo, Japan. $ CeO2, 99.9%, Shin-etsu Chemical Co. Ltd., Tokyo, Japan.

Starting materials were (i) co-precipitated, (ii) mixed, or (iii) fired samples. The first was prepared by co-precipitation of hydroxides from an

3. Results and discussion The first question to answer is whether the presence of high-pressure water influences the equilibrium or only increases the rate of reaction or crystallization in this system. Good agreement between the hydrothermal results in this work and previous results [4, 5, 7] by heating in air at high temperatures, as described later, indicates that the hydrothermal condition influences only the kinetics of the reaction in

M. Yoshirnura et al. / Phase equilibria in Z r O 2 - C e 0 2 below 1400°C

this system as well as in the systems Z r O 2 - C a O and Z r O 2 - Y 2 0 3 [13]. After the hydrothermal treatment with 15 wt% LiC1 solution at 1400°C under 100 MPa for 1 h, the 59 wt%(50 mol%) CeO2 sample fired at 1600°C, which was single-phase cubic SS with lattice parameter a = 5.260 ~ , was converted into a mixture of a tetragonal SS with a = 5.148 ~ and c = 5.235 ~ and a cubic SS with a = 5.340 ~ as shown in fig. 2. The mixed oxide also yielded similar products; a tetragonal SS with a = 5.150 ~ and c = 5.240 ,~ and a cubic SS with a = 5.318 ~ . The co-precipitated 48 wt%(40 mol%) Ce02 sample produced almost the same phases by the same hydrothermal treatment; a tetragonal SS with a = 5.146 ~ and c = 5.236 ~ and a cubic SS with a = 5.327 ~ . As the products at 1400°C were identical in both directions, from high temperature (1600°C fired sample) or low temperature (mixed sample and co-precipitated sample), the phase equilibrium appeared to be attained by these hydrothermai treatments. Thus, the tetragonal SS with 8 = 5.148(2) ~ and ~ = 5.237(3)]k and the cubic SS with a = 5.328(12) ~ are the stable phases at 1400°C in the system ZrO2-Ce02. They cor-

respond to the solid solution phases with 23 wt%(18 tool%) CeO2 and 79 wt%(73 mol%) C e O 2 , respectively, according to a linear relation between the composition and the lattice parameter [4,5]. Fig. 3 shows the lattice parameters of the solid solutions as a function of duration of treatment at 1200°C. Fig. 3 clearly indicates that phase equilibria appears to be established within 1 h and that the tetragonal and cubic SS were the stable phases in the equilibrium state at 1200°C. Small amounts of a monoclinic SS were sometimes observed in the products at 1200°C. This seems to be formed by the phase transformation from the tetragonal SS during cooling, because the tetragonai phase is not stable below 1000°C. Treatment at 800°C for 24 h yielded the same products; a mixture of a monoclinic and a cubic SS, both from the co-precipitated and the fired samples as seen in fig. 4. No tetragonal SS could be detected. The monoclinic and cubic SS, therefore, are the stable phases at 800°C. The lattice parameter of the monoclinic phase were always identical, a = 5.156(7) ~ , b = 5.220(4) ~ , c = 5.323(7) ~ and/3 = 99.2(1) °, while that of the cubic phase was scattered, a = 5.401 ~ from

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Fig. 3. Lattice parameters of the cubic and tetragonal phases produced by treatment at 1200°C from different starting materials.

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M. Yoshimura et al. / Phase equilibria in ZrO:--(~eOz heh)w 1400°("

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Fig. 4. Ptlase changes in the ZrO,-CeO2 san]pies after treatment at 800°C for 24 h under 100 MPa with 15 wt% LiCI solution. co-precipitated, 5.413 ]~ from mixed, 5.363 from fired at 1400°C or 5.373 A from fired at 1600°C, respectively, indicating a different composition in CeO2 content even after 24 h treatment at 800°C. These results indicate a slow kinetic process in these solid solutions even under hydrothermal conditions. At lower temperatures, as the kinetics becomes much slower, the lattice parameters of the cubic SS were no longer identical among the products particularly in fired samples, which are less active due to heating at high temperatures. Fig. 5 showing the X-ray diffraction peaks for (422) lines in cubic CeO2 SS after several treatments seems to indicate that the decomposition process was only half way to equilibrium even after treatment for 7 days at 700°C, and that such processes depend upon the concentration of mineralizer. The mixed samples were practically unchanged by the treatment at 700°C. The coprecipitated samples seemed to have the tendency to achieve equilibrium most easily. They, however, sometimes yielded a metastable tetragonal SS by hydrothermal treatment below 500°C. The confirmation of the equilibrium appears to be extremely difficult at

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such low temperatures, where the products depend upon thermal history of starting materials, and/or species and concentration of mineralizer [14]. In this regard, a thermochemical assessment has the potential of predicting true equilibria in such systems, though necessary thermochemical data are not always available. The phase boundaries among the tetragonal SS, tetragonal SS + cubic SS and cubic SS were calculated by a regular solution model using estimated thermochemical data: t~c = AGz,o2 2 5 7 3 - 1.0T,

G~x = 5150x(1 - x),

A G ct ~ c~ o 2- - - 0 - 1.OT, G'ex= 4850x(1 - x).

They are in good agreement with the experimental results determined from the average lattice parameter of each phase as shown in fig. 6. The detailed procedure of such calculations and extended experimental results will be given elsewhere [15]. Here, we would like to emphasize that the agreement between experimental and calculated values makes it possible

M. Yoshimura et al. / Phase equilibria in ZrO,,-Ce02 below 1400°C

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(3) The solubility limit of the tetragonal or cubic SS was 23 wt%(18 mol%) CeO2 or 79 wt%(73 mol%) CeO2, respectively, at 1400°C but was smaller at lower temperatures. (4) The solubility limit of the monoclinic or cubic SS was 5 wt% (3.5 mol%) CeO2 or 96 wt% (95 m o l % ) C e O 2 , respectively, at 800°C. (5) Thermochemical calculation appears to make it possible to predict true equilibria in this system.

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Fig. 6. Proposed phase diagram of the system ZrO,--CeO2 in the present study. - - o - - Experimental results, ------calculated boundary, - - - metastable boundary, - - - - - - after R o u a n e t [8]; they are the projection of liquidus observed and supposed solidus in air.

to predict a complete phase diagram by reasonable extrapolation. In this regard, the stability field of the tetragonal SS for the composition with 7-23 wt% (5-18 mol%) CeO2 below 1000°C would be metastable, even though a reversible phase transformation with hysteresis could be observed during heating and cooling for the samples with less CeO2 content [4, 5].

4. Summary (1) Phase equilibria were attained in the system ZrOz-CeO: by hydrothermal techniques at temperatures below 1400°C, particularly below 1200°C for the first time. (2) The tetragonal SS was no longer stable below 1000°C and decomposed into the monoclinic SS and cubic SS which are stable in this temperature range.

[1] S.F. Parguev, S.I. Alyamovskii and Z.S. Volchenkova, Russian J. Inorg. Chem. 4 (1959) 1158. [2] B.R. Rossing and H.K. Bowen, in: Materials limiting problems in energy production, ed. C. Stein (Plenum Press, New York, 1976) pp. 33,5-356. [3] K.W. Browall and R.E. H a n n e m a n , G.E. Report, No. 75 C R D 012 (1975). [4] P. Duwez and F. Odell, J. A m . Ceram. Soc. 33 (1950) 274. [5] V. Longo and L. Podda, Ceramurgia 1 (1971) 83. [6] V. Longo and D. Minichelli, J. A m . Ceram. Soc. 56 (1973) 6(X). [7] T. Negas, R.S. Roth, C.L. McDaniel, H.S. Parker and C.D. Olson, 12th Rare Earth Research Conference, Vail, Colorado, July 1976. [8] A. Rouanet, Compt. Rend. Acad. Sci. (Paris) C 266

(1968) 908. [9] M. Yoshimura and H.K. Bowen, Bull. Am. Ceram. Soc. 56 (1977) 301. [10] L. K a u f m a n and H. Bernstein, in: Phase diagrams, materials science and technology, Vol. 1, ed. A.M. AIper (Academic Press, New York, 1970) pp. 45-113. [1 I] K. Ando, M. A k i y a m a and Y. Oishi, J. Nucl. Mat., to be published. [12] S. S6miya, S. Hirano, T. Fukuda and M. Sawada, K6atsu Gasu 10 (1973) 368. [13] V.S. Stubican and S.P. Ray, J. A m . Ceram. Soc. 60 (1977) 5?,4. [14] E. Tani, M. Yoshimura and S. S6miya, T h e 25th Jinko Kobutsu Toronkai (Symposium of Artificial Minerals), Nagoya, Oct. 16, 1980. [15] E. Tani, M. Yoshimura and S. S6miya, to be published.