Journal of Crystal Growth 75 (1986) 630—632 North-Holland, Amsterdam
LETTER TO THE EDITORS FLUX GROWTh OF YTfRIA-STABILIZED ZIRCONIA CRYSTALS Mitsunori YOKOYAMA Nagoya Economics University, 61 Uchikubo, Inuyama 484, Japan
and Toshitaka OTA and Iwao YAMAI Ceramic Engineering Research Laboratory Nagoya Institute of Technology, 10-6 Asahigaoka, Tajimi 507, Japan
Received 19 January 1986; manuscript received in final form 21 March 1986
Flux growth of stabilized zirconia single crystals is reported. Among some conventional stabilizers, only yttria formed a cubic zirconia solid solution. Various fluxes previously employed to grow monoclinic zirconia crystals were investigated. The system KF—Na 2B4O7 proved to be suitable for crystal growth of an yttria-stabilized zirconia solid solution. Starting compositions and growth conditions for the KF—Na2B4O7 system are reported.
Stabilized zirconia single crystals have been grown by the skull melting method near the melting point of Zr02 (about 2300—2700°C).Yttriastabilized zirconia single crystals were grown by Alexandrov et al.  and Nassau . The zirconia rare earth oxide crystals, Ln~Zr1 x°2x/2 (Ln = La, Nd, Sm, Gd, Dy, Yb), were grown by Michel et al. .With respect to lower temperature growth techniques for cubic zirconia, only hydrothermal growth of yttria-stabilized zirconia crystals has been investigated by Nakamura et al. . Although flux growth of monoclinic zirconia crystals has been widely investigated at temperatures below the monoclinic tetragonal transition temperature (about 1050°C) [5—9],cubic solid solution crystals have not been reported to grow from the flux, probably because of the complexity of the melt behavior. This paper reports modification of previously published starting compositions for flux growth of monoclinic zirconia in order to grow stabilized cubic zirconia single crystals. The chemicals used as raw materials and fluxes were all of reagent grade. The raw materials (as shown in table 2) were well mixed and were put into 46 or 23 ml platinum crucibles. The crucible
was loosely capped with a platinum plate and was placed in an electric furnace. For the evaporation method, the furnace was previously heated to a given temperature. In other cases, the furnace was heated from room temperature to the maximum temperature in 2—3 h and then slow cooling was started. Crystals grown in the melt were separated by dissolving the flux in 30% hot nitric acid and were identified by X-ray powder diffraction. CaO, MgO and Y203 as the stabilizer agent and the systems K2O—Mo03, PbF2—PbO, KF—Na2B4 07, B203—V205 and NaA1F6 as the flux were investigated to grow cubic Zr02 crystal. Only Y203stabilized Zr02 crystal could be grown from a K20—MoO3, PbF2—PbO or KF—Na2B4O7 flux.
Table I Y203 content in cubic Zr02 solid solution crystals grown by Flux
Starting composition of Y201 (mol%)
Y203 content in crystal (mol%)
PbF2PbO Li20—K2MoO4 KF-Na2B4O7
33 18 50
29 18 14
0022-0248/86/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
M. Yokoyama et al.
/ Flux growth of yttria-stabilized zirconia crystals
Fig. 2. Y
40 60 Time (h)
203-stabilized cubic Zr02 crystals grown from the solution of 49KF.33Na2B4O79ZrO29Y203 at 1100°C for 24 h.
Fig. 1. Evaporation isotherms of the samples of 49KF 33Na2B4O7 . 9ZrO29Y203.
The amounts of Y203 required for the growth of the cubic phase were different corresponding to the fluxes being chosen. The Y203 contents of the grown crystals were also different from those of the starting composition. Table 1 shows the Y203 contents of the starting materials and of the cubic Zr02 crystals calculated from lattice parameters after Stubican et al. .The system KF—Na2B407 was the most suitable flux for bulk crystal growth using the evaporation method. Fig. 1 shows evaporation isotherms of the samples of 49KF~33Na2B407 9Zr0~ 9Y203. The rate of evaporation in the early stage was rapid at every temperature. The quenched sample of the run at 1100°C for 8 h showed a glassy state without any crystals in it. At this temperature, cubic ZrO2 crystals began to grow only when the holding time was over 24 h, but their shape -was a typical dendrite (fig. 2). Dendritic growth might .
be caused by a large evaporated amount of above 40 wt% which is related to high supersaturation. The evaporation rate was small at 900°C, and evaporation seemed to proceed in a nearly constant melt composition especially if the melt was held for over 24 h. The crystals obtained at this temperature were not dendritic but were bulk.
Fig. 3. Y201-stabilized cubic Zr02 crystals grown by slow cooling method (No. 2 in table 2).
Table 2 Conditions and results of flux growth of Y203-stabilized cubic Zr02 crystals using KF—Na2 B407 flux No.
1 2 3
Starting composition (mol%)
49 52 50
33 30 34
9 9 8
9 9 8
LC SC EV
LC: Localized cooling; SC: Slow cooling; EV: Evaporation.
Notes on the products
900 1100—1000 910
72 336 447
amount (wt%) 5.6 26.9 35.2
Monoclinic and cubic 0.2 mm, octahedron Max. 1 mm, bulk
M. Yokoyama et al.
/ Flux growth of yttria-slabilized zirconia crystals longer holding times at lower temperatures below 1000°C to suppress the too rapid evaporation. This resulted in the growth of Y203-stabilized crystals of a maximal size of 1 mm from a melt of
approximately 13 ml (fig. 4).
References ~ Fig. 4. Y,O~-stahihzedcubic ZrO~crystals grossn by evaporation method at lower temperature of 910°C(No. 3 in table 2).
Typical results of the crystal growth by evaporation, localized cooling and slow cooling using KF—Na2B407 flux were shown in table 2. Evaporation was suppressed by employing capped crucibles in the runs of localized cooling and slow cooling method. In the above cases, bulk crystals of octahedron shape were grown in spite of the relatively higher temperatures of 1000 to 1100°C (fig. 3). In order to obtain larger crystals by the evaporation method, it was necessary to adopt
 V.1. Alexandrov et al., US Patent 4,153,469, May 8 (1979).  K. Nassau, Lapidary J. 9 (1981) 1194.  D. Michel, M. Perez y Jorba and R. Collongues, J. Crystal Growth 43 (1978) 546.  K. Nakamura, S. Hirano and S. Somiya, Am. Ceram. Soc. Bull. 56 (1977) 513.  A.B. Chase and J.A. Osmer, Am. Mineralogist 51(1966) 1808.  A.B. Chase and J.A. Osmer, J. Am. Ceram. Soc. 50 (1967) 325  M. Ushio and Y. Sumiyoshi, Nippon Kagaku Zasshi 12 (1973) 2295.  Y. Fujiki and Y. Suzuki, J. Crystal Growth 24/25 (1974) 661.  S. Shimada, K. Kodaira and T. Matsushita, Z. Anorg. Allg. Chem. 469 (1980) 128. (10] VS. Stubican, R.C. Hink and S.P. Ray, J. Am. Ceram. Soc. 61(1978)17.