Role of thermodynamic miscibility gaps in phase selection in sol-gel synthesis of yttrium silicates |
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Affiliation: | 1. Japan Fine Ceramics Center, 2-4-1 Mutsuno, Atsuta-ku, Nagoya, 456-8587, Japan;2. The University of Tokyo, 2-11-16 Yayoi, Bunkyo-ku, Tokyo 113-8656, Japan |
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Abstract: | Yttrium monosilicate and disilicate are important materials for environmental barrier coatings. The two silicates were synthesized by sol-gel route and their phase selection upon calcination and thermal exposure was studied. First products of crystallization were the monosilicate and yttria. Amorphous silica precipitated out at 1300 °C as apatite phase. During prolonged high temperature treatment, up to 100 h at 1400 °C, the apatite disappeared and the disilicate appeared, only to disappear itself as the system approached equilibrium. Thermodynamic calculations performed using Thermo-Calc software show the presence of a metastable miscibility gaps in the amorphous (liquid) phase field. As a consequence, phase separation in the amorphous phase prior to crystallization is responsible for the formation of yttria-rich and silica-rich phases during crystallization. Multiple phase formation during both, yttrium monosilicate and disilicate synthesis is consistent with the presence of the amorphous phase miscibility gaps around the silicate compositions. |
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Keywords: | CALPHAD Metastable phases Phase separation Spinodal decomposition Yttrium silicates |
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