Stability of yttria stabilized zirconia in molten oxy-fluorite flux for the production of silicon with the solid oxide membrane process

Solar grade silicon can be formed using a YSZ solid oxide membrane (SOM). The SOM membrane is exposed to a complex fluoride flux with dissolved silica at high temperature and electrochemically separated into silicon and oxygen. A failure mode of the SOM membrane by the formation of ‘inner cracks’ was studied, and attributed to yttria depletion in the YSZ, leading to phase transformation from cubic to tetragonal phase. The roles of silica and YF3 in the flux were studied, and it was shown that silica attacks the SOM membrane, while YF3 retards the attack. A detailed mechanism of the yttrium depleted layer (YDL) formation, and its role in the formation of inner cracks is proposed. Based on this study, a new flux composition was designed and tested. The flux composition did not attack the SOM membrane, and Si crystals were produced, demonstrated long-term viability of the Si–SOM process.