Investigation of B4C for inhibiting crystallization in silica at high temperatures

Amorphous silica has numerous high temperature applications due to its inherent thermal shock resistance and low coefficient of thermal expansion (CTE). However, at the high temperatures required for processing (>1200 degrees C), the metastable beta-cristobalite phase preferentially forms and is accompanied by a volume change, and potential cracking, upon conversion to the low temperature alpha-cristobalite phase. The CTE of the final crystalline phase is an order of magnitude higher than its amorphous counterpart. Here experimental results demonstrate that small additions of B4C (3.5 wt%) effectively inhibit silica crystallization in powder and sintered gel-cast forms up to 22 h at temperatures as high as 1500 degrees C as confirmed via x-ray diffraction. The oxidation of B4C to B2O3 and its subsequent melt and evaporation disrupts the nucleation and growth of the cristobalite phase. The mechanism for crystallization inhibition is further explored through optical microscopy to probe changes in surface morphology.