Evaporation Behavior of SiO<SUB>2</SUB> in N<SUB>2</SUB>-O<SUB>2</SUB>-H<SUB>2</SUB>O Atmospheres

Abstract

In order to clarify the influence of water vapor on evaporation and crystallization of an amorphous SiO 2 scale formed on SiO 2 -formers, fused silica was exposed for up to 49 h at 1373-1673 K in N 2 -O 2 -H 2 O atmospheres. The water vapor concentration was regulated to 19.6 and 46.7% (vol%). Under conditions of low water vapor concentration, the mass change in the fused silica was negligible. On the other hand, the mass linearly decreased with the duration of exposure under high water vapor concentration. From the temperature dependence of the evaporation rate, it was speculated that the main volatile species were Si(OH) 4 and SiO(OH) 2 in N 2 -H 2 O and N 2 -O 2 -H 2 O, respectively. Moreover, although crystallization of fused silica was observed in both atmospheres, air and N 2 -O 2 -H 2 O, the rate of crystallization in the N 2 -O 2 -H 2 O atmosphere was much higher than that in air. These results indicate that crystallization is accelerated by the presence of water vapor.