Thermochemical and Spectroscopic Studies of Chemically Vapor-Deposited Amorphous Silica

Two kinds of low pressure CVD SiO₂samples, heterogeneous film and homogeneous “snow,” have been collected on stainless steel substrates at three different deposition temperatures: 523, 643, and 703 K. They have been characterized thermodynamically by transposed temperature drop and high temperature solution calorimetry. Both film and “snow” samples are metastable—in terms of enthalpy relative to bulk Si0₂glass—by up to 101 kJ-mol¹. The excess enthalpy increases with decreasing deposition temperature and is larger for “snows” than for films. On annealing at high temperatures, it gradually disappears. IR and Raman spectroscopies indicate that both Si-H and Si-OH species are present in the as-deposited samples but disappear upon annealing. All as-deposited samples have densities comparable to bulk Si0₂glass, ie. between 2.0 and 2.2 g-cm^-3. Annealing tends to increase the density of all samples.