18O diffusion through amorphous SiO2 and cristobalite

Abstract

Secondary ion mass spectrometry was used to profile the diffusion of oxygen in polycrystalline β-cristobalite and vitreous SiO2. The tracer concentration profiles of cristobalite are consistent with a model based on two mechanisms: bulk and short-circuit diffusion. The profiles of partially crystallized samples containing vitreous SiO2 and β-cristobalite were fitted using the sum of two complementary error functions and taking account of some interstitial-network exchange. The bulk oxygen diffusivity, in the temperature range 1240–1500 °C, is about five times greater for vitreous silica than for β-cristobalite.

Microstructure and interfaces of HfO2 thin films grown on silicon substrates

“…Although no data are available for amorphous HfO 2 , a faster diffusion in amorphous HfO 2 is expected due to the lower mass density than in crystalline HfO 2 (typical values of 7 g/cm 3 for amorphous films as compared with 9.7 g/cm 3 for the monoclinic phase [23]). This conclusion is supported by oxygen diffusion data in amorphous and crystalline SiO 2 [26,27], where the oxygen diffusion rate is a factor of 5 higher in the amorphous state (but this was reported for rather high temperatures of 1240-1500 C) [26]. According to our structural investigations, the amorphous phase is gradually replaced by crystalline monoclinic phase within the temperature range from 400 C to 500 C. The decrease in the rate of the oxygen diffusion in the film becomes stronger due to the crystallization than the increase due to the temperature increase.…”

Oxidation of silicon carbide and the formation of silica polymorphs

“…The rates of oxygen diffusion in crystalline SiO 2 are slower than in amorphous SiO 2 . Oxygen diffusion was experimentally determined to be slower in ␤-cristobalite than in vitreous silica, with diffusion coefficients equal to 1.96 × 10 −12 cm 2 s −1 and 3.35 × 10 −12 cm 2 s −1 , respectively at 1800 K. 30 The crystallization of the oxide grown over chemically vapordeposited SiC at 1573 K is responsible for a decrease in the oxidation rate by a factor of ∼30 8 . The nature of the silica scale should then significantly modify the oxidation behavior and affect the oxidant diffusivity.…”

Microstructure and oxidation behavior of in situ formed TaSi2–Si3N4 nanocomposite coating grown on Ta substrate

“…The volume fraction of Ta [36] at 1300 C) within the oxide scale formed on the TaSi 2 -Si 3 N 4 nanocomposite coating is much smaller than that on the monolithic TaSi 2 coating. This can be explained by the role of Si 3 N 4 phase in Eq.…”