Kinetics and Mechanism of Phase Transformations from Quartz to Cristobalite
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The largely debated silica polymorphism has in this thesis been investigated for the transformation from β-quartz to β-cristobalite. It is done as a step in understanding and improve the furnace operations of the silicon process.The goals of this thesis has been to describe the kinetics and the mechanism for phase transformations from β-quartz to β-cristobalite. Qz 29, which is a Faleq, has been investigated after heating to 1600°C and 1700°C in a rapid heating furnace and later analyzed with XRD. The amorphous phase has in the XRD analysis been quantified by the internal standard method. This method is found to be relatively accurate. In addition, has a sample been studied during heating to 1600°C in a wettability furnace. The volume expansion was also calculated during heating.Quartz samples obtained from a furnace excavation has been investigated to study size effect in particles exposed to the environment of a silicon furnace.It has been found that the transition to the amorphous phase is rapid, and the rate limiting factor for this transition seem to be the heat distribution in the particle. The further transition to β-cristobalite is slow and is for the transformation from β-quartz to β-cristobalite the rate limiting transformation.A size effect is found for particles heated to 1700°C, but not in samples heated to 1600°C and samples from furnace excavation. It is therefore in this thesis concluded that a size effect could be seen in samples exposed to a fast heating rate and high temperatures.