Nucleation cristobalite

Fig. 2. Devitrification rate of vitreous silica for surface-nucleated cristobalite as a function of temperature (99). Growth rate is proportional to the square...

To investigate the kinetic explanation for the step rule, we model the reaction of three silica polymorphs — quartz, cristobalite, and amorphous silica — over time. We consider a system that initially contains 100 cm3 of amorphous silica, the least stable of the polymorphs, in contact with 1 kg of water, and assume that the fluid is initially in equilibrium with this phase. We include in the system small amounts of cristobalite and quartz, thereby avoiding the question of how best to model nucleation. In reality, nucleation, crystal growth, or both of these factors might control the nature of the reaction we will consider only the effect of crystal growth in our simple calculation. 

In order for the foundry core or mold to become weak after heating and cooling, it is helpful to have crystalline silica such as cristobalite formed throughout the binder mass by spontaneous nucleation at high temperatures. Such nucleation apparently occurs at the surface of... 

Nucleation of mullite occurs at the Al203-Si02interfece. At 1450°C, the viscosity of SiCh glass decreases and hence diffusion through the viscous phase is enhanced, more so because the sizes of the diffusing complexes decrease. Above 1500°C, the mullite formation occurs by the reaction between cristobalite and a- AI2O3. 

At some temperature above IGOO C, devitrification may begin by nucleation of crystals of cristobalite. This is strongly catalyzed by traces of impurities, especially... 

If this phase transformation is a sequential process where even crystalline phases are transformed to other crystalline phases, characteristic changes observable in the stability range (D) of phase I must correspond to the incubation period (BO for the second crystalline phase (II), where the nucleation of the subsequent product takes place. As a consequence, the recrystallization process (period E) of phase I and the growing period (CO of phase II are of the same time scale. This sequential recrystallization process finally leads to thermodynamically more stable products such as cristobalite and quartz. 

Applying this to the nucleation of cristobalite in fused silica as an example we shall use the viscosity data of Fontana and Plummer ( ), and take... 

Calculated Nucleation Frequency Versus Temperature for Cristobalite Crystals in Si02... 

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