Preparation Of Silica Crucible And The Research On The Crystallization Behavior

Fused silica crucible has many good properties, for example, good thermalstability, low thermal expansion coefficient and thermal conductivity. Furthermore, itwill not react with silicon at high temperature. Therefore, silica crucible is widelyused in the production of polycrystalline silicon ingots. Although tremendousprogress has been made in dry shaping techniques in recent years, the most widelyused method for the preparation of silica crucible is still colloidal preparationtechniques. Hence, the research on slurry rheology is of great significance. Besides,fused silica ceramics tend to precipitate cristobalite during the sintering process athigh temperature, resulting in the decline of the comprehensive performance of silicacrucible, the occurrence of crack and even the damage of the silica crucible. Therefore,the research on the crystallization behavior of the silica crucible also has importantand real meaning.Powder preparation method plays an important role in the process of slip casting.The raw silica powders used in the preparation of silica crucibles can be prepared bywet and dry ball milling. Compared with dry ball milling, wet ball milling has highermilling efficiency. However, there exist significant differences in the surface chemicalstate and particle shape for the silica powders milled by wet and dry ball milling. Theabove two factors have a significant effect on the slurry rheology, but few studiesabout it were carried out. Hence, the present study is intended to illustrate the effect ofsurface chemical state and the particle shape on the rheological property of thesuspensions. SEM、FTIR、XPS、zeta potential analyzer and viscometer were employedto research the particle surface chemical state and particle shape on the slurryrheology. The results showed that, compared with the powders milled by dry ballmilling, there were more silanol groups on the surface of the powders milled by wetball milling and the particle shape was more irregular, resulting in the higher viscosity and instability of the suspension. Besides, the silanol groups on the particle sufacecould enhance the shear thickening behavior of the suspension.The present study also researched the effect of particle size, the adding of Si3N4and aqueous PVA solution, surface impurities and heat treatment on crystallization insilica crucible. The results showed that the finer the particles, the easier thecrystallization. When the median diameter of the silica powders was7.2μm，the greenbody gained the maximum density,1.83g/cm3, and after sintering at1200°C, therewas little of cristobalite in the sintered body, which met the application requirements.The adding of aqueous PVA solution also had an effect on the inhibition of the silicacrucible. In the present study, when the adding amount of PVA was0.27wt.%, theprecipitation of the cristobalite in silica crucible could be better inhibited. Besides, theadding of Si3N4could also affect the precipitation of the cristobalite. Also, the resultsshowed that the surface impurities greatly promoted the precipitation of thecristobalite. And heat treatment before the sintering process on the bodies played aninhibitory effect on the precipitation of the cristobalite.