In Si and FeSi production, the main Si source is SiO2, in the form of quartz. Reactions with SiO2 generate SiO gas that further reacts with SiC to Si. During heating, quartz will transform to other SiO2 modifications with cristobalite as the stable high-temperature phase. Transformation to cristobalite is a slow process. Its rate has been investigated for several industrial quartz sources and has been shown to vary considerably among the different quartz types. Other differences in behavior during heating between these quartz sources, such as softening temperature and volume expansion, have also been studied. The quartz-cristobalite ratio will affect the rate of reactions involving SiO2. The industrial consequences and other implications of the observed difference between quartz types are discussed. In the current work, a new experimental method has been developed, and an investigation of several new quartz sources has confirmed the earlier observed large variation between different sources. The repeatability of the data has been studied and the effect of gas atmosphere investigated. The results from the earlier work are included as a basis for the discussion.
Fused quartz has excellent thermal and chemical properties as crucible material for single crystal growth from melt, and its high purity and low cost makes it especially attractive for the growth of high-purity crystals. However, in the growth of certain types of crystals, a layer of pyrolytic carbon coating is needed between the melt and the quartz crucible. In this article, we describe a method for applying pyrolytic carbon coating by vacuum vapor transport. The method is shown to be effective in yielding relatively uniform coating on a wide range of crucible sizes and shapes. The resultant pyrolytic carbon coating is characterized by optical attenuation measurements. In each coating process, the thickness of coating is shown to approach a terminal value with an exponential tail as the duration of pyrolysis increases, and the average thickness roughly increases linearly with the ratio of the volume of available hexane vapor to the surface area of pyrolytic coating. Quartz crucibles coated by this process have been used to successfully grow up to 2-in-diameter Nal single crystals, and the surface quality of Nal crystal was found to improve as the thickness of coating increases
Post time: Aug-29-2023