Pyrolytic carbon coating of fused quartz by vacuum vapor transport

Abstract

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 NaI single crystals, and the surface quality of NaI crystal was found to improve as the thickness of coating increases.