Investigation of bubble distribution and evolution in solar cell quartz crucibles
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Quartz crucibles are, among other applications, used to produce monocrystalline silicon ingots for the solar cell industry. The crucibles must be of high purity as any gas bubbles or impurities close to the crucible-melt interface may end up in the silicon melt and influence the silicon ingot quality. This master thesis looks at three different fused quartz crucible types, including one reference. The crucibles have been compared with respect to bubble distribution and evolution which then have been related to their quartz sand quality and impurity content. The focus has been on the bubble distribution and evolution in the bubble free (BF) layer. Samples have been taken from the crucible wall and both new and industrially used samples were investigated. Heat treatments in a vertical gradient freeze (VGF) furnace and Czochralski puller have been performed to investigate the secondary bubble growth, the OH content and the micro-bubble formation as a function of heat treatment and crucible type. The results indicate that the sand quality, but also manually controlled processing steps such as crucible fusion, has influence on the bubble growth and BF layer bubble content. A reduction in OH content seems to correlate with increasing bubble growth. The reference (STD) crucible has the largest variation in bubble content and the largest average bubble growth for the samples studied in this work. Crucible B, which has the finest particle size distribution (PSD) and chemistry, seems to be the best choice for silicon ingot production as it experiences, on average, the lowest BF layer bubble growth and has the lowest density of bubbles below detection limit.