Researchers from Norway and Germany investigated the influence of impurities in fused quartz crucibles used in silicon PV ingot production, specifically the physical properties and the uniformity of hydroxyl impurities.
The research was enabled by infrared microscopy and viscosity testing.
“Understanding the weaker points in the crucible structure could potentially lead to improving them,” corresponding author, Gabriela Kazimiera Warden, told pv magazine. “We believe that our research should be of interest to the crucible manufacturers, as it shows that some of the aspects of the production process can influence the final crucible properties. It is also relevant for them to understand the quality of the ingot, the contamination of it and what they can expect from the crucibles during use.”
The raw material used in Czochralski puller crucibles is natural quartz sand (SiO2) that has been processed to 99.997% purity. A crucible’s performance is dependent on properties, such as viscosity, which in turn is dependent on the presence of impurities, such as the hydroxyl (OH) groups content, according to the researchers.
Too much OH leads to low viscosity. “It is preferable for the crucible to have high viscosity, which implies a greater mechanical stability at elevated temperatures, and consequently leads to better durability. This means that the crucible can be used for a longer time,” they stressed.
The experimental setup involved studying half of a fused quartz crucible manufactured from two types of sand. Samples were extracted from crucibles and prepared for the viscosity testing and IR microscopy examination. Typically, OH groups are present in quartz sand used in the fusion process and water inclusions.
The investigation of OH groups distribution in the crucible sought to identify any potentially weak regions in the crucible. The OH groups’ content was mapped and the viscosity at 1500 C was measured. “Local variations in the OH groups content of on average 26.7 ppm difference between the top and the bottom of the crucible were detected,” said the researchers.
The viscosity measurements further confirmed the crucible’s inhomogeneity as no clear trends or direct correlations to the OH groups content, according to the team.
Additionally, four samples originating from four different crucible types were also investigated. The samples originating from different crucibles were extracted and all characterized at the same layer.
The researchers examined the differences between the crucibles and attributed them to the different sand purification treatments. In two of the crucibles, the team noted an increased OH groups content in the boundary between the bubble-free and the bubble-containing layer, while others showed an increased and maximum OH groups content in the bubble-containing layer.
Furthermore, the link between the sand types and the OH groups content was confirmed.
“The source of the local inhomogeneities and the two different trends of OH groups distribution can possibly be attributed to the sand quality, the sand particle size, and the difference in the manufacturing process, such as thermal history or distribution of sand,” said the researchers.
The differences in the OH groups distribution were “most probably” linked to the different process conditions, including the thermal history, of vacuum and sand distribution, the quality of sand used, and the particle size of the sand, they noted.
A further correlation they found was between the type of sand used for manufacturing the crucible and its final OH groups content, which they said could influence the crucible’s viscosity.
“Both the chemical treatment and differences in chemical size are significantly affecting the final OH group content, which was shown to be the lowest in the crucible that was fused using only refined quartz sand, followed by a mixture of a fine and coarse, and the highest in the crucibles manufactured from coarser sand,” concluded the team.
And finally, the researchers stressed that in “contrast to previous studies,” they could not see a clear correlation between the OH groups content and the viscosity values within the OH group content variations in the samples. “This further proves that the crucibles are not uniform, and that the viscosity is affected by more factors than the OH groups content,” they said.
The research is detailed in “Investigation of uniformity in fused quartz crucibles for Czochralski silicon ingots,” published in Journal of Crystal Growth.
The team came from three Norwegian organizations Norwegian University of Science and Technology (NTNU), research institute SINTEF Industry, and The Quartz Corp, plus the Germany-based Max Planck Institute for Sustainable Materials
Looking ahead, Warden said that there is “still a lot to explore,” for example the potential influence of trace elements on the crucible properties.
“We hope that our research will bring some attention to the importance of high-quality quartz crucibles for the monocrystalline silicon production. We are looking forward to more groups getting involved with research on this material, as we think it is critical in order to improve productivity and hence bring the ingot cost down, making solar energy more affordable, as well as reduce the amount of waste,” she said.
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