HighVis - Development of a new Al-enriched quartz sand product for next generation solar crucibles with superior viscosity.

During the first 10 months of the project, 2 trial campaigns were carried out at Glatt in Germany. In the first campaign, coating and granulation conditions, binders and Al precursor type and content were optimized. During this campaign, 2 types of binder were used: polyvinyl alcohol (PVA) and silica sol. The addition of the second binder was necessary to maintain the stability of the granule at high temperatures even though granulation with PVA only was very effective. Furthermore, aluminum nitrate (Al (NO3) 3) and boehmite (AlOOH) were tested as Al precursors. During the second campaign, products with different Al- (using the procedure developed during the first campaign) were synthesized. Both coated samples as well as granulated samples with different Al concentration and precursors were prepared. Subsequently, obtained by the Glatt samples were characterized at NTNU using electron microscopy (SEM EDS) as well as micro-XRF, Raman spectroscopy and ICP MS. Both the coated and granulated products showed good Al distribution, and the particle size distribution of granulated samples was similar to the original TQC product. This is important if this new product is to be used for fusion purposes. During 2021, SINTEF has focused on developing the molecular model for silica glass interactions with Al. The models showed that Al rather replaces silicon in the glass network instead of be located in the channels. This was in contrary to some models recently reported in the literature, but it is in line with the general understanding of aluminum behavior in glasses. Overall, all activities were according to plan, except for SINTEF's modeling of viscosity and purchase of a melting furnace. Due to covid-related staff absences and longer test periods than expected for furnaces, these activities will be realized in 2022. This will not affect the implementation of the project.

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In this project we want to formulate a new high purity quartz product for high-tech applications with improved viscosity properties by homogeneous distribution of a defined concentration of Aluminium in currently available quartz materials. This material should be very attractive on the market, in particular for producing crucibles used in wafer production in the photovoltaic (solar) industry. Crucibles made from high viscosity quartz will allow pulling of higher volume monocrystalline silicon ingots, since they can be made larger without deformation or collapse during extended service time at >1420 deg., allowing multiple silicon refills through "hot charging". An added advantage of high viscosity is the reduced risk of bubble formation in the crucible. No company has yet introduced a high viscosity product on the market. However our competitors are likely to attempt a similar innovation. We therefore wish to act as fast as possible, but as the process will include a series of delicate and critical steps, we have initiated collaboration with internationally renowned researchers and laboratories through our project partners to mitigate risk and improve our chance of success. The high level of risk and associated R&D cost means that the project could not be carried by a company of TQC's size alone without NRC funding, which will be a direct decision factor. The project will follow an iterative process where a series of recipes are simulated and optimized based on advanced modelling. Al-doped quartz is then produced at an experienced research lab using a so-called incipient wetness method. The produced 1 kg material batches are then taken through a series of advanced tests in different stages of treatment to evaluate if the process was successful as planned. The most successful material batches will be reproduced in higher quantities of approx 200 kg and fused into actual crucibles that will be evaluated through real industry applications.