Popular Science Presentation
High Purity Quartz Sand (HPQS) is an essential material in modern high-tech applications, such as optical fibers, semiconductors for the electronic industry and solar cells. One of its main applications is fabrication of crucibles for the process of growing a single crystal ingot of silicon by the Czochralski (CZ) method.
The Quartz Corp (TQC) already has access to the source of the purest natural quartz ever found on Earth, in Spruce Pine, NC, USA. We hold a significant market share in the global quartz sand market for crucible production, offering quartz (SiO2) purities as high as 99.998%. However, we have identified an emerging need for a product that delivers higher viscosity quartz glass. There are two main market trends behind this:
• Demand for larger crucibles, which may lead to loss of structural integrity and deformation (sag) while the crucible is being used at high temperatures, thereby ruining the production process.
• Demand for crucibles that can spend longer time in the hot zone. Once cooled down, the crucible can never be used again due to surface cristobalite formation and subsequent cracking. “Hot charging” is therefore desirable, meaning the crucible must maintain integrity for up >500 hours without deformation.
The primary objective of the R&D HighVis project was therefore to develop an effective process route for Al-doped quartz formulation for fusion of PV glass crucibles with enhanced viscosity as well as to:
• Understand/simulate how quartz viscosity properties are influenced by the aluminum content.
• Homogeneously distribute aluminum in quartz sand with the desired shape/size.
• Characterization of material in form of a "toolbox" including experimental characterization and simulation, improves understanding of the new quartz sand properties and underlying mechanisms.
• Demonstrate tuned quartz crucibles properties through testing and analysis.
These goals were pursued in a consortium with NORSUN, NTNU and SINTEF.
In the course of the project, it was found that addition of Al to quartz enhances the mechanical properties of high purity quartz glass by increasing viscosity at high temperatures.
The mechanism was revealed by using atomistic modeling methods as well advanced characterization ‘toolbox’ which includes ICP-MS, Electron Microscopy, FTIR Spectroscopy as well as viscosity tests. It was concluded that the underlying mechanism of high viscosity is not only related to the presence of Al but also to structural changes in SiO2 network triggered likely also by oxygen vacancies and noticeably lower stability of OH groups.
The obtained coated material, with homogenously distributed Al, was suitable for crucibles fusion, and the results confirmed that the crucibles with addition of Al had higher, than reference, viscosity and lower cristobalite growth rate. Nevertheless, production of the silicon ingots was not possible due to withdrawal of the partner in the project (NORSUN) as well as the significant contamination of the material produced at the industrial scale using fluidized bed method at Glatt.
During the project 3 manuscripts were written. In addition, TQC has disseminated the project findings internally among the technical, quality and process teams, as well as the results were presented during Norwegian Solar Cell Conference 2024 in Son, Norway.
As a continuation of the High Vis project, it is planned to test also other cost efficient (and clean) doping methods as well as other network forming elements to investigate if they can improve viscosity and be less detrimental for the silicon.
The project delivered five major (impactful) outcomes:
1. Development of a new future product with homogenously Al doped quartz which will help TQC to have larger products portfolio to meet the market expectations for crucibles with long lifetime due to higher viscosity
2. Understanding of mechanism responsible for high viscosity of Al doped quartz glass which open future opportunities for further development even better products
3. Further development of a 'Tool Box' on the product analysis and characterization with cooperation with SINTEF and NTNU.
4. Development of crucibles testing methodologies like viscosity, fictive temperature, OH groups as well as fluorescence
5. Building of internal TQC's laboratory for innovation, with high-end equipment and experienced stuff to assure TQC's major role within high purity quartz sand market.
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.
