Metallurgical Silicon Purification via Unidirectional Solidification
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Solution Overview
Problem
The photovoltaic industry faces a shortage of solar-grade silicon due to the high cost and limited availability of ultra-high purity electronic grade silicon, and existing purification methods are inefficient in achieving the required low boron and phosphorus purity levels from metallurgical grade silicon.
Innovation Solution
A process involving unidirectional solidification of low-purity metallurgical grade silicon in a mould with insulated walls and electromagnetic stirring, controlling the solidification rate, and removing the impurity-enriched liquid to produce higher-purity solid polycrystalline silicon with reduced boron and phosphorus content.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If ultra-high purity electronic grade silicon is used for photovoltaic applications, then the purity level is sufficient (99.9999999%), but the cost becomes prohibitive and availability is limited
Solution Approach 1:
The patent applies parameter changes by adjusting the purification process parameters to achieve solar-grade silicon purity (99.9999% or 6N) which is sufficient for photovoltaic applications but requires less stringent purification than electronic grade (9N). This involves optimizing crystallization conditions, temperature gradients, and purification stages to achieve the required purity level without the excessive cost of full electronic grade production
Solution Approach 2:
The patent implements partial action by performing purification steps that achieve just enough purity for photovoltaic applications rather than complete purification to electronic grade. The process stops at solar-grade silicon (6N purity) which provides sufficient performance for solar cells while avoiding the prohibitively expensive additional purification steps required for electronic grade silicon
2Ease of manufacture
If conventional purification methods are used on metallurgical grade silicon, then the process is simpler, but the boron and phosphorus purity levels are insufficient for photovoltaic applications
Solution Approach 1:
The patent utilizes phase transitions, specifically the crystallization of silicon from molten state, to separate and remove boron and phosphorus impurities. By controlling the solidification process and utilizing the different crystallization behaviors of silicon versus impurities, the method achieves effective purification of boron and phosphorus to levels suitable for photovoltaic applications
Solution Approach 2:
The patent replaces complex chemical purification systems with a controlled crystallization process. Instead of using multiple chemical treatment steps, the method substitutes a physically-based crystallization approach that naturally separates impurities through phase change, simplifying the overall manufacturing process while achieving the required purity levels
3Quantity of substance
If the supply of electronic grade silicon scraps increases, then the availability for photovoltaic industry improves, but the semiconductor industry growth and improved chip productivity have decreased the scrap supply
Solution Approach 1:
The patent applies preliminary action by producing solar-grade silicon directly from metallurgical grade silicon through controlled crystallization, rather than relying on post-production scraps from the semiconductor industry. This proactive approach creates a dedicated supply chain for photovoltaic silicon that is independent of semiconductor industry productivity and scrap generation
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This process effectively reduces metal contaminants by at least 90% and phosphorus by 45% in the resulting silicon, achieving the necessary purity levels for solar-grade silicon production while minimizing carbon and oxygen impurities, making it suitable for large-scale, cost-effective production.
Implementation Method 1
a silicon melt is stirred electromagnetically in a mould
Implementation Method 2
The mould has insulated bottom and side walls and an open top surface
Implementation Method 3
solidifying the melt by unidirectional solidification from the open top towards the insulated bottom wall of the mould
Data Source
Figure 1A
Figure 1B
Figure 2
AI summary
A process for purifying low-purity metallurgical grade silicon, containing at least one contaminant and obtaining a higher-purity solid polycrystalline silicon is provided. The process includes containing a melt of low-purity metallurgical grade silicon in a mould having insulated bottom and side walls, and an open top; solidifying the melt by unidirectional solidification from the open top towards the bottom wall while electromagnetically stirring the melt; controlling a rate of the unidirectional solidification; stopping the unidirectional solidification when the melt has partially solidified to produce an ingot having an exterior shell including the higher-purity solid polycrystalline silicon and a center including an impurity-enriched liquid silicon; and creating an opening in the exterior shell of the ingot to outflow the impurity-enriched liquid silicon and leave the exterior shell which has the higher-purity solid polycrystalline silicon.