Controlled Silicon thin Casting

Elkem is a global supplier of silicon and silicon related products that our customers process further in their facilities. For example, it is the main ingredient both for production of high purity silicon for solar cells and for chemical processing converting silicon to silicone. The material properties will strongly affect the downstream process efficiency (energy consumption, productivity). Therefore, the product requirements are steadily increasing and the need for new product development in the market is strong. One important precondition to improve the material properties is to understand how to control the microstructure that constitutes the building bricks of the material. The microstructure consists of grains and intermetallic particles that are decisive for the properties of the material. During the solidification of silicon, almost pure silicon will crystallize first and reject other elements (mainly Al, Ca, Fe and Ti) in the melt. When the melt is saturated, these elements will form intermetallic particles (for example Al3FeSi2 which contains 3/6 Aluminum, 1/6 Iron and 2/6 Silicon). Therefore, it is important to understand how the process conditions and alloy chemistry affect the microstructure. The project is developing a mathematical model, SiStruc (Silicon Structure), able to predict the complex relationship between process conditions, chemical composition and microstructure in order to optimize the desired properties. The SiStruc software has been modernized and provided with a more user-friendly interface. Based on thermodynamic equilibrium data found in the literature, the model parameters have been calibrated. But the equilibrium state is not reached in practice as the element diffusion is too slow during the casting. Therefore the project team has started to investigate how to take into account the cooling rate and grain size effects. Indeed the new technology developed in the project will enable faster cooling rate and smaller grain size. In order to better understand the microstructure formation and assess the quality of the model prediction, casting experiments are carried out and samples are analyzed with different techniques. Characterization of samples with different thermal histories shows very different microstructures. The characterization of areas with different thermal histories show distinct different microstructures. This information will be very important regarding the selection of new casting technology that will improve the capability to cover the demands for more tailormade products. New methods for the measurement of specific microstructure parameters have been developed which enable to clearly distinguish microstructures with different thermal history. One challenge has been to analyze large samples automatically in order to drastically increase the amount of data available. It is especially important to identify and quantify the intermetallic particles. It requires a high resolution scanning to measure precisely the local composition (local ratio of different chemical elements). Today, it has become possible to scan the full sample by close cooperation with Bruker on the AMICS software. Knowledge from the casting experiments and microstructural characterization has been systematized and implemented in SiStruc. An updated version with a new interface has been released and a user forum for SiStruc will be established.

Over the last 15 years Elkem has focused on silicon and silicon related products. The traditional products have been Silgrain, metallurgical grade silicon, ferrosilicon and foundry products, but since introduction of Elkem Solar Silicon® in 2009, a continuous work on product development has taken place including a BIA project focusing on Si to anodes in Li batteries. A general trend for the new markets is the need for small sized particles and the consequence for Elkem is to find robust technologies for making homogenous particles with desired properties. Traditional casting techniques cause segregation and uneven distribution of dope and alloying elements. They are also associated with significant generation of dust and fumes. The regulations on dust exposure in working environment will in coming years force the industry to change the process. Both challenges can be handled by thin casting methods. This opens a new window of opportunities on controlling the process for tailor-making product properties. Redesigning of casting equipment for thin casting opens up to improvements: i) towards more controllable solidification and cooling stages, ii)of the process yield by reducing the need for crushing. A thin cast material has a different microstructure and thereby different product properties than thicker cast materials. The subsequent crushing step after casting will also benefit from the thin cast microstructure through the need of less energy for crushing, i.e. the degree of size reduction is smaller. Therefore, less low grade material called fines will be generated and the volume of high quality silicon will increase. The main objective of the R&D program is to develop a tool for designing controllable, homogeneous microstructures in silicon, silicides and silicon alloys which make the products suitable for further processing to use in new applications like Li batteries and thermoelectric generators.