Characterization and mechanical properties of solar grade silicon in granular and nanopowder form

Mohamad B Zbib, Purdue University

Abstract

Polycrystalline silicon is mainly used for solar cell applications, structures in micro-electromechanical systems, and production of single crystal Si. One of the relatively new methods for producing large quantities of polysilicon is fluidized bed reactor (FBR), where two main morphologies are produced, granular solid (1-3 mm) and nanopowders (30-300 nm). Grinding and fracture occurs in the granular solid during shipping and handling which can affect the final product properties and create safety issues. The microstructure and the morphology of both the granular and the nanopowder forms of Si were examined using scanning and transmission electron microscopes (SEM and TEM). The fracture toughness of the granular silicon was studied, using microindentation and nanoindentation techniques, at different annealing processes, and with different hydrogen concentrations during production. Hydrogen defects in silicon were analyzed using infrared spectroscopy to develop a new relationship between hydrogen and toughness. Based on the microstructural analysis it was shown that the granular Si are mostly crystalline with some amorphous regions linked to small pores, while the nanopowders are mostly amorphous with some crystalline bits; the porosity in the granular Si ranges between 1-4 volume percentage. It was proposed that the primary mechanism in FBR for the granular Si formation is chemical vapor deposition with minor agglomeration associated with pores. It was found that the lower the hydrogen in the production, the higher the fracture toughness where it can be improved up to 45% (from 0.6 to 0.86 MPa.m0.5), and lead for less dust during physical contact. New attrition parameters were proposed in order to better understand the fracture mechanisms of Si granules and other brittle microspheres. These parameters provide a relationship between the mechanical properties (indentation techniques), fracture behavior and failure mechanisms using both crushing tests and impact tests. Part of this thesis also focused on making a beneficial use of the Si nanopowders that are considered secondary products from FBR. The powders were processed into metal-coated carbon composites, using electroplating to improve electrical resistivity. This method can be used to enhance light trapping of solar cells in coated composites.

Degree

Ph.D.

Advisors

Bahr, Purdue University.

Subject Area

Materials science

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