Thermodynamics and kinetics of direct synthesis of solar grade silicon from metallurgical silicon wafer by liquid phase migration in solid silicon

Sakiko Kawanishi, Kunitoshi Matsunaga, Takeshi Yoshikawa, Kazuki Morita

Research output: Contribution to journalArticlepeer-review

Abstract

We propose a process for the direct synthesis of solar grade Si from a metallurgical Si wafer focusing on the fact that its microstructure is composed of almost pure Si grains and grain boundaries enriched with impurities. Principally, heating a metallurgical grade Si wafer above its eutectic temperature and applying a temperature gradient allows the grain boundaries to be melted and causes them to migrate to the high-temperature direction. The liquid phases are finally terminated at the end surface, resulting in the upgrading of the Si and making it more favorable for solar cells. In the present paper, to determine the purification effect during the liquid phase migration process, thermodynamic assessment was performed using CALPHAD method. Liquid phase migration experiments were also conducted using synthetic MG-Si (Si-Fe alloy) to determine the reaction time for the process. A maximum migration velocity of 8.17 × 10−7 m/s was obtained at 1623 K, which allows the migration process to be accomplished within 3 min for a 150-μm wafer.

Original languageEnglish
Pages (from-to)1571-1580
Number of pages10
JournalMaterials Transactions
Volume58
Issue number11
DOIs
Publication statusPublished - 2017
Externally publishedYes

Keywords

  • Diffusion coefficient
  • Silicon
  • Solvent refining
  • Temperature-gradient zone melting

ASJC Scopus subject areas

  • Materials Science(all)
  • Condensed Matter Physics
  • Mechanics of Materials
  • Mechanical Engineering

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