Silicon-based alloys

The fabrication of solar cell grade silicon (SOG-Si) feedstock involves processes where liquid and solid phases contact directly. Thermodynamic knowledge of phase equilibria of the Si-based system is desired for producing and refining SOG-Si to an ultra-high purity. TCSI1 is a thermodynamic database for the application of such materials. It is developed using the CALPHAD approach based on experimental and First Principles results. The impurity solubility in silicon is critically assessed in binary systems. The solubility in ternary and higher order systems can be calculated by Thermo-Calc through the extrapolation from binary systems.

By using Thermo-Calc and compatible thermodynamic databases, a vast amount of information can be obtained of relevance for Si-based alloys.

Examples of equilibrium properties that can be predicted: 

  • Solid solubility of impurity elements in silicon
  • Segregation coefficient of impurity elements in the solid Si phase vs in the liquid phase
  • Effect of third element on the equilibrium distribution coefficient of the primary impurity
  • Eutectic reaction involving silicon solid solution
  • Stability of relevant silicides and phase relations in Si-rich domain  
  • Phase amount and constitution as a function of composition
  • Phase amount and constitution as a function of temperature
  • Liquidus and solidus
  • Driving force for nucleation and growth e.g. of precipitates 

The solidification of Si-based alloys during casting happens in a non-equilibrium way. The non-equilibrium solidification path can be predicted rather accurately by using the Scheil-Gulliver module available in Thermo-Calc.

From a multicomponent Scheil-Gulliver calculation, one can obtain: 

  • Solidification temperature range
  • Incipient melting point
  • Phase amount and its composition
  • Phase formation sequence
  • Micro-segregation of solute elements
  • Latent heat evolution 

By using the Diffusion module (DICTRA) and combining corresponding thermodynamic and kinetic databases, typical diffusion-controlled phase transformations in Si-based alloys can be simulated under arbitrary heat treatment conditions.

Examples of applications that can be studied using the Diffusion module (DICTRA) include:

  • Homogenisation of pure silicon solution
  • Growth or dissolution of silicides
  • Cooling effects in solidification
  • Coarsening of precipitate phases

By using the Precipitation module (TC-PRISMA) and combining thermodynamic and kinetic databases, the concurrent nucleation, growth and coarsening of precipitates can be simulated.

Precipitation module (TC-PRISMA) simulation results include the temporal evolution of:

  • Mean radius 
  • Number density
  • Volume fraction
  • Particle size distribution