“The furnace’s small size lets
us evaluate feedstocks and crystalgrowth
techniques with less material
and faster turnarounds,” says SPI
Senior Casting Engineer Chenlei
Wang.
Algor Inc., Pittsburgh, Pa.,
provides the FEA software.
The minicaster loads silicon into
a vitreous-quartz crucible supported
and shielded by graphite plates and
surrounded by resistive heaters. A
moving shield controls cooling for
the molten silicon and directs crystal
growth during solidification.
“To assess the design of the minicaster’s
hot zone before fabricating its
components, we simulated the silicon
melting and solidification,” says Wang.
“We first modeled the furnace in 3D
using Autodesk Inventor, then modeled
the cross-sectional geometry of
the hot zone in multiphysics software.”
Wang defined the temperature-dependent
orthotropic material properties
for the feedstock, crucible, heaters, and
insulation. He also defined thermal
loads for internal heat generation, surface
radiation at the outside surfaces,
and body-to-body radiation between
exposed internal surfaces, as well as
specified fluid velocities on surfaces
surrounding the silicon.
“Natural convection due to buoyancy
plays an important role for
transport phenomena inside the
silicon melt,” says Wang. “Strong
velocity fields cannot be neglected.
Therefore, we used the multiphysics
program to couple the calculation of
temperature and flow fields, which
accounts for natural convection.
Then a steady-coupled thermal and
fluid-flow analysis produced temperature
contours throughout the
minicaster and fluid-velocity vectors
within the silicon melt.”
That took care of the melting
phase analysis, says Wang. “For the
solidification phase, we used a lower
internal heat-generation value to simulate lower temperatures while
cooling. The transient heat-transfer
analysis let us better understand silicon
solidification in the minicaster.”
Simulation predicted the metal
would solidify from the bottom up,
with unwanted effects. Sure enough,
when the company cast the first
ingot, most of its surface was flat
and smooth, but regions at the top
showed erratic solidification. “We
contiuned analyzing transient-heat
transfer in the solidification phase
to determine the heater’s best placement
and output power. Adjusting
these parameters and the insulationlift
distance led to a slightly convex
solidification interface to the melt,
the most beneficial approach for
high-quality silicon-crystal growth.”
Make contact
Algor, algor.com
SPI, solarpowerindustries.com