Modeled Geometry

1
Modeled Geometry
Equivalent to the experimental setup assuming
uniform current above and below the sample.
Half the sample is modeled, due to symmetry.
Mesh 25 steps/cm in r and z
2
Modeled Equations
Fouriers heat transfer
Electrostatic
Second order Arrhenius kinetics
3
Results 3cm radius, 90-10 MoSi2

• Current passes primarily through the die.
• MoSi2 reacts first, a narrow wave travels toward
  the center.
• SiC reacts second, with a wider wave.
• MoSi2 reacts fastest in the layer with the most
  MoSi2, SiC reacts fastest in the layer with the
  most SiC.

Temperature
Current
MoSi2 conversion
10 mol MoSi2
SiC conversion
30 50 70 90
Reaction direction
4
Results 3cm radius, 90-50 MoSi2

• More current travels through the sample than
  previously, but not in the SiC region.
• Electrical conductivity in the sample is greater.
• The MoSi2 conversion slope is less steep, because
  the difference in concentration between layers is
  less.
• The SiC reaction is slower and less energetic.

Temperature
Current
MoSi2 conversion
50 mol MoSi2
60 70 80 90
SiC conversion
5
Results 1cm radius, 90-10 MoSi2

• In 1cm samples, MoSi2 reacts as a wave, but in
  volume mode in 0.5 cm samples.

• SiC reacts in volume mode when the radius is 1cm
  or less.

10 mol MoSi2
SiC conversion, after 4.4 s
30 50 70 90
sample
die
0.5cm radius, 4.2 s
6
Results total current

• The total current is one parameter that may be
  measured experimentally and compared to the
  model.
• During the MoSi2 reaction, the samples with
  higher MoSi2 concentration draw much more current
  (90-50).

Less MoSi2 More MoSi2

• Current varies with total sample conductivity.
• Initially, current is just in the die, so the
  curves are identical.
• After the reaction, the conductivities are nearly
  the same.

Less MoSi2 More MoSi2
7
Model Results

• Ignition occurred near the die surface and
  reaction waves propagated to the center in large
  samples, due to temperature gradients caused by
  Joule heating in the die.
• The current density in the samples was controlled
  by the conductivity of the least conductive
  layer.
• The reaction rate of each product in each layer
  varied with product composition and overall
  electrical conductivity of the sample.