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Figure 1
: Effect of annealing on the relative fractions of intermetallic compounds [15]
Softwares have been developed to forecast the composition and amount of the
intermetallic at a given impurity level. The PC-program SiStruc
®
was developed in
2009 [16]. Its main purpose consists of calculating the composition, amount and
melting ranges of intermetallic phases in silicon, given the weight percentages of Fe,
Ca, Al, Ti, B and P. The calculation developed in the program are based on
directional solidification laws, such as Scheil equation. These conditions are not very
close to the industrial solidification, but they can at least give a good expectation.
Forecasting the composition of silicon is important for a high final product quality.
Silicon undergoes a reaction with methyl-chloride during the Direct process to
produce dimethyl-dichlorosilane. There have been several attempts to describe the
effect on the Direct process of the different intermetallic compounds, but there is no
complete consensus about it so far. Many parameters interfere with reactivity and
selectivity of the reaction. For this reason, experiments are difficult to reproduce, and
results between different authors are difficult to compare. The results from these
works have been gathered in Table 1.
Table 1
: Effect on reactivity and selectivity of the intermetallic phases.
Legenda: + = Positive effect; ÷ = Negative effect; n.p. = Analysis not performed
Phase
Reference
Reactivity
Selectivity
Reference
Sørheim [17]
Reference
Sørheim [17]
FeSi
2
[15]
÷
0
÷
0
CaSi
2
[15]
+
0
÷
÷÷
FeSi
2
Ti
[15]
÷
0
÷
0
Al
8
Fe
5
Si
7
[18]
+
++
÷
÷
Al
2
CaSi
2
[15,18]
++
+++
÷÷
÷÷
Al
6
CaFe
4
Si
8
[15,18]
0
÷
+
++
Al
3
FeSi
2
[19]
++
n.p.
0
n.p.
Al
4
CaFe
4
Si
6
[19]
÷
n.p.
+
n.p.
In this case of study, samples from a MG-Si thin cast will undergo different
thermal histories and casting conditions to investigate the effect on the phase
composition. A relation between the variation in phase composition and expected
effects on MCS production for each phase will be presented. The software SiStruc
®
will predict the phases present and confirm that industrial solidification is not
happening at equilibrium conditions. An hypothesis about the mechanisms of the
transformation can be made by looking at literature and results from previous works.
3CaAl
2
Si
2
+ 4FeSi
2
֞
Al
6
CaFe
4
Si
8
+ 2CaSi
2
+ Si
85