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Equilibrium calculation example: Reaction analysis between SiC-SiO2

Between SiC and SiO2, a violent reaction occurs at about 1800°C or higher in Ar of 1 atm. In this case, SiO2 is in the liquid phase, but there are many possible reactions that can occur as follows. In addition, many other types of gas are possible.

  •   SiC+2SiO2 --> 3SiO+CO

  •   2SiC+SiO2 --> 3Si+2CO

  •   SiO2+Si --> 2SiO

  •   ・・・

It is possible to find the equilibrium constant of each reaction and study it, but in reality it is quite difficult, and you may not be able to understand what the overall reaction will be. In fact, we have investigated this reaction in detail before. Summarizing the results, the reaction between SiC and SiO2 isapproximately,

  • 2SiC+3SiO2 → 4SiO+2CO+Si

I found that it can be arranged, but I could not understand why it was so.This reaction is actually a reaction that occurs during the production of metallic silicon by carbon reduction of silica, and many research papers have been published. In other words, it's just that it's hard to organize. So let's examine this reaction using CaTCalc.

■ Calculation procedure
The calculation procedure is as follows.

1. Specify elements, select and load thermodynamic data
2. Setting calculation conditions

3. Calculation execution
Four. Analysis of results

1. Specify elements, select and load thermodynamic data

Use the System button or the menu [Page]-[System] to display the System screen.

Click the mouse to select Si, O and C in the periodic table.
Select RICT-Ceram.EDB from the files displayed below (click the leftmost cell to display + and select it).

Press the Load button to load the thermodynamic data.

SiC-SiO2 fig1.png

2. Setting calculation conditions

In Feed/Activity Conditions, press the [Add Feed] button to add two lines, and manually enter SiC and SiO2 in the [Species] column as shown below. Next, select Equilibrium Calc as the calculation type and press the Set Default Values button. In addition, set the x value to 0.5 as shown in the right condition setting panel.

SiC-SiO2 fig2.png

3. Calculation execution

Press the Calculate button to execute the calculation. A progress bar will be displayed, so if you want to stop, press the X button on the right.

Four. Analysis of results

The calculation results are automatically displayed in a graph.

SiC-SiO2 fig3.png

It can be seen that at 1815°C or higher, it becomes a liquid phase of gas, SiC, and metal Si.
Next, select and display the [Axis] tab on the left panel as shown below. Select "Gas Pressures" as the Variable for that Y_Axis setting and press the "Apply" button. Gas pressure and Activity are displayed logarithmically by default, so uncheck the Logarithmic check box at the bottom here.

SiC-SiO2 fig4.png

As can be seen from this figure, the main components of the gas are SiO and CO.

So, in order to examine the ratio in detail, click the [List] tab to display the list.

SiC-SiO2 fig5.png

Select [Edit]-[Copy to ClipBoard] from the menu to copy the result to the clipboard and paste it into other spreadsheet software (Excel, etc.). In the spreadsheet software, the ratio of SiO and CO components in the gas phase was calculated and graphed as shown in the figure below.

SiC-SiO2 fig6.JPG

Summarizing the results, the gas pressure rises above 1 atm above about 1815°C, so the reaction progresses rapidly in argon at 1 atm, for example. Therefore,

  • The equilibrium partial pressure ratio of SiO/CO is approximately 2 at 1815°C and 1 atm.

  • As a result, approximately 2SiC+3SiO2 → 4SiO+2CO+Si

That's what I can say.

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