ИСТИНА

The key process of fabrication is a reaction between Silicon, that is added to Silicon Carbide, and Nitrogen 3Si(s,l,g) + 2N2(g) = Si3N4 (s) (1), that goes with a big exothermal effect ΔQ (1400оС) = 720 kJ/mol (2). Silicon may react with Nitrogen in solid, liquid and gaseous state. The reaction (1) proceed with positive weight and volume effects Δ m/m = m (Si3N4)/ 3 m (Si) = (3*28+4*14)/ (3*28) = 1,67 (3) ΔV/V = V (Si3N4)/ 3 V (Si) = 43,89/ (3*12,17)= 1,2 (4) , that means, that in the process of fabrication the green shapes of SiC blocks gain extra weight, while the resulting Silicon Nitride grains occupy spaces between the grains of Silicon Carbide, decreasing the porosity. Si3N4-SiC materials have considerable strength, yet in many industrial applications the decay of N-SiC materials takes place due corrosion or oxidation. The compressive and crushing strength of Si3N4-SiC materials diminish with decreasing amount of Silicon Nitride in the compositions, that is obvious, because the Silicon Nitride grains keep the grains of Silicon Carbide. To some extent the increasing amount of Silicon Nitride decrease the porosity. However we doubt to say, that there is no certain optimum of concentrations between Silicon Carbide and Silicon Nitride, that gives the optimum properties. N-SiC lining materials in reduction cells should be corrosive resistant to electrolyte and oxidation resistant. The priority of oxidation resistance of Silicon Carbide over Silicon Nitride leaves many questions. The same may be said about corrosion resistance of Silicon Carbide and Silicon Nitride. The question on priorities of oxidation and corrosion resistance of one over another has aspect not only in uncertain optimum of concentrations, but also in slight porosity and concentration gradients. In Si3N4-SiC materials the porosity gradients appear due to the fact, that some part of Silicon evaporates due to exothermal effect in the middle of the shapes and moves to the edges the shapes, where reacts with Nitrogen. From 3 to 15-17 % of Silicon in the green mixture may react with Nitrogen in gaseous state. The origin of the porosity gradients in N-SiC materials is in the excessive heat in the middle of the shapes due to the exothermal effect, that give rise to the temperature gradients between the middle of the shapes and the edges. The difference in porosity may be from 0,5-1% to 6-8%.