Aspect silicon nitrides

The thermodynamics of the above-elucidated SiC/C and SijN Si composites are determined by the decomposition of silicon carbide and silicon nitride, respectively, into their elements. The chemistry of ternary Si-C-N composites is more complex. If producing Si-C-N ceramics for applications at elevated temperature, reactions between carbon and silicon nitride have to be considered. Figure 18.2, which exhibits a ternary phase diagram valid up to 1484°C (1 bar N2) displays the situation. The only stable crystalline phases under these conditions are silicon carbide and silicon nitride. Ceramics with compositions in the three-phase field SiC/Si3N4/N are unknown (this is a consequence of the thermal instability of C-N bonds). Although composites within the three-phase field SiC/Si3N4/Si are thermodynamically stable even above 1500°C, such materials are rare. The reasons are difficulties in the synthesis of the required precursors and silicon melting above 1414°C. The latter aspect is of relevance, since liquid silicon dramatically worsens the mechanical properties of the derived ceramics. 

Silicon nitride, oxynitride and silicon aluminium oxynitride (SiAlON) compounds are of considerable technical interest as advanced ceramics because of their stability and chemical inertness at elevated temperatures, and their excellent mechanical properties. Si MAS NMR has proved to be useful in studying various aspects of these compounds, including their formation and structure, the processes by which they are... 

Lai and Tien [26] (Fig. 15) have shown that the flexure strength and fracture toughness of silicon nitride ceramics depends on the aspect ratio... 

Fig. 34. The effect of glass composition in the Si3N4-Y203-Mg0-Si02-Ca0 system on silicon nitride aspect ratio. Open circles indicate glass composition producing low aspect ratio grains. Solid circles represent compositions resulting in silicon nitride with a high aspect ratio.

The materials with 10 mole % of Mg0/Nd203 in the 1 1 ratio have an aspect ratio of silicon nitride grains equal to 3.3. With the same molar ratio, but a 17 mole % additive level, the aspect ratios are about 6. With the same total additive level (17 mole %), but a higher molar ratio (4 1), grain size is similar (about 0.4 jum), but the aspect ratio is slightly lower at 5.4 [34]. 

The composition of the IGF is important in determining overall creep behavior. For example, using Y2O3 as a sintering aid for silicon nitride ceramics has been found to be superior to using MgO. Other important aspects of the microstructure are the grain size and the volume fraction of liquid present. 

The needle-like grain shape seems to be kinetically determined and therefore not the equilibrium crystal shape [30,38], The estimated equilibrium aspect ratio (ratio of grain length to thickness) is 1.3, not as high as 20, as was found to be the case for the silicon nitride materials. The microstructural formation must therefore be explained on the basis of growth mechanisms. 

Etched silicon probes provide the highest aspect ratio and the most consistent tip sharpness of the probes supplied at present. The silicon nitride probe is inexpensive, durable, and suitable for contact mode imaging. 

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