Modifications, silicon nitrides

Fig. 7.9. Measurements of the degree of conversion of ct p silicon nitride at a fixed time and various temperatures are thought to show the strong influence of shock modification on the high temperature dissolution [84B01].

Rebenne, H. E., and Bhat, D. G., Effect of Diffusion Interface on Adhesion and Machining Performance of TiN-Coated Silicon-Nitride Cutting Tools, Proc. ThirdInt. Conf. on Surface Modification Technologies, Neuchatel, Switzerland (Aug., 1989)... 

Silicon nitride (95% of a-modification) with 2 wt% of alumina and 5 wt% of yttria as sintering aids (layers with index 1, bulk compressive stress)... 

Liquid phase modifications. Alternatively a porous membrane can be reduced in pore size by a liquid deposition prcx ess where the membrane is dipped into a solution or sol to form deposits inside the membrane pores. For example, a silicon nitride tube with a mean pore diameter of 0.35 pm is first immersed in a solution of aluminum alcoholate (aluminum isopropylate or 2aluminum tris(ethyl acetoacetate) or ethyl acetoacetate aluminum diisopropylate) in an organic solvent (hexane, cyclohexane, benzene, isopropanol, etc.). It is then treated with saturated water vapor to hydrolyze the alcoholate or chelate to form bochmite inside the pores, thus changing the pore diameter to as small as 20 nm [Mitsubishi Heavy Ind., 1984a and 1934b]. Upon calcining at 800X, boehmite transforms into transition-alumina. 

In order to raise the stability of nonmetal nitride network structures the element phosphoms has to be exchanged against silicon. In phosphoms nitrides the relative amount of (N ) never seems to exceed a value of 2/5 and in most examples it is zero. In contrast, in silicon nitrides even all of the nitrogen atoms each may connect three silicon atoms (N ). This topological situation is realized in the two modifications of binary silicon nitride Si3N4 (Fig. 2). 

Liden, E. et al.. Surface modification and dispersion of silicon nitride and silicon carbide powders, J. Eur. Ceram. Soc., 7, 361, 1991. 

As for the preparation of silicon nitride, the modification of the silica surface by NH3 is well investigated NH3 can chemisorb to produce Si-NHa at moderate temperature (> 673 K), silazane Si-NH-Si at higher temperature (> 873 K), and nitridated silica surface (silicon-oxinitride, SiaNaO) at more higher temperature (> 1473 K) [13]. On the other hand, it is also reported that the strained siloxane bridge on silica activated by evacuation at high temperature can easily react with NH3 to form Si-NHa [14]. We expected that the sites such as Si-NH2 formed through NH3 adsorption at moderate temperature on the activated sites function as base sites in catalysis. 

Modification of grain boundary glass chemistry in silicon nitride has profound effects on properties. Summarising ... 

We have discussed the formation of P-SigN crystals. However, silicon nitride has another modification - -Si3N4, which exists till 1500 C. 

Table 7.1 Crystallographic characteristics of polymorphic modifications of silicon nitride.

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