Silicon densification

In the area of preceramic polysilazanes, sufficient progress has been made to produce precursors for silicon nitride fibers, coatings and as binders for silicon nitride powder. However, particular problems still remain to be solved particularly with regard to reducing impurity levels and improving densification during pyrolysis. 

Because of the role of precursor structure on film processing behavior (consolidation, densification, crystallization behavior), the reaction pathways are typically biased through the use of the catalyst, which is simply an acid or a base. This steers the reaction toward an electrophilic or nucleophilic attack of the M—OR bond.1,63 Hydrolysis sensitivity of singly or multiply hydrolyzed silicon alkoxides is also influenced by the catalyst, which contributes to the observed variations in oligomer length and structure. Figure 2.3b illustrates... 

Table 10 summarises all methods for the densification of Si3N4 used at present. The resulting Si3N4 ceramics classified according to the densification routes are also listed together with several remarks on manufacturing characteristics, properties and applications. For comparison with the sintered qualities, information on reaction bonded silicon nitride ceramics are also included but will be treated in more detail in Sect. 8. 

Silicon nitride is a highly covalent bonded compound with self-diffusion coefficient of the nitrogen atoms of 6.3 x 10 2(1 cm2/s at 1400°C.22 Therefore, densification without any sintering additives is nearly impossible. In 1961, Deeley and Herbert.23 was the first to report that Si3N4 ceramics could be... 

As one may have expected, theoreticians have gathered numerous possibilities to verify Moore s Law. For instance, they have announced the end of the silicon s densification (silicon brick wall). Similarly, they have predicted the point at which silicon-based devices may not be further reduced. All these theoretical predictions had, however, to be continually revised. This is because the basic models behind these theses was the inability to process ever smaller structures in silicon. In that sense, all predictions were based on technological limitations rather than upon a physical science barrier. It might be serendipitous that engineers have overseen the theoretical predictions hinting to the end of silicon. Instead, they continued to build smaller devices and denser integrated circuits. 

A new type of liquid-phase sintered SiC using yttria [1314-36-9J, Y20, as the oxide additive and submicrometer SiC powder for enhanced densification, produces a material which can be densified without the application of pressure (13). This material, sintered from cold isostatically pressed billets, appears to be comparable to silicon nitride in strength and fracture toughness. 

Silicate ceramics are well suited for structural applications because of their strength, which originates in the partially ionic, strong silicon-oxygen bonds in the tetrahedral orthosilicate anion. This structural unit appears in naturally occurring minerals and clays, which are fashioned into ceramic pieces through sintering and densification processes. 

Figure 2. (a) The surface of an alumina ceramic from which all porosity has been removed during sintering the microstructure consists of grains and the boundaries between them, (b) The sintering of silicon results in the formation of a continuous network of solid material (white) and porosity (black). The microstructural change is not accompanied by any densification. (Reprinted from R. J. Brook, ed., Concise Encyclopedia of Advanced Ceramic Materials, eopyright ( 1991, p. 4, with kind permission from Elsevier Science Ltd.)... 

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