Silicon nitride whiskers

HIPing is considerably more complex and expensive than hot-pressing but for monolithic silicon nitride it normally produces a superior product with near-isotropic properties at low additive contents. However, with silicon nitride whisker composites, whisker bridging and the formation of whisker nests can be more of a problem.5... 

Silicon nitride whiskers are also produced by variations of processing conditions in these S3mthesis methods. Silicon nitride is used for toolbits for cutting cast iron and other high-temperatm% wear parts including burner nozzels. 

The formation of silicon nitride whiskers was observed in several different reactions, including vapor deposition, CVD, and growth from a melt. However, only the following techniques are considered to have commercial significance nitriding of metallic silicon or silicon-silica mixture, carbothermal reduction of silica with simultaneous nitridation, and thermal decomposition of silicon halides. 

Hirata, Y, Nakagama, S.. and Ishihara, Y., Dispersion and consolidation of silicon nitride whisker in aqueous suspension, J. Mater. Res., 5, 640, 1990. 

Four examples were given in the patent, all using supercritical propane with polysilane and aluminum isopropoxide, so-called precursor ceramic materials. In one instance scanning electron microscopy was used to demonstfate that polysil me was deposited as a smooth surtace film on the alumina fibers and fine silicon nitride whiskers. A silicon carbide material with internal pore openings as small as 10 microns was penetrated by supercritical propane laden with aluminum isopropoxide. Weight gains of up to 43% can be obtained depending on the density of the initial host ceramic. 

A low cost-high performance silicon nitride-BAS composite can be fabricated via pressureless sintering. In this composite, the BAS glass-ceramic serves as an effective liquid-phase-sintering aid, to attain full densification and complete the a-(3 Si,iN4 phase transformation, and remains as a structural matrix that is reinforced by the silicon nitride whiskers, Si3N4 whiskers grow in random directions in a near completely crystallized matrix of hex-acelsian BAS. 

The structure property relationships observed in the Si-N system [17], are reminiscent of those observed for silicon nitride whiskers grown by metal catalyzed chemical vapor deposition (Chapter 2.2). The difficulty in obtaining binary (i.e., boron or silicon nitride) fibers with exactly required stoichiometry seems to have so far precluded the production of single crystal fibers by this route. 

Silicon nitride whiskers n. Very fine fibers of Si3N4 prepared by vapor-phase reaction of silicon and a silicate in nitrogen and hydrogen at 1400°C. Density is 3.2g/cm, ultimate strength is 4.8 GPa (700kpsi), and modulus is 276 GPa. They are used as reinforcements in specialty laminates. 

Nitric oxide may interact with the surface of mineral fibres. Cigarette smoke which increases the risk of asbestos-induced lung cancer contains up to 600 [ig NO per cigarette (lARC 1985) raised the fibre-bound NO from 34 to 85 pg NO/g fibre (Leanderson et al. 1997). NO was found in different amounts on chrysotile B, crocidolite, amosite and siUcon carbide whiskers. There was a strong correlation between the amount of NO and the specific surface area of these fibres (r = 0.98). NO could not be demonstrate on rockwool fibres, manmade vitreous fibres MMV21 and MMV22 or silicon nitride whiskers. 

Table 3.2 shows the physical and chemical properties of a and p silicon nitride whiskers produced by Chemical Corporation of Japan and Ube Industries, respectively. 

Figure 3.2 shows the morphology of silicon nitride whiskers. ... 

Figure 6. Plot of relative toughness as a function of crack length for cordierite glass ceramic containing silicon nitride whiskers at the volume fractions indicated. Adding whiskers enhances the toughness over the base material by over a factor of two, approximately 1 MP m / for this material in air.

Silicon Nitride Whiskers n Very fine fibers of Si3N4 prepared by vapor-phase reaction of silicon and a silicate in S... 

Meek and co-workers have used a variety of 2Kklitives to achieve initial heating. Sodium nitrate and glycerol were used to accomplish coupling with the microwave field for alumina-silicon carbide whisker and alumina-silicon nitride whisker composites and a pyrex-silicon carbide... 

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