Ceramic powders silicon nitride

Schwier, G., On the Preparation of Fine Silicon Nitride Powders, inProgress in Nitrogen Ceramics, (F. Riley, ed.), pp. 156-157, MartinusNijhoff, Boston, MA (1983)... 

Bauer,R., Smulders,R, Geus,E., vanderPut, J., and Schoomman, J., Laser Vapor Phase Synthesis of Submicron Silicon and Silicon Nitride Powders from Halogenated Silanes, Ceram. Eng. Sci. Proc., 9(7-8) 949-956(1988)... 

J. J. Burke, ed., Powder Metal High-Performance Applications, Proceedings of the 18th S agamore Army Material Research Conference, Syracuse University Press, Syracuse, N.Y., 1972. Review on silicon carbide—silicon nitride ceramics. 

Besides the chemical industry, silicon is used as a powder in the ceramics (qv) industry for the production of silicon carbide and silicon nitride parts (see Advanced CERAMICS). Silicon powder is also used as an explosive for defense applications and in the refractory industry for plasma spraying with other oxide mixtures (see Refractory coatings). 

Considerable recent activity in the area of ceramic processing is aimed toward the formulation of materials with high strengths, comparable to the room temperature strength of metal alloys, at high temperatures (of the order of 2000 K). The impetus comes from the significant gains made in the last 20 years with materials formed from submicron powders of silicon nitride and silicon carbide and the promise of similar improvements in the near future. 

A c-BN - silicon nitride ceramic composite can be produced directly by sintering a mixture of c-BN powder and Si powder in N2 atmosphere. The composites have high resistance against heat, oxidation, and thermal shock [279]. 

Silicon nitride ceramics are not merely only one material but several classes of materials. All of them are multiphased, i.e., they exhibit a heterogeneous microstructure which has formed during sintering (Sect. 6). Therefore in all classes a large variety of properties is predominant and as a consequence also a large variety of potential applications (Sect. 10). Often little variations in the powders and the processing parameters cause remarkable changes in the microstructure which have a pronounced effect on properties (Sects. 6 and 7). 

Silicon nitride ceramics are mostly produced from oc-rich Si3N4 powders which transform during sintering by a solution reprecipitation process to / solid... 

Woetting G, Feuer H, Gugel E (1993) The Influence of Powders and Processing Methods on Microstructure and Properties of Dense Silicon Nitride. In Chen IW, Becher PF, Mitomo M, Petzow G, Yen TS (eds) Silicon Nitride Ceramics, Mat Res Soc Symp Proc 287. Mat Res Soc, Pittsburgh, p 133... 

Emoto H, Hirotsuri H (1999) Microstructure Control of Silicon Nitride Ceramics Fabricated from a Powder Containing Fine /(-Nuclei. In Niihara K, Sekino T, Yasuda E, Sasa T (eds) The Science of Engineering Ceramics II Key Eng Mat 161-163. Trans Tech Publications, Switzerland, p 209... 

Exposure limits for silicon carbide and powders of zirconium compounds (including zirconium dioxide) have been established by ACGIH. TLV —TWA s are 10 mg/m3 and 5 mg/m3, respectively. OSHA guidelines for zirconium compounds call for a PEL of 5 liig/m3. There are no exposure limits for silicon nitride powder, but prudent practice suggests a TLV—TWA of 0.1 mg/m3. The solid ceramics present no apparent health hazard. In machining such ceramics, however, care should be taken to prevent inhalation of respirable particles in amounts in excess of established limits. Disposal should be in approved landfills the materials are inert and should pose no danger to the environment. 

In some cases, the reaction between a gas and a solid gives a ceramic of interest, and this reaction can be used to densify the green body. The classic example is reaction sintered SisN4, also called reaction bonded Si3N4. The gas—solid reaction used to make reaction bonded silicon nitride [132] is between silicon metal powder and nitrogen ... 

The industrial ceramics produced by conventional sintering of 8-phase Si3N4 powders synthesized by SHS (Petrovskii et al, 1981) can be used as high-temperature articles with attractive dielectric properties (tan6=4.4 10 at/= 10 Hz, and dielectric strength, Ea=9.2 kV/mm). Also, silicon nitride powders with a relatively high a-phase content ( 80%) have been used for the production of advanced structural ceramics with good mechanical properties ([Pg.109]

Petrovskii, V. Y, Gorvits, E. 1., Borovinskaya, 1. P., and Martinenko, V. M., SHS silicon nitride An attractive powder for dielectric ceramic production. In Problems of Technological Combustion. Chemogoiovka, 5 (1981). 

The ceramic products of these pyrolyses were completely amorphous. Pyrolysis carried out at 1500 °C or lower yields only amorphous products. Amorphous materials can be converted to crystalline form by heating above the transition temperature. Pyrolysis must be carried out at 1550 °C or higher to obtain crystalline products. Thus, pyrolysis of the transamination product of Tris with ammonia at 1550 °C or higher gave high-purity, a-phase silicon nitride (33), as analyzed by powder X-ray difiraction (Figure 1 and Table I). 

For the preparation of technically important metal carbide and metal nitride materials the application of organosilicon compounds as preceramic precursors is advantageous under certain conditions [1-5]. Compared with the conventional metallurgical powder process, one benefit is the utilization of very low process temperatures for the preparation of individual ceramic materials. Another improvement is the high purity of the ceramics obtained from tailor-made preceramic precursors. Usually, after pyrolysis organosilicon compounds afford silicon-containing ceramic powders Likewise, they can also be used under certain conditions for the production of silicon carbide or silicon nitride fibers. 

Metallurgical grade silicon is marketed in a coarsely crushed form or as a finely ground powder in different particle sizes. Powders with increased purity due to acid washing, particularly for the removal of metallic impurities, are specialty products. They are utilized, for example, in the manufacture of silicon nitride powder or reaction-bonded silicon nitride ceramic components and are therefore the starting materials for engineering ceramic specialties. 

The practical utilization of silicon carbide and silicon nitride ceramics or of SiAlON in the above-mentioned application sectors has steadily increased in recent years. Ca. 400 t/a of SN-powder is currently consumed in the manufacture of SN-components. The main applications are for cutting tools, roller bearings, dosing and deliver pipes for aluminum processing, as well as a multiplicity of other components which enjoy the advantages of SN-ceramics. 

The solid SisN4 forms as a smoke. It is captured as a powder, mixed with a carefully controlled amount of MgO additive, placed in an enclosed mold, and sintered at 1850°C under a pressure of 230 atm. The resulting ceramic shrinks to nearly full density (no pores). Because the material does not flow well (to fill a complex mold completely), only simple shapes are possible. Hot-pressed silicon nitride is impressively tough and can be machined only with great difficulty and with diamond tools. 

For many traditional ceramics such as structural elements (tiles, bricks, etc.), white-wares, (tableware, sanitaryware, etc.), and common refractories, the raw materials are naturally occurring minerals, and moderate levels of impurities are tolerated. More specialized technical ceramics such as electronic ceramics (substrates, electronic packages, capacitors, inductors, etc.) or high performance structural ceramics (silicon carbide, silicon nitride, etc.) demand low or controlled levels of impurities and make use of higher purity powders often made by more specialized techniques. 

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