Silicon nitride decomposition

The reinforcing fibers are usually CVD SiC or modified aluminum oxide. A common matrix material is SiC deposited by chemical-vapor infiltration (CVI) (see Ch. 5). The CVD reaction is based on the decomposition of methyl-trichlorosilane at 1200°C. Densities approaching 90% are reported.b l Another common matrix material is Si3N4 which is deposited by isothermal CVI using the reaction of ammonia and silicon tetrachloride in hydrogen at 1100-1300°C and a total pressure of 5 torr.l" " ] The energy of fracture of such a composite is considerably higher than that of unreinforced hot-pressed silicon nitride. 

Kohtoku Y (1987) Developments in Si3N4 powder prepared by the imide decomposition method. In Somiya S, Mitomo M, Yoshimura M (eds) Silicon Nitride I. Elsevier, London, p 71... 

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. 

The synthesis of processable precursors for Si-B-N-C ceramics became a goal of intensive investigations as soon as the outstanding thermal and mechanical properties of this system were reported [1,2]. The amorphous phase of Si-B-N-C ceramics can show excellent thermal stability up to 2000 °C without mass loss or crystallization. The role of boron is believed to be to increase the high-temperature stability and to prevent the crystallization and decomposition of silicon nitride above 1500 °C. Primarily, the atomic ratio and chemical environment of boron in Si-B-N-C precursors seem to affect the thermal behavior of resulting ceramic materials. 

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. 

The crystalline properties of silicon, silicon nitride, and silicon carbide nanoparticles produced in a laboratory aerosol reactor were measured by Cannon et al. (1982). Particles were produced using a COi laser to irradiate aerosol precursor gases. For example, silane (SiHj) u.sed to produce silicon particles could be healed adiabatically to the reaction temperature as long as the gas pressure wa.s maintained above 0,05 atm. At lower pressures, beat conduction to the cell walls balanced the heat absorbed by the gases, Silicon particles were generated at about 100() C by silane decomposition ... 

These first LEED experiments have not yet led to understanding of NHg decomposition or synthesis on W. Effects of H2 or Ng coadsorbing with NHg or NHj still remain to be investigated and one can anticipate many more interesting experiments before the work is completed. It is appropriate to mention briefly that LEED study of NHg decomposition on a Si(lll) surface (293) reveals some similarities to the tungsten experiments. Low energy electron diffraction patterns attributed to N atoms on the surface are developed by heating a room temperature deposit above 700°C. These patterns could not be interpreted in terms of thin layers of silicon nitride. 

The thermal nitration of a H-terminated Si surface and the CVD of silicon nitride were studied in situ by FTIR-IRRAS [160, 161]. The adsorption and thermal decomposition of phosphine (PH3) on a Si surface was also studied by IR absorption depending on the coverage and the exposure to the flux, PH3 adsorbs both nondissociatively and dissociatively, and the IR absorption peaks... 

The decomposition of ammonia increases above 850°C, a temperature at virhich nitridation begins to increase. As a result active hydrogen breaks the Si-0 bonds while active nitrogen reacts with unbonded silicon to form silicon nitride fibers (Equation 8b). The process does not only yield straight fibers but also well defined, coiled fibers. 

Silicon nitride has been obtained by the pyrolysis (in a stream of NHj) of the perhydropolysilazane prepared by the ammonialysis of the H SiClj-pyridine adduct [23, 24]. The gas stream employed during the pyrolysis of the preceramic polymer plays a crucial role [25]. Pyrolysis of [B, H,j diamine] polymers in an NHj stream gives BN [26]. TiN is similarly obtained by the pyrolysis of an amine precursor [27]. TiN has been prepared from titanazane [28]. Pyrolysis of Nicalon in NHj is reported to give SijN [24]. Besides single-phase ceramics, multiphase ceramics (e.g. composites of SiC and TiC, BiN and SijN ) have been prepared from precursors [29, 30]. Group 13 metal nitrides (GaN, AIN, hiN) have been prepared by the decomposition of urea complexes [31]. This method has been extended for the synthesis of BN, TiN and NbN [32]. 

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