Preparation of silicon nitride

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. 

Liu J, Liu K, Wang HS, Gao F, liao R (2012) Preparation of silicon nitride porous ceramics. In Pan W, Gong JH (eds) High-ptaformance craximics Di, Parts 1 and 2. Trans Tech Publications Ltd, Stafa-Zurich, pp 824—827... 

If equation (6.2) applies, then silicon diffuses via a liquid or gaseous state towards the N2 molecule and steric hindrance from N=N may result in the more strained a-form. Impurities which reduce the likelihood of atomic nitrogen being present—i.e., O2, H2, and H2O, which scavenge N atoms—lead to a preponderance of a-Si3N4 in powder preparations of silicon nitride. 

Yoshino H., Kamiya K., Nasu H. IR study on the structure evolution of the sol-gel derived Si02 gels in the early stage of conversion to glasses. J. Non-Crystalline Solids 1990 126 68-76 Zhang S.C., Cannon W.G. Preparation of silicon nitride from silica. J. Am. Ceram. Soc. 1984 67 691-695... 

The metal dichalcogenides formed are usually amorphous but can be crystallized on heating (> 400°C). Gas-phase exchange reactions are exemplified by the preparation of silicon nitride from silicon tetrachloride and ammonia (equation 2) (6). 

Poly(carbosilane) fibres have been employed recently as precursors to silicon nitride and silicon oxynitride fibres. The preparation of silicon nitride fibres involves a two-stage process. The precursor fibres are first crosslinked by electron irradiation then heated in ammonia at temperatures up to 1400 °C. Electron irradiation is used to effect the crosslinking reaction without incorporation of oxygen. The nitridation reaction takes place between 500 and 800 °C. At a pyrolysis temperature of 1400 °C, essentially all the carbon in the precursor is lost, giving a composition close to that of pure silicon nitride. At this temperature, however, the fibre was extensively crystallized and the tensile strength was very low. The tensile strength of the fibre was shown to reach a maximum level of about 1.4 GPa at a nitridation temperature of 1300°C. 

Parr NL, Martin GF, May ERW (1960) Preparation, Microstructure and Mechanical Properties of Silicon Nitride. In Popper P (ed) Special Ceramics. Heywood, London, p 102... 

Thompson, D.P. (1989), Preparation and Properties of Silicon Nitride Based Materials, ed. D.A. Bonnell and T.Y. Tien, Materials Science Forum, Vol. 47, Trans Tech Publications, Aedermannsdorf, Switzerland. 

Comparison of silicon nitrides with carbon additions prepared by hot isostatic pressing and pressureless sintering... 

Okamura and co-workers (18) have taken air-cured PCS polymer and, through pyrolysis in the presence of ammonia, prepared essentially carbon-free silicon oxynitride fibers (equation 13). However, if the PCS polymer fiber is cured by electron beam radiation (to prevent oxygen addition), the same ammonia pyrolysis conditions provide nearly stoichiometric quantities of silicon nitride fibers (equation 14). 

Polysilazanes prepared by these processes are readily converted to silicon nitride or silicon-nitride-containing ceramic compositions upon pyrolysis at temperatures up to 1600 °C. High-purity a-Si3N4 (a-phase crystalline form of silicon nitride) has been efficiently prepared by these processes,... 

The reaction is surprisingly clean under these vigorous hydrosilation conditions, with no evidence for the formation of l,2-bis[tris(dimethyl-amino)]ethane (32). Thus, very satisfactory processes have been developed for both Tris and vinyl-Tris that do not involve chlorosilane intermediates. Both monomers are suitable intermediates for the preparation of polysilazane preceramic polymers that could be converted thermally to silicon nitride and mixtures of silicon nitride and silicon carbide. 

In the mass titration method, the PZC is determined as the natnral pH of a concentrated dispersion. A detailed description of the experimental procedure can be found in [667], Mass titration become popular in the late 1980s [668,669], but the same method was already known in the 1960s as the pH drift method [183], Usually, a series of natural pH values of dispersions with increasing solid loads is reported, but only the natural pH of the most concentrated dispersion is actually used. The only role of the data points obtained at lower solid loads is to confirm that a plateau was reached in pH as a function of solid load that is, a further increase in the solid load is unlikely to bring about a change in pH. The mass titration method is based on the assumption that the solid does not contain acid, base, or other surface-active impurities. This is seldom the case, thus mass titration often produces erroneous PZCs. In this respect mass titration is similar to the potentiometric titration without correction illustrated in Figure 2.7, only the solid-to-liquid ratio is different. The experimental conditions in mass titration (solid-to-liquid ratio, time of equilibration, nature and concentration of electrolyte, and initial pH) can vary, but little attention has been paid to the possible effects of experimental conditions on the apparent PZC. The effect of an acid or base associated with solid particles on the course of mass titration was studied in [670], To this end, a series of artificially contaminated samples was prepared by the addition of an acid or base to a commercial powder. The apparent PZC of silicon nitride obtained in [671] by mass titration varied from 4.2 (extrapolated to zero time of equilibration) to 8.2 for time of equilibration longer than 20 days. The method termed mass titration was used in [672], but it was different from the method discussed above. 

If a plasma is used to generate ions or radicals that recombine to give the desired film, the process is plasma-enhanced CVD (PEC VD). In PECVD it is possible to use much lower substrate temperatures because the plasma provides energy for the reaction to proceed. A major commercial application of PECVD is the formation of silicon nitride films for passivation and encapsulation of semiconductor devices. At this stage of the fabrication process the device caimot tolerate temperatures much above 300°C. High temperatures would still be required if crystalline or epitactic films were required. Many nitrides have been prepared in thin-film form by PECVD, including AIN, GaN, TiN, and BN. A more complete list of films deposited by PECVD is given in Table 28.3. 

Kanyal, S.S., Hahe, T.T., Cushman, C.V., Dhunna, M., Roychowdhury, T., Farnsworth, P.B., Morlock, G.E., and Linford, M.R. 2015. Microfabrication, separations, and detection by mass spectrometry on ultrathin-layer chromatography plates prepared via the low-pressure chemical vapor deposition of silicon nitride onto carhon nanotube templates, J. Chromatogr. A, 1404 115-123. 

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