Silicon carbonitride

Boron-containing nonoxide amorphous or crystalline advanced ceramics, including boron nitride (BN), boron carbide (B4C), boron carbonitride (B/C/N), and boron silicon carbonitride Si/B/C/N, can be prepared via the preceramic polymers route called the polymer-derived ceramics (PDCs) route, using convenient thermal and chemical processes. Because the preparation of BN has been the most in demand and widespread boron-based material during the past two decades, this chapter provides an overview of the conversion of boron- and nitrogen-containing polymers into advanced BN materials. 

Polysilazanes that can serve as precursors for silicon carbonitride have been prepared using a Ru3(CO)i2-catalyzed Si-H/N-H dehydrogenative coupling process by workers at SRI [21]. Thus the ammonolysis product of CH3SiHCl2, whose composition approximates [CH3Si(H)NH]n, could be crosslinked by heating at 40 °C with a catalytic quantity of Ru3(CO)i2. Other polysilazanes were prepared by this procedure ... 

Summary A brief review of the preparation of silicon containing preceramic polymers to prepare silicon carbide and silicon carbonitride fibers is given. Methylchlorodisilanes are converted to polysilanes and polysilazanes which yield ceramic fibers after meltspinning, curing, and pyrolysis. 

The polysilazanes were also melt spun, cured, and pyrolyzed to give silicon carbonitride fibers (Eq. 7). The carbon content of these fibers depends on the molecular composition of the polysilazane and the pyrolysis gas. When ammonia is used as reactive gas pure silicon nitride fibers will be obtained (Eq. 8) [14]. 

The Miiller-Rochow-Synthesis [16,17] (direct synthesis of methylchlorosilanes) provides as byproduct a high boiling fraction consisting essentially of 1,1,2-trimethyltrichlorodisilane and 1,2-dimethyltetrachlorodisilane [18]. Starting with these disilanes Wacker-Chemie has developed different ways to produce silicon carbide [19, 21] and silicon carbonitride [22] fibers. 

Chemical and phase purity are not always desirable. For example, H- and N-doped silicon carbide films behave as high temperature semiconductors, while silicon carbonitride glasses offer properties akin to glassy carbon with room temperature conductivities of 103 2 cm-118. Additional reasons for targeting materials that are not chemically or phase pure stem from the desire to control microstructural properties. 

Various SiC-type fibres with elemental compositions of Si-C, Si-N-C-O, Si-B-N, Si-C-0 and Si-Ti-C-0 are commercially available. These fibres are made from polymeric precursors. A multifilament SiC fibre, called Tyranno, is produced by Ube Industries, Japan [29], This fibre is made by the pyrolysis of poly(titano carbosilanes) and contains 1.5-40 titanium by weight. Another multifilament fibre is called silicon carbonitride, trade name HPZ, produced by Dow Coming Corporation, USA. 

Combining aspects of carbon and silicon chemistry while at the same time expanding the tool box of the periodic table, recently, a first report on the templated synthesis of mesoporous silicon oxycarbide (SiOC) and silicon carbonitride (SiCN) as analogs of the well-known mesoporous silica materials discussed in many chapters of the book has appeared (Fig. 25.5),58 opening an even wider horizon for the exploration of SiC-related nanomaterials in the fields covered in this book. 

Bis-trimethylsilylcarbodiimide is used as a monomer in the synthesis of silicon car-bodiimide polymers used as precursors for ceramics. Also, hard silicon carbonitride films are obtained by RF plasma-enhanced chemical vapor deposition of bis(trimethyl-silyl)carbodiimide. ... 

Initial explanations of the extraordinary high-temperature stability of precursor-derived Si-B-C-N ceramics were presented previously by Jalowiecki et al. 13 The authors investigated the microstructure of boron-doped silicon carbonitride composites by HR-TEM and found... 

In the pyrolysis of a preceramic polymer, the maximum temperature used is important. If the maximum temperature is too low, residual functionality (C-H, N-H, and Si-H bonds in the case of polysilazanes) will still be present. On the other hand, too high a pyrolysis temperature can be harmful because of solid-state reactions that can take place. For instance, if the polysilazane-derived silicon carbonitride contains a large amount of free carbon, a high-temperature reaction between carbon and silicon nitride (equation 1) (7) is a possibility. 

However, in many cases the pyrolysis products are amorphous. For instance, the polysilazane-derived silicon carbonitride mentioned above crystallizes only when it is heated above 1450 C, and its characterization is difficult. Elemental analysis poses problems, in part because the pyrolysis product is very porous. As a result of its high surface area, the material adsorbs moisture and volatiles very readily, and improper prior handling and preparation for analysis can result in misleading results. 

Fainer NI, Kosinova ML, Rumyantsev YM, Kuznetsov FA (1999) RPECVD thin silicon carbonitride films using hezamethyldisilazane. J de Physique IV, 9, Pr8-769-... 

R. Kumar, F. Phillipp, and F. Aldinger, Oxidation Induced Effects on the Creep Properties of Nano-Crystalline Boron Doped Silicon Carbonitrides, Mater. Sci. and Eng. A, 445 - 446, 251 - 258 (2007). 

Pyrolyzability, nature of product - 3°C min" to 1,200°C under high purity N2 Silicon carbonitride (1)... 

D. Bahloul, M. Pereira, P. Goursat, N. S. Choong Kwet, and R. J. P. Corriu, Preparation of silicon carbonitrides from an organosilicon polymer I, Thermal decomposition of the cross-linked polysilazane, J. Am. Ceram. Soc. 1993, 76, 1156-1162. 

In recent years silicon-based polymers were investigated as precursors for SiC and Si3N4 ceramics, as well as for crystalline or amorphous Si/C/N and SiC/Si3N4 composite materials [1, 2]. This is due to the very interesting chemical and thermomechanical properties of silicon carbonitrides, such as high hardness, toughness and corrosion resistance. In most of these studies polycarbosilanes, polysilazanes and polycarbosilazanes were applied [3]. 

Non-oxide preceramic polymers which are expected to yield, under convenient thermal and chemical conditions, boron-containing amorphous or crystallized ceramics including boron nitride (BN), boron carbide (B C), boron carbonitride (B-C-N), and boron silicon carbonitride Si-B-C-N. 

Si-C-N(O) fibers derived from HPZ precursor fibers are nanoporous and heterogeneous with a skin/core structure. The composition changes from SiOxCy in the external porous surface to SiNxC, in the core. The molecuiar formuia of this fiber is close to 4 mol. >4 SiOa, 81 mol.% SiNxCy (x = 1.02, y = 0.23) and 15 mol.% free C [22]. The presence of complex tetrahedral units is supported by the Si NMR spectrum which shows a broad signal covering the chemical shift region expected for silicon oxycarbide, siiicon oxynitride and silicon carbonitride units [21]. The occurrence of free carbon, expected from the nature of the precursor, is supported by the C Is XPS pattern [22]. 

The synthesizing of this catalyst consists of three process steps which are construction of carbon nanotubes (CNTs) on carbon fibers support, coating of polymer-derived silicon carbonitride (SiCN) on CNTs and finally decoration of transition metal on surface. The rate of hydrogen generation has been reported as 75 L mim g" [100]. 

Ceramic monolithic structures made from inverted opal silicon carbonitride (see Figure 6.6) were presented by Mitchell et al. [460]. The inverse opal structure was achieved using polystyrene templates. They prepared monoliths of 74% porosity, typically 350-pm wide, 100-pm high and 3-mm long. Propane steam reforming was then successfully performed in the reactor (see Section 7.1.2). 

The ceramic monolithic stmctures made from inverted opal silicon carbonitride (see Section 6.2), which had been developed by Mitchell et al. [460], were applied for propane steam reforming. A 5 wt.% mthenium catalyst was coated onto the monoliths. Full conversion was achieved at fairly high reactor temperatures of 900 °C. Despite the low S/C ratio of 1.33, stable operation of the catalyst and reactor at 800 °C and 60% conversion was achieved through several hours of operation and more than 15 thermal cycles without apparent coke formation. 

Gerardin, C., Taulelle, F., and Livage, J. Pyrolysis of a polyvinylsUazane, polymeric precursor for silicon carbonitride Structural investigation by H, C, Si, N, and N nuclear magnetic resonance. J. Chim. Phys. PCB 1992 89 461-467. 

Schiavon, M.A., Soraru, G.D., and Yoshida, l.V.P. Synthesis of a polycyclic sila-zane network and its evolntion to silicon carbonitride glass. 

Trassl, S., Suttor, D., Motz, G., Rdssler, E., and Ziegler, G. Structural characterisation of silicon carbonitride ceramics derived from polymeric precnrsors. 

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