Si-ceramic

This relation is shown in Fig. 6.3.4 for the example of Si ceramics. The threshold parameters of plastic deformation in hardness tests can be determined to a good approximation according to Evans and Law (1977) from... 

This criterion, which is product rather than precursor-property driven, is critical to the design and synthesis of new precursors. The need for high ceramic yields arises because of the excessive volume changes associated with pyrolytic conversion to ceramic materials. Scheme 1 illustrates these changes for a SiC precursor with an 80% ceramic yield of phase pure SiC (3.2 gml-1). Most precursors densities are close to 1 gml-1, whereas most Si ceramic densities range from 2.5 to 3.5 gml-1. 

Clays have layers of linked (Al, Si)04 tet-rahedra combined with layers of Mg(OH)2 or AI(0H)3- Clays are very important soil constituents and are used in pottery, ceramics, as rubber, paint, plastic and paper fillers, as adsorbents and in drilling muds. 

Fig. 1. Stress and temperature ranges of apphcation for Zr02 (—), Si N (-), and SiC (--) advanced stmctural ceramics. To convert MPa to psi,...

The shaping of these fine, submicrometer powders into complex components and their subsequent consoHdation into dense ceramic parts of ideally zero porosity is a major technological challenge. The parts formed need to be consoHdated to near-net shape because Si N machining requires expensive diamond grinding. Additionally, Si N dissociates at or near the typical densiftcation temperatures used in the fabrication of stmctural ceramics and, therefore, special measures have to be taken to preserve the compositional integrity of the material. 

K. Weber and N. Hakim, 1st Int. Ceramic Science and Technology Congress, Paper 4-SI-89C, (Anaheim, Calif., October, 1989). 

Li Si O enstatite [14681-78-8] MgSiO diopside [14483-19-3], CaMgSi O and woUastonite [14567-57-2], CaSiO. A number of sihcate glass-ceramic compositions are given in Table 1. 

Sihcon nitride occurs in two forms, a-Si N and P-Si N. Pure Si N is white, but the colors of commercial materials may be tan, gray, or black because of residual siUcon or impurities. Si N may be prepared by nitriding siUcon powder at 1200—1400°C or, for extremely fine-grained Si N, by the reaction of SiCl or SiH and N2 or NH (see also Advanced ceramics). 

Polymers, metals, ceramics, and glasses may be utilized as biomaterials. Polymers (see Ppolymerprocessing), an important class of biomaterials, vary gready in stmcture and properties. The fundamental stmcture may be one of a carbon chain, eg, in polyethylene or Tedon, or one having ester, ether, sulfide, or amide bond linkages. PolysiHcones, having a —Si—O—Si— backbone, may contain no carbon. 

Cordierite [12182-53-5] Mg Al Si O g, is a ceramic made from talc (25%), kaolin (65%), and Al O (10%). It has the lowest thermal expansion coefficient of any commercial ceramic and thus tremendous thermal shock resistance. It has traditionally been used for kiln furniture and mote recently for automotive exhaust catalyst substrates. In the latter, the cordierite taw materials ate mixed as a wet paste, extmded into the honeycomb shape, then dried and fired. The finished part is coated with transition-metal catalysts in a separate process. 

Titanium siUcides are used in the preparation of abrasion- and heat-resistant refractories. Compositions based on mixtures of Ti Si, TiC, and diamond have been claimed to make wear-resistant cutting-tool tips (157). Titanium siUcide can be used as an electric—resistant material, in electrically conducting ceramics (158), and in pressure-sensitive elastic resistors, the electric resistance of which varies with pressure (159). 

---