CVI

S. Hensley, Vacuum Insulated Pipe, CVI, Inc., Columbus, Ohio, personal communication, 1994. 

Reactive Hquid infiltration (45,68,90,93,94) is similar to the CVI process used to make RBSN. Driven by capillarity, a reactive Hquid infiltrates a porous preform and reacts on free surfaces. Reactive Hquid infiltration is used to make reaction bonded siHcon carbide (RBSC), which is used in advanced heat engines and as diffusion furnace components for semiconductor wafer processing. 

Qiu et al. [241 have reported the synthesis of macromolecules having 4-tolylureido pendant groups, such as poly(N-acryloyl-N -4-tolylurea-cvi ethyl acrylate) [po-ly(ATU-co-EA)] 18, and poly(N-methacryloyl-A/ -4-tol-ylurea-co-EA) [poly(MTU-co-EA)] 19, from the copolymerization of ATU and MTU with EA, respectively. Graft copolymerization of acrylamide onto the surface of these two copolymer films took place using the Ce(lV) ion as initiator. The graft copolymerization is proposed as Scheme (12). 

A reaction which has proved to be of much use in synthetic organic chemistry is the formation of the ortho and/or the para isomers of a hydroxyketone (CVI and CVTI) by treatment of a phenolic ester (CV) with an acid catalyst, viz. 

CVI is a special CVD process in which the gaseous reactants penetrate (or infiltrate) a porous structure which acts as a substrate and which can be an inorganic open foam or a fibrous mat or weave. The deposition occurs on the fiber (or the foam) and the structure isgradually densified to form a composite.The chemistry and thermodynamics of CVT are essentially the same as CVD but the kinetics is different, since the reactants have to diffuse inward through the porous structure and the by-products have to diffuse out.f l Thus, maximum penetration and degree of densification are attained in the kinetically limited low-temperature regime. 

A limitation of CVI is the necessity of interdiffusion of reactants and reaction products through relatively long, narrow, and sometimes tortuous channels. To avoid rapid deposition and choking of the entrance end of the channels, conditions are chosen to ensure deposition in the kinetically limited regime. This is a slow process which may take as much as several weeks before densification is achieved. In fact, full densification is almost impossible to obtain due to the formation of closed porosity. [ " 1... 

Lackey, W. J., Review, Status and Future of the CVI Process for Fabrication of Fiber-Reinforced Ceramic Composites, Ceram. Eng. Sci. Proc., 10(7-8) 577-584 (1989)... 

Present oxidation-protection systems are based on silicon carbide (SiC), which is applied by pack cementation or by chemical-vapor infiltration (CVI) (see Ch. 4).d ] Boron, zirconium, and other... 

Other ceramic cutting-tool materials include alumina, Si-Al-0-N, alumina-carbide composites and, more recently, a composite of silicon nitride reinforced with silicon carbide whiskers. This last material can be produced by chemical-vapor infiltration (CVI) and has high strength and toughness as shown in Table 18.3.Cl... 

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. 

If the composite is left only partially densified, it can be used as a filter for high temperature filtering systems with high collection efficiency as required in direct coal-fired gas and steam turbines. Similar systems are considered for particulate filtering in diesel engines by a carbon foam or felt coated with silicon carbide by CVI. 

Fraser has extensively discussed this relationship between laboratory work and clinical needs (Fraser and Hyltoft Peterson 1993) and has recently addressed the role of documented analytical quality as derived from measurements of RMs (Fraser and Hyltoft Peterson 1999). Among the concepts proposed by Fraser and his colleagues, it is suggested that analytical imprecision should be <0.50 CVi and bias should be <0.25 (CVi + where CVj is the within-subject biological variation (i.e. changes from day to... 

Das C,-Acetat XXXIV erhalt man aus dem Mono-Lithiumsalz des Diacetylens und Acrolein iiber das Carbinol CVI und das Bromid CVII. 

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