Chemical Vapor Infiltration CVI

Chemical vapor infiltration (CVI) is a special CVD process in which the gaseous reactant infiltrates a porous material such as an inorganic open foam or a fibrous mat or weave. The deposition occurs on the fiber (or the foam), and the structure is gradually densified to form a composite.i l 

CVI has the same chemistry and thermodynamics as conventional CVD, but the kinetics is different since the reactants have to diffuse inward through the porous structure and the by-products of the reaction have to diffuse out.f f The process is used extensively in the production of carbon-carbon materials, reviewed in Ch. 

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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. 

Pyrolytic carbon was inserted into the pores of these silica matrices by chemical vapor infiltration (CVI). The silica template was contacted with a flow of propylene Pr, (2.5 vol%) diluted in argon at 750°C during 15 hours. A quite uniform pore filling can be obtained by CVI. At the end, the carbon represents about 50 wt% of the C/Si02 material. Since the deposition... 

MOCVD as, 22 153-154 in silicon carbide fiber manufacture, 22 534 thermally activated, 24 744-745 Chemical vapor infiltration (CVI), 26 767 ceramics and, 5 664 Chemical warfare, 5 813-840 defense against, 5 830-837 Chemical warfare agents, detection of, 22 716-717... 

The molded plate with the large volume of pores was further coated by chemical vapor-infiltrated (CVI) graphitic carbon at 1,500°C with 5 kPa... 

The processing techniques used for CMCs can be quite exotic (and expensive), such as chemical vapor infiltration (CVI), or through pyrolysis of polymeric precursors. Their maximum use temperatures are theoretically much higher than most MMCs or PMCs, exceeding 1800°C, although the practical use temperature is often much lower... 

Due to the fact that industrial composites are made up of combinations of metals, polymers, and ceramics, the kinetic processes involved in the formation, transformation, and degradation of composites are often the same as those of the individual components. Most of the processes we have described to this point have involved condensed phases—liquids or solids—but there are two gas-phase processes, widely utilized for composite formation, that require some individualized attention. Chemical vapor deposition (CVD) and chemical vapor infiltration (CVI) involve the reaction of gas phase species with a solid substrate to form a heterogeneous, solid-phase composite. Because this discussion must necessarily involve some of the concepts of transport phenomena, namely diffusion, you may wish to refresh your memory from your transport course, or refer to the specific topics in Chapter 4 as they come up in the course of this description. 

In Section 3.4.2, we introdnced the concept of chemical vapor infiltration, CVI, in which a chemical vapor deposition process is carried out in a porous preform to create a reinforced matrix material. In that section we also described the relative competition between the kinetic and transport processes in this processing technique. In this section we elaborate npon some of the common materials used in CVI processing, and we briefly describe two related processing techniques sol infiltration and polymer infiltration. 

Chemical Vapor Infiltration (CVI). Recall from Section 3.4.2 that CVI is primarily nsed to create ceramic matrix composites, CMCs. Fabrication of CMCs by CVI involves a sequence of steps, the first of which is to prepare a preform of the desired shape and fiber architecture. This is commonly accomplished by layup onto a shaped form of layers from multifilament fibers using some of the techniques previously described, such as filament winding. 

In addition to the Pd-based membranes, microporous silica membranes for hydrogen permeation [8] can be produced by a special type of chemical vapor deposition [140] named chemical vapor infiltration (CVI) [141], A large amount of studies have been carried out on silica membranes made by CVI for hydrogen separation purposes [8,121], CVI [141] is another form of chemical vapor deposition (CVD) [140] (see Section 3.7.3). CVD involves deposition onto a surface, while CVI implies deposition within a porous material [141], Both methods use almost similar equipment [140] and precursors (see Figure 3.19) however, each one functions using different operation parameters, that is, flow rates, pressures, furnace temperatures, and other parameters. 

A combined analytical and numerical method is employed to optimize process conditions for composites fiber coating by chemical vapor infiltration (CVI). For a first-order deposition reaction, the optimum pressure yielding the maximum deposition rate at a preform center is obtained in closed form and is found to depend only on the activation energy of the deposition reaction, the characteristic pore size, and properties of the reactant and product gases. It does not depend on the preform specific surface area, effective diffusivity or preform thickness, nor on the gas-phase yield of the deposition reaction. Further, this optimum pressure is unaltered by the additional constraint of prescribed deposition uniformity. Optimum temperatures are obtained using an analytical expression for the optimum value along with numerical... 

Chemical vapor infiltration (CVI) is widely used in advanced composites manufacturing to deposit carbon, silicon carbide, boron nitride and other refractory materials within porous fiber preforms. " Because vapor phase reactants are deposited on solid fiber surfaces, CVI is clearly a special case of chemical vapor deposition (CVD). The distinguishing feature of CVI is that reactant gases are intended to infiltrate a permeable medium, in part at least, prior to... 

EVD), chemical vapor infiltration (CVI), CVD of diamond films, and atomic layer deposition (ALD). 

Aerosol-assisted CVD introduces rapid evaporation of the precursor and short delivery time of vapor precursor to the reaction zone. The small diffusion distance between the reactant and intermediates leads to higher deposition rates at relatively low temperatures. Single precursors are more inclined to be used in AACVD therefore, due to good molecular mixing of precursors, the stoichiometry in the synthesis of multicomponent materials can be well controlled. In addition, AACVD can be preformed in an open atmosphere to produce thin or thick oxide films, hence its cost is low compared to sophisticated vacuum systems. CVD methods have also been modified and developed to deposit solid phase from gaseous precursors on highly porous substrates or inside porous media. The two most used deposition methods are known as electrochemical vapor deposition (EVD) and chemical vapor infiltration (CVI). 

For preforms of fiber reinforcements, a thin coating is applied to the fibers using chemical vapor infiltration (CVI). This coating step is essential both to protect the fiber from chemical attack by the strongly reducing aluminum alloy and to provide for a weak fiber/matrix interface in the composite. Because the coating is thin, the CVI step requires only a few hours, unlike CVI matrix formation processes, where long times are necessary to achieve sufficient densification. 

Golecki I (2003) Industrial carbon chemical vapor infiltration (CVI) processes. In Delhaes P (ed) Fibre and composites. Taylor Francis, London, ppl 12-138... 

Coating and thin films can be applied by a number of methods. In thermal or plasma spraying, a ceramic feedstock, either a powder or a rod, is fed to a gun from which it is sprayed onto a substrate. For the process of physical vapor deposition (PVD), which is conducted inside an enclosed chamber, a condensed phase is introduced into the gas phase by either evaporation or by sputtering. It then deposits by condensation or reaction onto a substrate. A plasma environment is sometimes used in conjunction with PVD to accelerate the deposition process or to improve the properties of the film. For coatings or films made by chemical vapor deposition (CVD), gas phase chemicals in an appropriate ratio inside a chamber are exposed to a solid surface at high temperature when the gaseous species strike the hot surface, they react to form the desired ceramic material. CVD-type reactions are also used to infiltrate porous substrates [chemical vapor infiltration (CVI)]. For some applications, the CVD reactions take place in a plasma environment to improve the deposition rate or the film properties. 

Wider use of fiber-reinforced ceramic matrix composites for high temperature structural applications is hindered by several factors including (1) absence of a low cost, thermally stable fiber, (2) decrease in toughness caused by oxidation of the commonly used carbon and boron nitride fiber-matrix interface coatings, and (3) composite fabrication (consolidation) processes that are expensive or degrade the fiber. This chapter addresses how these shortcomings may be overcome by CVD and chemical vapor infiltration (CVI). Much of this chapter is based on recent experimental research at Georgia Tech. 

The materials used in this investigation are always C/SiC composites, which were processed by the classical chemical vapor infiltration (CVI). CVl processing is used to deposit the PyC interphase on the carbon fibers and infiltrate the SiC matrix into the perform pores. 

The chemical vapor infiltration (CVI) of porous pre-forms - also a low temperature/low pressure route - is also applied to make composite matrices or in combination with LPI to raise the density of the pre-infiltrated structures. The mechanical properties achieved by CVI are better compared with the ones obtained by LPI, but CVI takes long processing time and is therefore not usable for big plane and complex structures [253, 254],... 

Next generations of nuclear reactors will require structural materials that retain excellent mechanical properties at high temperatures, in a hostile environment. The SiC/SiC composites made via Chemical Vapor Infiltration (CVI) reinforced with recently developed near stoichiometric fibers appear as promising candidates [1,2]. 

Chemical vapor infiltration (CVI) is a CVD variant capable of internally coating porous objects, e.g., an object made out of carbon fibers, with a ceramic material. Silicon carbide (SiC) or boron carbide (B4C) are examples of ceramic matrix materials that are used in combination with carbon fibers. Strong, light, durable, wear-resistant, and biocompatible joint prostheses made of ceramic-ceramic composites are manufactured by means of CVI. Figure 6.18 shows how the degree of penetration is affected by temperature and pressure. Clearly, to get deposit deep in the interior of the porous object low temperatures are necessary for reaction limitation and low pressures for helping the diffusion. Under these conditions growth rates are low. 

The extent to which gas-phase diffusion can be prevented from controlling the deposition rate is of considerable importance for chemical vapor infiltration (CVI). Low pressures and low temperatures (conditions in the catalytic regime) favor penetration. Both factors slow the deposition rate, however, and very long reaction times would be necessary for this way of doing CVI. Consequently, thermal gradients and forced reactant gas flows are sometimes applied to increase deposition rates. 

SiC- Sic and SiC-C (Continuous Fiber-Reinforced SiC Matrix Composites) Three different processes are commonly used to manufacture carbon fiber-reinforced SiC materials (i) chemical vapor infiltration (CVI) [340] (ii) liquid polymer infiltration (LPI also termed polymer infiltration and pyrolysis, PIP) [341]) and (iii) melt infiltration or liquid silicon infiltration (MI/LSI) [342]. 

Laser assisted chemical vapor deposition (LCVD) yields continuous or discontinuous low diameter fibers directly from the vapor phase by tip growth. Chemical vapor deposition (CVD) on the surface of a small diameter, preferably sacrificial, precursor fiber yields a large diameter fiber or microtube. Chemical vapor infiltration (CVI) can change the chemistry of precursor fibers by infiltration of a chemically reactive vapor species. Finally, laser vaporization (LV) of carbon-metal mixtures yields highly entangled mats of nearly endless nanotube ropes. 

Chemical vapor infiltration (CVI) is a process whereby reactive chemical species are generated in the vapor phase and allowed to react with a solid substrate thus modifying its chemistry. A successful use of this process requires (1) an absolutely continuous and stoichiometric conversion of the initial to the final chemistry in the solid state, and (2) a continuous and eventually complete conversion of the morphology, density, surface tension, mechanical as well all other properties. Discontinuous or incomplete results cause a steep drop in strength, the premier measure of uniformity. By combining the complexities of chemical vapor deposition (this chapter) with those of fiber formation from a precursor fiber (Chapters 8 to 12), the process is therefore intrinsically more difficult to control than any other. 

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