Thermal gradient CVI

As shown in Figure 5.27 the I-CVI process is performed in a hot-wall reactor, in which both the heat and mass are transferred from the outer region to the inner region of the preform. As a result it leads to a severe density gradient and forms an impermeable skin which impedes the further densification of the inner part of the preform. However, thermal gradient CVI (TG-CVI) is a cold-wall technique, although the densification is also dependent on the mass diffusion. A temperature gradient can be deliberately established across the fibre preform. Unlike the I-CVI model, TG-CVI has a mobile infiltration front within the preform, caused by a... 

Tang ZH, Qu DN, Xiong J, Zou ZQ (2003) Effect of infiltration conditions on the densification behavior of carbon/carbon composites prepared by a directional-flow thermal gradient CVI process. Carbon 41 2703-2710... 

The forced flow-thermal gradient CVI process (FCVI) has been shown to permit the rapid consolidation of SiC matrix composites. Recently, the FCVI process has been extended to the fabrication of carbon fiber-carbon matrix composites. Using 2D carbon cloth preforms, composite disks 0.8 cm thick have been fabricated in less than three hours a small fraction of the time required for either the resin/pitch or conventional CVI processing. Further, the FCVI process facilitates the incorporation of oxidation inhibitors within the carbon matrix and may permit obtaining a preferred crystallographic orientation that yields the high thermal conductivity required for thermal management applications. 

Fabrication by CVI This is performed by the deposition of SiC vapor inside a porous preform (40-60% porosity) made from high-strength C- or SiC- fibers, and results in a composite with 10% residual porosity. The fracture mode is noncatastrophic, and typical flexural strength of 300-400 MPa and toughness values of over 20 MPam are obtained. Using the forced flow thermal-gradient CVI process developed at Oak... 

A new technique [204] for studying the progression of densification during the fabrication of composites by CVI has been introduced, which involves the preparation of a carbon-carbon composite and momentarily interrupting the carbon infiltration process (forced flow thermal gradient CVI) at various times to permit the deposition of very thin layers of SiC. Microscopic examination of these layers on a polished cross-section permitted determination... 

A recent modification to the CVI process has been development of a forced flow-thermal gradient chemical vapor infiltration technique (FCVI), which has been examined for propylene, propane, and methane gas precursors. ... 

In a cold-wall CVD only the substrates are heated either inductively or resistively and the wall of the reactor is colder than that of the substrate. Therefore, the deposition mainly occurs on the heated substrate, and negligible deposition on the walls of the reactor. Cold-wall reactors are mainly used for continuous deposition of fibres and depositions where a thermal gradient is required to facilitate CVI. Hot-wall CVD reactors represent one of the major categories of CVD reactors. In such systems, the chamber containing the parts is heated by a furnace from outside. In general, hot-wall reactors have the advantages of being... 

According to the controlling parameters used in the process, CVI approaches can be classified into five typical categories [9] isothermal/isobaric CVI (I-CVI), forced-flow CVI (F-CVI), thermal gradient/isobaric CVI (TG-CVI), pulsed CVI (P-CVI) and liquid immersion CVI (LI-CVI). There are more than ten CVI techniques if a particular method is coupled with plasma, microwave or a catalyst to enhance the process [10], Some representative techniques are discussed in this chapter as follows. 

The discussion in Section 5.4 clearly indicates that a temperature gradient has a significant influence on the densification of the preform. In order to increase the densification rate and improve the density, it is necessary to establish a very steep thermal gradient within the preform. In 1984 Houdayer el al. [52] described a method for rapid densification by immersing the preform into a liquid precursor at high temperatures to fabricate the C/C composites. This densification process is also called liquid-immersion CVI (LI-CVI). 

The in-depth model of the deposition process is another important aspect of gaining a deep understanding of an LI-CVI process as well as a TG-CVI process. Under a strong thermal gradient the relationship between the deposition rate on a fibre (udep) and the densification rate inside the preform (u/ront) can be calculated by using the one-dimensional model [42],... 

Zhao JG, Li KZ, Li HJ, Wang C (2006) The influence of thermal gradient on pyrocarbon deposition in carbon/carbon composites during the CVI process. Carbon 44 786-791... 

In order to overcome the above limitations of the F-CVI technique, alternative techniques using thermal gradients or pressure gradients have been examined for several years [11]. In the thermal gradient process, the core of the fibrous preform is heated in a cold wall reactor. The heat loss by radiation is favorable to get a colder temperature in the external surface. The densification front advances progressively from the internal hot zone toward the cold side of the preform. In the P-CVI process, the source gases are introduced during short pulses [11]. The P-CVI process is appropriate to the deposition of thin films. 

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. 

Schematic diagram of the thermal and pressure gradient method for CVI of composites. Source Reprinted with permission from Stinton DP, Caputo AJ, Lowden RA, Am Ceramic Soc Bull, 65, 347-350, 1986. Copyright 1986, The American Ceramics Society.

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