CVI processes

There are several types of CVI processes which can be used to process carbon-carbon  

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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. 

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)... 

The capacitance values of the carbons from propylene, i.e. CPr in the three electrolytic media are lower than for the materials from sucrose CS (Table 4), and they decrease with the total surface area of the carbon materials, i.e. with the filling rate of carbon in the silica porosity [18], These results are not surprising since the carbon filling is quite uniform during the CVI process, and consequently, the fraction of micropores formed is much... 

The CVI procedure can lead to some very interesting composite materials. Lackey et al. [17] have produced composites of carbon fibers in a laminated SiC/carbon matrix (see Figure 3.37). The alternating SiC/carbon layers were produced by CVI of two different reactions the decomposition of methyltrichlorosilane (MTS) in the presence of hydrogen and the decomposition of propylene in hydrogen, respectively. There is also an excellent opportunity to model these processes with the intent at gaining better control of the CVI process [18]. 

The following operating conditions are typically nsed for this CVI process ... 

Compare your results. Is this CVI process reaction-controlled, diffusion-controlled, or a combination of both ... 

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. 

The possibility to measure the permeance of several gases in-situ has proven to be very convenient for following the CVI-process. 

In addition to the reactor scale, which is measured in meters, vapor-phase mass transport effects can also be important in CVD at a much smaller scale, one measured in micrometers. This is often referred to as the feature scale . On this scale, the gas is generally in the transition or molecular flow regimes, rather than continuum flow. Mass transport on this scale plays an important role in the CVI processes discussed in Chapter 6. These phenomena are also important in CVD involving high-aspect ratio features, which can occur unintentionally in some growth morphologies and deliberately in microelectronics applications. 

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. ... 

Chemical vapour infiltration (CVI) is an extension of CVD processes only when a CVD process occurs on an internal surface of a porous substrate (especially for the fibre preform). As compared with CVD, the CVI process for ceramics is much more effective and important because it is the optimal technique to fabricate fibre reinforced ceramics and particularly carbon fibre reinforced carbon and advanced ceramic matrix composites. Both CVI and CVD techniques share some common features in overall chemistry, however, the CVI is much more complex than the CVD process in mass transport and chemical reactions. 

Permeability is a parameter defined to measure the physical influence of a porous structure on fluid flow, and for a CVI process it is an important physical parameter for fibre preforms. Another important parameter for porous structure is the porosity, which is the most important geometrical property. According to Darcy s law, the volumetric flow rate Q of a fluid through a porous medium is proportional to the hydrostatic pressure difference (AP) across the structure (see Figure 2.16), the permeability and the cross-section area, and is also inversely proportional to the length of the structure and the viscosity of the fluid, as given by [26]... 

In such conditions the brittle ceramic fibres, such as Nicalon SiC and A1203 fibres, remain undamaged during the CVI process. However, conventional techniques for the fabrication of ceramic-matrix composites such as hot pressing take place at extremely high temperatures (2000°C) and under high mechanical stresses (30 MPa), which usually severely damage the fibres. 

Moreover, the service temperature of CVI materials can be much higher than CVI processing temperatures. The reason for this advantageous feature is that the ceramic matrix produced by CVI is much purer than that obtained with the hot-pressing method, in which sintering additives are generally needed. 

For I-CVI processes, the driving force for mass transport is the concentration gradient of the reactant gaseous species as shown in Figure 5.3. During the densification process the precursor gases flow over the preforms at a reduced pressure. Then they diffuse into the porous and fibrous preforms, react and form a... 

To understand the I-CVI process, a one-dimensional model is widely used to demonstrate the principle as shown in Figure 5.3b. Based on this simple schematic diagram, it is possible and essential to establish its process model under steady conditions. A mass-conservation equation model is established based on the fundamental conservation of matter as this lies at the heart of all CVI processes. Under steady conditions the governing transport-reaction equation is written as [12-14]... 

For isothermal/isobaric CVI processes there is no forced flow, i.e. the convection term cannot be taken into consideration. Assuming the first-order reaction, Equation (5.2) becomes... 

According to Equation (5.7) the increase in processing temperature and preform thickness leads to larger values of Valid poor density uniformity. As stated previously, CVI processes preferably operate in a chemical-reaction-controlled regime where the ratio of k/D is small. For Fick diffusion, discussed in Section2.3.1, the diffusivity D is inversely proportional to the pressure and thus operates at lower pressures. Furthermore, coarser pore structures correspond to more uniform deposition. Figure 5.5 shows the microstructures of C/SiC composites prepared at different 9 numbers. 

For I-CVI processes the overall densification kinetics of a preform with initial density (po) follows an exponential pattern to a final density value (pj) as shown in Figure 5.7, which can be expressed as follows [10] ... 

The way to achieve the final required density is through several approximately exponential infiltration steps as shown in Figure 5.7. For I-CVI processes, the... 

Figure 5.7. Plot of the density of a thick preform against time in an I-CVI process...

From an economical point of view the overall conversion efficiency of the precursor is another important parameter for cost considerations. This efficiency is defined as the ratio of infiltrated mass to the precursor mass introduced into the reaction chamber. Investigations indicate that the conversion efficiency is dependent on the mass transport method, residual time of the precursor, pressure and temperature, etc. As shown in Figure 5.9 the conversion efficiency of propylene (C3H6) increases with an increase in residence time but decreases with the partial pressure of the precursor gas. In particular, it is worth noting that the conversion efficiency in the I-CVI process is rather low and generally is in the range of 0.78 to 2.14% [17],... 

In a forced flow CVI (F-CVI) process, the precursor gases are allowed to flow through the fibre preform, rather than relying on diffusion transport as with I-CVI processes. An F-CVI process offers the advantages of much reduced processing... 

F-CVI processes take place far from the thermodynamic equilibrium. The dimensionless Peclet number, Pe, is used to describe this process [39] ... 

For an I-CVI process the surfaces of preforms require grinding in several stages during a complete process in order to open the blocked channels or pores in the outer region of the preform for further infiltration. In contrast, F-CVI is a one-step... 

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