Forced CVI

Figure 5.20 is reprinted from Journal of Nuclear Materials, Vol. 219, T M Besmann, J C McLauglin and H T Lin, Fabrication of ceramic composites forced CVI, pp. 31-35, 1995, with permission from Elsevier. 

At the 1990 International Task Force for Vaccine Development meeting in New York, a children s vaccine initiative (CVI) was set up with the aim of developing an ideal vaccine. The following criteria were established although the list may not be totally inclusive. 

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. ... 

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. 

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... 

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... 

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... 

Figure 5.24 shows the simulated results of F-CVI under isothermal conditions. The forced effect on infiltration is very limited if the Pe number is small. Uniform infiltration is obtained when the Pe number reaches 10 at relatively the low temperature of 1223°K. The reason is the convective flow can force much more gaseous species to penetrate deeply into the pores of the preform. However, the influence becomes minimal at high temperatures (1373°K) even for high a Pe number (10.0). 

The overall model of a P-CVI process is as shown in Figure 5.43. During a P-CVI process the mass transport of gaseous species can be divided into two stages. In the first stage mass transport takes place by forced convection within a very short period of a few hundredths or tenths of a second. In the second stage of the duty cycle the mass transport is dominated by the diffusion from the free space of the reaction chamber into the pores of the fibre preform. The temperature is kept... 

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. 

Besides the good quality of the C/SiC composites processed by chemical vapour infiltration, one of the major advantages of this manufacturing route is that is allows the control of the fibre/matrix interphase. Therefore, C/SiC composites are shortly deposited in a first step with carbon (e.g. by the deposition of methane gas, CH4) to govern the fibre/matrix bonding forces. In summary, the manufacture of C/SiC components via the gradient-CVI process comprises the following steps ... 

Electroacoustic phenomena. They are electrokinetic phenomena that have recently gained interest, both experimentally and theoretically. In the ESA (electrokinetic sonic amplitude) technique, an alternating electric field is applied to the suspension and the sound wave produced in the system is detected and analyzed. The colloid vibration potential (CVP) or colloid vibration current (CVI) is the reciprocal of the former a mechanical (ultrasonic) wave is forced to propagate through the system, and the resulting alternating potential difference (or current) is measured. 

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. 

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... 

Lower-extremity chronic venous insufficiency (CVI) is caused by venous hypertension [1], Most patients develop venous hypertension from the hydrostatic forces produced by reflux that results from primary valvular insufficiency [2] Venous obstruction, muscular pump failure, and congenital anomalies are much less common causes. In addition, 85%-90%... 

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