Composites Densification

Ewsuk K G 1992 Effects of trapped gases on ceramic-fiiied-giass composite densification Solid State Phenomena voi 25-26, ed A C D Chakiader and J A Lund (Brookfieid, VT Trans-Tech) pp 63-72 (Proc. Sintering 91)... 

The maximum value of this ratio is 2, which occurs when v , = 0, so for v, less than 0.10, the composite densification rate is predicted to deviate from the rule of mixtures by not more than 10%. 

Carbon—carbon composites for rocket nozzles or exit cones are usually made by weaving a 3D preform composed of radial, axial, and circumferential carbon or graphite fibers to near net shape, followed by densification to high densities. Because of the high relative volume cost of the process, looms have been designed for semiautomatic fabrication of parts, taking advantage of selective reinforcement placement for optimum thermal performance. 

Ceramics. The properties of ferroelectrics, basically deterrnined by composition, are also affected by the microstmcture of the densifted body which depends on the fabrication method and condition. The ferroelectric ceramic process is comprised of the following steps (10,24,25) (/) selection of raw oxide materials, (2) preparation of a powder composition, (J) shaping, (4) densification, and (5) finishing. 

Fig. 7. Thermal conductivity of composites obtained using various densification processes.

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. 

Thermal-Gradient Infiltration. The principle of thermal-gradient infiltration is illustrated in Fig. 5.15b. The porous structure is heated on one side only. The gaseous reactants diffuse from the cold side and deposition occurs only in the hot zone. Infiltration then proceeds from the hot surface toward the cold surface. There is no need to machine any skin and densification can be almost complete. Although the process is slow since diffusion is the controlling factor, it has been used extensively for the fabrication of carbon-carbon composites, including large reentry nose cones. 

The majority of work done on VGCF reinforced composites has been carbon/carbon (CC) composites [20-26], These composites were made by densifying VGCF preforms using chemical vapor infiltration techniques and/or pitch infiltration techniques. Preforms were typically prepared using furfuryl alcohol as the binder. Composites thus made have either uni-directional (ID) fiber reinforcement or two-directional, orthogonal (0/90) fiber reinforcement (2D). Composite specimens were heated at a temperature near 3000 °C before characterization. Effects of fiber volume fraction, composite density, and densification method on composite thermal conductivity were addressed. The results of these investigations are summarized below. 

The third step is to heat the preform in a sealed chamber and pass a mixture of gases into the chamber that will react when they contact the hot fibers to form and deposit the desired chemical constitnents of the matrix. The deposition rate is very slow and becomes even slower as the thickness of the deposit increases and the permeability of the preform decreases. To achieve high levels of densification, the partially densified part is removed from the CVl chamber, the surface is machined to reopen pore channels, and the part is returned to the chamber for further infiltration. This procedure is typically repeated a number of times to achieve a composite density of over 80% (<20% porosity). 

The thus obtained high-density Mn-Zn ferrite was investigated in detail from the view of physical and mechanical properties, that is, the relationships between the composition of metals (a,) ) and <5 the magnetic properties such as temperature and frequency dependence of initial permeability, magnetic hysteresis loss and disaccommodation and the mechanical properties such as modulus of elasticity, hardness, strength, and workability. Figures 3.13(a) and (b) show the optical micrographs of the samples prepared by the processes depicted in Fig. 3.12(a) and (b), respectively. The density of the sample shown in Fig. 3.13(a) reached up to 99.8 per cent of the theoretical value, whereas the sample shown in Fig. 3.13(b) which was prepared without a densification process, has many voids. 

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