Infiltration Machine

The sensible heat gain is determined by taking into account all the heat gains in a space, such as solar, fabric, infiltration, machines, processes, occupancy, lighting, etc. 

Many metals are naturally brittle at room temperature, so must be machined when hot. However, particles of these metals, such as tungsten, chromium, molybdenum, etc., can be suspended in a ductile matrix. The resulting composite material is ductile, yet has the elevated-temperature properties of the brittle constituents. The actual process used to suspend the brittle particles is called liquid sintering and involves infiltration of the matrix material around the brittle particles. Fortunately, In the liquid sintering process, the brittle particles become rounded and therefore naturally more ductile. 

Isothermal Infiltration. Several infiltration procedures have been developed, which are shown schematically in Fig. 5.15.P3] In isothermal infiltration (5.15a), the gases surround the porous substrate and enter by diffusion. The concentration of reactants is higher toward the outside of the porous substrate, and deposition occurs preferentially in the outer portions forming a skin which impedes further infiltration. It is often necessary to interrupt the process and remove the skin by machining so that the interior of the substrate may be densified. In spite of this limitation, isothermal infiltration is used widely because it lends itself well to simultaneous processing of a great number of parts in large furnaces. It is used for the fabrication of carbon-carbon composites for aircraft brakes and silicon carbide composites for aerospace applications (see Ch. 19). 

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 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 application of ceramics has infiltrated almost all fields in the last 20 years, because of their advantages over metals due to their strong ionic or covalent bonding. But it is just this bonding nature of ceramics that directly results in their inherent brittleness and difficulty in machining. In other words, ceramics show hardly any macroscopic plasticity at room temperature or at low temperatures like metals. Hence, superplasticity at room temperature is a research objective for structural ceramics. In recent years, many researches have been carried out to investigate nanophase ceramic composites. 

The figure also shows that the deposition gradient exists under any conditions. These experimental results agree with the theoretical calculations from the onedimensional model discussed in Equation (5.6). In order to obtain uniformly dense composite material machining is necessary to open the blocked openings for further infiltration. 

Porous tungsten can be infiltrated either to machine it to rigid dimensional tolerances, or to fabricate composite materials. Copper, which is used as infiltrant for machining, can be completely removed afterward by heating in vacuum by evaporation. 

At room temperature the composites behave in a brittle manner. However, they are readily machinable using cemented carbide tools (ISO KIO). The arcing contacts can be clamped, brazed, or directly infiltrated onto the circuit-breaker components (commonly pure copper or copper alloys). 

The composites were fabricated by mixing the inorganic additives with the resin, infiltrating fiber preforms with this mixture, pressing in a hydraulic press, and curing. Test specimens were then machined from the composites. Although mechanical properties of the four new composites were not measured, similar previously fabricated materials had tensile strengths and moduli of approximately 200 MPa and 8 GPa, respectively. 

The gas species are conveyed through the porous preform mainly by diffusion. The driving force is the concentration gradient between the interior and the surface ofthe preform, which reduces the densification rate. When CVl conditions that shorten the densification time are selected non uniform deposition of the matrix is enhanced. Intermediate cycles of surface machining are thus required to open the pores that have been sealed. A few alternative CVl techniques have been proposed to increase the infiltration rate [11]. These techniques require more complicated CVl chambers, and are not appropriate to the production of large or complex shapes, or large quantities of pieces. 

The product is a macro porous open cell foam with a skeletons consisting of p-SiC bound with the initial a-SiC powders and Si. [16] Three batches of Si-SiC foams with different Si content were made. The carbonaceous foam for aU the three types are the same and just the amount of Si used for infiltration changes. Test pieces are cut into bars of (15x25xl70)mm 0.05. The lower and upper surfaces which are in contact with the bending fixtures are machined in order to satisfy the flatness and parallehsm conditions for a bending test. 

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