Melt infiltration

At the start of the melting process, the pressure in the channel is relatively low and the solid bed may not be fully compacted. In this case, molten resin from all films has the ability to flow into the voids between the individual pellets. This process is often referred to as melt infiltration. A photograph of a cross section of a Maddock solidification experiment at the start of the melting process is shown by Fig. 6.21. For this figure, the molten material prior to the solidification was black. Melt infiltration is shown by the black resin that has flowed from the films and in between the pellets. The flow of resin into the solid bed will likely cause the pressure in the films to decrease. 

Figure 6.21 Photograph of a cross section from a Maddock solidification experiment at the start of the melting process. The black material shows the melt films and the regions where the melt infiltrated the loosely packed solid bed...

Let us consider two hypothetical phases in our composite, A and B, without specifying their physical state. They conld be a polymer melt and a glass fiber reinforcement during melt infiltration processing, a metal powder and ceramic powder that are being snbjected to consolidation at elevated temperatnre and pressure, or two immiscible polymer melts that will be co-extruded and solidified into a two-phase, three-dimensional object. In any case, the surface that forms between the two phases is designated AB, and their individual surfaces that are exposed to their own vapor, air, or inert gas (we make no distinction here) are labeled either A or B. The following three processes are defined as these surfaces interact and form ... 

In the general case of a resin filling a fiber-packed mold, snch as is fonnd in melt infiltration (see Fignre 4.14), flnid flow is equivalent to flow throngh porons... 

Figure 4.14 Schematic diagram of the melt infiltration process for flow past a unidirectional fiber assembly.

Whiskers can be incorporated into the metallic matrix using a number of compositeprocessing techniques. Melt infiltration is a common technique used for the production of SiC whisker-aluminum matrix MMCs. In one version of the infiltration technique, the whiskers are blended with binders to form a thick slurry, which is poured into a cavity and vacuum-molded to form a pre-impregnation body, or pre-preg, of the desired shape. The cured slurry is then fired at elevated temperature to remove moisture and binders. After firing, the preform consists of a partially bonded collection of interlocked whiskers that have a very open structure that is ideal for molten metal penetration. The whisker preform is heated to promote easy metal flow, or infiltration, which is usually performed at low pressures. The infiltration process can be done in air, but is usually performed in vacuum. 

A great advantage of any silica-based glass is its ease of fabrication, which allows processes such as melt infiltration and compression molding to be used. 

It depends on the nature of the active phase, whether the hydrogenated form (i.e., metal hydride) or the dehydrogenated form (i.e., metal) is more suitable for the synthesis. In the first case, it has to be considered that hydrides can decompose before or during the melting process (e.g., NaAlHJ, so it might be necessary to conduct the infiltration process under hydrogen pressure. For a successful melt infiltration, the following key requirements should be met ... 

Ylv = 0.581 N m and rm = 667°C), the capillary pressure reaches up to 1000bar for a pore radius of 1 nm. Obviously, capillary forces of such magnitude can strongly enhance the infiltration process. Since pore filling is only promoted in the case of 0 < 90°, specific surface modification might be essential for a successful melt infiltration process. 

Figure 23 Chondrite-normalized abundances of REEs in representative harzburgites from the Oman ophiolite (symbols—whole-rock analyses), compared with numerical experiments of partial melting performed with the Plate Model of Vemieres et al. (1997), after Godard et al. (2000) (reproduced by permission of Elsevier from Earth Planet. Set Lett. 2000, 180, 133-148). Top melting without (a) and with (b) melt infiltration. Model (a) simulates continuous melting (Langmuir et al., 1977 Johnson and Dick, 1992), whereas in model (b) the molten peridotites are percolated by a melt of fixed, N-MORB composition. Model (b) is, therefore, comparable to the open-system melting model of Ozawa and Shimizu (1995). The numbers indicate olivine proportions (in percent) in residual peridotites. Bolder lines indicate the REE patterns of the less refractory peridotites. In model (a), the most refractory peridotite (76% olivine) is produced after 21.1% melt extraction. In model (b), the ratio of infiltrated melt to peridotite increases with melting degree, from 0.02 to 0.19. Bottom modification of the calculated REE patterns residual peridotites due to the presence of equilibrium, trapped melt. Models (c) and (d) show the effect of trapped melt on the most refractory peridotites of models (a) and (b), respectively. Bolder lines indicate the composition of residual peridotites without trapped melt. Numbers indicate the proportion of trapped melt (in percent). Model parameters...

As it provides easy access to a variety of metasomatized mantle rocks, the Lherz massif has been recently the focus of detailed geochemical studies—as well as the source of debates—concerning melt infiltration and melt-rock interaction processes in wall rocks of... 

Smith D., Griffin W. L., Ryan C. G., and Sie S. H. (1991) Trace element zonation in garnets from the thumb heating and melt infiltration below the Colorado Plateau. Contrib. Mineral. Petrol. 107, 60-79. 

The results of practical tests with glasses and slags indicate that with porous refractories melt infiltration takes place which may change the mineralogical (phase) composition in zones even quite distant from the interface. If the melt penetrates to points of lower temperature, reactions with the melt may create new minerals, in the form of quite distinct zones parallel to the surface. The infiltration can be effected not only through pores but also by faster dissolution of the finer bond between the Coarser grog grains. Such a transformed surface layer may cause faster destruction. 

The simplest corrosion test is based on placing the respective melt into a hole drilled in a brick of the respective refractory. After thermal exposure, the brick is cut cross-wise and the depth of melt infiltration, enlarging of the hole or other signs of corrosive attacks at the interface are assessed on the cross section. 

Compared with conventional techniques for fabricating ceramic-matrix composites, such as hot pressing (HP), reactive melt infiltration (RMI) and polymer impregnation and pyrolysis (PIP), CVI techniques have distinct advantages, which can be summarised as follows [8, 9] ... 

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