Dense-matrix composites

One should note that the terms porous-matrix composite and dense-matrix composite are often used to differentiate between families of composite behavior and are used loosely with regard to the actual density of the material. Current technology composites of the porous-matrix type are not fully sintered and have approximately 35-50% matrix porosity/25-40% total porosity (see Table 2 for examples and references). (The composite consists of a volume fraction of fibers,/. The remainder of the volume, (1-/) is considered to be matrix. The term matrix porosity (p ) refers to the pore or void fraction of the matrix, so that, as the fibers are assumed to be fully dense, the composite or total porosity is r= For example if/= 0.4 a 30% matrix porosity would correspond to 18% total... 

The possibility to obtain a uniformly dispersed composite powder was shown for the a-Fe-Al203 system where metal particles with an average size of 55 nm were formed in an amorphous/nano alumina matrix.18 Other studies attempting to obtain dense bulk composites based on the sol-gel route using conventional pressure-assisted sintering ( 1400°C and an applied force of 10 MPa) resulted in a coarse microstructure.16 However, if reaching theoretical density is not a necessary requirement, a porous ceramic microstructure containing nanometer-sized metal particles can be used as a catalytic material.19 Certain combinations of composite materials demand... 

Roeder et al. [14] used dilute mixtures (8 vol-% solids) for the preparation of dense alumina composites with CeC>2-Zr02 and AhCb-platelets. As main alumina powder (the matrix), AKP-30 was used with an average particle diameter of 0.4 pm and a surface area of 6.5 m /g. The resulting specimens were sintered at 1600°C. No further densification behaviour was mentioned. 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

Comparing the type of information obtained on suspensions with that obtained on composites gives useful insight into the types of mechanisms that control creep of ceramic matrix composites. The very large increase in creep resistance of dense particulate composites, i.e., more than 65vol.% particles, suggests that the particle packing density is above the percolation threshold. Creep of particulate composites is, therefore, controlled by direct interparticle contract, as modified by the presence of relatively inviscid matrices. Mechanisms that control such super-threshold creep are discussed in Section 4.5. 

In the early stages of bone formation, the osteons dominate the bone structure to make an overall structure of fiber-matrix composite. While the primary bone has a dense structure, the secondary bone structure is this composite. As a result, the cortical bone structure becomes very complex. It is microscopically porous, has a lamellar structure, and is also a fiber-matrix composite. Size and packing of osteons and canals, and their orientation, determine the mechanical properties of these bones. 

Carbon fibre/BN matrix composites were prepared by dipping the fibre bundles into a solution of (5) followed by pyrolysis under NH3/N2 up to 1200°C. Fig. 3-a shows that oligomer (5) has thoroughly wetted each carbon filament and has not been stripped during pyrolysis. The carbon fibres are distributed in a dense BN matrix, which follows exactly the shape of the bundle (Fig. 3-b). 

The combination of infiltration and reaction that characterizes DMO has been exploited to make a number of composites. As long ago as 1953, it was shown that silica containing refractories were reduced by molten aluminum to form alumina and silicon [4], Subsequently [27], the displacement reaction was extended to the formation of composites of alumina with residual Al-Si. More recently, the Al-Si02 displacement reaction has been used in the infiltration of dense preforms of silica [28] and mullite [29,30] by molten aluminum. Extension of the reactive infiltration process to porous silica-containing preforms [31,32] has resulted in the fabrication of metal-matrix composites in which the silica was replaced by a mixture of about 65% alumina and 35% metal, while the pores were infiltrated by molten alloy. In contrast to DMO, the displacement reaction appears to proceed at a critical temperature of 1100-1200°C and without the need for a volatile solute element or oxygen. Borosilicate glass has also been used as an initiator to enable the infiltration of Al-Si alloys into alumina preforms [33]. 

F. Olevsky, P. Mogilevsky, E. Y. Gutmanas, and I. Gotman, Synthesis of in situ TiB2/TiN ceramic matrix composites from dense BN-Ti and BN-Ti-Ni powder blends, Metall. Mater. Trans. 1996, 27A, 2071-2079. 

Fabrication by Liquid Silicon Infiltration (reaction bonding) (LSI) A leading candidate for use in industrial gas turbine engine is a SiC matrix composite named toughened Silcomp [175]. It is produced by melt infiltration of molten silicon into a porous preform containing carbon as well as BN-coated SiC fibers (e.g. Textron SCS - 6). The composites thus produced consist of a fully dense matrix of SiC + Si, reinforced with continuous SiC fibers. Moreover, the melt infiltration process is net shape and fast. Ultimate strength and strain at ultimate strength are 220 MPa and 0.8 /o, respectively at room temperature (LSI-SiC/SiC Si). 

As with all materials that have been recently developed, the health risks associated with SiC whiskers are not well known however, because their sizes and shapes are similar to that of asbestos, therefore they are considered hazardous. Airborne dispersion and subsequent inhalation are the most serious health hazards. However, with proper worl lace handling requirements and procedures, large quantities of SiC whiskers can be safely processed. No release of whiskers has been observed from dense ceramic matrix composites during fracture or wear processes. Material Safety Data Sheets (MSDS) are packaged with all products containing loose whiskers and the directions in the data sheets should be followed. At the present time, the American Society for Testing of Materials (ASTM) has developed procedures and handling practice standards for SiC whiskers [14]. 

The mechanical behavior of porous-matrix composites has been modeled, but the compromises between matrix properties and toughness are not yet fully quantified. Initial estimates indicated that a matrix porosity of 30% is needed for crack deflection within a porous matrix [4, 43]. Subsequent efforts have been aimed at better quantifying the transition between porous and dense CMC failure behavior [44] however, further investigation is essential, especially considering the temperature dependent nature of the porous microstructure. 

Matrix porosity on the order of 40% leads to overall porosity near 25%, resulting in the relatively low composite densities shown in Table 2. Specific surface area measurements of the GEN-IV composite show 30 m /g. This is 100 times greaterthan dense matrix CMC s and is indicative of fine interconnected porosity [30]. Such high matrix porosity is undesirable where hermetic fluid containment is required and also leads to concerns with use in erosive environments [76]. 

A. R. Bhatti and P. M. Farries, Preparation of Long-fiber-reinforced Dense Glass and Ceramic Matrix Composites, in Carbon/carbon, Cement and Ceramic Matrix Composites, R. Warren, Ed., Elsevier Science Ltd., Oxford, (2000). p. 645-667. 

Hot-pressing of infiltrated fibre tapes or fabric lay-ups is the most extensively used technique to fabricate dense fibre-reinforced glass matrix composites. Figure 5 shows a... 

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