Porous ceramic matrices

Perfusion systems have also been used for successful scale-up of MoAb production. During the culture period, cell growth occurs exponentially until the cell density reaches a maximum. At that point, the medium needs a continuous supplementation of fresh nutrients and elimination of waste. In perfusion systems, fresh nutrients are supplied and wastes are removed continuously so that the medium meets the physiological needs of the cells. At steady state, the cell concentration is determined by space and other limitations. High cell densities have been achieved by immobilizing the cells in porous ceramic matrices or hollow fiber devices. Intermediate cell densities have been achieved by perfusion reactors with a spin filter, or in a fluidized bed reactor in which the cells are embedded in sponge-like... 

Evans, W. D., J. R. Howell, and P. L. Varghese. 1991. The stability limits of methane combustion inside a porous ceramic matrix. AIAA Paper No. 91-1966. 

E. E. Lange, F. W. Zok and C. G. Levi, Processing and Properties of Porous Ceramic Matrix Composites, Comprehensive Composite Materials, Vol. 4 Carbon/Carbon, Cement and Ceramic Matrix Composites, Richard Warren, Vol. Ed., Anthony Kelly and Carl Zweben, Editors-in-Chief, Pergamon Press, Elsevier Science Ltd., Oxford, 2000. 

Generally, there are two main types of HA-containing composites under development (i) ceramics reinforced with small particles, discrete or continuous fibres and (ii) biocompatible polymers reinforced with small ceramic particles and satisfy to the aim of the tissue engineering approach, e.g. to be of porous microstructure and of load-bearing capacity [16]. An alternative approach is to introduce a polymer into a porous ceramic matrix to obtain composites with a continuous ceramic skeleton, in contrast to those reported in [16]. The biological and mechanical properties of such ceramic/polymer composites can differ markedly from those of the polymer/ceramic materials. 

Cera.micA.bla.tors, Several types of subliming or melting ceramic ablators have been used or considered for use in dielectric appHcations particularly with quartz or boron nitride [10043-11 -5] fiber reinforcements to form a nonconductive char. Fused siHca is available in both nonporous (optically transparent) and porous (sHp cast) forms. Ford Aerospace manufactures a 3D siHca-fiber-reinforced composite densified with coUoidal siHca (37). The material, designated AS-3DX, demonstrates improved mechanical toughness compared to monolithic ceramics. Other dielectric ceramic composites have been used with performance improvements over monolithic ceramics (see COMPOSITE MATERIALS, CERAMIC MATRIX). 

Infiltration (67) provides a unique means of fabricating ceramic composites. A ceramic compact is partially sintered to produce a porous body that is subsequently infiltrated with a low viscosity ceramic precursor solution. Advanced ceramic matrix composites such as alumina dispersed in zirconia [1314-23-4] Zr02, can be fabricated using this technique. Complete infiltration produces a homogeneous composite partial infiltration produces a surface modified ceramic composite. 

Directed Oxidation of a Molten Metal. Directed oxidation of a molten metal or the Lanxide process (45,68,91) involves the reaction of a molten metal with a gaseous oxidant, eg, A1 with O2 in air, to form a porous three-dimensional oxide that grows outward from the metal/ceramic surface. The process proceeds via capillary action as the molten metal wicks into open pore channels in the oxide scale growth. Reinforced ceramic matrix composites can be formed by positioning inert filler materials, eg, fibers, whiskers, and/or particulates, in the path of the oxide scale growth. The resultant composite is comprised of both interconnected metal and ceramic. Typically 5—30 vol % metal remains after processing. The composite product maintains many of the desirable properties of a ceramic however, the presence of the metal serves to increase the fracture toughness of the composite. 

Molten Carbonate Fuel Cell The electrolyte in the MCFC is a mixture of lithium/potassium or lithium/sodium carbonates, retained in a ceramic matrix of lithium aluminate. The carbonate salts melt at about 773 K (932°F), allowing the cell to be operated in the 873 to 973 K (1112 to 1292°F) range. Platinum is no longer needed as an electrocatalyst because the reactions are fast at these temperatures. The anode in MCFCs is porous nickel metal with a few percent of chromium or aluminum to improve the mechanical properties. The cathode material is hthium-doped nickel oxide. 

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... 

The principle of ceramic matrix modules is the same as for hollow-fibre systems, with the exception that cells are not separated from the medium circulation by a membrane. Cells are held in the highly porous matrix and are supplied with... 

Organic or inorganic entities as well as polymer particles can also be used as template agents in the preparation of porous ceramic membranes following either the polymeric or the colloidal sol-gel route. The strategy to control microstructure in porous material is illustrated in Fig. 7.13. The template agents are trapped during matrix formation and eliminated in a second step with the aim to define the pore size in the final material. 

Chemical vapour infiltration (CVI) is an extension of CVD processes only when a CVD process occurs on an internal surface of a porous substrate (especially for the fibre preform). As compared with CVD, the CVI process for ceramics is much more effective and important because it is the optimal technique to fabricate fibre reinforced ceramics and particularly carbon fibre reinforced carbon and advanced ceramic matrix composites. Both CVI and CVD techniques share some common features in overall chemistry, however, the CVI is much more complex than the CVD process in mass transport and chemical reactions. 

Ceramic coatings on fibers and powders have a variety of uses. For example, porous ceramic coatings on nanoscale metallic or ceramic particles can improve the catalytic properties of a powder. Also, the carbon fibers used as reinforcement in metallic matrices can be coated with a thin ceramic film (such as SiC or TiN) to reduce the rate of interdiffusion that may occur between the matrix materials and the fibers, and enhance the wetting of the fiber surface by metals. ... 

M.K. Cinibulk, K.A. Keller, and T.I. Mah, Effect of yttrium aluminum garnet additions on alumina-fiber-reinforced porous-alumina-matrix composites. J. Am. Ceram. Soc. 87(5), 881-887 (2004). 

Extensive testing of metal sulfide electrode materials was performed in cells of the type shown in Figure 1. The metal sulfide electrode case and current take-off rod were made of dense graphite to avoid any contact and possible contamination of the active material by metallic constituents. The active material (metal sulfide powder in a porous graphite matrix or a mixture of metal sulfide and graphite powder) was contained in the cavity (2.5 cm in diameter X 0.6 cm deep) of the dense graphite electrode case. A porous ceramic separator was placed over the active material and secured with high purity alumina pins. 

The inert ceramic matrix which holds the electrolyte in place between the cathode and the anode serves two purposes first, it holds the electrolyte by capillary action and prevents the molten salts from completely flooding the porous electrodes second, the membrane acts to prevent the bulk diffusion of gases between the cathode and the anode side of the cell. If the electrolyte was not in chemical equilibrium with the process gas, localized density changes in the electrolyte caused by reaction (20) would cause the membrane to crack and allow bulk mixing of the process and sweep gas streams. 

M. Schmiicker, P. Mechnich, All-Oxide Ceramic Matrix Composites with Porous Matrices, in W. Krenkel (ed.) "Ceramic Matrix Composites, Fibre-Reinforced Ceramics and their application" Wiley-VCH, Weinheim, 2008, 205-229... 

Palladium cermets have a number of advantages over thin films of Pd supported by porous ceramics. For systems which wet, sintering cermets at very high temperatures (well above membrane operating temperatures) produces dense, pinhole-free composites (see Fig. 8.5). Because Pd is closely confined within a matrix of ceramic and because small, individual, micron-size Pd crystallites already possess a small surface-to-volume ratio of low surface energy, the Pd has relatively low driving... 

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