Monolith, ceramic structures

Scientists from Politecnico di Milano and Ineos Vinyls UK developed a tubular fixed-bed reactor comprising a metallic monolith [30]. The walls were coated with catalytically active material and the monolith pieces were loaded lengthwise. Corning, the world leader in ceramic structured supports, developed metallic supports with straight channels, zig-zag channels, and wall-flow channels. They were produced by extrusion of metal powders, for example, copper, fin, zinc, aluminum, iron, silver, nickel, and mixtures and alloys [31]. An alternative method is extrusion of softened bulk metal feed, for example, aluminum, copper, and their alloys. The metal surface can be covered with carbon, carbides, and alumina, using a CVD technique [32]. For metal monoliths, it is to be expected that the main resistance lies at the interface between reactor wall and monolith. Corning... 

Initially, packed beds were also used. They, however, were no success, and at present monoliths are applied exclusively. This should not be misunderstood. Monolith means literally a single stone. However, metal-based analogues are also included in the definition of monolith. In fact, for catalytic converters in cars, in addition to ceramics, metal-based monoliths have been and still are used. A major advantage of metal was the thin wall thickness that could be achieved. Later, industry succeeded in manufacturing ceramic structures of comparable wall thickness. In view of their higher resistance against corrosion, ceramic monoliths are now more generally applied than metal ones. 

In the last 10 years, significant advances in fibrous monolithic ceramics have been achieved. A variety of materials in the form of either oxide or nonoxide ceramic for cell and cell boundary have been investigated [1], As a result of these efforts, FMs are now commercially available from the ACR company [28], These FMs are fabricated by a coextrusion process. In addition, the green fiber composite can then be wound, woven, or braided into the shape of the desired component. The applications of these FMs involve solid hot gas containment tubes, rocket nozzles, body armor plates, and so forth. Such commercialization of FMs itself proves that these ceramic composites are the most promising structural components at elevated temperatures. 

Trice, R.W., and Halloran, J.W. (1999), Influence of micro structure and temperature on the fracture energy of silicon nitride/boron nitride fibrous monolithic ceramics , J. Am. Ceram. Soc., 82(9) 2502-2508. 

It should be noted that this review concentrates on thermal shock (i.e. a single thermal cycle) and no attempt is made to incorporate and describe the effects of cyclic thermal loading (cyclic thermal shock, thermal shock fatigue, etc.) on the behaviour of CMCs. For information regarding cyclic thermal loading of ceramics and CMCs the reader is advised to consult the extensive review of Case (2002). Additionally, recent studies have shown that laminated ceramic-metal systems (Sherman, 2001) and layered ceramic-structures (Vandeperre etal, 2001) exhibit better resistance to thermal shock compared with monolithic materials. However, such systems are also beyond the scope of this contribution. 

It is truly remarkable that catalysts can function so well in the exhaust of the modem highspeed vehicle. This fact has raised confidence in industry to use different monolithic (ceramic and metal) structures as supports for catalysts for other environmental applications such as diesel exhausts, power and chemical plants, restaurants, and even on widebody aircraft. 

Compared to washcoating of ceramic structure, washcoating of metallic monolith is more difficult. It is possible to washcoat metallic monoliths with or without a prior oxidation. In the former case, the adhesion of the washcoat layer is better [48-51,53]. 

FIGURE 6.5 Details of the structure of a monolithic ceramic membrane element with filtrate conduits, from CeraMem. 

Melting. A ceramic structure is heated with a pitch carbon in an inert atmosphere, resulting in melting of the pitch and penetration of the pores of the monolith structure with the carbonized pitch. 

The physical form of the support has to be chosen with a view to the type of reactor in which its use is intended. Silica and alumina are available as coarse granules or fine powders, and may be formed into various shapes with the aid of a binder (stearic acid, graphite) they can then be used in fixed bed reactors. For fluidised beds, or for use in liquid media, fine powders are required. Ceramic monoliths having structures resembling a honeycomb are used where (as in vehicle exhaust treatment) very high space velocities have to be used, but they are made of a non-porous material (a-alumina, muUite) and have to have a thin wash-coat of high area alumina applied, so that the metal can be firmly affixed. 

The cellular micro- and macrostructure pseudomorphs with naturally grown wood tissue show a complex mechanical behavior, which is governed hy the unique arrangement of cells. In some aspects, the fracture behavior in biomorphous ceramics is similar to that of fibrous monolithic ceramics, as well as that of laminate composite ceramics showing a noncatastrophic stress-strain behavior. A pronounced anisotropy of fracture behavior is a characteristic feature which depends on the loading conditions with respect to the orientation of the cell-packing structure [357]. 

Ceramic matrix composites (CMCs), in which carbon or ceramic fibers are embedded in a ceramic matrix, have been designed to overcome the intrinsic brittleness of monolithic ceramics with a view toward structural uses at extremely high service temperatures. The most commonly used are carbon (C/C) and SiC matrix composites (C/SiC and SiC/SiC). Ceramic matrix composites with a silica based glass or glass-ceramic matrices have also been studied [12] [53-56]. 

The most studied and applied particulate trap is the wall flow monolith [82-84]. It consists of a ceramic structure with parallel channels, of which half are closed at the upstream end in an alternate, checkerboard manner, and the other half are closed at the downstream end. Thus, exhaust gases are forced to flow through the porous walls, which then act as filters. 

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