Ceramic structural elements

Naturally, fibers and whiskers are of little use unless they are bonded together to take the form of a structural element that can carry loads. The binder material is usually called a matrix (not to be confused with the mathematical concept of a matrix). The purpose of the matrix is manifold support of the fibers or whiskers, protection of the fibers or whiskers, stress transfer between broken fibers or whiskers, etc. Typically, the matrix is of considerably lower density, stiffness, and strength than the fibers or whiskers. However, the combination of fibers or whiskers and a matrix can have very high strength and stiffness, yet still have low density. Matrix materials can be polymers, metals, ceramics, or carbon. The cost of each matrix escalates in that order as does the temperature resistance. 

In 1937, dost presented in his book on diffusion and chemical reactions in solids [W. lost (1937)] the first overview and quantitative discussion of solid state reaction kinetics based on the Frenkel-Wagner-Sehottky point defect thermodynamics and linear transport theory. Although metallic systems were included in the discussion, the main body of this monograph was concerned with ionic crystals. There was good reason for this preferential elaboration on kinetic concepts with ionic crystals. Firstly, one can exert, forces on the structure elements of ionic crystals by the application of an electrical field. Secondly, a current of 1 mA over a duration of 1 s (= 1 mC, easy to measure, at that time) corresponds to only 1(K8 moles of transported matter in the form of ions. Seen in retrospect, it is amazing how fast the understanding of diffusion and of chemical reactions in the solid state took place after the fundamental and appropriate concepts were established at about 1930, especially in metallurgy, ceramics, and related areas. 

Since their early development, the geometrical and structural characteristics of ceramic membrane elements have readily changed (Figure 6.2). Originally, they were prepared as single tubes with an inside diameter ranging from 6 to 15 mm and a wall thickness of about 2 mm. These ceramic tubes are still available from some suppliers, but the main handicaps with such... 

FIGURE 6.1 Schematic representation of the multichannel structure of a ceramic membrane element. 

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

These fibers are, due to their high thermal stability, particularly suitable for applications in high temperature thermal insulation and for the manufacture of metal matrix and ceramic matrix composites. They arc clearly superior to metal materials due to their lower weight, particularly in the lightweight construction of accelerated structure elements for which the basic material should represent an improve-... 

For many traditional ceramics such as structural elements (tiles, bricks, etc.), white-wares, (tableware, sanitaryware, etc.), and common refractories, the raw materials are naturally occurring minerals, and moderate levels of impurities are tolerated. More specialized technical ceramics such as electronic ceramics (substrates, electronic packages, capacitors, inductors, etc.) or high performance structural ceramics (silicon carbide, silicon nitride, etc.) demand low or controlled levels of impurities and make use of higher purity powders often made by more specialized techniques. 

The ceramic catalyst is a homogeneous mixture of all components. After mixing with binders, plastifiers, and glass fiber the mass is extruded to honey-comb-structured elements, which are dried and calcined (Fig. 10.1). 

Ceramics, by definition, are composed of at least two elements, and consequently their structures are, in general, more complicated than those of metals. While most metals are face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal close-packed (HCP), ceramics exhibit a much wider variety of structures. Furthermore, and in contrast to metals where the structure is descriptive of the atomic arrangement, ceramic structures are named after the mineral for which the structure was first decoded. For example, compounds where the anions and cations are arranged as they are in the rock salt structure, such as NiO and FeO, are described to have the rock salt structure. Similarly, any compound that crystallizes in the arrangement shown by corundum (the mineral name for AI2O3) has the corundum structure, and so forth. 

Perovskites have also received much attention since 1986 because the superconducting oxide YBCO contains perovskite structural elements. The importance of this structure was again realized in 1993 when the phenomenon of colossal magnetoresistance (CMR) was discovered in a range of manganate ceramics with a layered perovskite structure similar to that found in YBCO and other high-temperature superconductors. 

FIGURE 9.2 Evolution of the geometry of ceramic membrane elements, (a) Conventional cylindrical-shaped channels, (b) Flower-like designed channels, (c) Honeycomb-type structure. 

In aircraft, aU internal textiles such as seating, internal decor, and blankets require defined levels of flame or fire resistance to internationally recognised standard levels. Higher levels of fire and heat resistant textiles and standards are required in engine insulation (e.g. ceramic fabric structures around combustion chambers), reinforcements for composites (e.g. carbon fibre reinforcanents for major structural elements), ara-mid honeycomb reinforcement for wall and floor structures, and fuselage acoustic and fire/heat insulation. 

Figure 8.1 The structural elements of ceramics.Reprinted with permission from Ref [20] 1997, Elsevier Ltd.

Structural elements in brittle ceramics fail with little or no plastic deformation, often without warning. Since brittle fracture may lead to catastrophic results, it has been studied more intensively. Begin by assuming that a sohd is perfect and that it breaks... 

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