Structures of Ceramics

The Solid State, however, kept its grains of microstructure coarsely veiled until X-ray diffraction pierced the Crystal Planes That roofed the giddy Dance, the taut Quadrille Where Silicon and Carbon Atoms will Link Valencies, four-figured, hand in hand With common Ions and Rare Earths to fill The lattices of Matter, Glass or Sand With tiny Excitations, quantitatively grand. 

The previous chapter dealt with how atoms form bonds with one another. This chapter is devoted to the next level of structure, namely, the arrangement of ions and atoms in crystalline ceramics. This topic is of vital importance because many properties, including thermal, electrical, dielectric, optical, and magnetic ones, are quite sensitive to crystal structures. 

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. 

is the most common of the binary structures, with over one-half of the 400 compounds so far investigated having this structure. In this structure, the coordination number (defined as the number of nearest neighbors) for both cations and anions is 6. In the CsCl structure (Fig. 3.1 ), the coordination number for both ions is 8. ZnS exists in two polymorphs, namely, the zinc blende and the wurtzite structures shown in Fig. 3.1r and d, respectively. In these structures the coordination number is 4 that is, all ions are tetrahedrally coordinated. 

One critical distinction should be drawn right at the beginning. It is that between predominantly ionic ceramics and those which are predominantly covalent in their bonding. 

We will first examine the simple structures given by ionic and covalent bonding, and then return to describe the microstructures of ceramics. 

The archetype of the ionic ceramic is sodium chloride ( rocksalt ), NaCl, shown in Fig. 16.1(a). Each sodium atom loses an electron to a chlorine atom it is the electrostatic attraction between the Na ions and the CF ions that holds the crystal together. To achieve the maximum electrostatic interaction, each Na has 6 CF neighbours and no Na neighbours (and vice versa) there is no way of arranging single-charged ions that does better than this. So most of the simple ionic ceramics with the formula AB have the rocksalt structure. 

Magnesia, MgO, is an example (Fig. 16.1b). It is an engineering ceramic, used as a refractory in furnaces, and its structure is exactly the same as that of rocksalt the atoms pack to maximise the density, with the constraint that like ions are not nearest neighbours. 

This packing argument may seem an unnecessary complication. But its advantage comes now. Consider cubic zirconia, ZrOj, an engineering ceramic of growing importance. The structure (Fig. 16.1c) looks hard to describe, but it isn t. It is simply an f.c.c. packing of zirconium with the ions in the tetrahedral holes. Since there are two tetrahedral holes for each atom of the f.c.c. structure, the formula works out at ZrOj. 

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Catalytic A catalytic-membrane reactor is a combination heterogeneous catalyst and permselective membrane that promotes a reaction, allowing one component to permeate. Many of the reactions studied involve H2. Membranes are metal (Pd, Ag), nonporous metal oxides, and porous structures of ceramic and glass. Falconer, Noble, and Sperry [in Noble and Stern (eds.), op. cit., pp. 669-709] review status and potential developments. 

Tubular composite (X-AI2O3 -based supports for Pd-containing metal membrane have been developed. Their distinction consists in using metal nickel for the modification of the porous structure of ceramic supports. Nickel is analog of palladium in many respects it is also effective catalyst for molecular hydrogen... 

SUPERCHAINS AND PLANES. The atomic structure of ceramic superconductors contains chains and planes of copper and oxygen atoms. Both chains and planes appear to contribute to high-temperature superconductivity in some of the new materials. (Courtesy Argonne National Laboratory.)... 

The porous structure of ceramic supports and membranes can be first described using the lUPAC classification on porous materials. Thus, macroporous ceramic membranes (pore diameter >50 nm) deposited on ceramic, carbon, or metallic porous supports are used for cross-flow microfiltration. These membranes are obtained by two successive ceramic processing techniques extrusion of ceramic pastes to produce cylindrical-shaped macroporous supports and slip-casting of ceramic powder slurries to obtain the supported microfiltration layer [2]. For ultrafiltration membranes, an additional mesoporous ceramic layer (2 nm<pore diameter <50 nm) is deposited, most often by the solgel process [11]. Ceramic nanofilters are produced in the same way by depositing a very thin microporous membrane (pore diameter <2 nm) on the ultrafiltration layer [4]. Two categories of micropores are distinguished the supermicropores >0.7 nm and the ultramicropores <0.7 nm. 

Describe the structure of ceramic materials and the ways in which they are formed (Section 22.2). 

The modelling of gas permeation has been applied by several authors in the qualitative characterisation of porous structures of ceramic membranes [132-138]. Concerning the difficult case of gas transport analysis in microporous membranes, we have to notice the extensive works of A.B. Shelekhin et al. on glass membranes [139,14] as well as those more recent of R.S.A. de Lange et al. on sol-gel derived molecular sieve membranes [137,138]. The influence of errors in measured variables on the reliability of membrane structural parameters have been discussed in [136]. The accuracy of experimental data and the mutual relation between the resistance to gas flow of the separation layer and of the support are the limitations for the application of the permeation method. The interpretation of flux data must be further considered in heterogeneous media due to the effects of pore size distribution and pore connectivity. This can be conveniently done in terms of structure factors [5]. Furthermore the adsorption of gas is often considered as negligible in simple kinetic theories. Application of flow methods should always be critically examined with this in mind. 

The crystal structure of ceramics, i.e., the periodic arrangement of atoms in space, ranges from simple to complex. The complexity arises from the occurrence of both covalent and ionic bonding and from the diverse compositions of ceramics, especially when multiple cations or anions are present. In order of... 

Figure 2. Crystal structures of ceramic high Tc superconductors. (a) K2NiF4 structure showing layers of distorted octahedra (4+2). (b) YBa2Cu307 structure with nearly...

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