The crystal structure of ceramics

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... 

In this section, the crystal structures of ceramics (confined to oxides for simplicity) and the impHcations of crystal geometry to dislocation behavior will be examined. 

Frequently, the crystal structure of ceramics is more complex than that of metals. Even an elementary ceramic, like diamond, does not crystallise in the cubic or hexagonal structure typical of metals. Because carbon in diamond is covalently bound with a valency of 4, each carbon atom has four nearest neighbours. A unit cell of the forming three-dimensional network is shown in figure 1.13. As can be seen, the structure of the diamond lattice is cubical, but it is not a Bravais lattice because it does not look the same from each atomic site. 

In compound materials - in the ceramic sodium chloride, for instance - there are two (sometimes more) species of atoms, packed together. The crystal structures of such compounds can still be simple. Figure 5.8(a) shows that the ceramics NaCl, KCl and MgO, for example, also form a cubic structure. Naturally, when two species of atoms are not in the ratio 1 1, as in compounds like the nuclear fuel UO2 (a ceramic too) the structure is more complicated (it is shown in Fig. 5.8(b)), although this, too, has a cubic unit cell. 

All of the steps preceding "Shape", step 10, Figure 1, are intended to make magnesium oxide and zirconia particles smaller and to mix them evenly. Figure 2 contains an illustration of what a classical "well mixed" pre-ceramic mixture might look like. Figures 3 and 4 are represenations of the crystal structures of magnesium oxide and zirconium (IV) oxide. 

Glass-Ceramics Based on Silicate Crystals. The principal commercial glass-ceramics fall into this category. These can be grouped by composition, simple silicates, fluorosilicates, and aluminosilicates, and by the crystal structures of these phases. 

Rutile Ceramic Pigments. Structurally, all rutile pigments are derived from the most stable titanium dioxide structure, ie, rutile. The crystal structure of rutile is very common for AX2-type compounds such as the oxides of four valent metals, eg, Ti, V, Nb, Mo, W, Mn, Ru, Ge, Sn, Pb, and Te as well as halides of divalent elements, eg, fluorides of Mg, Mn, Fe, Co, Ni, and Zn. 

The interatomic bonds that produce the crystalhne structures of minerals are briefly discussed first. This is followed by general mles used in constmcting models of crystal structures of phosphate minerals, then the crystal structures of orthophosphate mineral forms. The discussion is brief because the emphasis of the book is on practical aspects of novel phosphate ceramics and cements. Readers interested in more details are referred to Corbridge et al. [1] and Kanazawa [2]. 

To understand why ceramics have particular structures and why certain defects form in these structures, it is really important to understand Pauling s rules. These rules require you to visualize a tetrahedron and an octahedron and to see how they fit together. To understand properties such as piezoelectricity or the mechanisms of phase transformations, you must be able to visualize the crystal structure of the material. This is particularly important when we want to predict the properties of single crystals. We summarize the features of crystallography that we use throughout the text and give references to more specialized resources for rigorous proof of theorems and more detailed discussion. 

Thus, this contribution is aimed at the state of the art in boride ceramics with their problems in densification, microstructural peculiarities and exceptional mechanical properties. Starting with the unique interaction of metallic, covalent and ionic types of bonding and the crystal structures of technically important compounds, phase diagrams will be presented as far as they are of technical interest. The major part consists of the description of the synthesis and properties of ceramics and cermets, reflecting the development of suitable sintering procedures and the consequent improvement of the thermal and mechanical properties. 

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