Crystal structures ceramics

In the ceramics field many of the new advanced ceramic oxides have a specially prepared mixture of cations which determines the crystal structure, through the relative sizes of the cations and oxygen ions, and the physical properties through the choice of cations and tlreh oxidation states. These include, for example, solid electrolytes and electrodes for sensors and fuel cells, fenites and garnets for magnetic systems, zirconates and titanates for piezoelectric materials, as well as ceramic superconductors and a number of other substances... 

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. 

Aluminum oxide, A1203, is known almost universally as alumina. It exists with a variety of crystal structures, many of which form important ceramic materials (see Section 14.22). As a-alumina, it is the very hard, stable, crystalline substance corundum impure microcrystalline corundum is the purple-black abrasive known as emery. Some impure forms of alumina are beautiful, rare, and highly prized (Fig. 14.25). A less dense and more reactive form of the oxide is y-alumina. This form absorbs water and is used as the stationary phase in chromatography. 

Amperometric cells, sensors using, 22 271 Amperometric measurements, 14 612 Amphetamine, 3 89-90 Amphibole asbestos, 1 803 3 288 crystal structure, 3 297-298 exposure limits, 3 316 fiber morphology, 3 294-295 silicate backbone, 3 296 Amphibole potassium fluorrichterite, glass- ceramics based on, 12 637 Amphiphile-oil-water-electrolyte phase diagram, 16 427-428 Amphiphile-oil-water phase diagrams,... 

Here again certain trends were observed, and the most influential factor was the crystal structure which the superconducting material adopted. The most fruitful system was the NaCl-type structure (also referred to as the B1 structure by metallurgists). Many of the important superconductors in this ceramic class are based on this common structure, or one derived from it. Other crystal structures of importance for these ceramic materials include the Pu2C3 and MoB2 (or ThSi2) prototypes. A plot of transition temperature versus the number of valence electrons for binary and ternary carbides shows a broad maximum at 5 electrons per atom, with a Tc maximum at 13 K. 

However, at this point we should note that the electron-doped superconductors are not strictly single phase. Electron diffraction studies on both single crystals and ceramic superconducting samples have shown that they almost invariably, contain two phases one phase has the undistorted T -Nd2Cu04 structure,... 

Figure 1.42 The perovskite crystal structure of CaTiOs. From W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics. Copyright 1976 by John Wiley Sons, Inc. This material is used by permission of John Wiley Sons, Inc.

As with metals, ceramic crystals are not perfect. They can contain all of the same types of defects previously described in Sections 1.1.3-1.1.5. What is unique about ceramic crystals, particularly oxide ceramics, is that the concentration of point defects, such as vacancies and interstitials, is not only determined not only by temperature, pressure, and composition, but can be influenced greatly by the concentration of gaseous species in which they come in contact (e.g., gaseous oxygen). The concentration of gaseous species affects the crystal structure, which in turn can affect physical properties such... 

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. 

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