Ionic ceramics

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. 

Fig. 16.1. Ionic ceramics, (a) The rocksalt, or NoCl, structure, (b) Magnesia, MgO, has the rocksalt structure. It can be thought of as an f.c.c. packing with Mg ions in the octahedral holes. ( ) Cubic zirconia ZrOj an f.c.c. packing of Zr with O in the tetrahedral holes, (d) Alumina, AljOj a c.p.h. packing of oxygen with Al in two-thirds of the octahedral holes.

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. 

In ionic ceramics it is not the chemical but the electrochemical potential that defines equilibrium. 

Antistructure disorder or misplaced atoms. These are sites where one type of atom is found at a site normally occupied by another. This defect does not occur in ionic ceramics, but it has been postulated to occur in covalent ceramics like SiC. The notation for such a defect would be Si or C j, and the corresponding defect reaction is... 

Technologically, one important use of ionic ceramics is for potentiometric sensors. These are solids that, by virtue of being predominantly ionic conductors, are capable of measuring the absolute thermodynamic activities of various species. 

From the preceding sections it should be clear that in ceramics we do not usually have pure ionic bonds or pure covalent bonds but rather a mixture of two, or more, different types of bonding. Even so it is still often convenient and a frequent practice to call predominantly ionically bonded ceramics ionic ceramics and predominantly covalently bonded ceramics covalent ceramics. ... 

Climb dissociation occurs when the stacking fault does not lie parallel to the glide plane of the partial dislocations. The phenomenon has not been seen in pure fee metals, but it can occur in intermetallics. It is found in both covalent and ionic ceramics. We can make two comments here ... 

For simple ionic ceramics (e.g., NaCl and CaF2) Tf 10-100MPa lO p-lO- p. 

The importance of perovskites became apparent with the discovery of the valuable dielectric and ferroelectric properties of barium titanate, BaTiOj, in the 1940s. This material was rapidly employed in electronics in the form of capacitors and transducers. In the decades that followed, attempts to improve the material properties of BaTiOj lead to intensive research on the structure - property relations of a large number of nominally ionic ceramic perovskite-related phases with overall compositions ABOj, with a result that vast numbers of new phases were synthesised. 

Intrinsic Stacking fault energy in silicon, 249 Inverse spinels, 285 Ion beam cleaning, 126 effect on hardness, 126-127 Ion implantation, 126-127 in AI2O3, 262-263 Ionic carbides, 294-296 Ionic ceramics, 209 Indium phosphide (InP), hardness, 83 anisotropy, 77... 

Summarizing the above, it can be said that there are three ways the environment can be important in determining the observed hardness of a ceramic first, through zeta potential, and hence surface potential control, by adsorption and chemisorption and hence affecting the energy required for slip through dislocation movement. This will be most obvious with ionic ceramics. Second, through mechanochemical processes as the combined... 

Would you expect Frenkel defects for anions to O exist in ionic ceramics in relatively large concentrations Why or why not ... 

Most ionic ceramics and polymers are insulators at room temperature. Electrical conductivities range between about 10 and 10 (n m) by way of comparison, for most metals, a is on the order of 10 (H-m) . ... 

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