Defects in crystalline solids

5 Sintering and Densification (I)—Conventional Sintering Technologies Self-interstitial 

Defects in ceramics can be charged, which are different from those in metais. For a simple pure ionic oxide, with a stoichiometric formula of MO, consisting of a metal (M) with valence of +2 and an oxygen (O) with valence of -2, the types of point defects could be vacancies and interstitials of both the M and O, which can be either charged or neutral. Besides the single defects, it is also possible for the defects to associate with one another to form defect clusters. Electronic defects or valence defects, consisting of quasi-free electrons or holes, are also observed in crystalline solids. If there are impurities, e.g., solute atoms Mf, substitutional or interstitial defects of Mf could be formed, which can also be either charged or neutral. 

Similarly to chemical reactions, the formation of defects or defect reactions can be constmcted according to the following three conservation rules, i.e., (i) mass conservation or mass balance, (ii) electroneutrafily or charge balance, and (iii) site ratio conservation or site balance. 

For instance, if MgO is used to dope AI2O3, because the ionic radii of Mg and Al with coordination number of six are very close, the Mg ions can enter the lattice of AI2O3 to form solid solution as substitutional defects. AI2O3 has the corundum structure, in which one-third of the octahedral sites formed by the close-packed O ions are vacant, so that it is also highly possible for the Mg ions to sit on the interstitial sites. The defects with lower energy are more favorable. In AI2O3, the cation sites and anion sites have a number ratio of 2 3. If substitutional defects are formed, every two Mg atoms on cation sites will replace two A1 sites and two O sites are involved. In this case, the third O site should be a vacancy for site conservation. Therefore, on the basis of mass and site balance, the defect reaction is given by  

When the defects are fully ionized, with conservation of electroneutrality, there is  

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Field-ion Microscopy , Defects In Crystalline Solids Series, 2, North Hoiland Pub Co. (1970) 43) J.A. Swift, Electron Microscopes , Barnes Noble Publ (1970) 44) W.E. Voreck,... 

Figure 1.1 Defects in crystalline solids (a) point defects (interstitials) (b) a linear defect (edge dislocation) (c) a planar defect (antiphase boundary) (d) a volume defect (precipitate) (e) unit cell (filled) of a structure containing point defects (vacancies) and (/) unit cell (filled) of a defect-free structure containing ordered vacancies. ...

Lidiard, A. B., Report on the Conference on Defects in Crystalline Solids held at Bristol University in July, 1954, p. 283, Physical Society, London, 1955. 

Bloembergen, N., Rept. Bristol Conf. on Defects in Crystalline Solids, 1964 p. 1... 

G. Wyon and P. Lacombe, In Defects In Crystalline Solids, p. 187, Physical Society, 1954. 

Figure 2.25. Illustration of the difference between (a) interstitial and (b) substitutional defects in crystalline solids.

S. Andrew, E. R., in Report of the Bristol Conference on Defects in Crystalline Solids. The Physical Society, London, 1954. 

Henderson, B. Defects in crystalline solids. London Edward Arnold 1972... 

Fromhold, A.T. In "Theory of metal oxidation I, series Defects in crystalline solids Amelinckx, S. Gevers, R. Nihoul J., Eds. North Holland Publ. Comp. Amsterdam, 1976 Vol. IX, p. 289. 

For the purposes of the discussion given here, we will only consider point defects in crystalline solids. Our main objective in this section is to provide the semantic backdrop for the remainder of the chapter. Our starting point is the perfect crystal since this is the reference state against which the defected crystal is measured. In fig. 7.9, we provide a visual catalog of some of the key types of point defects that can perturb the uninterrupted regularity of the perfect crystal. 

Ion Implantation Defects in Crystalline Solids" North Holland, Amsterdam, 1973, 8. 

Defects in crystalline solids are important because they modify important properties. For example, just a trace of chromium impurity changes colourless aluminium oxide into ruby. Metals are ductile when linear defects called dislocations are free to move. Crystals dissolve and react at increased rates at points where dislocations intersect the surface of the crystal. Thus, it is necessary to have an idea of the types of defect that form and the role that they play in the control of properties in order to understand the behaviour of solids. 

B. Henderson, Defects in Crystalline Solids, Arnold, London, 1972. 

The principal difference between a dielectric loss experiment and an impedance spectrum is that the former usually utilizes temperature as the independent variable and measurements are made at several fixed frequencies. A typical example of the use of dielectric loss measurements to obtain data about the relaxations of defects in crystalline solids is the paper by Wapenaar et al. [1982], who studied LaFs-doped... 

Defects in crystalline solids occur for structural reasons, because the atoms (or ions) are not arranged ideally in the crystal when all the lattice sites are occupied, and for chemical reasons, because inorganic compounds may deviate from the fixed composition determined by the valence of the atoms. There are different types of structural defects in a crystalline solid which are normally classified into three groups (1) point defects, (2) line defects, and (3) planar defects. Point defects are associated with one lattice point and its immediate vicinity. They include missing atoms or vacancies, interstitial atoms occupying the interstices between atoms, and substitutional atoms sitting on sites that would normally be occupied by another type of atom. These point defects are illustrated in Fig. 7.2 for an elemental solid (e.g., a pure metal). The point defects that are formed in pure crystals (i.e., vacancies and interstitials) are sometimes referred to as intrinsic or native defects. 

The other major defects in crystalline solids occupy much more of the volume in the lattice. They are known as line defects. There are two types viz. edge dislocations and screw dislocations (Figure 1.4). Line defects play an important role in determining crystal growth and secondary nucleation process (Chapter 5). 

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