Point Defects in Ceramics

Vacancies, interstitials, and substitutional defects can all be charged. The special point defect in ceramics is the charged vacancy. Frenkel and Schottky defects are overall neutral. 

There are several ways that we can create point defects in ceramics. We have seen already that point defects can be produced in nonstoichiometric oxides, such as ZnO,... 

Inasmuch as electrical charges are associated with atomic point defects in ceramic materials, defects sometimes occur in pairs (e.g., Frenkel and Schottky) in order to maintain charge neutrality. 

We have discussed point defects in elements (A) and in nearly stoichiometric compounds having narrow ranges of homogeneity. Let us extend this discussion to the point defect thermodynamics of alloys and nonmetallic solid solutions. This topic is of particular interest in view of the kinetics of transport processes in those solid solutions which predominate in metallurgy and ceramics. Diffusion processes are governed by the concentrations and mobilities of point defects and, although in inhomogeneous crystals the components may not be in equilibrium, point defects are normally very close to local equilibrium. 

Point defects play a central role in the use of zirconia ceramics in such applications as oxygen sensors and fuel cells. As a result, point defects in these materials have been extensively studied. 

After this brief introduction to defects and their notation, it is pertinent to ask why point defects form in the first place. However, before the more complicated case of defects in ceramics is tackled in Sec. 6.2.3, the simpler situation involving vacancy formation in elemental crystals such as Si, Ge or pure metals is treated. 

At this point, the slightly more complicated problem of defects in ceramics is dealt with. The complications arise because, as noted above, the charges on the defects preclude their forming separately — they always form in bunches so as to maintain charge neutrality. In the following section, defect formation in ceramics is dealt with by writing down balanced-defect... 

Diffusion of oxygen in oxidation scales occurs along grain boundaries. Corrosion of metals is controlled by formation and diffusion of point defects in the ceramic. Corrosion of polycrystalline ceramics also occurs most quickly along grain boundaries. 

This chapter is one of the most basic in ceramics and for the student, the easiest to leam. You need to know the different types of point defects and their names, the thermodynamic principles leading to the calculation of point defect concentrations, and how point defects make diffusion possible. The next level of complexity concerns how point defects interact with one another. We touch on much of this in the examples of real materials where we bring in the fact that the point defects are often (usually) charged. The importance of the topic is that point defects not only affect the properties of materials but, in many cases, determine and control the properties that interest us. An interesting question is how does this discussion change for nanoceramics—are point defects in nanoceramics important The initial answer is that the surface (a two-dimensional defect) dominates everything when the particle is small, but one point defect in a nanoparticle can be a very high concentration ... 

In this chapter we examine four key properties of ceramic materials all of which we can classify as optical. (1) Ceramics can be transparent, translucent, or opaque for one particular composition. (2) The color of many ceramics can be changed by small additions additives, dopants, or point defects. (3) Ceramics can emit light in response to an electric field or illumination by light of another wavelength. (4) Ceramics can reflect and/or refract light. We will discuss why these effects are special for ceramics and how we make use of them. 

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. 

Geng, L. and Yang, W. (2006) Agglomeration of point defects in ferroelectric ceramics under cyclic electric field. Moded. Simul. Mater. Sci. Eng., 14, 137-155. 

To summarize. Fig. 3.5 presents all the above point defects in one figure, showing the possible configurations in ceramics. 

Point Defects in Amorphous Ceramics and Their Strengthening (Effect)... 

Most of our discussion will be confined to point defects in ionic solids (ceramics). Line defects, commonly referred to as dislocations, are characterized by displacements in the periodic structure of the lattice in certain directions. They play their most important role in the plastic deformation of metals. Planar defects include free surfaces, grain boundaries, stacking faults, and crystallographic shear planes. 

To understand the behavior of electronic ceramics, we need to understand the relationship between the observed material properties and the underlying physical phenomena responsible for those properties. For example, the presence of oxygen vacancy point defects in Zr02 ceramics leads to their use as oxygen sensors in... 

It has been demonstrated that the classical equihbiimn defect chemical concepts derived for binary compounds can be apphed to ternary and multinary compoimds. In the case of multicomponent materials, the space charge effects will become very important in cases in which the dimensions are no longer large compared with the thickness of the space charge layers, as in extremely thin films or in stractirral and functional ceramics with crystallites of nanometer dimensiorts. The formation of latent images in silver halide photography represents a prelude to effects of point defects in nanostractured materials, and is related to enlarged concentrations of point defects in botmdary layers. 

WS Williams. Resistivity as tool for characterizing point defects in nonstoichiomelric metallic ceramics. Mater Res Soc Symp Proc 411 169, 1996. 

---