Defects in ceramics

The need for reliable estimation of a defect in ceramic products of such a complicated structure as the grinding wheel (Fig. 5.22) follows from the mode of its use, since it can be dangerous to those in its vicinity. At the same time, the cost of the product and the requirement of 100% inspection... 

Trocha and Koros [1994] applied a diffusion-controlled caulking procedure with colloidal silica to plug large pores or defects in ceramic membranes. An important feature of this proc ure is the proper selection of the colloid particle size to eliminate or reduce large, less selective pores while minimizing deposition in the small desirable pores. The technique has b n successfully proven with anodic alumina membranes (having the majority of the pores about 2(X) nm in diameter) using 10-20 nm silica colloids. 

Over the years, several schemes have been proposed to denote defects in ceramics. The one that is now used almost universally is the Kroger-Vink notation and is thus the one adopted here. In this notation, the defect is represented by a main symbol followed by a superscript and a subscript. 

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... 

The really special feature of ceramics is that they can contain charged defects metals cannot. There are some definite differences when we compare defects in ceramics to similar ones in face-centered cubic (fee) metal crystals such as Cu. 

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,... 

The diffusion equations are summarized by Pick s laws. One assumption that is explicit in the derivation of these equations is that there is a well-defined value of D. We will see Pick s first law later that because of the high concentrations of defects in ceramic materi- Pick s second law als, there will be many important situations in... 

The structures of all defects in ceramic materials can be more complex than those of similar defects in the more thoroughly studied pure metals because of the presence of two or more ionic species on two sublattices. The implica-... 

Example 1 GBs are probably the most important defect in ceramic superconductors. They can dramatically... 

Uematsu K (2014) Processing defects in ceramic powdCTs and powder compacts. Adv Powder Technol 25 154-162... 

Uematsu K (1996) Immersion microscopy for detailed characterization of defects in ceramic powders and green bodies. Powder Technol 88 291-298... 

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. 

Tsurekawa, S., Kurishita, H., and Yoshinaga, H. (1989) Lattice Defects in Ceramics (eds T. Suzuki and S. Takeuchi), Japanese Journal of Applied Physics, Tokyo, p. 47. 

The theories outlined above are based upon a through-thickness crack in a large plate (see Figure 14.3). Defects in ceramics, however, are typically in the form of surface... 

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