Ceramics defects

In contrast to pure metals and elemental crystals for which point defects are rather straightforward to describe (because only one type of atom is involved and charge neutrality is not an issue), the situation in ceramics is more complex. One overriding constraint operative during the formation of ceramic defects is the preservation of electroneutrality at all times. Consequently, the defects occur in neutral bunches and fall in one of three categories ... 

Table 14.1 Expressions for stress intensity factor for common ceramic defect types. Internal penny-shaped defect ...

One overriding constraint operative during the formation of ceramic defects is the preservation of electrical neutrality. All the point defects fall in three categories. They are discussed in the following text. 

Addresses bonding, structures of ceramics, defects in ceramics, and ceramics microstructures... 

Both anatase and mtile are broad band gap semiconductors iu which a fiUed valence band, derived from the O 2p orbitals, is separated from an empty conduction band, derived from the Ti >d orbitals, by a band gap of ca 3 eV. Consequendy the electrical conductivity depends critically on the presence of impurities and defects such as oxygen vacancies (7). For very pure thin films, prepared by vacuum evaporation of titanium metal and then oxidation, conductivities of 10 S/cm have been reported. For both siugle-crystal and ceramic samples, the electrical conductivity depends on both the state of reduction of the and on dopant levels. At 300 K, a maximum conductivity of 1 S/cm has been reported at an oxygen deficiency of... 

Processing is key to the reproducible manufacture of ceramics. The tolerance of a finished ceramic to defects determines the raw materials selected, and the control that must be exercised during processing. More expensive advanced ceramics require higher quaUty, more expensive raw materials coupled with more carefully controlled manufacturing processes. 

The piopeities of a ceramic material that make it suitable for a given electronic appHcation are intimately related to such physical properties as crystal stmcture, crystallographic defects, grain boundaries, domain stmcture, microstmcture, and macrostmcture. The development of ceramics that possess desirable electronic properties requires an understanding of the relationship between material stmctural characteristics and electronic properties and how processing conditions maybe manipulated to control stmctural features. 

Semiconducting Ceramics. Most oxide semiconductors are either doped to create extrinsic defects or annealed under conditions in which they become non stoichiometric. Although the resulting defects have been carefully studied in many oxides, the precise nature of the conduction is not well understood. Mobihty values associated with the various charge transport mechanisms are often low and difficult to measure. In consequence, reported conductivities are often at variance because the effects of variable impurities and past thermal history may overwhelm the dopant effects. 

In the production of ceramic ware the shape of the ware must be retained after drying and the ware must be free from cracks and other defects. Controlled drying helps to minimize defects. In general, clays containing moderate amounts of nonclay minerals are easier to dry than those composed whoUy of clay minerals. Furthermore, clays composed of iUite, chlorite, and kaolinite are relatively easier to dry than those composed of montmorillonite. 

Other Ceramic Calcium Phosphate Materials. Other ceramic calcium phosphate materials for repairing bony defect iaclude p-tricalcium phosphate (P-TCP) [7758-87-4], P-Ca2(PO, and biphasic calcium phosphate (BCP) ceramics which consist of both P-TCP and HA. Unlike ceramic HA, P-TCP resorbs ia the tissue (293). The in vivo dissolution of BCP ceramic implants was shown (296) to iacrease with increasing P-TCP/HA ratio ia the implants. Both P-TCP and BCP can lead to new bone growth to various extents depending on the appHcations and the type of materials used (293,296). 

Many inorganic solids lend themselves to study by PL, to probe their intrinsic properties and to look at impurities and defects. Such materials include alkali-halides, semiconductors, crystalline ceramics, and glasses. In opaque materials PL is particularly surface sensitive, being restricted by the optical penetration depth and carrier diffusion length to a region of 0.05 to several pm beneath the surface. 

The work on colour centres outlined in Section 3.2.3.1, much of it in the 1930s, and its consequences for understanding electrically charged defects in insulating and semiconducting crystalline materials, helped to stimulate ceramic researches in the electrical/electronic industry. The subject is enormous and here there is space only for a cursory outline of what has happened, most of it in the last 80 years. 

The microstmcture of conventional ceramics contains flaws readily visible under optical microscopes the microstmcture of advanced ceramics is far more uniform and typically is examined for defects under electron microscopes capable of magnifications of 50,000 times or more. 

As with advanced ceramics, chemical reactions play a cracial role in the fabrication of ceramic composites. Both defect-free ceramic fibers and optimal chemical bonds between fiber... 

Given the potential future importance of ceramics in areas as diverse as electronics (see Chapter 4), machine tools, heat engines, and superconductors (see Chapter 4), the United States can ill afford to surrender technical leadership to its competitors. The dominant trend in the field is toward materials with finer microstractures, fewer defects, and better interactions at interfaces (particularly in composites). Chemical processes provide important tools to capture the promise of ceramics for the benefit of our society and to maintain our international competitive position in technology. 

Advanced materials systems based on polymers, ceramics, and composites are constmcted by assembling components to create stmctures whose properties and performance are determined by the form, orientation, and complexity of the composite stmcture. The properties of these assemblages are determined not by the sum of weighted averages of the components but rather by synergistic effects in intercoimected phases. For this reason, the study of fabrication of hierarchical assemblages of materials, as well as the study of mechanisms for repairing defects in assembled stmctures, must be supported by fundamental research. 

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