Covalent ceramic

The ultimate covalent ceramic is diamond, widely used where wear resistance or very great strength are needed the diamond stylus of a pick-up, or the diamond anvils of an ultra-high pressure press. Its structure, shown in Fig. 16.3(a), shows the 4 coordinated arrangement of the atoms within the cubic unit cell each atom is at the centre of a tetrahedron with its four bonds directed to the four corners of the tetrahedron. It is not a close-packed structure (atoms in close-packed structures have 12, not four, neighbours) so its density is low. 

Fig. 16.3. Covalent ceramics, (a) The diamond-cubic structure each atom bonds to four neighbours.

The first Section of this Chapter deals with predominantly covalent ceramics (Table 7.1), with a particular emphasis on SiC for which a recent detailed study of wetting by liquid metals combined with a study of surface chemistry has been reported. In the second Section, wetting by liquid metals of metal-like ceramics is presented. 

This review of the experimental results for metal/covalent ceramic systems shows that the intrinsic wetting properties in these systems are not well established at... 

At temperatures of 1000°C and above, A1 and Si wet covalent ceramics rather well with contact angles close to 50° for both non-reactive (A1/A1N and Si/SiC) and reactive systems (Al/SiC and Al/BN). This behaviour relates well to theoretical studies indicating the formation of metallic or covalent chemical bonds at the interfaces between A1 or Si and covalent ceramics. The ability of A1 and Si to bond strongly with ceramic surfaces appears to correlate with the degree of covalence (or, equivalently, with the degree of ionicity) of the ceramic, as shown by the data in Table 7.9 for Si on non-reactive solids. A similar tendency is observed for A1 on various solids, including solid Ai considering the metallic bond in solid Al as a homopolar Al-Al bond (Table 7.9). 

The building block of silicon-based covalent ceramics, which include among others the silicates (dealt with separately in the next section) SiC and Si3N4, is in all cases the Si tetrahedron Si04 in the case of silicates, SiC4 for SiC, and SiN4 for Si3N4. The reasons Si bonds tetrahedrally were discussed in the last chapter. 

Misplaced atoms types of atoms found at a site normally occupied by other types. This defect is only possible in covalent ceramics, however, where the atoms are not charged. 

Figure 6.1 Various types of defects typically found in ceramics. Misplaced atoms can only occur in covalent ceramics due to charge considerations.

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... 

The processes occurring during cross linking and ceramization as well as the structure of the preceramic network and the amorphous covalent ceramic depend strongly on the chemical structure of the synthesized precursor molecule. The influence of the type of the polymer molecule, of the catalysts, steric effects of the side groups, bond stren s, as well as radical stabilities are crucial factors. 

Beyond all doubt, the state of the amorphous covalent ceramics forms a specific and, by knowing the preparation route, a more or less exactly definable state which occurs as an intermediate during the transformation of the silicon organic polymer into the final crystalline ceramics. This state appears to be stable at room temperature for a long time. And, of course, materials in this state are distinguished by specific properties different from the state of crystalline ceramics. 

Hitherto we have discussed the formation of amorphous covalent ceramics only on the basis of polymer derived materials. In Sects. 2.2 and 4.2.2.2, thin amorphous, hydrogen stabilized SiC layers (a-SiC H) are also considered which are formed, first of all, by gas phase processes (CVD, PVD). They represent another type of amorphous covalent ceramics. And though it is not expected that properties of such layers agree completely with those of the polymer derived ACC, the basic ideas of stability and transformability of the ACC state discussed above should be transferable to this type of amorphous covalent ceramics, too. 

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. ... 

Misplaced atoms If an atom is present on a crystal site that should be occupied by a different atom, that atom is a misplaced atom and may be called an antisite defect. Antisite defects usually form in covalent ceramics such as AIN and SiC, but can also occur in complex oxides that have several different types of cation, for example, spinels and garnets. (We do not expect to see cations on anion sites and vice versa.)... 

For complex ionic (e.g., AI2O3) and covalent ceramics (e.g., SiC) Tf 1000 MPa 10 p. Dislocations have low mobility and lattice resistance is the main obstacle. 

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