Lecture # 8 Structure and properties of ceramics Application and processing of ceramics Intended learning Outcomes: 1- Structure of ceramic materials. 2- Properties of ceramics and the crystal structure of them. 3-Given the chemical formula for ceramic compound and the ionic radii of its component ions, predict the crystal structure. 4- Impurities in ceramics. 5-Mechanical properties of ceramics. 6- Application and processing of ceramics.

CERAMIC CRYSTAL STRUCTURES  ceramics are composed of at least two elements, and often more, their crystal structures are generally more complex than those for metals.  The atomic bonding in these materials ranges from purely ionic to totally covalent; many ceramics exhibit a combination of these two bonding types, the degree of ionic character being dependent on the electronegativities of the atoms.  Table 3.2 presents the percent ionic character for several common ceramic materials;

 With regard to the first characteristic, the crystal must be electrically neutral; that is, all the cation positive charges must be balanced by an equal number of anion negative charges.  The chemical formula of a compound indicates the ratio of cations to anions, or the composition that achieves this charge balance.  For example, in calcium fluoride, each calcium ion has a 2 charge (Ca2), and associated with each fluorine ion is a single negative charge (F). Thus, there must be twice as many F as Ca2 ions, which is reflected in the chemical formula CaF2  The second criterion involves the sizes or ionic radii of the cations and anions, rC and rA, respectively. Because the metallic elements give up electrons when ionized, cations are ordinarily smaller than anions, and, consequently, the ratio rC/rA is less than unity  All in contact with that cation, as illustrated in Figure 3.4. The coordination number (i.e., number of anion nearest neighbors for a cation) is related to the cation–anion radius ratio. For a specific coordination number, there is a critical or minimum rC/rA ratio for which this cation–anion contact is established (Figure 3.4), which ratio may be determined from pure geometrical considerations.

1-AX-TYPE CRYSTAL STRUCTURES 2-AmXp-TYPE CRYSTAL STRUCTURES

3-AmBnXp-TYPE CRYSTAL STRUCTURES

SILICATE CERAMICS:  Silicates are materials composed primarily of silicon and oxygen, the two most abundant elements in the earth’s crust; consequently, the bulk of soils, rocks, clays, and sand come under the silicate classification. Rather than characterizing the crystal structures of these materials in terms of unit cells, it is more convenient to use various arrangements of an SiO4tetrahedron (Figure 3.10). Each atom of silicon is bonded to four oxygen atoms, which are situated at the corners of the tetrahedron; the silicon atom is positioned at the center. Since this is the basic unit of the silicates, it is often treated as a negatively charged entity.

Coordination # increases with Issue: How many anions can you arrange around a cation? Adapted from Table 12.2, Callister 6e. Adapted from Fig. 12.2, Callister 6e. Adapted from Fig. 12.3, Callister 6e. Adapted from Fig. 12.4, Callister 6e. COORDINATION # AND IONIC RADII

Frenkel Defect -- a cation is out of place. Shottky Defect -- a paired set of cation and anion vacancies. Adapted from Fig , Callister 5e. (Fig is from W.G. Moffatt, G.W. Pearsall, and J. Wulff, The Structure and Properties of Materials, Vol. 1, Structure, John Wiley and Sons, Inc., p. 78.) See Fig , Callister 6e. DEFECTS IN CERAMIC STRUCTURES

Impurities must also satisfy charge balance Ex: NaCl Substitutional cation impurity Substitutional anion impurity IMPURITIES

Room T behavior is usually elastic, with brittle failure. 3-Point Bend Testing often used. --tensile tests are difficult for brittle materials. Determine elastic modulus according to: Adapted from Fig , Callister 6e. MEASURING ELASTIC MODULUS

3-point bend test to measure room T strength. Flexural strength: Typ. values: Si nitride Si carbide Al oxide glass (soda) Adapted from Fig , Callister 6e. Data from Table 12.5, Callister 6e. MEASURING STRENGTH

Elevated Temperature Tensile Test (T > 0.4 T melt ). Generally,... MEASURING ELEVATED T RESPONSE

TYPES OF CERAMICS

FABRICATION OF CERAMIC MATERIALS