Why Study Structures And Properties Of Ceramics?

Some of the properties of ceramics may be explained by their structures. For example: (a) The optical transparency of inorganic glass materials is due, in part, to their noncrystallinity (b) the hydroplasticity of clays( i.e., development of plasticity upon the addition of water) is related to interactions between water molecules and the clay structures (c) the permanent magnetic behaviors of some ceramic materials are explained by their crystal structures .

Ceramic Structures:

Because 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 12.1 presents the percent ionic character for several common ceramic materials; these values were determined using Equation below and the electronegativities in Figure 2.7.

1

Electronegativity a measure of how willing atoms are to accept electrons (subshells with one electron low electronegativity; subshells with one missing electron high electronegativity). Electronegativity increases from left to right. The atomic bonding in ceramics is mixed, ionic and covalent, the degree of ionic character depends on the difference of electronegativity between the cations (+) and anions (-).

Ceramic Crystal Structures:

ceramics that are predominantly ionic in nature have crystal structures comprised of charged ions, where positively charged (metal) ions are called cations, and negatively charged (non-metal) ions are called anions. The crystal structure for ceramics depends upon two characteristics:

1. the magnitude of electrical charge on each component ion, recognizing that the over all structure must be electrically neutral.

2. the relative size of the cation(s) and anion(s),which determines the type of interstitial site(s) for the cation(s) in an anion lattice.

Stable ceramic crystal structures: anions surrounding a cation are all in contact with that cation. For a specific coordination number there is a critical or minimum cation-anion radius ratio rC/rA for which this contact can be maintained.

2

- -

- -+

stable

- -- -+

unstable

- -- -+

stable

The structures of ceramics fall into two main groups:

1- Simple crystal structures:

containing ionic or covalent bonds, or a mixture of the two. Examples are magnesium oxide (used for furnace linings) which is an ionic compound with cubic structure, and silicon carbide, with covalent bonds and a tetrahedral structure similar to that of diamond. Alumina has a close packed hexagonal structure, with a mixture of covalent and ionic bonds, with one-third of the potential aluminum sites vacant in order to satisfy the valency requirements of the two elements.

2- Complex silicate structures:

The majority of ceramic materials, in particular those derived from clay, sand or cement, contain the element silicon in the form of silicates. The arrangements are many, involving both chains of silicate ions (SiO4)-2, double chains and links in sheet form. With the last of these, found in clays, cross-linking between adjacent sheets occurs when the clay is baked.

AX-type crystal structure:

1-Cesium Chloride (CsCl) structure:

CN = 8 (NOT a BCC structure) (2 interpenetrating simple cubics). (Cl -) anions at corners of unit cube, (Cs+) cations at centers of unit cube (we can think of this as a simple cubic structure in which 2 atoms associated with each lattice point).

3

2- Rock Salt (NaCl) structure:

Sodium chloride (NaCl) is the most common Rc/Ra =0.414-0.732 CN=6 for both cations and anions Unit cell: FCC arrangement of anions with one cation at center of each of 12 cube edges. Two interpenetrating FCC lattices common materials possessing this structure: NaCl, MgO, MnS, LiF, FeO.

3- Zinc Blende (ZnS) structure:

CN = 4 (0.225 < rc/ra < 0.414) all ions are tetrahedrally coordinated each atom is bonded to 4 atoms of the opposite type corner and face sites are occupied by anions (S-) interior tetrahedral sites are occupied by cations (Zn+) the bonding is mostly covalent other compounds possessing this structure: ZnTe, SiC.

4

Am-Xp ceramic crystal structures:

Suppose the electrical charge on the cation and ion is not the same. In this case, the stoichiometry of the crystal cannot be 1:1; in order to achieve charge neutrality, the ratio of cations to anions must ≠ 1. We classify these crystals AmXp, where m &/or p ≠ 1.

Example:

AX2⇒CaF2

rc/ra≅ 0.8 → CN = 8

cations (Ca+2) are positioned at the centers of unit cubes while anions (F-) occupy corner sites. Since there are half as many Ca+2 as there as F-, only half the center cube positions are occupied. A unit cell consists of 8 such cubes, shown at right. Other such compounds: UO2, PuO2 and ThO2.

5

AmBnXp ceramic crystal structures:

Many ceramic materials contain more than one type of cation; as long as charge neutrality is maintained, there is no absolute limit to the number of cations (or anions) that a crystal may possess. In the case of two cations (A and B) and one species of anion (X), the chemical formula is designated as AmBnXp.

Example: Barium titanate (BaTiO3). “Perovskite” structure, one species of cation (Ba+2) is positioned at the unit cube corners; a single second cation (Ti+4) is located at the cube center. The anions (O-2) occupy the face center positions. CN (O-2) = 6; CN (Ba+2) = 12; CN (Ti+4) = 6

Ceramic density:

6

Ceramic phase diagrams:

Phase diagrams for ceramic materials obey the same rules as for metal systems. An important difference is that the terminal end phases are usually themselves binary compounds, rather than pure elements. Anumber of binary systems contain oxygen as a common element. We will now look at a few of the more important pseudo-binary systems as examples:

Cr2O3 - Al2O3

SiO2 - Al2O3

MgO - Al2O3

CaZrO3 - ZrO2

1-Cr2O3 - Al2O3:

One of the simplest pseudo- binary phase diagrams. Complete solubility: Al, Cr atoms possess similar size and chemical valence Both oxides have the same crystal structure Al+3 substitutes for the Cr+3 ion in Cr2O3 (and vice versa).

7

2-SiO2 - Al2O3:

Most commercial refractories are based on the silica-alumina system fireclay refractories contain 25-45 wt. % Al2O3 typically used in furnace construction (insulation) silica refractories (< 7 wt. % Al2O3) are used as load-bearing supports for large industrial furnaces note the absence of solubility on either endpoint one intermediate compound exists: mullite (3Al2O3 - 2SiO2).

Imperfections in Ceramics:

1- Include point defects and impurities. 2- Charge neutral defects include the Frenkel defects(a vacancy- interstitial pair of

cations) and Schottky defects (a pair of nearby cation and anion vacancies).3- Defects will appear if the charge of the impurities is not balanced.

8

9

Shottky Defect:

Frenkel Defect

10

11