CHAPTER 12:
STRUCTURES AND PROPERTIES OF CERAMICS
(Written Report)

GROUP MEMBERS

Alzate, Niel
Aquino, Stef
Bartolazo, Ronald M.
Belen, Sinotriss
Benjamin, Jessa

BSIE III-I

CHAPTER 12:
STRUCTURES AND PROPERTIES OF CERAMICS
(Written Report)

Bartolazo
INTRODUCTION
Ceramic materials are inorganic and nonmetallic materials. Most ceramics are compounds between metallic and nonmetallic elements for which the inter-atomic bonds are either totally ionic, or predominantly ionic but having some covalent character. The term “ceramic” comes from the Greek word keramikos, which means “burnt stuff”, indicating that desirable properties of these materials are normally achieved through a high temperature heat treatment process called firing. Up until the past 60 years or so years, the most important materials in this class was termed “traditional ceramics (china, porcelain, bricks, tiles, glasses and high temperature ceramics)”, those for which the primary raw material is clay. Of late, significant progress has been made in understanding the fundamental character of these materials and of the phenomena that occur in them that are responsible for their unique properties. Consequently, a new generation of these materials has evolved, and the term ceramic has taken on a much broader meaning. To one degree or another, these new materials have a rather dramatic effect on our lives; electronic computer, communication, aerospace and a host of other industries rely on their use.

CERAMIC STRUCTURE
Because ceramics are composed of at least two elements, and often more, their crystal structures are generally more complex than those 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 electro negativities of the atoms.

CRYSTAL STRUCTURES For those ceramic materials for which the atomic bonding is predominantly ionic, the crystal structures may be thought of as being composed of electrically charged ions instead of atoms. The metallic ions or cations are positively charged because they have given up their valence electrons to the nonmetallic ions or anions, which are negatively charged. Two characteristics of the component ions in crystalline ceramic materials influence the crystal structure; the magnitude of the electrical charge on each of the component ions and the relative sizes of the cations and anions. 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. The second criterion involves the sizes or ionic radii of the cations and anions, ra and rc respectively. Because the metallic elements give up electrons when ionized, cations are ordinarily smaller than anions and consequently, the ratio is less than unity.
AQUINO
* AX-TYPE CRYSTAL STRUCTURE
Some of the common ceramic materials are those in which there are equal numbers of cations and anions. These are often referred to as AX compounds, where A denotes the cation and X the anion. There are several different crystal structures for AX compounds; each is normally named after a common material that assumes the particular structure. * Rock-salt Structure: the most common AX crystal structure is the sodium chloride or rock salt type. The coordination number for both cations and anions is 6; the cation-anion radius ratio is between approximately 0.414 and 0.732. * Cesium Chloride Structure: coordination number is 8 for both ion types. * Zinc Blende Structure: coordination number is 4 for both ion types.

* AmXp-TYPE CRYSTAL STRUCTURE
If the charges in the cations and anions are not the same, a compound can exist with the chemical formula AmXp where m and p is not equal to 1. This type of crystal structure is found in fluorite. * AmBnXp-TYPE CRYSTAL STRUCTURE
Are ceramic compounds having more than one type of cation.

CERAMIC DENSITY COMPUTATIONS Formula: ρ=n'AC+AAVCNA

Where: n’ = the number of formula units within the unit cell. ΣAC= the sum of the atomic weights of all cations in the formula unit.
ΣAA= the sum of the atomic weights of all anions in the formula unit.
VC = the unit cell volume
NA = Avogadro’s number 6.023 X 1023 units/mol

SILICATES 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. * SILICA (Silicon Dioxide)-the most simple silicate material. * SILICA GLASSES- Silica can also be made to exist as a non-crystalline solid or glass, having a high degree of atomic randomness, which is characteristic of the liquid; such a material is called fused silica, or vitreous silica.

CARBON Carbon is an element that exists in various polymorphic forms, as well as in the amorphous state. This group of materials does not really fall within any one of the traditional metal, ceramic, polymer classification schemes. * DIAMOND- is a metastable carbon polymorph at room temperature and atmospheric pressure. The physical properties of diamond make it an extremely attractive material. It is extremely hard (the hardest known material) and has a very low electrical conductivity; these characteristics are due to its crystal structure and the strong inter-atomic covalent bonds. * GRAPHITE- Another polymorph of carbon is graphite; it has a crystal structure distinctly different from that of diamond and is also more stable than diamond at ambient temperature and pressure. Desirable properties of graphite include the following: high strength and good chemical stability at elevated temperatures and in non-oxidizing atmospheres, high thermal conductivity, low coefficient of thermal expansion and high resistance to thermal shock, high adsorption of gases, and good machinability. * FULLERNESS- polymorphic form of carbon discovered in 1985. It exists in discrete molecular form and consists of a hollow spherical cluster of sixty carbon atoms.
ALZATE
IMPERFECTIONS IN CERAMICS * DEFECT STRUCTURE- used to designate the types and concentrations of atomic defects in ceramics. * ELECTRONEUTRALITY- is the state that exists when there are equal numbers of positive and negative charges from the ions. * FRENKEL DEFECT- a type of defect that involves a cation-vacancy and a cation-insterstitial pair. * SCHOTTKY DEFECT- a type of defect found in AX materials that has a cation vacancy-anion vacancy pair. * STOICHIOMETRY- defined as a state for ionic compounds wherein there is the exact ratio of cations to anions as predicted by the chemical formula. * NONSTOICHIOMETRY- occurs for some ceramic materials in which two valences (or ionic) states exist for one of the ion types.

IMPURITIES IN CERAMICS
Impurity atoms can form solid solutions in ceramic materials much as they do in metals. Solid solutions of both substitutional and interstitial types are possible. For an interstitial, the ionic radius of the impurity must be relatively small in comparison to the anion. Since there are both anions and cations, a substitutional impurity will substitute for the host ion to which it is most similar in an electrical sense: if the impurity atom normally forms a cation in a ceramic material, it most probably will substitute for a host cation.

DIFFUSION IN IONIC MATERIALS For ionic compounds, the phenomenon of diffusion is more complicated than for metals inasmuch as it is necessary to consider the diffusive motion of two types of ions that have opposite charges. Diffusion in these materials usually occurs by a vacancy mechanism. In order to maintain charge neutrality in an ionic material, the following may be said about vacancies: 1) ion vacancies occur in pairs (as with Schottky defects) 2) they form in non-stoichiometric compounds 3) They are created by substitutional impurity ions having different charge states than the host ions

BENJAMIN
CERAMIC PHASE DIAGRAM Phase diagrams have been experimentally determined for a large number of ceramic systems. For binary or two-component phase diagrams, it is frequently the case that the two components are compounds that share a common element, often oxygen. These diagrams may have configurations similar to metal–metal systems and they are interpreted in the same way.

* Al2O3–Cr2O3 SYSTEM- aluminum oxide–chromium oxide system. * MgO–Al2O3 SYSTEM- magnesium oxide–aluminum oxide system. * ZrO2-CaO SYSTEM- zirconium oxide (zirconia) and calcium oxide (calcia) system. * SiO2–Al2O3 SYSTEM- silica–alumina system.

MECHANICAL PROPERTIES OF CERAMICS * Brittle Fracture Process- consists of the formation and propagation of cracks through the cross section of material in a direction perpendicular to the applied load. Crack growth in crystalline ceramics may be either transgranular (through the grains) or intergranular (along grain bounadaries); for transgranular fracture, cracks propagate along specific crystallographic (or cleavage) planes, planes of high atomic density. * Fractographic Study- is normally a part of such an analysis, which involves examining the path of crack propagation as well as microscopic features of the fracture surface. * Flexural Strength (modulus of rupture, fracture strength or the bend strength)- a flexure test for the stress of fracture of ceramics; an important mechanical parameter for brittle ceramics.

At room temperature, virtually all ceramics are brittle. Microcracks, the presence of which is very difficult to control, result in amplification of applied tensile stresses and account for relatively low fracture strengths (flexural strengths). This amplification does not occur with compressive loads, and, consequently, ceramics are stronger in compression. Fractographic analysis of the fracture surface of a ceramic material may reveal the location and source of the crack-producing flaw, as well as the magnitude of the fracture stress. Representative strengths of ceramic materials are determined by performing transverse bending tests to fracture.
BELEN
MECHANISMS OF PLASTIC DEFORMATION Most ceramic materials suffer fracture before the onset of plastic deformation, a brief exploration into the possible mechanisms is worthwhile. Plastic deformation is different for crystalline and non-crystalline ceramics. * CRYSTALLINE CERAMICS- For crystalline ceramics, plastic deformation occurs, as with metals, by the motion of dislocations. One reason for the hardness and brittleness of these materials is the difficulty of slip (or dislocation motion). For crystalline ceramic materials for which the bonding is predominantly ionic, there are very few slip systems (crystallographic planes and directions within those planes) along which dislocations may move. This is a consequence of the electrically charged nature of the ions. For slip in some directions, ions of like charge are brought into close proximity to one another; because of electrostatic repulsion, this mode of slip is very restricted, one another; because of electrostatic repulsion, this mode of slip is very restricted. On the other hand, for ceramics in which the bonding is highly covalent, slip is also difficult and they are brittle for the following reasons: (1) the covalent bonds are relatively strong, (2) there are also limited numbers of slip systems, and (3) dislocation structures are complex. * NONCRYSTALLINE CERAMICS- Plastic deformation does not occur by dislocation motion for non-crystalline ceramics because there is no regular atomic structure. Rather, these materials deform by viscous flow, the same manner in which liquids deform; the rate of deformation is proportional to the applied stress. In response to an applied shear stress, atoms or ions slide past one another by the breaking and reforming of inter-atomic bonds. However, there is no prescribed manner or direction in which this occurs, as with dislocations. The characteristic property for viscous flow, viscosity, is a measure of a non-crystalline material’s resistance to deformation. For viscous flow in a liquid that originates from shear stresses imposed by two flat and parallel plates, the viscosity is the ratio of the applied shear stress and the change in velocity dv with distance dy in a direction perpendicular to and away from the plates.

MISCELLANEOUS MECHANICAL CONSIDERATION
Many ceramic bodies contain residual porosity, which is deleterious to both their moduli of elasticity and fracture strengths. In addition to their inherent brittleness, ceramic materials are distinctively hard. Also, since these materials are frequently utilized at elevated temperatures and under applied loads, creep characteristics are important. * Influence of Porosity- a mechanical consideration that will have a deleterious influence on both the elastic properties and strength of ceramics. Porosity is deleterious to the flexural strength for two reasons: (1) pores reduce the cross-sectional area, across which a load is applied, and (2) they also act as stress concentrators—for an isolated spherical pore, an applied tensile stress is amplified by a factor of 2. * Hardness- One beneficial mechanical property of ceramics is their hardness, which is often utilized when an abrasive or grinding action is required; in fact, the hardest known materials are ceramics. * Creep- is a result of exposure to stresses that usually compressive at elevated temperatures. High-temperature compressive creep tests are conducted on ceramic materials to ascertain creep deformation as a function of temperature and stress level.