Scientific Principles

Introduction:

Ceramics have characteristics that enable them to be used in a wide variety of applications including:

• high heat capacity and low heat conductance

• corrosion resistance

• electrically insulating, semiconducting, or superconducting

• nonmagnetic and magnetic

• hard and strong, but brittle

The diversity in their properties stems from their bonding and crystal structures.

Atomic Bonding:

Two types of bonding mechanisms occur in ceramic materials, ionic and covalent. Often these mechanisms co-exist in the same ceramic material. Each type of bond leads to different characteristics.

Ionic bonds most often occur between metallic and nonmetallic elements that have large differences in their electronegativities. Ionically-bonded structures tend to have rather high melting points, since the bonds are strong and non-directional.

The other major bonding mechanism in ceramic structures is the covalent bond. Unlike ionic bonds where electrons are transferred, atoms bonded covalently share electrons. Usually the elements involved are nonmetallic and have small electronegativity differences.

Many ceramic materials contain both ionic and covalent bonding. The overall properties of these materials depend on the dominant bonding mechanism. Compounds that are either mostly ionic or mostly covalent have higher melting points than compounds in which

neither kind of bonding predominates.

Table 1: Comparison of % Covalent and Ionic character with several ceramic compound's melting points.

Ceramic Compound	Melting Point ˚C	% Covalent character	% Ionic character
Magnesium Oxide	2798˚	27%	73%
Aluminum Oxide	2050˚	37%	63%
Silicon Dioxide	1715˚	49%	51%
Silicon Nitride	1900˚	70%	30%
Silicon Carbide	2500˚	89%	11%

Classification:

Ceramic materials can be divided into two classes: crystalline and amorphous (noncrystalline). In crystalline materials, a lattice point is occupied either by atoms or ions depending on the bonding mechanism. These atoms (or ions) are arranged in a regularly repeating pattern in three dimensions (i.e., they have long-range order). In contrast, in amorphous materials, the atoms exhibit only short-range order. Some ceramic materials, like silicon dioxide (SiO2), can exist in either form. A crystalline form of SiO2 results when this material is slowly cooled from a temperature (T>TMP @1723˙C). Rapid cooling favors noncrystalline formation since time is not allowed for ordered arrangements to form.

Figure 1: Comparison in the physical strucuture of both crystalline and amorphous Silicon dioxide

The type of bonding (ionic or covalent) and the internal structure (crystalline or amorphous) affects the properties of ceramic materials. The mechanical, electrical, thermal, and optical properties of ceramics will be discussed in the following sections.