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Properties of Ceramics

Ceramics are best known as brittle solids particularly suited for withstanding high temperatures but, in fact, the different materials used in ceramics can give them a wide range of properties. The classic properties of ceramics include durability, strength and brittleness, high electrical and thermal resistance, and an ability to withstand the damaging effects of acids, oxygen, and other chemicals because of their inertness (chemical unreactivity). But not all ceramics behave in this way. For example, graphite is a very soft ceramic and conducts electricity well, whereas diamond is a very good conductor of heat. Ceramics called ferrites are particularly good conductors of electricity and superconductors have almost no electrical resistance at all. Ceramic matrix composites, made by embedding fibers of a strengthening material in what is known as a ceramic matrix, are not at all brittle.

The properties of a particular ceramic depend not just on the materials from which it is made but also on the way they are joined together—in other words, on its crystalline structure. Diamond is strong because all of its carbon atoms are bonded tightly to other carbon atoms. Graphite (such as that used in pencil "leads") shears because it is made up from different layers. Although the carbon atoms are tightly bonded within a given layer, the different layers are held together only by much weaker bonds. China clay (also called kaolin) behaves in a similar way to graphite, with its constituent aluminum, silicon, oxygen, and hydrogen atoms tightly bonded into flat sheets. But the weak bonds between those sheets are easily broken when water surrounds them and it is this that makes wet clay so easy to mold. When china clay is fired, heat removes the water, and the chemicals inside the clay rearrange themselves into crystals of aluminum silicate tightly bonded by silicate glass, which is overall very much stronger.