How Materials Are Classified

Materials can be classified into four broad categories metals, ceramics, semiconductors, and polymers. Metals, because of their detached electrons, are good reflectors of light, good conductors of heat and electricity, and tend to be ductile. Ceramics generally are poor conductors of electricity because their valence electrons are tied up in chemical bonds, although the recent discovery of ceramic superconductors is a notable exception. Their 

Glasses and composites are important forms of materials but are not considered as specific classes of materials. Composites are simply a mixture of two or more of the other types of materials combined in such a way as to reinforce the favorable qualities and reduce the unfavorable qualities of the components for a specific application. Most metals and ceramics form a polycrystaUine structure when they solidify. However, it is possible to cause both metals and ceramics to solidify in an amorphous state without the order associated with the crystalline structure. Such an amorphous solid is called a glass. 

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