This chapter demonstrates approaches to the treatment of macroscopically inhomogeneous materials. The conception and criteria of granularity are introduced and experimental examples of metal-insulator transition are presented. The nature of the transition is tightly connected to the phenomenon of Coulomb blockade: electron tunneling between the grains is blocked because charging of small grains requires Coulomb energy larger than the temperature. The concept of a granular material is not restricted to cermets — a mixture of metallic and insulating grains of different sizes but quasi-spherical in shape. It can be extended to materials with fractal grains where the insulating volume resembles a loose glomus of thin insulating sheets.
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