材料科学与工程导论双语课件教学课件 ppt 作者 陈克正 王玮 刘春延 等编Chapter 8.ppt

ELECTRICAL CONDUCTION IN IONIC CERAMICS Ionic Materials In ionic materials, the band gap is large and only very few electrons can be promoted to the valence band by thermal fluctuations. Cation and anion diffusion can be directed by the electric field and can contribute to the total conductivity: ótotal= óelectronic + óionic High temperatures produce more Frenkel and Schottky defects which result in higher ionic conductivity. ELECTRICAL CONDUCTION IN POLYMERS Polymers Polymers are typically good insulators but can be made to conduct by doping. A few polymers have very high electrical conductivity - about one quarter that of copper, or about twice that of copper per unit weight. CAPACITANCE When a voltage V is applied to two parallel conducting plates, the plates are charged by +Q, –Q, and an electric field E develops between the plates. ++++++ -------- The charge remains on the plates even after the voltage has been removed. The ability to store charge is called capacitance and is defined as a charge Q per applied voltage V: C = Q / V [Farads] DIELECTRIC MATERIALS The dielectric constant of vacuum is 1 and is close to 1 for air and many other gases. But when a piece of a dielectric material is placed between the two plates in capacitor the capacitance can increase significantly. A dielectric material is an insulator in which electric dipoles can be induced by the electric field (or permanent dipoles can exist even without electric field), that is where positive and negative charge are separated on an atomic or molecular level DIELECTRIC MATERIALS Dipole formation induces additional charge Q’ on plates: total plate charge Qt= |Q+Q’|. Therefore, C = Qt / V has increased and dielectric constant of the material ?r = C / Cvac 1. The process of dipole formation/alignment in electric field is called polarization and is described by P = Q’/A. DIELECTRIC MATERIALS Mechanisms of polarizat