ELECTRICAL CHARACTERIZATION OF GRAIN BOUNDARIES IN (ZINC-OXIDE)-BASED VARISTORS
The material of the ZnO-based varistor (MOV) consists primarily of ZnO which is mixed with small amounts of other metal oxides, among these being oxides of Bi, Sb, Mn, Co, Ni and Cr. The resulting microstructure of the fired mixture consists of ZnO grains, formed during liquid phase sintering, which are surrounded by a discontinuous intergranular phase; the intergranular phase being made up primarily of the addivites to ZnO. Electrically, MoV's have been found to exhibit 1. symmetrical and nonlinear current-voltage characteristics which show a reversible Zener-type breakdown, 2. low voltage room temperature resistivities in the range 10('9)-10('12) ohm-cm, 3. activation energies for low voltage leakage conduction in the range 0.3-0.8 eV, 4. dielectric constants which are anomalously large compared to homogeneous metal oxides, and 5. AC resistances showing large dispersions with frequency. In contrast, crystalline or highly sintered pure ZnO 1. is ohmic (n-type), 2. has a room temperature resistivity of 1 ohm-cm. 3. possesses an activation energy for conduction of 0.05 eV which is attributed to ionization of point defects, and 4. has a relative dielectric constant of 8.5 which is representative of ZnO lattice and electronic polarization. The anomalous behavior of the varistor is attributed to insulating potential barriers of width t existing in the grain boundary regions between relatively conductive grains of diameter d. The nonohmic behavior is determined by the response of these barriers to an applied voltage, while the activation energy for conduction is determined by the barrier height. The large MOV dielectric constant is determined by the geometric capacitance, or insulating width, of the grain boundary which, when incorporated into the equivalent circuit for the total microstructure, enhances the dielectric constant of the single grain boundary by a factor d/t. The following varistor electrical properties were observed experimentally: (1) low voltage DC current-voltage characteristic, (2) high current pulsed current-voltage characteristic, (3) small signal admittance in the frequency range 0.1 Hz to 100 MHz, (4) admittance-bias voltage response at 1MHz, and (5) the instability to bias of the leakage currents. The experimental parameters varied were temperature, composition and type of post-sinter heat-treatment. The results of each experimental technique were best described by a model of back-to-back depletion layers existing at the ZnO/ZnO grain interface. The depletion layers form barriers to conduction which determine the magnitude and temperature dependence of the varistor leakage currents. The reversible current breakdown occurs vial tunneling through these barriers. Under AC conditions ionized donors in the depletion region act as trapping centers for free carriers in the grain, and thus determmine the dependence of varistor impedance on frequency and temperature. It was determined from experimental techniques (2), (3) and (5), above, that the depletion layer donor density is derived from the oxide additives to ZnO. The post-sintering heat-treatments were found 1. to change the forms of the leakage and breakdown regions of the I-V curve, 2. to change the dependence of capacitance on bias, and 3. to produce a more uniform electrical response among varistors.
WILLIAM CLIVE RICHMOND,
"ELECTRICAL CHARACTERIZATION OF GRAIN BOUNDARIES IN (ZINC-OXIDE)-BASED VARISTORS"
(January 1, 1980).
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