Electrical and microstructural properties of microwave sintered SnO2-based varistors

Abstract

An investigation was made of the microstructural and electrical properties of SnO 2 -based varistors microwave sintered at 1200 ºC, applying a heating rate of 120 ºC/min and treatment times of 10, 20, 30, 40, 50 and 60 min. The system used in this study was (98.95-X)%SnO 2 .1.0%CoO.0.05%Cr 2 O 3 .X%Ta 2 O 5 , where X corresponds to 0.05 and 0.065 mol%. Sintering was carried out in a domestic microwave oven (2.45 GHz) fitted for lab use. Silicon carbide was placed in a refractory vessel to form a heating chamber surrounding the sample holder. The pellets were examined by scanning electron microscopy, X-ray diffractometry, direct current measurements and impedance spectroscopy. The parameters of density, medium grain size, coefficient of nonlinearity, breakdown electrical field, leakage current, and height and width of the potential barrier were analyzed.

Basic models in dielectric spectroscopy of heterogeneous materials with semiconductor inclusions

“…Interpretation of frequency dependence of the complex dielectric permittivity in heterogeneous dielectrics with semiconductor inclusions is based on the models of the migration polarization of the Maxwell-Wagner type (Yu and Ang, 2002; Shin and Kwon, 2011; Wang et al , 2012; Macutkevic et al , 2014; Morozov et al , 2014; Panda et al , 2014) or concepts known for the near-surface region in a semiconductor (Tonkoshkur, 1978; Tonkoshkur et al , 1988; Liu et al , 2005; Bueno et al , 2008; Ahmad et al , 2012; Furtado et al , 2012; Wong et al , 2013; Chen et al , 2014). In the first case, the presence of the space charge in a semiconductor and electronic processes in it are ignored.…”

“…In the second case, as a rule, some selected physical concepts, not related to each other and only related to particular situations, are considered. For example, in Glot et al (1979), Liu et al (2005), Bueno et al (2008), Ahmad et al (2012), Cheng et al (2012), Furtado et al (2012), Lyashkov et al (2013), Wong et al (2013), Chen et al (2014) and Macutkevic et al (2014) only the capacitance of the near-surface space charge region is taken into account, in Tonkoshkur (1978) only the recharging of the localized states is discussed, in Tonkoshkur et al (1988) and Degtyar’ov et al (2006) only the relaxation of the “built-in” space charge in the dielectric component is considered. Such models are not of a systemic nature, what makes difficult to use them in the experiment.…”