Trans. Nonferrous Met. Soc. China 28(2018) 1377−1385 Dielectric characteristics and nonlinear properties of ZnO−polypyrrole composites Fariba TABRIZI 1 , Mojtaba PARHIZKAR 1 , Hassan BIDADI 1 , Mohammad GHAFOURI 2,3 1. Faculty of Physics, University of Tabriz, Tabriz, Iran; 2. Department of Basic Science, Shabestar Branch, Islamic Azad University, Shabestar, Iran; 3. Faculty of Physics, Islamic Azad University of Shabestar, Shabestar, Iran Received 27 May 2017; accepted 28 September 2017 Abstract: The nonlinear properties and frequency characteristics of ZnO−polypyrrole composites were investigated at 200 Hz− 5 MHz frequency interval with different zinc oxide contents. Samples were prepared using hot press method at 130 °C. Results show an optimum point for breakdown voltage at ZnO content of 70%. Breakdown voltage decreases from 590 to 380 V and after that tends to increase from 450 to 740 V due to the absence of polypyrrole at grain boundaries. No matter how breakdown voltage behaves, nonlinear coefficient increases from 4.2 to 9 by increasing ZnO content because of the increase in acceptor-like states at grain boundaries by increasing ZnO content. The electrical parameters such as dielectric constant, dielectric loss and series resistance of samples show a strong dependence on frequency especially below 1 kHz. These parameters fall off by increasing frequency up to 1 kHz, which is related to charge transportation through the Schottky barrier at grain boundaries. The high dielectric constant of samples below 1kHz is related to the Maxwell−Wagner polarization at grain boundaries. The presence of different anomalies at different frequency intervals is related to interfacial polarization because of different structures of grains and intergranular layer with a huge difference in conductivity. Key words: dielectric characteristics; electrophysical properties; composite varistor; frequency dependence 1 Introduction ZnO-based ceramics with a few metal oxide additives such as Bi 2 O 3 , CoO, MnO and TiO 2 , are known for their high nonlinear properties against surges and over voltages [1]. This is because: 1) they can do protection several times without destruction, and 2) they are highly capable of absorbing surge energy [2−4]. The nonlinearity of a varistor is related to its microstructure and formation of potential barriers (Schottky barriers) at grain boundaries (GBs) [5−11]. The presence of additives increases the density of trap states at the GBs which could help to block charge carriers movements and results in producing Schottky barrier [12−15]. Although ZnO-based varistors have lots of advantages, there are some disadvantages for them such as: 1) complicate microstructure, 2) high concentration of additives, 3) low permittivity (it is not suitable to make low-voltage varistors), and 4) low stability against degradation [16−20]. These issues caused that scientists look for other materials such as SnO 2 . But, another common difficulty of ceramic varistors is their high sintering temperature and heat treatment (950−1150 °C). One solution to these kinds of problems is changing primary materials. This stems from the fact that the I−V (I is the electric current, V is the applied voltage) characteristics of a varistor depend on its initial mixture. To this end, composite varistors have also been developed recently. Simple microstructure, low breakdown voltage possibilities (~40 V), low preparation temperature (130 °C), and good nonlinear coefficient (~15), make composite varistors an acceptable candidate for a new generation of technologies [21−25]. In general, a composite varistor is made of an inorganic material (e.g., Si), and an organic one (e.g., polyaniline). Although conduction mechanism and microstructure of composite varistors are completely different from ceramic ones, intergranular layer plays an important role in both of them [21]. In composite varistors, the intergranular layer is a mixture of conducting polymer and some thermoplastic polymers. Corresponding author: Mohammad GHAFOURI; E-mail: ghafouri.sar@gmail.com DOI: 10.1016/S1003-6326(18)64776-4