Study of the Electrophysical Properties of Composite Varistors Based on Zinc Oxide and Polymer (Polyaniline)

In this experimental work, current-voltage characteristics of composite varisrors prepared on the base of zinc oxide with different weight percentages of polyaniline and temperature dependence of the samples characteristics have been studied. The activation energy of donor levels as well as the existence of hysteresis loop has been investigated. The comparison of experimental results shows that by increasing the polymer percentage in the varistor structure as long as the nonlinear behavior is conserved, the threshold voltage increases. This comparison also indicates that increasing the polymer content in the varistor structure causes the temperature dependence of threshold voltage to decrease. It is also concluded that, by the increase of polymer content in the varistor structure, the activation energy of donor levels and the hysteresis value are increased. 1. Introduction Nowadays, for protecting the electronic and microelectronic circuits, and electrical equipment from overvoltages, the nonlinear resistors based on composite materials are widely utilized. By conversion of microelectronics into nanoelectronics, the need for low-voltage limiters will grow even more [1]. Zinc oxide varistors are electronical polycrystalline ceramic materials whose electronical behavior is determined by grain boundaries. These materials have important technological applications in electronic circuits for preventing them from overvoltages [2]. Pure zinc oxide is a nonstoichiometric n-type semiconductor whose current-voltage behavior is linear and additives such as Al2O3, CrO, and Bi2O3, are used to make it become nonlinear. Adding these materials causes a number of crystal imperfections to be formed in grains and also grain boundaries. These imperfections have the role of donors in grains and acceptors in grain boundaries [3]. As a result of imperfections due to covalent bonding, certain surface states in band gap are created that affect the electrical properties of varistors [4]. In a varistor, transition from a high resistance region (pre-breakdown region) to a lower resistance region (upturn region) is not sharp, but the relation between current and voltage is nonlinear which follows the form where α is called the coefficient of nonlinearity. The above relation shows that for a constant current, with increasing value, the voltage changes in the transition region decreases. The higher the value, the better the quality of varistor. The nonlinear coefficient ( ) is very important for protecting the electrical equipment. The advantage of ZnO varistors over the SiC varistors