Nonlinear properties of ZnO-polymer composites prepared by solution-casting method

In this experimental work, ZnO-polyaniline composite thin films, to which we added some polyvinyl alcohol, have been prepared using solution-casting technique. Then their current voltage characteristics were studied. The results show that the films have varistor behavior and can be used to protect circuits from 100 V up to 350 V over voltages. In addition, it is found that the higher the amount of ZnO in the varistor structure, the higher the breakdown voltage. Films with higher amount of polymer matrix have more nonlinearity and their nonlinearity coefficients (alpha) are higher. Each film has hysteresis which increases by increasing the ZnO content in the composites. This causes the varistors to have lower lifetime due to their high degradation. Studying the UV spectra shows that there are two recognizable energy gaps for each sample; one is related to the impurity levels and the other to the band gap. These two energy gaps may be calculated using Tauc relation for semiconductors. The UV spectrum also shows that by reducing the ZnO content in the varistors, their absorptions decrease and the corresponding band gaps increase. (C) 2012 Elsevier Ltd. All rights reserved.