Influence Of Preparation Process On Electrical Performance Of ZnO Varistor

ZnO varistors with highly nonlinear characteristic, excellent surge withstandingcapability are used in the fields of power systems and electrical circuits as voltage surgeprotection device. At present, the rapid development of power systems and electrical circuitsmakes the pressing requirement for the ZnO varistors with more security, reliability andminiaturization. So the preparation of ZnO varistor with high performance has become aresearch focus. To a great extent, the performance of ZnO varistor materials is determined bythe quality of raw materials, ingredients and the preparation process. In order to develop ZnOvaristors with high performance, the influence of preparation process on electricalperformance of ZnO varistors and the optimization of preparation process were studied in thispaper. The main contents and results are as follows:1. The influence of the amount of dispersant and milling time on electrical performanceof ZnO varistor was studied. The results showed that appropriate amount of ammoniumpolyacrylate and appropriate milling time can effectively increase the milling efficiency ofZnO powder and additives and improve the electrical properties of ZnO varistor.2. The polyvinyl alcohol (PVA) discharging thermal process and the influence of PVAdischarging process on electrical performance of ZnO varistor were studied. The resultsshowed that the residue moisture of ZnO varistor powder evaporates at25~150℃. Theadhesive PVA descomposes and volatilizes at200~350℃. Sb2O3is oxidated at500~600℃and the oxidation of Sb2O3results in large volume shrinkage. Reasonable debinding processcan reduce the oxygen vacancy defects in the depletion layer and the donnor concentrationand increase the barrier height, then it improves the nonlinear coefficient. In addition,debinding process also can increase the density and the uniformity of microstructure of ZnOvaristor materials and reduce leakage current and improve the pluse current withstandingcapability.3. The sintering thermal process of ZnO varistor materials with different potentialgradient and the influence of sintering process on the electrical properties of ZnO varistorwere studied. The results showed that the sintering processes of ZnO varistor materials withdifferent compositions are differernt. The decrease of Sb2O3and the increase of TiO2canreduce the liquid forming temperature and the density of materials. In addition, when thesintering temperature exceeds1150℃, the Bi2O3phase of ZnO varistor materials withdifferent compositions volatilizes seriously. It was also found that the sintering process greatlyaffects the electrical properties of ZnO varistor materials. The voltage gradient decreases with the increase of sintering temperature or sintering time. But the test results of the othercharacteristics showed that the best sintering conditions can be obtained for the materials withspecific formulation and under the best sintering conditions, the electrical properties of thematerials are best. Unfavorable sintering conditions can increase the leakage current anddeteriorate the nonliner characteristic and the pluse current withstanding capability.