600 فيز

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..effect of Gd₂O₃ additives on the microstructure and  electrical properties of Pr₂O₃, Co₂O₃, Cr₂O₃ doped ZnO varistor. 
    Zinc Oxide (ZnO) varistors are semiconductor ceramics which behave as highly non ohmic resistors. They are fabricated by sintering of ZnO powders with small amounts of metal oxides additives such as Pr₆O₁₁, Co₂O₃, Cr₂O₃ and rare earth oxides to improve device performance. The sintering process gives rise to a microstructure, which consists of semiconducting n–type ZnO grains surrounded by thin insulating intergranulare layers. These layers give rise to double Schottky barriers across ZnO grain boundaries, which are essentially formed by segregation of varistor- forming oxides.                                                
      ZnO varistors exhibit a highly nonlinear current ( I ) – voltage ( V ) relationship expressed by the equation I = KV^a, where K is a constant and ais a nonlinear coefficient which characterizes the nonlinear properties of varistors. This nonlinearity attributed to the presence of double Schottky barriers at grain   boundaries.                                                                                                                         
 
       ZnO Varistors used as protecting devices against voltage transient in electronic and industrial equipments and as surge arrestors.                                                  
      
       The electrical properties are directly dependent on the microstructure ( the grain boundary, the depletion region and the size of grains), which would be influenced by the kind and amount of additives , sintering temperature and time, cooling rate and sintering technique.
       The objective of the present research is to investigate the influence of Gd₂O₃ additives on microstructure,  and electrical properties of ZnO – Pr₆O₁₁ – Co₃O₄ – Cr₂O₃ – Gd₂O₃ (ZPCCG) – based varistors.
       The samples will be prepared by the conventional ceramic processing technique in the material group research labs. The lattice constant and the grain size of prepared samples will be obtained by XRD and SEM. The d – c measurements (nonlinear properties , capacitance – voltage characteristics) and dielectric properties at different temperature and in frequency range 20Hz to 3MHz will be determined