ZnO Nano-powders Synthesized By Spray Pyrolysis And ZnO Varistor

In recent years,as a significant wide-bandgap semiconductor material,ZnO had been extensively studied.Due to the direct band gap of 3.37eV and large excitation binding energy of 60 meV,ZnO had various applications in optical and electronic field such as light emitting diodes（LEDs）,varistors,gas sensors,photo-catalysts,transparent electronic,and solar cell.A series of methods for synthesizing ZnO particles were investigated.In this thesis,we designed a spray pyrolysis equipment based on spray pyrolysis principle,and prepared ZnO powders and ZnO composite powders for varistor.The effect of concentration of precursor solution,pyrolysis temperature of first and second temperature zones on the crystalline structure,morphology and composition of the powder was investigated by X-ray diffractometer,scanning electron micrograph,then the optimum concentration of precursor solution and the optimum pyrolysis temperature were determined.In addition,ZnO varistors were prepared by ZnO powders and ZnO composite powders using spray pyrolysis method,and by comparing the performance of varistors,the optimum doping concentration was determined.The main results are as follows:（1）The spray pyrolysis equipment was designed and improved.The pats of the equipment and the use of methods were introduced,and the components and use of methods of nano spray generator were emphatically introduced.The equipment has the following advantages:1.It combined many functions in one,2.Auto matization,compact structure,appearance,easy to oprate economic energy consumption,3.The equipment had wider application in nano powder preparation.（2）The dried plum-like ZnO assemblies consisted of ZnO nanoparticle was synthesized by a spray pyrolysis method.With increasing the solution concentration,the crystallite size increased and the average size of the dried plum-like ZnO particles increased from 300 nm to 870 nm.In addition,with increasing the pyrolysis temperature of first temperature zone and second temperature zone,the miscellaneous phase gradually disappeared,the crystallite size of the dried plum-like ZnO particles increased.The particle sizes were uniformly distributed and concentrated in 500 nm to 600 nm.Moreover,the optimum concentration of precursor solution was 0.05 mol/L and the optimum pyrolysis temperature were 500℃（first temperature zone）and 700℃（second temperature zone）.（3）The ZnO varistors were prepared using ZnO nano powder prepared by spray pyrolysis method with different concentration of precursor solution（0.025 mol/L,0.05mol/L,0.075 mol/L）and other doping oxides,and compare with ordinary ZnO.The ZnO varistor consisted of ZnO phase,Bi₂O₃ rich phase,Zn₇Sb₂O₁₂2 spinel phase,Bi₃Zn₂Sb₃O₄pyrochlore-type phase and Sm rich phase.The density of ZnO varistor prepared by ZnO powder prepared at concentration at 0.075 mol/L was biggest,which was 5.5276 g/cm³,moreover,the minimum varistor voltage was 553.4 V/mm,which was about 55.6%lower than that of the varistor produced by ordinary ZnO,and nonlinear coefficient was 10.2,the leakage current was 52μA.（4）The ZnO varistor composite powders were prepared by spray pyrolysis method doped with different concentration of Sm₂O₃.With increasing the doped concentration of Sm₂O₃,the crystallite size increased and the particle sizes had no change.With increasing the doped concentration of Sm₂O₃,the crystallite size and density of voristor increased,and the performance of varistor improved.The density of ZnO varistor prepared by ZnO varistor composite powders which doped concentration at 0.3 mol/L was biggest,which was 5.3662 g/cm³,moreover,the minimum varistor voltage was 880.4 V/mm,which was about 11%lower than that of the varistor produced by ZnO varistor composite powders that no doped Sm₂O₃,and nonlinear coefficient was 9.7,the leakage current was 59μA.