Preparation Of ZnO Varistor Ceramics By Reduction-reoxidation Method And The Research Of Key Technology Of Multilayer Varistors

ZnO MLVs are widely utilized for protecting low-voltage integrated circuits from electrostatic discharge because of their excellent nonlinear I-V characteristics.For ZnO and the additives are easy to be reduced when sintered in an atmosphere with low oxygen pressure,ZnO MLVs are required to be sintered in air.At this circumstance,the noble metal Ag/Pd alloys with high melting point and resistance to oxidation are adopted as the inner electrodes of ZnO MLVs,which led to the cost of production staying a high level.At the same time,the diffusion of Ag and the chemical reactions of Pd with Bi₂O₃ deteriorated the electrical properties of ZnO MLVs.In this paper,the base metal Ni was adopted as the inner electrodes of ZnO MLVs.For Ni is easy to be oxidized when sintered in air,the reduction-reoxidation method was adopted to prepare the ZnO MLVs.In which,the samples were firstly sintered in a protection atmosphere?N2?or a reducing atmosphere?N2-H2?to protect Ni from oxidizing and then re-oxidized in air at a relatively lower temperature to control the nonlinear properties of ZnO MLVs.The main studies as follows:The sintering properties of ZnO and binary system ZnO-Bi₂O₃ sintered in N2 and N2-H2 were studied,it was found that the ZnO and Bi₂O₃ were easy to be reduced thereby volatilized.As a result,the densification and grain growth of ZnO were inhibited?the average grain sizes were lower than 0.6?m?.In order to resolve the problems above,boron nitride?BN?was proposed as an anti-reduction additive to prevent ZnO-Bi₂O₃ based ceramics form reducing and evaporating.The results showed that the addition of BN inhibited the evaporation of ZnO and the additives,and the dense ceramics were obtained by sintering the samples in N2-H2?99.5%-0.5%?at 1000 0C.In which,the relative density of ceramics as-sintered in N2-H2 was 95.9%,the average grain size of them was 10.6 ?m and the mass loss of them was 2.4%.According to the back-scatted SEM images and the XRD results of the quenched samples,liquid phase Bi-B-O was formed in ceramic bodies in sintering process since BN absorbed the oxygen released by ZnO and additives.It was deduced that the liquid phase inhibited the evaporation of bismuth?Bi?and zinc?Zn?.On the other hand,it promoted the densification and grain growth of ZnO ceramics by coating ZnO particles in sintering process and preventing them from reducing.For Bi₂O₃ were reduced to metallic Bi and the O escaped from grainboundaries,the acceptor states in grainboundaries were deficient and the Double Schottky barriers?DSBs?were nonexistent.Thus the I-V characteristic was linear.In re-oxidation process,the metallic Bi absorbed oxygen and formed Bi₂O₃-rich phase.The re-segregation of Bi₂O₃-rich phase at grain-boundaries and multi-grain junctions promoted the transmission and absorption of oxygen.As a result,DSBs were formed and samples re-oxidized in air at 850? obtained nonlinear properties.The influence of Sb₂O₃ on the effective re-oxidation temperature and electrical properties of ZnO-Bi₂O₃ based varistors was studied.It was found that the addition of Sb₂O₃ shifted the effective re-oxidation temperature to a higher temperature.For the Sb₂O_3-free samples,the effective re-oxidation temperature was as low as 800?,while that of Sb₂O₃-added samples was 850?.The EDAX results showed that the content of metallic Sb dissolved in metallic Bi phase increased with the increasing addition of Sb₂O₃.For the oxidation of metallic Sb was prior to metallic Bi,it absorbed oxygen and formed Sb₂O₃?Sb2O5?thereby inhibited the oxidation of metallic Bi in re-oxidation process.In addition,the formed Sb₂O₃?Sb2O5?reacted with ZnO or Bi₂O₃,and formed spinel or pyrochlore phase with the consumption of oxygen.Then,the second phase particles pinned at the multi-grain junctions and grain-boundaries,therewith,led to the block of transmission paths of oxygen and improved the barrier of oxygen diffusion.As a result,the effective re-oxidation temperature of Sb2o3-added samples increased.To improve the electrical properties of ZnO-based varistors,the effects of transition metal oxides MnO2 and Co₂O₃ on improving the properties of ZnO-Bi₂O₃ based varistors were studied.It was found that the doped of MnO2 or Co₂O₃ promoted the grain growth,improved the nonlinear coefficient and reduced the leakage current density of ZnO-Bi₂O₃ based varistors.The results of First-principle calculations demonstrated that the co-doped of MnO2 and Co₂O₃ prevented the elements Co and Mn from dissolving into ZnO grains and promoted them segregating to the grain-boundaries.The segregation of Co and Mn at grain-boundaries enhanced the absorption of oxygen and improved the density of acceptor VZn in re-oxidation process.Thus,the double schottky barriers?DSBs?at grain-boundaries were enhanced and the nonlinear properties of ZnO-Bi₂O₃ based varistors were improved.Green bodies of ZnO-Bi₂O₃ based MLVs with base metal Ni inner electrodes were prepared by tape-casting method.The co-firing compatibility of Ni inner electrodes with ZnO varistor ceramics and the properties of ZnO-Bi₂O₃ based MLVs were studied.It was found that Ni inner electrodes and ZnO varistor ceramics was able to co-fire,and the re-oxidation temperature had important influence on the electrical properties of ZnO-Bi₂O₃ based MLVs.At the re-oxidation temperatures lower than 650?,the grain-boundaries were not oxidized and the electrical properties of ZnO-Bi₂O₃ based MLVs were poor.However,the oxidation of Ni inner electrodes deteriorated the electrical properties of ZnO-Bi₂O₃ based MLVs at the re-oxidation temperatures higher than 750?.The equivalent model of the samples with the oxidation of Ni was built.Furthermore,the influence mechanism of NiO layer on the electrical properties of ZnO-Bi₂O₃ based MLVs was studied by the amendatory RC circuit model.Moreover,the influence of re-oxidation time on the electrical properties of ZnO-Bi₂O₃ based MLVs was studied.The results indicated that 1)the re-oxidation temperature should be higher enough to promote the formation of Bi₂O₃ and then re-segregate to multi-grain junctions and grain-boundaries;2)the re-segregation of Bi₂O₃ was a slow process which required enough time.In this paper,the optimal re-oxidation temperature and time were 700? and 2 hours respectively.In present work,ZnO MLVs co-fired with Ni inner electrodes obtained the electrical properties of V1mA=16.3 V,a=26.5 and IL=0.68 ?A by reduction-reoxidation method,It provided a new way to replace Ag/Pd alloys by base metal Ni to cut down the cost.