Relationship Between Electrical Performances And Grain Boundary Characteristics Of Oxide Varistor Ceramics

Relationship between electrical performances and grain boundary characteristics ofoxide varistor ceramics was investigated. The main achievements are as follows:1. The effect of the electrical loading history on the teriminal capacitance of ZnOvaristor samples was tested and analyzed. Dynamic change of the defect structure in the grainboundaries of ZnO varistor materials was investigated. The teriminal capacitance is stronglyinfluenced by the electrical loading. The changing of the measured terminal capacitance is areflection of the changing of the depletion layer thickness in the grain boundaries. Twomechanisms have been proposed. One is the capture and emission of electrons by the interfacestates, and the other is impact ionization and recombination (trapping of electrons) of the deeplevel donor defects in the depletion layer. Some electrons are captured by the interface stateswhen the varistor sample is tested with a electric field, and the captured electrons are emittedafter disconnecting of the electric field. The electron capturing in the interfaces increase thenegative charge and also the depletion layer thickness, and opposite process will happen withthe captured electrons emitting. When the varistor sample is subjected to a strong electricalimpulse, in addition to the electron capturing in the interfaces, impact ionization of deep leveldonor defects will happen in the depletion layer. The ionizing of the deep level donor defectswill increase the positive charge concentration in the depletion layer, the depletion layer willthus decrease. The captured electrons emitting in the interface and the electrons trapping inthe depletion layer are two opposite factors influencing the dynamic change of the defectstructure and the recovering process of the depletion layer thickness after the surge currenttest. The non-monotonic changing process of the terminal capacitance is attributed to thedifferent time constants of the captured electrons emitting in the interfaces and the electronstrapping by the ionized deep level donor defects in the depletion layers.2．Different approaches for estimation of the Schottdy barrier height in the grain boundaries of oxide varistor ceramic materials were investigated. The C-V method (based onthe modified Mott-Schottky equation) is reasonable in physical principle. But it is difficult toobtain a true value of the barrier height because microstructure and defect structure of varistormaterial is much more complicated than a metal-semiconductor contact junction. TheSchottdy barrier's parameters including the the barrier height can be calculated according tothe J-E-T method (based on the Schottky emission equation) without concern for themicrosturcture or defect structure. But the ln(J0)~1/T plots aren't usually straight lines. Thecalculted barrier heights are different with different temperature range choosing. The lowelectrical field lnρ-1/T plot method (based on the Arrhenius equation) is easy and feasible toestimated the Schottdy barrier height. The barrier height can be reasonbly approcimated as theactivation energy calculated according to the low electrical field lnρ-1/T plot because theconduction mechanism in low field is solely electrons thermal activation acrossing theSchottdy barriers.3. Different from the results presented in literature, the starting ZnO particle sizes have alimited impact on the grain sizes and the varistor voltage gradients of the ZnO varistorceramics. Varistor samples were fabricated by using the nano-ZnO powder synthesized byplasma vapor-phase reaction (PVPR) process and submicron-ZnO powder manufactured byFrench process, respectively. The sintering activity of the samples prepared with thenano-ZnO powder is higher, but there is only a little difference between the mean grain sizesof the samples sintered at the same temperature. The current–voltage (I–V) characteristics inthe low-current region are similar. However, when subjected to surge currents, the residualvoltage ratio of the samples prepared using the PVPR ZnO powder is much lower.Comparative analysis of the dielectric property and the grain-boundary barrier height revealsthat the donor concen-tration of the ZnO crystal grains in the sample prepared using thePVPR ZnO powder is higher than that prepared using French process ZnO powder. A higher donor concentration gives rise to a higher grain conductivity, a higher nonlinear coefficient inthe high-current region and a better protection level in applications.4. Ceramic samples of CaCu3Ti4O12(CCTO) without additive were prepared and thevoltage–current characteristics were investigated. Extremely strong nonlinear behavior,similar to the reported results in literature [Nature Materials3[11]774–8(2004); Appl. Phys.Lett.,89[19]191907(2006); Appl. Phys. Lett.,89[21]212102(2006); Appl. Phys. Lett.,91[9]091912(2007)] was observed. However, we found that this is a measurement illusion due to athermionic emission current feedback amplification effect. Compared to that of ZnO varistor,the thermionic emission current of CCTO sample is about six orders of magnitude higher andit is feedback amplified due to strong negative temperature coefficient resistance (NTCR)behavior when the CCTO sample is tested with a voltage (or current) source. Eliminating thethermionic emission current feedback amplification effect, the true value of nonlinearcoefficient of the CCTO sample is much lower. The nonlinearity of all variants of CCTO(CCTO with different dopants) should be measured carefully to avoid the thermionic emissioncurrent amplification effect.