Power loss transition of stable ZnO varistor ceramics: Role of oxygen adsorption on the stability of interface states at the grain boundary

Highly stable ZnO varistor ceramics with steadily decreasing power loss have been put into applications in electrical and electronic systems for overvoltage protections, even with the absence of general understandings on their aging behaviors. In this paper, we investigated their aging nature via conducting comparative direct current (DC) aging experiments both in air and in nitrogen, during which variations of electrical properties and interface properties were measured and analyzed. Notably, continuously increasing power loss with severe electrical degradation was observed for the sample aged in nitrogen. The power loss transition was discovered to be closely related to the consumption of oxygen adsorption at the grain boundary (GB), which could, however, remain constant for the sample aged in air. The interface density of states (DOS) Ni, which is crucial for pinning the potential barrier, was proved to decrease in nitrogen, but keep stable in air. Therefore, it is concluded that the oxygen adsorption at the GB is significant for the stability of interface states, which further correlates to the long-term stability of modern stable ZnO varistor ceramics.

Microstructure, electrical and dielectric properties, and aging behavior of ZPCCA varistor ceramics with Er_2O_3 doping

The microstructure, electrical and dielectric properties, and DC-accelerated aging of the ZPCCA （ZnO-Pr6O11-CoO- Cr203-A1203） ceramics were investigated with various contents of Er203. The ceramic phases consisted of a bulk phase of ZnO grains, and a minor secondary phase of mixture of Pr6O11 and Er203. The increase of the content of doped Er203 increased the densities of sintered pellet from 5.66 to 5.85 g/cm3, and decreased the average grain size from 9.6 to 6.3 μm. With the increase of the content of doped Er203, the breakdown field increased from 2390 to 4530 V/cm, and the nonlinear coefficient increased from 28.4 to 39.1. The sample doped with 0.25 mol.% Er203 exhibited the strongest electrical stability; variation rates for the breakdown field measured at 1.0 mA/cm2, and for the non-ohmic coefficient were -3.4% and -23,8%, respectively, after application of a stress of 0.95 Eu/125 ℃/24 h.

Electrical properties of structures based on varistor ceramics and polymer nanocomposites with carbon filler

The characteristics of a two-layer structure on the basis of the layers of varistor ceramics and polymeric PPTC nanocomposite being in thermal contact for the purpose of using it as a limiter of constant voltage and long-term varying electrical overvoltages are analyzed.A theoretical model of such a structure has been developed,and its main electrical characteristics are simulated.It is shown that the provision of the required output voltage limitation is performed by selecting the classification voltage of the varistor layer.The maximum current of the varistor layer required for heating the structure is determined by the intensity of heat transfer to the environment.It has established a satisfactory agreement between the theoretical and experimental electrical characteristics for the structure based on the layers used in commercial varistors and PPTC fuses.

Significantly enhanced varistor properties of CaCu_(3)Ti_(4)O_(12) based ceramics by designing superior grain boundary:Deepening and broadening interface states

Significantly enhanced varistor properties via tailoring interface states were obtained in Ca_(1-2x/3)Y_(x)Cu_(3)Ti_(4)O_(12)-SrCu_(3)Ti_(4)O_(12) composite ceramics.The breakdown field was improved to 35.8 kV cm^(-1) and the nonlinear coefficient in 0.1-1 mA cm^(-2) was enhanced to 14.6 for Ca_(0.67)Y_(0.5)Cu_(3)Ti_(4)O_(12)-SrCu_(3)Ti_(4)O_(12).Noticeably,the withstand voltage of single grain boundary reached up to 24 V while the reported ones were constant to about 3 V.Greatly improved properties were attributed to the formation of superior grain boundary rather than the reduced grain size.Surprisingly,with distinct discrepancy of nonlinear performance in the composites,the resistance and activation energy of grain boundary exhibited little differences.Based on the double Schottky barrier at grain boundary and the field-assisted thermal emission model,it was found that the excellent electrical nonlinearity arose from the formation of deeper and broader interface states at grain boundary.In this case,interface states were not easily entirely filled and the barrier could maintain its height under applied voltage.This work provides a novel routine for enhancing the varistor properties of CaCu_(3)Ti_(4)O_(12) based ceramics by manipulating interface states at grain boundary.

Nonlinear behavior of Tb_4O_7-modified ZnO-Pr_6O_(11)-based ceramics with high breakdown field

The effect of Tb407 on electrical behavior of the ZnO-Pr6Oll-based varistor ceramics was investigated. Microstructural analysis indicated that the addition of Tb407 decreased average grain size from 3.6 to 3.2 and increased the sintered density from 5.58 to 5.68 g/cm3. As the amount ofTb407 increased, the breakdown field increased from 9393 to 12437 V/cm and the nonlinear coefficient increased from 50 to 65. The varistor ceramics added with 0.5 mol.% in the amount of Tb407 exhibited an excellent stability by exhibiting 0. 1% in the variation rate of the breakdown field, 0% in the variation rate of the nonlinear coefficient, and 8.8% in the variation rate of the leakage current density for DC-accelerated aging stress of 0.85 E1 mA/115 ℃/24 h.

Effect of sintering temperature on microstructure and nonlinear electrical characteristics of ZnO varistor

The nonlinear properties of ZBMCCS-based varistors,which are composed of ZnO-Bi_(2)O_(3)-MnO_(2)-Cr_(2)O_(3)-Sb_(2)O_(3)-Co_(3)O_(4) and SiO_(2) are studied inrelation to sintering temperature,in the range of 1280–1350℃.The samples are investigated for grain morphology by using scanning electron microscope(SEM).These samples were examined by using X-ray diffraction patterns(XRD)and DC electrical measurements.X-ray diffraction analysis of the samples show the presence of ZnO,Zn_(2)SiO_(4) willemite phase and Co_(2.33)Sb_(0.67)O_(4) spinel phases.The average grain size of ZnO increased as the sintering temperature increased from 2.57 to 6.84
m.In the examined temperature range,the breakdown field decreased from 2992 to 127 V/cm with the increase of sintering temperature.This system gives a relatively high nonlinearity coefficientα=33.61(at a sintering temperature of 1280℃)with a low leakage current of 0.21 mA/cm^(2).

High-frequency capacitance-voltage characteristics of the heterogeneous structure based on the model of spherical semiconductor particles in a dielectric

The dependence of the parameters of the capacitance effect in heterogeneous dispersed two-component structures based on semiconductors from the bulk fraction of the semiconductor component is modeled.The used method for determining the changes of the energy bands bending on the surface of the spherical semiconductor particle by applying dc electric field allowed to calculate the changes of the dipole moment and effective(taking into account the polarization of the free charge)dielectric constant of this semiconductor particle.This result allowed to use the known models of the dielectric constant of two-component structures for the description of the capacitance field effect in the heterogeneous structures.The relations allowing to estimate the value of the bulk donor concentration in the semiconductor component of the matrix of the heterogeneous system and the statistical mixture have been obtained.The approbation of the obtained calculation relations to evaluate the donor concentration in the ZnO grains of zinc oxide varistor ceramics leads to the correct values that are consistent with estimates of other methods and models.It is established that the sensitivity of the relative dielectric constant to the applied dc electric field is dependent on the bulk fraction of the semiconductor particles in the heterogeneous structures.The bulk fraction of the semiconductor particles significantly affects on the dielectric constant beginning with the values from
0:8 for matrix systems and
0:33 for statistical mixtures.