This paper focuses on the electrical properties and stability against DC accelerated aging stress of ZnO-V2O5-MnO2- Nb2O5-Bi2O3-Co3O4-Dy2O3 (ZVMNBCD) varistor ceramics sintered at 850 - 925 ℃. With the increase of sintering temperature, the average grain size increases from 4.4 to 11.8 mm, and the density of the sintered pellets decreases from 5.53 to 5.40 g/ cm3 due to the volatility of V2O5, which has a low melting point. The breakdown field abruptly decreases from 8016 to 1,715 V/cm with the increase of the sintering temperature. The maximum non-ohmic coefficient (59) is obtained when the sample is sintered at 875 ℃. The samples sintered at below 900 oC exhibit a relatively low leakage current, less than 60 mA/cm2. The apparent dielectric constant increases due to the increase of the average grain size with the increase of the sintering temperature. The change tendency of dissipation factor at 1 kHz according to the sintering temperature coincides with the tendency of the leakage current. In terms of stability, the samples sintered at 900 ℃ exhibit both high non-ohmic coefficient (45) and excellent stability, 0.8% in ΔEB/EB and -0.7% in Δα/α after application of DC accelerated aging stress (0.85 EB/85 oC/24 h).

This study describes the doping effect of Yb2O3 on microstructure, electrical and dielectric properties of ZnO-V2O5- MnO2-Nb2O5 (ZVMN) ceramic semiconductors sintered at a temperature as low as 900°C. As the doping content of Yb2O3 increases, the ceramic density slightly increases from 5.50 to 5.54 g/cm3; also, the average ZnO grain size is in the range of 5.3-5.6 μm. The switching voltage increases from 4,874 to 5,494 V/cm when the doping content of Yb2O3 is less than 0.1 mol%, whereas further doping decreases this value. The ZVMN ceramic semiconductors doped with 0.1 mol% Yb2O3 reveal an excellent nonohmic coefficient as high as 70. The donor density of ZnO gain increases in the range of 2.46-7.41×1017 cm−3 with increasing doping content of Yb2O3 and the potential barrier height and surface state density at the grain boundaries exhibits a maximum value (1.25 eV) at 0.1 mol%. The dielectric constant (at 1 kHz) decreases from 592.7 to 501.4 until the doping content of Yb2O3 reaches 0.1 mol%, whereas further doping increases it. The value of tanδ increases from 0.209 to 0.268 with the doping content of Yb2O3.

We have examined the co-doping effects of 1/2mol% NiO and 1/4mol% Cr2O3 (Ni:Cr=1:1) on the reaction,microstructure, and electrical properties, such as the bulk defects and the grain boundary properties, of ZnO-Bi2O3-Sb2O3 (ZBS;Sb/Bi=0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Ni,Cr-doped ZBS, ZBS(NiCr) varistors werecontrolled using the Sb/Bi ratio. Pyrochlore (Zn2Bi3Sb3O14), α-spinel (Zn7Sb2O12), and δ-Bi2O3 were detected for all ofcompositions. For the sample with Sb/Bi=1.0, the Pyrochlore was decomposed and promoted densification at lowertemperature by Ni rather than by Cr. A homogeneous microstructure was obtained for all of the samples affected by α-spinel.The varistor characteristics were not dramatically improved (non-linear coefficient, α=5~24), and seemed to formZni..(0.17eV) and Vo.(0.33eV) as dominant defects. From impedance and modulus spectroscopy, the grain boundaries werefound to have been divided into two types, i.e., one is tentatively assigned to ZnO/Bi2O3 (Ni,Cr)/ZnO (0.98eV) and the otheris assigned to a ZnO/ZnO (~1.5eV) homojunction.

In this study we aimed to examine the co-doping effects of 1/6mol% Co3O4 and 1/4mol% Cr2O3 (Co:Cr=1:1)on the reaction, microstructure, and electrical properties, such as the bulk defects and the grain boundary properties, of ZnO-Bi2O3-Sb2O3 (ZBS; Sb/Bi=0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Co,Cr-doped ZBS, ZBS(CoCr)varistors were controlled using the Sb/Bi ratio. Pyrochlore (Zn2Bi3Sb3O14), α-spinel (Zn7Sb2O12), and δ-Bi2O3 were formed inall systems. Pyrochlore was decomposed and promoted densification at lower temperature on heating in Sb/Bi=1.0 by Cr ratherthan Co. A more homogeneous microstructure was obtained in all systems affected by α-spinel. In ZBS(CoCr), the varistorcharacteristics were improved (non-linear coefficient, α=20~63), and seemed to form Zni..(0.20eV) and Vo.(0.33eV) asdominant defects. From impedance and modulus spectroscopy, the grain boundaries were found to be composed of anelectrically single barrier (0.94~1.1eV) that is, however, somewhat sensitive to ambient oxygen with temperature. The phasedevelopment, densification, and microstructure were controlled by Cr rather than by Co but the electrical and grain boundaryproperties were controlled by Co rather than by Cr.

We aimed to examine the co-doping effects of 1/6mol% Mn3O4 and 1/4mol% Cr2O3 (Mn:Cr=1:1) on the reaction,microstructure, and electrical properties, such as the bulk defects and grain boundary properties, of ZnO-Bi2O3-Sb2O3 (ZBS;Sb/Bi=0.5, 1.0, and 2.0) varistors. The sintering and electrical properties of Mn,Cr-doped ZBS, ZBS(MnCr) varistors werecontrolled using the Sb/Bi ratio. Pyrochlore (Zn2Bi3Sb3O14), α-spinel (Zn7Sb2O12), and δ-Bi2O3 (also β-Bi2O3 at Sb/Bi≤1.0)were detected for all of the systems. Mn and Cr are involved in the development of each phase. Pyrochlore was decomposedand promoted densification at lower temperature on heating in Sb/Bi=1.0 system by Mn rather than Cr doping. A morehomogeneous microstructure was obtained in all systems affected by α-spinel. In ZBS(MnCr), the varistor characteristics wereimproved dramatically (non-linear coefficient, α=40~78), and seemed to form Vo.(0.33eV) as a dominant defect. Fromimpedance and modulus spectroscopy, the grain boundaries can be seen to have divided into two types, i.e. one is tentativelyassigned to ZnO/Bi2O3 (Mn,Cr)/ZnO (0.64~1.1eV) and the other is assigned to the ZnO/ZnO (1.0~1.3eV) homojunction.

등온용량과도분광법(Isothermal Capacitance Transient spectroscopy)을 이용하여 ZnO 바리스터의 포획준위를 결정하였다. 여기서 등온용량과도분광기는 YHP 4192A 임피던스 Analyzer와 데이터해석을 위한 개인용 컴퓨터로 구성된다. 이 실험에서 우리는 ZnO-Bi2O3에 MnO 및 CoO를 첨가한계에서 -40˚C~60˚C 온도범위에서 0.28, 0.48, 0.50, 0.94eV 등의 입계포획준위가 존재함을 볼 수 있었다. 또한, ZnO-Bi2O3계는 CoO를 첨가하면 hole에 의한 emission특성을 나타내고, MnO를 첨가하면 전자에 의한 emission특성을 나타냄을 알 수 있었다. 그리고 비 직선저항계수 α는 도너농도의 감소에 직접적으로 비례하였으나, 포획준위의 밀도와는 별다른 비례관계를 발견할 수 없었다. 결론적으로 ZnO-Bi2O3-MnO계에 CoO를 첨가함에 따라 α값이 증가하는 한편, 포획준위밀도는 CoO의 첨가로 감소함을 알 수 있었다.