In this study we aims to examine the effects of Co3O4 and NiO doping on the defects and electrical properties in ZnO-Bi2O3-Sb2O3 (Sb/Bi=0.5) varistors. It seemed to form □(0.20 eV) and □(0.33 eV) as dominant defects in Co and Ni co-doped ZBS system, however only □appeared in Co- or Ni-doped ZBS. Even though the same defects it was different in capacitance (1.5∼4.5 nF) and resistance (0.3∼9.5 kΩ). The varistor characteristics were improved with Co and Co+Ni doping (non-linear coefficient, α= 36 and 29, relatively) in ZBS. The various parameters (Nd= 1.43∼2.33×1017 cm-3, Nt=1.40∼ 2.28×1012 cm-2, Φb= 1.76∼2.37 V, W= 98∼118 nm) calculated from the C-V characteristics in our systems did not depend greatly on the type of dopant, which were in the range of a typical ZnO varistors. It should be derived a improved C-V equation carefully for more reliable parameters because the variation of the varistor capacitance as a function of the applied dc voltage is depend on the defect, frequency, and temperature.

Citations

• Defect-controlled charge transport and switching in PLD-grown Zn0.90Ni0.10O MIS capacitors for non-volatile memory applications
  Sunena Subhash, Sudheendran Kooriyattil, T.S. Akhil Raman, S. Shyam Shankar, K.C. James Raju
  Physica B: Condensed Matter.2026; 739: 418941.     CrossRef

In this study we aims to examine the co-doping effects of 1/3 mol% Mn3O4+Co3O4 (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,Co-doped ZBS, ZBS(MCo) varistors were controlled by Sb/Bi ratio. Pyrochlore (Zn2Bi3Sb3O14) was decomposed and promoted densification at lower temperature on heating in Sb/Bi=1.0 by Mn rather than Co. Pyrochlore on cooling was reproduced in all systems however, spinel (α- or β-polymorph) did not formed in Sb/Bi=0.5. More homogeneous microstructure was obtained in Sb/Bi≥1.0. In ZBS(MCo), the varistor characteristics were improved drastically (non-linear coefficient, α=30∼49), and seemed to form (0.17 eV) and (0.33 eV) as dominant defects. From impedance and modulus spectroscopy (IS & MS), the grain boundaries have divided into two types, i.e. the one is tentatively assign to ZnO/Bi2O3(Mn,Co)/ZnO (0.47 eV) and the other ZnO/ZnO (0.80∼0.89 eV) homojunctions.

In this study we aims to evaluate the effects of 1/3 mol% Co3O4 addition on the reaction, microstructure development, resultant electrical properties, and especially the bulk trap and grain boundary properties of ZnO-Bi2O3-Sb2O3 (Sb/Bi=2.0, 1.0, and 0.5) system (ZBS). The samples were prepared by conventional ceramic process, and characterized by XRD, density, SEM, I-V, impedance and modulus spectroscopy (IS & MS) measurement. In addition of Co3O4 in ZnO-Bi2O3-Sb2O3 (ZBSCo), the phase development, density, and microstructure were controlled by Sb/Bi ratio. Pyrochlore on cooling was reproduced in all systems. The more homogeneous microstructure was obtained in ZBSCo (Sb/Bi=1.0) system. In ZBSCo, the varistor characteristics were improved drastically (non-linear coefficient α=23∼50) compared to ZBS. Doping of Co3O4 to ZBS seemed to form V*o (0.33 eV) as dominant defect. From IS & MS, especially the grain boundary of Sb/Bi=0.5 system is composed of electrically single barrier (0.93 eV) and somewhat sensitive to ambient oxygen with temperature.

In this study, we have investigated the effects of Co doping on I-V curves, bulk trap levels and grain boundary characteristics of ZnO-Bi2O3 (ZB) varistor. From I-V characteristics the nonlinear coefficient (a) and the grain boundary resistivity (ρgb) decreased as 32→22 and 18.4→0.6×10(9) Ωcm with sintering temperature (900∼1,300℃), respectively. Admittance spectra and dielectric functions show two bulk traps of zinc interstitial, Zn(i)·· (0.16∼0.18 eV) and oxygen vacancy, Vo· (0.28∼0.33 eV). The barrier of grain boundaries in ZBCo (ZnO-Bi2O3-Co3O4) could be electrochemically single type. However, its thermal stability was slightly disturbed by ambient oxygen because the apparent activation energy of grain boundaries was changed from 0.93 eV at the 460∼580 K to 1.13 eV at the 620∼700 K. It is revealed that Co dopant in ZB reduced the heterogeneity of the barrier in grain boundaries and stabilized the barrier against the ambient temperature.