Influence of Ag doping on the microstructure and electrical properties of ZnO-Bi₂O₃-based varistor ceramics

Abstract

Silver in the form of AgNO₃ was added to ZnO-based varistor ceramics prepared by the solid-state reaction method. The effects of AgNO₃ on both the microstructure and electrical properties of the varistors were studied in detail. The optimum addition amount of AgNO₃ in ZnO-based varistors was also determined. The mechanism for grain growth inhibition by silver doping was also proposed. The results indicate that the varistor threshold voltage increases substantially along with the AgNO₃ content increasing from 0 to 1.5mol%. Also, the introduction of AgNO₃ can depress the mean grain size of ZnO, which is mainly responsible for the threshold voltage. Furthermore, the addition of AgNO₃ results in a slight decrease of donor density and a more severe fall in the density of interface states, which cause a decline in barrier height and an increase in the depletion layer.

Keywords

ceramic materials / varistors / silver / microstructure / electrical properties / zinc oxide

References

[1] Gupta T.K. Application of zinc oxide varistors. J. Am. Ceram. Soc., 1990, 73(7): 1817.

[2] Bernik S., Maček S., Bui A. The characteristics of ZnO-Bi₂O₃-based varistor ceramics doped with Y₂O₃ and varying amounts of Sb₂O₃. J. Eur. Ceram. Soc., 2004, 24(6): 1195.

[3] Peiteado M. Zinc oxide-based ceramic varistors. Bol. Soc. Esp. Ceram. Vidrio, 2005, 44(2): 77.

[4] Fan J., Freer R. The electrical properties and d.c. degradation characteristics of silver doped ZnO varistors. J. Mater. Sci., 1993, 28(5): 1391.

[5] Leach C. Grain boundary structures in zinc oxide varistors. Acta Mater., 2005, 53(2): 237.

[6] Fan J.W., Freer R. The roles played by Ag and Al dopants in controlling the electrical properties of ZnO varistors. J. Appl. Phys., 1995, 77(9): 4795.

[7] Nahm C.W. Effect of cooling rate on degradation characteristics of ZnO·Pr₆O₁₁·CoO·Cr₂O₃·Y₂O₃-based varistors. Solid State Commun., 2004, 132(3–4): 213.

[8] Bernik S., Daneu N. Characteristics of ZnO-based varistor ceramics doped with Al₂O₃. J. Eur. Ceram. Soc., 2007, 27(10): 3161.

[9] Branković Z., Branković G., Bernik S., Žunić M. ZnO varistors with reduced amount of additives prepared by direct mixing of constituent phases. J. Eur. Ceram. Soc., 2007, 27(2–3): 1101.

[10] Peiteado M., Fernández J.F., Caballero A.C. Varistors based in the ZnO-Bi₂O₃ system: Microstructure control and properties. J. Eur. Ceram. Soc., 2007, 27(13–15): 3867.

[11] Bernik S., Branković G., Rustja S., et al. Microstructural and compositional aspects of ZnO-based varistor ceramics prepared by direct mixing of the constituent phases and high-energy milling. Ceram. Int., 2008, 34(6): 1495.

[12] Yang C.C., Chen M.S., Hong T.J., Wu C.M. Preparation of (100)-oriented metallic LaNiO₃ thin films on Si substrates by radio frequency magnetron sputtering for the growth of textured Pb(Zr_0.53Ti_0.47)O₃. Appl. Phys. Lett., 1995, 66(20): 2643.

[13] Morris W.G. Physical properties of the electrical barriers in varistors. J. Vac. Sci. Technol., 1976, 13(4): 926.

[14] Weast R.C. Handbook of Chemistry and Physics, 1989 Boca Raton, CRC Press, B-68.

[15] Mukae K., Tsuda K., Nagasawa I. Non-ohmic properties of ZnO-rare earth metal oxide-Co₃O₄ ceramics. J. Appl. Phys., 1977, 16(8): 1361.

[16] Nahm C.W. The effect of sintering temperature on electrical properties and accelerated aging behavior of PCCL-doped ZnO varistors. Mater. Sci. Eng. B, 2007, 136(2–3): 134.

[17] Pianaro S.A., Bueno P.R., Olivi P., et al. Effect of Bi₂O₃ addition on the microstructure and electrical properties of the SnO₂·CoO·Nb₂O₅ varistor system. J. Mater. Sci. Lett., 1997, 16(8): 634.

[18] Gupta T.K., Miller A.C. Improved stability of the ZnO varistor via donor and acceptor doping at the grain boundary. J. Mater. Sci., 1988, 3(4): 745.

[19] Zhou Z., Kato K., Komaki T., et al. Effects of dopants and hydrogen on the electrical conductivity of ZnO. J. Eur. Ceram. Soc., 2004, 24(1): 139.

[20] Mukae K., Tsuda K., Nagasawa I. Capacitance-vs-voltage characteristics of ZnO varistors. J. Appl. Phys., 1979, 50(6): 4475.