PROCESSING AND CHARACTERISATION OF ZNO-BASED CERAMIC MATERIALS

Abstract

Zinc oxide (ZnO) has been of considerable interest to the optical and electronic industries because of its unique optical, electrical and acoustic characteristics. It is a promising material for many different applications, such as solar cells, gas sensors and semiconductors. Sintered zinc oxide exhibits a wide range of unique electrical properties when eloped with minor constituents. For example, zinc oxide-based varistors are high-resistance, semiconducting devices which, because of their highly non-linear current-voltage characteristics, are used to mitigate power surges ( e.g. lightening strikes) in electronic circuits and electric power transmission lines by providing an emergency conduit to ground (1,2). Nanosized zinc oxide powders have been prepared from inverse single microemulsion and double microemulsion systems consisting of Zn(NO3)2 solution as the aqueous phase, petroleum ether (PE, boiling point: l 00-120°C) as the oil phase and poly(oxyethylene), nonyl phenol ether (NP5) as the non-ionic surfactant. Zinc hydroxide (Zn(OH)2) precursor was obtained by carrying out the precipitation reaction between Zn(NO3)2 and ammonium hydroxide (NH4OH) in the microemulsion domains. Thermal analysis was then performed on the hydroxide precursor particles. The desired zinc oxide powders were obtained by subsequently calcining the hydroxide precursors. For comparison, zinc oxide powders were also prepared via the conventional precipitation and in the non-microemulsion region of the phase diagram. The zinc oxide powders obtained were characterised for phases present, particle size, BET specific surface area and particle morphology. A study was also carried out using oxalic acid (H2C2O4) instead of ammonium hydroxide as the precipitant in preparing fine zinc oxide powders from the microemulsion. The resulting zinc oxalate (ZnC,O4) precursor was calcined in order to obtain the desired zinc oxide powder, which was then fully characterised. The sinterability of the zinc oxide powders obtained via the single microemulsion route and those doped with minor constituents for varistor applications have been studied. Their gram sizes and densities at various sintering temperatures and times were analysed and their non-linear current-voltage characteristics and dielectric properties were also investigated. Some of the results in this project have been published in Ceramics International, 24, ( 1998) 205. The article can be referred to in Appendix D.