The Effect of TiO 2 and B 2 O 3 Additions on the Grain Growth of ZnO
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INTRODUCTION
ZINC oxide-based materials have attracted much interest because of their unique properties for wide-ranging applications such as gas sensors,[1] piezoelectric transducers,[2] solar cells,[3] transparent conductive thin films,[4] LED-laser diodes,[5] biosensors,[6] and varistors.[7] The n-type electrical conductivity of undoped ZnO is caused by stoichiometric deviations in the form of interstitial zinc atoms and/or oxygen vacancies. In the application of ZnO as an electronic material, control of these native defects is essential since the electrical properties of these materials are largely affected by the native defects and extrinsic dopants.[8–10] The control of microstructure development is crucial to obtain ZnO ceramics with desirable electronic properties. The electrical properties of ZnO-based materials are directly related to the ratio of compositions as well as the microstructural properties such as grain size, secondary phases, and porosity. Recently, most studies are focused on the effect of various metal oxide additions such as Bi2O3,[11] Sb2O3,[12] Al2O3,[13] V2O5,[14] MnO,[15] TiO2,[16] and B2O3[8] on the microstructure and electrical properties of ZnO-based materials. It is well known that liquid phase sintering can enhance the densification and grain growth by accelerating mass transfer. Some metal oxide additions create the formation of a liquid phase in ZnO ceramics during sintering. For instance, the addition of Bi2O3 forms the liquid phase through a eutectic melting with ZnO at 1011 K (738 C).[17] Similarly, the effect of B2O3 addition which melts at 723 K (450 C) on the grain GO¨KHAN HARDAL and BERAT YU¨KSEL PRICE are with the Metallurgical and Materials Engineering Department, Engineering Faculty, Istanbul University, Avcilar, Istanbul 34320, Turkey. Contact e-mail: [email protected] Manuscript submitted August 29, 2016. Article published online February 6, 2017 2090—VOLUME 48A, APRIL 2017
growth of ZnO is reported in our previous study.[8] Hng and Halim reported that liquid phase sintering with the addition of V2O5 to ZnO ceramics above 1173 K (900 C) enhanced the densification for the ZnO-V2O5 binary system.[18] Another important observation in the literature is that the presence of a secondary phase in ZnO ceramics may give rise to the grain growth inhibition of ZnO ceramics for some binary system. The Sb2O3 addition acts as a grain growth inhibitor because of the formation of an orthorhombic spinel Zn7Sb2O12 phase at grain boundaries.[19,20] It is also reported that the grain growth of ZnO-based ceramics is hindered by the addition of Al2O3 due to the formation of a spinel ZnAl2O4 phase.[21] In recent years, considerable attention has been given to the ZnO-TiO2 system due to its application in many fields such as varistors,[22] gas sensor,[23] and microwave dielectric materials.[24] There are few works designated to lowering the sintering temperature of the ZnO-TiO2 system.[16,25,26] Nie et al. investigated the effect of V2O5 addition on the ZnO-TiO2 system to create liqu
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