Vanadium Doped Nanostructured TiO 2 Dielectrics
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Vanadium Doped Nanostructured TiO2 Dielectrics Fatih Dogan1, Sheng Chao1, Jing Peng2, and Steve G. Greenbaum2 Department of Materials Science and Engineering, Missouri University of Science and Technology, Rolla, MO 65401, U.S.A 2 Department of Physics, Hunter College of CUNY, New York, NY 10065, U.S.A.
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ABSTRACT Dielectric properties of titanium oxide ceramics are strongly influenced by the microstructural features and concentration of dopants and impurity ions. Electrical conductivity (via insulation resistance) of vanadium doped nanostructured titanium dioxide (TiO2) ceramics was measured as a function of donor concentration and temperature. In order to further clarify the effect of the dopants on the microstructural development and resultant dielectric properties of TiO2, electron paramagnetic resonance (EPR) spectroscopy was employed. Vanadium-doped TiO2 exhibited well-defined hyperfine splitting characteristics of the 51V nuclei indicating that the dopant ions are dispersed within the grains and not preferentially segregated at the grain boundaries. INTRODUCTION The dielectric properties of TiO2 are affected by defects and impurities (whether “native” or dopants) [1] as well as processing conditions [2]. Macroscopically, the presence of oxygen vacancies creates an overall reduced state of the TiO2 ceramics in the bulk, surface and grain boundaries. Upon annealing, the creation of defects within the bulk is associated with the creation of color centers and Ti+3 interstitial ions, and an overall reduced stoichiometry as a result of the loss of oxygen atoms. This effect occurs on the surface and grain boundaries as well, where the presence of an oxygen vacancy site leaves an exposed Ti+3 ion which may be visualized as a Ti+4 ion associated with a somewhat localized electron [3]. The nonstoichiometry of the main structural phase of TiO2 (rutile) and its natural n-type conductivity is due to oxygen deficiency, which has been explained in terms of native defects, i.e. oxygen vacancies and titanium interstitials (attributed to a shallow donor) [4-6]. Nanostructured TiO2 has shown promise as a dielectric material for high energy density ceramic capacitors because of its high dielectric breakdown strength and dielectric constant [7]. Strategies to increase the insulation resistance or to reduce the leakage current of TiO2 include doping with transition metal ions. The effect of sintering temperature on Mn-doped nanostructured TiO2 dielectrics was studied by impedance spectroscopy, DC measurements and electron paramagnetic resonance (EPR) techniques [8]. It was shown that the insulation resistance of the sample sintered at 900 °C was higher than that of the sample sintered at 1100 °C. Impedance spectroscopy revealed that the grain boundary resistance of 900 °C sample with smaller grain size was significantly higher as compared to the 1100 °C sample while bulk resistivities of grains were similar for both samples. EPR studies confirmed that improvement of insulation resistance is correlated with a nonuniform distribut
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