Study of Relationships Among Microstructural, Electrical and Photoelectrolytic Properties of Yttrium-Doped Barium Titanate Ceramics

• Main Authors: Chun-Ming Huan, 黃俊銘
• Other Authors: Tzong-Lin Jay Shieh
• Format: Others
• Language: zh-TW
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/39137697387569714824

博士 === 國立臺灣大學 === 材料科學與工程學研究所 === 100 === Intricate connections among the microstructural effect, semiconducting tendency and charge compensation behavior of yttrium (Y3+) dopants in near-stoichiometric barium titanate (BaTiO3; Ba/Ti atomic ratio = 0.999) ceramics sintered at 1460 ◦C in air are examined. It is found that with increasing Y3+ doping from 0 to 2.0 mol%, the microstructure of BaTiO3 evolves from a liquid-phase-assisted dense-sintered microstructure to a highly porous microstructure characterized by connected pores and loose lattices of fused submicrometre grains. When Y3+ doping is increased progressively from 0.02 to 0.2 mol%, the (negative) majority carrier concentration and conductivity are increased substantially by 4-8 orders of magnitude. This increase in n-type semiconductor characteristics is contributed not only by the increasing substitution of Y3+ for Ba2+ in host BaTiO3, but also by the formation of yttrium-rich and/or oxygen-deficient precipitates at the grain boundaries. The grain boundary phases would therefore stabilize the mechanism of free electron compensation and enable the transportation of electrons through the grain boundaries. Through a combined interpretation of the characterization data, Y3+ doping at 1.0 mol% is found to be the critical doping amount separating different site-occupying behaviors of Y3+ in the BaTiO3 cation sites, which eventually lead to different charge compensation mechanisms and semiconductor properties. As the amount of Y3+ doping is increased from 0 to 0.1 mol%, the Curie temperature for the cubic-to-tetragonal phase transition (Tc) increases due to the reduction in Schottky defects. At the Y3+ doping of 0.2 mol%, Tc starts to decrease due to the formation of low-temperature second phases. Finally, when Y3+ doping is increased to 1.0-2.0 mol%, Tc decreases further due to the increase in oxygen vacancy concentration and the formation of low-temperature second phases. Experimental data from the electron paramagnetic resonance, cathodoluminescsnce and photocatalytic characterizations show that 0.1 mol% Y-doped BaTiO3, which possesses polarons and shallow donor or acceptor energy levels, is suitable for photocatalytic applications under visible light irradiation; while 0.2 mol% Y-doped BaTiO3, which has a higher overall defect concentration, is suitable for photocatalytic applications under UV irradiation.