Grain boundary structure in TiO 2 -excess barium titanate
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Grain boundary structure in TiO2 -excess barium titanate Takahisa Yamamoto, Yuichi Ikuhara, Katsuro Hayashi, and Taketo Sakuma Department of Materials Science, The University of Tokyo, 7-3-1 Hongo, Bukyo-ku, Tokyo 113, Japan (Received 2 January 1998; accepted 6 March 1998)
Grain boundary structure was examined in 0.1 mol % TiO2 -excess BaTiO3 by high-resolution electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). Their grain boundaries were mostly faceted with h210j type habit. The faceted boundaries were characterized to be associated with an extra Ti–O2 bond with the rutile-like structure. The grain growth behavior in a small TiO2 -excess BaTiO3 is discussed from the viewpoint of grain boundary structure.
I. INTRODUCTION
BaTiO3 is one of the most important materials used for electroceramic devices such as thermistors, sensors, and condensers. Their electrical properties are often regarded as grain boundary phenomena.1 The positive temperature coefficient of resistivity (PTCR) effect in a donor-doped BaTiO3 is a typical example.2 From the analysis of grain orientation relationship, we found that the PTCR effect is dependent on the grain boundary structure.3 More detailed analysis on the grain boundaries from a viewpoint of atomic boundary state is essential to understand the electrical properties such as PTCR effect. In the present study, the grain boundary structure and chemical bonding state in coarse-grained BaTiO3 with a small excess of TiO2 was examined using a highresolution electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS). II. EXPERIMENTAL PROCEDURE
The starting material used in this study is high-purity and fine-grained commercial-based BaTiO3 powders (BT01 Lot. No. 000001, Sakaikagaku Co. Ltd., Japan) fabricated by the hydrothermal method. The nominal grain size, purity, and BayTi ratio are 0.1 mm, 99.98%, and 1.000, respectively. Excess TiO2 was added using a Ti-containing solution prepared by dissolving Ti(i –OC4 H9 )4 in C4 H9 OH. The BaTiO3 raw powders and the solution were ball-milled for 4 h to obtain 0.1 mol % TiO2 -excess BaTiO3 compound. Subsequently, the mixture was calcined at 900 ±C for 2 h, and ball-milled again for 6 h. After drying, they were sieved with a 250 mm mesh, and uniaxially pressed at 15 MPa in a cemented carbide die into a square bar with 5 3 5 3 25 mm3 in size. They were further isostatically pressed under a pressure of 150 MPa. The green compacts were sintered at 1300 ±C for 5 h in air. Grain boundary structure was characterized using a conventional transmission electron microscope (CTEM, H-800, HITACHI, Japan) and a high resolution J. Mater. Res., Vol. 13, No. 12, Dec 1998
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transmission electron microscope (HRTEM, EM-002B, TOPCON, Japan), both operated at 200 kV. For TEM observations, the specimens were prepared by a standard procedure including ion-thinning method (MODEL 600, GATAN). Electron energy loss spectroscopy (EELS) was
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