Abstract
The role of liquid phase in the enhancement of the PTCR (positive temperature coefficient of resistance) effect in (Ba 0.7 Sr 0.3 )TiO 3 (BST) with the addition of AST (4Al 2 O 3 . 9SiO 2 .3TiO 2 ) is investigated in this paper. The AST-BST samples were characterized with optical microscopy, transmission electron microscopy, energy-dispersive spectroscopy, and impedance spectroscopy. Microscopic observations showed that slower cooling might facilitate the precipitation of the (Ba,Sr)TiO 3 phase from the liquid phase on matrix grains since the amount of liquid phase was reduced with a decreasing cooling rate. Impedance spectroscopy indicated that this variation accompanied the change in the intrinsic properties of grain boundaries, which could not be explained by well-known oxidation effects. With the aid of a brick-layer model and high-resolution transmission electron microscopy (HRTEM), it appeared that the change in electrical characteristics of grain boundaries with decreasing cooling rate originated from the precipitation of (Ba,Sr)TiO 3 . Finally, the effect of precipitated (Ba,Sr)TiO 3 on the PTCR characteristics is discussed in terms of the acceptor-state density and the polarization state at grain boundaries.
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