Estimation of PTCR effect in single grain boundary of Nb-doped BaTiO3

Abstract

Semiconducting BaTiO3 ceramics exhibit a resistivity anomaly around the ferroelectric Curie temperature of about 130 °C, which is called the positive temperature coefficient of resistivity (PTCR) effect [1]. The PTCR effect is a phenomenon associated with grain boundaries because the effect does not appear in single crystals [2]. This effect is usually explained using Heywang's model [3] which assumes double Schottky barriers in grain boundaries. However, it is not easy to explain the observed temperature dependence of resistivity quantitatively and to find the origin of barriers because the PTCR effect is sensitive to various factors such as fabrication process and type of additive [4]. Measurements of temperature dependence of resistivity for a single grain boundary may be worthwhile to understand the origin of PTCR effect. Several preliminary trials have been made to examine the PTCR effect of single grain boundaries, and have shown that the resistivity change depends on grain boundaries [5, 6]. However, the difference in the PTCR effect with grain boundaries has not been explained yet. The present paper reports that the PTCR effect appears in ordinary boundaries but not in twin boundaries, and is grain boundary structure dependent. The raw materials used are high-purity, finegrained commercial BaTiO 3 powders fabricated through hydrothermal method (Sakaikagaku Co. Ltd., BT01, Lot. No. 0000010). Nb s+ ion was chosen as donor dopant. The 0.1 mol % Nb was doped into BaTiO3 with a dilute solution of Nb(O-n-C4H9) and C2H5OH. The Nb-doped BaTiO3 polycrystals were fabricated through pressureless sintering at 1300 °C for 5 h in air. Sinters with a grain size of about 2/xm were obtained. Thin plates with a size of 4 × 3 × 0.2 mm 3 were machined from the sinters. They were annealed at 1380 °C for 3 h in air for grain growth, and furnace cooled. The grain size of more than 300/~m was obtained after annealing. The resistance was measured with two probes in the temperature range between room temperature and 300 °C at a rate of 3 °C/min through the digital voltmeter (ADVANTEST R6551). The micro-manipulator was used to connect electrodes and leads on particular grains. Ohmic Ag paste and two Pt wires with 25/xm diameter were fixed at 500 °C for 1 h on two adjacent grains. Fig. 1 shows an example of the fixed electrodes. The size of each electrode is about 100/xm in diameter and the distance between the two electrodes is about 250/xm. Using these electrodes, the resistance across the grain boundary A-A must be