Positive temperature coefficient resistance effect in Ba 1-x Sr x TiO 3 ceramics modified with Bi 2 O 3 and PbO by a vap
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Positive temperature coefficient resistance effect in Ba1ⴚxSrxTiO3 ceramics modified with Bi2O3 and PbO by a vapor-doping method Jianquan Qi, Zhilun Gui, Longtu Li, and Yajing Wu Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China (Received 24 July 1998; accepted 29 April 1999)
Ba1−xSrxTiO3-based positive temperature coefficient resistance (PTCR) ceramics were prepared by use of a vapor-doping method. When doped with Bi, the PTCR effect is improved; when doped with lead, however, the effect is weakened. The different influences of Bi and Pb doping on the ceramic properties are discussed in terms of the defect chemistry.
I. INTRODUCTION
For some semiconducting BaTiO3 ceramics, the resistivity increased with temperature above the Curie temperatures, a phenomenon known as positive temperature coefficient resistance (PTCR).1 Such an effect often occurs in BaTiO3-based ceramics doped with rare earth elements or Bi3+, Nb5+, Sb3+ ions. Investigations have revealed that apart from the type of dopants, the procedure of sample preparation, particularly the way of adding dopants, has also an important influence on the PTCR properties of BaTiO3 materials.2 In conventional ceramic processing, dopants are added into the main composition either before or after calcination. A special way, which is called vapor doping, however, was also used in the sample preparation of our previous work. In that way, the dopants were not put directly into the composition; instead, they were used to provide an atmosphere during sintering and through diffusion, the doped ions entered the ceramic.3 In this study, we use this method to prepare Bi and Pb-doped BaTiO3-based PTCR ceramics. The electrical properties of these samples are also investigated.
II. EXPERIMENTAL
The samples were prepared by the vapor-doping method. First, powders with composition of Ba1−xSrxTiO3 + 0.5 mol% Y2O3 + 0.06 mol% MnO2 + 0.5 mol% SiO2 were synthesized through conventional ceramic processing. After milling and drying, the powders were pressed into 2-mm-thick pellets and 10 mm in diameter. These pellets were put into three covered alumina crucibles marked with A, B, and C, respectively. In crucible A, there was nothing else near the pellets. In crucible B, about 0.1 g Bi2O3 powders were put near the pellets to provide an atmosphere of Bi during sintering. 3328
J. Mater. Res., Vol. 14, No. 8, Aug 1999
In crucible C, about 0.1 g PbO powders were put beside the pellets to form lead atmosphere. The pellets in each of three crucibles weighed about 25 g. All the ceramics were sintered at 1350 °C for 1 h. III. RESULTS
The direct current resistance of the ceramic samples were measured from room temperature up to 250 °C. As shown in Fig. 1, the three samples exhibit different resistivity-temperature properties. For sample A, the resistivity jumping is in about 6 orders of magnitude. For samples B and C, the values of resistivity jumping are about 8 and 5 orders of magn
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