PTCR MECHANISM IN NEWLY DEVELOPED Bi_4Ti_3O_<12> CERAMIC HEATER

本文研究了Bi_4Ti_3O_3<12>陶瓷（Tc=675 ℃）的制备工艺，并阐明了其PTCR机理。结果表明，<12>含5mol%Nb的Bi_4Ti_3O_3陶瓷在270 ℃时具有3-4个数量级的快速电阻率增长（PTCR性能），其电阻温度系数比目前使用的BaTiO_3热敏电阻高10-1000倍。但是PTCR的温度与它们的居里温度并不对应。BaTiO_3陶瓷的Heywang模型不能适用于该系统，表现PTCR性能的样品的显微结构是：样品表面由TiO_2和Bi_2Ti_4O_2组成<11>，Bi金属分散在几μ m的样品内部。Bi金属的尺寸在样品的外部较大，而在内部较小。阻抗分析表明，介电常数在270 ℃时迅速下降，这对应于Bi金属的熔化温度（Tm）。因此，PTCR性能出现的机理是Bi金属的存在在Bi金属熔点以下形成低阻电路，而在Tm以上转变为高阻电路。研究发现，SrBi_4Ti_4O_3基体相<15>的加入提高了PTCR性能的稳定性。

The purpose of the present studies is to establish the preparataion techniques of Bi_4Ti_3O_<12> ceramics (Tc=675ﾟC) as PTC thermistors, and to clarify the PTCR mechanisms.As a result, Bi_4Ti_3O_<12> ceramics with 5mol%Nb as semiconducting element show the rapid resistivity increase (PTCR properties) with high-order of magnitude of 3-4 at 270ﾟC.The temperature coefficient of resistivity is by 10-1000 times higher than that of BaTiO_3 presently used thermistor. But the PTCR temperature does not correspond to their Curie temperature. Then the proposed Heywang model for BaTiO_3 ceramic is unable to apply to the system.The microstructure of the sample which shows PTCR properties are such that surface of the sample consists of TiO_2 and Bi_2Ti_4O_<11>, and the Bi metals are dispersed inside the sample of several mum. The size of Bi metals have a lager size at outer of the sample and finer size at inside. The impedance analysis indicates that the permittivity decreases rapidly at 270ﾟC,which corresponds to the melting temperature (Tm) of Bi metals. Therefore, the mechanism of the appearance of PTCR properties are such that existence of Bi metals forms the low resistivity electrical circult below the melting temperature of Bi metals, but it changes to high electrical circuit above Tm. Then the total resistivity of the sample is subject to the matrix phase of SrBi_4Ti_4O_<15>.It is also found that the addition of Sr oxides improved stabilities of PTCR properties.