PTCR MECHANISM IN NEWLY DEVELOPED Bi_4Ti_3O_<12> CERAMIC HEATER

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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.