Study On Lead-free High Curie Temperature BaTiO₃-based PTCR

The BaTiO3-based PTCR (positive temperature coefficient of resistance) is a kind of useful electrical functional ceramics. The high Tc (Curie temperature) PTCR is widely used as heating units and current limiters and so on, because of the low cost and high reliability. But lead element in the materials is harmful to human health and environment due to the vaporation of lead in the process of preparation and end product use. The rules and regulations have been instituted by developed countries such as Europe and America and so on, in which the lead material will be forbidden all-around after June 2006. So it is an urgent and important research on high Tc lead-free BaTiO3-based PTCR. The item was supported by the postgraduate innovation fund of HUST. In order to prepare the lead-free BaTiO3-based PTCR, the research focused on reducing the room-temperature resistance, increasing the Curie point and so on, the main contents are as following:Na0.5Bi0.5TiO3 is a kind of perovskite type structure material with compound ions at A site, and its Curie temperature is at 320℃. The Na0.5Bi0.5TiO3 powders were acted as the accelerator of Curie temperature in the experiments. The Na0.5Bi0.5TiO3 powders were prepared with de-ionized water and absolute alcohol respectively by solid state reaction. The reasonable wet-grinding medium and preparation technology were confirmed. The analytically pure Y2O3 and Nb2O5 were used as donors respectively. The BaTiO3-based PTCR ceramics were prepared by solid state reaction, the typical U curves of resistance were obtained, and the semiconducting mechanism was studied in detail and the best doping contents of donors were obtained by mathematical analysis. The solid state reaction temperature of Na0.5Bi0.5TiO3 and BaTiO3 was set above 1300℃by the differential thermography and thermogravimetic analysis (TGA) of Na0.5Bi0.5TiO3 and BaTiO3. Alternating Current Impedance (AC Impedance) and its application in PTCR research were studied. The different equivalent circuits based on the brick model were obtained by using the different response time between the bulk and boundary responsing to the different frequencies. The experiments of doping the Ba1-x(NaBi)x/2TiO3 with Nb2O5, Y2O3, La2O3 were investigated respectively. The properties of ceramics were tested, such as the tests and measurements of capacitance and disspation to temperature, ferroelectric performance, AC Impedance, XRD, SEM, energy spectrum analysis and resistance to temperature etc. It was analysed that sodium in Na0.5Bi0.5TiO3 by solid phase reaction was the active element and harmed to ceramics'performance. Na0.5Bi0.5TiO3 depressed the grain growth.