Lead-free PTCR-ceramics: charge transport and ceramic technology

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The aim of the project is to develop new lead-free functional ceramics with positive temperature coefficient of resistance (PTC thermistors). It is necessary to develop synthesis protocols as well as to study charge transport properties and mechanisms, the effect of dopants, the defect chemistry and the microstructure of this new class of materials. PTC thermistor ceramics are needed for efficient heating systems in the automotive industry.PTC thermistors are based on the PTC effects: if the temperature exceeds a certain critical temperature, the resistance of the material is increased by some orders of magnitude. After providing an operating voltage, the thermistor is heated to its critical temperature. The system is self-regulating: if heat is transported away from the thermistor, a low-resistance working point with high power consumption is maintained. As soon as the heat transport is reduced, the PTC thermistor changes in to its high-resistance state and limits the power consumption. Ceramics with PTC effect for heaters traditionally are made of Pb-substituted BaTiO3 with Curie temperatures in the range of 120-200°C. Ferroelectric insulating BaTiO3 is transferred into a semiconducting state by appropriate donor doping. The proposed project aims at investigating lead-free PTC thermistor materials based on the system (1-x)Bi0.5Na0.5TiO3--xBaTiO3 (BNBT) which show high Curie temperatures compare to BaTiO3. The main problem is to tailor optimum PTC characteristics through doping and microstructural design. Synthesis routes for the fabrication of homogeneously doped BNBT ceramics are needed. Optimum doping concentrations of donor and acceptors need to be identified as well as their mechanisms need to be clarified. The correlation between microstructure and properties of these PTC thermistors should be carefully looked at. Deviations from stoichiometry as well as the defect chemistry of the materials need to be measured and modeled. Th effect of dopant and point defect concentrations on the electrical properties of the material and especially on the grain and grain boundary regions needs to be understood.The proposal addresses engineering problems of preparation of a new generation of PTC ceramics and their property optimization as well as more basic problems of charge transport, defect chemistry and microstructure-properties correlations in lead-free PTC thermistors in the system BNBT.