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

该项目的目标是开发具有正电阻温度系数的新型无铅功能陶瓷（PTC热敏电阻）。这类新型材料的合成方案、电荷输运性质和机理、掺杂剂的影响、缺陷化学和微观结构等方面的研究是十分必要的。PTC热敏电阻陶瓷是汽车工业中高效加热系统所需要的材料。PTC热敏电阻是基于PTC效应：如果温度超过某一临界温度，则材料的电阻增加几个数量级。在提供工作电压后，热敏电阻被加热到临界温度。系统是自调节的：如果热量从热敏电阻中转移出去，则保持低电阻工作点，功耗高。一旦热传输减少，PTC热敏电阻变化到其高电阻状态并限制功耗。具有PTC效应的陶瓷加热器传统上是由pb取代的BaTiO3制成的，居里温度在120-200℃之间。通过适当的供体掺杂将铁电绝缘体BaTiO3转移到半导体状态。该项目旨在研究基于（1-x）Bi0.5Na0.5TiO3—xBaTiO3 （BNBT）体系的无铅PTC热敏电阻材料，该材料与BaTiO3相比具有较高的居里温度。主要问题是通过掺杂和微结构设计来定制最佳PTC特性。需要制备均匀掺杂BNBT陶瓷的合成路线。需要确定供体和受体的最佳掺杂浓度，并澄清其机制。这些PTC热敏电阻的微观结构和性能之间的关系应该仔细观察。从化学计量的偏差以及材料的缺陷化学需要测量和建模。掺杂剂和点缺陷浓度对材料电性能的影响，特别是对晶粒和晶界区域的影响需要了解。该方案解决了新一代PTC陶瓷的制备及其性能优化的工程问题，以及BNBT体系中无铅PTC热敏电阻的电荷输运、缺陷化学和微结构-性能相关性等更基本的问题。

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.