Electrocaloric effect in lead-free relaxor ceramics and composites

本提案的重点是寻找在室温附近表现出最大电热效应的块状无铅材料及其表征。在项目的第一个阶段，我们建立了两个装置来直接测量电热效应。一个是基于差示扫描量热计的等温装置另一个是定制的绝热装置。这两种设置都经过了调整，以获得可靠的可重复结果。我们发现，通常间接计算的电热系统的冷却功率与其实际换热性能之间存在很大差异。50%的差异在松弛剂中是典型的。在项目的第二阶段，我们打算系统地研究无铅弛豫铁电体的电热效应。弛豫态提供了额外的自由度，有助于外加电场下的熵变。弛豫器的最大优点是在类似冷却能力的温度范围内较宽。与经典铁电系统相比，这提供了更大的制冷剂容量。我们选择的系统，除了弛豫态，相变发生在室温附近。在观察电热应用时，这是一个非常有价值的特性。不仅是成分的影响，还有微观结构对热效应的影响，即：陶瓷的晶体取向、晶粒尺寸和密度。为了最大限度地冷却，必须施加大电场。为了能够做到这一点，陶瓷必须具有非常高的抗击穿性能。我们正通过高密度化来寻求这一特性。第二种方法使用复合。我们开发了一种化学途径来制备BaTiO3及其邻近松弛系统的疏水纳米和微粉末。这些被纳入到铁电聚合物体系PVDF-TrFE本身是一个很好的电热材料。两种材料，填料和基体，提供接近的居里点。我们的希望是，这将允许在室温或接近室温下构建具有更高冷却能力的材料。最大的优点是聚合物的大击穿强度，我们希望能够将其转移到复合材料中。研究结果将与“优先计划”其他小组开发的理论模型进行比较。我们认为我们在schwerpunktrp计划中的角色是块状材料的电热特性的中心测量项目。我们看到了建立可靠装置所必需的工作质量，我们认为这是我们对整个Schwerpunkt最重要的贡献。此外，我们还提供热传输、电学和结构表征。尤其是压电显微镜是我们的强项之一。此外，我们还为schwerpunktprogram的近设备项目提供最好的材料。

This proposal is focused on the search for bulk lead free materials exhibiting maximum electrocaloric effect in the vicinity of room temperature and their characterization. During the course of the first project phase, we have built two set-ups for the direct measurements of the electrocaloric effect. One is an isothermal set-up based on a differential scanning calorimeter the other one is a custom built adiabatic set-up. Both set-ups have been tuned for reliable reproducible results. We found that there is a large difference between the typically indirectly calculated cooling power of electrocaloric systems and their real heat changing properties. 50% difference is typical in relaxors. In the second project phase, we intend to systematically study the electrocaloric effect in lead free relaxor ferroelectrics. The relaxor state offers additional degrees of freedom contributing to entropy change under applied electric fields. The big advantage of relaxors is the broad temperature range of similar cooling capacity. This provides larger refrigerant capacity in comparison to classical ferroelectric systems. We have chosen systems where, additionally to the relaxor state, the phase transitions occur in the vicinity of room temperature. This is a highly valuable property when looking at electrocaloric application.Not only the effect of composition, but also the influence of the microstructure on the electrocaloric effect will be addressed, namely: crystallographic orientation, grain size, and density of ceramics.Large electric fields must be applied for maximum cooling. In order to be able to do so, the ceramics must show very high breakdown resistance. We are seeking this property by high densification. A second approach uses composites. We have developed a chemical route to generate hydrophobic nano- and micropowders of BaTiO3 and its vicinal relaxor systems. These are incorporated into the ferroelectric polymer system PVDF-TrFE which itself is a good electrocaloric material. Both materials, filler and matrix, offer close Curie points. Our hope is that this will permit to construct materials of even higher cooling power at or near room temperature. The big advantage is the large breakdown strength of the polymer which we hope to be able to transfer to the composite. The results will be compared with theoretical models developed by other groups of the Priority Program.We consider our role within the Schwerpunktrpogramm as a central measurement project for the electrocaloric characterization of bulk materials. Having seen the quality of work that is necessary to built reliable set-ups, we consider this our most important contribution to the overall Schwerpunkt. Furthermore, we offer thermal transport, electrical, and structural characterization. Particularly piezoforce microscopy is one of our strengths. We furthermore offer our best materials to the near device projects of the Schwerpunktprogramm.