Electrocaloric effect in lead-free relaxor ceramics and composites

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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.