Electrocaloric effect in ferroelectric relaxors: The road to solid-state cooling
Creators: name: Le Goupil, Florian
Publisher: name: Imperial College London
Year: 2013

description: This thesis describes the potential of relaxor ferroelectrics for solid state cooling based on the electrocaloric effect. The core of this investigation is to identify the reliable methods to correctly evaluate the electrocaloric effect and develop materials with the properties required for commercial electrocaloric cooling. A thorough review of the state-of-the-art electrocaloric research reveals that too many research groups still rely on the indirect evaluation of the electrocaloric effect (ECE) from polarization measurements and highlights the need for direct ECE measurements. A direct electrocaloric effect measurement set-up based on a modified-differential scanning calorimeter, allowing the acquisition of both ther- mal (ECE, heat capacity) and electrical (P-E loops, leakage current) information simultaneously, has successfully been constructed and benchmarked. Direct ECE measurements have been performed on normal ferroelectrics, such as barium titanate, but also well-known relaxor ferroelectrics, such as the PMN- PT system, for fundamental understanding of the electrocaloric effect. These results highlight the importance of the polar direction of the electrocaloric mate- rials with regard to the direction of applied electric field. A region with negative ECE, which could be exploited to increase the efficiency of electrocaloric cooling cycles, has been identified for &lt;001 &gt;-oriented PMN-30PT by both direct and indirect measurements. This negative ECE is observed in the vicinity of the low temperature field-induced structural phase transition, which forms intermediate lower-symmetry monoclinic phases. The occurrence of this phenomenon requires the combination of several parameters related to the direction of application of the electric field. The results on PMN-PT also show how the chemical disorder in ferroelectric relaxors provides important entropy changes over the ferroelectric to paraelectric transition which enables an extended cooling regime as the ECE maximum can be extended over several tens of degrees. Direct ECE measurements have therefore been performed on novel relaxor fer- roelectrics, including perovskite, Aurivillius phase and tungsten bronze structures, with a focus on lead-free, for environmental purposes, highly disordered materi- als. For most of these systems, the direct ECE measurement presented here are the first ever reported. The presence of a dual electrocaloric peak, sometimes far above the ferroelectric to paraelectric transition, is confirmed in all the studied relaxor ferroelectrics. This peak was attributed to the extra contribution to the field-induced entropy change by the polar nanodomains. The presence of this ex- tra ECE peak confirms the great potential of relaxor ferroelectrics for solid-state electrocaloric cooling over a range of temperature broad enough for commercial applications. Comparisons between direct and indirect measurements are performed on nu- merous systems throughout this thesis, in order to identify the domain of validity of the indirect method still overly used in the literature. It is shown that the indi- rect ECE method, although it gives satisfactory results for normal ferroelectrics, is unreliable for strong relaxor ferroelectrics above the ferroelectric to paraelectric phase transition, where the dual peak is observed by direct measurements. These limitations are attributed to the inability of the indirect method to account for the field-induced entropy contribution of the polar nanodomains to the electrocaloric effect.