Abstract
The need for more energy-efficient and environmentally-friendly alternatives in the refrigeration industry to meet global emission targets has driven efforts towards materials with a potential for solid state cooling. Adiabatic depolarisation cooling, based on the electrocaloric effect (ECE), is a significant contender for efficient new solid state refrigeration techniques. Some of the highest ECE performances reported are found in compounds close to the morphotropic phase boundary (MPB). This relationship between performance and the MPB makes the ability to tune the position of the MPB an important challenge in electrocaloric research. Here, we report direct ECE measurements performed on MPB tuned NBT-06BT bulk ceramics with a combination of A-site substitutions. We successfully shift the MPB of these lead-free ceramics closer to room temperature, as required for solid state refrigeration, without loss of the criticality of the system and the associated ECE enhancement.
Highlights
Rhombohedral, tetragonal and cubic phases co-exist[15]
The temperature of depolarisation (Td) of NBT-06BT or onset of the electrocaloric effect (ECE) occurs above 100 °C, too high for household refrigeration applications
The few values of electric field studied in this work did not allow for an accurate estimation of the critical electric field
Summary
The corresponding high-field ECE responsivity for 5%-Li is 0.31 × 10−6 K.m/V, which suggests that the ECE enhancement observed near the MPB composition in NBT-06BT can be recovered at lower temperatures when a critical concentration of lithium is introduced. ΔSmax increases linearly with E in a logarithmic scale, which makes the comparison between the different compositions easier We show that both the absolute values and field-dependence of the 5%-Li-doped ceramic are similar to that of undoped MPB composition NBT-06BT. The strong field-dependence of ECE peak position in the Li-rich compositions means that the maximum ΔSmax would be obtained 100 °C higher in the 15%Li-doped system compared to the 5%Li-doped one This last observation highlights that, the broad peak induced by the relaxor properties is attractive for cooling applications, the ECE reduction and the significant shift of ΔTmax to high. It will allow the use of the materials with the highest ECE performances without any loss in efficiency at any required temperature of operation, whether it is near 25 °C for household refrigeration or 200 °C for on-chip cooling in advanced electronics
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