It’s not about the mass

Electrocaloric cooling devices are traditionally based on sub-millimetre-thick ceramic working bodies. Using instead a flexible polymer that is one order-of-magnitude thinner yields lightweight devices that have now been stacked to pump heat across a relatively wide temperature span

Electrocaloric effects in multilayer capacitors for cooling applications

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Multicalorics

Magnetocaloric, electrocaloric and mechanocaloric effects are nominally reversible thermal changes that occur in magnetically, electrically and mechanically responsive materials when subjected to changes in applied magnetic, electric and mechanical field, respectively. These caloric effects are typically large near phase transitions, and analogous to the pressure induced thermal changes in fluids that have been exploited for many decades in refrigeration and air-conditioning systems. However, caloric effects promise high energy efficiencies without greenhouse gases

Large electrocaloric effects in single-crystal ammonium sulfate.

Electrocaloric (EC) effects are typically studied near phase transitions in ceramic and polymer materials. Here, we investigate EC effects in an inorganic salt, namely ammonium sulfate (NH4)2SO4, with an order-disorder transition whose onset occurs at 223 K on cooling. For a single crystal thinned to 50 μm, we use a Maxwell relation to find a large isothermal entropy change of 30 J K(-1) kg(-1) in response to a field change of 400 kV cm(-1) The Clausius-Clapeyron equation implies a corresponding adiabatic temperature change of 4.5 K.This article is part of the themed issue 'Taking the temperature of phase transitions in cool materials'.Royal SocietyThis is the author accepted manuscript. It is currently under an indefinite embargo pending publication by Royal Society Publishing

Landau Theory of Barocaloric Plastic Crystals

We present a simple Landau phenomenology for plastic-to-crystal phase transitions and use the resulting model to calculate barocaloric effects in plastic crystals that are driven by hydrostatic pressure. The essential ingredients of the model are (i) a multipole-moment order parameter that describes the orientational ordering of the constituent molecules, (ii) coupling between such order parameter and elastic strains, and (iii) the thermal expansion of the solid. The model captures main features of plastic-to-crystal phase transitions, namely large volume and entropy changes at the transition, and strong dependence of the transition temperature with pressure. Using solid C$_{60}$ under $0.60\,$GPa as case example, we show that calculated peak isothermal entropy changes of $\sim 58 \,{\rm J K^{-1} kg^{-1}}$ and peak adiabatic entropy changes of $\sim 23 \,{\rm K}$ agree well with experimental values.Comment: 17 pages, 3 figure

Electrocaloric Cooling Cycles in Lead Scandium Tantalate with True Regeneration via Field Variation

There is growing interest in heat pumps based on materials that show thermal changes when phase transitions are driven by changes of electric, magnetic or stress field. Importantly, regeneration permits sinks and loads to be thermally separated by many times the changes of temperature that can arise in the materials themselves. However, performance and parameterization are compromised by net heat transfer between caloric working bodies and heat transfer fluids. Here we show that this net transfer can be avoided-resulting in true, balanced regeneration-if one varies the applied electric field while an electrocaloric (EC) working body dumps heat on traversing a passive fluid regenerator. Our EC working body is represented by bulk PbSc0.5Ta0.5O3 (PST) near its first-order ferroelectric phase transition, where we record directly measured adiabatic temperature changes of up to 2.2 K. Indirectly measured adiabatic temperature changes of similar magnitude were identified, unlike normal, from adiabatic measurements of polarization, at nearby starting temperatures, without assuming a constant heat capacity. The resulting high-resolution field-temperature-entropy maps of our material, and a small clamped companion sample, were used to construct cooling cycles that assume the use of an ideal passive regenerator in order to span $\leq$20 K. These cooling cycles possess well defined coefficients of performance that are bounded by well defined Carnot limits, resulting in large ($>$50%) well defined efficiencies that are not unduly compromised by a small field hysteresis. Our approach permits the limiting performance of any caloric material in a passive regenerator to be established, optimized and compared; provides a recipe for true regeneration in prototype cooling devices; and could be extended to balance active regeneration.Gates Cambridge, the Winton Programme for the Physics of Sustainabilit

High-contrast imaging of 180° ferroelectric domains by optical microscopy using ferroelectric liquid crystals

Ferroelectric liquid crystals (FLCs) couple the direction of their spontaneous electric polarization to the direction of tilt of their optic axis. Consequently, reversal of the electric polarization by an electric field gives rise to an immediate and lasting optical response when an appropriately aligned FLC is observed between crossed polarizers, with one field direction yielding a dark image, and the opposite direction yielding a bright image. Here this peculiar electro-optic response is used to image, with high optical contrast, 180{\deg} ferroelectric domains in a crystalline substrate of magnesium-doped lithium niobate. The lithium niobate substrate contains a few domains with upwards electric polarization surrounded by regions with downward electric polarization. In contrast to a reference non-chiral liquid crystal that is unable to show ferroelectric behavior due to its high symmetry, the FLC, which is used as a thin film confined between the lithium niobate substrate and an inert aligning substrate, reveals ferroelectric domains as well as their boundaries, with strong black and white contrast. The results show that FLCs can be used for non-destructive read-out of domains in underlying ferroelectrics, with potential applications in e.g. photonic devices and non-volatile ferroelectric memories.Royal Society. Royal Commission for the Exhibition of 1851

Elastic anomalies associated with domain switching in BaTiO3 single crystals under in-situ electrical cycling

The elastic response of BaTiO3 single crystals during electric field cycling at room temperature has been studied using in-situ Resonant Ultrasound Spectroscopy (RUS), which allows monitoring of both the elastic and anelastic changes caused by ferroelectric polarization switching. We find that the first ferroelectric switching of a virgin single crystal is dominated by ferroelastic 90° switching. In subsequent ferroelectric switching, ferroelastic switching is reduced by domain pinning and by the ferroelectric domains, as confirmed by polarized light microscopy. RUS under in-situ electric field therefore demonstrates to be an effective technique for the investigation of electromechanical coupling in ferroelectrics