Band-like charge transport is observed in lead halide perovskite field-effect transistors.

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 PbSc 0.5 Ta 0.5 O 3 (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 for Electrocaloric cooling cycles in lead scandium tantalate with true regeneration via field variation

We show that scanning thermal microscopy (SThM) can measure reversible electrocaloric (EC) effects in <40 μm-thick ceramic films of the relaxor ferroelectric 0.9Pb(Mg 1/3 Nb 2/3)O 3-0.1PbTiO 3 , with the substrate present. We recorded roughly the same non-adiabatic temperature change (0.23 K) for a thinner film that was driven harder than a thicker film (31 V μm-1 across 13 μm versus11 V μm-1 across 38 μm), because the thicker film lay relatively closer to the substantially larger adiabatic values that we predicted by thermodynamic analysis of electrical data. Film preparation was compatible with the fabrication of EC multilayer capacitors (MLCs), and therefore our measurement method may be exploited for rapid characterisation of candidate films for cooling applications. Thermal changes arise when ferroelectric phase transitions are driven by changes of electric field ΔE, but these EC effects are limited to a few kelvin in bulk ceramics 1-4 because |ΔE| is limited by breakdown to ~10 V μm-1. By contrast, ceramic and polymer films of thickness 1 μm show order-of-magnitude larger breakdown fields, resulting in order-of-magnitude larger EC effects (e.g. 12 K with |ΔE| ~ 100 V μm-1) 5,6 that are typically evaluated indirectly 7 from variable-temperature measurements of ferroelectric polarization P(E). Direct measurements of a single film are difficult because small active volumes yield small thermal changes, and these must be both driven and measured on short time scales. Both volume and timescale problems are exacerbated by the unwanted thermal mass of substrates, which are typically present. Here we use non-contact thermometry to directly measure non-adiabatic temperature change ΔT* in single ceramic films on substrates. Predictions of adiabatic temperature change ΔT based on the indirect

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 PbSc 0.5 Ta 0.5 O 3 (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 passive regeneration, this net transfer has been noted explicitly in a seminal paper 1 , and elsewhere 22 .The key figure of merit for any cooling device is its coefficient of performance COP = Q/W, where in each cooling cycle, work W is done to pump heat Q from a cold load at T c towards a hot sink at T h . The effect of the net-transfer problem is to partially subsume regenerators into sinks or loads, with two consequences. First and foremost, it becomes impossible to realize intended cooling cycles, thus compromising device COPs. Second, the active material undergoes net heat exchange at temperatures lying away from T c and T h , thus compromising the calculation of the Carnot limit T c /(T h -T c ) from measured values of T c and T h . Given that COPs and Carnot limits are thus compromised, it immediately follows that device efficiencies are neither optimized nor accurate (efficiency = COP/Carnot). To solve the nettransfer problem, we will construct valid cooling cycles on detailed field-temperature-entropy maps, and we will accurately evaluate the resulting COPs, Carnot limits and efficiencies.These parameters contrast with COPs for materials in isothermal cycles 23 , and the related parameter of materials efficiency 24 that describes reversible caloric transitions driven isothermally in just one direction.The net-transfer problem has been hitherto masked in EC cooling cycles because the zerofield and finite-field isofields are rendered parallel by assuming peak performance at all points in any given cycle [25][26][27] . However, even for relaxors that show large EC effects over wide ranges of temperatures 6,20,21 , this assumption is only reasonable for small temperature spans T h -T c that would only be useful when exploiting converse EC effects for pyroelectric energy harvesting 28 . The net-transfer problem has also been masked in cooling cycles based on mC materials, either by likewise assuming peak performance 29 or by assuming isothermal conditions 23 . In t...

A multilayer capacitor comprising 19 layers of 38 μm-thick 0.9Pb(Mg1/3Nb2/3)O3–0.1PbTiO3 has elsewhere been shown to display electrocaloric temperature changes of 2.2 K due to field changes of 24 V μm−1, near ∼100 °C. Here we demonstrate temperature changes of 1.2 K in an equivalent device with 2.6 times the thermal mass, i.e., 49 layers that could tolerate 10.3 V μm−1. Breakdown was compromised by the increased number of layers, and occurred at 10.5 V μm−1 near the edge of a near-surface inner electrode. Further optimization is required to improve the breakdown strength of large electrocaloric multilayer capacitors for cooling applications.