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

Abstract

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...

Giant electrocaloric materials energy efficiency in highly ordered lead scandium tantalate

“…1b ) combined with thermal hysteresis, in zero-field specific heat C p measurements made by DSC (see Methods section) indicate the presence of a first-order phase transition in PST at the transition temperature T 0 = 300 K. A constant C p of 300 J kg −1 K −1 which corresponds to the background of the zero-field C p measurements in Fig. 1b , is measured and in good agreement with the literature 14 , 15 . The peak at T 0 is the latent heat Q 0 due to the first-order transition from FE–PE (on heating) or PE–FE (on cooling).…”

Origin of large negative electrocaloric effect in antiferroelectric PbZrO3

“…(1) (see Supplemental Material [8] and Refs. [9][10][11][12][13]). The interest in this anomalous effect was further enhanced by the report of indirectly measured "giant" negative electrocaloric effects in antiferroelectric thin films [10,14].…”

Near‐Room‐Temperature Large Electrocaloric Effect in Barium Titanate Single Crystal Based on the Electric Field–Temperature Phase Diagram

“…Due to the high polarizability of Pb 2þ , the lead-based FE materials usually exhibit outstanding polarization characteristics, yielding excellent EC effects. [11,13,30,32] However, they are severely restricted due to the toxicity of Pb on ecological and human health. This emphasizes the urgency of exploiting the large EC effect in lead-free materials, among which BaTiO 3 -based FE materials are one of the promising candidates.…”