Electrocaloric effects across room temperature in multilayer capacitors | Nature

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Subjects

- Ferroelectrics and multiferroics

- Phase transitions and critical phenomena

- Thermodynamics

Abstract

A growing number of cooling devices1,2,3,4 exploit large electrocaloric effects associated with a supercritically driven first-order ferroelectric phase transition in multilayer capacitors of PbSc0.5Ta0.5O3 (PST)5. However, these multilayer capacitors only operate above the room-temperature Curie temperature and require an energetically expensive 42-day anneal for high B-site order to maximize latent heat. Here we show that exaggerating valence mismatch through dilution with PbMg0.5W0.5O3 (PMW) maintains high B-site order and latent heat with no anneal, while disrupting dipolar order to reduce the Curie temperature as low as 230 K. Our multilayer capacitors of PST–PMW show supercritical electrocaloric effects of about 3 K across and well below room temperature owing to 17.1 V μm−1 fields we apply >107 times without breakdown. Using our multilayer capacitors in an ideal fluid regenerator and assuming work recovery yields cycle efficiencies of 70–90%. Taken together, our findings imply that multilayer capacitors of PST–PMW should now replace multilayer capacitors of PST in electrocaloric prototypes to permit electrocaloric refrigeration.

Main

Highly reversible thermal changes can arise when ferroelectric phase transitions are driven by applied electric fields above the Curie temperature TC. These electrocaloric (EC) effects6,7 are typically observed near and above room temperature because most ferroelectrics have relatively high Curie temperatures8,9. Moreover, EC effects can be large in thin layers as breakdown fields can be large10,11,12,13,14 and assemblies of thin ceramic/polymer layers (multilayer capacitors (MLCs))1,2,3,4,15,16,17,18,19,20,21,22,23,24, and flexible polymer bilayers25,26,27,28,29/monolayers30,31, have been exploited in a growing number of EC prototypes that cool loads down to temperatures near room temperature. However, EC cooling of a load through room temperature remains elusive, even though such cooling is important for food, beverages, the built environment, medicine and so on.

EC effects in existing material systems cannot be exploited for cooling through room temperature. For PST modified by B-site disorder32 or solid solution33, sub-room-temperature EC effects were reported in thin single layers and are small (≤1.6 K). For other ceramics, sub-room-temperature EC effects are sometimes larger (≤1–4 K) but they arise in thin single layers14 or decay to zero and change sign on crossing room temperature—as seen for antiferroelectric PMW34,35,36—such that it is thermodynamically impossible to use PMW to cool to or through room temperature (Supplementary Note 1) (ref. 14 also demonstrated an MLC that was compromised by substantial inactive thermal mass and only measured at room temperature).

Here we describe large-active-volume unannealed MLCs based on until now unexplored solid solutions of (1 − x)PST–xPMW, in which PMW disruption of PST dipolar order suppresses TC without overly disrupting B-site order, such that the latent heat of the ferroelectric phase transition remains large37. Supercritically driving this transition yields large EC effects over a wide range of operating temperatures that extends above and well below room temperature.

Preliminary screening in 0.05 ≤ x ≤ 0.25 led us to select x = 0.15 (85PST–15PMW) and x = 0.10 (90PST–10PMW) to maintain EC performance while suppressing TC as measured on heating down to 230 K and 242 K, respectively. The transition is strongly first order in our solid solutions because B-site order37 between high-valence cations (Ta5+, W6+) and low-valence cations (Sc3+, Mg2+)—observed using X-ray diffraction (XRD) and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM)—is facilitated by valence/size cation mismatch and achieved after low-temperature sintering without the long, expensive anneal. Despite inactive thermal mass in the active area (outermost PST–PMW layers, inner electrodes), the effective temperature change is large (|ΔTeff| ≈ 3 K), and similar to repeatably driven temperature changes in the MLC working bodies5 of EC prototypes1,2,3,4.

Our measurements of EC temperature change, performed with a small thermocouple, are consistent with indirect measurements based on dense adiabatic electrical polarization data and heat capacity data38. These indirect measurements are used to construct entropy maps on which we identify entropy changes consistent with direct measurements of isothermal EC heat (the high-field leakage is so low that even our sensitive calorimetry does not detect Joule heating except at very high temperatures).

If MLCs of PST5 in prototypes1,2,3,4 were replaced with MLCs of PST–PMW and likewise driven with 600 V, one could cool down to near 230 K, not 295 K, and slightly increase efficiency as seen by permuting variables in our entropy maps and constructing