S. Crossley

Giant Electrocaloric Strength in Single‐Crystal BaTiO3

Over the last fi fteen years, the discovery of giant magnetocaloric effects near room-temperature phase transitions in various magnetic materials [ 1 , 2 ] has led to suggestions of energy-effi cient and environmentally friendly household and industrial refrigeration. However, these large changes in isothermal entropy Δ S and adiabatic temperature Δ T require large changes in magnetic fi eld Δ H , which are challenging to generate economically. In contrast, it is straightforward to generate changes in electric fi eld Δ E in order to drive electrocaloric (EC) effects near ferroelectric phase transitions. Recently, giant EC effects near nominally second-order transitions have been reported in ferroelectric thin fi lms, [ 3 , 4 ] as thin fi lms can support large driving fi elds. However, two issues arise as follows. Firstly, measurements of heat Q and temperature change Δ T are typically indirect [ 3 , 4 ] as the direct measurement of fi lms is challenging. There is thus scope for error (e.g., because the possible role of thermal and electrical hysteresis is typically ignored). Secondly, the EC effects in fi lms are disproportionately small with respect to the large driving fi elds, and so EC strengths |Q |/| E | and | T |/| E | tend to be relatively small. Here we address both of these issues by presenting direct measurements of both Q and Δ T in single-crystal BaTiO 3 (BTO) near the ferroelectric phase transition at Curie temperature T C . We fi nd EC strengths |Q |/| E | and | T |/ | E | that are giant because the fi rst-order ferroelectric phase transition is very sharp. The observed EC effects are reversible at any temperature above the hysteretic transition regime. Giant EC strengths near sharp fi rst-order phase transitions with a large latent heat could therefore contribute to the future development of cooling devices with a high frequency of operation.

The Electrocaloric Efficiency of Ceramic and Polymer Films

Efficiency is defined as η = |Q|/|W| in order to investigate the electrical work |W| associated with electrocaloric heat |Q|. This materials parameter indicates that polymer films are slightly more energy efficient than ceramic films, and therefore both species of material remain candidates for future cooling applications.

New developments in caloric materials for cooling applications

Caloric materials are in the spotlight as candidates for future environmentally friendly cooling technologies. We describe stimulating recent developments in the three caloric strands that are now being studied collectively, namely magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects.

Direct electrocaloric measurements of a multilayer capacitor using scanning thermal microscopy and infra-red imaging

We present two techniques for directly measuring electrocaloric temperature change in a multilayer capacitor based on BaTiO3. Scanning thermal microscopy with resolution 80 mK, and infra-red imaging with resolution 25 mK, each record electrocaloric temperature changes of ∼0.5 K that match within error. We find that scanning thermal microscopy is more suitable for detecting giant electrocaloric effects in thin films with substrates present.

Giant barocaloric effects at low pressure in ferrielectric ammonium sulphate

Caloric effects are currently under intense study due to the prospect of environment-friendly cooling applications. Most of the research is centred on large magnetocaloric effects and large electrocaloric effects, but the former require large magnetic fields that are challenging to generate economically and the latter require large electric fields that can only be applied without breakdown in thin samples. Here we use small changes in hydrostatic pressure to drive giant inverse barocaloric effects near the ferrielectric phase transition in ammonium sulphate. We find barocaloric effects and strengths that exceed those previously observed near magnetostructural phase transitions in magnetic materials. Our findings should therefore inspire the discovery of giant barocaloric effects in a wide range of unexplored ferroelectric materials, ultimately leading to barocaloric cooling devices.

Energy harvesting performance of piezoelectric ceramic and polymer nanowires.

Energy harvesting from ubiquitous ambient vibrations is attractive for autonomous small-power applications and thus considerable research is focused on piezoelectric materials as they permit direct inter-conversion of mechanical and electrical energy. Nanogenerators (NGs) based on piezoelectric nanowires are particularly attractive due to their sensitivity to small-scale vibrations and may possess superior mechanical-to-electrical conversion efficiency when compared to bulk or thin-film devices of the same material. However, candidate piezoelectric nanowires have hitherto been predominantly analyzed in terms of NG output (i.e. output voltage, output current and output power density). Surprisingly, the corresponding dynamical properties of the NG, including details of how the nanowires are mechanically driven and its impact on performance, have been largely neglected. Here we investigate all realizable NG driving contexts separately involving inertial displacement, applied stress T and applied strain S, highlighting the effect of driving mechanism and frequency on NG performance in each case. We argue that, in the majority of cases, the intrinsic high resonance frequencies of piezoelectric nanowires (∼tens of MHz) present no barrier to high levels of NG performance even at frequencies far below resonance (<1 kHz) typically characteristic of ambient vibrations. In this context, we introduce vibrational energy harvesting (VEH) coefficients ηS and ηT, based on intrinsic materials properties, for comparing piezoelectric NG performance under strain-driven and stress-driven conditions respectively. These figures of merit permit, for the first time, a general comparison of piezoelectric nanowires for NG applications that takes into account the nature of the mechanical excitation. We thus investigate the energy harvesting performance of prototypical piezoelectric ceramic and polymer nanowires. We find that even though ceramic and polymer nanowires have been found, in certain cases, to have similar energy conversion efficiencies, ceramics are more promising in strain-driven NGs while polymers are more promising for stress-driven NGs. Our work offers a viable means of comparing NG materials and devices on a like-for-like basis that may be useful for designing and optimizing nanoscale piezoelectric energy harvesters for specific applications.

Direct electrocaloric measurement of 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3 films using scanning thermal microscopy

We show that scanning thermal microscopy can measure reversible electrocaloric (EC) effects in <40 μm-thick ceramic films of the relaxor ferroelectric 0.9Pb(Mg1/3Nb2/3)O3-0.1PbTiO3, 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, and therefore our measurement method may be exploited for rapid characterisation of candidate films for cooling applications.

Finite-element optimisation of electrocaloric multilayer capacitors

We used finite element analysis to model the flow of heat in multilayer capacitors (MLCs) that comprise good electrocaloric materials and high-thermal-conductivity electrodes. An ideal heat pump based on these MLCs could develop a sustained cooling power of 81.7 kW m−2. This supersedes the 36 kW m−2 predicted by a lumped thermal model [S. Kar-Narayan and N. D. Mathur, Appl. Phys. Lett. 95, 242903 (2009)]. We show that geometrical optimization increases predicted cooling power to 215 kW m−2.

Progress on electrocaloric multilayer ceramic capacitor development

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.

Improper ferroelectricity in lawsonite CaAl2Si2O7(OH)2?H2O: hysteresis and hydrogen ordering

Ferroelectric hysteresis measurements on ceramic lawsonite show a temperature dependence of the remanent polarization P(r) = P(o)Θ(s)(cothΘ(s)/T - cothΘ(s)/T(c)) ∼ Q(2), Θ(s) = 26 K, where Q is the thermodynamic order parameter of the phase transition Pmcn-P 2(1)cn. This almost linear temperature evolution of P(r) proves the improper nature of ferroelectricity in lawsonite. The Curie temperature is T(c) = 124 K. The phase transition is strictly continuous, with a weak conjugated field near the transition point, and hydrogen ordering is discussed as the primary driving mechanism.