Sohini Kar-Narayan

Caloric materials near ferroic phase transitions

A magnetically, electrically or mechanically responsive material can undergo significant thermal changes near a ferroic phase transition when its order parameter is modified by the conjugate applied field. The resulting magnetocaloric, electrocaloric and mechanocaloric (elastocaloric or barocaloric) effects are compared here in terms of history, experimental method, performance and prospective cooling applications.

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.

Direct and indirect electrocaloric measurements using multilayer capacitors

We report the discovery of serendipitous electrocaloric (EC) effects in commercial multilayer capacitors based on ferroelectric BaTiO3. Direct thermometry records ~0.5 K changes due to 300 kV cm−1, over a wide range of temperatures near and above room temperature. Similar results are obtained indirectly, via thermodynamic analysis of ferroelectric hysteresis loops. We compare and contrast these two results. Optimized EC multilayer capacitors could find applications in future cooling technologies.

Predicted cooling powers for multilayer capacitors based on various electrocaloric and electrode materials

We argue that the multilayer capacitor (MLC) geometry is ideal for electrocaloric cooling. Thermal modeling predicts that a commercially available MLC, which serendipitously shows electrocaloric effects, could constitute the heart of an idealized heat pump delivering a continuous cooling power of 22.5 W kg−1. This figure could be increased via materials optimization to ∼2875 W kg−1, such that an MLC array whose sheet area is just ∼0.56 m2 would deliver the ∼20 kW cooling power of typical air-cooled chillers for air-conditioning at residential and commercial sites. Expensive materials are not required, and performance could be further enhanced via geometrical improvements.

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.

PST thin films for electrocaloric coolers

Relaxor behaviour in a thin film of partially ordered PbSc0.5Ta0.5O3 (PST) was confirmed via slim P–E loops and the frequency dependence of the temperature at which the dielectric constant is maximum. Indirect measurements of the electrocaloric effect suggest that removing a field of 774 kV cm−1 yields a temperature change of −3.5 °C to −6.9 °C over a broad range of operating temperatures near room temperature (1–127 °C), with a correspondingly large refrigerant capacity of 662 J kg−1. In addition to low electrical hysteresis, there is negligible thermal hysteresis. PST thin films are therefore promising for EC cooling near room temperature.

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.

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.

Polymer-based nanopiezoelectric generators for energy harvesting applications

Abstract Energy harvesting from ambient vibrations originating from sources such as moving parts of machines, fluid flow and even body movement, has enormous potential for small power applications, such as wireless sensors, flexible, portable and wearable electronics, and bio-medical implants, to name a few. Nanoscale piezoelectric energy harvesters, also known as nanogenerators (NGs), can directly convert small scale ambient vibrations into electrical energy. Scavenging power from ubiquitous vibrations in this way offers an attractive route to provide power to small devices, which would otherwise require direct or indirect connection to electrical power infrastructure. Ceramics such as lead zirconium titanate and semiconductors such as zinc oxide are the most widely used piezoelectric energy harvesting materials. This review focuses on a different class of piezoelectric materials, namely, ferroelectric polymers, such as polyvinlyidene fluoride (PVDF) and its copolymers. These are potentially superior energy harvesting materials as they are flexible, robust, lightweight, easy and cheap to fabricate, as well as being lead free and biocompatible. We review some of the theoretical and experimental aspects of piezoelectric energy recovery using Polymer-based NGs with a novel emphasis on coupling to mechanical resonance, which is relevant for efficient energy harvesting from typically low frequency (<1 kHz) ambient vibrations. The realisation of highly efficient and low cost piezoelectric polymer NGs with reliable energy harvesting performance could lead to wide ranging energy solutions for the next generation of autonomous electronic and wireless devices.

Electroactive polymers for sensing

Electromechanical coupling in electroactive polymers (EAPs) has been widely applied for actuation and is also being increasingly investigated for sensing chemical and mechanical stimuli. EAPs are a unique class of materials, with low-moduli high-strain capabilities and the ability to conform to surfaces of different shapes. These features make them attractive for applications such as wearable sensors and interfacing with soft tissues. Here, we review the major types of EAPs and their sensing mechanisms. These are divided into two classes depending on the main type of charge carrier: ionic EAPs (such as conducting polymers and ionic polymer–metal composites) and electronic EAPs (such as dielectric elastomers, liquid-crystal polymers and piezoelectric polymers). This review is intended to serve as an introduction to the mechanisms of these materials and as a first step in material selection for both researchers and designers of flexible/bendable devices, biocompatible sensors or even robotic tactile sensing units.