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.

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nanowires, [ 7 ] PbZr x Ti 1-x O 3 nanowires [ 8 ] and nanoribbons [ 9,10 ] and poly(vinylidene fl uoride) (PVDF) nanofi bers [ 11 ] have all revealed promising energy harvesting performance. There has since been an ongoing concerted effort in developing this relatively new research fi eld, connecting nanotechnology with the fi eld of energy. [ 12 ] Piezoelectric nanowires are particularly attractive for energy harvesting due to their robust mechanical properties and high sensitivity to typically small ambient vibrations. [ 13 ] The implications of these properties, in fact, go beyond energy harvesting, as nanowirebased nanogenerators have recently been shown to function as bio mechanical sensors, [ 14 ] sensitive pressure sensors [ 15 ] and precision accelerometers. [ 16 ] The challenge lies in the large scale production of low-cost piezoelectric nanowires that can offer reproducible and reliable energy harvesting and/or sensor performance.The polymer PVDF [(CH2-CF2) n ] exhibits good piezoelectric and mechanical properties with excellent chemical stability and resilient weathering characteristics. [ 17,18 ] PVDF thin fi lms are thus commonly used as sensors and actuators. [ 19 ] However, the piezoelectric performance of PVDF is dependent on the nature of the crystalline phase present. Typically, PVDF occurs in the α, β and γ crystalline phases [20][21][22] and needs to be electrically poled (using an electric fi eld of the order of 100 MV m −1 ) and/ or mechanically stretched [ 20,22 ] to achieve the polar β-phase that shows the strongest piezoelectric behavior. [ 21 ] is a co-polymer that crystallizes more easily into the β-phase due to steric factors, [ 22 ] an advantage that we exploit in this work. Nanowires of PVDF and its copolymers have been previously incorporated into piezoelectric nanogenerators [ 11 ] but the relatively complex electrospinning fabrication process employed requires high voltages (5-50 kV) and specialized equipment. The associated high electric fi elds and stretching forces result in poled nanowires, however this fabrication process often suffers from poor control over nanowire size-distribution and alignment, and is yet to be conveniently and cost-effectively scaled up. [ 23 ] Here we report the growth of aligned P(VDF-TrFE) nanowires with a narrow size distribution using a simple, cost-effective and easily scalable template-wetting method, [ 24,25 ] where the template-induced space confi nement promotes high crystallinity and preferential orientation of the lamellar crystals in the polymer nanowires. [ 26,27 ] This results in the enhancement of piezoelectric properties, even without the need for electrical poling. A nanogenerator fabricated using template-grown, self-poled P(VDF-TrFE) nanowires is shown to have excellent electrical output when subjected to periodic vibrations. Using a circuit comprising a rectifi er to convert its AC output to DC, and a bank of capacitors to store the harvested energy, the nanogenerator is shown to be capable of lighting a commercial light emitting ...

We report the discovery of serendipitous electrocaloric effects in commercial multilayer capacitors based on ferroelectric BaTiO 3 . 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. Optimised electrocaloric multilayer capacitors could find applications in future cooling technologies.

Piezoelectric polymers are promising energy materials for wearable and implantable applications for replacing bulky batteries in small and flexible electronics. Therefore, many research studies are focused on understanding the behavior of polymers at a molecular level and designing new polymer-based generators using polyvinylidene fluoride (PVDF). In this work, we investigated the influence of voltage polarity and ambient relative humidity in electrospinning of PVDF for energy-harvesting applications. A multitechnique approach combining microscopy and spectroscopy was used to study the content of the β-phase and piezoelectric properties of PVDF fibers. We shed new light on β-phase crystallization in electrospun PVDF and showed the enhanced piezoelectric response of the PVDF fiber-based generator produced with the negative voltage polarity at a relative humidity of 60%. Above all, we proved that not only crystallinity but also surface chemistry is crucial for improving piezoelectric performance in PVDF fibers. Controlling relative humidity and voltage polarity increased the d 33 piezoelectric coefficient for PVDF fibers by more than three times and allowed us to generate a power density of 0.6 μW·cm –2 from PVDF membranes. This study showed that the electrospinning technique can be used as a single-step process for obtaining a vast spectrum of PVDF fibers exhibiting different physicochemical properties with β-phase crystallinity reaching up to 74%. The humidity and voltage polarity are critical factors in respect of chemistry of the material on piezoelectricity of PVDF fibers, which establishes a novel route to engineer materials for energy-harvesting and sensing applications.