Among the various forms of natural energies, heat is the most prevalent and least harvested energy. Scavenging and detecting stray thermal energy for conversion into electrical energy can provide a cost-effective and reliable energy source for modern electrical appliances and sensor applications. Along with this, flexible devices have attracted considerable attention in scientific and industrial communities as wearable and implantable harvesters in addition to traditional thermal sensor applications. This review mainly discusses thermal energy conversion through pyroelectric phenomena in various lead-free as well as lead-based ceramics and polymers for flexible pyroelectric energy harvesting and sensor applications. The corresponding thermodynamic heat cycles and figures of merit of the pyroelectric materials for energy harvesting and heat sensing applications are also briefly discussed. Moreover, this study provides guidance on designing pyroelectric materials for flexible pyroelectric and hybrid energy harvesting.

The electrocaloric effect (ECE), which refers to changes in the temperature of a material when an electric field is applied to or removed from the material, is one of the key phenomena of future highly efficient solid-state refrigeration devices.

A magneto-mechano-electric (MME) generator comprising a cantilever structured magnetoelectric (ME) composite having a magnet-proof mass is an ideal candidate for powering autonomous Internet of Things (IoT) sensor networks by scavenging electric energy from ambient magnetic noise. However, charging an energy storage device in a short time using an MME generator from low-intensity magnetic noise flux spreading in radial directions, e.g., around power cables, requires the generator volume to be enlarged. Here, we demonstrated a novel method to enhance the output power of the given MME generator by concentrating a magnetic flux around the circumference of an MME generator structure using a magnetic flux concentrator (MFC) fabricated from a high-permeability magnetic material. The output power of the MME generator was significantly enhanced by optimizing the MFC structure in terms of its shape, aspect ratio, magnetic permeability, and the number of layers, both through finite element analysis and experiments. The MME generator with the MFC was observed to generate a maximum power of 3.33 mW, which was 285% of that of the condition without the MFC (1.17 mW) under a weak magnetic field of 8 Oe (= 8 G = 800 μT in air), which was sufficient to charge the 2.2 mF storage capacitor within 13 s and continuously operate an IoT sensor for a long period. The stored energy is also sufficient enough to turn on 100 blue LEDs with high intensity. Furthermore, we demonstrated a realistic practical application by installing the self-powered wireless environmental monitoring module consisting of the MME generator with the MFC at a power substation, which successfully scavenges the power from the magnetic fields around the power cables and continuously operates a temperature wireless sensor. These results indicated that the MFC has great potential to enhance the performance of an MME generator as the power source of standalone IoT sensors without enlarging the MME generator.

Here we review the current status of magnetoelectric (ME) multiferroics and ME composite thin/thick films. The magnitude of ME coupling in the composite systems is dependent upon the elastic coupling occurring at the interface of piezoelectric and magnetostrictive phases. The multiferroic ME films in comparison with bulk ME composites have some unique advantages and show higher magnitude of ME response. In ME composite films, thickness of the films is one of the important factors to have enough signal. However, most of all reported ME nanocomposite structured films in literature are limited in overall thickness which might be related to interface strain resulting from difference in thermal expansion mismatch between individual phases and the substrate. We introduced noble ME composite film fabrication technique, aerosol deposition (AD) to overcome these problems. The success in AD fabrication and characterization of ME composite films with various microstructure such as 3-2, 2-2 connectivity are discussed.