Symposium EN05 : Field-Responsive Composites for Sustainable Energy

High dielectric constant and low dielectric loss are desirable electrical properties for next generation polymer dielectrics that show promise for applications in pulsed power, power electronics, and printable electronics. Unfortunately, the dielectric constant of polymers is often limited to 2-5, much lower than that of inorganic dielectrics, because of the nature of hydrocarbon covalent bonds for electronic and atomic polarizations. It is essential to understand the fundamental physics of different types of polarization and the associated loss mechanisms in polymers. In this presentation, we discuss the characteristics of each polarization and explain how to enhance the polarization using rational molecular designs without causing significant dielectric losses. Among various approaches for high dielectric constant and low loss polymers, the multilayer film technology is of particular interest because a multilayer film is a unique one-dimensional system with tailored material choices, layer thicknesses, and interfaces. By minimizing the disadvantageous polarizations and enhancing the advantageous polarizations, multilayer films hold promise as advanced dielectrics for future polymer film capacitors.

Nanodielectrics are extremely promising materials for use in high voltage cable transmission and as capacitor materials. It is now well accepted that the high volume of interfacial material creates opportunities for trapping of carriers, a reduction in conductivity, and a concomitant increase in dielectric breakdown strength and endurance strength. It has also become clear that the dispersion of the nanofiller is critical to optimization of both breakdown strength and permittivity. Thus, we have developed a ligand engineering approach that uses a bimodal population of molecules to control both dispersion and trapping behavior. A low graft density of long polymer chains ensures compatibility with the matrix and potentially entanglement or crosslinking with the matrix. A high graft density of charge trapping molecules modifies the composite trapping behavior. Using a variety of characterization methods including: PEA (Pulsed Electroacoustic Analysis), UV vis, microscopy, dielectric spectroscopy, breakdown strength, and endurance measurements, we have characterized the trapping and dielectric behavior of a variety of nanodielectrics. We have also developed a multiscale model that ranges from the nanoscale trapping behavior to the macroscopic breakdown behavior to attempt to create a methodology for nanodielectric design.

As part of a broader approach, we are developing dielectric data and tools for a new open source nanocomposite data resource, NanoMine. NanoMine has a growing set of data, microstructure quantification and reconstruction tools, and models that specifically incorporate interface behavior. We have run a comprehensive set of experiments that vary the mixing energy, shear stress, and strain rate in a polymer melt, and use a large set of TEM images, and NanoMine tools to quantify microstructure descriptors such as interfacial area, cluster size and volume fraction. We are using the data in NanoMine to correlate quantitative microstructure descriptors with dielectric properties with the goal of being able to develop design tools that identify the appropriate fillers, dispersion, and processing methods to meet specific combinations of properties. Such an approach to simulation and design could eventually accelerate the process of nanocomposite adoption for dielectric applications.

Weight and volume are often at a premium in engineering; any material that does not contribute to the load-carrying capacity is structurally parasitic. Current engineering design pursues optimisation of the individual components by utilising materials with improved specific properties. The alternative is to formulate multifunctional materials which can perform two or more functions simultaneously. Here, we develop multifunctional composite materials that can simultaneously carry mechanical loads whilst storing (and delivering) electrical energy. This work was initially motivated by the recognition that carbon-based materials are often used both for electrochemical devices, and high performance structural composites; in addition, both electrochemical devices and structural fibre-reinforced polymer composites are usually assembled in a laminated form. Thus, the goal was to produce a multifunctional structural supercapacitor built around laminated structural carbon fibre fabrics. Each cell of the proposed structural supercapacitor consists of two modified structural carbon fibre fabric electrodes, separated by a structural glass fibre fabric or polymer membrane, infused with a multifunctional polymeric electrolyte.
In order to simultaneously maximise the mechanical and electrical performance, the reinforcing fibres must be modified to increase electrochemical surface area whilst maintaining their intrinsic performance. Rather than using conventional activated carbon fibres, structural carbon fibres were treated to produce a mechanically robust, high surface area material, using a variety of methods, including direct etching, carbon nanotube sizing, and carbon nanotube in situ growth. One of the most promising approaches is integrate a porous bicontinuous monolithic carbon aerogel throughout the matrix. This nanostructured matrix both provides a dramatic increase in active surface area of the electrodes, and has the potential to address mechanical issues associated with matrix-dominated failures. The conflicting requirements for matrix stiffness and ionic conductivity can also be addressed using bicontinuous architectures.
Working structural supercapacitor composite prototypes have been produced and characterised electrochemically using impedance spectroscopy, cyclic voltammetry and charge-discharge measurements. The effect of introducing the necessary multifunctional resin on mechanical properties has also been assessed. Larger scale demonstrators have been produced, including a full size car boot/trunk lid as part of the STORAGE consortium.

Polymer nanocomposite dielectrics with high energy density and low loss are major enablers for a number of applications in modern electronic and electrical industry. Conventional fabrication of nanocomposites by solution routes involves equilibrium process which is slow and results in structural imperfections, hence high leakage current and compromised reliability of the nanocomposites. We propose and demonstrate that a nonequilibrium process which synergistically integrates electrospinning, hot-pressing and thermal quenching, is capable of yielding nanocomposites of very high quality. In the nonequilibrium nanocomposites of poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) and BaTiO3 nanoparticles (BTO_nps), an ultrahigh Weibull modulus β of ∼30 is achieved, which is comparable to the quality of the bench-mark biaxially oriented polypropylene (BOPP) fabricated with melt-extrusion process by much more sophisticated and expensive industrial apparatus. Favorable phase composition and small crystalline size are also induced by the nonequilibrium process, which leads to concomitant enhancement of electric displacement and breakdown strength of the nanocomposite hence a high energy density of ~ 21 J/cm3. Study on the polarization behavior and phase transformation at high electric field indicates that BTO_nps could facilitate the phase transformation from α to β polymorph at low electric field.

Dielectric polymer nanocomposites comprising one dimensional (1D) high-k nanowires have received strong interest because of their potential application in electrical energy storage. However, the large contrast in dielectric constant/electrical conductivity between polymer and nanowires usually result in low breakdown strength and high dielectric loss, leading to lower capability of electrical energy storage. Engineering the high-k nanowires surface can tailor the interfacial region of the nanocomposites, leading to significantly enhanced electrical energy storage capability. This work summarize the recent progress on the role of surface engineering in electrical energy storage of 1D high-k nanowires based dielectric polymer nanocomposites, including bio-inspired organic coating and inorganic coating. It was found that one can optimize the electrical energy storage capability of the nanocomposites by enhancing the matrix/nanowires compatibility, adjusting the coating layer thickness and tailoring the dielectric constant/electrical conductivity of the coating layer. The methods provided in this presentation offer important clues to design and fabricate high-energy-density polymer nanocomposites.

Multifunctional three-dimensional (3D) nano-architectures integrating all components of an entire device within tens of nanometers are intriguing for next generation energy storage, but have remained challenging to achieve. The lack of appropriate synthesis methods with precise spatial control over multiple distinct materials' phases in 3D on the nanoscale is a key issue holding back the development of such intricate architectures. Here we present chemical pathways to such systems based on the bottom-up synthesis of penta-continuous nanohybrid monoliths with four functional components integrated in a triblock terpolymer derived core-shell double gyroid architecture. Two distinct 3D interpenetrating redox-active cathode electrode and current collector networks are separated by continuous ultrathin polymer electrolyte shells from a carbon anode. All periodically ordered domains are less than 20 nm in their layer dimensions and integrated throughout the macroscopic monolith. Initial electrochemical measurements exhibit reversible battery-like charge-discharge characteristics with orders of magnitude decreases in footprint area over theoretical flat, three layer designs.

Reference: J. G. Werner, G. G. Rodríguez-Calero, H. D. Abruña, U. Wiesner, Block Copolymer Derived Multifunctional Gyroidal Monoliths for 3-D Electrical Energy Storage Applications, arXiv (2017), asXiv:1706.02134 [physics.chem-ph].

Piezoelectric and electro-active composites are investigated as new generation self-powered energy harvesting devices for a wide range of applications from the industrial to the medical field while maintaining high reliability, durability and sensitivity over wide range of frequencies. Three-phase piezoelectric composites are economically more feasible and non-toxic. The criteria that govern the applicability of piezoelectric composites depend upon electromechanical properties such as capacitance, impedance, conductance, resistance and dielectric constant. The desired electrical properties may be enhanced by including electro-active and conductive inclusions which play a vital role in enhancing the piezoelectric and dielectric characteristics of the composites. The present work investigates the role of graphene and zinc oxide inclusions distributed in an epoxy matrix to fabricate three-phase composites and studies the influence of several factors on the effective electromechanical properties of the composites. The materials under investigation will be comprised of Zinc Oxide, Epofix Cold-Setting Embedding Resin and Graphene, i.e. the electro-active, epoxy and conductive inclusions respectively. Our works seeks to understand the fabrication process parameters and suggest the inclusion of graphene enhancing piezoelectric properties which can be measured using the impedance analyzer and the scanning electron microscope (SEM).

Smart windows that can switch transparency-opacity in response to surrounding environment have been demonstrated from chromogenic materials, polymer dispersed liquid crystals, and suspended particle devices driven by electrical field, light, and temperature. Previously, we show a stretchable smart window film that is responsive to mechanical stimuli. However, there is a need to fine-tune the range of the strain and the degree of the transparency change in a low-cost fashion. Here, we report a new design of smart window consisting of a elastomer film from polydimethylsiloxane (PDMS) with wrinkles on one side and silica nanoparticles (NPs) on the other side of the film. It allows us to take advantage of the transparency/opacity change of individual components and/or their combinations on demand. Since silica and PDMS have similar refractive indices, when stretching the composite film below the wrinkle pre-strain, the increase of transparency is mainly attributed to the change of wrinkle geometries. When the film is further stretched, air-pockets are created around the silica NPs, leading to increased opacity due to the scattering effect from air/silica NP interface. In the meantime, secondary wrinkles are created in the direction of stretching, thereby further increasing opacity. Together, a larger yet tunable drop in transparency is observed compared to that from either the pure wrinkled film or a simple NP/PDMS composite film. We also investigate how the transmittance of the films can be varied by the size of silica NPs and/or wavelength of the wrinkles.

Broad implementation of the Internet of Things, portable electronics or inside-the-body sensors has been hindered by the limitations of conventional batteries. Advances in energy harvesting can play a crucial role towards the application of those technologies. This includes piezoelectric energy harvesting for the use of local mechanical vibrations and their transformation into electrical sources for low-power electronics. For this purpose, innovative engineered materials with high piezoelectric response and capable to work at low frequencies are required.
In this research, we show our progress in the development of Niobium-doped Pb(Zr_x-1Ti_x)O₃ ceramics (PNZT), close to the so-called Morphotropic Phase Boundary (MBP), containing ZrO₂ micro particles. Characterization via Scanning Electron Microscopy and Electron Backscatter Diffraction (EBSD) revealed a preferential orientation of the PNZT grains distinctive of textured ceramics, which are usually obtained with more complex techniques, such as templated grain growth (TGG) and tape casting. Despite that X-ray diffraction showed a single tetragonal perovskite phase, which is typically related with lower electromechanical properties, further characterization uncovered high piezoelectric coefficient d₃₃ ~ 1000 pm/V at frequencies up to 1 Hz and a piezoelectric voltage constant g₃₃ ~ 34 x 10^-3 Vm/N. By decreasing the dimension of the ZrO₂ particles, a systematic appearance of the monoclinic phase was found accompanied by a decrease of the piezoelectric properties. Therefore, a PNZT-ZrO₂ composite that displays high piezoelectric behavior at low frequencies has been fabricated by a simple and inexpensive method, compared to other highly-oriented ceramics. Understanding of the mechanisms that are responsible for such improvement in the piezoelectric performance would help the application of this methods to other (lead-free) materials, making it promising for energy harvesting applications.

Atmospheric pressure and ambient temperature micro-plasmas have been used for polarization of piezo-composites towards alignment of the electric dipoles. Dielectric polarization is caused when a dipole moment is formed in an insulating material because of an external electric field. When a dielectric interacts with an electric field a shift in charge distribution takes place, aligning the positive and negative ions with the electric field. By this mechanism important circuit elements such as capacitors can be developed. The same phenomenon of plasma micro-discharge can also be used for surface modification of piezoelectric-composites towards activation and enhancement of electrical properties of the material surface. This can be achieved by chain polymerization of the surface in organic composite thin films and modification of surface energy that changes the surface bonding characteristics and towards enhancing surface charge density. The current work focuses on the development and use of a corona discharge setup for surface modification of piezoelectric flexible composites. Three quasi-stable voltage driven micro-discharge regimes with a current of 0.1 mA and voltages of 3, 3.5 and 4 kV and a pulsed regime with a voltage 2.5 kV and current of 0.5 mA has been identified for surface modification. ZnO-Epoxy-Graphene composites are prepared with a variation of ZnO volume fraction from 10% to 70%. The capacitance, resistance, impedance and dielectric constant of these composite thin films are characterized using an impedance analyzer. The micro-structure and elemental properties are characterized using a Scanning Electron Microscope, EDS and a Raman spectrometer. The surface topography of the thin films are also characterized using a Profilometer. A 35-50% enhancement in the capacitance and dielectric constant values are observed for the quasi-stable corona discharge regimes.

The systems used in EMR applications are very strongly impacted by environmental conditions during their installation, operation or maintenance phase. The optimization of their functioning and their dimensioning can only be done by controlling the interaction between the medium and the components or structures of the systems. Thus, the development of numerical and experimental tools and methodologies capable of simulating the impact of wind, wave, current and behavior of these systems in a coupled way constitutes a major dimensioning challenge. The validation of these sizing processes is made possible by in-situ measurement, which requires a strategy adapted to the application envisaged. This aspect constitutes a major stake at the dawn of the deployment of pilot farms of fixed and laid wind turbines and prototype of hydro-turbines. Indeed, the aim of this phase is to validate the choice of a solution and to identify the optimization paths that will allow to go on the commercial phase with a substantial gain of LCOE (Levelized Cost of Energy).
The recovery of energy from the kinetics of marine or river currents is particularly interesting because it constitutes an immense and almost inexhaustible source. By installing marine or inland waterways, it is possible to recover some of this kinetic energy. During operation and taking into account the fluid - structure couplings, the machines are generally stressed by cyclic mechanical forces, repeated shocks and / or progressive rotation. The goals of structural service, machine efficiency and turbine performance pose significant challenges for designers, mechanics of solids and fluids.
Wind turbines and hydro-wind turbines are the two most advanced technologies, but the hydro-turbines still require optimization studies. In particular, the blades and the nozzle are on the critical path of machine life. Thus, they should be designed as safely as possible and be able to withstand the applied loads and the hostile environment. It has become common practice to find structural parts of composite materials such as blades, nozzles, etc. on energy recovery techniques. Moreover, the behavior of composites in service (shock, fatigue) remains difficult to predict.
The aim is to equip design departments with the dimensioning of composite structures with tools enabling them to choose materials (fiber / matrix), fibrous architectures (tablecloth, fabrics), stacking sequence of strata minimizing sensitivity to applied loads of working structures.

To face energy crisis and environmental pollution, clean renewable energy sources provide the most promising way for energy and environmental sustainability. Piezoelectric energy harvesters can be practically assembled using low-cost solution assembly methods to convert ambient mechanical energy, such as waves, wind, and others, into electricity. In this study, we demonstrate the assembly of fully functional piezoelectric energy harvester fabricated by vertically grown hydrothermal ZnO nanorods/nanosheets on aluminum foil as a composite. The advantages of this device are the process simplification of plating electrodes and the high-temperature heat treatment with aluminum foil substrate. Both factors are beneficial to improve the power output and efficiency of piezoelectric energy harvesters with ZnO thin films. Firstly, the zinc oxide seed layer is fabricated on the aluminum foil by sol-gel dip-coating method at different annealing temperatures. Subsequently, ZnO nanocrystals arrays are grown by hydrothermal method on ZnO seed layers. From the results, it shows that while the seed layer is annealed with atmosphere furnace at 400°C and then grown with hydrothermal method, mixed nanorods and nanosheets in the morphology of ZnO thin film are obtained and presented with the best piezoelectric property. By measuring the output voltage and output power of this device, it shows that with the vibration amplitude of 0.5cm and the frequency of 10Hz, the output voltage and output power can be reached at 3.70V and 268mW, respectively. More details of the influence of processing parameters on the nanostructures and piezoelectric properties of ZnO-based energy harvester are demonstrated in this study.

Thermal diodes, or devices that transport thermal energy asymmetrically, analogous to electrical diodes, hold promise for thermal energy harvesting and conservation, as well as for phononics or information processing. Thermal diodes enabled by phase change materials are ideal candidates for such applications due to their potential to efficiently operate at ambient conditions under low temperature biases (< 20 K). Theory and experiment for high-performance, ambient thermal diodes, enabled by junctions of phase change materials, is presented. It is shown that such diodes possess ideal temperature biases and material dimensions for optimal operation and design, and their performance greatly exceeds that of ambient thermal diodes operating under low temperature biases in the literature to date. In addition, the application of such thermal diodes for ambient energy harvesting and conservation applications is addressed both experimentally and theoretically.

Kinetic energy harvesters have been extensively studied due to their high-power density given by powerful vibration energy. There are many sources of vibration energy such as human motions, automobiles and other natural motions. For a decade, one dimensional (1D) nanostructures have been widely approached for miniaturization in a piezoelectric field, however, there are still critical limitations of insufficient piezoelectricity and unstable standing on a substrate. In this study, BaTiO₃ nanorod bundle arrays as a 1D novel structure were designed to obtain effective piezo-strain change and stable standing on fluorine-doped tin oxide (FTO) glass, simultaneously. In order to develop the nanorod bundle array, TiO₂ nanostructures as framework were hydrothermally synthesized with adjusting area density, aspect ratio and free-standing via investigation of pH and precursor effects. It is illustrated that area density was enlarged as an increase in Ti precursor concentration or increase in pH, respectively. Optimum TiO2 nanorod bundle arrays were optimized for non-aggregation of 1D nanostructures on high area density over 60 % with bundle diameter of 100nm consisted of rod diameter of 10nm. Next, M-shaped TiO₂ nanorod bundles were developed via chemical etching process to conduct complete phase transformation of BaTiO₃. During chemical etching, top surface was found to be more etched compared to side wall because of the higher surface energy of (001) with Z-axis than (110) with X and Y axis. After BaTiO₃ phase transformation from the M-shaped nanostructures, conversion ratio was found to be a higher magnitude of 92.3 % compared to 78.4 % conversion ratio of non-etched TiO₂ nanostructure. BaTiO₃ conversion ratio was defined as follow; Conversion ratio (%) = [XRD peak of BaTiO₃] / [XRD peak of TiO₂ + XRD peak of BaTiO₃] × 100, where the representative peaks of TiO₂ and BaTiO₃ were shown as (002) at 2θ = 62.8° and (110) at 2θ = 31.3°, respectively. It is noted that the M-shaped nanostructure can offer large diffusion sites of Ba²⁺ ions determining perovskite (ABO₃) phase with high piezoelectricity. The effective BaTiO₃ phase conversion using M-shaped nanostructure was confirmed with volume expansion and ion mapping by SEM and STEM analysis, respectively. Finally, the progress of BaTiO₃ phase transformation from M-shaped TiO₂ nanostructure was confirmed by morphology characterization with variations of Ba²⁺ ion concentration. During an early stage of phase transformation, BaTiO₃ was predominantly formed along Z axis in empty space. As the Ba²⁺ ion increased, (001) growth along the perpendicular direction to the FTO substrates occurred. On the contrary, (110) growth along X and Y axis hardly occurred owing to low diffusion of the Ba²⁺ ion. In conclusion, BaTiO₃ nanorod bundle structures were successfully synthesized with high conversion ratio, and the novel structure can be a strong candidate for high power piezoelectric devices.

Lightweight, flexible, and wearing comfortable personal thermal management (PTM) device with desirable performance becomes prevalent because of its potential to wisely adjust our body temperature to a thermally-safe and comfort state. In this work, we report a freestanding, flexible/foldable, and large-area ultrathin graphene papers (GPs) with high thermal conductivity and sensitive electro-thermal response, and their application to wearable bifunctional PTM devices for heating and cooling. The heating part is achieved by taking advantages of its joule heating, while the cooling part benefits from its high thermal conductivity which from the ultrathin and compact lateral structure of the GPs. The promising electrical conductivity grantees the superior Joule heating for extra warmth of 42 °C using a low supply voltage around 3.2 V. Besides, based on its high thermal conductivity, the graphene paper provides passive cooling via thermal transmission from the human body to the environment within 7s. The cooling effect of graphene paper is superior compared with that of the normal cotton fabric, and this advantage will become more prominent with the increased thickness. The present bifunctional graphene paper possesses high durability against bending cycles over 500 times and wash time over 1500 min, suggesting its great potential in wearable PTM. Furthermore, we integrated the GPs into textiles by the techniques of plain weave, co-woven, hollow-out, and kirigami. These wearable technology designs achieve not only the personal thermal management, but also the breathability of the PTM devices. It can bring inspiration to the development of intelligent clothing in the future.

Recently, Poly(vinylidene fluoride) (PVDF) based magnetoelectric (ME) composites have been studied with strong interests for feasible applications using high flexibility, non-brittle, low-temperature processing and good price competitiveness. In order to improve ME characteristics, polymer based ME composites were supposed to be developed with an effective transition of β phase exhibiting electrically asymmetric structure in the piezoelectric matrix. Therefore, various approaches were addressed for high β phase transition via inorganic filler addition, thermal annealing, uniaxial stretching, melting-quenching, electrical poling and solvent casting. Even though most of above approaches were simultaneously used during ME sample preparation, any research group has never been scrutinized exact roles of nanofillers on PVDF phase transition at each processing step.
In this study, the effects of two-type magnetostrictive nanofillers on PVDF phase transition were investigated by FT-IR analysis at each processing step of solvent casting, annealing, stretching and electrical poling, respectively. As magnetostrictive nanofillers CoFe₂O₄ were synthesized with two different structures of nanoparticles and nanorods by a hydrothermal method. Then neat PVDF (S1), PVDF/nanoparticles (S2) and PVDF/nanorods (S3) films were fabricated by solvent casting. Those films were found to exhibit β phase fraction of S1=77.1%, S2=77.4% and S3=78.6%. After thermal annealing of 210^oC for 10min, the β phase fraction was decreased to S1=29.5%, S2=29.9% and S3=35.8%. From two processes of solvent casting and thermal annealing, it is described that nanorods can serve confinement effect preventing α-phase transition from β-phase via mechanically hindering of PVDF chain shrinkage. After stretching process with stretch ratios 5, the β phase fraction was increased up to S1=72.5%, S2=78.5% and S3=76.7%. That is because two critical influences of stress concentration effect and active area occurred. The S2 and S3 films exhibited higher β-phase through stress concentration given by nanofiller structures in PVDF matrix. As well, the S2 films showed maximum β-phase since nanoparticles have large surface area compared with nanorods, which can be an active area expressing dominant concentration effect. After poling process following thermal annealing, the β phase fraction was increased up to S1=40.4%, S2=46.5% and S3=37.3%. During the poling process, nanorods served as high electrical leakage source in the S3 films. On the other hand, the S2 films have less leakage current with maximum transition, which might be induced by an effective surface charge. We believe that this study about confinement effect, stress concentration effect, active area and leakage current in terms of magnetostrictive filler’s roles can be a meaningful reference to optimize magnetoelectric composites.

Three-phase, lead-free, biocompatible barium titanate-epoxy-zinc oxide (BaTiO₃-Epoxy-ZnO) electroactive composite films were prepared. The volume fraction of the BaTiO₃ phase was held constant at 0.4, while the volume fraction of the ZnO phase was varied from 0.01 to 0.10. The dipoles of the electro-active phases were aligned using a plasma-microdischarge (Corona) poling technique. The piezoelectric strain coefficients, dielectric constant, dielectric loss tangent, capacitance, impedance, resistance, and conductance of the samples were measured and compared as a function of poling regime. The impedance and dielectric spectra of the composites were recorded over a frequency range of 20 Hz to 10 MHz. The fractured surface morphology and distribution of the phases were observed with the aid of Electron Dispersion Spectroscopy (EDS) and a Scanning Electron Microscope (SEM), which showed the inclusions embedded in the epoxy matrix. The electromechanical properties of the composites were optimized towards the development of sensors and energy harvesters for self-powered pace makers.

Microwave sintering has been successfully employed to rapidly sinter ceramics, metals, and ceramic-metal composites (cermets) at reduced temperatures, yielding dense materials with fine grain structures. However, one of the greatest challenges with microwave sintering is that the material needs to have suitable dielectric and magnetic characteristics such that it couples with the microwaves and heats the material effectively.
Most commercial microwave sintering techniques employ external susceptors, or thermal sources, to heat the sample to the targeted temperature. Alternatively, internal microwave susceptors can aid in sintering ceramic materials. This can be accomplished by combining microwave-transparent materials with microwave-susceptible materials. For this reason, SiC-B₄C composites are being investigated (as SiC is microwave-susceptible and B₄C is microwave-transparent). The light weight, high strength, high corrosion resistance, and high temperature stability of SiC-B₄C make it suitable for potential armor applications. In addition, desirable thermoelectric properties (i.e. high electrical conductivity, low thermal conductivity, high Seebeck coefficient, etc.) make them strong candidates for thermoelectric applications.
The incorporation of carbon precursors to SiC-B₄C composites has the potential to further enhance thermoelectric properties while also serving as an additive to improve sinterability. Therefore, microwave sintering has been performed using SiC, B₄C, and carbon black as starting materials. Powders of 85% SiC, 10% B₄C and 5% carbon black were ball milled to obtain homogenous mixtures, which were then pressed into pellets and cold isostatically pressed. In order to determine the optimum sintering conditions, samples were processed in a 2.45 GHz single mode microwave cavity using various magnetic/electric field ratios. In this study, microwave sintered composites will be compared to conventionally sintered composites using scanning electron microscopy, x-ray diffraction, and micro-hardness measurements. In the future, this technique could be applied to a variety of applications, such as thermoelectric energy storage ceramics, nanocomposites, and multi-functional material systems.

Temperature-sensitive materials (TSMs) attract a lot of attention in recent years. The physical properties of TSMs can be tuned by temperature. Among TSMs, room-temperature switchable materials (RTSMs), which responses automatically around room temperature, is more interesting for researchers. Because they have great potentials in smart switches, smart building, high sensitive sensors, automatic filters, energy storage and can be used in daily life.
In this paper, we prepared the room-temperature switchable materials by dispersing chemical functionalized CNTs into hexadecane. The DC conductivity, thermal conductivity and dielectric constant of RTSMs can be regulated simultaneously in a narrow temperature range around phase change point. By changing the volume fraction of octadecylamine-grafted MWCNTs in hexadecane, the switching ratio of DC conductivity, thermal conductivity and dielectric constant reaches 5 orders of magnitude, 3 times and 106.4 respectively. To our best knowledge, it is the highest switching ratio at room temperature in existing materials. The multifunctional switch is caused by the rearrangement of the fillers in hexadecane matrix during phase transition. During the freezing course, the fillers are squeezed into the grain boundaries of solid matrix to form percolating conducting networks. The conducting networks enhance the electrode injection, improve the interior electron migration and decrease the interface resistance at the same time. The interface between filler and solid matrix give rise to strong Maxwell-Wagner relaxation. These reasons make the DC conductivity, thermal conductivity and dielectric constant of frozen sample increase a lot. When the composite is remelting, the re-dispersion of fillers results in the DC conductivity, thermal conductivity and dielectric constant of sample decreasing again. The switch shows good stability during temperature cycles. The switching ratio can be easily regulated by the volume fraction of filler and temperature ramping rate. By using different long chain alkane as matrix, other multifunctional switchable composites with various trigger temperature can be easily achieved.

In this presentation, discussion will be focused on enhancing the dielectric and mechanical properties of polyvinylidene fluoride (PVDF) and its copolymers via different strategies including interfacial chemistry and additives. The effect of different additives and interfacial chemical treatment on the dielectric breakdown strength, dielectric constant and loss factor is investigated. Systematic study is carried out towards achieving high dielectric constant while mitigating the possible reduction of dielectric breakdown strength. This is a highly desirable and relevant to important applications such as in actuators, capacitors and electric caloric devices. Another discussion is on an unusually large enhancement of PVDF film toughness and ductility using nanoparticles at very low concentration of 0.1%. Mechanistic explanations are also provided.

Lead-free piezoelectric nanowires (NWs) show strong potential in sensing and energy harvesting applications due to their flexibility and ability to convert mechanical energy to electric energy. Currently, most lead-free piezoelectric NWs are produced through low yield synthesis methods and result in low electromechanical coupling, which limit their efficiency as energy harvesters. In order to alleviate these issues, a scalable method is developed to synthesize perovskite type 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (BZT–BCT) NWs with high piezoelectric coupling coefficient. The piezoelectric coupling coefficient of the BZT–BCT NWs is measured by a refined piezoresponse force microscopy (PFM) testing method and shows the highest reported coupling coefficient for lead-free piezoelectric nanowires of 90 ± 5 pm V−1. Flexible nanocomposites utilizing dispersed BZT–BCT NWs are fabricated to demonstrate an energy harvesting application with an open circuit voltage of up to 6.25 V and a power density of up to 2.25 μW/cc. The high electromechanical coupling coefficient and high power density demonstrated with these lead-free NWs produced via a scalable synthesis method shows the potential for high performance NW based devices.

With its light weight, low cost and high efficiency even at low operation frequency, the triboelectric nanogenerator (TENG) is considered a potential solution for self-powered sensor networks and large-scale renewable blue energy. As an energy harvester, its output power density and efficiency are dictated by the triboelectric charge density. Plenty of efforts have been devoted to improving the power output by maximizing the surface charge density. Here we first demonstrated that with the improved soft-contact and fragmental structure, the triboelectric charge density can be increased from 50 to 120 μC/m² in air when compared to a conventional TENG with only hard contact. However, due to high-voltage air breakdown, most of the enhanced surface charge density brought by material/surface optimization or external ion injection is not retainable or usable for electricity generation during continuous operation of contact-separation mode TENGs. We experimentally validated the existence of the air breakdown effect in a contact-separation mode TENG with a low threshold surface charge density of ~40–50 μC/m² under the high impedance external load, and conducted the theoretical study of the maximized effective energy output as limited by air breakdown under different air pressures and gas compositions. Subsequently, we applied high vacuum (~10^-6 torr) to eliminate the limitation of air breakdown, which boosted the charge density to 660 μC/m². With the coupling of surface polarization from triboelectrification and hysteretic dielectric polarization from a ferroelectric material, the charge density can further jump to 1003 μC/m², which elevates the maximum output power density of a conventional TENG from 0.75 to 50 W/m² even at a low-motion frequency of about 2 Hz, the normal frequency of human walking and ocean waves. These findings open more possibilities for TENGs both as highly efficient mechanical energy harvesters for large-scale energy sources such ocean waves, and as self-powered modules integratable with devices beyond wearable electronics and sensors. Our findings may also give new insights into long-lasting debates over the mechanism of triboelectrification and its kinetics.
Refs:
[1] J. Wang, C. Wu (co-first author), Y. Dai, Z.L. Wang, et al. Achieving ultrahigh triboelectric charge density for efficient energy harvesting. Nat. Commun. 8, 88 (2017)
[2] Y. Zi, C. Wu (co-first author), W. Ding, Z.L. Wang, Maximized effective energy output of contact-separation-triggered triboelectric nanogenerators as limited by air breakdown, Adv. Funct. Mater. 27, 1700049 (2017)

Triboelectric nanogenerators (TENGs) have rapidly risen to the forefront of mechanical energy harvesting technologies in the last five years, showing the potential to generate high power outputs at high efficiency and low cost. The maximum instantaneous power density of these devices has been reported to exceed 500 W/m². [1] TENGs have been demonstrated for their use as energy harvesters and self-powered active sensors which operate on ambient energy sources such as wind, machine vibrations and human movement, hence providing the pathway for sustainable energy generation. [1] However, the lack of knowledge in the fundamental working principles of TENGs has impeded the progress in this field. [2]
The classical theoretical models explaining TENGs are based on parallel plate capacitors, which makes it challenging to comprehensively describe the working principles of these devices. Furthermore, numerous circuit element based models are derived for different TENG types. These explanations are limited to planar geometries with parallel TENG layer arrangements, and some of the output predictions show significant deviations from experimental observations. [2]
Herein, we present the first analytical model to fully describe the working principles of different TENG types using Maxwell’s equations. [2] The new model is based on the distance-dependent electric field (DDEF) concept, derived using the spatial field variations of charged TENG layers. The DDEF model is capable of accurately predicting the output behaviour of vertical charge polarization TENGs including vertical contact and separation mode, single electrode mode and free standing TENG layer mode, hence encompassing the majority of existing TENG structures. The current, voltage, charge and power output of different TENG types are predicted using the DDEF model which show an excellent agreement with the experimental TENGs, indicating significant improvements over the existing models. Unlike the previous models, this model is not constrained to a planar geometry, and can be universally applied to complex geometries and surface topographies found in practical TENGs. Furthermore, a number of unique relationships between the TENG device parameters and power output is revealed using the DDEF model and verified experimentally, enabling the design of more efficient TENG structures for different applications.
In conclusion, this work, for the first time, presents a unified theoretical framework for different TENG working modes along with a systematic study on the device optimization, providing critical guidance for the design and construction of efficient TENG structures.
1. Wang et. al., Energy Environ. Sci. 8 (2015), 2250-2282.
2. Dharmasena et. al., Energy Environ. Sci. 10 (2017), 1801-1811.

The ubiquitous sensors have accelerated the realization of Internet of Things (IoT) but also raised challenges to the current overcrowding radio frequency (RF) based communications. The optical wireless communications (OWC) which utilizes the resourceful optic bandwidth can well solve the spectrum crisis and be an appealing complementary solution to the IoT applications. However, the additional direct current (DC) power supply and complicated modulating and power management circuits may limit the large-scale deployment of OWC systems. In this paper, we proposed the self-powered OWC driven by triboelectric nanogenerator (TENG). By integration with TENG devices, the light-emitting diode (LED) could be directly transformed into a wireless transmitter which conveys the information associated with mechanical stimuli without additional power supply. With the customized TENG devices and the help of advanced image processing and machine learning techniques, three demonstrations with functions of remote control, pressure sensing, and security authentication, were implemented in the laboratory environment. The concept and results in this paper may greatly broaden the application of IoT through the integration of OWC and TENG.

Compared to commonly used conventional energy storage devices such as batteries and fuel cells, polymer-based capacitors can have high energy densities, which are due to the fast charging and discharging capability. This high energy density arises from concerns of integration, especially in electrical and electronic systems that require pulses of energy in their operation mode. Another field of application is the development of soft generators for the harvesting of wearable electronics. In such a way, composite-elastomers (dielectric elastomer generators: DEGs) constitute a solution to convert ambient mechanical energy into electricity. One common factor is mandatory to develop polymer-based capacitors and DEGs: the increase in the dielectric constant of the polymer as this value is generally low for polymers dedicated to industrial applications. The solution proposed consists in benefiting both the insulating nature of polymeric materials thus ensuring a fairly high electric field and the addition of micro/nano particles (ceramics, oxides, metallic, carbon nanotubes…) inside the polymer matrix in order to obtain high dielectric constant. There is a profusion of scientific publications on the dielectric properties of micro/nano composites polymers showing a strong increase in the dielectric constant. Unfortunately, concrete applications have been slow to materialize. The major drawbacks are linked to the fact that the increase in the dielectric constant (necessary to enhance the value of the capacitance) is accompanied by a drastic increase in the dielectric losses and a collapse of the dielectric strength. One reason of that is the neighboring of particles inside the polymer matrix (without considering the percolation) which promotes conducting paths and reinforcing fields at the interfaces. The main goal of our study was to develop polymer composite dielectrics generators for wave energy converters (WECs) and for energy harvesting devices to exploit the human energy kinetic while walking. For that, carbon nanotubes (CNT) were mixed inside a polymer matrix (Dow Corning silicone Sylgard 184) to realize a 100 µm thick active layer. In order to reduce the drawbacks evoked above, a coating of CNT was made with a specific polymer. Investigations in terms of dielectric constant; loss factor, dissipation factor, ac-conductivity and dielectric strength were compared for uncoated-NTC and coated-NTC polymer composites with various weight contents (0.2%, 0.5%, 1%, 2% and 4%). The evidence is unequivocal: for example, for 1 wt%, uncoated-NTC polymer composites present a dielectric strength of 40 kV/mm against 85 kV/mm with coated-NTC. In same time, the dielectric constant in the range 0.1Hz-1MHz was around 6.5 and 8 for the uncoated-NTC and coated-NTC respectively. Lastly, the percolation threshold was strongly increased with coated-NTC polymers which is also benefit to increase more the dielectric constant for applications like polymer-based capacitors.

Development of high-performance power sources that comply with the continuously decreasing microscale dimensions of electronic devices is one of the major challenges in the IoT era. To address this challenge, we are working on new inorganic-organic thin-film materials for next-generation flexible energy harvesting and storage technologies. An elegant, yet industrially feasible way to fabricate such materials is to combine the ALD (Atomic Layer Deposition) technique originally developed to deposit high-quality thin films of simple inorganic materials with MLD (Molecular Layer Deposition) cycles based on organic precursors. This enables the atomic/molecular layer-by-layer production of inorganic-organic hybrid thin films through sequential self-limiting gas-surface reactions with high precision for the film thickness and composition.

Our hybrid inorganic-organic materials consisting of well-defined nanoscale layers of both components are promising candidates for enhanced thermoelectric materials, as such a layered structure provides us with the means to suppress the material’s thermal conductivity as a whole without significantly hindering the electrical transport properties of the individual inorganic layers. Moreover, the ALD/MLD fabrication technique employed allows the deposition of these materials directly on complex/flexible surfaces thus enabling the integration with e.g. textiles.

Another exciting application possibility for our hybrid thin films is in the all-solid-state organic thin-film microbattery. Organic electrode materials contain light, abundant and environmentally benign elements only, and show very high specific capacities, but in bulk form their poor electronic conductivity hinders the rate performance significantly. In a thin-film form this obstacle can be superbly circumvented. For our all-ALD/MLD fabricated microbattery with ultrathin Li-benzoquinone electrode and LiPON electrolyte layers, ultrahigh redox reaction rates are realized, such that it actually is able to combine the high energy density of batteries with the high power density of supercapacitors.

Harvesting otherwise wasted ambient mechanical energy from high frequency sources (>10 Hz) can be effectively done using piezoelectric and triboelectric energy harvesters. However, mechanical energy sources at low frequencies (<5 Hz) and static loading conditions pertaining to ambient human interactions remain inaccessible to these conventional platforms. Here I will discuss a new class of electrochemical-mechanical energy strain harvester with operation at low frequencies from 5 Hz to below 10 mHz. This harnesses the mechanical-electrochemical coupling in battery materials that we have recently demonstrated experimentally. [1] Specifically, I will discuss energy harvesting using three types of electrode materials: (i) sodiated black phosphorus, (ii) lithiated aluminum and (iii) sodium co-intercalated multilayered graphene at a broad range of low frequencies. A comparison of these three electrodes provides the ability to carefully tune frequency capability by modulating rate kinetics of the mechano-electrochemical response measured by the energy harvester. Overall our results provide a framework for utilizing the coupling between mechanics and electrochemistry in these different battery electrodes for developing efficient low frequency ambient energy harvesters and sensors.
References: [1] N. Muralidharan et al. ACS Nano 11, 6243-6251 (2017); [2] N. Muralidharan, M. Li et al. ACS Energy Lett. 2, 1797-1803 (2017).

Polymer-based composites were heralded in the 1960s as a new paradigm for materials. By dispersing strong, durable, multifunctional nanomaterials in a polymer matrix, high-performance lightweight polymer nanocomposites could be developed and tailored to individual applications. However, the challenges that remain to achieve good dispersion and uniform distribution of nanomaterials pose significant obstacles to these goals. Here, we introduce an innovative nanocomposite for energy application. Using an advanced aerosol-jet printing technique, an unprecedented dispersion and distribution of the nanoparticles (NPs) are realized for polymer-based nanocomposites, with significantly low concentration of NPs, allowing for precise control of various physical properties of interest. Perfect dispersion of NPs in the solvent can be preserved in the polymer matrix as well due to the dual atomizing and printing technique. The uniformity can be finely controlled up to 10 micro-meter scale, which is the resolution of the printer. The resultant nanocomposite contributes to the enhancement of piezoelectric and ferroelectric properties of polymer materials. Correspondingly, a triboelectric nanogenerator device based on highly-uniform and well-dispersed polymer nanocomposite showed a ~ 200% increase in output power density as compared to a neat polymer-based triboelectric generator, when subjected to identical mechanical excitations.

In an effort to create a paintable/printable thermoelectric material, comprised exclusively of organic components, polyaniline (PANi), graphene, and double-walled carbon nanotubes (DWNT) were alternately deposited from aqueous solutions using the layer-by-layer assembly technique. Graphene and DWNT are stabilized with an intrinsically conductive polymer, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). A 1 µm thick film, composed of 80 PANi/graphene-PEDOT:PSS/PANi/DWNT-PEDOT:PSS quadlayers (QL) exhibits electrical conductivity (σ) of 1.88 x 10⁵ S/m and a Seebeck coefficient (S) of 120 µV/K, producing a thermoelectric power factor (S²●σ) of 2710 µW/(m●K²). This is the highest value ever reported for a completely organic material measured at room temperature. Furthermore, this performance matches or exceeds that of commercial bismuth telluride. These outstanding properties are attributed to the highly ordered structure in the multilayer assembly. The thermoelectric power output increased with the number of cycles deposited, yielding 8.5 nW at 80 QL for ΔT = 5.6 K. A simple thermoelectric generator was prepared with selectively-patterned, fabric-based system. The electric voltage generated by each TE device increased in a linear relationship with both ΔT and the number of TE legs, producing ~ 5 mV with just five legs and a ΔT of 9.7 K, as shown in Figure 1. By stabilizing, nanotubes and graphene with nitrogen-rich molecules, n-type multilayer thin films with relatively high power factor have also been produced. This unique TE coating system is water-based and uses only organic components. For the first time, there is a real opportunity to harness waste heat from unconventional sources, such as body heat to power devices in an environmentally-benign way.

The electrocaloric effect (ECE) refers to reversible thermal changes of a polarizable material when an electric field is applied or removed. It can be used for developing all-solid-state refrigeration that are highly efficient and environmentally friendly. Ferroelectric polymers and ceramics are leading EC materials, but each of those materials have limitations. In this work, a ferroelectric polymer/ceramic nanocomposite is presented and it displays greatly enhanced ECE at room temperature. Also, the compositions of the polymer and the lead-free ceramic are modified, hence both of the matrix and fillers exhibit apparent ferroelectric relaxor feature near room temperature. Owing to this feature, the nanocomposite can generate sizable ECE under both low and high electric fields, and it also displays very good stability of the EC performance in the temperature interval between 15 to 50 °C. The giant ECE and ultrahigh EC strength, which is ΔT_{low E} = 6.47 K, ΔT/E = 0.129 Km/MV at 50 MV/m, and ΔT_{high E} = 44.32 K, ΔT/E = 0.222 Km/MV at 200 MV/m, can be observed at 35 °C. The results indicate that this nanocomposite is a promising candidate for the next-generation refrigeration.

Porous medium electromagnetic (EM) heat exchangers are devices which absorb EM radiation and convert its energy into thermal energy for a specific purpose, such as to power a turbine. They have recently been of growing interest, yet the field is predominantly studied with thermal resistance network models and is in need of more rigorous continuum modeling. Electromagnetic heating in which the wavelengths are much longer than the microscale has application to microwave and millimeter wave heating where the microstructures of heterogeneous material are comparably small. Homogenization methods average over the microscale to obtain a macroscopic description of the material. Homogenization has been used in low-frequency electromagnetics to describe macroscopic behavior of traveling waves. While dielectric material parameters vary with temperature, coupling the energy equation with Maxwell’s equations, little effort has been made toward homogenization techniques that capture the effects of this dependence, which is necessary to accurately model porous medium heat exchangers. We consider a laminate geometry composed of alternating layers of lossy dielectric material and lossless fluid channels in the homogenization limit. This model advances the designing of materials that facilitate efficient collection of energy in electromagnetic heat exchangers.