Matthew R. Gadinski

Flexible high-temperature dielectric materials from polymer nanocomposites

Dielectric materials, which store energy electrostatically, are ubiquitous in advanced electronics and electric power systems. Compared to their ceramic counterparts, polymer dielectrics have higher breakdown strengths and greater reliability, are scalable, lightweight and can be shaped into intricate configurations, and are therefore an ideal choice for many power electronics, power conditioning, and pulsed power applications. However, polymer dielectrics are limited to relatively low working temperatures, and thus fail to meet the rising demand for electricity under the extreme conditions present in applications such as hybrid and electric vehicles, aerospace power electronics, and underground oil and gas exploration. Here we describe crosslinked polymer nanocomposites that contain boron nitride nanosheets, the dielectric properties of which are stable over a broad temperature and frequency range. The nanocomposites have outstanding high-voltage capacitive energy storage capabilities at record temperatures (a Weibull breakdown strength of 403 megavolts per metre and a discharged energy density of 1.8 joules per cubic centimetre at 250 degrees Celsius). Their electrical conduction is several orders of magnitude lower than that of existing polymers and their high operating temperatures are attributed to greatly improved thermal conductivity, owing to the presence of the boron nitride nanosheets, which improve heat dissipation compared to pristine polymers (which are inherently susceptible to thermal runaway). Moreover, the polymer nanocomposites are lightweight, photopatternable and mechanically flexible, and have been demonstrated to preserve excellent dielectric and capacitive performance after intensive bending cycles. These findings enable broader applications of organic materials in high-temperature electronics and energy storage devices.

High Energy and Power Density Capacitors from Solution‐Processed Ternary Ferroelectric Polymer Nanocomposites

Concurrent improvements in dielectric constant and breakdown strength are attained in a solution-processed ternary ferroelectric polymer nanocomposite incorporated with two-dimensional boron nitride nanosheets and zero-dimensional barium titanate nanoparticles that synergistically interact to enable a remarkable energy-storage capability, including large discharged energy density, high charge-discharge efficiency, and great power density.

Solution-processed ferroelectric terpolymer nanocomposites with high breakdown strength and energy density utilizing boron nitride nanosheets

The development of high-performance capacitive energy storage devices is of critical importance to address an ever-increasing electricity need. The energy density of a film capacitor is determined by the dielectric constant and breakdown strength of dielectric materials. With the highest dielectric constant among the known polymers, poly(vinylidene fluoride)-based ferroelectric terpolymers are of great potential for high energy density capacitors. However, their energy storage capability has long been limited by the relatively low breakdown strength. Here we demonstrate remarkable improvements in the energy density and charge–discharge efficiency of the ferroelectric terpolymers upon the incorporation of ultra-thin boron nitride nanosheets (BNNSs). It is found that BNNSs function as a robust scaffold to hamper the onset of electromechanical failure and simultaneously as an efficient insulating barrier against electrical conduction in the resulting polymer nanocomposites, resulting in greatly enhanced breakdown strength. Of particular note is the improved thermal conductivity of the terpolymer with the introduction of BNNSs; this is anticipated to benefit the stability and lifetime of polymer capacitors. This work establishes a facile, yet efficient approach to solution-processable dielectric materials with performance comparable or even superior to those achieved in the traditionally melt-extruded ultra-thin films.

Ferroelectric polymer networks with high energy density and improved discharged efficiency for dielectric energy storage

Ferroelectric polymers are being actively explored as dielectric materials for electrical energy storage applications. However, their high dielectric constants and outstanding energy densities are accompanied by large dielectric loss due to ferroelectric hysteresis and electrical conduction, resulting in poor charge-discharge efficiencies under high electric fields. To address this long-standing problem, here we report the ferroelectric polymer networks exhibiting significantly reduced dielectric loss, superior polarization and greatly improved breakdown strength and reliability, while maintaining their fast discharge capability at a rate of microseconds. These concurrent improvements lead to unprecedented charge-discharge efficiencies and large values of the discharged energy density and also enable the operation of the ferroelectric polymers at elevated temperatures, which clearly outperforms the melt-extruded ferroelectric polymer films that represents the state of the art in dielectric polymers. The simplicity and scalability of the described method further suggest their potential for high energy density capacitors.

Ferroelectric Polymer Nanocomposites for Room‐Temperature Electrocaloric Refrigeration

Solution-processable ferroelectric polymer nanocomposites are developed as a new form of electrocaloric materials that can be effectively operated under both modest and high electric fields at ambient temperature. By integrating the complementary properties of the constituents, the nanocomposites exhibit state-of-the-art cooling energy densities. Greatly improved thermal conductivity also yields superior cooling power densities validated by finite volume simulations.

Acid-Functionalized Polysilsesquioxane−Nafion Composite Membranes with High Proton Conductivity and Enhanced Selectivity

A series of new Nafion-based composite membranes have been prepared via an in situ sol-gel reaction of 3-(trihydroxylsilyl)propane-1-sulfonic acid and solution casting method. The morphological structure, ion-exchange capacity, water uptake, proton conductivity, and methanol permeability of the resulting composite membranes have been extensively investigated as functions of the content of sulfopropylated polysilsesquioxane filler, temperature, and relative humidity. Unlike the conventional Nafion/silica composites, the prepared membranes exhibit an increased water uptake and associated enhancement in proton conductivity compared to unmodified Nafion. In particular, considerably high proton conductivities at 80 and 120 degrees C under 30% relative humidity have been demonstrated in the composite membranes, which are over 2 times greater than that of Nafion. In addition to a remarkable improvement in proton conductivity, the composite membranes display lower methanol permeability and superior electrochemical selectivities in comparison to the pure Nafion membrane. These unique properties could be exclusively credited to the presence of pendant sulfonic acid groups in the filler, which provides fairly continuous proton-conducting pathways between filler and matrix in the composite membranes and thus facilitates the proton transport without the anticipated trade-off between conductivity and selectivity. This work opens new opportunities of tailoring the properties of Nafion-the benchmark fuel cell membrane-to obviate its limitations and enhance the conductive properties at high temperature/low humidity and in direct methanol fuel cells.

Sandwich-structured polymer nanocomposites with high energy density and great charge–discharge efficiency at elevated temperatures

Significance Polymers are the materials of choice for high-energy capacitive storage devices due to their inherent advantages such as being lightweight, their ease of processing, and their high dielectric strength. Yet, their performance deteriorates significantly with increasing operating temperature, which falls short of emerging energy applications under harsh conditions. Here we demonstrate the sandwich-structured polymer nanocomposites with greatly improved energy densities, high power densities, and remarkable charge–discharge efficiencies that far exceed those of the existing polymer-based dielectrics at 150 °C, a temperature oriented toward electric vehicle applications. The development of polymer-based dielectric materials capable of high-temperature operation represents a key element in meeting the technological challenges and fulfilling the requirements of advanced electronics and electrical power systems. The demand for a new generation of high-temperature dielectric materials toward capacitive energy storage has been driven by the rise of high-power applications such as electric vehicles, aircraft, and pulsed power systems where the power electronics are exposed to elevated temperatures. Polymer dielectrics are characterized by being lightweight, and their scalability, mechanical flexibility, high dielectric strength, and great reliability, but they are limited to relatively low operating temperatures. The existing polymer nanocomposite-based dielectrics with a limited energy density at high temperatures also present a major barrier to achieving significant reductions in size and weight of energy devices. Here we report the sandwich structures as an efficient route to high-temperature dielectric polymer nanocomposites that simultaneously possess high dielectric constant and low dielectric loss. In contrast to the conventional single-layer configuration, the rationally designed sandwich-structured polymer nanocomposites are capable of integrating the complementary properties of spatially organized multicomponents in a synergistic fashion to raise dielectric constant, and subsequently greatly improve discharged energy densities while retaining low loss and high charge–discharge efficiency at elevated temperatures. At 150 °C and 200 MV m−1, an operating condition toward electric vehicle applications, the sandwich-structured polymer nanocomposites outperform the state-of-the-art polymer-based dielectrics in terms of energy density, power density, charge–discharge efficiency, and cyclability. The excellent dielectric and capacitive properties of the polymer nanocomposites may pave a way for widespread applications in modern electronics and power modules where harsh operating conditions are present.

Suppression of energy dissipation and enhancement of breakdown strength in ferroelectric polymer–graphene percolative composites

The percolative polymer composites have recently exhibited great potential in energy storage due to their high dielectric permittivities in the neighborhood of the percolation threshold. Yet high energy dissipation and poor voltage endurance of the percolative composites resulting from electrical conduction are still open issues to be addressed before full potential can be realized. Herein we report the percolative composites based on ferroelectric poly(vinylidene fluoride-co-chlorotrifluoroethylene) as the matrix and SiO2 coated reduced graphene oxide nanosheets as the filler. By capitalizing on the SiO2 surface layers which have high electrical resistivity and breakdown strength, the composites exhibit superior dielectric performance as compared to the respective composites containing bare reduced graphene oxide nanosheet fillers. In addition to greatly reduced dielectric loss, little change in dielectric loss has been observed within the medium frequency range (i.e. 300 kHz–3 MHz) in the prepared composites even with a filler concentration beyond the percolation threshold, indicating significantly suppressed energy dissipation and the feasibility of using the conductor–insulator composites beyond the percolation threshold. Moreover, these composites exhibit a remarkable breakdown strength of 80 MV m−1 at the percolation threshold, which far exceeds those of conventional percolative composites (lower than 0.1 MV m−1 in most cases) and thus enables the applications of the percolative composites at high electric fields. This work offers a new avenue to the percolative polymer composites exhibiting high permittivity, reduced loss and excellent breakdown strength for electrical energy storage applications.

High Energy Density and Breakdown Strength from β and γ Phases in Poly(vinylidene fluoride-co-bromotrifluoroethylene) Copolymers

Poly(vinylidene fluoride) PVDF-based copolymers represent the state of the art dielectric polymers for high energy density capacitors. Past work on these copolymers has been done with limited emphasis on the effects of copolymer composition and with a limited range of defect monomers, focusing primarily on the commercially available poly(vinylidene fluoride-co-chlorotrifluoroethylene), P(VDF-CTFE), and poly(vinylidene fluoride-co-hexafluoropropylene), P(VDF-HFP), and the processing thereof. To expand on this area of research, copolymers of VDF and bromotrifluoroethylene (BTFE) were synthesized examining the composition range where uniaxial stretching was possible. It is found that P(VDF-BTFE) copolymers with small BTFE contents (< 2 mol %) stabilize the γ phase, compared to P(VDF-CTFE)s and P(VDF-HFP)s that are largely α phase in composition. Furthermore, different from P(VDF-CTFE)s and P(VDF-HFP)s, whose energy storage capabilities depend on the reversibility of the α to β phases transformation, high discharged energy densities (i.e., 20.8 J/cm(3) at 716 MV/m) are also achievable through the β and γ phases in P(VDF-BTFE)s without significantly reducing crystallinity and breakdown strength. This study demonstrates new avenues to the development of high energy density ferroelectric copolymers via manipulation of the γ phase through variation of the structure and content of comonomers.

Controlling Chain Conformations of High-k Fluoropolymer Dielectrics to Enhance Charge Mobilities in Rubrene Single-Crystal Field-Effect Transistors.

A novel photopatternable high-k fluoropolymer, poly(vinylidene fluoride-bromotrifluoroethylene) P(VDF-BTFE), with a dielectric constant (k) between 8 and 11 is demonstrated in thin-film transistors. Crosslinking P(VDF-BTFE) reduces energetic disorder at the dielectric-semiconductor interface by controlling the chain conformations of P(VDF-BTFE), thereby leading to approximately a threefold enhancement in the charge mobility of rubrene single-crystal field-effect transistors.