This paper presents a progress update with the development of nanodielectric composites with electric field tunability for various high energy, high power electrical applications. It is demonstrated that nonlinear electrical/dielectric properties can be achieved via the nanostructure and interface engineering. A high level summary was given on the progress achieved as well as challenges remaining in nanodielectric engineering towards high energy density capacitors for energy storage and conversion, nonlinear dielectrics for tunable device, and high voltage varistor for surge suppression.
The rapid expansion of renewable energy applications demands higher efficiency and higher density energy storage and energy conversion systems [
All these advanced apparatus and future electric power infrastructure rely on the breakthroughs of material engineering and better understanding of the device physics [
In this investigation, polyetherimide (PEI), silicone, cyanoethyl cellulose, polyimide, and poly(vinylidene difluoride) were used as the polymer matrices in the investigation. Nanoparticles of interest include oxides of silicon, niobium, aluminum, zinc, bismuth, antimony, cobalt, titanium, and barium titanate and lead zirconates, with the particle size in the range of 10 to 100 nm. Polymer nanocomposites were prepared by first dissolving a polymer resin in a solvent at room temperature with a magnetic stirrer and then mixing with nanoparticles of 2–50 vol% in a high-energy sonicator (Sonics & Materials, Inc. Newtown, CT). The films were solvent cast onto a glass slide and dried at 100°C for two hours followed by a vacuum dry at 120°C to 150°C overnight. Ceramic composites were formulated with at least 85 mol% ZnO and other oxide additives. A conventional mixed oxide process was used for powder processing. Green compacts of 1′′ diameter were pressed using a uniaxial hydraulic press and then sintered at temperatures of 850–1000°C.
DC breakdown test was conducted following ASTM D149 (method A) using a ball-plane electrode configuration. The sample was immersed into insulation oil and DC voltage was applied at a ramp rate of 500 V/s until the sample failed. Dielectric responses were measured using a broadband dielectric spectrometer from Novacontrol GmbH. Scanning electron microscopy (SEM) imaging was done using a Zeiss Supra 55VP (Carl Zeiss AG, Oberkochen, Germany). Transmission electron microscopy (TEM) imaging was taken using a FEI Tecnai F20 transmission electron microscope.
High energy density capacitor dielectrics, field tunable composite dielectrics, and metal oxide varistors (MOVs) are taken as examples in this section to demonstrate the achievable electrical/dielectric properties via the nanodielectric and interface engineering to fulfill the aforementioned development needs.
Capacitors represent a big family of non-Faradic energy storage components used in applications ranging from decoupling in circuit board to series capacitive compensation banks at power transmission class. The state-of-art polymeric film capacitors are based primarily on low permittivity polymers such as polypropylene or polyester. One natural approach for higher energy density capacitor is the synthesis of higher permittivity composites with higher dielectric strength by incorporating ceramic fillers into polymer matrix. This is because the energy density of a film capacitor takes the simple form of
Although the dielectric constant can be increased to some extent at higher particle loading level, increasing the dielectric strength remains a great challenge. Instead, the addition of particles exceeding 10 wt% usually results in the considerable loss of dielectric strength of the polymer. Figure
The breakdown strength of PEI nanocomposites with various fillers of about 5 wt% loading. PZ stands for lead zirconate. All polymer films were processed using a solvent cast.
With decreasing particle size, interfacial fraction increases and the particle-polymer interface adhesion also becomes critical. Poor particle dispersion and interaction with polymer matrix is shown in Figures
TEM image of 5 vol% nanoparticles dispersing in polymer films. (a) Dry Al2O3, (b) dry silica, (c) colloidal SiO2, and (d) colloidal SiO2 in a polyimide.
In order to minimize the particle surface effect, colloidal particles that were already well dispersed in a solvent were procured and mixed with a polymer resin in the wet state. Figure
The success of nanodielectric engineering depends on several factors, including (1) higher dielectric constant polymer matrix, (2) high permittivity ceramic fillers with low (hysteresis) loss, (3) proper dispersion, and (4) good interfacing between filler and polymer matrix [
The dielectric constant of nanocomposites as a function of volume pertentage loading of
In order to enable the nonlinear tunability of polymer, we leveraged ferroelectric BaTiO3 and antiferroelectric lead zirconate nanoparticles (nPZ). The electric filed tunable behavior of the composites was further studied under high electric fields. The nonlinear dependence of the dielectric constant of the polymer composites was fully exhibited as the electric field is increased exceeding the coercive field of lead zirconate (nPZ) as shown in Figure
Room temperature dielectric constant under an increased electric field. Values were obtained from the hysteresis loop measurements of a PVDF polymer containing nPZ particles (10 Hz).
TEM imaging shows the great dispersion and particle distribution in the polymer achieved using properly processed particles and mixing method (Figure
TEM image of polyimide containing 15 vol% 40 nm BaTiO3 nanoparticles through in situ polymerization of 2,2-bis[4-(3,4-dicarboxyphenoxy) phenyl]propane dianhydride (BPADA) with 4,4′-oxydianiline (ODA). Film was solution cast and dried and completely imidized at elevated temperatures.
High capacity, high density power system may be subject to high voltage transients and surge protection devices are widely used. Compared with other protection devices such as TVS, metal oxide varistor (MOV) as an example of ceramic-based nonlinear dielectric material at low voltage provides a good combination of high voltage scalability, peak current carrying capability, and fast response speed [
SEM images of microstructures of a commercial metal oxide varistor (a) and nanoenabled MOV sintered at 850°C (b).
Higher breakdown voltage (
Nanoenabled MOVs with higher breakdown voltage after sintering at lower temperatures. The commercial MOVs made of coarse powder precursors were tested for comparison.
It is demonstrated that novel electrical/dielectric properties can be achieved via the nanodielectric engineering and interface engineering. Nanodielectric composites can be processed to host variety of ceramic particles and great particle dispersion and bonding with the host polymers can be fulfilled. The loading fraction and particle morphology, however, need to be controlled not to dramatically lower the dielectric strength. Electrically tunable composites were processed and a nonlinearly dependent dielectric constant was demonstrated by mixing polar nanoparticles with either low-K polyimide polymer or high-K PVDF polymers. The challenges are making the composite tunable at very low electric fields. High energy density (15 J/cm3) can be achieved in nanodielectric composites containing nonlinear ferroelectric particles, with proper particle morphology, dispersion, and particle-polymer adhesion. 10x increase of voltage withstanding capability with low leakage current and higher nonlinearity can be realized in nanoenabled varistors. Refined and uniform submicron structures can be further explored in polymer based tunable resistors.
Further optimization of the performance of these electric device/component based such principles is to be undertaken.
The authors declare that they have no conflict of interests regarding the publication of this paper.
Part of the work was sponsored by US Department of Defense under Contract of FA9451-08-C-0166.