Haixiong Tang

Ultra high energy density nanocomposite capacitors with fast discharge using Ba0.2Sr0.8TiO3 nanowires.

Nanocomposites combining a high breakdown strength polymer and high dielectric permittivity ceramic filler have shown great potential for pulsed power applications. However, while current nanocomposites improve the dielectric permittivity of the capacitor, the gains come at the expense of the breakdown strength, which limits the ultimate performance of the capacitor. Here, we develop a new synthesis method for the growth of barium strontium titanate nanowires and demonstrate their use in ultra high energy density nanocomposites. This new synthesis process provides a facile approach to the growth of high aspect ratio nanowires with high yield and control over the stoichiometry of the solid solution. The nanowires are grown in the cubic phase with a Ba0.2Sr0.8TiO3 composition and have not been demonstrated prior to this report. The poly(vinylidene fluoride) nanocomposites resulting from this approach have high breakdown strength and high dielectric permittivity which results from the use of high aspect ratio fillers rather than equiaxial particles. The nanocomposites are shown to have an ultra high energy density of 14.86 J/cc at 450 MV/m and provide microsecond discharge time quicker than commercial biaxial oriented polypropylene capacitors. The energy density of our nanocomposites exceeds those reported in the literature for ceramic/polymer composites and is 1138% greater than the reported commercial capacitor with energy density of 1.2 J/cc at 640 MV/m for the current state of the art biaxial oriented polypropylene.

Highly efficient synthesis of graphene nanocomposites.

Graphene consists of a monolayer of sp(2) bonded carbon atoms and has attracted considerable interest over recent years due to its extreme mechanical, electrical, and thermal properties. Graphene nanocomposites have naturally begun to be studied to capitalize upon these properties. A range of complex chemical and physical processing methods have been devised that achieve isolated graphene sheets that attempt to prevent aggregation. Here we demonstrate that the simple casting of a polymer solution containing dispersed graphene oxide, followed by thermal reduction, can produce well-isolated monolayer reduced-graphene oxide. The presence of single layer reduced-graphene oxide is quantitatively demonstrated through transmission electron microscopy and selected area electron diffraction studies and the reduction is verified by thermogravimetric, X-ray photoelectron spectroscopy, infrared spectrum, and electrical conductivity studies. These findings provide a simple, environmentally benign and commercially viable process to produce reduced-graphene oxide reinforced polymers without complex manufacturing, dispersion or reduction processes.

Nanocomposites with increased energy density through high aspect ratio PZT nanowires

High energy storage plays an important role in the modern electric industry. Herein, we investigated the role of filler aspect ratio in nanocomposites for energy storage. Nanocomposites were synthesized using lead zirconate titanate (PZT) with two different aspect ratio (nanowires, nanorods) fillers at various volume fractions dispersed in a polyvinylidene fluoride (PVDF) matrix. The permittivity constants of composites containing nanowires (NWs) were higher than those with nanorods (NRs) at the same inclusion volume fraction. It was also indicated that the high frequency loss tangent of samples with PZT nanowires was smaller than for those with nanorods, demonstrating the high electrical energy storage efficiency of the PZT NW nanocomposite. The high aspect ratio PZT NWs showed a 77.8% increase in energy density over the lower aspect ratio PZT NRs, under an electric field of 15 kV mm(-1) and 50% volume fraction. The breakdown strength was found to decrease with the increasing volume fraction of PZT NWs, but to only change slightly from a volume fraction of around 20%-50%. The maximum calculated energy density of nanocomposites is as high as 1.158 J cm(-3) at 50% PZT NWs in PVDF. Since the breakdown strength is lower compared to a PVDF copolymer such as poly(vinylidene fluoride-tertrifluoroethylene-terchlorotrifluoroethylene) P(VDF-TreEE-CTFE) and poly(vinylidene fluoride-co-hexafluoropropylene) P(VDF-HFP), the energy density of the nanocomposite could be significantly increased through the use of PZT NWs and a polymer with greater breakdown strength. These results indicate that higher aspect ratio fillers show promising potential to improve the energy density of nanocomposites, leading to the development of advanced capacitors with high energy density.

Relationship between BaTiO3 Nanowire Aspect Ratio and the Dielectric Permittivity of Nanocomposites

The aspect ratio of barium titanate (BaTiO3) nanowires is demonstrated to be successfully controlled by adjusting the temperature of the hydrothermal growth from 150 to 240 °C, corresponding to aspect ratios from 9.3 to 45.8, respectively. Polyvinylidene fluoride (PVDF) nanocomposites are formed from the various aspect ratio nanowires and the relationship between the dielectric constant of the nanocomposite and the aspect ratio of the fillers is quantified. It was found that the dielectric constant of the nanocomposite increases with the aspect ratio of the nanowires. Nanocomposites with 30 vol % BaTiO3 nanowires and an aspect ratio of 45.8 can reach a dielectric constant of 44.3, which is 30.7% higher than samples with an aspect ratio of 9.3 and 352% larger than the polymer matrix. These results demonstrate that using high-aspect-ratio nanowires is an effective way to control and improve the dielectric performance of nanocomposites for future capacitor applications.

High energy density nanocomposite capacitors using non-ferroelectric nanowires

A high energy density nanocomposite capacitor is fabricated by incorporating high aspect ratio functionalized TiO2 nanowires (NWs) into a polyvinylidene-fluoride matrix. These nanocomposites exhibited energy density as high as 12.4 J/cc at 450 MV/m, which is nine times larger than commercial biaxially oriented polypropylene polypropylene capacitors (1.2 J/cc at 640 MV/m). Also, the power density can reach 1.77 MW/cc with a discharge speed of 2.89 μs. The results presented here demonstrate that nanowires can be used to develop nanocomposite capacitors with high energy density and fast discharge speed for future pulsed-power applications.

Scalable Synthesis of Morphotropic Phase Boundary Lead Zirconium Titanate Nanowires for Energy Harvesting

Lead zirconium titanate (PZT) nanowires are synthesized using a scalable two-step hydrothermal reaction. The piezo-electric coupling coefficient of the PZT NWs shows the highest value for PZT nano-wires in the literature (80 ± 5 pm/V). A PZT-NW-based nanocomposite is fabri-cated to demonstrate an energy-harvesting application with an open-circuit voltage up to 7 V and a power density up to 2.4 μW/cm(3) .

Vertically aligned arrays of BaTiO3 Nanowires

Barium titanate (BaTiO3) nanowires have gained considerable research interest due to their lead-free composition and strong energy conversion efficiency. However, most research has focused on free-standing BaTiO3 nanowires, which are hard to apply for sensing and energy harvesting. Here, a novel method for the growth of vertically aligned BaTiO3 nanowire arrays on a conductive substrate is developed, and their electromechanical coupling behavior is directly evaluated to yield the strain coupling coefficient. The preparation of vertically aligned BaTiO3 nanowire arrays is based on a two-step hydrothermal reaction by first growing oriented rutile TiO2 nanowire arrays and then converting them to BaTiO3 while simultaneously retaining their morphology. A refined piezoelectric force microscopy (PFM) testing method is applied to demonstrate the piezoelectric behavior of BaTiO3 nanowires in the longitude direction. The piezoelectric response (d33 = 43 ± 2 pm/V) of the BaTiO3 nanowires is measured to demonstrate their potential application in sensors, energy harvesting, and micro-electromechanical systems.

Relationship between orientation factor of lead zirconate titanate nanowires and dielectric permittivity of nanocomposites

The relationship between the orientation of lead zirconate titanate (PZT) nanowires dispersed in nanocomposites and the resulting dielectric constants are quantified. The orientation of the PZT nanowires embedded in a polymer matrix is controlled by varying the draw ratio and subsequently quantified using Hermans Orientation Factor. Consequently, it is demonstrated that the dielectric constants of nanocomposites are improved by increasing the orientation factor of the PZT nanowires. This technique is proposed to improve the dielectric constant of the nanocomposites without the need for additional filler volume fraction since the nanocomposites are utilized in a wide range of high dielectric permittivity electronic components.

Controlled synthesis of ultra-long vertically aligned BaTiO3 nanowire arrays for sensing and energy harvesting applications

A novel approach for the synthesis of ultra-long (up to ∼45 μm) vertically aligned barium titanate (BaTiO3) nanowire (NW) arrays on an oxidized Ti substrate is developed. The fabrication method uses a two-step hydrothermal reaction that firstly, involves the growth of ultra-long aligned sodium titanate NW arrays and secondly, involves the transfer of these precursor sodium titanate NW arrays to BaTiO3 NW arrays while retaining the shape of the template nanowires. The ion-exchange during the second hydrothermal reaction in barium hydroxide solution results in the structural transformation from single-crystal sodium titanate NW arrays to BaTiO3 NW arrays. This synthesis approach is low-cost, scalable, and enables control over the morphology and aspect ratio of the resulting BaTiO3 NW arrays by tuning the hydrothermal reaction parameters. In addition to the synthesis methods reported here, the energy harvesting behavior of the BaTiO3 NW arrays is evaluated as a function of their aspect ratio and demonstrated to produce significant impact on the energy produced. The newly developed hydrothermal synthesis process for controlled growth of ultra-long, vertically aligned BaTiO3 NW arrays provides a promising method for their efficient utilization in nano-electromechanical system-based sensors, energy harvesters, and nano-scale electronic devices.

Hydrothermal growth of highly textured BaTiO3 films composed of nanowires

Textured barium titanate (BaTiO(3)) films are attracting immense research interest due to their lead-free composition and excellent piezoelectric and dielectric properties. Most synthesis methods for these films require a high temperature, leading to the formation of a secondary phase and an overall decrease in the electrical properties of the ceramic. In order to alleviate these issues, a novel fabrication method is introduced by transferring oriented rutile TiO(2) nanowires to a textured BaTiO(3) film at temperatures below 160 °C. The microstructure and thickness of the fabricated BaTiO(3) films were characterized by scanning electron microscopy, and the crystal structure and degree of orientation were evaluated by x-ray diffraction patterns using the Lotgering method. It is shown that the thickness of the BaTiO(3) film can be controlled by the length of TiO(2) nanowire array template, and the degree of orientation of the textured BaTiO(3) films is highly dependent on the film thickness; the crystallographic orientation has been measured to reach up to 87%. The relative dielectric constant (ε(r) = 1300) and ferroelectric properties (P(r) = 2.7 μC cm(-2), E(c) = 4.0 kV mm(-1)) of the textured BaTiO(3) films were also characterized to demonstrate their potential application in sensors, random access memory, and micro-electromechanical systems.