Kechao Zhou

Improved Dielectric Properties and Energy Storage Density of Poly(vinylidene fluoride-co-hexafluoropropylene) Nanocomposite with Hydantoin Epoxy Resin Coated BaTiO3

Energy storage materials are urgently demanded in modern electric power supply and renewable energy systems. The introduction of inorganic fillers to polymer matrix represents a promising avenue for the development of high energy density storage materials, which combines the high dielectric constant of inorganic fillers with supernal dielectric strength of polymer matrix. However, agglomeration and phase separation of inorganic fillers in the polymer matrix remain the key barriers to promoting the practical applications of the composites for energy storage. Here, we developed a low-cost and environmentally friendly route to modifying BaTiO3 (BT) nanoparticles by a kind of water-soluble hydantoin epoxy resin. The modified BT nanoparticles exhibited homogeneous dispersion in the ferroelectric polymer poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix and strong interfacial adhesion with the polymer matrix. The dielectric constants of the nanocomposites increased significantly with the increase of the coated BT loading, while the dielectric loss of the nanocomposites was still as low as that of the pure P(VDF-HFP). The energy storage density of the nanocomposites was largely enhanced with the coated BT loading at the same electric field. The nanocomposite with 20 vol % BT exhibited an estimated maximum energy density of 8.13 J cm(-3), which was much higher than that of pure P(VDF-HFP) and other dielectric polymers. The findings of this research could provide a feasible approach to produce high energy density materials for practical application in energy storage.

Hydroxyapatite Nanoparticles as a Novel Gene Carrier

Hydroxyapatite crystalline nanoparticles were created by a precipitation hydrothermal technique and the majority of crystal particles were in the size range of 40–60nm and exhibited a colloidal feature when suspended in water. The gastric cancer SGC-7901 cell line cells were cultivated in the presence of10–100 μg ml−1 hydroxyapatite nanoparticle suspension and verified by MTT evaluation for their biocompatibility in vitro. The agarose gel electrophoresis analysis demonstrated that the HA nanoparticles potentially adsorb the green fluorescence protein EGFP-N1 plasmid DNA at pH 2 and 7, but not at pH 12. The DNA–nanoparticle complexes transfected EGFP-N1 pDNA into SGC-7901 cells in vitro with the efficiency about 80% as referenced with Lipofectmine TM 2000. In vivo animal experiment revealed no acute toxic adverse effect 2weeks after tail vein injection into mice, and TEM examination demonstrated their biodistribution and expression within the cytoplasm and also a little in the nuclei of the liver, kidney and brain tissue cells. These results suggest that the HA nanoparticle is a promising material that can be used as gene carrier, vectors.

Copolymers from benzodithiophene and benzotriazole: synthesis and photovoltaic applications

Two new alternating low bandgap copolymers from benzodithiophene and benzotriazole units, namely poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b; 3,4-b]dithiophene-2,6-diyl-alt-2-octyl-4,7-di(thiophen-2-yl)-2H-benzo[d][1,2,3]triazole-5′,5′′-diyl} (PBDTDTBTz) and poly{4,8-bis(2-ethylhexyloxy)benzo[1,2-b;3,4-b]dithiophene-2,6-diyl-alt-2-dodecylbenzotriazole-4,7-diyl} (PBDTBTz), were designed and synthesized by a typical Stille coupling polymerization method. The copolymers were characterized by thermogravimetric analysis, UV-vis absorption and cyclic voltammetry. PBDTDTBTz and PBDTBTz possess moderate molecular weights and excellent thermal properties with a 5% weight loss temperatures (Td) around 300 °C. They exhibited good optical absorption, with peaks at 527 nm and 562 nm in the film state, respectively. Photovoltaic properties of the copolymers blended with [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) or [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as electron acceptors, were investigated. The photovoltaic device with the PBDTDTBTz/PC71BM shows a power conversion efficiency of 1.7% with a short circuit current density of 4.5 mA cm−2 and a good fill factor of 0.62, while PBDTBTz demonstrated a moderate power conversion efficiency of up to 1.4%, under the illumination of AM 1.5, 100 mW cm−2 with a device structure of ITO/PEDOT: PSS/polymer: PC71BM (1 : 4)/Ca/Al. All the above information highlighted that this kind of the copolymers is promising for the application of polymer solar cells.

Aligned porous barium titanate/hydroxyapatite composites with high piezoelectric coefficients for bone tissue engineering

It was proposed that the piezoelectric effect played an important physiological role in bone growth, remodelling and fracture healing. An aligned porous piezoelectric composite scaffold was fabricated by freeze casting hydroxyapatite/barium titanate (HA/BT) suspensions. The highest compressive strength and lowest porosity of 14.5MPa and 57.4% with the best parallelism of the pore channels were achieved in the HA10/BT90 composite. HA30/BT70 and HA10/BT90 composites exhibited piezoelectric coefficient d33 of 1.2 and 2.8pC/N, respectively, both of which were higher than the piezoelectric coefficient of natural bone. Increase of the solid loading of the suspension and solidification velocity led to the improvement of piezoelectric coefficient d33. Meanwhile, double-templates resulted in the coexistence of lamellar pores and aligned macro-pores, exhibiting the ability to produce an oriented long-range ordered architecture. The manipulation flexibility of this method indicated the potential for customized needs in the application of bone substitute. An MTT assay indicated that the obtained scaffolds had no cytotoxic effects on L929 cells.

Ultra-high discharged energy density capacitor using high aspect ratio Na0.5Bi0.5TiO3 nanofibers

Ceramic/polymer nanocomposites are attractive for energy storage applications due to their ability to exploit the high permittivity of ceramic fillers and high breakdown strength of the polymer matrix. One challenge for the development of high performance nanocomposites based on ceramic particulates or fibers in a polymer matrix is that they often require a high volume fraction (>50%) to achieve a high permittivity, which is often at the expense of a reduction in dielectric strength and mechanical flexibility. In this paper we demonstrate by both experiment and finite element simulation that high aspect ratio nanofiber fillers offer an effective approach to achieve high energy density and dielectric strength. Lead-free ferroelectric Na0.5Bi0.5TiO3 (BNT) nanofibers with a high aspect ratio (>200) are synthesized by a hydrothermal method and dispersed in a poly(vinylidene difluoride-co-hexafluoropropylene) (P(VDF-HFP)) matrix. The increased fraction of β-phase and the alignment of BNT nanofibers perpendicular to the direction of the applied electric field lead to an enhanced dielectric strength, compared to spherical BNT/P(VDF-FHP) nanoparticles and pure P(VDF-HFP), and experimental measurements are compared with numerical simulations. The results demonstrate that the nanofiber nanocomposites exhibited an ultra-high discharged energy density (12.7 J cm−3) and provide an innovative approach to produce high-energy storage density materials.

Effects of rheological properties on ice-templated porous hydroxyapatite ceramics

Freeze casting of aqueous suspension was investigated as a method for fabricating hydroxyapatite (HA) porous ceramics with lamellar structures. The rheological properties of HA suspensions employed in the ice-templated process were investigated systematically. Well aligned lamellar pores and dense ceramic walls were obtained successfully in HA porous ceramics with the porosity of 68-81% and compressive strength of 0.9-2.4 MPa. The results exhibited a strong correlation between the rheological properties of the employed suspensions and the morphology and mechanical properties of ice-templated porous HA ceramics, in terms of lamellar pore characteristics, porosities and compressive strengths. The ability to produce aligned pores and achieve the manipulation of porous HA microstructures by controlling the rheological parameters were demonstrated, revealing the potential of the ice-templated method for the fabrication of HA scaffolds in biomedical applications.

Highly enhanced dielectric strength and energy storage density in hydantoin@BaTiO3–P(VDF-HFP) composites with a sandwich-structure

A sandwich-structured composite consisted of a pure poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF-HFP)) central layer and two BaTiO3–P(VDF-HFP) neighboring layers was developed for high energy storage density. The sandwiched structure effectively enhanced dielectric properties and energy storage capacity of the composites. Breakdown strength and energy storage density of the composite with 35 vol% P(VDF-HFP) central layer reached 315 kV mm−1 and 5.22 J cm−3, which were enhanced by 68% and 125% compared to 187 kV mm−1 and 2.32 J cm−3 of the single layer composite, respectively. The dielectric constant of the composites with sandwich-structure was approximately 3 times higher than the pure P(VDF-HFP) and the dielectric loss was as low as 0.026 at 1 kHz. These results demonstrated that the sandwich-structured ceramics/polymer composite was an effective way to produce high energy density composites.

Sliding wear behavior of copper-based composites reinforced with graphene nanosheets and graphite

Abstract The mechanical and tribological properties of hot-pressed copper-based composites containing different amounts of graphene nanosheets (GNSs) are compared with those of copper–graphite (Gr) composites fabricated by the same method. The results show that the Cu–GNSs composites exhibit higher relative density, microhardness and bending strength compared with Cu–Gr composites with the same volume fraction of GNSs and Gr. Moreover, the friction coefficients and wear rates reduce significantly by the addition of GNSs, whereas the limited impact on reducing friction and wear is found on graphite. The abrasive and delamination wear are the dominant wear mechanisms of the composites. It is believed that the superior mechanical and tribological performances of Cu–GNSs composites are attributed to the unique strengthening effect as well as the higher lubricating efficiency of graphene nanosheets compared with those of graphite, which demonstrates that GNS is an ideal filler for copper matrix composites, acting as not only an impactful lubricant but also a favorable reinforcement.

High performance capacitors via aligned TiO2 nanowire array

Capacitors generally suffer from low energy density, which seriously limits their applications in energy storage devices. In this letter, TiO2 nanowire arrays were synthesized by a hydrothermal method and the morphologies were tuned by modulating the concentration of the Ti source. High energy density was realized after incorporating TiO2 nanowire arrays into a polyvinylidene fluoride matrix. The alignment of TiO2 nanowires with the direction of the applied electric field played an important role in achieving high breakdown strength and energy density, i.e., 380 kV/mm and 23.21 J/cm3, respectively. The findings provide a route to increase the energy density of polymer capacitors.

Significantly enhanced energy storage density of sandwich-structured (Na0.5Bi0.5)0.93Ba0.07TiO3/P(VDF–HFP) composites induced by PVP-modified two-dimensional platelets

Two-dimensional (Na0.5Bi0.5)0.93Ba0.07TiO3 (NBBT) platelets with a size of up to ca. 5 μm and thickness of 0.2–0.5 μm were introduced as fillers into a polymer matrix to prepare energy storage composites for the first time. The NBBT platelets were treated with an aqueous solution of H2O2 and coated with polyvinylpyrrolidone (PVP) before mixing with poly(vinylidene fluoride-co-hexafluoropropylene) (P(VDF–HFP)). The final composite was denoted as NBBT@PVP/P(VDF–HFP). Composites were prepared with NBBT@PVP loadings from 1 to 30 vol%. The relative permittivity of the composites increased significantly with increasing NBBT@PVP loading, while the breakdown strength decreased. To improve the breakdown strength of the composites, a sandwich-structure of multilayer films was developed, which used NBBT@PVP/P(VDF–HFP) composites with 1 vol% NBBT loadings as central hard layers and the composites with 30 vol% NBBT loadings as neighboring soft layers. The five-layered film, which contained three central hard layers and neighboring soft layers, showed excellent energy storage properties. The breakdown strength and the maximum energy storage density of the film reached 258 kV mm−1 and 14.95 J cm−3, respectively. The energy efficiency remained 0.9 at an electric field of 200 kV mm−1. The findings provide a new approach to produce energy storage materials with high performance.