Assembly of highly stable aqueous dispersions and flexible films of nitrogen-doped graphene for high-performance stretchable supercapacitors

Here, we propose a route for production of highly stable nitrogen-doped graphene (NG) aqueous dispersions and flexible NG films. The NG dispersions are formed through sonication of NG hydrogels in methyl orange solution and stabilized through electrostatic repulsion without using any polymeric or surfactant stabilizer. The flexible NG films are formed by vacuum filtration of as-prepared NG dispersions and transferred onto stretchable and adhesive polyacrylate for the fabrication of stretchable supercapacitors. The supercapacitor exhibits high gravimetric specific capacitances of 315 F g^?1 under no strain and 300 F g^?1 under 30% applied strain at a current density of 1 A g^?1. The fabrication of aqueous dispersions and stretchable NG films makes NG materials attractive for a diversity of applications in stretchable electronics.     

三维成球培养优化间充质干细胞的研究进展

间充质干细胞(mesenchymal stem cells,MSCs)源于发育早期的中胚层,因其来源广泛、具有多向分化潜能、低免疫原性和自我更新能力,在组织工程和再生医学应用中显示出巨大的潜力,也是当前基础研究和临床研究中应用最多的一类干细胞。然而,间充质干细胞的临床应用面临许多挑战,比如治疗所需细胞数量巨大,细胞质量存在异质性,细胞体内移植后存活率低,以及二维(two-dimensional,2D)贴壁培养导致间充质干细胞特征衰减等。三维(three-dimensional,3D)成球培养可以更好地模拟体内微环境,且大量的研究证明,3D成球培养增强了间充质干细胞的细胞存活和因子分泌能力,促进了干细胞特征维持、细胞迁移和血管生成,在临床医学领域具有广阔的应用前景。基于此,综述了体外3D成球培养的方法、3D成球培养优化的间充质干细胞的生物学特性及应用,并对3D成球培养未来的研究方向进行展望。     

Comparison of commercial supercapacitors and high-power lithium-ion batteries for power-assist applications in hybrid electric vehicles: I. Initial characterization

Commercial supercapacitors, also known as ultracapacitors or electrochemical capacitors, from Saft, Maxwell, Panasonic, CCR, Ness, EPCOS, and Power Systems were tested under constant current and constant power discharges to assess their applicability for power-assist applications in hybrid electric vehicles (HEVs). Commercial lithium-ion batteries from Saft and Shin-Kobe were also tested under similar conditions. Internal resistances were measured by electrochemical impedance spectroscopy (EIS), as well as by the “iR drop” method. Self discharge measurements were also recorded. Compared with earlier generations of supercapacitors, the cells showed improved current and power capability. However, their energy densities are still too low to meet goals set by Partnership for a New Generation of Vehicles (PNGV) for HEV propulsion. Cells that use acetonitrile as the electrolyte solvent yield better performance, although safety issues need to be addressed. New high-power lithium-ion batteries show high energy densities, with high power capabilities.     

Melt growth of CdTe single crystals with controlled deviation from stoichiometry

CdTe single crystals were grown by an improved vertical Bridgman method with the Cd or Te reservoir chamber at the lower part of the growth tube. The reservoir is provided for regulating the partial pressures exactly and in consequence, for controlling the stoichiometry of the grown crystals. The effectiveness of the proposed method was confirmed from the electrical conductivities of the grown crystals which changed drastically depending on the controlled partial pressures.     

ICP刻蚀损伤对金属场板结构SiC肖特基二极管电学性能的影响

本文通过电感耦合等离子体（ICP）系统地研究了各种刻蚀参数对4H-SiC刻蚀的影响,并进一步研究了刻蚀损伤对金属场板结构4H-SiC肖特基二极管电学性能的影响。研究表明刻蚀速率和SiC表面形貌都会受到ICP功率、RF功率、压强和刻蚀气体流量的影响。在高的ICP偏压下,观察到了刻蚀损伤（刻蚀坑和刻蚀锥）的形成。更深入的研究表明,这些刻蚀损伤的形成和SiC自身的缺陷有关。这些刻蚀损伤的存在会导致SiC肖特基二极管正反向I-V性能发生恶化。在刻蚀损伤严重的情况下,对比正反向I-V测试结果发现,在0~50V的绝对电压范围内,正向电流甚至远小于反向电流。     

Material properties and optical guiding in InGaAs-GaAs strained layer superlattices—a brief review

Due to the absence of lattice-matching requirements, strained-layer superlattices offer a large tunability in bandgap and other material properties suitable for device applications. Encouraging progress has been made in the molecular-beam epitaxial and metalorganic-vapor-phase-epitaxial growth of strained-layer superlattices and in their characterization. These have been briefly reviewed here. Since a strained-layer superlattice allows the use of In_xGa_1−xAs layers with x-values up to ∼ 1.0, a large variation of the refractive index from that in GaAs occurs due to mismatch strain and alloying. This variation in refractive index has been calculated. The increase in refractive index can be used to form optical guides in the SLS and such guides with good vertical confinement is demonstrated. Preliminary measurements of the impact-ionization parameters and deep-level traps in these materials are also reported. α/β values close to and slightly greater than unity are measured. A single electron trap with thermal activation energy equal to 0.16 eV is identified.     

Growth,doping and characterization of epitaxial thin films and patterned structures of AlN,GaN, and AlxGa1−xN

Advancements in the doping of GaN and Al_xGa_1−xN thin films, and the growth of GaN and Al_xGa_1−xN structures on patterned heterostructure substrates via metalorganic vapor phase epitaxy are reported. The acceptor-type behavior of Mg-doped GaN films grown in N₂ diluents is presented. Net ionized impurity concentrations up to 8×10¹⁸ cm⁻³ and Hall mobilities up to ≈14 cm² V⁻¹ s⁻¹ were measured for Mg-doped films grown in N₂ in the as-grown condition. Donor and acceptor doping of Al_xGa_1−xN alloys was performed. Acceptor doping of Al_xGa_1−xN for x≤0.13 and donor doping for x≤0.58 were achieved for films deposited at 1100 °C. Lateral epitaxial overgrowth of GaN and Al_xGa_1−xN layers was investigated. The growth and coalescence of GaN and Al_xGa_1−xN stripes patterned in SiO₂ and/or SiN_x masks deposited on GaN, including aligned second lateral epitaxial overgrowth on initial laterally overgrown GaN layers, are discussed.     

Mechanical property improvement and microstructure observation of SiCw–AlN composites

SiCw–AlN composites were produced with different sintering additives. Eight wt% Y₂O₃ was found a suitable sintering additive for mechanical property improvement of SiCw-AlN composites. With the sintering additive, SiCw–AlN composites with whisker content ranging from 10–30 wt% were hot-pressed to high density, and the flexural strength and fracture toughness were significantly improved with the increase of SiC whisker content. SEM and TEM observation indicated that several kinds of toughening mechanism contributed to the increasing toughness, including crack deflection, crack branching and whisker bridging process. The interfacial boundaries of the composites were also discussed in detail. ©     

Electrostatic self-assembly as a means to create organic photovoltaic devices

Recently, there has been a significant amount of work done on making photovoltaic devices (solar cells) from thin films of conjugated polymers and other organic systems. The advantages over conventional inorganic systems include the potential to create lightweight, flexible, and inexpensive structures. The challenge, however, has been to create more highly efficient devices. To date, the primary photovoltaic device mechanism that has been utilized is that of photoinduced charge transfer between an electron donor and acceptor. In this study, similar photovoltaic devices are fabricated using a water-based electrostatic self-assembly procedure, as opposed to the more conventional spin-coating and/or vacuum evaporation techniques. In this process, layers of oppositely charged species are sequentially adsorbed onto a substrate from an aqueous solution and a film is built up due to the electrostatic attraction between the layers. The technique affords molecular level control over the architecture and gives bilayer thickness values of the order of tens of angstroms. By repeating this process a desired number of times and utilizing different cations and anions, complex architectures can be created with very accurate control over the thickness and the interfaces. We have examined a number of systems built from a variety of components including a cationic PPV precursor, functionalized C₆₀, and numerous other polyelectrolytes. We report on the device characteristics of these films and on the overall applicability of this technique to the fabrication of photovoltaic devices.     

Post-impact compressive behaviour of low temperature curing woven CFRP laminates

The polymeric matrix in a fibre-reinforced composite serves to bind the fibres together, transfer load to the fibres and protect them against environmental attack and damage due to handling. The matrix has a strong influence on several mechanical properties of the composite such as transverse modulus and strength, impact resistance, shear properties and properties in compression. This paper describes the results of an experimental study to determine the effect of resin (matrix) on the post-impact compressive behaviour of carbon fibre woven laminates. Three new low temperature cure (50–125°C) epoxy resins are examined: an unmodified (LTM12), a rubber-modified (LTM25) and a thermoplastic toughened epoxy resin (MT8E). Note, however, that the first two are post-cured at 190°C. Velocities and impact energies were used to simulate momenta typical of low velocity impact hazards associated with aircraft in-service. Measurements of impact damage and damage growth during compression are made using ultrasonic C-scanning and penetrant-enhanced X-ray radiography techniques. For low impact energies the superior performance of the thermoplastic toughened epoxy is confirmed. Its residual compressive strength compares favourably with that obtained for high strength carbon fibre/epoxy laminates manufactured from unidirectional sheets cured at 190°C.