Electronic structure and absolute band edge position of tetragonal AgInS2 photocatalyst: A hybrid density functional study

Energy bands, effective mass of carriers, absolute band edge positions and optical properties of tetragonal AgInS₂ were calculated using a first-principles approach with the exchange correlation described by B3LYP hybrid functional. The results indicate that tetragonal AgInS₂ has a direct band gap of 1.93 eV, which reproduce well experimental value. Calculated effective masses of electrons and holes are both small which are beneficial to separation and migration of electron and hole pairs. This implies that AgInS₂ has good photocatalytic performance. The calculated optical characteristics indicate that AgInS₂ has a slight anisotropy for both the real and imaginary parts of the dielectric function and exhibits large optical absorption in the visible light region. Furthermore, the calculated band edge positions in (100), (010) and (001) surfaces indicate that tetragonal AgInS₂ is beneficial to the reduction and oxidation of water to hydrogen and oxygen under visible light irradiation.     

In situ growth of three-dimensional graphene coatings on arbitrary-shaped micro/nano materials and its mechanism studies

In the past decade, there have been great advances in the controllable growth of two-dimensional (2D) graphene sheets. However, the preparation of 3D structured graphene such as graphene coatings on arbitrary-shaped micro/nano materials still remains a formidable challenge. Herein, we have proposed a general strategy for the in situ growth of 3D graphene coatings on the micro/nano particles with arbitrary shapes. Inspired by the CVD growth mechanism of 2D graphene sheets on the bulk metal substrates, we have in situ constructed a nanometer-thick catalytic interface on the micro/nano particle surface by introducing a trace amount of transition metal salts and solid carbon sources with strictly-controlled content and ratio. Growth of 3D graphene coatings is accomplished through a solid-state reaction. Under the catalysis of the in situ formed catalytic interface consisting of highly-ordered metal nanoislands, the nano-thick amorphous carbon layer which arousing from the pyrolysis of carbon sources can be effectively transformed into a continuous and uniform graphene coating throughout the material surface based on a “dissolution–precipitation” mechanism. 3D graphene coatings have been successfully grown on lithium iron phosphate, silver, copper and silicon particles. The growth mechanism of the 3D graphene coatings has been studied in detail and a growth model is also proposed.     

Fabrication and microwave absorption properties of magnetite nanoparticle–carbon nanotube–hollow carbon fiber composites

Absorbents with “tree-like” structures, which were composed of hollow porous carbon fibers (HPCFs) acting as “trunk” structures, carbon nanotubes (CNTs) as “branch” structures and magnetite (Fe₃O₄) nanoparticles playing the role of “fruit” structures were prepared by chemical vapor deposition technique and chemical reaction. Microwave reflection loss, permittivity and permeability of Fe₃O₄–CNTs–HPCFs composites were investigated in the frequency range of 2–18 GHz. It was proven that prepared absorbents possessed the excellent electromagnetic wave absorbing performances. The bandwidth with a reflection loss less than −15 dB covers a wide frequency range from 10.2 to 18 GHz with the thickness of 1.5–3.0 mm, and the minimum reflection loss is −50.9 dB at 14.03 GHz with a 2.5 mm thick sample layer. Microwave absorbing mechanism of the Fe₃O₄–CNTs–HPCFs composites is concluded as dielectric polarization and the synergetic interactions exist between Fe₃O₄ and CNTs–HPCFs.     

Hydrogen from wet air and sunlight in a tandem photoelectrochemical cell

A solid-state photoelectrochemical (SSPEC) cell is an attractive approach for solar water splitting, especially when it comes to monolithic device design. In a SSPEC cell the electrodes distance is minimized, while the use of polymer-based membranes alleviates the need for liquid electrolytes, and at the same time they can separate the anode from the cathode. In this work, we have made and tested, firstly, a SSPEC cell with a Pt/C electrocatalyst as the cathode electrode, under purely gaseous conditions. The anode was supplied with air of 80% relative humidity (RH) and the cathode with argon. Secondly, we replaced the Pt/C cathode with a photocathode consisting of 2D photocatalytic g-C₃N₄, which was placed in tandem with the photoanode (tandem-SSPEC). The tandem configuration showed a three-fold enhancement in the obtained photovoltage and a steady-state photocurrent density. The mechanism of operation is discussed in view of recent advances in surface proton conduction in absorbed water layers. The presented SSPEC cell is based on earth-abundant materials and provides a way towards systems of artificial photosynthesis, especially for areas where water sources are scarce and electrical grid infrastructure is limited or nonexistent. The only requirements to make hydrogen are humidity and sunlight.     

Delayed failure of ceramic matrix composites in tension at elevated temperatures

Ultimate tensile strength of five different continuous fiber-reinforced ceramic matrix composites (CMCs), including SiC_f/BSAS (two dimensional (2D), 2 types), SiC_f/MAS (2D), SiC_f/SiC (2D), and C_f/SiC (2D, 2 types), was determined as a function of test rate at 1100–1200 °C in air. All five CMCs exhibited a significant dependency of ultimate tensile strength on test rate such that the ultimate tensile strength decreased with decreasing test rate. The dependency of ultimate tensile strength on test rate, the applicability of preload technique, and the predictability of life from one loading configuration (constant stress-rate loading) to another (constant stress loading) all suggested that the overall, phenomenological delayed failure of the CMCs would be governed by a power-law type of slow crack growth.     

Synthesis of β-Ag2Se from organo-selenium precursor and silver nitrate in DMF via simultaneous reduction of selenium and silver salt

Reaction of selenium powder and silver nitrate in aqueous N, N′-dimethylformamide (aq. DMF) leads to formation of silver selenide. Similar reaction with cycloalkeno-1,2,3-selenadiazole, an organic compound of selenium upon thermolysis, also produces silver selenide. It is believed that DMF activates selenium and reduces silver salt via in-situ generated hydrogen ion simultaneously. Slightly broadened X-ray diffraction pattern for pure beta silver selenide (β-Ag₂Se) was observed. Scanning electron microscopy (SEM) indicated that the agglomerated particles could be in the range of about 100 to 200 nm in dimension. EDAX analysis showed non-stoichiometric elemental ratio between silver and selenium. It is found that the weight percent of Ag is slightly higher than Se. However, free amorphous selenium is also detected by XPS analysis when the product is made by use of elemental selenium. Thermal analysis (TGA) revealed a small (4%) weight loss due to the presence of organics or the solvent from the reaction medium.     

Microstructure and mechanical behaviour of semi-solid die-casting AZ91D magnesium alloy

Microstructures and mechanical properties of an AZ91D magnesium alloy prepared with semi-solid die-casting (SSDC) were characterized in as-cast conditions. The SSDC alloy exhibits a unique microstructure featuring primary α-Mg globules uniformly distributed in the matrix of fine secondary α-Mg grains and β-Mg₁₇Al₁₂ intermetallic. High ultimate tensile strength and elongation have been achieved before fracture. Observations on the vertical-section microstructure of the fractured sample by scanning electron microscopy (SEM) show that the crack mainly originated from the brittle fracture of the eutectic phase causes the interface decohesion of the ductile Mg phase, making fracture a rather critical event. Before that, the deformation of ductile α-Mg phase in the matrix as well as the “pulling out” of primary α-Mg phase combines to provide the SSDC alloy a certain strain.     

调质工艺对汽车大梁钢700L组织性能的影响

摘 要: 通过金属摆锤冲击和显微硬度试验,采用OM、SEM、TEM等表征手段,研究了不同调质工艺对700L汽车大梁钢组织和力学性能的影响。结果表明,随着淬火温度的提高,粒状贝氏体(GB)组织有所减少,板条状贝氏体铁素体(BF)数量逐渐变多,板条宽度增加,铁素体基体及边界上的白色析出物数量增多;随着回火温度的提高,块状铁素体有所长大,马奥岛组织和残余奥氏体分解现象明显,且出现了数量较少的等轴状铁素体,回火析出物数量增多,回火温度超过600 ℃后粒子出现粗化长大现象。低温冲击功在不同淬火与回火条件下均表现为上下波动的状态,这主要与第二相粒子及基体组织规律性的变化有关;试验钢在经600 ℃回火后具有最佳低温冲击韧性,其主要原因是钢基体中存在数量较多的具有高密度位错的贝氏体铁素体(BF)与尺寸合适、分布均匀的第二相纳米粒子。