Li-feng Hou

Facile Synthesis of Hierarchical Mesoporous Honeycomb-like NiO for Aqueous Asymmetric Supercapacitors

Three-dimensional (3D) hierarchical nanostructures have been demonstrated as one of the most ideal electrode materials in energy storage systems due to the synergistic combination of the advantages of both nanostructures and microstructures. In this study, the honeycomb-like mesoporous NiO microspheres as promising cathode materials for supercapacitors have been achieved using a hydrothermal reaction, followed by an annealing process. The electrochemical tests demonstrate the highest specific capacitance of 1250 F g(-1) at 1 A g(-1). Even at 5 A g(-1), a specific capacitance of 945 F g(-1) with 88.4% retention after 3500 cycles was obtained. In addition, the 3D porous graphene (reduced graphene oxide, rGO) has been prepared as an anode material for supercapacitors, which displays a good capacitance performance of 302 F g(-1) at 1 A g(-1). An asymmetric supercapacitor has been successfully fabricated based on the honeycomb-like NiO and rGO. The asymmetric supercapacitor achieves a remarkable performance with a specific capacitance of 74.4 F g(-1), an energy density of 23.25 Wh kg(-1), and a power density of 9.3 kW kg(-1), which is able to light up a light-emitting diode.

Well-shaped Mn3O4 tetragonal bipyramids with good performance for lithium ion batteries

Well-shaped Mn3O4 tetragonal bipyramids with a high reversible capacity of 822.3 mA h g−1 are synthesized by a simple hydrothermal method without any surfactants or coordination compounds. The structural features and morphology of the final product are investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HRTEM). The SEM and HRTEM results reveal that all the eight exposed facets of the Mn3O4 tetragonal bipyramids are indexed to the high-energy {101} planes. The tetragonal bipyramids with high-energy facets provide the Mn3O4 anode material with a high initial discharge capacity (1141.1 mA h g−1). In addition, the anode displays a good fast rate performance, delivering a reversible capacity of 822.3 mA h g−1 (the theoretical capacity: 937 mA h g−1) at a current density of 0.2 C after 50 cycles. Moreover, the coulomb efficiency for the first cycle reaches about 66% and remains at about 100% during the subsequent cycles. A relatively detailed growth mechanism of these tetragonal bipyramids is proposed in this manuscript.

Corrosion behaviour of die-cast AZ91D magnesium alloys in sodium sulphate solutions with different pH values

The corrosion behaviours of die-cast AZ91D magnesium alloys were investigated in 0.1 mol/L sodium sulphate (Na2SO4) solutions with different pH values. The corrosion rates, morphologies, and compositions of the corrosion products were studied by means of scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), and X-ray diffractometry (XRD). The results indicate that the order of corrosion rates in Na2SO4 solutions with various pH values is pH2＞pH4＞pH7＞pH9＞pH12. The corrosion rates in acidic solutions are higher than those in alkaline solutions, and the corrosion products are mainly magnesium hydroxide (Mg(OH)2) and hydrated sulphate pickeringite (MgAl2(SO4)4•22H2O). The results also indicate that the solution pH can influence the corrosion rate and morphology of corrosion products. Chloride ions and sulphate ions have different pitting initiation time.

Microstructure and Mechanical Properties of TWIP Steel Joints

As a new type of high manganese steel, the twinning induced plasticity (TWIP) steels have attracted a growing interest in the automotive industry due to their good performance. Thin plates of TWIP steel were welded by laser beam welding (LBW) and gas tungsten arc welding (GTAW). The microstructure result shows that GTAW joint has obvious heat-affected zone (HAZ), while the HAZ of LBW joint is almost invisible. The X-ray diffraction result shows that the phase compositions of both joints are austenitic and no phase transition occurs. Energy dispersive spectrometry result shows that there is violent evaporation of Mn element in LBW joint, while the proportion of Mn element in GTAW joint is almost unchanged. Tensile tests and micro-hardness measurements were performed to take into account the mechanical properties of joints manufactured by the two different processes. The micro-hard-ness profiles of both joints present a typical saddle distribution, and the hardness of GTAW seam is lower than that of LBW seam. The failure positions of LBW joints are al located in base metal while the GTAW joints are al at the weld toe due to the softening of HAZ. By means of scanning electron microscopy, a typical ductile fracture is observed in LBW joint, while a brittle fracture with quasi-cleavage fracture characteristic is observed in GTAW joint.

Formation Mechanism of Discoloration on Die-Cast AZ91D Components Surface After Chemical Conversion

A notebook (NB) computer component was manufactured from AZ91D Mg alloy by a die-casting process. After chemical conversion treatment, a discoloration was noted on the component surface. The source of this discoloration has been studied in detail by scanning electron microscopy, energy dispersive spectroscopy, and spark atomic absorption spectroscopy. The corrosion resistance was also measured by potentiodynamic polarization, hydrogen evolution and salt spray testing. The formation mechanism for the discoloration which was caused by the residue left behind by excess mold release agent sprayed during the die-casting was discussed in detail. After chemical conversion treatment, the residual-baked mold release agent was apparent on the component surface as “white ash.” Consequently, it degraded seriously both the appearance and the corrosion resistance of the manufactured component.

Evaluation on Fatigue Performance and Fracture Mechanism of Laser Welded TWIP Steel Joint Based on Evolution of Microstructure and Micromechanical Properties

The fatigue performance and fracture mechanism of laser welded twinning induced plasticity (TWIP) steel joint were investigated experimentally based on the evolution of microstructure and micromechanical properties. The optical microscopy was used to analyze the evolution of microstructure. The variation of composition and phase structure of fusion zone were detected by energy dispersive X-ray and X-ray diffraction spectrometers. The micromechanical behaviors of the various zones were characterized using nanoindentation. The static tensile test and high cycle fatigue test were performed to evaluate the mechanical properties of welded joint and base metal. The microstructures, tensile properties and fatigue strength of base metal as well as welded metal were analyzed. The fatigue fracture surfaces of base metal and welded joint were observed by means of scanning electron microscopy, in order to identify fatigue crack initiation sites and propagation mechanisms. Moreover, the fatigue fracture characteristics and mechanisms for the laser welded TWIP steel joints were analyzed.

Corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in gear oil

Abstract The corrosion fatigue behavior of epoxy-coated Mg–3Al–1Zn alloy in gear oil was investigated. The corrosion and the fracture surfaces after fatigue test were analyzed by scanning electron microscopy (SEM) and the corrosion compositions were measured by energy-dispersive spectrometry (EDS). The fatigue properties and the crack initiation mechanisms of the specimens before and after epoxy coating treatment were discussed. The results indicate that the fatigue limit after epoxy coating treatment in gear oil is higher than that of the uncoated specimens. The epoxy coating is an excellent way to prevent direct contact between the Mg–3Al–1Zn alloy and surrounding environments. The mechanical properties of the epoxy coating layer are lower than that of magnesium alloy, which is the main reason for the fatigue crack initiation on the epoxy coating layer. In addition, the gear oil lubrication could lead to the flaking off of the epoxy-coated layer.

Microstructural Evolution of Surface Layer of TWIP Steel Deformed by Mechanical Attrition Treatment

A nanocrystallihe layer was synthesized on the surface of TWIP steel samples by surface mechanical attrition treatment (SMAT) under varying durations. Microhardness variation was examined along the depth of the deformation layer. Microstructural characteristics of the surface at the TWIP steel SMATed for 90 min were observed and analyzed by optical microscope, X-ray diffraction, transmission and high-resolution electron microscope. The results show that the orientation of austenite grains weakens, and armartensite transformation occurs during SMAT. During the process of SM AT, the deformation twins generate and divide the austenite grains firstly; then α-martensite transformation occurs beside and between the twin bundles; after that the martensite and austenite grains rotate to accommodate deformation, and the orientations of martensite and between martensite and residual austenite increase; lastly the randomly oriented and uniform-sized nanocrystalline layers are formed under continuous deformation.

Effect of erbium on microstructures and properties of Mg-Al intermetallic

Abstract The effect of the rare earth element Er on the microstructures and properties of Mg-Al intermetallic were studied in this experiment. Metallographic and X-ray diffraction (XRD) results showed that the microstructures of Mg-Al-Er alloys varied with Er content. The Mg-44Al-0.5Er and Mg-43.8Al-1.0Er alloys were both composed of Mg 17 Al 12 matrix and Al 3 Er phase, whereas Mg-43Al-3.0Er and Mg-42Al-5.0Er were composed of Mg 17 Al 12 matrix, Al 3 Er phase, and Mg-Mg 17 Al 12 eutectic. The Mg-42Al- 5.0Er alloy showed the highest microhardness, and the values remained nearly stable as Er content increased from 1.0 wt.% to 5.0 wt.%. The dispersed second phase Al 3 Er caused the grain refinement of the Mg-Al-Er alloy, which was the main reason for the improvement in microhardness. The corrosion resistance of the Er-containing alloys initially increased and then decreased with increasing Er content. All the Er-containing alloys had the ability to suppress hydrogen evolution, which was the main reason for the higher corrosion resistance of the modified alloys than that of the Mg-44.3Al alloy. Considering the higher hardness and dispersity of the Al 3 Er phase, Mg-43.8Al-1.0Er exhibited higher wear resistance than the as-cast Mg-44.3Al alloy.

Structural characteristics of nanocrystalline copper after carbon ion implantation.

A gradient structure was produced in a pure copper plate by means of surface mechanical attrition treatment (SMAT). The microstructure of the surface layer was reduced to nanoscale and the grain size increased gradually along the depth of the treated sample. In situ transmission electron microscopy (TEM) and high resolution transmission electron microscopy (HRTEM) observation was performed on the nanocrystalline copper after implantation of carbon. Carbon atoms first precipitated along the edges of the copper substrate or at the surface, then formed amorphous carbon layers. Subsequently, onion-like fullerenes were formed under electron-beam irradiation. The effects of ion implantation, electron beam irradiation, nanostructure of the substrate and interaction of C and Cu atoms on the formation of the onion-like fullerenes are discussed.