Hydrogen reduction behavior and microstructural characteristics of WO3 and WO3-NiO powders

The characterization and understanding of the hydrogen reduction and sintering behavior of powder mixtures prepared from WO₃ and WO₃-NiO have been investigated. The nano-sized W and W-Ni powders were prepared by ball milling and hydrogen reduction of oxide powders. The reduction behavior is analyzed by temperature-programmed reduction method with different heating rates in Ar-10% H₂ atmosphere. X-ray diffractometry analysis revealed that the oxide powders are changed to W and W-Ni powders with an average particle size of about 100 nm by hydrogen reduction at 800 °C for 1 h. The hydrogen reduction kinetics was evaluated by the amount of peak shift with heating rates. The activation energies for the reduction of pure WO₃ and WO₃-NiO, estimated by the slope of the Kissinger plot, were measured as 87.4–117.4 kJ/mol depending on reduction steps. The consolidated W-Ni by spark plasma sintering has relatively dense and large grains with neck growth by enhanced mass transport due to the addition of Ni. These results are help to optimize the powder synthesis process and to understand the hydrogen reduction behavior and Ni addition effect related to microstructure of powders and sintered bodies.     

Properties of graphene nanopowder and multi-walled carbon nanotube-filled epoxy thin-film nanocomposites for electronic applications: The effect of sonication time and filler loading

Graphene nanopowder (GNP) and multi-walled carbon nanotube (MWCNT)-filled epoxy thin-film composites were fabricated using ultrasonication and the spin coating technique. The effect of sonication time (10, 20 and 30 min) and GNP loading (0.05–1 vol%) on the tensile and electrical properties of GNP/epoxy thin-film composites was investigated. The addition of GNP decreased the material’s tensile strength and modulus. However, among the tested samples, the GNP/epoxy composites produced using 20 min of sonication time had a slightly higher tensile strength and modulus, with a lower electrical percolation threshold volume fraction. The effect of sonication time was supported by morphological analysis, which showed an improvement in GNP dispersion with increased sonication time. However, GNP deformation was observed after a long sonication time. The GNP/epoxy composites at different filler loadings showed higher electrical properties but slightly lower tensile properties compared with the MWCNT/epoxy composites fabricated using 20 min of sonication time.     

Simultaneous strengthening and toughening of reduced graphene oxide/alumina composites fabricated by molecular-level mixing process

Reduced graphene oxide/alumina composite powders were prepared by mixing of graphene oxides and aluminum ions at the molecular-level. It was found that the composite consolidated from the powders showed that reduced graphene oxide were homogeneously dispersed and strongly bonded with the alumina matrix by oxygen atoms presenting at reduced graphene oxide/alumina interfaces. Both the hardness and the toughness of the composites were enhanced simultaneously by the addition of reduced graphene oxide, which act as bridges to restrain the propagation of cracks in the alumina matrix. It is clarified that graphenes can be utilized as promising reinforcements for enhancement in mechanical properties of ceramic materials when the molecular-level mixing process is applied.     

Self-limited growth of nanocrystals in phosphosilicate melts during cooling

In this Letter, we demonstrate that the spontaneous nanophase-separation can greatly enhance the heterogeneous nucleation in the investigated phosphosilicate melts. The two separated phases are found to be the phosphate-rich phase as the floppy domain and the silicate-rich phases as rigid phase. We found that sodium phosphate nanocrystals form in the phosphate-rich phase during melt cooling. The growth of these nanocrystals are self-limited, i.e., limited by the surrounding silicate-rich phase with higher viscosity, and hence lower ionic diffusion compared to the phosphate-rich phase. Our results show that the substitution of B₂O₃ or Al₂O₃ for partial Na₂O enhances the spontaneous nucleation, although the viscosity of silicate-rich matrix phase is increased by such substitution. This implies that the compositional substitution enhances nanophase separation and thereby lowers the activation energy for non-isothermal crystallization. This work indicates that nanophase separation is crucial for fabrication of transparent glass-ceramics from phosphosilicate melts.     

Hollow graphene spheres coated separator as an efficient trap for soluble polysulfides in LiS battery

Hollow graphene spheres are successfully prepared and employed as the separator coating materials for lithium-sulfur batteries. The hollow graphene spheres coated separator has been proven an efficient trap to adsorb and block polysulfide, greatly alleviating the shuttle effect. In the case of using elemental sulfur as cathode active material and the weight of the diaphragm is only increased by 10.3%, the lithium-sulfur battery with hollow graphene spheres coated separator delivers a high initial specific capacity of 1172.3 mAh g⁻¹ at the current density of 0.2 C, and the discharge capacity remains at 829.6 mAh g⁻¹ after 200 cycles with a capacity decay of 0.146% per cycle, showing excellent electrochemical performance.     

Carbon particles modified macroporous Si/Ni composite as an advanced anode material for lithium ion batteries

Carbon particles modified macroporous Si/Ni composite (MP-Si/Ni/C) is easily obtained via a facile fabrication of porous Si/Ni precursor by dealloying SiNiAl alloy followed by a surface growth of carbon nanoparticles. MP-Si/Ni/C composite possesses the multiply conductivity modification that are built through mixing Ni dispersoid and growing one layer of carbon particles. Coupled with the structural advantages of interconnected network backbone, rich voids, and the coated carbon particles, MP-Si/Ni/C exhibits dramatically enhanced lithium storage performances with excellent reversible capacity, enhanced rate performance, as well as outstanding cycling stability compared with pure MP-Si and MP-Si/Ni. Especially, the reversible capacity remains up to 1113.1 and 708.8 mA h g⁻¹ at the current densities of 200 and 1000 mA g⁻¹ after 120 cycles, respectively. Besides, it shows excellent rate capability even when continuously cycled at high current density of 3000 mA g⁻¹. With the advantages of unique structure, excellent performances, and facile preparation, the as-made MP-Si/Ni/C composite shows promising application potential as an alternative anode for lithium ion batteries.     

Fabrication of BSCF-based mixed ionic-electronic conducting membrane by electrophoretic deposition for oxygen separation application

Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3-δ (BSCF), which exhibits a high mixed oxide ionic-electronic conduction, was used for the fabrication of an oxygen separation membrane. An asymmetric structure, which was a thin and dense BSCF membrane layer supported on a porous BSCF substrate, was fabricated by the electrophoretic deposition method (EPD). Porous BSCF supports were prepared by the uniaxial pressing method using a powder mixture with BSCF and starch as the pore-forming agent (0–50 wt.%). The sintering behaviors of the porous support and the thin layer were separately characterized by dilatometry to determine the co-fired temperature at which cracking did not occur. A crack-free and thin dense membrane layer, which had about a 15 μm thickness and >95% relative density, was obtained after optimizing the processes of EPD and sintering. The dense/porous interface was well-bonded and the oxygen permeation flux was 2.5 ml (STP) min⁻¹ cm^-2 at 850 °C.     

Finite-time stability and stabilization for stochastic markov jump systems with mode-dependent time delays

This paper is concerned with the problems of finite-time stability and stabilization for stochastic Markov systems with mode-dependent time-delays. In order to reduce conservatism, a mode-dependent approach is utilized. Based on the derived stability conditions, state-feedback controller and observer-based controller are designed, respectively. A new N-mode algorithm is given to obtain the maximum value of time-delay. Finally, an example is used to show the merit of the proposed results.     

Realization of direct flow control with load pressure compensation on a load control valve applied in overrunning load hydraulic systems

This paper presents the realization of direct proportional flow control with load pressure compensation feature on a LCV (load control valve). Proportional flow control performance means the flow through the LCV is proportional to the pilot pressure in the control stroke. Proportional flow control decides the overrunning load lowering speed control performance of the whole system. The load pressure compensation feature means when the load pressure is too high, the flow of the LCV can be restricted about the maximum rated flow. The load pressure compensation feature is important to the safety of the system. That is because large flow means undesired fast lowering speed, which will cause accident in applications, especially those large mass overrunning load systems. In this paper, the flow control performance was simulated and the parameter relationship of the orifices was derived, which is the base for the optimizing of the compensation orifice. In addition, load pressure compensation feature was simulated and the compensation orifice size was optimized. Finally, an LCV built according to above methods was tested on a test rig. Experiment data validates the methods presented and the realization of direct flow control with load pressure compensation feature gives guidance for the direct flow control performance development of other valves.     

Reactive sintering cBN-Ti-Al composites by spark plasma sintering

When synthesizing polycrystalline cubic boron nitride (PcBN) at normal pressure, cBN had a trend of hexagonal transformation, which reduces the hardness and strength of PcBN. The cBN-Ti-Al composite was prepared by spark plasma sintering with introducing Ti and Al to absorb hexagonal boron nitride (hBN) transformed from cBN. By the results of X-ray diffraction (XRD), Ti and Al reacted with BN and forming TiN, TiB₂, and AlN, which combined cBN as the binder by chemical bonding. The mechanical properties of the prepared composite increased as the increment of sintering temperature. The threshold temperature for preparing composite without hBN phase was at 1400 °C. The composite with optimal mechanical properties was prepared at 1400 °C, and the relative density, the bending strength, hardness, and fracture toughness were 98.9 ± 0.1%, 390.7 ± 4.4 MPa, 14.1 ± 0.5 GPa, and 7.6 ± 0.1 MPa·m^0.5, respectively.