Effects of milling conditions on hydrogen storage properties of graphite

Two milling modes (shearing and impact) were applied to investigate the hydrogen storage properties of graphite. It was found that the shearing mode leads to 0.613 wt.% hydrogen absorbed in graphite, while impact mode leads to 2.718 wt.%. X-ray diffraction was used to investigate the structure of the as-milled and subsequent annealed samples. Differential scanning calorimetry was used to study thermally induced transformations in the as-milled samples. Infrared spectrometry was carried out to investigate the interaction between carbon and hydrogen atoms. Results are compared and discussed in conjunction with Laser desorption time-of-flight mass spectrometry results obtained earlier.     

Improvement in hydrogen storage characteristics of magnesium by mechanical alloying with nickel

The hydriding and dehydriding kinetics of Mg are reviewed. It is reported that the hydriding and dehydriding reactions of Mg are nucleation-controlled under certain conditions and progress by a mechanism of nucleation and growth, and that the hydriding rates of Mg are controlled by the diffusion of hydrogen through a growing Mg hydride layer.The hydriding and dehydriding kinetics of Mg can be improved in consequence by a treatment such as mechanical alloying, which can facilitate the nucleation by creating defects and shorten diffusion distances by reducing the effective particle size of Mg.The hydriding and dehydriding characteristics of mechanically-treated Mg and mechanically-alloyed mixtures with the compositions Mg-x wt Ni (x=5, 10,25 and 55) are studied.The Mg₂Ni phase develops in the mechanically-alloyed mixtures. The Mg-10wt% Ni and Mg-25 wt% Ni mixtures are activated easily, show much larger hydrogen storage capacities and much higher hydriding rates, and higher dehydriding rates, than other magnesium- based alloys or mixtures.     

Thermal cycling of iridium coatings on isotropic graphite

The surface morphology observed on the iridium-coated isotropic graphite substrate varied widely between 300 and up to 2173 K thermal cycling and heat-treatment testing. The columnar structure was retained after thermal cycling between 300 and 1873 K. At high temperatures between 300 and 1973–2173 K thermal cycling, the columnar grain structure was replaced by dense equiaxed grains and the grain size increased with time and temperature. The structure obtained contains pores going outwards from the coating after thermal cycling between 300 and 1873 K; on the other hand, high-temperature thermal cycling porosities were diminished. Transmission electron microscopy of the specimens showed very little difference in grain size between as-deposited coatings and those thermally cycled between 300 and 1873 K. X-ray diffraction analysis indicated that the preferred orientation of columnar structure was destroyed by thermal cycling. In addition, there was no loss in weight after thermal cycling and heat treatment testing in nitrogen.     

Hydrogen accumulation in graphite and etching of graphite on hydrogen desorption

Studies of Atomic Hydrogen accumulation in highly oriented pyrolytical graphite (HOPG) have been performed using scanning tunnelling microscope (STM) and atomic force microscope (AFM). It is found that after intercalation atomic hydrogen is stored among graphene layers in H₂ gas form, captured inside graphene blisters. On desorption of hydrogen, some lateral etching of upper graphene layers takes place. Significant information about intake, retention and possibility of manifold accumulation of hydrogen in HOPG has been found.     

Improvement of the anode performance of graphite particles through surface modification in RF thermal plasma

Mesocarbon microbeads (MCMB) powders, which are known to be highly crystallized carbon materials and have a mean particle size of 11 μm, were treated in RF thermal plasma. Through the plasma treatment, the surface morphology, structure, and chemical composition of the powder were modified. The plasma-induced modification made the surface of MCMB particles disordered, and gave rise to an improvement in the thermal stability and electrochemical properties of the powders, such as the discharge capacity and first charge/discharge efficiency. It also made the powders suitable for further use as an anode in lithium-ion rechargeable batteries. Powders obtained without air exposure showed further improvement in anode performance.     

The influence of tetrahydrofuran treatment on hydrogen storage properties of the magnesium

The hydriding and dehydriding properties of the tetrahydrofuran (THF) treated magnesium, along with its electronic energy states, crystalline structure and micro morphology have been investigated. The THF treated magnesium absorbs 6.3 wt% hydrogen at 723 K and 3.5 MPa. After hydrogenation, in addition to the expected hydride MgH₂, a new less-stable hydride phase appears at 673 K, but not at a lower temperature. Desorption produces 5.5 wt% hydrogen at 723 K against a back-pressure of 1.3 Pa after 20 cycles of hydriding–dehydriding. The THF treatment improves the kinetics of hydrogen absorption and desorption. The THF treated Mg exhibited reasonable reaction rates with hydrogen at 623 K. XPS (X-ray Photo-electron Spectroscopy) studies show that THF treatment causes the electronic energy state of the magnesium atoms to shift, but the XRD (X-Ray Diffractometer) studies show the crystal structure remains unchanged. It is believed that the chemical state of magnesium surface is activated by THF treatment in favor of hydrogen absorption and desorption. Metallographic observation of the magnesium hydrides reveals some interesting features during hydrogenation.     

Effects of moisture and thermal cycling on in-plane shear properties of graphite fibre-reinforced cyanate ester resin composites

The weight change and retention of in-plane shear (±45°) strength of graphite fibre-reinforced cyanate ester resin matrix composites have been estimated on exposure to high humidity and thermal cycling, respectively. Cyanate ester resin matrix composites absorbed a remarkably small amount of moisture on exposure to high humidity. However, the degree of moisture absorption underwent a rather sudden increase to a new equilibrium level after prolonged exposure. A morphology study showed the occurrence of extensive cracking in the matrix/interface region in the form of delamination between plies as well as translaminar cracking within plies. The phenomenon is believed to be caused by weakening of the fibre-matrix interface, which was confirmed by microscopic analysis of fracture surfaces. A sudden moisture gain associated with extensive matrix/interface cracking was found to reduce the in-plane shear strength and fatigue lifetime at a given stress amplitude. The slope of the S-N curve was lower for wet specimens, implying a higher growth rate of local cracks as well as delamination. The rate of in-plane shear strength degradation was also measured on static exposure to dry heat as well as after thermal cycling to a peak temperature of 150 or 204°C. At a frequency of 10 min/cycle and for a relatively short duration, the effect of thermal cycling seems to be represented by the cumulative sum of thermal oxidation effects at the peak temperature.     

Improvement of mechanical properties in aluminum/CNTs nanocomposites by addition of mechanically activated graphite

Carbon nanotubes reinforced aluminum nanocomposite was prepared by ball milling route. CNTs were initially mixed with mechanically amorphized graphite. Specimens were analyzed by X-ray diffractometry and Raman spectroscopy. Crystallite size and dislocation density were calculated by modified Warren–Averbach method. Carbide formation was semi-quantitatively investigated via Raman spectroscopy. A band located in 950 cm⁻¹ was considered to be corresponded to Al₄C₃. Hardness of the samples was also evaluated using a Vickers micro-hardness tester. The hardness strengthening contributions were modeled to evaluate interfacial bonding between CNTs and the aluminum matrix. In specimens, including amorphized graphite, hardening was due to both work hardening and second phase strengthening otherwise, only due to work hardening. It was deducted that the amorphized graphite has a major role for mechanical properties improvement. This seems to be due to the formation of aluminum carbide at the interface which consequently increases adhesion of CNTs to aluminum.     

氢化燃烧合成镁基储氢合金的电化学性能的研究

研究了氢化燃烧合成Mg2NiH4产物的电化学性能，并探索了机械球磨处理对产物电化学性能的影响。电化学测试表明，HCS产物不经任何处理，最大放电容量仅为45.13mAh／g；产物球磨后最大放电容量和高倍率放电能力得到提高，如产物经球磨1h后，最大放电容量增至259．24mAh／g，产物添加3％（质量分数）的石墨球磨5h，最大放电容量增加了10倍以上，达到481．50mAh/g。     

Improvement of tensile properties of Al-Si alloys through directional solidification

This study examines the tensile properties in directionally solidified (DS-route) Al-Si alloys and in conventionally die casting (DC-route) of the same composition. An attempt has been made to correlate the results with the fracture and microstructure of the alloys. One of the most important findings in this work is that a marked improvement in ductility is observed for DS samples over their DC-processed counterparts.     

Preparation of calcium and magnesium hydrogen phosphates from natural dolomite and their sorptive properties

The reaction of dolomite with 10–20% phosphoric acid was studied by x-ray diffraction and chemical analysis in a wide range of dolomite concentrations in the reaction system. The structure, composition, and sorption capacity of the reaction products were determined. The reaction was shown to yield a mixture of calcium hydrogen phosphate, CaHPO₄ · 2H₂O, with the brushite structure and magnesium hydrogen phosphate, MgHPO₄ · 3H₂O, with the newberyite structure. The sorption capacity of the reaction product for Pb ions was up to 19 mg-equiv/g. With 10% H₃PO₄ at an initial dolomite concentration of 0.17 g/ml, a near-theoretical reaction yield was attained.     

Improvement of formability and mechanical properties of magnesium alloys via pre-twinning: A review

Twinning can generate the change of texture and a large of twin boundaries, which can greatly influence the mechanical properties of magnesium alloys. Thus, pre-twinning can be considered to be a simple and feasible method to improve the mechanical properties of magnesium alloys. Recently, some studies have confirmed that pre-twinning can be an effective way to enhance the strength, formability and mechanical anisotropy of magnesium alloys. Based on these results, some aspects of the present research on the improvement of mechanical properties via pre-twinning are reviewed. The relevant mechanisms have been summarized. Finally, for this research field, a few critical scientific problems are also proposed.     

The interaction of hydrogen with oxidic promoters of hydrogen storage in magnesium hydride

The present work is devoted to monitor the reactivity with hydrogen of various oxide systems (Nb₂O₅, WO₃ and a mixed Mg/Nb/O oxide) in the aim of understanding the role of oxides as promoters for hydrogen storage in MgH₂. The reactivity of the oxides has been tested using either molecular hydrogen or “nascent” hydrogen produced by reaction of zinc with hydrochloric acid. Thermal desorption-mass spectrometry experiments indicate that hydrogen adsorbed in Nb₂O₅ and Mg/Nb/O mixed oxides is, in part, reversibly released and, for a second fraction, released as water. By contrast, water is the only product desorbed by WO₃ after contact with hydrogen. This could explain the higher performances of Nb₂O₅ as kinetic promoter of hydrogen storage in comparison with WO₃.     

Controlling graphite oxide bandgap width by reduction in hydrogen

Transformation of the chemical composition and electron structure of graphite oxide (GO) nanolayers as a result of their annealing in hydrogen has been studied by X-ray photoelectron spectroscopy using synchrotron radiation. It is established that both the chemical composition and bandgap width of GO can be controlled by varying the temperature and duration of heat treatment. By this means, the properties of GO nanolayers can be smoothly changed from dielectric to semiconductor.