Materials Science Forum Vols. 561-565

Abstract: NaAlH4 has a theoretical hydrogen capacity of 5.6 wt. % with two-step reaction, and the control of the reaction temperature and reversibility is a critical issue for onboard application. To clarify nano-structural details of decomposition of NaAlH4, the in-situ annealing experiment was carried out in a high resolution microscope. It was confirmed that NaAlH4 decomposed at between 200 and 300°C, resulted in formation of many gas bubbles at interface between the particle and oxide film. A reactive intermediate, Na3AlH6, may decompose in this temperature range. Sodium alanate particle was originally agglomeration of small nano-sized crystal with the size of 10 – 20 nm, and the crystal grain grew to 110 nm in diameter after completing decomposition at around 400°C. This is the first step for examination of the microstructural response of catalysts on hydrogen storage materials.

Abstract: Mg-Ni-B system alloys were prepared by the mechanical alloying (MA) method. Body centered cubic (BCC) structure alloys are obtained in some of the Mg-Ni-B compositions after the starting mixtures of raw elements were ball milled for 200 h. Mg50Ni45B5 and Mg48Ni48B4 alloys after ball milling are with single BCC structure, which is confirmed by electron diffraction patterns. From the results of X-ray diffraction and transmission electron microscope, the crystallite size of the alloys is calculated into nanometer scale. Mg50Ni45B5 and Mg48Ni48B4 BCC alloys can absorb hydrogen at 373 K with higher rate than Mg50Ni50 alloy prepared in the same conditions. And these two samples can reach a hydrogen absorption capacity of 1.94 and 1.93 mass%, respectively at 373 K without any activation process.

Abstract: The structure of melt-spun and crystallized Mg-10%Ni and Mg-10%Ni-5%La alloys is studied using HRTEM, coupled with ED and EELS techniques, for specimens subjected to hydrogenation and dehydrogenation. The presence of nano-sized (5-10nm) Mg2Ni grains dispersed in the matrix of Mg nano-grains is observed before hydrogenation. This structure is almost preserved after hydrogenation and dehydrogenation at 300°C. In the hydrogenated specimen, nanoboundaries lying between MgH2 and Mg2NiH4 nano-grains are observed. They appear to provide main routes for the hydrogen transport in these nanostructured materials.

Abstract: Thermoelectric power has demonstrated a capability for rapid hydrogen assessment and can achieve the equivalent of the pressure-composition-temperature (activity) diagram. Effective use of hydrogen storage materials occurs in the alpha+beta two-phase region of the activity diagram. A thorough assessment of the content of each phase in this two-phase region can optimize the performance of hydrogen storage materials. The use of thermoelectric power measurements as a hydrogen sensor for reversible batteries is discussed.

Abstract: The hydrogen desorption of Sn/MgH2 nanocomposite which is formed by ball milling of MgH2 and tin compounds (Sn, Sn(C4H9)4 or SnCl2), has been studied. The hydrogen desorption properties (desorption temperature and enthalpy of dehydriding) were significantly improved as a result of a Sn/MgH2 nanocomposite formation. TDS (thermal desorption spectrometry), TG (thermogravimetry) and DSC (differential scanning calorimeter) measurements exhibited the existence of at least two types of hydrogen species in the Sn/MgH2 nanocomposite resulting from ball milling of MgH2 with Sn; the one was hydrogen in the newly formed Sn/MgH2 nanocomposite and the other hydrogen derived from MgH2 remaining in Sn/MgH2.

Abstract: Porous equiatomic Nickel-Titanium (NiTi) is a strong candidate material for bone engineering applications because its mechanical properties are within the range of bone and its porosity allows for biologic interlock of the material to the surrounding tissue. Self-propagating high-temperature synthesis (SHS) is one method for producing porous NiTi. Nickel and titanium powders, -325 mesh, were mixed for 24 hours then pressed into cylindrical pellets (0.5 inch diameter, 0.5 inch height) to a theoretical green density of approximately 53%. The pellets were preheated in flowing argon for one hour then ignited using a tungsten coil. Scanning electron microscopy and electron dispersive spectroscopy (EDS) show localized differences of stoichiometry suggesting variations in the crystal structure where the Ni to Ti atomic ratio varied between 48.5:51.5 and 50.7:49.3. X-ray diffraction (XRD) (Philips X’Pert PRO) confirmed the presence crystalline equiatomic NiTi as well as other intermetallic compounds including NiTi2 and Ni4Ti3. Nanoindentation (MTS Nano Indenter XP) of this heterogeneous material indicates a mean range indentation modulus of 89.6 ± 9.4 GPa. This is on the same order of magnitude as bone, which has an elastic modulus range of 14-20 GPa.

Abstract: The mechanical properties of sintered timing wheel in contact with chain wheels were analysed using Finite Element Methods (FEM), in which the timing wheel is modelled as a metal powder. The mechanical properties of sintered timing wheel were investigated as a function of sintered density. Tensile strength and Young’s modulus increased with a decrease in porosity. Current methods of calculating gear contact stresses use Hertz’s equations, which were originally derived for contact between sintered timing wheel and chain wheels. The results of the 2D dimensional FEM analyses from ANSYS are presented. The relationship between relative density of P/M steels and mechanical behavior is also obtained from FEM and compared with the experimental data. Good agreement between the experimental and FEM results is observed, which demonstrates that FEM can capture the major features of the P/M steels behaviour during loading. This indicates that the FEM model is accurate.

Abstract: This research article describes the newly developed composite material using the artificial pellets made of incineration ashes and recycled aluminum alloys. The factor affecting its various properties was investigated and discussed. Through trial and error, the hybrid preform with good soundness and preferable dispersion of the pellets could be obtained. The density and compression strength and thermal conductivity were measured in comparison of other structural materials.

Abstract: Nanoporous gold was fabricated by dealloying and their pore characteristics were further modified by thermal or acid treatment. The fabricated nanoporous gold had a ligament size of approximately 5 nm. Thermal treatment on the nanoporous gold increased the ligament size to approximately 500 nm. During the thermal treatment, ligaments are bonded across the cracks which had been generated during the dealloying. Acid treatment also increased the ligament size to approximately 500 nm; however, the acid treatment had a different effect on the pore characteristics from the thermal treatment. As a result, nanoporous gold prism microassembly with anisotropic structure was spontaneously fabricated by the acid treatment. The mechanical properties of nanoporous gold were also examined. It is estimated that the yield strength of nanosized ligaments in nanoporous gold is very high and close to the ideal strength of gold.

Abstract: Porous copper specimens with relative densities of 0.22–0.96 were produced by spacer method and their compressive properties were investigated. In the low relative density range (relative density < 0.5–0.6), porous copper showed a density exponent n of 2.3, where n represents the relative density dependence of yield strength. In this range, the bending and buckling of cell walls and the formation of macroscopic deformation bands were observed. On the other hand, porous copper with a higher relative density (0.5–0.6 < relative density < 0.9–1) had an n value of approximately 1, where the dominant deformation mode of cell walls was yielding and no clear deformation band was observed. Also, in the highest relative density range (relative density is very close to 1), the compressive properties degraded markedly with decreasing density, indicating that stress concentration around the minimal pores occurred in this density range.