Sung-Wook Cho

A new method for the identification and quantification of magnetite-maghemite mixture using conventional X-ray diffraction technique.

The electrical explosion of Fe wire in air produced nanoparticles containing the binary mixture of magnetite (Fe(3)O(4)) and maghemite (γ-Fe(2)O(3)). The phase identification of magnetite and maghemite by the conventional X-ray diffraction method is not a simple matter because both have the same cubic structure and their lattice parameters are almost identical. Here, we propose a convenient method to assess the presence of magnetite-maghemite mixture and to further quantify its phase composition using the conventional peak deconvolution technique. A careful step scan around the high-angle peaks as (511) and (440) revealed the clear doublets indicative of the mixture phases. The quantitative analysis of the mixture phase was carried out by constructing a calibration curve using the pure magnetite and maghemite powders commercially available. The correlation coefficients, R(2), for magnetite-maghemite mixture was 0.9941. According to the method, the iron oxide nanoparticles prepared by the wire explosion in this study was calculated to contain 55.8 wt.% maghemite and 44.2 wt.% magnetite. We believe that the proposed method would be a convenient tool for the study of the magnetite-maghemite mixture which otherwise requires highly sophisticated equipments and techniques.

The hydrogen storage characteristics of Ti–Cr–V alloys

Abstract The crystal structures, the lattice parameters and the characteristics of hydrogen storage at 303 K have been investigated in ternary alloys of the Ti–Cr–V system. All of these alloys, in the range of this study, have shown a BCC structure. The hydrogen storage capacities and the effective hydrogen storage capacities of the alloys were strongly dependent on the composition ratio of Ti/Cr, showing their maximum values at a Ti/Cr ratio of about 0.75. It was also found that the lattice parameters of the alloys increased linearly with increasing Ti/Cr ratio. The differences in affinities to hydrogen and the atomic radius of each element could explain the Ti/Cr ratio dependence of the lattice parameter and hydrogen storage capacity of the alloys.

Effect of Fe addition on hydrogen storage characteristics of Ti0.16Zr0.05Cr0.22V0.57 alloy

Abstract The effect of Fe addition on hydrogen storage characteristics of Ti 0.16 Zr 0.05 Cr 0.22 V 0.57 alloy has been studied at 303 K. The X-ray diffraction (XRD) patterns of the alloy powders showed the typical patterns of BCC structure as a main phase in all of the alloys. With increasing Fe content, the lattice parameters of the BCC phases decreased linearly in accordance with Vegard’s law. With regard to the maximum hydrogen storage capacities of the alloys, there was a noticeable decrease between alloys that contained 3 and 5 at% of Fe. The reason for this can be deduced from three factors: the decrease in the lattice parameter of BCC phase, the decrease of the amount of BCC phase itself in the alloy, and the excess of the electron-to-atom ratio over 5.1 (5.09 at 3 at% Fe and 5.16 at 5 at% Fe) with increasing Fe content. The Fe addition remarkably increased the second plateau pressures in the pressure composition (P–C) isotherms. However, the first plateau was not observed in all the alloys under the conditions of the present study. From the relation between the hydrogen storage capacities and the lattice parameters of the alloys, it was found that controlling the lattice parameter similarly to that of V (3.03 A), i.e. within the range of about 3.02–3.04 A, is of fundamental importance to obtain the largest effective hydrogen storage capacity in this alloy systems under the present conditions.

Fabrication of Nanostructured Ti from Ti and TiH2 by Rapid Sintering and Its Mechanical Properties

Titanium has good deformability, high hardness, high biocompatibility, excellent corrosion resistance and low density. Due to these attractive properties, it has been used in many industrial applications. Dense nanostructured Ti was sintered from mechanically activated Ti and TiH2 powders by high frequency induction heating under pressure of 80 MPa. The advantage of this process is that it allows very quick densification to near theoretical density and inhibition of grain growth. TiH2 powder was decomposed to Ti during sintering. The hardness of Ti increased and the average grain size of Ti decreased with increasing milling time. The average grain sizes of Ti samples sintered from Ti and TiH2 powder milled for 5 hrs were about 26 nm, 44 nm, respectively. The hardness of Ti sintered from Ti and TiH2 powder milled for 5 h was 504 kg/mm and 567 kg/mm, respectively. (Received June 1, 2011)

Isotope effect on structural transitions of Ti1.0Mn0.9V1.1HX(DX) and Ti1.0Cr1.5V1.7HX(DX) with hydrogenation

Abstract We have investigated the structural transitions of Ti 1.0 Mn 0.9 V 1.1 H X (D X ) and Ti 1.0 Cr 1.5 V 1.7 H X (D X ) upon hydrogenation at 293 K and discussed the effect of hydrogen isotope on their crystal structures. The various hydride samples used for X-ray diffraction (XRD) investigation were obtained after measurement of the P–C isotherms by taking them out of the reactor. The crystal structures, phase abundance and lattice parameters of the hydrides were determined by the Rietveld method using XRD data. Because the hydrides of the alloy Ti 1.0 Mn 0.9 V 1.1 revealed complex peak profiles, we double-checked the structures through transmission electron microscope (TEM) investigations. The results of the TEM observation agreed well with those of XRD data. The crystal structures of corresponding isotope hydrides, the phase abundance and the lattice parameters do not depend on the kind of hydrogen isotope, but only on the hydrogen content. That is, if the corresponding isotope hydrides have the same hydrogen contents, they have also the same crystal structures, although they show a large difference between the equilibrium pressures in their P–C isotherms. At the experimental temperature, the Ti 1.0 Mn 0.9 V 1.1 alloy and Ti 1.0 Cr 1.5 V 1.7 alloy revealed different structural transition processes upon hydrogenation although the crystal structures of these two alloys are both body centered cubic (BCC). The structural transitions of the alloys Ti 1.0 Mn 0.9 V 1.1 and Ti 1.0 Cr 1.5 V 1.7 can be summarized by BCC ( a =3.0183(1) A)→body centered tetragonal (BCT) ( a =2.874(3) A, c =3.89(1) A)→face centered cubic (FCC) ( a =4.311(8) A) in alloy Ti 1.0 Mn 0.9 V 1.1 and BCC ( a =3.0212(9) A)→FCC ( a =4.261(4) A) in alloy Ti 1.0 Cr 1.5 V 1.7 . The Ti-rich phases with NiTi 2 structure and α-Ti with hexagonal close packed (HCP) structure absorbed hydrogen at relatively low hydrogen pressures and the phase abundance remained almost constant. From this fact, it can be deduced that it is desirable to decrease their amount as far as possible in order to increase the effective hydrogen storage capacities of the alloys.

Hydrogen isotope effects in Ti1.0Mn0.9V1.1 and Ti1.0Cr1.5V1.7 alloys

Abstract The hydrogen isotope effects in Ti 1.0 Mn 0.9 V 1.1 and Ti 1.0 Cr 1.5 V 1.7 alloys have been investigated at 313 and 353 K and at 313 and 338 K, respectively, using protium and deuterium. The crystal structures including the phase abundance and the lattice parameters were determined by the Rietveld method. Using SEM and EDX, the morphology was examined and an elemental analysis was carried out. In both alloys and regardless of temperature, the amounts of deuterium absorption were larger than those of protium. The hydrogen isotope effects of Ti 1.0 Mn 0.9 V 1.1 and Ti 1.0 Cr 1.5 V 1.7 were fairly large in comparison with that of LaNi 5 . The protide of the Ti 1.0 Mn 0.9 V 1.1 alloy was more stable than the corresponding deuteride, although at 313 K the equilibrium pressures of deuterium and protium showed slightly opposite behaviour. In the Ti 1.0 Cr 1.5 V 1.7 alloy, the deuteride was always more stable than the corresponding protide in the experimental temperature range. The Ti 1.0 Cr 1.5 V 1.7 alloy absorbed more deuterium and protium than the Ti 1.0 Mn 0.9 V 1.1 alloy.

Making of tantalum powder using the hunter process

Tantalum powder was made from potassium heptafluorotantalate (K2TaF7) using sodium as a reductant based on the Hunter metallothermic reduction method. The apparatus for the experiment was designed and built specifically for the present study. The tantalum particle size greatly decreased as the reduction temperature decreased from 860 to 740°C. The particle size was fairly uniform, varying from 2–3 μm to submicron sizes depending on the reaction temperature. Holding the reactants for more than 10 minutes after the completion of a sodium feeding caused irregular particle growth, thereby affecting particle size uniformity. It was found to be possible to agglomerate the powder to tens of micrometers in size by a vacuum heat treatment at 1350°C. The formation of Ta2C during the heat treatment shows that care must be taken to prevent contamination from the furnace environment.

Mechanical Properties and Fabrication of Nanostructured 1.5TiAl-Al2O3 Composite by Pulsed Current Activated Sintering

Nano-powders of 1.5TiAl and Al2O3 were synthesized from 1.5TiO2 and 3Al powders by high energy ball milling. Nanocrystalline Al2O3 reinforced composite was consolidated by pulsed current activated sintering within 2 minutes from mechanochemically synthesized powders of Al2O3 and 1.5TiAl. The relative density of the composite was 99.5%. The average hardness and fracture toughness values obtained were 1250 kg/mm and 10 MPa ·m, respectively. (Received September 5, 2011)

Rapid Sintering of TiCu by Pulsed Current Activated Heating and its Mechanical Properties

Nanopowder of TiCu was synthesized by high-energy ball milling. A dense nanostructured TiCu was consolidated using a pulsed-current activated sintering method within 1 minute from mechanically synthesized powders of TiCu and horizontally milled powders of Ti+Cu. The grain size and hardness of the TiCu sintered from horizontally milled Ti+Cu powders and high-energy ball-milled TiCu powder were 68 nm, 27 nm and 490 kg/mm, 600 kg/mm, respectively. (Received August 17, 2010)

Numerical simulation of hydrogen desorption from high-density metal hydride hydrogen storage vessels

Metal hydride (MH) alloys are a promising type of material in hydrogen storage applications, allowing for low-pressure, high-density storage. However, while many studies are being performed on enhancing the hydrogen storage properties of such alloys, there has been little research on large-scale storage vessels which make use of the alloys. In particular, large-scale, high-density storage devices must make allowances for the temperature variations caused by the heat of reaction between hydrogen and MH alloys, which may impact the storage characteristics. In this study, we propose a numerical model for the design and evaluation of hydrogen storage devices using MH alloys. Hydrogen desorption reaction behavior for an alloy is observed in terms of temperature and reaction rate. This behavioral correlation is used as the basis for a comprehensive simulation model of the alloy system. Calculated results are found to be in good agreement with experimentally measured data, indicating that the model may be applied to multiple system geometries, scales, and alloy compositions.