Myoung-Youp Song

Improvement in hydrogen sorption properties of Mg by reactive mechanical grinding with Cr2O3, Al2O3 and CeO2

Abstract We tried to improve the hydrogen sorption properties of Mg by mechanical grinding under H 2 (reactive mechanical grinding) with oxides Cr 2 O 3 , Al 2 O 3 and CeO 2 . The hydriding rates of Mg are reportedly controlled by the diffusion of hydrogen through a growing Mg hydride layer. The added oxides can help pulverization of Mg during mechanical grinding. A part of Mg is transformed into MgH 2 during reactive mechanical grinding. The Mg+10wt.%Cr 2 O 3 powder has the largest transformed fraction 0.215, followed in order by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2 O 3 . The Mg+10wt.%Cr 2 O 3 powder has the largest hydriding rates at the first and fifth hydriding cycle, followed in order by Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . Mg+10wt.%Cr 2 O 3 absorbs 5.87wt.% H at 573 K, 11 bar H 2 during 60 min at the first cycle. The Mg+10wt.%Cr 2 O 3 powder has the largest dehydriding rates at the first and fifth dehydriding cycle, followed by Mg+10wt.%CeO 2 and Mg+10wt.%Al 2 O 3 . It desorbs 4.44 wt.% H at 573 K, 0.5 bar H 2 during 60 min at the first cycle. All the samples absorb and desorb less hydrogen at the fifth cycle than at the first cycle. It is considered that this results from the agglomeration of the particles during hydriding–dehydriding cycling. The average particle sizes of the as-milled and cycled powders increase in the order of Mg+10wt.%Cr 2 O 3 , Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . The quantities of hydrogen absorbed or desorbed for 1 h for the first and fifth cycles decrease in the order of Mg+10wt.%Cr 2 O 3 , Mg+10wt.%Al 2 O 3 and Mg+10wt.%CeO 2 . The quantities of absorbed or desorbed hydrogen increase as the average particle sizes decrease. As the particle size decreases, the diffusion distance shortens. This leads to the larger hydriding and dehydriding rates. The Cr 2 O 3 in the Mg+10wt.%Cr 2 O 3 powder is reduced after hydriding–dehydriding cycling. The much larger chemical affinity of Mg than Cr for oxygen leads to a reduction of Cr 2 O 3 after cycling.

Hydrogen sorption of Mg-based mixtures elaborated by reactive mechanical grinding

The use of mechanical grinding (MG) under H2 of magnesium powder improves the hydrogen sorption properties. The hydrogenation of Mg starts in situ during the milling process that allows suppressing the activation procedure generally requested for Mg. The effects of the addition of various elements or compounds have been studied. The hydriding is a two-step process: nucleation and diffusion. A direct relationship exists between the nucleation duration and the specific surface. A critical milling time exists below which the diffusion process is improved and above which no further improvement is observed (the maximum internal stress in the powder is also reached at this critical time). The diffusion is controlled by the number of crystallites per particle that can be reduced by increasing the milling time up to 10 h. The addition of Co (catalyst), YNi (hydrogen pump) or oxides (abrasive element and nucleation centre) leads to an improvement of the hydrogen sorption properties (but a strong dependence upon the milling time is reported). Finally, the sorption properties of our mixtures are comparable with thus reported for MgH2–metal mixtures.

Improvement in the cycling performance of LiMn2O4 by the substitution of Fe for Mn

Abstract We tried to improve the cycling performance of LiMn2O4 by substituting Fe for Mn. LiFe0.25Mn1.75O4 is the optimum composition which has a relatively large discharge capacity and the largest discharge capacity at n=30, and the smallest percentage of capacity deterioration at n=30, especially with (C1−C30)×100/C1=5.6%. The variation of discharge capacities for the LiFeyMn2−yO4 (y=0.0, 0.1, 0.25, 0.38, and 0.5) with cycling can be well expressed by the following equation: C n /C 0 =e −kt n where Cn is the discharge capacity at nth cycle, C0 the theoretical capacity, t=t1+t2+…+tn (tn=discharging time at nth cycle), and k and n constants.

Pressure–composition isotherms in the Mg2Ni–H2 system

Abstract A relatively pure Mg2Ni intermetallic compound was prepared by partial melting and sintering. Absorption and desorption pressure–composition isotherms for the Mg2Ni–H2 system were obtained. The relationships between the equilibrium plateau pressure (Peq) and the temperature were log P|=−3,211/T+6.2220 for absorption , and log P|=−3,300/T+6.2612 for desorption . The procedure to obtain the pressure–composition isotherms was explained and a method to calculate the composition for pressure–composition isotherms (“the summation method”) was also suggested.

Improvements of hydrogen storage properties of Mg-based mixtures elaborated by reactive mechanical milling

The influence of mechanical grinding under hydrogen (Reactive Mechanical Grinding=RMG) on the chemical properties (crystallographic and phase composition) and on the hydrogen storage properties of Mg-based mixtures is examined. Different additives were studied such as: Co, YNi, oxides, BN. All these additives lead to an unequal improvement in the hydriding properties. For Cr2O3, the effect of both the morphology and crystallinity were studied by elaborating Cr2O3 by supercritical fluid process.

A study of joining Ag/BSCCO superconducting tapes

Abstract A superconducting joint between Ag/BSCCO tapes has been fabricated by using a combination of etching, pressing and annealing techniques. The joined area, fabricated with a load of 10 ton, showed 80% current carrying capacity of the tapes without join-interface. I c of the joined area in the low-field region drops more quickly with magnetic field than the unjoined area. It is considered that the lower magnetic resistivity of I c and I c degradation in the joined area resulted from the heterogeneous deformation in the joined area. The joined area was mechanically weak.

Phase separation of Mg2Ni by hydriding–dehydriding cycling

Abstract The intermetallic compound Mg 2 Ni was prepared by sintering in a closed alumina tube furnace in order to reduce the quantity of evaporated Mg. A small amount of MgO and MgNi 2 was formed in the Mg 2 Ni as prepared, and their quantities increased as the number of cycles increased. Electron diffraction analysis showed that the particle on the sample surface was MgO. The magnetic measurements showed that Ni segregated after hydriding–dehydriding cycling and its quantity increased as the number of cycles increased. A small amount of O 2 in the H 2 reacts with Mg 2 Ni to form MgO and to segregate Ni: Mg 2 Ni+O 2 →2 MgO+Ni. The segregated Ni reacts with Mg 2 Ni to form MgNi 2 : Mg 2 Ni+3Ni→2MgNi 2 . These two reactions decrease hydrogen-storage capacity of Mg 2 Ni as the number of the hydriding–dehydriding cycles increases.

Hydriding kinetics of Mg2Ni below the polymorphic transformation temperature of Mg2NiH4

The hydriding kinetics of Mg2Ni are studied below the polymorphic transformation temperature of Mg2NiH4 under hydrogen pressures from 4.0 to 10.1 bar. The hydriding reaction of Mg2Ni progresses with the nucleation and growth mechanism under these conditions. The nucleation of Mg2Ni hydride is considered to control the hydriding rates. Heat transfer also decreases the hydriding rates.

A study of the hydriding and dehydriding kinetics of a mixture of 2 Mg and Co

Abstract Hydriding and dehydriding rates of a mixture of 2 Mg and Co were measured under nearly constant hydrogen pressures. The rate-controlling steps for its hydriding and dehydriding reactions were determined.

Improvement in the electrochemical properties of ZrMn2 hydrides by substitution of elements

The hydrogen-storage properties and the electrochemical properties are investigated for the alloys ZrMn2Nix, ZrMnNi1+x, Zr0.5Ti0.5Mn0.4V0.6Ni1−xFex and Zr0.5Ti0.5Mn0.4V0.6Ni0.85M0.15. The C14 Laves phase forms in all the alloys ZrMn2Nix (x=0.0, 0.3, 0.6, 0.9 and 1.2). Among the alloys ZrMn2Nix, ZrMn2Ni0.6 has the largest discharge capacity (29 mAh/g) and a relatively good cycling performance, and shows a relatively easy activation. The C14 Laves phase also forms in all the alloys ZrMnNi1+x (x=0.0, 0.1, 0.2, 0.3 and 0.4). Among the alloys ZrMnNi1+x, ZrMnNi1.0 has the largest discharge capacity (42 mAh/g) and a relatively good cycling performance, and shows the easiest activation. Zr0.5Ti0.5Mn0.4V0.6Ni1−xFex (x=0.00, 0.15, 0.30, 0.45 and 0.60) has the C14 Laves phase hexagonal structure. Their hydrogen storage capacities do not show significant differences. The discharge capacity just after activation decreases with an increase in the amount of the substituted Fe but the cycling performance is improved. The discharge capacity after activation of the alloy with x=0.00 is about 240 mAh/g at the current density 60 mA/g. Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 is the best composition with a relatively large discharge capacity and a good cycling performance. The increase in the discharge capacity of Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 with the increase in the current density (from 60 mA/g to 125 mA/g) is considered to result from the self-discharge property of the electrode. Zr0.5Ti0.5Mn0.4V0.6Ni0.85M0.15 (M=Fe, Co, Cu, Mo and Al) alloys also have the C14 Laves phase hexagonal structure. The alloys with M=Co and Fe have relatively larger hydrogen storage capacities. The discharge capacities just after activation are relatively large in the case of the alloys with M=Co and Fe. The Zr0.5Ti0.5Mn0.4V0.6Ni0.85Co0.15 alloy is best with a relatively large discharge capacity (257 mAh/g at the current density 250 mA/g for the 12th cycle) and a good cycling performance. During activation form Ni-rich and Fe-rich regions on the surface of the Zr0.5Ti0.5Mn0.4V0.6 Ni0.85Fe0.15 alloy. They may act as the active sites for the electrochemical reaction. With the increase in the number of charge-discharge cycles for the Zr0.5Ti0.5Mn0.4V0.6Ni0.85Fe0.15 alloy, the quantities of the Zr and Fe dissolved in the electrolyte solution increase.