The Zr based AB2 alloys ZrMnFe0.5Ni0.5, ZrMnFe0.5Co0.5 and mischmetal (Mm) based AB5 alloy MmNi3.5Al0.5Fe0.5Co0.5 have been prepared and characterized by means of powder x-ray diffractograms. The hydrogen absorption kinetics of these alloys have been studied in the temperature and pressure ranges 450-650 °C and 10-100 mbar respectively with a maximum H to host alloy formula unit ratio of 0.01, using a pressure reduction technique. The diffusion coefficient of the hydrogen interstitials has been determined from hydrogen absorption kinetics experiments. The dependence of the diffusion coefficient on the alloy content has been discussed. For Mm based MmNi3.5Al0.5Fe0.5Co 0.5 alloy, the diffusion coefficient is about an order of magnitude higher than that of the Zr based alloys. © 2005 IOP Publishing Ltd.

Sundara Ramaprabhu

Department Of Physics

Narayanaswami Mani

Absorption

Diffusion

Pressure effects

Reaction kinetics

Zirconium alloys

ABSORPTION KINETICS

Diffusion coefficients

MISCHMETALS

Pressure reduction

Hydrogen

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IIT Madras

Diffusion of hydrogen interstitials in Zr based AB2 and mischmetal based AB5 alloys

Journal of Physics Condensed Matter

The hydrogen diffusion studies in MmxTb1-xCo2-H (x = 0, 0.1 and 0.2) have been performed in the hydrogen solid solution phase (α-phase) by measuring the time dependence of hydrogen pressure during the hydrogen absorption process. The measurements were carried out between 500 and 700 °C, and pressures 50, 70 and 90 mbar, respectively, and the diffusion coefficient data are described by an Arrhenius relation. Thermodynamic parameters namely partial enthalpy and the chemical potential of dissolved hydrogen have been determined from the pressure composition isotherm. The dependence of diffusion coefficient, activation energy and the chemical potential of the dissolved hydrogen on the alloy content in MmxTb1-xCo2-H (x = 0, 0.1 and 0.2) have been discussed. © 2006.

Journal of Alloys and Compounds

Interstitial hydrogen diffusion in MmxTb1-xCo2-H (x = 0, 0.1 and 0.2)

The diffusion of hydrogen in the tetrahedral interstitials of bulk spherical Laves phase ZrMn0.85Cr0.1V0.05Fe0.5Ni0.5 and ZrMn0.85Cr0.1V0.05Fe0.5Ni0.5 + 1 wt% B alloys has been studied in the α-phase (solid-solution) region over the temperature range 450-650 °C, for hydrogen pressures up to 100 mbar using Sieverts-type apparatus. The diffusion constants have been determined from the gas-solid reaction, where the gas pressure dependence on time has been measured at fixed temperature. The results have been discussed on the basis of Fick's law of diffusion. The dependence of diffusion constant on alloy composition and initial pressure has been evaluated. Activation energy has been obtained from the temperature dependence of diffusion using Arrhenius relation. © 2007.

Hydrogen diffusion studies in Zr-based Laves phase AB2 alloys

The diffusion of hydrogen in cubic (C15-type) Laves phase Ho1 - x Mmx Co2 (x = 0, 0.2 and 0.4, Mm = mischmetal) alloys was measured in the α-phase (solid solution) region over the temperature range of 500 - 800 {ring operator} C using Sieverts-type apparatus. The alloys and its hydrides have been characterized by powder X-ray diffraction (XRD) Rietveld method. The diffusion constants have been determined from the gas-solid reaction, where the gas pressure dependence on time has been measured at fixed temperature. The results have been discussed on the basis of Fick's law of diffusion. The dependence of diffusion constant on alloy composition and initial pressure has been evaluated. Activation energy is obtained from the temperature dependence of diffusion using Arrhenius relation. © 2007 International Association for Hydrogen Energy.

International Journal of Hydrogen Energy

Diffusion of hydrogen in cubic Laves phase Ho1 - x Mmx Co2 (x = 0, 0.2 and 0.4) alloys

Hydrogen absorption isotherms for the CaCu5 hexagonal structured alloys MmNi3.8Al0.4Fe0.4Co 0.4, MmNi3.5Al0.5Fe0.5Co 0.5, MmNi3.4Mn0.4Al0.4Fe 0.4Co0.4, MmNi3.368Mn0.333V 0.333Al0.3Fe0.333Co0.333 and MmNi3Mn0.333V0.333Al0.333Fe 0.333Co0.333Cu0.333 with Fe and Co in the mole ratio 1:1 have been obtained in the temperature and pressure ranges 30 ≤ T/°C ≤ 100 and 0.1 ≤ P/bar ≤ 30 using a high pressure unit based on the pressure reduction method. The powder X-ray diffractograms of the unannealed alloys show the formation of single phase. The lattice constants and the unit cell volume of these alloys increase with substitution of Mn, V, Cu, Fe, Al and Co at the Ni site in MmNi5. The hydrogen absorption isotherms show the presence of a single plateau region (α+β) in the temperature and pressure ranges studied. The hydrogen absorption studies show no marked change in the plateau slope due to the presence of Fe and Co in 1:1 mole ratio. The maximum hydrogen intake capacity (r = nH/n f.u.) is around 4.75 in MmNi3.8Al0.4Fe 0.4Co0.4 at 30 °C and at 30 bar. The dependence of the thermodynamics of dissolved hydrogen in these alloy hydrides on the hydrogen concentration shows the different phase regions α, α+β and β seen in the hydrogen absorption isotherms. At any particular temperature investigated, the chemical potential of dissolved hydrogen in these alloy hydrides decreases with the substitution of Mn, V, Cu, Fe, Al and Co at the Ni site, which has been correlated with the increase in the volume of the unit cell of the alloys. The desorption isotherms of MmNi 3.8Al0.4Fe0.4Co0.4 at 30 and 50 °C show that the hysteresis is very small with free energy loss per cycle about 0.4 kJ/mol H at 30 °C. The kinetics of hydrogen absorption at 30, 50, 75 and 100 °C have been studied for MmNi3.8Al 0.4Fe0.4Co0.4. The different phases identified by both kinetic and thermodynamic studies confirm those seen in the hydrogen absorption isotherms. The average activation energies Ea in the α, (α+β) and β phases are found to be 0.38 eV, 0.32 eV and 0.054 eV, respectively. The diffusion coefficient at 303 K in MmNi 3.8Al0.4Fe0.4Co0.4-H is about 2.55 × 10-10 cm2 s-1. The powder X-ray diffractograms of the alloy hydrides show that these alloys do not undergo any structural transformation upon hydrogenation. © 2003 Elsevier B.V. All rights reserved.

Effect of substitutional elements on hydrogen absorption properties in Mm-based AB5 alloys

Hydrogen absorption pressure-composition isotherms have been investigated for the ZrMnFe1 - xNix (x=0.2, 0.4, 0.5 and 0.6) alloys having the C14 type hexagonal structure in the temperature and pressure ranges 30≤T≤100°C and 0.1≤P≤40 bar using a pressure reduction method. Powder X-ray diffractograms show that the lattice constants and the unit cell volume of ZrMnFe1 - xNix (x=0.2, 0.4, 0.5 and 0.6) alloys decrease with increasing Ni content. The P-C isotherms show the presence of a single plateau region (α+β) in the temperature and pressure ranges studied and it was found that the plateau pressure and plateau slope at any given temperature increase with increasing Ni content except for ZrMnFe0.5Ni0.5, where the plateau slope is found to be small due to the atomic ordering in the alloy due to presence of Fe and Ni in a 1:1 molar ratio. The desorption isotherms of ZrMnFe0.5Ni0.5 at 30 and 50°C show that the hysteresis is very small. The dependence of the thermodynamics of dissolved hydrogen in ZrMnFe1 - xNix-H (x=0.2, 0.4, 0.5 and 0.6) systems in the temperature range 30-50°C on the hydrogen concentration have shown the existence of α, α+β and β phase regions as seen in the isotherms. The powder X-ray diffractograms of the alloy hydrides show that these alloys do not undergo a structural transformation upon hydrogenation. In ZrMnFe0.5Ni0.5, the kinetics of hydrogen absorption during activation and after several cycles at 30 °C have been studied and compared. © 2002 Elsevier Science B.V. All rights reserved.

Hydrogen storage properties of ZrMnFe1 - x Nix (x=0.2, 0.4, 0.5 and 0.6) alloys

Hydrogen absorption isotherms have been obtained for the C14-type hexagonal-structured alloys ZrMnFe1-xCox (x = 0.2, 0.4, 0.5 and 0.6) in the temperature and pressure ranges 30 ≤ T/°C ≤ 100 and 0.1 ≤ P/bar ≤ 40 using a high-pressure unit based on the pressure reduction method. The powder x-ray diffractograms show that the lattice constants and the unit-cell volume for ZrMnFe1-xCox (x = 0.2, 0.4, 0.5 and 0.6) alloys decrease with increase of XCo. The hydrogen absorption isotherms show the presence of a single plateau region (α + β) in the temperature and pressure ranges studied and it is found that the plateau pressure and plateau slope at any given temperature increase with increase of the XCo-Content, except in the case of ZrMnFe0.5Co0.5 wherein the hydrogen absorption plateau slope is found to be smaller due to the ordering of the alloy. The dependence of the thermodynamics of the dissolved hydrogen in ZrMnFe1-xCox-H (x = 0.2, 0.4, 0.5 and 0.6) systems in the temperature range 30-50 °C on the hydrogen concentration shows the different phase regions (α, α + β, β) seen in the hydrogen absorption isotherms. The powder x-ray diffractograms of the hydrides of ZrMnFe0.8Co0.2 show that the formation of different phases is manifested in the isotherms without there being any structural change and the inclusion of hydrogen in the interstitial sites expands the unit-cell volume by about 21%. Comparison of the hydrogen absorption kinetics of ZrMnFe1-xCox with that of the corresponding Ni alloys in similar experimental conditions shows that the kinetics is slow in the ZrMnFe1-xCox alloys.

Journal	Journal of Physics Condensed Matter
ISSN	09538984
Open Access	No