The structural changes of the Laves phases pseudo-binary (Ho1-xAx)Co2 alloys (A = Mm and Ti) upon hydrogen absorption have been investigated by means of powder X-ray diffraction (XRD) and differential scanning calorimetry (DSC). The hydrogenation of alloys at different hydrogenation pressure and temperature has been performed using Sieverts-type apparatus. The effect of dehydrogenation and formation of hydride phases on the structural properties have been studied and results are interpreted with respect to their unhydrogentaed alloys. The pressure and temperature-dependent hydrogen-induced transformation from crystalline to amorphous and decomposition into constituents have been studied and discussed. The preferential occupancy of tetrahedral interstitial sites for hydrogen and relative stability of crystalline, amorphous and elemental hydrides have been reported. Further, the effect of Mm and Ti substitution upon amorphization conditions has been discussed. © 2007.

Sankaranarayanan V

Department Of Physics

Sundara Ramaprabhu

Binary alloys

Hydrogenation

Phase stability

Phase structure

X ray powder diffraction

AMORPHIZATION CONDITIONS

ELEMENTAL HYDRIDES

HYDROGEN-INDUCED AMORPHIZATION

TETRAHEDRAL INTERSTITIAL SITES

Hydrogen

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

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IIT Madras

Journal of Alloys and Compounds

The hydrogen absorption behavior of Laves phase Ho1-xTixCo2 (x=0.1-0.6) alloys has been investigated by pressure-concentration (PC) isotherms and cyclic-, temperature- and pressure-dependent absorption kinetics. The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.01-1 bar and temperature range 50-200 °C using Sievert's-type apparatus. The drastic changes in the induction period and particle size during the activation process have been discussed based on the kinetics of repeated hydrogenation cycles and scanning electron microscopy (SEM) images of the hydrides at different hydriding cycles, respectively. The experimental results of kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model, and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions in Ho1-xTixCo2-H have been identified from the different slope regions of the first-order-type kinetic plots. The dependence of the reaction rate parameter on hydriding pressure and temperature in the (α+β)-phase region has been discussed. © 2007.

Journal of Physics and Chemistry of Solids

Thermodynamic and kinetic properties of Ho1-xTixCo2-hydrogen system

The effect of hydrogen absorption-desorption on the structural properties of Laves phase Dy1-xMmxCo2 (x = 0.1, 0.3 and 0.5; Mm = mischmetal, a natural mixture of the light rare earth metals containing 50 wt% Ce, 35 wt% La, 8 wt% Pr, 5 wt% Nd and 1.5 wt% of other rare earth elements and 0.5 wt% Fe) alloys has been investigated by means of hydrogen absorption-desorption pressure-composition (PC) isotherms, kinetics of hydrogen absorption and powder x-ray diffraction (XRD). The PC isotherms and kinetics of hydrogen absorption have been studied in the pressure range 0.001-1 bar and temperature range 50-200 °C using Sieverts-type apparatus. The experimental results of the kinetic curves are interpreted using the Johnson-Mehl-Avrami (JMA) model and the reaction order and reaction rate have been determined. The α-, (α+β)- and β-phase regions have been identified from the different slope regions of the PC isotherms and first-order type kinetic plots. The dependence of the reaction rate parameter upon hydriding pressure and temperature in the (α+β)-phase region has been discussed. The effect of hydrogenation pressure, temperature and Mm concentration on the hydrogen-induced transformation from crystalline Dy1-xMm xCo2-H to amorphous Dy1-xMmxCo 2-H and decomposition into crystalline (Dy, Mm)H2 and Co have been discussed in detail. Further, the effect of dehydrogenation on the recovery of the crystalline Laves phase structure of Dy1-xMm xCo2 from its decomposed state is presented. This hydrogenation-disproportionation-desorption-recombination (HDDR) process can be conveniently used in powder metallurgy. © 2008 IOP Publishing Ltd.

Journal of Physics Condensed Matter

Influence of hydrogen absorption-desorption on structural properties of Dy1-xMmxCo2 alloys

Thermodynamic and hydrogen-induced structural properties of hydrogenated Ho1-xMmxCo2 (x ≤ 0, 0.1, 0.2, 0.3 and 0.4, Mm ≤ mischmetal) alloys have been investigated. The hydrogen absorption pressure-composition isotherms were studied in the temperature range 50-200 °C and pressure range 0.001-1 bar using a Sieverts-type apparatus. The effect of Mm on the equilibrium plateau pressure and hydride stability has been discussed. Structural analyses and hydrogen-induced amorphization of Ho 1-xMmxCo2hydrides were performed using powder x-ray diffraction. The structural stability of these alloys upon repeated hydrogenation and dehydrogenation cycles has been investigated. At lower hydriding pressure and temperatures, Ho1-xMmxCo 2-hydrides retain their Laves C15 structure; however, the large lattice volume expansion about 24% is observed in the β-phase. The effect of hydrogenation pressure and temperature and Mm concentration on the hydrogen-induced transformation from crystalline Ho1-xMm xCo2-H to amorphous Ho1-xMmxCo 2-H and recrystallization into elemental hydrides have been studied and discussed. © 2007 IOP Publishing Ltd.

Journal of Physics D: Applied Physics

Thermodynamic and hydrogen-induced structural properties of Ho 1-xMmxCo2-hydrides

Hydrogen absorption pressure-composition isotherms were determined for the intermetallic compounds Zr1-xHoxCo2 (x=0.4, 0.6 and 0.8) belonging to the C-15-type Laves phase structure in the temperature range 50-200 °C at pressures up to 40 bar. The maximum hydrogen concentration (number of hydrogen atoms per formula unit) increases from 0.1 to 3.8 as the holmium concentration (x) is increased from 0 to 1. The ratio of the number density of hydrogen atoms in the alloy to the density of liquid hydrogen lies in the range 0.1-1.24. The absorption of hydrogen is accompanied by a maximum of 19% expansion in the unit cell volume of the host without any change in the C-15-type Laves phase structure. For higher values of x the materials are highly distorted when the hydrogen concentration exceeds about 3.0 and tend towards the amorphous state. The relative partial molar enthalpy (ΔHH) and entropy (ΔSH) of hydrogen absorption in these materials are found to lie in the range 7-43 kJ (mol H)-1 and 10-115 J K-1 (mol H)-1 respectively. The hydrogen absorption kinetics recorded at different temperatures for all the compositions indicate that the kinetics are fast for lower x values. © 1993.

Hydrogen absorption characteristics of the Zr-xHoxCo2 system in the pressure range 0-40 bar

Pressure dependence of hydrogen-induced amorphization in C14 Laves phase NdMn2

Hydrogen pressure dependence of structural changes in C15 Laves phase DyMn2 studied by pressure differential scanning calorimetry

Materials Science and Engineering: A

Pressure dependence of hydrogen-induced amorphization in C15 Laves phase ErFe2

Hydrogen-induced and A-site substitution-dependent structural properties of AB2-type (Ho1-xAx)Co2-hydrogen system (A = Mm and Ti)

Journal	Journal of Alloys and Compounds
ISSN	09258388
Open Access	No