Phase transitions in Lnx(1) Ln1−x(2) H2-H3 alloy-hydrogen systems (original) (raw)
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Modelling of phase equilibria in metal–hydrogen systems
Journal of Alloys and Compounds, 2003
The paper presents a model quantitatively describing phase equilibria in metal–hydrogen systems. It is based on a formal consideration of interstitial hydrogen dissolved in the metal matrix as a van der Waals lattice gas. The model describes the asymmetry of the experimental ‘pressure–composition’ isotherms. It postulates an existence of local fluctuations in the stoichiometric composition of the alloys, causing an appearance of statistical deviations of the correlated values of entropy and enthalpy from their corresponding mean values. The model describes temperature-dependent plateau slopes and smooth transitions between α-, (α+β)- and β-regions. It was applied in the approximations of both single (LaNi4.8Sn0.2–H2) and double plateau (TiCr1.9–H2) experimental PCT-diagrams.
Acta Materialia, 2021
The applicability of an alloy as a hydrogen storage media mostly relies on its pressurecomposition-temperature (PCT) diagram. Since the PCT diagram is compositiondependent, the vast compositional filed of high entropy alloys, complex concentrated alloys or multicomponent alloys can be explored to design alloys with optimized properties for each application. In this work, we present a thermodynamic model to calculate PCT diagrams of body-centered (BCC) multicomponent alloys. The entropy of the phases is described using the ideal configurational entropy for interstitial solid solutions with site blocking effect. As a first approximation, it is assumed that the H partial molar enthalpy of a phase is constant, so the enthalpy of H mixing varies linearly with the H concentration. Moreover, the H partial enthalpy of a phase for a multicomponent alloy was approximated by a simple ideal mixture law of this quantity for the alloy's components with the same structure. Experimental data and DFT calculations were used for parametrization of the enthalpy terms of eight elements (Ti, V, Cr, Ni, Zr, Nb, Hf, and Ta), which are the components of the alloys tested in this work. Experimental PCTs of six BCC multicomponent alloys of four different systems were compared against the calculated ones and the agreement was remarkable. The model and parameters presented here can be regarded as a basis for developing powerful alloy design tools for different hydrogen storage applications.
Inter‐Dependency Relationships in High‐Entropy Alloys: Phase Stability Criteria
Advanced Engineering Materials, 2019
In High Entropy Alloys (HEAs), the thermodynamics govern alloy formation, phase stability and overall phase transition characteristics. Over the past decade, many thermodynamic parameters have been proposed for predicting the stability of HEA systems. In this work, analysis of these parameters was carried out carefully with the available data from the literature. It was found that all these parameters are not fully independent from each other and there exists a bridge of interconnection between them. Thus, in this work, inter-dependency relationships between the thermodynamic parameters were derived for predicting the stability of solid solution in HEAs. It is found that parameters with large and complex equations are not essentially required; instead the simple parameters , Ω and δ are sufficient for predicting the stability of HEA systems.
Measurement of thermodynamic properties of some hydrogen absorbing alloys
International Journal of Hydrogen Energy, 2009
In this paper, the effect of hydrogen concentration on the reaction enthalpies of some metal hydride alloys during hydriding and dehydring is presented. Pressure-concentration-temperature characteristics of the metal hydride alloys are measured under nearly isothermal condition during both absorption and desorption. Reaction enthalpies and entropies of LaNi 5 , LaNi 4.7 Al 0.3 , LmNi 4.91 Sn 0.15 , Ti 0.99 Zr 0.01 V 0.43 Fe 0.09 Cr 0.05 Mn 1.5 and MmCo 0.72 Al 0.87 Fe 0.04 Ni 3.91 are estimated by constructing van't Hoff plots at different hydrogen concentrations. It is observed that the effect of hydrogen concentration on reaction enthalpies is more significant for the alloys having larger plateau slopes. At the initial stage of hydrogenation, metal hydrides are found to have larger reaction enthalpies which decrease gradually by about 5-15% at the end of the hydrogen absorption. At any given temperature, desorption enthalpies of LaNi 5 , LmNi 4.91 Sn 0.15 , MmCo 0.72 Al 0.87-Fe 0.04 Ni 3.91 , LaNi 4.7 Al 0.3 and Ti 0.99 Zr 0.01 V 0.43 Fe 0.09 Cr 0.05 Mn 1.5 are found to be higher by about 5, 8, 10, 28 and 32% than their respective absorption enthalpies. Reaction enthalpies of the selected metal hydride alloys are expressed as a function of hydrogen concentration by a fourth order polynomial equation obtained from fitting with the experimental data.
Determination of the transition to the high entropy regime for alloys of refractory elements
Journal of Alloys and Compounds, 2012
The development of high entropy alloys is currently limited to experimental work aimed at the determination of specific compositions that exhibit particular properties. The main feature of these alloys is their particular phase structure, which tends to be a continuous solid solution in spite of the large number of constituents which would otherwise form a large number of intermetallic phases. While it is known that equimolar concentrations and large number of elements are two necessary conditions for achieving high entropy behavior, not much is known regarding the transition to this regime in the presence of specific elements. Such knowledge would be useful when determining alloy compositions, as it would set boundaries for the necessary concentrations of each element in experimental situations. In this work, results of a computational modeling effort are presented, where a recently developed 5-element high entropy alloy of refractory elements is used as the foundation needed to examine such transition and determine the necessary lower bounds for the concentration of each element. Details of the phase structure of the quaternary combinations of W, Nb, Mo, Ta and V as they evolve upon the addition of a fifth element are discussed. The results are compared to the experimental case for the case of V added to W-Nb-Mo-Ta. Using these examples as a reference, the concept of critical concentrations for each element, signaling the transition to the high entropy regime, is developed, based on a simple analysis of only bulk properties of such alloys (lattice parameter, bulk modulus, and cohesive energy).
Int J Hydrogen Energy, 2008
Hydrogen storage properties of the La 0.23 Ni 0.34 Co 0.33 Nd 0.08 Ti 0.01 Al 0.01 have been systematically studied in the present work. Pressure composition (P .C.T ) isotherm has been investigated in the temperature and pressure ranges of 308 T 338 K and 10 P 140 psi, respectively. The studies show the presence of two single and phases with one mixed + phase. The maximum H/M ratio was found to be around 0.94 at 308 K and 90 psi. The introduction of interstitial hydrogen particles into a metallic lattice causes several changes in respect to its electrical resistance. Hence an attempt has been made to relate change in the resistance of material with the content of hydrogen. The resistance of the specimen was found to increase with increasing H/M. Thermodynamical parameters viz., the relative partial molar enthalpy ( H ) and relative partial molar entropy ( S) of dissolved hydrogen are found to be 14.50755 ± 0.8 K J/mol H 2 and 47.01152 ± 2.42 J/K/mol H 2 , respectively. The variation of enthalpy and entropy with hydrogen concentration has also been calculated to confirm the existence of different phases. ᭧
1984
Phase diagrams for representing complete equilibrium (CE) and paraequilibrium (PE) in ternary A-B-H systems are presented and discussed with special reference to the Ti-Cu-H system. Transitions from PE to CE resulting from hydriding-dehydriding cycles can be understood with the aid of the superimposed phase diagrams. Local equilibrium, which is an intermediate stage between CE and PE, gives rise to a new form of hysteresis, chemical hysteresis, in alloy-hydrogen systems. In these alloy systems chemical hysteresis will supplement plastic hysteresis, usually present in all low temperature transitions in metal-hydrogen systems.
Phase separation of metallic hydrogen-helium alloys
Physical Review B, 1977
Calculations are presented for the thermodynamic functions and phase separation boundaries of solid metallic hydrogen-helium alloys at temperatures between 0°K and 19,000°K and at pressures between 15 and 90 megabars. Expressions for the band structure energy of a randomly disordered alloy (including third order in the electron-ion interaction) are derived and evaluated. Short-and long-range order are included by the quasi-chemical method, and lattice dynamics in the virtual crystal harmonic approximation. We conclude that at temperatures below 4,000°K there is complete phase separation of hydrogen-helium alloys, and that a miscibility gap remains at the highest temperatures and pressures considered. The relevance of these results to models of the deep interior of Jupiter is briefly discussed.