Abstract
We analyzed the effect of the interfacial free energy on the thermodynamics of hydrogen sorption in nano-scaled materials. When the enthalpy and entropy terms are the same for all interfaces, as in an isotropic bi-phasic system, one obtains a compensation temperature, which does not depend on the system size nor on the relative phase abundance. The situation is different and more complex in a system with three or more phases, where the interfaces have different enthalpy and entropy. We also consider the possible effect of elastic strains on the stability of the hydride phase and on hysteresis. We compare a simple model with experimental data obtained on two different systems: (1) bi-phasic nanocomposites where ultrafine TiH2 crystallite are dispersed within a Mg nanoparticle and (2) Mg nanodots encapsulated by different phases.
Highlights
Nanoparticles are halfway between atomic world and bulk world
Material’s properties change significantly when system size becomes comparable to a characteristic length scale, such as the mean free path of particles or excitations that carry charges, energy and momentum
The thermodynamics of nano-scaled systems is strongly altered with respect to bulk material and size-dependent effects [1,2], becoming more and more complicated when dealing with compounds and heterogeneous ones [3]
Summary
Nanoparticles are halfway between atomic world and bulk world. A whole new and non-trivial variety of effects arise whenever a system is pushed towards the atomic scale. The high density of interfaces confers to nano-systems unique physical and chemical properties, and promotes their chemical reactivity. The thermodynamics of nano-scaled systems is strongly altered with respect to bulk material and size-dependent effects [1,2], becoming more and more complicated when dealing with compounds and heterogeneous ones [3]. Far from putting a limit to the exploitation of nanoparticles, this fascinating behavior allows for the tailoring of material properties by designing unique functional devices at nanoscale. This work aims to discuss hydride formation in composite nanomaterials, relating peculiar H-sorption properties to confinement effects [4] and to the physical properties of interfaces [5]
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