Abstract
Metal-hydrogen systems offer grand opportunities for studies on fundamental aspects of alloy thermodynamics. Palladium-hydrogen (Pd-H) thin films of nano crystalline, multi-oriented and epitaxial microstructures, electrolytically charged with hydrogen, serve as model systems. In these films thermodynamics of hydrogen absorption is modified by interface effects related to mechanical stress and to microstructural defects. Since in this respect hydrogen can be utilized to reveal the microstructural constituents of the films, we aim to investigate the distribution of sites (DOS) hydrogen occupies in the films’ solid solution regime. A σDOS model is proposed, taking the measured substrate-induced stress contribution to the chemical potential into account. This enables the determination of the different sites’ volume fractions and of pure site energy distributions by fitting measured isotherms. Interstitial sites, grain/domain boundary sites and deep traps are distinguished. Dislocations and vacancies are shown to have a minor impact on the films’ trapping of hydrogen atoms, while deep traps are related to the films’ surface. Enhanced binding energies in nano crystalline films can be ascribed to the tensile strain effect of grain boundaries acting on the grains. Measured surface trapping energies fit to the respective bulk values, while the trapping of hydrogen in grain/domain boundaries of the films is significantly increased. This can be interpreted with different grain/domain boundary structures. Different from octahedral interstitial site occupation, tetrahedral site occupation is suggested for grain/domain boundaries of the films.
Highlights
Metal-hydrogen systems offer grand opportunities for studies on fundamental aspects of alloy thermodynamics [1,2,3,4,5,6,7,8,9,10,11,12,13,14]
Palladium-hydrogen (Pd-H) thin films of nano crystalline, multi-oriented and epitaxial microstructures, electrolytically charged with hydrogen, serve as model systems. In these films thermodynamics of hydrogen absorption is modified by interface effects related to mechanical stress and to microstructural defects. Since in this respect hydrogen can be utilized to reveal the microstructural constituents of the films, we aim to investigate the distribution of sites (DOS) hydrogen occupies in the films’ solid solution regime
There, the original, uncorrected data of the 80 nm nano crystalline Pd-H thin film is AIMS Materials Science plotted together with the fit according to the quasi-thermodynamic approach of Eq 4 and with the fitted line according to Eq 14
Summary
Metal-hydrogen systems offer grand opportunities for studies on fundamental aspects of alloy thermodynamics [1,2,3,4,5,6,7,8,9,10,11,12,13,14]. The interaction of solutes with structural defects of the metal can be assessed [2,6,15,16,17,18], as well as precipitation and growth of phases [5,9,10,19,20,21] This is enabled by two unique properties of metal-hydrogen systems: First, the chemical potential of hydrogen in the metal can be controlled by the surrounding hydrogen gas pressure or by the electromotoric force in electrochemical loading experiments. Reducing the alloy size to the nano scale, the volume fraction of interfaces increases This changes the systems properties due to interface effects, which relate to microstructural defects as well as to mechanical stress [6,8,10,11,18,19,22,23,24]. Hydrogen can be utilized as a probe for the systems’ microstructure
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