25 - Hydrogen in Metals

Abstract

Hydrogen in metals attracts interest from scientists since many decades. Most of the interesting properties are related to the small size of hydrogen: its interstitial diffusion accompanied by quantum mechanical tunnel transport results in an extraordinary high mobility of hydrogen atoms in materials. For metals, H diffusivity may reach values as known for ions in aqueous solutions. Thus, thermodynamic equilibrium is reached within comparably short times even at room temperature. Therefore, metal–hydrogen systems are often used as model systems to study physical or chemical properties and their change with concentration (see, for example Oates and Flanagan, 1981, 1981a or Pundt and Kirchheim (2006)). In 1937, Lacher (1937) already used Pd–H (Flanagan and Oates, 1981, 1991) to study solute–solute interactions and interpreted it in the framework of a quasi-chemical approach (Lacher, 1937). The quantum mechanical tunneling as a diffusion mechanism also for atoms in solids was first discovered and discussed for hydrogen tunneling in metals (Flynn and Stoneham, 1970; Völk and Alefeld, 1975; Birnbaum and Flynn, 1976). Völk and Alefeld (1978), Zabel and Peisl (1979, 1980), and Steyrer and Peisl (1986) studied hydrogen density modulations that are related to the sample geometry; and Zabel and his colleagues, as published by Miceli et al. (1985), Uher et al. (1987), Song et al. (1996, 2000), and Uher et al. (1987), firstly used metal–hydrogen systems to study the behavior of systems with reduced dimensions and modulated hydrogen affinity. Kirchheim (1988) and colleagues extensively studied metal–hydrogen systems as representative for solute/solvent systems. The high mobility of hydrogen further allows studying the impact of defects that usually annihilate at elevated temperatures, see Gottstein (2001). It was, therefore, suggested to use hydrogen as a probe for defects (Cahn, 1990; Flanagan et al., 2001a, 2001b; Kirchheim, 2004) and perform site energy spectroscopy by gradually increasing the hydrogen chemical potential.