Hydrogen in Nano‐sized Metals

Abstract

Nano-sized metal systems have received growing attention during the last decade which is not only motivated by the technological interest in low-dimensional systems providing the size reduction of electronic devices, but also by fundamental interest since physical properties tend to show interesting changes when the size of the system is reduced. So called finite-size effects become relevant as soon as the system-size is in the order of the correlation length of the system. Surface-as well as interface-effects influence strongly the system behaviour when their relative fraction compared to the volume increases. Furthermore, often substrate contributions influence the system properties. All these contributions become more relevant when the system size goes down to the nanometer-range. This range is, especially with regard to applications, a major field of interest: low-dimensional patterning of electronic devices still can increase the information density or might reduce the size of the product of interest. On the other hand, surface related processes, for example catalysts demand for nano-sized materials providing a large amount of active surface area. Physical properties of these systems, for example phase boundaries and lattice structures, tend to be quite different from those of the large size ªbulkº system. Therefore, the understanding of general rules or trends occurring at minimisation of metal-systems is an important problem that still needs to be solved.Nano-sized metal systems are mostly found in the shape of thin films, rods or clusters. Also stacking of nano-sized systems in so-called multi-layers causes interesting sample morphologies with a high density of interface volume (see Fig. 1).The physical properties of such nano-sized systems can be studied by choosing systems with a highly mobile alloying partner: the metal-hydrogen systems. The big advantage of such a system is that alloying is possible just by increasing the partial hydrogen-gas pressure (or chemical potential). According to Sieverts the concentration c H (given by the number of H-atoms per metal-atoms H/M) inside a metal is connected with the gas-pressure p H2 expressed by the solubility S 0 , c H =S 0 Öp H2 . [1] This equation holds for the low-concentration-range for all metals at the most up to the concentration where a hydride is formed. This concentration is called the solubility limit. The solubility S 0 varies significantly, for example for bulk Yttrium (Y) or Niobium (Nb) it is up to 10 11 or 10 3 , respectively, down to small values of 10 ±1 for bulk Palladium (Pd) or 10 ±20 for Tungsten (W). This implies huge differences in the hydrogen concentration for different metals at the same hydrogen gas pressure (chemical potential). However, the solubility for the nano-sized system can be different compared to bulk.Because of its small size and its ability to tunnel between neighbouring interstitial lattice sites, hydrogen is highly mobile. Diffusion constants such as 3´10 ±7 cm 2 /s for Pd (at 300 K) are usual, resulting in diffusio...

Thermodynamic Properties, Hysteresis Behavior and Stress-Strain Analysis of MgH2 Thin Films, Studied over a Wide Temperature Range

“…All isotherms show very flat plateaus in both absorption and desorption. As a sloping pressure plateau in nano-sized materials is often related to kinetic problems [9,10], this is an additional proof that we indeed probe the equilibrium properties of the Mg/Ta/Pd films. It can be seen from Figure 1 that some of the desorption Pressure-Transmission-Isotherms (PTI's), namely at 333 K and 363 K show an unusual behavior-An increase of the lnTr/Tr M during a decrease of the hydrogen pressure.…”

“…The width of the plateau, (c β − c α ) in bulk systems is therefore close to 2. In thin films, due to a larger amount of grain boundaries [10] the α-and the β-phase solubility limits are usually affected by the high defect concentration, changing the actual plateau width. Indeed, recent electrochemical studies observe a substantial-2 orders of magnitude-increase of the α-phase solubility in Mg films as compared to bulk [27].…”

Achieving coherent phase transition in palladium–hydrogen thin films

“…Stress relaxation can be caused by misfit dislocation formation near the film-substrate interface or misfit dislocation loops forming at the interface of matrix and hydride precipitates [9]. Misfit dislocation formation near the substrate is not necessarily related to hydride formation; it depends on the lattice misfit between film and substrate, and on the film's grain size [12,13]. In metal-hydrogen systems, the misfit is modified by the hydrogen concentration, while the grain size determines the yield strength of the film.…”

Anisotropic etching of diamond by molten Ni particles

“…This effect is well known and relates to the melting point lowering effect of thin metal films and hydrogen diffusion into the metal. [17][18][19][20] The diameter of the resulting Ni particles is estimated to be approximately ten times larger than the initially evaporated Ni layer thickness.…”