Harald Züchner

Hydrogen diffusion in palladium based f.c.c. alloys

The macroscopic hydrogen diffusion coefficient D in homogeneous Pd1−xAgx alloys obtained by electrochemical current pulse time-lag measurements shows for small hydrogen concentrations a strong dependence on the alloy composition. For small silver concentrations up to 25% the diffusion coefficient remains nearly constant. Then it falls off drastically by about three orders of magnitude reaching a minimum value at 60% silver content before increasing again with rising silver content directly towards the value for pure silver, which is nearly the same as for Pd. The results could be fitted satisfactorily for the complete range of alloy composition by Monte Carlo simulations on the basis of a simplified model. In this model two different octahedral sites are assumed to exist showing different hydrogen occupation probabilities (i.e. different hydrogen solubilities). At smaller silver contents the silver atoms partly block the energetically favored diffusion paths in the Pd matrix leading to a round-about way diffusion. At high silver concentrations, Pd atoms act as traps for hydrogen in a silver matrix. The interplay of the hydrogen transport via two kinds of occupation sites with different hydrogen solubilities determines the shape of the curve for the macroscopic diffusion coefficient as a function of the alloy composition. If Ag is substituted by Ni or Cu an analogous behavior is observed for the f.c.c. phase region. When alloying V or Nb to Pd — both metals have a higher hydrogen solubility compared to Pd — these metals act as traps for hydrogen in the Pd matrix similar to Pd in a silver matrix.

Hydrogen-diffusion in Pd1−xAgx (0⩽x⩽1)

Abstract By further improvement of the electrochemical pulse time-lag technique it has now become possible to measure the hydrogen diffusion coefficient on the silver rich side of the series of PdAg-alloys, despite the extremely low hydrogen solubility in pure silver and the silver rich alloys. The measured macroscopic diffusion coefficient of hydrogen in Ag reaches values comparable to the diffusion in pure Pd, but is, by orders of magnitude, higher than in the Pd0.5Ag0.5 alloy. In order to describe this the diffusion process has been regarded theoretically from a microscopical point of view. This process has been simulated by computer calculations assuming hydrogen as a random walker in a Pd matrix (with high hydrogen solubility) where silver is distributed statistically, blocking the hydrogen diffusion paths to a great extent due to the extremely small hydrogen solubility. The calculated mean square distance shows an analogous behavior with increasing silver content as the measured macroscopic diffusion coefficient. This means that hydrogen diffusion through PdAg alloys takes place preferably by a roundabout way via “Pd-rich diffusion paths” avoiding Ag-rich regions.

The existence of more than one jump process of hydrogen in palladium-silver alloys — an NMR study

The hydrogen dynamics in the α-phase of Pd1−xAgxHy has been studied by proton nuclear magnetic resonance (NMR) measurements. Jump frequencies of the hydrogen atoms in the metal lattice have been deduced from the dipolar spin-lattice relaxation rates Γ1d by applying the BPP model. In PdHy, the temperature dependence of Γ1d is well described by a single jump process for all hydrogen atoms. In contrast, in the palladium–silver alloys Pd1−xAgxHy, the Γ1d data indicate more than one jump process with different activation energies. Two and three processes could be separated for x=0.1 and x=0.3, respectively. In both alloys, the diffusion process with the lowest activation energy (Ea=226 meV) is comparable to that in pure palladium. The jump processes with higher activation energies occur more frequently in Pd0.7Ag0.3 than in Pd0.9Ag0.1. This indicates higher energy-barriers for jumps between O-sites in silver-rich environments. Direct measurements of the long-range diffusion coefficients D were performed by pulsed-field-gradient (PFG)-NMR at temperatures up to 450 K. For both Pd1−xAgxHysamples, D(T) is well represented by a single Arrhenius law with the diffusion parameters D0=3.0×10−7 m2 s−1 and Ea=230 meV for x=0.1 and D0=2.1×10−7 m2 s−1 and Ea=260 meV for x=0.3. The diffusivities calculated from the jump frequencies of the hydrogen atoms are in good agreement with these PFG results and also with measurements using the time-lag technique.

Chemical structure and bonding characteristics of metal hydrogen systems studied by the surface analytical techniques SIMS and XPS

Abstract Secondary ion mass spectrometry (SIMS) as well as photoelectron spectroscopy (XPS) are powerful tools for studying special properties of metal hydrogen systems and the interaction of hydrogen and metals. SIMS experiments have now also been extended to transition metal hydrogen systems with small hydrogen solubilities by using intelligent mass spectra accumulation. As known from studies on other metal hydrogen systems (V–H, Nb–H etc.) the cluster ion Me 2 H + is particularly characteristic for transition metal hydrogen systems. In AB 5 -type alloy hydrogen systems a quite different behavior is observed. We have focussed our attention on studying the properties of the LaNi 5 -alloy with nickel partially substituted by Al or Ag. The mass spectra, especially the negative ones, show a strong Ni–H, a weaker Ag–H and almost no Al–H bond, which explains the decrease in hydrogen storage capacity when going from pure LaNi 5 to LaNi 5− X Ag X and LaNi 5− X Al X . Pressure–concentration like isotherms are obtained for the LaNi 5− X Al X D Y system by SIMS with highest spectral purity when applying a synchronous in situ gasvolumetric hydrogen charging procedure during the SIMS analysis under Ar + ion bombardment. XPS studies on metal hydrogen systems yield valuable information concerning chemical and physical properties, which are complementary to the SIMS results. A hydrogen induced chemical shift and changes in the line shape of the valence band and the core electron levels are observed in LaNi 5 H Y photoelectron spectra if H 2 + ion implantation is used as an alternative hydrogen charging technique. The XPS results indicate the existence of a Ni–H bond in excellent agreement with the SIMS measurements.

Ewald Wicke and his work on metal-hydrogen systems

Abstract Ewald Wicke born in Wuppertal, Germany, on August 17 th 1914 died on March 7 th 2000 in Munster, Germany after a fully scientific life. He was one of the pioneers in the field of metal hydrogen systems and he has formed the community of metal ‘hydriders’ by his manifold activities as an all-round physical chemist and scientist, as an organizer, a busy editor and a sound discussion partner.

SIMS investigations on the system NbHn(Dn) (0 < n < 1)

Abstract SIMS measurements have been carried out to study the emission behavior of hydrogen/deuterium specific secondary ions from NbH n (D n ) samples as a function of the hydrogen/deuterium content for 0 n The detection sensitivity of hydrogen/deuterium is extremely high, especially at small concentrations. For all concentrations, the Nb 2 H + (D + ) ion is the most intense hydrogen/deuterium specific signal when bombarding with inert gas ions (Ar + or Kr + ). The interpretation of the SIMS spectra obtained for high hydrogen/deuterium contents is obviously facilitated by the fact that the ion beam bombardment does not influence the surface composition, as is the case for samples with small concentrations, where an ion bombardment induced segregation of hydrogen/deuterium to the surface leads to a hydrogen/deuterium enrichment in the near-surface region.

Multi-layer problems in time-lag measurements applied on metal–hydrogen systems

Time-lag techniques are powerful tools to determine diffusion coefficients of hydrogen in metals. The quality and reliability of time-lag measurements depends strongly on an exact consideration of the experimentally established boundary conditions in the mathematical evaluation of the measured time-lag curves. This also means that a change of inner and/or outer boundary conditions can lead to another type of time-lag experiment with a different solution for the diffusion equation. In numerous cases the metal foil is coated by surface layers possessing hydrogen solubilities and diffusivities different from that of the enclosed metal. The surface layers determine the boundary conditions at the interface between the metal and the surface layer. In these cases solutions of the diffusion equation for multi-layer problems have to be found. Complete analytical solutions normally do not exist; however, numerical calculations allow to solve the problems reliably. We present results from those calculations for double (AB) and triple (ABA) layer systems for cases where A or B controls the time-lag, but also for the situations in between these two extreme cases. The simulation also describes the transition from one type of time-lag experiment to another one.