Dünne Palladium-Wasserstoff-Schichten als Modellsystem: Thermodynamik struktureller Phasenübergänge unter elastischen und mikrostrukturellen Zwangsbedingungen

Abstract

Using the formation of a hydride phase in palladium-hydrogen thin films with 5 nm to 2000 nm thickness as a model, this work investigates the impact of film thickness as well as microstructural and elastic constraints on the thermodynamics of 1st order phase transitions. Hydrogen absorption leads to volume expansion of the palladium lattice, increasing abruptly with hydride formation due to the concentration gap ∆c_H. The palladium films’ hydrogen induced lattice expansion results in large mechanical stress gradients at the film-substrate interface as well as at inner boundaries such as phase- and grain boundaries. These yield an additive contribution to the chemical potential μ_H of hydrogen, modifying the stability of the hydride phase. The film’s microstructure alters the stress impact on the chemical potential, providing different hydrogen absorption sites with a spectrum of site energies, and determining possible channels of stress relaxation by plastic deformation. This study separates the interfering contributions of microstructure and stress on the thermodynamics of hydride formation experimentally, and – referring to thermodynamic model assumptions – quantifies resulting deviations of thin film thermodynamic parameters from that of the respective bulk system. The hydrogen loading of palladium films with nano crystalline, multi-oriented and epitaxial microstructure prepared by choosing proper preparation conditions was carried out in steps. H-induced changes of the films’ stress state, hydride formation and plastic deformation were determined in-situ with XRD, measuring the substrate curvature, electrical resistivity and acoustic emission as well as with STM and proton-proton scattering analysis. The study identifies cascades of critical film thicknesses and stresses for hydrogen induced plastic deformation. Already in the regime of elastic film expansion discrete stress relaxation events were observed, and attributed to the movements of defects pre-existing in the films. Below a film thickness of 22-34 nm a new type of partial coherent phase transition was found, with phase boundaries remaining coherent throughout the complete phase transition. Resulting from the films’ different microstructures and different stress states a significant reduction of the attractive H-H interaction – the phase transition’s driving force – by 20-50 % with respect to bulk was observed. For the films E_HH is between 15 and 30 kJ/mol_H, compared to bulk with E_HH = 36.8 kJ/mol_H. The elastic contribution to the reduction of the H-H interaction amounts to 2-5 kJ/mol_H. It rapidly increases for films with partially coherent transformation. The respective enthalpies of hydride formation in thin films are higher up to 3 kJ/mol_H. This difference of the enthalpies drives films locally relaxed by the formation of buckles to spatially decompose into the hydride phase in the buckles and the solid solution α-phase in the remaining clamped (stressed) film fraction. Furthermore, the study shows that Pd-H thin films occasionally reveal sloped pressure-concentration isotherms in the two-phase field due to non-linear film stress. This observation brings about a modification of the boundary conditions determining the critical temperature of hydride formation. The increment ∂μ_H/∂c_H | T=T_c needs to be considered quantitatively. The resulting critical temperatures of the Pd-H thin films are reduced by up to 40 % compared to bulk. T_c is 340 K to 490 K for the films, compared to bulk with T_c = 563 K. For all films phase separation is still found at 300 K. In general, the observed modifications of thin film thermodynamics were directly attributed to the films’ microstructures and stress states, while finite-size effects solely scaling with film thickness are of minor importance.