1,206 research outputs found
Nanoskalige Metall- Wasserstoff- Systeme
No full textSoftcover, 155 S.Softcover, 17x24Was passiert mit einem Metall, wenn es in seiner Größe reduziert wird? Welche Änderungen sind zu erwarten, was ist zu berücksichtigen? Am Beispiel der Metall-Wasserstoff-Systeme (M-H-Systeme) werden hier Antworten zu diesen Fragen gegeben. Verschiedene materialphysikalische Experimente zu dünnen Schichten, Vielfachschichten und Clustern von wenigen Nanometern Größe werden vorgestellt. Die Ergebnisse werden im Hinblick auf mechanische Spannungen und Mikrostruktur (inklusive der Oberflächen) der jeweiligen Probe diskutiert. Der wichtige Einfluss des Stabilisators auf die physikalischen Eigenschaften des nanoskaligen Systems wird belegt, indem zunächst gezeigt wird, dass die nach der linearen Elastizitätstheorie zu erwartenden mechanischen Spannungen (einige GPa!) und Dehnungen in nanoskaligen M-H-Systemen tatsächlich auftreten. Viele Systeme können diesen jedoch durch Versetzungsbildung oder Ablösung vom Stabilisator ausweichen. Der Beitrag der Mikrostruktur (Korngrenzen, Versetzungen, Oberflächen, innere Grenzflächen) auf experimentell ermittelte Phasengrenzen wird abgeschätzt und es wird nachgewiesen, dass dieser Beitrag nicht zur Erklärung der experimentellen Daten genügt. Im Falle kleinster Cluster müssen neue Strukturen berücksichtigt werden, die die Phasendiagramme dieser M-H-Systeme deutlich verändern. Im Hinblick auf die zwei materialphysikalischen Schwerpunkte (Mikrostruktur und mechanische Spannungen) werden auch ältere Daten neu diskutiert, wobei für scheinbare Konflikte Lösungen angeboten werden. Das Buch gibt daher einen breiten Einblick in mögliche Veränderungen materialphyiskalischer Eigenschaften von M-H-Systemen durch die Nanoskalierung
Hydrogen in Nano‐sized Metals
No full textSystems with small sizes show significant changes compared toy the bulk system. These changes are of major interest regarding the size reduction of technological applications. The hydrogen-metal system can be used as model alloy to study small size features: shifted phase boundaries and sloped isotherms are found and, also, new materials structures. Most features can be attributed to surface- and interface contributions as well as to mechanical stress
Double-locked nucleation and growth kinetics in Nb-H thin films
No full textThe kinetics of hydride precipitation in epitaxial Nb films are studied by means of scanning tunneling microscopy (STM) using hydrogen gas loading. Due to the clamped state of thin films, hydride formation results in strong unidirectional out-of-plane film expansion that can be easily detected with STM. Hydrides are found to initially form with cylindrical morphology, leading to typical surface topographies. Their localized expansion allows the analysis of the hydride lattice matching, which is coherent (H1) at the initial stages and semicoherent (H2) at later stages. The volume fraction of H1 and H2 precipitates changes with time. At initial stages, the coherent precipitates dominate, while at later stages semicoherent precipitates become the dominant ones. The relative occurrence of H1 and H2 is bimodal. A maximum occurrence of 30-40 nm sized H1 hydrides is found, which is related to coherency stress between the hydride and the Nb matrix hindering a further hydride growth. It is further demonstrated that for Nb-H films adhered to substrates, the system can be locked in the two-phase region of the phase diagram (here at 10(-4) Pa at about 50% of hydride). This is different from bulk Nb-H, where the complete sample transforms into a hydride when the hydride formation equilibrium pressure is exceeded. Impact parameters on the lateral hydride arrangement are studied. The impact of the Pd-island surface coating and the intrinsic dislocation network on the precipitation density and arrangement appear to be negligible. However, the substrate miscut and, thus, the surface roughness exhibit a strong influence on hydride nucleation. The H1 hydride arrangement along (111) and the directed H2 hydride growth along ( 111) are governed by the elastically soft matrix lattice orientations.Deutsche Forschungsgemeinschaft [SFB 602, A9, B12
Mechanical stress impact on thin Pd1−xFex film thermodynamic properties
No full textThermodynamic properties of thin films deviate strongly from those of bulk. The deviations are reported to originate from microstructure and from mechanical stress, whereas the contribution of both is unknown in particular. Focussing on the mechanical stress contribution and by using Pd(1-x)Fe(x)-H as a model system, it is shown that mechanical stress strongly changes phase transition pressures. The measured loading pressures shift up to 400 mbars in contrast to 18 mbars for bulk. These shifts relate to the film bonding to the substrate and can be affected by film detachment. (c) 2008 American Institute of Physics
Buckling of thin niobium-films on polycarbonate substrates upon hydrogen loading
No full textHydrogen loading is used to generate compressive stresses between a metallic film and its substrate. Above a critical hydrogen concentration the loading leads to film detachment and buckling. Hydrogen loading enables us to generate buckles in a controlled way and to derive the adhesion energy between a film and its substrate. (C) 2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All. rights reserved
Electrical resistivity and hydrogen solubility of PdHc thin films
No full textDuring hydrogen gas loading, Pd thin films exhibit an anomalous reduction of resistivity change with decreasing film thickness. In this paper we show that this effect can mainly be attributed to a stress-dependent reduction of hydrogen solubility at a given hydrogen pressure. Different stress states of the thin films result from different bonding to a rigid substrate. Strongly buckled thin films show bulk-like pressure-resistivity isotherms. The resistivity changes as a function of hydrogen concentration appear to be independent of film thickness. The apparent Sieverts' constant seems to be larger for thin films compared to bulk, and increases with cycling of the thin films. (C) 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved
Combined impact of microstructure and mechanical stress on the electrical resistivity of PdHc thin films
No full textPalladium hydrogen thin films are used as a model system to investigate the impact of microstructure and mechanical stress release on the electrical resistivity of thin film metals and alloys that undergo structural phase transitions. The results are compared with bulk resistivity models. Nanocrystalline, multi-oriented and epitaxial films are investigated, yielding initial terminal resistivities rho(infinity)(0) = 152 - 200 Omega nm. The hydrogen-related resistivity changes of epitaxial films are shown to approach the predicted a-phase bulk increment Delta rho(H)/Delta c(H) = 451 Omega nm, while hydrogen trapping in nanocrystalline films strongly reduces the resistivity response. In the two-phase region, the resistivity is shown to be modified by the steric distribution and geometry of the hydride precipitates, yielding different proportions of serial and parallel conduction. Film delamination from the substrate strongly reduces the resistivity increment due to the Gorsky effect. (c) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.Deutsche Forschungsgemeinschaft (DFG) [PU 131/7-2
FEM simulation supported evaluation of a hydrogen grain boundary diffusion coefficient in MgH 2
No full textQuasi-thermodynamic model on hydride formation in palladium-hydrogen thin films: Impact of elastic and microstructural constraints
No full textThe impact of elastic and microstructural constraints on structural phase transitions is investigated by using (10-300) nm Pd-H films of different microstructures. Hydrogen induced stress mainly arises from the film's adhesion to a substrate. Stress changes the hydrogens' chemical potential mu(H), modifying the hydride phase stability. Microstructural constraints channel stress release in films. A thermodynamic model is proposed to deduce the H-H interaction energy E-HH and an effective critical temperature T-c(eff) of hydride formation in films. It allows for occasionally observed sloped plateaus of mu(H) below T-c(eff). EHH (between 15 and 30 kJ/mol(H)) and T-c(eff) (340 K to 490 K) are reduced by up to 50% compared to bulk (E-HH = 36.8 kJ/mol(H), T-c = 563 K), for all films. Concentration-dependent contributions of substrate-induced stress (of about (2-5) kJ/mol(H)) and microstructure (of about (5-8) kJ/mol(H)) are separated. For all films phase separation is still found at 300 K. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.Deutsche Forschungsgemeinschaft [PU 131/7-2, PU 131/9-1
Metal nanowires and hydrogen loading: an AFM-study
No full textMetal nanowires (Nb, V, Pd and Fe) were studied by atomic force microscopy (AFM). The wires were produced by metal deposition onto faceted sapphire substrates. Periodic arrangements of wires of about 7 x 10(-8) m width and 3 x 10(-8) in height, accompanied with a wire length of about 1 x 10(-3) m were analysed. The grain size was normally about 2 x 10(-8) in. Prolonged scanning in contact mode leads to massive material agglomeration on the back of the substrate facet. Short-run measurements successfully image the wire morphology but impede an exact determination of the phase transition pressures. At 10(-2) Pa hydrogen gas pressure, the formation of a hydride along the wire axis of Nb-wires was verified in short-run experiments by AFM images including contour-lines as well as height histograms. The lattice of a 30 ran thin Nb-wire locally expands upon hydride formation with about 2 nm in vertical direction. This result agrees with the predictions of a model assuming linear elasticity, an ideally rigid substrate, and infinitely strong adhesion of the nanowires. No lateral expansion was detectable. (C) 2007 Elsevier B.V. All rights reserved
- …
