Körnvergröberung in Gegenwart von flüssigen Filmen an den Korngrenzen

Der signigkante Einfluss der Umschmelzmechanismen beim Konzentrationsausgleich zur Einstellung der Gleichgewichtskonzentration im Temperaturgradienten wird mit den in dieser Arbeit durchgeführten Konzentrationsmessungen und quantitativen Gefügebetrachtungen nachgewiesen

LiBH4-MgH2 Komposite für die Wasserstoffspeicherung

In dieser Arbeit werden Reaktive Hydridkomposite aus Leichtmetallhydriden zur Wasserstoffspeicherung durch DSC, in- und ex-situ XRD sowie volumetrische Messungen charakterisiert. Der Reaktionsmechanismus wird in Abhängigkeit von den Reaktionsbedingungen detailliert beschrieben. Die Reaktionskinetik wurde durch Zugabe von Additiven auf Basis von Übergangsmetallen optimiert und ihre Wirkungsweise konnte mit der Keimbildung und Kornfeinung der Reaktionsprodukte in Verbindung gebracht werden.In this work, Reactive Hydride Composites of light-metal hydrides for hydrogen storage applications are characterized by DSC, in-and ex-situ XRD as well as volumetric measurements. The reaction mechanism is described in detail in dependency of the reaction parameters. Reaction kinetics were optimized by the addition of transition-metal based additives. The substantial effect of the additives was related to nucleation and grain refinement during the reactions

Characterization of metal hydrides by in-situ XRD

In-situ synchrotron radiation powder X-ray diffraction (SR-PXD) technique is a powerful tool to gain a deeper understanding of reaction mechanisms in crystalline materials. In this paper, the implementation of a new in-situ SR-PXD cell for solid–gas reactions is described in detail. The cell allows performing measurements in a range of pressure which goes from light vacuum (10−2 bar) up to 200 bar and temperatures from room temperature up to 550 °C. The high precision, with which pressure and temperature are measured, enables to estimate the thermodynamic properties of the observed changes in the crystal structure and phase transformations

Pressure and Temperature Influence on the Desorption Pathway of the LiBH4−MgH2 Composite System

The decomposition pathway in LiBH4−MgH2 reactive hydride composites was investigated systematically as a function of pressure and temperature. Individual decomposition of MgH2 and LiBH4 is observed at higher temperatures and low pressures (T ≥ 450 °C and p(H2) ≤ 3 bar), whereas simultaneous desorption of H2 from LiBH4 and formation of MgB2 was observed at 400 °C and a hydrogen backpressure of p(H2) = 5 bar. The simultaneous desorption of H2 from LiBH4 and MgH2 without intermediate formation of metallic Mg could not be observed. In situ X-ray diffraction (XRD) and infrared (IR) spectroscopy reveal the present crystalline and amorphous phases

Intermediate phases observed during decomposition of LiBH4

Lithium tetrahydridoboranate is among the materials with the highest hydrogen content and has great potential as a possible H-2-storage material, although, the release and uptake of H-2 is not fully understood. In this work, LiBH4 was studied by in situ synchrotron radiation powder X-ray diffraction (PXD) and solid state CP/MAS NNIR both at variable temperatures. This study revealed two new phases observed during dehydrogenation of LiBH4. Phase I is hexagonal, a = 4.93(2) and c = 13.47(3) angstrom and is observed in the temperature range -200-300 degrees C, and phase II is orthorhombic, a = 8.70(1), b = 5.44(1) and c = 4.44](8) angstrom and is observed in the temperature range similar to 300-400 degrees C applying a constant heating rate of 5 degrees C/min. Apparently, I transforms into II, e.g. at a constant temperature of T= 265 degrees C after 5 h. Furthermore, a third phase, III, is observed in the temperature range RT to 70 degrees C, and is caused by a reaction between LiBH4 and water vapor from the atmosphere. Hydrogen release is associated with the decomposition of III at ca. 65 degrees C. (C) 2007 Elsevier B.V. All rights reserved

Pressure Effect on the 2NaH+MgB2 Hydrogen Absorption Reaction

The hydrogen absorption mechanism of the 2NaH + MgB2 system has been investigated in detail. Depending on the applied hydrogen pressure, different intermediate phases are observed. In the case of absorption measurements performed under 50 bar of hydrogen pressure, NaBH4 is found not to be formed directly. Instead, first an unknown phase is formed, followed upon further heating by the formation of NaMgH3 and a NaH-NaBH4 molten salt mixture; only at the end after heating to 380 °C do the reflections of the crystalline NaBH4 appear. In contrast, measurements performed at lower hydrogen pressure (5 bar of H2), but under the same temperature conditions, demonstrate that the synthesis of NaBH4 is possible without occurrence of the unknown phase and of NaMgH3. This indicates that the reaction path can be tuned by the applied hydrogen pressure. The formation of a NaH-NaBH4 molten salt mixture is observed also for the measurement performed under 5 bar of hydrogen pressure with the formation of free Mg. However, under this pressure condition the formation of crystalline NaBH4 is observed only during cooling at 367 °C. For none of the applied experimental conditions has it been possible to achieve the theoretical gravimetric hydrogen capacity of 7.8 wt %.JRC.F.2-Cleaner energ

Femtosecond x-ray diffraction study of multi-THz coherent phonons in SrTiO3_3

We report generation of ultra-broadband longitudinal acoustic coherent phonon wavepackets in SrTiO$_3$ (STO) with frequency components extending throughout the first Brillouin zone. The wavepackets are efficiently generated in STO using femtosecond infrared laser excitation of an atomically flat 1.6 nm-thick epitaxial SrRuO$_3$ film. We use femtosecond x-ray diffraction at the European X-Ray Free Electron Laser Facility to study the dispersion and damping of phonon wavepackets. The experimentally determined damping constants for multi-THz frequency phonons compare favorably to the extrapolation of a simple ultrasound damping model over several orders of magnitude