| dc.description.abstracteng | The purpose of this study is to investigate hydrogen absorption properties of grain boundary rich pure magnesium film.
X-ray diffraction (XRD) measurements has been carried out to estimate microstructures, texture, lattice expansion and inner strain changes during hydrogen absorption. As-prepared sample showed (0001)-texture of the Mg layer, and columnar grain growth was recognized by TEM observation. Hydrogen was loaded by means of electrochemical method or with gaseous hydrogen. Both loading methods lead to a nucleation of β-MgH2 phase. They revealed crystallographic orientation of αMg-(0002)//βMgH2-(110), with 2°-4° tilt of the hydride.
By the electrochemical loading measurements for Mg-films, it was found that loading condition with a 1: 2 (vol.) mixture of H3PO4 (85%) and Glycerin (85%) as electrolyte, a polymer coating on the sample edges and a Pd capping layer thicker than 20 nm showed a successful and suitable condition for successful loading.
Measured potential versus concentration curves were strongly affected by the hydrogen loading current density. Smaller hydrogen loading current achieved thicker hydride content in the end. At small currents of i < 4×10^(-7) [A/(cm^2)], more than 1300 nm thick MgH2-layers can be built up. However, using a Pd layer between the Mg sample and the Si substrate leads to buckling of the film, for 1400 nm thick Mg films.
Step-by-step loading of Mg films prepared on Si substrates at room temperature revealed an increase of in-plane compressive stress by increasing hydrogen concentrations. In contrast, for annealed Mg films, an increase of in-plane tensile stress by increasing hydrogen concentrations was measured. These tendencies were clearly observed by three different stress measurements, which are the Bragg peak shift, the sin^2(ψ) method, and the curvature measurement, in both cases of electrochemical hydrogen loading measurements and gas loading measurements.
For Mg-films prepared on Pd-substrates, the inner stress developments were succesfully obtained by in-situ XRD measurements during the step-by-step electrochemical hydrogen loading from back side, using originally prepared sample stage in this work. The hydrogen solution up to a concentration of cH = 10^(-3) H/Mg was clearly observed from the Bragg peak shifts. In the higher concentration regime, the increases of the in-plane compressive stress in the α-Mg matrix were otained by the sin^2(ψ) method.
The inner stress dependency on the hydrogen concentration was also evaluated. For the gas-loaded samples, it was in GPa range in the solid solution region, and 1-2 orders of magnitude larger than that in the two phase region. The same tendency was also obtained for the electrochemically loaded samples. The stress dependency in the solid solution concentration range was able to be explained by the linear elastic theory.
The achived hydride thicknesses in the Mg-films loaded by gas were evaluated by three different methods, which are the profilometer, Bragg peak area, and the electrical resistivity measurements. All of those three measurements result in the larger hydride fraction for the samples loaded at lower gas pressure, with a maximum hydride thickness of 1.8 μm.
The film-Mg-H system was also investigated from the kinetical point of view, at room temperature. Electrochemical hydrogen permeation measurements and in-situ gas-loading XRD-measurements have been performed on poly-crystalline Mg-films. Hydrogen diffusion constants, the hydride volume content and the in-plane stress were determined, at T = 300 K. For low concentrations, a hydrogen diffusion constant of D(H-in-Mg) = 7(+/-2) x 10^(-11) [m2/s] was obtained, which is attributed to αMg-H.
For higher concentrations, different kinetic regimes with reduced apparent diffusion constants were observed. The apparent diffusion coefficients dropped to about 10^(-18) [m2/s]. This value is still two orders larger than that of bulk β-MgH2, because of the contribution of grain boundaries in the films, acting as pathways for faster hydrogen diffusion in poly crystalline Mg-films.
The different kinetics regimes are attributed to the spatial distribution of the hydrides. A hydride nucleation and growth model is suggested that relyes on half-spherical hydrides with a nuclei densities depending on the driving force. The model allows explaining the complex stress development, the different diffusion regimes and the blocking-layer thickness. The blocking-layer thickness inversely scales with the driving force.
The spatial distribution of hydrides and the hydrogen diffusion in Mg thin films strongly depends on the driving force. A small driving force allows hydriding a larger film volume when compared to a high driving force. This can be explained by the theory of nucleation and growth that directly links an increase of the nucleation rate with an increase of the driving force. A high nucleation rate or a small distance between neighboring hydrides yields a closed hydride layer in short time. Vice versa, a low nucleation rate or a large distance between neighboring hydrides yields a closed hydride layer after long time.
Further hydrogen loading leads to a growth of hydride grains. For all nanocrystalline films, the increase of in-plane stress was measured by XRD techniques (the peak shift method and the sin2ψ method) and by curvature measurements, at different hydrogenated states. In-plane stress changes, being increased according to higher hydrogen concentrations, reveal a good agreement with the values obtained by other methods and are, thus, considered to be reliable. Furthermore, it was observed that the strain in out-of-film-direction increases with hydrogen loading in the two-phase region.
Increases of thickness in highly loaded states were also directly measured by profilometer measurements, and the increase was comparable to the calculated value applying the double layer model. Corresponding increase of hydride fraction was in good agreement with values obtained by resistivity change and also with the Bragg peak area change .
The hydrogen induced in-plane stress and corresponding strain cause an increase of the elastic energy of the film. This increase of the elastic energy affects the activation energy of the magnesium film with further hydrogen. This effect is clearly seen in the result of measurements at elevated temperatures, namely in the van't Hoff Plot by a change of the slope. However, solution effect of Mg into Pd cannot be eliminated in these results.
In-situ gas-loading measurements at elevated temperatures up to T = 363 K have been performed for Mg-films. Unloading of the hydrided Mg-film at T = 363 K was possible within 15 minutes. Furthermore, the hydride decomposition enthalpy was calculated to be 62 ± 7 kJ/mol, which is by 24 kJ/mol smaller compared to that for bulk Mg. The influence by the in-plane compressive biaxial stress is suggested for this detabilisation of the hydride phase compared to the bulk material.
The pure impact of grain boundaries on the hydrogen sorption property was also focused in this study. For the hydrogen concentration at the solid solution limit, solubility ratios of 3500 or 2×10^4 were obtained from the apparent solubility limit of H/Mg. Furthermore, at high hydrogen concentration regime, the apparent diffusion coefficients obtained in this work imply that the grain boundaries act as pathways for hydrogen transport in Mg. Using rough assumptions, a diffusivity ratio of 2∙10^3 can be estimated.
To conclude, the driving force plays a major role on the hydrogen absorption in Mg thin films as it controls the morphologies of the hydrides and the formation of the blocking layer. Introducing grain boundaries lead to a visible increase of the hydrogen permeability and are, therefore, beneficial to reduce the "blocking effect" on the hydrogen absorption reaction of Mg. | de |