Showing papers by "R.P. Griessen published in 2004"

Highly absorbing black Mg and rare-earth-Mg switchable mirrors

Abstract: Apart from a reflecting and a transparent state, rare-earth-Mg alloys (RE-Mg) exhibit also a highly absorbing, black state during loading with hydrogen The occurrence of such a black state is due to the disproportionation into subwavelength size ${\mathrm{REH}}_{2+\ensuremath{\epsilon}}$ and Mg grains during the first hydrogen loading While the optical properties of ${\mathrm{REH}}_{x}$ change continuously with a further increase in hydrogen concentration x, Mg changes abruptly from a good reflector to a transparent insulator $({\mathrm{MgH}}_{2})$ Thin pure Mg films also show this black state when (un)loaded carefully at elevated temperatures By using the Bruggeman effective medium approximation in combination with the transfer matrix method it is shown that the coexistence of Mg and ${\mathrm{MgH}}_{2}$ grains is the cause of this high absorption Furthermore, we compare this phenomenon to the high absorption of light observed in metal-dielectric composites

Structural and optical properties of Mg2NiHx switchable mirrors upon hydrogen loading

Abstract: The structural, thermodynamic and optical properties of ${\mathrm{Mg}}_{2}\mathrm{Ni}$ thin films covered with Pd are investigated upon exposure to hydrogen. Similar to bulk, thin films of metallic ${\mathrm{Mg}}_{2}\mathrm{Ni}$ take up 4 hydrogen per formula unit and semiconducting transparent ${\mathrm{Mg}}_{2}\mathrm{Ni}{\mathrm{H}}_{4\ensuremath{-}\ensuremath{\delta}}$ is formed. The dielectric function $\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}$ of ${\mathrm{Mg}}_{2}\mathrm{Ni}$ and fully loaded ${\mathrm{Mg}}_{2}\mathrm{Ni}{\mathrm{H}}_{4\ensuremath{-}\ensuremath{\delta}}$ is determined from reflection and transmission measurements using a Drude-Lorentz parametrization. Besides the two ``normal'' optical states of a switchable mirror---metallic reflecting and semiconducting transparent---${\mathrm{Mg}}_{2}\mathrm{Ni}{\mathrm{H}}_{x}$ exhibit a third ``black'' state at intermediate hydrogen concentrations with low reflection and essentially zero transmission. This state originates from a subtle interplay of the optical properties of the constituent materials and a self-organized double layering of the film during loading. ${\mathrm{Mg}}_{2}\mathrm{Ni}{\mathrm{H}}_{4\ensuremath{-}\ensuremath{\delta}}$ preferentially nucleates at the film/substrate interface and not---as intuitively expected---close to the catalytic Pd capping layer. Using ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}}_{{\mathrm{Mg}}_{2}\mathrm{Ni}}$ and ${\stackrel{\ifmmode \tilde{}\else \~{}\fi{}}{ϵ}}_{{\mathrm{Mg}}_{2}\mathrm{Ni}{\mathrm{H}}_{4}}$ and this loading sequence, the optical response at all hydrogen concentrations can be described quantitatively. The uncommon hydrogen loading sequence is confirmed by x-ray diffraction and hydrogen profiling using the resonant nuclear reaction $^{1}\mathrm{H}(^{15}\mathrm{N},\ensuremath{\alpha}\ensuremath{\gamma})^{12}\mathrm{C}$. Pressure-composition isotherms suggest that the formation of ${\mathrm{Mg}}_{2}\mathrm{Ni}{\mathrm{H}}_{4\ensuremath{-}\ensuremath{\delta}}$ at the film/substrate interface is mainly due to locally enhanced kinetics.

Self-Organized Layered Hydrogenation in Black Mg2NiHx Switchable Mirrors

Abstract: In addition to a mirrorlike (Mg2Ni) and a transparent (Mg2NiH4) state, thin films of Mg2NiHx exhibit a remarkable black state with low reflection over the entire visible spectrum, essentially zero transmission and a low electrical resistivity. Such a black state is not explicable for a homogeneous layer since a large absorption coefficient always yields substantial reflection. We show that it results from a self-organized and reversible double layering of metallic Mg2NiH0.3 and semiconducting Mg2NiH4.

Mg–Ni–H films as selective coatings: Tunable reflectance by layered hydrogenation

Abstract: Unlike other switchable mirrors, Mg2NiHx films show large changes in reflection that yield very low reflectance (high absorptance) at different hydrogen contents, far before reaching the semiconducting state. The resulting reflectance patterns are of interference origin, due to a self-organized layered hydrogenation mechanism that starts at the substrate interface, and can therefore be tuned by varying the film thickness. This tunability, together with the high absorptance contrast observed between the solar and the thermal energies, strongly suggests the use of these films in smart coatings for solar applications.

Effect of the strong metal-support interaction on hydrogen sorption kinetics of Pd-capped switchable mirrors

Abstract: The morphology and electronic structure of Pd clusters grown on oxidized yttrium surfaces are investigated by scanning tunneling microscopy and ultraviolet photoelectron spectroscopy. The hydrogen sorption mediated by the Pd clusters is determined from the optically monitored switching kinetics of the underlying yttrium film. A strong thickness dependence of the hydrogen uptake is found. The electronic structure of the as-grown Pd clusters depends only weakly on their size. Strong changes of the photoemission spectra are found after hydrogenation, in particular the oxide peak shifts and the Pd peaks vanish. Both phenomena are due to a strong metal-support interaction (SMSI) state, characterized by a complete encapsulation of the clusters by a reduced yttrium oxide layer. Scanning tunneling spectroscopy confirms the SMSI state of small Pd clusters after hydrogen exposure. The SMSI effect is less important with increasing Pd thickness. This explains the critical thickness for the catalyzed hydrogen uptake by the Pd/ YOx / Y system. The results shed light on the mechanism of hydrogen absorption at the triple point gas-catalyst-oxide, which also plays an important role in today’s fuel cell technology.

Temperature dependence of magnetoresistance and Hall effect in Mg2NiHx films

Abstract: The electrical resistivity, Hall effect, and magnetoresistance of ${\mathrm{Mg}}_{2}{\mathrm{NiH}}_{x}$ $(0lxl4)$ films are investigated as a function of temperature, between 2 K and 280 K and magnetic fields up to 7 T. The overall features exhibited by the electrical resistivity $\ensuremath{\rho}$ and the charge-carrier density n at 4.2 K in ${\mathrm{Mg}}_{2}{\mathrm{NiH}}_{4\ensuremath{-}\ensuremath{\delta}}$ are those of a heavily doped semiconductor. As a function of x, accompanied by an increase of $\ensuremath{\rho},$ n decreases linearly. This is consistent with an ``anionic'' model for H in the host lattice: each added H atom removes one electron from the conduction band, resulting in an increased resistivity. Over the entire concentration range (for $0lxl4),$ we find approximately that $\ensuremath{\rho}(T)\ensuremath{\simeq}{\ensuremath{\rho}}_{0}\ensuremath{-}{\ensuremath{\rho}}_{1}\mathrm{ln}T+{\ensuremath{\rho}}_{2}{T}^{1.85}.$ The amplitude ${\ensuremath{\rho}}_{1}$ of the Kondo term and ${\ensuremath{\rho}}_{2}$ of the electron-electron scattering term vary by more than two orders of magnitude and exhibit power-law dependence as a function of n. The experimental data can be understood within an effective medium theory, where ${\mathrm{Mg}}_{2}{\mathrm{NiH}}_{x}$ samples disproportionate into metallic ${\mathrm{Mg}}_{2}{\mathrm{NiH}}_{0.3}$ and semiconducting ${\mathrm{Mg}}_{2}{\mathrm{NiH}}_{4\ensuremath{-}\ensuremath{\delta}}.$ In this picture, we show that the relatively low $\ensuremath{\rho}$ $(l4\mathrm{m}\ensuremath{\Omega}\mathrm{cm})$ measured in samples with $x\ensuremath{\lesssim}3.5$ is due to the peculiar geometry of the metallic inclusions (very flat oblate spheroids). The relation with the highly absorbing black state in ${\mathrm{Mg}}_{2}{\mathrm{NiH}}_{0.7}$ is discussed.

Hydrogen sorption mechanism of oxidized nickel clusters

Abstract: Using an optical technique to measure hydrogen sorption kinetics the catalytic activity of the NiOx clusters is determined. The thus measured temperature dependence reveals an activation energy of 0.58 eV. The catalytic activity of NiOx clusters is studied as a function of the oxygen concentration. The surface properties are analyzed by Auger-electron spectroscopy. It appears that the catalytic hydrogen sorption originates from the dissociative chemisorption of hydrogen on O:Ni, which is strongly suppressed by the presence of oxides.

Effect of hydrogen uptake and substrate orientation on the flux penetration in NbHx thin films

Abstract: By absorbing various amounts of hydrogen in the same niobium sample we are able to change the local superconducting properties thus introducing quenched disorder in a controlled manner. Its effect on the magnetic flux penetration in NbHx thin films on A-plane (1 1 2 0) and R-plane (1 1 0 2) sapphire substrates is systematically investigated using a magneto-optical technique. With increasing hydrogen content, flux penetration patterns are observed to become more irregular. At high hydrogen concentrations non-superconducting inclusions are formed. The substrate orientation also has a strong influence on the observed behavior, revealing line-shaped avalanches at 4.2 K for A-plane substrates and a more continuous but spatially irregular behavior for R-plane substrates.

EFFECT OF SUBSTRATE ORIENTATION AND HYDROGEN IMPURITIES ON FLUX PENETRATION IN Nb THIN FILMS

Abstract: Very different kinds of magnetic flux penetration patterns have been reported in the type-II superconductor Nb. Using our advanced magneto-optical setup we investigate the flux penetration in Nb thin films. We find that depending on the sapphire substrate orientation (either A-plane or R-plane) qualitatively different structures are observed. In particular, for the A-plane orientation we find fingering and branching, whereas for R-plane samples a rough but continuous flux front is observed. Since Nb easily accepts hydrogen atoms as interstitial impurities, the influence of static pinning centers on the flux penetration process can be investigated. We find that the flux penetration drastically changes, becoming more irregular. The possibility to add a well- controlled amount of disorder makes NbHx an ideal system to study the influence of quenched noise on roughening phenomena.