Optical switching properties of RCo2-type alloy hydride based solid state device

Correlation of structural differences between Nafion/polyaniline and Nafion/polypyrrole composite membranes and observed transport properties

Abstract: Polyaniline/Nafion and polypyrrole/Nafion composite membranes, prepared by chemical polymerization, are studied by scanning electron microscopy, infrared and nuclear magnetic resonance spectroscopy. Differences in vanadium ion diffusion through the membranes and in the membranes’ area specific resistance are linked to analytical observations that polyaniline and polypyrrole interact differently with Nafion. Polypyrrole, a weakly basic polymer, binds less strongly to the sulfonic acid groups of the Nafion membrane. Infrared spectroscopy results suggest that the hydrophobic polymer aggregates in the center of the Nafion channel rather than attaching to the hydrophilic walls containing sulfonic acid groups. This results in a drastically elevated membrane resistance and only slightly decreased vanadium ion diffusion compared to a Nafion membrane. Polyaniline, on the other hand, polymerizes along the sides of the Nafion pores and on the membrane surface, binding tightly to the sulfonic acid groups of Nafion, polyaniline's greater basicity possibly causing the difference in polymerization behavior. This leads to a more effective reduction in vanadium ion transport across the polyaniline/Nafion membranes and the increase in membrane resistance is less severe. The performance of selected polypyrrole/Nafion composite membranes is tested in a static vanadium redox cell. Increased coulombic efficiency, compared to a cell employing a pure Nafion membrane, further confirms the reduced vanadium ion transport through the composite membranes.

Electromagnetic wave propagation in spatially homogeneous yet smoothly time-varying dielectric media

Abstract: We explore the propagation and transformation of electromagnetic waves through spatially homogeneous yet smoothly time-dependent media within the framework of classical electrodynamics. By modelling the smooth transition, occurring during a finite period τ, as a phenomenologically realistic and sigmoidal change of the dielectric permittivity, an analytically exact solution to Maxwell׳s equations is derived for the electric displacement in terms of hypergeometric functions. Using this solution, we show the possibility of amplification and attenuation of waves and associate this with the decrease and increase of the time-dependent permittivity. We demonstrate, moreover, that such an energy exchange between waves and non-stationary media leads to the transformation (or conversion) of frequencies. Our results may pave the way towards controllable light–matter interaction in time-varying structures.

Density functional calculations of Ti nanoclusters in the metastable Mg-Ti system

Abstract: Mg-Ti-H thin films exhibit interesting optical and electrical properties, offering a wide range of possible applications from coatings on solar collectors and smart windows to optical hydrogen sensors and semiconductor devices. However, Ti is known to be immiscible in Mg, and the microstructure of Mg-Ti thin films is not fully understood. In this work density functional theory calculations were used to investigate the ${\text{Mg}}_{100\ensuremath{-}y}{\text{Ti}}_{y}$ system with $1.56\ensuremath{\le}y\ensuremath{\le}98.44$. The crystal structure, mixing enthalpy, and electronic structure were compared for two different distributions of Ti: quasirandom and segregated. It was found that although the crystal structures did not differ significantly, the formation enthalpy per Ti atom was lowered by up to $\ensuremath{\sim}0.5\text{ }\text{eV}$ when Ti was arranged in nanoclusters. This gives support to previous experimental findings of chemical short-range order in Mg-Ti thin films [R. Gremaud et al., Phys. Rev. B 77, 144204 (2008)]. As a consequence of the decrease in the mixing enthalpy upon clustering the occurrence of short-range chemical order in all reported metastable Mg-Ti alloys with extended solubility is proposed. Further inquires into the influence of the size of the clusters revealed that the mixing enthalpy reaches a minimum after which further growth makes little difference, indicating that the phenomenon of nanoscale clustering must be understood separately from the larger scale phase separation occurring in an equilibrium process. The relaxed crystal structures were compared to experimental values from several sources and lattice-parameter variations as well as deviations from Vegard's law were discussed. Dependence of the size and shape of the nanoclusters on synthesis method is offered as an explanation for the large variation among the experimental lattice parameters. Local density of states calculations illustrated how segregated Ti forms a local environment resembling pure Ti while it was necessary to perform Bader analysis in order to obtain the correct picture of the charge transfer between Mg and Ti. Randomly distributed Ti atoms affect the charge distribution severely and the further the Ti atoms are apart the larger is the charge transfer from Mg. It was shown that a very limited amount of Ti nearest neighbors is sufficient for Ti to experience an imitation of elemental state, removing the driving force for further nucleation.

Hydrogen Storage Materials

Abstract: Hydrogen is a clean fuel as it produces only water as a by-product and there is no emission of pollutants or greenhouse gases. Storage of hydrogen is one of the main issues in exploiting it as an economical fuel. Several intermetallic compounds and alloys absorb and desorb hydrogen at moderate temperature and pressure. This class of materials, known as metal hydrides, are being extensively studied for their hydrogen storage capacity, reversibility, kinetics and cyclic stability. Metal hydrides find applications in Ni–MH (metal hydride) batteries, on-board energy supply, hydrogen isotope separation, gas purification, heat pumps, sensors and actuators. It is also of fundamental interest to study the interaction of hydrogen with various materials and the changes in their physico-chemical properties.

Optical switching of Y-hydride thin film electrodes: a remarkable electrochromic phenomenon.

Abstract: The optical appearance of yttrium thin film electrodes can be electrochemically switched from mirrorlike to highly transparent by making use of the hydride-forming properties of Y. A strong alkaline solution is a suitable electrolytic environment for obtaining stable electrochromic electrodes. The presence of a thin Pd layer covering the Y electrode is essential in providing a sufficiently high electrocatalytic activity for the electrochemical charge-transfer reaction. Hydrogen is irreversibly bound in Y-dihydride and reversibly bound in Y-trihydride. The optical changes are reversible and are induced within a narrow hydrogen concentration range, making this typical electrode material interesting for application in a new type of electrochromic devices.

Handbook of inorganic electrochromic materials

Abstract: Part 1 Case study on tungsten oxide: bulk crystalline tungsten oxide tungsten oxide films - preparation, structure, and composition of evaporated films tungsten oxide films - preparation, structure, and composition of sputter-deposited films tungsten oxide films - preparation, structure, and composition of electrochemically and chemically prepared films tungsten oxide films - ion intercalation/deintercalation studied by electrochemical techniques tungsten oxide films - ion intercalation/deintercalation studied by physical techniques tungsten oxide films -ultraviolet absorption and semiconductor bandgap tungsten oxide films - optical properties in the luminous and near-infrared range tungsten oxide films - theoretical models for the optical properties tungsten oxide films - electrical properties. Part 2 Electrochromism among the oxides (except tungsten oxide): molybdenum oxide films miscellaneous tungsten- and molybdenum-oxide-containing films iridium oxide films titanium oxide films manganese oxide films vanadium dioxide films vanadium pentoxide films nickel oxide films cobalt oxide films niobium oxide films miscellaneous oxide films systematics for the electrochromism in transition metal oxides inorganic non-oxide electrochromic materials. Part 3 Electrochromic devices: transparent electrical conductors electrolytes and ion conductors ion storage materials - brief overview devices with liquid electrolytes devices with solid inorganic electrolytes and ion conductors devices with polymer electrolytes time-dependent device performance - a unified treatment.

Yttrium and lanthanum hydride films with switchable optical properties

Abstract: IN many substances, changes in chemical composition, pressure or temperature can induce metal-to-insulator transitions1. Although dramatic changes in optical and electrical properties accompany such transitions, their interpretation is often complicated by attendant changes in crystallographic structure2. Yttrium, lanthanum and the trivalent rare-earth elements form hydrides that also exhibit metal–insulator transitions3–5, but the extreme reactivity and fragility of these materials hinder experimental studies5,6. To overcome these difficulties, we have coated thin films of yttrium and lanthanum with a layer of palladium through which hydrogen can diffuse. Real-time transitions from metallic (YH2 or LaH2) to semiconducting (YH3 or LaH3) behaviour occur in these films during continuous absorption of hydrogen, accompanied by pronounced changes in their optical properties. Although the timescale on which this transition occurs is at present rather slow (a few seconds), there appears to be considerable scope for improvement through the choice of rare-earth element and by adopting electrochemical means for driving the transition. In view of the spectacular changes in optical properties—yttrium hydride, for example, changes from a shiny mirror to a yellow, transparent window—metal hydrides might find important technological applications.

Switchable mirrors based on nickel–magnesium films

Abstract: A new type of electrochromic mirror electrode based on reversible uptake of hydrogen in nickel magnesium alloy films is reported. Thin,magnesium-rich Ni-Mg films prepared on glass substrates by cosputtering from Ni and Mg targets are mirror-like in appearance and have low visible transmittance. Upon exposure to hydrogen gas or on reduction in alkaline electrolyte, the films take up hydrogen and become transparent. When hydrogen is removed, the mirror properties are recovered. The transition is believed to result from reversible formation of Mg2NiH4 and MgH2. A thin overlayer of palladium was found to enhance the kinetics of hydrogen insertion and extraction,and to protect the metal surface against oxidation.

Optical switches based on magnesium lanthanide alloy hydrides

Abstract: It is shown that thin layers of palladium coated magnesium lanthanide alloys reversibly go through an optical transition by variation of the hydrogen concentration, just as has recently been shown for pure lanthanides. With these layers optical switches can be constructed that exhibit three different optical states: a color-neutral transparent state at high hydrogen concentration, a nontransparent dark absorbing state at intermediate hydrogen pressures, and a highly reflective metallic state at low hydrogen pressures. The ratio in transmission between the transparent state and the reflecting state is more than 1000.

Optical switching of Mg-rich Mg–Ni alloy thin films

Abstract: Mg–Ni alloy thin films were prepared by dc magnetron sputtering using Mg and Ni targets, and their optical switching property for hydrogen exposure has been investigated. Pd-capped MgNix (0.1