Hydrogen sensors are of increasing importance in connection with the development and expanded use of hydrogen gas as an energy carrier and as a chemical reactant. There are an immense number of sensors reported in the literature for hydrogen detection and in this work these sensors are classified into eight different operating principles. Characteristic performance parameters of these sensor types, such as measuring range, sensitivity, selectivity and response time are reviewed and the latest technology developments are reported. Testing and validation of sensor performance are described in relation to standardisation and use in potentially explosive atmospheres so as to identify the requirements on hydrogen sensors for practical applications.

Hydrogen Sensors - A review

http://nathan.instras.com/MyDocsDB/doc-617.pdf

Electrochromic organic and polymeric materials for display applications

Magnetic flux can penetrate a type-II superconductor in the form of Abrikosov vortices (also called flux lines, flux tubes, or fluxons) each carrying a quantum of magnetic flux phi 0=h/2e. These tiny vortices of supercurrent tend to arrange themselves in a triangular flux-line lattice (FLL), which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-Tc superconductors (HTSCs) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSCs the FLL is very soft mainly because of the large magnetic penetration depth lambda . The shear modulus of the FLL is c66~1/ lambda 2, and the tilt modulus c44(k)~(1+k2 lambda 2)-1 is dispersive and becomes very small for short distortion wavelengths 2 pi /k<< lambda . This softness is enhanced further by the pronounced anisotropy and layered structure of HTSCs, which strongly increases the penetration depth for currents along the c axis of these (nearly uniaxial) crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may `melt` the FLL and cause thermally activated depinning of the flux lines or ofthe two-dimensional `pancake vortices` in the layers. Various phase transitions are predicted for the FLL in layered HTSCs. Although large pinning forces and high critical currents have been achieved, the small depinning energy so far prevents the application of HTSCs as conductors at high temperatures except in cases when the applied current and the surrounding magnetic field are small.

https://iopscience.iop.org/article/10.1088/0034-4885/58/11/003/pdf

The flux-line lattice in superconductors

Magnetic flux can penetrate a type-II superconductor in form of Abrikosov vortices. These tend to arrange in a triangular flux-line lattice (FLL) which is more or less perturbed by material inhomogeneities that pin the flux lines, and in high-$T_c$ supercon- ductors (HTSC's) also by thermal fluctuations. Many properties of the FLL are well described by the phenomenological Ginzburg-Landau theory or by the electromagnetic London theory, which treats the vortex core as a singularity. In Nb alloys and HTSC's the FLL is very soft mainly because of the large magnetic penetration depth: The shear modulus of the FLL is thus small and the tilt modulus is dispersive and becomes very small for short distortion wavelength. This softness of the FLL is enhanced further by the pronounced anisotropy and layered structure of HTSC's, which strongly increases the penetration depth for currents along the c-axis of these uniaxial crystals and may even cause a decoupling of two-dimensional vortex lattices in the Cu-O layers. Thermal fluctuations and softening may melt the FLL and cause thermally activated depinning of the flux lines or of the 2D pancake vortices in the layers. Various phase transitions are predicted for the FLL in layered HTSC's. The linear and nonlinear magnetic response of HTSC's gives rise to interesting effects which strongly depend on the geometry of the experiment.

The Flux-Line Lattice in Superconductors

Electrochromics for smart windows: Oxide-based thin films and devices

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.

Yttrium and lanthanum hydride films with switchable optical properties

New magnetic torque measurements on a high-quality ${\mathrm{Bi}}_{2}$${\mathrm{Sr}}_{2}$${\mathrm{CaCu}}_{2}$${\mathrm{O}}_{\mathit{x}}$ single crystal show that only a lower bound for the anisotropy parameter \ensuremath{\gamma}g150 can be given. For misorientations of the field H from the (a,b) plane larger than 0.5\ifmmode^\circ\else\textdegree\fi{}, the results can be rescaled to magnetization M versus H. The behavior of M can be consistently described by a recent theory of Bulaevskii, Ledvij, and Kogan.

/pdf/magnetic-anisotropy-of-a-bi2sr2cacu2ox-single-crystal-2s65q5qa10.pdf

Magnetic anisotropy of a Bi2Sr2CaCu2Ox single crystal.

Synthesis of yttriumtrihydride films for ex-situ measurements

The lattice expansion of Pd during the absorption of hydrogen has been investigated with a combined technique in which the length changes and hydrogen concentration were measured simultaneously and on the same sample in the single-phase region of the PdHc phase diagram above the critical point. For atomic ratios up to c=0.65, the hydrogen induced relative volume changes are found to be Delta V/V0=0.200 c-0.0493c2. This concentration dependence of the lattice expansion, as well as that of the partial molar enthalpy Delta HH of hydrogen in Pd, can be described successfully by taking into account the volume and concentration dependences of the characteristic band-structure energy (EF-Es) introduced in the semi-empirical heat of formation model of Griessen and Driessen. Using this model the thermally broadened density of states in PdHc, as a function of band fillings above the Fermi level in pure Pd, is deduced from the experimental data and it is shown that the strong concentration dependence of Delta V/V0 in PdHc originates from the difference between the volume dependences of the almost completely filled d band at low hydrogen concentrations and the occupied s-p band at higher hydrogen concentrations. In NbHc where such a d band to s-p band 'transition' does not occur the concentration dependences of Delta HH and Delta V/V0 are dominated by the volume dependence of the partially filled d band.

https://iopscience.iop.org/article/10.1088/0305-4608/16/12/008/pdf

Hydrogen induced lattice expansion and effective H-H interaction in single phase PdHc

Using projection optics we made a detailed study of the interaction of a spatially uniform 248 nm excimer laser beam and a 99% dense YBa2Cu3O6+x target. Below a threshold fluence of 1 J/cm2 the roughness of the irradiated target increases dramatically due to non‐stoichiometric ablation. The overall target surface composition becomes increasingly Y rich and Cu poor, while the opposite is found for the corresponding ablated thin films. Above the threshold fluence the composition of the ablated target surface is conserved. As a result of the energy homogeneity of the laser beam obtained by means of projection optics, the optimization of the deposition parameters has been improved leading to the reproducible fabrication of flat, stoichiometric YBa2Cu3O7 films with Tc0’s over 91 K.

D.G. de Groot

Papers

Yttrium and lanthanum hydride films with switchable optical properties

Magnetic anisotropy of a Bi2Sr2CaCu2Ox single crystal.

Synthesis of yttriumtrihydride films for ex-situ measurements

Hydrogen induced lattice expansion and effective H-H interaction in single phase PdHc

Laser ablation threshold of yba2cu3o6+x