H. Schreuders

Mg-Ti-H thin films for smart solar collectors

Mg–Ti–H thin films are found to have very attractive optical properties: they absorb 87% of the solar radiation in the hydrogenated state and only 32% in the metallic state. Furthermore, in the absorbing state Mg–Ti–H has a low emissivity; at 400K only 10% of blackbody radiation is emitted. The transition between both optical states is fast, robust, and reversible. The sum of these properties highlights the applicability of such materials as switchable smart coatings in solar collectors.

A reliable, sensitive and fast optical fiber hydrogen sensor based on surface plasmon resonance.

We report for the first time on the experimental response of a Surface Plasmon Resonance fiber optic sensor based on wavelength modulation for hydrogen sensing. This approach of measuring the hydrogen concentration makes the sensor insensitive to intensity fluctuations. The intrinsic fiber sensor developed provides remote sensing and enables the possibility of multi-points sensing. The sensor consists of a multilayer of 35 nm Au/180 nm SiO2/Pd deposited on a step- index multimode fiber core. The sensitivity and selectivity of the sensor are optimal at a Pd thickness of 3.75 nm. The sensor is sensitive to a hydrogen concentration ranging between 0.5 and 4% H2 in Ar, with a response time less than 15 s.

Monolithic fiber-top sensor for critical environments and standard applications

We present a monolithic device obtained by carving a cantilever on the top of a single-mode optical fiber. We show that the vertical position of the cantilever can be determined with accuracy comparable to atomic force microscopes and other commonly used scientific instruments. The device does not require any alignment procedure and can be used in critical environments as well as in standard applications.

An optical method to determine the thermodynamics of hydrogen absorption and desorption in metals

Hydrogenography, an optical high-throughput combinatorial technique to find hydrogen storage materials, has so far been applied only to materials undergoing a metal-to-semiconductor transition during hydrogenation. We show here that this technique works equally well for metallic hydrides. Additionally, we find that the thermodynamic data obtained optically on thin Pd–H films agree very well with Pd–H bulk data. This confirms that hydrogenography is a valuable general method to determine the relevant parameters for hydrogen storage in metal hydrides.

Quasifree Mg–H thin films

The thermodynamics of hydrogen absorption in Pd-capped Mg films are strongly dependent on the magnesium thickness. In the present work, we suppress such dependency by inserting a thin Ti layer between Mg and Pd. By means of optical measurements, we show that the surface energy contribution to the destabilization of MgH2 is negligible. The inserted Ti layer prevents Mg–Pd alloy formation at the Mg/Pd interface, leading to quasifree Mg films and enhancing the kinetics of hydrogen desorption. Our observations are important for the development of thin film devices.

Carving fiber-top optomechanical transducers from an optical fiber

In a recent paper (Iannuzzi et al 2006 Monolithic fiber-top sensor for critical environments and standard applications Appl. Phys. Lett. 88 053501) we have presented the principle of the fiber-top position sensor, having a monolytical structure carved out of a single-mode optical fiber. The device alleviates sensing in a critical environment via interferometric readout, and because of its monolithic construction, facilitates plug-and-play utilization without alignment. In this paper we describe the fabrication method of a similar device which, however, was also equipped with a pyramidal tip on the top of the cantilever, an important detail for future implementation in scanning probe microscopy. A silicon surface was then periodically put in contact and moved out of contact with the device. The output signal resembles force curves that can be similarly obtained with atomic force microscopes.

Hydrogen absorption kinetics and optical properties of Pd-doped Mg thin films

In order to develop optical fiber hydrogen sensors, thin film materials with a high optical contrast between the metallic and hydrided states are needed. Magnesium exhibits such a contrast but cannot be easily hydrogenated at room temperature. However, thin films of Pd-doped Mg (MgPdy with 0.023⩽y⩽0.125) prepared by magnetron dc sputtering can easily be hydrided at room temperature and 0.5bar H2 within a few minutes. The rate of first hydrogenation increases linearly with increasing Pd concentration. Hydrogenation induces high variations of transmission (ΔT up to 20%) and reflection (ΔR up to 70%) of light (0.5eV⩽ℏω⩽6.0eV corresponding to 2500nm⩾λ⩾210nm). The optical properties can be understood by considering Pd as a deep donor in semiconducting MgH2.

Mg/Ti multilayers: Structural and hydrogen absorption properties

Mg-Ti alloys have uncommon optical and hydrogen absorbing properties, originating from a “spinodal-like” microstructure with a small degree of chemical short-range order in the atoms distribution. In the present study we artificially engineer shortrange order by depositing Pd-capped Mg/Ti multilayers with different periodicities and characterize them both structurally and optically. Notwithstanding the large lattice parameter mismatch between Mg and Ti, the as-deposited metallic multilayers show good structural coherence. Upon exposure to H2 gas a two-step hydrogenation process occurs, with the Ti layers forming

Fibre-top cantilevers: design, fabrication and applications

Fibre-top cantilevers are a new generation of miniaturized devices obtained by carving tiny mechanical beams directly on the cleaved edge of an optical fibre. The light coupled from the other side of the fibre allows measurements of the position of the cantilever with sub-nanometre accuracy. The monolithic structure of the device, the absence of electronic contacts on the sensing head, and the simplicity of the working principle offer unprecedented opportunities for the development of scientific instruments for both standard applications and utilization beyond research laboratories. In this paper we review the results that our group has obtained over the last year in the development of this technology. We describe the working principle and the fabrication procedure, and we present a series of proof-of-concept experiments that demonstrate that fibre-top cantilevers can be used both for atomic force microscopy and for the detection of chemical species.

Electrohydrogenation of MgH2-thin films

Mg-metal hydride systems are potential hydrogen storage media and might be used in hydrogen switchable mirrors. However, absorption kinetics are too slow even with appropriate catalysts. The authors demonstrate the enhancement of the formation of MgH2 by an electric current, thus enabling the hydrogenation of Mg without an external heat source. The effect is explained by the local heating of the Mg together with an electromigration of H− ions. The electrically supported hydrogen uptake might also be a possibility to enhance the hydrogen uptake rate of complex hydrides due to a similar electronic structure.