J.F. Fernández

Rate determining step in the absorption and desorption of hydrogen by magnesium

The kinetics of hydrogen absorption and desorption by magnesium has been investigated by a volumetric technique. Experimental data have been analysed in order to find the rate determining step for both the absorption and desorption processes. It is shown that a nucleation and growth (NG) mechanism, with exponent values n52 for desorption and n50.5 to n5l for absorption provides suitable equations in order to fit the experimental data. The influence of hydrogen pressure and temperature on the process rate has been studied to obtain an expression for the driving force of the reaction and its activation energy. The driving force for desorption seems to follow a parabolic law though the experimental data are also compatible with a linear law. According to our data, the rate determining step in the desorption of hydrogen by magnesium, and probably also in the absorption process, seems to be the hydrogen diffusion through the b 21 phase. An activation energy for such a diffusion process of 100610 kJ mol H has been obtained from the desorption data.

Simultaneous TDS–DSC measurements in magnesium hydride

Decomposition of MgH2 has been studied by means of simultaneous thermal desorption spectroscopy (TDS) and differential scanning calorimetry (DSC). Measurements have been accomplished in an experimental set-up composed of a differential scanning calorimeter connected through a capillary tube to a mass spectrometer. The experimental system allows the simultaneous determination of the heat absorbed by and the hydrogen evolved from MgH2 during its thermal decomposition. The H-desorption from magnesium hydride samples non-exposed to air has been studied. The desorption curves are analysed with a nucleation and growth formalism and kinetic parameters are obtained.

Lead-Free Piezoceramics: Revealing the Role of the Rhombohedral–Tetragonal Phase Coexistence in Enhancement of the Piezoelectric Properties

Until now, lead zirconate titanate (PZT) based ceramics are the most widely used in piezoelectric devices. However, the use of lead is being avoided due to its toxicity and environmental risks. Indeed, the attention in piezoelectric devices has been moved to lead-free ceramics, especially on (K,Na)NbO3-based materials, due to growing environmental concerns. Here we report a systematic evaluation of the effects of the compositional modifications induced by replacement of the B-sites with Sb(5+) ions in 0.96[(K0.48Na0.52)0.95Li0.05Nb1-xSbxO3]-0.04[BaZrO3] lead-free piezoceramics. We show that this compositional design is the driving force for the development of the high piezoelectric properties. So, we find that this phenomenon can be explained by the stabilization of a Rhombohedral-Tetragonal (R-T) phase boundary close to room temperature, that facilities the polarization process of the system and exhibits a significantly high piezoelectric response with a d33 value as high as ∼400 pC/N, which is comparable to part soft PZTs. As a result, we believe that the general strategy and design principles described in this study open the possibility of obtaining (K,Na)NbO3-based lead-free ceramics with enhanced properties, expanding their application range.

Ordered three-dimensional interconnected nanoarchitectures in anodic porous alumina

Three-dimensional nanostructures combine properties of nanoscale materials with the advantages of being macro-sized pieces when the time comes to manipulate, measure their properties, or make a device. However, the amount of compounds with the ability to self-organize in ordered three-dimensional nanostructures is limited. Therefore, template-based fabrication strategies become the key approach towards three-dimensional nanostructures. Here we report the simple fabrication of a template based on anodic aluminum oxide, having a well-defined, ordered, tunable, homogeneous 3D nanotubular network in the sub 100 nm range. The three-dimensional templates are then employed to achieve three-dimensional, ordered nanowire-networks in Bi2Te3 and polystyrene. Lastly, we demonstrate the photonic crystal behavior of both the template and the polystyrene three-dimensional nanostructure. Our approach may establish the foundations for future high-throughput, cheap, photonic materials and devices made of simple commodity plastics, metals, and semiconductors.

Simultaneous differential scanning calorimetry and thermal desorption spectroscopy measurements for the study of the decomposition of metal hydrides

An innovative experimental method to investigate the thermal decomposition of metal hydrides is presented. The method is based on an experimental setup composed of a differential scanning calorimeter connected through a capillary tube to a mass spectrometer. The experimental system allows the simultaneous determination of the heat absorbed and the hydrogen evolved from a metal hydride during thermal decomposition. This arrangement constitutes a coupled differential scanning calorimetry (DSC) and thermal desorption spectroscopy (TDS) technique. It has been applied to metal hydride materials to demonstrate the capability of the experimental system. A method to obtain the heat of decomposition of metal hydrides is described. It involves the measurement of an apparent decomposition heat as a function of the carrier gas flow.

Ferroelectric domain wall motion induced by polarized light

Ferroelectric materials exhibit spontaneous and stable polarization, which can usually be reoriented by an applied external electric field. The electrically switchable nature of this polarization is at the core of various ferroelectric devices. The motion of the associated domain walls provides the basis for ferroelectric memory, in which the storage of data bits is achieved by driving domain walls that separate regions with different polarization directions. Here we show the surprising ability to move ferroelectric domain walls of a BaTiO3 single crystal by varying the polarization angle of a coherent light source. This unexpected coupling between polarized light and ferroelectric polarization modifies the stress induced in the BaTiO3 at the domain wall, which is observed using in situ confocal Raman spectroscopy. This effect potentially leads to the non-contact remote control of ferroelectric domain walls by light.

Influence of the martensitic transformation on the hydrogenation properties of Ti50−xZrxNi50 alloys

Abstract Ti50−xZrxNi50 alloys with 0≤x≤24 develop either austenitic or martensitic crystal structures when prepared by melt-spinning or induction melting, respectively. This outcome is a consequence of the particular alloy microstructure resulting from each preparation method, which induces a difference of 100°C on the martensitic transformation temperatures for alloys with the same composition. Austenitic alloys absorb hydrogen up to 1.5 hydrogen atoms per AB unit (H/AB) at 130°C and 20 bar, without displaying any plateau pressure for hydrogen pressures between 0.1 and 10 bar. In contrast, martensitic alloys exhibit a plateau pressure with hydrogen concentrations between 1 and 2.1 H/AB, and reach a maximum hydrogen concentration of 2.6 H/AB under the same thermodynamic conditions. Consequently, martensitic alloys form a dihydride compound that, for the representative case of Ti32Zr18Ni50 alloy, has a formation enthalpy of −12.3±0.2 kcal mol H2−1.

Hydriding/dehydriding properties of magnesium–ZrCr2 composites

Abstract Composites of Mg and the intermetallic compound (IMC) ZrCr 2 have been prepared by mechanical alloying. The crystallographic structure, composition and morphology of the composites were analysed by means of X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDX). XRD patterns show that no alloy formation takes place during milling. EDX data show that milling time is a key parameter to control the morphology of the composites. Short milling times of 10 min give composites with the IMC attached to the Mg particle surface, while it is irregularly distributed in the Mg bulk for longer milling times of 70 min. Hydriding/dehydriding properties of the composites were studied by the solid–gas reaction method. Composites with a distribution of the IMC in the Mg bulk are easily activated and show faster hydriding/dehydriding kinetics than untreated Mg, which indicates that ZrCr 2 acts as a catalytic compound for the Mg–hydrogen reaction.

Ultrasonic irradiation as a tool to modify the H-desorption from hydrides: MgH2 suspended in decane

Effects of ultrasonic irradiation on magnesium hydride (MgH(2)) suspended in decane were investigated with the purpose of improving its hydrogen desorption process. Firstly, we have found that the presence of MgH(2) improves the sonolysis of decane enhancing the amount of hydrogen evolved during the sonication process. The sonicated-MgH(2) maintains its microstructural properties practically unaltered but a drastic reduction of the particle size of MgH(2) (down to approximately 20mum) as well as a high pressure MgH(2) phase are observed. However, no substantial modifications of H-kinetic properties of hydride occur as is determined by thermal desorption measurements. This could be attributed to decomposition of decane during sonication which leads to the formation of carbon compounds that hinder the thermal decomposition of MgH(2).

Influence of the preparation conditions of titanium hydride and deuteride TiHx(Dx) (X ≈ 2.00) on the specific heat around the δ-ɛ transition

Abstract Titanium hydrides and deuterides (TiH x (D x )) with hydrogen or deuterium concentraions near the stoichiometric limit, X = 2.00, were prepared in a Sieverts type apparatus under different experimental conditions; degassing time, sample temperature at which the hydrogenation takes place, stabilization time, cooling rate, purity and geometrical shape of the titanium samples. After the preparation the samples were analyzed by differential scanning calorimetry (DSC) to determine the main characteristics of the cubic-tetragonal (δ-ɛ) phase transition and to correlate these characteristics with the conditions under which the preparation was done. It was found that the cooling rate on crossing the (β + δ) field during the sample preparation is the most important factor affecting the specific heat jump in the δ-ɛ phase transition whenever other factors (titanium purity or crystalline quality of the original titanium) are not changed.