Valerio Rossi Albertini

The kinetics of phase transitions observed by energy-dispersive X-ray diffraction

A new energy-dispersive X-ray diffraction (EDXD) method to study the kinetics of the phase transitions (PTs) is reviewed. It enables one to follow the evolution of systems undergoing structural transformations in real time. It is based on a novel approach to PTs and provides several advantages with respect to the conventional techniques used for the same purpose. A detailed treatment of the theory is accomplished to demonstrate the consistency and reliability of this method. Despite the initial complexity of the subject that involves the interactions between X-ray and matter, just very simple measurements are required. Furthermore, the data processing is trivial and the relevant information on the PTs can be straightforwardly obtained from the X-ray spectra. The applications of the EDXD-PT method to the systems studied until now, namely polymeric, biological and mineralogical samples, are finally reported.

Enhancement of photo/thermal stability of organic bulk heterojunction photovoltaic devices via gold nanoparticles doping of the active layer

This study focuses on the crucial problem of the stability of organic photovoltaic (OPV) devices, aiming to shed light on the photo and thermal degradation mechanisms during prolonged irradiation under ambient conditions. For this purpose, the stability enhancement of bulk heterojunction OPV devices upon embedding surfactant free Au nanoparticles (NPs) into the photoactive layer is investigated by in situ time-resolved energy dispersive X-ray reflectometry (EDXR), photoluminescence (PL) and Raman spectroscopy as well as device degradation electrical measurements. It is shown that besides the improved cell efficiency attributed to plasmon absorption and scattering effects, the embedded NPs act as performance stabilizers, giving rise to enhanced structural stability and, in turn, to reduced photodegradation rate of the respective OPV devices. It is particularly clarified that, in addition to further stabilization of the polymer-fullerene blend, the observed improvement can be ascribed to a NP-mediated mitigation of the photooxidation effect at the cathode-active layer interface. Our work suggests the exploitation of surfactant free NPs to be a successful approach to address aging effects in OPV devices.

Superhard properties of rhodium and iridium boride films.

Very recently, the superhard properties of rhenium and ruthenium boride films were reported, this research being inspired by the discovery of the ReB(2) bulk superhardness. In this paper, we report the first successful deposition and characterization of rhodium and iridium boride films, other possible candidates for superhard materials. The films were prepared, applying the pulsed laser deposition technique, and studied by X-ray diffraction, scanning electron and atomic force microscopies, and Vickers microhardness. The refined structural parameters for RhB(1.1) and IrB(1.1) films were obtained. The RhB(1.1) film is characterized by the submicrometer crystallite size, whereas for the IrB(1.1) film, the crystallite size is in the tens of nanometers range, and this latter film presents a slightly preferred orientation along the [004] direction. Both the films exhibit very similar morphology, being composed of dense globular aggregate texture. The RhB(1.1) film presents a homogeneously textured surface with an average roughness of 20-50 nm, whereas the IrB(1.1) film possesses a finer texture with an average roughness of 20-30 nm. The intrinsic hardness of both films lies in the superhardness range: the 1.0 microm thick RhB(1.1) film possesses a hardness of 44 GPa, whereas the 0.4 microm thick IrB(1.1) film has a hardness of 43 GPa.

Controlling photoinduced degradation in plastic photovoltaic cells: A time-resolved energy dispersive x-ray reflectometry study

The electrode-active layer interface of organic photovoltaic cells, a critical point in the development of organic devices, was studied by the energy dispersive x-ray reflectivity (EDXR) technique applied in situ. An EDXR-based protocol allowing discrimination between the possible mechanisms that produce the aging process at the interface was established. The study detects photoinduced oxidation of the electrode at the buried interface, to which fading of the device performances could be attributed. This conclusion was further confirmed by results obtained on a new cell, of selectively modified architecture, whose performances turned out to be stable in time.

Titanium and Ruthenium Phthalocyanines for NO2 Sensors: A Mini-Review

This review presents studies devoted to the description and comprehension of phenomena connected with the sensing behaviour towards NO2 of films of two phthalocyanines, titanium bis-phthalocyanine and ruthenium phthalocyanine. Spectroscopic, conductometric, and morphological features recorded during exposure to the gas are explained and the mechanisms of gas-molecule interaction are also elucidated. The review also shows how X-ray reflectivity can be a useful tool for monitoring morphological parameters such as thickness and roughness that are demonstrated to be sensitive variables for monitoring the exposure of thin films of sensor materials to NO2 gas.

Time/Space-Resolved Studies of the Nafion Membrane Hydration Profile in a Running Fuel Cell

2009 WILEY-VCH Verlag Gm The determination of the amount and spatial distribution of water in a polymeric membrane of a proton-exchange-membrane fuel cell (PEMFC) under working conditions is a fundamental task to address in PEMFC technology. Indeed, since proton transfer in such polymeric materials is known to be assisted by water, the fuel-cell (FC) performances depend on the proton-exchange membrane (PEM) hydration degree. However, the hydration degree is influenced not only by the electrochemical conditions the FC is submitted to, but also by many other independent parameters, such as the constriction exerted on the membrane by the other FC components, the electrical current flowing across it, the actual temperature, aging effects, etc, which are hard to take into account in theoretical calculations. In this work, an original method based on very-high-energy synchrotron-radiation X-ray diffraction is applied to carry out the first space/time-resolved measurements of the PEM hydration profile in a running FC. Due to their capability of effectively converting chemical into electrical energy, PEMFCs play a major role in the development of future environmentally friendly hydrogen-based technologies. Indeed, PEMFCs are considered promising candidates for automotive propulsion and for stationary applications. To optimize the performance and lifetime of a PEMFC, one major problem that must be solved is the water management, because the PEM proton conductivity is highly dependent upon its water content. On the other hand, an excess of water is detrimental, as it may produce cathode flooding, and a consequent reduction of gas supply. During the running of the cell, the membrane both absorbs water, which is produced by oxygen reduction at the cathode or carried by the humidified gas stream, and releases it, because of the evaporation induced by the gas flow and by heating occurring under operative conditions. Water transport through the membrane is caused mainly by the electro-osmotic drag of water by protons moving from the anode to the cathode, and by back-diffusion of the water produced at the cathode, towards the anode, as a consequence of the concentration gradients that build up upon operation. In steady conditions, equilibrium among these competitive mechanisms is reached. However, when the operative parameters are changed, complex water dynamics are observed. Several theoretical investigations have been carried out to describe water intake, release, and transport through Nafion membranes. Nevertheless, uncertainties are also present in calculations, due to several experimental effects and constraints, which further complicate the water dynamics and are difficult to model. On the other hand, only a few experimental techniques aimed at measuring the water distribution in the membrane are available, due to the intrinsic difficulty of isolating the signal coming from water molecules inside a relatively thick membrane assembled in a working cell (as required by an in situ investigation). X-ray techniques, small-angle neutron scattering (SANS), magnetic-resonance imaging andmicro-Raman, neutron radiography, electrical-resistance measurements, infrared absorption and fluorescence spectroscopy have been used for this purpose. Unfortunately, if one excludes micro-Raman measurements, all of these techniques exhibit rather low spatial resolutions (if any). Moreover, they have other severe limitations, such as slow response to the hydration-degree variations, weak signals (resulting in poor accuracy), and only indirect dependence on the quantity of interest. Here, we propose an alternative approach, based on veryhigh-energy (about 90 keV) X-ray diffraction (VHEXD), to measure the hydration degree of PEMs in a working cell in situ. The method consists of vertical stratigraphy of the membrane from one electrode to the other, corresponding to ideally ‘‘slicing’’ the membrane itself in a stack of layers. As a result, the time-dependence of the hydration degree in each layer has been determined at the highest accuracy ever achieved, in all the experimental conditions in which PEMFCs may operate, and in the presence of all the concomitant effects mentioned above. To apply the method discussed in the experimental section, preliminary tests were required to identify the right inclination of the cell (parallelism between the PEM plane and the X-ray beam) and the height at which the beam intersects only the membrane. With these tests, spurious contributions from the other components of the cell to the diffraction patterns can be prevented. The parallelism condition was met by taking a sequence of radiographies of the cell during a scan of the rocking angle, carried out to visualize its inner parts. Figure 1a shows the first of these radiographies, collected after the cell was placed in the beam trajectory. The components of the cell around the membrane can be easily distinguished. The

Real-time monitoring of the mechanism of poorly crystalline apatite cement conversion in the presence of chitosan, simulated body fluid and human blood.

In this study, the real-time monitoring of structural changes, occurring upon poorly crystalline apatite bone cement hardening in the presence of chitosan, simulated body fluid and human blood, was performed. Strong experimental evidence of octacalcium phosphate intermediate phase is provided. The energy dispersive X-ray diffraction was applied in situ to monitor the structural changes upon the transformation process, while the Fourier transform infrared spectroscopy and the scanning electron microscopy supplied information on the vibrational and morphological properties of the system. The cooperative action of chitosan and simulated body fluid induces the formation of a preferentially oriented hydroxyapatite phase, this process being similar to the oriented self-assembling process in collagen-apatite matrix in human plasma, occurring upon in vivo biomineralization. Conversely, the presence of blood does not induce any significant change in hardening kinetics and the final structure of the investigated cement.

Elucidation of Real-Time Hardening Mechanisms of Two Novel High-Strength Calcium Phosphate Bone Cements

Despite the numerous literature data available in the field of calcium phosphate bone cements, the mechanism and kinetics of their hardening, both of which are of great importance for cements application, in most cases, is unknown. In this work, the mechanism and kinetics of hardening of two novel high-strength calcium phosphate bone cements were studied using the energy dispersive X-ray diffraction technique, which allows rapid collection of the patterns. The phase transformations occurring on the setting and hardening processes were monitored in situ. Containing minimal quantity of components, whose mixing leads to the formation of cements with pH close to neutral, the cements under study are simple in handling. The main component of both formulations is tetracalcium phosphate. In both cements, the effect of the addition of high- and low-molecular weight chitosan on phase development and kinetics was investigated in detail. One of the cements has the compressive strength of about 70 MPa, whereas the strength of the other, containing Ca(3)Al(2)O(6), is much higher, about 100 MPa. This latter cement could be regarded as an alternative to the common low-strength bioresorbable brushite cements.

Effect of the x-ray beam collimation on the resolution of an energy dispersive diffractometer

Although the energy dispersive x-ray diffraction (EDXD) technique has proved to have several merits in comparison with its conventional angular dispersive counterpart, it has the serious drawback that its resolution is intrinsically lower than that of the latter. This makes EDXD unsuitable each time that high-resolution measurements are needed. However, a wide class of samples does not require high resolution since the diffraction peaks they produce are so wide that the further broadening due to the use of EDXD is negligible. Amorphous solids, liquids, semicrystalline materials, and nanocrystalline powders belong to such a category. In this case, it is not worth performing sophisticated simulations to calculate the angular transfer function of the diffractometer because a simplified model is sufficient to describe the effect of the angular divergence of the x-ray beam on peak broadening. The aim of the present work is to obtain an analytic function that can be used for this purpose, allowing the collimati...

Study of Magnetic Easy Axis 3-D Arrangement in L1

We report a study of angular magnetic properties of high-anisotropy L10 CoPt (111) films having a tilted magnetic easy axis configuration without an oblique-grain microstructure. In particular, we investigated the field dependence of remanent magnetization while rotating the magnetic field inside three intersecting planes. The out-of-plane tilting of the L10 c-axis (the easy axis of the tetragonal cell) was induced by using a Pt (111) underlayer deposited onto a single-crystal MgO substrate in a conventional frontal pulsed laser deposition (PLD). The observed behavior is consistent with the presence of four easy axes with mutually orthogonal in-plane projections, symmetrically tilted at 36deg with respect to the film plane. Such a system can be used, like a common single-axis tilted medium, to record information in perpendicular mode, lowering the writing field to approximately 75% of the value along the easy direction, while still maintaining the high thermal stability typical of the L10 alloy. Moreover, the in-plane charge compensation arising from this easy axis arrangement when a perpendicular writing field is applied may favor a media noise reduction and better performance with respect to a single-axis tilted system.