Lisandro Pavie Cardoso

Ambient pressure colossal magnetocaloric effect in Mn1−xCuxAs compounds

Magnetic refrigeration is a good alternative to gas compression technology due to higher efficiency and environmental concerns. Magnetocaloric materials must exhibit large adiabatic temperature variations and a large entropic effect. MnAs shows the colossal magnetocaloric effect under high pressures or with Fe doping. In this work the authors introduce a class of materials—Mn1−xCuxAs—revealing a peak colossal effect of −175J∕(Kkg) for a 5T field variation at 318K and ambient pressure.

Structural properties of heteroepitaxial systems using hybrid multiple diffraction in Renninger scans

A method of characterizing heteroepitaxial structures using hybrid multiple diffractions (hybrid MD) which appear in the layer Renninger scans (RS) together with the normal MD features, is reported. The three beam surface MD cases are used to provide high intensity and structural sensitivity. The RS peak measurements around the symmetry mirrors allow for the layer parallel lattice parameter determination. A simulation program was developed in order to account for the influence of the wavelength, incident beam divergence, sample mosaic spread, and substrate/layer lattice misorientation in the correct position and profile of the RS peaks. GaAs/Si samples with different layer thicknesses have been analyzed as an application of the method.

Characterization and shelf life of β-carotene loaded solid lipid microparticles produced with stearic acid and sunflower oil

Solid lipid microparticles were tested as microencapsulation systems for protecting β-carotene from degradation. Blends of long-chain (C18) solid lipids (70% stearic acid) and sunflower oil (30%) were used to produce lipid microparticles encapsulating the carotenoid. Polysorbate 80 (4%) was employed to stabilize the stearic acid microparticles. The concentration of β-carotene was monitored using spectrophotometry, the particle size distribution was measured by laser diffraction, the crystal structure was determined by wide angle X-ray diffraction (WAXD), and the thermal behaviour was characterized by differential scanning calorimetry (DSC) over a period of seven months. All of the systems had an average particle size smaller than 5 µm. To avoid β-carotene oxidation, α-tocopherol was added to the formulations and its action as an oxygen trap was crucial for the antioxidant effect. For stearic-acid microparticles with a-tocopherol, more than 90% of the initial amount of β-carotene was preserved after seven months under refrigerated storage (7-10°C) in the dark. Significant microstructural alterations were detected using WAXD and DSC only in the stearic acid microparticles without alpha-tocopherol. These results seemed promising and suggested that the blends of long-chain solid lipids and liquid lipids were suitable for the production of stable solid lipid microparticles.

Evidence for the monoclinic-tetragonal phase coexistence in Pb(Zr0.53Ti0.47)O3 thin films

The structure and ferroelectric properties of PbZr0.53Ti0.47O3 thin films were investigated in detail by using the x-ray diffraction technique. The surface morphology of the film was studied by using the atomic force microscopy technique, showing a film with a dense morphology and a smooth surface. Based on recent results reported by Pandey et al (2008 Acta Crystallogr. A64 192), Rietveld refinements of the structure were conducted considering different models proposed in the literature. Results suggested the monoclinic and tetragonal (M+T) phase coexistence, with P4mm and Cm space groups, respectively. The monoclinic phase (68 mol%) is dominant over the tetragonal phase (32 mol%) for this PZT film composition.

Hybrid multiple diffraction in Renninger scan for heteroepitaxial layers

In this paper, we report the successful observation of the hybrid multiple diffraction (hybrid MD) in Renninger scans (RS) [M. Renninger, Z. Kristallogr. 106, 141 (1937)]. These diffractions occur when the beam first diffracted by an inclined layer (or substrate) plane with respect to the sample surface crosses the interface layer/substrate to be rescattered by another substrate (or layer) plane towards the detector. The construction of an incidence diagram allows to establish the occurrence of the hybrid MD for heteroepitaxial layers. The layer imperfection (mosaic spread) is used to explain these extra features in RS together along the normal MD features. The case of a GaAs layer grown on a Si substrate is investigated.

Structural and magnetic study of the MnAs magnetocaloric compound

The temperature induced phase transition in MnAs is analyzed in this work using X ray Rietveld refinement. The results show the presence of the hexagonal phase (P63/mmc) at room temperature and the first-order structural-magnetic transition to the orthorhombic phase (Pnma) around 318 K was followed in detail. The MnAs magnetic characterization allowed to obtain the transition temperature and a maximum value of 47 J/(kg.K) for the measured magnetocaloric effect for a magnetic field variation of 5 T.

Plasma surface treatment of AISI 4140 steel for improved corrosion resistance

Alloy AISI 4140 steel was plasma-nitrided in a low-pressure abnormal glow discharge in 75% N2+25% H2 atmosphere (P=3 torr andT = 540°C for 2 h) using a home-made laboratory reactor. The corrosion behaviour was evaluated in a chloride environment by potentiostatic and potentiodynamic polarization as a function of thermal treatment and cooling cycle. Results from potentiodynamic polarization in NaCl (3%) indicated the presence of a low anodic dissolution current of about 1 μA cm−2 for ionitrided steel having layers thickness >5 μm as compared to 0.1 A cm−2 for untreated steel. In addition, the open circuit potential shifted to noble potential clearly indicating that the ionitridation process is a very efficient process for corrosion protection of AISI 4140 steel. Typical results obtained from samples oxidized during the cooling cycle showed an improvement in the corrosion resistance. The potential for pitting initiation and propagation as well as for pitting protection has been determined in a chloride environment. The use of electrochemical techniques as a control of standard parameters has been suggested.

Piezoelectric coefficients of l-arginine hydrochloride monohydrate obtained by X-ray multiple diffraction using synchrotron radiation

In this paper, a very useful development to improve the application of a method based on the multiple-diffraction technique to determine the piezoelectric coefficients of crystals is described. The idea is to choose any specific crystallographic direction (plane) through the secondary peaks in the Renninger scan instead of those directions related to the electric field by the piezoelectric tensor. For a set of multiple-diffraction planes, it is possible to obtain an equation system based on the ordinary peak position for each H secondary plane that appears in the Renninger scan. This useful development was successfully applied to the amino acid L-arginine hydrochloride monohydrate in order to provide the d 21 , d 22 , d 23 and d 25 piezoelectric coefficients by applying the electric field along the [010] crystallographic direction.

Spatial ordering in InP/InGaP nanostructures

We report the observation of a spatially-ordered bidimensional array of self-assembled InP quantum dots grown on slightly In-rich InGaP layers. The alignment of InP dots is observed along [100] and [010] directions. This effect is enhanced when 2° off vicinal substrates are used; it is also strongly dependent on growth temperature. Our results suggest that the density and size of CuPt-type atomically ordered regions as well as compositional modulation of InGaP layers play an important role on the spatial alignment of InP/InGaP quantum dots.

Observation of coherent hybrid reflection with synchrotron radiation

High resolution synchrotron radiation has been used to investigate the occurrence of coherent hybrid reflections (CHR) in the In0.49Ga0.51P/GaAs(001) structure. Several φ scans at the 002 layer reflection were carried out. The scanned φ intervals are correlated by the [001] axis symmetry and should present the same pattern. A break in the symmetry is observed due to constructive/destructive interference of the hybrid amplitudes with the amplitude from the 002 layer reflection. The effects of substrate miscut and interface distance are taken into account to explain the observed patterns. The application of CHR as a high sensitive tool to analyze epitaxial growth is discussed.