J. Agostinho Moreira

Effect of preparation method on the solid state properties and the deN2O performance of CuO–CeO2 oxides

The present work aims at investigating the catalytic decomposition of N2O over CuO–CeO2 single or mixed oxides prepared by different synthesis routes, i.e., impregnation, precipitation and exotemplating. To gain insight into the particular role of CeO2 as well as of CuO–CeO2 interactions, three different types of materials were prepared and tested for N2O decomposition both in the absence and presence of excess O2: (i) bare CeO2 prepared by precipitation and exotemplating, (ii) CuO/CeO2 oxides synthesized by the impregnation of CeO2 samples prepared in (i) with CuO, and iii) single stage CuO–CeO2 mixed oxides synthesized employing the co-precipitation and exotemplating methods. The corresponding commercial samples were also examined for comparison purposes. All materials were characterized by N2 adsorption at −196 °C, X-ray diffraction (XRD), H2 temperature-programmed reduction (H2-TPR), X-ray photoelectron spectroscopy (XPS), micro-Raman spectroscopy (micro-Raman) and scanning electron microscopy (SEM). The results demonstrated the key role of the preparation procedure on the direct catalytic decomposition of N2O. Among the bare CeO2 samples, the best performance was obtained with the samples prepared by the precipitation method, followed by exotemplating, while commercial CeO2 showed the lowest performance. All bare oxides demonstrated low N2O conversion, never exceeding 40% at 600 °C. Amongst the CuO–CeO2 oxides, the optimum performance was observed for those prepared by co-precipitation, which achieved complete N2O conversion at 550 °C. In the presence of excess oxygen in the feed stream, a slight degradation is observed, with the sequence of deN2O performance remaining unchanged. The superiority of the Cu–Ce mixed oxides prepared by precipitation compared to all of the other materials can be mainly ascribed to their excellent redox properties, linked to Ce4+/Ce3+ and Cu2+/Cu+ redox pairs. A redox mechanism for the N2O catalytic decomposition is proposed, involving N2O adsorption on Cu+ sites and their regeneration through Cu–ceria interactions.

Detection of colon cancer by terahertz techniques

Normal and cancer affected samples of colon tissue have been measured using transmission time-domain terahertz spectroscopy and continuous wave terahertz imaging. We show that it is possible to distinguish between normal and cancerous regions in the samples when they are fixed in formalin or embedded in paraffin. Plots of the refractive index of normal and cancer affected tissues as well as 2-D transmission THz images are shown. Experimental results will be presented and the conditions for discrimination between normal and affected tissue will be discussed.

Low-temperature dielectric response of NaTaO3 ceramics and films

In this work, NaTaO3 ceramics are prepared by conventional mixed oxide method and NaTaO3 films are deposited by RF magnetron sputtering on Si/SiO2/TiO2/Pt substrates. The dielectric response of the obtained NaTaO3 ceramics and films is analyzed as a function of frequency and temperature. Between 1 kHz and 1 MHz, the dielectric permittivity of NaTaO3 ceramics is frequency independent and increases on cooling up to ∼324, which is the highest value ever reported for NaTaO3 ceramics. In contrast, NaTaO3 films exhibit a dielectric relaxation between ∼20 and 30 K, following the Arrhenius law with activation energy ∼51 meV and pre-exponential term ∼10−15 s and attributed to polaron hopping.

Enhancement of tetragonality and role of strontium vacancies in heterovalent doped SrTiO3

The effect of Sr vacancies on the behavior of strontium titanate with trivalent dopants (La3+, Gd3+, and Y3+) substituting Sr2+ ions is reported. A remarkable shift of the antiferrodistortive transition temperature Ta is revealed by Raman spectroscopy for just a small content of dopant. It is shown that a unique linear dependence of Ta versus tolerance factor is obtained when Sr-vacancies are taken into account. A vacancy size value of ∼1.54 A is estimated, which is ∼7% larger than Sr2+ radius. This size difference enables explaining the unexpected increase of lattice parameter with increasing Bi3+ content in Sr1−1.5xBixTiO3.

Effect of Pt bottom electrode texture selection on the tetragonality and physical properties of Ba0.8Sr0.2TiO3 thin films produced by pulsed laser deposition

The effect of platinum (Pt) bottom electrode texture on the tetragonality, dielectric, ferroelectric, and polarization switching response of pulsed laser deposited Ba0.8Sr0.2TiO3 (BST) thin films has been studied. The x-ray diffraction and Raman analysis revealed the higher tetragonality of BST films when they were grown on higher (111) textured Pt layer. The properties like dielectric permittivity, polarization, switching time, and leakage currents were found to be correlated to tetragonality and orientation of the BST films. The polarization current was observed to be higher in BST films on Pt epitaxial layer and it exhibits exponential dependence on the electric field. The voltage-current measurements displayed Ohmic behavior of leakage current irrespective of Pt texture for low voltages (up to 1 V), whereas at higher voltages the conduction mechanism was found to be dependent on texture selection of bottom Pt electrode.

Structural and insulator-to-metal phase transition at 50 GPa in GdMnO 3

We present a study of the effect of very high pressure on the orthorhombic perovskite GdMnO3 by Raman spectroscopy and synchrotron x-ray diffraction up to 53.2 GPa. The experimental results yield a structural and insulator-to-metal phase transition close to 50 GPa, from an orthorhombic to a metrically cubic structure. The phase transition is of first order with a pressure hysteresis of about 6 GPa. The observed behavior under very high pressure might well be a general feature in rare-earth manganites.

Resistive switching in ferroelectric lead-free 0.5Ba (Zr0.2Ti0.8)O3-0.5(Ba0.7Ca0.3)TiO3 thin films

In this work, resistive switching in pulsed laser deposited ferroelectric lead-free 0.5Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (0.5BZT–0.5BCT) thin films was investigated. This study reveals that films grown at 5.5 J cm−2 have shown optimal ferroelectric and resistive switching response, which are attributed to high tetragonality, large grain size and less defect concentration. Au/0.5BZT–0.5BCT/Pt capacitors show the electroforming free resistive switching that is explained based on the polarization modulation of the Schottky-like barrier at the 0.5BZT–0.5BCT/Au interface. The polarization induced resistive switching is evidenced by its disappearance as the temperature increases to the Curie temperature. The capacitor based on film grown at 5.5 J cm−2 shows resistive switching characterized by high switching ratio of 106 at a low set/reset voltage ≈1 V, and by a stable memory window, which are highly required for memory applications.

Competing exchanges and spin?phonon coupling in Eu1?xRxMnO3 (R=Y, Lu)

This work is focused on the phase diagrams and physical properties of Y-doped and Lu-doped EuMnO3. The differences in the corresponding phase boundaries in the (x,T) phase diagram could be overcome by considering a scaling of the Y(3+) and Lu(3+) concentrations to the tolerance factor. This outcome evidences that the tolerance factor is in fact a more reliable representative of the lattice deformation induced by doping. The normalization of the phase boundaries using the tolerance factor corroborates previous theoretical outcomes regarding the key role of competitive FM and AFM exchanges in determining the phase diagrams of manganite perovskites. However, significant differences in the nature and number of phases at low temperatures and concentrations could not be explained by just considering the normalization to the tolerance factor. The vertical phase boundary observed just for Lu-doped EuMnO3, close to 10% Lu, is understood by considering a low temperature Peierls-type spin-phonon coupling, which stabilizes the AFM-4 phase in Lu-doped EuMnO3.

Raman scattering study of the phase transition sequence in the system

We have measured the polarized Raman spectra of betaine arsenate and deuterated betaine arsenate single crystals in the temperature range 12-300 K. On the basis of the comparative study of Raman spectra of protonated and deuterated compounds and by taking into account the relevant literature data, we proposed a mode assignment for . The isotopic effect in this system is discussed. The analysis of the internal modes of clearly shows the important role played by this unit in the phase sequence of . The strong coupling exhibited by the proton/deuteron modes with the internal modes of provides some evidence for the ordering of the hydrogen bonds at the phase transitions. In this analysis reveals the existence of two types of hydrogen bond related to the mechanisms underlying the phase transitions.

The role of sucrose in amino polymers synthesized by the strongly acid process

This article studies the incorporation of sucrose in amino polymers produced by the strongly acid process and its role on the physico-mechanical properties and aldehyde emission of the resulting particleboards. The incorporation of sucrose at different pH environments was studied and differences on molecular weight of resins were analyzed by gel permeation chromatography/size exclusion chromatography (GPC/SEC) and characteristic chemical bands by Raman spectroscopy. A reaction mechanism was proposed to explain the observed differences in GPC/SEC chromatograms and was supported by Raman spectra. It was observed that small amounts of sucrose incorporated in polymeric matrix are sufficient to improve the physico-mechanical properties of particleboards; the sucrose addition protocol plays a key role on these improvements.