Repositório Institucional da Universidade de Aveiro
Not a member yet
41903 research outputs found
Sort by
Photoluminescent Thermometer Based on a Phase-Transition Lanthanide Silicate with Unusual Structural Disorder
No full textThe hydrothermal synthesis of the novel Na[LnSiO(4)] (Ln = Gd, Eu, Tb) disordered orthorhombic system is reported. At 100 K, and above, these materials are best described in the centrosymmetric orthorhombic Pnma space group. At lower temperatures (structure solved at 30 K) the unit cell changes to body-centered with Imma symmetry. The materials exhibit unique photophysical properties, arising from both, this phase transformation, and the disorder of the Ln(3+) ions, located at a site with D-2d point symmetry. Na[(Gd0.8Eu0.1Tb0.1)SiO4] is an unprecedented case of a luminescent ratiometric thermometer based on a very stable silicate matrix. Moreover, it is the first example of an optical thermometer whose performance (viz., excellent sensitivity at cryogenic temperatures <100 K) is determined mainly by a structural transition, opening up new opportunities for designing such devices
Studies on polymorph conversion in a new cyclodextrin inclusion compound
No full textA novel beta-cyclodextrin (beta CD) inclusion compound was prepared using 4-phenylpyridine-N-oxide (PPNO) as the organic guest. The inclusion compound, beta CD.PPNO, was characterised both in solution and in the solid state using numerous techniques. H-1 NMR in aqueous solution allowed the determination of a 1 : 1 stoichiometry and an association constant of 164 +/- 21 M-1. Powder and single-crystal X-ray diffraction studies showed the formation of two distinct crystal phases, appearing at different timings. Over time, one of the crystal phases converts spontaneously into the other. This work is the first to monitor the conversion of different polymorphs of cyclodextrin inclusion compounds in real time
Visible light-induced diastereoselective E/Z-photoisomerization equilibrium of the C=C benzofuran-3-one-hydantoin dyad
No full textThe diastereoselective photodependent isomerization equilibrium of E/Z-1,3-ditolyl-5-[3-oxobenzofuran-2(3H)-ylidene] imidazolidine-2,4-dione (5) is reported. Both diastereomers E-5 and Z-5 are stereochemically stable in solid state but show significant photosensibility in solutions of halogenated solvent. The photoisomerization equilibrium of E/Z-5 is therefore deduced from the H-1 NMR profile after visible-light irradiation of both E-5 and Z-5 samples. The results of the kinetic study, monitored by UV-HPLC, reveal that the E/Z equilibrium is diastereoselective and photodependent, being the transformation E -> Z proceeding faster than that of Z -> E, and the E/Z ratio at the equilibrium depends on the used solvent, light source, and temperature. Both diastereomers are visible-light photosensitive tending to coexist together in equilibrium solutions at a determined ratio, which is always in favor of the Z-product assuming a minimum thermodynamic energy and an increased entropy of the system. Time-dependent density functional theory calculations suggest that the photoisomerization mechanism proceeds via a conical intersection involving the first-excited state: Upon irradiation, the E-5 isomer is excited to the S1 potential energy surface, where it relaxes through rotation of the C=C bond and reaches a conical intersection with the ground-state potential energy surface, thus yielding the Z-5 isomer. Copyright (C) 2014 John Wiley \& Sons, Ltd
Gelled graphite/gelatin composites for latent heat cold storage
No full textAqueous suspensions of graphite were shape stabilized by the additions of collagen and characterized as phase change materials for cold storage with enhanced thermal conductivity. Collagen addition stabilized the graphite suspensions in aqueous media without requirements for additional stabilizers or previous functionalization. Gelation at room temperatures prevented sedimentation of graphite particles. SEM of dried samples confirmed nearly random distribution of graphite particles. These gelled suspensions with enhanced thermal conductivity were tested for latent heat cold storage. The thermal response was retained upon repeated phase change cycles, and impedance spectroscopy was used to monitor these cycles. The response time upon phase change was somewhat longer than predicted by taking into account the values of thermal diffusivity and the estimated value of Stefan number, probably due to significant interfacial resistance to heat transfer, combined with discontinuities caused by volume expansion upon phase change. (C) 2012 Elsevier Ltd. All rights reserved
Increased surface area during sintering of calcium phosphate glass and sodium chloride mixtures
No full textGlass scaffolds were obtained by the salt sintering method, using mixtures of calcium phosphate glass, 25.42%SiO2-10.89% P2O5-32.68%CaO-31%Mg0(mol%), and salt, NaCl, pressed and sintered bellow the salt melting point, followed by leaching of the salt in water. The sintering behaviour of glass/salt powder mixtures was followed by dilatometry, BET and SEM. It was found that the specific surface area (SSA) of supports sintered between 700-750 degrees C and leached, can be 15-50 times higher than that of the glass precursor powder, a very surprising behaviour because it presents the opposite tendency of a typical sintering process. Dilatometric curves showed an unusual expansion from similar to 450 degrees C until similar to 700 degrees C, followed by shrinkage at higher temperatures. This expansion accompanies the observed increase of SSA and the microstructures showed a clear effect of reaction between the glass and the salt with remarkable wrinkling of the glass particle surfaces. This phenomenon was further investigated by using DTA, XRD and FTIR. (C) 2014 Elsevier Ltd. All rights reserved
Gelatine-assisted synthesis of magnetite nanoparticles for magnetic hyperthermia
No full textMagnetite nanoparticles were synthesized by the co-precipitation method exploring the use of gelatine and agar as additives. For comparison, magnetite nanoparticles were also prepared by standard co-precipitation, by co-precipitation with the addition of a surfactant (sodium dodecyl sulphate) and by the thermal decomposition method. The structure and morphology of the synthesized nanoparticles were investigated by powder X-ray diffraction and transmission electron microscopy. Their magnetic properties were studied by SQUID magnetometry and Fe-57 Mossbauer spectroscopy. The nanoparticles potential for applications in magnetic hyperthermia was evaluated through heating efficiency under alternating magnetic field. The results show that all synthesis methods produce Fe3-xO4 nanoparticles with similar sizes. The nanoparticles synthesized in the gelatine medium display the narrowest particle size distribution, the lowest oxidation degree, one of the highest saturation magnetization values and the best hyperthermia efficiency, proving that this gelatine-assisted synthesis is an efficient, environmental friendly, and low-cost method to produce magnetite nanoparticles
Chiral Copper(II) Bis(oxazoline) Complexes Directly Coordinated to Amine-Functionalized Phenylene/Biphenylene Periodic Mesoporous Organosilicas as Heterogeneous Catalysts
No full textCopper(II) complexes with commercial chiral bis(oxazoline) s were for the first time directly coordinated onto aminemodified mesoporous phenylene and biphenylene silicas. All final materials maintained the 2D ordered mesoporous structure. The copper metal coordinates directly to the amine groups within the walls of the materials. The materials were tested as asymmetric heterogeneous catalysts in the kinetic resolution of hydrobenzoin. They were active, selective, and enantioselective in this reaction. The material containing the (S)-(-)-2,2'-isopropylidene- bis(4-phenyl-2-oxazoline) (Me(2)PhBox) ligand, the best ligand in the homogeneous phase, presented the highest enantioselectivity of all the materials in the first cycle (73 %). The 2,2'-methylenebis[(4S)-4-phenyl-2-oxazoline] (PhBox) ligand rendered the catalyst more stable, independent of the anion or organic moiety in the periodic mesoporous organosilica material; this catalyst was reused over five cycles without any significant loss of catalytic activity or enantioselectivity. This type of ligand plays an important role in the stability of the corresponding copper(II) complex upon immobilization and in the robustness of the heterogeneous catalyst upon reuse
Enhancement of near infrared emission in La co-doped ZnO/Er nanoplates
No full textUndoped and rare earth doped Er, Er/Yb and Er/Yb/La ZnO nanoplates were synthesized in order to study the effect of La co-doping on near infrared (NIR) emission properties of the Er ions hosted in the semiconductor. Samples were characterized by X-ray diffraction, scanning electron microscope, energy-dispersive X-ray spectroscopy. Emission spectra of the La co-doped Er/Yb ZnO nanoplates showed an efficiency enhancement of Er ion emission in NIR region around 1550 nm which is a very important wavelength for optical communication technologies. The enhancement mechanism for the NIR emission is discussed in detail. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved
Mechanics of filled carbon nanotubes
No full textThe benefits of filling carbon nanotubes (CNTs) with assorted molecular and crystalline substances have been investigated for the past two decades. Amongst the study of new structural phases, defects, chemical reactions and varied types of host-guest interactions, there is one fundamental characterisation aspect of these systems that continues to be overlooked: the mechanical behaviour of filled CNTs. In contrast to their empty counterparts, the mechanics of filled CNTs is a subject where reports appear far and apart, this despite being key to the application of these materials in technological devices. In the following paragraphs, we review the work that has been carried out up to the present on the mechanics of filled CNTs. The studies discussed range from experimental resonant frequency essays performed within electron microscopes to modelling, via molecular dynamics, of three-point bending of nanotubes filled with gases. (C) 2014 Elsevier B.V. All rights reserved
New Cu3TeO6 Ceramics: Phase Formation and Dielectric Properties
No full textTargeting low temperature cofired ceramics (LTCC) applications and base-metal electrode multilayer ceramic capacitors (BME-MLCCs), ceramics of a new composition, tricopper tellurate (Cu3TeO6), are reported here. The crystal structure of Cu3TeO6 was determined to be cubic, Ia3, with the unit cell parameter a = 9.538 angstrom. The sequence of phase formation is proposed with the oxidation of tetravalent tellurium (Te4+) into hexavalent tellurium (Te6+) as a key step for the formation of Cu3TeO6. Ceramics sintered at 865 degrees C with densities of 94% exhibit two dielectric anomalies in the temperature dependence of the dielectric response, around -150 degrees C and +50 degrees C, respectively, accompanied by obvious frequency dispersion of the relative permittivity (epsilon(r)) and dielectric losses (tan delta), with an Arrhenius like behavior. A temperature stable dielectric region (near room temperature) formed between the two anomalies with epsilon(r) similar to 12 and tan delta similar to 0.01, and a very low positive temperature coefficient of the relative permittivity (TC epsilon(r)), 2.07 X 10(-4) degrees C-1, was obtained in the same region. The low temperature dielectric anomaly is associated with the possible mixed Cu+/Cu2+ valence in Cu3TeO6 ceramics, while the high temperature anomaly is attributed to point defect ordering, including V-O, Cu-Cu2+(+)',which might be formed during sintering. Therefore, Cu3TeO6 ceramics are of interest in view of not only the possible applications in BME-MLCCs, LTCC, and related technologies, but also for their possible compatibility with low cost abundant Cu electrodes