Repositório Institucional da Universidade de Aveiro
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Local probe studies in the weakly Jahn- Teller distorted LaMnO3.08 manganite
No full textThe LaMnO3.08 manganite system was studied as a function of temperature ranging from 10 up to 292K using the perturbed angular correlation local probe technique at ISOLDE-CERN facility. The electrical field gradient (EFG) measurements performed across the different temperatures revealed a single EFG distribution, corresponding to a unique probe local environment. In contrast with the small-polaron picture, no fully Jahn-Teller distorted Mn3+O6 octahedra was observed in this insulator sample. A local single-phase scenario with uniform Mn charge distribution is proposed. (C) 2013 WILEY-VCH Verlag GmbH \& Co. KGaA, Weinhei
Comparing spectroscopic and electrochemical properties of complexes of type Cp'M(eta(3)-C3H5)(CO)(2) (Cp' = Cp, Ind, Flu): A complementary experimental and DFT study
No full textA series of allyl complexes of the general formula [Cp'Mo(eta(3)-C3H5)(CO)(2)], where Cp' = Cp, CpMe, Cp*, Ind, IndMe, IndMe2, Flu, and three tungsten analogues, has been prepared and characterized by H-1, C-13, and Mo-95 NMR, cyclic voltammetry, and the structure of [IndMo(eta(3)-C3H5)(CO)(2)] was determined by Xray single crystal analysis. Two conformers, corresponding to the two extreme orientations of the allyl ligand (endo and exo), have been identified in solution by H-1 and Mo-95 NMR for all the complexes, except for [FluMo(eta(3)-C3F15)(CO)(2)], which only presents an exo conformation in solution. A Mo-95 NMR investigation also shows the influence of electron donor capability of the Cp' ligands on the Mo-95 chemical shifts of the Mo center. He I and He II photoelectron spectra probe the electron richness of the metal center and the electronic structure of the complexes. Cyclic voltammetry studies show one oxidation process, which is only reversible for the Cp derivatives. DFT calculations provided a reliable way to determine ionization energies, and reflected very well the trends of the other properties, from the Mo-95 NMR chemical shifts, to vibration frequencies, and oxidation potentials. (1) 2015 Elsevier BY. All rights reserved
Nanosized titania modified with tungsten and silver: Microstructural characterisation of a multifunctional material
No full textWith the ever-growing interest in nanotechnologies, the area of metal oxides is playing an increasingly dominant role. Amongst them, titanium dioxide (TiO2) nanoparticles are attracting increasing interest, owing to their unique physicochemical properties, and widespread applications - i.e. photocatalysis, energy materials, antibacterial agents, gas-sensors. We characterised the microstructure of titania nanopowders - synthesised via an aqueous sol-gel method and modified with silver and/or tungsten - using XRD data, through the whole powder pattern modelling (WPPM) procedure. An overall linear dependence of the lattice volume expansion was observed - the volume increased with the lowering of the crystalline domain size. Concerning the dislocation density, no specific trend depending on the modifying cation was reported. However, for the samples fired at 600 degrees C, the undoped titania sample had a much larger number of screw dislocations in the rutile phase, while the Ag-modified sample had a much greater number of edge dislocations in the anatase phase. (C) 2013 Elsevier B.V. All rights reserved
The influence of TiO2 nanoparticles and poliacrilonitrile fibers on the rheological behavior and hardened properties of mortars
No full textThis experimental research evaluates the individual and combined influence of poliacrilonitrile (PAN) fibers and nanoTiO(2) (nT) particles on the fresh and hardened properties of mortars up to 28 days. Samples with 0-0.125 wt% PAN, 0-1.0 wt% nT and 0.83-0.87 W/B were prepared. Samples with similar workability showed distinct rheological behavior along time and the yield stress was the best rheological parameter to represent such variations. The very high specific surface area and wettability of nT was dominant over the one induced by PAN fibers and severely limited the rheological behavior maintenance up to 0.050PAN + 1.00nT sample. The physical properties (apparent density, water absorption, apparent porosity) indicate that PAN is dependent on the correct correlation with nT particles. In addition, the optimal concentration of PAN is about 0.05 wt% to the flexural and compressive strength, while the most active NOx degradation reached similar to 77% to those containing 1 wt% nT, regardless the concentration of PAN. (C) 2014 Elsevier Ltd. All rights reserved
Molecular modeling and molecular dynamics simulation of the polarization switching phenomena in the ferroelectric polymers PVDF at the nanoscale
No full textThe molecular modeling and molecular dynamics of polarization switching for the ferroclectric Earns model of polyvinylidene fluoride (PVDF) are investigated at the nanoscale. We consider a molecular model of PVDF film, consisting of two and four a chains [-CH2-CF2-](n) limited by n=6 elementary units. The first-principle approach is applied to the switching and kinetics of these models. Two types of behavior were established for PVDF chains: simultaneous and sequential rotation in high and low electric fields. Kinetics of sequential polarization switching shows a homogeneous critical behavior in the low electric field with a critical point at Landau-Ginzburg-Devonshire (LGD) coercive field E=E-C. This type of kinetics demonstrates a kink-like behavior for polarization solitary wave propagation. The simultaneous type of kinetics demonstrates the total domain-like polarization switching. corresponding to exponential behavior of switching Lime in high electric field as for bulk samples. Corresponding LGD intrinsic coercive field for a two-chain and four-chains model is E-C similar to 2.0 GV/m with revealing size effect. Obtained results show common quantum nature of PVDF chains switching phenomena the quantum interaction of the PVDE. molecular orbitals under applied electric field at the nanoscale level. The results obtained are compared with experimental data. (C) 2013 Elsevier B.V. All rights reserved
Timesaving microwave assisted synthesis of insulin amyloid fibrils with enhanced nanofiber aspect ratio
No full textInsulin amyloid fibrils with enhanced aspect ratio, were prepared using a timesaving microwave assisted (MW) methodology, reducing the incubation time from 13 to 2 h. The fibrillation process was followed indirectly by Thioflavin T Fluorescence and UV-vis analysis, by measuring the amount of beta-sheets formed and the insulin present in solution, respectively. TEM and AFM analysis revealed that the insulin fibrils obtained through the MW method, have very similar lengths but are much thinner than the ones obtained using the conventional method (CM). Additionally, it was verified that the nature of the peptides present in the final insulin fibrils was not affected by microwave irradiation. These morphological differences might reflect on noticeably enhanced mechanical and optical properties that can exploited on the development of advanced bionanomaterials. (C) 2016 Elsevier B.V. All rights reserved
Bottom ash from biomass combustion in BFB and its use in adhesive-mortars
No full textBottom bed ash (BA) from forest biomass residue combustion generated in bubbling fluidized bed combustor (BFBC) was chemically, mineralogically and physically characterized and evaluated according to Decision 2003/33/CE and its landfill classification was determined based on criteria established by European and Portuguese legislations. Aiming to investigate its potential for being used as alternative aggregate in cementitious-adhesive mortar (CAM), formulations were produced by replacing commercial sand by an equivalent mass of 25,50 and 100 wt.% of bottom ash. CAM properties were measured in fresh and hardened conditions. The bottom ash effect on the tensile adhesion strength (TAS) was investigated on samples cured under four distinct conditions. Macro and microstructural analyses were carried out to identify the influence of wetting capability and transference area in the TAS. Results revealed that high ash levels cause relevant changes in TAS, particularly those evaluated after 30 min of open time. However, within reasonable limits, the effects are minor and acceptable. This recycling route seems feasible, since it combines environmental benefits with economic advantages for the adhesive mortar producers in the sense that they can use BA as raw material, saving a natural resource; this recycling option is also important since one can avoid waste landfilling. (C) 2014 Elsevier B.V. All rights reserved
Isolation of natural red colorants from fermented broth using ionic liquid-based aqueous two-phase systems
No full textThere is a growing demand for natural colorants. This is prompting the search for new alternative and \"benign\" separation systems allowing higher recoveries, extraction yields, and selectivities. This work investigates the use of aqueous two-phase systems (ATPS) based on ionic liquids as extraction processes for the recovery of red colorants from the fermented broth of Penicillium purpurogenum DPUA 1275. Several ATPS based on quaternary ammonium and imidazolium were studied in this work aiming at separating the red colorants produced from the remaining colorants and contaminant proteins present in the fermented broth. The results suggest that the red colorants can be isolated by an appropriate manipulation of some of the process conditions, such as the use of quaternary ammonium with short alkyl chains, alkaline media, and short tie-line lengths (extraction point systems with lower concentrations of ionic liquid). These conditions allow large partition coefficients for the red colorants (K (red) = 24.4 +/- A 2.3), high protein removal (60.7 +/- A 2.8 %) and selectivity parameters (S (red/prot) = 10.05)
Hydrothermal Synthesis of Si-doped Hydroxyapatite Nanopowders: Mechanical and Bioactivity Evaluation
No full textSi-doped hydroxyapatite nanoparticles (n-Si(x)HA) were prepared by hydrothermal synthesis from calcium nitrate tetrahydrate and diammonium hydrogen orthophosphate. A rod-like morphology was obtained for all the powders irrespective of the incorporated Si-doping level. But the crystallinity of the n-Si(x)HA powders, the density achieved upon sintering powder compacts and their mechanical properties (three-point-bending strength), as well as their biomineralization activity evaluated by immersing them into simulated body fluid (SBF) were found to be dependent on the Si-doping amount
Synthesis and antibacterial activity of pyridylselenium compounds: Self-assembly of bis(3-bromo-2-pyridyl)diselenide via intermolecular secondary and pi center dot center dot center dot pi stacking interactions
No full textVarious pyridylseleniums and their hydrochloride salts have been synthesized and evaluated for their antibacterial activities. SEM, TEM and enzyme profiling techniques have been used to investigate the mode of action of these compounds as antibiotics. Single crystal X-ray study of bis(3-bromo-2-pyridyl) diselenide (6b) has also been reported. The molecule 6b displays pi center dot center dot center dot pi stacking and pi center dot center dot center dot Br secondary interactions leading to a supramolecular self-assembly. A search on Cambridge Structural Database was undertaken to identify the pi center dot center dot center dot pi stacking interactions in various other bis(2-pyridyl) diselenides. (C) 2014 Elsevier B. V. All rights reserved