Coupling of magnetic, strain and electric polarization fields in the structure of multiferroic material

The comprehensive study on the coupling of magnetism, electrical polarization and the crystalline lattice with the off-stoichiometric effects in self-doped multiferroic hexagonal h-LuMnxO3±δ (0.92≤x≤1.12) ceramic oxides was carried out for the PhD work. There is a complex coupling of the three ferroic degrees. The cancelation of the magnetic moments of ions in the antiferromagnetic order, electric polarization with specific vortex/antivortex topology and lattice properties have pushed researchers to find out ways to disclose the underlying physics and chemistry of magneto-electric and magneto-elastic couplings of h-RMnO3 multiferroic materials. In this research work, self-doping of Lu-sites or Mn-sites of h-LuMnxO3±δ ceramics prepared via solid state route was done to pave a way for deeper understanding of the antiferromagnetic transition, the weak ferromagnetism often reported in the same crystalline lattices and the ferroelectric properties coupled to the imposed lattice changes. Accordingly to the aim of the PhD thesis, the objectives set for the sintering study in the first chapter on experimental results were two. First, study of sintering off-stoichiometric samples within conditions reported in the bibliography and also extracted from the phase diagrams of the LuMnxO3±δ, with a multiple firings ending with a last high temperature step at 1300ºC for 24 hours. Second, explore longer annealing times of up to 240 hours at the fixed temperature of 1300 ºC in a search for improving the properties of the solid solution under study. All series of LuMnxO3±δ ceramics for each annealing time were characterized to tentatively build a framework enabling comparison of measured properties with results of others available in literature. XRD and Rietveld refinement of data give the evolution the lattice parameters as a function to x. Shrinkage of the lattice parameters with increasing x values was observed, the stability limit of the solid solution being determined by analysis of lattice parameters. The evolution of grain size and presence of secondary phases have been investigated by means of TEM, SEM, EDS and EBSD techniques. The dependencies of grain growth and regression of secondary phases on composition x and time were further characterized. Magnetic susceptibility of samples and magnetic irreversibility were extensively examined in the present work. The dependency of magnetic susceptibility, Neel ordering transition and important magnetic parameters are determined and compared to observation in other multiferroics in the following chapter of the thesis. As a tool of high sensitivity to detect minor traces of the secondary phase hausmannite, magnetic measurements are suggested for cross-checking of phase diagrams. Difficulty of previous studies on interpreting the magnetic anomaly below 43 K in h-RMnO3 oxides was discussed and assigned to the Mn3O4 phase, with supported of the electron microscopy. Magneto-electric coupling where AFM ordering is coupled to dielectric polarization is investigated as a function of x and of sintering condition via frequency and temperature dependent complex dielectric constant measurements in the final chapter of the thesis. Within the limits of solid solubility, the crystalline lattice of off-stoichiometric ceramics was shown to preserve the magneto-electric coupling at TN. It represents the first research work on magneto-electric coupling modified by vacancy doping to author’s knowledge. Studied lattices would reveal distortions at the atomic scale imposed by local changes of x dependent on sintering conditions which were widely inspected by using TEM/STEM methods, complemented with EDS and EELS spectroscopy all together to provide comprehensive information on cross coupling of distortions, inhomogeneity and electronic structure assembled and discussed in a specific chapter. Internal interfaces inside crystalline grains were examined. Qualitative explanations of the measured magnetic and ferroelectric properties were established in relation to observed nanoscale features of h-LuMnxO3±δ ceramics. Ferroelectric domains and topological defects are displayed both in TEM and AFM/PFM images, the later technique being used to look at size, distribution and switching of ferroelectric domains influenced by vacancy doping at the micron scale bridging to complementary TEM studies on the atomic structure of ferroelectric domains. In support to experimental study, DFT simulations using Wien2K code have been carried out in order to interpret the results of EELS spectra of O K-edge and to obtain information on the cation hybridization to oxygen ions. The L3,2 edges of Mn is used to access the oxidation state of the Mn ions inside crystalline grains. In addition, rehybridization driven ferroelectricity is also evaluated by comparing the partial density of states of the orbitals of all ions of the samples, also the polarization was calculated and correlated to the off-stoichiometric effect.

Combined effects of chemical and natural stressors to soil organisms and plants

Nas últimas décadas, a Terra tem experimentado um aquecimento global e mudanças nos padrões de precipitação. Muitos estudos sobre a avaliação de risco de agrotóxicos em organismos não-alvo foram realizados com base em protocolos padronizados, com condições abióticas controladas. Mas, em campo, os organismos são expostos a flutuações de vários fatores ambientais, bem como a poluentes, que podem alterar os limites de tolerância dos organismos aos stressores naturais, bem como alterar a toxicidade ou biodisponibilidade do químico em causa. Considerando isso, o principal objetivo deste trabalho foi o de avaliar de que modo e em que medida os fatores ambientais (temperatura, humidade do solo e radiação UV) podem interagir uns com os outros ou afetar a toxicidade do carbaril para invertebrados do solo e plantas. Para isso, foram utilizadas quatro espécies padrão: Folsomia candida, Eisenia andrei, Triticum aestivum e Brassica rapa, e simulados diferentes cenários climáticos, com vários parâmetros letais e subletais analisados. A exposição combinada foi analisada utilizando, quando possível, a ferramenta MIXTOX, com base no modelo de referência de acção independente (IA) e possíveis desvios, assim como rácios sinergísticos/antagonísticos (a partir de valores de EC50/LC50), quando a dose-resposta de um dos stressores não foi obtida. Todos os fatores de stress aplicados isoladamente causaram efeitos significativos sobre as espécies testadas e sua exposição combinada com carbaril, apresentaram respostas diferenciadas: para as minhocas, a seca e temperaturas elevadas aumentaram os efeitos deletérios do carbaril (sinergismo), enquanto o alagamento e temperaturas baixas diminuíram sua toxicidade (antagonismo). Para os colêmbolos, o modelo IA mostrou ser uma boa ferramenta para prever a toxicidade do carbaril tanto para temperaturas altas como para as baixas. Para as duas espécies de plantas foram encontradas diferenças significativas entre elas: em termos gerais, as interações entre carbaril e os stressores naturais foram observadas, com sinergismo aparecendo como o padrão principal relacionado com a radiação UV, solos secos e temperaturas elevadas, enquanto o padrão principal relacionado com temperaturas baixas e stress de alagamento foi o antagonismo. Quando os efeitos de dois stressores naturais (radiação UV e humidade do solo) em plantas foram avaliados, uma interação significativa foi encontrada: a seca aliviou o efeito deletério da radiação UV em T. aestivum e o alagamento aumentou os seus efeitos, mas para B. rapa a adição de ambos os stresses de água causou um aumento (sinergismo) dos efeitos deletérios da radiação UV para todos os parâmetros avaliados. Portanto é necessário que as diferenças sazonais e latitudinais, bem como as mudanças climáticas globais, sejam integradas na avaliação de risco de contaminantes do solo.

Detection of non-steroidal anti-inflammatory drugs in aqueous effluents using ionic liquids

Significant improvements in human health have been achieved through the increased consumption of pharmaceutical drugs. However, most of these active pharmaceutical ingredients (APIs) are excreted by mammals (in a metabolized or unchanged form) into the environment. The presence of residual amounts of these contaminants was already confirmed in aqueous streams since treatment processes either wastewater treatment plants (WWTPs) or sewage treatment plants (STPs) are not specifically designed for this type of pollutants. Although they are present in aqueous effluents, they are usually at very low concentrations, most of the times below the detection limits of analytical equipment used for their quantification, hindering their accurate monitoring. Therefore, the development of a pre-concentration technique in order to accurately quantify and monitor these components in aqueous streams is of major relevance. This work addresses the use of liquid-liquid equilibria, applying ionic liquids (ILs), for the extraction and concentration of non-steroidal anti-inflammatory drugs (NSAIDs) from aqueous effluents. Particularly, aqueous biphasic systems (ABSs) composed of ILs and potassium citrate were investigated in the extraction and concentration of naproxen, diclofenac and ketoprofen from aqueous media. Both the extraction efficiency and concentration factor achievable by these systems was determined and evaluated. Within the best conditions, extraction efficiencies of 99.4% and concentration factors up to 13 times were obtained.