Structural and optical properties of europium doped zirconia single crystals fibers grown by laser floating zone

Yttria stabilized zirconia single crystal fibers doped with europium ions were developed envisaging optical applications. The laser floating zone technique was used in order to grow millimetric high quality single crystal fibers. The as-grown fibers are completely transparent and inclusion free, exhibiting a cubic structure. Under ultraviolet (UV) excitation, a broad emission band appears at 551 nm. The europium doped fibers are translucent with a tetragonal structure and exhibit an intense red emission at room temperature under UV excitation. The fingerprint transition lines between the 5D0 and 7FJ(0–4) multiplets of the Eu3+ ions are observed with the main emission line at ∼ 606 nm due to 5D0→7F2 transition. Photoluminescence excitation and wavelength dependent the photoluminescence spectra confirm the existence of different Eu3+ optical centers. © 2011 American Institute of PhysicsFCT-PTDC/CTM/66195/2006FCT-SFRH/BD/45774/200

Water-based polyurethane dispersions: chemistry, technology and applications

Over the past few decades, polyurethane-polyurea aqueous dispersions (PUDs) have developed a solid reputation for high performance applications, particularly in the field of adhesives and coatings. PUDs are mostly environmentally compatible products; they are totally devoid or contain only low amounts of volatile organic compounds (VOC). This is an important feature in view of the present environmental policies where governments and internal agencies are placing emphasis on developing sustainable processes, improving work conditions and reducing emissions of toxic and polluting substances into the atmosphere. In the past years, our research group has been involved in the development of polyurethane-polyurea aqueous dispersions for two main applications (footwear and indirect food contact). With this work we intend to review this theme and describe some of the achieved developments. Characterization of commercial dispersions will be presented and examples of synthesis will be described, following a modified pre-polymer process developed in our group

Laser assisted flow deposition: a new method to grow ZnO

Zinc oxide (ZnO) has been one of the most studied materials in the last decades. Either as bulk material, epilayers or nanostructures, this direct wide band gap semiconductor is known to possess great potential for fundamental science and modern technology applications

Development of a modified pre-polymer method to produce NMP-free polyurethane-urea aqueous dispersions

A modified pre-polymer process was developed for the synthesis of NMP (N-methyl-2-pyrrolidone)-free polyurethane-urea aqueous dispersions. The pre-polymer process is based on the use of dimethylol propionic acid (DMPA, hydrophilizing diol), which requires dissolution in NMP to be introduced in the reactive mixture. NMP is difficult to remove from the dispersion remaining in the final product. In this work we present a modified pre-polymer process composed by four main stages: (1) pre-polymer synthesis in two stages: (i) reaction of the isocyanate with the polyol at 80ºC, and (ii) introduction of DMPA pre-neutralized in acetone and proceeding of the reaction at 50ºC, (2) Pre-polymer dispersion in water, (3) Chain extension with a diamine; and (4) Co-solvent removal. Several dispersions were synthesized using different DMPA contents (3.0, 4.0, 5.0%, keeping the pre-neutralization degree at 100.0%), and different DMPA pre-neutralization degree (100.0 and 90.0%, using 5.0 % of DMPA). The effect of these variables in the particle size and dispersion stability was evaluated. Based on the obtained results, the modified pre-polymer process is a feasible alternative to obtain NMP-free PUD, thus fulfilling the restrictions imposed by the European Union.POCI-01-0145-FEDER-006984 (LA LSRE-LCM), funded by FEDER, through POCI-COMPETE2020 and FCT; Project NORTE-01-0145-FEDER-000006, funded by NORTE2020 under PT2020.info:eu-repo/semantics/publishedVersio

Pervasive gaps in Amazonian ecological research

Biodiversity loss is one of the main challenges of our time, and attempts to address it require a clear understanding of how ecological communities respond to environmental change across time and space. While the increasing availability of global databases on ecological communities has advanced our knowledge of biodiversity sensitivity to environmental changes, vast areas of the tropics remain understudied. In the American tropics, Amazonia stands out as the world's most diverse rainforest and the primary source of Neotropical biodiversity, but it remains among the least known forests in America and is often underrepresented in biodiversity databases. To worsen this situation, human-induced modifications may eliminate pieces of the Amazon's biodiversity puzzle before we can use them to understand how ecological communities are responding. To increase generalization and applicability of biodiversity knowledge, it is thus crucial to reduce biases in ecological research, particularly in regions projected to face the most pronounced environmental changes. We integrate ecological community metadata of 7,694 sampling sites for multiple organism groups in a machine learning model framework to map the research probability across the Brazilian Amazonia, while identifying the region's vulnerability to environmental change. 15%–18% of the most neglected areas in ecological research are expected to experience severe climate or land use changes by 2050. This means that unless we take immediate action, we will not be able to establish their current status, much less monitor how it is changing and what is being lost

Study of the formation of a segmented polyester-polyurethane

Segmented polyurethanes are block copolymers of the (AB)n type characterized by a microphase-separated morphology. They are an important subclass of the family of thermoplastic elastomers presenting a broad range of applications, including adhesives and coatings. Typically, phase separation can go along with polymerization, being of great importance the simultaneous study of these two phenomena. In this work the development of a methodology for the study of the formation of a segmented polyester-polyurethane is presented. In a first stage, the determination through FTIR in ATR mode of the kinetics and phase separation has been undertaken. In a second step, the experimental size distributions of hard segments, by an original selective hydrolysis procedure was carried out. The polyurethanes have been prepared starting with 4,4’-methylene-diphenylene diisocyanate (MDI), policaprolactone diol (PCL) and 1,4-butanediol (BD) as chain extender, at various initial mole ratios. Along the polymerization, the system has typically presented a transition from a homogeneous state, during which the chemical kinetics was well described by an overall second order law, into a system presenting phase separation. The critical point, where phase separation starts, was characterized in terms of isocyanate conversion and number and weight average hard segments sizes. It became clear that phase separation requires not only a certain lower bound of the average hard segment size but also at least some minimum value of the concentration of hard segments. The occurrence of phase separation along the formation reaction modifies the size distribution of hard segments with respect to the geometrical distribution prevailing at homogeneous conditions. Moreover, average size tends to decrease as a consequence of phase segregation

A novel view of the manufacture of polyurethane-polyurea aqueous dispersions

Over the past few decades, polyurethane-polyurea aqueous dispersions (PUDs) have developed a solid reputation for high performance applications, particularly in the field of adhesives and coatings. PUDs are mostly environmentally compatible products; they are totally devoid or contain only low amounts of volatile organic compounds (VOC). This is an important feature in view of the present environmental policies where governments and internal agencies are placing emphasis on developing sustainable processes, improving work conditions and reducing emissions of toxic and polluting substances into the atmosphere. Moreover, polyurethanes are known as “tailor-made” products with properties resulting from a wide diversity of raw-materials which can be combined in different ways during the synthesis. In recent years, our research group has been involved in the development of polyurethanepolyurea aqueous dispersions for various applications. With this work we intend to review this theme and describe the most recent developments. Characterization of industrial dispersions will be presented and examples of synthesis will be described

Solvent-free aqueous polyurethane dispersions

The industrial production of aqueous polyurethane dispersions (PUDs) is nowadays a well established technology. There are two main synthetic routes to produce PUDs: the acetone process (a former process developed by Bayer AG) and the pre-polymer process (developed as an alternative response to the patented acetone process). Comparatively to the acetone process, the pre-polymer process has one major advantage since it requires none or only small amounts of acetone. The pre-polymer process, at present, is being forced to readapt due to ongoing developments, partly motivated by process constraints, raw materials restrictions and the need to obtain a true solvent-free product. Allied to this fact it is worth mentioning the European REACH legislation, which is having a considerable influence on the PUD industry. Most of the industrially produced PUDs use dimethylol propionic acid (DMPA) as the internal emulsifier. DMPA is sparingly soluble in the reactive mixture and needs to be previously dissolved in an organic solvent, usually Nmethyl- 2-pyrrolidone (NMP). NMP is difficult to remove and will remain in the final product. There are some alternatives to achieve the NMP-free concept. Among them we can refer the direct NM P replacement by an equivalent solvent, the DMPA replacement by an equivalent hydrophilising diol but with better solubility in the reactive mixture and the preneutralization of DMPA prior to reaction with isocyanate

YSZ:Dy3+ single crystal white emitter

Dysprosium doped yttria stabilized zirconia single crystals grown by the laser floating zone method crystallize in the tetragonal form as confirmed by Raman spectroscopy. In addition to the role of the dysprosium ion as stabilizer, the ion optical activation constitutes an opportunity to explore the zirconia-based material in the photonics area. The spectroscopic characteristics of the doped fibers were analyzed as a function of the amount of the Dy 3+ concentration. The ratio of blue and yellow transition of the Dy3+ ions indicates that lightly doped samples should be considered for the production of the white emission. The analysis of the excitation population mechanisms provides information on the ions preferential excitation paths for the desired optical applications. It was found that under ultraviolet excitation the lightly doped fibers exhibit an intense and bright white luminescence which can be observed by naked eye at room temperature. © 2011 The Royal Society of Chemistry.PTDC/CTM/66195/200

Bright room-temperature green luminescence from YSZ:Tb3+

Partilhar o documento na coleção da comunidade Laboratório Associado I3NThe room temperature spectroscopic properties of terbium doped zirconia fibres grown by Laser Floating Zone method are reported. The fibres were found to be stabilised in the tetragonal crystalline phase as measured by X-ray diffraction and Raman spectroscopy techniques. The doping with terbium ions was performed during the growth process and their incorporation on the wide band gap oxide host assumes an important role on the stabilisation of the high temperature tetragonal crystalline phase. Under ultraviolet optical pumping a bright green luminescence was observed by naked eye at room temperature, corresponding to the 4f → 4f transitions between the crystal-field split energy levels of the 5D4 and 7F5 manifolds of the 4f 8 electronic configuration of the Tb3+ ions. These results suggest that YSZ:Tb3+ could be considered as a promising candidate for applications in green electroluminescent devices. © 2011 Elsevier B.V. All rights reserved.PTDC/CTM/66195/2006PTDC/CTM/100756/2008SFRH/BD/45774/200