Physical chemistry chemical physics : PCCP, Jan 14, 2015
Although steel production by molten oxide electrolysis offers potential economic and environmenta... more Although steel production by molten oxide electrolysis offers potential economic and environmental advantages over classic extractive metallurgy, its feasibility is far from being convincingly demonstrated, mainly due to inherent experimental difficulties exerted by harsh conditions and lack of knowledge regarding relevant mechanisms and physico-chemical processes in the melts. The present work was intended to demonstrate the concept of pyroelectrolysis at very high temperature near the minimum liquidus point of magnesium aluminosilicate, being conducted under electron-blocking conditions using yttria-stabilized zirconia cells, and to provide a new insight into electrochemistry behind this process. Significant current yields are possible for pyroelectrolysis performed in electron-blocking mode using a solid electrolyte membrane to separate the anode and the molten electrolyte. Parasitic electrochemical processes rise gradually as the concentration of iron oxide dissolved in the molt...
In this work, EuNbO 4 samples were prepared, in the shape of fibres, by the laser floating zone (... more In this work, EuNbO 4 samples were prepared, in the shape of fibres, by the laser floating zone (LFZ) technique. The fibres grown at pulling rates below 20 mm h À1 are transparent and monophasic, crystallizing in a monoclinic structure. At higher growth rates, non-stoichiometric europium orthoniobate phases were promoted. The physical properties analysed in this work were the electric, dielectric and photoluminescence (PL) properties. For the fibre grown at 2.5 mm h À1 , the dc conductivity shows a value of 5.97 Â 10 À12 S m À1 and the ac conductivity, at 1 MHz and 300 K, is 3.08 Â 10 À4 S m À1. Under the same experimental conditions the dielectric constant is $39 and the loss tangent $0.14, also presenting a relaxation phenomenon centred at $120 kHz. An ionic polarization type behaviour was identified from the dielectric measurements, which justifies the temperature independent behaviour of the dielectric constant. This type of polarization was assigned to the NbO 4 units. PL measurements with ultraviolet excitation reveal the intra-4f 6 transitions of Eu 3+ ions in the fibres, exhibiting narrow intraionic lines as expected for single crystal materials. From the PL spectra at low temperature, it was possible to identify multiple europium related centres, from different local site symmetry and/or environment. The luminescence of Eu 3+ was found to be preferentially excited via a broad excitation band peaked at $280 nm.
Volume 6: Nick Newman Symposium on Marine Hydrodynamics; Yoshida and Maeda Special Symposium on Ocean Space Utilization; Special Symposium on Offshore Renewable Energy, 2008
In this paper analytical and numerical tools are used to describe the behaviour of a new wave pow... more In this paper analytical and numerical tools are used to describe the behaviour of a new wave power device. This device extracts energy from sea waves by relative pitch motion between their two independent parts. Frequency-domain analysis is carried out assuming ...
ABSTRACT Textured Bi2Ca2Co1.7Ox ceramic rods were obtained by Electrically Assisted Laser Floatin... more ABSTRACT Textured Bi2Ca2Co1.7Ox ceramic rods were obtained by Electrically Assisted Laser Floating Zone (EALFZ) technique. The polarity effects of an external current, applied during the laser floating zone growth, have been investigated. Microstructure and thermoelectric properties have been studied and correlated with the current polarity. An important improvement of power factor was obtained for samples grown with the positive pole connected to the seed. This result evidences the advantages of EALFZ technique as a potential method for obtaining high performance thermoelectric materials. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.
ABSTRACT Nano/microstructures of zinc oxide (ZnO) were grown by the laser assisted flow depositio... more ABSTRACT Nano/microstructures of zinc oxide (ZnO) were grown by the laser assisted flow deposition (LAFD) method. This new process has proved to be very efficient, allowing high yield ZnO deposits at high-rate applicable to large-scale substrates. Laser local heating promotes fast ZnO decomposition and recombination under a self-catalytic vapour–liquid-solid mechanism for the nucleation and growth. Three types of ZnO morphologies were obtained according to the temperature/oxygen availability inside the growth chamber. The morphology can also be controlled adding rare-earth elements to the initial composition. Particularly, tetrapod morphology was obtained by europium oxide addition to the precursors. The structural and microstructural characterizations confirm the good crystallinity of the wurtzite structure. The photoluminescence spectroscopy revealed high optical quality of the as-grown ZnO. Specifically, the free exciton recombination and a strong near band edge recombination due to donor bound exciton transitions can be clearly recognized, although deep level emission in the green spectral region is present.
After well succeeded preliminary work, an upgrade is under way to enable laser sintering and lase... more After well succeeded preliminary work, an upgrade is under way to enable laser sintering and laser pattern sintering, by including cylindrical optics, beam scanning and XYZ sample positioning. Laser processing of materials is increasingly gaining importance in R&D and industrial activities. In our group, it was initially used for growing superconductors by laser floating zone (LFZ), for which a setup was home built, based on a Nd:Yag laser. A CO2 laser was added afterwards to expand the range of materials to be processed. Many oxide materials (mullite, zirconia, YAG, CCTO, manganites, ferrites, cobaltites, galates, etc.) were successfully grown by LFZ. The fibres are highly textured or even monocrystalline in selected cases. A pioneering growth parameter, the applied electrical current during growth (EALFZ), was used to effectively enhance/reduce texture. Eutectic and/or glass ceramics were also grown, their microstructure being strongly dependent on the growth rate. The system is fitted with controlled atmosphere capabilities to increase versatility and to allow processing of non-oxide materials. The peripherals (2 colour pyrometer, high magnification CCD camera) allow assessing the materials melting temperature and thermal shrinkage.
The crystallization induced by laser radiation is a very promising technique to promote glass/cer... more The crystallization induced by laser radiation is a very promising technique to promote glass/ceramic transformation, being already used to produce crystalline patterns on glass surfaces. In this work, a SiO2–Li2O–Nb2O5 glass, prepared by the sol–gel route, was submitted to CO2 laser radiation and conventional heat-treatments in order to induce the LiNbO3 crystallization. The structure and morphology of the samples prepared by both routes was analyzed as a function of exposure time, radiation power and heat-treatment temperatures by XRD, Raman spectroscopy and SEM. The results reveal a correlation between the crystallization degree of LiNbO3 particles and glass matrix with the heat treatment type and experimental parameters. An heat-treatment at 650 °C/4 h was necessary to induce crystallization in heat treatments samples while 4 W/500 s was enough for laser radiation ones, corresponding a reduction time processing of ∼14 000 s.► Crystallization of LiNbO3 nanocrystals in a SiO2 matrix by CO2 laser irradiation process. ► Samples heat-treated at 650 °C (4 h) and laser treated (4 W/500 s) show similar morphology. ► Glass–ceramics produced by laser process requires a very low processing time.
Physical chemistry chemical physics : PCCP, Jan 14, 2015
Although steel production by molten oxide electrolysis offers potential economic and environmenta... more Although steel production by molten oxide electrolysis offers potential economic and environmental advantages over classic extractive metallurgy, its feasibility is far from being convincingly demonstrated, mainly due to inherent experimental difficulties exerted by harsh conditions and lack of knowledge regarding relevant mechanisms and physico-chemical processes in the melts. The present work was intended to demonstrate the concept of pyroelectrolysis at very high temperature near the minimum liquidus point of magnesium aluminosilicate, being conducted under electron-blocking conditions using yttria-stabilized zirconia cells, and to provide a new insight into electrochemistry behind this process. Significant current yields are possible for pyroelectrolysis performed in electron-blocking mode using a solid electrolyte membrane to separate the anode and the molten electrolyte. Parasitic electrochemical processes rise gradually as the concentration of iron oxide dissolved in the molt...
In this work, EuNbO 4 samples were prepared, in the shape of fibres, by the laser floating zone (... more In this work, EuNbO 4 samples were prepared, in the shape of fibres, by the laser floating zone (LFZ) technique. The fibres grown at pulling rates below 20 mm h À1 are transparent and monophasic, crystallizing in a monoclinic structure. At higher growth rates, non-stoichiometric europium orthoniobate phases were promoted. The physical properties analysed in this work were the electric, dielectric and photoluminescence (PL) properties. For the fibre grown at 2.5 mm h À1 , the dc conductivity shows a value of 5.97 Â 10 À12 S m À1 and the ac conductivity, at 1 MHz and 300 K, is 3.08 Â 10 À4 S m À1. Under the same experimental conditions the dielectric constant is $39 and the loss tangent $0.14, also presenting a relaxation phenomenon centred at $120 kHz. An ionic polarization type behaviour was identified from the dielectric measurements, which justifies the temperature independent behaviour of the dielectric constant. This type of polarization was assigned to the NbO 4 units. PL measurements with ultraviolet excitation reveal the intra-4f 6 transitions of Eu 3+ ions in the fibres, exhibiting narrow intraionic lines as expected for single crystal materials. From the PL spectra at low temperature, it was possible to identify multiple europium related centres, from different local site symmetry and/or environment. The luminescence of Eu 3+ was found to be preferentially excited via a broad excitation band peaked at $280 nm.
Volume 6: Nick Newman Symposium on Marine Hydrodynamics; Yoshida and Maeda Special Symposium on Ocean Space Utilization; Special Symposium on Offshore Renewable Energy, 2008
In this paper analytical and numerical tools are used to describe the behaviour of a new wave pow... more In this paper analytical and numerical tools are used to describe the behaviour of a new wave power device. This device extracts energy from sea waves by relative pitch motion between their two independent parts. Frequency-domain analysis is carried out assuming ...
ABSTRACT Textured Bi2Ca2Co1.7Ox ceramic rods were obtained by Electrically Assisted Laser Floatin... more ABSTRACT Textured Bi2Ca2Co1.7Ox ceramic rods were obtained by Electrically Assisted Laser Floating Zone (EALFZ) technique. The polarity effects of an external current, applied during the laser floating zone growth, have been investigated. Microstructure and thermoelectric properties have been studied and correlated with the current polarity. An important improvement of power factor was obtained for samples grown with the positive pole connected to the seed. This result evidences the advantages of EALFZ technique as a potential method for obtaining high performance thermoelectric materials. Crown Copyright (C) 2012 Published by Elsevier B.V. All rights reserved.
ABSTRACT Nano/microstructures of zinc oxide (ZnO) were grown by the laser assisted flow depositio... more ABSTRACT Nano/microstructures of zinc oxide (ZnO) were grown by the laser assisted flow deposition (LAFD) method. This new process has proved to be very efficient, allowing high yield ZnO deposits at high-rate applicable to large-scale substrates. Laser local heating promotes fast ZnO decomposition and recombination under a self-catalytic vapour–liquid-solid mechanism for the nucleation and growth. Three types of ZnO morphologies were obtained according to the temperature/oxygen availability inside the growth chamber. The morphology can also be controlled adding rare-earth elements to the initial composition. Particularly, tetrapod morphology was obtained by europium oxide addition to the precursors. The structural and microstructural characterizations confirm the good crystallinity of the wurtzite structure. The photoluminescence spectroscopy revealed high optical quality of the as-grown ZnO. Specifically, the free exciton recombination and a strong near band edge recombination due to donor bound exciton transitions can be clearly recognized, although deep level emission in the green spectral region is present.
After well succeeded preliminary work, an upgrade is under way to enable laser sintering and lase... more After well succeeded preliminary work, an upgrade is under way to enable laser sintering and laser pattern sintering, by including cylindrical optics, beam scanning and XYZ sample positioning. Laser processing of materials is increasingly gaining importance in R&D and industrial activities. In our group, it was initially used for growing superconductors by laser floating zone (LFZ), for which a setup was home built, based on a Nd:Yag laser. A CO2 laser was added afterwards to expand the range of materials to be processed. Many oxide materials (mullite, zirconia, YAG, CCTO, manganites, ferrites, cobaltites, galates, etc.) were successfully grown by LFZ. The fibres are highly textured or even monocrystalline in selected cases. A pioneering growth parameter, the applied electrical current during growth (EALFZ), was used to effectively enhance/reduce texture. Eutectic and/or glass ceramics were also grown, their microstructure being strongly dependent on the growth rate. The system is fitted with controlled atmosphere capabilities to increase versatility and to allow processing of non-oxide materials. The peripherals (2 colour pyrometer, high magnification CCD camera) allow assessing the materials melting temperature and thermal shrinkage.
The crystallization induced by laser radiation is a very promising technique to promote glass/cer... more The crystallization induced by laser radiation is a very promising technique to promote glass/ceramic transformation, being already used to produce crystalline patterns on glass surfaces. In this work, a SiO2–Li2O–Nb2O5 glass, prepared by the sol–gel route, was submitted to CO2 laser radiation and conventional heat-treatments in order to induce the LiNbO3 crystallization. The structure and morphology of the samples prepared by both routes was analyzed as a function of exposure time, radiation power and heat-treatment temperatures by XRD, Raman spectroscopy and SEM. The results reveal a correlation between the crystallization degree of LiNbO3 particles and glass matrix with the heat treatment type and experimental parameters. An heat-treatment at 650 °C/4 h was necessary to induce crystallization in heat treatments samples while 4 W/500 s was enough for laser radiation ones, corresponding a reduction time processing of ∼14 000 s.► Crystallization of LiNbO3 nanocrystals in a SiO2 matrix by CO2 laser irradiation process. ► Samples heat-treated at 650 °C (4 h) and laser treated (4 W/500 s) show similar morphology. ► Glass–ceramics produced by laser process requires a very low processing time.
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Papers by Nuno Ferreira
Laser processing of materials is increasingly gaining importance in R&D and industrial activities. In our group, it was initially used for growing superconductors by laser floating zone (LFZ), for which a setup was home built, based on a Nd:Yag laser. A CO2 laser was added afterwards to expand the range of materials to be processed. Many oxide materials (mullite, zirconia, YAG, CCTO, manganites, ferrites, cobaltites, galates, etc.) were successfully grown by LFZ. The fibres are highly textured or even monocrystalline in selected cases. A pioneering growth parameter, the applied electrical current during growth (EALFZ), was used to effectively enhance/reduce texture. Eutectic and/or glass ceramics were also grown, their microstructure being strongly dependent on the growth rate. The system is fitted with controlled atmosphere capabilities to increase versatility and to allow processing of non-oxide materials. The peripherals (2 colour pyrometer, high magnification CCD camera) allow assessing the materials melting temperature and thermal shrinkage.
Laser processing of materials is increasingly gaining importance in R&D and industrial activities. In our group, it was initially used for growing superconductors by laser floating zone (LFZ), for which a setup was home built, based on a Nd:Yag laser. A CO2 laser was added afterwards to expand the range of materials to be processed. Many oxide materials (mullite, zirconia, YAG, CCTO, manganites, ferrites, cobaltites, galates, etc.) were successfully grown by LFZ. The fibres are highly textured or even monocrystalline in selected cases. A pioneering growth parameter, the applied electrical current during growth (EALFZ), was used to effectively enhance/reduce texture. Eutectic and/or glass ceramics were also grown, their microstructure being strongly dependent on the growth rate. The system is fitted with controlled atmosphere capabilities to increase versatility and to allow processing of non-oxide materials. The peripherals (2 colour pyrometer, high magnification CCD camera) allow assessing the materials melting temperature and thermal shrinkage.