João C C Abrantes | Instituto Politécnico de Viana do Castelo (original) (raw)
Papers by João C C Abrantes
Materials science & engineering. C, Materials for biological applications, 2019
Advances on the fabrication of sintering-free biphasic calcium phosphate (BCP)/natural polymer co... more Advances on the fabrication of sintering-free biphasic calcium phosphate (BCP)/natural polymer composite scaffolds using robocasting as additive manufacturing technique are presented in the present work. Inks with high amounts of BCP powders (45 vol%) containing different HA/β-TCP ratios, in presence of crosslinked polymer, were successfully fine-tuned for extrusion by robocasting. The non-existence of sintering step opened the possibility to obtain drug loaded scaffolds by adding levofloxacin to the extrudable inks. The drug presence induced slightly changes on the rheological behaviour of the inks, more emphasized for the BCP compositions with higher amounts of β-TCP, and consequently, on the microstructure and on the mechanical properties of the final scaffolds. The strong interaction of β-TCP with chitosan difficult the preparation of suitable rheological inks for printing. Drug delivery studies revealed a fast release of levofloxacin with a high burst of drug within the first 3...
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Sensors and Actuators B: Chemical, 1999
Porous samples were used to evaluate the dependence of electrical conductivity on the oxygen part... more Porous samples were used to evaluate the dependence of electrical conductivity on the oxygen partial pressure. Results obtained with these porous samples revealed that Nb for Ti substitution suppresses the p-type contribution even in oxidizing conditions, and enhances the n-type conductivity. The conductivity of porous SrTi1−yNbyO3+δ samples can be described by the power law dependence on p−1/4O2, except possibly in very reducing conditions. The response time of porous samples is short, as required for resistive oxygen sensors.
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Sensors and Actuators B Chemical
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Boletin de la Sociedad Espanola de Ceramica y Vidrio
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Impedance spectroscopy (IS) is powerful technique to study the electrical and electrochemical beh... more Impedance spectroscopy (IS) is powerful technique to study the electrical and electrochemical behaviour of materials, with special emphasis on microstructural effects. ISA is a software package to analyse IS data using the equivalent circuit approach. Alternative download at http://uidm.estg.ipvc.pt/isa/index.php?versao=uk
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A metodologia proposta para processar materiais cerâmicos celulares porosos consiste na extrusão ... more A metodologia proposta para processar materiais cerâmicos celulares porosos consiste na extrusão de emulsões cerâmicas preparadas por emulsificação de uma suspensão cerâmica em líquidos orgânicos, usando agentes surfatantes e gelificantes para garantir a estabilidade da emulsão. A solidificação da parafina impossibilita a coalescência das gotas de fase dispersa, permitindo controlar as características microestruturais da matriz durante a extrusão e a secagem. Estudou-se o processo de extrusão de cerâmicos celulares com diferentes matérias-primas. Após pirólise da fase orgânica dispersa e posterior sinterização as amostras cerâmicas resultantes possuem as seguintes características: i) porosidades 40-85 %; ii) tamanho médio de cavidade entre 1 e 100 µm; permeabilidade a líquidos na gama 10-5-10-7 m s-1 e a gases entre 5x10-10 a 10-14 m2; iv) resistência mecânica à compressão até 70 MPa e à flexão até 40 MPa.
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Solid State Ionics, 2014
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Porous ceramic materials with designed microstructures are often proposed for many applications. ... more Porous ceramic materials with designed microstructures are often proposed for many applications. However, processing methods are often ill suited for industrial mass production. Thus, this work was intended to demonstrate the ability to process highly porous celular bodies by extrusion of emulsified ceramic suspensions. The plasticity of the emulsified suspensions was assessed by ressdeformation curves, and showed that extrusion is well fitted by Benbow-Bridgwater model.
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Natural clays are often studied for different applications such as adsorbents, filter materials, ... more Natural clays are often studied for different applications such as adsorbents, filter materials, catalytic supports, etc., due to their abundance, low cost and optimal absorption capabilities for a variety of organic and inorganic compounds. Nevertheless, these functionalities are usually lost on sintering at high temperatures, and even on calcining at intermediate temperatures if one seeks strong agglomerates; this is caused mainly by elimination of hydrated groups from the aluminosilicates. Thus, the actual work proposes an alternative strategy based on processing of highly porous clay ceramic monoliths, using kaolin and other natural clays as raw materials, and subsequent functionalization by selective leaching of alumina and/or in-situ hydrothermal synthesis of bohemite. Subsequent transformation to -Al2O3 was achieved by controlled heat treatment. The highly porous monoliths were processed by emulsification of ceramic suspensions, and microstructural characteristics were adjus...
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Thermal conductivity (▲) and relative latent heat (■) of composites. Thermal conductivity vs volu... more Thermal conductivity (▲) and relative latent heat (■) of composites. Thermal conductivity vs volume fraction, fitted by P3D model, for kg:km=50, 100, 200, 400 and 1000. Microstructural characterization • SEM microstructures, suggest that interaction between paraffin and graphite plays a very important role on the microstructures of corresponding composites. • The fraction of graphite is clearly the determining factor on the average size of paraffin droplets. • The graphite content promotes a decrease in droplet size by about one order of magnitude on increasing the graphite contents from 2 wt% to 20 wt%. • Preferential location at the interface between paraffin and the aqueous medium during the emulsification step was found after drying the aqueous medium and subsequent volatilization of the paraffin phase, to reveal the self-organized cellular graphite. • The highest magnification also shows reasonable evidence that graphite platelets show preferential orientation. Thermal characte...
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Insufficient thermal conductivity of PCMs imposes strong limitations on heat charge/discharge and... more Insufficient thermal conductivity of PCMs imposes strong limitations on heat charge/discharge and affects prospective applications. Thus, one designed a concept of PCM-based composites with 3D network of highly conducting phase, and developed an innovative method to processes core-shell paraffin-graphite composites. This is based on emulsification of liquid paraffin in aqueous suspension containing graphite platelets, stabilized with addition of starch. Graphite platelets are, then, self assembled along the interfaces, yielding the conducting shell upon starch consolidation and drying. Preferential orientation also contributes to outstanding grains in thermal conductivity, which increases by at least one order of magnitude for 10 vol% of graphite.
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A core-shell model was derived for microstructural design of PCM-based composites with 3D organiz... more A core-shell model was derived for microstructural design of PCM-based composites with 3D organization of a conducting phase, and a novel method based on emulsification of graphite suspensions in melted paraffin was developed to process self-assembled core-shell composites for thermal regulation or heat storage, with remarkable enhancement in thermal conductivity. These composites remains stable upon repeated cycling above and below the melting temperature, and shape stabilization is retained, even without external encapsulation. The thermal conductivity is sufficient for fast latent heat discharge from relatively large spherical samples to surrounding air. PCM allow heat storage or thermal regulation, based on latent heat. However, insufficient thermal conductivity sets kinetic limitations. Thus, thermal response requires highly conducting inclusions. The contribution of conducting inclusions is very dependent on microstructural organization. Thus, one derived a 3D model and use a ...
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Effects of defloculant content on viscosity for different solid loads in Al 2 O 3 suspensions. De... more Effects of defloculant content on viscosity for different solid loads in Al 2 O 3 suspensions. Defloculation for 40 vol.% solid load with 7.5 wt.% collagen (relative to water) Rheological characterization • The alumina ceramic suspension were defloculated with Dolapix PC-67. For 50 vol.% of solids, 10 vol.% of defloculant is needed to obtain a stable suspension with viscosity 0.4-0.5 Pa s. • Additions of collagen cause increase in viscosity of the ceramic suspension, with corresponding effects on drop size in the emulsified system. • Further additions of defloculant may be used to adjust the final viscosity of the emulsion , with corresponding effects on droplet size and on micerostructural features of resulting cellular ceramics. Objetives Highly porous ceramics find applications in a wide variety of technologies such as filters for hot liquids or gases, supports for catalysts, bioreactors, biomaterials, porous supports for batteries or fuel cells, etc. The performance of these app...
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New oxide-ion conductors Ln(2+x)Ti(2-x)O(7-x/2) (Ln = Dy-Lu, x = 0.096; 35.5 mol. % Ln(2)O(3) 64,... more New oxide-ion conductors Ln(2+x)Ti(2-x)O(7-x/2) (Ln = Dy-Lu, x = 0.096; 35.5 mol. % Ln(2)O(3) 64,5 mol.% TiO2)-LANTIOX with a disordered pyrochlore structure are obtained by coprecipitation with next thermal annealing at 1600 degrees C. Their ionic conductivity in air at 740 degrees C attains 5(.)10(-3) S/cm for Ln(2.096)Ti(1.904)O(6.952) (Ln= Lu, Yb), 10(-3) S/cm for Ln(2,096)Ti(1,904)O(6,952) (Ln=Tm, Er, Ho) and 10(-4)S/cm for Dy2,096Ti1,904O6,952 due to the presence of defects in both the cation and anion sublattices. The materials contain similar to 4.8-10.6% Ln(Ti) anti-site defects. Ln(2.096)Ti(1.904)O(6.952) ceramics are shown to be stable in the temperature range of 1400 to 1700 degrees C. 1400 degrees C-samples Ln(2+x)Ti(2-x)O(7-x/2) have lower ionic conductivity then 1600 degrees C and 1690 degrees C samples.
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Materials science & engineering. C, Materials for biological applications, 2019
Advances on the fabrication of sintering-free biphasic calcium phosphate (BCP)/natural polymer co... more Advances on the fabrication of sintering-free biphasic calcium phosphate (BCP)/natural polymer composite scaffolds using robocasting as additive manufacturing technique are presented in the present work. Inks with high amounts of BCP powders (45 vol%) containing different HA/β-TCP ratios, in presence of crosslinked polymer, were successfully fine-tuned for extrusion by robocasting. The non-existence of sintering step opened the possibility to obtain drug loaded scaffolds by adding levofloxacin to the extrudable inks. The drug presence induced slightly changes on the rheological behaviour of the inks, more emphasized for the BCP compositions with higher amounts of β-TCP, and consequently, on the microstructure and on the mechanical properties of the final scaffolds. The strong interaction of β-TCP with chitosan difficult the preparation of suitable rheological inks for printing. Drug delivery studies revealed a fast release of levofloxacin with a high burst of drug within the first 3...
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Sensors and Actuators B: Chemical, 1999
Porous samples were used to evaluate the dependence of electrical conductivity on the oxygen part... more Porous samples were used to evaluate the dependence of electrical conductivity on the oxygen partial pressure. Results obtained with these porous samples revealed that Nb for Ti substitution suppresses the p-type contribution even in oxidizing conditions, and enhances the n-type conductivity. The conductivity of porous SrTi1−yNbyO3+δ samples can be described by the power law dependence on p−1/4O2, except possibly in very reducing conditions. The response time of porous samples is short, as required for resistive oxygen sensors.
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Sensors and Actuators B Chemical
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Boletin de la Sociedad Espanola de Ceramica y Vidrio
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Impedance spectroscopy (IS) is powerful technique to study the electrical and electrochemical beh... more Impedance spectroscopy (IS) is powerful technique to study the electrical and electrochemical behaviour of materials, with special emphasis on microstructural effects. ISA is a software package to analyse IS data using the equivalent circuit approach. Alternative download at http://uidm.estg.ipvc.pt/isa/index.php?versao=uk
Bookmarks Related papers MentionsView impact
A metodologia proposta para processar materiais cerâmicos celulares porosos consiste na extrusão ... more A metodologia proposta para processar materiais cerâmicos celulares porosos consiste na extrusão de emulsões cerâmicas preparadas por emulsificação de uma suspensão cerâmica em líquidos orgânicos, usando agentes surfatantes e gelificantes para garantir a estabilidade da emulsão. A solidificação da parafina impossibilita a coalescência das gotas de fase dispersa, permitindo controlar as características microestruturais da matriz durante a extrusão e a secagem. Estudou-se o processo de extrusão de cerâmicos celulares com diferentes matérias-primas. Após pirólise da fase orgânica dispersa e posterior sinterização as amostras cerâmicas resultantes possuem as seguintes características: i) porosidades 40-85 %; ii) tamanho médio de cavidade entre 1 e 100 µm; permeabilidade a líquidos na gama 10-5-10-7 m s-1 e a gases entre 5x10-10 a 10-14 m2; iv) resistência mecânica à compressão até 70 MPa e à flexão até 40 MPa.
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Solid State Ionics, 2014
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Porous ceramic materials with designed microstructures are often proposed for many applications. ... more Porous ceramic materials with designed microstructures are often proposed for many applications. However, processing methods are often ill suited for industrial mass production. Thus, this work was intended to demonstrate the ability to process highly porous celular bodies by extrusion of emulsified ceramic suspensions. The plasticity of the emulsified suspensions was assessed by ressdeformation curves, and showed that extrusion is well fitted by Benbow-Bridgwater model.
Bookmarks Related papers MentionsView impact
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Natural clays are often studied for different applications such as adsorbents, filter materials, ... more Natural clays are often studied for different applications such as adsorbents, filter materials, catalytic supports, etc., due to their abundance, low cost and optimal absorption capabilities for a variety of organic and inorganic compounds. Nevertheless, these functionalities are usually lost on sintering at high temperatures, and even on calcining at intermediate temperatures if one seeks strong agglomerates; this is caused mainly by elimination of hydrated groups from the aluminosilicates. Thus, the actual work proposes an alternative strategy based on processing of highly porous clay ceramic monoliths, using kaolin and other natural clays as raw materials, and subsequent functionalization by selective leaching of alumina and/or in-situ hydrothermal synthesis of bohemite. Subsequent transformation to -Al2O3 was achieved by controlled heat treatment. The highly porous monoliths were processed by emulsification of ceramic suspensions, and microstructural characteristics were adjus...
Bookmarks Related papers MentionsView impact
Thermal conductivity (▲) and relative latent heat (■) of composites. Thermal conductivity vs volu... more Thermal conductivity (▲) and relative latent heat (■) of composites. Thermal conductivity vs volume fraction, fitted by P3D model, for kg:km=50, 100, 200, 400 and 1000. Microstructural characterization • SEM microstructures, suggest that interaction between paraffin and graphite plays a very important role on the microstructures of corresponding composites. • The fraction of graphite is clearly the determining factor on the average size of paraffin droplets. • The graphite content promotes a decrease in droplet size by about one order of magnitude on increasing the graphite contents from 2 wt% to 20 wt%. • Preferential location at the interface between paraffin and the aqueous medium during the emulsification step was found after drying the aqueous medium and subsequent volatilization of the paraffin phase, to reveal the self-organized cellular graphite. • The highest magnification also shows reasonable evidence that graphite platelets show preferential orientation. Thermal characte...
Bookmarks Related papers MentionsView impact
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Insufficient thermal conductivity of PCMs imposes strong limitations on heat charge/discharge and... more Insufficient thermal conductivity of PCMs imposes strong limitations on heat charge/discharge and affects prospective applications. Thus, one designed a concept of PCM-based composites with 3D network of highly conducting phase, and developed an innovative method to processes core-shell paraffin-graphite composites. This is based on emulsification of liquid paraffin in aqueous suspension containing graphite platelets, stabilized with addition of starch. Graphite platelets are, then, self assembled along the interfaces, yielding the conducting shell upon starch consolidation and drying. Preferential orientation also contributes to outstanding grains in thermal conductivity, which increases by at least one order of magnitude for 10 vol% of graphite.
Bookmarks Related papers MentionsView impact
A core-shell model was derived for microstructural design of PCM-based composites with 3D organiz... more A core-shell model was derived for microstructural design of PCM-based composites with 3D organization of a conducting phase, and a novel method based on emulsification of graphite suspensions in melted paraffin was developed to process self-assembled core-shell composites for thermal regulation or heat storage, with remarkable enhancement in thermal conductivity. These composites remains stable upon repeated cycling above and below the melting temperature, and shape stabilization is retained, even without external encapsulation. The thermal conductivity is sufficient for fast latent heat discharge from relatively large spherical samples to surrounding air. PCM allow heat storage or thermal regulation, based on latent heat. However, insufficient thermal conductivity sets kinetic limitations. Thus, thermal response requires highly conducting inclusions. The contribution of conducting inclusions is very dependent on microstructural organization. Thus, one derived a 3D model and use a ...
Bookmarks Related papers MentionsView impact
Effects of defloculant content on viscosity for different solid loads in Al 2 O 3 suspensions. De... more Effects of defloculant content on viscosity for different solid loads in Al 2 O 3 suspensions. Defloculation for 40 vol.% solid load with 7.5 wt.% collagen (relative to water) Rheological characterization • The alumina ceramic suspension were defloculated with Dolapix PC-67. For 50 vol.% of solids, 10 vol.% of defloculant is needed to obtain a stable suspension with viscosity 0.4-0.5 Pa s. • Additions of collagen cause increase in viscosity of the ceramic suspension, with corresponding effects on drop size in the emulsified system. • Further additions of defloculant may be used to adjust the final viscosity of the emulsion , with corresponding effects on droplet size and on micerostructural features of resulting cellular ceramics. Objetives Highly porous ceramics find applications in a wide variety of technologies such as filters for hot liquids or gases, supports for catalysts, bioreactors, biomaterials, porous supports for batteries or fuel cells, etc. The performance of these app...
Bookmarks Related papers MentionsView impact
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New oxide-ion conductors Ln(2+x)Ti(2-x)O(7-x/2) (Ln = Dy-Lu, x = 0.096; 35.5 mol. % Ln(2)O(3) 64,... more New oxide-ion conductors Ln(2+x)Ti(2-x)O(7-x/2) (Ln = Dy-Lu, x = 0.096; 35.5 mol. % Ln(2)O(3) 64,5 mol.% TiO2)-LANTIOX with a disordered pyrochlore structure are obtained by coprecipitation with next thermal annealing at 1600 degrees C. Their ionic conductivity in air at 740 degrees C attains 5(.)10(-3) S/cm for Ln(2.096)Ti(1.904)O(6.952) (Ln= Lu, Yb), 10(-3) S/cm for Ln(2,096)Ti(1,904)O(6,952) (Ln=Tm, Er, Ho) and 10(-4)S/cm for Dy2,096Ti1,904O6,952 due to the presence of defects in both the cation and anion sublattices. The materials contain similar to 4.8-10.6% Ln(Ti) anti-site defects. Ln(2.096)Ti(1.904)O(6.952) ceramics are shown to be stable in the temperature range of 1400 to 1700 degrees C. 1400 degrees C-samples Ln(2+x)Ti(2-x)O(7-x/2) have lower ionic conductivity then 1600 degrees C and 1690 degrees C samples.
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