102 results on '"Seabra, Maria Paula"'
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2. Benzene and NOx photocatalytic-assisted removal using indoor lighting conditions
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Tobaldi, David Maria, Dvoranová, Dana, Lajaunie, Luc, Czikhardtová, Kristina, Figueiredo, Bruno, Calvino, José Juan, Seabra, Maria Paula, and Labrincha, João António
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- 2022
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3. Unexplored alternative use of calcareous sludge from the paper-pulp industry in green geopolymer construction materials
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Saeli, Manfredi, Senff, Luciano, Tobaldi, David M., Carvalheiras, João, Seabra, Maria Paula, and Labrincha, João A.
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- 2020
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4. Carbon-modified titanium oxide materials for photocatalytic water and air decontamination
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Silva, Mariana R.F., Lourenço, Mirtha A.O., Tobaldi, David Maria, da Silva, Carla F., Seabra, Maria Paula, and Ferreira, Paula
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- 2020
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5. Novel biomass fly ash-based geopolymeric mortars using lime slaker grits as aggregate for applications in construction: Influence of granulometry and binder/aggregate ratio
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Saeli, Manfredi, Senff, Luciano, Tobaldi, David M., Seabra, Maria Paula, and Labrincha, João A.
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- 2019
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6. Sol gel graphene/TiO2 nanoparticles for the photocatalytic-assisted sensing and abatement of NO2
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Giampiccolo, Andrea, Tobaldi, David Maria, Leonardi, Salvatore Gianluca, Murdoch, Billy James, Seabra, Maria Paula, Ansell, Martin P., Neri, Giovanni, and Ball, Richard J.
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- 2019
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7. Red mud-based geopolymers with tailored alkali diffusion properties and pH buffering ability
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Ascensão, Guilherme, Seabra, Maria Paula, Aguiar, José Barroso, and Labrincha, João António
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- 2017
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8. Review of recycling alternatives for paper pulp wastes
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Vilarinho, Inês Silveirinha, primary, Gameiro, Tânia, additional, Capela, Marinélia N., additional, Carvalheiras, João, additional, Caetano, Ana P. F., additional, Novo, Catarina, additional, Novais, Rui M., additional, Seabra, Maria Paula, additional, and Labrincha, João António, additional
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- 2022
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9. Porcelain stoneware tiles with antimicrobial action
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Seabra, Maria Paula, Grave, Luciana, Oliveira, Cláudia, Alves, Artur, Correia, António, and Labrincha, João António
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- 2014
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10. Aluminium Anodizing Slurries as an Additive for FLASH Sintering of Alumina
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Vilarinho, Inês Silveirinha, primary, Pinho, Rui, additional, Seabra, Maria Paula, additional, Vilarinho, Paula Maria, additional, and Senos, Ana, additional
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- 2022
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11. Bio-CaCO3 from Eggshell Waste as Raw Material for Eco-Ceramic Products
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Vilarinho, Inês Silveirinha, primary, Filippi, Elena, additional, and Seabra, Maria Paula, additional
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- 2022
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12. Valorization of Bauxite Residue in Ceramic Pastes
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Vilarinho, Inês Silveirinha, primary, Dias, Ana Cláudia, additional, Carneiro, Jorge, additional, Pinto, Carlos, additional, Labrincha, João António, additional, and Seabra, Maria Paula, additional
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- 2022
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13. Multifunctional Silver Coated Tio2 Transparent Thin Films for Photocatalytic and Antimicrobial Applications
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González Cuadra, Jaime, primary, Molina Prados, Sergio, additional, Minguez Vega, Gladys, additional, Estrada, Ana, additional, Trindade, Tito, additional, Seabra, Maria Paula, additional, António Labrincha, João, additional, Porcar García, Samuel, additional, Cadena Bareño, Robinson, additional, Fraga Chiva, Diego, additional, and Carda, Juan Bautista, additional
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- 2022
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14. Benzene and NOx photocatalytic assisted removal using indoor lighting conditions
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Tobaldi, David Maria, primary, Dvoranová, Dana, additional, Lajaunie, Luc, additional, Czikhardtová, Kristina, additional, Figueiredo, Bruno, additional, Seabra, Maria Paula, additional, Calvino, José Juan, additional, and Labrincha, João António, additional
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- 2021
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15. Immobilization of Hazardous Wastes on One-Part Blast Furnace Slag-Based Geopolymers
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Paz-Gómez, Daniela Carolina, primary, Vilarinho, Inês Silveirinha, additional, Pérez-Moreno, Silvia M., additional, Carvalheiras, João, additional, Guerrero, José Luis, additional, Novais, Rui Miguel, additional, Seabra, Maria Paula, additional, Ríos, Guillermos, additional, Bolívar, Juan Pedro, additional, and Labrincha, João António, additional
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- 2021
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16. Valorization of Fly Ashes and Sands Wastes from Biomass Boilers in One-Part Geopolymers.
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Vilarinho, Inês Silveirinha, Capela, Marinélia Neto, Pinho, Ana Sofia, Labrincha, João António, and Seabra, Maria Paula
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MORTAR ,FLY ash ,SAND ,BIOMASS ,BOILERS ,BIOMASS energy ,PAPER industry - Abstract
Fly ash (FA) and exhausted bed sands (sands wastes) that are generated in biomass burners for energy production are two of the wastes generated in the pulp and paper industry. The worldwide production of FA biomass is estimated at 10 million tons/year and is expected to increase. In this context, the present work aims to develop one-part alkali-activated materials with biomass FA (0–100 wt.% of the binder) and sands wastes (100 wt.% of the aggregate). FA from two different boilers, CA and CT, was characterized and the mortar's properties, in the fresh and hardened conditions, were evaluated. Overall, the incorporation of FA decreases the compressive strength of the specimens. However, values higher than 30 MPa are reached with 50 wt.% of FA incorporation. For CA and CT, the compressive strength of mortars with 28 days of curing was 59.2 MPa (0 wt.%), 56.9 and 57.0 MPa (25 wt.%), 34.9 and 46.8 MPa (50 wt.%), 20.5 and 13.5 MPa (75 wt.%), and 9.2 and 0.2 MPa (100 wt.%), respectively. The other evaluated characteristics (density, water absorption, leached components and freeze–thaw resistance) showed no significant differences, except for the specimen with 100 wt.% of CA. Therefore, this work proved that one-part geopolymeric materials with up to 90 wt.% of pulp and paper industrial residues (FA and sand) can be produced, thus reducing the carbon footprint associated with the construction sector. [ABSTRACT FROM AUTHOR]
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- 2022
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17. Waste-Based One-Part Alkali Activated Materials
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Gonçalves, Margarida, primary, Vilarinho, Inês Silveirinha, additional, Capela, Marinélia, additional, Caetano, Ana, additional, Novais, Rui Miguel, additional, Labrincha, João António, additional, and Seabra, Maria Paula, additional
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- 2021
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18. A New Added-Value Application for Steel Wire Drawing Mill Scale Waste in Stoneware Ceramic Products
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Vilarinho, Inês Silveirinha, primary, Lopes, Ana Luisa, additional, Carneiro, Jorge, additional, Pinto, Carlos, additional, Labrincha, João António, additional, and Seabra, Maria Paula, additional
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- 2021
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19. ALKALI-ACTIVATED MORTARS FOR SUSTAINABLE CONSTRUCTION MATERIAL: EFFECTS OF BINDER-TO- AGGREGATE RATIO AND CURING CONDITION
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SAELI, Manfredi, SEABRA, MARIA PAULA, LABRINCHA, JOAO A., SAELI, Manfredi, SEABRA, MARIA PAULA, and LABRINCHA, JOAO A.
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Construction, Mortar, Geopolymer, Biomass fly ash, Pulp-paper industry, Waste, Circular economy ,Settore ICAR/10 - Architettura Tecnica - Abstract
Valorisation and reuse of industrial wastes has become a worldwide compelling topic to improve the sustainability of processes and materials. This paper discusses an alternative way to recycle the biomass fly ash, generated by the kraft pulp industry, to manufacture novel geopolymeric mortars intended for applications in construction. Biomass fly ash was used as a raw material, in partial substitution of the commonly used metakaolin, natural siliceous sand as aggregate. The followed manufacture process is highly simple and reproducible. Various proportions binder to aggregate were tested to study the effect on the final mortars properties. The mortars mechanical resistance was also studied in relation to the temperature and duration of curing in order to define the best condition to gain the maximum mechanical resistance. Also submersed curing was tested. Moreover, the mechanical performance was investigated under the effect of natural ageing. The investigations indicate that the novel mortars can be used as structural material in construction and represent an efficient solution to reduce the environmental footprint associated with waste disposal in light of the circular economy.
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- 2019
20. Alkali-activated mortars for sustainable construction material: effects of binder-to-aggregate ratio and curing conditions
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Saeli Manfredi, Seabra Maria Paula, Labrincha João A., and Saeli Manfredi, Seabra Maria Paula, Labrincha João A.
- Subjects
Construction, Mortar, Geopolymer, Biomass fly ash, Pulp-paper industry, Waste, Circular economy ,Settore ICAR/10 - Architettura Tecnica - Abstract
Valorisation and reuse of industrial wastes has become a worldwide compelling topic to improve the sustainability of processes and materials. This paper discusses an alternative way to recycle the biomass fly ash, generated by the kraft pulp industry, to manufacture novel geopolymeric mortars intended for applications in construction. Biomass fly ash was used as a raw material, in partial substitution of the commonly used metakaolin, natural siliceous sand as aggregate. The followed manufacture process is highly simple and reproducible. Various proportions binder to aggregate were tested to study the effect on the final mortars properties. The mortars mechanical resistance was also studied in relation to the temperature and duration of curing in order to define the best condition to gain the maximum mechanical resistance. Also submersed curing was tested. Moreover, the mechanical performance was investigated under the effect of natural ageing. The investigations indicate that the novel mortars can be used as structural material in construction and represent an efficient solution to reduce the environmental footprint associated with waste disposal in light of the circular economy.
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- 2019
21. Development of Coloured Stoneware Bodies through the Incorporation of Industrial Cr/Ni Electroplating Sludge
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Vilarinho, Inês Silveirinha, primary, Carneiro, Jorge, additional, Pinto, Carlos, additional, Labrincha, João António, additional, and Seabra, Maria Paula, additional
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- 2021
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22. Experimental and Computational Analysis of NOx Photocatalytic Abatement Using Carbon-Modified TiO2 Materials
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Zhiltsova., Tatiana, primary, Martins, Nelson, additional, Silva, Mariana R. F., additional, Silva, Carla F. Da, additional, Lourenço, Mirtha A. O., additional, Tobaldi, David M., additional, Covita, Daniel, additional, Seabra, Maria Paula, additional, and Ferreira, Paula, additional
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- 2020
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23. Selection of Novel Geopolymeric Mortars for Sustainable Construction Applications Using Fuzzy Topsis Approach
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Saeli, Manfredi, primary, Micale, Rosa, additional, Seabra, Maria Paula, additional, Labrincha, João A., additional, and La Scalia, Giada, additional
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- 2020
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24. Photocatalytic Aided Removal of Nitrogen Oxides and VOCs from Outdoor Environment: Graphene as a Highway for Electron Mobility in TiO2 Nanoparticles
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Tobaldi, David Maria, primary, Dvoranova, Dana, primary, Lajaunie, Luc, primary, rozman, nejc, primary, Figueiredo, Bruno, primary, Seabra, Maria Paula, primary, sever skapin, andrijana, primary, Calvino, José, primary, Brezova, Vlasta, primary, and Labrincha, Joao Antonio, primary
- Published
- 2020
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25. TiO2 Surface Hybridisation with Noble Metals (Ag and CuxO) for Solar De-NOx and VOC Removal
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Tobaldi, David Maria, primary, Lajaunie, Luc, primary, Seabra, Maria Paula, primary, Arenal, Raul, primary, and Labrincha, Joao Antonio, primary
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- 2020
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26. Photo-Electrochemical Properties of CuO–TiO2 Heterojunctions for Glucose Sensing
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Tobaldi, David Maria, primary, Espro, Claudia, primary, leonardi, salvarore gianluca, primary, Lajaunie, Luc, primary, Seabra, Maria Paula, primary, Calvino, José, primary, Marini, Silvia, primary, Labrincha, Joao Antonio, primary, and neri, giovanni, primary
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- 2020
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27. Copper-Graphene-TiO2 Hybrid Materials for Photocatalytically Assisted H2 Generation
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Tobaldi, David Maria, primary, Koci, Kamila, primary, Edelmannova, Miroslava, primary, Lajaunie, Luc, primary, Figueiredo, Bruno, primary, Calvino, José, primary, Seabra, Maria Paula, primary, and Labrincha, Joao Antonio, primary
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- 2020
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28. Cooperative and Fully Reversible Photocatalytic Colour Switching Activation in Graphene-Copper-TiO2 Nanoparticles
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Tobaldi, David Maria, primary, Lajaunie, Luc, primary, Dvoranova, Dana, primary, Figueiredo, Bruno, primary, Seabra, Maria Paula, primary, Calvino, José, primary, Brezova, Vlasta, primary, and Labrincha, Joao Antonio, primary
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- 2020
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29. Waste-Based Pigments for Application in Ceramic Glazes and Stoneware Bodies
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Carneiro, Jorge, primary, Tobaldi, David Maria, additional, Capela, Marinélia Neto, additional, Seabra, Maria Paula, additional, and Labrincha, João António, additional
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- 2019
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30. Sol gel graphene/TiO2 nanoparticles for the photocatalytic-assisted sensing and abatement of NO2
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Giampiccoloa, Andrea, Tobaldi, David Maria, Leonardi, Salvatore Gianluca, Murdoch, Billy James, Seabra, Maria Paula, Ansell, Martin P., Neri, Giovanni, and Ball, Richard J.
- Abstract
No abstract available.
- Published
- 2019
31. Innovative Recycling of Lime Slaker Grits from Paper-Pulp Industry Reused as Aggregate in Ambient Cured Biomass Fly Ash-Based Geopolymers for Sustainable Construction Material
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Saeli, Manfredi, primary, Senff, Luciano, additional, Tobaldi, David Maria, additional, La Scalia, Giada, additional, Seabra, Maria Paula, additional, and Labrincha, João Antonio, additional
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- 2019
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32. Alkali-activated Fly Ash-based Mortars for Green Applications in Architecture and Civil Engineering
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Saeli, Manfredi, primary, Senff, Luciano, additional, Seabra, Maria Paula, additional, and Labrincha, João A., additional
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- 2019
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33. Mix design and mechanical performance of geopolymeric binders and mortars using biomass fly ash and alkaline effluent from paper-pulp industry
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Saeli, Manfredi, primary, Tobaldi, David M., additional, Seabra, Maria Paula, additional, and Labrincha, João A., additional
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- 2019
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34. A Semiconducting Material Exhibiting Visible-Light Promoted Photochromism, Photoluminescent and Photocatalytic Activity
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Tobaldi, David Maria, primary, Lajaunie, Luc, primary, López‐Haro, Miguel, primary, Ferreira, Rute, primary, Leoni, Matteo, primary, Seabra, Maria Paula, primary, Calvino, José, primary, Carlos, Luís, primary, and Labrincha, Joao Antonio, primary
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- 2018
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35. Impact of Rutile Fraction on TiO2 Visible-Light Absorption and Visible-Light-Induced Photocatalytic Activity
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Tobaldi, David Maria, primary, Lajaunie, Luc, primary, caetano, ana, primary, rozman, nejc, primary, Seabra, Maria Paula, primary, sever skapin, andrijana, primary, Arenal, Raul, primary, and Labrincha, Joao Antonio, primary
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- 2018
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36. Photo-electrochemical properties of CuO–TiO2 heterojunctions for glucose sensing.
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Tobaldi, David Maria, Espro, Claudia, Leonardi, Salvatore Gianluca, Lajaunie, Luc, Seabra, Maria Paula, Calvino, José Juan, Marini, Silvia, Labrincha, João António, and Neri, Giovanni
- Abstract
Electrochemical sensors for monitoring biochemical substances are attracting considerable attention. These devices are usually based on enzymes that are sensitive and very specific. Still, the activity of those enzymes is lost with changes in temperature or pH. Non-enzymatic electrochemical sensors – fabricated via the modification of the electrode surface with metal oxide nanoparticles – are an ideal answer to address that problem. In this study, we investigated the photo-electrochemical properties of CuO–TiO
2 heterojunctions for glucose sensing in alkaline media. A combination of high-resolution (scanning) transmission electron microscopy, spatially resolved electron energy-loss spectroscopy, energy-dispersive X-ray spectroscopy and X-ray powder diffraction, was used to study in detail the microstructures of the prepared specimens. These results highlighted the strong intertwining between the TiO2 nanoparticles and the Cu-based nanoparticles, which present a metallic core with a CuO rich surface. In addition, we showed that CuO, joint to TiO2 , has smaller size compared to pure CuO, which entails larger specific surface area available for the glucose electro-oxidation, which consequently enhanced the electrochemical features. The influence of Cu loading over the sensing performance of TiO2 was examined in detail carrying out electrochemical sensing tests under dark, laboratory and halogen lamp irradiation conditions. Results demonstrated that, under halogen lamp irradiation, modified CuO–TiO2 electrodes showed a specific response signal that is four times higher than that of pure CuO. Those increased photo-electrochemical properties in CuO–TiO2 heterojunctions are likely due to a synergistic effect between the microstructural characteristics and effective separation of photo-generated excitons created at the heterojunction interface. Results of this study offer applicable guidelines for designing photo-electrochemical screen-printed electrodes based on nano-sized CuO on titania for efficient detection of glucose. [ABSTRACT FROM AUTHOR]- Published
- 2020
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37. Hybrid Noble-Metals/Metal-Oxide Bifunctional Nano-Heterostructure Displaying Outperforming Gas-Sensing and Photochromic Performances
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Tobaldi, David Maria, primary, Leonardi, Salvatore Gianluca, additional, Movlaee, Kaveh, additional, Lajaunie, Luc, additional, Seabra, Maria Paula, additional, Arenal, Raul, additional, Neri, Giovanni, additional, and Labrincha, João António, additional
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- 2018
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38. Copper and rare earth TiO2 nano-heterostructure as a bifunctional material
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Seabra, Maria Paula, primary, Tobaldi, David Maria, additional, Ferreira, Rute A. S., additional, Carlos, Luís D., additional, and Labrincha, João A., additional
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- 2017
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39. Mix design and mechanical performance of geopolymer binder for sustainable construction and building material
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Saeli, Manfredi, primary, Novais, Rui M., additional, Seabra, Maria Paula, additional, and Labrincha, João A., additional
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- 2017
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40. Materiais baseados em La (Mg 0,5 Ti 0,5) O3 para aplicações nas microondas
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Seabra, Maria Paula da Silva and Ferreira, Victor Miguel Cameiro de Sousa
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Cerâmica electrónica ,Microondas ,Cerâmica dieléctrica ,Ciência dos materiais - Abstract
Doutoramento em Ciência e Engenharia dos Materiais Os sistemas de telecomunicações foram um dos sectores da indústria electrónica com maiores taxas de desenvolvimento nas últimas duas décadas. De modo a conseguir maior densidade e velocidade de tráfego de comunicações estas deslocaram-se para frequências superiores, as das microondas. A consequente necessidade de componentes para estes sistemas conduziu ao desenvolvimento de novos materiais dieléctricos com aplicações à frequência das microondas, onde é vital o controle das suas propriedades dieléctricas, nomeadamente, da permitividade (εr), das perdas e da variação da sua frequência de ressonância com a temperatura ambiente (τf). Uma forma de ajustar estas propriedades tem sido o recurso à formação de soluções sólidas ou misturas de compostos, de modo a conseguir cumprir os requisitos fundamentais, isto é, elevada εr, perdas dieléctricas diminutas e τf ≈ 0. Com o intuito de compreender os factores que afectam as propriedades dieléctricas à frequência das microondas, foram estudadas as relações entre o processamento, a estrutura e as referidas propriedades de materiais cerâmicos baseados em La(Mg0.5Ti0.5)O3 [LMT]. Este material cerâmico dieléctrico é promissor dado que exibe um factor de qualidade relativamente elevado, Q·fo = 114000 GHz a 7GHz, e εr = 27. No entanto, apresenta um valor elevado para o τf (-81 ppm/K), pelo que é imprescindível proceder ao seu ajuste. A mistura de materiais dieléctricos com valores de τf de sinal oposto tem sido a prática empírica corrente para o ajuste referido, no entanto, é necessário para um desenvolvimento mais eficaz de materiais para estas aplicações conhecer e compreender os mecanismos responsáveis pela variação destas propriedades. Alguns materiais com estrutura perovsquítica têm sido estudados para este tipo de aplicações, precisamente porque exibem um conjunto de características estruturais que podem influenciar as propriedades dieléctricas à frequência das microondas, constituindo-se assim como um excelente meio de estudo dos mecanismos determinantes. Neste trabalho foram usados para formar soluções sólidas de estrutura perovsquítica com o LMT, um grupo de titanatos de alcalino-terrosos (CaTiO3, SrTiO3 e BaTiO3) visando alterar a estrutura e as propriedades dieléctricas do material base. O processamento adequado dos pós condiciona de uma forma dramática as características dieléctricas dos cerâmicos e, assim, recorreu-se a um método de preparação por via química alternativo ao método convencional de mistura de óxidos. A partir dos pós obtidos por esta via, comparativamente à convencional, foi possível preparar cerâmicos densos, monofásicos e com microestruturas homogéneas a temperaturas bastante inferiores. Estes cerâmicos apresentam ainda perdas dieléctricas menores do que as obtidas em materiais preparados pela via convencional. Neste âmbito, foi optimizado o processamento de todas as composições das soluções sólidas [(1-x)LMT-xAT, com A = Ca, Sr e Ba; 0≤x95%), monofásicos e homogéneos, fundamental quer para os estudos de caracterização dieléctrica quer para as próprias propriedades dieléctricas finais destes materiais. A estrutura cristalina dos materiais perovsquíticos pode influenciar as suas propriedades dieléctricas pelo que, neste trabalho, procedeu-se a um estudo detalhado da estrutura das soluções sólidas referidas anteriormente, recorrendo a técnicas de difracção de raios-X, coadjuvadas por análise de refinamento de Rietveld e de microscopia electrónica de transmissão. O material base [LMT] pertence ao grupo espacial P21/n (a-a-c+) e, a formação de soluções sólidas, com a substituição parcial no sítio A da estrutura perovsquítica, conduziu a uma série de modificações estruturais diferentes para os três catiões (Ca, Sr, Ba) envolvidos. O sistema LMT-CT foi o que apresentou menores alterações tendo-se observado apenas o desaparecimento da ordenação catiónica para x>0.1, pertencendo estes materiais ao grupo espacial Pbnm (a-a-c+). Em relação ao sistema LMT-ST, este exibe diversas alterações estruturais à medida que o teor de ST aumenta [P21/n (a-a-c+, ordenado no sítio B) → Pbnm (a-a-c+) → Imma (a-a-c0) → I4/mcm (a0a0c-)]. Para o sistema LMT-BT observaram-se as variações estruturais mais acentuadas [P21/n (a-a-c+, ordenado no sítio B) → Pbnm (a-a-c+) → I4/mcm (a0a0c-) → Pm3m (a0a0a0)]. Com a finalidade de correlacionar o processamento, a estrutura cristalina com as propriedades dieléctricas dos materiais destes sistemas, estas últimas foram caracterizadas à frequências das microondas (GHz), à frequência rádio (MHz) e, para o estudo dos mecanismos responsáveis, foram complementadas por medidas a frequências superiores (espectroscopia no infravermelho longínquo). As propriedades dieléctricas variam ao longo das soluções sólidas. A variação da εr é monotónica e segue a regra clássica das misturas atingindo-se valores interessantes para as aplicações nas microondas (εr>40). O τf varia de forma semelhante tendo-se conseguido, para os três sistemas, valores nulos para composições próximas de x=0.5. Esta característica varia linearmente com a εr em toda a extensão das soluções sólidas LMT-CT e LMT-ST. Este facto é um forte indício de que a diluição da polarizabilidade iónica é o mecanismo que controla o τf. Para o sistema LMT-BT esta linearidade é interrompida para x>0.5, o que pressupõe que o ajuste do τf é também condicionado pelo aparecimento da rotação dos octaedros de oxigénio. As perdas dieléctricas, descritas frequentemente através do seu inverso, o factor de qualidade dos ressonadores (Q), varia de forma semelhante para os três sistemas. O aumento do teor de AT provoca uma diminuição do Q, que é mais acentuada para x=0.1, apresentando um valor mínimo para x=0.5 embora tecnologicamente interessante. A queda inicial acentuada de Q resulta da diminuição da ordenação catiónica da estrutura sendo o mínimo verificado para x=0.5 relacionado com a presença de ordem a curta distância confirmada por espectroscopia Raman. A espectroscopia no infravermelho longínquo revelou-se útil para avaliar os contributos de carácter intrínseco e extrínseco para a perda dieléctrica. Ainda em relação ao sistema LMT-BT, é de realçar a descoberta da existência de um comportamento relaxor na zona da solução sólida rica em BT. Este é um dado importante tendo em conta tratar-se de um sistema relaxor constituído por materiais sem chumbo. Telecommunications systems are one of the sectors of electronic industry showing more dramatic growth in the last two decades. In order to achieve the highest communication densities or traffic these have moved on to higher frequencies such as the microwave range. The increasing need for components for these systems lead to the development of new dielectric materials suitable for applications at microwave systems, where the control of dielectric properties like permittivity (εr), losses and temperature stability of resonant frequency is fundamental. One way to tune these properties has been achieved trough the formation of solid solutions or mixtures in order to fulfil the fundamental requirements, such as high permittivity, low dielectric losses and a temperature coefficient of resonant frequency (τf) close to zero. Aiming to understand the factors affecting microwave dielectric properties, the relationships between processing, structure and dielectric properties of La(Mg0.5Ti0.5)O3 [LMT] based materials were studied. This dielectric ceramic material is a promising one since it shows a high quality factor, Q·fo = 114000 GHz at 7GHz, and a εr = 27. Nevertheless, it presents a high τf (-81ppm/K) that needs to be adjusted. Mixing dielectric materials with opposite τf values has been the current practice but a more efficient materials development rather imposes the understanding of the responsible mechanisms controlling these relevant properties. In this work, perovskite solid solutions based on LMT and a group of earthalkaline titanates (CaTiO3, SrTiO3 and BaTiO3) were used in order to change the structure and dielectric properties of the base material. Powder processing has a great influence on the ceramic dielectric characteristics and so a chemical powder preparation was used instead of the conventional mixed oxide route. With the chemically prepared powders it was possible to prepare dense, single phase ceramics with homogeneous microstructures at lower temperatures. These ceramics even presented lower dielectric losses than the ones obtained by the conventional route. Processing of all compositions resulting from solid solution formation with calcium, strontium and barium titanates [(1-x)LMT-xAT, and A = Ca, Sr and Ba; 0≤x95%), single phase and homogeneous ceramics was achieved, which is fundamental for dielectric characterization studies as well as for the final dielectric properties of microwave materials. Crystallographic structure of perovskite materials may influence their dielectric properties which led to the detailed study in this work of those solid solutions structure, with the help of techniques such as X-ray diffraction, together with Rietveld refinements studies, and transmission electron microscopy. The base material [LMT] belongs to P21/n space group (a-a-c+ tilt system) and presents a 1:1 B-site ordered monoclinic structure, with in-phase and anti-phase tilting of the oxygen octahedra, and anti-parallel shift of the lanthanum cations (A site). Formation of solid solutions with A-site partial substitution leaded to a series of structural modifications which were different for the three used cations (Ca, Sr, Ba). LMT-CT system was the one that presented slighter changes since all compositions with x>0.1 belong to the same space group (Pbnm) and present anti-parallel of A-site cations, in-phase and anti-phase tilting of oxygen octahedra (a-a-c+ tilt system). Regarding LMT-ST system, it exhibits several structural changes with the increase of ST content (x) [P21/n (a-a-c+, B site ordered) → Pbnm (a-a-c+) → Imma (a-a-c0) → I4/mcm (a0a0c-)]. For LMT-BT solid solutions greater variations were observed [P21/n (a-a-c+, B site ordered) → Pbnm (a-a-c+) → I4/mcm (a0a0c-) → Pm3m (a0a0a0)]. These observations were confirmed by TEM with the help of electron diffraction patterns and darkfield imaging. Aiming to correlate processing, structure and dielectric properties of the materials in these systems, these last ones were measured at microwave frequencies (GHz), at radiofrequency (MHz) and, for the study of its responsible mechanisms, they were complemented by measurements at even higher frequencies (far infrared spectroscopy). Dielectric properties changes with the composition of these solid solutions. Permittivity variation is monotonic, following the classic mixture rule and interesting values for microwave frequencies (εr>40) were obtained. Temperature coefficient of resonant frequency (τf) varies in a similar way for the three systems and close to zero values were obtained for compositions near x=0.5. This property varies linearly with permittivity throughout the entire LMTCT and LMT-ST solid solutions. This behaviour strongly indicates that ionic polarizability dilution is the main τf controlling mechanism. For LMT-BT this linearity is cut off around x>0.5, which may indicate that τf tuning is also conditioned by the onset of oxygen octahedral tilting happening in this range. Dielectric losses, often described by the quality factor (Q) vary in a similar way for the three studied systems. The AT increase causes a Q decrease, which is more pronounced for x=0.1 and presenting a minimum for x=0.5, although with technological interesting values. The initial drop in Q values results from the decrease of the cation ordering whereas the minimum at x=0.5 is related with the presence of short range order as confirmed from Raman spectroscopy. Far infrared spectroscopy as revealed to be useful for the evaluation of intrinsic and extrinsic contributions to dielectric loss. Still regarding the LMT-BT system, it is worth to emphasise the appearance of a relaxor behaviour in the BT-rich compositions. This is particularly important since it is a lead-free material relaxor system.
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- 2004
41. Study of far infrared optical properties and, photocatalytic activity of ZnO/ZnS hetero-nanocomposite structure
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Zamiri, Reza, primary, Tobaldi, David Maria, additional, Ahangar, Hossein Abbastabar, additional, Rebelo, Avito, additional, Seabra, Maria Paula, additional, Belsley, Michael Scott, additional, and Ferreira, J. M. F., additional
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- 2014
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42. Compósitos de mulite, reforçada com carboneto de silício
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Seabra, Maria Paula da Silva
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Mulite ,Materiais compósitos - Teses de mestrado ,Matriz cerâmica ,Prensagem a quente ,Carboneto de silício ,Sinterização - Abstract
Mestrado em Engenharia de Materiais No trabalho efectuado estudou-se a densificação de compósitos de mulite reforçada com carboneto de silício. A tenacidade à fractura dos corpos de mulite pode ser aumentada através da introdução de inclusões densas, contudo estas afectam de forma significativa a densificação da matriz que as rodeia. Dadas as dificuldades encontradas na obtenção de corpos densos de mulite pura e, em particular, da mulite com inclusões densas, através da sinterização sem pressão, recorreu-se à prensagem a quente. A densificação do material monolítico foi estudada para diversas condições de prensagem a quente (1500°C - 1600°C; 15MPa - 25MPa), tendo-se avaliado a influência da temperatura e da pressão. Observou-se que se obtêm corpos de mulite pura (SA-193CR), com densidades próximas da teórica para temperaturas iguais ou superiores a 1550°C, suspeitando-se da presença de vestígios de fase líquida. Tentou-se ainda identificar quais os mecanismos responsáveis pela densificação da mulite durante a prensagem a quente. Os resultados sugerem que o mecanismo de densificação se altera com o aumento da temperatura de 1500°C para 1550°C. Para a temperatura de 1500°C obteve-se, para o expoente da tensão (n), o valor de 1, sugerindo que o mecanismo densificação predominante, a esta temperatura, é a fluência de Coble, a fluência de Nabarro-Herring ou o escorregamento na fronteira de grão. Aumentando a temperatura de 1500°C para 1550°C o valor do expoente da tensão passou de 1 para 3, sugerindo que a a 1550°C o mecanismo que controla a densificação da mulite é o fluxo plástico, a menos que este incremento de n se deva a alterações microestruturais súbitas. Estudou-se a influência das inclusões densas no empacotamento dos compactos em verde. Observou-se que as fibrilas de SiC dificultam o empacotamento originado um compacto com uma menor densidade em verde e poros de tamanho mais elevado do que o material monolítico. No caso dos compósitos de mulite reforçada com partículas ou com plaquetas constatou-se que as densidades dos compósitos em verde são superiores à exibida pela mulite; contudo a densidade da matriz destes compósitos é inferior à apresentada pelo material monolítico. Nos compósitos de mulite reforçada com 22.5% em volume de carboneto de silício estudou-se a influência da temperatura e da pressão no seu comportamento durante a densificação. O efeito da morfologia (partículas, plaquetas e fibrilas) e da quantidade (15%, 22.5% e 30% em volume) e da 2- fase na densificação dos compósitos, foi também um dos pontos abordados no trabalho efectuado. A adição de 15% em volume de partículas ou de fibrilas de SiC não altera de uma forma significativa o comportamento do compacto durante a densificação. Nos compósitos de mulite reforçada com 22.5% em volume de 2a fase, quer a adição de fibrilas quer a adição de partículas de SiC inibe a densificação do compacto. Este efeito é mais acentuado para as menores pressões e temperaturas de prensagem a quente. Para as temperaturas de 1500°C e 1550°C as partículas inibem mais a densificação do compósito do que as fibrilas. Pelo contrário, à temperatura de 1600°C as fibrilas apresentam um efeito inibidor mais acentuado do que as partículas. A adição de 22.5% em volume de plaquetas de SiC não inibe a densificação do compacto. Presume-se que os comportamentos observados têm origem nas impurezas existentes na superfície das inclusões, no empacotamento em verde, na fase líquida proveniente da mulite e num efeito da distância média entre as inclusões. Nos compósitos com 30% em volume de 2ª fase todas as morfologias ensaiadas inibem a densificação do compacto, sendo o maior efeito inibidor provocado pelas fibrilas e o menor pelas plaquetas. Determinou-se a microdureza de Vickers e a tenacidade à fractura da mulite e dos compósitos de mulite reforçada com SiC de modo a tentar avaliar o grau de tencificação conseguido e a influência da quantidade e da morfologia da fase de reforço nestas propriedades mecânicas. A microdureza de Vickers é pouco afectada pela presença da fase de reforço mais dura, independentemente da sua morfologia e da sua quantidade. Os resultados obtidos são muito inferiores aos que seriam previstos pela regra das misturas, pensando-se que este comportamento resulta da localização, junto à interface matriz/inclusões, da porosidade residual das amostras. A adição de inclusões densas de carboneto de silício à mulite promoveu um aumento da tenacidade à fractura, relativamente ao valor apresentado pelo material monolítico. A morfologia que parece ser mais eficaz é a exibida pelas plaquetas, apresentando o compósito de mulite reforçada com 30% em volume de plaquetas um incremento de 54%, relativamente ao material monolítico. A morfologia menos efectiva são as fibrilas. Para todas as morfologias de fase de reforço ensaiadas o aumento do conteúdo promove um incremento da tenacidade à fractura do compósito.
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- 1992
43. Relaxor Behavior of the 0.9BaTiO3-0.1La(Mg1/2Ti1/2)O3Solid Solution
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Salak, Andrei N., primary, Seabra, Maria Paula, additional, and Ferreira, Victor M., additional
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- 2004
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44. Benzene and NOxphotocatalytic-assisted removal using indoor lighting conditions
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Tobaldi, David Maria, Dvoranová, Dana, Lajaunie, Luc, Czikhardtová, Kristina, Figueiredo, Bruno, Calvino, José Juan, Seabra, Maria Paula, and Labrincha, João António
- Abstract
Modern life-style is creating an indoor generation: human beings spend approximately 90% of their time indoors, almost 70% of which is at home – this trend is now exacerbated by the lockdowns/restrictions imposed due to the COVID-19 pandemic. That large amount of time spent indoors may have negative consequences on health and well-being. Indeed, poor indoor air quality is linked to a condition known as sick building syndrome. Therefore, breathing the freshest air possible is of outmost importance. Still, due to reduced ventilation rates, indoor air quality can be considerably worse than outdoor. Heating, ventilation, and air conditioning (HVAC), air filtration systems and a well-ventilated space are a partial answer. However, these approaches involve only a physical removal. The photocatalytic mineralization of pollutants into non-hazardous, or at least less dangerous compounds, is a more viable solution for their removal. Titanium dioxide, the archetype photocatalytic material, needs UVA light to be ‘activated’. However, modern household light emitting diode lamps irradiate only in the visible region of the solar spectrum. We show that the surface of titanium dioxide nanoparticles modified with copper oxide(s) and graphene has promise as a viable way to remove gaseous pollutants (benzene and nitrogen oxides) using a common light emitting diode bulb, mimicking real indoor lighting conditions. Titanium dioxide, modified with 1 mol% CuxO and 1 wt% graphene, proved to have a stable photocatalytic degradation rate, three times higher than that of unmodified titania. Materials produced in this research work are thus strong candidates for offering a safer indoor environment.
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- 2022
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45. Relaxor Behavior of the 0.9BaTiO3-0.1La(Mg1/2Ti1/2)O3 Solid Solution.
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Salak, Andrei N., Seabra, Maria Paula, and Ferreira, Victor M.
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- *
SOLID solutions , *CERAMICS , *DIELECTRIC measurements , *LOW temperatures , *EQUATIONS of state , *SINTER (Metallurgy) , *MATERIALS at high temperatures - Abstract
Low-frequency dielectric response of air- and oxygen-sintered ceramics with the composition 0.9BaTiO3-0.1La(Mg½Ti½) O3 (0.9BT-0.1LMT) has been studied in the temperature range of 12-550 K. In comparison with pure BT, in 0.9BT0.1LMT the dielectric permittivity maximum is shifted by almost 300 K toward lower temperatures. Both real and imaginary parts of dielectric permittivity of the solid solution, in the range 12-150 K, show a strong frequency-dependent behavior, which is typical of relaxors. On the basis of the model of exponential cluster size distribution and the ColeCole equation, the degree of interaction between the polar clusters was estimated. It was shown that the oxygen vacancies arising during sintering at high temperatures did not affect noticeably the relaxor properties of the material. The role of heterovalent La3+/Ba+2 and Mg2+/Ti4+ substitutions in the relaxor behavior formation is discussed. [ABSTRACT FROM AUTHOR]
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- 2004
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46. Experimental and Computational Analysis of NO x Photocatalytic Abatement Using Carbon-Modified TiO 2 Materials.
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Zhiltsova., Tatiana, Martins, Nelson, Silva, Mariana R. F., Silva, Carla F. Da, Lourenço, Mirtha A. O., Tobaldi, David M., Covita, Daniel, Seabra, Maria Paula, and Ferreira, Paula
- Subjects
SOLAR spectra ,PHOTOCATALYTIC oxidation ,FLUID dynamics ,SURFACE reactions ,FLUID flow - Abstract
In the present study, two photocatalytic graphene oxide (GO) and carbon nanotubes (CNT) modified TiO
2 materials thermally treated at 300 °C (T300_GO and T300_CNT, respectively) were tested and revealed their conversion efficiency of nitrogen oxides (NOx ) under simulated solar light, showing slightly better results when compared with the commercial Degussa P25 material at the initial concentration of NOx of 200 ppb. A chemical kinetic model based on the Langmuir–Hinshelwood (L-H) mechanism was employed to simulate micropollutant abatement. Modeling of the fluid dynamics and photocatalytic oxidation (PCO) kinetics was accomplished with computational fluid dynamics (CFD) approach for modeling single-phase liquid fluid flow (air/NOx mixture) with an isothermal heterogeneous surface reaction. A tuning methodology based on an extensive CFD simulation procedure was applied to adjust the kinetic model parameters toward a better correspondence between simulated and experimentally obtained data. The kinetic simulations of heterogeneous photo-oxidation of NOx carried out with the optimized parameters demonstrated a high degree of matching with the experimentally obtained NOx conversion. T300_CNT is the most active photolytic material with a degradation rate of 62.1%, followed by P25-61.4% and T300_GO-60.4%, when irradiated, for 30 min, with emission spectra similar to solar light. [ABSTRACT FROM AUTHOR]- Published
- 2020
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47. SYNTHESIS AND CHARACTERISATION OF W-, Ag-, AND W/Ag- CO-DOPED TITANIA NANOPOWDERS, AND THEIR PHOTOCATALYTIC ACTIVITY.
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Labrincha, João Antonio, Tobaldi, David Maria, Seabra, Maria Paula, Pullar, Robert Carlyle, and Piccirillo, Clara
- Subjects
- *
CHEMICAL synthesis , *TITANIUM dioxide , *METAL powders , *PHOTOCATALYSIS , *SOL-gel processes , *DOPING agents (Chemistry) - Abstract
Tungsten, silver, and tungsten/silver co-doped titaniananopowders were synthesised via an aqueous sol-gel method. The dried gels were thermally treated at 2 different temperatures, and the occurrence of the amorphous phase was assessed, using the combined Rietveld-RIR method, on the X-ray powder diffraction data. Systematic studies of the crystal structure and of the optical properties of the powders - made with diffuse reflectance spectroscopy (DRS) - were performed. The energy band gap was calculated using the differential reflectance method; their morphology was investigated using field emission scanning electron microscopy and transmission electron microscopy analyses (FE-SEM and TEM, respectively). The photocatalytic activity of the samples was assessed in liquid-solid phase, under UVA-light and visible-light irradiation, monitoring the degradation of an organic dye. The influence of the phase composition, optical properties, dimensions, and specific surface area of the powders on the photocatalytic activity was thoroughly discussed. [ABSTRACT FROM AUTHOR]
- Published
- 2013
48. Valorização de cinzas de caldeiras de biomassa por incorporação em materiais cimentícios e adsorventes para aplicações ambientais
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Capela, Marinélia de Neto, Baptista, João António Labrincha, Tarelho, Luís António da Cruz, and Seabra, Maria Paula da Silva
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Supplementary cementitious material ,Self-hardening ,Adsorbents ,Waste recycling ,Biomass fly ash ,Filler ,Mortars - Abstract
Global awareness about environmental issues has led to the increasing use of renewable and sustainable sources of energy, as substitutes for fossil fuels, being the combustion of biomass one of the most used alternative sources of energy. Kraft pulp and paper plants are equipped with biomass cogeneration facilities to produce steam and power that are used in the pulp manufacturing processes and, in some cases, they also have biomass power stations to produce electricity. Industrial by-products from the production process and forestry residues are commonly used as fuel. However, this process produces large quantities of biomass fly ash (BFA) that is essentially landfilled, although in the context of the circular economy concept it should be recycled. This thesis investigates the potential of using BFA in the production of cement-based materials or as dye adsorbent. For this purpose, eight different types of biomass typically present in the fuel mixtures at the mill were combusted and the properties of the ash they generate were determined. These biomass ashes cannot be used alone as secondary raw material for clinker production since they exhibit low concentrations of Al₂O₃ and Fe₂O₃ and a CaO/SiO₂ mass ratio that is, in general, too high. Nevertheless, according to EN197–1:2011 and the value of Na₂O(eq), they can be used in cementitious formulations, with the incorporation amount depending on the biomass type. This knowledge will allow, in the future, optimizing biomass blends to be used as fuel aiming to generate ash with suitable properties for a better incorporation in cement-based materials. The possibility of using BFA as secondary raw material in a commercial screed mortar formulation, as filler (5.0, 7.5 e 10.0 wt.%), or as a cement substitute (17.0, 50.0 e 67.0 wt.%), was fully tested. Some simple pre-treatment processes were applied, such as milling and sieving, aiming to adapt ash characteristics to maximize its incorporation rate. The valorisation of 7.5 wt.% of BFA in the asreceived condition or up to 10.0 wt.% after grinding and sieving (
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- 2023
49. Valorização de resíduos em eco cimentos
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Guimarães, Gonçalo Marques, Seabra, Maria Paula da Silva, and Baptista, João António Labrincha
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Argamassas densas ,Argamassas porosas ,Resíduos ,Cinza de fundo - Abstract
A utilização de resíduos como substitutos de matérias-primas provenientes do “supermercado do planeta” resultam de uma visão futurista de crescimento sustentável. O estudo, que foi efetuado em colaboração com a Valorsul e a Mota-Engil, pretende estudar vias para valorizar as cinzas de fundo provenientes da queima de Resíduos Sólidos Urbanos. Este estudo preliminar tem como objetivo diminuir a quantidade de cimento Portland usado em formulações de argamassas através da sua substituição parcial por este resíduo ou usar o mesmo como agregado ou filler. Este trabalho, inicialmente, caracterizou, física e quimicamente, as cinzas de fundo. Para tal foi avaliada a sua distribuição granulométrica, composição química e mineralógica, comportamento térmico e morfologia. Como resultado desta caraterização concluiu-se que a substituição do cimento Portland por cinzas de fundo seria, no máximo, de 15% em volume. De seguida, foram preparadas formulações de argamassas densas (com até 15% em volume de cinzas como substituto do cimento) as quais exibiram uma diminuição significativa (30%) da resistência à compressão, devido ao comportamento inerte da cinza. No ensaio de durabilidade verificou-se que a temperatura e a presença de água promoveram as reações de hidratação e, como resultado, a resistência à compressão das amostras aumentou. A adição de cinza como filler resultou na diminuição da resistência a compressão. A substituição do agregado por cinzas de fundo também foi testada. Verificou-se que, no caso da cura dentro de água, ocorria o aparecimento de eflorescências à superfície das amostras e, consequentemente, a diminuição da sua resistência mecânica à compressão. Num terceiro momento, foi utilizado um agente porogénico (pó de alumínio) para preparar argamassas porosas. Verificou-se que o teor adequado é 0,25%, em massa, pois foi o que apresentou os melhores resultados nas propriedades mecânicas e térmicas. Recorreu-se, ainda, à utilização de argila expandida como agregado para a formulação de argamassas onde foram efetuados ensaios de absorção sonora. Os resultados obtidos foram promissores pois conseguiuse uma atenuação sonora significativa. The use of waste as substitutes for raw materials from the “supermarket of the planet” is the result of a futuristic vision of sustainable growth. The study, which was carried out in collaboration with Valorsul and Mota-Engil, aims to study ways to valorize bottom ash from the burning of Municipal Solid Waste. This preliminary study whose objective was to reduce the amount of Portland cement in mortar formulations through its partial replacement by this waste or to use it as aggregate or filler. This work, initially, characterized, physically and chemically, the bottom ash. To this end, its granulometric distribution, chemical and mineralogical composition, thermal behavior and morphology were evaluated. As a result of this characterization, it was concluded that the replacement of Portland cement by bottom ash would be, at most, 15% by volume. Firstly, dense mortar formulations were prepared (with up to 15% in volume of bottom ash as cement replacement), which exhibited a significant decrease (30%) of the compressive strength, due to the inert behavior of the ash. In the durability test, it was found that the temperature and the presence of water promoted the hydration reactions and, as a result, the compressive strength of the samples increased. Formulations with bottom ash as filler resulted in the decrease of mechanic proprieties. The substitution of the aggregate by bottom ash was also tested. It was found that, in the case of curing in water, efflorescence appeared on the surface of the samples and, consequently, a decrease in the compressive strength was observed. In a third moment, a porogenic agent (aluminum powder) was used to develop porous mortars. It was found that the appropriate content was 0.25% by mass, as this presented the best results in mechanic and thermal proprieties. Expanded clay was also used as aggregate for the formulation of mortars where sound absorption tests were carried out. The results obtained were promising as significant sound attenuation was achieved. Mestrado em Engenharia de Materiais
- Published
- 2022
50. Geopolimerização de resíduos industriais para a obtenção de estruturas leves
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Buruberri, Leire Hernando, Labrincha, João António, and Seabra, Maria Paula da Silva
- Subjects
Argamassas geopoliméricas ,Valorização de resíduos industriais ,Geopolimerização ,Estruturas leves ,Geopolímeros porosos - Abstract
O principal objetivo deste trabalho é a valorização de resíduos industriais, tais como lama de anodização de alumínio, cinzas da queima de biomassa florestal e de casca de arroz, na produção de geopolímeros com arquitetura porosa controlada e argamassas geopoliméricas que possam ser aplicadas em revestimentos exteriores, com a adequada durabilidade face à ação de agentes de degradação. Desta forma perspetivam-se formas viáveis de valorização de resíduos que são ainda maioritariamente depositados em aterro, contribuindo para o novo paradigma da economia circular. Em primeiro lugar foi realizada a caracterização físico química dos resíduos e o estudo da sua reatividade em hidróxido de sódio, condições que simulam as da reação de geopolimerização. Utilizou-se metacaulino comercial como precursor sólido de referência e também para otimizar as misturas. O seu potencial, como precursor geopolimérico, foi avaliado através da libertação de espécies reativas (Si e/ou Al) no meio alcalino. As cinzas da queima de biomassa florestal demostraram fraco desempenho na libertação destas espécies, 357 mg/L de Si e 337 mg/L de Al em 8 dias, enquanto o metacaulino forneceu 942 mg/L de Si e 931 mg/L de Al em 48 horas. A lama de anodização e as cinzas da queima da casca de arroz fornecem rapidamente quantidades elevadas de Al (1086 mg/L em 30 min) e de Si (3364 mg/L, 60 min), respetivamente. A rápida libertação de Al por parte da lama de anodização não se traduziu em elevado potencial de endurecimento, mas foi utilizada para o controlo da arquitetura porosa na fabricação de geopolímeros porosos à base de metacaulino, devido à sua capacidade em aumentar a viscosidade das pastas. A incorporação de até 6 % (em massa) de lama permitiu obter materiais com porosidade total de 84 %, sendo 70 % a fração de poros abertos, com consequente reduzida densidade (339 kg/m3), baixa condutividade térmica (0,082 W/m.K) e resistência mecânica à compressão de 2 MPa, valor suficiente para permitir manuseio, transporte e aplicação. As cinzas da queima da casca de arroz foram utilizadas como substituto parcial de silicato de sódio na formulação do ativador alcalino, com o intuito de reduzir a concentração deste reagente na mistura Na2SiO3/KOH e diminuir, desta forma, a pegada ecológica associada à preparação das formulações. A substituição parcial de silicato de sódio pela cinza (10, 20 % em massa), permitiu obter argamassas com menor permeabilidade ao vapor de água, coeficiente de capilaridade (< 0,4 kg/m2min0,5), e índice de secagem (< 0,13 kg/m2.h), do que as formuladas com ligantes comerciais, o que traduz melhor desempenho. A permeabilidade destas argamassas garante superior resistência à degradação perante mudanças de temperatura/humidade, simuladas em ensaios de gelo/degelo. A perda de massa das amostras após 25 ciclos de teste foi de 61, 27 e 23 % quando são utilizadas, na sua preparação, soluções de KOH 5, 7,5 e 10M e se promove a substituição de silicato de sódio por 20 % de cinza da queima da casca de arroz. The main objective of this work was the valorisation of several industrial wastes, such as the forest biomass fly ash, rice husk ash and aluminium anodizing sludge in the formulation of porous geopolymers with controlled porous architecture, and in the production of geopolymer mortars that can be applied as exterior coatings, since possess suitable durability against external agents. This way, viable recycling alternatives were achieved for wastes whose actual destination is still the landfilling, contributing to the circular economy new paradigm. Firstly, the chemical and physical characterization of the wastes was performed, as well as their reactivity potential in the sodium hydroxide solution, attempting to simulate the geopolymerization reaction. Commercial metakaolin was also tested as the standard solid precursor and to compose the formulations. The potential as geopolymer precursor was measured by the release of reactive species (Si and/or Al) into the alkaline medium. The forest biomass fly ash showed poor performance in releasing such species, 357 mg/L of Si and 337 mg/L of Al after 8 days, when compared with the commercial metakaolin (942 mg/L of Si and 931 mg/L of Al in 48 hours). The aluminium anodizing waste and the rice husk ash provided high amounts of Al (1086 mg/L in 30 min) and Si (3364 mg/L, 60 min) respectively. The rapid release of Al from the aluminium anodizing waste did not assure an expressive hardening potential but was used to control the pore architecture development of metakaolin-based geopolymers, since enhances the viscosity of the pastes. Formulations containing up to 6 wt.% sludge generate materials having total porosity = 84 % (70 % corresponding to open pores), which are lightweight (339 kg/m3), thermal insulating (conductivity = 0.082 W/m.K), and show compressive strength = 2 MPa, enough to enable handling, transportation and application. The rice husk ash was used as substitute for sodium silicate in the alkaline solution formulation (Na2SiO3/KOH), then contributing to reduce the ecological footprint of the formulations. The partial substitution of sodium silicate by the rice husk ash (10, 20 wt.%), allowed to obtain mortars with lower values of water vapour permeability, capillarity coefficient (< 0.4 kg/m2min0.5), and drying coefficient (< 0,13 kg/m2.h), than those achieved for mortars prepared with commercial reagents. This assures higher resistance to degradation against temperature/humidity changes, simulated by freeze thaw cycles. The mass loss of the samples after 25 cycles was 61, 27 and 23 % when KOH 5, 7.5 and 10M solutions were used in their preparation and rice husk ash replaced 20 wt.% of Na2SiO3. Programa Doutoral em Ciência e Engenharia de Materiais
- Published
- 2021
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