Processamento de nitreto de alumínio (AIN) em meio aquoso

Doutoramento em Ciência e Engenharia de MateriaisO presente trabalho teve como objectivo principal o estudo das condições experimentais conducentes à viabilização do processamento de pós de AlN em meio aquoso, e à sinterização das peças consolidadas a temperaturas inferiores (1750ºC) às normalmente usadas (1900ºC). As propriedades finais das amostras processadas em meio aquoso, designadamente a condutividade térmica, deveriam ser comparáveis às das amostras de AlN actualmente processadas em meio orgânico. Para o efeito, a superfície dos pós de AlN foi protegida contra as reacções de hidrólise usando duas substâncias de natureza hidrofílica: ácido fosfórico e di-hidrogenofosfato de alumínio. A ligação destas espécies químicas à superfície das partículas do AlN e a sua eficácia na protecção da hidrólise foi avaliada por espectroscopia de infravermelho, Ressonância Magnética Nuclear (RMN) e medição do pH de uma suspensão ao longo do tempo. Ambas as substâncias permitiram evitar as reacções de hidrólise, embora o di-hidrogenofosfato de alumínio, por possuir uma ligação à superfície mais forte, se tenha mostrado mais eficaz, sendo por isso a espécie adoptada para o tratamento do AlN e posterior prossecução do trabalho. Com o pó de AlN assim tratado, foi possível preparar suspensões estabilizadas contendo 50- vol.% de sólidos na presença de ligantes e plastificantes, que permitiram o processamento por várias técnicas coloidais: enchimento por barbotina (slip casting), granulação de pós por aspersão-congelamento-liofilização (freeze granulation) e preparação de cintas (tape casting). As peças obtidas foram posteriormente sinterizadas a 1750ºC, usando como aditivos de sinterização o CaF2 e o YF3. Testaram-se diferentes quantidades e proporções, tendo-se verificado que a mistura 3-wt% YF3 + 2-wt% CaF2 era aquela que, de um modo geral, permitia obter os valores de condutividade térmica mais elevados para as amostras preparadas pelas três técnicas de processamento. A microestrutura das amostras e as fases secundárias formadas foram avaliadas por Microscopia Electrónica de Varrimento (SEM), Microscopia Electrónica de Transmissão (TEM) e Difracção de raios-X (DRX). Os valores de condutividade térmica das amostras obtidas pelas diferentes técnicas de processamento e sinterizadas nas mesmas condições foram os seguintes: 115 W m-1 K-1 para as amostras obtidas por enchimento por barbotina, 140 W m-1 K-1 para as amostras prensadas a partir de grânulos e 200 W m-1 K-1 para as cintas. A presença de carbono, resultante da queima incompleta das substâncias orgânicas (ligantes e plastificantes) durante a pré-sinterização, e a distância a percorrer pelas espécies gasosas ricas em oxigénio são os principais factores que controlam a concentração final de oxigénio nas fronteiras de grão e na rede cristalina e, consequentemente, a condutividade térmica.The present work aims at processing AlN in aqueous media and sintering the obtained samples at lower temperatures (1750ºC) than those usually used (1900ºC), keeping or even improving the final properties, namely thermal conductivity, usually obtained from organic processing. In order to protect AlN powder against hydrolysis, two hydrophilic substances, phosphoric acid and aluminium di-hydrogenphosphate, were tested. The nature of the bonding of these species to the AlN particles surface and their ability to avoid hydrolysis reactions were evaluated by Fourier Transformed Infrared (FT-IR) and Nuclear Magnetic Resonance spectroscopy (RMN), and by monitoring the pH of the AlN suspension with time. Although both species allowed controlling hydrolysis reactions, aluminium di-hydrogenphosphate was selected for further progress of the work due to its stronger bonding to the AlN particles surface and consequently, to the higher stability of the astreated powder in aqueous media. It was possible to prepare stabilised AlN suspensions with 50-vol.% solids loading using AlN treated powder in presence of binders and plasticizers, which allows to process samples by different colloidal shaping techniques: slip casting, freeze powders granulation and tape casting. The obtained samples were then sintered at 1750ºC using CaF2 and YF3 as sintering aids. Different amounts and proportions were tested, with the highest thermal conductivity values being generally obtained for the mixture 3-wt% YF3 + 2-wt% CaF2 for the samples prepared by the different shaping techniques and sintered under the same conditions. The microstructural features and the secondary phases of the sintered samples were evaluated by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and X-Ray Diffraction (DRX). The thermal conductivity of the samples prepared by slip casting, pressing of granules obtained by freeze granulation and tape casting are 115 W m-1 K-1, 140 W m-1 K-1 and 200 W m-1 K-1, respectively. The presence of carbon remaining from the burnout of the organic species (binder and plasticizer) and the diffusion path length that the oxygen rich species have to cross to be released from the grain boundaries were regarded as the main factors determining the final oxygen concentrations at the grain boundaries and crystalline lattice and, therefore, for the thermal conductivity values of the samples

3D-Printing of zirconia dental prostheses

Abstract in proceedings of the Fourth International Congress of CiiEM: Health, Well-Being and Ageing in the 21st Century, held at Egas Moniz’ University Campus in Monte de Caparica, Almada, from 3–5 June 2019.This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.info:eu-repo/semantics/publishedVersio

Entre fotografia e cinema: o papel da imagem-arquivo fotográfica no cinema de não-ficção

Esta dissertação aborda diferentes estratégias cinematográficas de reutilização de imagens-arquivo fotográficas no cinema de não-ficção, procurando perceber de que forma estas transformam as fotografias pré-existentes e afetam a perceção do seu estatuto documental. O trabalho baseia-se numa revisão da literatura sobre a relação entre fotografia e cinema, e sobre a reutilização de imagens de arquivo no cinema, aqui aplicada ao uso de materiais fotográficos, tendo em vista identificar conceitos teóricos relevantes para a análise de três casos de estudo: Luz Obscura de Susana Sousa Dias, Sob Céus Estranhos, de Daniel Blaufuks e Ascent, de Fiona Tan.This dissertation addresses different cinematographic strategies for the reuse of photographic archival images in non-fiction cinema, in order to understand how these transform pre-existing photographs and affect the perception of their documentary status. This work is based on a literary review on the relationship between photography and cinema, and on the reuse of archival images in cinema, applied here to the use of photographic materials, in order to identify relevant theoretical concepts for the analysis of three case studies: Luz Obscura by Susana Sousa Dias, Sob Céus Estranhos by Daniel Blaufuks and Ascent by Fiona Tan

Influence of ink rheology and post processing in the structural performance of silicon nitride-based ceramics fabricated by robocasting

The fabrication of complex shaped Si3N4 parts by conventional methods is challenging due to its high hardness and mechanical strength. Additive manufacturing (AM) appeared as a competitive way to attain three-dimensional complexity at lower costs. However, most of AM technologies still present limitations in producing high dense ceramics, mainly due to several difficulties in developing proper feedstock. This work presents the fabrication of dense silicon nitride-based ceramics by using an extrusion based additive manufacturing technique (direct ink writing, also known as robocasting) and cold isostatic pressing (CIP) as a post processing step. Silicon-nitride aqueous based inks containing different amounts of solids loading (36, 38 and 39 vol%) and proper rheological characteristics for the printing process were studied. The use of CIP (200 MPa) is suggested to reduce or even eliminate defects and porosity and consequently, to improve mechanical performance of the final parts. Relative density, microhardness and flexural strength increased as solids loading in the inks increased and were further improved when CIP pressing was used. Parts produced by robocasting with an ink containing 39 vol% and cold isostatic pressed at 200 MPa exhibited an average relative density around 99%, 1475 HV2 microhardness value and 650 MPa flexural strength, values similar to those of silicon nitride ceramics fabricated by conventional processing.publishe

Dualidades na lógica modal

Mestrado em Matemática e AplicaçõesO objectivo deste trabalho é desenvolver algumas ferramentas categoriais para provar teoremas de dualidades para categorias de álgebras relevantes na lógica (modal). O primeiro capítulo engloba os conceitos mais elementares de teoria das categorias. No segundo, analisamos adjunções, mónadas e algumas construções associadas, no sentido de determinar uma relação entre as meta-categorias das mónadas definidas numa categoria e das adjunções de Kleisli sobre a mesma categoria. Álem disso, mostramos que a construção de Vietoris é uma componente de uma mónada de Kock-Zöberlein. No terceiro capítulo provamos teoremas de dualidades para álgebras Booleanas com operador e reticulados distributivos com operador, como consequência de dualidades mais gerais de categorias de espaços e relações. Para finalizar, mostramos que a operação nas categorias de álgebras e “hemimorfismos” que corresponde ao produto cartesiano nas categorias de espaços e relações é o produto tensorial.The aim of this work is to develop some categorial tools for proving dualities for categories of algebras relevant in (modal) logic. The first chapter covers the most basic concepts of category theory. In the second, we analyze adjunctions, monads and some associated constructions, in order to determine a relationship between the meta-categories of monads defined on a category and of the Kleisli adjunctions on the same category. Moreover, we prove that the Vietoris construction is part of a Kock-Zöberlein monad. In the third chapter we prove duality theorems for Boolean algebras with operator and distributive lattices with operator as a consequence of more general dualities of categories of spaces and relations. Finally, we show that the operation in the categories of algebras and “ hemimorfismos” that corresponds to the cartesian product on the categories of spaces and relations is the tensor product

Spatial epidemiology: New approaches to old questions

Spatial Epidemiology is used to describe, quantify and explain the geographical variations of diseases, to evaluate the association between the incidence of diseases and potential risk factors and to identify spatial clusters of diseases. This article goes through the main aspects of spatial epidemiology, starting with an explanation of the importance of mapping health data, an historical perspective of the development of the discipline, a description of spatial data types, some methods of spatial statistics, and the importance of Geographical Information Systems (GIS) in the analysis of spatially-referenced data. Some applications of GIS regarding oral health are presented.info:eu-repo/semantics/publishedVersio

Assessment of groundwater contamination in an agricultural peri-urban area (NW Portugal): an integrated approach

The excessive use of pesticides and fertilisers in agriculture has generated a decrease in groundwater and surface water quality in many regions of the EU, constituting a hazard for human health and the environment. Besides, on-site sewage disposal is an important source of groundwater contamination in urban and peri-urban areas. The assessment of groundwater vulnerability to contamination is an important tool to fulfil the demands of EU Directives. The purpose of this study is to assess the groundwater vulnerability to contamination related mainly to agricultural activities in a peri-urban area (Vila do Conde, NW Portugal). The hydrogeological framework is characterised mainly by fissured granitic basement and sedimentary cover. Water samples were collected and analysed for temperature, pH, electrical conductivity, chloride, phosphate, nitrate and nitrite. An evaluation of groundwater vulnerability to contamination was applied (GOD-S, Pesticide DRASTIC-Fm, SINTACS and SI) and the potential nitrate contamination risk was assessed, both on a hydrogeological GIS-based mapping. A principal component analysis was performed to characterised patterns of relationship among groundwater contamination, vulnerability, and the hydrogeological setting assessed. Levels of nitrate above legislation limits were detected in 75 % of the samples analysed. Alluvia units showed the highest nitrate concentrations and also the highest vulnerability and risk. Nitrate contamination is a serious problem affecting groundwater, particularly shallow aquifers, especially due to agriculture activities, livestock and cesspools. GIS-based cartography provided an accurate way to improve knowledge on water circulation models and global functioning of local aquifer systems. Finally, this study highlights the adequacy of an integrated approach, combining hydrogeochemical data, vulnerability assessments and multivariate analysis, to understand groundwater processes in peri-urban areas

Successful aqueous processing of a lead free 0.5Ba(Zr0.2Ti0.8)O-3-0.5(Ba0.7Ca0.3)TiO3 piezoelectric material composition

We report on the successful aqueous processing of a lead free piezoelectric 0.5Ba(Zr0.2Ti0.8)O-3-0.5( Ba0.7Ca0.3)TiO3 (BZT-BCT) composition with the final functional properties of the materials unaffected by the various processing steps involved. X-ray diffraction results show a single tetragonal perovskite crystalline phase for the as-received sintered BZT-BCT powder. The purity of the perovskite phase for BZT-BCT powder was found to be controlled even after ageing the material in water for 24 h as a successful surface treatment against hydrolysis. An aqueous suspension of surface treated BZT-BCT powder with 50 vol% solid loading was successfully transformed into micro-sized granules via a freeze granulation (FG) method. Various structural, electrical and mechanical properties of sintered BZT-BCT-FG and BZT-BCT-NG ceramics consolidated from freeze granulated and non-granulated (NG) powders, respectively, were measured. The dielectric constant (epsilon(r)) values of the BZT-BCT-FG sample were found to be higher, with lower dielectric loss (tan delta) values in comparison with those of a sample prepared from the BZT-BCT-NG powder at all temperatures and with all frequency ranges tested. Nanoindentation results revealed that the ability to oppose deformation was nearly 10-fold higher for BZT-BCT-FG (6.93 GPa) than for BZT-BCT-NG ceramics (543 MPa). The functional properties of BZT-BCT- FG samples confirmed the benefits of the aqueous processing approach in comparison with traditional dry pressing

Preparation of ceramic microlectromechanical systems (MEMS) using a gelcasting consolidation technique

The present work focuses the preparation and characterisation of suspensions withthree different powders, ferrite, PZT and alumina, for the fabrication of 3D-shapedceramic structures by gelcasting. The characterization of powders by particle size ispresented together with the preparation and characterization of the resultingsuspensions by rheological and zeta potential measurements. Using 1-wt% of DispexA-40 is possible to prepare concentrated PZT and ferrite suspensions to beconsolidated by gelcasting. The workability of gelcasting technique is demonstratedon a range of component geometries suitable for MEMS applications