Ciência & Tecnologia dos Materiais最新文献

Bismuth niobate (BiNbO₄) has attracted attention as a low-fired ceramics with promising microwave application potential. It belongs to the bismutocolumbite oxides with similarly to scheelite-like stibiotantalite structure (SbTaO₄) A³⁺B⁵⁺O₄. The aim of the present research was to fabricate BiNbO₄ ceramics by solid state reaction route from the mixture of simple oxides viz. Bi₂O₃, and Nb₂O₅, and study its phase composition as well as crystal structure by X-ray diffraction method. The Rietveld refinement method was utilized for analysis of diffraction patterns. It was found that the mass change effects finished at temperature T=500°C and the total mass change was about Δm=-0.78%. It was found that BiNbO₄ ceramics sintered in ambient air exhibited multiphase composition, i.e., apart from the major α-BiNbO₄ phase, the orthorhombic Bi₅Nb₃O₁₅, Bi₃Nb₁₇O₄₇ and cubic Bi₃NbO₇ phases were present. An increase in sintering temperature caused an increase in the amount of major α-BiNbO₄ phase which adopted an orthorhombic symmetry described well by Pnna(52) space group.

The investment casting of reactive Ti and TiAl alloys requires the use of selected ceramics in the face-coat layer to prevent the reaction between the cast metal and ceramic shell, avoiding the formation of a hard layer at the metallic components surface. This work aims to study the influence of ceramic shells composition in some of its characteristics such as flexural strength, friability and dimensional accuracy. The microstructure of the shells was evaluated by SEM. Changes in the face-coat and back-up ceramic shells composition determines the ceramic shell strength to withstand the casting stage with adequate mould permeability and thermal conductivity, and a compromise resistance for knock-out. All the non-conventional ceramic shell systems with interest for reactive alloys, based on fumed alumina binder and alumina sand for the back-ups, present higher dimensional stability (low shrinkage or expansion) compared with traditional systems based on colloidal silica binder and zircon and aluminosilicates back- ups. In this work, better mechanical strength and lower friability were obtained with non-conventional face-coats of alumina and polymer binders, both with yttria flour and stucco, followed by alumina back-ups. Selecting the right ceramic shell composition, it is possible to achieve adequate properties for casting titanium alloys.

Urbcork is an urban furniture line developed at the Faculty of Engineering of the University of Porto (FEUP) for the collection produced by Leets Urban Design. It can be highlighted for its sustainable properties and inclusive design concerns.

Urban furniture is a product to be used by a large number of people in a public space. It has a high level of wear and ageing due to the intensive use, solar exposure and weather conditions.

During the project development there was a conceptual idea about using cork as the main material for the seat and the back. The opportunity of developing a prototype brought the question about how to keep cork properties after a long exposure on exterior.

The main line of investigation was about how to protect cork keeping its natural appearance and extending its properties and conservation over time. Laboratory tests held at CTCOR and FEUP were conducted in order to understand the behaviour of cork on extreme conditions. The results are not yet successful, but with more tests, the product can be well accepted in the international market.

Adhesive bonding is a viable technique to reduce weight and complexity in structures. Additionally, this joining technique is also a common repair method for metal and composite structures. However, a generalized lack of confidence in the fatigue and long-term behaviour of bonded joints hinder their wider application. Suitable strength prediction techniques must be available for the application of adhesive bonding, and these can be based on mechanics of materials, conventional fracture mechanics or damage mechanics. These two last methodologies require the knowledge of the fracture toughness (G_C) of materials. Being damage mechanics-based, Cohesive Zone Modelling (CZM) analyses coupled with Finite Elements (FE) are under investigation. In this work, CZM laws were estimated in shear for a brittle adhesive (Araldite^® AV138) and high-strength aluminium adherends, considering the End-Notched Flexure (ENF) test geometry. The CZM laws were obtained by an inverse methodology based on curve fitting, which made possible the precise estimation of the adhesive joints’ behaviour. It was concluded that a unique set of shear fracture toughness (G_IIC) and shear cohesive strength (t_s⁰) exists for each specimen that accurately reproduces the adhesive layer behaviour. With this information, the accurate strength prediction of adhesive joints in shear is made possible by CZM.

In order to identify and follow the challenges in the use of materials with the capacity to absorb hydrogen for energy storage, many groups address a diversity of issues, where the need to study the properties of the material upon absorbing hydrogen is always present. Appropriate equipment and techniques are needed: besides the use of classical systems, many research groups identified recently the need to study in more detail the properties related to the macroscopic changes of volume of the hydride powder as hydrogen content is cycled. In this article, we present the equipment and techniques developed by our group: after the classical volumetric systems, we addressed the problem of volumetric changes by building a novel coaxial capacitive system. This system measures the volume and porosity of a small amount of free hydride powder as a function of hydrogen content, after applying a complex deconvolution algorithm on the primary AC electric measurements.

Deteriorations generated in service can cause catastrophic failure at the specific properties of the polymer composite materials. In view of this, scientists have drawn inspirations by natural biological systems and their unique ability to heal an external wound, to develop a similar repair system within a material. Carbon and glass fiber reinforced polymers were manufactured following the wet lay up or the prepreg process. Microcapsules at contents, 5% or 10% by weight, vascular networks from wax and steel wires and finally reversible polymers were implemented within a composite as a potential self-healing system. Inspection techniques, including Ultrasonic C-Scan and Infrared Thermography, were applied, where possible. Optical microscopy revealed the disruption of the composite structural integrity, regarding the observed ply waviness and the resin reach zones around the vascular structures. Three point bending experiments determined the knock down factor, expressed as a decrease on flexural strength and modulus values, for each case, compared to the reference material. The reduction ranged from 12%-64% depending mainly not only to the selected manufacturing method but also to the different implemented healing system.

In this work some of the key issues which affect the performance of catalysts for the anode and cathode electrodes in Direct Methanol Fuel Cells are analyzed. To deal with present challenges and overcome limitations different approaches have been implemented, which include catalyst support diversification and functionalization, control of particle size and the introduction of Pt alloying and heat treatment in order to enhance the rate of critical reactions such as CO electroxidation and oxygen reduction reaction and also reduce Pt loading. A catalyst design strategy has been devised which incorporates the mentioned approaches in order to tackle various critical aspects for both electroactivity and stability, considered essential to boost Direct Methanol Fuel Cells technology.

In this paper, an overview on the subject of materials and renewable energy, mainly from the research point of view, is carried out. Energy and materials are nowadays driving science and technology. There is a search for cleaner, cheaper and more efficient energy production, and this is obviously related to the development of new and innovative materials. As energy is a top European priority, materials research can enable Europe to meet its future energy and climate goals. The importance of raw materials for the energy sector and the future of advanced materials for low carbon energy are addressed. Materials-based solutions to the energy problem and guidance on research in this field are also the aim of this paper.

TiO₂/graphene nano platelets (GNP) nanocomposite cathodes have been synthesized through a simple ball mill process. TiO₂ anatase nanoparticles, around 16 nm in size, were encapsulated in the 2D graphene matrix. The synthesized samples are characterized using x-ray diffraction (XRD), DSC, impedance spectroscopy, and scanning electron microscope (SEM). The graphene nano platelets act not only to reduce the charge transfer resistance but can help to absorb deformation caused by divalent insertion. The electrochemical behavior of Mg metal was tested in dimethyl sulfoxide solution containing magnesium perchlorate salt. The obvious redox peaks on the cyclic voltammetric curves confirm Mg²⁺ inserts/extracts into/from TiO₂ through our simple electrolyte solution.

The water management is a critical problem to overcome in the PEM fuel cell technology. Models play an important role in fuel cell development since they enable the understanding of the influence of different parameters on the cell performance allowing a systematic simulation, design and optimization of fuel cells systems. In this work, a model previously developed and validated, is used to predict the water transport through the cell. The influence of membrane thickness and transport properties, reactants pressure and relative humidity and operation temperature, on the water content through the membrane and on the cell performance was studied. The model predicts the membrane water content and water concentration profiles across the membrane electrode assembly (MEA). This work represents a useful tool to set-up suitable operating conditions leading to an optimised water management producing a better performance for PEM fuel cells.