Bentonite from Porto Santo Island, Madeira Archipelago: surface properties studied by inverse gas chromatography

The present paper shows the importance of Inverse Gas Chromatography (IGC) for the determination of the surface properties of bentonites. These properties are dispersive surface energy, acid-base properties, surface heterogeneity, sorption isotherms, BET surface areas and heat of sorption, using different probe molecules. IGC can contribute to the interpretation, prediction and optimization of the studied materials’ properties. The paper focuses on two distinctive bentonite samples from Porto Santo Island, Madeira archipelago. In view of their potential value, achieved through their incorporation in products for dermopharmacy and dermocosmetics, particle size and particle surface properties are of fundamental importance.info:eu-repo/semantics/publishedVersio

Influência da incorporação de partículas de SiC na microestrutura e no comportamento tribológico de compósitos Al/SiCp com gradiente funcional

Devido à boa relação entre densidade e resistência mecânica apresentada pelos compósitos de matriz de alumínio, estes materiais têm-se destacado em algumas aplicações industriais. Com efeito, a adição de reforços cerâmicos às ligas de alumínio, nomeadamente partículas de SiC, resulta na melhoria das suas propriedades mecânicas com particular impacto no aumento da resistência ao desgaste, ampliando o seu campo de aplicação como tribo-materiais. O desenvolvimento complementar de gradientes funcionais de propriedades neste tipo de compósitos permite a melhoria de uma grande variedade de propriedades, possibilitando a utilização destes tribo-materiais na indústria automóvel e aeronáutica. Exemplos de aplicações são cilindros, válvulas e seguidores de cames em motores de combustão interna, onde estes materiais operam como parte integrante de diversos sistemas tribológicos. Este estudo teve como objectivo verificar a influência da incorporação de partículas de SiC na microestrutura da liga Al-10Si-4.5Cu-2Mg processada por fundição centrífuga, tendo como referência a matriz não reforçada mas também processada por fundição centrífuga e caracterizada por um gradiente na microestrutura. A fracção de volume de partículas de SiC adicionadas foi de 10%. O comportamento tribológico dos dois materiais foi avaliado a fim de compreender o modo como as alterações na microestrutura promovidas pela adição de partículas de SiC influencia a resposta tribológica. Os testes tribológicos foram realizados na ausência de lubrificação e na geometria esfera-disco. A carga normal aplicada e a velocidade de deslizamento foram mantidas constantes com os valores de 3 N e 0.5 ms-1, respectivamente. Como contra-corpo foi usado aço de rolamento AISI 52100. A microestrutura e os principais mecanismos de desgaste foram caracterizados por recurso a SEM/EDS. Os resultados demonstraram a existência de fadiga extrema e a predominância de fenómenos de abrasão como mecanismos de desgaste. A resposta tribológica dos materiais caracterizados indiciou poder ser fortemente influenciada pela quantidade de partículas de reforço presente na matriz de alumínio.Fundação para a Ciência e Tecnologia (FCT) - Programa POCTI (Projecto POCTI/CTM/46086/2002)

Wetting in Al composites reinforced with SiC particles

Aluminium matrix composites have been wide used essentially due to the good relation between weight and mechanical resistance. To develop a ceramic particle/matrix interface with good characteristics, it is essential to control the interface reactivity, avoiding the formation of undesirable reaction products such as Al4C3. Essentially, there are three methods to prevent the Al4C3 formation: Si addition to Al matrix, coating of the SiC particles and to promote a passive oxidation of SiC particles. The last method has been the most used due to its simplicity. In this work the formation of a SiO2 layer, in SiC particles surface, was made by exposing the particles to temperatures above 800ºC, in air. SiC particles with different grain size (12.8 and 37.6 µm), were used. The oxidation behaviour was followed by TGA. Also, the wettability between the SiC particles and the Al matrix (Al-10Si-4.5Cu-2Mg) was investigated by measuring the contact angle as a function of temperature and/or time. XRD, SEM, AFM and EDS techniques were used to characterize the SiC particles surface, before and after the tests.Fundação para a Ciência e a Tecnologia (FCT

Effect of silicon carbide particles on the microstructure and tribological behaviour of functional graded Al/SiCp composites

Aluminium matrix composites have been wide used essentially due to the good relation between weight and mechanical resistance. Also, the additions of ceramic reinforcement, mainly SiC particles, improve some properties of these composites, particularly wear resistance. Additionally, when functional gradients of properties are promoted in this type of composites, a wide range of properties are improved. This type of materials could be used as tribomaterials of high potential for automotive and aeronautic industries, particularly in applications such as cylinder liners, valves and came followers. In those situations a tribological system is created. In this work, the influence of the incorporation of SiCp on the microstructure of the Al matrix (Al-10Si-4,5Cu-2Mg) processed by centrifugal casting was evaluated by comparing the unreinforced alloy, in which a graded microstructure is present, with that obtained in the functionally graded Al-alloy composite reinforced with SiCp. The volume fraction of SiC particles in the precursor composite was 10%. Additionally, the tribological behaviour of the two materials was evaluated and compared in order to estimate how the changes in microstructure promoted by SiCp addition would influence the tribological response. Unlubricated tribological tests were performed in a pin-on-disc tribometer where the normal load, frequency and linear velocity were kept constant (3 N, 1 Hz, 0.5 m/s respectively). As conterbody an AISI 52100 steel was used. The microstructures and the wear mechanisms were identified by SEM/EDS analysis. Worn surfaces were also characterised by AFM. Results show that some extreme fatigue and abrasion phenomena dominate the wear mechanisms presented in this type of materials. However, the tribological response appears to be strongly influenced by the volume fraction of reinforcing particles. Also, the role of the matrix microstructure on the tribological behaviour, essentially in what concerns to the particles/matrix interfacial region is discussed.Fundação para a Ciência e aTecnnologia (FC

Estudos de molhabilidade em compósitos de Al reforçados com partículas de SiC

Os compósitos de matriz de alumínio têm-se mostrado extremamente atractivos principalmente devido a uma relação peso/resistência muito favorável. Também, a incorporação de reforços cerâmicos, nomeadamente partículas de SiC, nestes compósitos conduziu à melhoria significativa na resistência ao desgaste, propiciando possibilidades de novas aplicações. Para melhorar as características da interface partícula/matriz é essencial controlar a reactividade da interface, evitando a formação de produtos de reacção não desejados durante o processamento, tais como o Al4C3. Este composto é frágil e instável, promovendo a degradação das propriedades mecânicas e a sensibilidade dos compósitos a ambientes corrosivos. Existem essencialmente três métodos para prevenir a formação de Al4C3: adição de Si à matriz de Al, revestir as partículas de SiC e fazer uma oxidação passiva do SiC. Este último método tem-se destacado por ser simples e proporcionar excelentes resultados, consistindo na formação de um filme de SiO2 na superfície das partículas de SiC, pela sua exposição a temperaturas acima dos 800 ºC, ao ar. Assim, o objectivo deste trabalho consistiu na determinação das condições óptimas de oxidação passiva de partículas de SiC de diferentes granulometrias por recurso a ensaios de termogravimetria (DTG). Ao mesmo tempo, a molhabilidade entre as partículas de SiC e a matriz de Al (Al-10Si-4.5Cu-2Mg) foi avaliada por recurso a ensaios de medição que permitiram a avaliação da evolução do ângulo de contacto em função da temperatura e do tempo. As técnicas de XRD, SEM, AFM e EDS foram usadas para caracterizar a superfície das partículas de SiC, antes e depois de todos os ensaios

Unipolar resistive switching in metal oxide/organic semiconductor non-volatile memories as a critical phenomenon

Diodes incorporating a bilayer of an organic semiconductor and a wide bandgap metal oxide can show unipolar, non-volatile memory behavior after electroforming. The prolonged bias voltage stress induces defects in the metal oxide with an areal density exceeding 10(17) m(-2). We explain the electrical bistability by the coexistence of two thermodynamically stable phases at the interface between an organic semiconductor and metal oxide. One phase contains mainly ionized defects and has a low work function, while the other phase has mainly neutral defects and a high work function. In the diodes, domains of the phase with a low work function constitute current filaments. The phase composition and critical temperature are derived from a 2D Ising model as a function of chemical potential. The model predicts filamentary conduction exhibiting a negative differential resistance and nonvolatile memory behavior. The model is expected to be generally applicable to any bilayer system that shows unipolar resistive switching. (C) 2015 Author(s).Dutch Polymer Institute (DPI), BISTABLE [704]; Fundacao para Ciencia e Tecnologia (FCT) through the research Instituto de Telecommunicacoes (IT-Lx); project Memristor based Adaptive Neuronal Networks (MemBrAiNN) [PTDC/CTM-NAN/122868/2010]; European Community Seventh Framework Programme FP7', ONE-P [212311]; Dutch Ministry of Education, Culture and Science (Gravity Program) [024.001.035]info:eu-repo/semantics/publishedVersio

Dyeing of PA6.6 fibers - Effect of solvent and temperature on thermal properties

Dyeing fibers at low temperatures has many advantages such as savings in energy and avoiding alterations to the physical properties of the fibers being dyed or other fibers also present in blends, The problem of low temperature dyeing in synthetic fibers is that it difficults the dye diffusion into the fiber. In the case of polyamide 6.6 microfibers, by using benzyl alcohol as an auxiliary dyeing, it was shown that good diffusion was obtained for the dye exhaustion with metal complex dyes at temperatures more than 30°C below the normal dyeing temperature for the dye exhaustion with metal complex dyes. Using thermal analysis methods these results were shown to be caused by the lowering of the T g of the fiber when in the presence of benzyl alcohol.Fundação para a Ciência e a Tecnologia (FCT

Tribocorrosion studies in centrifugally cast al-matrix siCp-reinforced functionally graded composites

The present work reports results obtained from a series of preliminary experiments aiming at complementing the current knowledge about the wear behaviour of centrifugally-cast FGM Al/SiCp composites, through concurrent corrosion processes. Precursor MMC’s were prepared by rheocasting, using 118.8 µm SiC particles and an Al-10Si–2.2 Mg alloy. Those MMC’s were then molten and centrifugally cast in order to produce cylindrical FGMMC’s. Discs machined from the top surface of each sample were tested against nodular cast iron pins, using an inverted configuration pin-on-disc tribometer. Sliding tests took place at room temperature, over a 50000 m sliding distance, with a sliding speed of 0.3 m s-1, under a 5 N normal load; both dry-sliding and water-lubricated tests were performed. In order to elucidate the mechanisms involved, the wear coefficients were calculated for each condition, and the samples were subjected to morphological characterization via SEM/EDS. Concurrently, in the case of the water-lubrication tests, the corrosion potential of the tribological pair was monitored. The results obtained show an increase in material loss for the water-lubricated cases, although variations are registered depending on reinforcing particle volume fraction. At the same time, the open circuit potential response of the tribological pair may be correlated with the events of formation/destruction of the tribolayers.Fundo Social Europeu, Program PRODEP Fundação para a Ciência e Tecnologia - (FCT