Francisco Manuel Braz Fernandes

Al/SiCp Functionally Graded Metal-Matrix Composites Produced by Centrifugal Casting: Effect of Particle Grain Size on Reinforcement Distribution

Functionally graded materials (FGM’s), particularly in the form of Al-Si metal matrix composites (MMC’s) selectively reinforced at the surface with SiC particles, are advanced materials, combining high wear resistance with high bulk toughness or even a thermal barrier at the surface. Centrifugal casting is one of the most effective methods for processing this type of MMC, but accurate control of the ceramic particles distribution/gradient in the metallic matrix has not yet been completely attained. In this work, precursor Al/SiC composites were prepared by rheocasting, using SiC particles and an Al-10Si–2.2 Mg alloy. Morphology of the SiC particles was previously characterized by laser interpherometry and SEM. Differing grain sizes were selected as reinforcing elements. The MMC’s were then molten and centrifugally cast in order to produce the FGM composites, whose structure and properties were investigated by XRD, quantitative image analysis of optical micrographs and longitudinal hardness profiles. Therefore, it was possible to evaluate the influence of the particle grain size on the structure and properties of the FGM. Apart from the evaluation of the effects of particle grain size per se, its influence when combined with differing casting conditions are reported as well.

Multifocus Optical Microscopy Applied to the Study of Archaeological Metals

Studies on cultural metal artifacts can benefit greatly from microscopy techniques. The examination of microstructural features can provide relevant information about ancient manufacturing techniques, as well as about corrosion/degradation processes. In the present work, advantages of the use of multifocus imaging techniques in optical microscopy for the study of archaeological metals are presented. An archaeometallurgical study of a large collection of bronzes demonstrates the possibility of a microstructural study with no need for sample removal, which is a great advantage in the study of cultural objects. In addition, the study of mounted samples illustrates the advantages of the multifocus technique in the examination of particular corrosion features, with the possibility of three-dimensional reconstructions.

Structural characterisation of NiTi thin film shape memory alloys

Currently, microactuators are being developed using shape memory alloys (SMAs), which allow simple design geometries and provide large work outputs in restricted space. Several techniques have been used to produce NiTi shape memory alloy thin films, but from the practical point of view, only the sputter deposition method has succeeded so far. Vacuum evaporation of NiTi binary alloy entails the potential problem of the evaporation rates of each component not being the same due to differences in vapour pressure. Aiming to study the possible applications of SMAs to microfabrication, NiTi thin films were produced at CENIMAT by sputter and vacuum evaporation using raw materials from different sources. The films were analysed by differential scanning calorimetry (DSC) and X-ray diffraction (XRD) at room temperature, as well as in situ high temperature, in order to characterise the temperature ranges at which the different structural transformations occur.

Role of the deposition conditions on the properties presented by nanocrystallite silicon films produced by hot wire

The aim of this work is to study the role of hydrogen dilution and filament temperature on the properties of nanocrystalline silicon thin films (undoped and doped) produced by the hot wire technique. These deposition parameters are correlated to the films structure, composition and electro-optical properties with special emphasis on boron doped nanocrystalline silicon carbide reported here.

Microstructure Interpretation of Copper and Bronze Archaeological Artefacts from Portugal

In the present paper, a selection of micrographs showing some typical microstructures and corrosion layers developed in copper and bronze alloys from different archaeological contexts will be discussed. Metallurgic interpretation of the microstructures observed, based in the proper binary equilibrium phase diagrams is presented. Micro-EDXRF and SEM-EDS analysis were carried out to assess the alloy composition and to contribute to the understanding of the corrosion processes occurred during the long periods of burial of the archaeological metals. Examples of surface decuprification, strong intergranular corrosion, cuprite under green corrosion layers and copper redeposition indicate particular corrosion processes. The presence of copper oxides, sulphides inclusions, lead globules and porosities is also documented.

Microstructural Signatures of Bronze Archaeological Artifacts from the Southwestern Iberian Peninsula

This study concerns the elemental and microstructural characterization of proto-historic bronze rings from the southwestern Iberian Peninsula. Micro-EDXRF analyses demonstrate that the artifacts are binary bronze alloys (8–13% Sn) with arsenic and lead as the major impurities. Optical microscopy and SEM-EDS allowed the identification of common inclusions (e.g. copper sulphides) and alteration processes (redeposited copper, intergranular and intragranular corrosion). Microstructures consisting of fine dendrites, coarse and/or equiaxial grains were also identified, as well as the presence of (+) eutectoid, deformed inclusions, twinned grains and/or slip bands. The combination of these characteristics allowed establishing the metallurgical procedures (casting, forging and annealing) used in the production of the bronze rings. The identification of different thermomechanical operational sequences indicates that the metallurgical knowledge was well established in the southwestern Iberian Peninsula during those ancient times.

Application of X-Ray Microtomography to the Microstructural Characterization of Al-Based Functionally Graded Materials

This paper provides a brief overview of the possibilities offered by X-ray computed microtomography, and particularly synchrotron radiation X-ray microtomography, regarding metal matrix composite characterization, emphasis being placed in the case of Al-based functionally graded materials. Examples are provided concerning the characterization of the reinforcement population, interfacial properties, in-situ transformation and damage evolution. The specific needs of the technique and limitations to its widespread use are mentioned.

Structural Characterisation of Zinc Oxide Thin Films Produced by Spray Pyrolysis

In this work, we present a study of the effect of temperature, type and concentration of the dopant on the structural characteristics of ZnO thin films produced by spray pyrolysis; the crystallite size has been determined from profile peak shape analysis. These results are compared to the electrical characterisation performed on these materials. The effect of the dopant on the properties of ZnO thin films depends on its characteristics, mainly its ionic radius. Al, Ga and In have been studied as dopants, the best one being In, since it leads to the lowest resistivity. Introduction The development of low cost optoelectronic devices requires more and more the use of transparent conductive oxide (TCO) thin films, mainly for applications such as solar cells [1], liquid crystal displays [2], heat mirrors [3], multilayers, photothermal conversion system, gas sensors [4], optical position sensors, etc. From all the TCO materials studied, in the last years, zinc oxide (ZnO) has emerged as one of the most promising materials, due to its optical and electrical properties, high chemical and mechanical stability together with its abundance in nature, which makes it a lower cost material when compared to the most currently used TCO materials (ITO and SnO2). However ZnO undoped thin films are not stable due to changes in the surface conductance under oxygen chemisorption and desorption. Doping zinc oxide can reduce this disadvantage [5]. Besides, doping leads to an increase of the conductivity of the ZnO thin films. The ZnO doping is achieved by replacing Zn 2+ atoms with ions of higher valence such as In 3+ , Al 3+ and Ga 3+ [6]. The efficiency of the dopant element is a function of its electronegativity and of the difference between its ionic radius and the ionic radius of zinc. These type of oxide materials can be produced by several techniques such as sputtering [7], thermal evaporation, CVD [8] and spray pyrolysis [9], among others. Nevertheless, the spray pyrolysis technique is cheaper, simpler and more versatile than the others, which allow the possibility of obtaining films with the required properties for optoelectronic applications and also when large areas are needed. Due to the fact that this is a chemical technique, one of the most important deposition parameter is the temperature. In this work, we present a study of the effect of temperature and the type of dopant on the structural characteristics of these thin films, namely the morphological aspects and the crystallite size determined from profile peak shape analysis. These results are compared to the electrical characterisation performed on these materials. Experimental Details The spray pyrolysis is a cheap and simple technique based on chemical vapour deposition process (CVD). In this technique, the precursor of the material to be deposited is in solution and sprayed onto a heated substrate using argon as carrier gas. The solution used was made of zinc acetate with a concentration of 0.2 M dissolved in methanol. For the undoped thin films, the deposition temperature changed between 300oC and 500oC. However, for the doped ones, the deposition temperature was kept constant at 425oC. Key Engineering Materials Online: 2002-10-25 ISSN: 1662-9795, Vols. 230-232, pp 599-602 doi:10.4028/www.scientific.net/KEM.230-232.599

Performances of Nano/Amorphous Silicon Films Produced by Hot Wire Plasma Assisted Technique

This work reports on the performances of undoped and n doped amorphous/nano-crystalline silicon films grown by hot wire plasma assisted technique. The films structure (including the presence of several nanoparticles with sizes ranging from 5 nm to 50 nm), the composition (oxygen and hydrogen content) and the transport properties are highly dependent on the filament temperature and on the hydrogen dilution. The undoped films grown under low r.f. power (≍ 4 mWcm −2 ) and with filament temperatures around 1850 °K have dark conductivities below 10 −1 Scm −1 , optical gaps of about 1.5 eV and photo-sensitivities above 10 5 , (under AM3.5), with almost no traces of oxygen content. N- doped silicon films were also fabricated under the same conditions which attained conductivities of about 10 −2 Scm −1 .

XRD Study of NiTi Endodontic Files Using Synchrotron Radiation

Two types of endodontic files (ProFile and Mtwo) were studied in order to analyze the effect of geometry on the stress-induced martensitic (SIM) transformation during bending. The use of a fine beam spot of synchrotron radiation allowed a detailed structural characterization with a fine spatial resolution. Experimental evidence of the effect of the cross-section geometry on the stress localization giving rise to different degrees of SIM transformation is presented for the first time in the published literature.