Manuel F. Vieira

Bio: Manuel F. Vieira is an academic researcher from University of Porto. The author has contributed to research in topics: Microstructure & Diffusion bonding. The author has an hindex of 21, co-authored 115 publications receiving 1428 citations. Previous affiliations of Manuel F. Vieira include Faculdade de Engenharia da Universidade do Porto & National Institute of Statistics and Geography.

In situ TEM study of grain growth in nanocrystalline copper thin films.

Abstract: Nanocrystalline metals demonstrate a range of fascinating properties, including high levels of mechanical strength. However, as these materials are exposed to high temperatures, it is critical to determine the grain size evolution, as this process can drastically change the mechanical properties. In this work, nanocrystalline sputtered Cu thin films with 43 ± 2 nm grain size were produced by dc-magnetron sputtering. Specimens were subsequently annealed in situ in a transmission electron microscope at 100, 300 and 500 °C. Not only was grain growth more evident at 500 °C but also the fraction of twins found. An analysis of grain growth kinetics revealed a time exponent of 3 and activation energy of 35 kJ mol − 1. This value is explained by the high energy stored in the form of dislocation, grain boundaries and twin boundaries existing in nanocrystalline copper, as well as the high probability for atoms to move across grains in nanocrystalline materials.

Plastic behaviour of copper sheets during sequential tension tests

Abstract: Sequences of two uniaxial tension tests along different axes were performed on a polycrystalline copper sheet. In order to reach a better understanding of the physical mechanisms occurring during the second deformation, the effect of the strain path change on subsequent yield and flow behaviour has been investigated using optical and transmission electron microscopy. The value of the reloading yield stress is a function of the angle φ between the two tensile axes. For φ >15° the requirement of glide of dislocations with a new Burgers vector implies that a very low density of potentially mobile dislocations is available at the beginning of the reloading deformation. This effect is moderated at φ values of about 90° by the inverse activity on some slip systems during reloading. The transient observed in the work-hardening behaviour after the path change is concomitant with the disappearance of some dislocation walls developed during the prestrain. A more homogeneous dislocation substructure appears for φ > 15° as a result of the interactions between the mobile dislocations on the new active slip systems and the previous dislocations walls; the dissolution of the previous dislocation arrangement is also promoted by the inversion of the slip direction in some systems, depending on the value of the angle φ. This results in an increase in dynamic recovery rate during the early stage of reloading, particularly when φ approaches 90°.

Overview of transient liquid phase and partial transient liquid phase bonding

Abstract: Transient liquid phase (TLP) bonding is a relatively new bonding process that joins materials using an interlayer. On heating, the interlayer melts and the interlayer element (or a constituent of an alloy interlayer) diffuses into the substrate materials, causing isothermal solidification. The result of this process is a bond that has a higher melting point than the bonding temperature. This bonding process has found many applications, most notably the joining and repair of Ni-based superalloy components. This article reviews important aspects of TLP bonding, such as kinetics of the process, experimental details (bonding time, interlayer thickness and format, and optimal bonding temperature), and advantages and disadvantages of the process. A wide range of materials that TLP bonding has been applied to is also presented. Partial transient liquid phase (PTLP) bonding is a variant of TLP bonding that is typically used to join ceramics. PTLP bonding requires an interlayer composed of multiple layers; the most common bond setup consists of a thick refractory core sandwiched by thin, lower-melting layers on each side. This article explains how the experimental details and bonding kinetics of PTLP bonding differ from TLP bonding. Also, a range of materials that have been joined by PTLP bonding is presented.

Crystallographic Texture of Materials

Abstract: Many naturally occurring as well as man made materials comprise of large number of crystallites with a preferred orientation. The preferred orientation, popularly known as texture, governs various structural and mechanical properties of these materials. Texture may develop during variety of processes like solidification, plastic deformation, annealing and phase transformation. It is, therefore, possible to tailor texture in materials to enhance a particular property. Traditionally, X-ray and neutron diffraction had been used to study texture in materials. It has been very recently that other techniques based on synchrotron X-rays and SEM based Electron Backscattered Diffraction have been developed for complete characterization of texture in materials. In the present review, the authors discuss the pole figures and more comprehensive orientation distribution function methods for texture analysis. In addition, a brief account of texture evolution in various technologically important materials, ranging from common metals and alloys to intermetallics, ceramics and polymers along with some naturally occurring materials like rocks, ice, bones etc. has been given. The present review is particularly aimed at readers newly initiated in this field rather than the experts.

Nanostructured energetic composites: synthesis, ignition/combustion modeling, and applications.

Abstract: Nanotechnology has stimulated revolutionary advances in many scientific and industrial fields, particularly in energetic materials. Powder mixing is the simplest and most traditional method to prepare nanoenergetic composites, and preliminary findings have shown that these composites perform more effectively than their micro- or macro-sized counterparts in terms of energy release, ignition, and combustion. Powder mixing technology represents only the minimum capability of nanotechnology to boost the development of energetic material research, and it has intrinsic limitations, namely, random distribution of fuel and oxidizer particles, inevitable fuel pre-oxidation, and non-intimate contact between reactants. As an alternative, nanostructured energetic composites can be prepared through a delicately designed process. These composites outperform powder-mixed nanocomposites in numerous ways; therefore, we comprehensively discuss the preparation strategies adopted for nanostructured energetic composites and t...

Display apparatus and method of manufacturing the same

Abstract: A display apparatus includes a substrate and a plurality of pixels disposed on the substrate. Each pixel includes a gate electrode disposed on the substrate, a gate dielectric layer disposed on the substrate and the gate electrode, an oxide semiconductor pattern disposed on the gate dielectric layer, a first insulating pattern disposed on the oxide semiconductor pattern that overlaps the gate electrode, a second insulating pattern disposed on the oxide semiconductor pattern and spaced apart from the first insulating pattern, source and drain electrodes spaced apart from each other on the oxide semiconductor pattern, a pixel electrode pattern disposed on the second insulating pattern to make contact with the source electrode, and a channel area defined where the oxide semiconductor pattern overlaps the gate electrode. A high carrier mobility channel is formed in the channel area when a turn-on voltage is applied to the gate electrode.