Multi-technique characterisation of MOVPE-grown GaAs on Si

The heterogeneous integration of III-V materials on a Si CMOS platform offers tremendous prospects for future high speed and low power logic applications. That said this integration generates immense scientific and technological challenges. In this work multi-technique characterisation is used to investigate properties of GaAs layers grown by Metal-Organic Vapour Phase Epitaxy (MOVPE) on Si substrates - (100) with 4⁰ offset towards - under various growth conditions. This being a crucial first step towards the production of III-V template layers with a relatively lower density of defects for selective epitaxial overgrowth of device quality material. The optical and structural properties of heteroepitaxial GaAs are first investigated by micro-Raman spectroscopy and photoluminescence and reflectance measurements. High-resolution X-ray diffraction (HR-XRD) is used to investigate structural properties. Advanced XRD techniques, including double-axis diffraction and X-ray crystallographic mapping are used to evaluate degrees of relaxation and distribution of the grain orientations in the epilayers, respectively. Results obtained from the different methodologies are compared in an attempt to understand growth kinetics of the materials system. The GaAs overlayer grown with annealing at 735⁰C following As predeposition at 500⁰C shows the best crystallinity. Close inspection confirms the growth of epitaxial GaAs preferentially oriented along (100) embedded in a highly-textured polycrystalline structure

High temperature nanoindentation response of RTM6 epoxy resin at different strain rates

This paper explores the feasibility of characterizing the mechanical response of the commercial aerospace grade epoxy resin RTM6 by nanoindentation tests at varying temperatures and strain rates. Since glassy polymers exhibit time-dependent mechanical properties, a dynamic nanoindentation technique was used. This method consists on superimposing a small sinusoidal force oscillation on the applied force. Viscoelastic properties are then characterized by their storage and loss moduli, whereas the visco-plastic response of the material can be associated to its hardness. In such experiments, thermal stability of the measuring technique is critical to achieve a low thermal drift and it becomes increasingly important as the measuring temperature increases. Our results show that conventional methods applied for drift correction in nanoindentation of inorganic materials are not applicable to glassy polymers leading to physically inconsistent results. We propose a method for drift correction based on the hypothesis that viscoelastic modulus should be a function of the applied load and frequency but independent of the global strain rate. Using this method, it was possible to determine the viscoplastic properties of RTM6 between RT and 200 °C.Fil: Frontini, Patricia Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Mar del Plata. Instituto de Investigación en Ciencia y Tecnología de Materiales (i); Argentina. Universidad Nacional de Mar del Plata. Facultad de Ingeniería; ArgentinaFil: Lotfian, S.. IMDEA Materials; EspañaFil: Monclus, M.A.. IMDEA Materials; EspañaFil: Molina Aldareguia, J.M.. IMDEA Materials; Españ

The effect of soaking time after ultrafast heating on the microstructure and mechanical behavior of a low carbon steel

The main objective of this study is to understand the effect of the soaking time during the ultrafast heat treatment of a low carbon steel on its complex multi-phase microstructure, tensile mechanical behavior and properties of individual microconstituents. Tensile tests were performed to determine the macro-mechanical properties. Nanoindentation testing was carried out on individual microconstituents (martensite, recrystallized ferrite and non-recrystallized ferrite) identified a priori via EBSD analysis to measure their properties. It is shown that ultrafast heating combined with short soaking times results in improved macro-mechanical properties due to finer grain size and higher fraction of non-recrystallized ferrite, that has a higher nanohardness than recrystallized ferrite. Prolonged soaking times eliminate the advantages of the ultrafast heat treatment. This occurs because, even though a long soaking time promotes a higher volume fraction of martensite than a short one, it also induces substantial grain growth and complete recrystallization of the ferritic matrix. On the micro-scale, the ferritic grains show two different types of mechanical response. The recrystallized ferritic grains are prone to show pop-in events on the nanoindentation curves that are associated to dislocation nucleation events as a consequence of their low dislocation density, while non-recrystallized ferritic grains demonstrate a continuous response. The relationship between microstructure and mechanical properties on the macro- and micro-scales is discussed with respect to the microstructure, which in turn strongly depends on the applied heating rate and soaking time. A general recipe for microstructural design to improve the tensile mechanical behavior of low carbon steels implementing controlled heating and soaking conditions is outlined

Effect of layer thickness on the mechanical behavior of oxidation-strengthened Zr/Nb nanoscale multilayers

The effect of bilayer thickness (L) reduction on the oxidation-induced strengthening of Zr/Nb nanoscale metallic multilayers (NMM) is investigated. Zr/Nb NMMs with L = 10 and 75 nm were annealed at 350 °C for a time ranging between 2 and 336 h, and the changes in structure and deformation behaviour were studied by nanoscale mechanical testing and analytical electron microscopy. Annealing led to the transformation of the Zr layers into ZrO2 after a few hours, while the Nb layers oxidised progressively at a much slower rate. The sequential oxidation of Zr and Nb layers was found to be key for the oxidation to take place without rupture of the multilayered structure and without coating spallation in all cases. However, the multilayers with the smallest bilayer thickness (L = 10 nm) presented superior damage tolerance and therefore structural integrity during the oxidation process, while for L = 75 nm the volumetric expansion associated with oxidation led to the formation of cracks at the interfaces and within the ZrO2 layers. As a result, the nanoindentation hardness increase after annealing was significantly higher for the nanolaminate with L = 10 nm. Comparison between nanoindentation and micropillar compression behaviour of the oxidised NMMs demonstrates that the hardness increase upon oxidation arises from the contribution of the residual stresses associated with the volume increase due to oxidation and to the higher strength of the oxides

Selective oxidation-induced strengthening of Zr/Nb nanoscale multilayers

The paper presents a new approach, based on controlled oxidation of nanoscale metallic multilayers, to produce strong and hard oxide/metal nanocomposite coatings with high strength and good thermal stability. The approach is demonstrated by performing long term annealing on sputtered Zr/Nb nanoscale metallic multilayers and investigating the evolution of their microstructure and mechanical properties by combining analytical transmission electron microscopy, nano-mechanical tests and finite element models. As-deposited multilayers were annealed at 350 °C in air for times ranging between 1 and 336 h. The elastic modulus increased by ?20% and the hardness by ?42% after 15 h of annealing.Longer annealing times did not lead to changes in hardness, although the elastic modulus increased up to 35% after 336 h. The hcp Zr layers were rapidly transformed into monoclinic ZrO2 (in the first 15 h), while the Nb layers were progressively oxidized, from top surface down towards the substrate, to form an amorphous oxide phase at a much lower rate.The sequential oxidation of Zr and Nb layers was key for the oxidation to take place without rupture of the multi-layered structure and without coating spallation, as the plastic deformation of the metallic Nb layers allowed for the partial relieve of the residual stresses developed as a result of the volumetric expansion of the Zr layers upon oxidation. Moreover, the development of residual stresses induced further changes in mechanical properties in relation to the annealing time, as revealed by finite element simulations

Literarno-estetski doživljaj in novi mediji - prihodnost literature?

Avtor poveže tradicijo raziskovanja literarno-estetskega doživljaja v fenomenološki literari vedi in recepcijski estetiki s teorijami novomedijskega objekta Leva Manovicha, kibertekstualnosti Espena J. Aarsetha ter konceptoma postmedijskega in algoritmičnosti Petra Weibla, da bi opozoril na literarne vidike v polju novih medijev.The author compares the literary-esthetic experience as elaborated in phenomenological literary criticism and reader-response criticism with theories of the new media object (Lev Manovich), cybertext (Espen J. Aarseth),and the concepts of the postmedia and algorithmic (Peter Weibel) to emphasize the literary aspects of the new media