3,069 research outputs found
Carbon materials and their role as reinforcement in composite materials
No full textL'abstract è presente nell'allegato / the abstract is in the attachmen
An Insight into the Chemistry of Cement—A Review
No full textEven if cement is a well-consolidated material, the chemistry of cement (and the chemistry inside cement) remains very complex and still non-obvious. What is sure is that the hydration mechanism plays a pivotal role in the development of cements with specific final chemical compositions, mechanical properties, and porosities. This document provides a survey of the chemistry behind such inorganic material. The text has been organized into five parts describing: (i) the manufacture process of Portland cement, (ii) the chemical composition and hydration reactions involving a Portland cement, (iii) the mechanisms of setting, (iv) the classification of the different types of porosities available in a cement, with particular attention given to the role of water in driving the formation of pores, and (v) the recent findings on the use of recycled waste materials in cementitious matrices, with a particular focus on the sustainable development of cementitious formulations. From this study, the influence of water on the main relevant chemical transformations occurring in cement clearly emerged, with the formation of specific intermediates/products that might affect the final chemical composition of cements. Within the text, a clear distinction between setting and hardening has been provided. The physical/structural role of water in influencing the porosities in cements has been analyzed, making a correlation between types of bound water and porosities. Lastly, some considerations on the recent trends in the sustainable reuse of waste materials to form “green” cementitious composites has been discussed and future considerations proposed
Vision based navigation for autonomous planetary landing
No full textVision Based algorithms for relative navigation represent nowadays a trending topic in the Computer Vision field, but are still not widely exploited in space exploration missions due to their high computational costs and low TRL. In recent years anyway, a step in this direction has been taken by a lot of companies and agencies, with the development of dedicated algorithms and hardware (e.g. LVS and APLNav by NASA, SINPLEX by DLR), enhancing the possibility to adapt Computer Vision techniques to space applications. Vision Based algorithms for planetary landing represent a very promising tool: Cameras are cheap and reliable sensors with great potential in terms of obtainable accuracy in spacecraft state reconstruction. This paper presents a Vision Based algorithm for spacecraft Terrain Relative Navigation during landing designed from scratch at PoliMi DAER, based on a monocamera working in the visible spectrum. The Vision Based Navigation algorithm works processing images from the monocamera. First two frames are used for initialization: Features are extracted from the first image and tracked onto the second; a set of 2D to 2D correspondences is obtained, relative pose between the frames is calculated and a sparse map of 3D points is initialized exploiting triangulation. 2D features are then tracked for each subsequent frame and correlated to the 3D map: This way a set of 3D to 2D correspondences is obtained and used to solve the Perspective-n-Point problem, which along with a RANSAC routine set to delete incoming outliers (wrong match between target image and map), gives as result a first estimate of the relative position of the camera. Bundle Adjustment, an optimization technique widely diffused in Computer Vision, is applied both on the map and relative pose during initialization, and on each successive step to the obtained camera pose only to increase the overall navigation accuracy. Each time the number of tracked features drops below a fixed threshold, a new map is triangulated and merged with the existing one. Performance assessment of the navigation system using synthetic video sequences of different landing trajectories on the Moon surface is presented, along with preliminary results of the experimental calibration and verification campaign on PoliMI-DAER facility for GNC testing with HIL; the facility includes a Mitsubishi PA-10 robotic arm to simulate the lander's dynamics with monocamera mounted on tip, a calibrated 2.4 x4m Lunar surface diorama and a dimmable 5600K LED lighting system to provide a fully controllable illumination environment
Controlled oxidation of MWCNTs at low temperature
No full textCarbon nanotubes (CNTs) are an allotropic form of carbon, extremely interesting for their electrical and mechanical properties[1]. Used as reinforcement in different matrices, CNTs present many problems due to their agglomeration and non-wetting surface, that can be solved by chemical functionalization[2]. In particular, when dispersion in common solvents or matrices is considered, it is necessary to have polar functional groups on the surface. Since oxygen-containing groups are easy to graft on the CNTs surface, oxidation seems to be the best way to guarantee improved surface polarity. Oxidation through acid attack has been extensively studied in literature[3], since this changes the CNTs hydrophilicity and improves their dispersion in various solvents. However, the use of acids creates numerous drawbacks such as a heavy damaging of the CNTs structure due to the aggressive environment present during the oxidation reaction. This work originates from the need to find an oxidation method able to preserve the nanotubes structure while functionalizing the surface, in order to be able to disperse them in solvents or matrices but keeping the whole advantage given from the properties of the single CNTs. The idea was to perform a very simple oxidation of carbon nanotubes (CNTs) by thermal treatment at a specific temperature in controlled atmosphere. The conditions were chosen through a thermal gravimetric analysis (TGA) screening in a low oxygen (1-5%) atmosphere. The very initial phase of the thermal degradation starts between 350 C and 450 C, hence, the thermal degradation of CNTs was tested at lower temperatures with an isothermal treatment. By choosing the right temperature, time and amount of oxygen, it was possible to perform a precise and non-destructive oxidation of the CNTs, as demonstrated by several characterization techniques. This process enable the production of oxidized CNTs, which can be easily dispersed in aqueous solutions, without damaging the structure of the nanotubes
Preparation of hierarchical material by chemical grafting of carbon nanotubes onto carbon fibers
No full textA hierarchical material was produced by grafting multiwalled carbon nanotubes (MWCTNs) onto the surface of carbon fibers. A single oxidation treatment based on the combination of strong acids and ultrasounds was employed for both materials, while grafting was obtained by mean of a mild thermal treatment. The effectiveness of the oxidation treatment was evaluated by Thermogravimetric Analysis (TGA) and Fourier Transform Infrared Spectroscopy (FT-IR), while the success of the grafting was studied by TGA and Field Emission Scanning Electron Microscope (FESEM) observation. Different carbon fibers (CFs)/carbon nanotubes (CNTs) weight ratios have been used in the grafting process in order to assess how the amount of grafted CNTs can affect the final morphology of the hierarchical material. A novel hierarchical structure was observed and a mechanism for its production was proposed
Effect of oxidation of end-of-life tire rubber as aggregate substitute in cement mortars
No full textIn this paper the possible use of end-of-life tire rubber as a substitute for aggregate in cement mortars was explored. The aim of this paper was to investigate the effect of rubber surface modification by acidic treatment on the final properties of mortars. In fact, through treatment with sulfuric acid at moderate concentrations, a significant improvement in rubber wettability and interaction with cement has been observed. The compressive strength of mortars containing 15%w treated rubber as a replacement of natural aggregate is comparable to those of standard mortars. This suggests that surface modification of rubber plays a very important role in the possible integration of end-of-life tire rubber in mortar and concrete. The results presented in this paper confirm that recycling in mortar and concrete is a promising way for improving rubber waste management and sustainable innovation in the construction industry
Process for the production of dispersions of thermally oxidised carbon-based materials
No full textMethods of preparing dispersions of carbon-based materials are disclosed herein. In some embodiments, a method comprises exposing the carbon-based material to an atmosphere comprising between about 0.5% v/v and about 5.0% v/v of oxygen for a selected time at an oxidation temperature to obtain a thermally oxidized material; and dispersing the thermally oxidized material in a liquid medium
An assessment of the impact of possible CAP reform scenarios on Romanian agriculture
No full textUsing a simplified model, with key-variable the prices of two different possible scenarios of CAP reform after 2013 (moderate and radical), this paper present a comparison between the price effects of implementation of each reform scenario at 2015 horizon on Romanian agriculture. This short analysis shows that, under the presented hypotheses, the net welfare effect, due to the price changes, for the selected products, is positive in both reform scenarios, yet greater in the case of the radical reform. Integrated in the large context of Romanian development, it seems that the influence of CAP reform upon agriculture and rural areas will be most likely a gradual one: an interpenetration between the two scenarios is foreseeable, starting with the moderate reform that will dominate the period around 2013, the reform measures acquiring a more radical character afterwards.CAP reform, Romania, welfare effects, Agricultural and Food Policy,
Al(Si)/Al2O3 and NiAl(Si)/Al2O3 co-continuous composites obtained by low temperature reactive metal penetration of dense silica preforms
No full textInterpenetrating network Al(Si)/Al2O3 composites, obtained by a reactive metal penetration process, where pure aluminum metal reacts with a dense amorphous silica preform, are an interesting class of metal/ceramic composites, with two continuous networks of metal (Al-Si alloy) and ceramic (Al2O3). These composites have high stiffness and hardness, while retaining acceptable toughness and good thermal and electrical conductivity, and can be used in wear or thermal management applications. A second infiltration step with nickel allows to substitute the Al(Si) alloy with an intermetallic, obtaining an interpenetrating network NiAl(Si)/Al2O3 composite, with very high hardness and melting point. While in the standard process the temperature for obtaining the Al(Si)/Al2O3 composites is in the 1100–1200 °C range, in this paper we studied instead the 700–1000 °C temperature range and its effect on the microstructure of the final NiAl(Si)/Al2O3 composite. After the first infiltration step, the microstructure of Al(Si)/Al2O3 composites depends on both temperature and time of the treatment. At 700–800 °C and for reaction time of 1–2 h, the grain size is completely sub-micrometric. When temperature and time of reaction increase, islands with a coarser microstructure, of micrometric size, form, reaching a fully micrometric coarser microstructure at 1100–1200 °C. The islands formation is due to the transformation from transition aluminas, formed at low temperature and low reaction time, to α-alumina, while at high temperature α-alumina forms directly. In a second step, the Al(Si) metal network was replaced with an intermetallic one, by contacting the composite with liquid nickel, that reacted with the Al(Si) alloy forming an Al-Ni-Si intermetallic. The temperature and duration of the first infiltration step strongly influences not only the microstructure of the Al(Si)/Al2O3 composite but also of the final NiAl(Si)/Al2O3 one, that is finer when the first step (reaction of aluminum with silica) occurs at lower temperature
Facile photo-induced growth of polymeric nanostructures onto cellulose: The poly(ethylene glycol) methacrylate (PEGMA)@cellulose case study
No full textPhoto-polymerization is a polymerization method widely exploited in many technological fields, due to its high versatility and fast reaction rates. In this study, poly(ethylene glycol) methacrylate (PEGMA) nanostructures are grown onto cellulose following a simple and promising light-driven reaction mechanism involving benzophenone (as photo-initiator) and UV-curing. The effectiveness of the surface functionalization has been determined by means of morphological and physicochemical characterizations, showing the formation of very sharp PEGMA nanometric architectures
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