• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 12
  • 3
  • 3
  • 2
  • 2
  • 1
  • 1
  • 1
  • Tagged with
  • 32
  • 32
  • 8
  • 7
  • 6
  • 5
  • 5
  • 5
  • 4
  • 4
  • 4
  • 4
  • 4
  • 4
  • 4
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Alternative Carbon Fiber Reinforced Polymer (Cfrp) Composites for Cryogenic Applications

Lee, James Khian-Heng 08 May 2004 (has links)
A cheaper access to space is needed in current times and new technologies need to be developed to reduce the cost of space access to increase productivity. This thesis presents a study on carbon fiber reinforced polymer (CFRP) composites which is an enabling technology for cost reduction in space vehicles. A literature review of the behavior of CFRP composite has been conducted and it was found that the currently used IM7/977 carbon fiber reinforced epoxy composites do not microcrack at a lower number of thermal cycles. Nano-composites and Thermoplastic matrix composites have been found as two promising alternatives for cryogenic applications. With the use of nano sized inclusions in currently used epoxy resins, coefficient of thermal expansion can be reduced while increase in strength and fracture toughness can be achieved. Some thermoplastics were found to have non-linear stress-strain relationships with signs of ductility even at 4.2K. Both of these resin systems show promise in reducing microcracking at cryogenic temperatures.

Processing Routes for Aluminum based Nano-Composites

Yu, Hao 27 April 2010 (has links)
The term "Metal Matrix Nano-Composites (MMNCs)" broadly refers to a composite system that is based on metal or alloy substrate, combined with metallic or non-metallic nano-scale reinforcements. The main advantages of MMNCs include excellent mechanical performance, feasible to be used at elevated temperatures, good wear resistance, low creep rate, etc. In the recent past, MMNCs have been extensively studied, especially the method of fabrication as the processing of such composites is quite a challenge. Though a variety of processing methods have been explored and studied over the years, none have emerged as the optimum-processing route. The major issue that needs to be addressed is the tendency of nano-sized particles to cluster and also the challenge as to how to disperse them in the bulk melt. This work explored the feasibility of utilizing Lorentz forces to address both of these critical issues: clustering and dispersion. The work was carried out both theoretically as well as with accompanying validation experiments. The results indicate that Lorentz Forces may be viable and should be considered in the processing of MMNCs.

Observing and Reconstructing Subsurface Nanoscale Features Using Dynamic Atomic Force Microscopy

Maria Jose J. Cadena Vinueza (5929547) 03 January 2019 (has links)
<div>The atomic force microscope (AFM), traditionally known as a nanoscale instrument for surface topography imaging and compositional contrast, has a unique ability to investigate buried, subsurface objects in non-destructive ways with very low energy. The underlying principle is the detection of interactions between the AFM probe and the sample subsurface in the presence of an external wave or eld. The AFM is a newcomer to the field of subsurface imaging, in comparison to other available highresolution techniques like transmission or scanning electron microscopy. Nevertheless,</div><div>AFM offers signicant advantages for subsurface imaging, such as the operation over a wide range of environments, a broad material compatibility, and the ability to investigate</div><div>local material properties. These make the AFM an essential subsurface characterization tool for materials/devices that cannot be studied otherwise. </div><div><br></div><div><div>This thesis develops a comprehensive qualitative and quantitative framework underpinning the subsurface imaging capability of the AFM. We focus on the detection of either electrostatic force interactions or local mechanical properties, using 2nd-harmonic Kelvin probe force microscopy (KPFM) and contact-resonance AFM (CRAFM),</div><div>respectively. In 2nd-harmonic KPFM we exploit resonance-enhanced detection to boost the subsurface contrast with higher force sensitivity. In CR-AFM we use the dual AC resonance tracking (DART) technique, in which the excitation frequencies are near one of the contact resonance frequencies. Both techniques take advantage of the maximized response of the cantilever at resonance which improves the signal to noise ratio. These enable high-resolution subsurface mapping on a variety of polymer</div><div>composites.</div></div><div><br></div><div><div>A relevant challenge is the ability to reconstruct the properties of the subsurface objects from the experimental observables. We propose a method based on surrogate</div><div>modelling that relies on computer experiments using nite element models. The latter are valuable due to the lack of analytical solutions that satisfy the complexity of the geometry of the probe-sample system and sample heterogeneity. We believe this work is of notable interest because offers one of few approaches for the non-destructive characterization of buried features with sub-micron dimensions.</div></div>

Appport des nanotubes de carbone à la conduction électrique de matériaux organiques / Carbon nanotubes contribution to electrical conduction of organic materials

Combessis, Anthony 03 November 2011 (has links)
Ce travail de thèse propose, par une approche multi-échelles, une compréhension de certains mécanismes de constitution des réseaux percolants de nanotubes de carbone initialement dispersés au sein de polymères thermoplastiques. L'impact du phénomène de « percolation dynamique » sur les propriétés électriques d.c. et a.c. des nanocomposites a ainsi été étudié par l'établissement d'inter-relations entre l'organisation des charges et les propriétés résultantes. L'effet de cette auto-organisation des systèmes sur les paramètres critiques d.c. de la loi de percolation statistique sont discutés. Des origines à la percolation dynamique sont proposées et permettent d'envisager de nombreuses applications industrielles. A titre d'exemple, le contrôle sur plusieurs ordres de grandeur de la permittivité et de la conductivité est proposé, certaines valeurs n'étant pas accessibles avec les méthodes conventionnelles. / The present thesis proposes a multi-scale understanding of some mechanisms that govern the genesis of percolating networks constituted with carbon nanotubes in thermoplastic polymers. The effect of "dynamic percolation" on the d.c. and a.c. electrical properties of the resulting nanocomposites was studied by means of the identification of the relationships between the filler organization and the use properties. The consequences of this controlled self-organization on the statistic percolation law d.c. critical parameters are discussed. Two possible origins of the dynamic percolation are proposed. From an applicative point of view, thermal treatments were applied to design new materials. The range of accessible permittivity and conductivity values is also discussed.

Computational tools for preliminary material design of metals and polymer-ceramic nano composites

Kraus, Zachary 22 May 2014 (has links)
In this dissertation, algorithms for creating estimated potentials for metals and modeling of nano composites are developed. The efficacy of the algorithms for estimated potentials were examined. The algorithm was found to allow molecular dynamic and Monte Carlo modeling to be included in the potential building process. Additionally, the spline based equations caused issues with the elastic constants and Young’s modulus due to extra local minima. Two algorithms were developed for improved modeling of nano composites: one was a random number generation algorithm for initializing polymer, second was a bonding algorithm for controlling bonds between polymer and nano particle. Both algorithms were effective in their tasks. Additionally, the algorithms for improved nano composite modeling were used for preliminary material design of PMMA metal oxide nano composite systems. The results from the molecular dynamic simulations show the bonding between polymer matrix and nanoparticle has a large effect on the Young’s modulus and if this bonding could be controlled, the tensile properties of PMMA-metal oxide nano composites could be tailored to the applications’ requirements. The simulations also showed bonding had caused changes in the density of the material which than effected the energy on the polymer chain and the Young’s modulus. A model was than developed showing the relationship between density and the chain energy, and density and the Young’s modulus. This model can be used for a better understanding and further improvement of PMMA-metal oxide nano composites.

Propriétés optiques linéaires et non-linéaires de nanocomposites métal-diélectrique anisotropes

Lamarre, Jean-Michel 05 June 2008 (has links) (PDF)
Les matériaux nanocomposites formés à partir de particules métalliques incluses dans une matrice diélectrique présentent des propriétés inusitées que l'on ne retrouve pas dans les matériaux massiques étant donné la taille nanométrique des particules métalliques. Notamment, la résonance des électrons de conduction du métal sous l'excitation d'une lumière incidente, appelée résonance plasmon de surface (spr), donne lieu à l'apparition d'une bande d'absorption dans le visible ayant une influence drastique sur les propriétés optiques du nanocomposite. La position, la largeur et l'amplitude de cette bande d'absorption sont des fonctions complexes des caractéristiques nano-structurales du nanocomposite, notamment : la nature et la concentration du métal, la taille des nanoparticules, leur forme, l'indice de réfraction de la matrice, etc. Spécifiquement, la présence de particules asymétriques, comme par exemple des particules de forme nano ellipsoïdale, permet l'observation de bandes d'absorption spr multiples résultant en un matériau non-symétrique possédant des propriétés anisotropes. Dans cette thèse, nous présenterons le développement et l'analyse de techniques avancées de fabrication permettant le contrôle précis de la synthèse d'un nanocomposite formé de nanoparticules d'or incluses dans une matrice de silice. Nous avons développé une technique en trois étapes permettant de fabriquer des couches nanocomposites comprenant des particules de tailles et de formes contrôlées. Ces étapes sont : 1) dépôt hybride pulvérisation/dépôt chimique en phase vapeur assisté par plasma, 2) recuits thermiques, 3) irradiation à l'aide d'un faisceau d'ions de haute énergie. Subséquemment, la performance optique des matériaux fabriqués a été testée par différentes techniques afin d'en évaluer les propriétés linéaires et non-linéaires. Les premières ont été étudiées par ellipsométrie et spectrophotométrie alors que les deuxièmes l'ont été par les techniques P-Scan et Z-Scan. Plus précisément, nous nous sommes intéressés à la métrologie et au contrôle de l'anisotropie des propriétés du système nanocomposite formé par faisceau d'ions. Une nouvelle approche de mesure des propriétés optiques non-linéaires anisotropes sera présentée. Dans ce document, le lecteur retrouvera une description théorique exhaustive des propriétés optiques (linéaires et non-linéaires) des matériaux nanocomposites métal/diélectrique incluant des modèles détaillés pour la description des propriétés de chacun des constituants du nanocomposite : métal (Drude, Lorentz) et diélectrique (Cauchy).

Νανο-διηλεκτρικά εποξειδικής ρητίνης- BaTiO3 : ανάπτυξη, ηλεκτρική απόκριση και λειτουργικότητα

Πατσίδης, Αναστάσιος 18 June 2009 (has links)
Τα σύνθετα συστήματα πολυμερικής μήτρας – κεραμικών εγκλεισμάτων φαίνεται ότι μπορούν να αποτελέσουν μία νέα γενιά υλικών υψηλού τεχνολογικού ενδιαφέροντος. Από την άλλη μεριά η σημασία των νανουλικών και νανοδομημένων υλικών είναι ευρέως αποδεκτή στις μέρες μας, τόσο σε επίπεδο βασικής έρευνας όσο και σε τεχνολογικό επίπεδο. Το σύγχρονο αυτό ερευνητικό πεδίο περιλαμβάνει τη μελέτη των νανοσύνθετων ή πολυφασικών υλικών, στα οποία μια ή περισσότερες από τις χωρικές διαστάσεις κάποιας φάσης βρίσκεται στην περιοχή των νανομέτρων (1 nm = 10-9 m = 10 ). Αυτό που ξεχωρίζει τα νανοσύνθετα από τα άλλα συμβατά σύνθετα υλικά είναι η ικανότητα τους να συνδυάζουν ιδιότητες, οι οποίες είναι απαγορευτικές για τα παραδοσιακά υλικά, αλλά και η μεγάλη λειτουργικότητα που παρουσιάζουν. Η διασπορά μίκρο- και νάνο-κεραμικών εγκλεισμάτων στο εσωτερικό πολυμερούς, οδηγεί σε σύνθετα συστήματα με βελτιωμένη μηχανική απόκριση, που όμως διατηρούν την ηλεκτρική συμπεριφορά των εγκλεισμάτων. Υλικά υψηλής ηλεκτρικής διαπερατότητας (high-K materials) είναι απαραίτητα σε πολλές εφαρμογές της ηλεκτρονικής, επειδή είναι σε θέση να μειώνουν τα ρεύματα διαρροής, ενώ παράλληλα λειτουργούν και ως ενσωματωμένοι μικρο-πυκνωτές. Σύνθετα πολυμερικά υλικά που ενσωματώνουν σιδηροηλεκτρικά κεραμικά στοιχεία παρουσιάζουν μεγαλύτερο ενδιαφέρον καθώς, η ηλεκτρική τους απόκριση πέραν των αναμενόμενων εξαρτήσεων (περιεκτικότητα, γεωμετρία και τρόπος διασποράς των εγκλεισμάτων) σχετίζεται και με τη θερμοκρασιακά ελεγχόμενη μετάβαση των εγκλεισμάτων από τη σιδηροηλεκτρική στην παραηλεκτρική φάση. Τέτοιου είδους λειτουργικά σύνθετα, αναφέρονται συχνά και ως ευφυή συστήματα. Στην παρούσα εργασία παρασκευάσθηκαν συστήματα πολυμερικής μήτρας – μικρο- και νανο-σωματιδίων κεραμικού BaTiO3 και στη συνέχεια εξετάσθηκαν οι διηλεκτρικές τους ιδιότητες, με παραμέτρους την περιεκτικότητα σε BaTiO3, τη θερμοκρασία και τη συχνότητα του εφαρμοζόμενου πεδίου. Η διηλεκτρική φασματοσκοπία (Broadband Dielectric Spectroscopy) έχει αποδειχθεί ως ένα ισχυρό εργαλείο για την έρευνα της μοριακής κινητικότητας, των αλλαγων φάσης, των μηχανισμών αγωγιμότητας και των διεπιφανειακών φαινομένων στα πολυμερή και τα σύνθετα πολυμερικά συστήματα. Η διηλεκτρική απόκριση των νανοσυνθέτων εξετάστηκε με τη βοήθεια της διηλεκτρικής φασματοσκοπίας (BDS) στο εύρος συχνοτήτων 10-1-10 7 Hz και στο διάστημα θερμοκρασιών από 30οC έως 160οC. Από τα πειραματικά αποτελέσματα προκύπτει πως παρατηρούνται διηλεκτρικές χαλαρώσεις που οφείλονται τόσο στην πολυμερική μήτρα, όσο και στην ενισχυτική φάση. Τρεις διακριτοί τρόποι χαλάρωσης καταγράφηκαν στα φάσματα των συστημάτων που μελετήθηκαν και αποδίδονται στη διεπιφανειακή πόλωση (IP) μήτρας/εγκλεισμάτων, στην υαλώδη μετάβαση (α - χαλάρωση) των πολυμερών και στην κίνηση πλευρικών πολικών ομάδων (β - χαλάρωση) των αλυσίδων. Η λειτουργική συμπεριφορά των μίκρο- και νάνο-σύνθετων βασίζεται στην μετάβαση “αταξίας”- “τάξης” που παρατηρείται στο BaTiO3 στην κρίσιμη θερμοκρασία Curie (~130oC). Η μετάβαση από την σιδηροηλεκτρική φάση στην παραηλεκτρική μελετήθηκε τόσο μέσω της Διηλεκτρικής Φασματοσκοπίας όσο και με φάσματα ακτίνων-Χ (XRD). Η μεταβολή της πόλωσης και η δημιουργία κορυφής στα διαγράμματα του πραγματικού μέρους της ηλεκτρικής διαπερατότητας με τη θερμοκρασία μπορεί να αποτελέσει τη βάση ανάπτυξης ευφυών συστημάτων και νανο-διατάξεων καθώς δίνεται η δυνατότητα ελέγχου της αποθηκευόμενης ηλεκτρικής ενέργειας στη νανοκλίμακα και επιτυγχάνεται η λειτουργία ρυθμιστή πόλωσης. Τέλος, με την εισαγωγή της Διηλεκτρικής Συνάρτησης Ενίσχυσης διερευνάται η απόκριση των σύνθετων συστημάτων και προσδιορίζεται η βέλτιστη λειτουργική συμπεριφορά και η βέλτιστη συμπεριφορά ως προς την αποθήκευση ενέργειας. / Ceramic-polymer composites consisting of ferroelectric crystal particles, homogeneously distributed, in an polymer host represent a novel class of materials, with several interesting properties. The impact of nanomaterials and nanostructured materials is well known and widely accepteble in our days, not only in the basic research level but also in the area of technological applications. This modern field of scientific research includes the study of nano – composites or multiphase materials. Multiphase materials have at least one of the dimensions of the reinforcing phase in nano-scale. The main difference between nano-composites and conventional composites is their ability to achieve superior performance at a very low concentration of the filler. The majority of the active or potential applications of nano-systems is based on their thermo-mechanical behaviour, flame resistance and electrical properties. Under this point of view nano-composites exhibit properties or functions, which seem to be prohibited for traditional materials. High tech electronic devices require new high dielectric permittivity materials (known as high-K materials), which combine, at the same time, suitable dielectric properties, mechanical strength and ease processing. Recently ceramic-polymer composites have been studied in various applications including integrated capacitors, acoustic emission sensors and for the reduction of leakage currents. Furthermore, if the embedded ceramic particles are ferroeletric, functional properties can be added to the composite structure. Ferroelectric materials exhibit spontaneous polarization and are characterized by a temperature dependent disorder to order transition. Thus, besides all the expected influences (volume fraction, geometrical characteristics, type of distribution etc), the electrical response of ferroelectric particles – polymer matrix composites depends on the ferroeletric to paraelectric transition of the inclusions. In the present study composite systems of epoxy resin and ceramic BaTiO3 micro and nano – particles have been prepared, varying the volume fraction of the inclusions. The dielectric response of the composites was studied in a wide frequency and temperature range. Broadband Dielectric Spectroscopy (BDS) has been proved to be a powerful tool for the investigation of molecular mobility, phase changes, conductivity mechanisms and interfacial effects in polymers and complex systems. The dielectric response of nano-composites was examined by means of Broadband Dielectric Spectroscopy (BDS) in the frequency range10-1-107 Hz and temperature interval from 30 o C to 160 o C. Experimental results include relaxation phenomena arising from both the polymeric matrix and the filler. Three distinct relaxation modes were recorded in the spectra of all systems. They were attributed to interfacial polarization, glass transition (α-relaxation) and motion of polar side groups (β – relaxation). The functional behaviour of micro and nano – composites is based on the disorder to order transition of BaTiO3 in the characteristic Curie temperature (~130 oC). The transition from the ferroelectric to paraelectric phase has been studied via Dielectric Spectroscopy and with X – Ray Diffraction spectra. The change in polarization and the formation of peaks in the diagrams of the real part of dielectric permittivity versus temperature can provide the suitable basis for the development of smart systems and nano-devices, since it allows the control of the stored electrical energy in nanoscale level and achieves the function of polarization regulator. Finally, by introducing the Dielectric Reinforcing Function, the composite systems’ response can be studied and optimal functional behaviour as well as optimal energy storage and capability can be determined.


Soltannia, Babak 16 August 2013 (has links)
The effects of high loading rates (HLR), and nano reinforcement on the mechanical response of adhesively-bonded SLJs with composite adherends, subjected to different loading (strain) rates are systematically investigated. The results are then compared to those of neat thermoset resin and thermo-plastic adhesive. More specifically, nano-reinforced and neat resin bonded joints mating carbon/epoxy and glass/epoxy adherends were subjected to tensile loadings under 1.5 and 3 mm/min and tensile impacts at a loading rate of 2.04E+5 mm/min. In some cases, additional tests were conducted under 15, 150, and 1500 mm/min to obtain additional properties gained using the nano-reinforcements for use in the further numerical investigations. The HLR tests were conducted, using a modified instrumented pendulum equipped with a specially designed impact load transfer apparatus. The dispersion of nanoparticles was facilitated using a mechanical stirrer and a three-roll mill machine. The failure mechanisms were studied with a scanning electron microscope.


McCalla, Eric 09 December 2013 (has links)
The improvement of volumetric energy density remains a key area of research to opti-mize Li-ion batteries for applications such as extending the range of electric vehicles. There is still improvement to be made in the energy density in the positive elec-trode materials. The current thesis deals with determining the phase diagrams of the Li-Mn-Ni-O and Li-Co-Mn-O systems in order to better understand the structures and the electrochemistry of these materials. The phase diagrams were made through careful analysis of hundreds of X-ray di raction patterns taken of milligram-scale combinatorial samples. A number of bulk samples were also investigated. The Li-Mn-Ni-O system is of particular interest as avoiding cobalt lowers the cost of the material. However, this system is very complex: there are two large solid-solution regions separated by three two-phase regions as well as two three-phase regions. Comparing quenched and slow cooled samples shows that the system trans-form dramatically when cooled at rates typically used to make commercial materials. The consequences of these results are that much of the system must be avoided in order to guarantee that the materials remain single phase during cooling. This work should therefore impact signi cantly researchers working on composite electrodes. Two new structures were found. The first was Li-Ni-Mn oxide rocksalt structures with vacancies and ordering of manganese which were previously mistakenly identi ed as LixNi2xO2. The other new structure was a layered oxide with metal site vacancies allowing manganese to order on two superlattices. The electrochemistry of both these materials is presented here. Finally, the region where layered-layered composites form during cooling has been determined. These materials were long looked for along the composition line from Li2MnO3 to LiNi0.5Mn0.5O2 and the most significant consequence of the actual locations of the end-members is that one of the structures contains a high concentration of nickel on the lithium layer. Layered-layered nano-composites formed in this system are therefore not ideal positive electrode materials and it will be demonstrated that single-phase layered materials lead to better electrochemistry.


MARIO SERGIO SANT ANNA GONCALVES 26 October 2005 (has links)
[pt] O trabalho aborda a elaboração do compósito Co-ZnO por redução preferencial com hidrogênio. Inicialmente foi desenvolvida uma avaliação termodinâmica das principais reações para o processo tais como, a pirólise dos nitratos de cobalto e de zinco, responsável pela obtenção dos respectivos óxidos, e a redução destes pelo hidrogênio. O estudo expe rimental foi realizado de forma a se avaliar o efeito de variáveis importantes (fluxo, temperatura e pressão parcial do gás reagente) sobre a cinética das reações de redução. Observou-se a viabilidade de se estabelecer condições onde ocorresse a redução preferencial do óxido de cobalto em relação ao óxido de zinco. Como exemplo pode-se mencionar que na faixa de temperatura de realização dos experimentos de redução do óxido de cobalto (590K - 790K), foram obtidas conversões entre 20 e 80%, dentro de um intervalo de tempo compreendido entre 1 e 15 minutos e com fluxo de hidrogênio da ordem de 0,18L/min. Por sua vez, o compósito pode ser obtido em ensaio realizado na temperatura de 690K pelo tempo de 20 minutos mantido o mesmo fluxo. As caracterizações dos mate riais obtidos foram realizados por difração de raios-X, microscopia eletrônica de varredura/EDS e microscopia eletrônica de transmissão, sendo possível identificar óxidos relativamente puros e o compósito Co-ZnO bastante homogêneo e constituído por partículas na escala nanométrica, características estas, desejáveis para materiais desta natureza. / [en] This work covers the Co-ZnO nano-composite synthesis by hydrogen preferential reduction of these metals oxides which, in turn, were obtained by simultaneous dissociation of their nitrates. Initially it has been developed a thermodynamic evaluation of the main process reactions such as the zinc and cobalt nitra tes pyrolises and the respective oxides reduction by hydrogen. The experimental work was carried out in order to study the effect of the most important variables on the reduction reaction kinetics of the cobalt oxide as well as to investigate some experimental conditions for the formation of the Co-ZnO nanocomposite. It can be said that cobalt oxide is completely and preferentially reduced in 20 minutes at 690K, then allowing the composite formation at this temperature. The materials involved in the process were characterized by X-R diffraction (oxides and co-obtained oxides) and electronic microscopy (SEM/EDS, TEM), being possible to identify a Co-ZnO homogeneous and containing particles in the nano range scale.

Page generated in 0.0719 seconds