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Nanostructured carbon materials under extreme conditions
Luleå University of Technology, Department of Engineering Sciences and Mathematics, Material Science.
2014 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The discovery of the Buckminsterfullerene in 1985 by Richard Smalley, Robert Curl and Harold Kroto opened an era of great interest to the exploration of nanostructured carbon materials. These materials come in many forms and are often categorized by their dimensionality. Graphene is an ideal 2-dimensional (2D) sheet of sp2-bonded carbon atoms. A carbon nanotube (CNT) is a graphene sheet rolled up into a 1D structure and the Buckminsterfullerene is a 0D truncated icosahedron comprised of 60 carbon atoms. Graphene can be stacked in two, three or more layers on top of each other forming graphite as the number of layers approaches infinity. CNTs can consist of a single roll of graphene or one or more rolls inside another and are hence referred to as single, double or multi walled (SW/DW/MW) CNTs. Fullerenes are distinguished by their size and are referred to as Cn where n is the number of carbon atoms in the molecule. The Buckminsterfullerene, comprising 60 carbon atoms, is consequently denoted C60. Nanostructured carbon materials exhibit outstanding physical properties like extreme strength and hardness, high electrical and thermal conductivity, efficient optical emission to name just a few. Though these systems have been extensively studied during recent years many outstanding questions, in particular, their behaviour at extreme conditions like high temperature and high pressure still remain to be answered. Aim of this work is to probe nanocarbon materials at the limit of their structural stability using pressure and temperature as variables. High temperature can be reached in different ways but laser heating, being of practical importance during characterization of nanostructured carbon materials, is in focus here. High static pressures are generated in diamond anvil cells (DACs) while high dynamic pressure can be achieved in a shock wave assembly where the material is simultaneously exposed to high pressure and high temperature during microsecond time spans. The primary characterization technique is Raman spectroscopy and transmission electron microscopy (TEM) although complementary methods such as energy dispersive X-ray spectroscopy, thermo-gravimetric analysis, mass spectrometry and X-ray photoelectron spectroscopy have also been employed. We have studied the thermal and chemical stability of SWCNT bundles to high laser power in air and argon. The samples were exposed to 110 kWcm−2 of laser irradiation bringing the sample temperature up to 870º C and 550º C under argon and air respectively. The experiments in air show the importance of oxidation during laser heating. Our results demonstrate different temperature thresholds for the CNT destruction due to oxidation and pure overheating. Importantly, the small diameter CNTs are more easily destroyed than large diameter ones. The metallic nanotubes also tend to have lower thermal stability in comparison to semiconducting species. SWCNTs exposed to 35GPa of static compression were compared to material from the same batch exposed to 35GPa of dynamic compression. Raman studies indicate differences in the CNT destruction process between the two methods of pressurization. The SWCNTs exposed to high static compression were albeit highly defective partially retrieved whereas the SWCNTs exposed to high dynamic compression were essentially destroyed as further supported by transmission electron microscopy analysis. On the contrary, no significant differences were revealed between SWCNTs and DWCNTs recovered after exposure to dynamic compression in spite of the expected higher structural stability of the DWCNTs. A series of experiments was dedicated to an in-situ monitoring of DWCNTs response to a static pressure of up to 35GPa in a DAC via recording their Raman spectra. An onset of the CNTs cross section change was observed at 14GPa followed by their collapse at 24GPa. Strong hysteresis in the nanotubes’ cross-section recovery was revealed on pressure release and a complete reopening of the tubes occurred at 3.5GPa. Characterization of the material recovered after the high pressure experiments testifies for complete restoration of the DWCNT structure after the nanotubes’ collapse though the density of structural defects on the CNT surface increased. CNT polymerization at moderate pressures recently predicted theoretically was not confirmed in our experiments at both dynamic and static pressure. The resistance of polymerized tetragonal C60 (T-C60) was measured in situ at high pressure up to 33GPa in a DAC. The measurements reveal a sharp drop in sample resistance near 15GPa which may be an indication of and onset of a theoretically predicted phase transition into a metallic 3D polymer. T-C60 polymers retained their structural integrity after recovery from the high-pressure experiment thus demonstrating much higher resilience to high pressure/stress than monomeric C60 that, on the contrary, collapses already at about 22GPa.

Place, publisher, year, edition, pages
Luleå tekniska universitet, 2014.
Series
Doctoral thesis / Luleå University of Technology 1 jan 1997 → …, ISSN 1402-1544
Research subject
Experimental physics
Identifiers
URN: urn:nbn:se:ltu:diva-18532Local ID: 90c91cf8-1ba8-4f15-947e-4fad9fa0c2eeISBN: 978-91-7439-941-7ISBN: 978-91-7439-942-4 (PDF)OAI: oai:DiVA.org:ltu-18532DiVA: diva2:991541
Note
Godkänd; 2014; 20140509 (matmas); Nedanstående person kommer att disputera för avläggande av teknologie doktorsexamen. Namn: Mattias Mases Ämne: Fysik/Physics Avhandling: Nanostructured Carbon Materials Under Extreme Conditions Opponent: Professor Fernando Rodríguez, Department of Earth Sciences and Condensed Matter Physics, Facultad de Ciencias, Universidad de Cantabria, Santander, Spain Ordförande: Professor Alexander Soldatov, Avd för materialvetenskap, Institutionen för teknikvetenskap och matematik, Luleå tekniska universitet Tid: Torsdag den 12 juni 2014, kl 10.00 Plats: E231, Luleå tekniska universitetAvailable from: 2016-09-29 Created: 2016-09-29Bibliographically approved

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