Thermal insulation

Thermal insulation is the reduction of heat transfer (i.e., the transfer of thermal energy between objects of differing temperature) between objects in thermal contact or in range of radiative influence. Thermal insulation can be achieved with specially engineered methods or processes, as well as with suitable object shapes and materials. Heat flow is an inevitable consequence of contact between objects of different temperature. Thermal insulation provides a region of insulation in which thermal conduction is reduced, creating a thermal break or thermal barrier, or thermal radiation is reflected rather than absorbed by the lower-temperature body. The insulating capability of a material is measured as the inverse of thermal conductivity (k). Low thermal conductivity is equivalent to high insulating capability (resistance value). In thermal engineering, other important properties of insulating materials are product density (ρ) and specific heat capacity (c). Definition Th

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Eberhard Ulrich Stützel

Graphene, a single layer of carbon atoms, exhibits excellent charge transport properties. However, due to the absence of a band gap in this two-dimensional carbon nanostructure, graphene-based field effect transistors cannot be turned off. One strategy to increase the on/off ratio relies on patterning graphene into narrow stripes, so-called graphene nanoribbons (GNRs). The present thesis aimed at developing novel fabrication and chemical functionalization methods for GNRs. Along these lines, electrical transport studies and spectroscopic investigations were envisioned as a major tool to monitor the changes brought about by the functional groups attached to the GNRs. GNRs were fabricated with the aid of V2O5 nanofibers or CdSe nanowires as an etching mask during the plasma etching of graphene. The resulting GNRs exhibited good electrical conductivity for ribbon widths as small as 10 nm. Moreover, the transport gap in the GNRs was found to scale inversely with the ribbon width. Scanning tunneling microscopy and surface enhanced Raman spectroscopy testified a good structural quality of the GNRs. Electrical characterization of GNRs revealed a pronounced hysteresis, which was exploited for the fabrication of electrically switchable GNR memory cells. Dynamic pulse response measurements demonstrated reliable switching between two conductivity states for clock frequencies of up to 1 kHz and pulse durations as short as 500 ns for >10^7 cycles. As the most likely switching mechanism, charge trapping within a water layer in the GNR surrounding could be identified. Furthermore, the optoelectronic properties of individual GNRs were studied by scanning photocurrent microscopy. The pronounced photocurrent signal close to the nanoribbon/metal contacts was found to be directly proportional to the conductance of the devices, suggesting that a local voltage source is generated at the nanoribbon/metal interface by the photo-thermoelectric Seebeck effect. The dominance of this mechanism over charge separation via built-in electrical fields was attributed to strong local heating of the GNRs by the laser spot, combined with the reduced thermal conduction capability of the nanoribbons in comparison to extended graphene sheets. Chemical functionalization of graphene and GNRs was attempted via different gas- and liquid-phase approaches. While electrical transport and Raman measurements revealed the presence of significant doping effects, it was not possible to unequivocally prove the covalent attachment of atoms at the GNR edges, neither through changes in the charge transport characteristics, nor via scanning microscopy studies.

EPFL2013

Facilitating the incorporation of VIP into precast concrete sandwich panels

Andrea Bassi, Matthew Leslie Heitel, Thierry Voellinger

This research focuses on the development of thin, load bearing, composite concrete facade elements containing high performance thermal insulation. A pilot project involving the construction of a detached house has been completed resolving issues related to the construction, production and economic requirements of utilizing vacuum insulated panels (VIPs) within prefabricated concrete facade elements. During the project planning process it was discovered that particular VIP dimensions were necessary for avoiding wasted material and thermal bridging. VIPs produced for facade elements with widths corresponding to architectural modules of 15, 30, and 60 cm and lengths corresponding to 1/3rd of the project height would alleviate these issues. Within the building industry, VIPs are viewed as a viable solution for optimizing the space saving attributes of high performance walls. By correctly sizing mass-produced VIP elements for facade measurement modules, rather than expecting insulation along the border of each sandwich panel to be cut-to-fit, architects can more easily take advantage of the efficiency that VIPs offer when designing prefabricated fa ade panels. Currently, there is a demand for a new market catering to VIP inclusion within fa ade strategies that would facilitate the use of VIPs by architects and planners in building large-scale urban planning strategies. (C) 2014 Elsevier B.V. All rights reserved.

Elsevier Science Sa2014

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Eberhard Ulrich Stützel

EPFL2013

High performance thermal insulation - examples from the Swiss built environment

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

Facilitating the incorporation of VIP into precast concrete sandwich panels

Fabrication and Optoelectronic Properties of Graphene Nanoribbons

High performance thermal insulation - examples from the Swiss built environment

Facilitating the incorporation of VIP into precast concrete sandwich panels