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Browsing Department of Physics by Author "Granqvist, C. G."
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Item Advances in chromogenic materials and devices(2010-03) Granqvist, C. G.; Green, Sara V.; Niklasson, Gunnar A.; Mlyuka, N.R.; Von Kræmer, S.; Georen, PeterChromogenic materials allow the transmittance of visible light and solar energy to be varied under the action of an external stimulus. This paper first discusses buildings related energy savings that can be accomplished by chromogenic technologies, and their beneficial effects on comfort issues. We then summarize recent work on thermochromic VO2-based thin films with particular attention to multi-layers of VO2 and TiO2 and to new VO2:Mg films for which the doping gives significantly lowered absorption of visible light. The final part covers electrochromic materials and devices with foci on coloration efficiency and on durability issues for foil-type constructions based on films of WO3 and NiO.Item Band gap widening in thermochromic Mg-doped VO2 thin films: Quantitative data based on optical(2013-10) Li, S.-Y.; Mlyuka, N.R.; Primetzhofer, Daniel; Hallén, Anders; Possnert, Goran; Niklasson, Gunnar A.; Granqvist, C. G.hermochromic Mg-doped VO2 films were deposited by reactive DC magnetron sputtering onto heated glass and carbon substrates. Elemental compositions were inferred from Rutherford backscattering. Optical band gaps were obtained from spectral transmittance and reflectance measurements—from both the film side and the back side of the samples—and ensuing determination of absorption coefficients. The band gap of Mg-doped films was found to increase by 3.6 ± 0.6 eV per unit of atom ratio Mg/(Mg + V) for 0 < Mg/(Mg + V) < 0.21. The presence of ~0.45 at.% Si enhanced the band gap even more.Item Chromogenics for Sustainable Energy: Some Advances in Thermochromics and Electrochromics(2010-10) Granqvist, C. G.; Green, Sara V.; Li, S.-Y.; Mlyuka, N.R.; Niklasson, Gunnar A.; Avendaño, E.Chromogenic materials are able to change their optical properties in response to external stimuli such as temperature (in thermochromic materials) and electrical charge insertion (in electrochromic materials). Below we review some recent advances for these types of materials. Specifically we first discuss the limitations of thermochromic VO2 films for energy efficient fenestration and show from calculations that nanocomposites containing VO2 can have superior properties and display high luminous transmittance and large temperature-dependent solar transmittance modulation. Even better results may be found for nanoparticles of VO2:Mg. In the second part of the paper we survey some recent progress for electrochromic devices and show that W oxide films have increased coloration efficiency when some Ni oxide is added. We also present initial results for flexible electrochromic foils produced by roll-to-roll coating and continuous lamination.Item Mg doping of thermochromic VO2 films enhances the optical transmittance and decreases the metal-insulator transition temperature(2009-10) Mlyuka, N.R.; Niklasson, Gunnar A.; Granqvist, C. G.Thermochromic films of MgxV1-xO2 were made by reactive dc magnetron sputtering onto heated glass. The metal-insulator transition temperature decreased by similar to 3 K/at. %Mg, while the optical transmittance increased concomitantly. Specifically, the transmittance of visible light and of solar radiation was enhanced by similar to 10% when the Mg content was similar to 7 at. %. Our results point at the usefulness of these films for energy efficient fenestrationItem Progress in chromogenics: New results for electrochromic and thermochromic materials and devices(2009-12) Granqvist, C. G.; Lansåker, Pia; Mlyuka, N.R.; Niklasson, Gunnar A.; Avendaño, E.Chromogenic device technology can be used to vary the throughput of visible light and solar energy for windows in buildings as well as for other see-through applications. The technologies can make use of a range of “chromic” materials – such as electrochromic, thermochromic, photochromic, etc – either by themselves or in combinations. The first part of this paper points at the great energy savings that can be achieved by use of chromogenic technologies applied in the built environment, and that these savings can be accomplished jointly with improved indoor comfort for the users of the building. Some recent data are presented on a foil-type electrochromic device incorporating tungsten oxide and nickel oxide. In particular, we consider the possibilities of controlling the near-infrared transmittance and optimize this property for specific climates. To that end we discuss Au-based transparent conductors for electrochromics as well as high-transmittance thermochromic multilayer films incorporating VO2 andItem Thermochromic multilayer films of VO2 and TiO2 with enhanced transmittance(2009-09) Mlyuka, N.R.; Niklasson, Gunnar A.; Granqvist, C. G.Thermochromic films of VO2, as well as multilayer films of VO2 and TiO2, were made by reactive DC magnetron sputtering. Spectrophotometrically measured transmittance and reflectance were used to determine optical constants pertinent to temperatures below and above a temperature-induced structural change at τc≈60 °C. We then used computations to optimize multilayer films for specific applications and, specifically, demonstrated that TiO2/VO2/TiO2 films could display a luminous transmittance significantly higher than that of bare VO2 films, and that TiO2/VO2/TiO2/VO2/TiO2 films could yield a large change of solar transmittance for temperatures above and below τc. Our data can serve as starting points for developing novel coatings for windows with superior energy efficiency.Item Thermochromic VO2‐based multilayer films with enhanced luminous transmittance and solar modulation(2009-09) Mlyuka, N.R.; Niklasson, Gunnar A.; Granqvist, C. G.Vanadium dioxide (VO2) shows an abrupt and reversible change in optical and electrical properties when the temperature is raised beyond a critical point of ∼68 °C. Films made from this material have a potential to be used in energy efficient “smart” windows with temperature-dependent throughput of solar radiation. Two of the drawbacks of this material have been its low luminous transmittance and limited solar modulation of transmittance during switching. In this work we report calculations and experiments on multilayers of VO2 and TiO2, produced by reactive DC magnetron sputtering, that significantly improve the luminous transmittance and solar modulation of the films during switching. We also explore the angular-dependent transmittance of five-layer TiO2/VO2/TiO2/VO2/TiO2 films and demonstrate that the modulation of luminous and solar transmittance can be enhanced at non-normal angles of incidence.