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VO2-based Thermochromic and Nanothermochromic Materials for Energy-Efficient Windows: Computational and Experimental Studies
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Solid State Physics. (Division of Solid State Physics)
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

VO2-based films are thermochromic and exhibit high or low infrared transmittance when the temperature is below or above a critical temperature. The thermochromic switching is passive and reversible, and therefore VO2 based films are promising for energy-efficient window appli­cations. However the practicaluse of VO2 for energy-efficient windows has long been hampered by low luminous transmittance and low solar energy transmittance modulation. The main goal of this dissertation work is to address these issues.

The first half of the work proposes the concept of nanothermochromics for simultaneous improvement of luminous transmittance and modulation of solar energy throughput. nanoth­ermochromics considers VO2 nanoparticle composite layers, whose optical properties were modeled by effective medium theories. Calculations on VO2 spheroids have shown that VO2 nanoparticles, especially nanospheres, can offer dramatically improved luminous transmittance and solar transmittance modulation that are not possible for films. Calculations done on coreshell nanoparticles showed comparable improvements and offer an opportunity to reduce the material costs. It was also found that the composite of In2O3:Sn (ITO) and VO2 can yield moderately high luminous transmittance, solar transmittance modulation and low-emittance properties.

In the second half of the dissertation work, Mg-doped VO2 films were sputter deposited. Their band gaps and Mg-content were investigated by means of optical absorption measurement and Rutherford backscattering spectrometry, respectively. The band gaps of VO2 were found to increase by ∼3.9±0.5 eV per unit of atom ratio Mg/(Mg+V) for 0<Mg/(Mg+V)<0.21. Computations based on effective medium theory were done to estimate the performance of Mg­-doped VO2 films and nanoparticle composite layers. The results suggest that moderately doped VO2 films with 0<Mg/(Mg+V)<0.06 perform better than un-doped films and that the perfor­mance can be further enhanced with one layer of antireflection coating. The best results were achieved by un-doped VO2 nanospheres, closely followed by the VO2 nanospheres with low Mg-content.

Furthermore, the an experimental study on sputter deposited VO2 nanorods has identified the geometry of the oxygen gas inlet, the type of substrate, the substrate temperature and the layer thickness as important factors that influence the growth morphology.

Taken as a whole, nanothermochromics offered by VO2 nanoparticles was shown to be the best solution for VO2 based thermochromic energy-efficient window coatings.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2013. , 143 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1095
Keyword [en]
VO2, thermochromism, nanothermochromics, energy efficient windows, optical modeling, effective medium theory, thin film characterization, doping
National Category
Condensed Matter Physics
Identifiers
URN: urn:nbn:se:uu:diva-210016ISBN: 978-91-554-8801-7 (print)OAI: oai:DiVA.org:uu-210016DiVA: diva2:660598
Public defence
2013-12-13, Häggsalen, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala University, Polacksbacken, Uppsala, 13:15 (English)
Opponent
Supervisors
Funder
Swedish Research Council
Available from: 2013-11-21 Created: 2013-10-29 Last updated: 2014-01-23
List of papers
1. Nanothermochromics: Calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation
Open this publication in new window or tab >>Nanothermochromics: Calculations for VO2 nanoparticles in dielectric hosts show much improved luminous transmittance and solar energy transmittance modulation
2010 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 108, no 6, 063525- p.Article in journal (Refereed) Published
Abstract [en]

VO2-based films are thermochromic and show infrared reflectance above a "critical" temperature in the vicinity of room temperature. Implementations on energy efficient windows have been discussed for decades but have been severely curtailed since the luminous absorptance is undesirably large and the solar energy transmittance modulation is too small. Here we show by calculations based on effective medium theory that dilute composites with VO2 nanoparticles embedded in hosts with properties mimicking glass or polymer can yield significantly decreased luminous absorption jointly with much enhanced transmittance modulation of solar energy. These results demonstrate that VO2-based nanothermochromics opens new avenues toward energy efficient fenestration.

National Category
Physical Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-134150 (URN)10.1063/1.3487980 (DOI)000282646400043 ()
Available from: 2011-09-21 Created: 2010-11-22 Last updated: 2017-12-12Bibliographically approved
2. Nanothermochromics with VO2-based core-shell structures: Calculated luminous and solar optical properties
Open this publication in new window or tab >>Nanothermochromics with VO2-based core-shell structures: Calculated luminous and solar optical properties
2011 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 109, no 11, 113515- p.Article in journal (Refereed) Published
Abstract [en]

Composites including VO2-based thermochromic nanoparticles are able to combine high luminous transmittance T-lum with a significant modulation of the solar energy transmittance Delta T-sol at a "critical" temperature in the vicinity of room temperature. Thus nanothermochromics is of much interest for energy efficient fenestration and offers advantages over thermochromic VO2-based thin films. This paper presents calculations based on effective medium theory applied to dilute suspensions of core-shell nanoparticles and demonstrates that, in particular, moderately thin-walled hollow spherical VO2 nanoshells can give significantly higher values of Delta T-sol than solid nanoparticles at the expense of a somewhat lowered T-lum. This paper is a sequel to a recent publication [S.-Y. Li, G. A. Niklasson, and C. G. Granqvist, J. Appl. Phys. 108, 063525 (2010)].

National Category
Physical Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-156243 (URN)10.1063/1.3592350 (DOI)000292214700036 ()
Available from: 2011-09-23 Created: 2011-07-18 Last updated: 2017-12-08Bibliographically approved
3. A thermochromic low-emittance coating: Calculations for nanocomposites of In2O3:Sn and VO2
Open this publication in new window or tab >>A thermochromic low-emittance coating: Calculations for nanocomposites of In2O3:Sn and VO2
2011 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 99, no 13, 131907- p.Article in journal (Refereed) Published
Abstract [en]

Calculations based on the Bruggeman effective medium theory were applied to thin films comprising a heavily doped wide band gap semiconductor (specifically In(2)O(3):Sn (ITO)) and VO(2). Films with similar to 20 vol. % of VO(2) can combine a 10% thermochromic modulation of the solar energy throughput with a luminous transmittance of 50%-60% and low thermal emittance. The maximum thermochromic modulation is similar to 13% and occurs at similar to 35 vol. % VO(2). Coatings of ITO-VO(2) are of interest for energy efficient fenestration.

Keyword
coatings, indium compounds, nanocomposites, semiconductor thin films, thermo-optical effects, vanadium compounds, wide band gap semiconductors
National Category
Physical Sciences Engineering and Technology
Research subject
Physics with spec. in Atomic, Molecular and Condensed Matter Physics; Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-161596 (URN)10.1063/1.3641869 (DOI)000295618000027 ()
Note

Copyright 2011 American Institute of Physics. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the American Institute of Physics.

Available from: 2011-12-19 Created: 2011-11-15 Last updated: 2017-12-08Bibliographically approved
4. Thermochromic VO2 nanorods made by sputter deposition: Growth conditions and optical modeling
Open this publication in new window or tab >>Thermochromic VO2 nanorods made by sputter deposition: Growth conditions and optical modeling
Show others...
2013 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 114, no 3, 033516- p.Article in journal (Refereed) Published
Abstract [en]

Reactive dc magnetron sputtering onto glass-based substrates yielded deposits of thermochromic VO2 with well-developed nanorods and nanowires. Their formation was promoted by high substrate temperature (above similar to 500 degrees C), sufficient film thickness, proper inlet of the reactive gas, dispersed gold "seeds," and pronounced substrate roughness. Rutherford back scattering ascertained mass thicknesses, scanning electron microscopy depicted the nanostructures, and glancing incidence X-ray diffraction proved that single-phase VO2 was normally formed. Spectrophotometric measurements of total and diffuse transmittance and reflectance on VO2 thin films, at room temperature and similar to 100 degrees C, allowed us to determine complex dielectric functions below and above the "critical" temperature for thermochromic switching (similar to 68 degrees C). These data were then used in computations based on the Bruggeman effective medium theory applied to randomly oriented prolate spheroidal structural units to derive the optical properties of the deposits. Experimental and computed data on spectral absorptance were found to be in good qualitative agreement.

National Category
Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-208384 (URN)10.1063/1.4813876 (DOI)000322202700031 ()
Note

Correction in: Journal of Applied Physics, Vol. 114, Issue 23, article nr 239902.

DOI: 10.1063/1.4848999

Available from: 2013-09-30 Created: 2013-09-30 Last updated: 2017-12-06Bibliographically approved
5. Bandgap widening in thermochromic Mg-doped VO2 thin films: Quantitative data based on optical absorption
Open this publication in new window or tab >>Bandgap widening in thermochromic Mg-doped VO2 thin films: Quantitative data based on optical absorption
Show others...
2013 (English)In: Applied Physics Letters, ISSN 0003-6951, E-ISSN 1077-3118, Vol. 103, no 16, 161907- p.Article in journal (Refereed) Published
Abstract [en]

Thermochromic Mg-doped VO2 films were deposited by reactive direct current magnetronsputtering onto heated glass and carbon substrates. Elemental compositions were inferred fromRutherford backscattering. Optical bandgaps 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 bandgap of Mg-doped films was found to increase by 3.9 ± 0.5 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 bandgap even more.

National Category
Condensed Matter Physics Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-210015 (URN)10.1063/1.4826444 (DOI)000326148700032 ()
Available from: 2013-10-29 Created: 2013-10-29 Last updated: 2017-12-06Bibliographically approved
6. Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows
Open this publication in new window or tab >>Thermochromic undoped and Mg-doped VO2 thin films and nanoparticles: Optical properties and performance limits for energy efficient windows
2014 (English)In: Journal of Applied Physics, ISSN 0021-8979, E-ISSN 1089-7550, Vol. 115, no 5, 053513/1-/10 p.Article in journal (Refereed) Published
Abstract [en]

Undoped and Mg-doped thermochromic VO2 films with atom ratios z equivalent to Mg/(Mg + V) of 0 <= z < 0.21 were deposited by reactive DC magnetron sputtering onto heated glass and carbon substrates. Elemental compositions were found by Rutherford backscattering spectrometry. Optical constants were determined from transmittance and reflectance measurements and were used for modeling the optical properties of thin films and dilute nanoparticle composite layers below and above the critical temperature for thermochromic switching between a low-temperature infrared transparent state and a high-temperature infrared reflecting or absorbing state. Mg-doped films showed superior luminous transmittance T-lum and solar transmittance modulation Delta T-sol compared to undoped VO2 films, and both of these parameters could be further enhanced by anti-reflection. VO2-containing nanocomposites had much larger values of T-lum and Delta T-sol than VO2-based films. Mg-doping was found to erode the properties of the nanocomposites. Approximate performance limits are given on T-lum and Delta T-sol for thermochromic VO2 films, with and without Mg doping and antireflection coating, and also for VO2-containing dilute nanocomposites.

National Category
Condensed Matter Physics Engineering and Technology
Research subject
Engineering Science with specialization in Solid State Physics
Identifiers
urn:nbn:se:uu:diva-210014 (URN)10.1063/1.4862930 (DOI)000331645900019 ()
Projects
EU GRINDOOR
Funder
EU, FP7, Seventh Framework Programme, 267234Swedish Research Council
Available from: 2013-10-29 Created: 2013-10-29 Last updated: 2017-12-06Bibliographically approved

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