Thermochromism of VO 2 Nanoparticles: Calculated Optical Properties and Applications to Energy Efficient Windows
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Thermochromism of VO2 Nanoparticles: Calculated Optical Properties and Applications to Energy Efficient Windows S.-Y. Li, G. A. Niklasson and C. G. Granqvist Department of Engineering Sciences, The Ångström Laboratory, Uppsala University, P. O. Box 534, SE-75121 Uppsala, Sweden
ABSTRACT Thermochromic materials have temperature-dependent optical properties. This paper discusses the limitations of thermochromic VO2 films for energy efficient fenestration and shows 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. INTRODUCTION Some 30 to 40 % of the world’s primary energy is used in buildings [1], mostly for heating, cooling, lighting and ventilation. The percentage for USA is 39% [2]. The relative amount of electricity used in buildings can be 70% for the most industrialized countries. Most of this energy typically comes from fossil fuel and is associated with CO2 emissions and global warming. The energy used in buildings can be radically cut by using proper materials and technologies, and “green nanotechnologies” offer many possibilities for the built environment [3]. One of them employs thermochromic fenestration, i.e., windows whose solar energy transmittance drops reversibly as the temperature rises so that the energy for air cooling is diminished. We note that air conditioning uses ~14% of the energy in buildings in the USA [2]. This paper demonstrates that VO2-based nanoparticles give distinct advantages over thin films of this material. Thus we introduce the new concept of “nanothermochromics” [4] for decreasing the energy for maintaining a benign indoor climate. The presentation is based on an earlier conference paper of ours [5] but includes new data. RESULTS AND DISCUSSION General characteristics of VO2: Bulk and thin films Thermochromism is well known in VO2 [6], and thin films of this material have been discussed for temperature-dependent modulation of the solar energy transmittance in windows for many years [7]. VO2 has a reversible structural transformation at a “critical” temperature IJc; the material is monoclinic, semiconducting and rather infrared transparent for temperatures IJ < IJc
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and it is tetragonal, metallic and infrared reflecting for IJ > IJc. Thermochromism is highly reversible in thin films. There are three main reasons why VO2 films need to be further developed to be practically useful for windows: Firstly, IJc is ~68 °C, at least in bulk specimens, which obviously is too high for buildings-related applications. However doping with transition metal ions with a valency of 6+ and 5+ can decrease IJc. The most widely studied dopant is W6+, which can bring IJc to room temperature without significantly deteriorating the thermochromism [7]; the required amount of tungsten seems to depend on the degree of crystalline order in the VO2. The addition of some W6+ does not have any large effect on the opti
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