The Effect of Alkali Metal (Na, K) Doping on Thermochromic Properties of VO 2 Films
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MRS Advances © 2018 Materials Research Society DOI: 10.1557/adv.2018.70
The Effect of Alkali Metal (Na, K) Doping on Thermochromic Properties of VO2 Films Işıl Topa*, Johannes Schläferb, Russell Binionsa, Ioannis Papakonstantinoub, Sriluxmi Srimurugananthana, Michael Powellc, Ivan Parkinc, Claire J Carmaltc, Isaac Abrahamsd* a: School of Engineering and Materials Science, Queen Mary University of London, Mile End Road, E1 4NS, UK. b: Department of Electronic and Electrical Engineering, University College London, Torrington Place, WC1E 7JE, UK. c: Department of Chemistry, University College London, 20 Gordon Street, London. WC1H 0AJ, UK. d: School of Biological and Chemical Sciences, Queen Mary University of London, Mile End Road, E1 4NS,UK.
ABSTRACT:
This work reports the synthesis of undoped and alkali metal doped thermochromic vanadium dioxide thin films by sol-gel spin coating and subsequent low-temperature annealing at 450 °C in N2-H2 atmosphere. The effect of sodium and potassium on the phase transition temperature as well as on the solar modulations were investigated. A dopant concentration of 0.3 at% resulted in a reduction of the critical transition temperature (T c) from 62 °C to 57 °C and 47 °C for the sodium and potassium doped films, respectively. Moreover, both dopants improved the solar modulations (ΔTsol) of the undoped VO2 films from 3.81 to 9.44 and 5.43 %, respectively. *[email protected], [email protected]
INTRODUCTION: Smart windows refer to windows which can intelligently control the amount of light and heat passing through, when triggered by an external stimulus such as voltage (electrochromic), temperature (thermochromic), UV illumination (photochromic) and reductive or oxidizing gases (gasochromic). Vanadium oxides exhibit thermochromic properties, among which VO, V 2O3, VO2 and V2O5 undergo metal–insulator phase transitions at the critical temperatures (Tc) of -147.15 °C, -123 °C [1], 68 °C [2] and 280 °C [3], respectively. Among other vanadium oxides, VO 2 has received much interest in recent years, due to its potential application as a solar control coating. Since buildings account for ~40% of the total world energy consumption, primarily due to heating and cooling demands, and most of the energy loss occurs through windows, VO 2 coatings on windows can be used to reduce these energy demands [4]. This reduction in energy consumption could lead to lower greenhouse gas emissions, in line with the Paris
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agreement to limit human-induced climate change to less than 2 °C above pre-industrial levels. Thermochromic smart window technology is based on a temperature induced fully reversible structural phase change from a high temperature tetragonal rutile structure to a low temperature monoclinic structure [5]. A remarkable feature of the monoclinic phase is the
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