Thermochromic characteristics of WO 3 -doped vanadium dioxide thin films prepared by sol–gel method Jeong-Ho Cho a, * , Young-Jun Byun a , Jin-Ho Kim a , Young-Jin Lee a , Young-Hun Jeong a , Myoung-Pyo Chun a , Jong-Hoo Paik a , Tae Hyun Sung b a Electronic Components Center, Korea Institute of Ceramic Engineering and Technology, Seoul 153-023, Republic of Korea b Department of Electrical Engineering, Hanyang University, Seoul 133-791, Republic of Korea Available online 27 May 2011 Abstract Thin films of tungsten-doped vanadium oxide were fabricated on an alumina substrate by spin coating technology. A V 2 O 5 solution was prepared by an inorganic sol–gel method, which was a fairly cheap and effective process. As-coated V 2 O 5 films turned to VO 2 films during heat treatment in a reducing gas flow. Non-doped VO 2 film exhibited the best switching property of 4.0 orders of magnitude of electrical resistance and a small hysteresis of approximately 5 8C width. Tungsten in VO 2 led to a diffuse phase transition and weak jump of electrical resistivity. A reduction of the transition temperature by 15.5 8C/mol was observed for the tungsten doping in this study. # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Films; C. Electrical properties; E. Thermal applications; Thermochromic 1. Introduction Global warming is receiving worldwide attention, and means to alleviate its harmful consequences are much in focus. Major changes in energy technology will be necessary, and will impact the global economy [1]. Intelligent window coatings respond to an external stimulus with increasing sophistication on an ‘as needed’ basis, this includes thermochromic coatings. Thermochromic window coatings offer new possibilities for achieving energy efficiency in windows. The most widely studied thermochromic material is VO 2 , which is characterized by a semiconductor-to-metal transition occurring from a reversible change in the crystalline structure as a function of the temperature [1–4]. This change has been observed in transition-metal oxides such as Ti 2 O 3 , Fe 3 O 4 , Mo 9 O 26 and in several Magneli phases of vanadium oxide, V n O 2n1 . Among them, VO 2 has been received most attention because of the large reversible change of electric, magnetic, and optical properties at temperatures around 68 8C, which is, compared with the other materials, nearest to room tempera- ture. During the semiconductor–metal transition, the optical properties of vanadium dioxide are characterized by a sharp decrease in optical transmission in the infrared spectrum. This is coupled with an increase in its resistivity. Because of this anomalous behaviour, vanadium dioxide has been presented as an attractive thin film material for electrical or optical switches [5], optical storage [6], laser protection, and solar energy control for windows [7]. For practical use, however, it is necessary in most cases that the transition temperature be reduced to near the ambient, and efforts have been made to reduce it by doping with metals such as W, Nb, Mo, and Ta or non-metals such as F and P [8,9]. The hysteresis profile associated with the transition depends on the microstructure and crystallinity. Tungsten has been shown to be the most effective dopant ion in reducing the metal– semiconductor transition of VO 2 , it can optimally lower the T c to about 25 8C at 2 at.% loading [10]. A quite precise relationship between transition temperature and the W concentration has been reported for the V 1x W x O 2 films deposited by dual-target sputtering, which gave a T c reduction efficiency of 24 8C/at.% W [11]. Tungsten-doped VO 2 films have been previously prepared by sol–gel [2], physical vapour deposition methods [12] and CVD; both atmospheric pressure [13,14] and aerosol-assisted www.elsevier.com/locate/ceramint Available online at www.sciencedirect.com Ceramics International 38S (2012) S589–S593 * Corresponding author. Tel.: +82 2 3282 2424; fax: +82 2 3282 2410. E-mail address: goedc@kicet.re.kr (J.-H. Cho). 0272-8842/$36.00 # 2011 Elsevier Ltd and Techna Group S.r.l. All rights reserved. doi:10.1016/j.ceramint.2011.05.104