Microstructure of Laser-Fired, Sol-Gel-Derived Tungsten Oxide Films D. J. Taylor,* ,† J. P. Cronin, ‡ L. F. Allard, Jr., § and D. P. Birnie III † Department of Materials Science and Engineering, University of Arizona, Tucson, Arizona 85721; Donnelly Corporation, Advanced Technology Center, 4545 E. Fort Lowell Road, Tucson, Arizona 85712; and High Temperature Materials Laboratory, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 Received December 4, 1995. Revised Manuscript Received April 22, 1996 X Half-micron-thick tungsten oxide films were deposited by the sol-gel method onto indium tin oxide (ITO) coated soda lime silicate substrates. Following a 100 °C prebake, the samples were fired with a carbon dioxide laser at a variety of power densities and translation speeds. The laser-fired tungsten oxide films were characterized by spectrophotometry, electrochem- istry, multiangle ellipsometry, and transmission electron microscopy and compared to similar furnace-fired films. The data showed an increase in electrochromic response with increased firing temperature up to the point where crystallization of the tungsten oxide retarded electrochromic response. A window with graded electrochromic properties was made by laser firing. Introduction After coating, sol-gel-derived films require heat in order to densify, which includes hydrolysis, completion of condensation reactions, network formation, organic burnout, and sintering. This heating is normally ac- complished in a furnace. The disadvantage of furnace- firing is that the whole coated sample is subjected to the same firing temperature. Absorbed laser radiation provides another method of heatingsone that is local- ized to the point of irradiation. 1,2 Laser densification of sol-gel-derived coatings has been shown to be suc- cessful for making hard, protective coatings 3,4 and optical waveguides. 5-8 The localized heat source that lasers provide has two advantages in firing sol-gel coatings: (1) the substrate may only be heated slightly, allowing low-refractory materials to be used as sub- strates; (2) the coating may be selectively heated. The latter consideration allows areas of the same coating to have different properties based on varied firing condi- tions. A pattern can be written into a sol-gel coating by densification with a laser, and the remaining un- dense coating can be easily etched away, leaving the pattern. 9 Or, one of the laser firing parameters can be changed continuously as the coated body is fired, producing a film with graded properties. As reported in some of the electrochromic literature and in this work, firing conditions affect the electro- chromic behavior of tungsten oxide. Through laser firing, the properties of sol-gel tungsten oxide can be tailored in several ways that are not possible by furnace firing. By taking some of the advantages of laser-fired, sol-gel-derived coatings and applying them to electro- chromic films, patterns can be written into a tungsten oxide coating that will be electrochromic. The unfired sol-gel-derived tungsten oxide film does not exhibit electrochromism and etches orders of magnitude faster than the fired regions of the film. A sign or pattern that can be turned on or off can be laser written into the coating on a window. Also, since changing the firing conditions (e.g., incident laser power or translation speed of the laser beam) alters the microstructure, affecting the electrochromic behavior, coatings of graded electrochromic response can be made. Therefore, laser- fired tungsten oxide could be of interest for making graded electrochromic sunglasses, mirrors, or windows. Tungsten oxide has been extensively studied for its electrochromic properties and potential commercial applications. These properties were first reported by Deb 10 in the late 1960s, and since then many theories have been proposed as to the electrochromic mecha- † University of Arizona. ‡ Donnelly Corp. § Oak Ridge National Laboratory. X Abstract published in Advance ACS Abstracts, June 1, 1996. (1) Chia, T.; Hench, L. L.; Qin, C.; Hsieh, C. K. Laser densification modeling. In Better Ceramics Through Chemistry IV; Zelinski, B. J. J., Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Mat. Res. Soc. Proc. Vol. 180; MRS: Pittsburgh, PA, 1990; pp 819-824. (2) Taylor, D. J.; Birnie, III, D. P.; Fabes, B. D. Temperature calculation for laser irradiation of sol-gel films on oxide substrates. J. Mater. Res. 1995, 10, 1429-1434. (3) Taylor, D. J.; Fabes, B. D.; Steinthal, M. G. Laser densification of sol-gel coatings, in Better Ceramics Through Chemistry IV; Zelinski, B. J. J., Brinker, C. J., Clark, D. E., Ulrich, D. R., Eds.; Mat. Res. Soc. Proc. Vol. 180; MRS: Pittsburgh, PA, 1990; pp 1047-1052. (4) Taylor, D. J.; Fabes, B. D. Laser processing of sol-gel coatings. J. Non-Cryst. Solids 1992, 147 and 148, 457-462. (5) Zaugg, T. C.; Weisenbach, L.; Fabes, B. D.; Zelinski, B. J. J. Waveguide formation by laser irradiation of sol-gel coatings. In Submolecular Glass Chemistry and Physics; Bray, P., Kreidl, N. J., Eds.; SPIE: Bellingham, WA, 1991; Vol. 1590, pp 26-35. (6) Guglielmi, M.; Colombo, P.; Mancinelli Delgi Esposti, L.; Righini, G. C.; Pelli, S. Planar and strip optical waveguides by sol-gel method and laser densification. In Glasses for Optoelectronics II; Righini, G. C., Ed.; SPIE: Bellingham, WA, 1991; Vol. 1513, pp 44-49. (7) Guglielmi, M.; Colombo, P. Mancinell Delgi Esposti, L.; Righini, G. C.; Pelli, S.; Rigato, V. Characterization of laser-densified sol-gel films for the fabrication of planar and strip optical waveguides. J. Non-Cryst. Solids 1992, 147 and 148, 641-645. (8) Fabes, B. D.; Zelinski, B. J. J.; Taylor, D. J.; Weisenbach, L.; Boggavarapu, S.; Dent, D. Z. Laser densification of optical films. In Sol-Gel Optics II; Mackenzie, J. D., Ed.; SPIE: Bellingham, WA, 1992; Vol. 1758, pp 227-234. (9) Fabes, B. D.; Taylor, D. J.; Weisenbach, L.; Stuppi, M. M.; Klein, D. L.; Raymond, L. J.; Zelinski, B. J. J.; Birnie, III, D. P. Laser processing of channel waveguide structures in sol-gel coatings. In Sol- Gel Optics; Mackenzie, J. D., Ed.; SPIE: Bellingham, WA, 1990; Vol. 1328, pp 319-328. 1396 Chem. Mater. 1996, 8, 1396-1401 S0897-4756(95)00570-9 CCC: $12.00 © 1996 American Chemical Society + +