Structure and properties of laser-fired, sol-gel derived tungsten oxide films.

摘要

This investigation focuses on the use of laser radiation to fire sol-gel derived oxide films. The main emphasis of this work was to make high quality tungsten oxide films with good electrochromic properties. Laser firing was done with a carbon dioxide laser operated in continuous mode. The laser-fired tungsten oxide films were measured for density, composition, crystallinity and electrochromic behavior. Analytical tools included multi-angle ellipsometry, FTIR, TEM, XRD, spectrophotometry and electrochemistry. The effect of process variables (laser power, spot size and translation speed) on the extent of film densification and microstructural evolution was investigated. Thermal modeling of laser-heated sol-gel films was studied to further understand the laser firing process and to estimate firing temperatures. Temperature calculations were based on laser parameters, sample geometry and target materials. Properties characteristic of firing temperature were used to verify the thermal modeling. For laser-fired films, the properties at the calculated temperatures agreed well with the properties of similar furnace-fired films. The modeling also provided the thermal profiles seen by the laser heated materials. Laser firing was shown to be a feasible technique to make good quality electrochromic films. By precisely controlling the irradiation, the microstructure of tungsten oxide films was tailored to produce the desired electrochromic properties. Transmission electron microscopy showed film microstructures that varied from completely amorphous to fully crystalline. Corresponding optoelectrochemical measurements indicated a decrease in electrochromism with increasing crystallinity. The effects of density/porosity and coating composition are also discussed. It is proposed that laser firing of sol-gel derived films can be used for optics, sensors, graded index materials, and electrochromic windows. The ability to heat localized regions afforded by laser firing is advantageous for writing lines and patterns in these films. Windows with graded electrochromic properties can be made by dynamically changing the laser firing conditions as the beam rasters through a workpiece. Similarly, electrochromic signs can be written into a window--after the pattern is written by laser densification, the remaining film is etched away, leaving the pattern.