Tungsten Oxide Films Forming Method

Tungsten is among the transition metals which form electrochromic metal oxide films. Electrochromic materials have variable light transmittance in response to an applied electrochemical potential. Such metal oxide films are used in electrochromic display devices wherein the film changes color when subjected to an electrical potential. A tungsten oxide film is typically preferred for this application due to its highly visible color change.

Tungsten oxide films are of interest for use as a coating for glass, to produce windows with controllable light transmission. For example, the automobile industry might use such coated windows to lower the amount of sunlight-generated heat in the passenger compartment of a car. The tungsten oxide, WO3, film is normally a faint yellow, and when the film reacts with protons from an electrolyte it changes color. The corresponding electrochemical reaction is:

WO3 +xM+ +xe- ⃡Mx WO3

where M =H, Li, K or Na (faint yellow to colorless) (blue, tungsten bronze).

This electrochromic reaction is sometimes characterized as a change from a bleached, white or colorless state to a colored state. The bleached state has relatively high transmittance and the colored state, relatively low transmittance.

Tungsten oxide, WO3, has been reported to possess ferroelectric properties. Ferroelectric materials have potential for use in nonvolatile memory devices, that is, devices in which data is retained even when power is cut off. In addition, tungsten oxide may potentially be used for infrared temperature sensors.

Several methods are used to form tungsten oxide films. The methods include sputtering, chemical vapor deposition, and plasma enhanced chemical vapor deposition, in which sub-atmospheric pressure must be maintained. In these methods, large, complex and expensive equipment is needed, and the methods involve significant energy consumption and relatively high operating costs. The films produced by current methods are tungsten oxide films of WO3, without suboxides, and which are colorless or faint yellow, single color films.

This invention relates to forming a tungsten oxide film on a substrate by applying an alkyl amine tungstate compound thereon and pyrolyzing at least a portion of the alkyl amine tungstate compound to form the tungsten oxide film. More particularly this invention relates to such method utilizing a solution containing the alkyl amine tungstate compound to uniformly apply the compound onto the substrate, drying the solution to form a deposit and heating the deposit for a time and at a temperature sufficient to achieve a desired composition of the tungsten oxide film.

In addition, the method includes forming a color gradient. That is, regions of the film having different colors are made by varying the time and temperature of heating for respective regions of the film. The regions are faint yellow to deep brown, and electrochromic. The method also results in the formation of suboxides of tungsten oxide (WO3) at selected times and temperatures.


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