Relationship between particle size and photochromic characteristics of tungsten oxide films prepared by electric current heating method using tungsten wire

Tungsten oxide films were deposited on glass substrates placed above tungsten wires heated by electric current in air. The film thickness in a region just above the wire (region A) was thinner than that in other region (region B). The observation using scanning electron microscope revealed that the films consisted of particles. The shapes of the particles were sphere in the region A while smaller octahedron shape was found in the region B. The amount of the particles in the region B was greater than that in A. The ratio of the number of small particles with a diameter ≤500 nm to the total particle number in the region A increased with decreasing applied voltage during the electric current heating, whereas, in the region B, the size of particles was ≤500 nm and had no applied voltage dependence. The films exhibited photochromism; the reflectance in the near-infrared region was decreased by the ultraviolet irradiation. In region A, the photochromic effects increased with decreasing applied voltage to the wire. On the other hand, in region B, the photochromic effects did not show the applied voltage dependence. Consequently, it was found that the obtained films showed positive correlation between photochromic effect and the ratio of the number of small particles (≤500 nm) to the total particle number. The particles in region B are suitable for mass production of the photochromic material because the photochromic effect and the amount of particles in region B are greater than those in A.