Control of powder microstructure for improved infrared reflectance modulation of an electrochromic plastic device

The so-called plastic technology first developed for Li-ion batteries is demonstrated for its potential optical applications in infrared emissivity control. WO3.H2O powder embedded in a plastic matrix is used as the active component in the LiCoO2/Li electrolyte/WO3.H2O system. The role of the microstructure of WO3.H2O as an electrochromic material is investigated. For instance, platelet-shaped grains with a surface area as large as 15 mum(2) induce a large improvement in device contrast properties. Reflectance measurements show a 53% contrast between the bleached and colored states over the 8-12 mum range. This effect is likely rooted in the free electron mobility in addition to scattering and phonon effects. The optical modulation performance over the range 2.5-20 mum compares favorably with literature results on solid devices built from annealed and sputtered m-WO3 thin films.