EXAMINATION OF THE KINETICS AND PERFORMANCE OF A CATALYTICALLY SWITCHING (GASOCHROMIC) DEVICE

The features of a new type of smart window are evaluated. The 'gasochr omic' device uses a WO, layer that can be coloured by performing a rea ction with gaseous hydrogen via a catalyst. The kinetics of this react ion as a function of several parameters were determined fbr optimizati on purposes: The dependence of the colouration speed on the H-2 concen tration was found to be linear for the range 0.1-20%. The temperature dependence is of the Arrhenius type with a Gibbs energy of activation of 27 kJ/mol. For evaporated layers, the kinetics depend strongly on t he overall thickness of the catalyst layer. The shape of the curve for colouration versus time is different for layers with different micros tructures i.e. diffusion coefficients. A model accounting for these ef fects was developed. Within this model the kinetics are described by a surface-controlled process (such as the adsorption) and one-dimension al diffusion of protons and electrons within the bulk material. We fou nd that diffusion limits the speed for dense materials, while the surf ace process is the limiting one for porous materials. SEM (secondary e lectron microscopy) images of the examined layers reveal that not only the degree of amorphicity influences the diffusion, but also the stru cture on a scale of 100 nm (columns formed in the deposition process). The influence of the water content of the atmosphere and the layer on the kinetics and degradation of the device are also discussed. The sa mples are produced by e-beam evaporation, reactive magnetron sputterin g and reactive gas flow sputtering (RGFS) causing the different micros tructure of the WO3 layer. (C) 1998 Elsevier Science B.V. All rights r eserved.