Microstructures And Reflective Property Of In Situ Deposited Silver Containing ITO Films On Silica Fibres

Fibrous thermal insulation materials are of great interest in architectural and national defence fields owing to their low cost, low density and high efficiency. However, the high temperature insulation performance is decayed a lot due to the materials'high transmittance of radiation wave. In this research, silver embedded indium tin oxide (Ag-ITO) multilayer films were in situ deposited by sol-gel and layer-by-layer adsorption methods onto fibres to be as infrared reflective coating to decrease the radiation heat transfer. There are three main factors to influence the reflectivity, i.e. porosity, surface roughness and the spatial distributional state of Ag. The effects of thermal treatments including temperature, heating rate and holding time on the porosity, surface roughness of ITO film and Ag distribution of multilayer film were studied through scanning electron microscopy (SEM), X-ray diffraction (XRD), grazing incidence small angle X-ray scattering (GISAXS), grazing incidence X-ray reflectivity (GIXR) et al. Furthermore, the optical performance of films is obtained from the UV-VIS-NIR and FT-IR spectra. The main contents and conclusions are listed as follows:ITO films are prepared by sol-gel dip-coating method. ITO film is stannum-rich on the surface with the crystalline temperature of about 450℃. Pores in the films show fractal structure and ellipsoid along the film. There are three basic regions along the thickness of film. The layer access to the substrate shows dense and small pores while the middle layer shows higher porosity and bigger pores. When the temperature is higher than 800℃, the near surface layer even shows higher porosity and bigger pores than next while it show opposite when the temperature is not.The effect of thermal annealing process on the porosity of ITO films is studied through GISAXS technique. The densification is improved with the increase of annealing temperature. The porosity is decreased due to the shrinkage of big pores as well as elimination of small pores in low temperature in low temperatures and just the latter one in high temperatures. The porosity is decreased with the increase of heating rate due to the shorter crystalline time. The porosity will be increased due to the increasing near surface porosity with the holding time at high temperature. Thus, thermal treatments of ITO film were optimized to be that slow heating during the volatity of solvent and then high heating.Ag-ITO composite films were prepared by layer-by-layer adsorption method with an in situ reducing agent (stannuous chloride). The favorite concentration of silver sols is 0.1M to keep the continuous,small and not aggregated. It is found that the ITO is diffused through diffusion-limited aggregate dynamic and metal introduced formation. The silver atoms are controlled by exchange-reaction limited aggregation mechanism and the coalescence of silver particles is inhibited. The nucleation density of silver phase shows"V"with the increase of thermal annealing temperatures of composite films. The knee point is at 900℃which means an active temperature of silver atoms. During the heating period in thermal annealing process, the formation and growth of silver particles according to the diffusion-exchange-diffusion-exchange model, and during the holding period, the exchange mechanism increases the correlation distance of silver particles and decreases the fractal dimension.Films were prepared on the surface of fibers through layer-by-layer adsorption method. In order to increase the electronegativity of fibers, fibers were treated by a solution including sodium dodecylbenzene sulfonate and ammonia. Films on the fibers show dense, homogenous and have thermal stability. The porosity and surface roughness both influence the hemisphere reflectivities of ITO films. The former factor is major and the latter is minor. While the hemisphere reflectance of Ag-ITO multilayer film during 1-2.5μm shows"N"curves. This phenomenon is explained by activation tunnel effect theory. The best thermal annealing temperature is proved to be 800℃. The transimittance of coated fibers are decreased with the increase of annealing temperature and holding time through pelleting technique. The reduction of radiation heat transfer could be up to 30% after the coating on the fibers through Rosseland mean theory.