Carbon-in-silica composite selective solar absorbers: a determination of composition and dielectric properties

The Bruggeman and Maxwell-Garnett effective medium approximations have been used widely to investigate optical properties of many different composite materials. In most cases, the effective medium approximation assumptions are based on random unit cell models in which some metal particles are embedded in a dielectric medium. The shapes of the embedded particles can be varied between spherical, ellipsoidal and cylindrical shapes. A new and interesting structure of connected short chains of completely amorphous carbon intermixed with short chains of silica at nanoscale level has been observed recently. A generalised Bergman representation based on an arbitrary spectral density function is currently applied on these carbon-in-silica samples with a reasonable success of fitting between experiment and theory. The curve-fitting procedure adopted here has resulted in information such as volume fraction of carbon relative to silica, percolation threshold, the thickness and effective dielectric function of the composite layer.

[1]  D. Aspnes.  Optical properties of thin films , 1982 .

[2]  D. A. G. Bruggeman.  Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen. III. Die elastischen Konstanten der quasiisotropen Mischkörper aus isotropen Substanzen , 1937 .

[3]  E. Wäckelgård,et al.  Solar selective absorber functionality of carbon nanoparticles embedded in SiO2, ZnO and NiO matrices , 2008 .

[4]  J. Garnett,et al.  Colours in Metal Glasses and in Metallic Films. , 1904, Proceedings of the Royal Society of London.

[5]  G. Niklasson,et al.  Optical properties and solar selectivity of coevaporated Co‐Al2O3 composite films , 1984 .

[6]  B. Mwakikunga,et al.  Optical, thermal and structural characteristics of carbon nanoparticles embedded in ZnO and NiO as selective solar absorbers , 2008 .

[7]  E. Wäckelgård,et al.  Low Cost Selective Solar Absorber Coatings: Characteristics of Carbon-In-Silica Synthesized with Sol-Gel Technique , 2005 .

[8]  G. Katumba,et al.  FTIR and Raman Spectroscopy of Carbon Nanoparticles in SiO2, ZnO and NiO Matrices , 2008, Nanoscale Research Letters.

[9]  David J. Bergman,et al.  The dielectric constant of a composite material—A problem in classical physics , 1978 .

[10]  G. Niklasson.  CoAl2O3 selective solar absorbing films: structure and effective medium theory for the optical properties , 1988 .

[11]  D. A. G. Bruggeman.  Berechnung verschiedener physikalischer Konstanten von heterogenen Substanzen. I. Dielektrizitätskonstanten und Leitfähigkeiten der Mischkörper aus isotropen Substanzen , 1935 .

[12]  Ewa Wäckelgård,et al.  Selectively Solar-Absorbing Coatings , 2001 .

[13]  F. Bechstedt,et al.  Validity of effective-medium theory for optical properties of embedded nanocrystallites from ab initio supercell calculations , 2003 .

[14]  Gunnar A. Niklasson,et al.  OPTICAL PROPERTIES OF INHOMOGENEOUS TWO-COMPONENT MATERIALS , 1991 .