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.
We present a cheaper and environmentally friendly method to fabricate efficient spectrally selective solar absorber
materials. The sol-gel technique was used to fabricate carbon-silica (C-SiO2) and carbon-nickel oxide (C-NiO)
composite films on aluminium substrates. UV-Vis and FTIR spectrophotometers were used to determine the solar
absorptance and thermal emittance of the coatings. C-NiO coatings gave the best spectral characteristics. We show that
it is possible to achieve a solar absorptance of 0.94 and a thermal emittance of 0.12. So far, to our knowledge, no
commercial spectral solar absorber has these spectral responses.
Selective solar absorber coatings of carbon dispersed in SiO2, ZnO and NiO matrices on aluminium substrates have been
fabricated by a sol-gel technique. Spectrophotometry was used to measure the near-normal reflectance of the composite
coatings. Calculations of absorbed and emitted power, power retention, solar absorptance and thermal emittance were
performed from the reflectance curves. The root-mean-square (rms) deviations of the reflectance curves from the ideal
case were computed to determine the sample with the best performance characteristics. The thermal emittances of the
samples were 30% for the SiO2, 15% for the ZnO and 10% for the NiO matrix materials. The solar absorptances were
90%, 89% and 93% for SiO2, ZnO and NiO samples, respectively. Based on the results, NiO matrix samples had the
best solar selective behaviour, followed by ZnO and last were the SiO2 based samples.
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