Optical properties and solar selectivity of coevaporated Cohyphen;Al2O3composite films

摘要

Cohyphen;Al2O3composite films were produced by electronhyphen;beam coevaporation in a system with elaborate process control. The deposits were analyzed by transmission electron microscopy, electron diffraction, Auger electron spectroscopy, secondary ion mass spectroscopy, Rutherford backscattering, field ion microscopy, mechanical stylus measurements, and electrical dc and ac measurements. A uniform separatedhyphen;grain structure with regular hcp Co particles embedded in Al2O3was found for Co contents (fCosrsquo;) up to sim;30 volthinsp;percnt;. The complex dielectric permeability egr;macr; was evaluated in the 0.3lsim;lgr;lsim;40hyphen;mgr;m wavelength range for samples with 0.11lsim;thinsp;fColsim;0.60 by carefully selected combinations of spectrophotometric transmittance and reflectance data. Numerical accuracy and internal consistency were investigated. Effective medium theories for egr;macr; were derived by applying classical scattering theory to spherical random unit cells defined so as to properly represent a number of typical microgeometries. The formulations due to Maxwell Garnett, Bruggeman, and others were thus rederived in a unified way. Largehyphen;size limits of validity and extensions to nonspherical particles were treated. It was found that the Maxwell Garnett theory could reproduce the experimental egr;macr;rsquo;s in detail at lowfCoprovided that some Co was taken to be dispersed in the insulating matrix. At largerfCorsquo;s we found discrepancies which are likely to be due mainly to dipolehyphen;dipole coupling among adjacent particles. A comparison of the experimental egr;macr;rsquo;s and the rigorous Bergmanndash;Milton bounds, which hold irrespective of detailed microgeometry, gave several interesting results: the egr;macr;rsquo;s varied monotonically along one of the bounds for isotropic materials asfCowas increased, at largefCowe found evidence for anisotropy, and at lgr;ape;12.5 mgr;m we noted certain cases of disagreement with even the most generous bounds. The empirical egr;macr;rsquo;s were used to construct surfaces which combine a high solar absorptanceaswith a low hemispherical thermal emittanceeH. From a computer optimization study we found thatas=0.95 andeH=0.07 could be obtained with 0.07 mgr;m of Cohyphen;Al2O3(thinsp;fCoape;0.6) antireflected with 0.07 mgr;m of Al2O3and laid on Ni. These results were verified by measurements on samples which approximate the ideal design.