Solar absorptance and thermal emittance of cermets with large particles

We evaluate, theoretically, the solar absorptance and thermal emittance of cermets with large particles. In particular, we consider Co and Ni particles with radii from 0.05 to 0.13 µm, embedded in binders of alumina and silica, respectively. For these particle sizes, it is necessary to consider the effect of the multiple scattering of light in the material for the visible and near infrared parts of the spectrum. The response of the particles is calculated using the Lorenz-Mie theory, and the multiple scattering effects are taken into account through a four-flux radiative transfer model. In the medium infrared, the Lorenz-Mie theory cannot be used because of the high absorption of the binder. The reflectance of the cermets in this regime is calculated using the Maxwell-Garnett effective medium approximation. In general, moderate values of the absorptance-to-emittance ratio are found.