Spectrally selective absorber coatings, deposited on engineering material substrates such as
stainless steel, have been developed for service as efficient solar photothermal energy converters.
The selective solar absorber is based on a multilayer of thin films, produced by sputtering. The
main solar absorber is a metal/ceramic (cermet) composite, such as, Mo/Al2th or Mo/Si02, with a
graded metal concentration. Such a cermet layer, strongly absorbs radiation over most of the range
of the solar spectrum but is transparent to longer wavelength radiation. The cermet layer is
deposited on a highly reflecting infrared metal layer. Two more layers were added: An AhO
diffusion barrier layer which is deposited first on the substrate and an AI2O or a Si02 antireflection
layer which is deposited on the top of the cermet film. In order to better understand the spectral
reflectivity of the multilayered selective coating, a procedure for the calculation of the optical
properties was developed.
After the R&D development phase was successfully completed, a full scale production
coating machine was constructed. The production machine is a linear in line coater. The selective
coating is deposited on stainless steel tubes, translating in the coating machine while rotating about
their axes, along their axial direction. Measurements of reflectance, solar absorptivity, a, thermal
emissivity, C, and high temperature durability, are all parts of the quality control routine. The
results show values of a in the range 0.96 - 0.98. The thermal emissivity at 350CC is in the range
0.16 - 0.18. Thermal durability tests, show no degradation of the coating when subjected to up to
65O in vacuum for one month and when passed through a temperature cycling test which
includes 1200 cycles between temperatures of 150CC and 450CCfor a period of two months.