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Selective coating stable in air at 500ºC for stainless steel absorber pipes

CIEMAT has developed a new selective coating for stainless steel pipes. This coating is stable in air at 500ºC. Samples of this new coating have been tested in an open oven at 500ºC for six months and no degradation was detected because the absorptance and emittance remained unchanged after this period. Selective coatings commercially available at present for working temperatures of up to 400ºC are not stable in hot air and a rapid degradation occurs when the glass envelope is broken and the vacuum conditions between the steel pipe and the glass cover is lost.

The coating developed by CIEMAT is manufactured using the inexpensive technique of Sol-gel, using platinum as both infrared reflector and metallic component for the cermet. The amount of platinum is very small and therefore the price of this noble metal is not a barrier to be cost-effective.

The structure of the complete absorber is composed of several layers. First of all, nickel-tungsten and silica layers are used to protect stainless steel substrate against thermal degradation and the diffusion barrier is employed to avoid platinum diffusion in the former layers due to high temperature. Platinum reflective layer is used to assure low thermal emittance of the absorber and two platinum-alumina cermets, with different metal content, are the absorber of solar radiation. Finally a silica antireflective layer is deposited over the absorber to reduce front surface reflection in the absorber and to optimise solar absorptance.

Solar absorptance (AM1.5) of the coating developed is 0.95 and thermal emittance is 0.15 at 500ºC.

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Reported by

CIEMAT - Avda. Complutense 22
28040 Madrid
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