Cylindrical hollow magnetron cathode. Al-N selective coatings for solar collector absorbers

Hollow magnetron cathodes are particularly useful in coating parts with cylindrical symmetry and when deposition must be centripetal; this is the case when coating the outside of tubular and long substrates. A hollow magnetron cathode and its vacuum system has been designed and constructed to coat 2m long heat pipe absorbers and fluid pipe absorber tubes with a selective film of aluminum nitride. The pipe absorbers will be used in vacuum and non-vacuum compound parabolic concentrators (CPC) of different models for thermal solar collectors. The long copper substrates are displaced through the DC reactive glow discharge inside the hollow cathode. Coating occurs in a single run. Adequate values of discharge voltage and pressure were chosen and the relative nitrogen dose was optimized to get maximum absorbance of the film in the solar spectrum for several substrates. Reflectivities of less than 1% at the wavelength of 550nm and excellent adhesion were obtained. In these last tests the pull method has been used.