Radiation selective absorber coating for an absorber pipe, absorber pipe with said coating, and method of making same

A method of making an absorber pipe (13), said method comprising the steps of:

• a) providing a first oxide barrier layer (24a) on an outer surface of a steel pipe (1) by thermal oxidation, said first barrier layer (24a) consisting of oxidized components of the steel pipe;
• b) applying a second barrier layer (24b) over said first oxide barrier layer (24a) by a gas-phase physical deposition (PVD) of aluminum while supplying oxygen, said second barrier layer (24b) consists of at least one aluminum oxide compound and said at least one aluminum oxide compound are each of the formula AlxOy, wherein x=1 or 2 and y=1, 2, or 3;
• c) applying an infrared-range reflective layer (21) over the second barrier layer (24b) and the first barrier layer (24a) by a gas-phase physical deposition of a metal selected from the group consisting of gold, silver, platinum and copper;
• d) applying an absorption layer (22) over the infrared-range reflective layer (21) by a simultaneous gas-phase deposition of aluminum and molybdenum;
• and
• e) applying an antireflection layer (23) over the absorption layer (22) by a gas-phase physical deposition of silicon while supplying oxygen.

An absorber pipe (13), especially for a parabolic trough collector, said absorber pipe (13) comprises a steel pipe (1) with an outer surface and a radiation selective absorber coating on said outer surface of said steel pipe (1);

• wherein said radiation selective absorber coating comprises at least two barrier layers (24a, 24b) arranged over each other on said outer surface of said steel pipe (1), an infrared-range reflective layer (21) arranged on said at least two barrier layers (24a, 24b), an absorption layer (22) arranged over said infrared-range reflective layer (21) and an antireflection layer (23) arranged over said absorption layer (22);
• wherein said radiation selective absorber coating includes an additional barrier layer (24c) arranged between the infrared-range reflective layer (21) and the absorption layer (22);
• wherein said at least two barrier layers comprise a first barrier layer (24a) and a second barrier layer (24b), said first barrier layer (24a) consists of oxidized components of the steel pipe and is made by thermal oxidation of the steel pipe and said second barrier layer (24b) consists of at least one aluminum oxide compound and said at least one aluminum oxide compound are each of the formula AlxOy, wherein x=1 or 2 and y=1, 2, or 3;
• and
• wherein said additional barrier layer (24c) comprises at least one other aluminum oxide compound and said at least one other aluminum oxide compound are each of the formula AlxOy, wherein x is 1 or 2 and y is 1, 2, or 3.