For deposition of the SS–AlN cermet selective surfaces, the vacuum chamber was pumped to a base pressure of 6–8×10−3 Pa, taking about 10 min. An SS–AlN cermet adhesion layer was first deposited on the glass substrate using the method described above. After this adhesion layer was deposited, the chamber vacuum improved to around 1.5–1.8×10−3 Pa. Afterwards, an Al infrared reflector layer was
deposited in pure argon gas at a pressure of 0.1 Pa. Then a high metal volume fraction SS–AlN cermet layer and a low metal volume fraction SS–AlN cermet layer were deposited in succession using the method as described above. Finally, an AlN anti-reflection layer was deposited on the low metal volume fraction cermet layer by dc reactive sputtering. It takes around 35–40 min to complete the deposition of the solar selective coatings onto each batch of tubes. The cycle time to coat 32 tubes is 60–70 min.