Stabilization of absorber stacks containing Zr or Ti compounds on Ag

A family of selective photothermal absorber stacks based on succesively deposited thin films of silver and a substoichiometric nitride, carbonitride, oxynitride or oxycarbonitride of zirconium or titanium has been investigated by the Bureau of Mines. The spectral selectivities are quite promising, especially because of very low infrared emittances at elevated temperatures. Long-wave emittances of 0.035 to 0.050 at 600 K and ratios of short-wave absorptance to long-wave emittance near 20 are common. However, improvements of absorptance and thermal stability are being sought. One problem that has been observed is a tendency for the silver reflecting films to deteriorate after a period of time at temperatures above 600 K. It appears feasible to counteract the deterioration and to stabilize the silver films with a light undercoating of metallic oxide. Oxides of Cr, Ti, Zr and stainless steel have been tried with encouraging results. Another study concerned the thermal stability of absorbing semiconductor films. The absorbing films oxidize slightly in air at 400 K, but titanium compounds are more stable in air than zirconium compounds. In a good vacuum, most absorbing films are reasonably stable at temperatures up to about 800 K, and carbon apparently enhances the stability. A zirconium carbonitride film had the best combination of absorptance and thermal stability among the absorbers examined.