Solar selective absorber based on zinc oxide-nickel cermet

The efficiency of solar thermal collectors strongly depends on the optical properties of solar selective materials used in their design since the solar absorber surfaces should have high solar absorptance in the wavelength range of 0.3–2.5 µm and low thermal emittance in the infrared region (beyond 2.5 µm). Different design principles are developed for achieving of spectral selectivity. They are creation of intrinsic selective absorbers, multilayer absorbers, absorbers based on quantum size effect or on textured surfaces, absorber-reflector tandems (semiconductor-metal tandem or composite coatings), heat mirrors [1]. Different techniques were used for the obtaining of selective solar absorber surfaces such as sputtering, evaporation, electroplating, or they can be sprayed or spread as a paint [1]. Among the above mentioned designs of selective surfaces the composite coatings (also called cermets) attract significant attention due to their strong absorption in the solar spectrum range and transparency in the infrared. Usually, the cermet coatings consist of nanoscale metal particles embedded in a dielectric matrix, generally SiO
2
, Al
2
O
3
, and MgO, deposited on a highly infrared reflecting metal substrate like bulk Cu and Al plates or coated by Al or Cu thin films glass substrates. Particles of metals with high melting points such as Cu, Au, Ni, Mo, Cr, Co, etc. are commonly used as filler materials with dielectric matrixes. The absorbing cermet layer may have either uniform or graded metal content. An antireflective coating can be deposited on the top of cermet stack with the aim to reduce the refractive index mismatch between air and the absorbing layer. Intrinsic high resistive zinc oxide ZnO can be used as alternative to above mentioned conventional ceramic matrix materials [2]. ZnO has attracted increasing interest due to its unique ability to form a variety of nanostructures, especially in the form of vertically arranged one-dimensional (1D) nanorod arrays. The various methods have been developed for production of the well-ordered 1D ZnO nanostructures which are suitable for applications in various devices such as short-wavelength lasers, sensors, photocatalytic systems and for solar cells of third generation [2]. In comparison with high vacuum technologies pulse electrochemical deposition has high potential due to a more efficient material consumption and low investment costs. It is possible to control the preferred orientation of ZnO films and their morphology by changing such pulse parameters like the cathodic average current density or cathode on/off potentials, the pulse length, the pulse shape and the pulse period.