## 3 Results  
The fabrication of tungsten nitrides by means of reactive sputtering has been the topic of a previous study on the influence of the deposition power and nitrogen content on the properties of thin films as they are grown. Large variation in the electrical resistivity as a function of the power supplied to the W target has been observed, in the direction of an increase of metallic character with power, as expected in different regimes of operating target during nitridation. The metallic character of WN films increases and the crystal content decreases with increasing deposition power from 300 to 700 W. In particular, from a structural point of view, the 300 W film is crystalline with a cubic distorted lattice of W\({}_{2}\)N hosting nitrogen in interstitial sites. The 500 W target power determines a highly crystalline growth of W\({}_{2}\)N with a small amount of amorphous phase. When power is increased to 700 W, an amorphous phase appears together with the W\({}_{2}\)N phase. This behavior has been defined as an effect of re-sputtering phenomena that occur at higher powers. Stability being an important requisite of a solar thermodynamic coating, the object of previous work has been to select ideal experimental conditions for maximizing the content of a thermodynamically stable fcc phase of W\({}_{2}\)N. In any case, the various phases--and in particular \(\delta\)-WN and the porous nitrides brought about when nitrogen is in excess--seem to have better catalytic performance in HER [15]. Moreover, there are material applications where different phases coexist, for example, the graded cermet AlN-WN, utilized as an absorber in the spectrally selective double nitride coating previously mentioned. In the optimized reactive co-sputtering cermet fabrication process, starting from 700 W (used for the cermet layer in proximity to the metallic back reflector) and arriving at 300 W (used in the fabrication of cermet of low metallic content in proximity to the antireflective layer) various powers are supplied to the W target. This means that different WN\({}_{\mathrm{x}}\) phases can be originated and extra N atoms can be entrapped in lattices and/or at grain boundaries of these graded double nitride absorbers. For the purpose of studying the stability of tungsten nitride particles inside graded cermet in operating conditions of receiver tubes, single tungsten nitrides films, WN\({}_{\mathrm{x}}\), and thin films of AlN-WN cermet at three different powers (300 W, 500 W and 700 W) were produced, vacuum annealed and characterized. It is important to outline that the samples deposited at 300, 500, and 700 W were all of a similar thickness (about 100 nm). Single WN\({}_{\mathrm{x}}\) films were subjected to two consecutive thermal treatments, at 470 \({}^{\circ}\)C for 8 h and at 580 \({}^{\circ}\)C for 6 h in vacuum, and studied for the application of receiver tubes at medium (400 \({}^{\circ}\)C) and high (550 \({}^{\circ}\)C) temperatures, for the purpose of verifying the materials' thermal stability and to promote gas-entrapped desorption. The sheet resistance (Figure 1) of the 300 W film increased after annealing, suggesting the film's less conductive nature after annealing. In the 500 W and 700 W films, sheet resistance remained almost unchanged. The UV-VIS-NIR reflectance spectra analysis of films before and after the two annealing steps (Figure 2) shows that there were modifications in the optical films' properties. Such modifications were produced by the first annealing in the case of 300 W and 500 W, and were progressive in the 700 W film. In particular, for 300 W, there was a 2% decrease in the NIR reflectance only, starting from 1500 nm, due to a decrease in its metallic character  
Figure 1: Sheet resistance versus annealing time. (maybe due to W oxidation).