applied sciences  
Article
Thermal Stability of Sputtered Tungsten Nitrides for Solar Thermal Applications [PERSON] 1,*(c), [PERSON] 1 and [PERSON] 2  
1 ENEA, TERIN-STSN-SCIS, [PERSON] 1, 80055 Portcial, Italy 2 Physics Department \"[PERSON]. [PERSON]\", University of Salerno, Via Giovanni Paolo II, 132, 84084 Fisciano, Italy 2 Correspondence: [EMAIL_ADDRESS]; Tel. : +39-(0)817723358  
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good performance in terms of photo-thermal conversion of solar energy [4]. The choice of tungsten nitrides as constituents of double nitride cermet is due to their metallic behavior and to the scarce wettability of these compounds with the ceramic insulating AlN matrix, useful for spheroidizing the metallic component during the co-sputtering fabrication process and hence obtaining a good cermet. The whole process of \"double nitride\" formulation has been based on the chemistry of simultaneous nitridation of transition and earth alkaline metals [5]. Tungsten nitrides being a large family of compounds, their properties depend on the exact stoichiometry [6] and phase. Their crystal structure consists of a fcc sub-lattice of W atoms. Half the octahedral holes are filled with N atoms in the \(\beta\)-W\({}_{2}\)N structure. All the octahedral holes are filled with nitrogen in the \(\delta\)-WN structure. These arrangements produce a cubic crystal lattice for \(\beta\)-W\({}_{2}\)N and a hexagonal lattice structure for \(\delta\)-WN. Extra N atoms occur at grain boundaries between tungsten nitride crystallites. In Table 1, various structure types and phases are reported. As the receiver fabrication technology requires that the spectral selective coating works under a good vacuum at medium-to-high temperatures (673 K for diatomic oil plants, 823 K for molten salts ones), it needs a very high structural, physical and chemical stability in these conditions, without modification of the double nitride (AlN-WN) optical properties due to phase transitions of the constituents. In consideration of the fact that the cermet matrix AlN has already been well characterized [7] and the influence of process parameter on fabrication of WN has been the topic of a previous work [8], the aim of this paper is to add new studies on the thermal stability of tungsten nitride in vacuum. The thermal stability in air of thin films of tungsten nitrides and their characteristic nitrogen loss have been studied at temperatures of 473 K, 673 K and 873 K by [PERSON] [9] with the aim of using these materials as diffusion barriers in microelectronics or as electrodes in semiconductor devices, and by [PERSON] and [PERSON] [10] at temperatures of 1200 K for investigating nitrogen release. Moreover, tungsten nitrides have been attracting a lot of attention in other applications, such as water reduction catalysts in hydrogen production water splitting processes; for promoting hydrogen evolution reactions (HER) in alkaline media, replacing the much more expensive platinum [11]; as material for capacitive energy storage devices [12]; and for plasma facing material for fusion applications [13]. For example, in catalyst applications it is important to achieve a balance between electrical conductivity and energetics of absorption/desorption of reaction intermediates in HER activities, both being related (albeit inversely) to nitrogen content [14]. In this field, it can also be interesting to increase the density of active sites and to test conductivity and stability at high temperatures. As shown in the work mentioned above [8], which investigated the process parameters of tungsten nitrides' sputtering deposition, it is possible to obtain different WN\({}_{\text{x}}\) phases over a large composition range of x (from 0.5 to 2.0) and changes in electrical properties from conductive to insulating ones.