Structural degradation of tungsten sandwiched in hafnia layers determined by in-situ XRD up to 1520 °C

The high-temperature stability of thermal emitters is one of the critical properties of thermophotovoltaic (TPV) systems to obtain high radiative power and conversion efficiencies. W and HfO₂ are ideal due to their high melting points and low vapor pressures. At high temperatures and given vacuum conditions, W is prone to oxidation resulting in instantaneous sublimation of volatile W oxides. Herein, we present a detailed in-situ XRD analysis of the morphological changes of a 3-layer-system: HfO₂/W/HfO₂ layers, in a high-temperature environment, up to 1520 °C. These samples were annealed between 300 °C and 1520 °C for 6 h, 20 h, and 40 h at a vacuum pressure below 3 × 10^–6 mbar using an in-situ high-temperature X-ray diffractometer, which allows investigation of crucial alterations in HfO₂ and W layers. HfO₂ exhibits polymorphic behavior, phase transformations and anisotropy of thermal expansion leads to formation of voids above 800 °C. These voids serve as transport channels for the residual O₂ present in the annealing chamber to access W, react with it and form volatile tungsten oxides. An activation energy of 1.2 eV is calculated. This study clarifies the limits for the operation of W-HfO₂ spectrally selective emitters for TPV in high-temperature applications.