Comparative Study of The Reactivity of the Tungsten Oxides WO<sub>2</sub> And WO<sub>3</sub> With Beryllium at Temperatures up to 1273 K

Tungsten oxides play a pivotal role in a variety of modern devices e.g. switchable glasses, wastewater treatment and modern gas sensors and metallic tungsten is used as armour material for e.g. gas turbines and future fusion power devices. In the first case you want to keep the oxide as functional material, while in the second case oxides can lead to catastrophic failures and you want avoid oxidation of tungsten. In both cases it is crucial to understand the stability of the tungsten oxides against chemicals. In this study the different reactivity of tungsten oxides towards the highly oxophilic beryllium is studied and compared. Tungsten--(IV)--oxide and tungsten--(VI)--oxide layers are prepared on a tungsten substrate. In the next step a thin film of beryllium is evaporated on the samples. In consecutive steps the sample is heated in steps of 100 K from r.t. to 1273 K. The chemical composition is investigated after each experimental step by high resolution X-ray photoelectron spectroscopy (XPS) of all involved core levels as well as the valence bands. A model is developed to analyse the chemical reactions after each step. In this study, we found the tungsten trioxid is reduced already by beryllium at r.t. and starts to react towards the ternary compounds BeWO_3 and BeWO_4 at temperatures starting from 673 K. However, the tungsten dioxide sample is reduction resistant to tempartures up to 1173 K. In conclusion, we found the WO_2 surface to be much more chemical resistant towards the reduction agent Be than WO_3.