(380h) Tungsten Nitride ALD Nanofilms for Reducing Hydrogen Diffusion

Nuclear thermal propulsion is an attractive in-space propulsion technology due to its large specific impulse about twice that of a chemical combustion engine. This specific impulse is achieved by using energy released from uranium fission to heat hydrogen gas to over 2400ºC before expelling it out a supersonic nozzle. However, hydrogen diffuses into the nuclear fuel elements and embrittles them, hindering the use of nuclear thermal engines. Here, we investigate the use of nanoscale tungsten nitride (WN) films as hydrogen environmental barrier coatings to address this issue.

Ultrathin WN films were deposited by atomic layer deposition (ALD) on zirconia nanoparticles and yttria stabilized zirconia (YSZ) micropowders using a fluidized bed reactor. The as-deposited film contained tungsten (W) in a variety of oxidation states, but was shown to dissociate nitrogen and form a crystallized W film upon annealing. The film’s intended use as a hydrogen barrier coating motivated the investigation of hydrogen diffusion in W using density functional theory (DFT) and differential thermal analysis (DTA). The diffusion pathway was determined from the lowest energy adatom site and the hydrogen charge state at various sites along the pathway were calculated. DTA was used to investigate the efficacy of the W/WN films in preventing hydrogen attack experimentally. The temperature at which hydrogen reacted with the sample increased with film thickness, thereby indicating that the film inhibited this reaction. However, the existence of a hydrogen reaction peak in the thicker film indicated a material with greater than 2.0 eV energy barrier is necessary for a substantial decrease in hydrogen interaction with the substrate. This is the first study for ALD of WN on particles from bis(t-butylimido)bis(dimethylamino)tungsten(VI) and the use of WN ALD films as environmental barrier coatings (EBCs) for high-temperature H₂ above 1000^oC.