Zirconium Nitride Coating Fabrication via Fluidized Bed Chemical Vapor Deposition
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In order to reduce the transport and use of high enriched uranium for civilian applications, research reactor fuels are being converted to use low enriched uranium under the Reduced Enrichment for Research and Test Reactors program. A uranium-molybdenum (U-Mo) dispersion type fuel is under investigation for the higher performance reactors that require a higher uranium density than can be provided with current fuel technology. Interactions between U-Mo and the aluminum matrix necessitate the use of a diffusion barrier on the particle surfaces. A fluidized bed chemical vapor deposition system with an inverted reactor was used to create barrier coatings of zirconium nitride on the surface of U-xMo microspheres. The process utilized the metalorganic precursor tetrakis(dimethylamino)zirconium heated in a Swagelok stainless steel sample cylinder to 51 ± 2 °C. Experiments were performed over three phases: preliminary tests, system modifications, and parametric studies. The time-dependent studies analyzed coating produced after 2-8 days of operation at 100 mL/min precursor carrier flow rate and 500 mL/min fluidization flow rate. Flow rate-dependent studies produced coated samples using a total flow rate of 600 mL/min, with the precursor carrier flow rate ranging from 100-300 mL/min and the fluidization flow rate adjusted accordingly. Ultra high purity argon (99.999%) was used for the precursor carrier and fluidization gas. The CVD reaction was carried out at 280 ± 10 °C with precursor transport tubes heated to 60-75 °C. Coatings were qualitatively characterized using energy dispersive X-ray spectroscopy, wavelength dispersive X-ray spectroscopy, and X-ray distribution mapping. Zirconium-based coatings up to 2.2 ± 0.3 μm thick after 2 days of deposition. The estimated coating thickness was not significantly impacted by extending the duration of the deposition process or increasing the precursor carrier gas flow rate. Despite apparent precursor transport throughout the duration of the experiment, the majority of the coating deposition seems to have occurred within the first 24 hours of the experiment. Imaging of the microsphere cross-sections provided evidence of the formation of uranium oxide, zirconium oxide, and zirconium nitride layers on the surface of the particles, with nitrogen deposition becoming more favorable further from the bulk U-Mo surface.
Sudderth, Laura (2015). Zirconium Nitride Coating Fabrication via Fluidized Bed Chemical Vapor Deposition. Master's thesis, Texas A & M University. Available electronically from