# Investigation of In Situ and Ex Situ Passivation of Pyrophoric Uranium-Niobium Alloy Powder
## 4 Conclusions  
In situ and ex situ passivation methods were evaluated for their effectiveness in mitigating the pyrophoricity of U-6 Nb powders processed via the hydride-dehydride process and plasma spheroidization. Ex situ passivation produced uniform oxide layers (-20 nm) with consistent non-pyrophoric behavior, while in situ passivation resulted in variable oxide thicknesses (less than 10 nm to greater than 200 nm) and occasional pyrophoricity at lower oxygen flow rates. Despite requiring higher bulk oxygen content, in situ passivation enabled faster processing and safer handling immediately after production.  
Figure 4: HAADF STEM (**a**,**b**,**d**,**e**) and EDX (**c**,**f**) images of passivated particles: (**a**–**c**) ex situ-passivated particle with a thin, consistent oxide layer; (**d**,**f**) 5 SLPM in situ-passivated particle with a thick, variable oxide layer.  
While both in situ and ex situ passivation can create non-pyrophoric U-6 Nb powder, the trade-offs between the two methods emphasize the importance of application-specific considerations. Ex situ passivation ensures a uniform oxide layer with lower oxygen incorporation, while in situ passivation offers operational efficiency and precise control of bulk oxygen content. These findings can significantly enhance safety standards and material handling protocols for reactive metal powder production, supporting wider industrial and research applications.  
Future efforts should focus on reducing the variability of in situ passivation by optimizing plasma chamber conditions to improve oxide layer uniformity. For ex situ passivation, the development of monitoring and control techniques for oxygen exposure and temperature control to decrease the time required to passivate and regulate oxide growth more precisely could enhance its reliability and scalability. Balancing safety, efficiency, and material performance remains critical for advancing the use of reactive metal powders in emerging applications.  
The following supporting information can be downloaded at [[https://www.mdpi.com/article/10.3390/app15126431/s1](https://www.mdpi.com/article/10.3390/app15126431/s1)]([https://www.mdpi.com/article/10.3390/app15126431/s1](https://www.mdpi.com/article/10.3390/app15126431/s1)), Video S1: Sparkle test; Video S2: Plasma chamber; Figure S1: Sparkle test apparatus; Figure S2: Powder X-ray diffraction of in situ-passivated, ex situ-passivated, and unpassivated U-6 Nb powder.  
Conceptualization, E.M.C., H.B.H., R.L.S., and K.H.; investigation, E.M.C., H.B.H., E.S.E., T.T.L., L.D.W., R.L.S., and K.H.; writing--original draft preparation, E.M.C.; writing--review and editing, E.M.C., H.B.H., E.S.E., I.R.C., O.G.L., S.L.H., and K.H.; visualization, E.M.C.; supervision, K.H., and J.R.J.; project administration, R.L.S., and K.H.; funding acquisition, J.D.K. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
The original contributions presented in this study are included in the article/Supplementary Materials. Further inquiries can be directed to the corresponding author.  
This manuscript has been authored by Lawrence Livermore National Security, LLC, under Contract No. DE-AC52-07 NA27344 with the US. Department of Energy. The United States Government retains, and the publisher, by accepting the article for publication, acknowledges that the United States Government retains, a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.  
The authors declare no conflicts of interest.