Role of defects on the surface properties of HfC

Journal Article · 31 October 2019 · Applied Surface Science

DOI: https://doi.org/10.1016/j.apsusc.2019.07.242 · OSTI ID:1559513

Rimsza, Jessica M. ^[1]; Foiles, Stephen ^[1]; Michael, Joseph ^[1]; Mackie, William ^[2]; Larson, Kurt ^[1]

---

1. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
2. Applied Physics Technology (APTech), McMinnville, OR (United States)

HfC has shown promise as a material for field emission due to the low work function of the (100) surface and a high melting point. Recently, HfC tips have exhibited unexpected failure after field emission at 2200K. Characterization of the HfC tips identified faceting of the parabolic tip dominated by coexisting (100) and (111) surfaces. To investigate this phenomenon, we used density functional theory (DFT) simulations to identify the role of defects and impurities (Ta, N, O) on HfC surface properties. Carbon vacancies increased the surface energy of the (100) surface from 2.35 J/m² to 4.75 J/m² and decreased the surface energy of the carbon terminated (111) surface from 8.75 J/m2 to 3.48 J/m². Once 60% of the carbon on the (100) surface have been removed the hafnium terminated (111) surface becomes the lowest energy surface, suggesting that carbon depletion may cause these surfaces to coexist. The addition of Ta and N impurities to the surface are energetically favorable and decrease the work function, making them candidate impurities for improving field emission at high temperatures. Overall, DFT simulations have demonstrated the importance of understanding the role of defects on the surface structure and properties of HfC.

• Research Organization: Sandia National Laboratories (SNL-NM), Albuquerque, NM (United States)
• Sponsoring Organization: IARPA
• Grant/Contract Number: AC04-94AL85000
• OSTI ID: 1559513
• Report Number(s): SAND2019-9394J; 678377
• Journal Information: Applied Surface Science, Journal Name: Applied Surface Science Journal Issue: C Vol. 495; ISSN 0169-4332
• Publisher: ElsevierCopyright Statement
• Country of Publication: United States
• Language: English