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Title: Role of defects on the surface properties of HfC

Abstract

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/m 2 to 4.75 J/m 2 and decreased the surface energy of the carbon terminated (111) surface from 8.75 J/m2 to 3.48 J/m 2. 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.

Authors:
 [1];  [1];  [1];  [2];  [1]
  1. Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
  2. Applied Physics Technology (APTech), McMinnville, OR (United States)
Publication Date:
Research Org.:
Sandia National Lab. (SNL-NM), Albuquerque, NM (United States)
Sponsoring Org.:
IARPA
OSTI Identifier:
1559513
Report Number(s):
SAND2019-9394J
Journal ID: ISSN 0169-4332; 678377
Grant/Contract Number:  
AC04-94AL85000
Resource Type:
Accepted Manuscript
Journal Name:
Applied Surface Science
Additional Journal Information:
Journal Volume: 495; Journal Issue: C; Journal ID: ISSN 0169-4332
Publisher:
Elsevier
Country of Publication:
United States
Language:
English
Subject:
36 MATERIALS SCIENCE

Citation Formats

Rimsza, Jessica M., Foiles, Stephen, Michael, Joseph, Mackie, William, and Larson, Kurt. Role of defects on the surface properties of HfC. United States: N. p., 2019. Web. doi:10.1016/j.apsusc.2019.07.242.
Rimsza, Jessica M., Foiles, Stephen, Michael, Joseph, Mackie, William, & Larson, Kurt. Role of defects on the surface properties of HfC. United States. doi:10.1016/j.apsusc.2019.07.242.
Rimsza, Jessica M., Foiles, Stephen, Michael, Joseph, Mackie, William, and Larson, Kurt. Fri . "Role of defects on the surface properties of HfC". United States. doi:10.1016/j.apsusc.2019.07.242.
@article{osti_1559513,
title = {Role of defects on the surface properties of HfC},
author = {Rimsza, Jessica M. and Foiles, Stephen and Michael, Joseph and Mackie, William and Larson, Kurt},
abstractNote = {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.},
doi = {10.1016/j.apsusc.2019.07.242},
journal = {Applied Surface Science},
number = C,
volume = 495,
place = {United States},
year = {2019},
month = {11}
}

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This content will become publicly available on November 1, 2020
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