  Experimental observations of thermionic electron emission demonstrate a
smooth transition between TL and FSCL regions of the
emitted-current-density-versus-temperature (J-T) (Miram) curve and the
emitted-current-density-versus-voltage (J-V) curve. Knowledge of the
temperature and shape of the TL-FSCL transition is important in evaluating the
thermionic electron emission performance of cathodes, including predicting the
lifetime. However, there have been no first-principles physics-based models
that can predict the smooth TL-FSCL transition region for real thermionic
cathodes without applying physically difficult to justify a priori assumptions
or empirical phenomenological equations. Previous work detailing the nonuniform
thermionic emission found that the effects of 3-D space charge, patch fields,
and Schottky barrier lowering can lead to a smooth TL-FSCL transition region
from a model thermionic cathode surface with a checkerboard spatial
distribution of work function values. In this work, we construct a
physics-based nonuniform emission model for commercial dispenser cathodes for
the first time. This emission model is obtained by incorporating the cathode
surface grain orientation via electron backscatter diffraction (EBSD) and the
facet-orientation-specific work function values from density functional theory
(DFT) calculations. The model enables construction of two-dimensional emitted
current density maps of the cathode surface and corresponding J-T and J-V
curves. The predicted emission curves show excellent agreement with experiment,
not only in TL and FSCL regions but, crucially, also in the TL-FSCL transition
region. This model improves the understanding on the relationship between
thermionic emission and cathode microstructure, which is beneficial to the
design of vacuum electronic devices.
