RESUMEN
Herein, the work functions of group 4 and group 5 transition-metal (Ti, Zr, Hf, V, Nb, and Ta) carbides and transition-metal oxycarbides (TMCOs) were investigated by first-principles calculations for their potential application as electron emitters. The work functions of both groups decreased as the substitution of carbon atoms with oxygen proceeded, and the reduction in group 4 was more than that of group 5. In particular, ZrC1-x O x and HfC1-x O x (x ≥ 0.25) exhibited work functions of less than 3 eV, which were comparable with those of LaB6- and ZrO-coated tungsten. The reduction in the work functions could be explained by the rigid-band model of the electronic density of states. The increase in valence electrons increased the Fermi energy, while it demonstrated a less significant influence on the vacuum potential, resulting in a reduction in the work functions. The phonon dispersion curves indicated that the NaCl-type group 5 TMCOs were less stable than the group 4 TMCOs. This agrees with the experimental findings that TaC1-x O x was not synthesized and NbC1-x O x was synthesized only for smaller values of x (i.e., x < 0.28). From the viewpoints of the work functions and structural stabilities, group 4 (Ti, Zr, and Hf) TMCOs exhibit better potential for application as electron emitters than group 5 (V, Nb, and Ta) TMCOs.
RESUMEN
The electronic transport and field emission properties of a single-crystalline GdB44Si2 nanowire are studied. The atomic structure and elemental composition of the GdB44Si2 nanowire are characterized by transmission electron microscopy (TEM) using atomic imaging, energy-dispersive X-ray spectroscopy (EDS), and electron energy-loss spectroscopic (EELS) mapping. The electrical conductivity of the single GdB44Si2 nanowire is in the range of 46.8-60.1 S m-1. The in situ TEM field emission measurement reveals that it has a low work function of 2.4 eV. To realize a converged electron emission, a field evaporation pretreatment was used to clean the emission surface and to make a sharpened tip. The field emission probe measurement results show that the electron emission from the sharp GdB44Si2 nanowire is converged to a single field emission spot and it has a work function of 2.6 eV which is in agreement with the in situ TEM measurement. The stability of field emission current is also very good with a fluctuation of 1.4% in 20 min. With a low work function and stable emission current, the GdB44Si2 nanowire shows great promise for field emission applications.
RESUMEN
A single hafnium carbide (HfC) nanowire field-induced electron emitter with a sharp tip apex is fabricated by Pt deposition and focused ion beam (FIB) milling. The structure of the electron emitter is characterized by scanning transmission electron microscopy (STEM) and atom probe tomography (APT). The HfC nanowire is single-crystalline with a thin oxide layer on its tip surface. The field emission properties are determined by using both in situ transmission electron microscopy (TEM) and a field-emission probe in a high-vacuum chamber. A high current of 173 nA was obtained at a low extraction voltage of 631 V with an emission gap of 5 mm. The emission current is stable at 60 nA for 100 min with a fluctuation of 0.7%. The deduced work function was 3.1 eV. It is suggested that the implanted Ga ions and the oxide layer induce more downward dipoles that are beneficial for lowering the work function and creating a stable surface. When the low keV FIB processing is applied, it takes within 30 minutes to finish a HfC nanowire emitter, establishing an efficient procedure for the preparation of nanowire emitters. These results provide a controllable and fast production method for the fabrication of single nanowire field-emission point electron sources.
RESUMEN
The aim of this study was to investigate proton changes of the parotid gland after gustatory stimulation by semi-quantitative parameters and an empirical mathematical model (EMM) using high-temporal-resolution, double-echo, echo-planar imaging (EPI). Approved by a local institutional review board, this study examined 20 parotid glands from 10 healthy volunteers (male:female = 6: 4; age ± standard deviation =35.1 ± 14.1 years) with written informed consent obtained. All participants underwent 1.5-T, double-echo EPI with gustatory stimulation. Semi-quantitative parameters, including maximal drop ratio (MDR), time to peak (TTP), drop slope (DS), recovery slope (RS) and recovery ratio (RR), were calculated. The effect of temporal resolution on parotid functional parameters was evaluated. An EMM comprising an output function ( Sot=Aoe-kot+B) and an input function ( Sint=Ain1-e-kint) was also applied to fit all dynamic curves. Kruskal-Wallis test, Wilcoxon test, linear regression analysis and goodness of fit were used for statistical analysis. p < 0.05 was considered to be statistically significant. The signal intensity dropped significantly after gustatory stimulation on the proton density (PD) image (p < 0.01). MDR was 8.26% in the PD image. MDR and RR were negatively associated with time interval, whereas DS and TTP were significantly positively associated with time interval (all p < 0.05). EMM parametric values derived from PD-time curves of parotid glands were 12.04 ± 6.81%, 6.43 ± 4.23 min-1 , 88.73 ± 6.18%, 8.41 ± 4.86 min-1 and 1.09 ± 1.35 for Ao , ko , B, Ain and kin , respectively. Semi-quantitative functional parameters and EMM parameters using high-temporal-resolution, double-echo EPI allow the quantification of parotid proton changes after gustatory stimulation.