To test the performance of the field emission and measurement of current level, during the experiment,
the two MWCNT vacuum devices, a high vacuum chamber, and the learn more tip-off system were connected to the same vacuum level. MWCNT for the vacuum gauge was packaged by tip-off through a vacuum system at a pressure of 1.3 × 10-6 Torr. The vacuum gauge output was measured by using a source meter (Keithley 2400, Cleveland, OH, USA) and LabVIEW software (National Instruments Corp., Austin, TX, USA). Figure 1 Structure of MWCNT device and FE-SEM image of MWCNT paste after heat treatment. (a) Structure of the MWCNT device. (b) FE-SEM image of MWCNT paste printed on ITO glass substrate after heat treatment. Figure 2 Schematic of the high vacuum chamber with tip-off system. Results and discussion Figure 3a shows the field emission characteristic of printed CNT before and after vacuum packaging. The turn-on field required to reach a current density AICAR mouse of 10 μA/cm2 was 2.54 V/μm (610 V) and 2.5
V/μm (600 V) with tip-off (Sample 1) and vacuum chamber (Sample 2) processes, respectively. Figure 3b shows the Fowler-Nordheim (F-N) plot (ln(I/V 2 ) versus 1/V) and nonlinear slopes. At an applied voltage of 950V, the emission current of MWCNT film decreased from 0.9 to 0.7 mA after the tip-off. The reasons for this could be explained by vacuum level change due to outgassing inside the flat panel during tip-off process. Figure 3 Current versus voltage properties for the printed MWCNT paste film (a). The F-N plots (b). Figure 4 exhibits the plot of the current versus time of the packaged see more device which was loaded in the vacuum chamber tip-off system (Sample 1). In this experiment, applied voltage to the vacuum gauge was 1 V. The measurement of the current was initiated after saturation was reached by the rotary pump and the turbo pump. As the gauge was heated by the tip-off heater from 2,000 to 2,300 s, the current increased after heater was turned on and decreased gradually following the turning-off of the heater. This phenomenon can be probably explained by the fact that there is limit in the amount of outgas that can be removed by the pumps. When the vacuum
status approached many to 1.2 × 10-6 Torr, the device was tipped off. The tip-off process was as follows: glass tip was located on the heater, which was in the vacuum chamber, and heated. The heater made the temperature exceed the melting point of the glass in a few minutes. At this instance, melted glass was held together for a short time to close the glass tip and separated from the vacuum pump. The outgas generated by heating and field emission resulted in the increase of the current, i.e., the current increased upon exposure to field emission outgases. Figure 4 Current changes of the MWCNT device during tip-off process. Figure 5 shows the current of the MWCNT vacuum gauge at the device versus time inside high vacuum chamber (Sample 2).