A uniform film of the CNT/metal binder mixture with the thickness of approximately 20 μm was prepared on the copper tip after an annealing process at 900°C (Figure 6a). The magnified FESEM images of the CNT/metal
binder mixture (Figure 6b) show that vertically standing CNTs of different heights (Figure 6c) as well as CNTs lying on the side (Figure 6d) were formed on the surface. One end of the vertically standing CNTs was generally embedded in the binder film, suggesting strong adhesion to the coating. In contrast, agglomerates of amorphous carbons or CNTs (rectangular regions in Figure 6d) that were not bound to the coating materials were also observed. The agglomerates of amorphous carbons or CNTs were 3-deazaneplanocin A attributed to an incomplete purification process that was described in the ‘Methods’ section. These agglomerates exert negative effects on the stable operation of the field emitter. Figure 6 FESEM images of the fabricated CNT emitter on a copper tip substrate.
(a) FESEM image of a CNT/metal binder coated on a copper tip substrate using the metal mixture binder annealed at 900°C. (b) Cetuximab order Magnified FESEM image of the CNT/metal mixture binder shown in (a). (c, d) Magnified FESEM images of the regions marked in (b). In order to remove the loosely bound carbon agglomerates, the as-prepared CNT emitters were treated with electrical conditioning processes . Electrical conditioning is a process to induce arcing intentionally to remove the materials that negatively affect field emission. An electrical conditioning process was carried out by increasing the applied electric field at the emitters by 0.033 V/μm
(corresponding to 500 V in these experiments) to 0.83 V/μm (Figure 7a). The electric field at each step was maintained for 5 min, and three runs of the conditioning processes were performed for each CNT field emitter. It should be noted that the electric field (abscissa) shown in Figure ioxilan 7a was calculated by dividing applied voltage by the emitter-anode distance. However, actual electric fields are much higher than the abscissa values. This is because small metal tips (diameter, 1 mm) were used as the substrates of CNT emitters in our experiments and such small metal tips produce higher electric field than a flat substrate at the same applied voltage . While the electric field was increasing, many arcing events occurred because loosely bound materials on the surface were removed by the strong electric field [14–16]. After three runs of electrical conditioning processes, the loosely bound materials shown in Figure 6d were almost completely removed (Figure 7d). Meanwhile, arcing events inevitably occur during the field emission at emission current densities higher than a critical density of approximately 50 mA/cm2[22, 23]. This is because emitting CNTs are self-heated due to Joule heating, which can result in a thermal runaway over the critical current density.