These results indicate that silver NPs could not work as a good binder of a CNT emitter that can withstand against high-voltage arcing. To analyze the bad performance of the CNT emitter, the adhesion force between the silver NP binder and the tungsten substrate

was characterized with a pencil hardness test. For the characterization, the silver NPs were annealed on a tungsten sheet (10 × 10 mm2) at 750°C. The pencil hardness of the silver film attached to the tungsten sheet was 2B, which is a soft level as determined by ASTM D3363. Such poor adhesion of the silver film might be improved by changing the substrate, and thus, we prepared the silver film on other metal sheets such as SUS, titanium, kovar, and copper. However, the pencil hardness of the silver film did not exceed

1B, reflecting that the adhesive force of PD173074 clinical trial the silver binder is not so high on the metal substrates. Figure 2 FESEM Alvocidib supplier images and stability test of the fabricated CNT emitters using silver NPs. (a) FESEM image of the fabricated CNT emitter using silver NPs on tungsten metal tip. (b) Stability find more test of the fabricated CNT emitter with time. (c) FESEM image of the CNT emitter after emission stability experiment. Severe damage of the CNT/silver NP mixture was observed as compared with (a). As a candidate of a good binder, we tried to use a brazing filler material that is used to join two different metals. The brazing filler material is a metal mixture composed of silver, copper, and indium micro- and nanoparticles described in the ‘Methods’ section. Before using this material as a binder of the CNT emitters, the adhesion behavior of the material at different substrates was analyzed. As shown in Figure  3a,b,c,d, the metal mixture was melted at 750°C, but the Cobimetinib in vitro melted metal mixture was spherically aggregated on the tungsten, SUS, titanium, and silver substrates, suggesting a poor wettability to the substrates. However, thin films of metal mixture binders were uniformly

formed on kovar and copper substrates (Figure  3e,f, respectively). In addition, pencil hardness tests revealed that the hardness of the metal mixture films on the kovar and copper substrates were 4H. This indicates that the metal mixture films were very strongly attached to the substrate and the adhesive force to the substrate was remarkably enhanced compared to silver NPs. Figure 3 FESEM images of metal mixture binders on various tip substrates. (a) Tungsten, (b) SUS, (c) titanium, (d) silver, (e) kovar, and (f) copper. The annealing temperature was 750°C. Based on this fact, CNT emitters were fabricated on kovar and copper tips using the metal mixture as a binder. The metal mixtures were annealed at 750°C. FESEM images of the CNT emitter prepared on a kovar tip show that CNTs were uniformly coated on the kovar tip and vertically aligned CNTs were clearly observed (Figure  4a).