Robust high current density cathode using active complex electroplating.

Summary form only given. Carbon nanotubes (CNTs) are candidate for ideal electron field emitters due to their high aspect ratio, superior electrical and thermal conductivity, and relatively high chemical and mechanical stability. Recently, they have been extensively used for high current density emitters of X-ray tubes and THz imaging source to overcome the limitation of thermionic emitters such as high power consumption, lower brightness/resolution, complex additional facilities and short operating time. CNTs cathodes have been a subjected of studies for over a decade and are relatively easy to be made either by chemical vapor deposition in situ growth or by post-growth transfer onto a substrate. However, these methods have not been employed in practical devices because of expensive equipments. They could not be sufficiently satisfied with mass product. In this article, we will focus on a simple fabrication technique for high current density emitters without complex equipments. Figure 1 shows the detailed process and the shape of self-assembled composite emitter by capillary force and pulling-out effect. We can fabricate a high current density emitter from the colloidal solution, dispersing CNTs and dissolving Na₂WO₄middot2H₂O in DMF solvent. Tungsten rod (0.3 mm diameter) was prepared. Electrochemically etched tungsten tip was immersed in the colloidal solution until the solution in the vicinity of tip was stable. Then the tungsten tip was pulled out with the constant speed and DC voltage was simultaneously applied between tungsten tip and colloidal solution. This technique is called active complex electroplating (ACE). The ACE is able to synthesize composite emitters, consists of CNTs and tungsten oxide, on the apex of tungsten tip. The concentration of CNT, withdrawing speed and DC voltage play a role in controlling the diameter and length of the composite emitters. In figure 2, field emission characteristics of 20 um-diamater co- mposite emitter were investigated with respect to current density and emission stability. The maximum current density of annealed emitter was achieved to withstand continuously emission for 24 hours in the order of 380 A/cm² compared with the pristine emitter. The composite emitter fabricated by ACE will be great promise candidate for robust high current emitter of high-resolution X-ray and THz imaging source.