Superior Carbon Nanotube Field Emission for X-Ray Source via Metal Silicide-Induced Adhesion

Carbon nanotube field emitters (CNT FEs) have obtained increasing attention for vacuum electronics devices such as cold cathode X-ray tubes. However, the low adhesion of CNTs on the substrate thwarts their ability to achieve high field emission current density. To this end, the effects of Ni, Si, and Al₂O₃ fillers on their adhesion both in the paste and onto the Kovar (nickel-cobalt ferrous alloy) substrate are investigated. Chemical reactions between Ni, Si, and the Kovar constituents lead to the formation of micrometer-sized protruding particles. Si fillers are key in promoting their formation in the paste and on the substrate. During high-temperature vacuum annealing, the Si fillers reacted with the Ni fillers and the Kovar constituents, forming Ni₂Si in the paste and Fe₂NiSi on the substrate, both of which strengthened adhesion. The adhesion of CNT FEs with both Ni and Si fillers is better compared to those containing Ni or Si fillers alone. With the resulting retention of more CNTs on the substrate after tape activation, a current density of 30.9 A cm^-2 and stable field emission for 14 h at 500 mA cm^-2 are achieved, indicating the commercial potential of CNT FEs in vacuum electronics.