X-ray imaging from various carbon-nanotube-based cold cathodes

Abstract

X-ray radiography is used in a wide range of imaging applications such as diagnostic medical imaging, nano-device inspection, and security. The conventional thermionic cathode for x-ray tube is heated over 1000 oC to allow free electrons to escape from the surface and has several inherent limitations, such as high operating time and power consumption, slow response time, limited miniaturization, and short lifetime. On the other hand, x-ray radiography using a carbon-nanotube(CNT)-based cold cathode, which has mechanical and chemical stability as well as superior electron-emission properties, can overcome the problems that became apparent when thermionic cathode was used. Recently, we have attempted a screen-printing method of pasting CNTs to fabricate a CNT-cold cathode. However, this method showed a serious problem in adhesion between the substrate and the CNTs, which resulted in the destruction of the cold cathode and significantly degraded its long-term stability. In this study, we present x-ray imaging data obtained from cold cathodes using CNTs directly grown by inductively coupled plasma-chemical vapour deposition. Various CNT-based electron-emitters with a configuration of CNT/catalyst/buffer/sus-substrate were fabricated by altering the catalyst (Ni, Co, etc.) and buffer (TiN, Al/Ni/TiN, etc.) materials. Field-emission SEM, high-resolution TEM, and Raman spectroscopy were used to analyze the morphologies, microstructures, and crystal qualities of CNTs. The electron-emission characteristics of CNT-emitters as well as the spatial distributions of emitted electrons were measured. The CNT-emitters with a triode in structure were installed in the x-ray tube which was composed of gate-electrode meshes and electrostatic lens inside and was housed in a vacuum chamber pumped down to 3 10-7 Torr. X-rays radiated from the anode (Cu) was extracted passing through a beryllium (Be) window where the specimen was in contact with. The relationship between CNT-performance and x-ray image was examined, in terms of the CNT-emitters considered in this study.