Jun-Tae Kang

A digital miniature x-ray tube with a high-density triode carbon nanotube field emitter

We have fabricated a digital miniature x-ray tube (6 mm in diameter and 32 mm in length) with a high-density triode carbon nanotube (CNT) field emitter for special x-ray applications. The triode CNT emitter was densely formed within a diameter of below 4 mm with the focusing-functional gate. The brazing process enables us to obtain and maintain a desired vacuum level for the reliable electron emission from the CNT emitters after the vacuum packaging. The miniature x-ray tube exhibited a stable and reliable operation over 250 h in a pulse mode at an anode voltage of above 25 kV.

Fast and Stable Operation of Carbon Nanotube Field-Emission X-Ray Tubes Achieved Using an Advanced Active-Current Control

The advanced active-current control (ACC) with a push-up voltage source is proposed for the fast operation of carbon nanotube (CNT) field-emission x-ray tubes while preserving the advantages of improved stability and reliability. The push-up voltage source, composed of a voltage supply and a resistor, was found to reduce markedly the response time of CNT x-ray tubes down to several tens nanoseconds. The very fast operation of CNT x-ray tube with the advanced ACC could provide an improved x-ray imaging technology, including high-speed scanning and blur-less imaging.

Wall-Controlled Growth of Carbon Nanotubes Using Temperature Treatment

Large arrays of self-oriented, multiwalled carbon nanotubes were obtained by chemical vapor deposition using a metallic substrate as well as a Si substrate. We controlled the size of catalytic Fe nanoparticles in order to control the wall number of the carbon nanotubes (CNTs). The size and distribution of Fe nanoparticles varied according to the growth condition, heat exposure time, and the thickness of the catalytic metal layer, resulting in a change in the wall number of CNTs grown. A decrease in heat exposure time from 10 to 0 s decreased the average wall number of CNTs. A combination of atomic force microscopy, scanning electron microscopy and transmission electron microscopy images provided evidence of the crucial role in the surface distribution of metallic catalyst particles on the substrate during the growth of CNTs. We confirmed the vertically aligned growth of CNTs that have a small number of walls and synthesized CNTs on a stainless plate with a 5 mm diameter.

Highly adhesive carbon nanotube field emitters with a carbide filler

We have developed highly adhesive carbon nanotube field emitters with a carbide filler. The adhesion was reinforced by a chemical reaction between the carbide filler and the substrate. We observed the field emission properties to be improved by the increment of the effective field emitters through repeated surface treatments.

A carbon nanotube field emitter formed on W-wire with high current density for X-ray source

Field emission micro-focus X-ray source based on carbon nanotube (CNT) has attractive features for biomedical or clinical imaging. X-ray sources for micro-focus and/or miniature tube types require a mini-sized, high-density emission current in the order of 102~103 mA/cm2. We have previously reported that the optimized paste composition with a nano-scale metal filler enhanced adhesion between CNT emitter and cathode electrode at low temperature. In this study, a CNT-on-Tip cathode, having CNT emitter on a tungsten (W) wire of 100 μm in diameter, was developed in order to increase emission current density.

Brightness of electron beam from a small carbon nanotube emitter fabricated using the probe contact method

A novel method for fabricating a small-sized carbon nanotube (CNT) emitter was developed. The emitter was fabricated by contacting the tungsten probe covered with CNT paste to the surface of Kovar rod, and then the brightness of electron beam generated from the CNT paste emitter was measured. We obtained the virtual source size of 1.13 × 10^-10 m² and the brightness of 1.2 × 10⁷ A m^-2sr^-1 at an applied voltage of 3,100 V.

Highly reliable carbon nanotube field emitters for vacuum electronic devices

We fabricated highly reliable carbon nanotube (CNT) field emitters and evaluated their performances in a harsh environment of a tiny vacuum-sealed vessel for vacuum electronic devices. The CNT paste was formulated by using a ball-milling method with optimized fillers for high-temperature endurance of the resultant field emitters. The CNT field emitters in a triode configuration showed reliable operation for over 150 h at a high current density of 0.4 A/cm2.

Advanced active-current control for fast and stable operation of a carbon nanotube field-emission X-ray tube

The advanced active-current control with a push-up voltage source is proposed to operate a carbon nanotube (CNT) field-emission X-ray tube rapidly with maintaining the advantages of improved stability and reliability. The push-up voltage source, comprising of a resistor and a voltage supply, strongly reduced the response time of CNT X-ray tubes down to several μs.

The improvement of field emission characteristic after high temperature sealing process of carbon nano-tube x-ray tube

We have successfully developed the vacuum-sealed carbon nano-tube (CNT) x-ray tube. The remarkable improvement of field emission properties, life time and stability, was obtained after high temperature brazing process in a vacuum circumstance.