Development of a compact X-ray tube with CNT field emitters

Abstract

Field emission devices based on sharp cold cathodes such as carbon nanotubes (CNTs) have long been a hot issued research subject for advanced vacuum device applications. Especially, the utilization of these cathodes is an attractive alternative for the replacement of thermionic cathodes for generating X-rays [l, 2]. We have investigated the potential of screen printed CNT field emitters for improving the efficiency and reliability of compact X-ray tubes, and then we have fabricated and tested about our compact X-ray tube with ceramic body using ceramic brazing vacuum sealing technique as shown in Fig. 1. First of all, need-specific CNTs with an average diameter of 5nm were used for the preparation of CNT pastes. Formulation recipe includes; i)preparation of optimized polymer binder composing of Acrylate, solvent, isopropyl alcohol, Terpineol, ii)mixing CNTs, nano-metal particles(NPs), etc. with the binder, iii)3-roll milling for screen printable CNT pastes .. After screen printing on cathode electrode, drying of samples was done in air at 120°C for 10 min and then, heat treatment at about 260 °C was followed for more than 2 hours. Just before the emission test of prepared sample, surface treatment using sticky roller was done to set up surface CNT emitters up-right as shown in Fig.2[3]. Fig. 2 shows the developed CNT emitters with nano-scale metal particles that are melted during the CNT emitter process. The developed CNT paste and emitter process had so many vertical aligned CNT emitters and an improved adhesion to the cathode. During the field emission measurements, we could not observe any detachment of CNT emitters invoking arc damage. The field emission properties of the developed CNT emitters, applied anode field versus anode current characteristics in a diode configuration and its Fowler-Nordheim (F-N) plot, are shown in Fig. 3. The developed CNT emitter was observed to have a sufficient emission current under an apparent electric field of below 2.0 Y I�m along with good reliability. The large field emission current resulted from the optimization in compositional and morphological properties of the CNT paste [3]. The electron current density from the CNT emitters with gated triode structure was about 10 mA/cm 2 and was stable within 3% for continuous DC biasing time t > 12 h as shown in Fig. 4. The electrons passing through hole in the gate electrode were accelerated to the desired energy, and were made to impinge on the metal target (such as Ag, Cu, Mo, and W etc) for generating X-rays. The X-rays transmitted through Be-window were characterized using a dosimeter. At an acceleration voltage of Ya�10 kY and an emitted current of Ia�150 �A, the dose rates of X-rays were approximately 2mSv with low tube temperature (compared with thermionic X-ray tube). Our compact X-ray tube design utilizing CNTs cold cathodes with brazing technique may be a significant advance in X-ray technology development and could lead to portable and miniature X-ray sources for medical and industrial applications.