High-current field-emission of carbon nanotubes and its application as a fast-imaging X-ray source

Abstract

The high-current field-emission (FE) characteristics of carbon nanotubes (CNTs) are reported. Powders of bismuth (Bi) were mixed with the CNTs and baked at 400 °C, and because the melting point of Bi is low, the CNTs bonded strongly to the Bi blocks. In the FE characterization, the turn-on electric field was 1.54 V/μm, with current density 1 μA/cm², and the threshold electric field was 4.24 V/μm, with current density 1 mA/cm². Because the Bi blocks played a role in enhancing the adhesion of CNTs to the substrate and improving the FE performance, the stable FE current reached 88 mA when the electric field was increased to 12.75 V/μm; the current density was 11.2 A/cm². A fast-imaging X-ray tube with CNT field emitters was fabricated. The metal mesh was used as the gate electrode, which was designed to be cooled in oil in order to switch the electron beam rapidly. The X-ray image of a vibrating metronome was captured clearly when the exposure time was decreased to 20 ms, showing that high-current CNT field emitters can be used to obtain X-ray images of moving objects.

Introduction

Carbon nanotubes (CNTs) have excellent field-emission (FE) properties, and therefore have great potential for application in, for example, display devices [1], [2], microwave tubes and X-ray tubes. As the electron source in these devices, CNT field emitters need to provide a high emission current and current density. In recent years, efforts have been made to improve the FE performance of CNT emitters. CNTs have been decorated with In₂O₃ [3] and Ta [4] film to enhance the FE current. CNT yarn has been prepared by direct spinning through chemical vapor deposition (CVD) and then formed into a carpet structure by tying the yarn to a conductive substrate [5]. High FE performance from a CNT carpet has been reported. CNTs have been decorated with Er nanoparticles to decrease the work function, and the turn-on electric field and threshold electric field were clearly decreased compared with pristine CNTs [6]. Graphene has also been reported as a field emitter for improving emission performance [7], [8]. Although a current of a few microamperes has been obtained from a single CNT [9], it is a big challenge to get a high current from a CNT array with an area of a few square millimeters. Generally, the adhesion of emitters attached to a substrate and the non-uniformity of emission have seriously influenced the upper limit to achievable FE current [9].

By means of FE X-ray tubes with CNT emitters, it is possible to obtain an X-ray image with high speed and low power consumption [10], [11], and therefore these have been used in stationary tomosynthesis, biological imaging and security detection [12], [13], [14]. In recent years, the FE X-ray tube has attracted much research attention. Vacuum-sealed miniature X-ray tubes with CNT field emitters have been developed [10], [15], [16]. As field emitters, CNTs have been decorated to improve the performance stability of X-ray tubes [17], [18], [19]. CNTs are also integrated with photo diode, so the field-emission current can be modulated by laser pulse [20], [21]. The triode structure of focusing-functional gate (FFG) and the brazing process at an elevated temperature have been developed to improve the performance of the miniature X-ray tube [22]. To increase the field-emission current further, it is reported that the CNT paste are composed of multi-walled CNTs with a diameter of 4–8 nm, inorganic fillers of Cu alloy and Al₂O₃ particles, and an organic powder of ethylcellulose [23]. Finally, the FE current is as high as 50 mA (current density is 0.31 A/cm²) and the focal spot is only 0.3 mm. The highly adhesive uniform field emitters are fabricated from CNT pastes by reacting nanometer-scale SiC fillers on a Kovar substrate at a high temperature in vacuum [24]. The CNT emitters show a high emission current of over 4 mA, giving a large current density of around 150 mA/cm² at an applied electric field of 2.5 V/μm. The linear array of CNT cathodes has been developed to create a line-focused X-ray source [25]. Because the area of linear array of CNT cathodes is large, the tube current can reach 70 mA. It is also reported that the X-ray images of human breast cancer is obtained by digital breast tomosynthesis system with multi-wall CNTs [26]. Some efforts have also been made to decrease the exposure time in FE X-ray tubes [13], but the fast-imaging process using these tubes has not previously been studied intensively.

To capture the X-ray image of a moving object, the exposure speed of the X-ray tube must be much faster than the speed of the object. Therefore, the FE electron beam must be switched very rapidly in the X-ray tube. Simultaneously, a peak FE current with a narrow driving pulse must be large enough to maintain a clear X-ray image. Therefore, field emitters with an ultra-high emission current are required in fast-imaging X-ray tubes.

In this paper, Bi powders with a low melting temperature were introduced into CNT field emitters. Because the CNTs could bond to Bi blocks strongly after the baking process, an ultra-high FE current and current density could be obtained with the CNT emitters. Aided by the design of a cooling triode to modulate the FE electron beam rapidly, a fast-imaging X-ray tube with CNT emitters was fabricated.