Highly stable carbon nanotube field emitters on small metal tips against electrical arcing
© Ha et al.; licensee Springer. 2013
Received: 19 June 2013
Accepted: 8 August 2013
Published: 16 August 2013
Carbon nanotube (CNT) field emitters that exhibit extremely high stability against high-voltage arcing have been demonstrated. The CNT emitters were fabricated on a sharp copper tip substrate that produces a high electric field. A metal mixture composed of silver, copper, and indium micro- and nanoparticles was used as a binder to attach CNTs to the substrate. Due to the strong adhesion of the metal mixture, CNTs were not detached from the substrate even after many intense arcing events. Through electrical conditioning of the as-prepared CNT emitters, vertically standing CNTs with almost the same heights were formed on the substrate surface and most of loosely bound impurities were removed from the substrate. Consequently, no arcing was observed during the normal operation of the CNT emitters and the emission current remained constant even after intentionally inducing arcing at current densities up to 70 mA/cm2.
Carbon nanotubes (CNTs) are widely used as field emission electron emitters for X-ray tubes[1–4], field emission displays, and high-resolution electron beam instruments[6, 7] because of their excellent electron emission property, chemical inertness, and high electrical and thermal conductivity[8, 9]. In spite of these superior characteristics, practical applications of CNT field emitters to devices particularly requiring high-voltage operation are limited due to unstable electron emission properties of the CNT emitters. Electron beam current emitted from CNT emitters can be fluctuated or degraded because CNTs are damaged by the back bombardment of ions produced from the residual gas[10, 11] or CNTs are structurally deformed due to excessive Joule heating[12, 13]. More seriously, emission current can be abruptly dropped because CNTs are detached from a substrate. If a very high current (300 nA per single CNT) flows through a CNT, adhesion between the CNT and the substrate becomes weak due to resistive heating and accordingly the CNT can be peeled off from the substrate[14, 15], or a strong electric field exerts electrostatic force on CNTs, leading to the detachment of the CNTs[15, 16]. Weak adhesion of CNTs to a substrate deteriorates the removal of CNTs.
In addition, if CNT emitters are operated at a high voltage or at a high electric field, electrical arcing (or vacuum breakdown) can occur. Arcing can be initiated by the removed CNTs, impurities on the CNTs or substrates[18, 19], protrusion of CNTs, low operating vacuum, and a very high electric field[20–23]. Since arcing is accompanied with a very high current flow and it can produce a plasma channel near the emitter, CNTs are seriously damaged or sometimes CNTs are almost completely removed from the substrate by the arcing events[17, 20]. Detachment of CNTs from a substrate is an irreversible catastrophic phenomenon for a device operation. In addition to the detachment of CNTs, arcing induces a sudden voltage drop, and thus, device operation is stopped. Therefore, for a stable operation of a device using CNT emitters, arcing should be prevented. Particularly, CNT emitters on small metal tips (diameter < 1 mm) are necessary for miniature X-ray tubes[1–4] and micro-focus X-ray tubes[6, 7]. Small metal tips produce much higher electric field than flat substrates at the same applied voltage due to their sharp geometry. As a consequence, CNT emitters on small metal tips can suffer from much serious and frequent arcing, and hence, stable operation of the CNT emitters against arcing is a big issue[4, 14].
So far, few papers have been reported on CNT emitters to withstand arcing, although some methods to reduce arcing events have been reported, including the operation of the CNT emitters under ultrahigh vacuum (approximately 10−9 Pa)[24, 25], plasma treatment of the emitters[10, 26], and removal of organic impurities by firing. Here, we present an approach to fabricate CNT emitters on small metal tips that show extremely high stability against arcing. Using a metal alloy as a binder, CNT emitters can be strongly attached to a metal tip substrate. Due to the strong adhesion, CNTs emit constant currents even after intense arcing events. In addition, CNT emitters can be pre-treated with an electrical conditioning process with the help of strong adhesion, and almost no arcing events are observed during a normal operation.
The morphologies of the fabricated CNT emitters were characterized using a field emission scanning electron microscope (FESEM; Hitachi S-4800, Chiyoda-ku, Japan). The adhesive force of the CNT/metal binder coating on a substrate was measured by a pencil hardness test, which is described in American Society for Testing and Materials (ASTM) D3363. Field emission properties of the fabricated CNT emitters were characterized in a vacuum chamber, which is schematically shown in Figure 1b. A diode type with a copper disc (diameter, 30 mm) acting as an anode was employed for the field emission test. A negative high voltage of 0 ~ −70 kV was applied to the CNT emitter while the Cu anode was grounded. The distance between the CNT emitter and the anode was fixed to 15 mm. In order to protect the high-voltage power supply due to high-voltage arcing, a current-limiting resistor (resistance, 10 MΩ) was installed between the power supply and the emitter.
Results and discussion
CNT emitters were fabricated on copper tip substrates using a metal mixture that was composed of silver, copper, and indium micro- and nanoparticles as a binder. The metal mixture strongly attached CNTs to the tip substrate. Due to the strong adhesion, CNT emitters could be pre-treated with an electrical conditioning process without seriously damaging the CNTs even though many intense arcing events were induced at the small and sharp geometry of the tip substrate. Impurities that were loosely bound to the substrates were almost removed and CNT heights became uniform after the electrical conditioning process. Consequently, no arcing events were observed from the CNT emitters during the normal operation with the current density less than 50 mA/cm2. Moreover, even though arcing was induced at a higher current density of 70 mA/cm2, the emitters could withstand the arcing and the emission current remained constant with time. Due to the strong binding of the CNTs to the substrates, CNTs were not detached from the substrates even by the arcing events. Consequently, the fabricated CNT emitters exhibit very stable field emission properties, which are very useful for the realization of miniature X-ray tubes and small-sized electronic devices that require high-voltage operation.
This study was supported by the R&D Program of MKE/KEIT (10035553).
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