Degradation of Vertically Aligned Carbon Nanotubes at Growth Interface Joints at High Temperatures and Its Impact on Ele
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Degradation of Vertically Aligned Carbon Nanotubes at Growth Interface Joints at High Temperatures and Its Impact on Electron Emission Properties Feng Jin, Yan Liu, Christopher M Day and Scott A Little Department of Physics and Astronomy, Ball State University, Muncie, IN 47306 ABSTRACT Degradation of carbon nanotubes (CNTs) significantly affects CNT’s electron emission capability and limits their applications as electron emitters. In this study, CNTs were placed in controlled environment and their degradations were studied. Two types of CNT degradations, thermal and chemical degradations were identified and their effects on electron emission were studied. It was found that CNTs placed in low vacuum environment at high temperature were subject to oxidation that led to severe degradation of CNTs and their electron emission ability, whereas, CNTs treated at the same high temperatures in an ultrahigh vacuum environment only suffered from morphological changes that had minor impact on electron emission. The CNT samples used in this study were grown on tungsten substrates via plasma enhanced chemical vapor deposition (PE-CVD). The surface morphology changes of the CNT emitters were examined using scanning electron microscopy (SEM). The field emission properties of the CNTs were measured and correlated to the morphology changes. INTRODUCTION Carbon nanotubes (CNTs) are natural field emitters. The field emission properties of CNTs have been extensively studied in recent years. The unique geometry and high aspect ratio of CNTs give rise to a high field enhancement factor β which makes CNTs an ideal candidate for field emitters. Their applications as electron emitters have been demonstrated in a variety of devices, including field emission flat panel displays, electron microscopes, gas discharge tubes, and x-ray and microwave generators.[1-6] Despite of their great potential, their reliability and robustness in real device operating environment is a serious issue. Although CNTs have demonstrated long-term field emission stability in ultrahigh vacuum (< 10-9 Torr), their emission is not stable and degrades over the time under various environmental and operating conditions. [7, 8, 9, 10] The degradation of CNTs significantly impacts the long term performance and the stability of the devices that they operate in. One of our research projects focus on CNTs’ application as field enhanced thermionic emitters in vacuum electronic devices [11], and in such as application, CNT degradation is particularly important, as it directly limits the lifetime of the devices. Many environmental variables such as vacuum level, temperature, and electric field and current are the factors that can cause CNT degradation and emission instability. In this study, vertically aligned CNT samples were placed in various controlled thermal, vacuum environment and electrical operating conditions to study the cause and effect of the CNT degradations. Scanning electron microscopy (SEM) was employed to study the morphological changes of the CNT th
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