Field emission microscopy of carbon nanotube fibers: Evaluating and interpreting spatial emission

Abstract

In this work, we quantify field emission properties of cathodes made from carbon nanotube (CNT) fibers. The cathodes were arranged in different configurations to determine the effect of cathode geometry on the emission properties. Various geometries were investigated including: 1) flat cut fiber tip, 2) folded fiber, 3) looped fiber and 4) and fibers wound around a cylinder. We employ a custom field emission microscope to quantify I-V characteristics in combination with laterallyresolved field-dependent electron emission area. Additionally we look at the very early emission stages, first when a CNT fiber is turned on for the first time which is then followed by multiple ramp-up/down. Upon the first turn on, all fibers demonstrated limited and discrete emission area. During ramping runs, all CNT fibers underwent multiple (minor and/or major) breakdowns which improved emission properties in that turn-on field decreased, field enhancement factor and emission area both increased. It is proposed that breakdowns are responsible for removing initially undesirable emission sites caused by stray fibers higher than average. This initial breakdown process gives way to a larger emission area that is created when the CNT fiber sub components unfold and align with the electric field. Our results form the basis for careful evaluation of CNT fiber cathodes for dc or low frequency pulsed power systems in which large uniform area emission is required, or for narrow beam high frequency applications in which high brightness is a must.

Confirmation of Transit-Time Limited Field Emission in Advanced Carbon Materials with Fast Pattern Recognition Algorithm

“…This sample was formed by braiding and twisting 90 µm fibers, then folding the resulting yarn multiple times and inserting it in a metal tube. 2 Scanning electron microscopy (SEM) images of the samples are given in Fig. 8.…”

“…Many studies [1][2][3][4][5][6] have convincingly demonstrated that the electron emission from a large surface area field emission cathode placed in a macroscopic electric field E is not uniform. The emission area is only a small portion of the total surface area of a cathode since most of the emission is confined to a small number of emission spots randomly distributed over the cathode surface.…”

“…Second is fundamental: only a properly established j-E (and not I-V ) relationship can be used to define the validity range of a classical Fowler-Nordheim (FN) emission and clarify the role of other mechanisms that could cause deviation from the FN emission, i.e. cause non-conventional behavior that is being observed across a large body of experimental work [1][2][3][4][5][6][7][8][9][10] .…”

FEbeam: Cavity and Electron Emission Data Conversion, Processing and Analysis. A Freeware Toolkit for RF Injectors

“…There is a number of physical mechanisms at play and this is currently a subject of intense research. Thus, a routine searching for a knee point was developed based on an earlier idea implemented for dc case 6 . It is included as an optional data processing tool.…”

Fast Pattern Recognition for Electron Emission Micrograph Analysis

“…Many studies [1][2][3][4][5][6] have convincingly demonstrated that electron emission from a large surface area field emission cathode placed in a macroscopic electric field E is not uniform. The area contributing to emission is only a small portion of the total surface area available for emission.…”