Field electron emission cathodes were constructed from knitted fabrics comprised entirely of carbon nanotube (CNT) fibers. The fabrics consisted of a top layer array of ∼2 mm high looped structures and a bottom layer that was 1 mm thick with a flat underlying surface. Field emission (FE) experiments were performed on 25.4 mm diameter CNT fabric cathodes in both direct current (DC) and pulsed voltage (PV) modes, and the results were compared to those obtained from a CNT film cathode. The DC measurements were performed at a maximum voltage of 1.5 kV. The CNT fabric cathode emitted 20 mA, which was an 8× increase over the emission current from the CNT film cathode. The DC results were analyzed using the corrected form of the Fowler–Nordheim FE theory initially developed by Murphy and Good, which allows for the determination of the formal emission area and effective gap-field enhancement factor. The PV experiments resulted in Ampere level emission currents from both CNT fabric and CNT film cathodes. For a 25 kV, 500 ns voltage pulse, the CNT fabric cathode emitted 4 A, which was 2× more current than the CNT film cathode. Scanning electron microscopy imaging after PV testing revealed that the fibers remained intact after >5000 pulses. These results indicate that knitted CNT fabrics offer a promising approach for developing large area, conformable, robust FE cathodes for vacuum electronic devices.

Carbon Nanotube (CNT) fibers and films have demonstrated excellent field emission (FE) properties and thus hold significant potential for use as electron sources for vacuum electronic devices VEDs). FE cathodes made from 100μm diameter CNT fibers were fabricated on an industrial grade 3D knitting machine. A 1” diameter cathode was tested using 30kV, 30A power supply with a 300ns pulse width. An applied field strength of 3V/μm produced a measured current level of approximately 15A. A CNT film cathode was fabricated from a 20mm wide film arranged in a corrugated geometry. This cathode was tested in a low impedance linear transformer driver (LTD) system operating at 30kV with a 200ns pulse width. The measured current level was approximately 2kA for an applied field strength of 6V/μm.

In the previous chapter, building on recent efforts to characterize carbon nanotube fibers (CNFs) as efficient electron emission sources suitable for compact, high-power, high-frequency vacuum electronic devices, an exhaustive approach toward optimizing CNF field electron emission (FE) properties was proposed. It consists of a platform of scientific enquiry geared toward a meaningful comparison between different CNF-based emitters. The platform envisages an iterative procedure involving (a) the growth, processing, and functionalization of CNFs; (b) full investigation of the CNF material properties before and after FE diagnosis; and (c) multiscale modeling of FE properties, including self-heating, shielding effects and beam characteristics for both CNFs, and emitting carbon nanotubes (CNTs) at the fiber apexes. The modeling would be applicable to a wide variety of CNFs and wirelike sources and would provide essential feedback to the growth, processing, and functionalization of CNFs, in order to optimize their FE properties, especially long-term stability, low noise and maximum emission current, current density, emittance, and brightness.

Wet spun carbon nanotube fibers were characterized using both field emission and electron energy distribution measurements. Fowler-Nordheim analysis of the field emission results showed that the carbon fibers demonstrated a large effective emission area, 2 x 10-12 m2, which resulted in a reduced brightness of 1.84 x 1010 A/m2/sr/V. By considering the emission and number of carbon nanotube emitters it can be shown that the brightness is consistent with previous reports for single nanotube emitters. Additionally, using the effective emission area determined from the Fowler-Nordheim analysis an emittance value around 0.70 μm was found. These characteristics are useful metrics in determining the applicability of using wet spun carbon nanotube fibers for field emission devices.

Building on recent efforts [1-4] to characterize carbon nanotube fibers (CNFs) and electron emission [5,6] suitable for compact, high power, high frequency, vacuum electronic devices, this paper describes a proposed exhaustive approach towards optimizing CNF field emission (FE) properties. It outlines how a platform geared towards meaningful comparisons between different CNF-based emitters can be developed. The platform envisages an iterative procedure involving (a) the growth, processing, and functionalization of CNFs, (b) full investigation of the CNF material properties before and after FE diagnosis, and (c) multi-scale modeling of FE properties, including self-heating, shielding effects and beam characteristics in the CNFs and in the emitting carbon nanotubes (CNTs) at the fiber apexes. The modeling would be applicable to a wide variety of CNFs and wire-like sources, and would provide essential feedback to the growth, processing, and functionalization of CNFs, in order to optimize their FE properties (especially long-term stability, low noise, and maximum emission current, current density and brightness).

The authors regret to inform that the Acknowledgement section of the paper has incorrect grant numbers. Here is how it is now in the paper: Acknowledgments Work supported by US Air Force contract FA8650-11-D-5401 at the Materials & Manufacturing Directorate (AFRL/RXAP). The authors thank Lt Col Victor Putz of AFOSR/EOARD and Jason Marshall at AFOSR.D.G and M.S. wish to thank the EPSRC (EP/K503241/1). Here is how it needs to read Acknowledgments This material is based upon work supported by the Air Force Office of Scientific Research under award numbers FA9550-17RXCOR428 and FA9550-17-1-0317. The authors also wish to thank the EPSRC (EP/K503241/1). Authors would like to apologize for the inconvenience caused.

Reducing the emission of secondary electrons from anode materials is critical to improved efficiency and increased performance in high power vacuum electronics for defense systems. The focus of this proposed effort is to leverage advances in materials technology, specifically thin films, to reduce secondary electron generation and outgassing from anode surfaces. By using advanced thin film deposition techniques, hybrid materials can be developed that provide the thermal and electrical conductivity required for operation, while reducing secondary electrons and desorption of gas species from the anode surface. Proposed solutions to these issues need to be robust, yet cost effective and applicable through available manufacturing processes. In this paper, we will introduce an improved mathematical expression for the secondary emission yield as a function of the impact voltage which is an extension of the formula first introduced by Vaughan [1]. Our expression gives a better fit to some of our experimental data of secondary emission yield versus impact voltage for polycrystalline Copper for which the maximum secondary emission yield is only slightly larger than unity.

Multipactor and vacuum breakdown are serious problems for RF systems that utilize vacuum electronic devices. These devices contain metallic surfaces that serve as the source of unwanted particles released into vacuum. The primary culprits are desorbed gas molecules and secondary electrons. Outgassing results from stimulated or thermal desorption of molecular species in metals and can lead to undesired plasma formation in the vacuum gap. Secondary electrons (SEs) can lead to the multipactor effect when an avalanche of SEs obtain resonance with the internal RF field. Here we discuss surface treatments for mitigating these problems. Laser surface melting (LSM) entails irradiating a metallic sample with the output of a high energy, continuous laser beam, thereby causing melting, flow and re-solidification of the material. LSM processing reduces H outgassing by decreasing the number of grain boundaries through which H can diffuse. We have demonstrated a 50x reduction over an untreated stainless steel surface1. For SE reduction, both surface roughening and patterning have been investigated. Surface roughening was accomplished by using specific grades of metallographic polishing pads imparting controlled levels of roughness and surface features. Electron beam bombardment experiments showed that the secondary electron yield (SEY) increased with surface roughening2. Surface patterning with micro-pores has also been shown to reduce SEY. Modeling has shown that the amount of reduction depends on the aspect ratio of the pore (the ratio of the pore height to the pore diameter), with the greatest reductions occurring for aspect ratios less than two. In order to validate these models, a variety of micro-porous gold surfaces were designed and fabricated using photolithography and electroplating processes, and the models were validated with experimental results3, These results will be discussed.

High Pulsed Power Electromagnetic (HPEM) devices are used in numerous applications including vacuum electronics, particle accelerators, and microwave generators. Stable, long term HPEM device operation is presently constrained by pulse shortening due to plasma formation in the anode-cathode gap region. Plasma is formed through collisions between secondary electrons and gas molecules, both of which are produced by high energy electron impact at the anode; hydrogen is the most abundant species desorbed from metal surfaces by high energy electron impact. The purpose of the work described here was to determine the feasibility of using anodes of 304 stainless steel (SS), processed by laser surface melting (LSM), to reduce hydrogen outgassing. The LSM technique entails irradiating a sample with the output of a high energy, continuous laser beam, thereby causing melting, flow and re-solidification of the material as the laser beam is scanned across the anode surface. When compared to more conventional processing techniques, such as electropolishing, LSM processing introduces significantly fewer contaminants (especially hydrogen) into the anode surface and has the potential to reduce outgassing by decreasing the number of grain boundaries through which hydrogen can diffuse.

When (on average) an electron is field emitted from above the Fermi level there is a cooling effect, called Hendersoncooling; when (on average) it is emitted from below the Fermi level then Nottingham-heating occurs. This work compares our earlier used simple estimation to predict this emitted electron energy difference (average energy of emitted electron minus the Fermi level in the cathode) during field emission (FE) from a carbon nanotube [1,2] with a more numerically intensive formula that better captures the effects of Henderson-cooling and Nottingham-heating and the transition between the two regimes [3-5].

A multiscale array model has recently been presented to describe field electron emission (FE) from carbon nanotubes (CNTs) on the apex of a carbon nanofiber (CNF) fabricated as a CNT aggregate. The model considered Joule heating, radiative cooling and Henderson/Nottingham cooling/heating, assumed CNT destruction at a specified tip temperature, and predicted the CNT fraction destroyed as a function of macroscopic field, and related current-voltage characteristics. The model reproduces features observed in recent experiments on FE from CNFs, including the emission current order-of-magnitude (mA range), the low turn-on field (fraction of V/μm), deviation at high field from the pure Fowler-Nordheim behavior expected for an undamaged CNT array, loops in current-voltage characteristics, and spatial variation of temperature along the CNF. Here, this array model is used to explore how some details of the CNT arrangement at the fiber apex (especially statistical variations in CNT geometry) might affect overall emission characteristics.

Recently, we presented a multiscale model of field emission (FE) from carbon nanotube fibers (CBFs) taking into account Joule heating within the fiber and radiative cooling and the Nottingham effect at the tip of the individual carbon nanotubes in the array located at the fiber tip [1]. The model was used to predicts the fraction of carbon nanotubes (CNTs) being destroyed as a function of the applied external electric field and reproduces many experimental features observed in some recently investigated carbon nanotube fibers such as, order of magnitude of the emission current (mA range), low turn on electric field (fraction of V/μm), deviation from pure Fowler-Nordheim behavior at large applied electric field, hysteresis of the FE characteristics, and a spatial variation of the temperature along the CNF axis with a maximum close to its tip of a few hundred °C. In this work, we report the simulations of the field emission properties from small arrays of carbon nanotube fibers in the presence of shielding effects. The latter are modeled using the line charge model recently developed by Harris et al. [2-5]. The average total emission current and its variance for linear arrays composed of seven carbon nanotube fibers are calculated to show their sensitivity to the morphology of the apex of the individual fibers which are modeled as random arrays of CNTs. In practice, the FE properties of the latter can be strongly dependent of the cutting technique used to form the fiber apexes.

Metal anodes in high power source (HPS) devices erode during operation due to hydrogen outgassing and plasma formation, both of which are thermally driven phenomena generated by the electron beam impacting the anode s surface. This limits the lowest achievable pressure in an HPS device, which reduces its efficiency. Laser surface melting the 304 stainless steel anodes by a continuous wave fiber laser showed a reduction in hydrogen outgassing by a factor of ~4 under 50 keV electron bombardment, compared to that from untreated stainless steel. This is attributed to an increase in the grain size (from 40 - 3516 micrometer2), which effectively reduces the number of characterized grain boundaries that serve as hydrogen trapping sites, making such laser treated metals excellent candidates for use in vacuum electronics.

Dept of Physics and Astronomy, Ohio University Athens, OH USA One of the problems associated with long term operation of high pulsed power, vacuum electronic devices is pulse shortening, which is caused by hydrogen outgassing and by secondary electron emission from the anode. We recently showed the feasibility of Laser Surface Melting (LSM) of stainless steel (SS) anodes to reduce hydrogen outgassing from SS samples subjected to 50 keV electron bombardment. The results showed a reduction in outgassing from LSM-treated SS. This was attributed to a reduction in the number of grain boundaries, which serve as trapping sites for hydrogen. We have since measured the hydrogen depth profiles of treated and untreated samples by Elastic Recoil Detection in order to more completely understand the mechanism for reduced outgassing. The results indicate a significant reduction in residual hydrogen within the melt depth $( \sim 15 \mu \mathrm {m})$ of LSM-treated samples due to the small solubility of hydrogen in molten steel. We describe here a more complete model of the mechanism for reduced hydrogen outgassing that includes both a reduction in the number of trapping sites as well a reduction in the residual hydrogen concentration within the melt depth of LSMtreated samples. We conclude by describing the use of vacuum arc re-melted steel as an anode material and describe the effect of laser patterning of such samples to reduce secondary electron yield.

A multiscale model of field emission (FE) from carbon nanotube fibers (CNFs) is developed, which takes into account Joule heating within the fiber and radiative cooling and the Nottingham effect at the tip of the individual carbon nanotubes (CNTs) in the array located at the fiber tip. The model predicts the fraction of CNTs being destroyed as a function of the applied external electric field and reproduces many experimental features observed in some recently investigated CNFs, such as order of magnitude of the emission current (mA range), low turn on electric field (fraction of V/μm), deviation from pure Fowler-Nordheim behavior at large applied electric field, hysteresis of the FE characteristics, and a spatial variation of the temperature along the CNF axis with a maximum close to its tip of a few hundred °C.

LaB 6 eutectic materials show promise as a replacement for common thermionic cathode materials. This eutectic system belongs to a class of materials referred to as directionally solidified eutectics (DSEs). LaB 6 DSEs consist of a LaB 6 matrix with a second phase, a metal di-boride, forming cylindrical rods in the matrix. Previous investigations on this material were focused on its use as a high temperature structural material. Use as a thermionic emitter remains largely unexplored.

Recently, we presented a multiscale model of field electron emission (FE) from carbon nanotube fibers (CNFs), taking into account Joule heating within the fiber, radiative cooling from its surface and Henderson/Nottingham-type cooling and heating effects at the tips of the individual carbon nanotubes (CNTs) located on the fiber apex [1]. The model was used to predict the CNT fraction destroyed as a function of the applied external electric field. The model reproduces many experimental features observed in recent investigations of CNFs, including the emission current order-of-magnitude (mA range), the low turn-on macroscopic field (fraction of V/μm), deviation from pure Fowler-Nordheim behavior at large applied electric field, irreversibility loops in the FE characteristics, and a spatial variation of the temperature along the CNF axis, with a maximum a few hundred °C close to its apex. In this paper, this model is used to investigate how electrostatic screening and details of the CNT array at the fiber apex might affect the overall emission characteristics.

Acid spun carbon nanotube (CNT) fibers were investigated for their field emission properties and performance was determined to be dependent on fiber morphology. The fibers were fabricated by wet-spinning of pre-made CNTs. Fiber morphology was controlled by a fabrication method and processing conditions, as well as purity, size, and type of the CNT starting material. The internal fiber structure consisted of CNT fibrils held together by van der Waals forces. Alignment and packing density of the CNTs affects the fiber's electrical and thermal conductivity. Fibers with similar diameters and differing morphology were compared, and those composed of the most densely packed and well aligned CNTs were the best field emitters as exhibited by a lower turn-on voltage and a larger field enhancement factor. Fibers with higher electrical and thermal conductivity demonstrated higher maximum current before failure and longer lifetimes. A stable emission current at 3 mA was obtained for 10 h at a field strength of <1 V μm−1. This stable high current operation makes these CNT fibers excellent candidates for use as low voltage electron sources for vacuum electronic devices.

Field emission (FE) measurements are reported from carbon nanotube (CNT) fibers and laser-patterned free standing films fabricated by direct online condensation from a floating catalyst chemical vapor deposition reactor. Fiber and film cathodes showed stable emission in the 1–2 mA current (I) range at maximum cathode temperatures less than 1000 °C; film cathodes show localized heating at the triangular tips and higher maximum temperatures than the fibers. Fowler-Nordheim (FN) analysis indicated a change in the morphology of the emitters with increasing external electrical field (Eext). Fiber cathode I-Eext data are interpreted as FN emission from the fiber tip which is eventually limited by space-charge effects. At higher Eext, FN emission from the fiber sidewall occurs. The single fiber cathode stopped emitting abruptly when field induced self-heating effects became significant. For CNT films, self-heating effects can destroy a portion of the film, but FE can still occur from other areas.