The field electron emission properties of carbon nanotube (CNT) films composed of densely packed and highly aligned CNTs were investigated. The CNT films were produced by a continuous film casting process and are spooled into long lengths with the CNTs aligned lengthwise in the film. The anisotropic nature of the CNT film morphology was confirmed by performing specific conductivity measurements in directions both parallel and perpendicular to the aligned CNT microstructure. Field emission experiments were performed on 5 and 10 mm wide films that were mechanically cut into small samples and then vertically mounted so that the emission occurred from the film edge. The films were mounted with the aligned CNT microstructure oriented either parallel or perpendicular to the direction of the applied electric field. The highest emission currents were produced by films mounted in the parallel alignment configuration. Additional experiments were performed on films that were folded, which eliminated surface irregularities at the film edge due to the cutting process. SEM imaging performed at the ridge of the folded film before and after field emission (FE) experiments showed that films mounted in the parallel alignment configuration had minimal surface damage after FE, while films mounted in the perpendicular alignment configuration showed substantial damage. The effective emission area and field enhancement factor were extracted from the FE data using the orthodox Fowler–Nordheim theory. Folded CNT film cathodes mounted in the parallel alignment configuration produced the highest emission currents, while demonstrating a larger emission area and lower field enhancement factor.

Laser melting the surface of 304 stainless steel allows controlled grain growth in the direction of the laser scan [1]. We demonstrate the application of laser surface melting with a Yb fibre laser as a technique for single crystal grain refinement and security marking in polycrystalline metals via localized grain nucleation. Single crystals were achieved after three consecutive passes with constant laser parameters throughout the length of the laser raster at 19.17 kJ/cm(2) average energy at 300K, 0.1% O-2 environment. The depth of localized grain nucleation was measured to be approximately 20 mm for a single pass, making hidden messages in the bulk of the material possible after mechanically removing the immediate surface melt. The patterns are undetectable by conventional optical microscopy but can be viewed with interferometric microscopy due to fine height differences between untreated and laser treated surface regions, this way establishing a metal security marking technique.

Structures with a sharp apex amplify an applied macroscopic field, FM, substantially and generate significant field electron emission (FE). The apex barrier field, Fa, is related to FM by the apex field enhancement factor (aFEF), γa≡Fa/FM. In this Letter, we provide a theoretical explanation for extremely high-effective FEFs (104 ≲ γeff ≲ 105) recently extracted from an orthodoxy theory analysis of the emission current–voltage characteristics of looped carbon nanotube (CNT) fibers, making them promising candidates for FE applications. In this work, we found a dependence of γa on the geometrical parameters for an isolated conductive looped CNT fiber, modeled via the finite element technique. The aFEF of looped CNT fibers is found to scale as γa=2+[hf/rfiber][ln (2h/rfiber)]−1, where f≡1+θ[rfiber/b]α[ln (2h/rfiber)−1], in which h is the height of a looped fiber standing on an emitter plate, b is its base length, rfiber is the radius of the fiber, and θ and α are fitting parameters that have a nonlinear dependence on the scaling parameter h/b. Our results show that the scaling law predicts that 10 ≲ γa ≲ 100 for looped CNT fibers with parameters: 10 μm ≤rfiber≤ 100 μm, 0.4 ≤h/b≤ 2, and d/h≥1, where d is the distance between the apex of the looped fiber and the anode. However, scanning electron microscopy images reveal the presence of microfibrils protruding from the looped CNT fiber surface close to its apex. We show that the modeling of a combined two-stage structure (looped CNT fiber + fibrils) leads to aFEF values in excellent agreement with an orthodoxy theory analysis of FE experiments performed on these fibers.

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.

This paper presents field emission (FE) from a single carbon nanotube (CNT) fiber with varying anode- cathode (AK) gap distances. We found that the field enhancement factor depends strongly on the finite AK gap distance, due to the combination of geometrical effects and possible fiber morphology change. The slope drop in the Fowler-Northeim (FN) plot of the FE data in the high voltage is related to the electrical contact resistance between the CNT fiber and the substrate.

The authors forgot to include the following disclaimer in Acknowledgements section.

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.

LaB 6 –ZrB 2 composites obtained by directional solidification at eutectic composition have been investigated by low-energy electron microscopy (LEEM) and thermal emission electron microscopy (ThEEM). The transitions from the mirror electron microscopy mode to the LEEM mode for the hexa- and diborides indicate lower work functions of the two phases when embedded in the composite compared to the corresponding single phases. In the composite, the work function of the ZrB 2 fibers is similar to that of the matrix and ThEEM images display a brighter contrast for the fibers. This is explained by the thermally activated diffusion of La on the fiber surface.

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.

LaB6-ZrB2 composites obtained by directional solidification at eutectic composition have been investigated by low-energy electron microscopy (LEEM) and thermal emission electron microscopy (ThEEM). The transitions from the mirror electron microscopy mode to the LEEM mode for the hexa- and diborides indicate lower work functions of the two phases when embedded in the composite compared to the corresponding single phases. In the composite, the work function of the ZrB2 fibers is similar to that of the matrix and ThEEM images display a brighter contrast for the fibers. This is explained by the thermally activated diffusion of La on the fiber surface.

This paper studies field emission (FE) from a single carbon nanotube (CNT) fiber with different anode-cathode (AK) gap distances. It is found that the field enhancement factor depends strongly on the finite AK gap distance, due to the combination of geometrical effects and possible fiber morphology change. The geometrical effects of AK gap distance on the field enhancement factor are confirmed using COMSOL simulations. The slope drop in the Fowler-Northeim (FN) plot of the FE data in the high voltage is related to the electrical contact resistance between the CNT fiber and the substrate. It is found that even a small series resistance to the field emitter (<30% of the emission gap impedance) can strongly modify the FE characteristics in the high voltage regime, inducing a strong deviation from the linear FN plot.