Active vacuum brazing of CNT films to metal substrates for superior electron field emission performance

We report enhanced life-time stability for the carbon nanotubes (CNTs) by coating ultrananocrystalline diamond (UNCD) or hybrid granular structured diamond (HiD) films. Electron field emission (EFE) properties of UNCD/CNTs emitters show a turn-on field of 4.86 V/μm and an emission current density of 0.6 mA/cm2 at an applied field of 8.7 V/μm. There are no notable current degradations or fluctuations over a period of τUNCD/CNTs=228 min for UNCD/CNTs emitters at an applied current of 45 μA. The robustness of UNCD/CNTs emitter is overwhelmingly superior to the lifetime stability of bare CNTs emitters (τCNTs=40 min), even though the bare CNTs possess much better EFE properties (E 0 =0.73 V/μm, J e =1.1 mA/cm2 at 1.05 V/μm, with). The HiD/CNTs emitters exhibited even better EFE properties, i.e., turn-on field of 3.5 V/μm and EFE current density of 0.64 mA/cm 2 at 5.0 V/μm with longer lifetime of tHiD/CNTs=275 min. Furthermore, the plasma illumination (PI) property of a parallel-plate microplasma device fabricated using the UNCD/CNTs (or HiD/CNTs) as cathode shows a high Ar plasma current density of 1.56 mA/cm 2 (or 1.56 mA/cm 2 ) at an applied field of 5600 V/cm with lifetime plasma stability of 130 min (or 180 min). The diamond films coated CNTs emitters, which possess marvelous EFE and PI properties with improved lifetime stability, have great potential for the applications as cathodes in flat panel displays and microplasma display devices.

A single step in-situ process for improvement in electron emission properties of surface-modified carbon nanotubes (CNTs): Titanium dioxide nanoparticles attachment

Optimizing field-emission devices: Advancements in stability and performance with well-oriented and dense integrated carbon nanotube assemblies

Summary form only given. We have reviewed the recent progresses in simulations on the field electron emission (FE) from carbon nanotubes of physical sizes. We propose a theoretical model which reveals the origin of the low turn-on field of FE of carbon nanotubes

The problem of evaluating the field amplification factor in electron field emitters on the basis of carbon nanotubes (CNT) has been considered. The electrostatic problem of determination of the field amplification factor for CNTs having various structures of the tip, depending on the tilting angle in regard to the cathode plane as well as on the inter-electrode distance, has been resolved. The dependence of the field amplification factor for vertically aligned CNTs comprising an array on the intertube distance has been obtained. This dependence is used for evaluation of the optimal surface density of CNTs in the array providing the maximum emission current density. The current-voltage characteristic of an array calculated with taking into account the statistical spread of the orientation angle of CNTs is compared with that obtained recently with taking into account a spread in geometrical parameters of CNTs.

The electrophoretic deposition (EPD) method was used to deposit polyethylenimine (PEI) functionalized multiwall carbon nanotube (CNT) films onto the surface of individual S-2 glass fibers. By varying the processing parameters of EPD following Hamaker's equation, the thickness of the CNT film was controlled over a wide range from 200 nm to 2 μm. The films exhibited low electrical resistance, providing evidence of coating uniformity and consolidation. The effect of the CNT coating on fiber matrix interfacial properties was investigated through microdroplet experiments. Changes in interfacial properties due to application of CNT coatings onto the fiber surface with and without a CNT-modified matrix were studied. A glass fiber with a 2 μm thick CNT coating and the unmodified epoxy matrix showed the highest increase (58%) in interfacial shear strength (IFSS) compared to the baseline. The increase in the IFSS was proportional to CNT film thickness. Failure analysis of the microdroplet specimens indicated higher IFSS was related to fracture morphologies with higher levels of surface roughness. EPD enables the thickness of the CNT coating to be adjusted, facilitating control of fiber/matrix interfacial resistivity. The electrical sensitivity provides the opportunity to fabricate a new class of sizing with tailored interfacial properties and the ability to detect damage initiation.

Electron emission signifies an important mechanism facilitating the enlargement of devices that have modernized large parts of science and technology. Today, the search for innovative electron emission devices for imaging, sensing, electronics, and high-energy physics continues. Integrating two materials with dissimilar electronic properties into a hybrid material is an extremely sought-after synergistic approach, envisioning a superior field electron emission (FEE) material. An innovation is described regarding the fabrication of a nanostructured carbon hybrid, resulting from the one-step growth of boron-doped nanocrystalline diamond (BNCD) and carbon nanospikes (CNSs) by a microwave plasma-enhanced chemical vapor deposition technique. Spectroscopic and microscopic tools are used to investigate the morphological, bonding, and microstructural characteristics related to the growth mechanism of these hybrids. Utilizing the benefits of both the sharp edges of the CNSs and the high stability of BNCD, promising FEE performance with a lower turn-on field of 1.3 V/μm, a higher field enhancement factor of 6780, and a stable FEE current stability lasting for 780 min is obtained. The microplasma devices utilizing these hybrids as a cathode illustrate a superior plasma illumination behavior. Such hybrid carbon nanostructures, with superb electron emission characteristics, can encourage the enlargement of several electron emission device technologies.

Experimental Fowler-Nordheim plots taken from orthodoxly behaving carbon nanotube (CNT) field electron emitters are known to be linear. This shows that, for such emitters, there exists a characteristic field enhancement factor (FEF) that is constant for a range of applied voltages and applied macroscopic fields $F_\text{M}$. A constant FEF of this kind can be evaluated for classical CNT emitter models by finite-element and other methods, but (apparently contrary to experiment) several past quantum-mechanical (QM) CNT calculations find FEF-values that vary with $F_\text{M}$. A common feature of most such calculations is that they focus only on deriving the CNT real-charge distributions. Here we report on calculations that use density functional theory (DFT) to derive real-charge distributions, and then use these to generate the related induced-charge distributions and related fields and FEFs. We have analysed three carbon nanostructures involving CNT-like nanoprotrusions of various lengths, and have also simulated geometrically equivalent classical emitter models, using finite-element methods. We find that when the DFT-generated local induced FEFs (LIFEFs) are used, the resulting values are effectively independent of macroscopic field, and behave in the same qualitative manner as the classical FEF-values. Further, there is fair to good quantitative agreement between a characteristic FEF determined classically and the equivalent characteristic LIFEF generated via DFT approaches. Although many issues of detail remain to be explored, this appears to be a significant step forwards in linking classical and QM theories of CNT electrostatics. It also shows clearly that, for ideal CNTs, the known experimental constancy of the FEF value for a range of macroscopic fields can also be found in appropriately developed QM theory.

Carbon nanotube (CNT) wires are light-weight and robust alternatives to conventional metal conductors. The weight savings, increased flexure tolerance, and corrosion resistance of CNT conductors make them a viable solution for a variety of space, defense, and power transmission applications. An individual CNT has orders of magnitude greater electrical conductivity than conventional metal conductors, but this has not been realized in bulk CNT networks. Recently, the use of chemical dopants has resulted in bulk CNT wire conductivities approaching 10 MS/m. As the electrical conductivity of CNT wires continues to approach that of conventional metals, deployment of these CNT conductors will require an understanding of how these CNT conductors and chemical dopants behave during practical operation.In the present work, commercially scaled CNT wires are physically densified through radial drawing dies and chemically doped with KAuBr4 resulting in an improvement in electrical conductivity to greater than 1 MS/m, a 6x improvement compared to the as-received CNT wire. The current density at failure of CNT wires increases with KAuBr4 doping and densification by 67% to 35 MA/m2. The electrical performance retention is determined by applying increasing current densities with intermittent rest steps and low current I-V sweeps. The low current I-V sweeps allow for changes in electrical conductivity due to permanent material degradation caused by previous high current density exposure to be determined. The instantaneous electrical conductivity averaged over the last 5 seconds of each current “on” step is used to determine the temperature dependent effects on electrical conductivity caused by Joule heat during high current application. The KAuBr4 doped and densified samples experience no permanent change in initial electrical conductivity at current densities up to ~32 MA/m2, 4x greater than the as-received CNT wires.The enhanced electrical conductivity retention of KAuBr4 doped and densified CNT wires at high current density was further probed though an analysis of the thermal stability of these doped CNT conductors. Thermogravimetric analysis indicated that minimal degradation of the KAuBr4 dopant occurs prior to thermal oxidation of the CNT materials at 400 °C. A variety of KAuBr4 doping and 400 °C thermal oxidation procedures were performed on CNT wires to understand how thermal degradation of the dopant relates to changes in the electrical conductivity and high current density stability. Energy dispersive X-ray (EDX) spectroscopy further related the changes in electrical conductivity to the elemental spatial analysis of gold and bromine in the KAuBr4-doped CNT samples before and after 400 °C thermal oxidation. This work concludes that the thermal stability of KAuBr4 results in improved electrical conductivity and stability at high current densities. Trace bromine and gold nanoparticles are maintained at temperatures below the onset of CNT degradation, thus providing residual doping until ultimate CNT wire failure.Recently chemical dopants such as I2 and IBr have also emerged as premiere dopants to improve the electrical conductivity of bulk CNT networks. These dopants will be used to further explore the effect doping procedures and the presence of excess dopant have on the electrical conductivity and high current stability of doped CNT wires. Developing a relationship between dopant thermal stability and electrical conductivity retention will allow for the development of a mechanism explaining dopant degradation at high current densities.

Measurements of the electronic conductivity of lithium ion coatings are an important part of electrode development, particularly for thicker electrodes and in high power applications. A resistance measurement system with 46 probes has been used to characterise lithium ion electrodes, with different formulations and coat weights. The results show that the total through plane resistance is dominated by the interface resistance between the coating and the metal foil, rather than the volumetric resistivity of the coating. For coatings containing carbon nano-tubes, the in plane resistivities in the coating and perpendicular directions are different. A finite volume model was developed to help analyse and interpret the resistivity data.

We have characterized the field electron emission from well-aligned carbon nanotubes (CNTs) synthesized in methanol using cobalt (Co) as a catalyst on silicon (Si) substrates. In order to prepare CNTs aligned vertically to the substrate, the physicochemical state of the Co catalyst was controlled by annealing in air at a temperature of 900 °C as a thermal preoxidation prior to the liquid-phase growth. For the CNT growth, the Co-loaded Si substrate was heated by applying a direct current through the substrate in methanol. Vertically well-aligned CNTs of different crystallinities were synthesized at temperatures of 600 and 900 °C. Field electron emission characteristics of the synthesized well-aligned CNTs were measured using diode-type electrodes in a high-vacuum chamber. A low-threshold electric field of approximately 0.9 V·µm-1 and a very stable emission were observed in the CNTs with high crystallinity grown at a temperature of 900 °C.

Electron field emission (FE) from carbon nanotubes (CNTs) in atmospheric pressure Ar gas was measured. The FE measurements were carried out using a needle for anode and a CNT specimen for cathode. The FE current usually appeared at several hundred volts at applied voltage to the anode at the gap length from 15 to 100 μm. When the applied voltage was lower than a conventional gas breakdown voltage around 300 V, the FE in Ar gas was stable and reproducible even at atmospheric pressure. There was no damage on the surface of CNT cathode in SEM observation after the FE operation in Ar gas. When the applied voltage was increased to more than the gas breakdown voltage, a conventional gas discharge took place resulting in destroying the CNT cathode surface.

This article presents a model of electron field emission from quantum states arising from the tight confinement of quasi-free electrons on a nanotube hemispherical cap. The model outlines the possibility of inhomogeneous electron field emission for very thin carbon nanotubes at high emission levels and the appearance of peculiar ring-shaped field emission images. The conclusions qualitatively agree with existing experimental evidence, therefore supporting the hypothesis that part of the electrons on the cap of the emitter may behave as quasi-free in a high emission level/high-temperature regime.

Printable electron field emitters could lead to cheap and scalable large area electron sources. This paper presents work on electron field emission from water-based multiwall carbon nanotube (MWCNT) dispersions, and introduces new results on emission from different substrates. We summarize work in which MWCNTs are deposited onto paper, glass, and plastic substrates, and show that the field emission characteristics can be tailored by controlling the underlying morphology as well as by post-laser irradiation. We also show that engineering the work function of MWCNTs can significantly enhance field emission, and that resonant tunneling effects may be induced by suitable surface functionalization.

In this work, the electron field emission behaviour of electrodes formed by carbon nanotubes (CNTs) grown onto monolithic vitreous carbon (VCarbon) substrates with microcavities is presented. Scanning electron microscopy was used to characterize the microstructure of the films. Tungsten probes, stainless steel sphere, and phosphor electrodes were employed in the electron field emission study. The CNT/VCarbon composite represents a route to inexpensive excellent large area electron emission cathodes with fields as low as 2.1 V μm−1. In preliminary lifetime tests for a period of about 24 h at an emission current of about 4 mA cm−2, there is an onset degradation of the emission current of about 28%, which then stabilizes. Electron emission images of the composites show the cavity of the samples act as separate emission sites and predominantly control the emission process. The emission of CNTs/VCarbon was found to be stable for several hours.