Field Emission Tubes Research Articles

Technique to Make Nano Carbon Tubes Buried Inside Rubber Composite Visible Using Femtosecond Pulse Laser Ablation

A method to observe the dispersion structure of carbon nanotubes buried inside a molding composite is described in this communication. The idea is that by utilizing the difference in ablation temperature between the tubes and the composite material, only the composite material, which has a lower ablation temperature, is ablated and buried tubes appear. This was applied to a rubber composite mixed with vapor-grown carbon fibers. A 150-fs pulse laser was prepared as an ablation source. In the ablation images observed with a field-emission secondary electron microscope, linear tubes, clusters, and connections of the tubes could be easily found in their original state.

Large‐signal and small‐signal electronic equivalent circuits for a field electron emitter

AbstractLarge‐signal and small‐signal electronic equivalent circuits are developed to describe signal generation by a field electron emitter, with and without a ballast resistor. It is shown that different small‐signal equivalent circuits are needed for the cases where a small alternating voltage variation is applied and where the tunneling process is directly modulated, e.g. by directing a laser at the field emitter. The role of circuit resistances and of the parasitic capacitance between the emitter and its surroundings is crucially different in the two cases, with low‐pass filter behaviour exhibited in the first case and high‐pass filter behaviour in the second. Some consequences for the technical use of a field electron emitter as a non‐linear device are discussed, particularly in applications that involve laser‐assisted field electron emission and in the context of testing field emission tubes. Copyright © 2004 John Wiley & Sons, Ltd.

Prototypes using metal, carbon fiber and composite field emission sources modulated by a laser beam

Field emission of electrons from a variety of metallic, carbon fiber and composite metal–insulator micropoint cathodes was employed in this study. Tungsten, carbon fiber and ZrC tips, were studied using a field emission microscope. These cathodes were characterized and the current–voltage ( I– V) characteristics were determined. A variety of surface treatment procedures were carried out to increase the stability of emission. These electron sources were mounted in sealed prototype field emission tubes, while others were tested under medium, high and UHV conditions. The emission current switch-on phenomenon was found with all non-metallic cathodes. The emitters were then subjected to a square wave-modulated, maximally focused laser diode beam ( λ=658 nm, 30 mW). The beam impedance (∼1 GΩ) and the anode capacitance (∼10 pF) act as a low-pass filter.

Influence of silicon doping density on vacuum field emission triode at high frequency range

Silicon FEA will affect the high frequency application of field emission tubes when it works at the microwave frequency range. This article shows that the electron emitting will be influenced by the majority carrier response time in semiconductor silicon. The surface capacitance and delay time of n-type and p-type silicon are calculated by using semiconductor theory. The result shows that the semiconductor conductivity will determine the maximum work frequency of device. The maximum work frequency (no considering other effects such as Cgc, gm etc.) will be decreased from about 200 GHz to 2 GHz when the resistivity of p-type silicon is increased from 0.1 Ω · cm to 10 Ω cm.

Field desorption and surface diffusion of sodium on germanium (100) and (111) planes

Field desorption and surface diffusion of sodium on atomically clean germanium (100) and (111) single-crystal planes has been investigated by means of the field emission method. Sealed-off field emission tubes with a channel multiplier were used. The surface density of sodium adatoms on the germanium surface was determined by using a standard tungsten (112) crystal plane. The temperature dependence of the desorption field has been studied with the aim of determining the binding energy of sodium adatoms at zero coverage. The values of 1.83 ± 0.05 eV and 1.72 ± 0.05 eV were obtained for the germanium (100) and (111) planes, respectively. The correlation between the adsorbate and substrate properties and the properties of alkali adlayers on a semiconductor surface is discussed on the basis of experimental results.

Surface diffusion of potassium on (100) and (111) germanium planes

Surface diffusion of potassium on atomically clean (100) and (111) germanium single crystal planes has been investigated by means of the field emission method. Sealed-off field emission tubes with a channel multiplier were used. The surface density of potassium adatoms on the germanium surface was determined by using a standard tungsten crystal plate, movable between the adsorbate atom flux and a low-energy electron gun. The influence of the external electric field on the surface diffusion of potassium has been studied with the aim to determine the dipole moment of the potassium adatoms on the germanium surface. The obtained values of 4.5 ± 0.2 and 4.1 ± 0.2 debye for potassium adatoms on (100) and (111) germanium planes, respectively, are in good agreement with initial dipole moments calculated from work function measurements. The influence of the atomic relief and the electronic structure of the substrate on the properties of an alkali adlayer on a semiconductor surface is discussed on the basis of experimental results.

Work function of the adsorption system of potassium on tungsten

The work functions φ hkl of tungsten single crystal planes as functions of the surface densities N hkl of the adsorbed potassium have been measured by means of the field emission method. Sealed-off field emission tubes with a Faraday collector and rotatable emitter were used. Another, special tube was made in order to determine the surface density of potassium. The unequilibrated adlayers on {110}, {112}, {100} and {111} tungsten single crystal planes have been investigated. For all the planes investigated the φ hkl ( N hkl ) dependence exhibited a distinct minimum. An attempt has been made to compare the experimental results with the theoretical models suggested recently.

The Use op Field Emission Tubes in X-Ray Analysis

Recently developed, miniature, steady state, field emission tubes are finding application in several areas of x-ray analysis. These tubes require only a high voltage, low current power supply to produce relatively intense beams of x-rays. Since anodes can be fabricated from almost any element, and since the tubes can be operated at potentials up to about 70 kV, many different output x-ray spectra are available. Miniaturized battery operated x-ray sources of this type, occupying a volume of about one liter, have several advantages over radioisotope sources. These include cost, safety, and controllable output spectra and intensity. X-ray sources for energy dispersive fluorescence analyzers are designed so that no scattered characteristic radiations will interfer with the analysis of the sample fluorescence. Sources which are essentially monoenergetic can be fabricated for use in non-dispersive x-ray fluorescence analyzers. Because of the intensity and safety of the field emission tubes, such analyzers can be made which are sensitive while compact, portable, and inexpensive. In x-ray absorption analysis the measurement of absorption edge jump ratios provides a quantitative measure of sample impurities. Field emission tubes whose output spectra consist primarily of bremsstrahlung are particularly well suited to such measurements. The techniques involved in using these tubes in x-ray analysis are described.

Field Emission in Liquids

Field emission from tungsten emitters into liquid H2, D2, He, Ar, N2, O2, and C6H6 has been carried out in modified field emission tubes, which permitted switching from ultrahigh vacuum to very pure liquids and back to vacuum. In all cases space-charge-limited behavior was found at >10−9 A, from which the following mobilities (in centimeter2 volt−1·second−1/units) could be estimated: 3 × 10−2 cm2 V−1·sec−1 for H2 at 21°K, 0.9∼10−2 for D2 at 21°K, 4 × 10−2 for He at 4.2°K, 0.36 for He at 1.8°K, 8 × 10−3 for O2, 8 × 10−3 for N2, and 4 × 10−3 for C6H6. In the case of Ar a limiting velocity of 5 × 106 cm/sec rather than a field-dependent mobility was observed. For He and Ar onset of high emission (10−9 A) is abrupt, indicating electron avalanching with concomitant gas-bubble formation at the tip which leads to sputtering. In the case of O2 electrochemical etching occurred even at low currents. For H2 and D2 a true Fowler–Nordheim field emission regime could be observed for i≤10−10 A, from which an energy barrier of 0.1–0.5 eV to the solution of free electrons in H2 was estimated.

Noise measurements on field-emission tubes

An attempt has been made to measure space-charge noise suppression in field emission. The attempt was unsuccessful, but in the process we extended the diffusion noise and shot noise measurements to two orders of magnitude larger currents, and we learned something about the pre-breakdown condition in field-emission tubes.

The generation of recurring high-voltage X-ray impulses

Voltage-multiplication devices, such as the Marx and similar inductance circuits, have been operated at high recurrence frequencies and used as voltage sources for X-ray generation. The merits of this arrangement and the problem of obtaining optimum X-ray efficiency are discussed in relation to the design of voltage network and X-ray tube.The operation of the generator depends upon the production of short rectangular pulses, and this has been achieved by incorporating delay lines in each stage of the generator. In consequence, the generator has a characteristic impedance which, for optimum efficiency, must equal the X-ray tube impedance. The latter is non-linear, but it is shown that this is an advantage rather than a disadvantage because it suppresses multiple reflections.The X-ray-tube problem differs from that of the ordinary constant-voltage tube in that a higher peak emission is required. The possibilities of using thermionic, field-emission or low-pressure gas-filled tubes are discussed.