The effect of cesium and potassium atoms deposited onto multiwalled carbon nanotubes grown in an electrical arc on their emission characteristics was studied. The current–voltage characteristics of the field electron emission of specimens with cesium or potassium doped multiwalled carbon nanotubes of this type were revealed to retain their linear character in the Fowler–Nordheim coordinates within several orders of magnitude of change in the emission current. The deposition of cesium and potassium atoms was shown to lead to a considerable increase in the emission current and a decrease in the work function φ of studied emitters with multiwalled nanotubes. The work function was established to decrease to φ ~ 3.1 eV at an optimal thickness of coating with cesium atoms and to φ ~ 2.9 eV in the case of doping with potassium atoms. Cesium and potassium deposition conditions optimal for the attainment of a maximum emission current were found.

Исследовано влияние атомов цезия и калия, нанесенных на многостенные углеродные нанотрубки, выращенные в электрической дуге, на их эмиссионные характеристики. Обнаружено, что вольт-амперные характеристики автоэлектронной эмиссии образцов с многостенными углеродными нанотрубками данного типа, легированных цезием или калием, сохраняют свою прямолинейность в координатах Фаулера-Нордгейма в пределах нескольких порядков изменения эмиссионного тока. Показано, что нанесение атомов цезия и калия приводит к значительному увеличению эмиссионного тока и уменьшению работы выхода varphi исследуемых эмиттеров с многостенными нанотрубками. Установлено, что работа выхода уменьшается при оптимальной толщине покрытия атомами цезия до varphi~3.1 eV, а в случае легирования калием --- до varphi~2.9 eV. Найдены оптимальные условия нанесения цезия и калия для достижения максимального эмиссионного тока. DOI: 10.21883/FTT.2017.04.44289.344

A small-sized X-ray tube with a carbon nanotube planar field electron emitter has been manufactured and tested. Field electron emitter was grown by the chemical vapor deposition (CVD) method on a nickel substrate. During long time tests (more than 50 hours) the X-ray tube has demonstrated stable characteristics of the emitter current in the dc mode. Relative emission current fluctuations were about 0.3% at the emitter emission current equal to 550 μA.

Planar field electron emitters containing carbon nanotubes that demonstrate stable emission currents with densities about 1 A/cm2 have been fabricated using the chemical vapor deposition method. The analysis of field emission characteristics based on Fowler–Nordheim theory allowed one to calculate not only the field amplification coefficient β, but also the total emission area A of all the emitting nanotubes. Using the value of A and an estimate of the emission area of an individual nanotube fulfilled in this work, the approximate number of emitting nanotubes N before and after the flow of the high density emission current, has been calculated. It has been found that the concentration of the emitting nanotubes N in such field emitters after the high density emission current flow was about 105 cm−2. Besides this, the location of light radiating nanotubes heated by high density emission currents has been investigated. This has revealed the regions of the emitter surface that gave the main contribution to the electron emission.

A compact X-ray tube with a field emitter based on carbon nanotubes is developed. Over a long time interval, the X-ray tube maintains an anode current of 300 μA, an anode voltage of 10 kV, and the stable characteristics of the field emitter.

For carbon layers with very long and sparse nanotubes (nanofilaments) field electron emission at very low average electric field was observed. Field emission current of 10 mu A was obtained at the average electric field E-av = 0,16V/mu m and the value of the field amplification coefficient beta reached 45,000. At high emission currents (exceeding 30-50 mu A) one or several luminous nanotubes (nanofilaments) heated by the emission cuurent have been observed in the gap between the sample and the anode.

Planar field emitters based on carbon nanotubes are studied in an ultrahigh-vacuum chamber and sealed-off vacuum devices. In the sealed-off devices, an emission current of 1 mA is obtained at a mean electric field of 5.2–5.8 V/μm and the relative fluctuation of the emission current is 0.2–0.4%. The emission characteristics of the devices remain unchanged over a storage period of one year.

Field emitters on the base of carbon nanotubes can be used in various electron vacuum devices. We describe the construction and parameters of a prototype of an X-ray tube with the carbon nanotube field emitter. The X-ray tube is a glass device 40 mm long and 16 mm in diameter

Field-emission characteristics of carbon layers with very long (up to several mm) and sparse nanotubes (nanofilaments) have been investigated. For such layers field emission current of 10μA is registered at very low average electric field Eav=0.16V∕μm and the values of the field amplification coefficient β reach 45 000. It has been found that, at electric fields corresponding to the onset of the field emission, the emitting nanotubes are stretched towards the anode. At high emission currents (exceeding 30–50μA), one or several luminous filaments have been observed in the gap between the sample and the anode. These luminous filaments are carbon nanotubes (nanofilaments) heated by the emission current.

Field emission characteristics of single-walled carbon nanotube layers have been investigated at room and low temperatures. For these layers the emission current density of 10mA/cm(2) was obtained at the average field E-av = 1.6-3.8 V/mum. Current-voltage characteristics in Fowler-Nordheim coordinates have a break at emission current about 10(-8) A. Cooling of samples only insignificantly changed the form of current-voltage characteristics. This indicates, that investigated single-walled nanotubes have the metal type conductivity.

Nanotubes were fabricated by carbon evaporation at high gas pressure (gas–nitrogen or nitrogen–argon, pressure up to 1300 atm). Three main types of tubes were observed and are characterised as follows: (1) Multilayered surface modulated micro- and nanotubes (SMMTs and SMNTs, respectively) with fluctuating diameters caused by periodically interrupted movement of catalytic particles during tube formation. All tubes of this type have a common principle of structural organization: the main part of the walls is formed by continuous graphene layers to which the side layers of inner caps are connected. An extreme form of SMNTs was observed in which the carbon nanotube walls were spheroidal. (2) Stacked conical layer carbon nanotubes (CLNTs). These nanotubes have open edges and were found to readily absorb argon. (3) Multiple walled carbon nanotubes (MWNTs) consisting of concentric cylindrical layers. The inner channels of these nanotubes were sometimes observed to be filled with Fe carbides. The growth of (1) and (2) type nanotubes is promoted by Fe catalytic particles. At reduced catalyst concentration and at high temperature, most of the observed nanotubes are type (3). In addition the presence of N2 in the gaseous mixture was found to enhance the rate of graphite evaporation and increased the nanotube yield.

The structural components and the internal organization of carbon cathodic deposits fabricated using an arc with the conditions adjusted for the effective production of nanotubes have been characterized by transmission electron microscopy (TEM), high resolution electron microscopy (HREM) and scanning electron microscopy (SEM). Typically, such deposits are columnar structures oriented along the growth direction. Three main components were observed: multiwalled nanotubes, multilayer polyhedral particles and curved graphitic formations. The measured distributions and relative quantities of the components depended on the deposition regimes. Column interiors were composed of a mixture of all these components, while the outer covering of the columns was formed predominantly of nanotubes. Multiwalled nanotubes formed a mesh-like arrangement around these columns and their growth surfaces, with smaller amounts of nanotubes present inside the columns. Nanotubes in various fragments of the deposits were mainly oriented at high angles to the deposit axes. In the column coverings, groups of tubes oriented at angles >45° to the axes were present with no sets of nanotubes or bundles aligned along the deposit axes seen. Finally, a mechanism of deposit formation is proposed in connection with the recorded data.

Layers (similar to 1 mu m thick) of crooked carbon nanotubes 20-60 nm in diameter were obtained at 800 degrees C by pyrolysis of polyethylene in He atmosphere using a Ni plate as a catalytic substrate. They were identified by electron microscopy and electron diffraction. Tips of some tubes (similar to 10(6)/cm(2)) come out of the layer. We studied their field electron emission at 10(-9) torr vacuum. 1-10 mA/cm(2) current density was observed at 4-10 V/mu m (V similar to 100-250 V). (C) 1998 Elsevier Science Ltd. All rights reserved.

Carbon nanotube layers, deposited on Ni substrates by chemical vapour deposition from polyethylene, have been shown to be low voltage field emitters of electrons.

The work is dedicated to the extension of the ability to identify fullerene and nanotube forms of carbon in solid objects by X-ray emission spectroscopy applied under severe conditions of low dose (less than or equal to 5x10(-2) C/cm(2)) irradiation of samples by a primary electron beam in the course of spectrum registering. Besides spectrum smoothing, extraction of fine structures connected with density of the valence state characteristics is used. This is fulfilled by deconvolution and Fourier analysis methods.

Carbon nanoparticles of fullerites and single layer nanotubes were detected in Fe-Ni-C, Ni-Fe-C and Fe-Ni-Co-C alloys, manufactured from powders of corresponding metals and graphite. Their identification was fulfilled by methods of X-ray emission spectroscopy in conjunction with scanning electron microscopy, and by high resolution transmission electron microscopy.

It is shown that X-ray emission spectroscopy can be effective tool for identification of new allotropic forms of carbon especially in inhomogeneous samples. Smoothing of spectra registered at low doses of electron beam irradiation (less than 0.5 C/cm(2)) and spectral resolution improvement by deconvolution method are used to increase the reliability of analysis.

Thin films of disordered graphite wore obtained by chemical vapor deposition (CVD) on silica substrate using hydrocarboneous polymers as starting materials. Several experimental methods revealed the presence of unusual form of carbon in these films. HREM images confirmed definitely the formation of single layer nanotube bundles and fullerite nanocrystals. Hence, the possibility of formation of carbon nanostructures at relatively low temperatures (600 degrees C to 700 degrees C) has been established. It is demonstrated that CVD method can be used in principle for the production of carbon nanostructure films on silica substrate.

In carbon inclusions in Fe(25 %)-Ni(70 %)-C(5 %) alloys of a specially made composition, where the X-ray emission spectroscopy (CKalpha line) has shown the tubelene- and/or fullerene-like nearest atomic order, the structure of skeleton carbon, that is characteristic for fullerenes and tubulenes of various types, has been found by the high resolution transmission electronmicroscopy (HREM). Such carbon forms have been synthesized in the specific environment of the closed space of micropores in Fe-Ni-C alloys.