Cold Cathodes based on Carbon Nanotubes for micro-propulsion systems and Ion beam neutralisation
Cold cathodes based on carbon nanotubes are proposed for micro-propulsion and neutralization systems. They are characterized by outstanding electron emission properties due to their conductivity and high aspect ratio that helps to focus the electric field on their tip. By combining catalysed CVD, proper substrate preparation choice and synthesis parameters it is possible to determine the structural characteristics of the nanotube deposits to produce highly optimised self-focusing electron beams.
Cold cathodes based on carbon NanoTubes (CNT) can be divided in 2 main classes, single-walled (SWCNT) and multi-walled (MWCNT). They are characterized by outstanding properties of electron emission by the Field Emission (FE ) mechanism, due to their conductivity and high aspect ratio that helps to focus the electric field on their tip and thus to emit electrons at lower voltages. During the last decade the process of electron emission from carbon nanostructures (mostly carbon nanotubes) has deeply been studied and innovative solutions together with enabling methodologies for the manufacturing of “cold cathodes“ with designed geometries and defined functional properties have been developed by the donor. The growth of the nanotube arrays is made by a catalysed Chemical Vapor Deposition (CVD) technique starting from hydrocarbon/hydrogen mixtures, at temperatures in the range of 600-800°C and pressures in the range of 20-80 Torr. The substrates can be Si, metals oxides, alloys. A proper choice of substrate preparation (nature and structure of the catalyst, patterning) and of synthesis parameters (hydrocarbon/hydrogen ratio, flowing rate of gaseous mixture, deposition temperature and pressure) determines the structural characteristics of the nanotube deposits (SWCNT or MWCNT) as well as height and density of the bundles. The novel concept of electron source based on carbon nanotubes, in some cases coupled with diamond nano-grains, is exploited in the donor labs by producing cathodes with various geometries and different distribution of the emitters. The options go from nanostructured flat substrates with aligned arrays of CNT bundles, to the control of bundle density and of their orientation, to the growth of packed CNT bundles, each one emitting a tiny beam of electrons, on reduced areas and nano-sized spots patterned by lithography, to the growth of CNT on metal needles and wires (Ta, W, steel…), to the assembling of self-standing micro-sized fibres (diameter of 100-200 micron) consisting of aligned CNT. A further kind of self-standing cathodes can be fabricated by growing vertical arrays of CNT bundles inside the pits of honeycomb Al oxide templates and by subsequent substrate removal. The refining and improving of the cathode design are addressed to establish the effectiveness in producing self-focusing electron beams. The nanotube technology offers the way to produce ideal electron emitters. The structure of the nanotubes, consisting in sp2-coordinated no-polar covalent bonds between C atoms, is responsible for the outstanding mechanical and thermal properties of CNT: Young modulus up to 1.8 TPa, thermal stability up to 2000°C in vacuum, at about 900°C in oxygen atmosphere. Their small diameters and the high aspect ratio provide the focusing of the electric field on the tips and the lowering of the required voltages. The emission is from nano-sized point sources instead of a spread from a single, large point as in conventional systems. As a result, the electron emission from nanotubes is characterized by low turn-on and threshold fields and by a low energy spread of about 0.2 eV. Nanotubes can carry high current densities without damaging The structure of the fabricated cathodes can be tailored and the performances can be improved by a proper packaging of the device. As an example, in the triode configuration the modulating grid can be integrated with the anode. Cold cathodes assembled with CNT can be fabricated using easy, low-cost and scalable methodologies. Also the coupling of CNT with nano-diamonds is obtained by already settled processes, using a variety of substrates and of cathode geometries.
Innovations & Advantages
Nowadays the devices used for advanced applications require reliability, cost effectiveness and high performances. Conventional cathodes utilize electron beams produced by thermo-ionic emission from heated metal filaments. The emission by the thermo-ionic mechanism is characterized by slow response times, high power consumption, short lifetime of the emitter and broad spatial distribution of the radiation. In recent years, a new scenario has been opened by the availability of nanostructures characterized by efficient FE and feasibility to fabricate the so-called “cold cathodes”. Initially nanostructured surfaces with arrays of tips emitting tiny beams of electrons were produced using metallic nanowires. However, metals are limited in carrying/emitting high currents densities by metal atoms electro-migration, that leads to structural failures and strongly shorts the working life of the emitters. Moreover, thermal expansion/contraction processes can cause shear strains at the end boning producing detachment of the wires from the substrate. Significant advances have been made by the use of carbon nanotubes, characterized by a number of versatile electrical, mechanical and thermal properties.
The use of different types of CVD reactors and of proprietary post-synthesis steps allowed to engineer emitting electron sources for specific application, such as the fabrication of miniaturized X-ray tubes or for vacuum m tube amplifiers for THz radiation. Other interesting applications regarded the use of cathodes for MW amplifiers, vacuum electronics and display technologies. A CNT-based electron gun was used in the frame of the CANTES-INFN project to inject electrons in ECR plasmas. The novel concept of electron source based on carbon nanotubes, in some cases coupled with diamond nanograins, was exploited by investigating several geometries of the cathodes and different distribution of the emitters. The studies were addressed to establish the effectiveness in producing self-focusing electron beams. The options go from continuous and homogeneous deposits on flat substrates of nanotubes with controlled bundle density and orientation/alignment, to the growth of packed CNT bundles on reduced areas and nanosized spots patterned by lithography, to the growth of CNT on metal needles and wires, to the assembling of self-standing microsized fibres(diameter in the range of 100-200 micron ) consisting in an ensemble of aligned CNT.A further kind of cathodes was fabricated by growing vertical arrays of CNT bundles inside the etched pits of honeycomb Al oxide templates. Moreover, FE properties and lifetime of nanocrystalline diamond, in form of films, whiskers, pillars, or of hybrid systems made by coupling thin layers of nanodiamonds with CNT arrays have been explored as well.
Current and Potential Domains of Application
The proposed use for the cold CNT cathodes is as electron sources to provide propellant ionization and ion beam neutralization for high efficient and high versatile electrically powered micro-propulsion systems.