Thermionic emission from multiwalled carbon nanotubes MWNTs was investigated by using MWNT yarns. The work function of MWNT and thermionic emission constant of the yarn sample were both calculated from the thermionic emission data. The measured work function of MWNT is about 4.54-4.64 eV. The emission constant is larger than conventional thermionic cathode. Thermionic emission electron energy spectra at various temperatures have also been measured by using the MWNT yarn as the thermionic cathode in an electron momentum spectra instrument. The full width at half maximum of the spectra show a linear relation with the temperature. The carbon nanotube yarn may be used as thermionic cathode to replace the conventional cathode for their special properties.

An efficient method has been developed to fabricate field electron emitters by cutting a continuous multiwalled carbon nanotube MWNT yarn into segments. The cross section of each segment was composed of open-ended MWNTs which serve as field emitters. The emission current can reach several milliamperes, and the emitters can work stably for a long time. The excellent field emission properties are attributed to the large enhancement factor over 100000, dense emitting centers, and the robust structure. The emitters might be easily manipulated and assembled into devices, such as electron guns, x-ray tubes, pixel tubes, etc.

The multiwalled carbon nanotube MWCNT yarns converted from superaligned MWCNT arrays are selfassembled sheetlike structure in which MWCNTs are aligned along the stretching direction and will shrink into a tight fiber after passing through volatile solvents. In this paper, we report the comparative studies on the physical properties of the MWCNT sheets and shrunk yarns. The major differences between them lie in the surface area and intertube interactions, which result in different mechanical and electrical properties. Their surface areas were measured according to the energy conservation law and Stefan-Boltzmann law, manifesting that the surface area of the sheet is around 30 times larger than that of the shrunk yarn. With the measured surface area, the thermionic emission from them was further studied. There is no difference in their work function, emission efficiency, as well as Richardson's emission constant which is approximately equal to the theoretical value of 120.4 A cm−2 K−2. The electronic transport properties were studied in low temperature ranging from 3 to 390 K and high temperature around 1100-2300 K. In all the temperature range, there exists a resistance reduction from the MWCNT sheet to the shrunk yarn which was attributed to the increase of intertube crosstalk points between neighboring MWCNTs. It is also found that R decreases with increasing temperature until 1800 K above which R increases due to the electron-phonon scattering at high T.

We report controlled syntheses of super-aligned carbon nanotube (CNT) arrays with the desired tube-diameter, number of walls, and length for spinning continuous unidirectional sheets to meet a variety of industrial demands. The tube-diameter distribution of super-aligned arrays is well controlled by varying the thicknesses of catalyst films, and the length of them is tuned by the growth time. Further investigation indicates that the physical properties of the unidirectional sheets, such as electrical transport, optical transmittance, and light emission properties, can be well tuned by the tube-diameter-and length-controlled growth. This work extends the understanding of the super-aligned CNT arrays and will be very helpful in developing further applications.

Barium-functionalized multiwalled carbon nanotube yarns were fabricated by drawing and twisting multiwalled carbon nanotube forests through a solution containing barium nitrate. After heat activation under vacuum, the functionalized yarns were enriched in barium oxide due to the high surface-to-volume ratio of the nanotubes. The cathodes exhibited good thermionic properties, with a work function as low as 1.73-2.06 eV and thermionic current density that exceeded 185 mA/cm2 in a field of 850 V/5 mm at 1317 K. The barium-functionalized yarns had high tensile strength of up to 420 MPa and retained strength of 250 MPa after a 2 h activation process.

Nano is one of the hottest topics in current science and technology. Characterizations of nanomater ials by high resolution transmission electron microscopy (TEM) are becoming indispensable today. To gain better performance of TEM, people are expecting a novel TEM grid of which the supporting film should be highly conductive, ultrathin but robust, and preferably nanoholey. Here we report a method of mass producing such a kind of carbon nanotube TEM grids. The supporting films are made by cross stacking ultrathin superaligned carbon nanotube films, resulting in a large number of nanosized holes and numerous effective edges. Together with the robustness, good conductivity, and strong adsorbability inherited from carbon nanotubes, these TEM grids show much better performance than conventional amorphous carbon grids.

We found that very thin carbon nanotube films, once fed by sound frequency electric currents, could emit loud sounds. This phenomenon could be attributed to a thermoacoustic effect. The ultra small heat capacity per unit area of carbon nanotube thin films leads to a wide frequency response range and a high sound pressure level. On the basis of this finding, we made practical carbon nanotube thin film loudspeakers, which possess the merits of nanometer thickness and are transparent, flexible, stretchable, and magnet-free. Such a singleelement thin film loudspeaker can be tailored into any shape and size, freestanding or on any insulating surfaces, which could open up new applications of and approaches to manufacturing loudspeakers and other acoustic devices.

The authors report the synthesis of multiwalled carbon nanotubes on transparent substrates by utilizing a diode laser to locally heat the catalysts to high temperature. Well-aligned bamboo-shaped multiwalled carbon nanotubes can be synthesized on glass substrate covered with a carbon black layer. If the carbon black was substituted by the commercial graphite inner coating, randomly oriented but high quality multiwalled carbon nanotubes were obtained with excellent field emission performance. This approach facilitates the fabrication of multiwalled carbon nanotube field emitters for field emission flat panel displays without limitations on the geometry size and temperature requirement of the substrate.

A novel and simple growth mark method was developed to make marks during the growth process of carbon nanotube arrays. These marks can be read out under scanning electron microscope or optical microscope. Based on this method, the growth rates at different temperatures and under different acetylene partial pressures were measured, from which the activation energy and the order of reaction were determined. Based on our experimental results, the growth of carbon nanotube arrays in our experimental condition could not be diffusion-limited. The measured activation energy could possibly be attributed to the heterogeneous decomposition of acetylene over the catalyst particle. Furthermore the marked array with special segmental structure may be found some applications in the future.

The effect of carbon deposits on the reactor wall during the growth of multi-walled carbon nanotube (MWCNT) arrays was studied. It was found that the carbon deposits were helpful in converting the continuous catalyst film into carbon-containing nanoparticles before growth started, which was beneficial for the nucleation of MWCNT arrays. As a result, MWCNT arrays synthesized in a reactor with carbon deposits were well aligned with high nucleation density. Further study revealed that this effect could be probably ascribed to some activated species adsorbed in the carbon deposits. These results will be helpful in the repeatable synthesis of high quality MWCNT arrays and in understanding the nucleation process of MWCNT arrays.