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.