Date of Award
Doctor of Philosophy (PhD)
Ganapati R. Myeni
James L. Cox
J. Wallace Van Orden
A number of sensitive applications would be greatly benefited by the development of better cold cathodes that employ the electron field emission process. Among the many kinds of field emitters that could be tried, carbon nanotubes (CNT) have a number of distinct advantages because of their unique geometrical structure, chemical inertness, mechanical stiffness, and high thermal and electrical conductivities. This dissertation describes research in which CNT cathodes were fabricated and their emission characteristics were measured.
Multi-walled carbon nanotubes (MWNT) were grown by chemical vapor deposition (CVD) on various substrates: Ni and Hastelloy gauze, 304 stainless steel (SS) plates, and Ni-coated Si wafers. Either C2H2/Ar (or N2) source gases were used in a temperature range from 650–780°C. Nanotubes were produced with diameters that varied from 20nm to 300 nm, depending on the substrate and temperature. Structures of these nanotubes were characterized by Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), Atomic Force Microscopy (AFM), and Raman scattering.
Field emission performance for samples of carbon nanotubes deposited on the various substrates was intensively investigated. Nanotubes grown on the Ni substrates were found to have turn-on fields of 1.0–2.0 V/μm, the lowest obtained. The emission from all individual samples was reproducible within 3% among operation cycles. Variations of less than 7% among different batches were found for MWNT grown on SS substrates. MWNT on the gauze substrates were very stable emitters up to a pressure of 10−6 Torr in air. Our experiments revealed that there exist absorption dominated, intermediate, and intrinsic emission regimes caused by three different gas-surface reaction processes. Operation of CNT emitters in a hydrogen atmosphere was found to improve emission stability. Tube deformation, elastic or plastic, was found to occur for high electric fields. Emission performance was also characterized by surface emission mapping and by emission pattern imaging. Experiments suggest that the emission current from a single carbon nanotube could be greater than 20 μA for the sample grown on the Hastelloy substrate and ∼4 μA for tubes grown on SS.
With the help of computer simulation, an optimum design for an ion gauge with a CNT electron source was developed. This gauge was built and its operation was investigated. A total emission current of 64 μA was obtained for a CNT cathode on Ni substrate at an acceleration gate voltage of 310 V. Electron transmission through the gate grid was found to be 70–75%, only ∼10% lower than the gate transparency. This ion gauge had excellent linearity from 10−6 to 10−10 Torr, with gauge sensitivity between 2 and 2.5/Torr for nitrogen. This gauge will find application in ultra-high vacuum and extreme-high vacuum (UHV/XHV) applications.
"Field Emission Based Sensors Using Carbon Nanotubes"
(2003). Doctor of Philosophy (PhD), Dissertation, Physics, Old Dominion University, DOI: 10.25777/cfn6-n568