Date on Master's Thesis/Doctoral Dissertation

5-2012

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Chemical Engineering

Committee Chair

Sunkara, Mahendra K.

Author's Keywords

Carbon nanotubes; Doping; Diamond; Thermionic emission; Chemical vapor deposition (CVD); Field emission

Subject

Nanotubes; Waste heat; Heat recovery; Nanoparticles

Abstract

More than 50% of the total energy produced is typically rejected in the form of waste heat from various processes. The grand challenge is that most of this waste heat is released at temperatures much lower than 1000 C, which makes it difficult to recover it using traditional methods which require higher operating temperatures for thermal energy conversion. In addition, these traditional methods involve different intermediate processes and do not offer direct conversion into electricity. In this regard, thermal energy conversion through thermionic emission can offer direct conversion of waste heat into electricity with highest theoretical efficiency. So, waste heat recovery through thermionic emission energy conversion is of great interest and is the motivation for the present work. However the greatest challenge involves the discovery or availability of the material with appropriate work functions and stability criteria. To address the need for developing suitable materials toward thermionic energy conversion, we investigated phosphorus doping in individual diamond nanocrystals, conical carbon nanostructures (CCNTs) and diamond nanocrystals supported on conical carbon nanostructures. Hybrid architectures, diamond nanocrystals supported on high aspect ratio structures will allow the study of true performance of nanocrystals free from grain boundaries and also offer field enhancements. First, the synthesis of CCNTs over large area and flat substrates is investigated. From the experimental results, we successfully synthesized CCNTs on planar graphite and tungsten foil substrates with areas as large as (>0.5 cm2). A detailed underlying nucleation and growth mechanism was also demonstrated supported with regrowth experiments and kinetic growth model. Secondly, selective nucleation of the diamond crystals on the tips or complete coating on CCNTs was demonstrated and a likely mechanism for the nucleation and growth of diamond crystals is also presented. Thirdly, the field and thermionic emission characteristics from the as synthesized CCNTs have shown to exhibit enhanced emission characteristics such as low turn-on voltages, large field enhancement factor and lower work function values owing to their higher aspect ratios and optimum density overcoming field screening effects. Finally, phosphorus doping into these individual diamond crystals and diamond films was performed and thermionic emission characteristics were studied. Work function values as low as 1.8 eV from diamond films and 2.2 eV from diamond crystals was obtained. In summary, the main outcomes of this work include growth of large area CCNTs on flat substrates, discovery of the enhanced field and thermionic emission characteristics of CCNTs, selective nucleation and phosphorus doping of individual diamond nanocrystals on CCNTs free from grain boundaries and work function value as low as 2.2 eV from thermionic emission from these crystals.

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