Synthesis and field emission properties of carbon nanostructures

This dissertation focuses on developing carbon nanostructures for application as the electron emissive material in novel back-gated triode field emission devices. The synthesis, characterization, and field emission properties of carbon nanostructures, including 1-D carbon nanofibers (CNF), 2-D carbon nanosheets (CNS), and chromium oxide coated carbon nanosheets (CrOx-CNS), are presented in this work.;First, we have fabricated aligned carbon nanofiber based back-gated triode field emission devices and confirmed the operation of these devices. 1-D carbon nanofibers were directly synthesized on blank TiW substrates using direct current plasma enhanced chemical vapor deposition. It was found that the morphology of carbon nanofibers could be tuned from spaghetti-like to aligned by adjusting the applied plasma power. Field emission properties of spaghetti-like and aligned carbon nanofibers on blank TiW substrates were studied using the cartridge holder assembly. Results demonstrated that spaghetti-like carbon nanofibers had better field emission performance than aligned carbon nanofibers, however, the electrostatic simulation of the triode device demonstrated that aligned carbon nanofibers should yield the best device performance.;Second, we have demonstrated that carbon nanosheets, a 2-D carbon nanostructure developed by our group, were a competitive electron emissive material for application as the cold cathode in vacuum microelectronic devices. Carbon nanosheets were synthesized on a variety of substrates, without the need for catalysts, by radio frequency plasma enhanced chemical vapor deposition. Materials characterization results revealed that carbon nanosheets consisting of vertically oriented ultra-thin graphitic sheets terminating with 1-3 graphene layers were hundreds of nanometers in length and height but less than 4 nm in thickness. By using the diode holder assembly, field emission properties of carbon nanosheets were studied from a broad perspective, including turn-on and threshold field, maximum total current, emission lifetime and stability, and emission uniformity. The results revealed that the threshold field of nanosheets ranged from 3.5 to 5.2 V/mum, which was in the same range as 1-D carbon nanotubes and 3-D diamond. Moreover, the lifetime of nanosheets showed milliampere current emission (1.5 mA in a dc mode and 13 mA in a slow pulse mode) for hundreds of hours without significant current degradation after the conditioning process. However, the emission uniformity of nanosheets was quite poor due to the existence of hot runners during PEEM and FEEM observations. Further, the effectiveness of carbon nanosheet based back-gated triode field emission device was briefly studied.;Third, we have demonstrated that the emission uniformity of nanosheets could be improved by incorporating a thin chromium oxide coating. The chromium oxide coated carbon nanosheets were fabricated by vacuum evaporating thin chromium films on carbon nanosheets and sequentially exposing them to the atmosphere. The stoichiometry of the oxide was estimated to be 0.37, very close to Cr2O3. PEEM and FEEM observations showed excellent emission uniformity of chromium oxide coated carbon nanosheets. The field emission properties of chromium oxide coated carbon nanosheets were dependent on the coating thickness. The enhanced field emission performance of chromium oxide coated carbon nanosheets was observed with an appropriate thickness (from 1.5 nm to 15 nm). An explanation for this thickness dependence is suggested

Fatigue-creep life analysis for powder metallurgy material with inclusion

AbstractTo predict the life of the life limit parts is an important work for the airworthiness compliance. The compliance work of life limit parts involves typical flight profile, heat transfer analysis, stress analysis, materials data, and life predict method. In this study, fatigue-creep interaction behaviors of powder metallurgy materials with inclusion have been performed with the Kachanov-Rabotnov damage law to show the life compliance. On the constant fatigue-creep loading, different types of inclusion are investigated in the work. The research shows that the type of inclusion has strong influence on the damage of matrix. Generally, the elliptical inclusion has much more damage than the round inclusion. Furthermore, the fatigue-creep life of powder metallurgy materials is calculated by the damage model, which can be considered for the design of the engine life limit parts

The Adoption of advanced technologies for Hong Kong construction industry

Thesis (B.Sc)--University of Hong Kong, 2005.published_or_final_versio

Nanomechanical characterization of quantum interference in a topological insulator nanowire

The discovery of two-dimensional gapless Dirac fermions in graphene and topological insulators (TI) has sparked extensive ongoing research toward applications of their unique electronic properties. The gapless surface states in three-dimensional insulators indicate a distinct topological phase of matter with a non-trivial Z2 invariant that can be verified by angle-resolved photoemission spectroscopy or magnetoresistance quantum oscillation. In TI nanowires, the gapless surface states exhibit Aharonov-Bohm (AB) oscillations in conductance, with this quantum interference effect accompanying a change in the number of transverse one-dimensional modes in transport. Thus, while the density of states (DOS) of such nanowires is expected to show such AB oscillation, this effect has yet to be observed. Here, we adopt nanomechanical measurements that reveal AB oscillations in the DOS of a topological insulator. The TI nanowire under study is an electromechanical resonator embedded in an electrical circuit, and quantum capacitance effects from DOS oscillation modulate the circuit capacitance thereby altering the spring constant to generate mechanical resonant frequency shifts. Detection of the quantum capacitance effects from surface-state DOS is facilitated by the small effective capacitances and high quality factors of nanomechanical resonators, and as such the present technique could be extended to study diverse quantum materials at nanoscale.Comment: 15+16 pages, 4+11 figure