Development of carbon nanotubes with a diamond interlayer for field electron emission and heat transfer applications

Carbon Nanotubes (CNTs) have great potentials for Field Electron Emission (FEE) and Flow Boiling Heat Transfer (FBHT) applications. However, their weak adhesion on metallic substrates limits the development of CNTs in both applications. Diamond has high thermal conductivity and develops strong bonding with CNTs. The development of a diamond interlayer between CNTs and substrates is a feasible approach to address the adhesion problems. The purpose of this research was to develop a new CNT-based materials with a diamond interlayer for FEE and FBHT applications by focusing on four objectives: (1) enhancement of diamond thin film adhesion on a Cu substrate, (2) improvement of the CNT FEE stability, (3) reduction of the CNT FEE turn-on field, and (4) investigation of the FBHT performance of CNT based structures. The CNTs and diamond thin films in this thesis were prepared by Microwave Plasma enhanced Chemical Vapor Deposition (MPCVD) and Hot Filament enhanced Chemical Vapor Deposition (HFCVD). The structure and chemical states of the diamond films and CNTs were characterized by Scanning Electron Microscopy (SEM), cross-sectional Transmission Electron Microscopy (TEM), X-Ray Diffraction (XRD), Raman spectroscopy, synchrotron based X-ray Absorption Spectroscopy (XAS). To deposit diamond thin films on a Cu substrate with sufficient adhesion strength, a sandblasting pretreatment and alloying with a tiny amount of Al were investigated. The adhesion of diamond thin films to substrates was evaluated by Vickers micro-hardness indentation. The FEE stability and turn-on field were measured by a Keithley 237 high voltage measuring unit. The FBHT property of the structures was tested repeatedly at different flow velocities to explore the dependence of heat transfer performance on certain parameters, including the flow patterns, Critical Heat Flux (CHF), and stability. The results show that sandblasting pretreatment increases the surface roughness and surface defect density, thereby increasing diamond nucleation density and adhesion to the Cu substrate. Al alloying appears to inhibit the formation of graphite at the interface between diamond and the Cu substrate, which improves the chemical bonding between diamond and the Cu substrate and increases the adhesion strength between them. The FEE testing results show that ultra-high FEE stability (more than 5000 minutes) was achieved for the CNTs with a diamond interlayer. This is attributed to the good contact at the diamond-CNT and diamond-substrate interfaces. The main factors that affect the CNT FEE turn-on field were also studied. By optimizing the structure, an FEE turn-on field of 5.1 V/μm was achieved and an emission barrier model for CNTs with a diamond interlayer on Cu substrate was used to explain the results. FBHT testing was done on CNTs with different structures and the results show that high heat transfer efficiency can be achieved on CNTs with a diamond interlayer at low mass fluxes