| Summary: | 博士 === 國立清華大學 === 材料科學工程學系 === 96 === Vertically aligned nano-scale diamond tips have been successfully synthesized on rugged polycrystalline diamond substrates and micro-size diamond particles. Nano-scale diamond tips, ~ 1 um in height and the diameter at the bottom of a nanotip is ~200 nm, were synthesized on diamond film for 2 hr. The thermal properties of a diamond heat spreader made of nano-scale diamond tips on a diamond/Si substrate were measured. The nano-scale diamond tips act as a nano-scale “fin structure”, which may provide extended surface area to dissipate heat away. A concept of “quasi thermal resistance” is developed to characterize the thermal spreading properties of the nano-scale diamond tips/diamond/Si structures at various air flow rates in a wind tunnel. Experimental results indicate that nano-scale diamond tips help to improve the thermal spreading properties, especially at zero air flow rate. At an air flow rate higher than zero, the nano-scale diamond tips slightly effect on improving heat spreading property since the air flow would be limited among nano-scale diamond tips and the extended surface area would be neglected. When nano-scale diamond tips are deposited on the diamond/Si substrate, the quasi thermal resistances are reduced further by ~7.3 % at 0 CFM, ~2 % at 3 CFM and ~1 % at 6 CFM.
Diamond nanotips with high aspect ratio can be synthesized on the diamond/Si substrates by planar microwave plasma enhanced chemical vapor deposition. The field emitter made of the as-grown diamond nanotips suffers with high turn-on voltage and low emission current density. A nitrogen PIII treatment can improve the field emission properties. A low turn-on field of 3 V/um and a high current density of 4 mA/cm2 at 9 V/um are achieved.
Vertically oriented nano-scale diamond tips were synthesized on the micro-size diamond particles in a planar microwave plasma enhanced chemical vapor deposition chamber by varying synthesis time, bias voltage, and gas ratio. The optimum bias voltage to achieve nano-scale diamond tips and the density of nano-scale diamond tips can be tuned primarily by varying the gas ratio of Ar/CH4/H2.
The quasi thermal resistances of both micron-size and nano-size diamond structures were determined at various flow rates in a wind tunnel. Experimental results indicate that micron-size diamond is much more effective in heat dissipation than nano-size diamond. With 350-420 um diamond particles on top of the aluminum plate, the quasi thermal resistance reduces by 13 % independent of air flow rate. The reduction of quasi thermal resistance is attributed to the increase of the uncovered diamond surface area. Diamond particles act as a fin structure in the design of a heat spreader. Experimental results support that nano-size diamond structures play no role in the reduction of quasi thermal resistance at various air flow rates. No heat dissipation is contributed from the nanoscale channels among nano-size diamond structures.
In appendix A, Carbon nanosheets were success to grow uniformly on flat Si and pyramidal Si in a plasma-enhanced chemical vapor deposition process. By replacing flat Si(100) by pyramidal Si(100) as the substrate, the field emission performance of carbon nanosheets was greatly improved. A low turn-on field of 3.2 V/um and a high current density of 2.5 mA/cm2 at 7.0 V/um were achieved for the field emitter of carbon nanosheets/pyramidal Si(100). The enhancement factor β increases from ~4400 to ~9500 at low electric field when carbon nanosheets are synthesized on pyramid Si(100) rather than flat Si(100).
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