奈米流體熱傳導係數的研究及奈米磁性流體於變壓器上的應用

Abstract

本論文主要討論奈米流體的熱傳導係數及奈米磁性流體的應用。在第一個主題中，我們討論了基礎流體的黏滯性對奈米流體的熱傳導係數的影響。實驗結果顯示，在低黏度的基礎流體中，奈米顆粒對奈米流體的熱傳導係數有明顯的增益。對於低黏度的奈米流體，量測到的熱傳導係數高於Maxwell模型所估計的值。當基礎流體的黏滯性增加時，量測到的奈米流體的熱傳導係數會越來越趨近於Maxwell模型的估計值，這表示了奈米流體的黏滯性會影響它們的熱傳導係數，以及懸浮的奈米顆粒的布朗運動大大地增進了奈米流體的熱傳導係數。此外，由於奈米磁性流體具有一些特殊的性質，因此衍生了新的應用。在第二個主題中，奈米磁性流體及四氧化三鐵塊被用來作為變壓器的磁芯。本論文使用的變壓器建構於毛細管及晶圓上。我們針對不同磁芯的變壓器的效能做了量測及模擬。雖然四氧化三鐵的存在增加了電感值及耦合係數，但是由於外加磁場與材料磁化有相位差的關係，電阻值也跟著增加而影響效能。最後，我們提出了一個製造固態磁芯的製程。在低於4MHz頻率下，具有固態磁芯的變壓器的效能會高於空氣芯變壓器的效能。

The thermal conductivity of nanofluids and the application of ferrofluids are investigated. With respect to the first topic, the effect of the viscosity of base fluids on the thermal conductivity of nanofluids is discussed. Experimental results reveal an obvious enhancement on thermal conductivity of nanofluids with low viscous base fluids. The measured thermal conductivity of low viscous nanofluids markedly exceeds that predicted by Maxwell prediction model. As the viscosity of the base fluid increases, the measured thermal conductivity of the nanofluid gradually approaches the value predicted by Maxwell prediction model, indicating that the viscosity of nanofluids influences their thermal conductivity, and the Brownian motion of suspended particles importantly enhances the thermal conductivity of nanofluids. Moreover, while the first topic is investigated, some special properties of ferrofluid are found. Therefore, a new application is derived. With respect to the second topic, ferrofluids and bulk Fe3O4 are applied as the magnetic cores of transformers. The transformers used in this thesis are constructed on a capillary or on a wafer. The performance of transformers with different magnetic cores is measured and simulated. Although Fe3O4 increases the inductance and coupling coefficient, it also increases the resistance owing to a lag between the external magnetic field and the magnetization of the material. Finally, a new process for fabricating a solid magnetic core is proposed, in which ferrofluids are used to deliver ferro-nanoparticles into microchannels. A transformer with a solid magnetic core outperforms the same transformer that with an air core below a frequency of 4 MHz.