General Remarks

Nanofluids that comprise suspended nanoparticles in a base fluid has been used in numerous engineering applications, including microfluidic devices, heat exchangers, and optical switches. Choi [1] first coined the term “nanofluid” in 1995. Experimental results [2-7] reveal that a nanofluid has better heat transfer properties than the conventional heat transfer fluid or the corresponding base fluid with microparticles.

However, the mechanism of improvement of the performance of a system using nanofluid should be carefully studied to optimize the performance of the system. This thesis investigates the mechanism of enhancement of the thermal conductivity of a base fluid upon the addition of nanoparticles. Then, due to the special properties of ferrofluid, a ferrofluid is used as a magnetic core of a capillary power transformer and of a MEMS power transformer. This application of nanofluid has not been investigated in prior studies.

Suspended nanoparticles in base fluid are expected to increase the thermal conductivity of the base fluid because the thermal conductivity of the suspended particles greatly exceeds that of the base fluid. However, numerous experimental studies

have shown that the enhancement of thermal conductivity of the base fluid greatly exceeds the enhancement that is predicted by traditional models of the thermal conductivity of composite materials. Even at a very low volume fraction, nanoparticles significantly enhance the thermal conductivity of a base fluid. This study presents evidence of the mechanism of enhancement of the thermal conductivity of base fluid by suspended nanoparticles.

In the second part of this thesis, due to the special properties of ferrofluid we found in the first part, the magnetic core of a miniature power transformer is replaced with ferrofluid. Ferrofluid is a colloidal mixture of ferro-nanoparticles, surfactant and base fluid. The ferro-nanoparticles are typically oxides of Fe, Co, Ni or a combination of different metals with an average diameter of approximately 10 nm. The ferro-nanoparticles are coated with surfactant to prevent their aggregation. This surfactant also modifies their surfaces, making them hydrophilic or hydrophobic, causing the ferro-nanoparticles to be uniformly suspended in the base liquid, like water or oil. The ferrofluid does not exhibit magnetism and the orientation of the ferro-nanoparticles is random when no external magnetic field is applied. When an external magnetic field is applied, the ferro-nanoparticles become polarized and their magnetic moments align with the magnetic force lines. Once the external magnetic field is removed, the orientation of ferro-nanoparticles returns to random. Although bulk

Fe3O4 exhibits ferromagnetism, the ferro-nanoparticles with a diameter of less than 50 nm exhibit super-paramagnetism. Therefore, the ferrofluid also exhibits super-paramagnetism. For decades, ferrofluid has been used as the heat transfer fluid in loudspeakers [8], as the damper in stepper motors and shock absorbers, and as the low-friction seal in rotating shaft motors and computer disk drives to keep out contaminants [9]. Ferrofluid also has some interesting properties. When an external magnetic field is applied to it, it adopts special shapes [10].

A transformer is an electrical device that typically contains two or more copper coils. The main purposes of transformers are to step up or down the voltage of the AC source, to change its effective impedance and separate the circuits. The transformer applies Faraday's law of induction to transfer electrical energy from one coil to another.

The law states that the induced electromotive force (EMF) in any closed circuit equals the rate of change of the magnetic flux through the circuit. The AC source in the primary winding generates an alternating magnetic flux through the core of the transformer. Then, the alternating magnetic flux goes passes through the secondary winding, inducing an alternating EMF in it. This effect is called mutual induction. One of the most important parts of a transformer is the magnetic circuit, which provides a path of least magnetic resistance. A magnetic circuit typically includes a ferromagnetic material, such as metal and oxides of Fe, Co and Ni. These materials have a high

relative permeability, which ranges from hundreds to thousands, even reaching 20000, and so provide a path of low magnetic resistance. However, such solid materials exhibit hysteresis. Whenever the magnetic flux is reversed, some energy is lost by hysteresis in the magnetic core. As the frequency increases, the hysteresis loss increases proportionally. Another special transformer is the air core transformer, which is commonly used in radio-frequency circuits. This transformer has a non-magnetic core, and therefore none of the undesirable properties of a ferromagnetic core, such as eddy current loss, hysteresis loss, saturation and others, but the coupling coefficient between windings is lower than that of an iron core transformer. Both types of transformer have shortcomings. Transformers have a wide range of sizes, from a micro-sized transformer to a huge unit that weighs tons. Although range of designs is extensive, their basic operating principles are identical.

Both of the above topics have attracted considerable attention and have been extensively investigated in recent decades. However, there are no researches or convincing investigations about the integration of above topics. The aim of this study is to elucidate the effect of the viscosity of base fluid on the thermal conductivity of nanofluids, and the application of ferrofluids on MEMS chip transformers. To investigate thermal conductivity, water-based and oil-based nanofluids are adopted. The mechanism of enhancement of thermal conductivity of nanofluids is discussed with

reference to experimental results. Ferrofluids are applied in two transformers. One is constructed on a capillary, and the other is constructed on a wafer by the MEMS process.

The performance of the transformers is measured using a precision impedance analyzer and simulated by performing an Ansoft HFSS 3D full-wave electromagnetic field simulation.