Investigation of electrical and magnetic properties of ferro-nanofluid on transformers
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This study investigated a simple model of transformers that have liquid magnetic cores with different concentrations of ferro-nanofluids. The simple model was built on a capillary by enamel-insulated wires and with ferro-nanofluid loaded in the capillary. The ferro-nanofluid was fabricated by a chemical co-precipitation method. The performances of the transformers with either air core or ferro-nanofluid at different concentrations of nanoparticles of 0.25, 0.5, 0.75, and 1 M were measured and simulated at frequencies ranging from 100 kHz to 100 MHz. The experimental results indicated that the inductance and coupling coefficient of coils grew with the increment of the ferro-nanofluid concentration. The presence of ferro-nanofluid increased resistance, yielding to the decrement of the quality factor, owing to the phase lag between the external magnetic field and the magnetization of the material.
KeywordsFe3O4 Quality Factor Fe3O4 Nanoparticles Coupling Coefficient Magnetic Core
transmission electron microscope
vibrating sample magnetometer.
In coming decades, new generations of electronic products such as mobile phones, notebooks, and e-paper will be developed with the primary goals of mobilization and miniaturization. New CMOS fabrication technology will be applied to fabricate the miniaturized IC of electronic products on silicon substrates, including on-chip micro-transformers. Several issues of on-chip micro-transformers have been investigated for many years [1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21]. Some researches focused on the material of the magnetic core [1, 2, 3, 4, 5, 6, 7, 8, 9, 10] and the geometry of the transformer [11, 12, 13, 14]. Some papers discussed the parasitic effect of the conductive substrates. Transformer losses become dramatic at high frequencies and limit the performance of the transformers. Previous studies have discussed in detail the causes of transformer losses such as parasitic capacitance, ohmic loss, and substrate loss [15, 16, 17, 18]. Core loss from the solid magnetic core significantly affected the performance of the transformers. The solutions for the solid magnetic core loss were proposed [19, 20, 21].
Consequently, only a few studies addressed transformers with liquid magnetic cores. The liquid magnetic core, ferro-nanofluid, with its distinguishing features of low electric conductivity and super-paramagnetism is regarded as a solution to the core losses of eddy current and hysteresis. In this study, a ferro-nanofluid was applied as a liquid magnetic core in a transformer. The performance of the transformer with the ferro-nanofluids was measured, simulated, and compared with that of a transformer with an air core.
The ingredients of ferro-nanofluid used in this study were Fe3O4 nanoparticles, oleic acid, and diesel oil. The oil-based Fe3O4 nanofluid was synthesized by co-precipitation, surface modification, nanoparticles dispersing, and base-fluid phase changing .
Results and discussion
Different magnetic cores, air, and Fe3O4 nanofluids of 0.25, 0.5, 0.75, and 1 M were applied as the magnetic core of transformers. The inductance (L), coupling coefficient (K), resistance (R), and quality factor (Q) were measured by an Agilent 4294A Precision Impedance Analyzer. In this study, the simulation of the transformer was also established with HFSS 3D Full-wave Electromagnetic Field Simulation. By applying measured permeability, permittivity, and magnetic tangent loss and setting exciting sources, the impedances will be calculated by the finite element method. Both the frequencies of measurement and simulation range from 100 kHz to 100 MHz.
In this study, different concentrations of ferro-nanofluids were applied to the magnetic cores of transformers. The performance of transformers with magnetic cores of air and Fe3O4 nanofluids of 0.25, 0.5, 0.75, and 1 M were measured, simulated, and compared. The experimental results indicated that the presence of Fe3O4 improved the inductance and the coupling coefficient of the coils. Due to phase lag on the material magnetization behind the external magnetic field at high frequencies, the resistance increased larger and faster than inductance, thus yielding a lower quality factor. For a micro-transformer, if a solid magnetic core is needed for higher inductance, it could be achieved by adding ferro-nanofluid and removing the base fluid repeatedly. This method has a lower thermal budget than the processes that sputtered or electroplated materials on chips. It is compatible with the MEMS process.
The authors deeply appreciate the financial support provided by the National Science Council in Taiwan under the grant numbers of NSC 96-2628-E-002-194-MY3 and NSC 98-3114-E-002-002-CC2.
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.