奈米磁流體在外加直流與交變磁場作用之磁光物理特性與磁動力學機制研究

Magneto-Dynamics and Magneto-Optical properties of Nano-Ferrofluids Exposed to AC and DC Magnetic fields

鍾旻峰(Min-Feng Chung)

指導教授 ： 傅昭銘

國立臺灣大學/理學院/物理研究所/博士(2013年)

https://doi.org/10.6342/NTU.2013.03003

摘要

奈米磁流體是由奈米級磁性顆粒均勻懸浮分散於載液中所組成。磁流體具有顆粒流體與磁性材料的雙重性質，可藉由磁場作用而操控磁性顆粒分佈狀態，使顆粒排列結構從無序而到有序，進而影響磁流體的物理特性．由於奈米級磁性顆粒具有異於較大尺度磁顆粒物理特性，探討奈米磁性流體受外加磁場調控下的動態行為及物理特性乃有其意趣。因此本論文著手探討奈米磁流體在外加磁場作用下的磁動力學機制，實驗研究乃藉由光散射機制與磁光效應進行分析，並利用數值模擬方法進行磁顆粒結構序化機制作探討。本研究對奈米磁流體動力學機制與磁光特性的瞭解，可作為探討以奈米磁流體作為生醫檢測應用的基礎。
本論文研究以化學合成法製作含四氧化三鐵(Fe3O4)奈米顆粒的水基磁流體，其四氧化三鐵顆粒在SEM量測下尺寸介於15~35 nm，且在SQUID測量下具有超順磁性。以FTIR量測，四氧化三鐵奈米顆粒表面具有-OH官能基，所以在Zeta -potential量測下顆粒表面電位為35 mV，因此在溶液中形成良好的分散性。製成的奈米磁流體填充入平行玻片樣品盒，以自行設計建製的磁光系統探討磁流體在外加磁場作用下的磁顆粒結構變化。在外加磁場作用下，磁流體中的奈米磁性顆粒會隨著外加磁場方向排列而形成鏈狀微結構．由於奈米磁顆粒的排列結構變化不易以顯微鏡觀察，本實驗以量測透光率與光散射變化來分析奈米磁顆粒排列結構演化．
實驗中觀察到，磁流體在外加磁場作用下，其透光率隨時間變化行為，會先下降至最小值而隨繼再緩慢上升至飽和值．藉由磁流體的光散射效應，觀察到有圓形繞射條紋的產生。本研究進一步嘗試建構經驗方程式作分析實驗結果。實驗與模式的擬和分析得到，散射強度隨時間的變化係與奈米磁顆粒形成鏈狀結構演化動態行為有關。本研究進一步利用磁光效應(Magneto-optical effect)來了解磁流體在外加交流磁場作用下的顆粒結構演化。交流磁光性質乃源自由於奈米級磁性顆粒隨著外加交流磁場而形成的鏈狀結構所導致的磁二色性所致．因此，當磁場增強時，光散射將影響磁二色性強度．實驗觀察到，磁流體的磁光強度會隨著交流磁場頻率的增加而逐漸遞減其強度，並發現磁光震盪頻率為外加磁場頻率之兩倍。進一步以經驗模式分析磁光訊號上升與下降特徵時間，由分析結果中得到，上升與下降特徵時間皆隨交變磁場頻率的增加而增加。
此外，本研究亦嘗試著以數值模擬方法來模擬奈米磁性顆粒在外加場下的演化行為。研究方法乃採用蒙地卡羅模擬法(Monte Carlo Simulation)，在考慮粒子間相互作用力，如凡得瓦力，靜電排斥力與磁偶極交互作用力等貢獻，成功地模擬磁流體在外加場下磁性顆粒之演化行為．本研究提供對奈米磁流體動力學機制的瞭解，亦可作為探討奈米磁性顆粒與分子結合，作為從事生醫檢測應用的基礎。

關鍵字

磁流體 ； 奈米級磁顆粒 ； 磁光效應 ； 蒙地卡羅模擬

並列摘要

Ferrofluid is colloidal mixtures composed of nanoscale magnetic particles suspended in a carrier fluid. Because ferrofluid displays both liquid behaviors and the magnetic characteristics of solid magnetic materials, it has attracted attention from scholars. The ferrofluid has an ordered structure as it exposed to external magnetic field, which has special light scattering and magneto-optical effect when light transport through it. Thus, it is critical to understand the optical properties and dynamic behaviors of ferrofluid that are exposed to external magnetic fields. Therefore, in this dissertation, we examine the dynamic behaviors of ferrofluid by using the optical transmittance, light scattering and magneto-optical effect methods, and a Monte Carlo simulation to compare against the experimental results. This research not only gives an understanding for the micro-properties of ferrofluid, but also it is essential to give possibility to use in the optical detection in biomedical diagnostic.
This dissertation comprises four parts. First, we manufactured water-based ferrofluid by using the chemical method at room temperature. The sizes of the magnetic nanoparticles (MNP; Fe3O4) ranged from 15 nm–35nm measured by XRD and they possessed super-paramagnetic properties measured by SQUID. The MNPs were suspended in DI water, and the zeta-potential of the ferrofluid was 35 mV. Moreover, the surfaces of the MNPs contained an –OH functional group, possessing a strong ability to avoid the aggregation between particles and change the surface properties to bio-functional properties.
Second, we studied the variation of transmittance in ferrofluids that were exposed to external magnetic field. After applying the magnetic field, the magnetic particles aligned with the magnetic field, forming a train-like structure. This variation is difficult observe directly by using an optical microscope; thus, we used the optical transmittance method to examine the structural evolution. We determined that the diffraction pattern varied over time while the magnetic field was applied. According to these physical phenomena, we proposed an equation to analysis the experimental results. A successful fitting would allow us to analyze the variation time according to the diffraction and the train-like structures.
Third, we researched the magneto-optical effect of ferrofluid on the DC magnetic field, analyzing the additional AC magneto-optical effects of ferrofluid. We found that the magneto-optical strength decreases with the increasing frequency of the AC magnetic field, and the resonance frequency of the magneto-optical signal was doubled to the applied frequency of the magnetic field for a weak magnetic field strength (<40 Oe). Further, we analyzed the characteristic times of rising and falling magneto-optical signals, these characteristic times increase with increasing magnetic field frequencies. These AC properties result from the aggregation behavior of the magnetic nanoparticles.
Finally, we used the Monte Carlo method to simulate the structure of the magnetic nanoparticles that were exposed to an external magnetic field. This simulation successfully explained the experimental results. This research not only gives an understanding for the micro-properties of ferrofluid, but also it is essential to give possibility to use in the optical detection in biomedical diagnostic.

並列關鍵字

Ferrofluid ； Magnetic nanoparticles ； Magneto-optical effect ； Monte Carlo Simulation

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