磁性流體 (Magnetic Fluid) 是將奈米級粒徑的磁性顆粒 (Nano-magnetic Particles) 經表面活性劑處理後，安定分散於水或有機溶媒中，而製備之懸浮液，其磁性顆粒不因磁場或重力場之作用，產生凝聚而與液體分離。將磁性流體置於磁場中，由於磁場之作用，懸浮液中之磁性顆粒即往強磁場方向移動，同時亦帶動溶媒分子一起移動，其行為與鐵、鎳等固態強磁性物體相同，為液態之強磁性物體，因此磁性流體是兼具磁性與流體之性質。
　　磁性流體是1960年代中，美國航空太空總署 (NASA) 科學家Papell發明，Papell將磁鐵礦 (Magnetite) 與表面活性劑混合，經由長時間的研磨而形成超微細之顆粒 (粒徑在10 nm以下)，並使表面活性劑被覆於顆粒表面，使磁性顆粒安定分散於溶媒中，形成磁性流體。由於磁性流體具有許多特殊的物理性質，如在外加磁場的作用下，磁性流體可固著於轉動軸與軸套部份之間成為液體軸承，可完全密封，且無摩擦及熱產生；亦可作潤滑油，硬碟機之塵封元件，揚聲器之阻尼器等。
　　本研究利用化學共沉法合成奈米級粒徑的Mn/Zn鐵氧化物粉末，在室溫下，將0.5 M之Mn2+、Zn2+和Fe3+ 溶液先予混合，〔(Mn2++Zn2+)/Fe3+ = 1/2〕，再將2 M之NaOH溶液，以5 ml/min的速率緩慢滴入混合液中，同時使用直棒型攪拌子以500 rpm轉速攪拌，進行共沉反應，在pH值達到10左右完成共沉。所產生的錳鋅鐵氧化物經48小時之陳化，粒徑約為12 nm，經X光繞射鑑定得知為尖晶石相，飽和磁化量可達50 emu/g左右。磁化曲線的測定結果顯示無殘留磁化量 (Br)，亦無矯頑磁力 (Hc)，呈現超順磁性之性質。
　　共沉反應所得之錳鋅鐵氧化物經過濾及淋洗，去除殘餘之未反應陰、陽離子後，在pH 10下，添加適量的油酸，可在顆粒表面達成化學與物理方式之雙層吸附，獲得安定分散之水基磁性流體。水基磁性流體可以酒精為替換媒介將水去除，再轉換成煤油為介質之油基磁性流體。由表面活性劑之吸附測定顯示，水基磁性流體之穩定是由表面活性劑之雙層吸附造成，而油基磁性流體為單層吸附。
油基磁性流體之固體重量濃度為63％～67％，其黏度為37.70～55.40 mPa.s，呈現牛頓流體之行為，顯示已經完全分散。

　　Magnetic fluid is a suspension of nano-sized magnetic particles in water or organic solvents. The magnetic particles are first coated with proper surfactant so as to be dispersed in the solvent and will not settle or flocculate in the magnetic or gravitational field. When subjected by a magnetic field, the ultra-fine particles will move to the higher gradient region. In the same time the fluid is also carried with the particles to move. Thus it behaves like a solid ferromagnetic body, such as iron and nickel, but it is liquid. Therefore the magnetic fluid has both magnetic and liquid-like characteristics.
Papell of NASA invented magnetic fluid in the 1960’s by very long time grinding of a magnetite and surfactant mix to make magnetite particles below 10 nm. Magnetic fluid has many specific physical characteristics. Under magnetic field, magnetic fluid can be fixed between the rotating axis and casing and becomes a liquid bearing. It doesn’t produce friction and heat, can be used as lubricating fluid, dust-proof component of computer hard disks, dampers of loudspeakers, etc.
　　This study utilized co-precipitation to synthesize nano-sized Mn/Zn ferrite particles. At room temperature, 0.5 M Mn2+, Zn2+, Fe3+ solutions were mixed〔(Mn2++Zn2+)/Fe3+ = 1/2〕, then 2 M NaOH solution was titrated into Mn/Zn/Fe solutions at the rate of 5 ml/min, while the solution was constantly stirred using a magnet bar at 500 rpm. The co-precipitation reaction proceeded until the pH value reached 10. After aged for 48 hours, the particle sizes of the Mn/Zn ferrite obtained are around 12 nm. The X-ray diffraction analysis showed that the crystal is of spinel form. The saturated magnetization is about 50 emu/g. The hysteresis curves show that there is no retentivity (Br) and coercivity (Hc), i.e. it displays superparamagnetism.
　　After filtering and washing to eliminate residual cation and anion, the wet Mn/Zn ferrite slurry at pH 10 was added with proper amount of oleic acid. Well-dispersed stable magnetic fluid can be obtained. The water-based magnetic fluid can be transformed to kerosene-based using alcohol as a intermediate media. The adsorption isotherm of oleic acid showed that the water-based magnetic fluid is stabilized by double layer coverage, while the oil-based magnetic fluid is by single layer coverage.
　　The solid weight concentration of the oil-based magnetic fluid between 63％～67％, and the viscosity between 37.70～55.40 mPa.s. The fluid displays a Newtonian behavior showing complete dispersion.

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