Preparation and Properties of an Aqueous Ferrofluid W
Preparation and Properties of an Aqueous Ferrofluid W
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发表时间：
1999
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通讯作者：
P. Berger;N. Adelman;K. Beckman;D. Campbell;A. Ellis;G. Lisensky
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作者：
P. Berger;N. Adelman;K. Beckman;D. Campbell;A. Ellis;G. Lisensky
想象一下一种可以由磁场控制的液体的生产和应用。创造一个强磁性液体不像熔化一个强磁性固体那么容易，因为磁性固体在居里温度以上失去了大部分的磁性，因为热能压倒了它们的电子在磁域(具有类似取向的电子自旋的区域)排列的趋势。居里温度远低于已知磁性材料的熔点(1-3)。磁流体是磁性材料在液体介质中的胶体悬浮液，是液体对外部磁场做出响应的一个例子。液体和磁性行为的耦合意味着液体的位置可以通过外加磁场来操纵。20世纪60年代，NASA的斯蒂芬·帕佩尔首次开发和分类了磁流体，作为一种控制太空流体的方法(4)。NASA最初在卫星上使用它们作为旋转轴封，现在它们在从离心机到计算机硬盘驱动器(1，2)的各种机器上都有同样的用途。它们被结合到扬声器的音圈间隙中，以抑制不需要的振动和冷却。利用在磁场作用下流体中出现的密度变化，铁流体也被用于从矿石中分离金属。一家南非公司甚至一直在利用磁流体从海滩沙子中分离钻石(5)。在医学上，已经提出了一种用于植入型人工心脏的铁流体致动器(1)。这种致动器可以简单地通过施加外部磁场来驱动。有可能将药物附着在磁性颗粒的表面，并使用磁场将药物保持在需要的位置(3)。水磁流体成功地定向了生物组件，例如烟草花叶病毒，使得能够获得关于病毒螺旋结构的信息(6)。最近，磁流体与微接触印刷和毛细管填充一起被用于制造微米级的磁性材料的图案化结构(7)。例如，产生超细磁性颗粒图案的能力具有重要的技术应用，因为磁带上的信息密度与颗粒的大小成反比。已经进行了研究，探索将磁流体用作喷墨打印的磁性墨水(2)。磁性油墨目前被用于印刷美国纸币，这可以从一张真正的美元钞票被强大的磁铁吸引而得到证明(图1)(8)。
Imagine the production and applications of a liquid that can be controlled by a magnetic field. Creating a strongly magnetic liquid is not as easy as melting a strongly magnetic solid, since magnetic solids lose much of their magnetism above what is known as the Curie temperature, as thermal energy overwhelms the tendency of their electrons to align in magnetic domains (regions of similarly oriented electron spins). The Curie temperature is well below the melting point for known magnetic materials (1–3). Ferrofluids, which are colloidal suspensions of magnetic material in a liquid medium, are an example of a liquid that responds to an external magnetic field. The coupling of liquid and magnetic behavior means that the liquid’s location may be manipulated by an applied magnetic field. Ferrofluids were first developed and classified in the 1960s by Stephen Pappell at NASA as a method for controlling fluids in space (4). NASA initially used them as rotating shaft seals in satellites, and they now serve the same purpose in a wide variety of machines, ranging from centrifuges to computer hard disk drives (1, 2). They are incorporated into the voice coil gap of loudspeakers for damping undesired vibrations and for cooling. Ferrofluids have also been used in the separation of metals from ores by taking advantage of a density change that appears in the fluid under application of a magnetic field. One South African company has even been utilizing ferrofluids to separate diamonds from beach sand (5). In medicine, a ferrofluidic actuator has been proposed for an implantable artificial heart (1). This actuator would be driven simply by applying an external magnetic field. It is possible to attach drugs to the surface of the magnetic particles and use magnetic fields to hold the drug at the site where it is needed (3). Aqueous magnetic fluids have successfully oriented biological assemblies such as the tobacco mosaic virus, enabling information concerning the helical structure of the virus to be obtained (6 ). Recently, ferrofluids have been utilized in conjunction with microcontact printing and capillary filling to fabricate patterned structures of magnetic materials on the micron scale (7 ). The ability to produce patterns of ultrafine magnetic particles has important technological applications, since the information density on tapes, for example, is inversely proportional to the size of the particles. Research has been conducted exploring the use of ferrofluids as magnetic inks for ink jet printing (2). Magnetic inks are currently used in printing United States paper currency, as can be demonstrated by the attraction of a genuine dollar bill to a strong magnet (Fig. 1) (8).