MATERIALS PROCESSING WITHAMAGNETIC LEVITATION FURNACE

强梯度磁场使抗磁性材料在重力作用下悬浮。这种悬浮状态等同于微重力，因为磁力抵消了重力。因此，磁悬浮有望为材料加工提供一种新的技术。无容器熔炼是磁悬浮最实用、最有用的应用之一。磁悬浮炉的设计需要几个具体的特点：首先是在强磁场中加热样品。第二种是将加热炉安装在内径为52 mm的混合磁体中的一个狭小空间内。三是观察悬浮样品在炉内的行为。CO_2激光加热炉就是满足上述要求的加热方式之一。激光束不受磁场的影响，仅对样品进行局部照射就可以在样品附近使用CCD摄像机。然而，激光熔炉在加热悬浮样品方面也有缺点。当样品加热不均匀且样品的导热系数很小时，样品被部分加热，导致热分解。当样品的抗磁化率随着温度的升高而增加时，样品会向上移动，失去水平稳定位置，从而与磁铁孔壁接触。使用电炉来获得均匀的加热。为了观察样品在炉内的行为，使用了耐热内窥镜。这种新的炉子能够在混合磁体中观察高达800℃的悬浮样品。我们进行了环烯烃聚合物的无容器熔融实验，成功地制备了球形样品，研究了磁悬浮条件下水中的传热行为，发现强梯度磁场对热对流有明显的抑制作用。

Strong gradient magnetic fields allow levitation of diamagnetic materials against the gravity. This levitation state is equivalent to the microgravity because the magnetic force counterbalances the gravity. The magnetic levitation is thus expected to provide a novel technique for materials processing. Containerless melting is one of the most practical and useful applications of magnetic levitation. The design of a magnetic levitation furnace requires several specific features : The first is to heat samples in high magnetic fields. The second is to install the furnace in a confined narrow space in the hybrid magnet with a 52 mm bore. The third is to observe the behavior of a levitating sample in the furnace. A CO_2 laser furnace is one of heating methods satisfying above requirements. The laser beam is not affected by magnetic fields, and the local irradiation just on a sample allows use of a CCD camera near the sample. Laser furnaces, however, have also disadvantages in heating a levitating sample. When a sample was heated inhomogeneously and thermal conductivity of the sample was very small, the sample was partially heated, causing thermal decomposition. When the diamagnetic susceptibility of a sample increased with increasing temperature, the sample moved upward and lost the horizontal stable position, resulting in a contact with the wall of a magnet bore.We developed a new magnetic levitation furnace. An electric furnace was employed to get homogeneous heating. For the observation of the sample behavior in the furnace, a heat-proof bore scope was used. This new furnace enabled to observe the levitating sample up to 800℃ in the hybrid magnet. We performed containerless melting experiments of a cycloolefin polymer, and succeeded in preparing a spherical sample.We investigated the heat transfer behavior in water under the magnetic levitation conditions and found that strong gradient magnetic fields considerably suppress thermal convection.