Heat and Mass Transport in Undercooled Melts of Germanium and Silicon

通过悬浮法的无容器处理已用于材料科学和技术应用，例如熔体的快速凝固、纯化和热物理性质测量。电磁悬浮法（EML）可用于在给定气氛中悬浮相对大尺寸的金属熔体。然而，独立控制的温度和空间位置的试样是困难的，强洛伦兹力产生电磁搅拌的熔体和不稳定的形状的熔体最近，一种新的方法，用于悬浮的金属熔体由电磁线下的静磁场被开发，以阻尼熔体中的对流和振动的洛伦兹力。在本研究中，新的EML方法成功地稳定了硅和锗悬浮熔体的位置和形状，并且每个熔体中的对流被强磁场强烈阻尼。测量了纯硅熔体的密度和热导率在很宽的温度范围内由EML进行，而锗的那些性质由于熔体的较高蒸发速率而显示出大的数据分布。熔体的发射率几乎与温度保持恒定。硅熔体的密度与温度呈线性关系，在熔点附近没有异常增加，热膨胀系数与金属液体模型计算值吻合较好。由于熔体的热导率与维德曼-弗朗茨定律估计的值大致一致，因此硅熔体中的自由电子明显影响热导率。结果表明，硅熔体即使在过冷状态下也表现出金属特性。

Containerless processing by a levitation method has been used for materials science and technological applications such as rapid solidification, purification and thermophysical properties measurement of melts. An electromagnetic levitation method (EML) can be used to levitate a relatively large size of a metallic melt in a given atmosphere. However, independent control for the temperature and spatial position of the specimen is difficult and the strong Lorentz force yields electromagnetic stirring in the melt and destabilizes the shape of the melt Recently, a novel method for levitating a metallic melt by EML under a static magnetic field was developed in order to damp the convection and vibration in the melt by the Lorentz force. In the present study, stabilizing of position and shape of a levitated melt for silicon and germanium was succeeded by the new EML method, and convection in each melt was strongly damped by the strong magnetic field.Measurements of the density and thermal conductivity of a pure silicon melt over a wide temperature range were performed by the EML, while those properties for germanium showed large data spreads due to higher evaporation rate of the melt. The emissivity of the melt showed almost constant to temperature. The determined density for silicon melt showed a linear relation with temperature, and no anomalous increase near the melting point was observed In addition the thermal expansion coefficient agreed well with the value calculated based on the metallic liquid model. Since the thermal conductivity of the melt agreed roughly with values estimated by the Wiedemann-Franz law, free electrons in a silicon melt obviously affect the thermal conductivity. In the result it was concluded that silicon melt even in an undercooled state showed metallic characteristic.