Thermophysical Property Measurements and Processing of High-Temperature Melts Using Magnetic Fields, Thermophysical Properties and Structual Study on High-Temperature Melts Using Current levitation Techniques
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A levitation method using simultaneous imposition of the static and the alternating magnetic fields has been developed to investigate solidification behavior from the under cooled melt. The surface vibration and the convection in the levitated melt decreased with increasing the static magnetic field. The metallic melt was stably levitated like a hard sphere when intensity of the static magnetic field was more than 1 T. Since the convection was sufficiently reduced by the static magnetic field, the heat transfer in the levitated melt was controlled by the conduction.
Some experimental conditions for diffusion experiment in a static magnetic field were derived by a numerical
simulation. Based on the calculated results, diffusion coefficient in In Sn melt was measured precisely by utilization of a strong static magnetic field from the melting point to 5 K. The obtained coefficients near the melting point agreed well with those estimated by a hard sphere model. However, the experimental values at high temperature was higher that expected by the model.
The transient hot wire method incorporating with a static magnetic field has been developed to measure thermal conductivities of molten metals. Measurements were conducted on liquid mercury. Prior to the measurements, effect of an alumina-coated hot wire on the measurements was evaluated. Natural convection was effectively suppressed by the Lorentz force acting on the mercury in a static magnetic field. The thermal conductivity of liquid mercury is .5 W/mK at 94 K.
Systems with free melt surfaces are frequently affected by gravity independent, capillary flows due to the tempera- ture and /or concentration dependence of the surface tension.For low Prandtl number liquids like metal or semicon- ductor melts, small temperature differences are already suffi cient to cause a time-dependent flow regime, which in- duces fluctuation of the temperature and the velocity field of the melt. The high electrical conductivity of the low Prandtl number substances makes the use of magnetic fields the first choice for flow control and the elimination of undesirable flow structures. Static magnetic fields or dynamic ones are in use.
In radiation-heated float zones, the time-dependent natural convection results in irregular temperature fluctuations in the melt, which reach peak-to-peak values of 1 K; their frequency range is between 0.05 and 0.2 Hz. Numerical simulations reveal flow velocities in the range of 15 cm /s.Static axial magnetic fields result in crystals free of (detectable) microsegregation. The radial dopant distribution is distorted by the magnetic field due to the establishment of stationary flow cells. Static magnetic fields can induce thermoelectromagnetic flows ahead of the solid-liquid inter- face, which cause strong compositional irregularities in the µm to mm range.Applying rotating magnetic fields, the convectively induced temperature fluctuations can be reduced by more than one order of magnitude.The microsegregation is nearly eliminated and the radial dopant distribution is improved.
Thermophysical properties of molten refractory metals have been measured using an electrostatic levitation method. An electrostatic levitation furnace has been developed and could stably levitate molten samples at temperature exceeding
,000 degrees C. In addition, non-contact thermophysical properties measurement techniques have been implemented to the levitator furnace, and such thermophysical properties as density, heat capacity, heat of fusion, surface tension, and viscosity have been measured over wide temperature ranges including the undercooled region.
A review of thermophysical property measurements of metallic alloys by electromagnetic levitation methods per- formed under reduced gravity conditions is given. These methods have been developed for measurements on high temperature reactive alloys where chemical reactions by contact of the specimen with container walls will invariably affect the results such obtained. Non-contact electromagnetic induction calorimetry was developed for the measurement of the specific heat capacity, the enthalpy of fusion, and of thermal transport properties such as the total hemispherical emissivity and an effective thermal conductivity. The method is based on radio-frequency induction heating and non-contact pyrometric temperature measurement. For quantitative applications it is essential that the mutual inductivity between the radio-frequency oscillating circuit and the specimen allows the determination of the inductive power input to the specimen. These techniques have been applied to the determination of the thermodynamic functions of reactive metallic alloys in the stable and undercooled liquid phase as well as to the measurement of their transport properties. Other applications of electromagnetic levitation processing include the measurement of the surface tension and the viscosity by the oscillating drop method. In the future, these techniques shall be applied to the measurement of the thermophysical properties of industrial alloys.
The thermophysical properties and the structure of molten semiconductors and metals in undercooled state have been observed using several levitation techniques. Two groups reported the structure data of undercooled molten silicon by using different levitation technique. Each data were completely different. The decrease of coordination number with temperature decreasing in undercooled state was obtained in gas-jet levitation experiments. On the other hand the in- crease of coordination number with decrease of temperature was obtained in electromagnetic levitation experiments. The difference of two data was discussed in the viewpoint of undercooled molten silicon structure. Based on the results of previous studies, we have been developed the precise measurement technique of structure of levitated melts in order to clarify the structural change of melts in undercooled state.
Under the micro-gravity environment, holding technology of molten metal is important to manufacture new
materials. In the present study, the technology to handle the material in space by the ultrasonic wave is developed. At first, the characteristics of bubble in water held by the ultrasonic standing wave under normal gravity and reduced gravity environment are investigated. Secondly, the characteristics of the droplets in air held by the ultrasonic standing wave under normal gravity environment are investigated. Finally, it is clarified that the surface tension and viscous coefficient of the droplet are estimated by measuring surface oscillation and the damping constant of the droplet.
Sample rotation control capability is a must for a contactless processing facility for scientific as well as technologi- cal reasons. Sample rotation is preferable during material processing as it offers better temperature homogeneity and its control helps the sample to maintain a spherical shape, which simplifies the data analysis for density and surface tension measurements. Rotation is also an advantage, due to its spin stabilizing effect, when processing electrostatically levitated materials, in particular those having a tendency to form oxide or nitride layers. These advantages were illustrated in this paper with the measurement of the thermophysical properties of Hf and Si. In addition, rotation contributes to fining (bubble migration) and could eventually be used to produce hollow spheres. Besides the obvious disadvantages of excessive rotation for material processing (e.g. sample deformation, instability, induced sedimentation), even a low rotation rate could induce non-negligible g-forces that could be detrimental if the experiment is carried out in microgravity. Hence, rotation control is important not only for ground-based experiments but also to fully exploit microgravity environment.
The meta-stable phase formed from the undercooled melt was reviewed in metallic and oxide materials. Three differ- ent conditions have to be satisfied for the meta-stable phase to be formed from the undercooled melt. The first one is deep undercooling below the hypothetical melting temperature of the meta-stable phase. The second one is the solid-liquid interface energy of the meta-stable phase to be less than that of the stable phase. The last one is that the growth velocity of the meta-stable phase is larger than that of the stable phase. In Fe-based alloys without the y loop such as Fe76 ix o 4 x, the meta-stable b phase can be formed when the melt of hyper-eutectic composition is undercooled below the critical temperature where the energy barrier for the critical nucleus of the b phase to be formed is less than that of the y phase. In rare-earth ferrites such as YFe0 , the meta-stable phase can be formed provided that the same conditions as those in the metallic materials are satisfied.
A microgravity experiment on Marangoni convection in a liquid bridge onboard the sounding rocket 6 launched in September 1997, is reviewed. The experiment aimed at clarifying in detail the unsteady behaviors of the convection at a high Marangoni number by means of a combination of sophisticated measuring techniques. However, the experiment suffered from an unexpected and crucial problem with liquid-bridge formation, making scientific interpretation of the data obtained diffi cult. Nevertheless, the data analysis well demonstrated the power of instrumentations adopted in this experiment, and to be used in the future experiments in the International Space Station, for the study of unsteady behaviors of Marangoni convections in liquid bridges.
The Fluid Dynamics Experiment Equipment Type-II (FTX-II) was developed and launched for marangoni convection experiment used in the sounding rockets TR-I A #4 and #6 . This paper introduces the lessons learned through
development of the FTX-II from a viewpoint of equipment development.