An improved method of noncontact measurement of the surface tension of a molten material has been developed, partly to overcome the disadvantages of contact measurement techniques as described in the preceding article, "Noncontact Measurement of Resistivity of Molten Material" (NPO-20369). This method also overcomes the primary disadvantage of an older noncontact method in which a nonrotating levitated sample is set into vibration, the frequencies of vibrational resonances are measured, and the surface tension is determined from the known relationship among the surface tension, frequencies, and other relevant quantities. The validity of the older method is limited to viscosities less than about 1 poise. The present method works over the full range of viscosities encountered in the thermal processing of metals, glasses, and metallic glasses.
Like the method of measuring resistivity described in the preceding article, the present method of measuring surface tension involves electrostatic levitation and noncontact heating of the sample in a vacuum chamber, plus the use of a rotating magnetic field to apply torque to the sample. In this case, the application of torque is metered and timed to introduce a predetermined amount of angular momentum. The magnetic field is then turned off and the sample allowed to settle into a steady state, in which it rotates as a rigid body. The shape and the frequency of rotation of the sample are measured in the steady state. Then by use of a computational model of a rotating liquid drop, the surface tension is computed for the measured shape and frequency. This method has been verified experimentally on electrostatically levitated molten drops of aluminum and tin.
This work was done by Won-Kyu Rhim and Takehiko Ishikawa of Caltech forNASA's Jet Propulsion Laboratory. NPO-20367
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Noncontact measurement of surface tension of molten material
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Overview
This document outlines a novel non-contact method for measuring the surface tension of molten materials, particularly focusing on metallic and semiconductor melts with high viscosities. Developed by Takehiko Ishikawa and Won-Kyu Rhim at NASA's Jet Propulsion Laboratory, this technique addresses the limitations of existing methods, which are primarily effective for low-viscosity melts (less than 1 poise).
The traditional contact measurement techniques can induce crystallization in highly undercooked liquids, making accurate measurements challenging. The new method utilizes electrostatic levitation to keep the sample in a clean environment, avoiding physical contact that could lead to crystallization. It employs a rotating magnetic field to introduce a predetermined amount of angular momentum to the levitated melt. Once the sample reaches a steady state of rigid body rotation, the magnetic field is turned off, allowing for precise measurements of the drop's shape and rotational frequency.
The surface tension is then calculated using a computational model of rotating liquid drops, based on the measured shape and frequency. This innovative approach has been experimentally validated using molten aluminum and tin, demonstrating its effectiveness across a wide range of viscosities encountered in the thermal processing of metals, glasses, and metallic glasses.
The document emphasizes the significance of this technique for materials science and engineering applications, particularly during the glass formation process, where understanding surface tension is crucial. The ability to measure surface tension in high-viscosity melts opens new avenues for research and development in the field, enhancing the understanding of material properties and behaviors during processing.
In summary, this document presents a significant advancement in the measurement of surface tension in molten materials, particularly for high-viscosity samples. The method's non-contact nature, combined with its applicability to a broader range of viscosities, positions it as a valuable tool for researchers and engineers working with metallic and semiconductor melts. The work is a collaboration between Caltech and NASA, showcasing the integration of advanced technology in materials science research.
