It is now possible to determine the electrical resistivity of a molten sample of a pure, electrically conductive material (a metal or semiconductor), without contact between the sample or any solid object. Once the electrical resistivity has been determined, the thermal conductivity can be estimated by use of the Wiedemann-Franz-Lorenz law. (For molten materials, thermal conductivities estimated in this way are often more accurate than are thermal conductivities determined by direct measurements, because direct thermal measurements are often distorted by convection.)
It is necessary to prevent contact with the sample because typically, the molten material can become contaminated by chemical reaction with a container or other solid object. In addition, if one seeks to characterize a deeply undercooled molten material, then contact is undesirable because it can induce crystallization and thereby terminate the undercooled state.
The present method of noncontact measurement of electrical resistivity involves electrostatic levitation and noncontact heating of the sample in a vacuum chamber. The interior of the chamber is subjected to a rotating magnetic field, which exerts a torque on the sample, in essentially the same manner in which torque is generated in an induction motor. From the values of torque measured at various temperatures, one can compute the relative resistivities at those temperatures, by use of an established equation for an induction motor.
This work was done by Won-Kyu Rhim and Takehiko Ishikawa of Caltech forNASA's Jet Propulsion Laboratory. NPO-20369
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Noncontact measurement of resistivity of molten material
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Overview
The document discusses a novel non-contact technique developed for measuring the electrical resistivity of molten metals and semiconductors, particularly in high-temperature environments. This method, created by researchers Won-Kyu Rhim and Takehiko Ishikawa at NASA's Jet Propulsion Laboratory, utilizes electrostatic levitation and a rotating magnetic field to avoid physical contact with the molten sample, which is crucial for maintaining the purity of the material and preventing contamination.
Traditional methods of measuring electrical and thermal conductivities in molten materials are often hindered by the reactivity of these materials with container walls and the challenges posed by gravitational effects, especially in high-temperature scenarios. The new technique allows for the measurement of electrical resistivity in a vacuum chamber, where the sample is levitated, thus avoiding any interaction with solid objects that could lead to contamination or crystallization.
The process involves applying a rotating magnetic field to the levitated molten drop, generating torque that can be measured. From this torque data, the electrical resistivity can be calculated at various temperatures using established equations related to induction motors. This innovative approach not only facilitates the measurement of resistivity but also enables the estimation of thermal conductivity through the Wiedemann-Franz-Lorenz law, which relates these two properties in conductive materials.
The document highlights the advantages of this non-contact method, particularly its ability to accurately measure the properties of deeply undercooled molten materials, which are typically difficult to characterize using conventional techniques. The researchers validated their method using pure aluminum around its melting temperature, demonstrating its effectiveness.
Overall, this advancement represents a significant step forward in the characterization of high-temperature metallic melts, with potential applications in various industrial and scientific fields. The ability to measure electrical resistivity and estimate thermal conductivity without contamination opens new avenues for research and development in materials science, particularly for materials that are reactive or difficult to handle in traditional settings.
