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