RESEARCH
Levitated metals yield thermodynamic data Rice University group uses lévitation calorimetry to study liquid metals and alloys Lévitation calorimetry is yielding new high-temperature thermodynamic data never before determined experimentally for liquid metals and alloys. Enthalpy increments for conducting liquids, being measured by a group at Rice University, will help establish precise information about thermodynamic properties of the liquids at high temperatures. The Rice scientists expect to use lévitation calorimetry for studying a large number of materials at temperatures above 1500° K. By doing so they hope to fill a major gap in current knowledge of thermodynamic properties. The technique is especially attractive, says Dr. John L. Margrave, who heads the Rice University group, because small samples, usually 0.5 to 1.5 grams, can be studied. Lévitation calorimetry combines lévitation and electromagnetic heating with conventional isothermal drop calorimetry, explains Dr. Margrave. A sample is placed in a nonuniform, rapidly alternating (about 450 kilocycles per second) electromagnetic field. Eddy currents result which cause heating as well as mixing when the metal sample melts. The magnetic field supports the sample inside the specially designed coil, thereby eliminating need for a container for the sample. The lévitation coils are made from copper tubing wound first clockwise and then counterclockwise to give the best magnetic field configuration. Water is pumped through the tubing at high velocity to cool the coil. The lévitation force is proportional to the square of the current put through the coil. Current and coil design make it possible to use the method for any metal regardless of density or conductivity. With data obtained via lévitation calorimetry, heats of fusion of refractory metals can be calculated, most of which previously were estimated, says Dr. Margrave. The Rice group has levitated many metals and thus far obtained enthalpy increments for platinum, nickel, titanium, and cobalt over a wide range of temperatures above and below their melting points. The Margrave team (which now includes David W. Bonnell and Sumner Hunter and with which Dr. R. B. Badachhape of Prairie View A&M, Dr. A. K. 36 C&EN OCT. 28, 1968
APPARATUS. Dr. John L. Margrave (left) and David W. Bonnell inspect the glass vessel in which a sample is levitated before dropping
Chaudhuri of Rensselaer Polytechnic Institute, and A. L. Ford of the University of Texas were formerly associated) has used lévitation calorimetry on copper and obtained data which agree well with literature values, thereby confirming the reliability of the technique. For their studies, Dr. Margrave and coworkers use argon as an inert gas inside a glass vessel surrounding the lévitation coil. Other gases could be used if reaction of the metal with a gaseous material were desired. Control of the flow of the inert gas also helps to control sample temperature.
After a sample has been heated and allowed to reach equilibrium its temperature is measured with an optical or photoelectric pyrometer. The sample is then allowed to drop into the calorimeter by suddenly cutting off the field. The calorimeter contains a sleeve made of the same material as the sample. This avoids a correction for heat gained or lost in a possible reaction of the hot sample with some other material after falling into the calorimeter. The calorimeter opening also has a water-cooled gate to minimize energy leaks before and after drops.
The enthalpy increment for a typi cal sample, such as platinum at 2500° Κ., Η25οο-Η298? is about 22,000 calories per mole. Preliminary results of work with platinum give a heat of fusion of 5320 calories per mole at 2043° K., Dr. Margrave says. This value, he notes, is somewhat greater than the values (4700 to 5200 cal./mole) estimated by other scientists by comparison with determinations on low-melting met al s. The technique of lévitation calorimetry was proposed several years ago by other scientists, including Dr. A. E. Jenkins and associates at the University of South Wales in Australia. It has, apparently, not been used until recently to determine thermodynamic properties of metals and other conducting substances. Besides requiring only small samples and p:)sing no containment problem, the technique has other advantages, Dr. Margrave points out. He no les that the choice of atmospheres is broad, that samples can be heated quickly (usually in less than three minutes) to the desired temperature, that melting points give a builtin temperature calibration, and that very high temperatures are attainable. Furthermore, the Rice scientist adds, enthalpies are easily obtained for conducting liquids and solids and heats of fusion are easily derived. The technique, however, has disadvantages as well, Dr. Margrave admits. Samples must conduct electricity and a new sample must be supplied for each determination. Excessive vaporization or decomposition of the sample can occur, heating may be uneven because of a "skin effect," and emissivity corrections may be needed. Thus the applicability of lévitation calorimetry varies with the kind of sample involved and the conditions needed for the experiment. Another problem now being studied for all kinds of samples concerns the heat losses by radiation and conduction when the sample is dropped. The losses appear to be small but could be the major reason for a ±2% deviation in runs. In conventional drop calorimetry, a blank is run to estimate such corrections or a standard reference sample is used. The technique of lévitation calorimetry will be useful for measurement of the heats of fusion of the most refractory metals—tungsten and tantalum—and for the refractory and conducting carbides, borides, nitrides, suicides, and the like, Dr. Margrave says. In addition, alloys could be studied by this method. These data for conducting liquids at high temperatures should be of interest to theorists seeking data for model systems.
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