Institute of Metals Division - Observations Concerning Zone Refining and Thermal Treatment of Molybdenum from Low Temperature Resistance Measurements

High-pu.1-ity molybdenum has been heat treated, melted, and zone refined and the physical and chemical changes that are produced by these processes have been studied by making resistance ratio measurements. Pronounced effects are observed that are associated with recrystalliza-tion at about 1200°C and with melting. Melting alone is found to result in a substantial purification by the evaporation of impurities such as iron. Zone refining is found to be reasonably effective with four passes producing a three-fold decrease in resistance ratio and a minimum ratio of 0.0003. With the aid of low temperature measurenzents, it has been shown that the build-up of a deposit of the material being zone refined on the radio frequency coil used for heating can lead to contamination of the sample. CONSIDERABLE interest in high-purity refractory metals for the study of mechanical properties has developed during recent years. Also the need for highly pure metals in certain fundamental studies, such as electronic band structure, has become quite widely appreciated. Molybdenum can now be produced with sufficiently high purity to make it attractive for such investigations. The most suitable measure of chemical purity of metals used for many of these studies is its resistance ratio, R,.,o,/Ro, During the purification of molybdenum by zone refining and its subsequent evaluation by this technique, a number of informative observations were made. These observations concern the effectiveness of zone refining, the volatilization of impurities at the melting point, and the recrystallization of molybdenum and appear to be pertinent to other refractory metals. Also, the usefulness of low-temperature resistance measurements as an analytical tool for evaluating the presence of chemical impurities and physical defects in metals is again illustrated. Samples of high-purity molybdenum of known mechanical history were heated to various temperatures between 800°c and their melting point. Resistance ratios of the samples were measured after they had been given a specified treatment. The data were used, along with mass spectrographic analysis, as an aid in interpreting the nature of the processes that occurred in conjunction with each treatment. Changes in the concentrations of chemical impurities and physical defects are typical of things that effect the low-temperature resistivity. SAMPLES The starting material was obtained from Westing-house Corp., Bloomfield, N.J. It was in the form of molybdenum rods, usually 1/8 or 3/16 in. in diameter. These had been previously processed from bars, 5/8 in, in diameter, originally reduced from 4 in. diam arc-cast ingots, by hot swaging at about 1300°c. In the final step, the temperature was lowered to about l lOO°C which resulted in the production of elongated grains. Typical grains were about 0.03 mm thick and 0.6 mm long. The molybdenum was quite pure as is indicated by the results of mass spectro-scopic analysis1 given in Column 2 of Table I. Other nongaseous elements not listed in the table were not detected. ZONE REFINING AND THERMAL TREATMENT OF SAMPLES Several sample rods, 8 to 10 in. long, were zone refined or were heat treated at several temperatures. Both types of operations were performed using 3 me-