A New Thermoelectric Effect of Bismuth Single Crystals Grown in Magnetic Fields

Citation

Hasler, Maurice Fred (1930) A New Thermoelectric Effect of Bismuth Single Crystals Grown in Magnetic Fields. Master's thesis, California Institute of Technology. doi:10.7907/3bj6-wf95. https://resolver.caltech.edu/CaltechTHESIS:08132024-222020494

Abstract

Production of crystals.

Crystals of bismuth of any desired orientation were grown, one half of each normally, the other half in a magnetic field. The orientations predetermined by a seed crystal were not affected by this process.

Thermoanalysis of Crystals.

A method was developed to measure and localize any changes of thermoelectric properties along the lengths of crystals without applying mechanical stresses to them. Distortions or imperfections in normal crystals were detectable. Changes in crystal structure caused by the application or variation of forces, such as magnetic fields, heat gradients, etc., at the zone of crystallization, were likewise detectable.

Magnetic Effect as a Function of the Orientation.

Results obtained on crystals having different orientations indicate that thermal e.m.f's exist between the two halves of crystals, unmagnetic and magnetic when the trigonal axis, the axis of least diamagnetism, in forced to grow normal to the lines of force. The mechanical instability of these orientations due to magnetic forces is discussed.

Effect as a Function of the Method of Growth.

The sign of the magnetic-thermoelectric effect was found to be a function of the growing conditions prevailing at the moment when the field was energized. Continuously grown crystals, those whose growth was not interrupted to apply the field, showed differently shaped thermoanalysis curves from discontinuously grown crystals, those remelted half way to apply the field. These differences are discussed on the basis of instability due to heat gradient forces.

Effect as a Function of the Impurities in the Bismuth Used.

It was found that the magnitude of the effect varied greatly with the amount of impurity present in the bismuth, being large for impure metal and small for pure. A spectroscopic analysis of the four kinds of bismuth used was completed. The total amount of impurity present in the worst sample was less than 0.5% and yet the effect for this bismuth was ten times greater than that for the purest electrolytic metal.

Effect as a Function of the Field-strength.

It was found that a complicated relation exists between the magnetic effect, the field strength and growing conditions. Field strengths up to 21,000 gauses were employed. For one case investigated, the effect increased with increasing field strength up to 15,000 gauses above which, however, it dropped off.

Effect as a Function of the Temperature.

The magnetic-thermoelectric effect, investigated as a function of the temperature, indicates the presence of an allotropic modification of bismuth with a transition at some temperature between 70° and 90°.

Effect as a Function of Annealing.

Annealing crystals above the allotropic-modification transition temperature had no influence on the variation in crystal structure producing the effect, as no diminution of the thermal e.m.f. was observable.

Thesis Files