It has been demonstrated that the ultrapurification of thorium by electrotransport refining is highly sensitive to the background gas density in which the refining takes place. The electrotransport purification of thorium has been studied over a total pressure range (predominantly H20 and CO) of 2 X 10 -2 to 2 X 10 -i2 Torr. Polycrystalline thorium samples, with an initial resistance ratio of 35, were purified to a ratio of 2040. Thorium refined a second time, yielded metal with a ratio of 4200, which is the purest thorium ever reported. Purification experiments conducted within a prescribed background of nitrogen (4 X 10 -1~ and 5 X 10 -9 Torr) show a significantly higher resistance ratio than normally achieved at such pressures having a hydrogen and CO background. It is postulated that the adsorbed nitrogen inhibits the adsorption and incorporation of CO and, therefore, limits the carbon concentration in the thorium bulk. During the purification process (1500~176 gravitationally induced grain slippage was observed to occur at the boundaries of the thorium grains. A distributed magnetic force system was applied to the sample to counteract the gravitational force, resulting in a 90% reduction of the grain slippage.The migration of solutes in solid metals under the influence of a direct current has been known since 1928 when Coehn and Specht (1) observed hydrogen transport in palladium. In this process a constant voltage is applied across the metal sample which yields an internal electric field and a constant current which heats the sample. These combined effects produce the electrotransport phenomenon. The solutes are stimulated by the charged carriers to move toward one of the ends of the sample, thereby leaving a major portion of the sample virtually free of impurities. Many of the earlier investigators of electrotransport of interstitial solutes interpreted the direction and velocity of migration on the basis of a charge on the solute particle. In 1953, Seith and Wever (2) showed this interpretation to be incomplete and one cannot separate the contributions to the force moving the solute which arise from the charge on the solute in an electric field and the momentum transfer from the scattering of electrons or holes. Reviews by Heuman (3), Verhoeven (4), Huntington (5) consider both the electrostatic force and the force resulting from moving electrons being scattered by the solute. The potential for applying electrotransport to the ultrapurification of metals has been described by Verhoeven (6) and Peterson (7). This technique has been used extensively at the Ames Laboratory, DOE, for removing interstitial solutes from many of the refractory and rare earth metals. These investigations have been summarized in a recent publication in which the direction of solute movement in an electric field is empirically correlated with the atomic radius of the solvent metal at the temperature of transport (8).However, electrotransport processing has been limited by the presence of the residual gases in the vacuum system in w...
