Purification electrotransport

The method of choice for the preparation of Th metal is reduction of the tetrachloride (Section II,B) by Mg (55), followed by refinement using electrotransport purification (Section III,D) (87, 88, 90). 

The overall purities increase by 0.5at.% to >99.9 with respect to all impurities after electrotransport purification with a 5- to 10-fold reduction in the above mentioned impurities. A similar 5- to 10-fold increase in the resistance ratio P300/P4.2 is observed. These changes are sufficient to allow de Haas—van Alphen measurements to be made on single crystals of Sc, Y, Pr, Gd, Tb and Lu (Young 1979, Gschneidner 1980a) and refinement of some of their physical properties which are sensitive to impurities (Tsang et al. 1985, Hill et al. 1987). 

The passage of a narrow molten zone along a solid rod has been successfully used to purify semiconductor materials and less reactive metals for over 25 years. Just as for the electrotransport purification method, zoning was unsuccessful until vacua of 10 Torr or better became standard practice in the laboratory. In this technique a molten zone is repeatedly passed in one direction, usually 20 passes are required to reach steady-state conditions. Impurities which raise the melting point of the solvent are left behind during solidification as the liquid zone moves forward. Thus with continued passes these impurities are moved to the beginning end of the rod. Those impurities which lower the melting point tend to remain in the molten zone... 

The first study of electrotransport purification of a rare earth metal which met the pure atmosphere condition was done by Peterson and Schmidt (1%9) on the purification of Lu. A Lu sample with a resistance ratio of 151 compared to 21 for the starting metal was prepared by electrotransport in a vacuum of 2 x 10 Torr (2.7 X 10" Pa). Carlson et al. (1975) reported the O and N content had been lowered in Lu from 118 and 27 to 13 and 6wt. ppm, respectively. They did not observe any movement of C in their study of Lu. 

In an electrotransport purification study Jordan et al. (1974) grew a Gd crystal 0.6 cm diameter by 2.5 cm long which had a resistance ratio of 250. A small sphere approximately 0.3 cm diameter was spark machined from this crystal, polished and etched. X-ray Laue patterns of the sphere showed it to have a maximum mosaic spread of approximately 1 minute of arc. 

Refining of Vanadium. In addition to the purification methods described above, vanadium can be purified by any of three methods iodide refining (van Arkel-deBoer process), electrolytic refining in a fused salt, and electrotransport. 

Electron-beam melting of zirconium has been used to remove the more volatile impurities such as iron, but the relatively high volatiUty of zirconium precludes effective purification. Electrorefining is fused-salt baths (77,78) and purification by d-c electrotransport (79) have been demonstrated but are not in commercial use. 

If an actinide metal is available in sufficient quantity to form a rod or an electrode, very efficient methods of purification are applicable electrorefining, zone melting, and electrotransport. Thorium, uranium, neptunium, and plutonium metals have been refined by electrolysis in molten salts (84). An electrode of impure metal is dissolved anodically in a molten salt bath (e.g., in LiCl/KCl eutectic) the metal is deposited electrochemically on the cathode as a solid or a liquid (19, 24). To date, the purest Np and Pu metals have been produced by this technique. 

For further purification of the rare earth metals electrotransport [or solid-state electrolysis (SSE) as it is sometimes called] has been the most successful technique. But it has only been demonstrated on the low vapor pressure rare earth metals (Sc,... 

The first report of purification of a rare earth by electrotransport was that given by Williams and Huffine (1961) for Y. They showed that O and N are quite mobile in Y and move to the anode end of the rod. The electrotransport parameters of C, N, and O in Y were determined by Carlson et al. (1966). Metals purified by electrotransport are frequently too pure to measure the impurities present quantitatively. As a means of indicating total purity both with respect to interstitial impurities and other lattice imperfections, the ratio of the resistance at room temperature to the resistance at 4.2 K (R300/R4.2) is reported. The Y... 

The electrorefining of Ce was studied by Marchant et al. (1%2) in an inert gas chamber. This early study showed Fe, C, Cu and Si could be moved toward the anode by the electric field. The importance of the high vacuum or ultra-pure inert atmosphere is evident from this study, since the O content of their metal increased during electrorefining. Moore et al. (1%5) in their study of Ce noted a movement of the Mn, Fe, Co, Ni and Cu impurities, but they apparently did not have a sufficiently clean atmosphere to purify the Ce with respect to O, N or C. A small movement of C was noted, but not sufficient to be termed purification . Amonenko et al. (1966) studied the purification of Ce with a combination of zone melting and electro transport. They showed that Ce containing 0.11 wt.% O could be reduced to 0.045% O by electrotransport or by the combination of the two methods. Their study was also done under an insufficiently pure atmosphere. 

Peterson, D.T., 1971, Experimental factors in the purification of metals by electrotransport, in Lidding, A. and T. Lagerwall, eds. Atomic Transport in Solids and Liquids (Verlag der Zeitschrift fiir Naturforschung, Tubingen) p. 104. 

Inspite of all the theoretical predictions about the Fermi surface geometry of rare earth metals, there was scanty experimental confirmation until recently. The difficulty in carrying out the measurements was due to the unavailability of crystals of high purity. Early experiments used the method of positron annihilation, which did not require high purity crystals. Unfortunately, the information obtained this way is not sufficient to map out the Fermi surface. With the development of the electrotransport method of sample purification, see ch. 2 sections 2.3 and 5.7, it is now possible to grow high quality crystals of a number of rare earth metals for de Haas-van Alphen experiments. We will review these results with emphasis on the latest findings. 

---