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Preparation of high purity thorium and thorium single crystals
JOURNAL OF THE L~-~O~ON
223
MI3’rAJ.S
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
PREPARATION CRYSTALS
D. T. PETERSON
OF HIGH
AND
PURITY THORIUM
AND THORIUM
SINGLE
F. A. SCHMIDT
Institute for Atomic Research and Department of Metallurgy, Iowa State Universit,y, Ames, Iowa 50010
(U.S.A.) (Received January lSth, 1971)
SUMMARY
Ultra-high purity thorium has been prepared using electrotransport as the refining method. This metal has a total impurity content of less than 50 p.p.m. and a resistance ratio of greater than 1000. A procedure was also developed by which single crystals of the high purity thorium could be prepared by repeated heating of the specimen through its alpha to beta phase transformation followed by prolonged heating just below the transformation temperature.
INTRODUCTION
Thorium may be of considerable importance in the atomic energy program because it is a fertile material which can be converted to 233U. Research on its fundamental physical and chemical properties has been handicapped by the difficulty in preparing very high purity specimens. The primary problem has been the almost inevitable introduction of some amounts of carbon, nitrogen and oxygen during processing. The high affinity of thorium for these elements makes their removal nearly impossible once they have entered the metal lattice. The ~gration of a solute in a metal at high temperature when a d.c. current is passed has been known for many years’. This migration has been proposed as a means of purifying metals but has seldom been demonstrated to be an effective method of achieving really high purity materials. The authors have previously determined the electrotransport mobilities and diffusion coefficients of carbon, oxygen and nitrogen in thorium between 1440” and 1685’C’. The mobilities were found to be large and Verhoeven3 computed the degree of purification of a rod for realistic values of the pertinent parameters. He showed that it could be possible to lower the nitrogen content to 3 x 10-” and the carbon content to 1 x lo-’ of its original concentration by electrotransport after only five days of processing. These values are for an ideal experiment -* Work was performed in the Ames Laboratory of the U.S. Atomic Energy Commission. Contribution No. 2920. J. Less-Common Metals, 24 (1971) 223-228
D. T. PETERSON, F. A. SCHMIDT
224
in which no impurities are introduced from the environment, and they demonstrate the potential of electrotransport as a purification procedure. It was the purpose of this investigation to utilize the mobility and diffusion data of the interstitial impurities in thorium and to develop the electrotransport process so that thorium could be prepared in very high purity form. In conjunction with this work, a method was also sought by which single crystals of pure thorium could be prepared since they are essential in various characterization studies and experimental investigations. PROCEDURE AND RESULTS
The thorium used in this study was prepared by a magnesium intermediate alloy process developed by the authors4. This material had been triple electron-beam melted and contained less than 250 p.p.m. total impurities. The major impurities were oxygen, 80 p.p.m.; nitrogen, 35 p.p.m.; carbon, 50 p.p.m.; and less than 100 p.p.m. of total metallic impurities. Purijkation of thorium by electrotransport In the initial experiments in which electrotransport was to be evaluated as a method of purifying thorium, specimens, 10 cm in length and 0.254 cm in diam., were heated with a d.c. current for a length of time sufficient to approach the steadystate distribution. At the steady state, the interstitial solutes have migrated with the flow of electrons until the electrotransport fluxes are balanced by the diffusion fluxes due to the concentration gradients. The thorium rods were treated for 90 h at 1600°C under a pressure of 6 x 10e8 torr. The purified rods were sectioned into four equal lengths and the resistance ratio of each section measured. Typical values obtained were 120,90,75 and 20 which represent a profile along the rod in the same direction as the electron flow, that is, from the cathode to the anode end of the rod. The thorium, initially, had a resistance ratio of 33. It was apparent that the vacuum system and chamber being used were incapable of providing a non-contaminating environment for the specimens and, as a result, only partial purification of the rods was being achieved. An all-metal apparatus was constructed which was bakeable to 400°C and had a vacuum system capable of obtaining a pressure of 5 x lo- l1 torr. The apparatus consisted of a stainless-steel sample chamber that was equipped with two electrodes, a sight glass, a titanium sublimation pump and a flange for mounting on an ionization pump. The chamber was 15 cm in diameter and 27 cm long. A glass to metal seal was used between the electrodes and the chamber. A sorption pump was used to evacuate the system to 1 x lo-’ torr after which the titanium sublimation pump and the Ultek 100-l/see differential ionization pump were used to achieve and maintain the low pressure throughout the run. The specimens were thorium rods, 16.5 cm long and 0.254 cm in diameter. They were threaded and screwed into tantalum “U” shaped adapters attached to the ends of the electrodes. This arrangement not only accomodated the thermal expansion of the sample but greatly reduced the thermal gradients at the ends of the specimen. This was accomplished by adjusting the cross-sectional areas of the adapter so that they were heated to nearly the same temperature as the specimen. The temperature was measurJ. Less-Common Metals,24 (1971)
223-228
225
HIGH PURITY THORIUM
ed, using an optical pyrometer, and the observed temperature corrected for the emissivity of the sample and for absorption by the viewing window. The d.c. current was supplied by a saturable core, step-down transformer which provided a full-wave rectified current with a 15 % ripple. A standard resistance and a potentiometer were used to measure the current. The specimens were heated to 1575°-16000C using a current density of 1850-1950 A/cm’, and the pressure in the chamber was maintained at l-5 x lo- lo torr. The specimens were heated for 70-100 h which was sufficient to produce the steady-state condition. The purified rods were cut into four equal lengths and the resistance ratio of each section determined. Resistance ratio values of 400 to 600 were consistently obtained for the cathode sections. It was found that the end of the cathode section attached to the adapter always had a lower resistance ratio than the rest of the section. This end undoubtedly contained a significant concentration of impurities which had not been removed by electrotransport because of the slightly lower temperature at the end of the rod. These impurities were released slowly during the run and helped prevent the concentration in the first half of the rod from attaining as low a value as predicted by the steady-state calculation, A notable improvement in the purity of the thorium was achieved by cutting off 2 cm of the cooler portion of the cathode end and 3 cm of the anode end and then running the center portion of the rod for the time necessary to insure a steady-state condition. The anode section was removed because it was heavily contaminated by electrotransport of the impurities. The thorium prepared by this duplex refining procedure had a resistance ratio of 800-1300 and a hardness of 27-3 1 DPH. The effect of the flux of interstitial solutes from the cooler ends of the rod was also reduced by butt welding a 3 cm length of thorium which had been previously purifield by electrotransport to the cathode end of the specimen. The cool end then, essentially, contained no interstitial solute. In this case, a larger temperature gradient was used and the cool end of the rod was near room temperature. This modification in the procedure produced thorium having a resistance ratio of 700-1000 after heating for only 80 h at 1575°C. The build-up of interstitial impurities at the end of the rod can be easily observed after the pu~~~ation run has been completed. If suflicient impu~ties are present in the starting material, a band, containing precipitated particles of thorium oxide and thorium nitride is formed. The oxide-rich band appears black in color because most of the surface is covered by substoichiometric thorium oxide. The thorium oxide band is formed further from the anode end than the thorium nitride-containing band because of the lower solubility of oxygen in thorium. Figure 1 is a photomi~rograph showing the interface between the thorium oxide precipitated at temperature and the liner particles of oxide precipitated upon cooling the specimen to room temperature. Figure 2 shows an area of the rod closer to the anode end that contains both the coarse thorium oxide particles and the star-like particles of thorium nitride. Considerable difficulty has been encountered with “grain slippage” when thorium is heated for more than 100 hat 1525°-f6000C. The thorium rod develops a “bamboo” beta grain structure and the weight of the rod is sufficient to cause grain-boundary sliding. Sometimes this slippage is so extensive that it results in the actual parting of the rod. It was found that this problem can be overcome if the specimen is tubular in shape since in this geometry the grain boundaries do not extend across the sample J. Less-Common Met&,
24 (1971) 223-228
D. T. PETERSON, F. A. SCHMIDT
226
Fig. 1. Anode section of thorium rod showing interface between thorium oxide in and out of solution at temperature. As polished. ( x 75) Fig. 2. Anode section of thorium rod showing thorium oxide and thorium nitride (star-like) particles. As polished. ( x 250)
but are randomly distributed around the tube. In a single experiment, a thorium tube, 0.47 cm outside diam., 0.22 cm inside diam, and 16.25 cm long, was heated to 1575°C for 168 h. Very little grain slippage was observed. The resistivity ratio of this specimen was 30 before purification and 930 after purification. The resistance ratio is an easily obtained parameter which reflects the total amount of solutes and impurities in solution. However, it is not helpful in identifying the impurities which are present. For example, the room temperature resistivity of high purity thorium is about 15.8 x 1O-6 s2 cm, therefore a specimen with a resistance ratio of 1000 will have a resistivity at 4’K of 15.8 x lo-’ Szcm. Carbon in thorium has been shown’ to increase the 4OK resistance of thorium by 5.5 x 10m9 B cm per one p.p.m. carbon by weight. Such a sample could contain at most 3 p.p.m. of carbon if all other impurities were absent. Because we cannot analyze for carbon accurately enough at this concentration level, it cannot be established whether the limiting impurity is carbon, nitrogen or oxygen ; indeed, it might be none of these but a substitutional metal solute which could not be treated by electrotransport. In an effort to determine if this was the case, thorium rods prepared from several different ingots were refined by the electrotransport process. After identical treatment of heating to 1550°C for 75 h, the rods were evaluated by determing their resistance ratio. It was found that rods prepared from the same ingots resulted in refined metal having almost identical ratios. However, these ratios varied from 250 to 1200, depending upon the ingots used as the parent material. Comparative mass spectrographic analysis of these specimens before and after purification revealed that no substitutional impurity, with the possible exception of silicon, was removed by electrotransport. In previous work, the authors6 have shown that the resistance of vanadium is increased by the presence of silicon. The effect of silicon on the resistance of thorium has not been studied. The concentrations of sodium, magnesium, aluminum, potassium calcium, chromium and copper in the thorium rods decreased from about 2-10 p.p.m. to less than 1 p-p-m. under the fowJ. Less-Common Met&,
24 (1971) 223-228
227
HIGH PURITY THORI~
pressure environment during electrotransport. Since the resistance ratio of the parent materials was unchanged, when heated under the same conditions with an a.c. current, the effect of these volatile impurities on the resistance ratio is considered to be negligible. At the present time, the reason for different limiting resistance ratios in different ingots of thorium is not known. Preparation
of thorium
single crystals
Many properties of solid substances can be investigated more easily and completely if measurements can be made on single crystals of that material. Thorium has a phase transformation upon heating from face centered cubic to body-centered cubic at 1345’C’. This transformation has made the growing of thorium single crystals impossibly by several of the well established techniques. We found in our experiments that single crystals of alpha thorium can be prepared if the electrotransport-purified specimen is cycled through the alpha to beta transformation and then heated for a prolonged time just below this temperature. After the purification process was finished, the temperature of the rod was lowered to 1290°C and, subsequently, raised to 1450°C through the transformation. This cycle was repeated twice after which the rods were cooled to 1300”-1325°C and held at this temperature for 60 h. The phase transformation was detected by measuring the resistance of the sample by means of a potentiometer and a voltage divider circuit. The optical sight glass was always somewhat coated with metal after a long electrotransport run and the optical pyrometer readings were subject to an uncertain absorption correction By observing the sample resistance, a temperature very close to the transformation temperature could be used for the prolonged high alpha anneal, without inadvertently heating into the beta range. We have prepared single crystals up to 17 mm in length and 2.5 mm in diameter by this procedure. Because the purification and crystal growth steps were done in the same degassed vacuum system, without an intermediate exposure to the atmosphere, contamination of the specimen during crystal growth should have been reduced to the greatest possible extent. In a separate series of experiments, this cycling technique was used to prepare several single crystals, 1.3 cm in diam. and 1.9 cm long, from the 99.9.SP; pure thorium used as the parent material in the electrotransport studies. The crystals were prepared by sealing cylindrical specimens of the polycrystalline thorium in tantalum, rapidly heating and cooling them through the transformation temperature four times, and then heating at 1320°C f 10°C for two days. The crystals were examined by optical microscopy and by neutron diffraction to establish that they were indeed single crystals. CONCLUSIONS
(1) Electrotransport has been demonstrated as a method of preparing very high purity thorium. (2) The method is simple but requires careful attention to the design of the specimen, the time and temperature of the experiment and the prevention of significant contamination by the environment. (3) Single crystals of the high purity thorium can be prepared by heating the sample several times through the alpha to beta transformation and then heating it for approximately 48 h at 1320” & 10°C. J. Less-Common
Metals, 24 (1971) 223-228
D. T. PETERSON. F. A. SCHMIDT
228
(4) The thermal cycling procedure has also been used to prepare large single crystals of thorium measuring 1.3 cm in diameter and 1.9 cm long. ACKNOWLEDGEMENT
The authors would like to thank Dr. J. D. Verhoeven for his consultation throughout this work and Mr. M. Thompson for his meticulous work in performing the experiments. Acknowledgement is also made to Mr. J. E. Ostenson for measuring the resistivity of the various samples and to Inorganic and Physical Chemistry Group VII for performing the mass spectrographic analyses.
REFERENCES 1 A. COEHNAND W. SPECHT,Z. Physik, 62 (1930) 1. 2 D. T. PETERSON, F. A. SCHMIDTAND J. D. VERHOEVEN,Trans. AIME,
3 4 5 6 7
236 (1966) 1311. J. D. VERHOEVEN,J. Metals, 18 (1966) 26. D. T. PETERSON,W. E. KRUPP AND F. A. SCHMIDT,J. Less-Common Metals, 10 (1966) 1. D. T. PETERSON,D. F. PAGE, R. B. RUMP AND D. K. FINNEMORE,Phys. Rev., 153 (1967) 701. 0. N. CARLS~N AND F. A. SCHMIDT, U.S. At. Energy Commission Rept. IS-RD-10, 1967, p. 3. P. CHIOTH AND G. J. DOOLEY, J. iVuc1. Mater., 23 (1966) 45.
J. Less-Common
Metals, 24 (1971) 223-228
223
MI3’rAJ.S
Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands
PREPARATION CRYSTALS
D. T. PETERSON
OF HIGH
AND
PURITY THORIUM
AND THORIUM
SINGLE
F. A. SCHMIDT
Institute for Atomic Research and Department of Metallurgy, Iowa State Universit,y, Ames, Iowa 50010
(U.S.A.) (Received January lSth, 1971)
SUMMARY
Ultra-high purity thorium has been prepared using electrotransport as the refining method. This metal has a total impurity content of less than 50 p.p.m. and a resistance ratio of greater than 1000. A procedure was also developed by which single crystals of the high purity thorium could be prepared by repeated heating of the specimen through its alpha to beta phase transformation followed by prolonged heating just below the transformation temperature.
INTRODUCTION
Thorium may be of considerable importance in the atomic energy program because it is a fertile material which can be converted to 233U. Research on its fundamental physical and chemical properties has been handicapped by the difficulty in preparing very high purity specimens. The primary problem has been the almost inevitable introduction of some amounts of carbon, nitrogen and oxygen during processing. The high affinity of thorium for these elements makes their removal nearly impossible once they have entered the metal lattice. The ~gration of a solute in a metal at high temperature when a d.c. current is passed has been known for many years’. This migration has been proposed as a means of purifying metals but has seldom been demonstrated to be an effective method of achieving really high purity materials. The authors have previously determined the electrotransport mobilities and diffusion coefficients of carbon, oxygen and nitrogen in thorium between 1440” and 1685’C’. The mobilities were found to be large and Verhoeven3 computed the degree of purification of a rod for realistic values of the pertinent parameters. He showed that it could be possible to lower the nitrogen content to 3 x 10-” and the carbon content to 1 x lo-’ of its original concentration by electrotransport after only five days of processing. These values are for an ideal experiment -* Work was performed in the Ames Laboratory of the U.S. Atomic Energy Commission. Contribution No. 2920. J. Less-Common Metals, 24 (1971) 223-228
D. T. PETERSON, F. A. SCHMIDT
224
in which no impurities are introduced from the environment, and they demonstrate the potential of electrotransport as a purification procedure. It was the purpose of this investigation to utilize the mobility and diffusion data of the interstitial impurities in thorium and to develop the electrotransport process so that thorium could be prepared in very high purity form. In conjunction with this work, a method was also sought by which single crystals of pure thorium could be prepared since they are essential in various characterization studies and experimental investigations. PROCEDURE AND RESULTS
The thorium used in this study was prepared by a magnesium intermediate alloy process developed by the authors4. This material had been triple electron-beam melted and contained less than 250 p.p.m. total impurities. The major impurities were oxygen, 80 p.p.m.; nitrogen, 35 p.p.m.; carbon, 50 p.p.m.; and less than 100 p.p.m. of total metallic impurities. Purijkation of thorium by electrotransport In the initial experiments in which electrotransport was to be evaluated as a method of purifying thorium, specimens, 10 cm in length and 0.254 cm in diam., were heated with a d.c. current for a length of time sufficient to approach the steadystate distribution. At the steady state, the interstitial solutes have migrated with the flow of electrons until the electrotransport fluxes are balanced by the diffusion fluxes due to the concentration gradients. The thorium rods were treated for 90 h at 1600°C under a pressure of 6 x 10e8 torr. The purified rods were sectioned into four equal lengths and the resistance ratio of each section measured. Typical values obtained were 120,90,75 and 20 which represent a profile along the rod in the same direction as the electron flow, that is, from the cathode to the anode end of the rod. The thorium, initially, had a resistance ratio of 33. It was apparent that the vacuum system and chamber being used were incapable of providing a non-contaminating environment for the specimens and, as a result, only partial purification of the rods was being achieved. An all-metal apparatus was constructed which was bakeable to 400°C and had a vacuum system capable of obtaining a pressure of 5 x lo- l1 torr. The apparatus consisted of a stainless-steel sample chamber that was equipped with two electrodes, a sight glass, a titanium sublimation pump and a flange for mounting on an ionization pump. The chamber was 15 cm in diameter and 27 cm long. A glass to metal seal was used between the electrodes and the chamber. A sorption pump was used to evacuate the system to 1 x lo-’ torr after which the titanium sublimation pump and the Ultek 100-l/see differential ionization pump were used to achieve and maintain the low pressure throughout the run. The specimens were thorium rods, 16.5 cm long and 0.254 cm in diameter. They were threaded and screwed into tantalum “U” shaped adapters attached to the ends of the electrodes. This arrangement not only accomodated the thermal expansion of the sample but greatly reduced the thermal gradients at the ends of the specimen. This was accomplished by adjusting the cross-sectional areas of the adapter so that they were heated to nearly the same temperature as the specimen. The temperature was measurJ. Less-Common Metals,24 (1971)
223-228
225
HIGH PURITY THORIUM
ed, using an optical pyrometer, and the observed temperature corrected for the emissivity of the sample and for absorption by the viewing window. The d.c. current was supplied by a saturable core, step-down transformer which provided a full-wave rectified current with a 15 % ripple. A standard resistance and a potentiometer were used to measure the current. The specimens were heated to 1575°-16000C using a current density of 1850-1950 A/cm’, and the pressure in the chamber was maintained at l-5 x lo- lo torr. The specimens were heated for 70-100 h which was sufficient to produce the steady-state condition. The purified rods were cut into four equal lengths and the resistance ratio of each section determined. Resistance ratio values of 400 to 600 were consistently obtained for the cathode sections. It was found that the end of the cathode section attached to the adapter always had a lower resistance ratio than the rest of the section. This end undoubtedly contained a significant concentration of impurities which had not been removed by electrotransport because of the slightly lower temperature at the end of the rod. These impurities were released slowly during the run and helped prevent the concentration in the first half of the rod from attaining as low a value as predicted by the steady-state calculation, A notable improvement in the purity of the thorium was achieved by cutting off 2 cm of the cooler portion of the cathode end and 3 cm of the anode end and then running the center portion of the rod for the time necessary to insure a steady-state condition. The anode section was removed because it was heavily contaminated by electrotransport of the impurities. The thorium prepared by this duplex refining procedure had a resistance ratio of 800-1300 and a hardness of 27-3 1 DPH. The effect of the flux of interstitial solutes from the cooler ends of the rod was also reduced by butt welding a 3 cm length of thorium which had been previously purifield by electrotransport to the cathode end of the specimen. The cool end then, essentially, contained no interstitial solute. In this case, a larger temperature gradient was used and the cool end of the rod was near room temperature. This modification in the procedure produced thorium having a resistance ratio of 700-1000 after heating for only 80 h at 1575°C. The build-up of interstitial impurities at the end of the rod can be easily observed after the pu~~~ation run has been completed. If suflicient impu~ties are present in the starting material, a band, containing precipitated particles of thorium oxide and thorium nitride is formed. The oxide-rich band appears black in color because most of the surface is covered by substoichiometric thorium oxide. The thorium oxide band is formed further from the anode end than the thorium nitride-containing band because of the lower solubility of oxygen in thorium. Figure 1 is a photomi~rograph showing the interface between the thorium oxide precipitated at temperature and the liner particles of oxide precipitated upon cooling the specimen to room temperature. Figure 2 shows an area of the rod closer to the anode end that contains both the coarse thorium oxide particles and the star-like particles of thorium nitride. Considerable difficulty has been encountered with “grain slippage” when thorium is heated for more than 100 hat 1525°-f6000C. The thorium rod develops a “bamboo” beta grain structure and the weight of the rod is sufficient to cause grain-boundary sliding. Sometimes this slippage is so extensive that it results in the actual parting of the rod. It was found that this problem can be overcome if the specimen is tubular in shape since in this geometry the grain boundaries do not extend across the sample J. Less-Common Met&,
24 (1971) 223-228
D. T. PETERSON, F. A. SCHMIDT
226
Fig. 1. Anode section of thorium rod showing interface between thorium oxide in and out of solution at temperature. As polished. ( x 75) Fig. 2. Anode section of thorium rod showing thorium oxide and thorium nitride (star-like) particles. As polished. ( x 250)
but are randomly distributed around the tube. In a single experiment, a thorium tube, 0.47 cm outside diam., 0.22 cm inside diam, and 16.25 cm long, was heated to 1575°C for 168 h. Very little grain slippage was observed. The resistivity ratio of this specimen was 30 before purification and 930 after purification. The resistance ratio is an easily obtained parameter which reflects the total amount of solutes and impurities in solution. However, it is not helpful in identifying the impurities which are present. For example, the room temperature resistivity of high purity thorium is about 15.8 x 1O-6 s2 cm, therefore a specimen with a resistance ratio of 1000 will have a resistivity at 4’K of 15.8 x lo-’ Szcm. Carbon in thorium has been shown’ to increase the 4OK resistance of thorium by 5.5 x 10m9 B cm per one p.p.m. carbon by weight. Such a sample could contain at most 3 p.p.m. of carbon if all other impurities were absent. Because we cannot analyze for carbon accurately enough at this concentration level, it cannot be established whether the limiting impurity is carbon, nitrogen or oxygen ; indeed, it might be none of these but a substitutional metal solute which could not be treated by electrotransport. In an effort to determine if this was the case, thorium rods prepared from several different ingots were refined by the electrotransport process. After identical treatment of heating to 1550°C for 75 h, the rods were evaluated by determing their resistance ratio. It was found that rods prepared from the same ingots resulted in refined metal having almost identical ratios. However, these ratios varied from 250 to 1200, depending upon the ingots used as the parent material. Comparative mass spectrographic analysis of these specimens before and after purification revealed that no substitutional impurity, with the possible exception of silicon, was removed by electrotransport. In previous work, the authors6 have shown that the resistance of vanadium is increased by the presence of silicon. The effect of silicon on the resistance of thorium has not been studied. The concentrations of sodium, magnesium, aluminum, potassium calcium, chromium and copper in the thorium rods decreased from about 2-10 p.p.m. to less than 1 p-p-m. under the fowJ. Less-Common Met&,
24 (1971) 223-228
227
HIGH PURITY THORI~
pressure environment during electrotransport. Since the resistance ratio of the parent materials was unchanged, when heated under the same conditions with an a.c. current, the effect of these volatile impurities on the resistance ratio is considered to be negligible. At the present time, the reason for different limiting resistance ratios in different ingots of thorium is not known. Preparation
of thorium
single crystals
Many properties of solid substances can be investigated more easily and completely if measurements can be made on single crystals of that material. Thorium has a phase transformation upon heating from face centered cubic to body-centered cubic at 1345’C’. This transformation has made the growing of thorium single crystals impossibly by several of the well established techniques. We found in our experiments that single crystals of alpha thorium can be prepared if the electrotransport-purified specimen is cycled through the alpha to beta transformation and then heated for a prolonged time just below this temperature. After the purification process was finished, the temperature of the rod was lowered to 1290°C and, subsequently, raised to 1450°C through the transformation. This cycle was repeated twice after which the rods were cooled to 1300”-1325°C and held at this temperature for 60 h. The phase transformation was detected by measuring the resistance of the sample by means of a potentiometer and a voltage divider circuit. The optical sight glass was always somewhat coated with metal after a long electrotransport run and the optical pyrometer readings were subject to an uncertain absorption correction By observing the sample resistance, a temperature very close to the transformation temperature could be used for the prolonged high alpha anneal, without inadvertently heating into the beta range. We have prepared single crystals up to 17 mm in length and 2.5 mm in diameter by this procedure. Because the purification and crystal growth steps were done in the same degassed vacuum system, without an intermediate exposure to the atmosphere, contamination of the specimen during crystal growth should have been reduced to the greatest possible extent. In a separate series of experiments, this cycling technique was used to prepare several single crystals, 1.3 cm in diam. and 1.9 cm long, from the 99.9.SP; pure thorium used as the parent material in the electrotransport studies. The crystals were prepared by sealing cylindrical specimens of the polycrystalline thorium in tantalum, rapidly heating and cooling them through the transformation temperature four times, and then heating at 1320°C f 10°C for two days. The crystals were examined by optical microscopy and by neutron diffraction to establish that they were indeed single crystals. CONCLUSIONS
(1) Electrotransport has been demonstrated as a method of preparing very high purity thorium. (2) The method is simple but requires careful attention to the design of the specimen, the time and temperature of the experiment and the prevention of significant contamination by the environment. (3) Single crystals of the high purity thorium can be prepared by heating the sample several times through the alpha to beta transformation and then heating it for approximately 48 h at 1320” & 10°C. J. Less-Common
Metals, 24 (1971) 223-228
D. T. PETERSON. F. A. SCHMIDT
228
(4) The thermal cycling procedure has also been used to prepare large single crystals of thorium measuring 1.3 cm in diameter and 1.9 cm long. ACKNOWLEDGEMENT
The authors would like to thank Dr. J. D. Verhoeven for his consultation throughout this work and Mr. M. Thompson for his meticulous work in performing the experiments. Acknowledgement is also made to Mr. J. E. Ostenson for measuring the resistivity of the various samples and to Inorganic and Physical Chemistry Group VII for performing the mass spectrographic analyses.
REFERENCES 1 A. COEHNAND W. SPECHT,Z. Physik, 62 (1930) 1. 2 D. T. PETERSON, F. A. SCHMIDTAND J. D. VERHOEVEN,Trans. AIME,
3 4 5 6 7
236 (1966) 1311. J. D. VERHOEVEN,J. Metals, 18 (1966) 26. D. T. PETERSON,W. E. KRUPP AND F. A. SCHMIDT,J. Less-Common Metals, 10 (1966) 1. D. T. PETERSON,D. F. PAGE, R. B. RUMP AND D. K. FINNEMORE,Phys. Rev., 153 (1967) 701. 0. N. CARLS~N AND F. A. SCHMIDT, U.S. At. Energy Commission Rept. IS-RD-10, 1967, p. 3. P. CHIOTH AND G. J. DOOLEY, J. iVuc1. Mater., 23 (1966) 45.
J. Less-Common
Metals, 24 (1971) 223-228