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The melting temperature of uranium at high pressures
2272
TECHNICAL NOTES
of the low sound velocity in the h e a v y bismuth compounds. For the bismuth orthogermanate one should expect similar values for the attenuation. But since the piezoelectric coupling coefficient is still lower in this material, the signals obtained were below the capability of our apparatus, and no measurements were possible.
been reported on the effect of pressure on the melting temperature o f uranium. Asami e t a/.[l] record a most strange behavior for the melting of uranium. Their results show that the melting temperature of uranium increases with pressure from its atmospheric value o f 1132~ to a broad maximum at circa 1180 ~ in the pressure range of 30-40 kbar and that, with increasing presAcknowledgement--The authors wish to thank P. Janak sure, the melting temperature of uranium for his help in preparing the samples and performing some decreases. This unexpected behavior has of the measurements, and also D. Bortfeld who critically prompted our reexamination of the effect of read the manuscript. pressure on the melting of uranium. R C A Laboratories Ltd., W. REHWALD Unfortunately, uranium, like iron, alloys Zurich, R. WIDMER with almost every metal which melts at a Switzerland higher temperature and, further, uranium has REFERENCES an extremely low oxidation potential (Rand 1. NITSCHE R., Jr. appl. Phys. 36, 2358 (1965). and Kabuchewski [2]). An eutectic is shown 2. BORTFELD D. P. and MEIER H., Jr. appl. Phys. between uranium and uranium oxide (Elliot 43, 5110 (1972). 3. PHILIPSBORN H. VON., J. crystal Growth 11, 348 [3]). H a n s e n and A n d e r k o [4] show that both (1971). tungsten and tantalum have relatively low 4. SCHWEPPE H., I E E E Trans. Sonics and Ultrason. solubility in uranium at the melting temperaSU-16, 219 (1969). 5. In the notation used, the first symbol gives the direc- ture. Unfortunately, the details of these two tion of propagation, the second the direction of polar- binary phase diagrams have not been comization of the sound wave. 6. REHWALD W., In Proc. Intern. Conf. on Phonon pletely worked out. Klement e t al. [5] found Scattering in Solids, (Edited by H. J. Albany), p. 45. tantalum to have a small effect on the subParis, (1972). solidus phase transformations of uranium at 7. BOEMMEL H. E. and DRANSFELD K., Phys. Rev. high pressures in contrast to other investi117, 1245 (1960). 8. KLERK J. de, Phys. Rev. 139, A1635 (1965). gated encapsulating materials. Tantalum also 9. The preliminary value given in Ref. [6] for the longitud- is easily machinable and deforms well under inal wave attenuation proved to be slightly too high. cold compression at high pressures. Thus, we have elected to investigate the J. Phys. Chem. Solids, pp. 2272-2274. effect of pressure on the melting temperature or uranium encapsulated in tantalum, knowing that the resulting melting curve will be The melting temperature of uranium at high somewhat modified owing to the slight pressures* alloying effects of tantalum and uranium oxide. H o w e v e r , inasmuch as there is very (Received5 February 1973) little oxygen available to the sample in our experimental setup, and the solubility of THE EFFECT of pressure on the melting tem- tantalum in uranium is extremely low, it is perature of most of the pure elements have hoped that the latent heat and volume change been studied and published over the last on melting of our somewhat alloyed sample several years. A single prior observation has would not differ significantly from that of pure uranium. Consequently we should ex*Publication # 1154, Institute of Geophysics and pect our melting curve to be virtually parallel Planetary Physics, University of California, Los Angeles, to that of pure uranium. Ca. 90024, U,S.A.
TECHNICAL NOTES
2273
EXPERIMENTAL METHOD
RESULTS
The experiments we report here were carried out in a piston-cylinder apparatus essentially similar to that described by Cohen et al.[6]. Our pressure cell consisted of an outer talc sleeve separated from a concentric graphite furnace by pyrex glass tubing. The sample, which was in the form of a small cylinder (0.125in. thick, 0.125in. dia., 99.9 per cent pure) was stored in alcohol. The sample was polished gently with emery paper to remove surface oxidation, and was tightly press-fitted to the tantalum capsule. The space inside the graphite furnace, not occupied by the encapsulated uranium sample, was filled with boron nitride parts fired at 850~ for 24 hr. The pressure cell is analogous to that described and illustrated by Akella et al. [7]. The temperatures of melting were determined on a compression cycle by differential thermal analysis signal. Platinumplatinum 10 per cent rhodium thermocouples were used. Frictional losses were determined to be around 10 per cent of the nominal pressure by intercomparison with the melting curve of NaC! determined on a compression cycle in the same pressure cell. Corrections for the effect of pressure on the emf of thermocouples were made according to the resuits of Getting and Kennedy [8]. Hansen and Anderko [4] report the 1 atm melting point of uranium to be at 1132 ~ Our 1 arm melting point of uranium to be at 1132~ Our 1 arm results in our apparatus were consistantly lower than this value; however, the melting temperature was found to settle down to a steady value in about 30-40 min time after the sample was heated to the neighborhood of the melting temperature. Thus, it became apparent that in our experimental configuration alloying was sufficiently rapid so that there was little hope of obtaining the melting point of pure uranium at high pressures. Thus, we permitted the sample to stabilize at high temperatures in the field of both melt and solid for approximately 1 hr before determining the melting point.
Our results are shown in Table 1 in which we report the pressure, corrected for friction, and temperature corrected for the effect of pressure on the e.m.f, of thermocouples. Each data point is a result of the averaging 3-5 determinations scattered over a temperature interval of approximately 4 ~. All these data were obtained on a single experiment; thus depression of the melting point owing to contamination should be the same throughout, except as modified by the effect of pressure on the system. The slope of the melting curve fitted to these data, as shown in Fig. 1, is 4.1~ kbar. The approximate melting curve of pure uranium is generated by fitting this slope to the atmospheric melting temperature of the pure metal as reported by Hansen and Anderko [4] which is 27 ~ higher than shown by the extrapolation of our experimental curve to zero pressure. Our results are in serious disagreement with those of Asami et al., who determined the melting curve of uranium to 80 kbar by electrical resistivity measurements. These investigators reported a broad melting temperature maximum near 35 kbar and the initial slope of their melting curve to this maximum is about 2.8~ as compared to ours of around 4-l~ Our value, however, is in precise agreement with that determined by Asami et al., from thermochemical data at atmospheric pressure. In their experimental Table 1. Experimental results on melting of uranium Pressure kbar
Temp. ~
9 13-5 18-0 23.4 28.9 36-0 41.4
1144 1162 1180 1199 1221 1254 1276
2274
TECHNICAL NOTES
~500
for pure Uranium
1250 cJ 1200 o 1150 ~
m
e
n
~
o
l melting curve for Uranium. Te + 0 impuri'ties
rl00 1050 0
5
Io
1'5
2o
2~
3o
3'5
40
4."5
5'0
P, kbar Fig. 1. Melting curve of uranium.
set up they may well have had an influx of water from the dehydration of talc and pyrophyllite, which constitute significant part of their pressure cell. Thus progressive oxidation of this sample might take place. In addition, they did not correct for the pressure effect on the e.m.f, of the thermocouple (Pt-Pt 13% Rh), and used erroneous calibration points (e.g. Bi 88 kbar instead of Bi 77 kbar, Ba 59 kbar, instead of Ba 55 kbar) in their pressure calibration. Also A1203, which they used as insulating material around the uranium sample could to some extent reduce the melting temperature of uranium by virtue of the reaction A1203 = 2AI (uranium, melt) + 3/20z (cf. Hansen). Acknowledgements-We wish to acknowledge partial financial support from our AEC A T (11-1)-34) Grant. Dr. R. G. McQueen of the Los Alamos Scientific Laboratory kindly supplied the sample of uranium and Dr. D. Stephens of the Lawrence Livermore Laboratory called
the work of Asami et al. to our attention and provided us with a translation of the original article.
Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Ca. 90024, U.S.A.
J. G A N G U L Y G. C. K E N N E D Y
REFERENCES 1. ASAMI N., YAMADO M. and TAKAHASHI S., Nippon Kinzoku Gakkaishi 31, 389 (1961). 2. RAND M. H. and KABACHEWSKI O., The Thermochemical Properties of Uranium Compounds, pp. 1-96. Wiley-Interscience, New York (1963). 3. E L L I O T P. E., Constitution of Binary Alloys. First supplement, p. 877. McGraw-Hill, New York (1965). 4. H A N S E N M. and A N D E R K O K., Constitution of Binary Alloys, p. 1224. McGraw-Hill, NewYork (1958). 5. K L E M E N T W., J A Y A R A M A N A. and K E N N E D Y G. C., Phys. Rev. 179, 1971 (1963). 6. C O H E N L. H., K L E M E N T W. and K E N N E D Y G. C., J. Phys. Chem. Solids 27, 179 (1966). 7. A K E L L A J., V A I D Y A S. N. and K E N N E D Y G. C., Phys. Rev. 185, 1135 (1969). 8. G E T T I N G I. C. and K E N N E D Y G. C., J. appl. Phys. 41, 4552 (1970).
TECHNICAL NOTES
of the low sound velocity in the h e a v y bismuth compounds. For the bismuth orthogermanate one should expect similar values for the attenuation. But since the piezoelectric coupling coefficient is still lower in this material, the signals obtained were below the capability of our apparatus, and no measurements were possible.
been reported on the effect of pressure on the melting temperature o f uranium. Asami e t a/.[l] record a most strange behavior for the melting of uranium. Their results show that the melting temperature of uranium increases with pressure from its atmospheric value o f 1132~ to a broad maximum at circa 1180 ~ in the pressure range of 30-40 kbar and that, with increasing presAcknowledgement--The authors wish to thank P. Janak sure, the melting temperature of uranium for his help in preparing the samples and performing some decreases. This unexpected behavior has of the measurements, and also D. Bortfeld who critically prompted our reexamination of the effect of read the manuscript. pressure on the melting of uranium. R C A Laboratories Ltd., W. REHWALD Unfortunately, uranium, like iron, alloys Zurich, R. WIDMER with almost every metal which melts at a Switzerland higher temperature and, further, uranium has REFERENCES an extremely low oxidation potential (Rand 1. NITSCHE R., Jr. appl. Phys. 36, 2358 (1965). and Kabuchewski [2]). An eutectic is shown 2. BORTFELD D. P. and MEIER H., Jr. appl. Phys. between uranium and uranium oxide (Elliot 43, 5110 (1972). 3. PHILIPSBORN H. VON., J. crystal Growth 11, 348 [3]). H a n s e n and A n d e r k o [4] show that both (1971). tungsten and tantalum have relatively low 4. SCHWEPPE H., I E E E Trans. Sonics and Ultrason. solubility in uranium at the melting temperaSU-16, 219 (1969). 5. In the notation used, the first symbol gives the direc- ture. Unfortunately, the details of these two tion of propagation, the second the direction of polar- binary phase diagrams have not been comization of the sound wave. 6. REHWALD W., In Proc. Intern. Conf. on Phonon pletely worked out. Klement e t al. [5] found Scattering in Solids, (Edited by H. J. Albany), p. 45. tantalum to have a small effect on the subParis, (1972). solidus phase transformations of uranium at 7. BOEMMEL H. E. and DRANSFELD K., Phys. Rev. high pressures in contrast to other investi117, 1245 (1960). 8. KLERK J. de, Phys. Rev. 139, A1635 (1965). gated encapsulating materials. Tantalum also 9. The preliminary value given in Ref. [6] for the longitud- is easily machinable and deforms well under inal wave attenuation proved to be slightly too high. cold compression at high pressures. Thus, we have elected to investigate the J. Phys. Chem. Solids, pp. 2272-2274. effect of pressure on the melting temperature or uranium encapsulated in tantalum, knowing that the resulting melting curve will be The melting temperature of uranium at high somewhat modified owing to the slight pressures* alloying effects of tantalum and uranium oxide. H o w e v e r , inasmuch as there is very (Received5 February 1973) little oxygen available to the sample in our experimental setup, and the solubility of THE EFFECT of pressure on the melting tem- tantalum in uranium is extremely low, it is perature of most of the pure elements have hoped that the latent heat and volume change been studied and published over the last on melting of our somewhat alloyed sample several years. A single prior observation has would not differ significantly from that of pure uranium. Consequently we should ex*Publication # 1154, Institute of Geophysics and pect our melting curve to be virtually parallel Planetary Physics, University of California, Los Angeles, to that of pure uranium. Ca. 90024, U,S.A.
TECHNICAL NOTES
2273
EXPERIMENTAL METHOD
RESULTS
The experiments we report here were carried out in a piston-cylinder apparatus essentially similar to that described by Cohen et al.[6]. Our pressure cell consisted of an outer talc sleeve separated from a concentric graphite furnace by pyrex glass tubing. The sample, which was in the form of a small cylinder (0.125in. thick, 0.125in. dia., 99.9 per cent pure) was stored in alcohol. The sample was polished gently with emery paper to remove surface oxidation, and was tightly press-fitted to the tantalum capsule. The space inside the graphite furnace, not occupied by the encapsulated uranium sample, was filled with boron nitride parts fired at 850~ for 24 hr. The pressure cell is analogous to that described and illustrated by Akella et al. [7]. The temperatures of melting were determined on a compression cycle by differential thermal analysis signal. Platinumplatinum 10 per cent rhodium thermocouples were used. Frictional losses were determined to be around 10 per cent of the nominal pressure by intercomparison with the melting curve of NaC! determined on a compression cycle in the same pressure cell. Corrections for the effect of pressure on the emf of thermocouples were made according to the resuits of Getting and Kennedy [8]. Hansen and Anderko [4] report the 1 atm melting point of uranium to be at 1132 ~ Our 1 arm melting point of uranium to be at 1132~ Our 1 arm results in our apparatus were consistantly lower than this value; however, the melting temperature was found to settle down to a steady value in about 30-40 min time after the sample was heated to the neighborhood of the melting temperature. Thus, it became apparent that in our experimental configuration alloying was sufficiently rapid so that there was little hope of obtaining the melting point of pure uranium at high pressures. Thus, we permitted the sample to stabilize at high temperatures in the field of both melt and solid for approximately 1 hr before determining the melting point.
Our results are shown in Table 1 in which we report the pressure, corrected for friction, and temperature corrected for the effect of pressure on the e.m.f, of thermocouples. Each data point is a result of the averaging 3-5 determinations scattered over a temperature interval of approximately 4 ~. All these data were obtained on a single experiment; thus depression of the melting point owing to contamination should be the same throughout, except as modified by the effect of pressure on the system. The slope of the melting curve fitted to these data, as shown in Fig. 1, is 4.1~ kbar. The approximate melting curve of pure uranium is generated by fitting this slope to the atmospheric melting temperature of the pure metal as reported by Hansen and Anderko [4] which is 27 ~ higher than shown by the extrapolation of our experimental curve to zero pressure. Our results are in serious disagreement with those of Asami et al., who determined the melting curve of uranium to 80 kbar by electrical resistivity measurements. These investigators reported a broad melting temperature maximum near 35 kbar and the initial slope of their melting curve to this maximum is about 2.8~ as compared to ours of around 4-l~ Our value, however, is in precise agreement with that determined by Asami et al., from thermochemical data at atmospheric pressure. In their experimental Table 1. Experimental results on melting of uranium Pressure kbar
Temp. ~
9 13-5 18-0 23.4 28.9 36-0 41.4
1144 1162 1180 1199 1221 1254 1276
2274
TECHNICAL NOTES
~500
for pure Uranium
1250 cJ 1200 o 1150 ~
m
e
n
~
o
l melting curve for Uranium. Te + 0 impuri'ties
rl00 1050 0
5
Io
1'5
2o
2~
3o
3'5
40
4."5
5'0
P, kbar Fig. 1. Melting curve of uranium.
set up they may well have had an influx of water from the dehydration of talc and pyrophyllite, which constitute significant part of their pressure cell. Thus progressive oxidation of this sample might take place. In addition, they did not correct for the pressure effect on the e.m.f, of the thermocouple (Pt-Pt 13% Rh), and used erroneous calibration points (e.g. Bi 88 kbar instead of Bi 77 kbar, Ba 59 kbar, instead of Ba 55 kbar) in their pressure calibration. Also A1203, which they used as insulating material around the uranium sample could to some extent reduce the melting temperature of uranium by virtue of the reaction A1203 = 2AI (uranium, melt) + 3/20z (cf. Hansen). Acknowledgements-We wish to acknowledge partial financial support from our AEC A T (11-1)-34) Grant. Dr. R. G. McQueen of the Los Alamos Scientific Laboratory kindly supplied the sample of uranium and Dr. D. Stephens of the Lawrence Livermore Laboratory called
the work of Asami et al. to our attention and provided us with a translation of the original article.
Institute of Geophysics and Planetary Physics, University of California, Los Angeles, Ca. 90024, U.S.A.
J. G A N G U L Y G. C. K E N N E D Y
REFERENCES 1. ASAMI N., YAMADO M. and TAKAHASHI S., Nippon Kinzoku Gakkaishi 31, 389 (1961). 2. RAND M. H. and KABACHEWSKI O., The Thermochemical Properties of Uranium Compounds, pp. 1-96. Wiley-Interscience, New York (1963). 3. E L L I O T P. E., Constitution of Binary Alloys. First supplement, p. 877. McGraw-Hill, New York (1965). 4. H A N S E N M. and A N D E R K O K., Constitution of Binary Alloys, p. 1224. McGraw-Hill, NewYork (1958). 5. K L E M E N T W., J A Y A R A M A N A. and K E N N E D Y G. C., Phys. Rev. 179, 1971 (1963). 6. C O H E N L. H., K L E M E N T W. and K E N N E D Y G. C., J. Phys. Chem. Solids 27, 179 (1966). 7. A K E L L A J., V A I D Y A S. N. and K E N N E D Y G. C., Phys. Rev. 185, 1135 (1969). 8. G E T T I N G I. C. and K E N N E D Y G. C., J. appl. Phys. 41, 4552 (1970).