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Thermal anomalies in supercooled parahydrogen in Vycor porous glass
PHYSICA
Physics B 194-196 (1994) 687-688 North-Holland
Thermal Anomalies in Supercooled Parahydrogen In Vycor Porous Glass D.F. Brewer, J.C.N. Rajendra and A.L. Thomson ~ Physics Laboratory, University of Sussex, Brighton, BN1 9QH, U.K. We describe measurements of the specific heat of parahydrogen in Vycor porous glass, using two different methods, between 6K and 15K. Results from the first method are very similar to those from other laboratories, showing broad peaks at solidification and melting at temperatures well below the bulk, and decreasing smoothly at lower temperatures. The second set were carried out very slowly and we believe are much closer to thermodynamic equilibrium. They show new phenomena not resolved in the previous measurements, namely three distinct peaks in the freezing anomaly, and at lower temperatures a small cusp-like featuxe which is reproducible in four separate experiments over a period of several weeks. Below the anomaly the specific heat follows a roton-type function, but these measurements cannot determine whether superfluid is present.
For many years there have been speculations that parahydrogen ( p - tt2) will undergo a Bose condensation to superfluidity, and Ginsburg and Sobyanin 1 calculated the transition temperature from ideal Bose gas theory as 6.7K. Unfortunately the normal solidification is 13.8K, so it is necessary to achieve substantial supercooling if this prediction is to be tested. The Brown University group investigated the possibility of supercooling, noting that it is encouraged in highly porous material with high surface/volume ratio. They showed that solidification is lowered to 9.9K in vycor glass with 54A diameter pores, but their heat capacity and torsional oscillator experiments gave 2 no indication of a transition to superfluidity down to low temperatures. Bretz and Thomson s were also unsuccessful in detecting superflow through a porous Vycor plug. We have now measured the heat capacity of p - H2 in Vycor porous glass using two different methods described below. In the second we took particular care to ensure thermodynamic equilibrium and it revealed thermal anomalies not previously observed. 1. A P P A R A T U S
AND RESULTS
The Be-Cu calorimeter contained four Vycor discs each about 2.5cm diameter and 0.9mm thick and of pore diameter 601k. This cell was surrounded by a vacuum case (IVCII) which was always evacuated, then by another (IVCI) at0921-4526/94/$07.00 © 1994
SSDI 0921-4526(93)E0924-6
-
tached to a liquid 'tHe pot and finally by an outer vacuum case immersed in liquid 4He at 4.2K. By suitable manipulation this system allowed the residual heat exchange with the cell to be controlled very sensitively. Hydrogen at atmospheric pressure was condensed into the cell at 20K which was then closed off with a needle value. The first set of observations was made by the standard method of measuring the temperature increase caused by the addition of a known quantity of heat superimposed on a slow background of warming or cooling. The results of this were very similar to earlier measurements, 4 showing broad peaks at the melting and solidification points at different temperatures but both well below the bulk triple point (13.8K). Data from our warming experiment is presented in Figure 1. At lower temperatures (below 10K) the specific heat was higher than bulk solid, which we attribute to the continued presence of liquid. This is supported by estimates of the entropy under the melting peak (2.5 J m o l . - 1 K - 1 ) w h i c h is only about one third of the bulk value. In this set of measurements there were very long thermal constants, of the order of one hour, and although the results were similar to other laboratories their accuracy w a s questionable. Consequently, in the second set of experiments we used a much slower procedure in which the innermost vacuum can, IVCII, was left evacuated, but its temperature left at 4.2K by exchange gas in the other two. The background
Elsevier Science B.V. All rights reserved
688 60
4O
3O 2 ,~ 2 o °°
°
°o
°
°o°
o°
•
......~...~.....'"'"'.-~" 8
9
10
ii
12
13
z4
15
T~
Figure 1. The specific heat of confined hydrogen while warming (see text). heat leak Qb from the calorimeter to surroundings was measured as a function of temperature by adding various quantities of heat to keep its temperature constant. Then to measure the total heat capacity C as a function of temperature the cell was allowed to cool slowly to 7K over a period of 20 hours, giving C = Q b / ( d T / d t ) . The temperature drift rates were less than 10mK per minute, much less than in previous work, and we believe that these measurements are much closer to equilibrium conditions. The resulting heat capacity in a cooling experiment is shown in Figure 2 for four separate experiments with different starting temperatures, carried out over a period of several weeks. The excellent reproducibility of the results indicates that there is no effect of ortho to para conversion, and we believe that the sample is in essentially the 100% para state. The three sharp
peaks around 10.8K are associated with solidification but are more likely to be due to the pore size distribution in Vycor than intrinsic to p - H2. The more interesting result is the small anomaly s at 9.6K. This is also observed at the saxne temperature in a warming experiment, although somewhat reduced in height and broadened towards higher temperatures. Its origin is therefore different from the melting/solidification processes which invariably show hysteresis. It should be recalled that at the LT19 conference we presented results 4 showing that in the temperature region 4.2K < T < 9K the confined hydrogen displays a specific heat with a rotontype temperature variation, C ~ T - ] e - A / k r with A ~ 23K. This compares with A ~ 5.6K for helium in Vycor. e The larger roton gap for hydrogen would be consistent with a higher superfluid transition temperature. However, these experiments cannot detect superfluidity, but do suggest that torsional oscillator measurements with a very long period would be worthwhile. ACKNOWLEDGEMENTS We are grateful to N. P a t h m a n a t h a n and E. Nyeanchi for assistance with the Figures, to T.P. Spiller and D. Waxrnan for discussions. The work was carried out under SERC grant G/R52845. REFERENCES
1.
38 (1974) 329. 2.
160
3. Ik"
.}; $ioo
4.
t
,¢ eo
5.
6o o 40 i 2O
9.5
lO
lO. 5
T ¢K,
Figure 2. The specific heat of confined hydrogen measured while cooling (see text).
V.L. Ginsburg and A.A. Sobyanin, Soy. Phys. J E T P Left. 15 (1972) 242; Soy. Phys. J E T P
6.
R.H. Torii, H.J. Masis and G.H. Scidel, Phys. Rev. B41 (1990) 7161. M. Bretz and A.L. Thomson, Phys. Rev. B24 (1981) 467. D.F. Brewer, J. Rajendra, N. Sharma, and A.L. Thomson, Physica B165 and 166 (1990), 569. A similar effect was observed in an experiment at Brown which could not, however, be confirmed and was not published (H.J. Maris, private communication). D.F. Brewer, J. Rajendra, N. Sharma and Jin Xin, Physica B165 and 166 (1990) 551.
Physics B 194-196 (1994) 687-688 North-Holland
Thermal Anomalies in Supercooled Parahydrogen In Vycor Porous Glass D.F. Brewer, J.C.N. Rajendra and A.L. Thomson ~ Physics Laboratory, University of Sussex, Brighton, BN1 9QH, U.K. We describe measurements of the specific heat of parahydrogen in Vycor porous glass, using two different methods, between 6K and 15K. Results from the first method are very similar to those from other laboratories, showing broad peaks at solidification and melting at temperatures well below the bulk, and decreasing smoothly at lower temperatures. The second set were carried out very slowly and we believe are much closer to thermodynamic equilibrium. They show new phenomena not resolved in the previous measurements, namely three distinct peaks in the freezing anomaly, and at lower temperatures a small cusp-like featuxe which is reproducible in four separate experiments over a period of several weeks. Below the anomaly the specific heat follows a roton-type function, but these measurements cannot determine whether superfluid is present.
For many years there have been speculations that parahydrogen ( p - tt2) will undergo a Bose condensation to superfluidity, and Ginsburg and Sobyanin 1 calculated the transition temperature from ideal Bose gas theory as 6.7K. Unfortunately the normal solidification is 13.8K, so it is necessary to achieve substantial supercooling if this prediction is to be tested. The Brown University group investigated the possibility of supercooling, noting that it is encouraged in highly porous material with high surface/volume ratio. They showed that solidification is lowered to 9.9K in vycor glass with 54A diameter pores, but their heat capacity and torsional oscillator experiments gave 2 no indication of a transition to superfluidity down to low temperatures. Bretz and Thomson s were also unsuccessful in detecting superflow through a porous Vycor plug. We have now measured the heat capacity of p - H2 in Vycor porous glass using two different methods described below. In the second we took particular care to ensure thermodynamic equilibrium and it revealed thermal anomalies not previously observed. 1. A P P A R A T U S
AND RESULTS
The Be-Cu calorimeter contained four Vycor discs each about 2.5cm diameter and 0.9mm thick and of pore diameter 601k. This cell was surrounded by a vacuum case (IVCII) which was always evacuated, then by another (IVCI) at0921-4526/94/$07.00 © 1994
SSDI 0921-4526(93)E0924-6
-
tached to a liquid 'tHe pot and finally by an outer vacuum case immersed in liquid 4He at 4.2K. By suitable manipulation this system allowed the residual heat exchange with the cell to be controlled very sensitively. Hydrogen at atmospheric pressure was condensed into the cell at 20K which was then closed off with a needle value. The first set of observations was made by the standard method of measuring the temperature increase caused by the addition of a known quantity of heat superimposed on a slow background of warming or cooling. The results of this were very similar to earlier measurements, 4 showing broad peaks at the melting and solidification points at different temperatures but both well below the bulk triple point (13.8K). Data from our warming experiment is presented in Figure 1. At lower temperatures (below 10K) the specific heat was higher than bulk solid, which we attribute to the continued presence of liquid. This is supported by estimates of the entropy under the melting peak (2.5 J m o l . - 1 K - 1 ) w h i c h is only about one third of the bulk value. In this set of measurements there were very long thermal constants, of the order of one hour, and although the results were similar to other laboratories their accuracy w a s questionable. Consequently, in the second set of experiments we used a much slower procedure in which the innermost vacuum can, IVCII, was left evacuated, but its temperature left at 4.2K by exchange gas in the other two. The background
Elsevier Science B.V. All rights reserved
688 60
4O
3O 2 ,~ 2 o °°
°
°o
°
°o°
o°
•
......~...~.....'"'"'.-~" 8
9
10
ii
12
13
z4
15
T~
Figure 1. The specific heat of confined hydrogen while warming (see text). heat leak Qb from the calorimeter to surroundings was measured as a function of temperature by adding various quantities of heat to keep its temperature constant. Then to measure the total heat capacity C as a function of temperature the cell was allowed to cool slowly to 7K over a period of 20 hours, giving C = Q b / ( d T / d t ) . The temperature drift rates were less than 10mK per minute, much less than in previous work, and we believe that these measurements are much closer to equilibrium conditions. The resulting heat capacity in a cooling experiment is shown in Figure 2 for four separate experiments with different starting temperatures, carried out over a period of several weeks. The excellent reproducibility of the results indicates that there is no effect of ortho to para conversion, and we believe that the sample is in essentially the 100% para state. The three sharp
peaks around 10.8K are associated with solidification but are more likely to be due to the pore size distribution in Vycor than intrinsic to p - H2. The more interesting result is the small anomaly s at 9.6K. This is also observed at the saxne temperature in a warming experiment, although somewhat reduced in height and broadened towards higher temperatures. Its origin is therefore different from the melting/solidification processes which invariably show hysteresis. It should be recalled that at the LT19 conference we presented results 4 showing that in the temperature region 4.2K < T < 9K the confined hydrogen displays a specific heat with a rotontype temperature variation, C ~ T - ] e - A / k r with A ~ 23K. This compares with A ~ 5.6K for helium in Vycor. e The larger roton gap for hydrogen would be consistent with a higher superfluid transition temperature. However, these experiments cannot detect superfluidity, but do suggest that torsional oscillator measurements with a very long period would be worthwhile. ACKNOWLEDGEMENTS We are grateful to N. P a t h m a n a t h a n and E. Nyeanchi for assistance with the Figures, to T.P. Spiller and D. Waxrnan for discussions. The work was carried out under SERC grant G/R52845. REFERENCES
1.
38 (1974) 329. 2.
160
3. Ik"
.}; $ioo
4.
t
,¢ eo
5.
6o o 40 i 2O
9.5
lO
lO. 5
T ¢K,
Figure 2. The specific heat of confined hydrogen measured while cooling (see text).
V.L. Ginsburg and A.A. Sobyanin, Soy. Phys. J E T P Left. 15 (1972) 242; Soy. Phys. J E T P
6.
R.H. Torii, H.J. Masis and G.H. Scidel, Phys. Rev. B41 (1990) 7161. M. Bretz and A.L. Thomson, Phys. Rev. B24 (1981) 467. D.F. Brewer, J. Rajendra, N. Sharma, and A.L. Thomson, Physica B165 and 166 (1990), 569. A similar effect was observed in an experiment at Brown which could not, however, be confirmed and was not published (H.J. Maris, private communication). D.F. Brewer, J. Rajendra, N. Sharma and Jin Xin, Physica B165 and 166 (1990) 551.