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Thermodynamic analysis of hydrogen solubility in graphite
jourual of
Journal of Nuclear Materials 200 (1993) 218-222 North-Holland
Thermodynamic
nuclear
materials
analysis of hydrogen solubility in graphite
Yoshirou Shirasu, Shinsuke Yamanaka and Masanobu Miyake Department of Nuclear Engineering, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565, Japan
Received 3 July 1992; accepted 2 December 1992
The hydrogen solubility in isotropic graphites IS0 880U and EK 98 has been measured in the temperature range of 700-1000°C at pressures below 2 x lo4 Pa. The solubility data obtained closely obeyed Sieverts’ law. The hydrogen solubility and the enthalpy of solution for IS0 88OU and EK 98 graphites were compared with those for isotropic graphites IG 1lOU and POCO AXF-SQ. The hydrogen solubility in a highly oriented pyrolytic graphite PGCCL has also been measured at 1000°C. It was an order of magnitude lower than that in isotropic graphites. Partial thermodynamic functions of hydrogen in isotropic graphites were obtained by a dilute solution mode1 and discussed.
and validity of the thermodynamic ated.
1. Introduction Graphite has recently emerged as one of the promising materials for a limiter and first wall in a fusion reactor because of its low atomic number, and excellent thermal and mechanical properties. Graphite will be exposed as the plasma-facing material to tritium gas as well as to energetic tritium ions and atoms. Tritium recycling and retention are crucial factors for a future use of graphite in a fusion reactor. Several authors [l-4] have studied on the retention of hydrogen isotopes implanted as neutral atoms and ions into various graphite samples. In these studies, the retention of hydrogen isotope has been closely related to the type of graphite and to implantation conditions such as temperature and energy. For a clear understanding of the interaction of hydrogen isotope gas and ion with graphite, it is required to obtain equilibrium data for sorption capacity for hydrogen isotopes in graphite. However, only a few studies have been performed on the hydrogen solubility measurement and its thermodynamic analysis [5-81. In our previous study [9,10], solubility measurement has been performed for hydrogen and deuterium in isotropic graphites (IG 11OU and POCO AXF-SQ), and a thermodynamic model for hydrogen solubility in graphite was proposed. In the present study, the hydrogen solubility in isotropic graphites (IS0 880U and EK 98) and a highly oriented pyrolytic graphite (PGCCL) was measured, 0022-3115/93/$06.00
model was evalu-
2. Experimental Graphite samples employed in the present study were two types of isotropic graphite: IS0 880U (Toy0 Tanso Co.) and EK 98 (Ringsdorff-Werke GmbH), and highly oriented pyrolytic graphite (PGCCL, Le Carbone-Lorraine, 10 X 10 mm2 in area of basal plane and 2 mm in thickness). The hydrogen solubility in the isotropic graphite samples was measured by a constant volume method. Prior to solubility measurement, isotropic graphite samples were mechanically polished and degassed at 1400°C in a vacuum below 1O-3 Pa. The measurement was performed in the temperature range of 700 to 1000°C at pressures ranging from 2 X lo3 to 2 X lo4 Pa. Details of the experimental apparatus and procedure were described eleswhere [9]. The hydrogen solubility in the PGCCL sample was measured by thermal desorption spectroscopy (TDS), because the hydrogen solubility in the sample was much lower than the measurable amount due to the apparatus based on the constant volume method. The graphite sample was degassed at 1000°C for 5 h in a vacuum below 10e6 Pa before the solubility measurement, and then it was exposed to hydrogen gas at 1000°C under 1.33 X lo4 Pa for 40 h. After the expo-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
219
Y. Shirasu et al. / Thermodynamic analysis of hydrogen solubility in graphite Temperalure
( ‘C )
-115-
0
IG 1lOU
APoco AXF-50 0 IS0 0
1
2
3
4
5
-14.51
6
7
c~+xlo‘(HIC)
Fig. 1. Sieverts’ plot for hydrogen in IG 11OU graphite at
700-1ocWc.
sure, the sample was quenched to room temperature. The total amount of hydrogen gas dissolved in the sample was analyzed by TDS at a heating rate of 20”C/min up to lOOO”C, and hydrogen solubility was determined from a total amount of released hydrogen. In this apparatus the amount of 5 X lOWE mol H, can be measured reproducibly. From the reproducibility of the solubility data the hydrogen content in the sample is estimated to be at equilibrium. Details of the procedure and apparatus will be published eleswhere [ll].
3. Results Solubility data obtained for hydrogen in IS0 880U and EK 98 graphites closely followed Sieverts’ law; the hydrogen solubility was proportional to the square root of the equilibrium pressure: C, = K,Pe,
(1)
where C, is the hydrogen solubility in atom ratio (H/C), Pt-r, is the equilibrium hydrogen pressure in Pa and K, is the Sieverts’ constant. Fig. 1 indicates a representative of the linear relation between the square root of the equilibrium pressure and hydrogen content. Since Sieverts’ law applied to the isotropic graphite samples, hydrogen gas is dissolved into the graphite as atomic hydrogen. Fig. 2 shows the temperature dependence of Sieverts’ constant K, for hydrogen in IS0 880U and EK
I
,
8
9
_
88OU I
10
11
lO’/T(K-‘)
Fig. 2. Temperature dependence of Sieverts’ constant K H’
98 graphites together with K, for IG 1lOU and POCO AKF-SQ graphites reported in our previous study [lo]. It is apparent from this figure that the temperature dependence of Sieverts’ constant can be expressed by In K, = A + B/T,
(2)
where A and B are constants and T is the sample temperature in K. In table 1, the values of A and B are given together with the values of enthalpy of solution AH &J/g-atom) for hydrogen in the isotropic graphites derived from the value of B. It is found from fig. 2 that the hydrogen solubility at a constant pressure is larger for IG 1lOU graphite than those for POCO AKF-SQ, IS0 880U and EK 98 graphites. It is obvious from table 1 that the enthalpies of solution for IG 1lOU and POCO AKF-SQ graphites are nearly
Table 1 Temperature dependence of Sieverts’ constant for hydrogen in graphites Graphite
A
B
IG 1lOU POCO AXF-5Q IS0 88OU
-14.5 - 15.6 -15.8 - 18.5 -14.7 - 18.5
2190 2590 2640 5800 1620 5730
AH w/g-
Temperature range (“0
atom)
EK 98
-18.2 - 21.5 - 22.0 -48.2 -13.5 -47.6
700-1000 700-1000 700- 900 900-1000 700- 800 800-1000
220
Y. Shirasu et al. / Thermodynamic
constant at 700-lOWC, whereas the enthalpies of solution for IS0 88OU and EK 98 graphites in a higher temperature range are negatively larger than those in a lower temperature ranged. The enthalpies of solution for IS0 880U and EK 98 in the lower temperature range are almost similar to those for IG 1lOU and POCO AXF-SQ. The negative enthalpy of solution suggests that isotropic graphite exothermically dissolves hydrogen gas. The hydrogen solubility in PGCCL obtained at 1000°C was 1.4 x lo-’ H/C at a hydrogen pressure of 1.33 x lo4 Pa, which was much lower than that in isotropic graphites. The hydrogen solubility in IS0 88OU and EK 98 graphites of this study and the deuterium solubility in IS0 88OU at 850-1ooo”C obtained by Atsumi et al. [6] were different in the temperature dependence. The enthalpy of solution for IS0 880U and EK 98 in the lower temperature range of this study is fairly similar to - 19 H/gram-atom for IS0 88OU [6], but the enthalpy of solution of these graphites in the higher temperature range is negatively larger than that value.
4. Discussion It has been reported by Hoi&is [7] that sorption of deuterium in the nuclear grade graphitic Matrix A3-3 at 900°C at pressures below 1.3 X 10’ Pa was dissociative adsorption, and that the saturation concentration was 140 appm. However, in this study the sorption data for hydrogen in isotropic graphites in the temperature range of 700 to 1000°C at pressures up to 2 X lo4 Pa closely obeyed Sieverts’ law, and the saturation was not observed. The results for solubility measurement suggest that the hydrogen solubility depends on the type of graphite. The hydrogen solubility in PGCCL was an order of magnitude lower than that in isotropic graphites. PGCCL is a highly oriented pyrolytic graphite and a well graphitized material. Artificial isotropic graphites were found from Raman spectroscopic analysis [12] to contain a large amount of ungraphitized carbon atoms, the amount of which was strongly influenced by the type of graphite. Hydrogen solubility appears to be associated with the ungraphitized carbon existing in an artificial graphite. The area containing ungraphitized carbon in a graphite sample may serve as interactive sites for hydrogen. Hydrogen solubility is influenced by the amount of the ungraphitized carbon and the enthalpy of solution is determined by the type of interactive sites. Possible interactions between graphite and
analysis of hydrogen soiubility in graphite
hydrogen are dissolution into an interstitial site of a graphite matrix, formation of chemical bonds with ungraphitized carbon atoms and adsorption on a graphite surface. It is unknown which interaction is dominant. The dilute dissociative adsorption and the dilute solution can be represented by the same expression with a difference of the implication of an enthalpy term and an entropy term. Taking into consideration the application to both the dilute dissociative adsorption and the dilute solution, a dilute solution model [13,14] was modified. The hydrogen solubility in graphite was analyzed by the modified dilute solution model. The chemical potential of hydrogen in the solid solution &, is written by &=HH-T(SH+S”),
(3)
where pn is the partial molar enthalpy of hydrogen. The 3, is the partial molar excess entropy of hydrogen and consists of a thermal entropy term, an electronic entropy term and a configurational entropy term the last of which arises from deviation from the reference configurational entropy term shown below. The $, in eq. (3) is the reference partial molar configurational entropy of hydrogen. On the assumption that IZ~ hydrogen atoms have access to one interactive site per carbon atom in graphite, the sn is defined as follows: *“=
-k
ln(n,/(Nc-n,)),
(4)
where k is Boltzmann’s constant and Nc is the total number of carbon atoms in graphite. Setting n,/N, corresponding to the hydrogen solubility in graphite experimentally measured at a constant hydrogen pressure to be C,, the following expression is obtained for a dilute solution (C, +X 11, ??fl= -k
In C,.
(5)
The chemical potential of atomic hydrogen in the gas phase is represented as [14] ~6 = kT In Pg
+ kT
ln( AH2T-7/4)
+ Eiz,
(6)
is one half of the dissociation energy of where -E& the hydrogen molecule at 0 K. The An2 refers to partition functions of hydrogen gas [13], the value of which can be evaluated from spectroscopic data for hydrogen gas [l&16]. At equilibrium, the chemical potentials are equal, I&=&.
(7)
I’. Shirasu et al. / Thermodynamic analysis of hydrogen solubility in graphite
0 IG 1lCYJ 0.0 0 I
I
s
7
88ou
I
IO
9
lO’/T(
IS0
I,
K-l)
Fig. 3. Thermodynamic analysis of hydrogen solubility in graphites.
Consequently, the following solubility obtained for a dilute graphite-hydrogen ln(C,T7/4/P~~~rr,)
= - ( pH - &)/kT
equation was solid solution, + S,/k. (8)
Eq. (8) was applied to the solubility data for hydrogen in isotropic graphites. Plots of In (CuT7j4/ PAcAHz) versus l/T are shown in fig. 3, indicating good straight lines. The values of the slope and intercept give --(flu -E&)/k and the S,/k, respectively. In table 2, the partial molar enthalpy Brr and the partial molar excess entropy 3, for hydrogen in isotropic graphites are given. The partial molar enthalpy for IS0 880U and EK 98 graphites in the lower temperature range is almost similar to that for IG 1lOU and POCO AXF-SQ graphites at 700-lOOO”C, whereas the partial molar
Table 2 The partial molar enthalpy RH and the partial molar excess entropy s, for graphites Graphite Temperature HH %I &J/mol) (J/mol K) range (“C) IG 1lOU POCO AXF-SQ IS0 88OU EK 98
-
218.1 221.4 222.3 246.3 214.6 246.6
11.6 2.5 0.6 - 20.0 9.1 - 20.4
700-loo0 7oc-loo0 700- 900 900-1000 700- 800 800-loo0
221
enthalpy in the higher temperature range is negatively larger than that in the lower temperature range. The volume contribution to the partial molar enthalpy of hydrogen for a dilute solution is negligibly small, and accordingly the partial molar enthalpy of hydrogen for graphite is approximately equal to the partial molar energy of hydrogen at constant volume and can be discussed on the basis of the energy level of the interactive site for graphite. The energy level of the interactive site for IS0 880U and EK 98 graphites in the lower temperature range is almost similar to that for IG 1lOU and POCO AXF-SQ graphites, while the energy level of the interactive site for IS0 880U and EK 98 graphites is negatively larger in the higher temperature range than in the lower temperature range. This implies the presence of two types of interactive sites for hydrogen in IS0 880U and EK 98 graphites. A dissociative adsorption energy of hydrogen on a graphite surface at 0 K reported by Hoinkis is 2.5 eV/D, [7], which corresponds to the partial molar energy of hydrogen at constant volume for a dissociative adsorption -336.7 kJ/mol, the value of which is negatively much larger than the partial molar enthalpy of hydrogen for IG llOU, POCO AXF-SQ, IS0 880U and EK 98 graphites. The absolute value of the partial molar enthalpy of hydrogen for these four types of graphite is quite small in comparison with the bond energies of C-H at 0 K which are 335 kJ/mol for C-H, 421 kJ/mol for HC-H, 453 kJ/mol for H&-H and 432 kJ/mol for H&-H [16]. This suggests that interactions of hydrogen with graphite may not be attributed to formation of chemical bonds. The partial molar excess entropy of hydrogen in the higher temperature range is almost the same for both IS0 880U and EK 98 graphites. However, the different partial molar excess entropies of hydrogen were obtained for IG llOU, POCO AXF-SQ, IS0 880U and EK 98 graphites in the lower temperature range. Assuming that there exist two types of interactive sites for IS0 880U and EK 98 graphites and that an interactive site in the lower temperature range is similar to that for IG. 1lOU and POCO AXF-5Q graphites, the partial molar excess entropy in the lower temperature range appears to change with the type of graphite in contrast with that in the higher temperature range. If nonconfigurational contributions to the partial molar excess entropy in the same type of interactive site do not change with the type of graphite, a configurational contribution to the partial molar excess entropy in the lower temperature range may be strongly influenced by the type of graphite in comparison with that in the higher temperature range.
222
I’. Shirasu et al. / 7’hermodynamic analysis of hydrogen solubility in graphite
5. Conclusions
The hydrogen solubility in isotropic graphites IS0 880U and EK 98 has been measured at temperatures between 700 and lOOO”C,and solubility data obtained closely obeyed Sieverts’ law. These data were compared with those in isotropic graphites IG 1lOU and POCO AXF-SQ reported in our previous study. The hydrogen solubility in IG 1lOU graphite was larger than that in the other isotropic graphites. Two different enthalpies of solution for IS0 880U and EK 98 graphites were observed at 700-1000°C. The enthalpy of solution for IS0 880U and EK 98 graphites in the lower temperature range was almost similar to that for IG 1lOU and POCO AXF-SQ graphites at 700-lOOO”C, whereas the enthalpy of solution for IS0 880U and EK 98 graphites was negatively larger in the higher temperature range than in the lower temperature range. The hydrogen solubility in a highly oriented pyrolytic graphite PGCCL has been measured at 1000°C. The hydrogen solubility in PGCCL graphite was an order of magnitude lower than that in isotropic graphites. By comparison of hydrogen solubility in PGCCL with those in isotropic graphites, it can be concluded that the amount of ungraphitized carbon depending on the type of artificial graphite appears to influence the hydrogen solubility in graphite. A dilute solution model was applied to the solubility data, and the partial thermodynamic functions of hydrogen in the isotropic graphites were estimated. The differences in the hydrogen solubility for the isotropic graphites appear to result from the partial molar excess entropy including the effect of ungraphitized carbon in graphite. Two different partial molar enthalpies for IS0 880U and EK 98 graphites were obtained in
each temperature range. There was no marked difference in the partial molar enthalpy between the two graphites. This suggests that there exist two types of interactive sites for IS0 880U and EK 98 graphites.
References t11 B.L. Doyle, W.R. Wampler and D.K. Brice, J. Nucl. Mater. 103 & 104 (1981) 513.
t21R.A. Causey and K.L. Wilson, J. Nucl. Mater. 138 (1986) 57. [31 R.A. Causey, M.I. Baskes and K.L. Wilson, J. Vat. Sci. Technol. A4 (1986) 1189. [41 W. Moller, J. Nucl. Mater. 162-164 (1989) 138. [51 R.A. Causey, T.S. Elleman and K. Verghese, Carbon 17 (1979) 323. 61 H. Atsumi, S. Tokura and M. Miyake, J. Nucl. Mater. 155-157 (1988) 241. [71 E. Hoinkis, J. Nucl. Mater. 182 (1991) 93. Dl E. Hoinkis, J. Nucl. Mater. 183 (1991) 9. [91 Y. Shirasu, S. Yamanaka and M. Miyake, Technol. Rep. Osaka Univ. 40 (1990) 35. [lOI Y. Shirasu, S. Yamanaka and M. Miyake, J. Nucl. Mater. 179-181 (1991) 223. [HI Y. Shirasu, S. Yamanaka and M. Miyake, J. Alloys and Compounds 190 (1992) 87. [121 T. Tanabe, Kakuyuugou Kenkyuu 61 (1989) 367. 1131 R.B. McLellan, Phase Stability in Metals and Alloys (McGraw-Hill, New York, 1967). 1141S. Yamanaka, T. Tanaka and M. Miyake, J. Nucl. Mater. 167 (1989) 231. [151 G. Herzberg, Molecular Spectra and Molecular Structure, vol. 1 (Van Nostrand, New York, 1950). [16] M.W. Chase, Jr., C.A. Davies, J.R. Downey, Jr., D.J. Frurip, R.A. McDonald and A.N. Syverud, JANAF Thermochemical Tables, 3rd. ed. (American Institute of Physics, New York, 1985).
Journal of Nuclear Materials 200 (1993) 218-222 North-Holland
Thermodynamic
nuclear
materials
analysis of hydrogen solubility in graphite
Yoshirou Shirasu, Shinsuke Yamanaka and Masanobu Miyake Department of Nuclear Engineering, Faculty of Engineering, Osaka University, Yamadaoka 2-1, Suita, Osaka 565, Japan
Received 3 July 1992; accepted 2 December 1992
The hydrogen solubility in isotropic graphites IS0 880U and EK 98 has been measured in the temperature range of 700-1000°C at pressures below 2 x lo4 Pa. The solubility data obtained closely obeyed Sieverts’ law. The hydrogen solubility and the enthalpy of solution for IS0 88OU and EK 98 graphites were compared with those for isotropic graphites IG 1lOU and POCO AXF-SQ. The hydrogen solubility in a highly oriented pyrolytic graphite PGCCL has also been measured at 1000°C. It was an order of magnitude lower than that in isotropic graphites. Partial thermodynamic functions of hydrogen in isotropic graphites were obtained by a dilute solution mode1 and discussed.
and validity of the thermodynamic ated.
1. Introduction Graphite has recently emerged as one of the promising materials for a limiter and first wall in a fusion reactor because of its low atomic number, and excellent thermal and mechanical properties. Graphite will be exposed as the plasma-facing material to tritium gas as well as to energetic tritium ions and atoms. Tritium recycling and retention are crucial factors for a future use of graphite in a fusion reactor. Several authors [l-4] have studied on the retention of hydrogen isotopes implanted as neutral atoms and ions into various graphite samples. In these studies, the retention of hydrogen isotope has been closely related to the type of graphite and to implantation conditions such as temperature and energy. For a clear understanding of the interaction of hydrogen isotope gas and ion with graphite, it is required to obtain equilibrium data for sorption capacity for hydrogen isotopes in graphite. However, only a few studies have been performed on the hydrogen solubility measurement and its thermodynamic analysis [5-81. In our previous study [9,10], solubility measurement has been performed for hydrogen and deuterium in isotropic graphites (IG 11OU and POCO AXF-SQ), and a thermodynamic model for hydrogen solubility in graphite was proposed. In the present study, the hydrogen solubility in isotropic graphites (IS0 880U and EK 98) and a highly oriented pyrolytic graphite (PGCCL) was measured, 0022-3115/93/$06.00
model was evalu-
2. Experimental Graphite samples employed in the present study were two types of isotropic graphite: IS0 880U (Toy0 Tanso Co.) and EK 98 (Ringsdorff-Werke GmbH), and highly oriented pyrolytic graphite (PGCCL, Le Carbone-Lorraine, 10 X 10 mm2 in area of basal plane and 2 mm in thickness). The hydrogen solubility in the isotropic graphite samples was measured by a constant volume method. Prior to solubility measurement, isotropic graphite samples were mechanically polished and degassed at 1400°C in a vacuum below 1O-3 Pa. The measurement was performed in the temperature range of 700 to 1000°C at pressures ranging from 2 X lo3 to 2 X lo4 Pa. Details of the experimental apparatus and procedure were described eleswhere [9]. The hydrogen solubility in the PGCCL sample was measured by thermal desorption spectroscopy (TDS), because the hydrogen solubility in the sample was much lower than the measurable amount due to the apparatus based on the constant volume method. The graphite sample was degassed at 1000°C for 5 h in a vacuum below 10e6 Pa before the solubility measurement, and then it was exposed to hydrogen gas at 1000°C under 1.33 X lo4 Pa for 40 h. After the expo-
0 1993 - Elsevier Science Publishers B.V. All rights reserved
219
Y. Shirasu et al. / Thermodynamic analysis of hydrogen solubility in graphite Temperalure
( ‘C )
-115-
0
IG 1lOU
APoco AXF-50 0 IS0 0
1
2
3
4
5
-14.51
6
7
c~+xlo‘(HIC)
Fig. 1. Sieverts’ plot for hydrogen in IG 11OU graphite at
700-1ocWc.
sure, the sample was quenched to room temperature. The total amount of hydrogen gas dissolved in the sample was analyzed by TDS at a heating rate of 20”C/min up to lOOO”C, and hydrogen solubility was determined from a total amount of released hydrogen. In this apparatus the amount of 5 X lOWE mol H, can be measured reproducibly. From the reproducibility of the solubility data the hydrogen content in the sample is estimated to be at equilibrium. Details of the procedure and apparatus will be published eleswhere [ll].
3. Results Solubility data obtained for hydrogen in IS0 880U and EK 98 graphites closely followed Sieverts’ law; the hydrogen solubility was proportional to the square root of the equilibrium pressure: C, = K,Pe,
(1)
where C, is the hydrogen solubility in atom ratio (H/C), Pt-r, is the equilibrium hydrogen pressure in Pa and K, is the Sieverts’ constant. Fig. 1 indicates a representative of the linear relation between the square root of the equilibrium pressure and hydrogen content. Since Sieverts’ law applied to the isotropic graphite samples, hydrogen gas is dissolved into the graphite as atomic hydrogen. Fig. 2 shows the temperature dependence of Sieverts’ constant K, for hydrogen in IS0 880U and EK
I
,
8
9
_
88OU I
10
11
lO’/T(K-‘)
Fig. 2. Temperature dependence of Sieverts’ constant K H’
98 graphites together with K, for IG 1lOU and POCO AKF-SQ graphites reported in our previous study [lo]. It is apparent from this figure that the temperature dependence of Sieverts’ constant can be expressed by In K, = A + B/T,
(2)
where A and B are constants and T is the sample temperature in K. In table 1, the values of A and B are given together with the values of enthalpy of solution AH &J/g-atom) for hydrogen in the isotropic graphites derived from the value of B. It is found from fig. 2 that the hydrogen solubility at a constant pressure is larger for IG 1lOU graphite than those for POCO AKF-SQ, IS0 880U and EK 98 graphites. It is obvious from table 1 that the enthalpies of solution for IG 1lOU and POCO AKF-SQ graphites are nearly
Table 1 Temperature dependence of Sieverts’ constant for hydrogen in graphites Graphite
A
B
IG 1lOU POCO AXF-5Q IS0 88OU
-14.5 - 15.6 -15.8 - 18.5 -14.7 - 18.5
2190 2590 2640 5800 1620 5730
AH w/g-
Temperature range (“0
atom)
EK 98
-18.2 - 21.5 - 22.0 -48.2 -13.5 -47.6
700-1000 700-1000 700- 900 900-1000 700- 800 800-1000
220
Y. Shirasu et al. / Thermodynamic
constant at 700-lOWC, whereas the enthalpies of solution for IS0 88OU and EK 98 graphites in a higher temperature range are negatively larger than those in a lower temperature ranged. The enthalpies of solution for IS0 880U and EK 98 in the lower temperature range are almost similar to those for IG 1lOU and POCO AXF-SQ. The negative enthalpy of solution suggests that isotropic graphite exothermically dissolves hydrogen gas. The hydrogen solubility in PGCCL obtained at 1000°C was 1.4 x lo-’ H/C at a hydrogen pressure of 1.33 x lo4 Pa, which was much lower than that in isotropic graphites. The hydrogen solubility in IS0 88OU and EK 98 graphites of this study and the deuterium solubility in IS0 88OU at 850-1ooo”C obtained by Atsumi et al. [6] were different in the temperature dependence. The enthalpy of solution for IS0 880U and EK 98 in the lower temperature range of this study is fairly similar to - 19 H/gram-atom for IS0 88OU [6], but the enthalpy of solution of these graphites in the higher temperature range is negatively larger than that value.
4. Discussion It has been reported by Hoi&is [7] that sorption of deuterium in the nuclear grade graphitic Matrix A3-3 at 900°C at pressures below 1.3 X 10’ Pa was dissociative adsorption, and that the saturation concentration was 140 appm. However, in this study the sorption data for hydrogen in isotropic graphites in the temperature range of 700 to 1000°C at pressures up to 2 X lo4 Pa closely obeyed Sieverts’ law, and the saturation was not observed. The results for solubility measurement suggest that the hydrogen solubility depends on the type of graphite. The hydrogen solubility in PGCCL was an order of magnitude lower than that in isotropic graphites. PGCCL is a highly oriented pyrolytic graphite and a well graphitized material. Artificial isotropic graphites were found from Raman spectroscopic analysis [12] to contain a large amount of ungraphitized carbon atoms, the amount of which was strongly influenced by the type of graphite. Hydrogen solubility appears to be associated with the ungraphitized carbon existing in an artificial graphite. The area containing ungraphitized carbon in a graphite sample may serve as interactive sites for hydrogen. Hydrogen solubility is influenced by the amount of the ungraphitized carbon and the enthalpy of solution is determined by the type of interactive sites. Possible interactions between graphite and
analysis of hydrogen soiubility in graphite
hydrogen are dissolution into an interstitial site of a graphite matrix, formation of chemical bonds with ungraphitized carbon atoms and adsorption on a graphite surface. It is unknown which interaction is dominant. The dilute dissociative adsorption and the dilute solution can be represented by the same expression with a difference of the implication of an enthalpy term and an entropy term. Taking into consideration the application to both the dilute dissociative adsorption and the dilute solution, a dilute solution model [13,14] was modified. The hydrogen solubility in graphite was analyzed by the modified dilute solution model. The chemical potential of hydrogen in the solid solution &, is written by &=HH-T(SH+S”),
(3)
where pn is the partial molar enthalpy of hydrogen. The 3, is the partial molar excess entropy of hydrogen and consists of a thermal entropy term, an electronic entropy term and a configurational entropy term the last of which arises from deviation from the reference configurational entropy term shown below. The $, in eq. (3) is the reference partial molar configurational entropy of hydrogen. On the assumption that IZ~ hydrogen atoms have access to one interactive site per carbon atom in graphite, the sn is defined as follows: *“=
-k
ln(n,/(Nc-n,)),
(4)
where k is Boltzmann’s constant and Nc is the total number of carbon atoms in graphite. Setting n,/N, corresponding to the hydrogen solubility in graphite experimentally measured at a constant hydrogen pressure to be C,, the following expression is obtained for a dilute solution (C, +X 11, ??fl= -k
In C,.
(5)
The chemical potential of atomic hydrogen in the gas phase is represented as [14] ~6 = kT In Pg
+ kT
ln( AH2T-7/4)
+ Eiz,
(6)
is one half of the dissociation energy of where -E& the hydrogen molecule at 0 K. The An2 refers to partition functions of hydrogen gas [13], the value of which can be evaluated from spectroscopic data for hydrogen gas [l&16]. At equilibrium, the chemical potentials are equal, I&=&.
(7)
I’. Shirasu et al. / Thermodynamic analysis of hydrogen solubility in graphite
0 IG 1lCYJ 0.0 0 I
I
s
7
88ou
I
IO
9
lO’/T(
IS0
I,
K-l)
Fig. 3. Thermodynamic analysis of hydrogen solubility in graphites.
Consequently, the following solubility obtained for a dilute graphite-hydrogen ln(C,T7/4/P~~~rr,)
= - ( pH - &)/kT
equation was solid solution, + S,/k. (8)
Eq. (8) was applied to the solubility data for hydrogen in isotropic graphites. Plots of In (CuT7j4/ PAcAHz) versus l/T are shown in fig. 3, indicating good straight lines. The values of the slope and intercept give --(flu -E&)/k and the S,/k, respectively. In table 2, the partial molar enthalpy Brr and the partial molar excess entropy 3, for hydrogen in isotropic graphites are given. The partial molar enthalpy for IS0 880U and EK 98 graphites in the lower temperature range is almost similar to that for IG 1lOU and POCO AXF-SQ graphites at 700-lOOO”C, whereas the partial molar
Table 2 The partial molar enthalpy RH and the partial molar excess entropy s, for graphites Graphite Temperature HH %I &J/mol) (J/mol K) range (“C) IG 1lOU POCO AXF-SQ IS0 88OU EK 98
-
218.1 221.4 222.3 246.3 214.6 246.6
11.6 2.5 0.6 - 20.0 9.1 - 20.4
700-loo0 7oc-loo0 700- 900 900-1000 700- 800 800-loo0
221
enthalpy in the higher temperature range is negatively larger than that in the lower temperature range. The volume contribution to the partial molar enthalpy of hydrogen for a dilute solution is negligibly small, and accordingly the partial molar enthalpy of hydrogen for graphite is approximately equal to the partial molar energy of hydrogen at constant volume and can be discussed on the basis of the energy level of the interactive site for graphite. The energy level of the interactive site for IS0 880U and EK 98 graphites in the lower temperature range is almost similar to that for IG 1lOU and POCO AXF-SQ graphites, while the energy level of the interactive site for IS0 880U and EK 98 graphites is negatively larger in the higher temperature range than in the lower temperature range. This implies the presence of two types of interactive sites for hydrogen in IS0 880U and EK 98 graphites. A dissociative adsorption energy of hydrogen on a graphite surface at 0 K reported by Hoinkis is 2.5 eV/D, [7], which corresponds to the partial molar energy of hydrogen at constant volume for a dissociative adsorption -336.7 kJ/mol, the value of which is negatively much larger than the partial molar enthalpy of hydrogen for IG llOU, POCO AXF-SQ, IS0 880U and EK 98 graphites. The absolute value of the partial molar enthalpy of hydrogen for these four types of graphite is quite small in comparison with the bond energies of C-H at 0 K which are 335 kJ/mol for C-H, 421 kJ/mol for HC-H, 453 kJ/mol for H&-H and 432 kJ/mol for H&-H [16]. This suggests that interactions of hydrogen with graphite may not be attributed to formation of chemical bonds. The partial molar excess entropy of hydrogen in the higher temperature range is almost the same for both IS0 880U and EK 98 graphites. However, the different partial molar excess entropies of hydrogen were obtained for IG llOU, POCO AXF-SQ, IS0 880U and EK 98 graphites in the lower temperature range. Assuming that there exist two types of interactive sites for IS0 880U and EK 98 graphites and that an interactive site in the lower temperature range is similar to that for IG. 1lOU and POCO AXF-5Q graphites, the partial molar excess entropy in the lower temperature range appears to change with the type of graphite in contrast with that in the higher temperature range. If nonconfigurational contributions to the partial molar excess entropy in the same type of interactive site do not change with the type of graphite, a configurational contribution to the partial molar excess entropy in the lower temperature range may be strongly influenced by the type of graphite in comparison with that in the higher temperature range.
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I’. Shirasu et al. / 7’hermodynamic analysis of hydrogen solubility in graphite
5. Conclusions
The hydrogen solubility in isotropic graphites IS0 880U and EK 98 has been measured at temperatures between 700 and lOOO”C,and solubility data obtained closely obeyed Sieverts’ law. These data were compared with those in isotropic graphites IG 1lOU and POCO AXF-SQ reported in our previous study. The hydrogen solubility in IG 1lOU graphite was larger than that in the other isotropic graphites. Two different enthalpies of solution for IS0 880U and EK 98 graphites were observed at 700-1000°C. The enthalpy of solution for IS0 880U and EK 98 graphites in the lower temperature range was almost similar to that for IG 1lOU and POCO AXF-SQ graphites at 700-lOOO”C, whereas the enthalpy of solution for IS0 880U and EK 98 graphites was negatively larger in the higher temperature range than in the lower temperature range. The hydrogen solubility in a highly oriented pyrolytic graphite PGCCL has been measured at 1000°C. The hydrogen solubility in PGCCL graphite was an order of magnitude lower than that in isotropic graphites. By comparison of hydrogen solubility in PGCCL with those in isotropic graphites, it can be concluded that the amount of ungraphitized carbon depending on the type of artificial graphite appears to influence the hydrogen solubility in graphite. A dilute solution model was applied to the solubility data, and the partial thermodynamic functions of hydrogen in the isotropic graphites were estimated. The differences in the hydrogen solubility for the isotropic graphites appear to result from the partial molar excess entropy including the effect of ungraphitized carbon in graphite. Two different partial molar enthalpies for IS0 880U and EK 98 graphites were obtained in
each temperature range. There was no marked difference in the partial molar enthalpy between the two graphites. This suggests that there exist two types of interactive sites for IS0 880U and EK 98 graphites.
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