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Tritium absorption–desorption characteristics of LaNi4.25Al0.75
Journal of Alloys and Compounds 441 (2007) 359–363
Tritium absorption–desorption characteristics of LaNi4.25Al0.75 Wei-du Wang ∗ , Xing-gui Long, Gui-jun Cheng, Shu-ming Peng, Ben-fu Yang Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China Received 14 July 2006; received in revised form 20 September 2006; accepted 28 September 2006 Available online 31 October 2006
Abstract As a part of the program to develop metal hydride technology for storing and processing tritium, the tritium absorption–desorption of LaNi4.25 Al0.75 was investigated. The pressure–composition–temperature (P–C–T) curves, activation characteristics and the absorption–desorption rate of tritium from LaNi4.25 Al0.75 Tx at different temperature were measured. And standard enthalpy Hθ and entropy Sθ changes for T2 absorption and desorption were calculated from van’t Hoff plots based on the measured P–C–T relations. The calculated value of Hθ and Sθ during the formation of the LaNi4.25 Al0.75 T2.5 phase is (−31.8 ± 0.8) kJ mol−1 and (−76.5 ± 2.4) J mol−1 K−1 , and the value of desorption is (−36.5 ± 0.5) kJ mol−1 and (−84.2 ± 1.5) J mol−1 K−1 , respectively. Composition difference between the origin tritium gas and tritium released from 7-day-aged LaNi4.25 Al0.75 tritide was analyzed by quadrupole mass spectrometer. © 2006 Elsevier B.V. All rights reserved. Keywords: LaNi4.25 Al0.75 ; Metal tritide; P–C–T curves; Thermodynamic parameters
1. Introduction Metal hydride is widely used for tritium storage and processing in nuclear energy industry. Typical metals used are uranium, titanium, zirconium and the AB5 type alloy, i.e. LaNi5 . For tritium processing applications, the metal should meet specified requirements, such as low equilibrium pressure, high absorption capacity and delivery of high purity 3 He-free tritium without the use of pumps [1]. In addition, the properties of poison resistance and lifetime are always considered. Many attempts have been made to improve these properties. Intermetallic compounds of LaNi5−x Alx (x = 0.1–1) series have been developed rapidly in recent years. Substitution of Al for Ni in LaNi5 alloys leads to remarkable improvements in hydrogenation and dehydrogenation characteristics. LaNi4.25 Al0.75 tritide is a favoured material with several above advantages, and as the tritium storage medium has been used more than 20 years. Aging effects in 5a and 13a-aged LaNi4.25 Al0.75 tritide were under investigation by Nobile et al. [1,2] and Shanahan et al. [3], respectively. They reported that the material captures nearly all of the 3 He formed from natural radioactive decay of the solid tritide in 5 years, and some of 13-year-old sample, the
isotherms show a complete loss of the typical plateau structure, show a decreasing absorption capacity, and one 13-year-old sample has shown 3 He release behavior. Although many studies have been published concerning tritium aging effects in LaNi4.25 Al0.75 , little work has been published on the basic tritium absorption–desorption characteristics. In the research described here, activation characteristics, pressure–composition–temperature (P–C–T) curves, absorption–desorption rate and purity of released tritium were measured, also some thermodynamic parameters were calculated. 2. Experimental 2.1. Materials LaNi4.25 Al0.75 alloy is supplied by Institute of Metal Research, CAS, was prepared by vacuum melting of the lanthanum and nickel components with subsequent addition of aluminum to the melt, and the composition was determined by method of chemical analysis. LaNi4.25 Al0.75 granules of <2 mm in diameter were chosen as sample, which was taken from the interior of the alloy ingot. Tritium and deuterium gases are stored on uranium beds and delivered as needed. The purities of T2 and D2 are 98% and 99%, respectively.
2.2. LaNi4.25 Al0.75 activation ∗
Corresponding author. Tel.: +86 816 2487043; fax: +86 816 2494852. E-mail address: [email protected] (W.-d. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.122
About 1.0087 g LaNi4.25 Al0.75 has been put into a stainless steel reactor, and connected to an all metal UHV experimental apparatus developed by ourselves,
360
W.-d. Wang et al. / Journal of Alloys and Compounds 441 (2007) 359–363
and the activation characteristics was measured at 21–100 ◦ C with deuterium. The sample was evacuated at 10−3 to 10−4 Pa and at 250 ◦ C beforehand. The pressure was monitored with 1000 Torr high-accuracy sensor. The temperatures were monitored with CHINO type K thermocouples. These data were collected by computer logging with special software installed on a 1.8 GHz PC.
2.3. P–C–T measurement The apparatus employed for P–C–T measurement are similar to sample activation. A sample of 0.1034 g was evacuated to 250 ◦ C, and activated with deuterium. After three activation cycles, the sample was evacuated to 10−3 Pa and at 250 ◦ C, and the P–C isotherms were experimentally made by employing a volumetric analysis method. The reactor is heated at different temperatures and dehydrogenation characteristics have been monitored.
2.4. Absorption–desorption rate After the P–C–T measurement, the sample was evacuated and heated at given temperature, and at the initial tritium pressure of 50.0 kPa, the absorbing rates of tritium from LaNi4.25 Al0.75 at different temperatures were measured. And then, the sample was re-evacuated and saturated with tritium and the atomic ratio is controlled to 5, and the experimental apparatus was pumped out at 10−3 Pa, and the sample was heated at given temperature, and the pressure–time isotherms of LaNi4.25 Al0.75 T5 desorption were then plotted.
2.5. Compare of absorption–desorption tritium Composition difference between the origin tritium gas and the tritium released from 7-day-aged LaNi4.25 Al0.75 tritide were analyzed by quadrupole mass spectrometer. The temperature of dehydrogenation was controlled at 250 ◦ C.
3. Results and discussion 3.1. LaNi4.25 Al0.75 activation Table 1 shows experimental data of LaNi4.25 Al0.75 activated by deuterium. The primary observation is measured at 51.3 kPa deuterium pressures at room temperature (21 ◦ C). After 1 h, the partial equilibrium is achieved, and the atomic ratio [D]/[La] = 0.05. The reactor is then heated at 100 ◦ C, the equilibrium time is 1.5 h and [D]/[La] = 1.07. After the temperature returned to room temperature and the equilibrium is achieved, [D]/[La] = 3.92. And then, the sample was reheated to 250 ◦ C and hold up for 10 min, a vacuum of 4.1 ×10−4 Pa was obtained, these results are understandable because a majority of deuterium was released from the sample. The sample temperatures of second thermal cycles were controlled at 75, 50 and 21 ◦ C, the primary pressure of deuterium gas was 58.2 kPa, the achiev-
Fig. 1. Absorption and desorption P–C–T curves for tritium in LaNi4.25 Al0.75 Tx . Ab. temp. (◦ C): (-) 20, () 50, (䊉) 80, () 110; De. temp. (◦ C): () 24, () 50, () 80, (♦) 110.
ing partial equilibrium took about 2 h, and [D]/[La] increased gradually and up to 4.26. Here, all of the alloy granules became exiguous pulverizing. It is suggested that most of LaNi4.25 Al0.75 was accomplished activation. It is obvious that LaNi4.25 Al0.75 is easily activated. After the activated LaNi4.25 Al0.75 deuteride has been exposed to atmosphere for 0.5 h, the material still shown hydrogen absorption capability, but the rate slowed obviously. When the vacuum re-evacuated, the material almost resumed its absorption capacity and absorption–desorption rate. This indicates that the material has high tolerance to the poison gases. 3.2. P–C–T curves and phase analysis The measured P–C–T curves for tritium in LaNi4.25 Al0.75 are shown in Fig. 1. As shown in the P–C–T curves, the equilibrium pressure increases with the increasing of the equilibrium temperature, and isotherm plateau slope also shows a slight increase. It can be seen that the desorption isotherm is under the absorption, this shows a significant hysteresis. The plateau pressure was around 1 kPa and the absorption capacity ([T]/[La]) is more than 5.0 at room temperature. In the temperature range 20–110 ◦ C, clear plateau pressures could be reproducibly obtained between approximate [T]/[La] = 1.0 and 3.5,
Table 1 Experimental data of LaNi4.25 Al0.75 activated by deuterium Cycles and times
Starting pressure (kPa)
Ending pressure (kPa)
Activation temperature (◦ C)
Atomic ratio [D]/[La]
Equilibrium times (h)
Cycle 1
1 2 3
51.3 51.1 49.8
51.1 49.8 46.0
21 100 21
0.05 1.07 3.92
1.0 1.5 2.0
Cycle 2
1 2 3
58.2 53.6 53.1
53.6 53.1 52.5
75 50 21
3.41 3.78 4.26
2.0 2.0 2.0
W.-d. Wang et al. / Journal of Alloys and Compounds 441 (2007) 359–363
361
Fig. 2. ln(PT2 /P0 ) − 1/T plots of tritium absorption (a) and desorption (b) in LaNi4.25 Al0.75 Tx . () 1.5, () 2.0, () 2.5, (夽) 3.0.
these facts indicates that the coexistence of the ␣-solid solution and the -tritide LaNi4.25 Al0.75 . In the temperature of 110 ◦ C, the plateau shows a notable narrowing down. As shows in Fig. 1, the ␣-solid solution is the main phase in [T]/[La] < 1.0, while the curves approach saturation after [T]/[La] = 3.5, the fraction of the -tritide LaNi4.25 Al0.75 increases rapidly. 3.3. Thermodynamic properties The change of standard free energy Gθ for tritiding reaction in tritium storage alloys can be expressed as: Gθ = H θ − TS θ
(1)
where T is the absolute temperature. The change of standard free energy Gθ is related to the dissociation pressure (PT2 /P0 ) for two-phase metal hydrides, according to the relation: PT2 (2) Gθ = RT ln P0 Assuming that the changes of standard enthalpy and entropy are independent of the temperature. The equilibrium pressures PT2 is read from the P–C–T curves, its unit is Pa, P0 = 101,325 Pa, R is the gas constant, Eq. (1) using these relations gives: PT2 H θ S θ ln = − (3) P0 RT R The plots of ln(PT2 /P0 ) versus 1/T (van’t Hoff plots) in Fig. 2 shows that ln(PT2 /P0 ) are linear with 1/T, the related coefficient of the lines are more than 0.9994. The changes of standard enthalpy Hθ and entropy Sθ were calculated from
corresponding slopes and intercepts according to Eq. (3). The obtained experimentally thermodynamic parameters are shown in Table 2. The entropy denotes the reaction trend of hydrogenation, for an alloy of the same type, the larger absolute value of entropy is implies that the pressure of desorption is lower and the alloy hydride is more stable. The enthalpy is heat of formation of alloy hydride; the larger absolute value denotes the more stable of alloy hydride. The reported [4,5] value of Hθ and Sθ of the LaNi4.25 Al0.75 hydride formation are equal to −23.4 kJ mol−1 and −73.9 J mol−1 K−1 , these values are smaller than that of LaNi4.25 Al0.75 tritide. It may indicate that LaNi4.25 Al0.75 tritide is more stable than LaNi4.25 Al0.75 hydride. The experimental plateau pressure of LaNi4.25 Al0.75 tritide is lower in comparison with its hydride, shows positive-hydrogen-isotopes effect as reported in Ref. [6], and it is another evidence of the difference in the thermodynamic stabilities. According to these thermodynamic parameters and van’t Hoff relations, if we assumed that the sample temperature is 25 ◦ C, it can easily extrapolate that the equilibrium pressures of tritium absorption and desorption of LaNi4.25 Al0.75 tritide are 2.6 and 1.4 kPa, respectively. Results show that calculations are in good agreement with experimental data. Contrarily, if we know the equilibrium pressure, the result of this work can be used to calculate the sample temperature. For example, the corresponding sample temperature of desorption pressure of 200 kPa is 181 ◦ C. 3.4. Absorption–desorption rate Fig. 3 gives the results of the experiment. As shown in Fig. 3, the material possesses the characteristics of high-speed tritium
Table 2 Hθ and Sθ of tritium absorption and desorption in LaNi4.25 Al0.75 Tx [T]/[La]
Ab.Hθ (kJ mol−1 H2 )
Ab.Sθ (J mol−1 K−1 H2 )
De.Hθ (kJ mol−1 H2 )
De.Sθ (J mol−1 K−1 H2 )
1.50 2.00 2.50 3.00
−31.7 −32.0 −31.8 −31.8
−74.9 −76.4 −76.5 −76.9
−35.0 −35.7 −35.6 −35.9
−81.2 −82.8 −84.2 −85.4
± ± ± ±
0.5 0.3 0.8 0.6
± ± ± ±
1.5 1.0 2.4 1.7
± ± ± ±
0.5 0.5 0.5 0.4
± ± ± ±
1.5 1.5 1.5 1.2
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W.-d. Wang et al. / Journal of Alloys and Compounds 441 (2007) 359–363
Fig. 3. Tritium absorption (a) and desorption (b) speed of LaNi4.25 Al0.75 Tx at different temperature (◦ C): 1–110, 2–80, 3–50, 4–20, 5–24.
absorption–desorption rate, and the range of half-time of saturation is 8–30 s. Unfortunately, because of tritium greatly overfeeding, the influence of temperature on tritium absorption is unobtrusively so that the curves have not an obvious regularity. However, the equilibrium speed of tritium desorption is accelerated with a rise of sample temperature and has a concomitance of the decreasing equilibrium time, furthermore, the volume of released tritium increases with the temperature.
HT may be released from stainless steel tube at high temperature. Besides this, content of the other species did not change, and 3 He release behavior is not observed. Overall, the results reveal that the absorbed and the released tritium not have much difference in purity.
3.5. Compositions of absorption–desorption tritium
The changes of standard enthalpy and entropy during the formation of the LaNi4.25 Al0.75 T2.5 phase are (−31.8 ± 0.8) kJ mol−1 and (−76.5 ± 2.4) J mol−1 K−1 , and the value of desorption are (−36.5 ± 0.5) kJ mol−1 and (−84.2 ± 1.5) J mol−1 K−1 , respectively. In the temperature range 20–110 ◦ C, both the equilibrium pressure of absorption and desorption increase with a rise of the equilibrium temperature, and the isotherm of desorption is under that of absorption and presents a significant hysteresis. The plateau pressure was around 1 kPa and the absorption capacity ([T]/[La]) is more than 5.0 at room temperature. The influence of temperature on tritium absorption is unobtrusively in the temperature range 20–80 ◦ C, but the equilibrium speed of tritium desorption increases with a rise of sample temperature and has a concomitance of the equilibrium time decreasing. LaNi4.25 Al0.75 is easily activated. In addition, the absorption–desorption processing is rapid and does not need a long equilibrium time. It can be concluded that LaNi4.25 Al0.75 is a good material for hydrogen isotopes storage and pump over.
Composition difference between the origin tritium gas and tritium released from 7-day-aged LaNi4.25 Al0.75 tritide are analyzed by quadrupole mass spectrometer. Fig. 4 provides results of the analysis, the intensity of mass number over eight is around baseline and cut-out. As can be seen from the spectrum, comparing the released gases with the origin gases, the species of mass number of two and four increase, a possible explanation of this is that the H2 and
4. Conclusions
Acknowledgements
Fig. 4. Mass spectra of absorbed (a) and desorbed (b) tritium for LaNi4.25 Al0.75 tritide.
This work is supported by the Foundation of China nature science under contract number 50121050 and 50371079. Institute of Metal Research, CAS, is gratefully acknowledged for their sample. The authors wish to thank Prof. Xiao-hua Cao, Dr. De-min Chen and M.Sc. Xiao-song Zhou for helpful discussion.
W.-d. Wang et al. / Journal of Alloys and Compounds 441 (2007) 359–363
References [1] A. Nobile, R.T. Walters, W.C. Mosley, J. Less-common Met. 172–174 (1991) 1352–1362. [2] A. Nobile, J.R. Wermer, R.T. Walters, Fusion Technol. 21 (1992) 769–774. [3] K.L. Shanahan, J.S. H¨older, D.R. Bell, et al., J. Alloys Compd. 356–357 (2003) 382–385.
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[4] S.M. Peng, P.J. Zhao, S.J. Yao, et al., At. Energy Sci. Technol. 31 (1997) 351–354. [5] S.M. Peng, et al., China Nuclear Science and Technology Report, Atomic Energy Press, Beijing, 1999. [6] R.H. Wiswall, J.J. Reily, Inorg. Chem. 11 (1972) 1691.
Tritium absorption–desorption characteristics of LaNi4.25Al0.75 Wei-du Wang ∗ , Xing-gui Long, Gui-jun Cheng, Shu-ming Peng, Ben-fu Yang Institute of Nuclear Physics and Chemistry, China Academy of Engineering Physics, Mianyang 621900, China Received 14 July 2006; received in revised form 20 September 2006; accepted 28 September 2006 Available online 31 October 2006
Abstract As a part of the program to develop metal hydride technology for storing and processing tritium, the tritium absorption–desorption of LaNi4.25 Al0.75 was investigated. The pressure–composition–temperature (P–C–T) curves, activation characteristics and the absorption–desorption rate of tritium from LaNi4.25 Al0.75 Tx at different temperature were measured. And standard enthalpy Hθ and entropy Sθ changes for T2 absorption and desorption were calculated from van’t Hoff plots based on the measured P–C–T relations. The calculated value of Hθ and Sθ during the formation of the LaNi4.25 Al0.75 T2.5 phase is (−31.8 ± 0.8) kJ mol−1 and (−76.5 ± 2.4) J mol−1 K−1 , and the value of desorption is (−36.5 ± 0.5) kJ mol−1 and (−84.2 ± 1.5) J mol−1 K−1 , respectively. Composition difference between the origin tritium gas and tritium released from 7-day-aged LaNi4.25 Al0.75 tritide was analyzed by quadrupole mass spectrometer. © 2006 Elsevier B.V. All rights reserved. Keywords: LaNi4.25 Al0.75 ; Metal tritide; P–C–T curves; Thermodynamic parameters
1. Introduction Metal hydride is widely used for tritium storage and processing in nuclear energy industry. Typical metals used are uranium, titanium, zirconium and the AB5 type alloy, i.e. LaNi5 . For tritium processing applications, the metal should meet specified requirements, such as low equilibrium pressure, high absorption capacity and delivery of high purity 3 He-free tritium without the use of pumps [1]. In addition, the properties of poison resistance and lifetime are always considered. Many attempts have been made to improve these properties. Intermetallic compounds of LaNi5−x Alx (x = 0.1–1) series have been developed rapidly in recent years. Substitution of Al for Ni in LaNi5 alloys leads to remarkable improvements in hydrogenation and dehydrogenation characteristics. LaNi4.25 Al0.75 tritide is a favoured material with several above advantages, and as the tritium storage medium has been used more than 20 years. Aging effects in 5a and 13a-aged LaNi4.25 Al0.75 tritide were under investigation by Nobile et al. [1,2] and Shanahan et al. [3], respectively. They reported that the material captures nearly all of the 3 He formed from natural radioactive decay of the solid tritide in 5 years, and some of 13-year-old sample, the
isotherms show a complete loss of the typical plateau structure, show a decreasing absorption capacity, and one 13-year-old sample has shown 3 He release behavior. Although many studies have been published concerning tritium aging effects in LaNi4.25 Al0.75 , little work has been published on the basic tritium absorption–desorption characteristics. In the research described here, activation characteristics, pressure–composition–temperature (P–C–T) curves, absorption–desorption rate and purity of released tritium were measured, also some thermodynamic parameters were calculated. 2. Experimental 2.1. Materials LaNi4.25 Al0.75 alloy is supplied by Institute of Metal Research, CAS, was prepared by vacuum melting of the lanthanum and nickel components with subsequent addition of aluminum to the melt, and the composition was determined by method of chemical analysis. LaNi4.25 Al0.75 granules of <2 mm in diameter were chosen as sample, which was taken from the interior of the alloy ingot. Tritium and deuterium gases are stored on uranium beds and delivered as needed. The purities of T2 and D2 are 98% and 99%, respectively.
2.2. LaNi4.25 Al0.75 activation ∗
Corresponding author. Tel.: +86 816 2487043; fax: +86 816 2494852. E-mail address: [email protected] (W.-d. Wang).
0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.09.122
About 1.0087 g LaNi4.25 Al0.75 has been put into a stainless steel reactor, and connected to an all metal UHV experimental apparatus developed by ourselves,
360
W.-d. Wang et al. / Journal of Alloys and Compounds 441 (2007) 359–363
and the activation characteristics was measured at 21–100 ◦ C with deuterium. The sample was evacuated at 10−3 to 10−4 Pa and at 250 ◦ C beforehand. The pressure was monitored with 1000 Torr high-accuracy sensor. The temperatures were monitored with CHINO type K thermocouples. These data were collected by computer logging with special software installed on a 1.8 GHz PC.
2.3. P–C–T measurement The apparatus employed for P–C–T measurement are similar to sample activation. A sample of 0.1034 g was evacuated to 250 ◦ C, and activated with deuterium. After three activation cycles, the sample was evacuated to 10−3 Pa and at 250 ◦ C, and the P–C isotherms were experimentally made by employing a volumetric analysis method. The reactor is heated at different temperatures and dehydrogenation characteristics have been monitored.
2.4. Absorption–desorption rate After the P–C–T measurement, the sample was evacuated and heated at given temperature, and at the initial tritium pressure of 50.0 kPa, the absorbing rates of tritium from LaNi4.25 Al0.75 at different temperatures were measured. And then, the sample was re-evacuated and saturated with tritium and the atomic ratio is controlled to 5, and the experimental apparatus was pumped out at 10−3 Pa, and the sample was heated at given temperature, and the pressure–time isotherms of LaNi4.25 Al0.75 T5 desorption were then plotted.
2.5. Compare of absorption–desorption tritium Composition difference between the origin tritium gas and the tritium released from 7-day-aged LaNi4.25 Al0.75 tritide were analyzed by quadrupole mass spectrometer. The temperature of dehydrogenation was controlled at 250 ◦ C.
3. Results and discussion 3.1. LaNi4.25 Al0.75 activation Table 1 shows experimental data of LaNi4.25 Al0.75 activated by deuterium. The primary observation is measured at 51.3 kPa deuterium pressures at room temperature (21 ◦ C). After 1 h, the partial equilibrium is achieved, and the atomic ratio [D]/[La] = 0.05. The reactor is then heated at 100 ◦ C, the equilibrium time is 1.5 h and [D]/[La] = 1.07. After the temperature returned to room temperature and the equilibrium is achieved, [D]/[La] = 3.92. And then, the sample was reheated to 250 ◦ C and hold up for 10 min, a vacuum of 4.1 ×10−4 Pa was obtained, these results are understandable because a majority of deuterium was released from the sample. The sample temperatures of second thermal cycles were controlled at 75, 50 and 21 ◦ C, the primary pressure of deuterium gas was 58.2 kPa, the achiev-
Fig. 1. Absorption and desorption P–C–T curves for tritium in LaNi4.25 Al0.75 Tx . Ab. temp. (◦ C): (-) 20, () 50, (䊉) 80, () 110; De. temp. (◦ C): () 24, () 50, () 80, (♦) 110.
ing partial equilibrium took about 2 h, and [D]/[La] increased gradually and up to 4.26. Here, all of the alloy granules became exiguous pulverizing. It is suggested that most of LaNi4.25 Al0.75 was accomplished activation. It is obvious that LaNi4.25 Al0.75 is easily activated. After the activated LaNi4.25 Al0.75 deuteride has been exposed to atmosphere for 0.5 h, the material still shown hydrogen absorption capability, but the rate slowed obviously. When the vacuum re-evacuated, the material almost resumed its absorption capacity and absorption–desorption rate. This indicates that the material has high tolerance to the poison gases. 3.2. P–C–T curves and phase analysis The measured P–C–T curves for tritium in LaNi4.25 Al0.75 are shown in Fig. 1. As shown in the P–C–T curves, the equilibrium pressure increases with the increasing of the equilibrium temperature, and isotherm plateau slope also shows a slight increase. It can be seen that the desorption isotherm is under the absorption, this shows a significant hysteresis. The plateau pressure was around 1 kPa and the absorption capacity ([T]/[La]) is more than 5.0 at room temperature. In the temperature range 20–110 ◦ C, clear plateau pressures could be reproducibly obtained between approximate [T]/[La] = 1.0 and 3.5,
Table 1 Experimental data of LaNi4.25 Al0.75 activated by deuterium Cycles and times
Starting pressure (kPa)
Ending pressure (kPa)
Activation temperature (◦ C)
Atomic ratio [D]/[La]
Equilibrium times (h)
Cycle 1
1 2 3
51.3 51.1 49.8
51.1 49.8 46.0
21 100 21
0.05 1.07 3.92
1.0 1.5 2.0
Cycle 2
1 2 3
58.2 53.6 53.1
53.6 53.1 52.5
75 50 21
3.41 3.78 4.26
2.0 2.0 2.0
W.-d. Wang et al. / Journal of Alloys and Compounds 441 (2007) 359–363
361
Fig. 2. ln(PT2 /P0 ) − 1/T plots of tritium absorption (a) and desorption (b) in LaNi4.25 Al0.75 Tx . () 1.5, () 2.0, () 2.5, (夽) 3.0.
these facts indicates that the coexistence of the ␣-solid solution and the -tritide LaNi4.25 Al0.75 . In the temperature of 110 ◦ C, the plateau shows a notable narrowing down. As shows in Fig. 1, the ␣-solid solution is the main phase in [T]/[La] < 1.0, while the curves approach saturation after [T]/[La] = 3.5, the fraction of the -tritide LaNi4.25 Al0.75 increases rapidly. 3.3. Thermodynamic properties The change of standard free energy Gθ for tritiding reaction in tritium storage alloys can be expressed as: Gθ = H θ − TS θ
(1)
where T is the absolute temperature. The change of standard free energy Gθ is related to the dissociation pressure (PT2 /P0 ) for two-phase metal hydrides, according to the relation: PT2 (2) Gθ = RT ln P0 Assuming that the changes of standard enthalpy and entropy are independent of the temperature. The equilibrium pressures PT2 is read from the P–C–T curves, its unit is Pa, P0 = 101,325 Pa, R is the gas constant, Eq. (1) using these relations gives: PT2 H θ S θ ln = − (3) P0 RT R The plots of ln(PT2 /P0 ) versus 1/T (van’t Hoff plots) in Fig. 2 shows that ln(PT2 /P0 ) are linear with 1/T, the related coefficient of the lines are more than 0.9994. The changes of standard enthalpy Hθ and entropy Sθ were calculated from
corresponding slopes and intercepts according to Eq. (3). The obtained experimentally thermodynamic parameters are shown in Table 2. The entropy denotes the reaction trend of hydrogenation, for an alloy of the same type, the larger absolute value of entropy is implies that the pressure of desorption is lower and the alloy hydride is more stable. The enthalpy is heat of formation of alloy hydride; the larger absolute value denotes the more stable of alloy hydride. The reported [4,5] value of Hθ and Sθ of the LaNi4.25 Al0.75 hydride formation are equal to −23.4 kJ mol−1 and −73.9 J mol−1 K−1 , these values are smaller than that of LaNi4.25 Al0.75 tritide. It may indicate that LaNi4.25 Al0.75 tritide is more stable than LaNi4.25 Al0.75 hydride. The experimental plateau pressure of LaNi4.25 Al0.75 tritide is lower in comparison with its hydride, shows positive-hydrogen-isotopes effect as reported in Ref. [6], and it is another evidence of the difference in the thermodynamic stabilities. According to these thermodynamic parameters and van’t Hoff relations, if we assumed that the sample temperature is 25 ◦ C, it can easily extrapolate that the equilibrium pressures of tritium absorption and desorption of LaNi4.25 Al0.75 tritide are 2.6 and 1.4 kPa, respectively. Results show that calculations are in good agreement with experimental data. Contrarily, if we know the equilibrium pressure, the result of this work can be used to calculate the sample temperature. For example, the corresponding sample temperature of desorption pressure of 200 kPa is 181 ◦ C. 3.4. Absorption–desorption rate Fig. 3 gives the results of the experiment. As shown in Fig. 3, the material possesses the characteristics of high-speed tritium
Table 2 Hθ and Sθ of tritium absorption and desorption in LaNi4.25 Al0.75 Tx [T]/[La]
Ab.Hθ (kJ mol−1 H2 )
Ab.Sθ (J mol−1 K−1 H2 )
De.Hθ (kJ mol−1 H2 )
De.Sθ (J mol−1 K−1 H2 )
1.50 2.00 2.50 3.00
−31.7 −32.0 −31.8 −31.8
−74.9 −76.4 −76.5 −76.9
−35.0 −35.7 −35.6 −35.9
−81.2 −82.8 −84.2 −85.4
± ± ± ±
0.5 0.3 0.8 0.6
± ± ± ±
1.5 1.0 2.4 1.7
± ± ± ±
0.5 0.5 0.5 0.4
± ± ± ±
1.5 1.5 1.5 1.2
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W.-d. Wang et al. / Journal of Alloys and Compounds 441 (2007) 359–363
Fig. 3. Tritium absorption (a) and desorption (b) speed of LaNi4.25 Al0.75 Tx at different temperature (◦ C): 1–110, 2–80, 3–50, 4–20, 5–24.
absorption–desorption rate, and the range of half-time of saturation is 8–30 s. Unfortunately, because of tritium greatly overfeeding, the influence of temperature on tritium absorption is unobtrusively so that the curves have not an obvious regularity. However, the equilibrium speed of tritium desorption is accelerated with a rise of sample temperature and has a concomitance of the decreasing equilibrium time, furthermore, the volume of released tritium increases with the temperature.
HT may be released from stainless steel tube at high temperature. Besides this, content of the other species did not change, and 3 He release behavior is not observed. Overall, the results reveal that the absorbed and the released tritium not have much difference in purity.
3.5. Compositions of absorption–desorption tritium
The changes of standard enthalpy and entropy during the formation of the LaNi4.25 Al0.75 T2.5 phase are (−31.8 ± 0.8) kJ mol−1 and (−76.5 ± 2.4) J mol−1 K−1 , and the value of desorption are (−36.5 ± 0.5) kJ mol−1 and (−84.2 ± 1.5) J mol−1 K−1 , respectively. In the temperature range 20–110 ◦ C, both the equilibrium pressure of absorption and desorption increase with a rise of the equilibrium temperature, and the isotherm of desorption is under that of absorption and presents a significant hysteresis. The plateau pressure was around 1 kPa and the absorption capacity ([T]/[La]) is more than 5.0 at room temperature. The influence of temperature on tritium absorption is unobtrusively in the temperature range 20–80 ◦ C, but the equilibrium speed of tritium desorption increases with a rise of sample temperature and has a concomitance of the equilibrium time decreasing. LaNi4.25 Al0.75 is easily activated. In addition, the absorption–desorption processing is rapid and does not need a long equilibrium time. It can be concluded that LaNi4.25 Al0.75 is a good material for hydrogen isotopes storage and pump over.
Composition difference between the origin tritium gas and tritium released from 7-day-aged LaNi4.25 Al0.75 tritide are analyzed by quadrupole mass spectrometer. Fig. 4 provides results of the analysis, the intensity of mass number over eight is around baseline and cut-out. As can be seen from the spectrum, comparing the released gases with the origin gases, the species of mass number of two and four increase, a possible explanation of this is that the H2 and
4. Conclusions
Acknowledgements
Fig. 4. Mass spectra of absorbed (a) and desorbed (b) tritium for LaNi4.25 Al0.75 tritide.
This work is supported by the Foundation of China nature science under contract number 50121050 and 50371079. Institute of Metal Research, CAS, is gratefully acknowledged for their sample. The authors wish to thank Prof. Xiao-hua Cao, Dr. De-min Chen and M.Sc. Xiao-song Zhou for helpful discussion.
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