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The effect of the hydrogenation process on the production of lattice defects in Pd
Journal of Alloys and Compounds 414 (2006) 204–206
The effect of the hydrogenation process on the production of lattice defects in Pd K. Sakaki a,c,∗ , M. Mizuno a,b , H. Araki a,b , Y. Shirai a,b a
c
Department of Materials Science and Engineering, Osaka University, Yamada-oka 2-1, Suita Osaka 565-0871, Japan b Science and Technology Center for Atoms, Molecules and Ions Control, Graduate School of Engineering, Osaka University, Suita Osaka 565-0871, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-0035, Japan Received 25 February 2005; accepted 23 April 2005 Available online 6 September 2005
Abstract The effect of the hydrogen absorption and desorption process on the production of lattice defects in pure Pd was studied by means of positron lifetime spectroscopy. Pd hydrogenated and dehydrogenated at 623 K contains some dislocations but few metal vacancies. In contrast, a remarkable vacancy production was observed in Pd hydrogenated at 623 K and subsequently electrochemically dehydrogenated at ambient temperature. This result shows that vacancies are formed during the hydrogen desorption process at RT, and suggests that the vacancy formation is caused by the effect of stresses accompanied by the phase separation. © 2005 Elsevier B.V. All rights reserved. Keywords: Positron annihilation; Hydrogen storage materials; Vacancy; Dislocation
1. Introduction It has been found that large numbers of metal vacancies are produced when hydrogen storage alloys such as LaNi5 , NdNi5 , ZrMn2 and pure Pd absorb hydrogen at ambient temperature [1–3]. This vacancy production is difficult to explain by the currently accepted vacancy production mechanisms, namely, quenching, radiation and plastic deformation. On the other hand, Shirai and co-workers have found [4,5] that excess vacancies are produced by some phase transformations and they proposed a stress-induced vacancy formation mechanism. Excess vacancies are generated as a result of the nucleation and growth of a new phase having different lattice parameters and/or volume of the matrix. In order to clarify the defect formation mechanism in more details, the effect of hydrogen absorption and desorption process on the production of lattice defects in Pd has
∗
Corresponding author. E-mail address: [email protected] (K. Sakaki).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.04.214
been studied by positron lifetime spectroscopy. Above a critical temperature, Pd can absorb and desorb hydrogen without passing through the (␣ + ) two-phase region. The ␣ phase is the hydrogen solution phase and the  phase is the hydride phase. Defect formation during dehydrogenation was also investigated.
2. Experimental procedure Samples of pure Pd (99.99%) have a square surface of 10 mm × 10 mm and a thickness of 0.3 mm, and were fully annealed at 1173 K for over 4 h in a pure Ar atmosphere. Two sets of pure Pd samples were hydrogenated at 623 K without passing through the two-phase (␣ + ) region. For hydrogenation, hydrogen gas pressure was gradually increased up to 7 MPa, that was maintained for 2 h. One set of the Pd was dehydrogenated at 623 K by evacuation. The other set was cooled down to 296 K in the first place under hydrogen pressure higher than 5 MPa. After cooling down the sample, hydrogen gas pressure was released. Subsequently,
K. Sakaki et al. / Journal of Alloys and Compounds 414 (2006) 204–206
electrochemical hydrogen desorption experiments were carried out in order to discharge completely the absorbed hydrogen; hydrogen was desorbed from the Pd sample at 296 K by using 6 M KOH as electrolyte, with a Hg/HgO reference electrode and a NiOOH-Ni(OH)2 counter electrode; the currents were varied from 1 to 50 mA, down to the cut-off voltage. To investigate lattice defects contained in the two types of samples, isochronal annealing treatments were performed. After each annealing, the positron lifetime was measured at 296 K. The details of isochronal annealing conditions and positron lifetime measurements are described elsewhere [3].
3. Results and discussion Before the hydrogenation treatment, all samples showed a positron lifetime value of 106 ps, which is the same value as that calculated for Pd without defects [3]. This clearly shows that there were no detectable lattice defects in the starting samples.
205
As shown in Fig. 1, τ m is about 124 ps just after dehydrogenation at 623 K. This moderate increase in τ m from the bulk value 106 ps shows that some lattice defects were formed during absorption and/or desorption of hydrogen at 623 K. The τ m was almost constant in the annealing temperature range below 573 K and decreased to the positron lifetime value of fully annealed samples (106 ps) in the temperature range from 623 to 773 K. This recovery behavior indicates that the sample contains excess dislocations, but few vacancies. It is not clear whether vacancies are formed or not during the hydrogen absorption and desorption processes at 623 K, since the temperature is much higher than the migration temperature of vacancies in Pd. During hydrogenation at 623 K, hydride is expected to be formed homogeneously in the equilibrium state. Nevertheless, the above results show that some dislocations are formed even above a miscibility gap. Some nuclei of hydride that have a larger lattice constant than the matrix may be formed in a transitional stage of hydrogen absorption. Then, excess vacancies may also be formed, but they must be annealed out at 623 K. 3.2. Dehydrogenation at ambient temperature
3.1. Hydrogen absorption and desorption at 623 K Fig. 1 shows the change in the mean positron lifetime, τ m , during the recovery process for the samples hydrogenated and dehydrogenated at 623 K together with that for Pd hydrogenated at 296 K [3], whose recovery process can be divided into two temperature ranges from 373 to 573 K and from 673 to 923 K. The former recovery is attributed to the annealing out of excess vacancies which were formed during the hydrogenation process at 296 K and the latter to dislocations [3].
Before electrochemical desorption, the τ m was 128 ps, which is nearly equal to that for the sample hydrogen absorbed and desorbed at 623 K. So this increase in positron lifetime is presumably due to dislocations. After electrochemical desorption at RT, τ m increased dramatically from 128 to 166 ps. This result shows that numerous new lattice defects were produced during the hydrogen desorption process at 296 K. Fig. 2 shows the change in τ m of the sample dehydrogenated at 296 K by the electrochemical method. The
Fig. 1. Mean positron lifetime changes in Pd hydrogenated at 296 and 623 K on the isochronal annealing.
Fig. 2. Mean positron lifetime in Pd hydrogenated at 623 K and subsequently electrochemically desorbed at RT, as a function of annealing temperature.
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recovery process can be divided into two stages: one from 348 to 573 K and the other from 673 to 923 K, where the positron lifetime value returns to that of a fully annealed specimen. This positron lifetime behavior is similar to that for the sample hydrogenated at 296 K (see Fig. 1). As described there, the first and second stages correspond to vacancy and dislocation recovery processes, respectively. It is clear that both vacancies and dislocations are formed during the  → ␣ transformation. This demonstrates that vacancy formation occurs not only during the hydrogen absorption process but also during the desorption process. The formation of dislocations during the  → ␣ transformation was reported by Jamieson et al. [6], by using TEM observation.
4. Conclusion
3.3. Vacancy formation mechanism
Acknowledgement
Based on X-ray diffraction measurements, Fukai et al. [7] have reported that vacancies are formed in Pd at high temperature (over 900 K) under high hydrogen pressure (∼GPa). They propound that these are thermal vacancies whose concentration is enhanced by the hydrogen-vacancy binding energy. By contrast, present study shows that excess vacancies are generated even at 296 K, not only in the hydrogen absorption process but also in the hydrogen desorption process. In particular, the phenomenon of vacancy formation during the hydrogen desorption process cannot be explained by the decrease of vacancy formation enthalpy caused by the hydrogenvacancy binding. Therefore, the vacancy formation phenomena observed by present authors are probably due to different mechanisms. It has been found that some diffusional phase separations accompany excess vacancy formation and has been explained as a stress effect between two phases [4,5]. Observed vacancy formation in Pd during hydrogen desorption process can be explained by the same mechanism: the large coherency and volume strain between the ␣ and  phases is most likely to cause the excess vacancy generation, not only during the hydrogen absorption process but also during the desorption process.
The authors wish to thank Dr. Miyamura and Dr. Senoh for kind help concerning the electrochemical desorption method. This work was partly carried out at “Handai Frontier Research Center” and partly supported by a 21st Century COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Sports, Culture, Science and Technology of Japan.
The effect of the hydrogenation and the dehydrogenation process on the production of lattice defects was investigated by using positron lifetime spectroscopy. The following conclusions were drawn: (1) Dislocations were formed even in the hydrogen solution region above the miscibility gap. (2) During the  → ␣ a phase transformation at 296 K, excess vacancies are formed. (3) The mechanism of vacancy formation at ambient temperature may be due to stress generated as the result of the phase transformation.
References [1] Y. Shirai, H. Araki, T. Mori, W. Nakamura, K. Sakaki, J. Alloys Compd. 330–332 (2001) 125–131. [2] K. Sakaki, H. Araki, Y. Shirai, Mater. Trans. JIM 43 (7) (2002) 1494–1497. [3] K. Sakaki, T. Yamada, M. Mizuno, H. Araki, Y. Shirai, Mater. Trans. JIM 43 (11) (2002) 2652–2655. [4] Y. Shirai, F. Nakamura, M. Takeuchi, K. Watanabe, M. Yamaguchi, Positron annihilation, in: L. Dorikens-Vanpraet, M. Drikens, D. Segers (Eds.), Proceedings of the 8th International Conference on Positron Annihilation, World Scientific, Singapore, 1989, pp. 488–490. [5] P. Chalermkarnnon, H. Araki, Y. Shirai, Mater. Trans. JIM 43 (7) (2002) 1486–1488. [6] H.C. Jamieson, G.C. Weatherly, F.D. Manchester, J. Less-Common Met. 50 (1976) 85–102. [7] Y. Fukai, Y. Ishii, Y. Goto, K. Watanabe, J. Alloys Compd. 313 (2000) 121–132.
The effect of the hydrogenation process on the production of lattice defects in Pd K. Sakaki a,c,∗ , M. Mizuno a,b , H. Araki a,b , Y. Shirai a,b a
c
Department of Materials Science and Engineering, Osaka University, Yamada-oka 2-1, Suita Osaka 565-0871, Japan b Science and Technology Center for Atoms, Molecules and Ions Control, Graduate School of Engineering, Osaka University, Suita Osaka 565-0871, Japan National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-0035, Japan Received 25 February 2005; accepted 23 April 2005 Available online 6 September 2005
Abstract The effect of the hydrogen absorption and desorption process on the production of lattice defects in pure Pd was studied by means of positron lifetime spectroscopy. Pd hydrogenated and dehydrogenated at 623 K contains some dislocations but few metal vacancies. In contrast, a remarkable vacancy production was observed in Pd hydrogenated at 623 K and subsequently electrochemically dehydrogenated at ambient temperature. This result shows that vacancies are formed during the hydrogen desorption process at RT, and suggests that the vacancy formation is caused by the effect of stresses accompanied by the phase separation. © 2005 Elsevier B.V. All rights reserved. Keywords: Positron annihilation; Hydrogen storage materials; Vacancy; Dislocation
1. Introduction It has been found that large numbers of metal vacancies are produced when hydrogen storage alloys such as LaNi5 , NdNi5 , ZrMn2 and pure Pd absorb hydrogen at ambient temperature [1–3]. This vacancy production is difficult to explain by the currently accepted vacancy production mechanisms, namely, quenching, radiation and plastic deformation. On the other hand, Shirai and co-workers have found [4,5] that excess vacancies are produced by some phase transformations and they proposed a stress-induced vacancy formation mechanism. Excess vacancies are generated as a result of the nucleation and growth of a new phase having different lattice parameters and/or volume of the matrix. In order to clarify the defect formation mechanism in more details, the effect of hydrogen absorption and desorption process on the production of lattice defects in Pd has
∗
Corresponding author. E-mail address: [email protected] (K. Sakaki).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.04.214
been studied by positron lifetime spectroscopy. Above a critical temperature, Pd can absorb and desorb hydrogen without passing through the (␣ + ) two-phase region. The ␣ phase is the hydrogen solution phase and the  phase is the hydride phase. Defect formation during dehydrogenation was also investigated.
2. Experimental procedure Samples of pure Pd (99.99%) have a square surface of 10 mm × 10 mm and a thickness of 0.3 mm, and were fully annealed at 1173 K for over 4 h in a pure Ar atmosphere. Two sets of pure Pd samples were hydrogenated at 623 K without passing through the two-phase (␣ + ) region. For hydrogenation, hydrogen gas pressure was gradually increased up to 7 MPa, that was maintained for 2 h. One set of the Pd was dehydrogenated at 623 K by evacuation. The other set was cooled down to 296 K in the first place under hydrogen pressure higher than 5 MPa. After cooling down the sample, hydrogen gas pressure was released. Subsequently,
K. Sakaki et al. / Journal of Alloys and Compounds 414 (2006) 204–206
electrochemical hydrogen desorption experiments were carried out in order to discharge completely the absorbed hydrogen; hydrogen was desorbed from the Pd sample at 296 K by using 6 M KOH as electrolyte, with a Hg/HgO reference electrode and a NiOOH-Ni(OH)2 counter electrode; the currents were varied from 1 to 50 mA, down to the cut-off voltage. To investigate lattice defects contained in the two types of samples, isochronal annealing treatments were performed. After each annealing, the positron lifetime was measured at 296 K. The details of isochronal annealing conditions and positron lifetime measurements are described elsewhere [3].
3. Results and discussion Before the hydrogenation treatment, all samples showed a positron lifetime value of 106 ps, which is the same value as that calculated for Pd without defects [3]. This clearly shows that there were no detectable lattice defects in the starting samples.
205
As shown in Fig. 1, τ m is about 124 ps just after dehydrogenation at 623 K. This moderate increase in τ m from the bulk value 106 ps shows that some lattice defects were formed during absorption and/or desorption of hydrogen at 623 K. The τ m was almost constant in the annealing temperature range below 573 K and decreased to the positron lifetime value of fully annealed samples (106 ps) in the temperature range from 623 to 773 K. This recovery behavior indicates that the sample contains excess dislocations, but few vacancies. It is not clear whether vacancies are formed or not during the hydrogen absorption and desorption processes at 623 K, since the temperature is much higher than the migration temperature of vacancies in Pd. During hydrogenation at 623 K, hydride is expected to be formed homogeneously in the equilibrium state. Nevertheless, the above results show that some dislocations are formed even above a miscibility gap. Some nuclei of hydride that have a larger lattice constant than the matrix may be formed in a transitional stage of hydrogen absorption. Then, excess vacancies may also be formed, but they must be annealed out at 623 K. 3.2. Dehydrogenation at ambient temperature
3.1. Hydrogen absorption and desorption at 623 K Fig. 1 shows the change in the mean positron lifetime, τ m , during the recovery process for the samples hydrogenated and dehydrogenated at 623 K together with that for Pd hydrogenated at 296 K [3], whose recovery process can be divided into two temperature ranges from 373 to 573 K and from 673 to 923 K. The former recovery is attributed to the annealing out of excess vacancies which were formed during the hydrogenation process at 296 K and the latter to dislocations [3].
Before electrochemical desorption, the τ m was 128 ps, which is nearly equal to that for the sample hydrogen absorbed and desorbed at 623 K. So this increase in positron lifetime is presumably due to dislocations. After electrochemical desorption at RT, τ m increased dramatically from 128 to 166 ps. This result shows that numerous new lattice defects were produced during the hydrogen desorption process at 296 K. Fig. 2 shows the change in τ m of the sample dehydrogenated at 296 K by the electrochemical method. The
Fig. 1. Mean positron lifetime changes in Pd hydrogenated at 296 and 623 K on the isochronal annealing.
Fig. 2. Mean positron lifetime in Pd hydrogenated at 623 K and subsequently electrochemically desorbed at RT, as a function of annealing temperature.
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recovery process can be divided into two stages: one from 348 to 573 K and the other from 673 to 923 K, where the positron lifetime value returns to that of a fully annealed specimen. This positron lifetime behavior is similar to that for the sample hydrogenated at 296 K (see Fig. 1). As described there, the first and second stages correspond to vacancy and dislocation recovery processes, respectively. It is clear that both vacancies and dislocations are formed during the  → ␣ transformation. This demonstrates that vacancy formation occurs not only during the hydrogen absorption process but also during the desorption process. The formation of dislocations during the  → ␣ transformation was reported by Jamieson et al. [6], by using TEM observation.
4. Conclusion
3.3. Vacancy formation mechanism
Acknowledgement
Based on X-ray diffraction measurements, Fukai et al. [7] have reported that vacancies are formed in Pd at high temperature (over 900 K) under high hydrogen pressure (∼GPa). They propound that these are thermal vacancies whose concentration is enhanced by the hydrogen-vacancy binding energy. By contrast, present study shows that excess vacancies are generated even at 296 K, not only in the hydrogen absorption process but also in the hydrogen desorption process. In particular, the phenomenon of vacancy formation during the hydrogen desorption process cannot be explained by the decrease of vacancy formation enthalpy caused by the hydrogenvacancy binding. Therefore, the vacancy formation phenomena observed by present authors are probably due to different mechanisms. It has been found that some diffusional phase separations accompany excess vacancy formation and has been explained as a stress effect between two phases [4,5]. Observed vacancy formation in Pd during hydrogen desorption process can be explained by the same mechanism: the large coherency and volume strain between the ␣ and  phases is most likely to cause the excess vacancy generation, not only during the hydrogen absorption process but also during the desorption process.
The authors wish to thank Dr. Miyamura and Dr. Senoh for kind help concerning the electrochemical desorption method. This work was partly carried out at “Handai Frontier Research Center” and partly supported by a 21st Century COE Program (Project: Center of Excellence for Advanced Structural and Functional Materials Design) from the Ministry of Education, Sports, Culture, Science and Technology of Japan.
The effect of the hydrogenation and the dehydrogenation process on the production of lattice defects was investigated by using positron lifetime spectroscopy. The following conclusions were drawn: (1) Dislocations were formed even in the hydrogen solution region above the miscibility gap. (2) During the  → ␣ a phase transformation at 296 K, excess vacancies are formed. (3) The mechanism of vacancy formation at ambient temperature may be due to stress generated as the result of the phase transformation.
References [1] Y. Shirai, H. Araki, T. Mori, W. Nakamura, K. Sakaki, J. Alloys Compd. 330–332 (2001) 125–131. [2] K. Sakaki, H. Araki, Y. Shirai, Mater. Trans. JIM 43 (7) (2002) 1494–1497. [3] K. Sakaki, T. Yamada, M. Mizuno, H. Araki, Y. Shirai, Mater. Trans. JIM 43 (11) (2002) 2652–2655. [4] Y. Shirai, F. Nakamura, M. Takeuchi, K. Watanabe, M. Yamaguchi, Positron annihilation, in: L. Dorikens-Vanpraet, M. Drikens, D. Segers (Eds.), Proceedings of the 8th International Conference on Positron Annihilation, World Scientific, Singapore, 1989, pp. 488–490. [5] P. Chalermkarnnon, H. Araki, Y. Shirai, Mater. Trans. JIM 43 (7) (2002) 1486–1488. [6] H.C. Jamieson, G.C. Weatherly, F.D. Manchester, J. Less-Common Met. 50 (1976) 85–102. [7] Y. Fukai, Y. Ishii, Y. Goto, K. Watanabe, J. Alloys Compd. 313 (2000) 121–132.