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Agglomeration of hydrogen-induced vacancies in nickel
Journol of
ALLO~ AND COMI~DUND5 ELSEVIER
Journal of Alloys and Compounds 231 (1995) 41-45
Agglomeration of hydrogen-induced vacancies in nickel H. Osono a, T. Kino a, Y. Kurokawa b, Y.
Fukai b
aHiroshima-Denki Institute of Technology, Hiroshima 739-03, Japan bDepartment of Physics, Chuo University, Tokyo 112, Japan
Abstract Scanning electron microscope observations of Ni samples annealed after recovery from high temperature heat treatment in the hydride phase showed the presence of numerous holes 20-200 nm in size. From various features of the holes they are identified as voids formed by agglomeration of supersaturated vacancies (about 5 at.% in concentration) which have diffused from the surface to the interior of the sample during heat treatment. Keywords: Nickel; Hydrogen; Vacancy; Void
1. Introduction Recently it was discovered that the lattice parameter of NiH and PdH decreased gradually (in several hours) at high temperatures (700-800 °C) and this contraction could be retained after recovery to amblent conditions and even after degassing hydrogen. It was then proposed that this lattice contraction was caused by a large number of vacancies introduced in the hydride phase, amounting to about 20 at.% [1]. If the lattice contraction is indeed due to vacancies, they should be in a supersaturated state after recovery and degassing and must form clusters or voids by adequate heat treatment. The aim of this study is to observe such voids using a scanning electron microscope formed by the agglomeration of hydrogen-induced vacancies.
2. Experimental procedure The sample was a nickel disk 2 mm in diameter and 0.2 mm thick, hydrogenated under a hydrogen pressure of 5 GPa and heat treated at 800 °C for prescribed times of 10 min, 30 min and 5 h. After recovery to ambient conditions the sample was heated in vacuum at a rate of 20 °C rain -1 to 800 °C, maintained there for 15 min and cooled to room temperature within 20 min. Full details of the sample preparation are described elsewhere [1]. For observation by a scanning electron microscope (SEM) the sample surface was lightly 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(95)01835-2
polished with emery paper and subsequently electropolished at 15V in a solution of 20HCIO 4 + 80C2H5OH using a twin-jet method until a small hole was formed at the centre. The circumference of the sample thus prepared was close to the original surface and the centre of the electropolished sample close to the middle part of the original sample. Therefore the inner part of the original sample would be examined by surface observations of the electropolished sample.
3. Results and discussion Observed micrographs for samples prepared with three different holding times of the heat treatment are shown in Fig. 1 at a magnification of about 1000 times. These pictures were taken at positions approximately ½ from the periphery, i.e. at depths of ~-~11 from the original surface. Large cracks were observed both in the grains and on the grain boundaries (the grain size being 10-30 p.m). They were found in all samples independently of the time of heat treatment. These cracks are considered to be formed in the process of decomposition of the nickel hydride after recovery to ambient conditions. In addition to these cracks, many etch pits greater than 500 nm in size were seen in all samples, the origin of which has not been identified. Micrographs with a magnification of about 10 000 times are shown in Fig. 2. Only etch pits were seen in the sample heat treated for 10 min. In the sample heat treated for 3 0 imn , however, holes (H) distinctly
42
H. Osono et al. / Journal of Alloys and Compounds 231 (1995) 41-45
Fig. 1. Cracks and etch pits in samples heat treated at 800 °C for 10 min, 30 rain and 5 h at depths of ¼-13 from the surface. smaller than the etch pits (P) were observed. These small holes were observed clearly in the sample heat treated for 5 h. Higher magnification micrographs of the samples heat treated for 30 min and 5 h are shown in Fig. 3. Small holes 20-200 nm in size can be seen in both samples and their size and n u m b e r increase with the holding time of the heat treatment, This temporal variation in the appearance of small holes is consistent with the previous proposal [1] that a large n u m b e r of vacancies diffuse f r o m the surface to the interior of the hydride at high temperatures. The diffusion lengths of vacancies in N i H during heat treatment of 800 °C for 30 min and 5 h can be estim a t e d approximately f r o m the diffusivity of vacancies in metallic Ni (D v ~ 8 × 10 -13 m 2 s - l ) 1 to be about 0.04 and 0 . 1 2 m m respectively, Since these values are comparable with the sample 1The vacancy diffusivity in Ni at 5 GPa was calculated from Dv ~10-'e-13°V(kT) -~ mZs-~. The migration enthalpy of 1.3eV was obtained from the migration energy of 1.2 eV at ambient pressure (a middle value of the reported values of 1.0-1.4 eV) plus a pressure-dependent term of about 0.1 eV (assuming a vacancy migration volume of about 0.2 of the atomic volume) [2].
thickness (0.2 mm), it is naturally expected that the supply of vacancies to the observed region should have b e e n distinctly larger in the heat treatment for 5 h than for 30 min. The discussion can be m a d e m o r e quantitative by estimating the total volume fraction Vf from the m e a n size D and n u m b e r density n of holes in Fig. 3. Results are given in Table 1. For simplicity the holes are classified into two groups, those greater than 100 n m and smaller than 100 n m in size. T h e n u m b e r density is estimated from the m e a n distance between holes and the volume by approximating each hole as a sphere of diameter D. If we assume that the holes were formed by the agglomeration of vacancies in the hydride phase, the total volume fraction Vf should correspond to the concentration of vacancies remaining after recovery to ambient conditions. Thus we obtain the concentration of vacancies in the sample heat treated for 5 h at 800 °C to be as m a n y as several atomic per cent. This extraordinarily high value of the vacancy concentration is consistent with the previous estimate from the observed lattice contraction and the relaxation volume of a vacancy [1].
H. Osono et al.
Journal of Alloys and Compounds 231 (1995) 41-45
43
Fig. 2. Variation in appearance of small holes with holding of heat t r e a t m e n t for 10 min, 30 min and 5 h.
Table 1 Total volume fraction of holes for two different holding times of heat t r e a t m e n t Holding time
D ~> 100 n m
30min 5h
D ~< 100 n m
Total Vf
D (nm)
n (m 3)
Vf
D (nm)
n (m -3)
Vf
110 140
2 x 1017 2 x 1019
1 x 10 4 4 × 10 2
15 30
4 x 1019 7 x 102°
6 x 10 -5 1 x 10 -z
2 x 10 -4 5 x 10 -2
Table 2 Total volume fraction of holes at three different depths (related to Figs. 4(a)-4(c)) in sample Fig.
D >I 100 n m
D ~< 100 n m
Total
v,
4(a) 4(b) 4(c)
D (nm)
n (m 3)
Vf
D (nm)
n (m -3)
Vf
120 140 110
2 X 1019 2 X 10 19 2 × 10 TM
1 X 10 -2 4 X 10 -2 2 x 10 3
20 30 25
6 X 1020
2 x 10 -3 1 X 10 -2 3 x 10 -3
7 X 10 20
3 X 1020
2 x 10 -2 5 X 10 -2
5 x 10 -3
44
H. Osono et al. / Journal of Alloys and Compounds 231 (1995) 41-45
Fig. 3. Magnified micrographs of Fig. 2.
In order to investigate the depth dependence of the hole distribution, micrographs were taken at three different depths of the sample heat treated for 5 h. Results are shown in Fig. 4. Fig. 4(0) shows the appearance of the electropolished sample from the circumference (left side of the photograph) to the centre (right side of the photograph). As described before, different positions of the electropolished sample correspond to different depths from the original surface of the sample. Thus Figs. 4(a)-4(c) correspond to the near-surface, intermediate and middle parts of the sample thickness respectively. Estimated volume fractions of holes are given in Table 2. Whereas the number density, dominated by smaller holes, is nearly the same at all positions, the size of the holes varies with depth and the total volume fraction is definitely smaller in the middle part. Apparently, 5 h is not a sufficient time for vacancies in the hydride phase to attain a uniform distribution throughout the sample. The present observation of the hole distribution, i.e. its dependence on the holding time of the heat treatment and the depth in the sample, can only be understood in terms of transport of vacancies from the surface to the interior of the hydride. A naive expectation that the holes might be bubbles of hydrogen gas evolved in the course of hydride decomposition cannot explain the observed features. Decomposition of the
Fig. 4. Distribution of holes at three different depths of sample heat treated for 5 h at 800°C: (a), (b) and (c) correspond to near-surface, intermediate and middle parts of sample thickness respectively.
H. Osono et al. / Journal of Alloys and Compounds 231 (1995) 41-45
hydride led to the formation of cracks, as actually observed in all samples irrespective of high temperature heat treatment. The total volume fraction of holes also provides independent support to the amount of superabundant vacancies introduced in the process. The concentration of vacancies can be determined by measuring the density and lattice parameter changes using the method of Simmons and Bulluffi [3]. This has been done for Pd [4]. After similar heat treatment in the hydride phase, recovery and degassing, a vacancy concentration of about 18 at.% was obtained. For Ni the experiment of this type has been hampered so far by the formation of cracks, but may be done in the near future as the sample recovery method is improved. Evidence of superabundant vacancy formation is also provided by the formation of a vacancy-ordered structure in the hydride phase. The vacancy-ordered structure of L12 type (]~/13Vac) h a s b e e n o b s e r v e d i n PdH [4], P d - R h alloy hydride and more recently in NiH as well. These results will be published in due COUrSe.
45
The present status of our understanding of superabundant vacancy formation and its consequences and implications are discussed by Fukai [5].
Acknowledgment This work is supported in part by a Grant-in-Aid for General Scientific Research from the Ministry of Education, Science and Culture.
References [1] Y. Fukai and N. Okuma, Jpn. J. Appl. Phys., 32 (1993) L1256. [21 H.J. Wollenberger, in R.W. Cahn and P. Haasen (eds.), Physical Metallurgy, Elsevier, Amsterdam, 1983, p. 1139. [3] R.O. Simmons and R.W. Bulluffi, Phys. Rev., 117 (1960) 52. [4] Y. Fukai and N. Okuma, Phys. Rev. Lett., 73 (1994) 1640. [5] Y. Fukai, J. Alloys Comp., this issue.
ALLO~ AND COMI~DUND5 ELSEVIER
Journal of Alloys and Compounds 231 (1995) 41-45
Agglomeration of hydrogen-induced vacancies in nickel H. Osono a, T. Kino a, Y. Kurokawa b, Y.
Fukai b
aHiroshima-Denki Institute of Technology, Hiroshima 739-03, Japan bDepartment of Physics, Chuo University, Tokyo 112, Japan
Abstract Scanning electron microscope observations of Ni samples annealed after recovery from high temperature heat treatment in the hydride phase showed the presence of numerous holes 20-200 nm in size. From various features of the holes they are identified as voids formed by agglomeration of supersaturated vacancies (about 5 at.% in concentration) which have diffused from the surface to the interior of the sample during heat treatment. Keywords: Nickel; Hydrogen; Vacancy; Void
1. Introduction Recently it was discovered that the lattice parameter of NiH and PdH decreased gradually (in several hours) at high temperatures (700-800 °C) and this contraction could be retained after recovery to amblent conditions and even after degassing hydrogen. It was then proposed that this lattice contraction was caused by a large number of vacancies introduced in the hydride phase, amounting to about 20 at.% [1]. If the lattice contraction is indeed due to vacancies, they should be in a supersaturated state after recovery and degassing and must form clusters or voids by adequate heat treatment. The aim of this study is to observe such voids using a scanning electron microscope formed by the agglomeration of hydrogen-induced vacancies.
2. Experimental procedure The sample was a nickel disk 2 mm in diameter and 0.2 mm thick, hydrogenated under a hydrogen pressure of 5 GPa and heat treated at 800 °C for prescribed times of 10 min, 30 min and 5 h. After recovery to ambient conditions the sample was heated in vacuum at a rate of 20 °C rain -1 to 800 °C, maintained there for 15 min and cooled to room temperature within 20 min. Full details of the sample preparation are described elsewhere [1]. For observation by a scanning electron microscope (SEM) the sample surface was lightly 0925-8388/95/$09.50 © 1995 Elsevier Science S.A. All rights reserved SSDI 0925-8388(95)01835-2
polished with emery paper and subsequently electropolished at 15V in a solution of 20HCIO 4 + 80C2H5OH using a twin-jet method until a small hole was formed at the centre. The circumference of the sample thus prepared was close to the original surface and the centre of the electropolished sample close to the middle part of the original sample. Therefore the inner part of the original sample would be examined by surface observations of the electropolished sample.
3. Results and discussion Observed micrographs for samples prepared with three different holding times of the heat treatment are shown in Fig. 1 at a magnification of about 1000 times. These pictures were taken at positions approximately ½ from the periphery, i.e. at depths of ~-~11 from the original surface. Large cracks were observed both in the grains and on the grain boundaries (the grain size being 10-30 p.m). They were found in all samples independently of the time of heat treatment. These cracks are considered to be formed in the process of decomposition of the nickel hydride after recovery to ambient conditions. In addition to these cracks, many etch pits greater than 500 nm in size were seen in all samples, the origin of which has not been identified. Micrographs with a magnification of about 10 000 times are shown in Fig. 2. Only etch pits were seen in the sample heat treated for 10 min. In the sample heat treated for 3 0 imn , however, holes (H) distinctly
42
H. Osono et al. / Journal of Alloys and Compounds 231 (1995) 41-45
Fig. 1. Cracks and etch pits in samples heat treated at 800 °C for 10 min, 30 rain and 5 h at depths of ¼-13 from the surface. smaller than the etch pits (P) were observed. These small holes were observed clearly in the sample heat treated for 5 h. Higher magnification micrographs of the samples heat treated for 30 min and 5 h are shown in Fig. 3. Small holes 20-200 nm in size can be seen in both samples and their size and n u m b e r increase with the holding time of the heat treatment, This temporal variation in the appearance of small holes is consistent with the previous proposal [1] that a large n u m b e r of vacancies diffuse f r o m the surface to the interior of the hydride at high temperatures. The diffusion lengths of vacancies in N i H during heat treatment of 800 °C for 30 min and 5 h can be estim a t e d approximately f r o m the diffusivity of vacancies in metallic Ni (D v ~ 8 × 10 -13 m 2 s - l ) 1 to be about 0.04 and 0 . 1 2 m m respectively, Since these values are comparable with the sample 1The vacancy diffusivity in Ni at 5 GPa was calculated from Dv ~10-'e-13°V(kT) -~ mZs-~. The migration enthalpy of 1.3eV was obtained from the migration energy of 1.2 eV at ambient pressure (a middle value of the reported values of 1.0-1.4 eV) plus a pressure-dependent term of about 0.1 eV (assuming a vacancy migration volume of about 0.2 of the atomic volume) [2].
thickness (0.2 mm), it is naturally expected that the supply of vacancies to the observed region should have b e e n distinctly larger in the heat treatment for 5 h than for 30 min. The discussion can be m a d e m o r e quantitative by estimating the total volume fraction Vf from the m e a n size D and n u m b e r density n of holes in Fig. 3. Results are given in Table 1. For simplicity the holes are classified into two groups, those greater than 100 n m and smaller than 100 n m in size. T h e n u m b e r density is estimated from the m e a n distance between holes and the volume by approximating each hole as a sphere of diameter D. If we assume that the holes were formed by the agglomeration of vacancies in the hydride phase, the total volume fraction Vf should correspond to the concentration of vacancies remaining after recovery to ambient conditions. Thus we obtain the concentration of vacancies in the sample heat treated for 5 h at 800 °C to be as m a n y as several atomic per cent. This extraordinarily high value of the vacancy concentration is consistent with the previous estimate from the observed lattice contraction and the relaxation volume of a vacancy [1].
H. Osono et al.
Journal of Alloys and Compounds 231 (1995) 41-45
43
Fig. 2. Variation in appearance of small holes with holding of heat t r e a t m e n t for 10 min, 30 min and 5 h.
Table 1 Total volume fraction of holes for two different holding times of heat t r e a t m e n t Holding time
D ~> 100 n m
30min 5h
D ~< 100 n m
Total Vf
D (nm)
n (m 3)
Vf
D (nm)
n (m -3)
Vf
110 140
2 x 1017 2 x 1019
1 x 10 4 4 × 10 2
15 30
4 x 1019 7 x 102°
6 x 10 -5 1 x 10 -z
2 x 10 -4 5 x 10 -2
Table 2 Total volume fraction of holes at three different depths (related to Figs. 4(a)-4(c)) in sample Fig.
D >I 100 n m
D ~< 100 n m
Total
v,
4(a) 4(b) 4(c)
D (nm)
n (m 3)
Vf
D (nm)
n (m -3)
Vf
120 140 110
2 X 1019 2 X 10 19 2 × 10 TM
1 X 10 -2 4 X 10 -2 2 x 10 3
20 30 25
6 X 1020
2 x 10 -3 1 X 10 -2 3 x 10 -3
7 X 10 20
3 X 1020
2 x 10 -2 5 X 10 -2
5 x 10 -3
44
H. Osono et al. / Journal of Alloys and Compounds 231 (1995) 41-45
Fig. 3. Magnified micrographs of Fig. 2.
In order to investigate the depth dependence of the hole distribution, micrographs were taken at three different depths of the sample heat treated for 5 h. Results are shown in Fig. 4. Fig. 4(0) shows the appearance of the electropolished sample from the circumference (left side of the photograph) to the centre (right side of the photograph). As described before, different positions of the electropolished sample correspond to different depths from the original surface of the sample. Thus Figs. 4(a)-4(c) correspond to the near-surface, intermediate and middle parts of the sample thickness respectively. Estimated volume fractions of holes are given in Table 2. Whereas the number density, dominated by smaller holes, is nearly the same at all positions, the size of the holes varies with depth and the total volume fraction is definitely smaller in the middle part. Apparently, 5 h is not a sufficient time for vacancies in the hydride phase to attain a uniform distribution throughout the sample. The present observation of the hole distribution, i.e. its dependence on the holding time of the heat treatment and the depth in the sample, can only be understood in terms of transport of vacancies from the surface to the interior of the hydride. A naive expectation that the holes might be bubbles of hydrogen gas evolved in the course of hydride decomposition cannot explain the observed features. Decomposition of the
Fig. 4. Distribution of holes at three different depths of sample heat treated for 5 h at 800°C: (a), (b) and (c) correspond to near-surface, intermediate and middle parts of sample thickness respectively.
H. Osono et al. / Journal of Alloys and Compounds 231 (1995) 41-45
hydride led to the formation of cracks, as actually observed in all samples irrespective of high temperature heat treatment. The total volume fraction of holes also provides independent support to the amount of superabundant vacancies introduced in the process. The concentration of vacancies can be determined by measuring the density and lattice parameter changes using the method of Simmons and Bulluffi [3]. This has been done for Pd [4]. After similar heat treatment in the hydride phase, recovery and degassing, a vacancy concentration of about 18 at.% was obtained. For Ni the experiment of this type has been hampered so far by the formation of cracks, but may be done in the near future as the sample recovery method is improved. Evidence of superabundant vacancy formation is also provided by the formation of a vacancy-ordered structure in the hydride phase. The vacancy-ordered structure of L12 type (]~/13Vac) h a s b e e n o b s e r v e d i n PdH [4], P d - R h alloy hydride and more recently in NiH as well. These results will be published in due COUrSe.
45
The present status of our understanding of superabundant vacancy formation and its consequences and implications are discussed by Fukai [5].
Acknowledgment This work is supported in part by a Grant-in-Aid for General Scientific Research from the Ministry of Education, Science and Culture.
References [1] Y. Fukai and N. Okuma, Jpn. J. Appl. Phys., 32 (1993) L1256. [21 H.J. Wollenberger, in R.W. Cahn and P. Haasen (eds.), Physical Metallurgy, Elsevier, Amsterdam, 1983, p. 1139. [3] R.O. Simmons and R.W. Bulluffi, Phys. Rev., 117 (1960) 52. [4] Y. Fukai and N. Okuma, Phys. Rev. Lett., 73 (1994) 1640. [5] Y. Fukai, J. Alloys Comp., this issue.