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Identification of nickel hydride phase in nickel matrix by optical microscope observation
Results in Materials 5 (2020) 100066
Contents lists available at ScienceDirect
Results in Materials journal homepage: www.journals.elsevier.com/results-in-materials
Identification of nickel hydride phase in nickel matrix by optical microscope observation Noriyuki Takano *, Hiroki Yamamoto Kanazawa Institute of Technology, Department of Mechanical Engineering, 3-1 Yatsukaho, Hakusan, Ishikawa, 924-0838, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Nickel hydride Reflectivity Anisotropy Hydride decomposition Hydrogen embrittlement
It is well known that nickel transforms into nickel hydride under high hydrogen fugacity. However, distribution of the hydride formation is not necessarily uniform. It is proposed that the nickel hydride phase in the nickel matrix can be identified by the difference in contrast by optical microscope observation. That is to say that, the hydride phase is observed as a dark region in the matrix. The ratio of the observed dark regions, compared to the light regions, corelated with the ratio of the integrated intensity of the X-ray diffraction profile of hydride to nickel during the decomposition process of hydride formed by electrochemical cathode charging of hydrogen in pure nickel. Such agreement was also confirmed for the hydride distribution below the surface of the sample. The identification of nickel hydride in the present work concludes that the depth of hydride formation is different between crystal grains.
1. Introduction Nickel has been considered as a hydrogen storage material, in a method by which hydrogen is stored as nickel hydride. Many studies as to the efficacy of nickel hydride as a storage material have been conducted [1]. Active research has also been carried out as to a role of hydride in the hydrogen embrittlement of nickel and nickel alloys [2]. It is well known that nickel transforms into nickel hydride (β phase) when the ratio of hydrogen to nickel is over approximately 0.7 [3]. The lattice constant increases by about 6% in hydride [4]. The internal stress induced by its lattice expansion is considered to cause cracking when nickel hydride decomposes [5,6]. Hydride has been measured mainly by X-ray diffraction in those studies. It could also be detected using electrochemical potential or electrical resistance. These measurements give only the presence/absence of hydride in a sample or a ratio of hydride to the matrix. Neither 2D distribution of hydride on the surface and its 3D map in a specimen is given. Electron Back Scattering Diffraction (EBSD) spectroscopy is a useful tool to measure 2D distribution of microstructure on the surface. However, nickel hydride cannot be measured using EBSD, because nickel hydride decomposes in the required vacuum. Boniszewski et al. reported that the reflectivity of the hydride is lower than that of nickel using optical microscope observation[7]. A similar observation was also presented by Hagi et al. [8]. These studies as to difference in reflectivity of nickel, however, have been hardly reported. We also found
that dark and light regions exist on the surface after hydrogen charge using an optical microscope. In this present paper, it is reported that the dark regions correspond to nickel hydride formations. 2. Experimental procedure A basal plane of a cylindrical specimen (diameter: 9 mm, thickness: 6 mm) of pure Ni (99.99% pure) was polished by alumina powder (grain size: 0.05 μm). Hydrogen was charged electrochemically to the plane, which was a cathode electrode; in 0.05 mmol/m3 sulfuric acid added 1.4 kg/m3 thiourea during 16 h. A platinum plate was used for an anode electrode. The cathode current density was 100 A/m2. The surface was observed using an optical microscope (Olympus, PME3) and analyzed using X-ray diffraction (Co-Kα) (Rigaku, RINT 2200) in atmosphere. The observation and analysis were carried out repeatedly during the 80 h period after the hydrogen was charged. The specimen was left at room temperature in the atmosphere during this procedure. 3. Results Fig. 1 shows X-ray diffraction profiles of the specimen after hydrogen charge. There are 111 and 200 diffraction lines of nickel and nickel hydride. The intensity of diffraction lines of nickel hydride reduces with time in atmosphere after hydrogen charge. It shows that the nickel
* Corresponding author. E-mail addresses: [email protected] (N. Takano), [email protected] (H. Yamamoto). https://doi.org/10.1016/j.rinma.2020.100066 Received in revised form 12 December 2019; Accepted 28 December 2019 Available online 23 January 2020 2590-048X/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066
estimated using the least square method, is considered to be proportional to the area of each region. The integral of each term in eq. (1) is assumed to give the total number of pixels over each region. The ratio of the integral in the dark regions to one of the light regions, is defined by R ad fd ðxÞdx Id=1 ¼ R al fl ðxÞdx
(2)
is shown in Fig. 2 (solid circles). Every plot in this figure is the average of the ratios taken from 8 images and the error bar indicates their standard uncertainty. The 8 images immediately after charge are shown in Fig. 7 in the Appendix. Each image size is 2048 1563 pixels, its real dimensions are about 0.33 0.25 mm. Change of the ratio of the dark regions to light regions is the same as the ratio of the integrated intensity of X-ray diffraction profile of nickel hydride to one of nickel. 4. Discussion Fig. 2 indicates that the dark regions in optical microscope images correspond to nickel hydride formations. Two reasons for the dark appearance are considered. One is the surface roughness; the apparent brightness of an optical microscope image may be caused by the roughness of a surface of the specimen, due to the lattice constant of the hydride phase being about 6% larger than the constant of the nickel phase. Plastic deformation due to hydride formation on the surface has been reported by Hagi et al. [8] and Kitagawa et al. [5]. The roughness is also likely seen in the images in Fig. 3. Hydride decomposed with time in atmosphere, as shown in Fig. 1. The dark region gradually vanished over time as shown in Fig. 3, that is to say that the change of the brightness is correlated to the decomposition of the hydride phase. If the darkness was caused by plastic deformation of about 6%, it probably would not disappear even after hydride decomposition. Afterwards, a surface observation was carried out following removal of the surface layer, which has a thickness of a few micrometers, by repeated polishing using alumina powder. X-ray diffraction was also carried out multiple times during the process. The specimen was cooled frequently using liquid nitrogen during this procedure to prevent the diffusion and release of hydrogen. Dark and light regions also existed on the images of the section at a few micrometers beneath the surface, as shown in Fig. 5. To estimate the distance from the surface, an indentation was made using a Vickers hardness testing machine, and was estimated by a ratio of indentation width at the original surface and the grinded surface. It is considered that there is hardly roughness in them because they were observed immediately after polishing by aluminum powder. A little roughness, however, might be observed on the images as residual strain is released, induced by hydride formation after polishing. Fig. 6 shows the change of the ratio of the dark region to light regions, and the ratio of the integrated intensity of X-ray diffraction lines of nickel hydride to nickel, with the distance from the surface. The ratio was estimated using 5 images, and the error bar indicates their standard uncertainty. It once again indicates that the dark regions of optical microscope images correspond to nickel hydride formations. These observations below the surface of the specimen indicates that the reason for the darkness is not necessarily due to the surface roughness. Another reason for the dark regions is a change of the optical properties of the material by hydride transformation. The reflectivity of nickel may decrease due to the transformation to hydride. Metal glossiness is caused by high reflectivity. The reflectivity is related to dielectric dispersion and electron transition from a state below the Fermi energy to an empty state above it. The dielectric dispersion depends on the electron state, especially near Fermi energy. Nickel is ferromagnetic at room temperature, and its Fermi energy is located in d-bands. It is known that hydrogen lowers the energy of the d-bands to less than the Fermi energy in nickel so that nickel hydride becomes nonferromagnetic [9–11]. Its electron band structure implies that visible light can induce the electrons in d-bands below the Fermi energy to s-p bands above it so that the
Fig. 1. X-ray diffraction profiles of the specimen after hydrogen charge. Each specimen was left in atmosphere at room temperature during each time.
hydride formed by hydrogen charge was decomposing at room temperature in atmosphere. The ratio of the integrated intensity of nickel hydride profile to one of nickel profile is shown for 111 (solid line) and 200 (dashed line) in Fig. 2. Fig. 3 shows optical microscope images, which were expressed using 256 graduations in grayscale, of an area of the specimen’s surface with time after hydrogen charge. There are dark and light regions on the images. The dark regions decrease with time. That is to say that the change in darkness was reversible. Fig. 4 shows typical histograms of the contrast for the whole area, dark and light regions in Fig. 3 (b). They are expressed by G(x), fd(x), and fl(x) respectively, where x is from 0 to 256 graduation in grayscale. These functions are treated as a continuous function of x, while they are discrete in the original. Moreover, fd(x), and fl(x) are approximated by Gaussian function. The histogram, G(x), of a whole area is assumed to be equal to a linear combination of histograms of the dark, fd(x), and light region, fl(x), as follows; G(x) ¼ ad fd(x) þ al fl(x)
(1)
where ad and al are coefficients for the dark and light region. Functions fd(x), and fl(x) were estimated for every image because contrast was not the same in all photos. Each coefficient of linear combination, which is
Fig. 2. Ratio of the integrated intensity of nickel hydride profile to the nickel profile for each 111 (solid line) and 200 (dashed line) X-ray diffraction line, and ratio of the integral of the dark regions to the light regions on the optical microscope photo defined by Eq. (2) (solid circle) for elapsed time after hydrogen charge. The error bar indicates the standard uncertainty for 8 images. 2
N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066
Fig. 3. Optical microscope images of the specimen’s surface left in atmosphere after hydrogen charge.
reflectivity reduces in nickel hydride. Reduction of brightness by hydride transformation is probably caused by the light absorption due to the electron transition near the Fermi energy. Returning to the images in Fig. 5, it is seen that the surface immediately after hydrogen charge was covered almost completely by the hydride dark regions. The dark region reduced inside the specimen, but it was observed even relatively far beneath the surface. The depth of hydride formation most likely depends on the crystallographic orientation of the grain. That is, the rate of hydride formation is anisotropic. Some of the below reasons are considered. One is that the anisotropy of hydrogen absorption at the surface. Another is anisotropy of hydrogen diffusion [12]. Hydrogen diffusion is isotropic generally. This assumption is correct in low concentration of hydrogen. However, nickel hydride restricts hydrogen diffusion [13]. Further investigation of the relationship between crystallographic orientation and hydride formation is needed. 5. Conclusion A method for identification of nickel hydride formations using an optical microscope is proposed in the present work. It is proposed that the reflectivity of nickel hydride is lower than that of nickel matrix so that
Fig. 4. Typical histogram of gray scale contrast of an image of the specimen’s surface.
3
N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066
Fig. 5. Optical microscope images of the section of the specimen at each distance under the surface after hydrogen charge over 16 h.
hydride is observed as dark in an optical microscope compared with the surrounding nickel matrix. The conclusion drawn from optical observation is that hydride formation occurs anisotropic in the electrochemical cathodic hydrogen charging. The identification of nickel hydride is useful in the study of hydrogen embrittlement in nickel and nickel alloys. Contributions authors statement Noriyuki Takano: Conceptualization, Methodology, Software, Supervision, Writing - Original Draft, Hiroki Yamamoto: Validation, Investigation.
Appendix Optical microscope images of 8 areas used to estimate the ratio in Fig. 2 are shown in Fig. 7. Fig. 6. Ratio of the integrated intensity of nickel hydride profile to one of nickel profile for each 111 (solid line) and 200 (dashed line) X-ray diffraction line, and ratio of the integral in the dark region to one of the light region on the optical microscope image defined by Eq. (2) (solid circle) at the distance from the surface. The error bar indicates the standard uncertainty for 5 images. 4
N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066
Fig. 7. Optical microscope images of 8 areas used to estimate the ratio in Fig. 2. [4] A. Janko, J. Pielaszek, Lattice spacing determination for the a and b phases of nickel –Hydrogen and nickel– deuterium systems, Bull. Acad. Pol. Sci. Ser. Sci. Chim. 15 (1967) 569–572. [5] S. Kitagawa, Mechanism of surface crack formation by hydrogen in pure NIckel, Proc. JIMIS-2, Hydrog. Met. (1980) 497–500. [6] N. Takano, S. Kaida, Crack initiation by cathodic hydrogen charging in nickel single crystal, ISIJ Int. 52 (2012) 263–266, https://doi.org/10.2355/ isijinternational.52.263.
References [1] S.M.F. Baranowski B, 45 Years of nickel hydride, Pol. J. Chem. 79 (2005) 789–806. [2] S. Hinotani, Y. Ohmori, F. Terasaki, Effect of nickel on hydride formation and hydrogen embrittlement in NiCrFe alloys, Mater. Sci. Eng. 74 (1985) 119, https:// doi.org/10.1016/0025-5416(85)90425-2. [3] B. Baranowski, M. Smialowski, Penetration depth of cathodically evolved hydrogen into nickel films, Bull. Acad. Pol. Sci. Ser. Sci. Chim. 7 (1959) 206–207.
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N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066 [11] S. Kanagaprabha, A. Meenaatci, R. Rajeswarapalanichamy, K. Iyakutti, First principles study of the electronic structure, structural properties and superconductivity of nickel hydride, Walailak J. Sci. Technol. 9 (2012) 115–126, https://doi.org/10.2004/wjst.v9i2.231. [12] E. Torres, J. Pencer, D.D. Radford, Atomistic simulation study of the hydrogen diffusion in nickel, Comput. Mater. Sci. 152 (2018) 374–380. [13] Z. Szklarska-Smialowska, M. Smialowski, The kinetics of hydrogen desorption from plastically defromed nickel, Bull. Acad. Pol. Sci. Ser. Sci. Chim. Geol. Geogr. 6 (1958) 187–190.
[7] T. Boniszewski, G.C. Smith, A note on nickel hydride, J. Phys. Chem. Solid. 21 (1961) 115–118, https://doi.org/10.1016/0022-3697(61)90219-0. [8] H. Hagi, S. Asano, R. Otsuka, Hydrogen pick-up and hydride formation in nickel by cathodic polarization, Nippon Kinzoku Gakkaishi/J. Japan Inst. Met. 40 (1976) 796–801, https://doi.org/10.2320/jinstmet1952.40.8_796. [9] M. Gupta, J.P. Burger, Trends in the electron-phonon coupling parameter in some metallic hydrides, Phys. Rev. B 24 (1981) 7099–7111, https://doi.org/10.1103/ PhysRevB.24.7099. [10] P. Vargas, M.P.I. Metaliforschung, Band-structure calculations, Phys. Rev. B 35 (1987) 1993–2004, https://doi.org/10.1103/PhysRevB.35.1993.
6
Contents lists available at ScienceDirect
Results in Materials journal homepage: www.journals.elsevier.com/results-in-materials
Identification of nickel hydride phase in nickel matrix by optical microscope observation Noriyuki Takano *, Hiroki Yamamoto Kanazawa Institute of Technology, Department of Mechanical Engineering, 3-1 Yatsukaho, Hakusan, Ishikawa, 924-0838, Japan
A R T I C L E I N F O
A B S T R A C T
Keywords: Nickel hydride Reflectivity Anisotropy Hydride decomposition Hydrogen embrittlement
It is well known that nickel transforms into nickel hydride under high hydrogen fugacity. However, distribution of the hydride formation is not necessarily uniform. It is proposed that the nickel hydride phase in the nickel matrix can be identified by the difference in contrast by optical microscope observation. That is to say that, the hydride phase is observed as a dark region in the matrix. The ratio of the observed dark regions, compared to the light regions, corelated with the ratio of the integrated intensity of the X-ray diffraction profile of hydride to nickel during the decomposition process of hydride formed by electrochemical cathode charging of hydrogen in pure nickel. Such agreement was also confirmed for the hydride distribution below the surface of the sample. The identification of nickel hydride in the present work concludes that the depth of hydride formation is different between crystal grains.
1. Introduction Nickel has been considered as a hydrogen storage material, in a method by which hydrogen is stored as nickel hydride. Many studies as to the efficacy of nickel hydride as a storage material have been conducted [1]. Active research has also been carried out as to a role of hydride in the hydrogen embrittlement of nickel and nickel alloys [2]. It is well known that nickel transforms into nickel hydride (β phase) when the ratio of hydrogen to nickel is over approximately 0.7 [3]. The lattice constant increases by about 6% in hydride [4]. The internal stress induced by its lattice expansion is considered to cause cracking when nickel hydride decomposes [5,6]. Hydride has been measured mainly by X-ray diffraction in those studies. It could also be detected using electrochemical potential or electrical resistance. These measurements give only the presence/absence of hydride in a sample or a ratio of hydride to the matrix. Neither 2D distribution of hydride on the surface and its 3D map in a specimen is given. Electron Back Scattering Diffraction (EBSD) spectroscopy is a useful tool to measure 2D distribution of microstructure on the surface. However, nickel hydride cannot be measured using EBSD, because nickel hydride decomposes in the required vacuum. Boniszewski et al. reported that the reflectivity of the hydride is lower than that of nickel using optical microscope observation[7]. A similar observation was also presented by Hagi et al. [8]. These studies as to difference in reflectivity of nickel, however, have been hardly reported. We also found
that dark and light regions exist on the surface after hydrogen charge using an optical microscope. In this present paper, it is reported that the dark regions correspond to nickel hydride formations. 2. Experimental procedure A basal plane of a cylindrical specimen (diameter: 9 mm, thickness: 6 mm) of pure Ni (99.99% pure) was polished by alumina powder (grain size: 0.05 μm). Hydrogen was charged electrochemically to the plane, which was a cathode electrode; in 0.05 mmol/m3 sulfuric acid added 1.4 kg/m3 thiourea during 16 h. A platinum plate was used for an anode electrode. The cathode current density was 100 A/m2. The surface was observed using an optical microscope (Olympus, PME3) and analyzed using X-ray diffraction (Co-Kα) (Rigaku, RINT 2200) in atmosphere. The observation and analysis were carried out repeatedly during the 80 h period after the hydrogen was charged. The specimen was left at room temperature in the atmosphere during this procedure. 3. Results Fig. 1 shows X-ray diffraction profiles of the specimen after hydrogen charge. There are 111 and 200 diffraction lines of nickel and nickel hydride. The intensity of diffraction lines of nickel hydride reduces with time in atmosphere after hydrogen charge. It shows that the nickel
* Corresponding author. E-mail addresses: [email protected] (N. Takano), [email protected] (H. Yamamoto). https://doi.org/10.1016/j.rinma.2020.100066 Received in revised form 12 December 2019; Accepted 28 December 2019 Available online 23 January 2020 2590-048X/© 2020 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).
N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066
estimated using the least square method, is considered to be proportional to the area of each region. The integral of each term in eq. (1) is assumed to give the total number of pixels over each region. The ratio of the integral in the dark regions to one of the light regions, is defined by R ad fd ðxÞdx Id=1 ¼ R al fl ðxÞdx
(2)
is shown in Fig. 2 (solid circles). Every plot in this figure is the average of the ratios taken from 8 images and the error bar indicates their standard uncertainty. The 8 images immediately after charge are shown in Fig. 7 in the Appendix. Each image size is 2048 1563 pixels, its real dimensions are about 0.33 0.25 mm. Change of the ratio of the dark regions to light regions is the same as the ratio of the integrated intensity of X-ray diffraction profile of nickel hydride to one of nickel. 4. Discussion Fig. 2 indicates that the dark regions in optical microscope images correspond to nickel hydride formations. Two reasons for the dark appearance are considered. One is the surface roughness; the apparent brightness of an optical microscope image may be caused by the roughness of a surface of the specimen, due to the lattice constant of the hydride phase being about 6% larger than the constant of the nickel phase. Plastic deformation due to hydride formation on the surface has been reported by Hagi et al. [8] and Kitagawa et al. [5]. The roughness is also likely seen in the images in Fig. 3. Hydride decomposed with time in atmosphere, as shown in Fig. 1. The dark region gradually vanished over time as shown in Fig. 3, that is to say that the change of the brightness is correlated to the decomposition of the hydride phase. If the darkness was caused by plastic deformation of about 6%, it probably would not disappear even after hydride decomposition. Afterwards, a surface observation was carried out following removal of the surface layer, which has a thickness of a few micrometers, by repeated polishing using alumina powder. X-ray diffraction was also carried out multiple times during the process. The specimen was cooled frequently using liquid nitrogen during this procedure to prevent the diffusion and release of hydrogen. Dark and light regions also existed on the images of the section at a few micrometers beneath the surface, as shown in Fig. 5. To estimate the distance from the surface, an indentation was made using a Vickers hardness testing machine, and was estimated by a ratio of indentation width at the original surface and the grinded surface. It is considered that there is hardly roughness in them because they were observed immediately after polishing by aluminum powder. A little roughness, however, might be observed on the images as residual strain is released, induced by hydride formation after polishing. Fig. 6 shows the change of the ratio of the dark region to light regions, and the ratio of the integrated intensity of X-ray diffraction lines of nickel hydride to nickel, with the distance from the surface. The ratio was estimated using 5 images, and the error bar indicates their standard uncertainty. It once again indicates that the dark regions of optical microscope images correspond to nickel hydride formations. These observations below the surface of the specimen indicates that the reason for the darkness is not necessarily due to the surface roughness. Another reason for the dark regions is a change of the optical properties of the material by hydride transformation. The reflectivity of nickel may decrease due to the transformation to hydride. Metal glossiness is caused by high reflectivity. The reflectivity is related to dielectric dispersion and electron transition from a state below the Fermi energy to an empty state above it. The dielectric dispersion depends on the electron state, especially near Fermi energy. Nickel is ferromagnetic at room temperature, and its Fermi energy is located in d-bands. It is known that hydrogen lowers the energy of the d-bands to less than the Fermi energy in nickel so that nickel hydride becomes nonferromagnetic [9–11]. Its electron band structure implies that visible light can induce the electrons in d-bands below the Fermi energy to s-p bands above it so that the
Fig. 1. X-ray diffraction profiles of the specimen after hydrogen charge. Each specimen was left in atmosphere at room temperature during each time.
hydride formed by hydrogen charge was decomposing at room temperature in atmosphere. The ratio of the integrated intensity of nickel hydride profile to one of nickel profile is shown for 111 (solid line) and 200 (dashed line) in Fig. 2. Fig. 3 shows optical microscope images, which were expressed using 256 graduations in grayscale, of an area of the specimen’s surface with time after hydrogen charge. There are dark and light regions on the images. The dark regions decrease with time. That is to say that the change in darkness was reversible. Fig. 4 shows typical histograms of the contrast for the whole area, dark and light regions in Fig. 3 (b). They are expressed by G(x), fd(x), and fl(x) respectively, where x is from 0 to 256 graduation in grayscale. These functions are treated as a continuous function of x, while they are discrete in the original. Moreover, fd(x), and fl(x) are approximated by Gaussian function. The histogram, G(x), of a whole area is assumed to be equal to a linear combination of histograms of the dark, fd(x), and light region, fl(x), as follows; G(x) ¼ ad fd(x) þ al fl(x)
(1)
where ad and al are coefficients for the dark and light region. Functions fd(x), and fl(x) were estimated for every image because contrast was not the same in all photos. Each coefficient of linear combination, which is
Fig. 2. Ratio of the integrated intensity of nickel hydride profile to the nickel profile for each 111 (solid line) and 200 (dashed line) X-ray diffraction line, and ratio of the integral of the dark regions to the light regions on the optical microscope photo defined by Eq. (2) (solid circle) for elapsed time after hydrogen charge. The error bar indicates the standard uncertainty for 8 images. 2
N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066
Fig. 3. Optical microscope images of the specimen’s surface left in atmosphere after hydrogen charge.
reflectivity reduces in nickel hydride. Reduction of brightness by hydride transformation is probably caused by the light absorption due to the electron transition near the Fermi energy. Returning to the images in Fig. 5, it is seen that the surface immediately after hydrogen charge was covered almost completely by the hydride dark regions. The dark region reduced inside the specimen, but it was observed even relatively far beneath the surface. The depth of hydride formation most likely depends on the crystallographic orientation of the grain. That is, the rate of hydride formation is anisotropic. Some of the below reasons are considered. One is that the anisotropy of hydrogen absorption at the surface. Another is anisotropy of hydrogen diffusion [12]. Hydrogen diffusion is isotropic generally. This assumption is correct in low concentration of hydrogen. However, nickel hydride restricts hydrogen diffusion [13]. Further investigation of the relationship between crystallographic orientation and hydride formation is needed. 5. Conclusion A method for identification of nickel hydride formations using an optical microscope is proposed in the present work. It is proposed that the reflectivity of nickel hydride is lower than that of nickel matrix so that
Fig. 4. Typical histogram of gray scale contrast of an image of the specimen’s surface.
3
N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066
Fig. 5. Optical microscope images of the section of the specimen at each distance under the surface after hydrogen charge over 16 h.
hydride is observed as dark in an optical microscope compared with the surrounding nickel matrix. The conclusion drawn from optical observation is that hydride formation occurs anisotropic in the electrochemical cathodic hydrogen charging. The identification of nickel hydride is useful in the study of hydrogen embrittlement in nickel and nickel alloys. Contributions authors statement Noriyuki Takano: Conceptualization, Methodology, Software, Supervision, Writing - Original Draft, Hiroki Yamamoto: Validation, Investigation.
Appendix Optical microscope images of 8 areas used to estimate the ratio in Fig. 2 are shown in Fig. 7. Fig. 6. Ratio of the integrated intensity of nickel hydride profile to one of nickel profile for each 111 (solid line) and 200 (dashed line) X-ray diffraction line, and ratio of the integral in the dark region to one of the light region on the optical microscope image defined by Eq. (2) (solid circle) at the distance from the surface. The error bar indicates the standard uncertainty for 5 images. 4
N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066
Fig. 7. Optical microscope images of 8 areas used to estimate the ratio in Fig. 2. [4] A. Janko, J. Pielaszek, Lattice spacing determination for the a and b phases of nickel –Hydrogen and nickel– deuterium systems, Bull. Acad. Pol. Sci. Ser. Sci. Chim. 15 (1967) 569–572. [5] S. Kitagawa, Mechanism of surface crack formation by hydrogen in pure NIckel, Proc. JIMIS-2, Hydrog. Met. (1980) 497–500. [6] N. Takano, S. Kaida, Crack initiation by cathodic hydrogen charging in nickel single crystal, ISIJ Int. 52 (2012) 263–266, https://doi.org/10.2355/ isijinternational.52.263.
References [1] S.M.F. Baranowski B, 45 Years of nickel hydride, Pol. J. Chem. 79 (2005) 789–806. [2] S. Hinotani, Y. Ohmori, F. Terasaki, Effect of nickel on hydride formation and hydrogen embrittlement in NiCrFe alloys, Mater. Sci. Eng. 74 (1985) 119, https:// doi.org/10.1016/0025-5416(85)90425-2. [3] B. Baranowski, M. Smialowski, Penetration depth of cathodically evolved hydrogen into nickel films, Bull. Acad. Pol. Sci. Ser. Sci. Chim. 7 (1959) 206–207.
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N. Takano, H. Yamamoto
Results in Materials 5 (2020) 100066 [11] S. Kanagaprabha, A. Meenaatci, R. Rajeswarapalanichamy, K. Iyakutti, First principles study of the electronic structure, structural properties and superconductivity of nickel hydride, Walailak J. Sci. Technol. 9 (2012) 115–126, https://doi.org/10.2004/wjst.v9i2.231. [12] E. Torres, J. Pencer, D.D. Radford, Atomistic simulation study of the hydrogen diffusion in nickel, Comput. Mater. Sci. 152 (2018) 374–380. [13] Z. Szklarska-Smialowska, M. Smialowski, The kinetics of hydrogen desorption from plastically defromed nickel, Bull. Acad. Pol. Sci. Ser. Sci. Chim. Geol. Geogr. 6 (1958) 187–190.
[7] T. Boniszewski, G.C. Smith, A note on nickel hydride, J. Phys. Chem. Solid. 21 (1961) 115–118, https://doi.org/10.1016/0022-3697(61)90219-0. [8] H. Hagi, S. Asano, R. Otsuka, Hydrogen pick-up and hydride formation in nickel by cathodic polarization, Nippon Kinzoku Gakkaishi/J. Japan Inst. Met. 40 (1976) 796–801, https://doi.org/10.2320/jinstmet1952.40.8_796. [9] M. Gupta, J.P. Burger, Trends in the electron-phonon coupling parameter in some metallic hydrides, Phys. Rev. B 24 (1981) 7099–7111, https://doi.org/10.1103/ PhysRevB.24.7099. [10] P. Vargas, M.P.I. Metaliforschung, Band-structure calculations, Phys. Rev. B 35 (1987) 1993–2004, https://doi.org/10.1103/PhysRevB.35.1993.
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