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Formation of Mg2Ni alloy layers and kinetic studies in the binary Mg–Ni system
Thin Solid Films 517 (2009) 4745–4748
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Formation of Mg2Ni alloy layers and kinetic studies in the binary Mg–Ni system S.L. Cheng a,b,⁎, Y.Y. Chen b, S.W. Lee b, H.F. Hsu c a b c
Department of Chemical and Materials Engineering, National Central University, Chung-Li City, Taoyuan, Taiwan, ROC Institute of Materials Science and Engineering, National Central University, Chung-Li City, Taoyuan, Taiwan, ROC Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan, ROC
a r t i c l e
i n f o
Available online 13 March 2009 Keywords: Mg2Ni alloy Solid-state reaction Dominant diffusing species Diffusivity
a b s t r a c t We report here the study on the kinetics and growth of Mg2Ni alloy layers by solid-state reactions in the Mg–Ni system. After appropriate heat treatments, only polycrystalline Mg2Ni phase was found to form on the Mg/Ni interfaces and the grain sizes of Mg2Ni formed near the Mg side were found to be much smaller than those formed near the Ni side. Cross-sectional SEM examinations revealed that the Mg2Ni intermetallic layers grew following the diffusion-control process. During the formation of Mg2Ni layers, the dominant diffusing species was determined to be Ni by using the immobile trench markers. Furthermore, based on the marker analysis, the intrinsic and interdiffusion diffusivities for the Mg–Ni diffusion couples at different temperatures have been determined. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, magnesium-based alloys have been the subject of great interests because of their potential applications in energy storage media and switchable mirrors [1–3]. Among these Mg-based metal hydrides, Mg2Ni alloy is perhaps the most promising candidate for the hydrogen storage material due to its high specific hydrogen storage capacity, light weight, and low environmental impact [4–7]. To fabricate Mg2Ni alloy materials, various fabrication techniques such as mechanical milling, combustion synthesis, and melt casting have been developed [8–11]. However, these conventional methods are limited by the high operating temperature and the serial processing format. In addition, due to the high oxidation tendency of Mg, it is difficult to obtain pure Mg2Ni compound by the above-mentioned techniques. Previously, it has been demonstrated that a variety of pure metal compounds were successfully fabricated by the solid-state reactions between two different pure metallic materials [12–14]. Although various pure intermetallic compounds were produced by the solid-state reaction methods, the studies on the microstructures and kinetics of the pure Mg2Ni alloy layer formation in the Mg–Ni system are relatively rare [15]. Since the kinetic data can provide crucial insights to the fundamental understanding of the formation process of Mg2Ni intermetallic compounds, it is of much importance to determine the kinetic parameters such as diffusivities and dominant diffusing species in the Mg–Ni reaction couples under different experimental conditions. Diffusion-marker experiments have been widely utilized to determine the kinetic data in solid-state reactions involving two elements [16–18]. However, the marker drag and the barrier effects occurring during the
⁎ Corresponding author. Department of Chemical and Materials Engineering, National Central University, Chung-Li City, Taoyuan, Taiwan, ROC. Tel.: +886 3 4227151x34233; fax: +886 3 2804510. E-mail address: [email protected] (S.L. Cheng). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.011
growth of intermetallic compounds usually resulted in ambiguous results. Therefore, for the investigation of the kinetics of Mg2Ni intermetallic layers formation, the route for creating an immobile marker at the original interface of the Mg–Ni reaction couple is demanded to avoid the abovementioned problems. In the present study, results from a systematic investigation on the interfacial reactions between pure Mg and Ni metal laminas after different heat treatments and the kinetics of the Mg2Ni layers formation in the Mg–Ni reaction couples are reported. 2. Experimental procedures Commercial pure Mg and Ni metal laminas were used as the starting materials in this study. Both the pure Mg and Ni laminas were firstly cut into small pieces and then cleaned chemically by dipping in a 5% Nital solution (5 vol.% HNO3 in 95 vol.% C2H5OH) to remove the surface contaminants and native oxide layers and then rinsed in absolute ethanol and blew dried with N2 gas. For the marker experiments, shallow trenches were created on the smooth surfaces of the soft Mg laminas by using a sharp knife and served as the immobile markers. After the cleaning and marking processes, the cleaned Mg and Ni metal laminas were pressed together and held in contact with each other in a custom-built stainless steel jig. The jig with the Mg–Ni couple was then placed in a three-zone diffusion furnace and annealed isothermally at 480 °C and 510 °C for various time periods in high-purity N2 ambient. After the isothermal annealing, the cross-sectional samples of the Mg– Ni diffusion couples were prepared by the metallographic polishing technique. The thicknesses and morphologies of the produced Mg–Ni intermetallic layers were examined by using backscattered electron (BSE) scanning electron microscopy (SEM). X-ray diffractometry (XRD) (Cu Kα radiation, λ = 0.154 nm), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED) analysis were carried out
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S.L. Cheng et al. / Thin Solid Films 517 (2009) 4745–4748
Fig. 2. XRD diffraction spectra of the intermetallic compound layers formed from the 510 °C annealed Mg–Ni samples: (a)–(b) near the Ni side; and (c)–(d) near the Mg side. The annealing time: (a),(c) 1 h; and (b),(d) 4 h.
Fig. 1. (a) A cross-sectional BSE SEM image of an Mg–Ni reaction couple with a trench marker after annealing at 510 °C for 4 h. (b) A representative SEM image of an Mg–Ni intermetallic layer and the corresponding EDS spectra acquired from the regions at: (i) the top; (ii) the middle; and (iii) the bottom of the intermetallic layer.
Mg2Ni phase were detected in these peeled samples, indicating that the intermetallic compound layers were pure Mg2Ni. The microstructures of the intermetallic layers were investigated in more detail by planview TEM and SAED analysis. Figs. 3 (a) and (d) show the bright-field TEM images taken from the regions of the intermetallic layers formed near the Mg side and the Ni side, respectively. Figs. 3 (c) and (d) show the corresponding indexed SAED patterns. Based on the analysis of SAED patterns, it further confirmed that only hexagonal Mg2Ni phase was found to form at the interface between Mg and Ni and the crystal structures of the produced Mg2Ni layers were polycrystalline. Furthermore, from the TEM observation, the grain sizes of Mg2Ni formed near the Mg side (~5–10 nm) were found to be much smaller than those formed near the Ni side (~150– 250 nm), as shown in Figs. 3 (a)–(b). The results suggested that the Mg2Ni grains near the Ni side might be formed early compared to those near the Mg side and then the grain growth of Mg2Ni grains near the Ni side would be induced by the longer holding time during the isothermal annealing processes.
for phase identification and microstructure examination. A Link ISIS energy dispersion spectrometer (EDS) attached to the SEM was utilized to determine the chemical composition of local areas in the intermetallic layers. 3. Results and discussion From cross-sectional BSE SEM observations, continuous intermetallic compound layers were found to form in the Mg–Ni diffusion couples after annealing at different temperatures and time. During the formation of Mg–Ni intermetallic layers, the trench markers were always observed to locate on the original Mg/Ni interface. A representative BSE SEM image is shown in Fig. 1 (a). The result demonstrated that the shallow trenches were suitable to serve as the immobile markers in this experiment. After the formation of Mg–Ni alloy layers, EDS analysis was carried out to investigate the distribution of Mg and Ni atoms. An example is shown in Fig. 1 (b). Analysis of the spectra obtained by EDS indicated that the intermetallic layer is composed mainly of Mg and Ni. In addition, the quantitative EDS analysis gives the average value of the ratio of the atomic concentrations of Mg and Ni as about 2 which suggests that the intermetallic layer formed is Mg2Ni. For the examination of the phase formation in the reaction zones between Mg and Ni, the annealed samples were peeled into two half parts by mechanical means. The peeled surfaces were then analyzed by XRD. Figs. 2 (a)–(d) show the typical XRD spectra of the intermetallic layers formed near the Ni side and the Mg side, respectively. In Fig. 2, it is clearly revealed that only diffraction peaks corresponding to the hexagonal
Fig. 3. Bright-field TEM images of the intermetallic compound layers formed (a) near the Mg side and (b) near the Ni side for the Mg–Ni samples after annealing at 510 °C for 3 h. (c) and (d) show the corresponding indexed SAED patterns for (a) and (b), respectively.
S.L. Cheng et al. / Thin Solid Films 517 (2009) 4745–4748
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Table 1 Interdiffusion coefficients and intrinsic diffusion coefficients of Mg and Ni in Mg2Ni at 480 °C and 510 °C. Interdiffusion coefficient (cm2/s), D̃ 480 °C 510 °C
Fig. 4. (a) The thickness of the Mg2Ni layer as a function of annealing temperature. (b) The thickness of the Mg2Ni layer versus the square root of annealing time curves for the Mg–Ni samples annealed at different temperatures.
After a series of cross-sectional SEM examinations, the thicknesses of the Mg2Ni layers versus reaction time data for the Mg–Ni diffusion couples annealed at 480 °C and 510 °C were obtained, as shown in Fig. 4 (a). In Fig. 4 (a), it is revealed that the thicknesses of Mg2Ni layers increased parabolically with time. In addition, the growth rate of the Mg2Ni intermetallic layers was observed to increase with the annealing temperature. Fig. 4 (b) shows the plot of the square of the Mg2Ni layer thickness versus the annealing time for temperatures at 480 °C and 510 °C. The relation curves shown in Fig. 4 (b) are almost linear. The result clearly indicates that the growth of the Mg2Ni alloy layer is diffusion-controlled. Figs. 5 (a) and (b) show the representative cross-sectional BSE SEM images of the Mg–Ni reaction couples after annealing at 480 °C for 6 h and at 510 °C for 2 h, respectively. The original interfaces are
− 11
4.1 × 10 1.0 × 10− 10
Intrinsic diffusion coefficient (cm2/s) Mg in Mg2Ni, DMg − 12
7.5 × 10 1.9 × 10− 11
Ni in Mg2Ni, DNi 5.8 × 10− 11 1.5 × 10− 10
evident in these annealed Mg–Ni samples. As can be seen in Figs. 5 (a)–(b), the thicknesses of the Mg2Ni layers formed on the Mg side (i.e., X2 in Fig. 5) were measured to be much thicker than those formed on the Ni side (i.e., X1 in Fig. 5). The Mg2Ni compound thickness on the Ni side versus the Mg2Ni compound thickness on the Mg side data (i.e., X1 versus X2) in the Mg–Ni sample after different heat treatments is shown in Fig. 5 (c). It is known that for a singlelayer compound formation in an A–B diffusion couple, if more A diffuses into the B with the marker position fixed, the thickness of the A–B intermetallic layer on the B side would be thicker than that on the A side. Therefore, from Fig. 5, it can be concluded that Ni is the dominant diffusing species during the Mg2Ni intermetallic layer formation. The mechanism for the high mobility of Ni atoms during the Mg2Ni compound formation has been proposed by Hong et al. [15]. Their study pointed out that the faster diffusion process of Ni could be attributed to the existence of unique Ni-atom chain structures in the Mg2Ni compound. The Ni atoms can easily diffuse along the continuous chains of Ni atoms during the solid-state reaction in the Mg–Ni diffusion couple. On the other hand, to obtain the diffusivities from the marker experiments, the basic kinetic equations were adopted. In this study, only a single intermetallic compound Mg2Ni was found to form between Mg and Ni during annealing and the growth of the Mg2Ni layer was diffusion-controlled. Thus, the interdiffusion coefficient D̃˜ of the Mg2Ni layer can be simply expressed by: 2 ˜ X = 4 Dt
ð1Þ
where X is the thickness of the Mg2Ni layer, and t is the annealing time. The intrinsic diffusion coefficients of Mg (DMg) and Ni (DNi) in the Mg2Ni layer can be expressed by: DNi = ½2ðX2 = X1 ÞDMg
ð2Þ
where X1 and X2 are the thicknesses of the Mg2Ni layers formed on the Ni side and on the Mg side, respectively, as defined in Fig. 5. In addition, for a single-layer Mg2Ni intermetallic compound formation, the interdiffusion coefficient D̃ can also be expressed as: D˜ = NNi DMg + NMg DNi
ð3Þ
where NNi and NMg are the fractions of Ni and Mg atoms in the unit volume, respectively. Based on these kinetic equations and the results of our marker experiments, the interdiffusion and intrinsic diffusion coefficients for the binary Mg–Ni diffusion couples annealed at different temperatures can be readily determined. The diffusivities obtained from this study are summarized in Table 1. The observed results present the exciting prospect that with appropriate controls, the above solid-state reaction technique and the immobile trench marker scheme promise to be the effective methods for investigating the formation kinetics of other Mg-based hydrogen storage alloys. Fig. 5. Cross-sectional BSE SEM images of the Mg–Ni reaction couples annealed at (a) 480 °C for 6 h and (b) 510 °C for 2 h. X1 and X2 dimensions in (a) and (b) are the thicknesses of the Mg2Ni intermetallic layers formed on the Ni side and on the Mg side, respectively. (c) Plot of the thickness of the Mg2Ni layer formed on the Ni side versus that formed on the Mg side for the Mg–Ni samples after different heat treatments, illustrating a reliable linear relationship.
4. Summary and conclusions In summary, the present study has demonstrated that continuous pure Mg2Ni layers were successfully produced by solid-state reactions
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S.L. Cheng et al. / Thin Solid Films 517 (2009) 4745–4748
in the Mg–Ni system. The thicknesses of Mg2Ni alloy layers could be tuned from several to tens of micrometers by adjusting the annealing temperatures and time. TEM observation clearly revealed that the grain sizes of Mg2Ni formed near the Mg side were found to be much smaller than those formed near the Ni side. Cross-sectional SEM examinations show that the square of the thickness of Mg2Ni layer was proportional to the annealing time, which indicates that the growth of the Mg2Ni alloy layer is diffusion-controlled. Based on the results of SEM and marker analysis, Ni was found to be the dominant diffusing species during the formation of Mg2Ni alloy layers. Furthermore, the interdiffusion and intrinsic diffusion coefficients for the Mg–Ni samples at different temperatures have been determined by using the kinetic equations and the results of our marker experiments. Acknowledgment The research was supported by the National Science Council through grant No. NSC 96-2221-E-008-054.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
C. Iwakura, H. Inoue, N. Furukawa, S. Nohara, Electrochemistry 71 (2003) 776. K. Tajima, Y. Yamada, S. Bao, M. Okada, K. Yoshimura, J. Appl. Phys. 103 (2008) 013512. W. Oelerich, T. Klassen, R. Bormann, J. Alloys Compd. 315 (2001) 237. T.Z. Si, D.M. Liu, Q.A. Zhang, Int. J. Hydrogen Energy 32 (2007) 4912. R. Janot, L. Aymard, A. Rougier, G.A. Nazri, J.M. Tarascon, J. Phys. Chem. Solids 65 (2004) 529. A. Ludwig, J. Cao, B. Dam, R. Gremaud, Appl. Surf. Sci. 284 (2007) 682. H.Y. Shao, H.R. Xu, Y.T. Wang, X.G. Li, Nanotechnology 15 (2004) 269. L. Sun, G.X. Wang, H.K. Liu, D.H. Bradhurst, S.X. Dou, Electrochem. Solid State Lett. 3 (2000) 121. J.T. Yin, K. Tanaka, Mater. Trans. 43 (2002) 1732. R. Vijay, R. Sundaresan, M.P. Maiya, S.S. Murthy, Int. J. Hydrogen Energy 30 (2005) 501. Y. Kodera, N. Yamasaki, T. Yamamoto, T. Kawasaki, M. Ohyanagi, Z.A. Munir, J. Alloys Compd. 446 (2007) 138. W. Sprengel, M. Denkinger, H. Mehrer, Intermetallics 2 (1994) 127. H.T.G. Hentzell, R.D. Thompson, K.N. Tu, J. Appl. Phys. 54 (1983) 6923. R. Roy, S.K. Sen, S. Sen, J. Mater. Res. 7 (1992) 1377. Q.Z. Hong, F.M. d'Heurle, J. Appl. Phys. 72 (1992) 4036. M.J.H. van Dal, M.C.L.P. Pleumeekers, A.A. Kodentsov, F.J.J. van Loo, Acta Mater. 48 (2000) 385. S.Q. Wang, J.W. Mayer, Thin Solid Films 207 (1992) 37. J. Tardy, K.N. Tu, Phys. Rev., B 32 (1985) 2070.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Formation of Mg2Ni alloy layers and kinetic studies in the binary Mg–Ni system S.L. Cheng a,b,⁎, Y.Y. Chen b, S.W. Lee b, H.F. Hsu c a b c
Department of Chemical and Materials Engineering, National Central University, Chung-Li City, Taoyuan, Taiwan, ROC Institute of Materials Science and Engineering, National Central University, Chung-Li City, Taoyuan, Taiwan, ROC Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan, ROC
a r t i c l e
i n f o
Available online 13 March 2009 Keywords: Mg2Ni alloy Solid-state reaction Dominant diffusing species Diffusivity
a b s t r a c t We report here the study on the kinetics and growth of Mg2Ni alloy layers by solid-state reactions in the Mg–Ni system. After appropriate heat treatments, only polycrystalline Mg2Ni phase was found to form on the Mg/Ni interfaces and the grain sizes of Mg2Ni formed near the Mg side were found to be much smaller than those formed near the Ni side. Cross-sectional SEM examinations revealed that the Mg2Ni intermetallic layers grew following the diffusion-control process. During the formation of Mg2Ni layers, the dominant diffusing species was determined to be Ni by using the immobile trench markers. Furthermore, based on the marker analysis, the intrinsic and interdiffusion diffusivities for the Mg–Ni diffusion couples at different temperatures have been determined. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Recently, magnesium-based alloys have been the subject of great interests because of their potential applications in energy storage media and switchable mirrors [1–3]. Among these Mg-based metal hydrides, Mg2Ni alloy is perhaps the most promising candidate for the hydrogen storage material due to its high specific hydrogen storage capacity, light weight, and low environmental impact [4–7]. To fabricate Mg2Ni alloy materials, various fabrication techniques such as mechanical milling, combustion synthesis, and melt casting have been developed [8–11]. However, these conventional methods are limited by the high operating temperature and the serial processing format. In addition, due to the high oxidation tendency of Mg, it is difficult to obtain pure Mg2Ni compound by the above-mentioned techniques. Previously, it has been demonstrated that a variety of pure metal compounds were successfully fabricated by the solid-state reactions between two different pure metallic materials [12–14]. Although various pure intermetallic compounds were produced by the solid-state reaction methods, the studies on the microstructures and kinetics of the pure Mg2Ni alloy layer formation in the Mg–Ni system are relatively rare [15]. Since the kinetic data can provide crucial insights to the fundamental understanding of the formation process of Mg2Ni intermetallic compounds, it is of much importance to determine the kinetic parameters such as diffusivities and dominant diffusing species in the Mg–Ni reaction couples under different experimental conditions. Diffusion-marker experiments have been widely utilized to determine the kinetic data in solid-state reactions involving two elements [16–18]. However, the marker drag and the barrier effects occurring during the
⁎ Corresponding author. Department of Chemical and Materials Engineering, National Central University, Chung-Li City, Taoyuan, Taiwan, ROC. Tel.: +886 3 4227151x34233; fax: +886 3 2804510. E-mail address: [email protected] (S.L. Cheng). 0040-6090/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2009.03.011
growth of intermetallic compounds usually resulted in ambiguous results. Therefore, for the investigation of the kinetics of Mg2Ni intermetallic layers formation, the route for creating an immobile marker at the original interface of the Mg–Ni reaction couple is demanded to avoid the abovementioned problems. In the present study, results from a systematic investigation on the interfacial reactions between pure Mg and Ni metal laminas after different heat treatments and the kinetics of the Mg2Ni layers formation in the Mg–Ni reaction couples are reported. 2. Experimental procedures Commercial pure Mg and Ni metal laminas were used as the starting materials in this study. Both the pure Mg and Ni laminas were firstly cut into small pieces and then cleaned chemically by dipping in a 5% Nital solution (5 vol.% HNO3 in 95 vol.% C2H5OH) to remove the surface contaminants and native oxide layers and then rinsed in absolute ethanol and blew dried with N2 gas. For the marker experiments, shallow trenches were created on the smooth surfaces of the soft Mg laminas by using a sharp knife and served as the immobile markers. After the cleaning and marking processes, the cleaned Mg and Ni metal laminas were pressed together and held in contact with each other in a custom-built stainless steel jig. The jig with the Mg–Ni couple was then placed in a three-zone diffusion furnace and annealed isothermally at 480 °C and 510 °C for various time periods in high-purity N2 ambient. After the isothermal annealing, the cross-sectional samples of the Mg– Ni diffusion couples were prepared by the metallographic polishing technique. The thicknesses and morphologies of the produced Mg–Ni intermetallic layers were examined by using backscattered electron (BSE) scanning electron microscopy (SEM). X-ray diffractometry (XRD) (Cu Kα radiation, λ = 0.154 nm), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED) analysis were carried out
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S.L. Cheng et al. / Thin Solid Films 517 (2009) 4745–4748
Fig. 2. XRD diffraction spectra of the intermetallic compound layers formed from the 510 °C annealed Mg–Ni samples: (a)–(b) near the Ni side; and (c)–(d) near the Mg side. The annealing time: (a),(c) 1 h; and (b),(d) 4 h.
Fig. 1. (a) A cross-sectional BSE SEM image of an Mg–Ni reaction couple with a trench marker after annealing at 510 °C for 4 h. (b) A representative SEM image of an Mg–Ni intermetallic layer and the corresponding EDS spectra acquired from the regions at: (i) the top; (ii) the middle; and (iii) the bottom of the intermetallic layer.
Mg2Ni phase were detected in these peeled samples, indicating that the intermetallic compound layers were pure Mg2Ni. The microstructures of the intermetallic layers were investigated in more detail by planview TEM and SAED analysis. Figs. 3 (a) and (d) show the bright-field TEM images taken from the regions of the intermetallic layers formed near the Mg side and the Ni side, respectively. Figs. 3 (c) and (d) show the corresponding indexed SAED patterns. Based on the analysis of SAED patterns, it further confirmed that only hexagonal Mg2Ni phase was found to form at the interface between Mg and Ni and the crystal structures of the produced Mg2Ni layers were polycrystalline. Furthermore, from the TEM observation, the grain sizes of Mg2Ni formed near the Mg side (~5–10 nm) were found to be much smaller than those formed near the Ni side (~150– 250 nm), as shown in Figs. 3 (a)–(b). The results suggested that the Mg2Ni grains near the Ni side might be formed early compared to those near the Mg side and then the grain growth of Mg2Ni grains near the Ni side would be induced by the longer holding time during the isothermal annealing processes.
for phase identification and microstructure examination. A Link ISIS energy dispersion spectrometer (EDS) attached to the SEM was utilized to determine the chemical composition of local areas in the intermetallic layers. 3. Results and discussion From cross-sectional BSE SEM observations, continuous intermetallic compound layers were found to form in the Mg–Ni diffusion couples after annealing at different temperatures and time. During the formation of Mg–Ni intermetallic layers, the trench markers were always observed to locate on the original Mg/Ni interface. A representative BSE SEM image is shown in Fig. 1 (a). The result demonstrated that the shallow trenches were suitable to serve as the immobile markers in this experiment. After the formation of Mg–Ni alloy layers, EDS analysis was carried out to investigate the distribution of Mg and Ni atoms. An example is shown in Fig. 1 (b). Analysis of the spectra obtained by EDS indicated that the intermetallic layer is composed mainly of Mg and Ni. In addition, the quantitative EDS analysis gives the average value of the ratio of the atomic concentrations of Mg and Ni as about 2 which suggests that the intermetallic layer formed is Mg2Ni. For the examination of the phase formation in the reaction zones between Mg and Ni, the annealed samples were peeled into two half parts by mechanical means. The peeled surfaces were then analyzed by XRD. Figs. 2 (a)–(d) show the typical XRD spectra of the intermetallic layers formed near the Ni side and the Mg side, respectively. In Fig. 2, it is clearly revealed that only diffraction peaks corresponding to the hexagonal
Fig. 3. Bright-field TEM images of the intermetallic compound layers formed (a) near the Mg side and (b) near the Ni side for the Mg–Ni samples after annealing at 510 °C for 3 h. (c) and (d) show the corresponding indexed SAED patterns for (a) and (b), respectively.
S.L. Cheng et al. / Thin Solid Films 517 (2009) 4745–4748
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Table 1 Interdiffusion coefficients and intrinsic diffusion coefficients of Mg and Ni in Mg2Ni at 480 °C and 510 °C. Interdiffusion coefficient (cm2/s), D̃ 480 °C 510 °C
Fig. 4. (a) The thickness of the Mg2Ni layer as a function of annealing temperature. (b) The thickness of the Mg2Ni layer versus the square root of annealing time curves for the Mg–Ni samples annealed at different temperatures.
After a series of cross-sectional SEM examinations, the thicknesses of the Mg2Ni layers versus reaction time data for the Mg–Ni diffusion couples annealed at 480 °C and 510 °C were obtained, as shown in Fig. 4 (a). In Fig. 4 (a), it is revealed that the thicknesses of Mg2Ni layers increased parabolically with time. In addition, the growth rate of the Mg2Ni intermetallic layers was observed to increase with the annealing temperature. Fig. 4 (b) shows the plot of the square of the Mg2Ni layer thickness versus the annealing time for temperatures at 480 °C and 510 °C. The relation curves shown in Fig. 4 (b) are almost linear. The result clearly indicates that the growth of the Mg2Ni alloy layer is diffusion-controlled. Figs. 5 (a) and (b) show the representative cross-sectional BSE SEM images of the Mg–Ni reaction couples after annealing at 480 °C for 6 h and at 510 °C for 2 h, respectively. The original interfaces are
− 11
4.1 × 10 1.0 × 10− 10
Intrinsic diffusion coefficient (cm2/s) Mg in Mg2Ni, DMg − 12
7.5 × 10 1.9 × 10− 11
Ni in Mg2Ni, DNi 5.8 × 10− 11 1.5 × 10− 10
evident in these annealed Mg–Ni samples. As can be seen in Figs. 5 (a)–(b), the thicknesses of the Mg2Ni layers formed on the Mg side (i.e., X2 in Fig. 5) were measured to be much thicker than those formed on the Ni side (i.e., X1 in Fig. 5). The Mg2Ni compound thickness on the Ni side versus the Mg2Ni compound thickness on the Mg side data (i.e., X1 versus X2) in the Mg–Ni sample after different heat treatments is shown in Fig. 5 (c). It is known that for a singlelayer compound formation in an A–B diffusion couple, if more A diffuses into the B with the marker position fixed, the thickness of the A–B intermetallic layer on the B side would be thicker than that on the A side. Therefore, from Fig. 5, it can be concluded that Ni is the dominant diffusing species during the Mg2Ni intermetallic layer formation. The mechanism for the high mobility of Ni atoms during the Mg2Ni compound formation has been proposed by Hong et al. [15]. Their study pointed out that the faster diffusion process of Ni could be attributed to the existence of unique Ni-atom chain structures in the Mg2Ni compound. The Ni atoms can easily diffuse along the continuous chains of Ni atoms during the solid-state reaction in the Mg–Ni diffusion couple. On the other hand, to obtain the diffusivities from the marker experiments, the basic kinetic equations were adopted. In this study, only a single intermetallic compound Mg2Ni was found to form between Mg and Ni during annealing and the growth of the Mg2Ni layer was diffusion-controlled. Thus, the interdiffusion coefficient D̃˜ of the Mg2Ni layer can be simply expressed by: 2 ˜ X = 4 Dt
ð1Þ
where X is the thickness of the Mg2Ni layer, and t is the annealing time. The intrinsic diffusion coefficients of Mg (DMg) and Ni (DNi) in the Mg2Ni layer can be expressed by: DNi = ½2ðX2 = X1 ÞDMg
ð2Þ
where X1 and X2 are the thicknesses of the Mg2Ni layers formed on the Ni side and on the Mg side, respectively, as defined in Fig. 5. In addition, for a single-layer Mg2Ni intermetallic compound formation, the interdiffusion coefficient D̃ can also be expressed as: D˜ = NNi DMg + NMg DNi
ð3Þ
where NNi and NMg are the fractions of Ni and Mg atoms in the unit volume, respectively. Based on these kinetic equations and the results of our marker experiments, the interdiffusion and intrinsic diffusion coefficients for the binary Mg–Ni diffusion couples annealed at different temperatures can be readily determined. The diffusivities obtained from this study are summarized in Table 1. The observed results present the exciting prospect that with appropriate controls, the above solid-state reaction technique and the immobile trench marker scheme promise to be the effective methods for investigating the formation kinetics of other Mg-based hydrogen storage alloys. Fig. 5. Cross-sectional BSE SEM images of the Mg–Ni reaction couples annealed at (a) 480 °C for 6 h and (b) 510 °C for 2 h. X1 and X2 dimensions in (a) and (b) are the thicknesses of the Mg2Ni intermetallic layers formed on the Ni side and on the Mg side, respectively. (c) Plot of the thickness of the Mg2Ni layer formed on the Ni side versus that formed on the Mg side for the Mg–Ni samples after different heat treatments, illustrating a reliable linear relationship.
4. Summary and conclusions In summary, the present study has demonstrated that continuous pure Mg2Ni layers were successfully produced by solid-state reactions
4748
S.L. Cheng et al. / Thin Solid Films 517 (2009) 4745–4748
in the Mg–Ni system. The thicknesses of Mg2Ni alloy layers could be tuned from several to tens of micrometers by adjusting the annealing temperatures and time. TEM observation clearly revealed that the grain sizes of Mg2Ni formed near the Mg side were found to be much smaller than those formed near the Ni side. Cross-sectional SEM examinations show that the square of the thickness of Mg2Ni layer was proportional to the annealing time, which indicates that the growth of the Mg2Ni alloy layer is diffusion-controlled. Based on the results of SEM and marker analysis, Ni was found to be the dominant diffusing species during the formation of Mg2Ni alloy layers. Furthermore, the interdiffusion and intrinsic diffusion coefficients for the Mg–Ni samples at different temperatures have been determined by using the kinetic equations and the results of our marker experiments. Acknowledgment The research was supported by the National Science Council through grant No. NSC 96-2221-E-008-054.
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