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X-ray diffraction, scanning electron microscopy and magnetic characterization of hydrogenated Ni–Pd alloys
Journal of Alloys and Compounds 384 (2004) 152–156
X-ray diffraction, scanning electron microscopy and magnetic characterization of hydrogenated Ni–Pd alloys S.S.M. Tavares a,∗ , J.M. Pardal a , T. Gurova b , J.R.R. Bernardo c , J.M. Neto d a
Departamento de Engenharia Mecˆanica – PGMEC/UFF – Rua Passo da Pátria, 156, CEP 24210-240, Niterói-RJ, Brazil b Departamento de Engenharia Metalúrgica e Materiais, COPPE/EE/UFRJ, Brazil c FAETEC, Rio de Janeiro, Brazil d Instituto de F´ısica, UFRJ, Brazil Received 6 February 2004; received in revised form 8 April 2004; accepted 8 April 2004
Abstract The present work shows results on the surface effects produced by electrolytic hydrogenation and desorption of Ni–Pd alloys (60Ni–40Pd and 40Ni–60Pd, wt.%). The behaviour of these two alloys is rather similar to that of pure nickel rather than pure palladium. Intergranular and transgranular cracks were observed just after the electrolytic charging. During the natural aging the cracks become larger and propagate, as observed by scanning electron microscopy. The hydride phases formed are very unstable at room temperature and normal pressure. X-ray diffractograms were used to obtain a microstrain and microstress analysis of the NiPd solid solution and hydride phases. Magnetization curves of the Ni60–40Pd show that trapped hydrogen atoms remain in the NiPd phase after hydride decomposition and decrease the magnetization saturation and the magnetic permeability. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Metal hydrides; Electrochemical reactions; Magnetic measurements
1. Introduction In a previous work [1], surface features of thin foils of nickel and palladium hydrogenated by electrolytic charging were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). A very distinct behaviour of these two cfc metals was observed. Hydrogen insertion promotes the formation of the NiHx (x = 0.67) hydride and creates intergranular cracks. This hydride is very unstable at room temperature and normal pressure. Its decompostion into Ni and H2 starting by the grain boundaries promotes the opening and propagation of intergranular cracks. Pure palladium is easily hydrogenated by electrolytic charging and the kinetics of the decomposition of the hydride is slower than that of NiH0.67 [1]. Palladium is also much less susceptible to hydrogen embrittlement although a solid solution hardening occurs [2]. The stresses created during hydrogen absortion promote strong plastic deformation in paladium, but intergranular cracks are not observed ∗
Corresponding author. E-mail address: [email protected] (S.S.M. Tavares).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.103
just after the hydrogenation [1]. Palladium hydrides could not be distinguished from the pure Pd phase by scanning electron microscopy. The aim of this work was to identify the hydrogenated phases formed and to show the presence of surface effects produced during and after the electrolytic hydrogenation of 40wt.%Ni–60wt.%Pd and 60wt.%Ni–40wt.%Pd alloys. The results are compared with those obtained for pure nickel and pure palladium [1]. Microstrain analysis was performed with the X-ray diffraction data.
2. Experimental The Ni–60wt.%Pd and Ni–40wt.%Pd alloys were melted in an arc furnace from high purity raw materials. Then the alloys were cold rolled to 0.1 mm and annealed at 600 ◦ C for 1 h. The Ni–40%Pd and Ni–60%Pd foils were hydrogenated in a 1N H2 SO4 solution for 12 h with current densities in the range of 20–40 mA/cm2 . As2 O3 was used as catalyst at a concentration of 5 × 10−5 mol/l. The hydrogenated samples were analysed by X-ray diffraction and
S.S.M. Tavares et al. / Journal of Alloys and Compounds 384 (2004) 152–156
scanning electrom microscopy immediately after charging and during the desorption. Magnetization curves were measured in a vibrating sample EGG-PAR model 4500 at room temperature and normal pressure. XRD measurements were carried out in a PHILIPS® X-Pert diffractometer using Cu K␣ (1.54056 Å) radiation with step size of 0.02◦ and time per step of 3 s. The amount of hydride phase was quantified by the integration of the (2 0 0) and (1 1 1) peaks of each phase, using the equations proposed by Cullity [3], and considering the same structure
153
factor and absorption coefficient for the hydride and the NiPd phase.
3. Results and discussion Figs. 1 and 2 show the X-ray diffractograms of the hydrogenated 60Ni–40Pd and 40Ni–60Pd alloys obtained just after hydrogenation and during natural aging. The lattice parameters of the hydride phases were 3.776 Å for the
Fig. 1. X-ray diffractograms of 60Ni–40Pd alloy. Time after hydrogenation is indicated.
Fig. 2. X-ray diffractograms of 40Ni–60Pd alloy. Time after hydrogenation is indicated.
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Fig. 3. Volume fraction of hydride against aging time at room temperature.
60Ni–40Pd and 3,792 Å for the 40Ni–60Pd alloy. Taking the volume occupied by a hydrogen atom as 2.8 Å3 [4] the hydride phases formed were (Ni, Pd)H0.54 in the 60Ni–40Pd alloy and (Ni, Pd)H0.56 in the 40Ni–60Pd alloy. The amount of the hydride phase formed was 69% in the 60Ni–40Pd and 80% in the 40Ni–60Pd alloy. Fig. 3 shows that the kinetics of hydride decomposition is faster in the 60Ni–40Pd alloy than in the 40Ni–60Pd. This is coherent with the observation that the kinetics of decomposition of palladium hydrides is much slower than nickel hydrides in the pure metals [1]. Fig. 4a shows a backscattered electron image taken just after hydrogenation in the 60Ni–40Pd alloy. Many intergran-
ular and some transgranular cracks are observed. Fig. 4b was obtained in the same region 215 min later with the same operating conditions. It is possible to notice crack opening and propagation after the hydrogenation. Fig. 5a and b show this same behaviour in the 40Ni–60Pd alloy. In both cases, the crack opening is very fast in the first hour after hydrogenation, and becomes slower after that. In the pure nickel hydrogenation experiments it was possible to obtain contrast between the Ni and the NiH0.67 phases in the backscattered electron image. A careful examination of Fig. 4a may also show a very slight contrast inside the grains between the (Pd,Ni)H0.56 and the NiPd phases in the 60Ni–40Pd alloy. This is not observed in the 40Ni–60Pd
Fig. 4. Transgranular and intergranular cracks just after hydrogenation (a) and 215 min after hydrogenation (b) in the 60Ni–40Pd alloy.
S.S.M. Tavares et al. / Journal of Alloys and Compounds 384 (2004) 152–156
155
Fig. 6. Half height line width of (1 1 1) and (2 0 0) peaks of NiPd and (Ni, Pd)Hx phases in the 60Ni–40Pd (a) and 40Ni–60Pd (b) alloys. (A) – annealed; (H) – hydrogenated; H95, H155 and H215 — hydrogenated and aged for 95, 155 and 215 min, respectively. Fig. 5. Intergranular cracks just after hydrogenation (a) and 215 min after hydrogenation (b) in the 40Ni–60Pd alloy.
to β by [3]: alloy (Fig. 5a) due to its higher palladium content. The contrast between the hydride and the metal phase that is clearly observed in pure nickel foils [1] is decreased by increasing amounts of palladium. Fig. 6a and b show the variation of the half height peak width (β) upon hydrogenation and the natural aging of 60Ni–40Pd and 40Ni–60Pd alloys. In both cases, the hydrogenation promotes a very pronounced peak broadening in the solid solution NiPd, which can be attributed to nonuniform microstrains and microstresses, composition variations and increase of the dislocation density. The maximum microstrain and tensile microstress can be related
ε=
β 4tan θ
(1)
σ=
Eβ 4tan θ
(2)
In Fig. 6a and b, the value of β2 0 0 of the phase NiPd after charging is twice the value of β1 1 1 , indicating that in this phase the peak broadening due to the hydrogenation depends on the crystallographic direction. From Eqs. (1) and (2) the microstrain and microstress levels in the [2 0 0] direction is 1.70 higher than in the [1 1 1] direction. In contrast, the (1 1 1) and (2 0 0) peaks of the (Ni, Pd)Hx hydride (x = 0.54 and 0.56) present about the same β values (∼0.38)
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Fig. 7. Magnetization curves of the 60Ni–40Pd alloy: annealed (a), just after hydrogenation (b), 120 min after hydrogenation (c), and 300 min after hydrogenation (d).
just after the hydrogenation. A small increase of this value is then observed during the hydride decomposition. Fig. 6a and b also show that the peak broadening of the NiPd and hydride phases does not change during the hydride decomposition, which can indicate two important features: (1) the crystaline defect densities (dislocations, hydrogen trapped atoms) in the NiPd phase are maintained during the natural aging; (2) the microstrain and microstress level is maintained during the hydride decomposition. In the beginning, when the hydride amount is still high the microstresses promote the crack opening and propagation. Another effect produced by the hydrogenation is observed in the magnetization curves of the hydrogenated 60Ni–40Pd alloy (Fig. 7). Hydrogen decreases the magnetic moment of nickel in pure nickel and Ni–Pd alloys [5]. In the annealed condition the magnetization saturation (ms ) is 34.6 Am2 /kg (curve a). Just after the hydrogenation ms and the initial permeability (ρ) are minimum. The ms and ρ increase with the aging time, but even after the complete hydride decomposition (300 min, curve d) they are still lower than the values found in the annealed sample. In fact, the NiPd phase dissolves hydrogen atoms that remain trapped in lattice defects after hydride decomposition by natural aging. Trapped hydrogen atoms also contribute to the peak broadening observed in the (1 1 1) and (2 0 0) lines.
4. Conclusions The hydrogenation of 60Ni–40Pd and 40Ni–60Pd alloys by an electrolytic method in H2 SO4 solution promotes the formation of metastable hydrides (Ni, Pd)Hx , with estimated
x values of 0.54 and 0.56, respectively. After the hydrogenation two phases are clearly identified: (1)the NiPd solid solution with some H dissolved, and (2) (Ni, Pd)Hx . The rate of decomposition of the hydride at room temperature and normal pressure is faster in the 60Ni–40Pd alloy. During the hydrogenation intergranular and transgranular cracks are created in both alloys. These cracks propagate and become wider after the hydrogenation, during the decomposition of the hydrides. The large peak broadening produced by the hydrogenation can be attributed to microstresses, dislocations and hydrogen trapped atoms in the NiPd and hydride phases. The half height peak width (β) of the NiPd phase does not decrease with aging, indicating that the microstress level is maintained during the hydride decompostion. Acknowledgements The authors are grateful to the Brazilian research agencies (FAPERJ and CNPq) for financial support. References [1] S.S.M. Tavares, D. Fruchart, S. Miraglia, D.S. dos Santos, J. Alloys Compd. 347 (2002) 105. [2] R.J. Smith, D.A. Otterson, J. Less Common Mat. 24 (1971) 419. [3] B.D. Cullity, Elements of X-ray Diffraction, Addison–Wesley Publishing Company, 1956. [4] M. Shelyiapina, D. Fruchart, D. S. dos Santos, E. K. Hlil, S. Miraglia, S.S.M. Tavares, J. Tobola, J. Alloys Compd. 256/7C (2003) 218. [5] N. Tsulruda, K. Itoh, N. Moriolva, H. Ohkubo, E. Kuramoto, J. Alloys Compd. 293–295 (1999) 174.
X-ray diffraction, scanning electron microscopy and magnetic characterization of hydrogenated Ni–Pd alloys S.S.M. Tavares a,∗ , J.M. Pardal a , T. Gurova b , J.R.R. Bernardo c , J.M. Neto d a
Departamento de Engenharia Mecˆanica – PGMEC/UFF – Rua Passo da Pátria, 156, CEP 24210-240, Niterói-RJ, Brazil b Departamento de Engenharia Metalúrgica e Materiais, COPPE/EE/UFRJ, Brazil c FAETEC, Rio de Janeiro, Brazil d Instituto de F´ısica, UFRJ, Brazil Received 6 February 2004; received in revised form 8 April 2004; accepted 8 April 2004
Abstract The present work shows results on the surface effects produced by electrolytic hydrogenation and desorption of Ni–Pd alloys (60Ni–40Pd and 40Ni–60Pd, wt.%). The behaviour of these two alloys is rather similar to that of pure nickel rather than pure palladium. Intergranular and transgranular cracks were observed just after the electrolytic charging. During the natural aging the cracks become larger and propagate, as observed by scanning electron microscopy. The hydride phases formed are very unstable at room temperature and normal pressure. X-ray diffractograms were used to obtain a microstrain and microstress analysis of the NiPd solid solution and hydride phases. Magnetization curves of the Ni60–40Pd show that trapped hydrogen atoms remain in the NiPd phase after hydride decomposition and decrease the magnetization saturation and the magnetic permeability. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen absorbing materials; Metal hydrides; Electrochemical reactions; Magnetic measurements
1. Introduction In a previous work [1], surface features of thin foils of nickel and palladium hydrogenated by electrolytic charging were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). A very distinct behaviour of these two cfc metals was observed. Hydrogen insertion promotes the formation of the NiHx (x = 0.67) hydride and creates intergranular cracks. This hydride is very unstable at room temperature and normal pressure. Its decompostion into Ni and H2 starting by the grain boundaries promotes the opening and propagation of intergranular cracks. Pure palladium is easily hydrogenated by electrolytic charging and the kinetics of the decomposition of the hydride is slower than that of NiH0.67 [1]. Palladium is also much less susceptible to hydrogen embrittlement although a solid solution hardening occurs [2]. The stresses created during hydrogen absortion promote strong plastic deformation in paladium, but intergranular cracks are not observed ∗
Corresponding author. E-mail address: [email protected] (S.S.M. Tavares).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.04.103
just after the hydrogenation [1]. Palladium hydrides could not be distinguished from the pure Pd phase by scanning electron microscopy. The aim of this work was to identify the hydrogenated phases formed and to show the presence of surface effects produced during and after the electrolytic hydrogenation of 40wt.%Ni–60wt.%Pd and 60wt.%Ni–40wt.%Pd alloys. The results are compared with those obtained for pure nickel and pure palladium [1]. Microstrain analysis was performed with the X-ray diffraction data.
2. Experimental The Ni–60wt.%Pd and Ni–40wt.%Pd alloys were melted in an arc furnace from high purity raw materials. Then the alloys were cold rolled to 0.1 mm and annealed at 600 ◦ C for 1 h. The Ni–40%Pd and Ni–60%Pd foils were hydrogenated in a 1N H2 SO4 solution for 12 h with current densities in the range of 20–40 mA/cm2 . As2 O3 was used as catalyst at a concentration of 5 × 10−5 mol/l. The hydrogenated samples were analysed by X-ray diffraction and
S.S.M. Tavares et al. / Journal of Alloys and Compounds 384 (2004) 152–156
scanning electrom microscopy immediately after charging and during the desorption. Magnetization curves were measured in a vibrating sample EGG-PAR model 4500 at room temperature and normal pressure. XRD measurements were carried out in a PHILIPS® X-Pert diffractometer using Cu K␣ (1.54056 Å) radiation with step size of 0.02◦ and time per step of 3 s. The amount of hydride phase was quantified by the integration of the (2 0 0) and (1 1 1) peaks of each phase, using the equations proposed by Cullity [3], and considering the same structure
153
factor and absorption coefficient for the hydride and the NiPd phase.
3. Results and discussion Figs. 1 and 2 show the X-ray diffractograms of the hydrogenated 60Ni–40Pd and 40Ni–60Pd alloys obtained just after hydrogenation and during natural aging. The lattice parameters of the hydride phases were 3.776 Å for the
Fig. 1. X-ray diffractograms of 60Ni–40Pd alloy. Time after hydrogenation is indicated.
Fig. 2. X-ray diffractograms of 40Ni–60Pd alloy. Time after hydrogenation is indicated.
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Fig. 3. Volume fraction of hydride against aging time at room temperature.
60Ni–40Pd and 3,792 Å for the 40Ni–60Pd alloy. Taking the volume occupied by a hydrogen atom as 2.8 Å3 [4] the hydride phases formed were (Ni, Pd)H0.54 in the 60Ni–40Pd alloy and (Ni, Pd)H0.56 in the 40Ni–60Pd alloy. The amount of the hydride phase formed was 69% in the 60Ni–40Pd and 80% in the 40Ni–60Pd alloy. Fig. 3 shows that the kinetics of hydride decomposition is faster in the 60Ni–40Pd alloy than in the 40Ni–60Pd. This is coherent with the observation that the kinetics of decomposition of palladium hydrides is much slower than nickel hydrides in the pure metals [1]. Fig. 4a shows a backscattered electron image taken just after hydrogenation in the 60Ni–40Pd alloy. Many intergran-
ular and some transgranular cracks are observed. Fig. 4b was obtained in the same region 215 min later with the same operating conditions. It is possible to notice crack opening and propagation after the hydrogenation. Fig. 5a and b show this same behaviour in the 40Ni–60Pd alloy. In both cases, the crack opening is very fast in the first hour after hydrogenation, and becomes slower after that. In the pure nickel hydrogenation experiments it was possible to obtain contrast between the Ni and the NiH0.67 phases in the backscattered electron image. A careful examination of Fig. 4a may also show a very slight contrast inside the grains between the (Pd,Ni)H0.56 and the NiPd phases in the 60Ni–40Pd alloy. This is not observed in the 40Ni–60Pd
Fig. 4. Transgranular and intergranular cracks just after hydrogenation (a) and 215 min after hydrogenation (b) in the 60Ni–40Pd alloy.
S.S.M. Tavares et al. / Journal of Alloys and Compounds 384 (2004) 152–156
155
Fig. 6. Half height line width of (1 1 1) and (2 0 0) peaks of NiPd and (Ni, Pd)Hx phases in the 60Ni–40Pd (a) and 40Ni–60Pd (b) alloys. (A) – annealed; (H) – hydrogenated; H95, H155 and H215 — hydrogenated and aged for 95, 155 and 215 min, respectively. Fig. 5. Intergranular cracks just after hydrogenation (a) and 215 min after hydrogenation (b) in the 40Ni–60Pd alloy.
to β by [3]: alloy (Fig. 5a) due to its higher palladium content. The contrast between the hydride and the metal phase that is clearly observed in pure nickel foils [1] is decreased by increasing amounts of palladium. Fig. 6a and b show the variation of the half height peak width (β) upon hydrogenation and the natural aging of 60Ni–40Pd and 40Ni–60Pd alloys. In both cases, the hydrogenation promotes a very pronounced peak broadening in the solid solution NiPd, which can be attributed to nonuniform microstrains and microstresses, composition variations and increase of the dislocation density. The maximum microstrain and tensile microstress can be related
ε=
β 4tan θ
(1)
σ=
Eβ 4tan θ
(2)
In Fig. 6a and b, the value of β2 0 0 of the phase NiPd after charging is twice the value of β1 1 1 , indicating that in this phase the peak broadening due to the hydrogenation depends on the crystallographic direction. From Eqs. (1) and (2) the microstrain and microstress levels in the [2 0 0] direction is 1.70 higher than in the [1 1 1] direction. In contrast, the (1 1 1) and (2 0 0) peaks of the (Ni, Pd)Hx hydride (x = 0.54 and 0.56) present about the same β values (∼0.38)
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Fig. 7. Magnetization curves of the 60Ni–40Pd alloy: annealed (a), just after hydrogenation (b), 120 min after hydrogenation (c), and 300 min after hydrogenation (d).
just after the hydrogenation. A small increase of this value is then observed during the hydride decomposition. Fig. 6a and b also show that the peak broadening of the NiPd and hydride phases does not change during the hydride decomposition, which can indicate two important features: (1) the crystaline defect densities (dislocations, hydrogen trapped atoms) in the NiPd phase are maintained during the natural aging; (2) the microstrain and microstress level is maintained during the hydride decomposition. In the beginning, when the hydride amount is still high the microstresses promote the crack opening and propagation. Another effect produced by the hydrogenation is observed in the magnetization curves of the hydrogenated 60Ni–40Pd alloy (Fig. 7). Hydrogen decreases the magnetic moment of nickel in pure nickel and Ni–Pd alloys [5]. In the annealed condition the magnetization saturation (ms ) is 34.6 Am2 /kg (curve a). Just after the hydrogenation ms and the initial permeability (ρ) are minimum. The ms and ρ increase with the aging time, but even after the complete hydride decomposition (300 min, curve d) they are still lower than the values found in the annealed sample. In fact, the NiPd phase dissolves hydrogen atoms that remain trapped in lattice defects after hydride decomposition by natural aging. Trapped hydrogen atoms also contribute to the peak broadening observed in the (1 1 1) and (2 0 0) lines.
4. Conclusions The hydrogenation of 60Ni–40Pd and 40Ni–60Pd alloys by an electrolytic method in H2 SO4 solution promotes the formation of metastable hydrides (Ni, Pd)Hx , with estimated
x values of 0.54 and 0.56, respectively. After the hydrogenation two phases are clearly identified: (1)the NiPd solid solution with some H dissolved, and (2) (Ni, Pd)Hx . The rate of decomposition of the hydride at room temperature and normal pressure is faster in the 60Ni–40Pd alloy. During the hydrogenation intergranular and transgranular cracks are created in both alloys. These cracks propagate and become wider after the hydrogenation, during the decomposition of the hydrides. The large peak broadening produced by the hydrogenation can be attributed to microstresses, dislocations and hydrogen trapped atoms in the NiPd and hydride phases. The half height peak width (β) of the NiPd phase does not decrease with aging, indicating that the microstress level is maintained during the hydride decompostion. Acknowledgements The authors are grateful to the Brazilian research agencies (FAPERJ and CNPq) for financial support. References [1] S.S.M. Tavares, D. Fruchart, S. Miraglia, D.S. dos Santos, J. Alloys Compd. 347 (2002) 105. [2] R.J. Smith, D.A. Otterson, J. Less Common Mat. 24 (1971) 419. [3] B.D. Cullity, Elements of X-ray Diffraction, Addison–Wesley Publishing Company, 1956. [4] M. Shelyiapina, D. Fruchart, D. S. dos Santos, E. K. Hlil, S. Miraglia, S.S.M. Tavares, J. Tobola, J. Alloys Compd. 256/7C (2003) 218. [5] N. Tsulruda, K. Itoh, N. Moriolva, H. Ohkubo, E. Kuramoto, J. Alloys Compd. 293–295 (1999) 174.