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Nanostructured Ni-Al2O3 films prepared by DC and pulsed DC electroplating
Scripta mater. 44 (2001) 1813–1816 www.elsevier.com/locate/scriptamat
NANOSTRUCTURED Ni-Al2O3 FILMS PREPARED BY DC AND PULSED DC ELECTROPLATING Jan Steinbach and Hans Ferkel Department of Materials Science and Technology, Technical University Clausthal, Agricolastraße 6, D-38678 Clausthal-Zellerfeld, Germany (Received August 21, 2000) (Accepted in revised form December 20, 2000) Keywords: Electroplating; Transmission electron microscopy; Nickel-alumina composite; Magnetic methods; Structural properties 1. Introduction Nanocrystalline Ni/Al2O3 composites can easily be produced by co-deposition of nickel and alumina nanoparticles under direct current (DC) plating conditions from electrochemical nickel baths in which alumina nanoparticles are dispersed [1–9]. Recent investigations have also shown that alumina nanoparticles in the plating bath inhibit growth of nickel nuclei on the composite surface and enable the formation of Ni/Al2O3 films with an average matrix grain diameter down to around 30 nm using DC [8]. It was found by transmission electron microscopy (TEM) that the size distribution of the nanoparticles in the deposited films plated under DC conditions is essentially the same as the particle size distribution of the employed nanopowder prior to deposition [6]. This is of importance when uniform composite films are required. When films are plated from the same bath under pulsed direct current (PDC) conditions, the pulse length being the nickel nuclei growth limiting factor, large nanoparticles or parts of particle agglomerates might not be sufficiently embedded by growing nuclei during one pulse to be trapped on (in) the film. Therefore these particles might escape from the surface before the next pulse occurs, which does not necessarily promote a continuous growth of the same nuclei. First results on PDC plating of Ni/Al2O3 composite films have already shown that during PDC plating smaller nanoparticles are embedded more efficiently than larger particles and therefore a size selection of nanoparticles takes place. Furthermore Ni/Al2O3 composites plated under PDC conditions show a lower degree of particle agglomeration than films produced under DC conditions [10]. In this work results on DC and PDC plating of nanocrystalline Ni/Al2O3 films are presented. In order to investigate the structure and thermal stability of the films, specimens were inspected by TEM and measurement of the static coercivity Hc before and after thermal treatment. 2. Experimental Procedure The Ni/Al2O3 films were produced in a typical sulfamate bath of following composition: 485 g/l nickel-(II)-sulfamate (Ni(NH2SO3)), 22 g/l boric acid (H3BO3), 8 g/l nickel chloride (NiCl2 䡠6 H2O), 0.5 g/l surfactant, and different amounts of alumina nanopowder. The alumina nanopowder employed was a commercially available powder (NanoTek aluminium oxide 0100, Nanophase Technologies Corp.) produced by physical vapor synthesis (PVS). The particle size distribution of the powder can be 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00799-0
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Figure 1. TEM pictures of Ni/Al2O3 samples produced in a sulfamate bath under different conditions: (left) film produced under DC conditions at j ⫽ 770 A/m2 and cp ⫽ 150 g/l; (middle) film produced under DC conditions at j ⫽ 150 A/m2 and cp ⫽ 10 g/l; (right) film produced under PDC conditions at ⌬ton ⫽ 200 ms, jp ⫽ 1000 A/m2 and cp ⫽ 10 g/l.
characterized by a log-normal distribution with a median particle diameter dm ⫽ 25 nm and a geometric standard deviation ⫽ 1.7. The nanoparticles are of spherical shape and are crystallized in the ␥-phase. The dispersion of the particles in the bath at 25°C was assisted by ultrasonic treatment during plating on flat polished copper cathodes. The pH value of the bath was kept between 3.5 and 4.0. Under DC conditions films were plated at current density j ⫽ 770 A/m2 and powder concentration in the bath cp ⫽ 150 g/l and at j ⫽ 150 A/m2 and cp ⫽ 10 g/l. In the case of PDC plating cp was 10 g/l and the ratio between pulse length ⌬ton and pulse pause ⌬toff was kept constant at 1:2.5. The pulse current density jp was 1000 A/m2. Films formed at higher jp exhibited a rough film surface and dark gray nickelhydroxide deposits. The as plated films and films after thermal treatment were prepared for TEM investigations by dimple grinding and ion etching. Measurements of the static coercivity Hc were carried out on specimens of 40 ⫻ 9 ⫻ 0.5 mm. Results and Discussion In the left and middle part of Fig. 1 TEM pictures of Ni/Al2O3 films plated at DC conditions in the sulfamate bath at j ⫽ 770 A/m2 and cp ⫽ 150 g/l as well as at j ⫽ 150 A/m2 and cp ⫽ 10 g/l, respectively, are shown. The right part of the figure shows a Ni/Al2O3 film plated in the same bath under PDC conditions at ⌬ton ⫽ 200 ms, cp ⫽ 10 g/l and jp ⫽ 1000 A/m2. In the DC plated specimens ceramic particles larger than 100 nm can be seen clearly and the co-deposited particles are agglomerated. The smaller particles seem to be attached to the largest particles of the PVS nanopowder employed. The average matrix grain size of the sample in Fig. 1 (left) is around 30 nm and the grain size of the film shown in Fig. 1 (middle) is around 60 nm. In the case of layer produced under PDC conditions (right) no particles larger than 100 nm and no big particle agglomerates of the kind seen in the other pictures can be found. Furthermore, the average matrix grain size is around 20 nm. This very fine matrix grain size was expected since PDC plating of pure nickel has already been shown to promote the formation of nanocrystalline nickel with grain sizes less than 20 nm (11,12). Due to the strong contrast of the TEM picture in Fig. 1 (right), caused by the very fine matrix structure, most of the fine
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Figure 2. TEM pictures of Ni/Al2O3 samples after thermal treatment: (left) film produced under DC conditions at j ⫽ 770 A/m2 and cp ⫽ 150 g/l after 1050°C for 20 h; (right) film produced under PDC conditions at ⌬ton ⫽ 200 ms, jp ⫽ 1000 A/m2 and cp ⫽ 10 g/l after 850°C for 20 h.
ceramic particles are hidden in the film. However after a thermal treatment at 850°C for 20 h matrix grain growth is induced and these fine ceramic particles are visible as can be seen in the right part of Fig 2. Figure 2 compares the sample from Fig. 1 after a 20 h thermal treatment at 1050°C (left) and 850°C (right). In Fig. 2 (left) the large particles (and former large agglomerates) decorate the grain boundaries of the film. No particles smaller than 20 nm can be found. Smaller particles in the as-DC plated film are only found in agglomerates; these particles start sintering with other adjacent particles in the film at 1050°C and form larger particles, as already found in earlier investigations [7,9]. These large globular particles pin matrix grain boundaries very efficiently which explains the fine matrix grain size after excessive heat treatment. In the case of the PDC plated film the heat treatment at 850°C induces grain growth of the Ni matrix (right part of Fig. 2). Obviously, the smaller ceramic particles pin matrix grain boundaries less efficiently. In contrast to the untreated film in Fig. 1 (right) the small ceramic particles can now be seen. Furthermore, no particles larger than 50 nm can be seen in Fig. 2 (right). Obviously a particle size selection during PDC plating took place. In contrast to the sample in the left part of the figure the ceramic particles are dispersed into the matrix grain after the thermal treatment. Should dislocation plasticity be operative, these dispersoids will act as obstacles for dislocation glide during plastic deformation. In Fig. 3 the values of the coercivity Hc for three different nickel films after various heat treatments at 850°C are compared. Hc depends strongly on the matrix grain size of a ferromagnetic material [13] and therefore can be employed to trace the evolution of the nickel matrix grain size [7–9]. In the figure the Hc value for the specimens shown in Fig. 1 (left) and (right) and of pure nickel plated under DC conditions are given. A drop in Hc is observed after the first 20 h of heat treatment for all samples, reflecting a structural change of the nickel matrix, as also observed by TEM inspection. Whereas Hc of the composites remains constant after further annealing, Hc of the pure nickel continues decreasing. These results are consistent with microstructure investigations which gave no hint to structural changes of the composites after further heat treatments at 850°C, while structural investigation of the pure nickel did. The PDC plated Ni/Al2O3 film shows about half the Hc value of the Ni/Al2O3 film plated under
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Figure 3. Hc of nickel samples produced under different conditions in a sulfamate bath after various heat treatments at 850°C. (1) sample produced under DC conditions at j ⫽ 770 A/m2 and cp ⫽ 150 g/l; (2) sample produced under PDC conditions at ⌬ton ⫽ 140 ms and jp ⫽ 1000 A/m2 in a bath with cp ⫽ 10 g/l; (3) pure nickel sample produced under DC conditions at j ⫽ 770 A/m2.
DC conditions. This finding is consistent with the larger grain size of the PDC plated specimen observed (Fig. 2). This is caused by less pinning of nickel grain boundaries by small dispersoids at the beginning of heat treatment. However, the volume fraction of co-deposited alumina nanoparticles is not exactly known in both specimens and a difference in the amount of co-deposited particles will of course also influence the thermal stability of the matrix grain structure. In conclusion, it was shown that under PDC electroplating conditions nanocrystalline Ni/Al2O3 composites can be produced. In contrast to DC plating the incorporated alumina nanoparticles are less agglomerated in the deposited film. Furthermore, under the PDC plating conditions used here smaller alumina nanoparticles are embedded more efficiently than larger particles of the nanopowder employed. That is to say, a particle size selection of the co-deposited nanoparticles during PDC plating takes place. These smaller particles pin grain boundaries of the nickel matrix less efficiently than large particles do. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
F. K. Sautter, J. Electrochem. Soc. 110, 557 (1963). V. O. Nwoko and L. L. Shreir, J. Appl. Electrochem. 3, 137 (1973). R. R. Oberle, M. R. Scanlon, R. C. Cammarata, and P. C. Searson, Appl. Phys. Lett. 66, 19 (1995). S. Steinha¨user and B. Wielage, in Proceedings of ICCM-11, ed. M. L. Scott, vol. III, pp. 495–504, Gold Cost, Australia (1997). H. Ferkel, B. Mu¨ller, and W. Riehemann, Mater. Sci. Eng. A234 –236, 474 (1997). B. Mu¨ller and H. Ferkel, Nanostruct. Mater. 10, 1285 (1998). B. Mu¨ller and H. Ferkel, Z. Metallkd. 11, 868 (1999). B. Mu¨ller und H. Ferkel, in Verbundwerkstoffe und Werkstoffverbunde, ed. K. Schulte und K. U. Kainer, p. 658, Wiley-VCH, Weinheim, DGM-Tagungsband (1999). B. Mu¨ller and H. Ferkel, J. Metast. Nanocrystall. Mater. 8, 476 (2000). J. Steinbach and H. Ferkel, in Proceedings of the NATO Advanced Study Institute: Functional Gradient Materials and Surface Layers Prepared by Fine Particles Technology, NATO ASI Kiev, in press. N. Wang, Z. Wang, K. T. Aust, and U. Erb, Mater. Sci. Eng. A237, 150 (1997). H. Natter, M. Schmelzer, and R. Hempelmann, J. Mater. Res. 13, 1186 (1998) and references therein. G. Herzer, IEEE Trans. Magn. 26, 1397 (1990).
NANOSTRUCTURED Ni-Al2O3 FILMS PREPARED BY DC AND PULSED DC ELECTROPLATING Jan Steinbach and Hans Ferkel Department of Materials Science and Technology, Technical University Clausthal, Agricolastraße 6, D-38678 Clausthal-Zellerfeld, Germany (Received August 21, 2000) (Accepted in revised form December 20, 2000) Keywords: Electroplating; Transmission electron microscopy; Nickel-alumina composite; Magnetic methods; Structural properties 1. Introduction Nanocrystalline Ni/Al2O3 composites can easily be produced by co-deposition of nickel and alumina nanoparticles under direct current (DC) plating conditions from electrochemical nickel baths in which alumina nanoparticles are dispersed [1–9]. Recent investigations have also shown that alumina nanoparticles in the plating bath inhibit growth of nickel nuclei on the composite surface and enable the formation of Ni/Al2O3 films with an average matrix grain diameter down to around 30 nm using DC [8]. It was found by transmission electron microscopy (TEM) that the size distribution of the nanoparticles in the deposited films plated under DC conditions is essentially the same as the particle size distribution of the employed nanopowder prior to deposition [6]. This is of importance when uniform composite films are required. When films are plated from the same bath under pulsed direct current (PDC) conditions, the pulse length being the nickel nuclei growth limiting factor, large nanoparticles or parts of particle agglomerates might not be sufficiently embedded by growing nuclei during one pulse to be trapped on (in) the film. Therefore these particles might escape from the surface before the next pulse occurs, which does not necessarily promote a continuous growth of the same nuclei. First results on PDC plating of Ni/Al2O3 composite films have already shown that during PDC plating smaller nanoparticles are embedded more efficiently than larger particles and therefore a size selection of nanoparticles takes place. Furthermore Ni/Al2O3 composites plated under PDC conditions show a lower degree of particle agglomeration than films produced under DC conditions [10]. In this work results on DC and PDC plating of nanocrystalline Ni/Al2O3 films are presented. In order to investigate the structure and thermal stability of the films, specimens were inspected by TEM and measurement of the static coercivity Hc before and after thermal treatment. 2. Experimental Procedure The Ni/Al2O3 films were produced in a typical sulfamate bath of following composition: 485 g/l nickel-(II)-sulfamate (Ni(NH2SO3)), 22 g/l boric acid (H3BO3), 8 g/l nickel chloride (NiCl2 䡠6 H2O), 0.5 g/l surfactant, and different amounts of alumina nanopowder. The alumina nanopowder employed was a commercially available powder (NanoTek aluminium oxide 0100, Nanophase Technologies Corp.) produced by physical vapor synthesis (PVS). The particle size distribution of the powder can be 1359-6462/01/$–see front matter. © 2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. PII: S1359-6462(01)00799-0
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Figure 1. TEM pictures of Ni/Al2O3 samples produced in a sulfamate bath under different conditions: (left) film produced under DC conditions at j ⫽ 770 A/m2 and cp ⫽ 150 g/l; (middle) film produced under DC conditions at j ⫽ 150 A/m2 and cp ⫽ 10 g/l; (right) film produced under PDC conditions at ⌬ton ⫽ 200 ms, jp ⫽ 1000 A/m2 and cp ⫽ 10 g/l.
characterized by a log-normal distribution with a median particle diameter dm ⫽ 25 nm and a geometric standard deviation ⫽ 1.7. The nanoparticles are of spherical shape and are crystallized in the ␥-phase. The dispersion of the particles in the bath at 25°C was assisted by ultrasonic treatment during plating on flat polished copper cathodes. The pH value of the bath was kept between 3.5 and 4.0. Under DC conditions films were plated at current density j ⫽ 770 A/m2 and powder concentration in the bath cp ⫽ 150 g/l and at j ⫽ 150 A/m2 and cp ⫽ 10 g/l. In the case of PDC plating cp was 10 g/l and the ratio between pulse length ⌬ton and pulse pause ⌬toff was kept constant at 1:2.5. The pulse current density jp was 1000 A/m2. Films formed at higher jp exhibited a rough film surface and dark gray nickelhydroxide deposits. The as plated films and films after thermal treatment were prepared for TEM investigations by dimple grinding and ion etching. Measurements of the static coercivity Hc were carried out on specimens of 40 ⫻ 9 ⫻ 0.5 mm. Results and Discussion In the left and middle part of Fig. 1 TEM pictures of Ni/Al2O3 films plated at DC conditions in the sulfamate bath at j ⫽ 770 A/m2 and cp ⫽ 150 g/l as well as at j ⫽ 150 A/m2 and cp ⫽ 10 g/l, respectively, are shown. The right part of the figure shows a Ni/Al2O3 film plated in the same bath under PDC conditions at ⌬ton ⫽ 200 ms, cp ⫽ 10 g/l and jp ⫽ 1000 A/m2. In the DC plated specimens ceramic particles larger than 100 nm can be seen clearly and the co-deposited particles are agglomerated. The smaller particles seem to be attached to the largest particles of the PVS nanopowder employed. The average matrix grain size of the sample in Fig. 1 (left) is around 30 nm and the grain size of the film shown in Fig. 1 (middle) is around 60 nm. In the case of layer produced under PDC conditions (right) no particles larger than 100 nm and no big particle agglomerates of the kind seen in the other pictures can be found. Furthermore, the average matrix grain size is around 20 nm. This very fine matrix grain size was expected since PDC plating of pure nickel has already been shown to promote the formation of nanocrystalline nickel with grain sizes less than 20 nm (11,12). Due to the strong contrast of the TEM picture in Fig. 1 (right), caused by the very fine matrix structure, most of the fine
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Figure 2. TEM pictures of Ni/Al2O3 samples after thermal treatment: (left) film produced under DC conditions at j ⫽ 770 A/m2 and cp ⫽ 150 g/l after 1050°C for 20 h; (right) film produced under PDC conditions at ⌬ton ⫽ 200 ms, jp ⫽ 1000 A/m2 and cp ⫽ 10 g/l after 850°C for 20 h.
ceramic particles are hidden in the film. However after a thermal treatment at 850°C for 20 h matrix grain growth is induced and these fine ceramic particles are visible as can be seen in the right part of Fig 2. Figure 2 compares the sample from Fig. 1 after a 20 h thermal treatment at 1050°C (left) and 850°C (right). In Fig. 2 (left) the large particles (and former large agglomerates) decorate the grain boundaries of the film. No particles smaller than 20 nm can be found. Smaller particles in the as-DC plated film are only found in agglomerates; these particles start sintering with other adjacent particles in the film at 1050°C and form larger particles, as already found in earlier investigations [7,9]. These large globular particles pin matrix grain boundaries very efficiently which explains the fine matrix grain size after excessive heat treatment. In the case of the PDC plated film the heat treatment at 850°C induces grain growth of the Ni matrix (right part of Fig. 2). Obviously, the smaller ceramic particles pin matrix grain boundaries less efficiently. In contrast to the untreated film in Fig. 1 (right) the small ceramic particles can now be seen. Furthermore, no particles larger than 50 nm can be seen in Fig. 2 (right). Obviously a particle size selection during PDC plating took place. In contrast to the sample in the left part of the figure the ceramic particles are dispersed into the matrix grain after the thermal treatment. Should dislocation plasticity be operative, these dispersoids will act as obstacles for dislocation glide during plastic deformation. In Fig. 3 the values of the coercivity Hc for three different nickel films after various heat treatments at 850°C are compared. Hc depends strongly on the matrix grain size of a ferromagnetic material [13] and therefore can be employed to trace the evolution of the nickel matrix grain size [7–9]. In the figure the Hc value for the specimens shown in Fig. 1 (left) and (right) and of pure nickel plated under DC conditions are given. A drop in Hc is observed after the first 20 h of heat treatment for all samples, reflecting a structural change of the nickel matrix, as also observed by TEM inspection. Whereas Hc of the composites remains constant after further annealing, Hc of the pure nickel continues decreasing. These results are consistent with microstructure investigations which gave no hint to structural changes of the composites after further heat treatments at 850°C, while structural investigation of the pure nickel did. The PDC plated Ni/Al2O3 film shows about half the Hc value of the Ni/Al2O3 film plated under
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Figure 3. Hc of nickel samples produced under different conditions in a sulfamate bath after various heat treatments at 850°C. (1) sample produced under DC conditions at j ⫽ 770 A/m2 and cp ⫽ 150 g/l; (2) sample produced under PDC conditions at ⌬ton ⫽ 140 ms and jp ⫽ 1000 A/m2 in a bath with cp ⫽ 10 g/l; (3) pure nickel sample produced under DC conditions at j ⫽ 770 A/m2.
DC conditions. This finding is consistent with the larger grain size of the PDC plated specimen observed (Fig. 2). This is caused by less pinning of nickel grain boundaries by small dispersoids at the beginning of heat treatment. However, the volume fraction of co-deposited alumina nanoparticles is not exactly known in both specimens and a difference in the amount of co-deposited particles will of course also influence the thermal stability of the matrix grain structure. In conclusion, it was shown that under PDC electroplating conditions nanocrystalline Ni/Al2O3 composites can be produced. In contrast to DC plating the incorporated alumina nanoparticles are less agglomerated in the deposited film. Furthermore, under the PDC plating conditions used here smaller alumina nanoparticles are embedded more efficiently than larger particles of the nanopowder employed. That is to say, a particle size selection of the co-deposited nanoparticles during PDC plating takes place. These smaller particles pin grain boundaries of the nickel matrix less efficiently than large particles do. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
F. K. Sautter, J. Electrochem. Soc. 110, 557 (1963). V. O. Nwoko and L. L. Shreir, J. Appl. Electrochem. 3, 137 (1973). R. R. Oberle, M. R. Scanlon, R. C. Cammarata, and P. C. Searson, Appl. Phys. Lett. 66, 19 (1995). S. Steinha¨user and B. Wielage, in Proceedings of ICCM-11, ed. M. L. Scott, vol. III, pp. 495–504, Gold Cost, Australia (1997). H. Ferkel, B. Mu¨ller, and W. Riehemann, Mater. Sci. Eng. A234 –236, 474 (1997). B. Mu¨ller and H. Ferkel, Nanostruct. Mater. 10, 1285 (1998). B. Mu¨ller and H. Ferkel, Z. Metallkd. 11, 868 (1999). B. Mu¨ller und H. Ferkel, in Verbundwerkstoffe und Werkstoffverbunde, ed. K. Schulte und K. U. Kainer, p. 658, Wiley-VCH, Weinheim, DGM-Tagungsband (1999). B. Mu¨ller and H. Ferkel, J. Metast. Nanocrystall. Mater. 8, 476 (2000). J. Steinbach and H. Ferkel, in Proceedings of the NATO Advanced Study Institute: Functional Gradient Materials and Surface Layers Prepared by Fine Particles Technology, NATO ASI Kiev, in press. N. Wang, Z. Wang, K. T. Aust, and U. Erb, Mater. Sci. Eng. A237, 150 (1997). H. Natter, M. Schmelzer, and R. Hempelmann, J. Mater. Res. 13, 1186 (1998) and references therein. G. Herzer, IEEE Trans. Magn. 26, 1397 (1990).