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Thermal stability of carbon-encapsulated Fe–Nd–B nanoparticles
Journal of Alloys and Compounds 423 (2006) 74–76
Thermal stability of carbon-encapsulated Fe–Nd–B nanoparticles M. Bystrzejewski a,∗ , S. Cudziło b , A. Huczko a , H. Lange a a
b
Department of Chemistry, Warsaw University, Pasteur 1 Street, 02-093 Warsaw, Poland Department of Engineering, Chemistry and Technical Physics, Military University of Technology, Kaliskiego 1 Street, 00-908 Warsaw, Poland Available online 26 January 2006
Abstract Thermal stability of various magnetic nanomaterials is very essential, due to their prospective future applications. In this paper, thermal behaviour of the carbon-encapsulated Fe–Nd–B nanoparticles is studied. These nanostructures were produced by direct current arcing of carbon anodes filled with Nd2 Fe14 B material. The thermogravimetry and differential thermal analysis curves were recorded in an oxygen atmosphere. The thermal processes were monitored by X-ray diffraction to follow the changes in the phase composition. The investigated samples have been thermally stable up to 600 K. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructures; X-ray diffraction; Thermal analysis
1. Introduction It may be a dream of material scientists to handle and store metallic nanoclusters freely in air without oxidation. This would make the application of metallic nanoclusters wide and easy. Ferromagnetic nanoparticles are very useful for many prospective applications in various areas such as magnetic data storage [1], catalysis [2], xerography [3], magnetic resonance imaging [4], and biomedical applications [5]. However, the application of such nanoclusters is limited by the agglomeration into bigger objects and their sensitivity to air oxidation. These disadvantages disappear in the case of carbon-encapsulated nanoparticles (named also nanocapsules or encapsulates). They are consisted of the crystalline core (filled with the simple elements, alloys, or compounds) and the carbon coating (which could be amorphous or crystalline). The role of carbon is to isolate the nanoparticles magnetically from each other, thus avoiding the problems caused by interactions between closely compacted magnetic domains, and to provide the oxidation resistance of the bare metal nanoparticles. The encapsulates can be easily produced by using the carbon arc [6,7], RF [8] plasmas, and by using combustion synthesis [9]. In the presented study we show the results on the carbon encapsulation of a hard magnetic material (Fe14 Nd2 B), which is routinely used in the construction of permanent magnets. The thermal stability of these nanostruc∗
Corresponding author. Fax: +48 22 822 59 96. E-mail address: [email protected] (M. Bystrzejewski).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.12.047
tures was also studied by means of thermogravimetry (TG) and differential thermal analysis (DTA) methods. 2. Experimental Fe–Nd–B carbon-encapsulated nanoparticles were prepared by an arc discharge method described elsewhere [5,10,11]. Four different tests were performed under different pressure and Fe14 Nd2 B anode loading (Table 1). The discharge current and voltage was between 61 and 83 A and 17 and 28 V, respectively. The graphite anodes (6 mm in diameter, 100 mm in length) were drilled (3 mm in diameter, 30 mm in length) and filled with a mixture of Fe14 Nd2 B (Magnequench International) and graphite powder. The pure graphite rods were used as cathodes. The as-prepared electrodes were arc sublimated. Two kinds of products were obtained: (i) the resulted soot collected from the walls of the reactor, and (ii) the deposit grown on the cathode. All products were subjected to the purification procedure (to remove non-encapsulated metals and amorphous carbon). A sample of soot (around 1000 mg) was put into a flask, and boiled with 2 M HNO3 for 24 h. Then each sample was washed with distilled water several times, and finally dried in an oven in air atmosphere at 350 K. High-resolution transmission electron microscope (HRTEM, Jeol 3010) operating at 300 kV was used to reveal the internal structure of the nanocapsules. The HRTEM samples were prepared by dispersing the products in ethanol with ultrasonic bath and then drying a drop of the suspension on a copper grid. X-ray diffraction (Siemens Diffractometer D500) was employed to identify the phase composition. The thermal stability of the carbon-encapsulated Fe–Nd–B nanoparticles was investigated by TG–DTA measurements (LABSYS Setaram) at the heating rate 2 K min−1 in an oxygen atmposhere.
3. Results and discussion Fig. 1 presents the HRTEM images of the purified products. The core-shell structure is clearly seen. The core diameter
M. Bystrzejewski et al. / Journal of Alloys and Compounds 423 (2006) 74–76
75
Table 1 Anode composition and arc parameters Test no.
Fe14 Nd2 B content in anode (wt.%)
Voltage (V)
Current (A)
Static pressure (kPa)
1 2 3 4
27 42 27 42
17 19 27 28
83 61 79 65
13.3 60.0
was between 10 and 30 nm. The carbon coating thickness was between 2 and 10 nm, what means that it was composed of 5–30 graphene layers. The interlayer distance in the graphene coating estimated from HRTEM images was around 0.33 nm. The shape of the cores was generally spherical, but for the products from cathode deposits were more angular (Fig. 1). The EDS analysis showed that the cores are filled with Fe, Nd and B, or their alloys. All products exhibit ferromagnetic behaviour. The values of the coercivity (0.1–33 kA/m) and the saturation magnetization (2–32 × 10−3 Am2 /g) strongly depend on the operational parameters. The comparison of the literature data [12] concerning the magnetic characteristics of the carbon nanotube filled with Fe14 Nd2 B with our results is a indirect proof that the carbon-encapsulated material has the same stoichiometry as starting powder. The detailed structure and magnetic studies of these nanostructures were presented in our previous papers [5,10]. Fig. 2 shows TG and DTA curves for the soot. All TG and DTA curves of soots obtained in tests 1–4 are very similar. Below 400 K, a weight loss of 2–10 wt.% occurs in the soots. This endothermic process (a weak peak on DTA curve, Fig. 2) is attributed to the release of adsorbed gases and moisture on the surface of the nanocapsules. The weight increase of 2.1 wt.% in the temperature interval 420–570 K is related with oxidiation of not removed (during purification procedure) iron and neodymium. This exothermic phenomenon (a weak peak on DTA curve at around 480 K) was observed only for soots. In the temperature interval 600–720 K weight loss of 41.6 wt.% is observed. This decrease is accompanied by exothermic reactions (two peaks on DTA curve, at 578 and 663 K) and can be ascribed to the burning the carbon coatings, and finally the cores oxidation. The weight loss of 3.4 is observed in the temperature
Fig. 2. TG and DTA (oxygen atmosphere) curves of the soot obtained in test 2.
range 720–1100 K. This exothermic reaction is attributed to the burning the graphite microcrystals. The residue formed after burning has been investigated by X-ray diffraction. It contained Fe2 O3 , Fe3 O4 , FeNdO3 , and NdBO3 . This result indicates, that the nanocapsules cores were filled with iron, Fe–Nd or Nd–B alloys, presumably with the starting Fe14 Nd2 B. The in-depth studies (electron diffraction) are currently under way to further elucidate the composition of the core. The residue mass (42.1 wt.%) is directly related with the nanocapsules content, because it was generated from the nanocapsules cores. During the heat treatment (TG/DTA analysis) the core remains unchanged until the carbon coating is burned off and the oxidation temperature of elements is reached, which is followed by the formation of respective oxides. Fig. 3 shows TG and DTA curves of purified cathode deposit obtained in test 2 (the samples from other tests exhibit similar thermal behaviour). The cathode deposit during arc discharge grows at very high temperature (3300–3500 K) [11]. Under such conditions Fe, Nd, their carbides as well as carbon nanocapsules can easily evaporate. Thus, there were no mass increase below 400 K. The first exothermal peak on DTA curve, accompanied by weight decrease (3.5%) is located at 570 K. This effect is related with the burning of the carbon coatings of the nanocapsules. There is a mass decrease of 86.7 wt.% observed in the temperature interval 670–1020 K, connected with two exother-
Fig. 1. HRTEM images of obtained products (test 2) from soot and cathode deposit, left and right, respectively.
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coatings, the core, and finally the graphite microcrystals. The mass of the residue formed during oxidation is an important indicator of the nanocapsules concentration. Acknowledgements This work was supported by the Committee for Scientific Research (KBN) through the Department of Chemistry, Warsaw University, under Grant No. 3 T08D 012 28. Authors also thank Prof. J. Kozubowski and M. Wo´zniak for HRTEM images. References
Fig. 3. TG and DTA (oxygen atmosphere) curves of the cathode deposit obtained in test 2.
mal reactions (two peaks at 748 and 856 K). During this weight loss two reactions take place: (i) burning the nanocapsules cores, and (ii) burning the graphite microcrystals. The residue mass (9.5 wt.%), containing Fe2 O3 and NdBO3 , clearly indicates that the concentration of carbon nanocapsules in the cathode deposit is much lower than in soot. 4. Conclusions We have showed, that carbon nanocapsules formed in carbon arc are thermally stable in oxygen atmosphere up to 600 K. Below that temperature (i) the releasing of adsorbed gases, and moisture, and (ii) the burning of not dissolved Fe, and Nd occurs. The temperature increase leads to the oxidation of the carbon
[1] R.K. Kodama, J. Magn. Magn. Mater. 200 (1999) 359–372. [2] T. Oku, T. Hirata, N. Motegi, R. Hatakeyama, N. Sato, T. Mieno, N.Y. Sato, H. Mase, M. Niwano, M. Miyamoto, J. Mater. Res. 15 (10) (2000) 2182–2186. [3] H. Zhang, J. Chen, Y. He, X. Xue, S. Peng, J. Mater. Chem. Phys. 55 (1998) 167–172. [4] H. Song, X. Chen, Chem. Phys. Lett. 374 (2003) 400–404. [5] M. Bystrzejewski, A. Huczko, H. Lange, Sensors Actuat. B 109 (2005) 81–85. [6] Y. Saito, Carbon 33 (1995) 979–988. [7] H. Lange, P. Baranowski, P. Byszewski, A. Huczko, Rev. Sci. Instrum. 68 (1997) 3723–3727. [8] G. Cota-Sanchez, G. Soucy, A. Huczko, H. Lange, Trans. Mater. Res. Soc. Jpn. 25 (2004) 29–35. [9] S. Cudziło, M. Bystrzejewski, H. Lange, A. Huczko, Carbon 43 (2005) 1778–1782. [10] M. Bystrzejewski, A. Huczko, H. Lange, P. Baranowski, J. Kozubowski, M. Wo´zniak, M. Leonowicz, W. Kaszuwara, Solid State Phenom. 99/100 (2004) 273–278. [11] H. Lange, K. Saidane, M. Razafinimanana, A. Gleizes, J. Appl. Phys. D: Appl. Phys. 32 (2000) 1024–1030. [12] C.T. Kuo, C.H. Lin, A.Y. Lo, Diamond Relat. Mater. 12 (2003) 799–805.
Thermal stability of carbon-encapsulated Fe–Nd–B nanoparticles M. Bystrzejewski a,∗ , S. Cudziło b , A. Huczko a , H. Lange a a
b
Department of Chemistry, Warsaw University, Pasteur 1 Street, 02-093 Warsaw, Poland Department of Engineering, Chemistry and Technical Physics, Military University of Technology, Kaliskiego 1 Street, 00-908 Warsaw, Poland Available online 26 January 2006
Abstract Thermal stability of various magnetic nanomaterials is very essential, due to their prospective future applications. In this paper, thermal behaviour of the carbon-encapsulated Fe–Nd–B nanoparticles is studied. These nanostructures were produced by direct current arcing of carbon anodes filled with Nd2 Fe14 B material. The thermogravimetry and differential thermal analysis curves were recorded in an oxygen atmosphere. The thermal processes were monitored by X-ray diffraction to follow the changes in the phase composition. The investigated samples have been thermally stable up to 600 K. © 2005 Elsevier B.V. All rights reserved. Keywords: Nanostructures; X-ray diffraction; Thermal analysis
1. Introduction It may be a dream of material scientists to handle and store metallic nanoclusters freely in air without oxidation. This would make the application of metallic nanoclusters wide and easy. Ferromagnetic nanoparticles are very useful for many prospective applications in various areas such as magnetic data storage [1], catalysis [2], xerography [3], magnetic resonance imaging [4], and biomedical applications [5]. However, the application of such nanoclusters is limited by the agglomeration into bigger objects and their sensitivity to air oxidation. These disadvantages disappear in the case of carbon-encapsulated nanoparticles (named also nanocapsules or encapsulates). They are consisted of the crystalline core (filled with the simple elements, alloys, or compounds) and the carbon coating (which could be amorphous or crystalline). The role of carbon is to isolate the nanoparticles magnetically from each other, thus avoiding the problems caused by interactions between closely compacted magnetic domains, and to provide the oxidation resistance of the bare metal nanoparticles. The encapsulates can be easily produced by using the carbon arc [6,7], RF [8] plasmas, and by using combustion synthesis [9]. In the presented study we show the results on the carbon encapsulation of a hard magnetic material (Fe14 Nd2 B), which is routinely used in the construction of permanent magnets. The thermal stability of these nanostruc∗
Corresponding author. Fax: +48 22 822 59 96. E-mail address: [email protected] (M. Bystrzejewski).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2005.12.047
tures was also studied by means of thermogravimetry (TG) and differential thermal analysis (DTA) methods. 2. Experimental Fe–Nd–B carbon-encapsulated nanoparticles were prepared by an arc discharge method described elsewhere [5,10,11]. Four different tests were performed under different pressure and Fe14 Nd2 B anode loading (Table 1). The discharge current and voltage was between 61 and 83 A and 17 and 28 V, respectively. The graphite anodes (6 mm in diameter, 100 mm in length) were drilled (3 mm in diameter, 30 mm in length) and filled with a mixture of Fe14 Nd2 B (Magnequench International) and graphite powder. The pure graphite rods were used as cathodes. The as-prepared electrodes were arc sublimated. Two kinds of products were obtained: (i) the resulted soot collected from the walls of the reactor, and (ii) the deposit grown on the cathode. All products were subjected to the purification procedure (to remove non-encapsulated metals and amorphous carbon). A sample of soot (around 1000 mg) was put into a flask, and boiled with 2 M HNO3 for 24 h. Then each sample was washed with distilled water several times, and finally dried in an oven in air atmosphere at 350 K. High-resolution transmission electron microscope (HRTEM, Jeol 3010) operating at 300 kV was used to reveal the internal structure of the nanocapsules. The HRTEM samples were prepared by dispersing the products in ethanol with ultrasonic bath and then drying a drop of the suspension on a copper grid. X-ray diffraction (Siemens Diffractometer D500) was employed to identify the phase composition. The thermal stability of the carbon-encapsulated Fe–Nd–B nanoparticles was investigated by TG–DTA measurements (LABSYS Setaram) at the heating rate 2 K min−1 in an oxygen atmposhere.
3. Results and discussion Fig. 1 presents the HRTEM images of the purified products. The core-shell structure is clearly seen. The core diameter
M. Bystrzejewski et al. / Journal of Alloys and Compounds 423 (2006) 74–76
75
Table 1 Anode composition and arc parameters Test no.
Fe14 Nd2 B content in anode (wt.%)
Voltage (V)
Current (A)
Static pressure (kPa)
1 2 3 4
27 42 27 42
17 19 27 28
83 61 79 65
13.3 60.0
was between 10 and 30 nm. The carbon coating thickness was between 2 and 10 nm, what means that it was composed of 5–30 graphene layers. The interlayer distance in the graphene coating estimated from HRTEM images was around 0.33 nm. The shape of the cores was generally spherical, but for the products from cathode deposits were more angular (Fig. 1). The EDS analysis showed that the cores are filled with Fe, Nd and B, or their alloys. All products exhibit ferromagnetic behaviour. The values of the coercivity (0.1–33 kA/m) and the saturation magnetization (2–32 × 10−3 Am2 /g) strongly depend on the operational parameters. The comparison of the literature data [12] concerning the magnetic characteristics of the carbon nanotube filled with Fe14 Nd2 B with our results is a indirect proof that the carbon-encapsulated material has the same stoichiometry as starting powder. The detailed structure and magnetic studies of these nanostructures were presented in our previous papers [5,10]. Fig. 2 shows TG and DTA curves for the soot. All TG and DTA curves of soots obtained in tests 1–4 are very similar. Below 400 K, a weight loss of 2–10 wt.% occurs in the soots. This endothermic process (a weak peak on DTA curve, Fig. 2) is attributed to the release of adsorbed gases and moisture on the surface of the nanocapsules. The weight increase of 2.1 wt.% in the temperature interval 420–570 K is related with oxidiation of not removed (during purification procedure) iron and neodymium. This exothermic phenomenon (a weak peak on DTA curve at around 480 K) was observed only for soots. In the temperature interval 600–720 K weight loss of 41.6 wt.% is observed. This decrease is accompanied by exothermic reactions (two peaks on DTA curve, at 578 and 663 K) and can be ascribed to the burning the carbon coatings, and finally the cores oxidation. The weight loss of 3.4 is observed in the temperature
Fig. 2. TG and DTA (oxygen atmosphere) curves of the soot obtained in test 2.
range 720–1100 K. This exothermic reaction is attributed to the burning the graphite microcrystals. The residue formed after burning has been investigated by X-ray diffraction. It contained Fe2 O3 , Fe3 O4 , FeNdO3 , and NdBO3 . This result indicates, that the nanocapsules cores were filled with iron, Fe–Nd or Nd–B alloys, presumably with the starting Fe14 Nd2 B. The in-depth studies (electron diffraction) are currently under way to further elucidate the composition of the core. The residue mass (42.1 wt.%) is directly related with the nanocapsules content, because it was generated from the nanocapsules cores. During the heat treatment (TG/DTA analysis) the core remains unchanged until the carbon coating is burned off and the oxidation temperature of elements is reached, which is followed by the formation of respective oxides. Fig. 3 shows TG and DTA curves of purified cathode deposit obtained in test 2 (the samples from other tests exhibit similar thermal behaviour). The cathode deposit during arc discharge grows at very high temperature (3300–3500 K) [11]. Under such conditions Fe, Nd, their carbides as well as carbon nanocapsules can easily evaporate. Thus, there were no mass increase below 400 K. The first exothermal peak on DTA curve, accompanied by weight decrease (3.5%) is located at 570 K. This effect is related with the burning of the carbon coatings of the nanocapsules. There is a mass decrease of 86.7 wt.% observed in the temperature interval 670–1020 K, connected with two exother-
Fig. 1. HRTEM images of obtained products (test 2) from soot and cathode deposit, left and right, respectively.
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M. Bystrzejewski et al. / Journal of Alloys and Compounds 423 (2006) 74–76
coatings, the core, and finally the graphite microcrystals. The mass of the residue formed during oxidation is an important indicator of the nanocapsules concentration. Acknowledgements This work was supported by the Committee for Scientific Research (KBN) through the Department of Chemistry, Warsaw University, under Grant No. 3 T08D 012 28. Authors also thank Prof. J. Kozubowski and M. Wo´zniak for HRTEM images. References
Fig. 3. TG and DTA (oxygen atmosphere) curves of the cathode deposit obtained in test 2.
mal reactions (two peaks at 748 and 856 K). During this weight loss two reactions take place: (i) burning the nanocapsules cores, and (ii) burning the graphite microcrystals. The residue mass (9.5 wt.%), containing Fe2 O3 and NdBO3 , clearly indicates that the concentration of carbon nanocapsules in the cathode deposit is much lower than in soot. 4. Conclusions We have showed, that carbon nanocapsules formed in carbon arc are thermally stable in oxygen atmosphere up to 600 K. Below that temperature (i) the releasing of adsorbed gases, and moisture, and (ii) the burning of not dissolved Fe, and Nd occurs. The temperature increase leads to the oxidation of the carbon
[1] R.K. Kodama, J. Magn. Magn. Mater. 200 (1999) 359–372. [2] T. Oku, T. Hirata, N. Motegi, R. Hatakeyama, N. Sato, T. Mieno, N.Y. Sato, H. Mase, M. Niwano, M. Miyamoto, J. Mater. Res. 15 (10) (2000) 2182–2186. [3] H. Zhang, J. Chen, Y. He, X. Xue, S. Peng, J. Mater. Chem. Phys. 55 (1998) 167–172. [4] H. Song, X. Chen, Chem. Phys. Lett. 374 (2003) 400–404. [5] M. Bystrzejewski, A. Huczko, H. Lange, Sensors Actuat. B 109 (2005) 81–85. [6] Y. Saito, Carbon 33 (1995) 979–988. [7] H. Lange, P. Baranowski, P. Byszewski, A. Huczko, Rev. Sci. Instrum. 68 (1997) 3723–3727. [8] G. Cota-Sanchez, G. Soucy, A. Huczko, H. Lange, Trans. Mater. Res. Soc. Jpn. 25 (2004) 29–35. [9] S. Cudziło, M. Bystrzejewski, H. Lange, A. Huczko, Carbon 43 (2005) 1778–1782. [10] M. Bystrzejewski, A. Huczko, H. Lange, P. Baranowski, J. Kozubowski, M. Wo´zniak, M. Leonowicz, W. Kaszuwara, Solid State Phenom. 99/100 (2004) 273–278. [11] H. Lange, K. Saidane, M. Razafinimanana, A. Gleizes, J. Appl. Phys. D: Appl. Phys. 32 (2000) 1024–1030. [12] C.T. Kuo, C.H. Lin, A.Y. Lo, Diamond Relat. Mater. 12 (2003) 799–805.