TY - JOUR
AU - Suganuma,, Katsuaki
AB - Abstract Iron (Fe) nanoparticles coated with boron nitride (BN) nanomaterials were synthesized by using Fe4N and B powders as raw materials. The Fe4N was reduced to α-Fe during annealing at 1000°C for several hours with flowing 100 sccm N2 gas. The reaction was predicted by Ellingham diagram. The atomic structure and magnetic properties were investigated by high-resolution electron microscopy and vibrating sample magnetometer system. BN nanomaterial, Fe nanocapsule coated with BN layers, Bamboo BN nanotube, High-resolution electron microscopy Introduction Nanosized particles have been extensively studied not only for their fundamental interests but also for their unique magnetic and electronic transport properties, and they have much potential for the future applications [1–3]. Especially, metal nanoparticles of Fe, Co, and some Fe-based alloys have an advantage of showing high magnetization. However, oxidation- and wear resistances of the surface are weak points of these nanoparticles. In addition, most magnetic materials are accompanied by exothermic heat due to the eddy current loss in high frequency. As the solution, it is well known that increase in the resistance using insulators is good. Recently, boron nitride (BN) nanomaterials, such as nanocapsules, nanohorns and nanotubes, have been discovered and have attracted the attention among researches as an allotropic form of carbon nanomaterials [4–8]. The BN nanomaterials provide excellent protection against oxidation and wear, and the conductance is insulator (∼5 eV). Therefore, magnetic nanoparticles coated with BN nanomaterials would have significant advantages for technological applications. Fe and Co nanocapsules coated with BN layers have been synthesized by arc-discharge method [9–12]. However, the arc-discharge method is not suitable for mass production because of limitation of the plasma area, and it is difficult to control nanoparticles size and the number of BN layers. Fe nanocapsules coated with BN layers have been synthesized by annealing of boron and α-Fe2O3 powders [13,14]. However these samples contained oxygen in the starting materials. Therefore high annealing temperature was needed to remove the oxygen. In addition, magnetizations of these samples were low due to oxidation of Fe nanoparticles. The purpose of the present work is to synthesize Fe nanoparticles coated with BN nanomaterials, and to investigate the nano structures and magnetic properties. Ellingham diagram of nitride metals for N2 gas (per mol), which were calculated by HSC software, is shown in Fig. 1. Fe4N particles would be reduced to α-Fe completely by annealing with boron, because boron easily reacts with nitrogen than Fe. Similarly, several nitrides would be reduced to pure metals by reaction with boron. Fig. 1 Open in new tabDownload slide Ellingham diagram of Fe, Ni and Co nitrides for a N2 molecule. Fig. 1 Open in new tabDownload slide Ellingham diagram of Fe, Ni and Co nitrides for a N2 molecule. Methods Fe4N and B powders were used as raw materials. Their particle diameters were ∼50 and 45 μm, respectively. After the Fe4N and B (weight ratio [WR] = 5:5, 7:3, 9:1) were well mixed by a triturator, the samples were set on an alumina boat. The samples were annealed at 1000°C for 1 or 5 h with flowing 100 sccm nitrogen (N2) gas, and cooled down to room temperature in a furnace. Phases of the samples were determined by X-ray diffraction with Cu-Kα irradiation. To observe the morphology of samples, high-resolution electron microscopy (HREM) observation was performed with a 300 kV electron microscope (JEM-3000F). Magnetic properties were measured by a vibrating sample magnetometer system (VSM) under 0.8 mA m−1 DC field, and results of the saturation magnetization were compared. To investigate reaction process between Fe4N and B during annealing, differential thermal analysis (DTA) was carried out from 25 to 1100°C at an elevation rate of 10°C min−1 with flowing N2 gas (20 sccm). Results and discussion X-ray diffraction patterns of some samples were indicated in Fig. 2. Peaks of hexagonal BN and α-Fe were confirmed for all samples. Average particle diameters of Fe were summarized in Table 1, which were calculated from the half-widths of α-Fe (110) using Scherrer Equation. Table 1 Average particle diameters of Fe. Results of VSM measurement of Fe nanoparticles coated with BN nanomaterials at room temperature. Saturation magnetization (Ms*) and coercivity (Hc*) values of Fe nanoparticles coated with BN nanomaterials were measured after a PC test (120°C × 12 h, humidity 100%, 1 atm). Values of degauss coefficient were calculated according to the following equation: (Ms* − Ms)/Ms × 100% Composition of Fe4N:B Annealing time at 1000°C (h) Particle diameter (nm) Ms(emu g−1) Hc(Oe) Ms* (emu g−1) Hc* (Oe) Degauss coefficient (%) 5:5 1 24 95.0 24.4 78.3 42.5 −17.6 5:5 5 28 92.6 22.5 78.9 39.8 −14.8 7:3 1 — 134.2 20.9 117.4 33.8 −12.5 9:1 1 30 174.9 19.0 149.9 37.5 −14.3 Composition of Fe4N:B Annealing time at 1000°C (h) Particle diameter (nm) Ms(emu g−1) Hc(Oe) Ms* (emu g−1) Hc* (Oe) Degauss coefficient (%) 5:5 1 24 95.0 24.4 78.3 42.5 −17.6 5:5 5 28 92.6 22.5 78.9 39.8 −14.8 7:3 1 — 134.2 20.9 117.4 33.8 −12.5 9:1 1 30 174.9 19.0 149.9 37.5 −14.3 Open in new tab Table 1 Average particle diameters of Fe. Results of VSM measurement of Fe nanoparticles coated with BN nanomaterials at room temperature. Saturation magnetization (Ms*) and coercivity (Hc*) values of Fe nanoparticles coated with BN nanomaterials were measured after a PC test (120°C × 12 h, humidity 100%, 1 atm). Values of degauss coefficient were calculated according to the following equation: (Ms* − Ms)/Ms × 100% Composition of Fe4N:B Annealing time at 1000°C (h) Particle diameter (nm) Ms(emu g−1) Hc(Oe) Ms* (emu g−1) Hc* (Oe) Degauss coefficient (%) 5:5 1 24 95.0 24.4 78.3 42.5 −17.6 5:5 5 28 92.6 22.5 78.9 39.8 −14.8 7:3 1 — 134.2 20.9 117.4 33.8 −12.5 9:1 1 30 174.9 19.0 149.9 37.5 −14.3 Composition of Fe4N:B Annealing time at 1000°C (h) Particle diameter (nm) Ms(emu g−1) Hc(Oe) Ms* (emu g−1) Hc* (Oe) Degauss coefficient (%) 5:5 1 24 95.0 24.4 78.3 42.5 −17.6 5:5 5 28 92.6 22.5 78.9 39.8 −14.8 7:3 1 — 134.2 20.9 117.4 33.8 −12.5 9:1 1 30 174.9 19.0 149.9 37.5 −14.3 Open in new tab Fig. 2 Open in new tabDownload slide X-ray diffraction patterns of the annealed samples of Fe4N, which are (a) WR of Fe4N:B = 5:5 annealed at 1000°C for 1 h, (b) WR of Fe4N:B = 5:5 annealed at 1000°C for 5 h and (c) WR of Fe4N:B = 9:1 annealed at 1000°C for 1 h, respectively. Fig. 2 Open in new tabDownload slide X-ray diffraction patterns of the annealed samples of Fe4N, which are (a) WR of Fe4N:B = 5:5 annealed at 1000°C for 1 h, (b) WR of Fe4N:B = 5:5 annealed at 1000°C for 5 h and (c) WR of Fe4N:B = 9:1 annealed at 1000°C for 1 h, respectively. HREM images of samples are shown in Fig. 3. Figure 3a is an image of Fe nanocapsules coated with BN layers, which are produced by annealing the powder of Fe4N:B = 1:1 at 1000°C for 1 h. Bamboo BN nanotubes were also produced, as shown in Fig 3b. The length and width of bamboo BN nanotubes are ∼5–10 μm and 40 nm, respectively. Figure 3c is an image of the tip of bamboo BN nanotube, which is produced by annealing the powder of Fe4N:B = 1:1 at 1000°C for 5 h. The enlarged image is shown in Fig 3d. Samples annealed for 5 h were almost similar to the ones annealed for 1 h. Fe nanoparticle was observed at the one end of bamboo BN nanotube. The other side was amorphous boron. An HREM image of Fe4N:B = 9:1 annealed at 1000°C for 1 h is shown in Fig. 3e. Figure 3f is the enlarged image of Fig. 3e. Cap-stacked type BN nanotubes were also observed. Interval of the cells in the direction of tube length was ∼5 nm, which was narrower than the samples of Fe4N:B = 1:1 (Interval of the cells ≈ 30 nm). Fig. 3 Open in new tabDownload slide (a) Fe nanocapsules coated with BN layers. (b) Image of bamboo BN nanotubes. (c) Low magnification image of the end of bamboo BN nanotubes. (d) Enlarged image of (c). (e) Low magnification image of a cap-stacked type BN nanotubes. (f) Enlarged image of (e). (a–d) WR of Fe4N:B = 5:5. (e, f) WR of Fe4N:B = 9:1. Samples were annealed at 1000°C for (a, b, e, f) 1 h and (c, d) 5 h. Fig. 3 Open in new tabDownload slide (a) Fe nanocapsules coated with BN layers. (b) Image of bamboo BN nanotubes. (c) Low magnification image of the end of bamboo BN nanotubes. (d) Enlarged image of (c). (e) Low magnification image of a cap-stacked type BN nanotubes. (f) Enlarged image of (e). (a–d) WR of Fe4N:B = 5:5. (e, f) WR of Fe4N:B = 9:1. Samples were annealed at 1000°C for (a, b, e, f) 1 h and (c, d) 5 h. A magnetic hysteresis loop is shown in Fig. 4, which exhibits a soft magnetic characteristic. The saturation magnetization (Ms) and coercivity (Hc) values were 174.9 emu g−1 and 19.0 Oe, respectively. The Ms and Hc values are 80 and 88% of bulk Fe (217.6 emu g−1, 21.5 Oe). Results of the VSM measurements for some composition samples are summarized in Table 1. Fe powder that oxidized only the surface gives 130 emu g−1 of Ms value while Ms value of magnetite (Fe3O4) is 92 emu g−1. In case of Fe nanoparticles coated with BN nanomaterials, the Ms values were reduced due to the weight loss of BN layers. Therefore, the Ms values of Fe nanoparticles coated with BN nanomaterials are high enough values as pure Fe metal. To investigate the oxidation- and wear resistances, saturation magnetization (Ms*) and coercivity (Hc*) values of samples were measured by VSM, after pressure cooker test (PCT) was carried out under the following condition: 120°C × 12 h, humidity 100% and 1 atm. Magnetic properties after the PCT test are summarized in Table 1. The values of degauss coefficient were calculated according to the following equation: (Ms* − Ms)/Ms × 100%. The sample of Fe4N:B = 7:3 were more stable for oxidation- and wear resistances than other samples in the present work. Although Ms value is high for the increasing of Fe4N powder in the composition, appreciate amount of B powder is needed to keep the high Ms value for oxidation- and wear resistances by covering the Fe nanoparticles with BN layers. Fig. 4 Open in new tabDownload slide Hysteresis loop of Fe4N:B = 9:1 annealed at 1000°C for 1 h with flowing 100 sccm N2 gas. The values of Ms and Hc are 174.9 emu g−1 and 19.0 Oe, respectively. Fig. 4 Open in new tabDownload slide Hysteresis loop of Fe4N:B = 9:1 annealed at 1000°C for 1 h with flowing 100 sccm N2 gas. The values of Ms and Hc are 174.9 emu g−1 and 19.0 Oe, respectively. A DTA measurement is shown in Fig. 5 in order to reveal the reaction process. The first peak confirmed between 300 and 400°C corresponds to an exothermal reaction of Fe4N and B. In the process, Fe4N are reduced to Fe. These Fe particles are nitrided the surface above 500°C. Therefore, the second exothermal peak between 500°C and 950°C would be due to nitridation and reduction of Fe particles. In the present works, Fe nanocapsules coated with BN layers and bamboo BN nanotubes were synthesized as shown in Fig. 3. From the look of it, Fe nanoparticles are needed for the growth of BN layers. A Model for the growth of Fe nanocapsules coated with BN layers and bamboo BN nanotubes encaging Fe nanoparticles was shown in Fig. 6. Amorphous boron would be formed around the surface of Fe4N nanoparticles over 300°C. Amorphous boron changes to BN layers on the surface of the Fe4N particles with reducing it to α-Fe. Fe nanocapsules coated with BN layers were mainly formed when the Fe nanoparticle size is small (< 20 nm). On the other hand, bamboo BN nanotubes encaging Fe nanoparticles were formed when the Fe nanoparticle size is large (>100 nm). Formative BN layers do not react with Fe particles under 1350°C. Therefore, BN layers exfoliate easily from the surface of Fe particle before covering over it. Strip Fe particles react with B and N again, and BN layers are formed around the surface. Bamboo BN nanotubes would be formed by repeating of the reactions. Fig. 5 Open in new tabDownload slide DTA curve on the reaction of Fe4N and B (WR = 5:5). The rates of rising temperature and flowing N2 gas were 10°C min−1 and 20 sccm, respectively. Fig. 5 Open in new tabDownload slide DTA curve on the reaction of Fe4N and B (WR = 5:5). The rates of rising temperature and flowing N2 gas were 10°C min−1 and 20 sccm, respectively. Fig. 6 Open in new tabDownload slide Models for the growth of Fe nanoparticles coated with BN nanomaterials. (a) Fe nanocapsule coated with BN layers and (b) bamboo BN nanotube. Fig. 6 Open in new tabDownload slide Models for the growth of Fe nanoparticles coated with BN nanomaterials. (a) Fe nanocapsule coated with BN layers and (b) bamboo BN nanotube. Conclusion The present work provides the new process for Fe nanoparticles coated with BN nanomaterials in large quantities compared with an ordinary arc-discharge method. Ellingham diagram drawn in the present work is useful for the design of BN nanomaterial formation. Fe4N particles were reduced to α-Fe completely by the reaction. Fe nanoparticles coated with BN nanomaterials behaved as a good magnetic material against oxidation and wear. Cap-stacked type BN nanotubes were also produced in the present work. These unique structures would be suitable materials for H2 gas storage. The authors would like to thank Dr Munenori Yamashita for experimental help and warm encouragement. This work was supported by Research Fellowship of the Japan Society for the Promotion of Science for Young Scientist, Grant-in-Aid, Ministry of Education, Science, Sports and Culture of Japan. References 1 Kruis F E , Fissan H , Peled A . Synthesis of nanoparticles in the gas phase for electronic, optical and magnetic applications , J Aerosol Sci , 1998 , vol. 29 (pg. 511 - 535 ) Google Scholar Crossref Search ADS WorldCat 2 McHenry M E , Majetich S A , Artman J O , DeGraef M , Staley S W . Superparamagnetism in carbon-coated Co particles produced by the Kratschmer carbon arc process , Phys Rev , 1994 , vol. B49 (pg. 11358 - 11363 ) Google Scholar Crossref Search ADS WorldCat 3 Sun S , Murray C B , Weller D , Folks L , Moser A . 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Published by Oxford University Press on behalf of Japanese Society of Microscopy. All rights reserved. For permissions, please email: journals.permissions@oxfordjournals.org
TI - Synthesis and structures of iron nanoparticles coated with boron nitride nanomaterials
JO - Journal of Electron Microscopy
DO - 10.1093/jmicro/dfl005
DA - 2006-06-01
UR - https://www.deepdyve.com/lp/oxford-university-press/synthesis-and-structures-of-iron-nanoparticles-coated-with-boron-ehc5AmQOUm
SP - 123
EP - 127
VL - 55
IS - 3
DP - DeepDyve
ER -