Effect of carbon addition on synthesis of BN nanolayers encapsulating Fe fine particles and BN nanotubes

Article abstract of DOI:10.1016/j.ssc.2004.12.034

Boron nitride (BN) nanolayers encapsulating iron (Fe) fine particles and BN nanotubes were synthesized by annealing mixture powders of hematite, boron and carbon (C) at 1473 K for 2 h in a nitrogen atmosphere. The particles exhibited good soft magnetic properties. The thickness of BN nanolayers increased in the range of 10-100 nm with increasing C content in the mixture powders, which results in providing improvement of oxidation resistance of the particles. In the BN layers, no C was detected. BN nanotubes with diameters of ～100 nm were also synthesized, and they had cup-stacked and bamboo-like structures.

Full text of DOI:10.1016/j.ssc.2004.12.034

Solid State Communications 133 (2005) 681–685
www.elsevier.com/locate/ssc
Effect of carbon addition on synthesis of BN nanolayers
encapsulating Fe fine particles and BN nanotubes
a,
Hisato Tokoro , Shigeo Fujii , Takeo Oku
a
b
*
aHitachi Metals, Ltd, Advanced Electronics Research Laboratory, Mikajiri 5200, Kumagaya, Saitama 360-0843, Japan
bOsaka University, Nanoscience and Nanotechnology Center, Institute of Scientific and Industrial Research, Mihogaoka 8-1, Ibaraki,
Osaka 567-0047, Japan
Received 23 February 2004; received in revised form 1 October 2004; accepted 21 December 2004 by C.E.T. Gonc˛alves da Silva
Available online 11 January 2005
Abstract
Boron nitride (BN) nanolayers encapsulating iron (Fe) fine particles and BN nanotubes were synthesized by annealing
mixture powders of hematite, boron and carbon (C) at 1473 K for 2 h in a nitrogen atmosphere. The particles exhibited good
soft magnetic properties. The thickness of BN nanolayers increased in the range of 10–100 nm with increasing C content in the
mixture powders, which results in providing improvement of oxidation resistance of the particles. In the BN layers, no C was
detected. BN nanotubes with diameters of w100 nm were also synthesized, and they had cup-stacked and bamboo-like
structures.
q 2004 Elsevier Ltd. All rights reserved.
PACS: 75.50; 68.55
Keywords: A. Metals; B. Chemical synthesis; C. Scanning and transmission electron microscopy
1. Introduction
(BN) nanolayers have been prepared by arc-discharge and
annealing boric acid and urea [10]. Unfortunately, these
methods are not suitable to treat large quantity of
nanoparticles for industrial production.
Magnetic nanoparticles, such as iron (Fe), cobalt (Co),
and Fe-based alloys, have been paid much attention, and are
expected to have various magnetic applications [1–4] for
their high saturation magnetization compared to Fe oxide
nanoparticles. Since metal nanoparticles are easily oxidized
in air, several kinds of inorganic nanolayers encapsulating
them have been studied. Graphite nanolayers have been
synthesized by an arc-discharge [5], modified arc-discharge
[6], and electron beam irradiation [7] for the protection
layers. Oxide nanolayers have also been prepared by a slight
oxidation of core metals [8,9]. In addition, boron nitride
We have reported a promising method for mass-
production to synthesize BN nanolayers encapsulating Fe
fine particles as well as BN nanotubes (BNNTs) by using
hematite (a-Fe2O3) and boron (B) powders as starting
materials [11,12]. BN is known to exhibit good chemical
stability, and its nanolayers encapsulating Fe particles have
shown good oxidation resistance [12]. However, the
thickness of the BN nanolayers was only a several
nanometers. Thicker BN layers and control over their
thickness are desired in order to enhance the resistance. The
purpose of the present work is to investigate the effect of
carbon addition to the starting materials on the magnetic
properties, oxidation resistance and morphology of Fe fine
particles with BN nanolayers.
* Corresponding author. Tel.: C81 485311669; fax: C81
485337102.
E-mail
address:
hisato_tokoro@hitachi-metals.co.jp
(H. Tokoro).
0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.ssc.2004.12.034

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H. Tokoro et al. / Solid State Communications 133 (2005) 681–685
2. Experimental
a-Fe2O3, B and graphite carbon (C) powders were used
as starting materials, and were mixed well. Each average
diameter was 0.03, 21 and 5 mm, respectively. Weight ratio
of a-Fe2O3 to sum of B and C (denoted as B1KxCx) was
50%, that is a-Fe2O3: B1KxCxZ1:1. The ‘x’ was varied in
the range from 0 to 1.0. Samples were prepared by annealing
the mixtures at 1473 K for 2 h in nitrogen atmosphere.
Magnetic properties were measured by a vibrating
sample magnetometer (VSM) under a magnetic field of
1.6 kA/m. In order to examine oxidation resistance, the
samples were exposed to atmosphere of 393 K with a
humidity of 100% for 12 h, followed by measuring changes
of saturation magnetization before and after the exposure.
An X-ray diffractometer (Rigaku RINT-2500) was used to
detect metallurgical phases under an applied power of 50 kV
and 250 mA with a Cu Ka irradiation. Microstructures of the
samples were observed by high-resolution transmission
electron microscopy (HRTEM) with an acceleration voltage
of 200–300 kV. An energy dispersive X-ray analysis (EDX)
and an electron energy loss spectroscopy (EELS) equipped
with the HRTEM were also employed to analyze chemical
composition of the samples.
Fig. 1. Magnetization change of samples S1–S7 exposed at 393 K
for 12 h with a humidity of 100%.
that an addition of C improved oxidation resistance of Fe
fine particles possible because C addition facilitates
synthesis of the BN nanolayers and complete encapsulation
of the Fe particles.
Metallurgical phases of the samples were detected
macroscopically by an X-ray diffraction measurement.
Fig. 2 shows their X-ray diffraction patterns in the range
from 52 to 588 in 2q. The intensity of peaks assigned to
hexagonal BN (h-BN) or rhombohedral BN (r-BN)
decreases, while that assigned to graphite C becomes
stronger with increasing ‘x’. Peaks assigned to Fe–B
compounds, such as Fe2B and FeB, are detected in the S6
and S7 (xZ0.6 and 0.8). The compounds are considered as
intermediates under a synthesis of BN nanocoatings [11].
After the exposure test, peaks assigned to a-Fe2O3 and
Fe3O4 appeared in the sample S1 and S3–S7.
3. Results
Table 1 shows the magnetic properties of samples (S1–
S7). Saturation magnetization (Ms) increases from 44.7 to
97.9 A m2/kg with increasing ‘x’, which is equivalent to 21–
45% of bulk Fe magnetization [13]. Coercivity (Hc) shows
no dependence on ‘x’, and is in the range of 1.5–2.2 kA/m.
All samples in Table 1 exhibited superior soft magnetic
properties to Fe nanoparticles with C nanocoatings that have
a Hc of around 20 kA/m in the previous works [14,15].
In order to investigate oxidation resistance of the
samples, changes in Ms before and after an exposure test
was measured. The changes were normalized by Ms before
the test and expressed as percentage, and Fig. 1 shows the
changes as a function of ‘x’. When the carbon content
increases, the change of magnetization can be as small as K
10% for xZ0.5 and 0.6 (sample S5 and S6). This indicates
The morphology of S1 and S2 has been studied in the
previous work, and BN or C nanolayers encapsulating Fe
fine particles were produced [12]. The morphologies of S3,
S5 and S7 were observed by HRTEM in this study. From the
investigation of oxidation resistance, the sample S5 was
selected for detailed structure analysis, and Fig. 3(a) shows a
typical nanoparticle observed in the S5. The particle has
core-shell structure. The core has a diameter of w100 nm
Table 1
Magnetic properties of samples
No
x
(wt%)
Fe2O3
Ms
Hc
Reference
(B1KxCx)
B
C
(A m2/kg)
(KA/m)
S1
S2
S3
S4
S5
S6
S7
0.0
1.0
0.2
0.4
0.5
0.6
0.8
50
50
50
50
50
50
50
50
0
0
47.3
100.0
51.5
59.7
66.0
69.6
77.1
1.5
1.6
1.5
2.0
1.9
2.2
1.8
[11]
50
10
20
25
30
40
[11]
40
30
25
20
10
This work
This work
This work
This work
This work

H. Tokoro et al. / Solid State Communications 133 (2005) 681–685
683
Fig. 2. X-ray diffraction patterns of samples S1–S7 (xZ0–1.0).
Fig. 3. TEM image of a Fe particle with BN nanocoating (a),
magnified image of the BN nanocoatings (b), and EELS spectrum
obtained from the nanocoating (c).
and is composed of Fe by EDX analysis. The shell is a
nanocoating with w20 nm in thickness, where lattice
fringes correspond to the {002} plane of h-BN (Fig. 3(b)).
An EELS spectrum focused on the nanocoating is shown in
Fig. 3(c). Spectrum assigned to B and N can be detected, but
there is no sign of C. Spectrum indexed as p* and s* in a B–
K edge are similar to ones obtained from sp2-hybridized BN
[16], suggesting that the nanocoatings are h-BN. The
observation of w100 particles proved that diameters of Fe
particles and thickness of BN nanocoatings were in the
range of 50–500 nm and of 10–100 nm, respectively.
Similar morphology to Fig. 3(a) and (b) was observed in
the sample S3 and S7 as well. Half-value widths of the peak
assigned to Fe(110) in the X-ray diffraction patterns showed

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H. Tokoro et al. / Solid State Communications 133 (2005) 681–685
that there is no dependence of the particle size with the C
content, x. It is noted that all nanocoatings were composed
of only BN, and that no trace amount of C was detected in
them although C was used as a starting material.
work showed thickness of less than w10 nm [11]. Thicker
coatings could be attributed to enhancement of the growth of
BN layers by addition of C, by the mechanism proposed
below.
Besides the BN nanocoatings, two kinds of BN
nanotubes with diameters of w100 nm and lengths of
several micrometers were observed as shown in Fig. 4(a)
and (d). Fig. 4(a) shows a so called cup-stacked structure
[17], and a HRTEM image in Fig. 4(b) reveals that lattice
fringes of BN layers are inclined by 138 from nanotube axis.
An EELS spectrum focused on the BN layers in Fig. 4(b) is
shown in Fig 4(c). The spectrum can be assigned to a B–K
and N–K edge, suggesting that the nanotube consists of h-
BN without C. Intensity of a p* peak is stronger than that of
a s* peak, which suggests that lattice fringes of w0.3 nm
spacing in Fig. 4(b) can be indexed as {002} plane of h-BN.
Another type of nanotubes in Fig. 4(d) could be classified as
a bamboo-like structure [18]. Lattice fringes in Fig. 4(d) are
almost parallel to the nanotube wall as shown in Fig. 4(e).
An EELS spectrum is almost the same as that of the cup-
stacked nanotube (Fig. 4(f)), suggesting that this nanotube is
also composed of h-BN without C.
Yeadon and the coworkers have reported that BN
nanocages were formed by nitridation of Fe–B nanoparticles
[19]. Since Fe–B compounds were generated as intermedi-
ates of the reaction process in the present work, they are
considered as precursor for the growth of the BN
nanocoatings. In the present work, the thicker BN
nanocoatings are probably synthesized not only from the
Fe–B precursor but also from the reaction proposed as
follows:
2Fe2O3 C3B C3C/3Fe CFeB CB2O3 C3CO[
B2O3 C3CO CN2 /2BN C3CO2[
(1)
(2)
The formulas suppose that carbon monoxide (CO) is
generated besides boron oxide (B2O3) through a reduction
of Fe2O3, forming Fe–B compounds (formula (1)). CO
reduces B2O3 under N2 atmosphere, resulting in formation
of BN at high temperatures (formula (2)). C reacts with
neither B nor N in the formulas, as it was found in the
present experiment that C was not contained in either BN
nanolayers or nanotubes. Appearance of Fe–B compounds
(Fe2B, FeB) as shown in Fig. 2 would reveal that BN
nanolayers formed through the formula (2) encapsulated
Fe–B precursors before nitridation of them, and as a result,
the BN nanocoatings prevented them from reacting with N2
atmosphere.
4. Discussion
BN nanocoatings in the present work have thickness in
the range of 10–100 nm, which results in improvement of
oxidation resistance of Fe fine particles as shown in Fig. 1.
Those synthesized from only Fe2O3 and B in the previous
Fig. 4. TEM images of BN nanotubes analyzed by EELS; (a,d) low magnification images and (b,e) enlarged HREM images of white arrow area
in (a) and (d). (c,f) EELS spectra obtained from BN layers in (b) and (e).

H. Tokoro et al. / Solid State Communications 133 (2005) 681–685
685
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