I. Narita et al. / Solid State Communications 137 (2006) 44–48
45
et al. reported that continuous filling of BN nanotubes with fine
Co had been obtained by using substitution synthesis from
C nanotubes [15]. Although fine Co were encapsulated in BN
nanotubes or nanocapsules, the size and shape were irregular,
and high temperature (1450 8C) is necessary for reduction of
B2O3 powder. As a synthesis method of BN, Hu et al. reported
that nanocrystalline BN had been synthesized by reaction of
KBH4 and NH4Cl at 650 8C with 22 MPa [16]. This synthesis
method does not contain oxides in starting materials.
The purpose of the present work is to synthesize metal
magnetic nanocapsules coated with BN layers by annealing of
ammine complexes, and to investigate the structure and
magnetic properties. In the present work, [Co(NH3)6]Cl3 was
used as a starting material instead of NH4Cl to synthesize Co
nanocapsules coated with BN layers.
Fig. 1. X-ray diffraction pattern of the annealed samples (a) 700 8C, 2 h,
100 sccm of N2 gas, (b) 1000 8C, 2 h, 100 sccm of N2 gas, (c) 1000 8C, 2 h,
200 sccm of N2 gas.
2. Experimental
respectively. Number of BN layers covering fcc-Co nanopar-
ticles are in the range of 15–30 layers.
Co nanocapsules coated with BN layers were synthesized by
annealing of ammine complex. KBH4 and [Co(NH3)6]Cl3
powders were used as raw materials. After the KBH4 and
[Co(NH3)6]Cl3 (molar ratioZ3:1) were well mixed by
triturator, the samples were set on an alumina boat. Since
KBH4 powder was easily deliquesced in the atmosphere, all
manipulations were done in an inert gas. The samples were
annealed with flowing nitrogen (N2) gas (100 or 200 sccm) at
500–1000 8C for 2 h, and cooled down to room temperature in
the furnace. Annealed samples were well washed and filtrated
several times in distilled water and ethanol.
A magnetic hysteresis loop of a sample annealed at 1000 8C
for 2 h with flowing 100 sccm N2 gas is shown in Fig. 4, which
exhibits a soft magnetic characteristic. The values of saturation
magnetization (Ms) and coercivity (Hc) were 74.5 emu/g and
88 Oe, respectively. Results of VSM measurement are
summarized in Table 1. The Ms values of the samples annealed
at 1000 8C are larger than that of 700 8C. On the other hand, Hc
values decrease at elevated temperatures. This behavior can be
thought as an effect of particle size. Herzer had shown that Hc
value depend on magnetic particle size, and Ms value is
inversely proportional to the variance of Hc values [17].
Therefore, decrease of the Hc values at 1000 8C would be due
to increase of particle sizes.
Phases of the samples were determined by X-ray diffraction
with Cu–Ka irradiation. To observe the morphology of the
samples, high-resolution electron microscopy (HREM) was
performed with a 300 kV electron microscope (JEM-3000F).
Magnetic properties were measured by vibrating sample
magnetometer system (VSM) under 0.8 MA/m dc field. To
investigate reaction process between KBH4 and [Co(NH3)6]Cl3
during annealing, differential thermal analysis and thermo
gravimetric analysis (DTA–TGA) were carried out from 25 to
1000 8C at an elevation rate of 5 or 10 8C/min with flowing N2
gas (20 sccm).
To investigate oxidation- and wear-resistances, pressure
cooker test (PCT) was carried out under the following
condition: 120 8C!12 h, humidity 100% and 1 atm, and the
values of saturation magnetization ðMsÃÞ and coercivity ðHcÃÞ
were measured by VSM. Magnetic properties after the PCT are
summarized in Table 2. The values of degauss coefficient were
calculated according to the following equation:
ððMsà KMsÞ=MsÞ!100%. The samples annealed at 1000 8C
were more stable for oxidation and wear than that of 700 8C.
This would be due to that strong connection of boron and
nitrogen atoms were formed by annealing at 1000 8C.
3. Results and discussion
X-ray diffraction patterns are shown in Fig. 1. The reactions
for formation of BN layers have not ended in 700 8C yet, as
shown in Fig. 1(a). Obvious diffraction peaks of hexagonal BN
and fcc-Co were observed for the samples annealed at 1000 8C
as shown in Fig. 1(b) and (c). Average particle sizes of fcc-Co
calculated by Scherrer equation are listed in Table 1. Particle
sizes depend on annealing temperatures rather than flowing
rate of N2 gas.
A DTA–TG measurement was carried out in order to reveal
the reaction process as shown in Fig. 4(a). The first decrease of
gravity (200–300 8C) corresponds to a melting point of
Table 1
Averaged particle sizes of Co nanoparticles calculated by Scherrer equation
and the values of saturation magnetization (Ms) and coercivity (Hc), which is
measured by VSM at room temperature
Temperature Flow rate
Particle size
(nm)
Ms (emu/g)
Hc (Oe)
Fig. 2(a) is a low magnification image of a sample annealed
at 1000 8C for 2 h with flowing 100 sccm N2 gas. Fcc-Co
nanocapsules coated with BN layers and BN nanocages are
observed. HREM images of fcc-Co nanocapsules coated with
BN layers and BN nanocages are shown in Fig. 3(b) and (c),
(8C)
(sccm)
700
100
100
200
27
40
37
48.6
74.5
72.3
342.9
88.0
1000
1000
134.1
Narita, Ichihito
