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253
the magnetic properties of the alloy, but also improves the anti-
oxidation of the alloy. Hexagonal boron nitride (h-BN) has
received considerable attention due to its high thermal stability,
electrical conductivity, low dielectric constant and mechanical
strength [11,12], widely used in gaseous uptake, protective coatings,
advanced ceramic composites and catalyst support [13e19].
Therefore, the h-BN layers are ideal shells because they can protect
metal nanoparticles effectively against oxidation and limit the
growth of the alloy.
Several process, including magetron, ion-beam co-sputtering
[20] and spraying methods [21] have been developed to prepare BN
coated nanoparticles. However, these methods still exit all sorts of
drawbacks such as complicated working procedure and difficulty in
microstructure control. It has been urgent affairs to produce a large
number of nanocapsules with good property.
and microstructure of samples were investigated by transmission
electron microscopy (TEM, Tecnai G2 F20, Netherlands) and field
emission scanning electron microscopy (SEM, FEI Company, USA)
with an energy dispersive analysis of X-rays (EDAX). The oxidation
mechanism of samples were measured by thermal gravimetric
analysis (TGA) and scanning differential thermal analysis (SDTA)
during heating up from room temperature to 900 ꢀC with 5 ꢀC/min
in air. The magnetic properties of FeNi NPs and FeNi/BN were
evaluated by vibrating sample magnetometer (VSM, 7404, America)
run under 20000 Oe field.
3. Results and discussion
3.1. Crystal structure and functional groups analysis
The purpose of the present work is threefold. The first was to
develop a new and simple method for producing the FeNi/BN
samples, therefore, the mixture of FeNi alloy precursors and H3BO3
powders were selected. The second purpose was to investigate the
microstructure of FeNi NPs and FeNi/BN samples by means of X-ray
powder diffraction, Transmission electron microscopy and Field
emission scanning electron microcopy. The last was to compare the
oxidation temperature of pure FeNi NPs and FeNi/BN, these char-
acterization data will provide us with guideline for researching the
BN-coated nanostructure materials.
Fig. 1 illustrates the XRD patterns of the (a) FeNi NPs, (b) h-BN
powders and (c) FeNi/BN. The characterization diffraction peaks at
44.30ꢀ, 51.56ꢀand 75.90ꢀ are assigned to the (111), (200) and (220)
crystallographic plants of FeNi NPs (Fig. 1(a)). As exhibited in
Fig. 1(b), the peaks located at 25.86ꢀ, 41.78ꢀ and 76.32ꢀ matched
well the (002), (100) and (110) reflections of the crystalline plants of
h-BN [22e24]. After BN coated on the surface of FeNi NPs, as
Fig. 1(c) shown, it is obviously observed that the characterization
peaks of FeNi/BN are in good accordance with the bragg diffractions
of FeNi NPs and h-BN powders, indicating that FeNi NPs can be
relatively well combined with the h-BN. In addition, no peaks of
metal hydroxides and metal oxides could be found throughout out
the reaction and the good crystallinity of the samples were shown
by strong peak intensity and narrow peak width, demonstrating
that the purity of products were high and without oxidated, due to
the better reducibility of ammonia gas and the protection of boron
nitride layers.
2. Experimental procedures
2.1. Materials
Hexagonal boron nitride (h-BN) powders were synthesized via a
simple high-temperature approach (900 ꢀC). Iron nitrate hexahy-
drate, nickel nitrate hexahydrate, sodium borohydride, poly-
vinylpyrrolidone and ethanol were used in the experiment are
analytically pure grade. Deionized water (DI water) is produced by a
water purification machine.
Fig. 2 shows the FT-IR spectrum of (a) h-BN and (b) FeNi/BN. As
is shown in Fig. 2(b), FeNi/BN exhibits three main distinct peaks of
BeNeB, BeN and BeNH2/BeOH bending at 785 cmꢁ1, 1383 cmꢁ1
and 3451 cmꢁ1
, respectively. The high frequency peaked at
3451 cmꢁ1 is a typical eOH stretching vibration from surface at-
mosphere species [25]. Both of three peaks can be found in two
samples, predicating the existing of BN phase in the sample. It
should be noted that, the relative peak intensity of B-N-B and B-N
bending vibration are decreased significantly compared with h-BN
powders. It could be attributed to the presence of BN coating.
Moreover, the h-BN presents additional peaks at 1080 cmꢁ1 and
2.2. Preparation of FeNi Nps
Firstly, 1.16 g nickel nitrate hexahydrate and 0.404 g iron nitrate
hexahydrate were dissolved into a beaker with 60 ml alcohol for
mixing. Then, adding 1.6 g PVP-K30 and 0.6 g sodium borohydride
under stirring until dissolved, the black precipitate were obtained
and washed to neutral by deionized water and ethanol for several
times before dried at 60 ꢀC for 6 h in a vacuum. The dried mixture
(i.e., FeNi alloy precursors) was annealed with flowing ammonia at
900 ꢀC for 2 h to obtain FeNi NPs.
2.3. Preparation of FeNi/BN
The FeNi alloy precursors and H3BO3 powders with molar ratio
of 4:1 were well mixed by triturator. Then, the mixture powders
were placed in an alumina tube mounted in a tube furnace and
heated up to a temperature of 900 ꢀC with flowing ammonia for 2 h,
and naturally cooled (6 h) to room temperature to give a black
precipitate.
2.4. Characterization
The phases formed in the FeNi NPs and FeNi/BN were charac-
terized by X-ray diffraction (XRD, Japan, Rigaku smartlab) having
Cu-K
in the 2
a
radiation (
q
l
¼ 0.15405 Å) at a scanning rate of 8ꢀ per second
range from 10ꢀ to 90ꢀ. Chemical bonding and groups were
determined by fourier transform infrared spectroscopy (FTIR,
Vector 22, Germany) between 500 and 4000 cmꢁ1. The morphology
Fig. 1. XRD patterns of (a) FeNi NPs, (b) h-BN powders and (c) FeNi/BN.