Low-temperature solid state synthesis and in situ phase transformation to prepare nearly pure cBN

Article abstract of DOI:10.1039/c0dt01823f

Cubic boron nitride (cBN) is synthesized by a low-temperature solid state synthesis and in situ phase transformation route with NH₄BF ₄, B, NaBH₄ and KBH₄ as the boron sources and NaN₃ as the nitrogen

Full text of DOI:10.1039/c0dt01823f

View Article Online / Journal Homepage / Table of Contents for this issue
Dynamic Article Links
Dalton
Transactions
Cite this: Dalton Trans., 2011, 40, 6961
www.rsc.org/dalton
PAPER
Low-temperature solid state synthesis and in situ phase transformation to
prepare nearly pure cBN†
Gang Lian,a,b Xiao Zhang, Miao Tan, Shunjie Zhang, Deliang Cui* and Qilong Wang*
b
a
b
a
b
Received 23rd December 2010, Accepted 19th April 2011
DOI: 10.1039/c0dt01823f
Cubic boron nitride (cBN) is synthesized by a low-temperature solid state synthesis and in situ phase
transformation route with NH
4
BF
4
, B, NaBH
4
and KBH
4
as the boron sources and NaN as the
3
nitrogen source. Furthermore, two new strategies are developed, i.e., applying pressure on the reactants
during the reaction process and introducing the structural induction effect. These results reveal that the
relative contents of cBN are greatly increased by applying these new strategies. Finally, almost pure
◦
cBN (~90%) crystals are obtained by reacting NH
4
BF
4
and NaN
3
at 250 C and 450 MPa for 24 h, with
NaF as the structural induction material. The heterogeneous nucleation mechanism can commendably
illuminate the structure induction effect of NaF with face center cubic structure. In addition, the
induction effect results in the cBN nanocrystals presenting obvious oriented growth of {111} planes.
1
8
1
. Introduction
(liquid ammonia, hydrazine etc.), the conditions are still too
◦
harsh (>800 C & >1000 MPa). Thereby, it will be of high practical
value to develop a facile and effective route to fabricate cBN under
moderate conditions.
2
Boron nitride (BN) has many polymorphs, including sp -bonded
hexagonal BN (hBN), turbostratic BN (tBN) and sp -bonded
3
cubic BN (cBN) and wurtzite BN (wBN). Among them, cBN,
structurally analogous to diamond, has strong covalent bonds
which makes it an attractive material with excellent properties.
Only next to diamond, cBN possesses the second best hardness,
elastic modulus and thermal conductivity, but it is superior
to diamond for its higher oxidation temperature and chemical
inertness to molten ferrous metals. These merits make cBN the
best material for fabricating cutting tools, protective coatings and
heat-sinks of high power electronic devices etc. Besides, the wide
bandgap (~6.2 eV), high carrier mobility, p- and n- conductivity
modifications endow cBN with the functions of being fabricated
According to the phase diagram of BN, hBN and tBN are stable
phases at ambient pressure, and cBN is stable at high pressure.
So, generally, it is believed that the conventional low-temperature
solid state reaction route always results in the formation of
19–22
2
hBN or tBN
(sp -hybrid). However, we have successfully
obtained cBN nanocrystals by a modified low-temperature solid
state reaction route (namely the reactants pressed into dense
pellets). Furthermore, we also discuss what would be the result
if pressure was applied to the reactants during the synthesis
23–24
of BN. Actually, Kaner et al.
have successfully synthesized
some high-pressure phase nitrides with cubic structures (GaN,
TaN, CrN, Mo N) by adding pressure. In addition, the structural
1–8
into ultraviolet optoelectronic devices etc.
2
Conventionally, cBN was prepared by directly converting hBN
induction effect is successfully applied in promoting the deposition
9
under high-temperature and high-pressure. It is well known that
25–28
of the cBN film.
So, would this induction effect be useful to
hBN is a stable phase at ambient temperature and pressure, and
a high energy barrier exists between hBN and cBN, thus rather
promote the formation of cBN in our solid state system? Based on
these considerations, we developed a low-temperature solid state
synthesis and in situ phase transformation route, by applying a
constant pressure on the reactants and introducing the structural
induction effect during the synthesis of BN. As a result, nearly
pure cBN nanocrystals have been successfully prepared by a low-
temperature solid state route for the first time.
harsh conditions are required for such a phase transformation
◦
(
for example, 10 GPa & 3000 C was required for the direct
transformation). Although the temperature and pressure obvi-
ously decreased by utilizing some catalysts (Mg, Mg–Al, MgB
2
,
1
0–15
Mg
3
N
2
, Li
3
N, AlN etc),
replacing well crystallized hBN with
and introducing supercritical fluids
16–17
low ordered tBN or aBN
a
State Key Lab of Crystal Materials, Shandong University, Shanda
Nanlu No.27, Jinan, 250100, P. R. China. E-mail: cuidl@sdu.edu.cn;
Fax: (+86)531-88361856; Tel: (+86)531-88361856
School of Chemistry and Chemical Engineering, Shandong University,
Shanda Nanlu No.27, Jinan, 250100, P. R. China. E-mail: gang_lian2000@
yahoo.com.cn; Tel: (+86)531-88365899
2
. Experimental section
2.1 Synthesis of BN at low temperature with the reactants
b
pressed into dense pellets
All the chemicals were of analytical grade (A.R.) and without
further purification prior to use, and the experimental parameters
†
1
Electronic supplementary information (ESI) available. See DOI:
0.1039/c0dt01823f
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 6961–6967 | 6961

View Article Online
Table 1 Key parameters of the experiments
Table 2 Key parameters of preparing BN samples
◦
a
◦
a
No. Reactants
Pressed
T ( C) t (h)
Phases
No.
NH
4
BF
4
/NaN
3
T ( C)
t (h)
P (MPa)
Phases
S-1
S-2
S-3
S-4
S-5
1NH
1NaBH
1KBH : 3NaN
4
BF
4
: 4NaN
3
Y
Y
Y
Y
Y
250
250
250
250
250
24
24
24
24
tBN*+cBN
tBN*+cBN
tBN*+cBN
tBN*+cBN
S-6
S-7
S-8
S-9
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
1/4
220
220
220
220
220
250
250
250
250
250
300
300
300
300
300
24
24
24
24
48
24
24
24
24
48
24
24
24
24
48
0
t*BN+cBN
t*BN+cBN
t*BN+cBN
tBN+cBN
tBN+cBN*
t*BN+cBN
t*BN+cBN
t*BN+cBN
t*BN+cBN
tBN+cBN*
tBN
4
: 3NaN
3
125
300
450
450
0
125
300
450
450
0
125
300
450
450
4
3
1B: 3NaN
1B: 3NaN
3
3
24 ¥ 6 tBN+cBN*
S-10
S-11
S-12
S-13
S-14
S-15
S-16
S-17
S-18
S-19
S-20
a
*
Denotes the dominant phase(s).
can be found in Table 1. In a typical synthesis, 5 mmol ammonium
tetrafluoroborate (NH BF ) and 20 mmol sodium azide (NaN
were mixed together and pressed into dense pellets. Then the
pellets were placed in an N -flowing glove box (the concentration
of both moisture and oxygen was less than 1 ppm) to eliminate the
adsorbed water and oxygen, followed by sealing the pellets into
a stainless steel reaction autoclave. Afterwards, the autoclave was
heated at a rate of 3 C min to 250 C and held for 24 h, followed
by cooling the autoclave to room temperature naturally. Finally,
the product was washed successively with hydrochloric acid (HCl)
and deionized water several times to remove the byproducts and
4
4
3
)
t*BN+cBN
t*BN+cBN
t*BN+cBN
tBN+cBN
2
a
* Denotes the dominant phase in the sample.
◦
-1
◦
Furthermore, 5 mmol NaF, which was used as the structural
induction material, was introduced into the reactants, and the
synthesis was carried out at 250 C under 125 MPa and 450 MPa,
◦
◦
respectively.
impurities. After heating at 80 C for 8 h, the BN sample was
obtained. It was quite amazing that a small amount of the cubic
phase of BN was detected in the sample prepared at a temperature
as low as 250 C, and the cubic phase of BN was also observed in
2
.3 Characterization of the samples
◦
Phases of the sample were identified by X-ray powder diffraction
the samples prepared by the same route, using NaBH
B as the boron sources (ESI S-1†).
4
, KBH and
4
(XRD) patterns, which were obtained on a Rigaku D/max-gA
X-ray diffractometer with Ni filtered Cu-Ka radiation (V = 40 kV,
I = 50 mA). Besides, infrared (IR) absorption spectra, collected
on a Nicolet NEXUS 670 Fourier transformation infrared spec-
trometer, were also used to determine the structural information
of samples. On the other hand, the X-ray photoelectron spectra
2
.2 Preparing the cubic phase of BN by applying pressure on the
reactants and introducing the structural induction effect (in situ
phase transformation)
(
XPS), collected on a ThermoFisher ESCALAB 250 equipment,
In another typical synthesis, 5 mmol NH
4
BF
4
and 20 mmol NaN
3
were employed to verify the elemental and phase composition of
samples. The morphology of the samples was observed by a JSM-
6
were used as the reactants, and the experimental process was
similar to that described above, except that a constant pressure
was applied on the dense pellets during the reaction process
700F field emission scanning electron microscope (SEM, ¥200 k)
and a Hitachi H-800 transmission electron microscope (TEM,
(
Route 2 in Scheme 1). Using this method, a series of BN samples
1
2
2
50 kV). Lattice fringes were observed by a Phillips Tecnai Twin-
0U high resolution transmission electron microscope (HRTEM,
00 kV). Prior to the measurement of SEM, TEM and HRTEM,
were prepared by varying the temperature, pressure and reaction
duration, and the detailed experimental parameters are shown in
Table 2.
the samples were ultrasonically treated in ethanol for 30 min.
3
. Results and discussion
3.1 Identification of the phase compositions
Via the modified low-temperature solid state reaction route, we
have prepared BN using NH BF and NaN as the reactants, the
4
4
3
detailed parameters of the experiments are presented in Table 1
and the reaction can be expressed as follows:
2
NH
4
BF
4
+ 8NaN
3
→ 2BN + 8NaF + 11N
2
+ 2NH
3
+ H
2
Firstly, the XRD pattern has been employed to identify the
phase compositions of the sample, and the typical XRD pattern
of sample S-1 is shown in Fig. 1(a). Only two broad diffraction
◦
◦
peaks at 2q of 24–27 and 40–46 imply a typical tBN structure.
Although the two peaks match well the (002) and (100) plane
spacings of a crystalline hBN, the measured interlayer distance
Scheme 1 Schematic of the solid state reaction for synthesizing BN. Route
1
: modified low-temperature solid state reaction route; Route 2: modified
˚
˚
low-temperature solid state reaction route with assistance of pressure.
of d002 is ~3.51 A and is obviously larger than ~3.33 A, which is
6
962 | Dalton Trans., 2011, 40, 6961–6967
This journal is © The Royal Society of Chemistry 2011

View Article Online
2
to sp -bonded BN (low symmetric turbostratic phase) and the
higher bonding energies of B1s (191.5 eV) and N1s (398.8 eV) are
3
contributed by sp -bonded cBN (high symmetric cubic phase).
So, the XPS results are in good agreement with the former FTIR
analysis.
In order to examine the applicability of the modified solid state
reaction route, we tried to synthesize cBN by using other boron
sources such as NaBH
4
, KBH and B, and the reaction proceeded
4
◦
at 250 C for 24 h with the reactants pressed into dense pellets.
The results show that, besides the dominant phase of tBN in the
as-prepared samples S-2, S-3 and S-4, the cubic phase is also
detected (ESI S-1†). Comparatively, when the reaction duration is
prolonged to 144 h, then cBN becomes the major phase in sample
S-5 with B as the boron source. Unfortunately, the yield of BN
Fig. 1 Phase composition of sample S-1. (a) XRD pattern. Inset shows
◦
that the peak of 40~46 can be fitted to the (100) plane of tBN and (111)
plane of cBN; (b) FTIR spectra. Top: mixed phase (tBN+cBN) BN of
sample S-1 prepared by modified solid state reaction route; bottom: pure
phase tBN prepared by traditional solid state reaction route.
sample (tBN+cBN) is very poor due to the low activity of B at 250
◦
C. In consideration of the yield and activity of reactants, NH
is selected as the boron source in the following experiments.
4
BF
4
30
peculiar to well-crystalline hBN. The BN phase formation can
be further confirmed by the FTIR analyses, as depicted in Fig.
Based on the above experimental results, we believe that an
in situ phase transformation process achieves the formation of the
cubic phase of BN. So, we schematically depict the solid state
-
1
-1
1
(b). The dominant vibrations at ~1400 cm and ~784 cm are
assigned to the n(B–N) and d(B–N) modes of tBN, which verifies
that the major phase in the as-prepared sample S-1 is tBN. But,
except for the characteristic B–N vibrations of tBN, it is worth
reaction process in Scheme 1. Because the reaction temperature
◦
(
(
250 C) is lower than the decomposition temperatures of NH
4
BF
(>283 C), the exothermic reaction
and NaN might have proceeded slowly at
4
-
1
noting that a weak peak centered at ~1100 cm is observed, which
◦
◦
33
>307 C) and NaN
3
can be attributed to the characteristic absorption of the transverse
between NH
4
BF
4
3
31–32
optical (TO) phonon of cBN.
at 250 C.
It is quite exciting to obtain cBN
◦
2
50 C. Thus the pellet configuration is preserved after reaction
◦
(
Route 1 in Scheme 1). This phenomenon possibly leads to the
following results: (1) compared to the non-compact powders
Conventional solid state route in Scheme 1), the reactants more
In addition, XPS analysis is also used to determine the elemental
compositions and chemical bondings of sample S-1. Fig. 2(a)
shows the survey scan XPS spectrum. Besides the weak peaks
(
easily contact with each other and conveniently diffuse in the
pressed dense pellet. As a result, the BN crystallites grow larger,
but the low temperature conditions prohibited the improvement
of BN crystallinity. Thus poorly crystallized tBN micro-flakes are
obtained (ESI S-2†); (2) gas products of the reaction, such as
of O1s and C1s, from the CO
2
and O adsorbed on the surface of
2
the sample, two dominant strong peaks of B1s and N1s indicate that
the sample is composed of B and N elements. The binding energies
of B1s (190.3 eV) and N1s (398.0 eV) are in good agreement with the
29
values of bulk BN. The quantification of peaks confirms that the
atomic ratio of B : N is ~1.02 : 1, which is very close to the chemical
stoichiometry of BN. Fig. 2(b–c) shows the 1s core spectra of B
and N. The dissymmetric peaks suggest that there is more than one
type of bonding scheme for the B and N atoms. The XPS spectra
were therefore fitted to investigate possible chemical bonding and
phase compositions of sample S-1. The fitting results of the B1s
and N1s spectra give two group peaks centered at 190.2 eV &191.5
eV (inset in Fig. 2(b)), and 397.9 eV & 398.8 eV (inset in Fig. 2(c)),
respectively. According to the NIST database, the lower bonding
energies of B1s (190.2 eV) and N1s (397.9 eV) can be attributed
NH
3
, N
2
and H , could not easily escape from the pressed dense
2
pellet, and squash together to form high pressure tiny regions (see
the bottom-left inset in Scheme 1). Within these regions, freshly
formed tBN with poor crystallinity might partly convert to the
cubic phase by the in situ phase transformation process (see Route
1
in Scheme 1). It is revealed that because cBN is the stable
phase under high pressure, the close contact between reactants
and the existence of high pressure tiny regions in the pressed dense
pellets are beneficial to the formation of cBN by the in situ phase
transformation process. If the stress inside the pellets can be so
profitable, how about applying a constant pressure to the pellets,
Fig. 2 XPS spectra of sample S-1. (a) Survey spectrum; (b) spectrum of B1s. Inset is the fitting result for the B1s spectrum; (c) spectrum of N1s. Inset is
the fitting result for the N1s spectrum.
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 6961–6967 | 6963

View Article Online
as shown the Route 2 in Scheme 1? Otherwise, the escape of gas
products from the pellets can be greatly suppressed by applying a
direct pressure.
depicting the relationship between cBN concentration and the
pressure/temperature/duration is drawn in S-3 in the ESI.†
3
.3 Fabrication of nearly pure cBN nanocrystals under the
3
.2 Formation of cBN via a solid state synthesis and in situ phase
structural induction effect
transformation with pressure assistance
Furthermore, for obtaining pure cBN by the low-temperature
solid state synthesis and in situ phase transformation route, herein
we introduce the structural induction effect. NaF displays an
analogous face center cubic structure (fcc) to cBN and might
induce the tBN transforming into cBN. All the other experimental
parameters are the same as that of samples S-12 and S-14 (Table
In the following experiments, NH
4
BF
4
and NaN are selected as
3
the reactants, and a series of BN samples are prepared via Route
. The detailed experimental parameters are shown in Table 2.
2
We mainly investigate the effects of temperature, pressure and
duration for the phase transformation of turbostratic to cubic
phase.
2
). In our experiments, 5 mmol NaF were added into the reactants
The as-obtained BN samples (S-6 to S-20) were characterized by
-
1
(Table 3), and the FTIR spectra of the BN samples are shown
in Fig. 4. By comparing the spectra of samples S-12 and S-21,
FTIR spectra, as shown in Fig. 3. The peaks of 1386~1415 cm
-
1
and 772~805 cm are attributed to n(B–N) and d(B–N) modes
-
1
-
1
it is quite clear that the peak of cBN centered at 1103 cm is
rather weak without the assistance of the structural induction
effect, which indicates that only a small amount of cBN has
formed on this occasion. But when NaF is introduced into the
of tBN, respectively. The peaks of 1050~1108 cm come from
the absorption of the TO phonon of cBN.
the relative content of cBN rapidly increases with improving the
pressure and prolonging the reaction duration at 220 C, and
31–32
As is expected,
◦
-
1
reactants, this peak of cBN centered at 1098 cm becomes the
strongest peak and cBN is the dominant phase in this sample S-
finally, cBN becomes the dominant phase in sample S-10 prepared
◦
at 450 MPa & 220 C for 48 h. With increasing the temperature to
◦
◦
2
1. Furthermore, with increasing the pressure to 450 MPa, the
2
50 C and 300 C, a similar phase transformation trend from tBN
in situ phase transformation of tBN to cBN is performed more
completely. By evaluating the cBN concentration of sample S-22
via eqn (1), an almost pure cBN (~90%) sample is obtained.
to cBN can be also observed. But because the freshly synthesized
tBN at lower temperature presents poorer crystallinity, the in situ
◦
phase transformation extent of BN at 220 C is obviously stronger
◦
◦
than that of 250 C and 300 C at the same pressure.
In order to quantitatively analyze the variation of cBN concen-
trations in the as-prepared BN samples, we fitted the FTIR peaks
by multi-Gaussian curves and evaluated the relative content of
cBN via eqn (1):
Table 3 Key parameters of preparing BN samples under the structural
induction effect
◦
a
No.
4 4 3
NH BF /NaN /NaF T ( C) t (h) P (MPa) Phases
IcBN
S-12 1/3/0
S-21 1/3/0.5
S-22 1/3/0.5
250
250
250
24
24
24
125
125
450
t*BN+cBN
tBN+cBN*
tBN+cBN*
cBN% =
×%
(1)
IcBN + ItBN
-
1
-1
wherein, IcBN (at ~1079 cm ) and ItBN (at ~1400 cm ) stand for the
a
*
Denotes the dominant phase.
absorption peak areas of cBN and tBN, respectively. A diagram
Fig. 3 FTIR spectra of BN samples prepared by solid state synthesis and in situ phase transformation process under constant pressure. The peaks within
-1
-1
-1
1
050~1108 cm are attributed to the absorption of cBN, and those at 1386~1415 cm and 772~805 cm come from the absorption of tBN.
6
964 | Dalton Trans., 2011, 40, 6961–6967
This journal is © The Royal Society of Chemistry 2011

View Article Online
Fig. 5 XPS spectra of samples S-12 and S-22. (a) Survey spectrum of
sample S-22; (b) survey spectrum of sample S-12; (c) B1s spectra of samples
S-12 and S-22; (d) N1s spectra of samples S-12 and S-22. pP: p plasmon.
34–38
respectively.
Therefore, both the p plasmon and the values of
the binding energies support that sample S-22 is mainly composed
of cBN, while the dominant phase of S-12 is tBN. The conclusion
is also in good agreement with the FTIR results.
In addition, at another temperature and reactant system, the
structure induction material also presents a positive influence for
the in situ phase transformation of BN (ESI S-4†).
TEM was used to observe the micro-morphology of the almost
pure cBN phase of sample S-22. Fig. 6(a) exhibits the typical
images of cBN nanocrystals with diameters in the range of
30~60 nm. The corresponding SAED pattern taken from these
nanocrystals reveals the cubic structure characteristic of the BN
sample. The diffraction rings can be indexed to the (111), (200),
(220) and (311) planes of cBN, respectively. In addition, the
corresponding XRD pattern of sample S-22 is also presented in
the ESI (Fig. S5).†
Fig. 4 FTIR spectra of BN samples prepared under the structural
-1
-1
induction effect. The peaks at 1390~1413 cm and 772~805 cm come
-1
from the absorption of tBN, while those at 1095~1103 cm should be
attributed to the absorption of cBN. All the samples were prepared by
◦
reacting NH
4
BF
4
and NaN
3
at 250 C for 24 h. Besides, 5 mmol NaF was
used as the structural induction material.
The XPS spectra were employed to determine the chemical
bonding of samples S-12 and S-22, which are mainly composed of
tBN and cBN, respectively. The peaks of C1s, O1s, B1s and N1s can be
observed on the survey spectra shown in Fig. 5(a–b). The C1s peak
at 284.6 eV, which derives from surface contamination because the
samples have been exposed to air before the XPS measurements,
has been used to calibrate all the spectra due to the charging
effect. The quantification of peaks shows that the atomic ratios
of B : N are ~0.98 : 1 for S-22 and ~1.05 : 1 for S-12, respectively,
which are very close to the chemical stoichiometry of BN. Besides,
it is well known that XPS is an effective method to characterize
Fig. 6 (a) TEM image of cBN nanocrystals; (b) the corresponding SAED
pattern of cBN nanocrystals. The diffraction rings can be indexed to the
(111), (200), (220) and (311) planes of cBN, respectively.
2
cBN. It has been reported that only sp -bonded BN (tBN) shows
a bump of p plasmon (pP) loss peak at ~9 eV away from the B1s
3
and N1s spectra, but the peak does not appear in the sp -bonded
2
cBN phase. This difference is usually used to distinguish sp and
sp hybrid BN phases.
Besides, HRTEM was also used to identify the existence of cBN
nanocrystals in sample S-22. Fig. 7(a) presents the residual tBN of
3
34–36
After carefully comparing Fig. 5(c) and
˚
(
d), two pP loss peaks centered at ~9 eV away from B1s and N1s
sample S-22. The interlayer distance of (002) planes is ~3.5 A. Ac-
peaks of sample S-12 are obviously found. However, no such pP
peaks exist for sample S-22. Furthermore, the binding energies of
the B1s and N1s peaks of sample S-12 and S-22 are characteristic of
tually, the poor crystallinity can facilitate the in situ phase transfor-
mation of tBN to cBN. Fig. 7(b) displays the lattice fringes of cBN.
The regular fringes reveals good crystallinity of cBN nanocrystals
˚
the tBN phase (190.0 ± 0.1 eV for B1s and 398.0 ± 0.1 eV for N1s
)
with a d value of 2.08 A, which can be indexed to the (111) plane
and cBN phase (191.0 ± 0.1 eV for B1s and 398.6 ± 0.1 eV for N1s),
of cBN. Fig. 7(c) shows a typical cBN nanocrystal with ~60 nm in
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 6961–6967 | 6965

View Article Online
Fig. 7 (a) HRTEM image of residual tBN in sample S-22; (b) HRTEM image of cBN nanocrystals in sample S-22; (c) a typical cBN nanocrystal; (d)
high-magnification image of lattice fringes labeled by the box in (c). Inset is the corresponding structure model of cBN along the [111] direction.
size. The high-magnification of the HRTEM image presents well
the single crystal properties of the cBN nanocrystal and obvious
oriented growth of the {111}planes of cBN (Fig. 7(d)).
nucleation processes, respectively. Apparently, DGhom > DGhet, and
the smaller q is, the lower DGhet is required. This fact indicates
that a homogeneous nucleation process is more difficult than a
heterogeneous one.
These above measurement results indicate that almost pure
cBN phase can be fabricated via the low-temperature solid state
synthesis and in situ phase transformation route with the help
of the structural induction effect. This induction effect can be
explained by the heterogeneous nucleation model, which deals
with the nucleation barrier of a new born phase. If there are
no foreign particles, cBN would nucleate spontaneously via
phase transformation at the first step. It needs to overcome an
energy barrier of DGhom. On the contrary, with the existence of
foreign particles with fcc structure, cBN would prefer to nucleate
on the surfaces of them, and the nucleation process becomes
heterogeneous. In this case, the energy barrier reduces to DGhet
It is well known that cBN is a stable phase under high pressure,
and it is difficult to synthesize cBN under ambient pressure.
However, if foreign particles with fcc structure are introduced
into the reaction system, the nucleation of cBN changes from
homogeneous to heterogeneous, and the energy barrier decreases
from DGhom to DGhet. In other words, the formation of cBN
crystallites becomes much easier on this occasion. As a result, the
relative content of cBN obviously increases and an almost pure
cubic phase BN sample is obtained when the structural induction
material is added.
Except for the induction effect, NaF with fcc structure seems
to play an important role in the oriented growth of {111} planes
of cBN nanocrystals. In the NaF sponge, a lot of {111} planes
are expected to be randomly exposed. So, while inducing the
in situ phase transformation of tBN to cBN, the foreign NaF can
be used as a template and promote the oriented growth of cBN
nanocrystals on its surface. In contrary, when no foreign NaF is
added in the reacting system, so obvious oriented characteristics
of the cBN nanoparticles are not easily observed.
39
as follows:
2
⎡
⎢
⎢
⎣
⎤
⎥
⎥
⎦
(
2 + cosq)(1− cosq)
DGhet = DGhom
(2)
4
here q is the contact angle between the foreign particles and
◦
cBN, and is within the range of 0 < q < 180 . DGhom and DGhet
are the energy barriers of the homogeneous and heterogeneous
6
966 | Dalton Trans., 2011, 40, 6961–6967
This journal is © The Royal Society of Chemistry 2011

View Article Online
4
. Conclusion
10 V. L. Solozhenko and V. Z. Turkevich, J. Phys. Chem. B, 1999, 103,
8
137.
1
1
1
1 S. K. Singhal and J. K. Park, J. Cryst. Growth, 2004, 260, 217.
2 H. Lorenz and I. Orgzall, Diamond Relat. Mater., 1995, 4, 1046.
3 V. Turkevich, T. Taniguchi, A. Andreev and P. Itsenko, Diamond Relat.
Mater., 2004, 13, 64.
cBN is prepared by a low-temperature solid state synthesis and
in situ phase transformation route, and the cBN concentration
in the BN sample has been greatly increased by utilizing two
new strategies, i.e., applying pressure onto the reactants during
the reaction process and introducing the structural induction
14 J. Gijnna, H. J. Meurer, G. Nover, T. Peun, D. Schiinbohm and G. Will,
Mater. Lett., 1998, 33, 321.
5 X. C. Wang, X. P. Jia, T. C. Zhang, G. Z. Ren, H. J. Liu, C. Y. Zang, P.
W. Zhu, H. A. Ma and G. T. Zou, Diamond Relat. Mater., 2003, 12, 57.
1
effect. Finally, almost pure cBN sample is synthesized by reacting
◦
NH
4
BF
4
and NaN
3
at 250 C. In fact, the conventional solid state
16 M. M. Bindal, B. P. Singh, S. K. Singhal, R. K. Nayar and R. Chopra,
J. Cryst. Growth, 1994, 144, 97.
route is generally regarded as a facile and convenient route to
synthesize hBN or tBN, and it is rather difficult to prepare cBN
by this method. However, the modified solid state route reported
in this paper has been proved to be very effective for increasing
the relative content of cBN, but the actual yield of cBN is not
high. It is reasonable to believe that, by applying these strategies
and optimizing the preparation parameters, pure cBN will be
synthesized with high yield soon.
1
1
7 J. Y. Huang and Y. T. Zhu, Chem. Mater., 2002, 14, 1873.
8 V. L. Solozhenko, V. Z. Turkevich and G. Will, High Press. Res., 1996,
1
0, 135.
19 E. G. Gillan and R. B. Kaner, Chem. Mater., 1996, 8, 333.
2
2
0 L. Rao and R. B. Kaner, Inorg. Chem., 1994, 33, 3210.
1 V. L. Solozhemko, V. Z. Turkevich and G. Will, J. Am. Ceram. Soc.,
1
996, 79, 2798.
22 J. H. Ma, J. Li, G. X. Li, Y. G. Tian, J. Zhang, J. F. Wu, J. Y. Zheng, H.
M. Zhuang and T. H. Pan, Mater. Res. Bull., 2007, 42, 982.
3 C. H. Wallace, S. H. Kim, G. A. Rose, L. Rao, J. R. Heath, M. Nicol
2
and R. B. Kaner, Appl. Phys. Lett., 1998, 72, 596.
24 J. L. O’Loughlin, C. H. Wallace, M. S. Knox and R. B. Kaner, Inorg.
Chem., 2001, 40, 2240.
Acknowledgements
This work was supported financially by the Natural Science
Foundation of China (50672048, 21073107, 50990061), the China
Postdoctoral Science Foundation (20100481245), the Foundation
of Shandong Science & Technology Council (2008GG30003008),
Postdoctoral Innovation Foundation of Shandong Province
25 A. Spiesser, Y. M. Chong, K. M. Leung, G. Abel, G. G. Ross, M. J.
Walzak, R. Jachlin, W. M. Lau, W. J. Zhang and I. Bello, J. Phys. Chem.
C, 2007, 111, 12768.
2
2
2
6 X. W. Zhang, H. G. Boyen, N. Deyneka, P. Ziemann, F. Banhart and
M. Schreck, Nat. Mater., 2003, 2, 312.
7 W. J. Zhang, I. Bello, Y. Lifshitz, K. M. Chan, X. M. Meng, Y. Wu, C.
Y. Chan and S. T. Lee, Adv. Mater., 2004, 16, 1405.
8 D. D. Nataliya, U. Herr, H. J. Fecht, X. W. Zhang, H. Yin, H. G. Boyen
and P. Ziemann, Adv. Eng. Mater., 2008, 10, 482.
(
201003077), Independent Innovation Foundation of Shandong
University (11250070611039) and Postdoctor Foundation of
Shandong University (10000080962080).
29 J. F. Moulder, W. F. Stickle, P. E. Sobol and K. D. Bemben, Handbook
of X-ray Photoelectron Spectroscopy, Physical Electronics, Division,
Perkin-Elmer Corporation, 1992.
Notes and references
30 C. C. Tang, Y. Bando, Y. Huang, C. Y. Zhi and D. Golberg, Adv. Funct.
Mater., 2008, 18, 3653.
1
P. B. Mirkarimi, K. F. McCarty and D. L. Medlin, Mater. Sci. Eng., R,
997, 21, 47.
3
3
3
3
3
3
3
3
3
1 K. M. Leung, C. Y. Chan, Y. M. Chong, Y. Yao, K. L. Ma, I. Bello, W.
J. Zhang and S. T. Lee, J. Phys. Chem. B, 2005, 109, 16272.
1
2
3
I. Tsiaoussis and N. Frangis, J. Microsc., 2006, 223, 205.
J. Yu, Z. Zheng, H. C. Ong, K. Y. Wong, S. Matsumoto and W. M. Lau,
J. Phys. Chem. B, 2006, 110, 21073.
2 U. Vetter, P. Reinke, C. Ronning, H. Hofsass, P. Schaaf, K. Bharuth-
Ram and T. Taniguchi, Diamond Relat. Mater., 2003, 12, 1182.
3 B. E. Douglas, D. H. McDaniel and J. J. Alexander, Concepts and Model
of Inorganic Chemistry, 2nd Edition, Wiley, New York 1983.
4
5
6
7
8
O. Mishima, K. Era, J. Tanaka and S. Yamaoka, Appl. Phys. Lett.,
1
988, 53, 962.
4 K. S. Park, D. Y. Lee, K. J. Kim and D. W. Moon, Appl. Phys. Lett.,
O. Mishima, J. Tanaka, S. Yamaoka and O. Fukunaga, Science, 1987,
38, 181.
E. Knittle, R. M. Wentzcovitch, R. Jeanloz and M. L. Cohen, Nature,
989, 337, 349.
1
997, 70, 315.
2
5 L. D. Jiang, A. G. Fitzgerald, M. J. Rose, A. Lousa and S. Gimeno,
Surf. Interface Anal., 2002, 34, 732.
1
6 B. Angleraud, M. Cahoreau, I. Jauberteau, J. Aubreton and A.
Catherinot,, J. Appl. Phys., 1998, 83, 3398.
W. J. Zhang, X. M. Meng, C. Y. Chan, K. M. Chan, Y. Wu, I. Bello
and S. T. Lee, J. Phys. Chem. B, 2005, 109, 16005.
7 K. Nose, H. S. Yang, H. Oba and T. Yoshida,, Diamond Relat. Mater.,
J. B. MacNaughton, A. Moewes, R. G. Wilks, X. T. Zhou, T. K. Sham,
T. Taniguchi, K. Watanabe, C. Y. Chan, W. J. Zhang, I. Bello, S. T. Lee
and H. Hofsass, Phys. Rev. B, 2005, 72, 195113.
2
005, 14, 1960.
8 J. B. Lian, T. Kim, X. D. Liu, J. M. Ma and W. J. Zheng, J. Phys. Chem.
C, 2009, 113, 9135.
9
F. P. Bundy and R. H. Wentorf, J. Chem. Phys., 1963, 38,
9 C. H. L. Goodman, Crystal Growth Theory and Techniques, Plenum
Press-London-New York, 1974, Chapter 2.
1
144.
This journal is © The Royal Society of Chemistry 2011
Dalton Trans., 2011, 40, 6961–6967 | 6967