Comparative catalytic study on the carbothermic formationof hexagonal boron nitride with Li, Na, K and Ca carbonates
H. Erdem Camurlu • Ahmet Gencer •
Burcu Becer
Received: 30 May 2013 / Accepted: 30 August 2013 / Published online: 10 September 2013
� Springer Science+Business Media New York 2013
Abstract Catalytic effects of Li, Na, K and Ca carbon-
ates on carbothermic formation of hexagonal boron nitride
(hBN) were investigated. Experiments were conducted by
keeping the plain and catalyst added B2O3 ? C powder
mixtures at 1400 �C in flowing nitrogen for 40–160 min.
Products were subjected to quantitative and X-ray dif-
fraction analyses, scanning and transmission electron
microscopy examinations, and particle size measurements.
Investigated catalysts increased the amount, particle size
and crystallinity of the formed hBN. Average particle sizes
of the obtained hBN powders were in 150–350 nm range.
Catalytically, lithium and sodium carbonates were found to
be the most effective, whereas calcium carbonate was the
least effective. In the experiments conducted for 80 min,
the quantity of the formed hBN increased 13-fold when
40 wt% Li2CO3 was used, as compared to plain mixture.
The increases were ninefold and fourfold for the same
duration when 40 wt% K2CO3 and 10 % CaCO3 were
used, respectively.
Introduction
Hexagonal boron nitride (hBN) has a layered hexagonal
crystal structure, similar to that of graphite [1]. Industrially,
it is an important material that has been utilized in various
applications. Exceptional properties of hBN, such as
lubricating, high temperature and chemical stability, and
non reactivity to molten metals have rendered hBN crucial
for many applications [1]. hBN can be used in hot-pressed
monolithic form [2, 3]; in ceramic, metal or polymer
matrix composites [4–9], in the form of coatings [10, 11],
in the form of nanotubes [12] and other structures [13], in
lubricants [14, 15] for fulfilling functions such as enhanc-
ing high-temperature stability, wear resistance, thermal
shock resistance, and for lowering the friction coefficient
and the thermal expansion coefficient. It was also reported
to improve machinability in ceramic and metal systems
[16, 17]. Furthermore, hBN has been used in polymer
matrix composites in order to improve critical properties
such as toughness [18], wear properties and flame retar-
dance [19].
A number of methods have been used for the production
of hBN in commercial level. One method involves the
reaction of boron oxide and ammonia, which is called
direct nitridation [20, 21]. In this technique, process is
carried out at a relatively lower temperature (\1200 �C)
than the carbothermic method. Reaction of urea or mela-
mine and boric acid in nitrogen or ammonia atmosphere is
another relatively low-temperature method [20, 22]. When
the process temperature is low, the particle size is small
and crystallinity of the obtained hBN is poor. Thus, in these
methods, a second high-temperature calcination step is
required for increasing the crystallinity and particle size.
Carbothermic reduction and nitridation is another method
of hBN production, in which simultaneous reduction and
nitridation of B2O3 occur at relatively higher temperatures
([1400 �C) than the above-mentioned methods. This
results in larger hBN particles and better crystallinity [23–
27].
It has been the aim of many researchers to reduce the
hBN production temperature. Therefore, the use of
H. E. Camurlu (&)
Makine Muhendisligi Bolumu, Akdeniz Universitesi,
Dumlupınar Bulvarı, Kampus, 07058 Antalya, Turkey
e-mail: [email protected]
A. Gencer � B. Becer
Kimya Bolumu, Akdeniz Universitesi, Dumlupınar Bulvarı,Kampus, 07058 Antalya, Turkey
123
J Mater Sci (2014) 49:371–379
DOI 10.1007/s10853-013-7714-x
catalysts such as CaCO3 [24] MgO, BaCO3 [25], Na2CO3
[22, 28], Li2CO3, H3BO3 NH4Cl [26] and LiOH has been
investigated [28]. Effects of lithium carbonate and lithium
hydroxide on the formation of hBN from boric acid and
urea or by carbothermic method were examined by
Bartnitskaya et al. [29]. These additives were suggested to
increase the amount and augment the crystal structure of
the formed hBN. The catalytic effect of these additives was
proposed to be providing the formation of a lithium borate
melt. Then the formed hBN is suggested to dissolve and
crystallize in the melt [26, 29, 30]. Additional studies have
been performed by Camurlu and Sevinc with the aim of
clarifying the catalytic mechanism through which hBN
forms. In these studies, alkaline earth oxides or carbonates
such as CaCO3, MgO or BaCO3 have been added to the
B2O3 ? C mixtures at various amounts and the mixtures
were reacted in nitrogen atmosphere at 1500 �C [24, 25,
31]. When the mentioned catalysts were present in the
reactant mixture, hBN formation was suggested to proceed
by two different mechanisms [24, 25]. Some of the hBN
forms by carbothermic reduction and nitridation, as in the
case without the addition of a catalyst. Additionally, some
hBN forms through an ionic mechanism, which occurs by
the introduction of the catalysts. This mechanism consists
of (1) formation of an alkali borate melt by the reaction of
alkali oxide and boron oxide, (2) formation of borate ions
in the melt and dissolution of nitrogen in the form of
nitrogen anions in the melt and (3) formation of hBN from
the melt by the reaction of borate and nitrogen anions [22,
24, 25, 31]. A similar mechanism was proposed for the
formation of hBN from urea ? boric acid mixtures in
nitrogen or ammonia atmosphere, with the catalytic addi-
tion of Na2CO3. In this case, some of the hBN was sug-
gested to form through the reaction of boron and nitrogen
from urea or from the reaction atmosphere. Some hBN was
proposed to form by the ionic mechanism that takes place
in the borate melt, which becomes operative in the pre-
sence of Na2CO3 [22].
Hexagonal boron nitride is a technologically significant
material that can find applications in many industrial areas.
Thus, catalysts are utilized in each production method of
hBN, in order to enhance its formation efficiency and to
control its properties. Previously, effects of Li [29], Na
[28], Ca, Mg and Ba [24, 25] carbonates on carbothermic
formation of hBN have been investigated by different
groups in literature. However, it is not possible to compare
the results reported in various studies due to the different
experimental conditions. In addition to the differences in
the temperature and duration of the experiments, expres-
sion of the catalyst amount is different. For example, in
some studies the catalyst amount is expressed in wt% [24,
25, 27, 28], while in others it is reported in Li:B mole ratio
[29]. Therefore, in this study, experiments were conducted
for comparative purposes with B2O3 ? C mixtures con-
taining Li2CO3 and K2CO3. In addition, some experiments
were performed by using Na2CO3 and CaCO3 as catalysts
at 1400 �C for different durations.
Materials and methods
A planetary ball mill was utilized for dry mixing the B2O3
(Eti Mine, [98 %), C (Merck [99 %) and catalyst [and
Li2CO3, Na2CO3, K2CO3 or CaCO3 (Merck, [99 %)]
powders. Mixing was performed with steel balls (20 mm
diameter) at 250 rpm for 30 min. B2O3 was used 100 %
more than the amount required according to Reaction (1).
Weight of B2O3 ? C (12.5 g) in the reactants was kept
constant, and catalysts were added into this mixture in
varying percentages. A graphite crucible, inside of which
was lined with hBN powder, was used for containing the
mixtures. A tube furnace which was heated by SiC hot rods
and which had a mullite tube was used for the experiments.
The tube had inner and outer diameters of 50 and 60 mm,
respectively. The furnace was heated to 1400 �C and after
the temperature was stabilized, the graphite crucible with
its contents was drawn to the hot zone. 1400 �C is the
highest allowable working temperature for the mullite tube
of the furnace. Reactions were conducted by keeping the
crucible and its contents under flowing N2 (200 ml/min) for
predetermined durations. Quantities of the catalysts in the
reactants and experiment durations are presented in
Table 1. When the predetermined duration was elapsed, the
products were immediately drawn out of the furnace and
they were cooled under argon gas to prevent oxidation.
B2O3 lð Þ þ 3C sð Þ þ N2 gð Þ ¼ 2BN sð Þ þ 3CO gð Þ ð1ÞA Rigaku Multiflex unit was utilized in order to conduct
the XRD analyses. The Scherrer formula [32] was used for
calculating the interplanar spacing and the average crys-
tallite thickness, Lc values of the formed hBN from the
XRD data. Full width at half maximum (FWHM) values
were determined by Qantitative Analysis software, pro-
vided with the XRD device. Purification and quantitative
analysis of hBN were conducted as described previously
[23, 24]. This method is composed of successive leaching–
oxidation and leaching processes of the products. In the
first leach, boron oxide and borates in the products are
removed by mixing the products in 1/1 HCl/water for 15 h
on a magnetic stirrer. The leach residue contains hBN,
unreacted carbon and in some cases, B4C. In the second
step, the leach residue is heated in air at 800 �C for oxi-
dation of unreacted C to CO(g) and B4C to B2O3(l) ?
CO(g). The final leaching step is performed for removing
the B2O3 (which forms as a result of oxidation of B4C at
800 �C). Pure hBN is obtained at the end of the final leach.
372 J Mater Sci (2014) 49:371–379
123
The obtained hBN particles were subjected to particle size
analysis (Malvern Zetasizer ZS), scanning electron
microscopy (QUANTA 400F Field Emission SEM) and
transmission electron microscopy (JEOL Jem 2100F
HRTEM) examinations.
Results and discussion
Quantitative and XRD analyses
The initial experiments were performed without Li2CO3
and with 10 % Li2CO3 addition for durations up to
160 min. Amounts of hBN were determined by the previ-
ously described purification method. The results are given
in Table 1 and Fig. 1 [23, 24]. It can be seen that only
0.15 g hBN formed in 80 min from plain mixtures. The
amount of hBN formed in this experiment was too low to
be separated from the filter paper during the purification
process. Therefore, this product could not be subjected to
SEM, TEM and particle size analyses. In 160 min 0.57 g
hBN formed from plain mixture and this powder was used
for comparison with hBN formed from the catalyst added
mixtures. A significant amount of hBN was seen to form
when 10 % Li2CO3 added mixture was reacted for 80 min
(1.43 g) as compared to that formed from plain mixtures
(0.15 g). Amount of formed hBN increased with time at a
decreasing rate of formation and about 2 grams of hBN
formed in 160 min when 10 % Li2CO3 was used. Fol-
lowing experiments were performed by increasing the
Li2CO3 catalyst addition in the starting mixture. The same
amount of hBN (2 g) formed in 120 min when 20 %
Li2CO3 was used. When 40 % Li2CO3 was used as cata-
lyst, about 2 g hBN formed in 80 min. These results
indicate that the formation rate of hBN increases with the
increase in the addition of Li2CO3.
XRD patterns of the experiments conducted for 80 min
with plain and 10–40 % Li2CO3 containing mixtures are
presented in Fig. 2a–e. It can be seen that the intensity of
hBN peaks is very low when no catalyst was used and their
intensity increases with the increase in the addition of
Li2CO3. These results are in agreement with the results of
the quantitative analysis given in Fig. 1.
According to the XRD patterns given in Fig. 2a, b,
boron oxide (which transforms into boric acid, H3BO3,
during sample preparation) was present in the products
when no Li2CO3 was used and when 10 % Li2CO3 was
used. Li2CO3 is expected to decompose to Li2O and CO2 in
the initial stages of the experiment at 1400 �C, after the
mixture is placed in the hot zone of the preheated furnace.
The decomposition temperature of Li2CO3 is 1300 �C [33].
The formed Li2O is expected to dissolve in B2O3, which is
in liquid state at the reaction temperature. Therefore, a
lithium borate melt forms. Upon cooling, formation of
solid lithium borate phases (or also solid B2O3 phase) takes
place in accordance with the phase diagram given in Fig. 3
[34]. Thus, the formed phases are dependent on the amount
of Li2O in the Li2O–B2O3 system. Weight % of Li2O in
Li2O–B2O3 mixture was 4.8 % when 10 wt% Li2CO3 was
Table 1 Types and amounts of used catalysts, experiment durations, amounts and average particle sizes of obtained hBN
Experiment duration and type of catalyst Amount of catalyst in the starting mixture, wt%
0 10 20 30 40 60
Amount of formed hBN (g) and [average particle size (nm)]
40 min, Li2CO3 0.71 (232) 1.14 (251) 1.39 (301)
40 min, Na2CO3 1.43 (278)
40 min, K2CO3 [27] 0.37 (154) 0.86 (160) 0.87 (218) 0.51 (221)
80 min, Li2CO3 0.15 (na) 1.43 (261) 1.96 (282) 1.86 (302) 2.04 (333)
80 min, K2CO3 [27] 0.40 (222) 0.84 (247) 1.25 (295) 1.35 (292) 1.16 (301)
80 min, CaCO3 0.63 (200)
120 min, Li2CO3 1.85 (290) 2.08 (286)
160 min, Li2CO3 0.57 (150) 2.01 (346)
Fig. 1 Amounts of hBN formed as a function of duration from plain
mixtures and from mixtures containing various amounts of Li2CO3
J Mater Sci (2014) 49:371–379 373
123
added. This composition corresponds to Li2O�4B2O3 ?
B2O3 region in the Li2O–B2O3 phase diagram given in
Fig. 3. This fact is in agreement with the presence of boron
oxide peak in the XRD analysis in the 10 % Li2CO3 added
sample. The other expected phase, Li2O�4B2O3, was not
observed in the XRD analysis, most probably due to its
amorphous structure. This structure may be a result of rapid
cooling after the sample was quickly withdrawn from the
furnace. The sample having 20 % Li2CO3 is in the vicinity
of Li2O�4B2O3 line in the given phase diagram, with the
Li2O wt% of 9.2. Neither B2O3 nor 4Li2O�B2O3 peaks were
present in this composition. When the Li2O3 % was 30 and
40, Li2O wt% in the initial Li2O ? B2O3 system was 13.2
and 16.8, respectively. These values correspond to Li2O�4B2O3 ? Li2O�2B2O3 and Li2O�2B2O3 ? Li2O�B2O3
regions after cooling, respectively. In the XRD patterns,
Li2O�4B2O3 phase was not observed; however, crystalline
Li2O�2B2O3 ? Li2O�B2O3 phases were present. The
intensities of the Li2O�2B2O3 and Li2O�B2O3 phases were
seen to increase with the increase in the amount of Li2CO3
addition. These findings are in accordance with the expected
phase compositions in the Li2O–B2O3 phase diagram.
The average crystallite size value, Lc value of the hBN
which formed without Li2CO3 addition in 80 min, was
calculated as 9 nm. The Lc value increased to 19.3 nm
with the addition of 10 % Li2CO3. Lc values for the
experiments conducted for 80 min were 22, 23.7 and
38.7 nm for 20, 30 and 40 % Li2CO3 additions, respec-
tively. The quantitative and XRD analyses indicate that
formation rate and crystallinity of hBN are enhanced by the
catalytic effect of Li2CO3.
Quantities of hBN formed in 40 and 80 min by using
varying amounts of Li2CO3 and K2CO3 [27] are presented
in Fig. 4. Amounts of hBN formed with Li2CO3 in both 40
and 80 min series were higher than those formed with
K2CO3. Li2CO3 was so effective that the amount of hBN
formed with Li2CO3 in 40 min was similar to that formed
with K2CO3 in 80 min.
In a previous study, 10 wt% CaCO3 was found as the
optimum amount for increasing the hBN yield [24].
Therefore, in this study 10 % CaCO3 was used for
Fig. 2 XRD patterns of the products obtained without additive and
with 10–40 % Li2CO3, 40 % K2CO3 [27] or 10 % CaCO3 after
holding for 80 min in N2 at 1400 �C. (1 hBN, 2 H3BO3, 3 B4C, 4
Li2O�2B2O3 (ICDD # 18-0717), 5 LiBO2 (ICDD #51-0517)), 6
KB5O8.4H2O (ICDD #25-0624)
Fig. 3 Boron oxide-rich side of the Li2O–B2O3 phase diagram [34].
(Reprinted by permission of John Wiley & Sons, Inc)
Fig. 4 Quantities of hBN formed in 40 and 80 min as a function of
catalyst type (Li2CO3, Na2CO3, K2CO3[27] and CaCO3) and amount
374 J Mater Sci (2014) 49:371–379
123
comparing its catalytic effect with Li2CO3 and K2CO3 in
the same experimental conditions. 10 % CaCO3 resulted in
the formation of much less hBN as compared to the amount
of hBN formed when the optimum amount of K2CO3 was
used (30 %). Furthermore, catalytic effect of CaCO3 was
seen to be much lower than that of Li2CO3.
Effect of Na2CO3 was compared with the effects of
Li2CO3 and K2CO3 by conducting an experiment using
40 % Na2CO3. It was seen that similar amount of hBN
formed in 40 min when Na2CO3 or Li2CO3 was used. This
amount is about 50 % more than the hBN formed in
40 min with 40 % K2CO3 addition.
XRD patterns of the products of the experiments con-
ducted for 40 min with 40 % Li2CO3, Na2CO3 and K2CO3
[27] are presented in Fig. 5. In order to examine the cata-
lytic effects of the additives, the reaction products had to be
analyzed at an intermediate stage before complete con-
sumption of the reactants. 40-min duration served well for
this purpose. It can be seen that the relative intensities of
the hBN peaks are lower in the products obtained from
K2CO3 added mixture than those obtained by using Li2CO3
and Na2CO3. This result indicates that catalytic activities of
Li2CO3 and Na2CO3 are better than the catalytic effect of
K2CO3. This outcome is in parallel with the results of the
quantitative analyses given in Fig. 4. It can be seen that
lower amount of hBN was obtained when K2CO3 was used.
Li, Na and K borates were also present in the reaction
products of the experiments conducted for 40 min with
40 % additions. In the case of Na and K, borates were
detected in the form of borate hydrates, which form due to
hydration of borates during sample preparation. Average
crystallite sizes (Lc values) for hBN obtained in 40 min
from mixtures containing Li, Na and K carbonates were
similar, about 25 nm.
XRD patterns of the experiments conducted for 80 min
(Fig. 2a) with plain mixtures and with mixtures containing
40 % Li2CO3 (Fig. 2e), 40 % K2CO3 (Fig. 2f) [27] and
10 % CaCO3 (Fig. 2g) are presented in Fig. 2. The inten-
sities of hBN peaks in the products obtained from plain
mixtures are considerably lower than those obtained from
catalyst added mixtures. The diffuse form of the hBN peaks
observed in Fig. 2a indicates that the formed hBN has low
crystallinity or turbostratic nature. CaCO3 addition
(Fig. 2g) provided slightly narrower hBN peaks than plain
mixture. Li2CO3 (Fig 2e) and K2CO3 (Fig. 2f) addition
resulted in XRD peaks of higher intensity and lower
FWHM, indicating their superior catalytic activity over
CaCO3 in carbothermic formation of hBN. The relative
peak intensities in the XRD patterns of the products
obtained by using various catalysts are in agreement with
the quantitative analysis results given in Fig. 4. The aver-
age crystallite sizes (Lc values) of the hBN obtained in
80 min from plain, 40 % Li2CO3, 40 % K2CO3 and 10 %
CaCO3 added mixtures were 9, 38.7, 34 and 13.2 nm,
respectively. It can be inferred that alkaline carbonates are
more effective in improving the crystal structure of hBN as
compared to alkaline earth carbonates.
It was seen that K borate peaks decrease with the
increase in the experiment duration from 40 min (Fig. 5c)
to 80 min (Fig. 2g). In addition, boron oxide peaks appear.
This indicates that the composition of the K2O–B2O3 sys-
tem shifts to boron oxide-rich side of the phase diagram.
This also suggests that the vaporization of potassium
borates is rapid. The fact that the weight of the products
and also weight of boron oxide ? potassium borate
decrease with increasing amount of K2CO3 in the starting
mixture supports this indication. In the case of Li2CO3, the
relative heights of the peaks of the borate phase did not
change by duration (Figs. 5a, 2e). However, the back-
ground between 20 and 25� disappears, indicating the
consumption of an amorphous phase. The weight of the
reactants increases with the increase in the catalyst addition
in the initial B2O3 ? C mixtures. Thus an increase in the
weight of the products may be expected. However, the
weight of the products and weight of boron oxide ? lith-
ium borate in the products were found to be constant and to
be independent from the Li2CO3 content. The weights of
the products and the weights of the boron oxide ? borate
phases formed in 40 min from the mixtures containing
40 % Li, Na and K carbonates are presented in Fig. 6. The
weight loss was higher in K2CO3 or Na2CO3 added mix-
tures than the weight loss in Li2CO3 added mixtures. Thus
in these mixtures a higher rate of borate evaporation can be
inferred.
Fig. 5 XRD patterns of the samples containing 40 % Li2CO3,
Na2CO3 and K2CO3 [27], after holding for 40 min in N2 at
1400 �C. 1 hBN, 3 KB5O8.4H2O (ICDD #25-0624), 4 Na2B4O7.5H2O
(ICDD # 7-0277), 5 Li2O�2B2O3 (ICDD # 18-0717), 6 LiBO2 (ICDD
#51-0517)
J Mater Sci (2014) 49:371–379 375
123
SEM, TEM analyses and particle size measurements
SEM and TEM micrographs of the hBN powders obtained
in 160 min from the plain mixtures are presented in Figs. 7
and 8, respectively. The micrographs of the hBN powders
obtained in 40 and 80 min from the mixtures containing the
investigated catalysts are presented in the same figures.
Size of the hBN particles obtained from plain mixtures in
160 min are between 100 and 200 nm (Figs. 7, 8a). hBN
particles obtained in 40 min from 40 % Li, Na and K
carbonate added samples were larger than those obtained
from plain mixtures in 160 min (Figs. 7, 8b, d, f). In
addition, Li and Na carbonates resulted in larger hBN
particles, as compared to K carbonate in 40 min. Increasing
the experiment duration from 40 to 80 min resulted in a
considerable increase in the hBN particle size when Li and
K carbonates are used, as shown in Figs. 7, 8c, e. hBN
particles obtained in 80 min from 10 % CaCO3 added
mixture (Figs. 7, 8g) were seen to be much finer than those
obtained in 80 min from 40 % Li and K carbonate added
mixtures.
In order to gain more accurate information on particle
size distribution, hBN powders obtained in various dura-
tions by addition of various catalysts were subjected to
particle size distribution analyses via a Malvern Zetasizer
unit. The results are presented in Fig. 9. In addition, the
average particle size values are given in Table 1 in
parenthesis. The particle size analysis results, presented in
Table 1 and Fig. 9, are in agreement with the SEM and
TEM figures given in Figs. 7 and 8.
The average particle size of the hBN powder obtained in
160 min from plain mixture was 150 nm. Particle size was
346 nm when 10 % Li2CO3 was used and the experiment
was conducted for the same duration. In Fig. 9a the effect
of experiment duration on hBN particle size can be seen.
Experiments conducted for 80, 120 and 160 min with 10 %
Li2CO3 resulted in average hBN particle sizes of 261, 290
and 346 nm, respectively.
Effect of the change in Li2CO3 amount on hBN particle
size can be seen in Fig. 9b. Average particle size of hBN
increased from 261 to 333 nm when Li2CO3 content was
increased from 10 to 40 %. In the experiments conducted
for 40 min with 40 % Li, Na and K carbonates, it was seen
that Li2CO3 was the most effective on growth of hBN
particles (Fig. 9c). Average hBN particle sizes were 301,
Fig. 6 Quantities of products and B2O3 ? borate phase in the
products obtained in 40 min as a function of 40 % addition of
various catalysts
Fig. 7 SEM micrographs of hBN obtained from a plain mixture in
160 min, and from mixtures containing, b 40 % Li2CO3 in 40 min,
c 40 % Li2CO3 in 80 min, d 40 % K2CO3 in 40 min, e 40 % K2CO3
in 80 min, f 40 % Na2CO3 in 40 min and g 10 % CaCO3 in 80 min
376 J Mater Sci (2014) 49:371–379
123
278 and 218 for Li, Na and K carbonates, respectively. In
order to compare the effect of CaCO3, an experiment was
conducted for 80 min with 10 % CaCO3 addition. CaCO3
was used at this quantity since 10 % was found as the
optimum amount for CaCO3 in a previous study [24].
Average particle size of hBN obtained in 80 min by using
10 % CaCO3 was 200 nm. This value is higher than the
average particle size of the hBN obtained from plain
mixtures in 160 min (150 nm). In 80-min duration, Li and
K carbonates resulted in hBN particle sizes of 333 and
292 nm, respectively. These results indicate that alkaline
carbonates are significantly effective on growth of hBN
particles. It was found that the most effective one is Li2CO3
and it is followed by Na and K carbonates.
Carbothermic reaction given in Reaction (1) is the single
mechanism that takes place when no catalyst is present in
the B2O3–C mixture. It was seen that at 1400 �C this
reaction is very slow without catalysts, and only 0.57 g
hBN forms in 160 min. However, about 2 g hBN formed in
40 min when 40 % Li2CO3 was used at the same temper-
ature. Therefore, a significant enhancement in the forma-
tion rate is provided with the addition of the catalysts. It is
Fig. 8 TEM micrographs of
hBN obtained from a plain
mixture in 160 min, and from
mixtures containing, b 40 %
Li2CO3 in 40 min, c 40 %
Li2CO3 in 80 min, d 40 %
K2CO3 in 40 min, e 40 %
K2CO3 in 80 min, f 40 %
Na2CO3 in 40 min and g 10 %
CaCO3 in 80 min
J Mater Sci (2014) 49:371–379 377
123
known that addition of alkali or alkaline earth oxides to
B2O3 results in the dissolution of these oxides in B2O3(l).
Thus a borate melt forms [24, 25]. The borate melt is
composed of borate anions (such as BO33- or B2O5
4-).
The structure of these anions depends on the acidity of the
liquid [35]. During the reaction, N2(g) dissolves in the
formed borate liquid as N3- or N- ions. In the ionic
mechanism, formation of hBN is believed to take place in
the melt by the reaction of the borate and nitrogen ions, i.e.
through Reaction (2).
BO3�3
� �þ N3�� �
¼ BN sð Þ þ 3O2� ð2Þ
Recently, it was reported that when CaO bearing B2O3
melts were subjected to nitrogen atmosphere at 1500 �C,
hBN did not form, due to the nucleation barrier for the
homogenous nucleation of hBN [31]. However, when hBN
particles were added into the CaO–B2O3 melt, the amount
of hBN was seen to increase, indicating that the formation
of hBN takes place via the ionic mechanism when hBN
nuclei are present.
Thus, it can be suggested that when a basic catalyst is
introduced into the B2O3–C mixture, the formation of hBN
by the ionic mechanism takes place on the hBN particles
which initially form through the carbothermic reaction.
Formation of hBN is expected to take place by both the
carbothermic reaction and the ionic mechanism, when the
catalyst is present.
Conclusion
For the formation of hBN, experiments were performed by
adding Li, Na, K and Ca carbonates as catalysts into
B2O3 ? C mixtures and by keeping the mixtures at
1400 �C in nitrogen atmosphere. Amount, particle size and
crystallinity of the obtained hBN increased with the addi-
tion of the catalysts. Li2CO3 and Na2CO3 were found to be
the most effective. They were followed by K2CO3 and
CaCO3. When no catalyst was used, 0.15 and 0.57 g hBN
formed in 80 and 160 min, respectively, from a total of
12.5 g B2O3–C mixture. The particle size was 150 nm in
160 min. About 2 g hBN formed in 80 min when 40 %
Li2CO3 was used in the B2O3 ? C reactant mixture. The
average particle size of hBN was 333 nm. Catalytic func-
tion of the additives was proposed as providing the for-
mation of an alkali or alkaline earth borate liquid, in which
Fig. 9 Particle size distribution graphs of hBN obtained from a plain
mixture in 160 min and from mixtures containing 10 % Li2CO3 in
80–160 min, b 10–40 % Li2CO3 in 80 min, c 40 % Li2CO3 or
Na2CO3 or K2CO3 in 40 min, d 10 % CaCO3 and 40 % K2CO3 [27]
or Li2CO3 in 80 min
378 J Mater Sci (2014) 49:371–379
123
the hBN particles form. This mechanism takes place in
addition to the carbothermic formation reaction, which is
the single mechanism in the absence of a catalyst.
Acknowledgements Authors are grateful to The Scientific and
Technological Research Council of Turkey (TUBITAK) for sup-
porting this study with the Project Number 110M722.
References
1. Lipp A, Schwetz KA, Hunold K (1989) J Eur Ceram Soc 5:3
2. Drevet B, Eustathopoulos N (2012) J Mater Sci 47:8247
3. Eichler J, Lesniak C (2008) J Eur Ceram Soc 28:1105
4. Zhang G-J, Yang J-F, Ando M, Ohji T (2002) J Eur Ceram Soc
22:2551
5. Kodera Y, Toyofuku N, Yamasaki H, Ohyanagi M, Munir ZA
(2008) J Mater Sci 43:6422
6. Herrmann M, Rathel J, Hohna S, Eichler J, Michaelis A (2011) J
Eur Ceram Soc 31:2401
7. Ma X-D, Ohtsuka T, Hayashi S, Nakagawa Z (2006) Compos Sci
Technol 66:3089
8. Wei X-F, Wang R-C, Feng Y, Peng C-Q, Zhu X-W (2011) J Cent
South Univ Technol 18:1334
9. Wang Z, Iizuka T, Kozako M, Ohki Y, Tanaka T (2011) IEEE
Trans Dielectr Electr Insulation 18:1963
10. Tsunekawa Y, Ozdemir I, Okumiya M (2006) J Therm Spray
Technol 15:239
11. Smid I, Segall AE, Walia P, Aggarwal G, Eden TJ, Potter JK
(2012) Tribol Trans 55:599
12. Singhal SK, Srivastava AK, Pant RP, Halder SK, Singh BP,
Gupta AK (2008) J Mater Sci 43:5243
13. Sajjad M, Ahmadi M, Guinel MJ-F, Lin Y, Feng P (2013) J Mater
Sci 48:2543
14. Scharf TW, Prasad SV (2013) J Mater Sci 48:511
15. Demas NG, Timofeeva EV, Routbort JL, Fenske GR (2012)
Tribol Lett 47:91
16. Jin H-Y, Xu H, Qiao G-J, Gao J-Q, Jin Z-H (2008) Mater Sci Eng
A Struct 483–484:214
17. Tanakaa R, Yamane Y, Sekiya K, Narutaki N, Shiraga T (2007)
Int J Mach Tool Manuf 47:1971
18. Michael F (2000) In: Pearson RA (ed) Toughening of plastics:
advances in modeling and experiments. American Chemical
Society, USA
19. Madakbas S, Cakmakcı E, Vezir Kahraman M (2013) Thermo-
chim Acta 552:1
20. Paine RT, Narula CK (1990) Chem Rev 90:73
21. Ay N, Tore I (2007) Mater Sci Forum 554:207
22. Camurlu HE (2011) Ceram Int 37:1993
23. Camurlu HE, Sevinc N, Topkaya Y (2006) J Mater Sci 41:4920
24. Camurlu HE, Sevinc N, Topkaya Y (2008) J Eur Ceram Soc
28:679
25. Camurlu HE, Topkaya Y, Sevinc N (2009) Ceram Int 35:2271
26. Lyashenko VI, Ostrovskaya NF, Zelyavskii VB, Kurdyumov AV,
Bartnitskaya TS (2003) Powder Metall Met Ceram 42:189
27. Camurlu HE, Gencer A, Becer B, Ozdogan S (2013) Ceram Int.
doi:10.1016/j.ceramint.2013.04.032
28. Akyıldız U, Sevinc N (2011) Turkish J Eng Environ Sci 35:199
29. Bartnitskaya TS, Kurdyumov AV, Lyashenko VI, Ostrovskaya
NF (1998) Powder Metall Met Ceram 37:26
30. Ostrovskaya NF, Bartnitskaya TS, Lyashenko VI, Zelyavskii VB,
Kurdyumov AV (1996) Powder Metall Met Ceram 35:636
31. Ozkenter AA, Camurlu HE, Sevinc N (2012) J Ceram Process
Res 13:533
32. Cullity BD, Stock SR (2001) Elements of X-ray diffraction.
Prentice Hall, New Jersey
33. Weast RC (1987) Handbook of chemistry and physics, 67th edn.
CRC Press, USA
34. Sastry BSR, Hummel FA (1959) J Am Ceram Soc 42:216
35. Min DJ, Fruehan RJ (1990) Metall Trans B 21B:1025
J Mater Sci (2014) 49:371–379 379
123