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: erdemcamurlu@gmail.com

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