In situ molten phase-assisted self-healing for maintaining fiber morphology during conversion from melamine diborate to boron nitride

Article abstract of DOI:10.1039/c9ra10292b

C₃N₆H₆·2H₃BO₃ (M·2B) is a highly promising precursor of boron nitride (BN) fibers due to its eco-friendly and low-cost fabrication. However, it is still unclear why the fibers can maintain their morphology in spite of drastic weight loss (nearly 80 wt%) during M·2B-to-BN pyrolysis. Herein, an interesting cracking and self-healing behavior of the heated M·2B fibers was observed at initial pyrolysis. In situ formed molten boron oxide (B₂O₃) was figured out to be the healing agent for the cracks and subsequently merged into the continuous matrix enclosing melamine/melem molecules, which subsequently acted as a nitrogen source. The B₂O₃ matrix helped to keep the fiber morphology undamaged under the second weight-loss stage in the pyrolysis process. This strategy of taking advantage of the in situ formed molten phase for healing cracks offers detailed guidance to prepare defect-free M·2B-derived BN fibers and would be significant in defect repair for other ceramics.

Full text of DOI:10.1039/c9ra10292b

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In situ molten phase-assisted self-healing for
maintaining fiber morphology during conversion
from melamine diborate to boron nitride†
Cite this: RSC Adv., 2020, 10, 11105
a
a
b
Cheng Han,a Xiaoshan Zhanga
*
Chunzhi Wu,
Bing Wang,
Nan Wu,
a
*
and Yingde Wang
C3N6H6$2H3BO3 (M$2B) is a highly promising precursor of boron nitride (BN) fibers due to its eco-friendly
and low-cost fabrication. However, it is still unclear why the fibers can maintain their morphology in spite of
drastic weight loss (nearly 80 wt%) during M$2B-to-BN pyrolysis. Herein, an interesting cracking and self-
healing behavior of the heated M$2B fibers was observed at initial pyrolysis. In situ formed molten boron
oxide (B2O3) was figured out to be the healing agent for the cracks and subsequently merged into the
continuous matrix enclosing melamine/melem molecules, which subsequently acted as a nitrogen
source. The B2O3 matrix helped to keep the fiber morphology undamaged under the second weight-loss
stage in the pyrolysis process. This strategy of taking advantage of the in situ formed molten phase for
healing cracks offers detailed guidance to prepare defect-free M$2B-derived BN fibers and would be
significant in defect repair for other ceramics.
Received 8th December 2019
Accepted 21st February 2020
DOI: 10.1039/c9ra10292b
rsc.li/rsc-advances
propylamino)borazine].17 This implies that substantial release
occurs during thermal conversion from M$2B to the nal BN
Introduction
bers. Moreover, M$2B is a supra-molecule constructed by low-
strength hydrogen bonding19,20 that might fail to self-support at
a high temperature, resulting in structural collapse. These two
disadvantages may cause serious defects in the nal BN bers.
The fact, astonishingly, is that the M$2B-derived BN bers
display intact morphology basically inherited from initial
M$2B.13,21 It still remains unclear why the seemingly contra-
dictory phenomenon occurs.
In this work, we observed unique self-healing behavior aer
morphological cracks were caused by huge weight loss. This
self-healing behavior kept the ber morphology intact, while its
micro-structure was drastically broken and reconstructed
during pyrolysis. The molten boron oxide (B2O3) generated in
situ from the H3BO3 portion was proven to work for healing the
cracks and repairing the ber. Moreover, the boron oxide
network could enclose melamine molecules and its condensate
melem to prevent them from evaporation, which was crucial to
keep the bers intact as pyrolysis and nitridation proceeded in
the subsequent process.
Due to the excellent comprehensive performances of thermal
stability, chemical corrosion tolerance, oxidation resistance,
neutron absorption, high thermal conductivity, electrical insu-
lation and low dielectric constant,1–4 hexagonal boron nitride
(BN) holds great promise in various elds such as aerospace,
high-temperature ltration, nuclear industry, and microelec-
tronics. As typical BN materials, BN bers similar to anisotropic
BNNSs enjoy wide attention owing to their unique properties
provided by their high-aspect-ratio morphology.5,6
Up to now, numerous fabrication techniques have been
developed for the high-efficiency production of BN bers, which
are mostly based on the spinning process that requires strict
manipulations.7–11 In addition, the pyrolysis of a C3N6H6$2H3-
BO3 (M$2B) supra-molecule adduct is an alternative way and
promising for large-scale production owing to the facile and
eco-friendly fabrication of M$2B.12–14
Unfortunately, the ceramic yield of M$2B is much lower
(ꢀ20 wt%) than that of other reported polymeric precursors of
BN,15–18 for example, 57 wt% yield of poly[2,4,6-tris(iso-
aScience and Technology on Advanced Ceramic Fibers and Composites Laboratory,
College of Aerospace Science and Engineering, National University of Defense
Technology, Changsha, 410073, P. R. China. E-mail: bingwang@nudt.edu.cn;
wangyingde@nudt.edu.cn
bDepartment of Materials Science and Engineering, College of Aerospace Science and
Engineering, National University of Defense Technology, Changsha, 410073, P. R.
China
Experimental section
Synthesis of C3N6H6$2H3BO3
A mixture of C3N6H6 and H3BO3 (analytical reagents, Sino-
pharm Chemical Reagent Co., Ltd.) with a molar ratio of 1 : 2
was added into de-ionized water and dissolved at 85 ꢁC in
a water bath with vigorous stirring; the concentration of C3N6-
H6$(H3BO3) was set as 0.08 mol Lꢂ1 (0.16 mol Lꢂ1) to ensure
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra10292b
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complete dissolution. Then, the hot aqueous solution was
naturally cooled down to precipitate the C3N6H6$2H3BO3
(M$2B) occulent. Subsequently, the white occulent was
frozen and dried using a vacuum freeze-dryer (LGJ-FD, Beijing
Songyuan Huaxing Technology Develop Co., Ltd, China) to get
well-dispersed M$2B bers.
Thermal treatment of C3N6H6$2H3BO3
The thermal behavior of M$2B was analyzed using thermog-
ravimetry and differential scanning calorimetry (TG-DSC,
NETZSCH STA449F5, Germany); the heating rate was set as
10 C minꢂ1. Based on the results, the M$2B bers were sepa-
ꢁ
rately heated to 125 ꢁC, 150 ꢁC, 200 ꢁC, 400 ꢁC, 550 ꢁC, 650 ꢁC,
and 1000 ꢁC at the same temperature ramping rate and
immediately cooled down. The thermal treatments were con-
ducted in a quartz tubular furnace under a protective N2 ow.
The obtained samples were designated as M$2B125, M$2B150
,
Fig. 1 TG-DSC curves of (a) M$2B supramolecular fibers, (b) H3BO3
and (c) C3N6H6. In (a), the first weight-loss stage is in the range of 20–
400 ꢁC; the second weight-loss stage is in the range of 400–1000 ꢁC,
the inset SEM images demonstrate the morphological evolution of the
C3N6H6$2H3BO3 fibers during thermal treatment: a M$2B, b M$2B150, c
M$2B200, d M$2B400, e M$2B650, f M$2B1000. (d) Molecular trans-
formation of H3BO3 and (e) C3N6H6 in the heating process. In (d),
H3BO3 is converted to metaboric acid (HBO2) and further to tetraboric
acid H2B4O7 and finally to a boron oxide structure. In (e), C3N6H6
condenses to melem at 300–400 ꢁC.22
M$2B200, M$2B400, M$2B550, M$2B650, and M$2B1000. For
comparison, melamine and boric acid underwent the same TG-
DSC testing and were also heated at 150 ꢁC, 200 ꢁC, and 400 ꢁC.
To explore the effect of the self-healing process on the
morphology of the nal BN bers, the M$2B bers were also
heated under a stage-variable N2 or NH3 atmosphere. For
example, the M$2B ber was heated to 700 ꢁC for 1 hour under
ꢁ
NH3 and subsequently heated to 1000 C under switched N2.
Characterization and corrosion testing
ꢁ
slowly decreases from 500 to 1000 C, and a broad weak endo-
The morphologies of the samples were observed by eld-
emission scanning electron microscopy (SEM, Hitachi S-4800,
Japan), and the constituents were characterized through Four-
ier transform infrared spectroscopy (FT-IR, Frontier, Perki-
nElmer, USA), X-ray diffraction (XRD, Bruker AXS D8 Advance
diffractometer, Germany), and 13C nuclear magnetic resonance
(NMR) spectroscopy in a DMSO-d6 solvent (400 MHz Agilent
400/54/ASP, USA). The molten transition point of monophasic
H3BO3 was measured using the melting point apparatus (WRS-
2A, Shanghai INESA Physico-Optical Instrument Co., Ltd.).
The corrosion testing of the M$2BT ber (T ¼ 150, 200, 400,
and 550 ꢁC) was conducted using ethanol (EA) and ethylene
glycol (EG) as corrosive agents. Initially, two samples were
separately immersed in EA and EG for 1 minute and then taken
out for washing 3 times by de-ionized water via a vacuum
ltration apparatus. Aer that, the treated samples were dried
over-night in an oven at 80 ꢁC for observing morphology.
Furthermore, the M$2BT bers (0.02 g) were dipped in 4 ml
solvent (EA and EG) for 6 hours to estimate their solubility,
which reected the corrosion resistance of the bers.
thermic peak is located at 535 ꢁC. The insets show the
morphology of the M$2B ber ꢁand its ꢁthermal products aer
ꢁ
ꢁ
being heated separately at 150 C, 200 C, 400 C, 650 C, and
1000 ꢁC; they are designated as M$2B150, M$2B200, M$2B400
,
M$2B650, and M$2B1000, respectively. It was obvious that
a drastic variation in the ber's morphology occurred at the rst
stage; the ber cracked when heated at 150 ꢁC and self-healed at
200 ꢁC. Aer this, the ber got smoother at 400 ꢁC (inset (d) of
Fig. 1a) and was intact in the whole second stage (from inset (d)
to inset (f) of Fig. 1a). In the nal stage, the M$2B1000 ber
displayed compact morphology. Thus, self-healing was proven
to be the key to maintain the ber's morphology in spite of the
ultralow yield (ꢀ20 wt%) of M$2B.
The ber's cracking could undoubtedly be mainly attributed
to the intense weight loss, revealing substantial decomposition
at 150 C, but it was not clear why the self-healing behavior of
M$2B200 occurred. To explore the reason, the structural trans-
formation during cracking-healing was analyzed.
Considering that M$2B is an adduct of H3BO3 and C3N6H6
connected via hydrogen bonding,20 we studied the thermal
behavior of the two components separately. Fig. 1b and c
display the TG-DSC curves of H3BO3 and C3N6H6, respectively. It
can be found that the same-region weight loss of boric acid
occurs (Fig. 1b) as the rst-stage weight loss of M$2B. Mean-
while, C3N6H6 remained stable at 150–200 ꢁC, as implied by the
at TG curve (Fig. 1c) and the XRD patterns of the heated
products (Fig. S1b†).
ꢁ
Results and discussion
Morphology transformation
Fig. 1a indicates the thermal behavior of M$2B using TG-DSC;
the whole pyrolysis procedure can be divided into two obvious
weight-loss stages. In the rst stage, in the range of 125–200 ꢁC,
ꢁ
there is a sharp endothermic peak at 159.9 C due to intense
weight loss. During the second stage, the gravimetric curve
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The weight loss of boric acid could be divided into three structural disordering. Morphology cracking could be attrib-
stages according to three sharp endothermic peaks, separately uted to the drastic structure transformation and also the huge
corresponding to the different stages of the dehydration of release of the removed H2O. As for the micro-structure of the
H3BO3, as shown in Fig. 1d. These reaction equations were cracked ber, it should be noted that the sharp XRD peaks of
proved by the XRD patterns of the heating products (Fig. S1a†). M$2B125 correspond well with the (ꢂ110), (012), (111) and (102)
Thus, the rst-stage weight loss of M$2B and the corresponding crystal planes of melamine (ICSD PDF#39-1950) but not M$2B
morphology evolution were basically attributed to the dehy- (Fig. 3a). This means that there occurred the assembly of
dration of H3BO3 in M$2B. At the subsequent stage from 200 to melamine molecules to form crystals, while disordering
ꢁ
400 C, there was no obvious weight loss against the sublima- occurred in the initial M$2B crystal, allowing for the fact that
tion and condensation of melamine (Fig. 1c and e), which would C3N6H6 was molecularly isolated by H3BO3 in M$2B. Thus, the
be discussed later. This stable stage ensured that the double broad peaks of M$2B150 were proved to result from the
morphology of the M$2B400 ber became more dense and widening of the (ꢂ110) and (102) diffraction peaks and indi-
smooth rather than broken.
cated the existence of melamine micro-crystals in the M$2B150
Based on the analysis of the thermal transformation of the ber.
pure phases of C3N6H6 and H3BO3, the structural evolution of
From the cracked M$2B150 to the healed M$2B200, there also
M$2B involving morphological cracking–healing was further occurred intense structural transformation. In addition to the
studied using FT-IR spectroscopy and XRD. As shown in Fig. 2a, IR peaks belonging to –NH2, C–N, BO3, triazine ring, and B3O3,
plenty of sharp peaks disappear aer the conversion from M$2B the evolved peak of BO4 at 1050 cmꢂ1 indicates the formation of
to M$2B150, especially in the ngerprint region (marked by the a B2O3 network.23,26 Two undened peaks at 1240 cmꢂ1 and
light green rectangle areas) of the hydrogen-bonded structure. 1302 cmꢂ1 existing for M$2B200–400 may be related to the C–O–B
The FTIR peaks at 1186 cmꢂ1 and 786 cmꢂ1 that respectively structure formed between melamine and boron oxide,27 which
represents bending and twisting vibrations of B–OH, became can keep melamine non-volatile in the range of 300–400 ꢁC
feeble as to M$2B150. This indicated the drastic removal of (Fig. 1a and c). The 13C NMR spectra of monophasic melamine,
hydroxyl groups, which was in accordance with the dehydration M$2B, M$2B150 and M$2B200 (Fig. S2†) also proved the consis-
of H3BO3. Meanwhile, the highly crystallized M$2B was con- tent structural evolution, which especially revealed a slight
verted into an unordered phase of M$2B150 characterized as difference between M$2B150 and M$2B200. The change may be
double broad peaks (Fig. 2b).
caused by the newly formed carbonaceous structure.
M$2B125 displays an intermediate structure, revealing the
Besides, the double broad XRD peaks of M$2B150 merged
into an envelope peak of M$2B200–400. Interestingly, a broad
transformation process from M$2B to unordered M$2B150
,
which was characterized as weaker peaks located mainly in the peak belonging to g-B2O3 was located at 23ꢁ (Fig. S1a†) between
green ngerprint region in the IR spectrum and the co-existence the double broad peaks located at 16.3ꢁ and 26.2ꢁ. To study this
of sharp and broad peaks in the XRD pattern. Therefore, the in detail, we tted the XRD peaks of M$2B150, M$2B200, and
drastic removal of hydroxyl groups due to the dehydration of M$2B400 using that of g-B2O3 as a reference. As shown in Fig. 3b,
H3BO3 resulted in the breakage of the M$2B crystal and thus the envelope peak for the healed M$2B200 ber can be well tted
as the double peaks of M$2B150 and a broad peak of g-B2O3, and
the envelope peak of smoothed M$2B400 becomes more
Fig. 2 (a) FT-IR spectra and (b) XRD patterns of the M$2B supra-
molecule and its heated products M$2B125
, M$2B150, M$2B200,
M$2B400, M$2B550. The light green rectangle areas in (a) are the
fingerprint region of the hydrogen-bonded adduct structure, and the
absorption bands are labeled as follows: d(NH2): –NH2 bending,
1620 cmꢂ1; n(C–N): C–N stretching, 1552 cmꢂ1 and 1525 cmꢂ1; n(B–
O): 1450 cmꢂ1 to B–O asymmetric stretching in BO3 and 1380 cmꢂ1 to
B–O stretching in B3O3 ring; n(B–N): B–N stretching at 1400 cmꢂ1; d(– Fig. 3 (a) Analysis of the XRD pattern of M$2B125 in comparison with
OH): –OH bending, 1186 cmꢂ1; n(BO4): B–O stretching in the BO4 unit, that of melamine. (b) Peak fitting of the XRD patterns of M$2B150
,
1050 cmꢂ1; n(BO3): B–O symmetric stretching in the BO3 unit, M$2B200, and M$2B400 using B2O3 as a reference. (c) Photographs of
873 cmꢂ1; d(triazine ring): triazine ring bending at 808 cmꢂ1; d(B–N– the original H3BO3 and heat treatment products: HBO2 after 150 ꢁC
B): out-of-plane bending of B–N–B at 802 cmꢂ1; t(–OH): out-of- heating, B2O3 after 200–400 ꢁC heating, the insets are the corre-
plane –O–H twisting at 786 cmꢂ1
.
sponding molecular structures.
23–25
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integrated owing to the more intense peak of g-B2O3, while the
double peaks belonging to melamine decrease. Therefore, it was
conrmed that a g-B2O3 phase evolved in M$2B200–400, which
may be related to the self-healing of the ber. Fig. 3c displays
the photos of H3BO3 and its heated products. Metaboric acid
formed at 150 ꢁC was a white powder just like boric acid, but g-
B2O3 obtained at 200–400 ꢁC turned into a glass-like solid,
which revealed that it was melted. The result of the melting
point measurement of H3BO3 also proved that a viscous phase
evolved at 206.2–207.8 ꢁC. Allowing for the same boron–oxygen
network in M$2B200–400 as in pure B2O3, we have reason to
believe that the viscous B2O3 formed in situ played a key role in
healing the ber's cracks. Besides, melamine and condensed
melem displayed a powder state (Fig. S3†), indicating that the
solid state remained during heating. This melamine/melem
solid could prevent the excessive ow of viscous B2O3 and hel-
ped avoid the fusion of bers. In the nal stage, the self-healed
M$2B200–400 ber was composed of B2O3 glass and melamine/
melem.
Fig.
4
Schematic illustration of microstructural evolution corre-
sponding to the morphological transformation: cracking and self-
healing.
Moreover, to determine whether the B–O oligomer was
uniformly distributed in M$2B150 and B2O3 acted as a matrix in
M$2B200–400, the bers underwent corrosion testing by EG
(ethylene glycol) and EA (ethanol) considering that B2O3 could
be rapidly dissolved in EG while remaining stable in EA for
a long period. The testing went on merely for 1 minute for
avoiding complete dissolution, and the corroded bers were
separately designated as M$2BT-EG and M$2BT-EA (T ¼ 150, 200,
schematic illustration in Fig. 4 shows structure evolution cor-
responding to the staged morphology. Initially, M$2B with
intact brous morphology possessed
a hydrogen-bonded
lattice, in which C3N6H6 and H3BO3 were arranged uniformly
and orderly. Aer heating at 150 ꢁC, H3BO3 was dehydrated and
formed a B–O oligomer, while the C3N6H6 molecules aggregated
and assembled into tiny crystals. This resulted in the breakage
of the ordered hydrogen-bonded structure and the intact ber
decomposed into short-rod bundles. However, the prole of the
ber was maintained integrated owing to the existence of the
B–O oligomer at the interface of the rods. As a result, the
cracked structure was derived from the mismatched gaps of the
bundles. For M$2B200, the extended B2O3 network was formed
as a result of further dehydration, while the C3N6H6 domain
shrank and was isolated by the continuous B2O3 matrix. Thus,
the cracked structure of M$2B150 was healed by the in situ
viscous matrix. Our results provide a self-healing strategy for
inorganic materials, which is different from that for organic
polymers involving reversible bonding.28,29
To explore the effect of the “self-healing” process on the
morphology of the nal BN ber, we treated the M$2B bers
under different pyrolysis conditions with a stage-variable
atmosphere. As shown in Fig. 5, there are three sets of
contrasts. In Fig. 5a and b, the BN ber obtained aer treatment
under an NH3 atmosphere is defective, while that obtained
under N2 is smooth in spite of undergoing the same heating
process at 1000 ꢁC for 1 hour. The difference in the ber
morphologies could be attributed to “self-healing” between 200
and 400 ꢁC. As is well-known, the complex reaction between
B2O3 and NH3 occurs at a temperature close to 200 ꢁC, forming
a non-melting (B2O3)n$NH3 compound,30 which prevents the
cracked ber from self-healing, while the molten B2O3 phase
formed under a nitrogen atmosphere assists in healing the
cracks.
ꢁ
400 C).
As shown in Fig. S5e and f,† all the M$2BT-EG bers collapse
into pieces due to the severe corrosion in EG. These results
disclosed that the B–O oligomer or B2O3 was uniform
throughout the whole ber. Aer 6 hour corrosion, these bers
were completely dissolved, while the M$2B550 ber remained
undissolved due to the predominant BN phase (Fig. S4a†). In
contrast, the samples in EA were partially dissolved or undis-
solved (Fig. S4b†). Aer being corroded for 1 minute in EA,
M$2BT-EA maintained the ber's prole (Fig. S5b and S5c†)
despite slight etching on the surface of M$2B150-EA, which was
composed of parallel short rods (Fig. S5a†) resulting from the
decomposition of the intact M$2B ber. The partial dissolution
of the B–O oligomer at the interface between the rods disclosed
this unique morphology.
The cracks in M$2B150 might be the gaps between the mis-
matched rod bundles. As for M$2B200-EA, the ultrane bers
generated on the basis of the original M$2B200 ber (Fig. S5b†)
might be attributed to the assembly of C3N6H6 and H3BO3
(formed via the moisture absorption of supercial B2O3) in an
ethanol solution. M$2B400-EA maintained the morphology for
the initially smooth surface and the inactive melem. In
summary, the different corrosion behaviors of the M$2BT bers
in EA compared with that in EG conrmed that the B–O olig-
omer or B2O3 was the dominant phase in affecting the ber's
morphology. In other words, B2O3 acted as the matrix in the
M$2B200–400 ber in which melamine/melem was enclosed.
On the basis of the above-mentioned discussion, the struc-
tural conversion corresponding to “cracking and self-healing”
morphology transformation was discussed in detail. The
The same comparisons were performed at temperatures
lower than 1000 ꢁC. As shown in Fig. 5c and d, the M$2B green
ber is separately heated under an NH3 atmosphere and
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 51773226, 61701514), the Natural
Science Foundation of Hunan Province (2018JJ3603), Research
Project of National University of Defense Technology (ZK17-XX-
02) and Defense Industrial Technology Development Program
(JCKY2016).
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ꢁ
vacuum at 700 C. As shown in Fig. 5e and f, the M$2B green
ber is separately treated at 400 ꢁC under the different
combination of NH3/N2 atmosphere. Both these comparisons
indicated the same results as those obtained from Fig. 5a and
b; the bers pyrolyzed under NH3 were defective, while the
bers pyrolyzed under N2 or vacuum were smooth. In
summary, the signicance ꢁof self-healing assisted by the
molten B2O3 formed at 200 C is crucial to obtain defect-free
BN bers. The result is of great signicance to provide guid-
ance to fabricate dense BN bers when applying an active
atmosphere such as NH3 for better nitridation and complete
removal of carbon.
¨
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