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

Abstract

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.

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Introduction

Due to the excellent comprehensive performances of thermal stability, chemical corrosion tolerance, oxidation resistance, neutron absorption, high thermal conductivity, electrical insulation and low dielectric constant,^1–4 hexagonal boron nitride (BN) holds great promise in various fields such as aerospace, high-temperature filtration, nuclear industry, and microelectronics. As typical BN materials, BN fibers 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 fibers, which are mostly based on the spinning process that requires strict manipulations.^7–11 In addition, the pyrolysis of a C₃N₆H₆·2H₃BO₃ (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-propylamino)borazine].¹⁷ This implies that substantial release occurs during thermal conversion from M·2B to the final BN fibers. Moreover, M·2B is a supra-molecule constructed by low-strength hydrogen bonding^19,20 that might fail to self-support at a high temperature, resulting in structural collapse. These two disadvantages may cause serious defects in the final BN fibers. The fact, astonishingly, is that the M·2B-derived BN fibers display intact morphology basically inherited from initial M·2B.^13,21 It still remains unclear why the seemingly contradictory phenomenon occurs.

In this work, we observed unique self-healing behavior after morphological cracks were caused by huge weight loss. This self-healing behavior kept the fiber 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 fiber. Moreover, the boron oxide network could enclose melamine molecules and its condensate melem to prevent them from evaporation, which was crucial to keep the fibers intact as pyrolysis and nitridation proceeded in the subsequent process.

Experimental section

Synthesis of C₃N₆H₆·2H₃BO₃

A mixture of C₃N₆H₆ and H₃BO₃ (analytical reagents, Sinopharm 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 C₃N₆H₆·(H₃BO₃) was set as 0.08 mol L⁻¹ (0.16 mol L⁻¹) to ensure complete dissolution. Then, the hot aqueous solution was naturally cooled down to precipitate the C₃N₆H₆·2H₃BO₃ (M·2B) flocculent. Subsequently, the white flocculent 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 fibers.

Thermal treatment of C₃N₆H₆·2H₃BO₃

The thermal behavior of M·2B was analyzed using thermogravimetry and differential scanning calorimetry (TG-DSC, NETZSCH STA449F5, Germany); the heating rate was set as 10 °C min⁻¹. Based on the results, the M·2B fibers were separately 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 conducted in a quartz tubular furnace under a protective N₂ flow. The obtained samples were designated as M·2B₁₂₅, M·2B₁₅₀, 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 final BN fibers, the M·2B fibers were also heated under a stage-variable N₂ or NH₃ atmosphere. For example, the M·2B fiber was heated to 700 °C for 1 hour under NH₃ and subsequently heated to 1000 °C under switched N₂.

Characterization and corrosion testing

The morphologies of the samples were observed by field-emission scanning electron microscopy (SEM, Hitachi S-4800, Japan), and the constituents were characterized through Fourier transform infrared spectroscopy (FT-IR, Frontier, PerkinElmer, 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·2B_T fiber (T = 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 filtration apparatus. After that, the treated samples were dried over-night in an oven at 80 °C for observing morphology. Furthermore, the M·2B_T fibers (0.02 g) were dipped in 4 ml solvent (EA and EG) for 6 hours to estimate their solubility, which reflected the corrosion resistance of the fibers.

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 first 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 slowly decreases from 500 to 1000 °C, and a broad weak endothermic peak is located at 535 °C. The insets show the morphology of the M·2B fiber and its thermal products after 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 fiber's morphology occurred at the first stage; the fiber cracked when heated at 150 °C and self-healed at 200 °C. After this, the fiber 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 final stage, the M·2B₁₀₀₀ fiber displayed compact morphology. Thus, self-healing was proven to be the key to maintain the fiber's morphology in spite of the ultralow yield (∼20 wt%) of M·2B.

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 transformation 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

The fiber'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 transformation 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 first-stage weight loss of M·2B. Meanwhile, C₃N₆H₆ remained stable at 150–200 °C, as implied by the flat TG curve (Fig. 1c) and the XRD patterns of the heated products (Fig. S1b†).

The weight loss of boric acid could be divided into three stages according to three sharp endothermic peaks, separately corresponding to the different stages of the dehydration of H₃BO₃, as shown in Fig. 1d. These reaction equations were proved by the XRD patterns of the heating products (Fig. S1a†). Thus, the first-stage weight loss of M·2B and the corresponding morphology evolution were basically attributed to the dehydration of H₃BO₃ in M·2B. At the subsequent stage from 200 to 400 °C, there was no obvious weight loss against the sublimation and condensation of melamine (Fig. 1c and e), which would be discussed later. This stable stage ensured that the morphology of the M·2B₄₀₀ fiber became more dense and smooth rather than broken.

Based on the analysis of the thermal transformation of the pure phases of C₃N₆H₆ and H₃BO₃, the structural evolution of M·2B involving morphological cracking–healing was further studied using FT-IR spectroscopy and XRD. As shown in Fig. 2a, plenty of sharp peaks disappear after the conversion from M·2B to M·2B₁₅₀, especially in the fingerprint region (marked by the light green rectangle areas) of the hydrogen-bonded structure. The FTIR peaks at 1186 cm⁻¹ and 786 cm⁻¹ that respectively represents bending and twisting vibrations of B–OH, became feeble as to M·2B₁₅₀. This indicated the drastic removal of hydroxyl groups, which was in accordance with the dehydration of H₃BO₃. Meanwhile, the highly crystallized M·2B was converted into an unordered phase of M·2B₁₅₀ characterized as double broad peaks (Fig. 2b).

Fig. 2 (a) FT-IR spectra and (b) XRD patterns of the M·2B supramolecule 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 cm⁻¹; ν(C–N): C–N stretching, 1552 cm⁻¹ and 1525 cm⁻¹; ν(B–O): 1450 cm⁻¹ to B–O asymmetric stretching in BO₃ and 1380 cm⁻¹ to B–O stretching in B₃O₃ ring; ν(B–N): B–N stretching at 1400 cm⁻¹; d(–OH): –OH bending, 1186 cm⁻¹; ν(BO₄): B–O stretching in the BO₄ unit, 1050 cm⁻¹; ν(BO₃): B–O symmetric stretching in the BO₃ unit, 873 cm⁻¹; d(triazine ring): triazine ring bending at 808 cm⁻¹; d(B–N–B): out-of-plane bending of B–N–B at 802 cm⁻¹; t(–OH): out-of-plane –O–H twisting at 786 cm⁻¹.^23–25

M·2B₁₂₅ displays an intermediate structure, revealing the transformation process from M·2B to unordered M·2B₁₅₀, which was characterized as weaker peaks located mainly in the green fingerprint region in the IR spectrum and the co-existence of sharp and broad peaks in the XRD pattern. Therefore, the drastic removal of hydroxyl groups due to the dehydration of H₃BO₃ resulted in the breakage of the M·2B crystal and thus structural disordering. Morphology cracking could be attributed to the drastic structure transformation and also the huge release of the removed H₂O. As for the micro-structure of the cracked fiber, it should be noted that the sharp XRD peaks of M·2B₁₂₅ correspond well with the (−110), (012), (111) and (102) crystal planes of melamine (ICSD PDF#39-1950) but not M·2B (Fig. 3a). This means that there occurred the assembly of melamine molecules to form crystals, while disordering occurred in the initial M·2B crystal, allowing for the fact that C₃N₆H₆ was molecularly isolated by H₃BO₃ in M·2B. Thus, the double broad peaks of M·2B₁₅₀ were proved to result from the widening of the (−110) and (102) diffraction peaks and indicated the existence of melamine micro-crystals in the M·2B₁₅₀ fiber.

Fig. 3 (a) Analysis of the XRD pattern of M·2B₁₂₅ in comparison with that of melamine. (b) Peak fitting of the XRD patterns of M·2B₁₅₀, M·2B₂₀₀, and M·2B₄₀₀ using B₂O₃ as a reference. (c) Photographs of the original H₃BO₃ and heat treatment products: HBO₂ after 150 °C heating, B₂O₃ after 200–400 °C heating, the insets are the corresponding molecular structures.

From the cracked M·2B₁₅₀ to the healed M·2B₂₀₀, there also occurred intense structural transformation. In addition to the IR peaks belonging to –NH₂, C–N, BO₃, triazine ring, and B₃O₃, the evolved peak of BO₄ at 1050 cm⁻¹ indicates the formation of a B₂O₃ network.^23,26 Two undefined peaks at 1240 cm⁻¹ and 1302 cm⁻¹ existing for M·2B_200–400 may be related to the C–O–B structure formed between melamine and boron oxide,²⁷ which can keep melamine non-volatile in the range of 300–400 °C (Fig. 1a and c). The ¹³C NMR spectra of monophasic melamine, M·2B, M·2B₁₅₀ and M·2B₂₀₀ (Fig. S2†) also proved the consistent structural evolution, which especially revealed a slight difference between M·2B₁₅₀ and M·2B₂₀₀. The change may be caused by the newly formed carbonaceous structure.

Besides, the double broad XRD peaks of M·2B₁₅₀ merged into an envelope peak of M·2B_200–400. Interestingly, a broad peak belonging to g-B₂O₃ was located at 23° (Fig. S1a†) between the double broad peaks located at 16.3° and 26.2°. To study this in detail, we fitted the XRD peaks of M·2B₁₅₀, M·2B₂₀₀, and M·2B₄₀₀ using that of g-B₂O₃ as a reference. As shown in Fig. 3b, the envelope peak for the healed M·2B₂₀₀ fiber can be well fitted as the double peaks of M·2B₁₅₀ and a broad peak of g-B₂O₃, and the envelope peak of smoothed M·2B₄₀₀ becomes more integrated owing to the more intense peak of g-B₂O₃, while the double peaks belonging to melamine decrease. Therefore, it was confirmed that a g-B₂O₃ phase evolved in M·2B_200–400, which may be related to the self-healing of the fiber. 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·2B_200–400 as in pure B₂O₃, we have reason to believe that the viscous B₂O₃ formed in situ played a key role in healing the fiber'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 flow of viscous B₂O₃ and helped avoid the fusion of fibers. In the final stage, the self-healed M·2B_200–400 fiber was composed of B₂O₃ glass and melamine/melem.

Moreover, to determine whether the B–O oligomer was uniformly distributed in M·2B₁₅₀ and B₂O₃ acted as a matrix in M·2B_200–400, the fibers 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 fibers were separately designated as M·2B_T-EG and M·2B_T-EA (T = 150, 200, 400 °C).

As shown in Fig. S5e and f,† all the M·2B_T-EG fibers 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 fiber. After 6 hour corrosion, these fibers were completely dissolved, while the M·2B₅₅₀ fiber remained undissolved due to the predominant BN phase (Fig. S4a†). In contrast, the samples in EA were partially dissolved or undissolved (Fig. S4b†). After being corroded for 1 minute in EA, M·2B_T-EA maintained the fiber's profile (Fig. S5b and S5c†) despite slight etching on the surface of M·2B_150-EA, which was composed of parallel short rods (Fig. S5a†) resulting from the decomposition of the intact M·2B fiber. 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 mismatched rod bundles. As for M·2B_200-EA, the ultrafine fibers generated on the basis of the original M·2B₂₀₀ fiber (Fig. S5b†) might be attributed to the assembly of C₃N₆H₆ and H₃BO₃ (formed via the moisture absorption of superficial B₂O₃) in an ethanol solution. M·2B_400-EA maintained the morphology for the initially smooth surface and the inactive melem. In summary, the different corrosion behaviors of the M·2B_T fibers in EA compared with that in EG confirmed that the B–O oligomer or B₂O₃ was the dominant phase in affecting the fiber's morphology. In other words, B₂O₃ acted as the matrix in the M·2B_200–400 fiber in which melamine/melem was enclosed.

On the basis of the above-mentioned discussion, the structural conversion corresponding to “cracking and self-healing” morphology transformation was discussed in detail. The schematic illustration in Fig. 4 shows structure evolution corresponding to the staged morphology. Initially, M·2B with intact fibrous morphology possessed a hydrogen-bonded lattice, in which C₃N₆H₆ and H₃BO₃ were arranged uniformly and orderly. After 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 fiber decomposed into short-rod bundles. However, the profile of the fiber 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

Fig. 4 Schematic illustration of microstructural evolution corresponding to the morphological transformation: cracking and self-healing.

To explore the effect of the “self-healing” process on the morphology of the final BN fiber, we treated the M·2B fibers 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 fiber obtained after 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 fiber 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 fiber from self-healing, while the molten B₂O₃ phase formed under a nitrogen atmosphere assists in healing the cracks.

Fig. 5 (a) BN fibers obtained after heating at 1000 °C for 1 hour under NH₃ atmosphere; (b) heating at 1000 °C for 1 hour under N₂ atmosphere; (c) heating at 700 °C for 1 hour under NH₃ and subsequent heating at 1000 °C under N₂; (d) heating at 700 °C for 1 hour in vacuum and subsequent heating at 1000 °C under N₂; (e) heating at 400 °C for 1 hour under NH₃ and heating to 1000 °C under N₂; (f) heating at 400 °C for 1 hour under N₂ and subsequently at 700 °C for 1 hour under NH₃, and finally to 1000 °C under N₂.

The same comparisons were performed at temperatures lower than 1000 °C. As shown in Fig. 5c and d, the M·2B green fiber is separately heated under an NH₃ atmosphere and vacuum at 700 °C. As shown in Fig. 5e and f, the M·2B green fiber 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 fibers pyrolyzed under NH₃ were defective, while the fibers pyrolyzed under N₂ or vacuum were smooth. In summary, the significance of self-healing assisted by the molten B₂O₃ formed at 200 °C is crucial to obtain defect-free BN fibers. The result is of great significance to provide guidance to fabricate dense BN fibers when applying an active atmosphere such as NH₃ for better nitridation and complete removal of carbon.

Conclusions

In summary, the dehydration of H₃BO₃ and evolution of B₂O₃ were figured out to be responsible for the unique “cracking and self-healing” morphological transformation. The resulting fiber was composed of a B₂O₃ matrix, in which melamine/melem was enclosed. This compound structure ensured stable morphology at the subsequent pyrolysis and nitridation stages and laid the foundation for the BN fiber. The self-healing mechanism assisted by the in situ molten phase can be adopted to guide the preparation of high-performance M·2B-derived BN fibers and can also be significant for other precursor-derived ceramic materials, especially those with low yields.

Conflicts of interest

There are no conflicts 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).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ra10292b

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