A new class of boron nitride fibers with tunable properties by combining an electrospinning process and the polymer-derived ceramics route

Vincent Salles *, Samuel Bernard , Arnaud Brioude , David Cornu and Philippe Miele
Laboratoire des Multimatériaux et Interfaces, UMR 5615 CNRS – Université Lyon 1, Université de Lyon, 43 bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France. E-mail: vincent.salles@univ-lyon1.fr; Fax: +33 4 72 44 06 18; Tel: +33 4 72 43 16 08

Received 15th July 2009 , Accepted 16th October 2009

First published on 5th November 2009


Novel boron nitride (BN) fibers have been developed with diameters ranging from the nano- to microscale by thermal conversion of as-electrospun fibers from polyacrylonitrile and poly[B-(methylamino)borazine] blend solutions. Such a new class of ceramic fibers is seen as potential candidate for thermal management applications and filtration systems in harsh environments.


The preparation of sub-microscale and nanoscale fibers of controlled chemical composition is the subject of intense research due to their potential applications in a large variety of fields ranging from smart textile to biomedical, automotive and environmental sciences. In this context, a lot of effort has been focused on the electrospinning process due to its versatility, which permits the formation of polymeric fibers as well as oxide and non-oxide inorganic fibers at sub-micro- and nanoscales.1,2 This process, easily scalable to large amounts, has recently been employed to develop non-oxide ceramic nanowires. In this field, the synthesis of sub-microscale SiC,3,4 B4C,5 B4C/SiC6 and GaN7 fibers has been reported. Applications of ceramic nanofibers for air treatment has been recently described.8

Hexagonal boron nitride (h-BN) presents interesting properties such as high temperature stability, high dielectric breakdown strength, good thermal conductivity and low chemical reactivity. Such peculiar properties enhance the interest in obtaining fibers at the micro- and nanoscale to be used in composite materials and improve some of their properties.9,10 BN nanofibers have recently been produced by Qiu et al.11,12 In an initial paper,11 they used boron oxide as BN precursor and polyvinylbutyral (PVB) to enhance the spinnability of the boron-containing solution. As-formed green nanofibers were pyrolyzed to generate BN nanofibers with diameters as low as 100 nm. In a second paper,12 they coated electrospun polyacrylonitrile (PAN) fibers with boron oxide solutions to give BNnanofibers. Here, we propose an alternative process which avoids the use of oxygen-containing precursors which are known to produce boron nitride with a non-negligible amount of oxygen.13

Our group is developing chemical approaches for the fabrication of BN microscale fibers14 or micro- and nanotubes from vapour15 or liquid16 phases. Our latest research deals with the polymer-derived ceramics (PDCs) approach which is an efficient chemical route based on the use of pre-ceramic polymers and allows fine control of the chemical composition (especially for non-oxide materials) and the properties of the final ceramics.17–19

In the present letter, we report the synthesis of sub-microscale and nanoscale BN fibers by electrospinning of poly[B-(methylamino)borazine]/polyacrylonitrile (PAN) blend solutions followed by the pyrolysis of green fibers to achieve the polymer-to-ceramic conversion. Study of the polymer-to-ceramic conversion has been based on TGA and FT-IR experiments and the final material has been characterized by SEM/EDX and HR-TEM. We would like to point out that this new method combining the electrospinning process and PDCs route enables the production, in large amounts, of structure- and size-tunable BN fibers for industrial applications.

The poly[B-(methylamino)borazine] ([B3N5C3.3H13,2]n; empirical formula normalized to three B atoms) was synthesized with a quite low molecular weight (Mw = 900 ± 100 g mol−1), according to a procedure previously described by our group.20 We chose this polymer because it already contains the basal structural units of h-BN, i.e., hexagonal B–N rings, limiting the occurrence of complex structural rearrangements upon ceramic conversion. The choice of this precursor was also motivated by the possibility of preparing N,N-dimethylformamide (DMF) solutions in a large range of concentrations. However, we observed that solutions of pure poly[B-(methylamino)borazine] in DMF formed droplets, whatever the applied voltage and the electrode distance. We attributed this result to inappropriate solution viscosities.

As the increase of the precursor molecular weight implies its lower solubility, we suggested that PAN, a high-molecular-weight organic polymer which is soluble in DMF, could improve the viscoelastic properties of the solutions when mixed with the poly[B-(methylamino)borazine] to be electrospun. As a consequence, we have prepared solutions of a mixture of poly[B-(methylamino)borazine] with PAN (Mw = 150[hair space]000 g mol−1) in DMF. By varying the concentration of the blend in DMF, the poly[B-(methylamino)borazine]:PAN ratio and the electrospinning conditions (flow rate of the extruded solution, voltage and electrode distance), sub-microscale fibers of different diameters were obtained as shown in the SEM images (Hitachi S-800) reported in Fig. 1. In a typical experiment, a polymer solution was prepared by dissolving PAN powder into DMF, with a PAN content of 12 wt%, before adding the poly[B-(methylamino)borazine] in order to reach a 4[thin space (1/6-em)]:[thin space (1/6-em)]10 poly[B-(methylamino)borazine]:PAN weight ratio. It should be mentioned that electrospinning experiments were performed under an inert atmosphere (N2) as a consequence of the sensitivity to air and moisture of poly[B-(methylamino)borazine]. The solution was extruded with a liquid flow rate of 0.1 mL h−1 through a stainless-steel needle (gauge 21) placed at 6 cm from an aluminium target (13 cm diameter). A high-voltage power of 4.5 kV was applied to the device between the needle and the ground metallic target. The electrospun polymeric fibers were collected at the center of the aluminium target (Fig. 1a and 1b). The as-formed mat is composed by a network of polymeric fibers characterized by a smooth cylindrical surface, without beads, and a reduced diameter distribution (ø ≈ 500 nm).



          SEM images of electrospun fibers obtained from a poly[B-(methylamino)borazine]/PAN (4 : 10 wt ratio) blend solution (12 wt% of PAN): (a–b) as-electrospun fibers; TEM images of these fibers after c) pyrolysis under NH3 up to 1000 °C and (d) annealing under N2 at 1800 °C (scale bar = 5 nm, insets: SEM images of the corresponding fibers).
Fig. 1 SEM images of electrospun fibers obtained from a poly[B-(methylamino)borazine]/PAN (4[thin space (1/6-em)]:[thin space (1/6-em)]10 wt ratio) blend solution (12 wt% of PAN): (a–b) as-electrospun fibers; TEM images of these fibers after c) pyrolysis under NH3 up to 1000 °C and (d) annealing under N2 at 1800 °C (scale bar = 5 nm, insets: SEM images of the corresponding fibers).

In a second step, a pyrolysis was carried out at 1000 °C under ammonia, and then at 1800 °C under nitrogen to generate the desired boron nitride submicro- and nanofibers. The main shrinkage occurred during the first part under ammonia since the diameter of the fibers reduced homogeneously from 500 nm to 200 nm. This strong shrinkage was due to both the important decomposition of the poly[B-(methylamino)borazine] under ammonia up to 1000 °C through successive reactions including transamination which have been identified in our previously-published papers,21,22 and the complete decomposition of PAN in such an atmosphere. In particular, decomposition of the poly[B-(methylamino)borazine] involves both a weight loss and a density increase whereas decomposition of PAN induces complete carbon removal. Several studies have pointed out the gaseous evolution of carbon-based species due to the presence of ammonia, without any oxidizing atmosphere.21,23–26 Such predictions are reflected well by TGA experiments (Fig. 2, Setaram TGA 92 16.18) performed under ammonia at 1000 °C. The thermal behaviour of the poly[B-(methylamino)borazine] under ammonia is similar to that previously published.21 The polymer undergoes a continuous weight loss in two steps which are identified in the temperature ranges from 20 to 400 °C (Δm/m0 ∼ 35.0 wt%) and 400 to 1000 °C (Δm/m0 ∼ 18.0 wt%). TGA data reveal an overall weight loss of ∼55%, thus a ceramic yield of ca. 45% at 1000 °C. Decomposition of PAN under ammonia starts at 220 °C and is complete at 1000 °C. The weight loss (88 wt%) of the blend PAN + poly[B-(methylamino)borazine] is consistent with the theoretical value calculated taking into account the PAN and the poly[B-(methylamino)borazine] amounts, as well as the poly[B-(methylamino)borazine] ceramic yield. However, the FT-IR spectra presented in Fig. 3 (Nicolet 380 FT-IR spectrometer) revealed that the polymer-to-ceramic step is not completely achieved after thermal treatment at 1000 °C according to the presence of a band which is attributed to the C–N bonds (1095 cm−1, picked out by an arrow) in the as-prepared material after treatment at 1000 °C. A further treatment up to 1800 °C under nitrogen is required to generate BN free of carbon. This result is consistent with the white colour characterizing the final mat. In the FT-IR spectrum of the sample prepared at 1800 °C (Fig. 3), the strong and broad band centered at 1367 cm−1 and the sharp weak band at 806 cm−1 are representative of the specific types of B–N chemical bonds in the h-BN.27 After annealing at 1800 °C, the morphology of the fibers remained stable with a narrow size distribution (Insets in Fig. 1c and d). The average diameter size seems to slightly decrease between 1000 °C and 1800 °C, due to the ceramization and then the crystallization of the boron nitride material as evidenced by TEM (Topcon 002B) observations (Fig. 1c and d). The average crystallite size appears to increase with the annealing temperature and a preferential orientation of the (002) planes parallel to the nanofiber axis can be noticed. This result is similar to that already pointed out in a previous study dealing with BN nanotubes and demonstrating a high alignment of the planes after treatment at 1800 °C.16 In the present work, this orientation was lowered since the nanowires are composed of a core full of matter.



          TGA curve of the polyacrylonitrile (PAN), of the poly[B-(methylamino)borazine] and of a mixture poly[B-(methylamino)borazine]:PAN (corresponding to a weight ratio of 4 : 10).
Fig. 2 TGA curve of the polyacrylonitrile (PAN), of the poly[B-(methylamino)borazine] and of a mixture poly[B-(methylamino)borazine]:PAN (corresponding to a weight ratio of 4[thin space (1/6-em)]:[thin space (1/6-em)]10).


          ATR-FTIR
          spectra of electrospun fibers presented Fig. 1 (obtained from a poly[B-(methylamino)borazine]:PAN weight ratio of 4 : 10 with 12 wt% of PAN, and treated at 1000 °C and 1800 °C.
Fig. 3 ATR-FTIR spectra of electrospun fibers presented Fig. 1 (obtained from a poly[B-(methylamino)borazine]:PAN weight ratio of 4[thin space (1/6-em)]:[thin space (1/6-em)]10 with 12 wt% of PAN, and treated at 1000 °C and 1800 °C.

The main parameter controlling the fiber diameter is the poly[B-(methylamino)borazine]:PAN ratio, as illustrated by the SEM images of Fig. 1 and Fig. 4 presenting fibers prepared with different ratios (8[thin space (1/6-em)]:[thin space (1/6-em)]10, 4[thin space (1/6-em)]:[thin space (1/6-em)]10 and 1[thin space (1/6-em)]:[thin space (1/6-em)]10). The fiber section decreases with the poly[B-(methylamino)borazine]:PAN ratio from 400 nm to less than 100 nm leading to the formation of nanofibers. It should be mentioned that the concentration of PAN in the polymer solution was kept constant (12 wt%) in the three studied ratios. As a consequence, the behaviour was attributed to a decrease of the poly[B-(methylamino)borazine] amount in the solution, thereby causing a decrease in the ceramic yield of the polymer blend .



          SEM images of annealed fibers (1800 °C) prepared in the same experimental conditions as Fig. 1 except different poly[B-(methylamino)borazine]:PAN ratios: (a) (8 : 10) and (b) (1 : 10).); Inset: optical image of the as-prepared mat after treatment at 1800 °C.
Fig. 4 SEM images of annealed fibers (1800 °C) prepared in the same experimental conditions as Fig. 1 except different poly[B-(methylamino)borazine]:PAN ratios: (a) (8[thin space (1/6-em)]:[thin space (1/6-em)]10) and (b) (1[thin space (1/6-em)]:[thin space (1/6-em)]10).); Inset: optical image of the as-prepared mat after treatment at 1800 °C.

As-prepared samples are mats of submicro- or nanofiber network (inset of Fig. 4). It must be noticed that the variation of the dimensions observed at a macroscopic scale indicated a homogeneous 2D shrinkage of the matter during the polymer-to-ceramic conversion. Therefore, this phenomenon has to be anticipated for preparing near-net shape preforms.

To our knowledge, the as-prepared boron nitride sub-micro- and nanofibers, with a high purity, reported in the present paper are the first obtained using a non-oxide electrospinning route. In the working conditions presented in this communication, the production lab scale varies from 30 mg to 80 mg of technical polymer fibers per hour (with one needle). We think that these results are promising for the large-scale production of ultrafine BN fibers with tunable properties, if we take into account that electrospinning is a low-cost process and that it is possible to multiply the fiber sources (number of needles). This synthesis route allows the diameter of the final morphology of the fibers to be controlled. As an illustration, a large range of diameters can be achieved, from 400 nm to less than 100 nm in the studied conditions, with a high homogeneity which is necessary to obtain for some technical materials. The as-prepared mats can be used for numerous applications. The first one could be the elaboration of fiber-reinforced nanocomposites to enhance the mechanical properties of refractories without any deleterious effect on its thermal stability. To obtain a high tensile strength, we have to improve the BN plane alignment with the nanofiber axis. Beside these thermal and thermo-mechanical applications, such fibers could be exploited as fillers to confer enhanced and even original properties to polymer matrices for thermal management applications.

Acknowledgements

We gratefully acknowledge the CTμ (Centre Technologique des Microstructures) of the Université Lyon 1 for access to the SEM apparatus.

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Footnote

Electronic supplementary information (ESI) available: Experimental details and EDX results. See DOI: 10.1039/b9nr00185a

This journal is © The Royal Society of Chemistry 2010