UNCORRECTED PROOF Journal of Solid State Chemistry ] (]]]]) ]]]–]]] Evolution of structural features and mechanical properties during the conversion of poly[(methylamino)borazine] fibers into boron nitride fibers Samuel Bernard, Khaled Ayadi, Marie-Paule Berthet, Fernand Chassagneux, David Cornu, Jean-Marie Letoffe, and Philippe Miele* Laboratoire des Multimate´riaux et Interfaces, UMR CNRS 5615, Universite´Claude Bernard-Lyon 1, 43, Bd du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received 15 October 2003; received in revised form 9 January 2004; accepted 14 January 2004 Abstract Poly[(methylamino)borazine] (PolyMAB) green fibers of a mean diameter of 15 mm have been pyrolyzed under ammonia up to 1000  C and heat treated under nitrogen up to 2000  C to prepare boron nitride (BN) fibers. During the polymer-to-ceramic conversion, the mechanical properties of the green fibers increase within the 25–400  C temperature range owing to the formation of a preceramic material and remain almost constant up to 1000  C. Both the crystallinity and the mechanical properties slightly increase within the 1000–1400  C range, in association with the consolidation of the fused-B 3 N 3 basal planes. A rapid increase in tensile strength (s R ) and elastic modulus (Young’s modulus E) is observed in relation with crystallization of the BN phase for fibers treated between 1400  C and 1800  C. At 2000  C, ‘‘meso-hexagonal’’ BN fibers of 7.5 mm in diameter are finally obtained, displaying values of s R ¼ 1:480 GPa and E ¼ 365 GPa. The obtention of both high mechanical properties and fine diameter for the as-prepared BN fibers is a consequence of the stretching of the green fibers on a spool which is used during their conversion into ceramic. r 2004 Published by Elsevier Inc. Keywords: Boron nitride; Ceramic fiber; Preceramic polymer; Polymer-to-ceramic conversion; Ceramization; Crystallization 1. Introduction Although the ability of specific polymers to be converted into covalently bonded ceramics (polymer- derived ceramics, PDCs) is known for a long time [1], much attention has been focused recently on the preceramic polymer route for the production of shape- controlled ceramics [2–6]. One significant advantage of this method is that ceramic fibers with fine diameters can be elaborated from tractable polymers after spinning and pyrolysis steps [4,7]. Thus, following the approaches of Winter et al. [8] and Yajima et al. [9] on fibers around Si-based systems (SiC and SiC/Si 3 N 4 fibers), consider- able interest has been dedicated to the preparation of melt-processible preceramic polymers and their conver- sion into various non-oxide ceramic fibers. These polymers are usually composed of an inorganic back- bone bearing organic groups whose nature determines to a certain extent the polymer processability [10,11]. These pendent groups are partly released during the pyrolysis process to yield an amorphous and covalent ceramic at moderate temperature. Depending on the application, a subsequent optional high temperature heat treatment allows this preceramic material to be transformed into a stable crystalline ceramic [12–15]. As illustrations of this method, one could refer to the preparation of carbon fibers and silicon carbide fibers from polyacrilonitrile [16] and polycarbosilane [17,18], respectively. Many boron and nitrogen containing polymers have been described in the literature [19–24]. They have been synthesized for various applications in several forms such as boron nitride (BN)-based bulk materials, matrices, thin films and coatings [25–30]. In the case of BN fibers, poly[(alkylamino)borazines] have proven to be the most attractive precursors [31–34]. Kimura et al. have prepared a condensat from a mixture of B- ARTICLE IN PRESS 1 3 5 7 9 11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49 51 53 55 57 59 61 63 65 67 69 71 73 75 77 3B2v7:51c GML4:3:1 YJSSC : 10311 Prod:Type:FTP pp:128ðcol:fig::NILÞ ED:S:Gayathri PAGN: ashok SCAN: Jane *Corresponding author. Fax: +33-4-72-44-06-18. E-mail addresses: samuel.bernard@univ-lyon1.fr (S. Bernard), philippe.miele@univ-lyon1.fr (P. Miele). 0022-4596/$ - see front matter r 2004 Published by Elsevier Inc. doi:10.1016/j.jssc.2004.01.004