Curing green fibres infusible by electron beam irradiation for the preparation of SiBNC ceramic fibres
Introduction
Polymer-derived SiBNC ceramic has been developed as a new ultrahigh temperature material which can be alternatives to the binary system of SiC and Si3N4 due to the preservation of amorphous state at high temperatures. For the same reason, ceramic fibres made from this material do not exhibit grain boundaries and show the excellent high-temperature creep behaviour [1], [2], [3], [4], [5].
The polymer-derived route for the preparation of SiBNC fibres includes synthesis of precursor polymer, melt spinning, curing and ceramic conversion [6], [7], [8], [9]. Up to now, using polymer fibres to prepare SiBNC ceramic fibres, the curing methods are limited to thermal curing and chemical curing. Since various precursor polymers are rich in Si–H bonds and N–H bonds which have high reactivity, chemical curing with gaseous species is preferred. Generally, the curing atmosphere could be sorted in two types: (1) NH3 which results in Si–NH–Si cross-linking and (2) the classification of chlorides such as HSiCl3 and BCl3 which can easily crosslink with N–H bonds.
Using NH3 as a curing atmosphere was studied by S. Bernard et al. [7], [8], [10]. A boron-modified polyvinylsilazane precursor was successfully processed into green fibres. The shaped fibres were then cured in ammonia atmosphere for 15 h, which allowed to increase the ceramic yield of the polymer and avoid inter-fibre fusion during the further increase of the temperature. Chemical reactions such as transaminations play an important role on this cross-linking process by the introduction of thermal reactive N-H sites. This curing route is moderate but slow. By applying HSiCl3, curing the N-mehtylpolyborosilazane (PBS-Me) fibres was completed within a few seconds [2]. The sufficiently cured polymer fibres were then pyrolyzed by exposure to increasingly high temperature of 1500 °C and pure inorganic SiBN3C fibres are obtained with the yield of 55 wt%. The use of MeHSiCl2, H2SiCl2 and BCl3 as curing atmosphere was also investigated by Y Tang et al. [11], [12], [13]. However, the gaseous species above are all so active that the cross-linking reaction is difficult to control. In addition, NH3 and these chlorides are caustic atmospheres which lead to the corrosion and damage of the reactors and human bodies. Moreover, whether NH3 or above chlorides, the curing processes are gas-solid reactions which rely on the gaseous diffusion and therefore, the inhomogeneity of the cured fibres from the surface layer to the fibre core is inevitable. If curing insufficiently, for instance, the cured outer parts remained and conversed to ceramic after pyrolysis, while the uncured inner parts decomposed and the hollow pores would be formed [14]. Therefore, the homogeneity of the fibres is difficult to guarantee particularly in large scale.
Electron beam (EB), which has high energy and strong penetrating power, can make the irradiated material achieve internal irradiation treatment and has been widely used in organosilicon polymer irradiation curing [15], [16], [17], [18], [19], [20]. As is known, EB irradiation has been a valuable method to render green fibres infusible for non-oxidation ceramic fibres such as SiC, Si3N4 and carbon fibres [21], [22], which derive from PCS and PAN precursor fibres, respectively. Attributing the sites of Si–H, C–H and Si–C in PCS fibres or C≡N in PAN fibres as a function of EB irradiation, radical polymerization reactions rapidly take place with the cross-linking of Si–C and Si–CH2–Si or the transformation of C≡N to C˭N.
As non-oxidation ceramic, to the best of our knowledge, there has been no detailed study reported on the SiBNC ceramic fibres prepared from precursor fibres by EB irradiation curing. In this work, polyborosilazane precursor for SiBNC fibres which contained plenty of Si–H, C–H and N–H active sites was obtained and had been shaped into fibres as before [23]. EB irradiation was then applied to render the green fibres infusible for the preparation of SiBNC fibres.
Section snippets
General
All reactions were conducted under purified N2 using Schlenk techniques. Boron trichloride (BTC) was obtained from Guangming Special Gas Corporation and dissolved in n-hexane at a concentration of 5.0 M. Dichloromethylsilane (DCMS) and hexamethyldisilazane (HxMDZ) were purchased from Xinghuo Chemical Corporation and Guibao Chemical Corporation, and were each distilled before use. The as-prepared polyborosilazane (PBSZ) precursors were stored in an Ar glove box.
Synthesis of polymers
Synthesis of the PBSZ precursor was
Cross-linking process of the green fibres
One-pot synthesis of PBSZ precursors for SiBNC fibres was well-realised by reacting BTC and DCMS simultaneously with HxMDZ. As shown in Fig. 1, the PBSZ are constituted of BN3 (a) and -N-SiH(CH3)- (b) as polymer repeating units, as well as terminal -Si(CH3)3 groups (c). The random arrangement of the two repeating units forms the desired polymer precursor with a -Si-N-B- framework, accompanied with a few -Si-N-Si- and -B-N-B- units [23]. Green fibres were then obtained via melt spinning.
To
Conclusions
In this work, rendering the green fibres infusible by EB irradiation was applying to the preparation of SiBNC ceramic fibres. During EB irradiation process, the free radicals are formed at the C atoms and Si atoms on the -N-SiH(CH3)- main chain units and terminal -Si(CH3)3 groups. The radicals react with each other to produce cross-linking, coupling and grafting among PBSZ chains, which all contribute to improvement of the cross-linking density of green fibres. As a result, the cured fibres
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