| Polymer alloying is to mix different polymers to obtain the desired properties.It is widely used in industry because of its convenience,economy and adoptability.It is also one of the major interests in silicon carbide fibre production for the development of high-performance and low cost fibres.In this work,highly branched liquid polycarbosilane(LPCS)was used as the alloying polymer to modify the properties of polycarbosilane(PCS).The resultant blend is used as the precursor for silicon carbide ceramic fibres.The blend is melt-spun into precursor fibres,oxidation-cured in hot air,and finally converted into silicon carbide fibres by pyrolysis in nitrogen.The key scientific problems encountered in the processing are comprehensively studied in this thesis.LPCS has similar molecular structure to PCS.Therefore,they have good compatibility.A homogeneous blend can be obtained simply by mechanical mixing, making the expensive vacuum drying unnecessary.No obvious phase separation or chemical reaction is observed during the preparation.The blend is essentially a physical mixture of the two components.The addition of LPCS into PCS increases the Si-H concentration in the precursor. On the base of the FTIR spectra,the Si-H/Si/CH3 increases from 0.91 for PCS to 0.98 for 20%LPCS/PCS.From 1H-NMR,the Si-H/C-H increases from 0.096 for PCS to 0.14 for 20%LPCS/PCS.The high Si-H concentration is conducive to the subsequent oxidation curing.During melt-spinning,LPCS is partially reacted with PCS,consuming some Si-H. However,an overwhelming majority of them is preserved.In the precursor fibres, LPCS is stable during storage.LPCS reduces the spinning temperature significantly, from 285℃for PCS to 205℃for 20%LPCS/PCS.Because the LPCS prolongs the solidification of the extruded filament,the spinning ability of the precursor is markedly improved.The fibre diameter and the diameter distribution are also improved.For example,the fibre diameter decreases from 19.6±1.8μm for PCS to 15.4±0.5μm for 20%LPCS/PCS.The reduced melt spinning temperature is beneficial to the thermal stability of the precursor.The LPCS also improves the surface quality,reducing the surface defects.The major reactions occurred during oxidation-curing are the oxidation of Si-H bonds,forming Si-OH groups.Condensation reactions then takes place between the Si-OH groups,producing Si-O-Si linkage,which is responsible for curing.At higher curing temperature,Si-CH3 groups are also oxidized into Si-OH,further promoting the curing process.Water and formaldehyde are the major gaseous species evolved during oxidation curing.This is the first time to experimentally confirm the proposed oxidation mechanisms by Prof Yajima.LPCS promotes the oxidation curing because of its richness in Si-H groups. Therefore,the fibres can be rendered infusible at lower curing temperature.At 150℃,the gel fraction of the cured fibres from PCS is almost zero,while for 20% LPCS/PCS precursor fibres,it is 80%.When LPCS is over 10%in the precursor,the fibre shape can be retained after pyrolysis at 1250℃.But when it is＜5%,oxidation at＞150℃is needed to produce sufficient curing.LPCS addition improves the ceramic yield to 81%for 15% LPCS/PCS from 77.6%for PCS.For the same oxidation temperature,the LPCS-containing precursor gives the ceramic fibres with higher SiCxOy fraction,suppressing the growth ofβ-SiC crystallites.The introduction of LPCS into the precursor results in ceramic fibres with lower tensile strength(1.76GPa for 15%LPCS/PCS)in comparison with the PCS-derived ceramic fibres(2.81GPa).It stems from the non-uniform oxygen distribution because of the active nature of LPCS toward oxygen.However,the strength retention at 1400℃is almost 100%for the 15%LPCS/PCS-derived fibres, much higher than the PCS-derived ceramic fibres.The main reason is that there is an oxygen-rich surface layer in 15%LPCS/PCS-derived fibres,impeding the diffusion of oxygen into the interior of the fibre and hence slowing down further oxidation. |
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