Selective cross-linking of oligosilazanes to tailored meltable polysilazanes for the processing of ceramic SiCN fibres

Abstract

An interesting alternative for the processing of non-oxide ceramic fibres at lower costs than that for the current commercially available fibre types was developed by modifying different commercially available liquid oligosilazanes (ML33 and HTT1800) into polysilazanes by selective cross-linking via the N-H and Si-H groups with tetra-n-butylammoniumfluoride (TBAF) as a catalyst. Termination of the reaction with calcium borohydride allows the processing of meltable solid polysilazanes (ML33S and HTTS) with tailored chemical and thermal properties, to fulfil the requirements for the melt spinning of mechanically stable and homogeneous polymeric fibres. The chemical and thermal stability of the polysilazanes ML33S and HTTS were investigated by using GPC, DSC and rheological measurements.These techniques indicate the dependency of the molecular weight and glass temperature on the catalytical cross-linking conditions. Polymers with up to $10 000 g mol À1 show glass-liquid transition (T g ) between 65 and 81 C and viscoelasticity, which are essential properties for the melt-spinning process. The thermal stability of ML33S is ensured up to 220 C. In contrast the thermal stability of HTTS is limited to 170 C due to the presence of vinyl-groups. The viscoelastic behaviour of the polymer melts, measured by oscillatory rheometry, and the sufficient thermal stability allowed the continuous processing of stable green fibres by melt spinning in the temperature range of 110 to 130 C. After fast electron beam irradiation curing of the green fibres and pyrolysis of continuous amorphous ceramic SiCN fibres from both ML33S and HTTS polysilazanes were successfully synthesized, while a defined T g point influences the shape and the smoothness positively.

Processing and characterization of large diameter ceramic SiCN monofilaments from commercial oligosilazanes

“…Two different types of meltable polysilazanes with tailored properties were recently developed in our chair. 12,13 These polymers open new opportunities for cost effective processing of ceramic SiCN bers with larger diameters as demonstrated in this investigation.…”

“…Based on rheological measurements, this required a processing temperature of about 50 K higher than the glass temperatures of the polymers. 12,13 In addition to the viscosity and the spinneret hole diameter, the nal diameter of the green bers is controlled by the pull-off speed. Polymer monolaments having diameters between $35 and 150 mm were successfully processed by adjusting the temperature (and hence the viscosity of the melt) between 110 and 120 C and by varying the pull-off speed between 10 and 30 m min À1 .…”

“…Based on the molecular, thermal and rheological analysis of the polysilazanes manufactured by controlled cross-linking, 12,13 continuous green bers were processed in air, using the labscale melt spinning equipment. Aer the green bers were spun in air and le for 3 hours on the spool in contact with air, the oxygen content of the bers remains <0.5 wt%.…”

Ceramics for Sustainable Energy Technologies with a Focus on Polymer-Derived Ceramics

“…5 shows their chemical structures), into solid meltable polysilazanes by a selective cross-linking via dehydrocoupling of N-H and Si-H groups with tetra-n-butylammonium fluoride (TBAF) as a catalyst. The termination of the reaction with calcium borohydride as inhibitor allows the processing of meltable solid polysilazanes (ML33S and HTTS) with tailored chemical and thermal properties, to fulfill the requirements for the melt spinning of mechanically stable and homogeneous polymeric fibers (Flores et al 2013(Flores et al , 2014. These polymers have a defined glass transition temperature and viscoelastic melt behavior, which enables the continuous melt spinning of mechanically stable green fibers.…”

Polymer-derived ceramics route toward SiCN and SiBCN fibers: from chemistry of polycarbosilazanes to the design and characterization of ceramic fibers

“…Within this context, authors proceeded to a partial crosslinking of these two precursors to render them melt-spinnable. 142), 143) To do this, tetra-n-butylammoniumfluoride (TBAF) has been added dropwise to the solution containing the polymers dissolved in THF. TBAF acted as a catalyst to selectively crosslink the polymers via the NH and SiH groups while calcium borohydride played the role of an inhibitor of the reaction.…”