Si-Ti-C-O fibers

T. Yamamura, T. Harashima, M. Shibuya, and Y. Iwai, Development of Continuous Si-Ti-C-O Fiber With High Mechanical Strength and Heat Resistance, 6th World Congress on High Tech. Ceramics (CIMTEC), Milan, Italy, 1986. 

SiC fibers were produced using polycarbosilanes by Yajima et al. in 1975 [1,2]. Besides SiC fibers, Si-Ti-C-O fibers prepared from a polytitanocarbosi-lane have been obtained by adding a titanium tetrabutoxide to polycarbosilane or polysilane [3]. SiC fibers (Nicalon) and Si-Ti-C-O fibers (Tyranno) are manufactured on an industrial scale. Colorless silicon oxynitride fibers and silicon nitride fibers [4] have been obtained by the nitridation of polycarbosilanes in the author s laboratory. Polymers used for ceramic precursor and the resulting ceramic fibers are listed in Table 1. 

UBE Chemicals synthesized amorphous Si-Ti-C-O fibers from the PCS-titanium alkoxide compound polymer. These so called Tyranno fibers show excellent properties and can be spun thinner than the Nicalon fibers (see Table 4). However heating above 1000°C results in a crystallization of the fiber. In the mid 1990s the German company Bayer AG synthesized an amorphous Si-B-N-C fiber, by pyrolysis of a polyborosilazane polymer [56]. This SiBN3C fiber (see Fig. 11) has a tensile strength of 3 GPa and maintains its amorphous character up to 1800°C. The advantage of the production route from liquid to solid to produce SiC has also attracted attention for... 

M. Nakamura, M. Mabuchi, N. Saito, Y. Yamada, M. Nakanishi, K. Shimojima and I. Shigematsu, Joining of a Si-Ti-C-O Fiber-Bonded Ceramic and an Fe-Cr-Ni Stainless Steel with a Ag-Cu-Ti Brazing Alloy,... 

Hydrothermal formation of carbon on SiC was first observed in the course of corrosion studies of amorphous Si-Ti-C-O (Tyranno) fibers performed at 300-800°C at about 100 MPa [47,48,53]. Increasing downshift of the carbon D band in the Raman spectra with increasing reaction temperature from 1355-1360cm in microcrystalline graphite to 1350cm , 1336 cm , and 1330 3cm after hydro-thermal treatments at 300°C, 500°C, and 600-800°C respectively indicated the possible formation of sp -bonded carbon [50]. 

It is to be expected that such phases may also control the later crystallization of SiC by heterogeneous nucleation. Such effects have been described in [150, 165, 166] concerning the crystallization of Si-C-(O) precursors containing metals like Ti, Zr, Al, i.e., precursor systems from which different types of Tyranno fibers are formed [167]. In the case of the Al-containing system it was observed that - under the given unusual conditions - SiC polytype 2H is stabilized by AI2OC which is isostructural to SiC 2H. 

Titanium can be introduced in PCS precursors as titanium tetrabutoxide, Ti(OC4H9)4, to yield Si-C-O(Ti) ceramic fibers such as Tyranno (Equation 6) (48). Fibers produced from such PCS(Ti) precursors have a slightly higher pyrolysis temperature (Tp) than that previously mentioned for their pure PCS counterparts [49]. As a result, they also retain their amorphous state and hence their tensile strength to a slightly higher temperature. 

Polytitanosiloxanes were prepared in one stage by simultaneous controlled cocondensation of Si(OEt)4 and Ti(OPr )2(acac)2. A ladder polymer containing Si-0-Si and Si-O-Ti imits is formed. Their ratio depends on the conditions. This ratio determines the time to gel formation and if fiber ceramics can form upon annealing of the material (500-900°C). The structures, growth mechanism and the properties of siloxane composites containing the silicon, titanimn and mixed titanium-silicon phases have been thoroughly reviewed. ... 

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