Ceramics for Sustainable Energy Technologies with a Focus on Polymer-Derived Ceramics - Novel Combustion Concepts for Sustainable Energy Development - page 503

oxidation resistance up to 1,400 ° C. Ube Industries also launched the third-generation

Tyranno SA3 SiC fibers are obtained from polyaluminocarbosilane

by curing in air and further by pyrolysis up to 1,300

ber. These

° C under inert gas. The ceramic

Si fibers with excess of carbon and oxygen content of about 12 wt% are

converted into sintered SiC by decomposition of oxide phases and outgassing of CO
at temperatures from 1,500 to 1,700

Al C O

- - -

° C and sintering at temperatures >1,700 ° C.

Tyranno SA3 fibers show considerable improvement in creep resistance compared to

the earlier generations Tyranno S, LOX-M, and ZMI.
The company ATK COI Ceramics, USA, developed third-generation SiC

bers
based on the approach adopted by Ube Industries to make the Tyranno LOX-M.
Sylramic and Sylramic-iBN fibers are produced from PTC polymer bers. PTC

fibers are cured by oxidation and doped with boron. The cured doped bers are

pyrolyzed at around 1,400 C to eliminate the excess of carbon and oxygen as

volatile species and sintered at higher temperatures to form a near stoichiometric
SiC

°

fibers are considered
to have reduced creep and increased oxidation resistance in comparison with the
other commercial SiC

fiber with smaller grains of TiB 2 and B 4 C. Sylramic-iBN

bers. These fibers are the result of a further thermal treat-

ment of Sylramic fibers in a nitrogen-containing gas to allow the diffusion of boron

from the bulk to the fiber surface, where it reacts with nitrogen to form an in situ

BN coating needed for fiber-reinforced CMCs.

2.1.3 Polymer-Derived SiCN Fibers

In 1972, Verbeek synthesized a meltable carbosilazane resin for the processing of
ceramic

fibers in the SiCN systems. Since then, many studies concerning pol-
yorganosilazanes (SiCN(O)-based polymers) and metal-modi ed polyorganosila-

zanes (e.g., SiBCN-based polymers) were published and attempts done to
commercialize ceramic SiCN fibers. Polyorganosilazanes have higher thermal sta-

bility in comparison with polycarbosilanes due to the formation of a more stable
amorphous SiCNO phase. In this amorphous phase, the nitrogen delays the SiC
crystal nucleation, shifting the decomposition of ceramic SiCNO

bers — in com-

parison with oxygen-rich SiC to higher temperatures.

At the institute of Ceramic Materials Engineering, University of Bayreuth,
Germany, a method was developed to synthesize a solid and meltable polyorga-
nosilazane, named ABSE, by ammonolysis of 1,2-bis(dichloromethylsilyl)ethane
on a pilot plant scale. This polymer has a thermal stability up to 220

bers —

C and good
moisture resistance. Only at higher temperatures, the material starts to cross-link,
which takes place via condensation releasing ammonia, and changes the polymer to
an unmeltable resin (Motz et al. 2001 ; Hacker et al. 2001a ). In cooperation with the
Institute of Textile Chemistry and Chemical Fibers (ITCF) (Denkendorf, Germany),
further improvements concerning the rheological properties of ABSE were suc-
cessful to achieve a polymer melt with viscoelastic properties necessary for a stable
melt spinning process of

°

fl flexible green fibers. In contrast to Nicalon ber types,

very low doses of 300 kGy are necessary to cure ABSE-derived green fibers with

*