Item

Abstract

Inorganic random networks composed of silicon, boron, nitrogen,
and carbon constitute a novel class of ceramics with
outstanding durability at elevated temperatures. The only
approach known to their synthesis is the pyrolysis of
appropriate pre-ceramic polymers, which on their part are
either accessible via co-condensation of different molecular
species, or by polycondensation of single source precursors
exhibiting structural features that are desired to be present
in the final ceramics. In this report the different synthetic
routes known to date have been compiled and critically
assessed. Although the reliable knowledge of this rather young
class of materials is still limited, their fundamental features
with respect to structures and performance have already began
to shine through. The range of their properties - mainly
depending on the synthetic route, but to some extent also on
the composition - is quite considerable. The upper limits for
thermal loads with respect to the onset of weight loss
(decomposition) in an inert atmosphere, to the onset of
crystallization and phase separation, and to the resistance to
oxidation in air, seem to be similar to2000 degreesC, similar
to1900 degreesC, and similar to1500 degreesC, respectively.
Some representatives show unique combinations of properties,
relevant for an application at high temperatures, which clearly
surpass those of SiC or Si3N4. For instance, fibers made of
SiBN3C, derived from the single source precursor CL3Si(NH)BCl2,
keeps its high mechanical durability, as measured by creep
resistance and tensile strength, up to temperatures of similar
to1400 degreesC. This is related to the fact that these
amorphous fibers do not show any grain boundaries and are
virtually free of macroscopic pores. SiC, in contrast, is
crystalline and suffers from grain boundary sliding and
subcritical crack propagation at mechanical loading, at
elevated temperatures. By now the feasibility of scaling up the
synthesis of SiBN3C to technical dimensions has been
demonstrated by processing 150 kg-batches (of polymer) which
have been spun and pyrolyzed to ceramic long fibers.