The purposes of this study were to determine as a function of temperature
the thermal diffusivity and/or thermal conductivity of SiC-Mullite and SiC-SiC,
and to explain the observed behavior in terms of changes in temperature, microstructure,
composition, and/or orientation.
Materials used in the SiC-Mullite study consisted of single crystal SiC
whiskers (prepared from rice hulls or by the vapor-liquid-solid process) dispersed
within a polycrystalline mullite matrix. During measurement of thermal
diffusivity, the samples were heated to l500°C and cooled back to room temperature.
No hysteresis occurred. However, both thermal diffusivity and
conductivity exhibited maximum values at room temperatures, perpendicular to
the hot pressing direction, at high volume percentages of SiC whiskers, and when
VLS whiskers were employed.
The SiC-SiC samples consisted of a crossweave of polycrystalline SiC übers
that were coated with phenolic resin and surrounded by a chemically-vapordeposited
matrix of SiC. The two types of samples examined were prepared with
different amounts of resin. The matrices of the high resin samples were found to
be dominated by the presence of char. Samples were cycled to 1000, 1400, and l800°C; hysteresis occurred on some of the cycles. Thermal diffusivity was
highest parallel to one set of fibers.
These results allow the qualitative tailoring of the heat flow properties of
these ceramic composites, for particular applications, and set forth limitations on
the use of the SiC-SiC composites at high temperatures.
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