Biomedical Engineering Reference
In-Depth Information
1.1
Introduction
Dramatic improvements in toughness, strength, creep strength and
thermoresistance of ceramic-matrix composites have been achieved by
incorporating either nanocrystalline oxide/non-oxide ceramic particles or
their hybrid combination in a microcrystalline matrix. The other group of
nanophase ceramic composites is nanocrystalline matrix composites, also
called nanoceramics, in which the matrix grain size is below 100 nm. The
nano-nano type microstructure can also be formed when the second phase is
also nanoscaled. The main objective of this chapter is to present the
performance behaviour of ceramic nanocomposites under conditions of
thermal shock, i.e. when they are subjected to sudden changes in
temperature during either heating or cooling or may be in flame propagating
zones. Such conditions are possible in the high-temperature applications for
which ceramic nanocomposite materials can be selected (Ohnabe et al.
1991). The importance of the use of thermal shock resistant ceramic
nanocomposite materials was reported by Baste in 1993 during steady-state
operation of a gas turbine (Baste 1993). While thermal shock is not a
concern during steady-state operation of a gas turbine, it becomes of great
importance during emergency shut-downs, when cool air drawn from the
still spinning compressor is driven through the hot sections and can result in
a temperature decrease of more than 800
C at the turbine inlet within one
second. An additional factor is that such a situation may arise about 100
times during the lifetime of a modern gas turbine engine.
The use of thermal shock resistant ceramic nanocomposite materials was
also illustrated by Jones et al. (2002) for fusion energy applications. In the
nuclear industries, SiC reinforced with SiC fibres has been proposed as a
structural material for the first wall and blanket in several conceptual design
studies of future fusion power reactors. In this case, apart from the
moderate shocks inflicted during start-up and shut-down of the system, the
plasma-facing material can suffer rapid heating due to plasma discharge.
When a body is subjected to a rapid temperature change such that a non-
linear temperature gradient appears, stresses arise due to the differential
expansion of each volume element at a different temperature. The
temperature at each point changes with time at a rate that depends on the
surface heat transfer coefficient (HTC) between the media at different
temperature and the body, the shape of the body and its thermal
conductivity. High HTCs, large dimensions and low thermal conductivities
result in large temperature gradients and, thus, large stresses. The
dimensionless parameter, the Biot modulus (Bi), can be used to describe
the heat transfer condition (Kastritseas et al. 2006)
8
Bi
¼
lh
ðÞ
=
k
½
1
:
1
