Biomedical Engineering Reference
In-Depth Information
Thermal shock, cycling, and ageing behavior
Glasses and glass-ceramics are sensitive to sudden temperature variations
because they generally have poor thermal conductivity and low tensile
strength (Arnold et al., 1996). The brittle nature and internal stresses
produced due to thermal gradients usually result in the catastrophic failure
of glasses and glass-ceramics. CNTs possess high strength and thermal
conductivity and their incorporation in glasses/glass-ceramics can produce
thermally conductive composites with enhanced resistance to thermal shock.
Thermal shock resistance is experimentally found by subjecting a material
to a sudden change (e.g. quenching) in temperature from an initial
temperature (T
i
) to final temperature (T
f
) and measuring the change in
fracture strength. T
f
is generally kept constant (room temperature) while T
i
is gradually increased until the critical temperature (T
c
) is achieved, which is
the temperature at which a sudden drop in fracture strength is noticed due
to the formation of cracks in the specimen. The difference between two
temperatures (
T
f
) is the safe range wherein a specimen remains
unaffected by a thermal shock. The resistance to thermal shock is
characterized by a thermal shock resistance parameter (R) (Boccaccini
et al., 1999), which is related to the fracture strength (
Δ
T=T
c
σ
ν
), Poisson's ratio (
),
thermal expansion coefficient (
α
) and modulus of elasticity (E) of a material:
¼
sð
1
nÞ
R
½
7
:
10
a
E
A high value of R shows better resistance of a material to thermal shock.
Other factors, including specific heat capacity, fracture toughness, disper-
sion quality of reinforcing phase and the geometry of the composite, may
also affect thermal shock resistance.
No reports are available describing thermal shock, cycling and ageing
behaviors of CNT-glass/glass-ceramic matrix composites. However, unpub-
lished data (Subhani, 2012c) of a model CNT-SiO
2
composite showed that
the incorporation of CNTs in silica glass did not reduce the thermal shock
resistance of otherwise highly resistant silica glass after quenching from
500
8
C and 1000
8
C to room temperature (20
8
C); a further increase in
temperature to 1200
C initiated devitrification of silica glass, which reduced
the strength of CNT-SiO
2
composites. Figure 7.19 shows SEM images of
the surfaces of 2.5wt% CNT-SiO
2
composite specimens that had been
thermally shocked from 1000
8
C. Figure 7.19(a) shows no
evidence of cracks while Fig. 7.19(b) shows surface cracks due to cristobalite
formation, which was also witnessed in specimens heated to 1200
8
C and 1200
8
8
Cand
slowly cooled to room temperature without any thermal shock.
Repeating the thermal shock cycles (thermal cycling) on CNT-SiO
2
composites, i.e. heating up to 1000
8
C and quenching in water at 20
8
Cupto
