Biomedical Engineering Reference
In-Depth Information
1000°C, 4hours
1080°C, 4hours
(a)
(b)
10µm
50µm
1000°C, 8 hours
1000°C, 16 hours
(d)
(c)
50µm
10µm
Figure 3.8.
SEM mages of glass ceramics samples (SiO
2
-MgO-Al
2
O
3
-K
2
O-B
2
O
3
-F) heat treated
at (a) 1000 °C, 4 hours, showing no sign of crystallinity, (d) 1120 °C, 4 hours showing usual ran-
domly oriented interlocked mica fl akes. (c) SEM images of time variation samples (heat treated
at 1000 °C), showing single “Butterfl y” shaped crystals (soaking time 8 hours) (d) “Tree leave”
structure at the end of each mica rod (soaking time 16 hours)
40
.
with 40 °C temperature interval with heating rate of 10 °C/min for both sets of
experiments.
The results of critical heat treatment experiments have revealed scientifi -
cally interesting information about the crystal growth mechanisms. Figure 3.8
shows the microstructures of glass and glass ceramic, heat treated at different
temperatures of up to 1120 °C for four hours. The microstructure is characterized
by interlocked, randomly oriented mica plates when heat treated at different tem-
perature. When the base glass is heat treated for longer time period, such as more
than four hours, a new type of crystal morphology appears, whose growth pattern
can be approximately described by spherulitic-dendritic like crystallization habit.
Image “c” and “d” in Figures 3.8 represent the morphology of a novel shaped
crystal, which has a “butterfl y” like pattern with mica rods radiating from a
central nucleus.
The possible mechanism for the development of this unusual microstructure
is discussed with reference to a nucleation-growth kinetics based model. The
activation energy for crystal nucleation and Avrami index are computed to be
388 KJ/mol and 1.3 respectively, assuming Johnson-Mehl-Avrami model of
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