S. S. SRIKANT ET AL. 1059
power 1900 W. Hence when the sillimanite is exposed
for 25 minutes, a mullite is formed under these experi-
mental conditions. During microwave processing, the
potential energy exists to reduce processing time and en-
hance product quality as microwaves can transfer en-
ergy throughout the sample volumetrically.
3.3. Studies on Formation of Mullite from
Sillimanite Using Conventional Furnace
from
sillim arge
l furnace
an
that in the conventional fur-
na
central region of the pellets, resulted
in
The experimental results on the formation of mullite
anite using conventional furnace with the ch
sample containing sillimanite (60%) and Al2O3 (40%)
and additives with 40% SiC and 10% binder reveals that
it took three hours to form a mullite at 1300˚C.
It is important to mention here the observations once
again on the tests carried out with conventiona
d microwave furnace that a mullite formation is seen at
25 minutes of experimental duration with microwave
furnace and where as the formation of mullite is not seen
with conventional furnace during 25 minutes under op-
timum material variables. It has taken three hours of time
at 1300˚C furnace temperature for formation mullite us-
ing conventional furnace.
The heating mechanism through microwaves furnace
are distinctly different form
ce. Typical Figure 7 shows a schematic illustration of
the process steps and effect of the heating mechanism in
the carbothermal reduction reaction for microwaves fur-
nace and conventional furnace with the charge material
kaolin clay [2].
In the microwaves furnace, the heating with micro-
waves started in the
two distinct zones: darker central core completely sur-
rounded by lighter zone. It is also observed that the outer
zone for MWCR as shown in Figure 7 is primarily mul-
lite with a small percentage of Al2O3 and much less SiC
than in the central zone with primarily SiC and small per-
centages of mullite and Al2O3. Both CO and SiO gases
must diffuse out of the pellets. However, during conven-
tional heating as shown in Figure 7, the carbothermal
reaction began outer the pellets and then progressed into
Mullit e
Al O
the central core. The pellet showed two distinct zones:
lighter central region completely surrounded by a darker
zone. It was also observed that the outer zone was pri-
marily mullite with small percentages of SiC, whereas
the inner region was primarily mullite with a small per-
centage of SiO2 and much less SiC than in the outer
zone.
Thus it is concluded that during microwave processing,
the pot
2 3
SiC
Al
2
O
3
Mulli t
SiC
Mullite
Al
2
O
3
SiC
Al
2
O
3
SiC
Mulli t e
Figure 7. Effect of heating mechanism for the carbothermal
reduction for both CCR and MWCR.
additives such
as silliposite
s, it attracts Si and C atoms from the vapor to
th
ential energy exists to reduce processing time and
enhance product quality as microwaves can transfer en-
ergy throughout the sample volumetrically. Hence mi-
crowave heat source is much effective for value addition
to red sediment placer sillimanite to form mullite in com-
pare to conventional furnace.
3.4. Structural and Morphological
Characterization of Mullite
The XRD patterns of charge materials and
manite, alumina, silicon carbide and com
charge material (raw mullite) are shown in Figure 8. The
XRD patterns of mullite formation in both microwave
furnace and conventional furnace are also shown in Fig-
ure 9.
The XRD data indicate that the mullite formation in
both microwave furnace and conventional furnace are al-
most similar with reference to mullite phase concern. The
results evidenced that microwave is more efficient to
produce Al2O3/mullite/SiC composite at 25 min than
conventional furnace at 1300˚C/3 h duration.
The FESEM images for mullite formation from red
sediment placer sillimanite using microwave heat treated
is shown in Figure 10. It can clearly be seen that SiC
(massive form), and mullite in cluster structure, are ob-
served in the morphological features of mullite formed
from red sediment placer sillimanite using microwave
furnace.
It may be noted here that as the metal melts at high
temperature
e catalyst to form whiskers. When saturation of these
atoms occurred in the liquid catalyst, the growth is side
branched type, resulting in formation of fibrous structure
[11]. Apart from this, the alumina needles are also found
and seen in the SEM image. This alumina phase is very
helpful in microwave heating because it helps to isolate
silica from silicon carbide during mullite formation.
Typical SEM EDAX data and image mapping for Al,
Si, C and O of microwave mullite are shown in Figures
11 and 12 respectively.
The image mapping for Al, Si, carbon and oxygen for
typical mullite sample prepared from red sediment placer
sillimanite using microwave sintering furnace reveals that
the sample contain both mullite and SiC materials. Thus
the data obtained from XRD, SEM EDAX confirm the
findings on the presence of mullite and silicon carbide
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