Materials and methods 1 Materials
S. Sembiring, W. Simanjuntak, R. Situmeang, Buhani, Shella
SWUP
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C, cordierite is metastable and it is slowly transforms to -cordierite. It has also been
reported that -cordierite, MgAl
2
O
4
spinel and cristobalite present at 1300 °C, while the only -cordierite phase was observed at 1350
C and 1375
C Salje, 1987. By sol gel method, the initiation of the
cordierite transformation was achieved in the temperature range 1000- 1100 °C and high stability -cordierite only at 1200 °C Kumta, 1994.
Related to natural resources as raw materials for preparation of ceramics, rice husk is a waste material derived from agriculture residu, which makes its an alternative as silica
source. In our previous research, active silica from rice husk was obtained by simple acid leaching, and has been used to produce borosilicate Sembiring, 2011, cordierite
Simanjuntak Sembiring, 2011, carboxyl Simanjuntak et al., 2012, aluminosilicate Simanjuntak et al., 2013, mullite Sembiring Simanjuntak, 2012; Sembiring et al., 2014.
The present study was carried out with the aim of exploring the feasibility of rice husk silica to produce refractory cordierite precursor as an alternative to commonly used silica
synthetics. The precursor produced was then subjected to thermal treatment to investigate the phase development, physical and thermal properties as refractory cordierite. The
functionality change of as a function of heat treatments was investigated by FTIR spectroscopy, the structure was characterized by XRD and the microstructure was studied
using SEM.
2. Materials and methods 2.1 Materials
Raw husk used as a source of silica 97.5 was obtained from local rice milling industry in Bandar Lampung Province, Indonesia, and Al
2
O
3
95 and MgO 98 powders were taken from PT ELO KARSA UTAMA merck, kGaA, Damstadt, Germany. KOH, HCl, and absolute
alcohol C
2
H
5
OH used are reagent grade obtained from Merck.
2.2 Procedure
Preparation of silica powder from rice husk Rice husk silica was obtained using alkali extraction method adopting the procedure
reported in literature Sembiring et al., 2014. Typically, a sample of 50 g dried husk was mixed with 500 ml of 5 KOH solution in a beaker glass. The mixture was boiled for 30
minutes, and then allowed to cool to room temperature and left for 24 hours. The mixture was filtered through Millipore filter to separate the filtrate which contains silica silica sol.
To obtain solid silica, the sol was acidified by dropwise addition of 5 HCl solution until the sol was converted into gel. The gel was aged for three days, and then rinsed repeatedly with
deionized water to remove the excess of acid. The gel was oven dried at 110
C for eight hours and then ground into powder.
Preparation of cordierite The ratio of MgO:Al
2
O
3
:SiO
2
for cordierite ceramic preparation was 2:2:5 by mass obtained using method reported in literature Gonzalez-Velasco, 1999; Simanjuntak
Sembiring, 2011. The compounds were mixed with alcohol and placed under magnetic stirring and then grounded into powder by mortar with the size of 200 mesh. The powder
was pressed in a metal die with the pressure of 2 x10
4
Nm
2
to produce cylindrical pellet. Then, the pellets were sintered at temperatures of 1050, 1110, and 1170
C, using
Synthesis and characterization of refractory cordierite precursors from rice husk silica
SWUP
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temperature programmed with a heating rate of 5
o
C min and holding time of 4 hours at peak temperatures.
Characterization A Perkin Elmer FTIR-1752 was used for the investigation of functional groups of the
cordierite. The sample was mixed with KBr of spectroscopy grade, and scanned in the spectral range of 4000-400 cm
-1
. The structure of cordierite is examined using an automated Shimadzu XD-610 X-ray diffractometer at the Agency of Nuclear Energy National BATAN,
Serpong-Indonesia. The operating conditions used were CuK
radiation = 0.15418, produced at 40 kV and 30 mA, with a 0.15
receiving slit. Patterns were recorded over goniometric 2
ranges from 5-80
with a step size of 0.02, counting time 1sstep, and using post-diffraction graphite monochromator with a NaI detector. The diffraction data were
analyzed using search-match method Powder Diffraction File, 1997. Microstructural analysis was conducted using SEM Philips-XL, on polished and thermally-etched samples. The
porosity and density were examined according to Archimedes method Australian Standard, 1989. A Zwick tester was used to measure the Vickers hardness. At three measurements
were made for each loading position. Electrical conductivity of the samples was studied at ambient temperature by the four-probe method. Measurement was carried out on a sample
in the form of plate size of 2 cm x 2.5 cm x 1 cm, prepared by pressing sample placed in a stainless steel dics using hydraulic pressing 3 tones. The conduction was ohmic in nature and
the electrical conductivity was given by the equation:
= LR A Pantea, 2001, where R is the resistance
, A is the area of the sample cm
2
and L is the sample thickness cm.
3 Results and discussions 3.1 Characteristics of synthesized refractory cordierite
To study phase development, the samples subjected to sintering treatment at 1050, 1110, and 1170
C were characterized using FTIR, XRD, and SEM. The results of FTIR spectra for the synthesized and thermally treated at different temperatures are compiled in Figure
1.
Figure 1. FTIR spectra of sintered samples at different temperatures a 1050
o
C, b 1110
o
C, and c 1170
o
C.
It is known a broad absorption bands with the position of 1056-1100 cm
-1
Figure 1, corresponding to the presence of Si-O bonds of crystalline SiO
2
, as presented in the literature
S. Sembiring, W. Simanjuntak, R. Situmeang, Buhani, Shella
SWUP
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Nagai Hashimoto, 2001. These bands decrease with increasing temperature of sintering indicating the formation of Si-O-Mg-Al
. The band centered at 720 cm
-1
Fig 1a-b is broader than the band in the Fig 1c, probably corresponding to the vibration of Al-O and Mg-O
indicating the presence of Si-O-Mg-Al bonding as supported by previously study Janackovic et al., 1997; Petrovic et al., 2003.
The XRD patterns of the sintered samples at 1050
o
C, 1110
o
C, and 1170
o
C were collected and the formation of crystalline phases were compiled in Figure 2.
Figure 2. The x-ray diffraction patterns of the sintered samples at different
temperatures a 1050
o
C, b 1110
o
C, and c 1170
o
C, P= Spinel, Q=µ-Cordierite, R= -
Cordierite, S= Cristobalite.
The phases were identified with the PDF diffraction lines using search-match method Powder Diffraction File, 1997, showing the presence of spinel PDF-21-11520 with the most
intense peak at 2 = 36.92
µ-cordierite PDF-14-0249 at 2 = 13.45
, -cordierite PDF-13-
0294 at 2 = 10.48
, and cristobalite PDF-39-1425 at 2 = 21.2
. It was observed, that the crystallisation gets higher with increasing heat treatment temperatures. For the sample
sintered at 1050 C Figure 2a, the presence of cristobalite, µ-cordierite, and spinel clearly
detected, and µ-cordierite changed into -cordierite up to 1170 C. The presence of
cristobalite is most likely formed as a result of rice husk silica crystallisation during the heating, while the presence of µ-cordierite may be formed through inter-diffusion between
cristobalite and spinel, but spinel was formed by interaction of AlO
6
and MgO
6
octahedral, and
-cordierite may be formed through inter-diffusion between µ-cordierite and spinel, as has also been observed by others Naskar Chatterjee, 2004.
The morphology of the sintered samples was characterized using SEM. The images were shown in Figure 3. In all samples, crystallisation was detected after the heat treatment
at 1050 C, b 1110
C, and 1170
C. As displayed by the images in Figure 3a-c, the surface morphology of the samples is marked by different grain size and distribution. The
microstructure of the sample sintered at 1050 C Figure 3a show quite difference to that
of the sample treated at 1110 C Figure 3b. The sample prepared at 1050
C Figure 3a, is marked by small grains with less evident grain boundaries, compared to those observed
for the other two samples Figures 3b and 3c. In addition, it is obvious that the clusters in the sample prepared at 1050
C are surrounded by fine grains. The large clusters are most likely composed of µ-cordierite, while the middle and fine grains are spinel and cristobalite.
The surface of samples prepared at higher temperatures 1110 and 1170
C is most likely dominated by larger grains composed of
-cordierite clusters and covered some fine grains
Synthesis and characterization of refractory cordierite precursors from rice husk silica
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of cristobalite and spinel. Both samples are marked by initiated coalescence of -cordierite
which is crytallised. This feature suggests that at 1110 and 1170
C, phases of cristobalite and spinel continue to change and allowed for particles rearrangement of
-cordierite, before the formation of
-cordierite takes place that undetected at 1050 C as observed in the XRD results Figure 2a. The formation
-cordierite can be seen more clearly by inspecting the SEM micrograph of the sample treated at 1170
C Figure 2c, which displays relatively very uniform surface with small grain sizes, and covered the entire surface. Increasing sintering
temperature was found to intensifying the formation of -cordierite as indicated by XRD
result Figure 2b and 2c.
a b
c
Figure 3. The scanning electron microscopy SEM images of samples sintered at
different temperatures a 1050 C, b 1110 C, and 1170 C.
Figure 4 show the characteristics of density and porosity of the samples as a function of sintering temperature. The result reveals the density increased as the sintering
temperature increased, and porosity is inversely. As shown in Figure 4a, increased temperatures resulted in higher density, which is probably the homogeneity of -cordierite
and particles arrangement in the samples as a result of higher sintering temperatures applied, which is in accordance with the surface morphology of the samples, as seen in Figure
3. As sintering progresses the pores become smaller, it shows in Figure 4b the porosity decreasing by increases sintering temperature. Reduction of the pores make sample become
more compact. It is observed that the increment of sintering temperature increased the density but decreased the porosity.
Figure 4. Porosity a and density b as a function of sintering temperature.
S. Sembiring, W. Simanjuntak, R. Situmeang, Buhani, Shella
SWUP
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Figure 5 shows the characteristics of hardness and electrical resistivity of the samples as a function of sintering temperature. The result reveals the hardness and electrical
resistivity of the samples increased with increasing the sintering temperature.
Figure 5. a Hardness and b Electrical resistivity as a function of sintering temperature.
As shown in Figure 5b, increased temperatures resulted in higher hardness and electrical resistivity, which are in agreement with the sample more compact and the increase
of the relative amount of -cordierite Figures 2b and 2c. The electrical resistivity is increased slowly from sintering temperature 300 to 1000
C and increased sharply up to 1050 C. Increase of the amount of -cordierite caused the samples tend to act as electrical
insulator because -cordierite is known as good electrical insulator. This means that the electrical resistivity contributed by the -cordierite phase can be assumed to be negligible,
and therefore, electrical resistivity by the samples can be considered as fully due to the phase
-cordierite. From practical point of view, this finding demonstrates that the electrical resistivity of samples be controlled by controlling the -cordierite, to adjust the electrical
resistivity for specified application, such as insulator and conducting element in refractory device. Another factor is probably the homogeneity of
-cordierite and particles arrangement in the samples as a result of higher sintering temperatures applied, which is in
accordance with the surface morphology of the samples, as shown in Figure 3.