PROS Simon S, Wasinton S, Rudi S, Buhani, Shella Synthesis fulltext

Proceedings of the IConSSE FSM SWCU (2015), pp. BC.110–117

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ISBN: 978-602-1047-21-7

Synthesis and characterization of refractory cordierite precursors
from rice husk silica
Simon Sembiringa, Wasinton Simanjuntakb, Rudi Situmeangb, Buhanib, Shellac
Department of Physics,
Department of Chemistry,
c
Under Graduate Student of Physics Department,
Lampung University, Jl. Prof. Soemantri Brojonegoro No.1 Bandar Lampung 35145, Indonesia
Email: *simonsembiring2@gmail.com
a

b

Abstract
This study describes the production of refractory cordierite ceramics by mixing silica

extracted from rice husk, Al2O3 and MgO powder. The mixture was sintered at different
temperatures of 1050, 1110, 1170 C for 4 h. The phases formed and structure change
as a result of sintering were investigated using different characterisation technique of
Fourier transform infrared (FTIR), X-ray diffraction (XRD) and scanning electron
microscopy (SEM). Density, porosity, hardness and electrical resistivity were also
measured. Cordierite, spinel and cristobalite were the major phases in the samples
sintered at temperature range of 1050-1170 C. µ-cordierite was formed through the
intermediate phases of spinel and cristobalite at 1050 C. The final phase -cordierite
(indialite) was produced through µ-cordierite and cristobalite from 1110 to 1170 C. The
density, hardness and electrical resistivity were found to increase with increasing of
sintering temperature, as they are strongly influenced by microstructure of the material.
Keywords cordierite, rice husk, sintering, structure, refractory

1. Introduction
It is well known that cordierite (Mg2Al4Si5O18) ceramics is excellent insulator and highthermal resistant material, possessing low dielectric constant and thermal expansion
coefficient. The cordierite ceramics is a promising material as refractory material with the
highest melting temperature 1460 C among silicate glass-ceramics (Hamzawy & Ali, 2006).
Due to its low thermal expansion coefficient (Kai et al., 2010), excellent thermal shock
resistance (Oliveira & Fernandez 2002), high refractoriness (Zhu et al., 2007), therefore,
cordierite ceramic is considered as a very promises for structural materials and finds

applications as heat exchangers for gas turbine engines (Laokula & Maensirib, 2006),
electrical and thermal insulation (Gonzalez-Velasco, 1999; Evans et al., 1980). In previous
studies (Oliveira & Fernandez, 2002; Lim & Jang, 1993), have established that the excellent
thermal shock resistant material when it is subjected to rapid changes in temperature. They
found that fracture toughness increases with increasing sintering temperature from 1250 to
1300 C.
Several methods were developed and applied to synthesis cordierite ceramics. Among
other, solid state reaction (Ghitulica et al., 2007), sol gel (Simanjuntak & Sembiring, 2011),
spraying (Rohana et al., 2004), or crystallization from glasses (Rudolph et al., 1993). For
example, the studies of cordierite by solid reaction (Petrovic et al., 2001) have shown that
cordierite exist in three polymorphs, -cordierite, -cordierite and µ-cordierite. Below 1450
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S. Sembiring, W. Simanjuntak, R. Situmeang, Buhani, Shella

C, cordierite is metastable and it is slowly transforms to -cordierite. It has also been
reported that -cordierite, MgAl2O4/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 10001100 °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 Al2O3 (95%) and MgO (98%) powders were taken
from PT ELO KARSA UTAMA (merck, kGaA, Damstadt, Germany). KOH, HCl, and absolute
alcohol (C2H5OH) 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:Al2O3:SiO2 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 x104 N/m2 to produce cylindrical pellet.
Then, the pellets were sintered at temperatures of 1050, 1110, and 1170 C, using
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temperature programmed with a heating rate of 5 oC /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 1s/step, 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: = L/R A (Pantea, 2001), where R is the
resistance (), A is the area of the sample (cm2) 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 oC, (b)
1110 oC, and (c) 1170 oC.

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 SiO2, as presented in the literature
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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 oC, 1110 oC, and 1170 oC 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 oC, (b) 1110 oC, and (c) 1170 oC, 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-130294) 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 2(a)), 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 AlO6 and MgO6 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 3(a)) show quite difference to that
of the sample treated at 1110 C (Figure 3(b)). The sample prepared at 1050 C (Figure 3(a)),
is marked by small grains with less evident grain boundaries, compared to those observed
for the other two samples (Figures 3(b) and 3(c)). 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
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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 2(a)). The formation -cordierite can be seen more clearly by inspecting the

SEM micrograph of the sample treated at 1170 C (Figure 2(c)), 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 2(b) and 2(c)).

(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 4(a), 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 4(b) 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.

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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 5(b), 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 2(b) and 2(c)). 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.

4.

Conclusions

The synthesis and characterization of refractory cordierite precursors based on the rice
husk silica have been successfully demonstrated. XRD result on the sintered samples shows
the presence of µ and  cordierite, spinel and cristobalite. It was found that -cordierite
formation started to form at 1110 C which its occurs through µ-cordierite and cristobalite
from 1110 to 1170 C The existence of both ne and coarse grains of -cordierite was
revealed by microstructure analysis. The results revealed that the formation of -cordierite
is a function of sintering temperatures. Based on these electrical resistivity and hardness
values, the samples are considered as insulator, suggesting the potensial used of the
cordierite in recfractory device.

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Acknowledgments
The authors wish to thank and appreciate the Directorate General Higher Education
Republic of Indonesia (DIKTI) for research funding provided through Hibah Competence
Research Grant Batch I No: no: 050/SPH2/PL/Dit. Litabmas/II/2015 Program in 2015.

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