Microwave-assisted sintering of zircon

Article abstract of DOI:10.1016/j.jallcom.2007.02.032

Zircon samples were subjected to microwave heating (2.45 GHz and 900 W) in a domestic oven between 4 and 60 min and results compared to conventional heating (1300-1600 °C, holding for 120 min). Microwave heating of zircon with susceptor showed a higher temperature for any heating time compared to those heated in the absence of susceptor. To reach almost the same densification, microwave heating reduced the sintering time and temperature (16 min from room temperature to 1280 °C) compared to conventional heating (130 min from room temperature to 1300 °C and holding for 120 min at this temperature). The results also show that a long microwave heating (higher than 30 min) accelerates the decomposition of zircon. The extra phase formed acts a glassy phase during sintering of the samples; however, a lower average particle size (38.5%) was observed for the microwave-sintered samples.

Full text of DOI:10.1016/j.jallcom.2007.02.032

Journal of Alloys and Compounds 448 (2008) 246–249
Review
Microwave-assisted sintering of zircon
∗
T. Ebadzadeh , M. Valefi
Ceramic Division, Materials & Energy Research Center, P.O. Box 14155-4777, Tehran, Iran
Received 19 June 2006; received in revised form 4 February 2007; accepted 8 February 2007
Available online 11 February 2007
Abstract
Zircon samples were subjected to microwave heating (2.45 GHz and 900 W) in a domestic oven between 4 and 60 min and results compared
◦
to conventional heating (1300–1600 C, holding for 120 min). Microwave heating of zircon with susceptor showed a higher temperature for any
heating time compared to those heated in the absence of susceptor. To reach almost the same densification, microwave heating reduced the sintering
◦
◦
time and temperature (16 min from room temperature to 1280 C) compared to conventional heating (130 min from room temperature to 1300 C
and holding for 120 min at this temperature). The results also show that a long microwave heating (higher than 30 min) accelerates the decomposition
of zircon. The extra phase formed acts a glassy phase during sintering of the samples; however, a lower average particle size (38.5%) was observed
for the microwave-sintered samples.
©
2007 Elsevier B.V. All rights reserved.
Keywords: Zircon; Microwave; Sintering
Contents
1.
2.
3.
4.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
Experimental procedures. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246
Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 248
as a shortened processing time [5–7]. Moreover, a smaller grain
size can be obtained with microwave processing due to higher
heating rate and shorter sintering cycles [8]. In microwave pro-
cessing, energy is directly transferred to the material through the
interaction of electromagnetic waves with molecules leading to
heating. A dense material with improved microstructure can be
obtained by microwave heating in a much shorter time compared
to the conventional processes.
1
. Introduction
Microwave sintering of ceramics has been the subject of sev-
eral studies [1–3]. It is known that ceramic materials can be
heaped up at a very rapid rate and at a lower temperature using
microwave heating compared to the conventional sintering pro-
cess [4]. Microwave processing of materials is fundamentally
different from traditional techniques and has several advantages
over conventional sintering techniques.
In this study, the densification behaviour of zircon prepared
through microwave heating was investigated to determine the
advantages of microwave over the conventional heating process.
Microwave heating offers the potential of an enhanced sin-
tering process and a decreased densification temperature as well
2. Experimental procedures
The raw material was zircon powder (Zircosil 5; Cookson Matthey Ceramics
&
Materials) with average particle size of 4 ␮m (91.5% purity). The chemical
composition of zircon is reported in Table 1. Zircon powders were pressed at
3.7 MPa in a steel mold with a 20 mm diameter. The thickness of pressed pow-
6
ders was 1.4 mm. Samples were sintered in a domestic microwave (2.45 GHz)
and compared with those sintered through the conventional heating. Fig. 1 shows
the schematic diagram of microwave system used in the present work. A SiC cru-
cible was used as a susceptor since it efficiently absorbs the microwave energy
∗
Corresponding author. Tel.: +98 2616204131; fax: +98 2616201888.
E-mail address: t-ebadzadeh@merc.ac.ir (T. Ebadzadeh).
0
925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2007.02.032

T. Ebadzadeh, M. Valefi / Journal of Alloys and Compounds 448 (2008) 246–249
247
Table 1
Chemical composition (wt%) of zircon
SiO2
37.81
0.44
61.52
0.03
0.04
0.04
0.12
Al2O3
ZrO2
MgO
Fe2O3
CaO
TiO2
due to its high loss factor [9]. The SiC crucible indirectly heats the samples
to a high enough temperature and improves the sinterability of the samples.
Microwave heating was carried out in air at 900 W power between 4 and 60 min.
All runs were made by fast heating up to a maximum temperature measured
using a R-type thermocouple placed in contact with the sample surface quickly
Fig. 2. Microwave absorption efficiency by zircon accompanied by susceptor
(
A), susceptor (B) and zircon (C).
(
2 s) after turning off the power and samples were cooled down to the room tem-
tor (curve A) between regions I and II is nearly the same as
that of the susceptor (curve B). At the end of region II, the
slop of curve A increases rapidly and the temperature of the
perature. The maximum error in temperature measurement was between −10
◦
◦
and −15 C for temperatures lower and higher than 850 C, respectively. At
least five temperature measurements were carried out for each time of heating
and measured errors became greater as the temperature went up. The density
and porosity of sintered samples were measured by Archimedes method. In
the conventional heating, sintering was carried out in a furnace at temperatures
◦
sample reaches 1330 C (25.4% increase) after 20 min heating.
The thermal changes in susceptor (curve B) is different and
◦
remains nearly constant (5.5% increase) and reaches 1020 C
◦
between 1300 and 1600 C at a rate of 10 K/min with soaking at the maximum
temperature for a period of 2 h. As-sintered samples were used for X-ray analysis
(
oscan 360, Lecia Cambridge Instrument) was performed on thermally etched
(
determined from counts of 258 grains.
after the same heating regime. The above-mentioned shows the
microwave absorbing effect of zircon sample with susceptor
after 10 min heating. As Fig. 2 reveals, the increase of tem-
perature after 10 min heating was not observed in zircon and
susceptor which were individually heated. The thermal runaway
phenomenon is unlikely to occur by the use of hybrid heating
since the heating rate slows down at high temperatures, as indi-
cated in Fig. 2. The green density of as-pressed pellets was found
Siemens D500 with Cu K␣ radiation). Scanning electron microscopy (Stere-
◦
1300 C, for 20 min) samples. The average grain size of sintered samples was
3
. Results and discussion
3
to be about 2.57 g/cm (i.e., relative density 55%). In conven-
The microwave absorption efficiency by zircon and susceptor
◦
tional heating, zircon samples sintered at 1400 C with 120 min
was individually and together determined by the time evolu-
tion of temperature (Fig. 2). The temperature of zircon sample
with susceptor (curve A) increased rapidly at a rate of about
showed a relative density of 73.7%. When sintering was per-
◦
formed at 1600 C for 120 min, relative density increased to
◦
86.3% (Fig. 3). The variation in density as a function of sin-
tering temperature for the microwave process is depicted in
Fig. 4. Microwave-sintered samples exhibit enhanced densifica-
tion at a lower sintering temperature and time (relative density
1
90 C/min in region I which can be attributed to the exis-
tence of the susceptor as the curve B determines. SiC has a
higher tendency to absorb the microwave energy compared to
zircon sample (curve C) which is not a good microwave absorber
◦
◦
64.5% for samples heated from room temperature to 1087 C
[
10]. The heating rate decreases markedly to around 10.5 C/min
◦
in 12 min) compared to the conventional-sintered samples (rela-
for a sample temperature higher than 992 C (region II). As
Fig. 2 reveals, the absorption of microwave energy is limited in
region IV and the heating rate is approximately low (4 C/min).
◦
tive density 56.7% for samples sintered at 1100 C for 120 min).
◦
The temperature needed to achieve density of around 69.2%
◦
for the microwave-sintered samples (16 min heating) was 20 C
The change of the heating rate of zircon sample with suscep-
lower than that for the conventional-sintered samples (the con-
◦
ventional sintering was performed at 1300 C for 120 min). For a
Fig. 1. Schematic diagram of microwave system.
Fig. 3. Densification achieved by conventional process.

2
48
T. Ebadzadeh, M. Valefi / Journal of Alloys and Compounds 448 (2008) 246–249
Fig. 4. Densification achieved by microwave process.
relative density of 80%, the temperature difference increased to
◦
1
10 C between the microwave-sintered sample (heated from
◦
room temperature to 1390 C in 30 min) and conventional-
sintered sample (performed at 1500 C for 120 min). This
◦
implies that the temperature difference between microwave and
conventional-sintered samples increases by increasing the time
and temperature of heating. As Fig. 5 reveals, the dissociation
of zircon (ZrSiO4 → ZrO2 + SiO2) has been accelerated in the
microwave-sintered sample after 60 min heating. A low level of
dissociationisobservedinthesamplessinteredconventionallyat
◦
1
600 C (with 120 min soaking). Therefore, the microwave pro-
cess may also effectively assist the dissociation of zircon which
may be useful method for the synthesizing of ZrO2 powder.
The dissociation of zircon may also be contributed to the higher
density exhibited by samples heated for 60 min (relative den-
Fig. 6. Electron microscope images of samples sintered by microwave at
1
◦ ◦
460 C (A) and conventional process at 1600 C (B).
sity 87.6%, porosity 0.7%) compared to conventional-sintered
samples (relative density 86%, porosity 5.56%).
Scanning electron microscope images of samples sintered via
◦
microwave and conventional heating at 1460 and 1600 C are
given in Fig. 6. The rapidity of microwave method also avoids
undesirable grain growth and provides the finer microstructure
in spite of the existence of a glassy phase due to the dissociation
ofzircon(Fig. 6A). Theaveragegrainsizeofzirconparticleswas
0
.8 ± 0.5 ␮m for the microwave-sintered sample which is lower
than that for the conventional-sintered sample (1.3 ± 0.8 ␮m).
4. Conclusions
The results reveal that the use of 2.45 GHz microwave as a
heating source improves densification kinetics of zircon mate-
rials. Microwave heating of zircon with a susceptor showed a
higher temperature for any heating time compared to samples
heated in the absence of the susceptor. Long microwave heat-
ing (60 min) accelerates the dissociation of zircon while gives a
lower average particle size (38.5%) compared to conventional-
sintered samples.
References
Fig. 5. X-ray diffraction patterns of microwave heated samples for 60 min (A),
◦
3
0 min (B) and conventional sample sintered at 1600 C for 2 h holding time (C)
(
zircon: ꢀ and zirconia: ꢀ).
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