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Materials Science and Engineering A266 (1999) 211 – 220
Use of partially oxidized SiC particle bed for microwave sintering
of low loss ceramics
Peelamedu D. Ramesh a,*,1, David Brandon a, Levi Schächter b
a
b
Department of Materials Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
Department of Electrical Engineering, Technion-Israel Institute of Technology, Haifa 32000, Israel
Received 17 March 1998; received in revised form 17 November 1998
Abstract
A 1 kW hybrid microwave system was fabricated using a partially oxidized SiC powder bed and used to sinter small
components of low-loss insulating ceramics. Samples of 3% yttria stabilized zirconia/20% alumina (3YZA) and 99% alumina were
sintered to final densities of 99%(3YZA) and 95%(99% alumina). Partially oxidized silicon carbide (b-SiC) powder was used as
susceptor (preheater). By comparison, unoxidised b-SiC powder, which couples well with microwaves at room temperature,
exhibited thermal runaway above 400°C. It could be possible that at high oxidation levels the connectivity between SiC particles
in b-SiC powder bed might become depercolated, and a model similar to one proposed for silicon nitridation under microwaves
could be appropriate in understanding the microwave absorption phenomenon. Stable temperature measurements at various
positions in the system confirmed that the maximum temperature was at the center of the 3YZA sample. The plot of the relative
temperature difference between the sample and its surroundings as a function of sample temperature resulted in a bell-shaped
curve with a clear maximum at around 800°C, associated with the rapid increase in radiation heat transfer above this temperature.
Experiments also confirmed that sintering occurred at lower temperatures in a microwave field when compared to conventional
sintering. © 1999 Elsevier Science S.A. All rights reserved.
Keywords: Hybrid microwave heating; Oxidized silicon carbide; Depercolation; Sintering
1. Introduction
Sintering of ceramics is an attractive area of microwave processing [1]. The advantages include rapid
heating rates, short processing times, low power requirements and reduced reaction with the atmosphere.
Materials with a high loss tangent can couple with
microwaves at room temperature while less ‘lossy’ materials need higher initial temperatures in order to
couple. Some fine powders, such as carbon [2], silicon
carbide [3] and vanadium oxide [4] couple efficiently
with microwaves at room temperature, whereas others
such as zirconia [5] and alumina [6], only couple at high
temperatures. The dependence of dielectric constant (o%r)
and loss tangent (tand) on temperature are needed to
* Corresponding author. Tel.: +49-721-6087922; fax: + 49-721174263.
E-mail address: [email protected] (P.D. Ramesh)
1
Now at Institüt für Keramik im Maschinenbau, Universität Karlsruhe, Haid-und-neu-Str. 7, 76131 Karlsruhe, Germany.
predict the power absorption characteristics of materials in a microwave field.
Kitchen microwave ovens provide inexpensive multimode cavities for experimental materials processing.
The modes generated within such a cavity result in a
complex electric field distribution, so that the specimen
position within the cavity is an important variable in
the heating process. More uniform electric fields may be
achieved either by operating at higher frequencies or by
using mode ‘stirrers’. Larger specimens (a high filling
factor) absorb microwaves more efficiently and result in
higher heating rates, but have the disadvantage of field
non-uniformity and resonances within the specimen
(which may result in cracking of the sample). Heat
generation is also affected by the depth of penetration
(Dp), in the sample. As the temperature of the material
rises, microwave penetration decreases, leading to localized surface heating due to non-uniform power absorption [7,8]. Smaller workpieces experience a uniform
electric field, but the heating efficiency is then very low
0921-5093/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 9 2 1 - 5 0 9 3 ( 9 9 ) 0 0 0 1 7 - 9
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P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
Fig. 1. The microwave hybrid heating configuration. All dimensions are in mm. The temperature was measured at positions a, b, c, and d,
corresponding to the outer insulation, the inner insulation, the alumina fiber board and the sample, respectively. Measurement of temperature on
the alumina fiberboard surrounding the sample ‘c’ is a measure of susceptor temperature, since the sample is well-separated from the fiberboard.
due to the low mass [9] and can only be improved by
using a single mode cavity [10].
Since the dielectric loss properties increase with temperature, low loss materials can be made absorptive by
raising their temperature. Most commonly, a room
temperature susceptor surrounds the specimen and supplies thermal energy by heat transfer, simultaneously
immersing the specimen in both thermal and electric
fields2. Computer modeling of microwave heating indicates that, despite a decrease in total electric field
intensity, the field at the specimen is not significantly
screened by using a preheat susceptor [7].
Several preheating configurations have been suggested. De’et al. [11] used a SiC-lined susceptor around
the specimen to sinter Al2O3 compacts. An array of SiC
rods [12] has been used to sinter 8Y – ZrO2 (a ‘picket
fence’ arrangement). Using this configuration, densities
as high as 99% have been reported for specimen sintering temperatures of 1100°C, ca. 200°C lower than the
2
In this study radiation heat transfer within the microwave oven
assembly is taken to refer to thermal radiation and not to the
externally imposed microwave radiation field.
conventional sintering temperature of the same powder.
Carbon [13] has also been tried as a preheating source,
while LaCrO3 and mixtures of Al2O3 + LaCrO3 have
also proved to be suitable susceptor materials (Spotz et
al. [14]).
In this study, we report the use of a partially oxidized
b-SiC powder compact as a susceptor (pre-heater) to
sinter small components of low-loss ceramics (Al2O3
and ZrO2). The susceptor powder surrounds the specimen cavity and heats the low-loss specimen to temperatures of the order of 1250°C.
2. Experimental
2.1. The hybrid microwa6e heating system
Various configurations [11,12] of hybrid heating have
proved successful for ZrO2 and Al2O3 compacts. Poor
microwave absorbers such as porous magnesia (MgO)
and quartz (SiO2), form the container walls of the
refractory cavity, and alumina fiber board and fibrous
alumina are used as thermal barriers. A schematic of
P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
the present hybrid heating arrangement is shown in Fig.
1. The loss tangent values of MgO and SiO2 are very
low and these ceramics remain transparent to microwaves even above 1000°C. b-SiC powder (600 grit)
was used as the susceptor material. About 30 –40 g. of
the SiC powder was placed in the gap between the
alumina fiberboard inner cavity and the quartz wall.
The quantity of susceptor powder determines the electric field at the sample and was selected to ensure that
the specimen was adequately exposed to the electric
field at high temperatures, even though the microwave
penetration depth into the SiC powder bed decreases as
the temperature rises. Temperature measurements were
made at selected sites, labelled a, b, c and d in Fig. 1,
using a type R (Pt – Pt13%Rh) thermocouple within an
alumina support tube. The specimen temperature Td
was measured by inserting the thermocouple into a
small hole at the center. The temperature at the Al2O3
fiber board Tc was taken as an estimate of the temperature of the silicon carbide susceptor. Temperatures were
also determined across the thickness of the porous
MgO crucible Ta and Tb. The present system was
designed to generate a moderate thermal gradient in the
central region: a critical consideration for uniform sintering and crack prevention.
2.2. Temperature measurement
Simultaneous temperature measurement in a continuous electromagnetic field using a thermocouple is usually impossible as the thermocouple – field interaction
leads to gross errors. A very thin metal sheath surrounding the thermocouple could be used to overcome
this problem, but contact between the thermocouple tip
and the metal sheath must be avoided, while, shielding
reduces the response of the thermocouple. In this work,
the microwave energy was pulsed and an alumina covered thermocouple, with the tip uncovered, was used
for calibrating temperature measurements. Temperatures were measured using a Pt – Pt 13% Rh thermocouple (type R, 0.5 mm wire). The stable temperature
readings agreed with readings obtained with a platinum
sheathed thermocouple. While the temperature readings
are judged to be reliable, they are still average readings
which do not necessarily reflect the energy conversion
processes associated with the microwave power absorption. In particular, the relation between the measured
temperature and the effective temperature in the sintering process may not be linear.
213
Pmw = Pinc(1−p 2)(1− e − 2ad) W m − 3
(1)
where Pmw is the total power absorbed by the material.
The reflection coefficient, p, and attenuation constant,
a, mainly determine the total power absorbed in the
material. Low values of p and a correspond to insulating materials, in which a limited microwave absorption
results. Metals reflect microwaves and have high values
of p and a.
For a homogeneous material, reflection constant [15]
can be written as
p=
1−
o*r
(2)
1+ o*r
and attenuation constant a,
a=
! 2pf o0o ’r
c
2
1/2
1+
n
s
2pfo0o %r
"
2 1/2
1/2
−1
(3)
where o *r is the RMS value of the complex dielectric
constant, s is the electrical conductivity of the material,
f= 2.45 GHz, c= 3× 108 m s − 1, and o0 and o %r are the
free space permittivity and the relative dielectric constant of the material, respectively.
At a fixed microwave frequency, only the material
properties or = o0o %r and tand determine the total power
dissipated within the sample. Both these parameters
depend strongly on temperature [16].
Experimental time versus temperature curves for
fresh powders of SiC, Al2O3 and ZrO2 are given in Fig.
2 for increasing power loadings. Experiments were conducted with 50 g. of each powder sample in an alumina
crucible without any preheating powders. The figure
clearly indicates the strong absorption of the SiC powder in comparison with the other powders. A few
minutes exposure, even at low power levels (450 W),
raises the temperature of fresh SiC powder above
1600°C, with thermal runaway occurring after 90 min
of irradiation. More unexpectedly, after three successive
runs of the now partially oxidized SiC powder the
microwave heating curve exhibits much better coupling
at the lower temperatures with no runaway at high
temperatures.
All subsequent hybrid sintering experiments were
performed with a partially oxidized SiC powder susceptor for two reasons: (1). Repeated runs generated nearly
identical power/temperature/time conditions. (2) The
controlled coupling of the electric field together with
the moderate temperature maximum ($ 1250°C at 1
kW) fulfilled the basic prerequisites for a hybrid preheating system.
2.3. Absorption characteristics of SiC, ZrO2 and Al2O3
2.4. Penetration depth
Polar substances absorb microwaves to convert microwave energy to thermal energy. The generalized
equation for the absorbed power is written as
Microwave absorption for planar thick samples can
be characterized by a parameter termed the penetration
depth, DP. The primitive equation to calculate the
P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
214
depth of penetration for microwaves in a homogeneous
material is given by
DP =
lo
2p tand o%r
(4)
At a frequency of 2.45 GHz, the free space wavelength
lo =122.4 mm. The temperature dependence of Dp was
calculated from the data of o %r and o ¦r for Al2O3 and SiC
available in the literature [16]. At 1350°C, 2.45 GHz
microwaves penetrate 1 mm in SiC and 93 mm in solid
alumina. Alumina only absorbs microwaves significantly above 800°C, SiC absorbs at room temperature
but at high temperatures heat generation is limited to
the surface layer and the susceptor will screen the
sample. It follows that both the thickness and the
temperature of the susceptor layer must be limited.
In general, if the penetration depth exceeds sample
thickness, the material behaves transparent. Volumetric
heating results, if the penetration depth is comparable
with the sample thickness. Otherwise, if the sample is
too thick, absorption limits to the surface.
In the case of an oxide layer surrounding the susceptor powder particle, such as SiC powder particle surrounded by silica, the above Eq. (4) can not be
applicable. For a compositionally changing ceramic
system, a more complex equation involving terms for
connectivity and percolation, is needed to describe the
microwave penetration depth.
prepared by gelled tape-casting [17] as shown in the
flowchart (Fig. 3). Cylindrical pellets of YZA 15 mm in
diameter and 4–5 mm in thickness were obtained from
the gelled tape by uniaxial pressing. The measured
green density of these samples was 50–55% of theoretical. Al2O3 green samples were supplied in the form of
small cylinders (16 mm height and 3 mm diameter). A
maximum of three YZA pellets (approx. 15 gm.) and 15
Al2O3 specimens (ca. 5 g) were placed separately in the
fiber-board specimen cavity (Fig. 1).
A kitchen microwave oven was used (multimode
cavity, Sanyo Super Showerwave, 1000 W maximum
power). The microwave power was pulsed at a constant
rate, but with the pulse duration adjusted for different
power levels (at maximum power the power supply was
continuous). Stable temperatures were recorded in-situ
between pulses.
No ‘sparking’ associated with microwave–thermocouple interaction was observed. The hybrid pre-heating system (Fig. 1) was placed at the center of the
microwave oven cavity and insulated by alumina fiberboard. Even at high specimen temperatures the heatlosses due to convection from the insulated system were
low and no overheating of the oven was observed.
3. Results and discussion
3.1. Oxidation of SiC powder
2.5. Materials and methods
Specimens with two different compositions and of
various shapes (see Fig. 8) were used for the sintering
studies. The compositions used were 3Y2O3 –ZrO2/20
Al2O3 (YZA) and 99% Al2O3. The nominal particle size
of the YZA powder was 20 nm. All YZA samples were
When the SiC powder bed is microwave heated to
temperatures above 1000°C in ambient atmosphere,
oxidation phenomena takes place. The oxidation phenomena [18] that takes place in SiC is briefly discussed
below. The possible oxidation reactions for SiC could
be
Fig. 2. Effect of power-dependent microwave absorption on the heating of SiC, Al2O3 and ZrO2 powders without any preheater (susceptor).
P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
215
where b is the mass gain per unit volume of SiO2.
Values of Kpa were calculated for oxidation of silicon,
based on oxygen diffusion through a silica layer, and
compared with the experimental SiC oxidation data.
From a thermodynamical point of view, various possible reaction rate limiting steps in the SiC oxidation
reaction are discussed in detail elsewhere [18]. These
steps include (i) inward O2 diffusion through SiO2 layer
(ii) reaction at the SiC SiO2 interface and (iii) outward
diffusion of CO molecules. On observation of a higher
oxidation rate of silicon compared to SiC, it was suggested that more than one rate-limiting step is involved
in SiC oxidation, i.e. a mixed interface-reaction/diffusion controlled oxidation takes place in the case of SiC.
3.2. Absorption by different materials
Fig. 3. Flowchart for YZA pellet formation by tape-casting.
SiC (s)+ O2 (g)USiO2 (s) +C (s)
(a)
and
SiC (s)+3/2O2 (g)USiO2 (s) +CO (g)
(b)
The reaction rate constant, Kp, can be written as
Kp =
(Dx)2 2D 0,*
(p g −p iO2)
= O2 O2
t
a
(5)
where D 0,*
O2 is the tracer diffusion coefficient in silica, a
is the number of moles of oxygen required to form one
mole of silica and p gO2 and p iO2 are the oxygen partial
pressures at the oxygen silica and SiC silica interfaces,
respectively. Dx is the SiO2 layer thickness through
which oxygen diffusion takes place. Depending on the a
value, one of the above described reactions ((a) or (b))
is chosen.
The above Eq. (5) shows a parabolic behavior, which
can also be expressed in terms of SiO2 mass gain, Km,
Km =Kpb 2
(6)
Some attempt has been made to understand the
absorption characteristics of powders as opposed to
solid samples. An amount of 15 g of sample powder
was placed in an alumina crucible which was surrounded by 50 g of fresh SiC powder (as susceptor).
The heating curves for different sample powders at a
constant power level of 650 W are shown in Fig. 4. The
measured temperatures reflect both the microwave absorption by the powder sample (the sample temperature
was measured using a Pt–Pt13%Rh thermocouple
which was covered by a thin alumina tube in order to
avoid the thermocouple tip touching the sample powder) and mixed mode heat transfer from the SiC susceptor powder: firstly, if microwave absorption by the
powder sample were negligible, at all temperatures,
then all the heating curves should be similar, with the
sample temperature only dependent on heat transfer
through the sample powder. Secondly, the lower thermal conductivity of ZrO2 as compared to Al2O3 should
result in a higher Al2O3 temperature at a given time.
However, in Fig. 4, the ZrO2 samples reach higher
temperatures confirming that ZrO2 powder absorbs microwaves much more efficiently than Al2O3.
The vast difference in the heating curves between
unoxidized SiC powder bed and oxidized SiC powder
bed clearly suggests that in these two materials, the
microwave absorption nature is not identical. When a
SiC powder compact is microwave heated to temperatures above 1000°C in an ambient atmosphere, the
surface of SiC particle starts oxidizing leading to formation of a SiO2 layer. This system can now be viewed
as a compositionally changing ceramic system, which is
similar to nitridation of silicon under microwaves,
where a conducting silicon phase continuously gets
converted to a non-conducting silicon nitride phase. A
finite difference model developed by Skamser et al. [19]
gives a deeper insight into the microwave absorption
nature in a compositionally changing ceramic system.
Indeed, the model was successful in predicting the
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P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
temperature and composition distributions inside a reaction bonded silicon nitride specimen [20]. During the
nitridation reaction, silicon was gradually converted to
silicon nitride, and at the intermediate stage, the material was two phasic nature. Despite an insulating layer
formation, their experiments revealed the existence of
silicon connectivity at low conversion levels. At around
25% conversion, the electrical conductivity was maximum, thereafter started decreasing with more and more
conversion, and reached below the detection limit beyond 70% conversion. At this region, the silicon particles lost their connectivity and became completely
depercolated, clearly indicating that there was a neck
formation due to sintering of silicon particles, occurred
at the initial stages of reaction. As long as the connectivity among silicon particles persisted, silicon behaved
like a conductor, reflecting most of the microwave
energy. But, when the silicon particles lost their connectivity and became depercolated, the system behaved like
an insulator, absorbing more microwave energy. This
led to non-uniform local heatings in the specimen,
where the more nitrided regions heated to higher temperatures compared to the less nitrided ones. Thomas et
al. [20] have obtained a reasonably accurate empirical
model of the microwave heating characteristics of the
RBSN system by assuming it as a homogenous conductor at low conversion levels and like a conductor loaded
dielectric (CLD) at high conversion levels.
Let us consider the microwave heating process of
fresh SiC powder bed. At the initial stage, the unoxidized silicon carbide powder is homogeneous, having a
characteristic dielectric behavior. At the initial stages of
Fig. 4. Heating curves for various ceramic powders surrounded by fresh SiC susceptor powder (constant power loading).
Fig. 5. Experimental time-temperature profiles at various sites within the cavity (see Fig. 1) on a YZA sample.
P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
217
Fig. 6. Plot of [1 − Tc/Td] versus sample temperature [Td] (YZA specimen).
the heating process (i.e. before oxidising), nevertheless
the Eqs. (1)–(3) can be used to calculate the reflection
coefficient, the attenuation constant and the power
absorption. The temperature dependence of conductivity for yellow green coloured SiC shows a semiconductor behavior with an increased conductivity on heating
from room to high temperatures. This dielectric loss
factor increases sharply at temperatures beyond 400°C,
which makes the material to show a thermal runaway
behavior beyond this temperature. As the SiC powder
bed gradually oxidize to SiO2 (see section 3.1), the
system gradually slips away from homogeneity, consisting of two different solid phases coexisting with porosity. With the continuous increase in SiO2 volume
fraction during microwave heating, in a normal scenario, it can be assumed that the semiconducting SiC
particles are gradually isolated by an electrically insulating layer of SiO2. This would cause a gradual decrease in the tangent loss of the SiC, as their volume
fraction decreases during the oxidation phenomenon.
Since the above case is somewhat analogous to silicon nitridation under microwaves, we believe that a
similar modelling procedure could be possible. On the
same basis, we speculate that at the intermediate stage
(partially oxided), the conductivity of SiC could be of
the same order of magnitude as that of unoxidised SiC,
which might suggest that the silicon carbide particles
sintered partially and still remain well connected. But,
the silicon carbide phase would become completely
depercolated at higher oxidation levels resulting in a
very low conductivity. This could mean that at low
conversion levels, SiC might behave like a semiconductor and at high conversion levels, it might transform to
a semiconductor loaded dielectric (SLD) material.
The microwave absorption behavior of such a material is more complex and a mathematical treatment
similar to nitridation of silicon reaction could be helpful in understanding the microwave absorption nature
of this material. The above given Eqs. (2)–(4) for
reflection coefficient, attenuation constant and the penetration depth respectively, becomes invalid at high
conversion levels and a new set of equations similar to
the one proposed by Neelakanta and Park [21] might be
more appropriate. More importantly, in the energy
balance equation, the SiC depercolation, and chemical
phenomena of oxidation should properly be accounted
in order to get accurate temperature distributions in the
powder bed. This discussion is purely speculative and
no conductivity measurements were made in order to
confirm the percolation phenomenon.
3.3. Temperature profiles
Temperature versus time profiles measured as a function of power input for the different sites in the hybrid
system of Fig. 1 are shown in Fig. 5. The highest
temperatures were recorded at the center of the YZA
sample Td, (1340°C), whereas the highest temperature
measured in the alumina fiber board Tc, was 1250°C.
Curve Td diverges increasingly from curve Tc above
400°C, consistent with increasing microwave coupling
of the YZA specimen. This divergence reaches a maximum at 800°C and then decreases, consistent with
increasing radiation heat transfer. The relative temperature divergence was plotted as [1− Tc/Td] against the
sample temperature [Td], Fig. 6 and can be qualitatively
explained as follows. At low temperatures (B400°C)
there is no YZA specimen-microwave coupling, and the
measured temperature at the center of the sample is
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P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
Fig. 7. Radiant heat transfer from a YZA specimen calculated from the experimental temperature values.
predominantly the result of heat transfer from the
susceptor powder. Above 400°C [1− Tc/Td] increases as
the sample starts to couple and absorb microwave
energy. As heat transfer due to radiation, proportional
to T 4d, increases, above 800°C the temperature difference again decreases, leading to the observed bellshaped curve in the [1− Tc/Td] versus Td graph.
Heat transfer due to radiation can be calculated from
the appropriate values for the parameters in the radiation equation. The radiant heat loss from the sample
calculated from experiments as a function of the sample
temperature [Td] is shown in Fig. 7. As predicted, the
radiation loss increases rapidly above 800°C, but then
saturates as the temperature difference (Td − Tc) decreases at the highest temperatures.
effect was the same at all temperatures. Enhanced
densification in a microwave field was also observed for
ZrO2 –3% Y2O3 compacts by Nightingale et al.
[26].Even though there is still much to explain in microwave-field assisted densification, the effect appears
real and significant.
In the present work, specimen densities of 99% of the
theoretical value were achieved for YZA and about
95% for Al2O3 samples. Example of the various green
and sintered samples are given in Fig. 8. Uniform size
reduction (density) was observed when a batch of 15
Al2O3 samples were sintered together. The microstructure of the sintered YZA samples showed a uniform
distribution of the two phases (the Al2O3 grains are
dark, Fig. 9a). The grain size of the Al2O3 samples in
3.4. Sintering
In most reports on microwave sintering of Al2O3 or
ZrO2 ceramics, researchers have observed a ‘microwave
field effect’. Janney et al. [22] have sintered Al2O3 +
0.1% MgO compacts to densities of ca. 98% at 1200°C
using a 28 GHz microwave furnace, a temperature
nearly 300°C lower than that required for conventional
sintering. Using the ‘picket fence’ SiC susceptor arrangement, the same authors have reported [23] sintering of ZrO2 –8% Y2O3 compacts to densities above 99%
at 1195°C, ca. 150°C lower than the conventional firing
temperature. Fang et al. [24] have reported much reduced sintering times for hydroxyapatite ceramics
heated in a multimode cavity compared to conventional
sintering [25]. They showed that the same soaking time
and sintering temperature in the microwave field gave
higher densities and finer microstructures, and that this
Fig. 8. Sintered and unsintered samples of different composition used
in the present work.
P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
219
samples with different soaking times are shown in Fig.
10. The sintering temperature for YZA was 1340°C
(Fig. 5) with soaking times from 5 to 60 min. The
temperature of the Al2O3 sample was not recorded.
The faster sintering of the YZA samples is attributed
to greater microwave absorption in ZrO2 as compared
to Al2O3.
4. Conclusions
Fig. 9. Scanning electron micrographs of (a) YZA, r= 99%, (Al2O3
grains appear dark) and (b) Al2O3, r =95%.
Fig. 9b was about 1 mm, with no exaggerated grain
growth but some porosity at the triple junctions.
Microwave sintering curves for Al2O3 and YZA
Room temperature microwave absorption by silicon
carbide powder was utilised to construct a microwave
hybrid heating configuration. Partially oxidized SiC
powder proved a very satisfactory room temperature
preheater, generating reproducible power/time/temperature conditions. Since the SiC powder bed was continously oxidizing at high temperatures generated by
microwave energy, it could be considered as a compositionally changing ceramic system. It is suggested that
a model similar to the one proposed for silicon nitridation could be appropriate in explaining the observed
difference in the microwave absorption nature between
pure and partially oxidized SiC powder compacts.
Temperature measurements indicated that a zirconia
sample reached the highest temperature in the hybrid
system. The temperature increase of zirconia at low
temperatures (B 800°C) is primarily by thermal radiation from the susceptor. At higher temperatures radiation cooling of zirconia occurs, while the continuing
temperature increase is due to microwave power absorption by the zirconia. Using this simple hybrid
heating unit, it was possible to sinter small samples of
yttria stabilized zirconia and 99% alumina to densities
above 95%.
Fig. 10. Density change with sintering time for YZA (at 1340°C) and Al2O3 specimens.
220
P.D. Ramesh et al. / Materials Science and Engineering A266 (1999) 211–220
Acknowledgements
The authors acknowledge the Wolfson Centre for
Interface Science for providing a research fellowship
(PDR) and the Faculty of Materials Engineering for
providing the experimental facilities. The research was
supported by AFIRST. L. Schächter’s research on microwave ceramic sintering was supported by the Israeli
Ministry of Science.
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