Use of post-heat treatment to obtain a 2H solid solution in spark plasma sintering-processed AlN-SiC mixtures

Article abstract of DOI:10.1111/j.1551-2916.2008.02307.x

Dense aluminum nitride-silicon carbide (AlN-SiC) solid solutions with a 2H structure were fabricated by the post-heat treatment of dense AlN-SiC composites fabricated by spark plasma sintering (SPS). The changes in the relative density, microstructure, and phases present were investigated for the compositions of 25 mol% AlN-75 mol% SiC (AlN 25), 50 mol% AlN-50 mol% SiC (AlN 50), and 75 mol% AlN-25 mol% SiC (AlN 75). The AlN-SiC composites fabricated by the SPS were dense ceramics with fine microstructures and composed of 2H and 6H AlN-SiC solid solutions. The AlN 50 samples heat treated at 2200°C and the AlN 75 samples heat treated at 2100°C were also dense ceramics and composed of only the 2H AlN-SiC solid solution. In contrast, the AlN 25 samples heat treated at 2200°C were porous ceramics and composed of several AlN-SiC solid solutions (2H, 4H, 6H, and 15R), and the AlN 75 samples heat treated at 2200°C were decomposed into an Al melt. Dense AlN-SiC solid solutions composed of only the 2H phase can be obtained by controlling the heat-treatment temperatures, except for the AlN 25 sample.

Full text of DOI:10.1111/j.1551-2916.2008.02307.x

J. Am. Ceram. Soc., 91 [5] 1548–1552 (2008)
DOI: 10.1111/j.1551-2916.2008.02307.x
r 2008 The American Ceramic Society
ournal
J
Use of Post-heat Treatment to Obtain a 2H Solid Solution in Spark
Plasma Sintering-Processed AlN–SiC Mixtures
w
Ryota Kobayashi, Junichi Tatami, Toru Wakihara, Katsutoshi Komeya, and Takeshi Meguro
Graduate School of Environmental and Information Sciences, Yokohama National University, Hodogaya-ku,
Yokohama 240-8501, Japan
Takashi Goto and Rong Tu
Institute for Materials Research, Tohoku University, Aoba-ku, Sendai 980-8577, Japan
Dense aluminum nitride–silicon carbide (AlN–SiC) solid solu-
(HP) has been used to obtain AlN–SiC solid solutions. Howev-
er, firing at a high temperature (above 21001C) for a long time
(for 4 h or more) is required to obtain dense 2H AlN–SiC solid
solutions from powder mixtures of AlN and SiC with sintering
tions with a 2H structure were fabricated by the post-heat treat-
ment of dense AlN–SiC composites fabricated by spark plasma
sintering (SPS). The changes in the relative density, microstruc-
ture, and phases present were investigated for the compositions
of 25 mol% AlN–75 mol% SiC (AlN 25), 50 mol% AlN–50
mol% SiC (AlN 50), and 75 mol% AlN–25 mol% SiC (AlN
8,16
11
additives such as Y O .
2
Rafaniello et al. prepared AlN–SiC
3
solid solutions powders using the sol–gel process and sintered
them dense by HP, but these powders contained large amounts
of free carbon and silica. Although we also obtained 2H AlN–
SiC solid solutions at 20001C by firing for 1 h using pressureless
7
5). The AlN–SiC composites fabricated by the SPS were dense
ceramics with fine microstructures and composed of 2H and 6H
AlN–SiC solid solutions. The AlN 50 samples heat treated at
1
5
sintering (PLS) without sintering additives, dense ceramics
2
2001C and the AlN 75 samples heat treated at 21001C were
were not obtained. Even if sintering additives such as Y
2 3
O
also dense ceramics and composed of only the 2H AlN–SiC solid
solution. In contrast, the AlN 25 samples heat treated at 22001C
were porous ceramics and composed of several AlN–SiC solid
solutions (2H, 4H, 6H, and 15R), and the AlN 75 samples heat
treated at 22001C were decomposed into an Al melt. Dense
AlN–SiC solid solutions composed of only the 2H phase can be
obtained by controlling the heat-treatment temperatures, except
for the AlN 25 sample.
were used for the densification of AlN–SiC composites by PLS,
a long holding time was required to obtain AlN–SiC solid
1
7,18
solutions.
Furthermore, the sintering additives may affect
the microstructures and intrinsic properties such as electrical
conduction. It has been reported that spark plasma sintering
(SPS) can shorten the sintering time and lower the sintering
2 3
temperature of AlN–SiC composites with Y O as sintering ad-
1
9
20
ditive. Shirai et al. carried out low-temperature sintering of
SiC nanopowder with a small amount of AlN using SPS. We
have also reported that the SPS can provide dense AlN–SiC
composites without sintering additives, which have a fine mi-
crostructure with smaller grains but comprise several solid so-
I. Introduction
ILICON carbide (SiC) has excellent mechanical properties
and oxidation and corrosion resistance at high tempera-
S
2
1
6
lution phases. According to the phase diagram of AlN–SiC,
the SPSed dense AlN–SiC composites without additives should
be 2H single phase by heat treatment at a higher temperature.
Furthermore, the heat-treatment temperature and time are ex-
pected to be lower and shorter because a fine microstructure
should result in a higher rate of mass transfer.
tures. This implies that the SiC ceramics can be used for high-
temperature structural applications. In addition, SiC is a
semiconductor with a very wide bandgap (3.2 eV, which is
three times that of silicon). Therefore, SiC has a great po-
tential in high-performance semiconductor devices used in
power electronics. Aluminum nitride (AlN) has an excellent
thermal conductivity (320 W/mK for a single crystal, which is
comparable to that of Al) and excellent electrical insula-
In this study, we fabricated dense 2H AlN–SiC solid solutions
by SPS, followed by post-heat treatment. First, dense AlN–SiC
composites were fabricated by the SPS at a lower temperature
for a short sintering duration in order to obtain a finer micro-
structure. The dense 2H AlN–SiC solid solutions were then
fabricated by heat treatment up to 22001C.
1
,2
tion. Therefore, AlN has been used as a substrate materi-
al in power electronics. AlN is also used for manufacturing
semiconductors because of its high resistance to halogens
3
and plasma.
In 1978, Cutler et al. reported that AlN and SiC form a 2H
4
solid solution over a wide range of compositions. Rafaniello
et al. investigated the phase stabilities of the AlN–SiC solid so-
II. Experimental Procedure
5
AlN powder synthesized by the reduction–nitridation method
Grade F, Tokuyama Corp., Tokyo, Japan, mean particle
lutions and reported that spinodal decompositions occurred
when these solid solutions were annealed at lower temperatures.
Subsequently, Zangvil and Ruh presented a tentative AlN–SiC
(
6
size5 0.6 mm) and a-SiC powder synthesized by the Acheson
method (OY-20, Yakushima Denko Co. Ltd., Tokyo, Japan,
mean particle size5 0.55 mm) were used as the starting powders.
They were wet mixed in ethanol for 24 h using SiC balls and a
nylon pot. The starting compositions were 25 mol% AlN–75
mol% SiC (AlN 25), 50 mol% AlN–50 mol% SiC (AlN 50),
and 75 mol% AlN–25 mol% SiC (AlN 75). After drying, the
mixtures were sieved using a 250-mm nylon mesh to form gran-
ules. The granules wereplaced in graphite molds (inner diame-
ter 5 +25 mm) lined with a graphite foil. SPS was performed at
phase diagram. AlN–SiC solid solutions have potential applica-
7
–10
tions as not only structural materials
materials.
but also new functional
However, it is difficult to fabricate dense 2H
1
1–15
AlN–SiC solid solutions due to the high covalence and low
diffusion coefficients of AlN and SiC. In general, hot pressing
T. Parthasarathy—contributing editor
2
0001C for 10 min under an Ar atmosphere at 1 atm. A constant
uniaxial pressure of 30 MPa was applied during the SPS. After
the holding time, the power supply was shut down and the
Manuscript No. 23127. Received April 24, 2007; approved December 21, 2007.
Author to whom correspondence should be addressed. e-mail: tatami@ynu.ac.jp
w
1
548

May 2008
Use of Post-heat Treatment to Obtain 2H Solid Solution in AIN–SiC Mixtures
1549
Fig. 1. Relative densities of the SPSed and HTed samples.
Fig. 2. AlN contents of the SPSed and HTed samples. AlNinit means
the initial contents of AlN in the samples.
are light elements and the starting materials have high purity.
Thus, we can consider a change in the AlN content of a sample as
a change in its Al content. The calibration curves for determining
the Al contents of the samples were obtained using the mixtures
of raw powders of AlN and SiC. Their microstructures were
evaluated using a scanning electron microscope (SEM). The sam-
ples for the evaluation of the microstructures were polished with a
diamond paste (2–4 mm), and then chemically etched by boiling
Murakami’s reagent (10 g of K Fe(CN) , 10 g of KOH, and 100 g
uniaxial pressure was released immediately. Using a diamond
wheel and cutter, the sintered samples were cut into 3 mmÂ
3
mm  20 mm test pieces. These test pieces were placed in a
carbon crucible and molded in a packing powder with the same
composition as the test piece in order to prevent a composition
shift caused by evaporation or decomposition. They were heat
treated at 20001–22001C for 3 h under an Ar flow using an
induction-heating furnace to obtain the AlN–SiC solid solu-
tions. Subsequently, the surfaces of the heat treated samples
were ground to remove the adhered packing powder using a
diamond buffer.
3
6
of water). The phases present were evaluated using an X-ray
diffractometer with CuKa radiation.
We evaluated the relative densities, compositions, microstruc-
tures, and phases present in the sintered and heat treated samples.
The relative densities were measured using the Archimedes meth-
od. Determining the compositions (AlN content) of the samples,
elemental analyses were carried out by X-ray fluorescent analysis
III. Results
(
1) Relative Densities
Figure 1 shows the relative densities of the samples. For all the
compositions, the relative densities of the samples sintered at
20001C were over 99.5% of the theoretical densities; this is the
(XRF). Although Al and Si were detected by XRF, the anions (C,
N) and other impurities were not detected because these anions
Fig. 3. SEM images of the AlN 25 samples: (a) SPSed at 20001C for 10 min. (b) SPS-HTed at 21001C for 3 h. (c) SPS-HTed at 22001C for 3 h.

1
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Journal of the American Ceramic Society—Kobayashi et al.
Vol. 91, No. 5
Fig. 4. SEM images of the AlN 50 samples: (a) SPSed at 20001C for 10 min. (b) SPS-HTed at 22001C for 3 h.
same result as that obtained in our previous study (AlN 25:
9
and the AlN/AlNinit ratios of the AlN 25 samples decreased to
almost 0.4.
2
9.8%, AlN 50: 99.2%, and AlN 75: 99.6%). The samples heat
1
treated at 20001C were also dense materials (AlN 25: 98.7%,
AlN 50: 99.2%, and AlN 75: 99.3%). The relative densities of
the samples heat treated at 21001C decreased slightly, except for
the AlN 50 samples (AlN 25: 96.6%, AlN 50: 99.2%, and
AlN 75: 98.8%). After heat treatment at 22001C, the change
in the relative densities depended on the composition (AlN 25:
(
3) Microstructures
Figures 3–5 present SEM images of the polished and chemically
etched surfaces of the sintered and heat treated samples. Note
that the SEM image of the AlN 25 sample heat treated at
2
2001C was taken from the fractured surface. Although the
6
7.7% and AlN 50: 99.2%). Note that we could not evaluate the
samples sintered at 20001C had dense textures, their grain sizes
were different for different compositions. The AlN 25 and AlN
AlN 75 sample heat treated at 22001C because the sample
decomposed and the Al melt appeared. Although the weight
loss caused by the adhesion of the packing powders was not
measured, it is considered to be below 5% of that in our pre-
5
1
0 samples were composed of small grains (approximately
mm); in contrast, the AlN 75 sample was composed of larger
1
5
grains (approximately 2 mm). The samples heat treated at
0001C were dense ceramics, and the grain sizes were almost
vious study.
2
the same as those of the sintered samples. In contrast, the mi-
crostructures of the samples heat treated above 21001C were
different for different compositions. The AlN 25 and AlN 75
samples heat treated at 21001C were dense ceramics with slightly
larger grains than the samples sintered at 20001C (AlN 25:
approximately 2 mm, AlN 75: approximately 5 mm). Although
the AlN 50 samples heat treated at 22001C were also dense ce-
ramics, the samples were composed of large grains (approxi-
mately 10 mm). The AlN 25 samples heat treated at 22001C were
porous ceramics with large grains (approximately 10 mm).
(
2) Composition
Figure 2 shows the changes in the AlN contents of the samples
measured by XRF. In Fig. 2, the quantity AlN/AlNinit on the
y-axis denotes the ratio of the Al content of a sample after the
SPS or the heat treatment process to its initial AlN content
(
AlNinit). The AlN contents of the samples sintered at 20001C
were almost the same as those in the starting compositions (AlN
5: 26.0 mol%, AlN 50: 49.4 mol%, and AlN 75: 76.0 mol%);
2
therefore, the AlN/AlNinit ratios of the samples were close to 1.
The AlN contents of the samples heat treated at 20001C were
also close to their starting compositions (AlN 25: 25.0 mol%,
AlN 50: 51.6 mol%, and AlN 75: 76.3 mol%). Further, the AlN
contents of the samples heat treated at 21001C were also close to
the starting compositions, except for the AlN 25 sample (AlN
(
4) Phases Present
Figures 6–8 show the phases present in the samples. The phases
present in the samples sintered at 20001C were 6H and 2H; this
implied that the AlN and SiC did not form the AlN–SiC solid
solutions but formed the AlN–SiC composites. The phases pres-
ent in the samples heat treated at 20001C were also 6H and 2H,
and the diffraction intensity of the 6H phase was not changed.
The phases present in the samples heat treated at 21001C were
2
5: 16.9 mol%, AlN 50: 51.7 mol%, and AlN 75: 75.9 mol%),
and the AlN/AlNinit ratios of the AlN 25 samples decreased
below 0.7. Subsequently, the AlN contents of the samples heat
treated at 22001C were significantly different from their starting
compositions (AlN 25: 10.8 mol% and AlN 50: 52.3 mol%),
Fig. 5. SEM images of the AlN 75 samples: (a) SPSed at 20001C for 10 min. (b) SPS-HTed at 21001C for 3 h.

May 2008
Use of Post-heat Treatment to Obtain 2H Solid Solution in AIN–SiC Mixtures
1551
tive densities is mainly caused by the loss of AlN. Figures 1 and
show that the relative densities of the AlN 25 samples de-
2
creased with the loss of Al. This result indicates that the de-
composition or evaporation of AlN occurs in the AlN 25
sample. On the other hand, both the relative densities and the
AlN contents are substantially constant in the AlN 50 and AlN
75 samples. These results suggest that no decomposition or
evaporation of AlN occurs in these samples. The slight increase
in the AlN content of the AlN 50 sample is due to the incorpo-
ration of Al from packing materials.
We observed that the microstructures changed with the com-
positions and heat treatment temperatures. The grain sizes of the
AlN 25 sample drastically increased at 22001C, which changed
inversely with a decrease in the relative density. In contrast, the
grain sizes of the AlN 50 sample drastically increased at 22001C
without decreases in the relative densities. This tendency is also
shown in the AlN 75 sample heat treated at 21001C. Because the
sintered samples had dense and fine textures, it is expected that
the heat treated samples should also have dense textures if AlN
or SiC do not decompose. The typical grain growth mechanism
in the AlN 25 sample is stated as below. First, the open pores are
formed by the decomposition of the AlN–SiC solid solutions
and evaporation of AlN from the sample. Next, the evaporated
AlN is condensed on the grains of the residual AlN–SiC solid
solutions. The grain sizes of the AlN 25 samples increased due to
the repetition of these processes. The grain growths in the AlN
Fig. 6. XRD patterns of the SPSed and HTed AlN 25 samples: (a)
SPSed at 20001C for 10 min. (b) SPS-HTed at 20001C for 3 h. (c) SPS-
HTed at 21001C for 3 h. (d) SPS-HTed at 22001C for 3.
50 and AlN 75 samples might be driven not by evaporation–
condensation but by solid-phase diffusion because the textures
of the samples are dense at the heat treatment temperatures. As
1
5
we showed in our previous study, the differences of the grain
sizes between the AlN 50 and AlN 75 samples could be ex-
plained by the difference of the driving force in the formation of
the solid solutions.
Fig. 7. XRD patterns of the SPSed and HTed AlN 50 samples: (a)
SPSed at 20001C for 10 min. (b) SPS-HTed at 20001C for 3 h. (c) SPS-
HTed at 21001C for 3 h. (d) SPS-HTed at 22001C for 3 h.
V. Summary
different for different compositions. For the AlN 25 and AlN 50
samples, the 6H and 2H phases were also found, and the diffrac-
tion intensity of the 6H phase was slightly decreased in the AlN
In this paper, we fabricated dense AlN–SiC solid solutions by
the post-heat treatment of dense AlN–SiC composites and dis-
cussed the stability of AlN–SiC solid solutions. This paper can
be summarized as follows:
5
0 sample. In contrast, the AlN 75 sample was composed of only
the 2H phase. The phases present in the samples heat treated at
2001C changed drastically. Although the AlN 50 sample was
(
1) We obtained dense AlN–SiC solid solutions with fine
2
microstructures composed of only the 2H phase, except for the
AlN 25 sample.
composed of only the 2H phase similar to the AlN 75 sample
heat treated at 21001C, the AlN 25 sample was mixture of 6H,
(
2) The optimum heat treatment temperatures to fabricate
2H, 4H, and 15R phases.
dense 2H AlN–SiC solid solutions are 22001 and 21001C for the
AlN 50 and AlN 75 samples, respectively.
(3) The decomposition of the heat treated samples was re-
strained by the dense texture of the sintered samples.
IV. Discussion
In this study, the relative densities correlated with the AlN
contents. The phase stability calculations of AlN and SiC show
that 14 mol% of AlN and 0.2 mol% of SiC are decomposed at
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