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Materials Letters 59
Concentration gradient of solute ions within a-SiAlON grains
Hiroyuki Miyazaki*, Mark I. Jones, Kiyoshi Hirao
Synergy Materials Research Center, National Institute of Advanced Industrial Science and Technology, 2268-1 Shimo-shidami, Nagoya 463-8687, Japan
Received 19 December 2003; accepted 30 July 2004
Available online 2 October 2004
Abstract
The morphology and composition of a-SiAlON grains have been studied in a material prepared from a-Si3N4, AlN, Al2O3 and
Yb2O3 powders. TEM analysis has shown that some a-SiAlON grains contain an a-Si3N4 core, indicating that a-SiAlON grains are
nucleated from a-Si3N4 seed crystals. The initial precipitation on a-Si3N4 showed a higher content of Al and Yb than the subsequent
precipitation, indicating that the concentration of Al and Yb in the liquid increases temporarily at the initial stage of a-SiAlON
formation and decreases later.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Ceramics; Microstructure; SiAlON; Transmission electron microscopy
1. Introduction
a-SiAlON is the solid solution of a-Si3N4 with O, Al
and some other metal ions M. The general formula is
MxSi12-(m+n)Al(m+n)OnN16-n for a-SiAlON, where the
subscripts x , m and n are variables within their
respective solubility range and the relationship between
x and m is given by x=m/p where p is the valence of
the metal ion. A typical batch consists of a mixture of
Si3N4, AlN, Al2O3 and a metal oxide. The sintering
process begins with the native SiO2 present on the Si3N4
particles reacting with the oxide additives to form a
eutectic melt. SiAlON forms via a solution-reprecipitation
mechanism from an oxynitride melt, which is a transient
reaction product of the starting oxide and nitride powders
[1]. It is likely that the composition of the melt may vary
during the sintering according to a reaction sequence
between the oxide melt and nitride powders, which may
cause a compositional variation in the precipitated
SiAlON. The densification kinetics for h-SiAlON with
a-Si3N4, AlN, Al2O3 and Y2O3 as starting powders have
0167-577X/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.matlet.2004.07.045
* Corresponding author. Tel.: +81 52 736 7486; fax: +81 52 736 7405.
E-mail address: [email protected] (H. Miyazaki).
been delineated by Hwang and Chen [2]. They found
that AlN preferentially dissolves in the oxide melt
during the early stages of sintering and enriches the
melt composition in Al, triggering transient precipitation
of supersaturated h-SiAlON [2]. They attributed the
subsequent formation of h-SiAlON with a low Al(O)
content to the dilution of the Al concentration in the
melt due to the dissolution of Si3N4 at higher temper-
atures. Variation of the concentration of solute ions in
Y added a-SiAlON during sintering has also been
suggested by Sheu [3]. He found that X-ray diffraction
peaks of a-SiAlON shifted during the sintering from
low to higher 2h, indicating the formation of a-SiAlON
of different Al and/or Y concentrations at each stage of
the sintering. However, direct microscopic information of
the concentration of the ions was not reported in his
study. Although the morphology, composition and growth
defects of a-SiAlON have been examined by transmission
electron microscopy (TEM) [4,5], microscopic evidence
corresponding to the compositional variation of the
precipitated a-SiAlON has yet to be clarified. In this
study, using a TEM equipped with a thin-window
energy-dispersive spectroscope (EDS), we have directly
observed a variation in the concentration of solute ions
within Yb stabilized a-SiAlON grains. A possible
(2005) 44–47
Fig. 1. Bright field image showing an a-Si3N4 core within the a-SiAlON
grains. The diffraction patterns of the core and shell are shown in the insets
(B=[1̄21̄3]).
H. Miyazaki et al. / Materials Letters 59 (2005) 44–47 45
mechanism for this variation in concentration, from the
point of the reaction pathways in this system, is
proposed.
Fig. 2. EDS spectra taken from (a) a-Si3N4 core and (b) a-SiAlON shell. T
2. Experimental procedure
The target composition was a single phase a-SiAlON
material given by m=n=1.1 in the formula MxSi12-(m+n)
Al(m+n)OnN16-n, and the oxygen content of the nitride
powders was taken into consideration when calculating the
composition. The samples were produced by mixing
appropriate amounts of a-Si3N4 (E-10 grade, Ube Industries,
Japan), Al2O3 (AKP-50, Sumitomo Chemical, Japan), AlN
(F grade, Tokuyama, Japan) and Yb2O3 (99.9%, Nihon
Yttrium, Japan) in methanol using a Si3N4 pot and Si3N4
balls. The slurry was dried, and then passed through 125
mesh. The powders were hot-pressed at 1950 8C for 2 h with
an applied pressure of 40 MPa in a 0.9 MPa N2 atmosphere.
Foils for TEM were prepared from slices cut from the
sample. The slices were mechanically ground to less than
60 Am thick, followed by polishing, dimpling and ion
milling. A thin layer of carbon was evaporated onto the
foils to avoid surface charging under the electron beam.
Microscopy was performed using an analytical TEM
(JEOL 2010F, Japan) equipped with a thin-window
he core contains no Al, O or Yb. (C from the carbon coating film).
H. Miyazaki et al. / Materials Letters 59 (2005) 44–4746
energy-dispersive X-ray analysis (EDS, Thermo NORAN,
WI) system. EDS line scan analyses (Al, Yb, Si) were
performed within individual a-SiAlON grains using the
intensities of the Al-Ka, Yb-La1 and Si-Ka peaks. Due
to the overlapping of the Yb-Ma1 and Al-Ka and the
Yb-Mg peak and Si-Ka peaks, the width of the region of
interest (ROI) for both Al-Ka and Si-Ka peaks were
adjusted in order to minimize the contributions of Yb-
Ma1 on Al-Ka and Yb-Mg on Si-Ka. A line scan
consisted of 20 measurement points. The duration time of
each measurement point was 0.05 s and each line scan
was repeated 5000 times in order to obtain sufficient peak
intensity. In order to correct for possible drift of the foil
position during data collection, a computer program
(VISTA, Thermo NORAN, WI) was employed which
compensates for image drift.
Fig. 3. (a) STEM image showing a-SiAlON grains with an a-Si3N4 core.
EDS line scan analysis was conducted along with the straight line in the
figure. (b) EDS line scan which passes through the core.
3. Results and discussion
X-ray diffraction analysis revealed that the sample was
single phase a-SiAlON. Fig. 1 is a TEM micrograph
showing that one of the individual grains contains a core
with a contrast different from that of the surrounding shell
under bright-field (BF) imaging conditions. The selected
area diffraction patterns shown in the inset of this figure
indicate that the core and the shell have the same a-Si3N4
structure and crystallographic orientation. Fig. 2 shows the
EDS analysis of both the core and the shell. Whilst only
Si and N were seen in the EDS analysis of the core (Fig.
2a), analysis of the shell also indicated the presence of Al,
Yb and O (Fig. 2b). The presence of the strong electron-
scattering Yb in the shell explains the darker contrast of
this region when compared with the core region (Fig. 1).
The lack of elements other than Si and N in the core
identify it as Si3N4 whilst the shell can be identified as
SiAlON. These features are consistent with the core-shell
structure of Y-a-SiAlON reported by Fang-Fang et al. [4]
and Hwang and Chen [5]. They concluded that the
precipitation process proceeds by heterogeneous nuclea-
tion of a-SiAlON onto the a-Si3N4 particles. It is
reasonable to suppose that the a-SiAlON grains in the
present Yb system are also nucleated from a-Si3N4 seed
crystals.
Fig. 3(a) shows a STEM image of an a-SiAlON grain
containing an a-Si3N4 core. EDS line scan analysis was
performed on a line passing through the core as shown in
the figure. Fig. 3(b) shows the EDS line scan analysis.
The intensity of Al and Yb dropped suddenly at the core,
while the intensity of Si increased. The reason for
intensity of Al and Yb being not zero at the core was
attributed to signal contribution from the adjacent shell
beneath the core. The intensity of each line was affected
not only by the concentration of each element but also by
the thickness of the sample. In order to cancel the
contribution of the sample thickness, the intensity of the
Al and Yb were normalized to the Si intensity. These
relative normalized intensities are shown in Fig. 4. The
relative intensity of both Al and Yb in the regions
immediately adjacent to the core/shell interface was higher
than the value in the outer parts of the a-SiAlON grain.
This indicates that the initial precipitation on the a-Si3N4
seed is richer in Al and Yb content than subsequent
precipitation. From TEM observations, the initial precip-
itations were not separate phases but continuous with the
subsequent precipitation so that the concentration of Al
and Yb varied within the single grain.
The concentration gradients of Al and Yb within a-
SiAlON grains observed in this study is consistent with
the results of the progressive XRD study of phase
development during sintering of Y-a-SiAlON reported
by Sheu [3]. He found that a-SiAlON with larger lattice
constants is initially formed during the early stage of
phase development. He attributed the larger lattice
constants to the formation of a-SiAlON with higher
Fig. 4. Line scans (Al and Yb normalized by Si intensity) for a-SiAlON
grain containing an a-Si3N4 core.
H. Miyazaki et al. / Materials Letters 59 (2005) 44–47 47
concentration of Al and Y, because the lattice constants of
a-SiAlON increase with the m or n values in the Ym/3
Si12-(m+n)Al(m+n)OnN16-n formula.
The concentrations of Al and Yb in the oxide melt,
which is produced at the initial stage of sintering, are
relatively higher than that of Si. Extensive study of the
phase development of a-SiAlON during sintering with
various metal ions showed that the oxide melt in the Yb
containing system preferentially wets AlN at lower
temperatures and dissolution of Si3N4 occurs later [6]. It
is obvious that the melt retains a higher content of Al and
Yb than Si when the dissolution of AlN triggers the
precipitation of a-SiAlON since Si is not provided to the
melt from Si3N4 powder. It is therefore reasonable to
suppose that the mechanism for initial precipitation of Al-
and Yb-rich a-SiAlON on the a-Si3N4 seeds is due to the
melt composition being rich in Al and Yb. We believe
that Si3N4, dissolved at a later stage of sintering, dilutes
both the Al and Yb concentration in the melt and causes
the restoration of the concentration of Al and Yb during
subsequent precipitation.
This suggested mechanism is similar to that reported for
the variation of the solute composition within h-SiAlONgrains by Hwang and Chen [7]. They examined, by TEM
analysis, h-SiAlON grains grown from several fine-grained
Ym/3Si12-(m+n)Al(m+n)OnN16-n compositions with a-Si3N4,
AlN, Al2O3 and Y2O3 starting materials. In their report, it
was found that some of the h-SiAlON grains were nucleated
from h-Si3N4 seed crystals present in the a-Si3N4 starting
powder and that the content of Al and O during initial
precipitation on h-Si3N4 was higher. The reason for the
similarity of the mechanisms is probably that the reaction
pathway in the Yb–Si–Al–O–N system is similar to that in
the Y–Si–Al–O–N system [6].
4. Conclusion
TEM analysis suggested that a-SiAlON grains were
nucleated from a-Si3N4 seed crystals. The initial precip-
itation of a-SiAlON on the seed crystal was found to be rich
in Al and Yb, whereas a-SiAlON of lower Al and Yb
content was precipitated subsequently. The concentration
gradient of the solute ions within a-SiAlON was attributed
to the variation of the Al and Yb contents of the liquid
during sintering which resulted from the reaction sequence
between the melt and the nitride powders.
References
[1] T. Ekstrom, M. Nygren, J. Am. Ceram. Soc. 75 (1992) 259.
[2] S.-L. Hwang, I.-W. Chen, J. Am. Ceram. Soc. 77 (1994) 165.
[3] T.-S. Sheu, J. Am. Ceram. Soc. 77 (1994) 2345.
[4] X. Fang-Fang, W. Shu-Lin, L.-O. Nordberg, T. Ekstrom, J. Eur. Ceram.
Soc. 17 (1997) 1631.
[5] S.-L. Hwang, I.-W. Chen, J. Am. Ceram. Soc. 77 (1994) 1711.
[6] M. Menon, I.W. Chen, J. Am. Ceram. Soc. 78 (1995) 545.
[7] S.-L. Hwang, I.-W. Chen, J. Am. Ceram. Soc. 77 (1994) 1719.
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