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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: h-miyazaki@aist.go.jp (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

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[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.