# The Preparation and Characterization ofβ-SiAlON Nanostructure Whiskers

Authors: Pengli Dong; Xidong Wang; Mei Zhang; Min Guo; Seshadri Seetharaman
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

Two kinds ofβ-SiAlON nanostructure whiskers, rod-like and wool-like whiskers, were synthesized by pressure-less sintering method at 1773 K for 5 hours. The whiskers synthesized were characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and high-resolution electron microscope (HREM) techniques. It was found that diameter distribution of rod-like whiskers was about 80–250 nm, while it was about 45–55 nm in diameter for the wool-like whiskers. The growth mechanisms of β-SiAlON nanostructure whiskers are discussed by the
vapor-solid (VS) and vapor-liquid-solid (VLS)
mechanisms, respectively.


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## Body

## 1. Introduction

β-SiAlON is the solid solution of β-Si3N4, Al2O3, and AlN. The composition of β-SiAlON can be represented by the formula Si6-ZAlZOZN8-Z,
and the Z value varies constantly from zero to 4.2 [1, 2]. Hexagonal β-SiAlON
has a crystalline structure similar to β-Si3N4 and
exhibits excellent properties such as good corrosion resistance, antioxidation,
and high fracture toughness [3–6]. Therefore, β-SiAlON can be used
as refractory, cutter material, and high performance ceramics. It can also be
made into medical instruments, oxidation resistant, and corrosion resistant
materials at high temperature in aerospace [7–12].It is well known that nanowhiskers and fibers are significant in improving the
strength and toughness of composite structural materials, owing to their small
particle size and the high-aspect ratio [13–18]. β-SiAlON whiskers inserted
into matrix are expected to increase the fracture toughness of β-SiAlON composite
ceramics. To the knowledge of the authors, there are few works dealing with the
synthesis of abundant of β-SiAlON whiskers with nanostructure and its growth
mechanism. Jia et al. prepared a few β-SiAlON particles of about
30 nm in diameter using organic precursors [19]. The results show
that a few novel rod-like β-SiAlON whiskers were found at the surface of α-SiAlON
(5%) and β-SiAlON (95%) composites. They also did related analysis for α-SiAlON
whiskers [20–22]. Fu et al. [23] synthesized β-SiAlON whiskers by a combustion method, using of
silicon and aluminum powders as raw materials, with SiO2 and Y2O3 as additives and α-Si3N4 or β-SiAlON powders as diluents.
SEM results showed that the aspect ratio of whiskers was higher than 5. Dickon
fabricated single crystalline β-SiAlON nanowires with 100–500 nm in diameter
by sintering powder mixture of Al, Si, and SiO2 at 1873 K [24]. Chen synthesized β-SiAlON whiskers from silica fume and α-alumina
via carbothermal reduction and which formed by VLS growth mechanism involving
SiO, FeCl3, and other vapor species [25]. Other research works on α-SiAlON or β-SiAlON whiskers can be referred
to from references [26, 27].In the present paper, large quantity ofβ-SiAlON nanostructure whiskers were synthesized
by the pressure-less sintering technique under the proper conditions. The phase
composition and morphology of β-SiAlON nanostructure whiskers were investigated
by the XRD, SEM, TEM, HREM techniques. The growth mechanisms of the rod-like
and wool-like whiskers were  discussed.


## 2. Experiment

### 2.1. Preparation

Silicon (≥99.9%, 3.256μm, C.R., Beijing
Chemical Co., Beijing, China), aluminum (≥99.5%, 3.360 μm, C.R.,
Shanghai, China), and alumina (≥99%, 3.873 μm, A.R., Guangdong,
China) powders were used as raw materials to synthesize β-SiAlON (with the
formula Si6-ZAlZOZN8-Z) nanostructure
whiskers by pressure-less sintering technique. The starting materials with
proper ratio of Z=1.3 [mol (Al2O3 + AlN)/mol Si3N4 = 1.3:4.7]
were milled for 24 hours by using absolute ethyl alcohol as the medium. The
slurry was dried and pressed into pallets of 13 mm in diameter and 3.0 mm in
thickness at the pressure of 100 MPa; the pallets were placed
into an alumina crucible, covered with silicon nitride powders, sieved by 300-mesh,
and then sintered at 1773 K for 5 hours. The nitrogen gas with high purity was
introduced into alumina reaction tube, and the partial pressure was maintained at
0.1 MPa by keeping the flow rate of 1 mL/min.


### 2.2. Phase and Microstructure Characterization

The phase composition of the obtained products was identified by X-ray diffraction
measurement on a M21X-SRA X-ray diffractometer (manufactured by MAC Science Co. Ltd, Yokohama, Japan) equipped with graphite crystal
diffracted-beam monochromator. The accelerating voltage and current are 40 kV
and 150 mA, respectively. Scanning speed is 8°/min,
step wise is 0.02°, and scanning region is from 10° to 80°.Scanning electron microscope (ZJIESS CSM 950, Carl Zeiss Co.,
Oberkochen, Germany) was used to observe the morphology
and the particle size of the prepared samples by scanning on the cross-sections
of fracture. Amorphous carbon was sputtered on the section before SEM measurement.Both transmission electron microscope (HITACHI H-8100, Hitachi, Tokyo, Japan) and high
resolution electron microscope (JEM2010, Joel Ltd., Tokyo, Japan) operating at 200 kV are
used to certificate the phase and crystal morphology of the products.


## 2.1. Preparation

Silicon (≥99.9%, 3.256μm, C.R., Beijing
Chemical Co., Beijing, China), aluminum (≥99.5%, 3.360 μm, C.R.,
Shanghai, China), and alumina (≥99%, 3.873 μm, A.R., Guangdong,
China) powders were used as raw materials to synthesize β-SiAlON (with the
formula Si6-ZAlZOZN8-Z) nanostructure
whiskers by pressure-less sintering technique. The starting materials with
proper ratio of Z=1.3 [mol (Al2O3 + AlN)/mol Si3N4 = 1.3:4.7]
were milled for 24 hours by using absolute ethyl alcohol as the medium. The
slurry was dried and pressed into pallets of 13 mm in diameter and 3.0 mm in
thickness at the pressure of 100 MPa; the pallets were placed
into an alumina crucible, covered with silicon nitride powders, sieved by 300-mesh,
and then sintered at 1773 K for 5 hours. The nitrogen gas with high purity was
introduced into alumina reaction tube, and the partial pressure was maintained at
0.1 MPa by keeping the flow rate of 1 mL/min.


## 2.2. Phase and Microstructure Characterization

The phase composition of the obtained products was identified by X-ray diffraction
measurement on a M21X-SRA X-ray diffractometer (manufactured by MAC Science Co. Ltd, Yokohama, Japan) equipped with graphite crystal
diffracted-beam monochromator. The accelerating voltage and current are 40 kV
and 150 mA, respectively. Scanning speed is 8°/min,
step wise is 0.02°, and scanning region is from 10° to 80°.Scanning electron microscope (ZJIESS CSM 950, Carl Zeiss Co.,
Oberkochen, Germany) was used to observe the morphology
and the particle size of the prepared samples by scanning on the cross-sections
of fracture. Amorphous carbon was sputtered on the section before SEM measurement.Both transmission electron microscope (HITACHI H-8100, Hitachi, Tokyo, Japan) and high
resolution electron microscope (JEM2010, Joel Ltd., Tokyo, Japan) operating at 200 kV are
used to certificate the phase and crystal morphology of the products.


## 3. Results and Discussions

### 3.1. Composition Analysis

Figure1 shows the XRD patterns of β-SiAlON prepared by pressure-less sinter technique
at 1773 K for 5 hours. The diffraction pattern is in agreement with the data
reported by the JCPDS (48–1615) card. The
spectra identified that β-SiAlON is the main phase, and no impurities were
found within the analysis uncertainty limits. The sharper peaks of diffraction
illustrated that β-SiAlON presents
a high degree of crystallization.Figure 1
XRD pattern ofβ-SiAlON (Z=1.3) nanostructure whiskers prepared by pressure-less sintering
technique at 1773 K for 5 hours.


### 3.2. Morphology and Microstructure Observation

Two kinds of whiskers have been observed in the present products. It can be seen in
Figure2 that most whiskers illustrated in Figure 2(a) were straight and column-shaped
crystals which were denoted as rod-like whiskers. This kind of whiskers has a
wider distribution of diameter, and it is between 80 and 250 nm with high-aspect
ratio. In the small photo on the left bottom in the Figure 2(a), a spheric node
can be seen on the tip of rod-like whiskers, which might be the evidence for
the VLS mechanism of the rod-like whiskers. In Figure 2(b), a large quantity of
whiskers with smaller diameter and flexible microstructure denoted as wool-like
whiskers are also observed. The diameter of this kind of whiskers is quite
homogeneous and it is about 45–55 nm. The two kinds of whiskers interlaced each
other, which would improve the strength and toughness of the material.SEM micrographs ofβ-SiAlON nanostructure whiskers prepared by using silicon,
aluminum, and alumina powders as raw materials. (a) Most of rod-like
whiskers and (b) most of wool-like whiskers magnified image of rod-like whisker
on the left bottom of Figure 2(a)).
(a)(b)TEM micrograph ofβ-SiAlON nanostructure whiskers together with results of the
selected area election diffraction (SAED) investigation is shown in Figure 3.
The straight column-shaped whiskers with 160 nm in diameter could be observed
in Figure 3(a), their aspect ratio having reached more than 40. The diffraction
pattern, marked dots on the body, indicated that the nanostructure whisker is a
single crystal and it grew along the c-axis, corresponding to the single β-SiAlON
structure. At the same time, the diffractions on the tip of β-SiAlON
nanostructure whiskers show that they are multiphase crystal diffraction rings.
This phenomenon might result from the VLS growth mechanism. Figure 3(b) exhibited
a series of wool-like whiskers with high-aspect ratio intercross together. The
diffraction spots for the tip and end of whisker were proved to have the same
component corresponding to single-crystals of β-SiAlON, which is likely to explain
that the growth mechanism of wool-like whiskers is based on VS mechanism. EDS
analysis for one wool-like whisker carried out and it certificated that the
whiskers are β-SiAlON phase, and it can be seen in Figure 4. The surface of two
kinds of whiskers is very smooth during β-SiAlON whiskers growing process. HREM
was also employed to analyze the structure of the single crystal wool-like β-SiAlON
nanostructure whiskers. The results are shown in Figure 5. Lattice image indicated
that the plane distance of single β-SiAlON material is about 0.662 nm, corresponding
to (1000) plane of β-SiAlON.TEM micrographs ofβ-SiAlON nanostructure whiskers with the corresponding SAEDs analysis.
(a) Rod-like whiskers, (1) SAEDs analysis for the top of rod-like whiskers and diffraction rings include
Si, Al, and β-SiAlON amorphous, (2) SAEDs analysis for the top of rod-like
whiskers corresponding to single β-SiAlON, (b) wool-like whiskers and
diffraction spots for the top of wool-like whiskers corresponding to single
β-SiAlON).
(a)(b)Figure 4
EDS analysis for one wool-like whisker corresponding to TEM analysis.Figure 5
HREM results of wool-likeβ-SiAlON
nanostructure whiskers and plane distance are about 0.662 nm corresponding to
(1000) plane of β-SiAlON.


### 3.3. Mechanism of Whisker Growth

In the present work, vapor-solid (VS) and vapor-liquid-solid (VLS) mechanisms [28–30] were used to explain the growth mechanism of wool-like and rod-like whiskers
during sinter process, respectively.The vapor-solid (VS) mechanism, for the wool like whiskers, was assumed as the main
growth mechanism. The main reactions were listed from (1) to (3), and growth
schematic diagram of whisker is illustrated in Figure 6 for part I. At the starting
reaction step, silicon, aluminum, and alumina reacted and formed β-SiAlON seeds
under the flowing nitrogen atmosphere as (1). These β-SiAlON seeds act as
active centers and accelerate the growth of whiskers:(1)Si(s)+Al(s)+Al2O3(s)+N2(g)→β-SiAlON(seeds). The partial evaporation of silicon monoxide (SiO)
and aluminum suboxide (Al2O) is likely to be formed as (2), owing
to the low oxygen partial pressure under the reactant condition:(2)2Si(l)+Al2O3(s)=2SiO(g)+Al2O(g),4Al(l)+Al2O3(s)=3Al2O(g). The above vapor phases, SiO and Al2O,
react with flowing nitrogen gas and deposited on the preferential growth plane
of β-SiAlON seeds. With the continuous cycle, the β-SiAlON nanostructure whiskers,
with high-aspect ratio, finally formed. The reaction can be described as
(3)SiO(g)+Al2O(g)+N2(g)→β-SiAlON.seeds→β-SiAlON.particlesβ-SiAlON(whiskers).Figure 6
Simplified sketch of growth mechanism ofβ-SiAlON whiskers. (Process I is the
vapor-solid (VS) mechanism for formation of rod-like β-SiAlON whiskers, process
II is the vapor-liquid-solid (VLS) mechanism for formation of rod-like β-SiAlON
whiskers).The rod-like whiskers may form through a vapor-liquid-solid
(VLS) process. Mechanism analysis sketch diagram could be seen in Figure6 for
part II, and the procedure is described as follows. Based on reactions (2), the vapor phases may reacted with nitrogen and dissolve into the spherical
liquid droplets of silicon and aluminum and some impurities, although the content
of impurities is very small to form β-SiAlON containing liquid. When the content
of β-SiAlON in droplet became supersaturated, a new complete β-SiAlON crystalline
particle would precipitate from the droplet. With the procedure getting
repeated, rod-like β-SiAlON whiskers would grow up as(4)SiO(g)+Al2O(g)+N2→dissolvedin[Al],[Si]supersaturatedβ-SiAlON(s).A lot of spheric nodes on the tip of rod-like whiskers also
proved that VLS growth mechanism is interested for rod-like whiskers.


## 3.1. Composition Analysis

Figure1 shows the XRD patterns of β-SiAlON prepared by pressure-less sinter technique
at 1773 K for 5 hours. The diffraction pattern is in agreement with the data
reported by the JCPDS (48–1615) card. The
spectra identified that β-SiAlON is the main phase, and no impurities were
found within the analysis uncertainty limits. The sharper peaks of diffraction
illustrated that β-SiAlON presents
a high degree of crystallization.Figure 1
XRD pattern ofβ-SiAlON (Z=1.3) nanostructure whiskers prepared by pressure-less sintering
technique at 1773 K for 5 hours.


## 3.2. Morphology and Microstructure Observation

Two kinds of whiskers have been observed in the present products. It can be seen in
Figure2 that most whiskers illustrated in Figure 2(a) were straight and column-shaped
crystals which were denoted as rod-like whiskers. This kind of whiskers has a
wider distribution of diameter, and it is between 80 and 250 nm with high-aspect
ratio. In the small photo on the left bottom in the Figure 2(a), a spheric node
can be seen on the tip of rod-like whiskers, which might be the evidence for
the VLS mechanism of the rod-like whiskers. In Figure 2(b), a large quantity of
whiskers with smaller diameter and flexible microstructure denoted as wool-like
whiskers are also observed. The diameter of this kind of whiskers is quite
homogeneous and it is about 45–55 nm. The two kinds of whiskers interlaced each
other, which would improve the strength and toughness of the material.SEM micrographs ofβ-SiAlON nanostructure whiskers prepared by using silicon,
aluminum, and alumina powders as raw materials. (a) Most of rod-like
whiskers and (b) most of wool-like whiskers magnified image of rod-like whisker
on the left bottom of Figure 2(a)).
(a)(b)TEM micrograph ofβ-SiAlON nanostructure whiskers together with results of the
selected area election diffraction (SAED) investigation is shown in Figure 3.
The straight column-shaped whiskers with 160 nm in diameter could be observed
in Figure 3(a), their aspect ratio having reached more than 40. The diffraction
pattern, marked dots on the body, indicated that the nanostructure whisker is a
single crystal and it grew along the c-axis, corresponding to the single β-SiAlON
structure. At the same time, the diffractions on the tip of β-SiAlON
nanostructure whiskers show that they are multiphase crystal diffraction rings.
This phenomenon might result from the VLS growth mechanism. Figure 3(b) exhibited
a series of wool-like whiskers with high-aspect ratio intercross together. The
diffraction spots for the tip and end of whisker were proved to have the same
component corresponding to single-crystals of β-SiAlON, which is likely to explain
that the growth mechanism of wool-like whiskers is based on VS mechanism. EDS
analysis for one wool-like whisker carried out and it certificated that the
whiskers are β-SiAlON phase, and it can be seen in Figure 4. The surface of two
kinds of whiskers is very smooth during β-SiAlON whiskers growing process. HREM
was also employed to analyze the structure of the single crystal wool-like β-SiAlON
nanostructure whiskers. The results are shown in Figure 5. Lattice image indicated
that the plane distance of single β-SiAlON material is about 0.662 nm, corresponding
to (1000) plane of β-SiAlON.TEM micrographs ofβ-SiAlON nanostructure whiskers with the corresponding SAEDs analysis.
(a) Rod-like whiskers, (1) SAEDs analysis for the top of rod-like whiskers and diffraction rings include
Si, Al, and β-SiAlON amorphous, (2) SAEDs analysis for the top of rod-like
whiskers corresponding to single β-SiAlON, (b) wool-like whiskers and
diffraction spots for the top of wool-like whiskers corresponding to single
β-SiAlON).
(a)(b)Figure 4
EDS analysis for one wool-like whisker corresponding to TEM analysis.Figure 5
HREM results of wool-likeβ-SiAlON
nanostructure whiskers and plane distance are about 0.662 nm corresponding to
(1000) plane of β-SiAlON.


## 3.3. Mechanism of Whisker Growth

In the present work, vapor-solid (VS) and vapor-liquid-solid (VLS) mechanisms [28–30] were used to explain the growth mechanism of wool-like and rod-like whiskers
during sinter process, respectively.The vapor-solid (VS) mechanism, for the wool like whiskers, was assumed as the main
growth mechanism. The main reactions were listed from (1) to (3), and growth
schematic diagram of whisker is illustrated in Figure 6 for part I. At the starting
reaction step, silicon, aluminum, and alumina reacted and formed β-SiAlON seeds
under the flowing nitrogen atmosphere as (1). These β-SiAlON seeds act as
active centers and accelerate the growth of whiskers:(1)Si(s)+Al(s)+Al2O3(s)+N2(g)→β-SiAlON(seeds). The partial evaporation of silicon monoxide (SiO)
and aluminum suboxide (Al2O) is likely to be formed as (2), owing
to the low oxygen partial pressure under the reactant condition:(2)2Si(l)+Al2O3(s)=2SiO(g)+Al2O(g),4Al(l)+Al2O3(s)=3Al2O(g). The above vapor phases, SiO and Al2O,
react with flowing nitrogen gas and deposited on the preferential growth plane
of β-SiAlON seeds. With the continuous cycle, the β-SiAlON nanostructure whiskers,
with high-aspect ratio, finally formed. The reaction can be described as
(3)SiO(g)+Al2O(g)+N2(g)→β-SiAlON.seeds→β-SiAlON.particlesβ-SiAlON(whiskers).Figure 6
Simplified sketch of growth mechanism ofβ-SiAlON whiskers. (Process I is the
vapor-solid (VS) mechanism for formation of rod-like β-SiAlON whiskers, process
II is the vapor-liquid-solid (VLS) mechanism for formation of rod-like β-SiAlON
whiskers).The rod-like whiskers may form through a vapor-liquid-solid
(VLS) process. Mechanism analysis sketch diagram could be seen in Figure6 for
part II, and the procedure is described as follows. Based on reactions (2), the vapor phases may reacted with nitrogen and dissolve into the spherical
liquid droplets of silicon and aluminum and some impurities, although the content
of impurities is very small to form β-SiAlON containing liquid. When the content
of β-SiAlON in droplet became supersaturated, a new complete β-SiAlON crystalline
particle would precipitate from the droplet. With the procedure getting
repeated, rod-like β-SiAlON whiskers would grow up as(4)SiO(g)+Al2O(g)+N2→dissolvedin[Al],[Si]supersaturatedβ-SiAlON(s).A lot of spheric nodes on the tip of rod-like whiskers also
proved that VLS growth mechanism is interested for rod-like whiskers.


## 4. Conclusions

β-SiAlON
nanostructure whiskers were synthesized by pressure-less sintering method at 1773 K
for 5 hours under the nitrogen gas with pressure of 0.1 MPa. There were two
kinds of whiskers, rod-like nanostructure whiskers and wool-like structure,
produced during the process. It was found that the diameter distribution of
rod-like whiskers was about 80–250 nm, and most
of them are straight crystals. However, the diameter of wool-like whiskers is
relatively narrow and homogeneous, about 45–55 nm. These two
structures which interlaced each other might form a designing reinforced
structural material. The growth mechanism of the rod-like and wool-like
structure of β-SiAlON has been investigated through a series means of
microstructure and phase analyses with XRD, SEM, TEM, and HREM.



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*Source: 282187-2008-10-09.xml*