Rapid crystallization process of amorphous silicon nitride

Article abstract of DOI:10.1111/j.1551-2916.2011.04914.x

The crystallization of nanosized amorphous silicon nitride powder is one of the methods to produce sub-micrometer/nanosized α-Si₃N ₄ powder. The application of this method is still limited by the long crystallization time, low output, and high cost. This article invents a new crystallization method of amorphous silicon nitride involving the addition of Si powder. The new process reduces the complete crystallization time from more than 6 h to 30 min, thus allowing efficient production of sub-micrometer/ nanosized α-Si₃N₄ powder. Effects of factors such as additive, temperature, and duration on the crystallization process are investigated using XRD and FTIR. The experimental results showed that the added Si powder accelerates the crystallization process effectively. The final product is a mixture of α-Si₃N₄ and Si₂N ₂O. In this article, amorphous silicon nitride powder with added Si is annealed at 1450°C. Powder of nearly 100% crystalline phase content is produced either by adding 10% Si and annealing for 15 min or by adding 5% Si and annealing for 30 min.

Full text of DOI:10.1111/j.1551-2916.2011.04914.x

J. Am. Ceram. Soc., 94 [12] 4169–4173 (2011)
DOI: 10.1111/j.1551-2916.2011.04914.x
© 2011 The American Ceramic Society
ournal
J
Rapid Crystallization Process of Amorphous Silicon Nitride
Yanhui Li,‡ Li Wang,‡,† Shaowu Yin,‡ Fuming Yang,‡ and Ping Wu§
‡School of Mechanical Engineering, University of Science & Technology Beijing, Beijing, 100083 China
§Department of Physics, University of Science & Technology Beijing, Beijing, 100083 China
The crystallization of nanosized amorphous silicon nitride pow-
der is one of the methods to produce sub-micrometer/nanosized
a-Si3N4 powder. The application of this method is still limited
by the long crystallization time, low output, and high cost. This
article invents a new crystallization method of amorphous sili-
con nitride involving the addition of Si powder. The new pro-
cess reduces the complete crystallization time from more than
6 h to 30 min, thus allowing efficient production of sub-micro-
meter/nanosized a-Si3N4 powder. Effects of factors such as
additive, temperature, and duration on the crystallization pro-
cess are investigated using XRD and FTIR. The experimental
results showed that the added Si powder accelerates the crys-
tallization process effectively. The final product is a mixture of
a-Si3N4 and Si2N2O. In this article, amorphous silicon nitride
powder with added Si is annealed at 1450°C. Powder of nearly
100% crystalline phase content is produced either by adding
10% Si and annealing for 15 min or by adding 5% Si and
annealing for 30 min.
powder of about 80% crystalline phase content with an a/b
ratio of about 6 is produced by annealing nanosized amor-
phous silicon nitride powder at 1450°C for 360 min in nitro-
gen flow.16 The XRD result in early research shows that
some b-SiC formed and the transformation of a-Si3N4 to b-
Si3N4 also took place in the amorphous silicon nitride pow-
ders after heating at 1700°C.18
High quality silicon nitride ceramics are usually produced
from pure silicon nitride powder of high a phase content
with particle size in the micrometer range and a narrow
particle size distribution. Synthesis of nanocrystalline materi-
als from amorphous solids has been proposed for many
years.19–21 The crystallization of nanosized amorphous
silicon nitride powder is one of the methods to produce sub-
micrometer/nanosized a-Si3N4 powder. However, in earlier
researches, the crystallization process of amorphous silicon
nitride is very time consuming. The complete crystallization
time of nanosized amorphous powder is more than 6 h at
temperatures in the range 1400–1500°C,16 and the crystalliza-
tion time of amorphous films is nearly 20 h at 1390°C.22 The
application of this method is still limited by the long crystal-
lization time, low output, and high cost. Developing a means
to shorten the crystallization time is a topic of utmost impor-
tance from both a theoretic and practical point of view. The
aim of this work was to shorten the crystallization time and
study the crystallization behavior of amorphous silicon
nitride powder with additives.
I. Introduction
ILICON nitride ceramics of superior properties have been
S
widely used in the field of energy, metallurgy, machinery,
and aerospace industries.1–3 The quality of the silicon nitride
powder directly affects the performance of ceramic produc-
tions. Nanosized amorphous silicon nitride powder of high
surface energies offer significant advantages in terms of sin-
tering rate.4–6 In recent years, dense ceramics of more than
90% crystalline phase content were produced by direct sin-
tering of unannealed amorphous powder at temperatures in
the range 1600°C–1800°C.7–10 The amorphous powder exhib-
its larger shrinkage compared with crystallized ones.11 The
local densification and the formation of clusters happen dur-
ing sintering connected with the crystallization process.12
In the past two decades, several studies on the crystalliza-
tion process of amorphous silicon nitride powder13–16 and
thin films17 have been performed. During the crystallization
process of micrometer amorphous silicon nitride powder, the
chlorine impurities inhibit grain growth, resulting in a delay
of the initiation of crystallization. The activation energy cal-
II. Experimental Procedures
The additives may accelerate the crystallization process of
amorphous powder and shorten the crystallization time. In
this respect, special attention should be paid to the effects of
additives on the crystallization process of amorphous pow-
der. In this article, a-Si3N4 and Si powder are chosen as the
additives. The added a-Si3N4 powder may be the seed of the
crystallization process as well as the a-Si3N4 produced by the
added Si powder and nitrogen. Characteristics of the raw
materials used in the experiments are presented in Table I.
The raw materials are dried in a vacuum oven for 8 h at
100°C to remove adsorbed vapor before crystallization pro-
cess. The crystallization tests have been performed in a resis-
tance furnace filled with nitrogen gas. The experimental
conditions are listed in Table II. The results are analyzed
using XRD (Cu-Ka radiation) and FTIR.
culated using
a modified Avrami–Erofe’ev equation is
306 kJ/mol with chlorine impurities and 318 45 kJ/mol
without chlorine impurities.14,15 The ammonium chloride im-
purites in the silicon nitride powder and the nitrogen in the
furnace atmosphere cause the formation of whiskers.13
A
III. Results and Discussion
H.-J. Kleebe—contributing editor
(1) Effect of Additives Content
Figure 1 shows the XRD pattern of the product obtained
after annealing pure amorphous silicon nitride powder at
1450°C for 90 min (sample A). The diffractogram shows that
the amorphous powder did not crystallize. It can be inferred
that the crystallization of pure amorphous silicon nitride
Manuscript No. 30050. Received July 23, 2011; approved September 19, 2011.
The authors are grateful for the support of the National Natural Science Foundation
of China (No. 51076010).
†Author to whom correspondence should be addressed. e-mail: liwang@me.ustb.
edu.cn
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powder at 1450°C is very slow, and no obvious phase trans-
formation can be observed after annealing for 90 min. Fig-
ure 2 shows the XRD patterns of amorphous silicon nitride
powder added with 5% a phase and the product obtained
after annealing at 1520°C for 15 min (sample B and F). The
diffractograms show that the major phase is still amorphous,
and the crystal peaks correspond to the added a-Si3N4. Other
samples added with 5% a phase also show similar results.
Figure 3 shows the XRD patterns of the products
obtained after annealing amorphous silicon nitride powder
added with 5% and 10% Si at 1450°C for 15 min (sample J
and M). Evident crystallization has been observed in both
samples. The diffractogram shows that the original amor-
phous powder has transformed to crystalline phases domi-
nated by a-Si3N4. Furthermore, Si2N2O has been formed
because of the adsorbed O2 at the surfaces of amorphous
powder and Si powder. The residual Si content decreases
with an accompanying nitridation process. The crystalline
phase content of sample M is nearly 100%. All reactions
involved are shown as follows16:
Table I. Characteristics of the Raw Materials
Raw material
Mean particle size [lm]
Purity [wt%]
Amorphous silicon nitride
a-Si3N4
0.02
1
99.0
99.0
99.5
Si
2.7
Table II. Experimental Conditions
No.
Amorphous Si3N4 [wt%]
Additive
T [°C]
t [min]
A
B
C
D
E
F
G
H
I
100
95
95
95
95
95
95
95
95
95
95
95
90
—
a-Si3N4
a-Si3N4
a-Si3N4
a-Si3N4
a-Si3N4
Si
1450
—
90
—
15
15
15
15
—
15
8
15
30
15
15
1400
1440
1480
1520
—
1400
1450
1450
1450
1500
1450
Si
Si
Si
Si
Si
Si
amorphous ꢀ Si3N4ðsÞ ! a ꢀ Si3N4ðsÞ
3Siðs=lÞ þ 2N2 ! a ꢀ Si3N4ðsÞ
ð1Þ
ð2Þ
J
K
L
M
Fig. 1. The XRD pattern of sample A, the product obtained after annealing pure amorphous silicon nitride powder at 1450°C for 90 min.
Fig. 2. The XRD patterns of amorphous silicon nitride powder added with 5% a phase and the product obtained after annealing at 1520°C for
15 min (sample B and F).

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Fig. 3. The XRD patterns of the products obtained after annealing amorphous silicon nitride powder added with 5% and 10% Si at 1450°C
for 15 min (sample J and M).
The results indicate that the added a-Si3N4 has no evident
2Si3N4ðsÞ þ 1:5O2ðgÞ ! 3Si2N2OðsÞ þ N2ðgÞ
ð3Þ
effect on crystallization process; however, the added Si pow-
der accelerates the crystallization process considerably. Fur-
ther study is required to understand the reaction mechanism.
The added Si powder will react with N2 completely, and the
final product will be in a multiphase of a-Si3N4 and Si2N2O
at the proper conditions.
Figure 4 shows the FTIR spectra of amorphous silicon
nitride powder added with 5% Si and the product obtained
after annealing at 1450°C for 15 min (sample G and J). It is
evident that the main Si–N bond peaks of amorphous silicon
nitride centered at 967 cmꢀ1 and 474 cmꢀ1 have been drasti-
cally broadened after heating at 1450°C. In addition, the
Si–O bond peak centered at 1083 cmꢀ1 disappeared. The
remaining “Si–N” peaks may be further decomposed into
(2) Effect of Temperature
Earlier research reveals that temperature affects the crystalli-
zation significantly.13 In this study, temperature affects the
nitridation of added Si powder as well as the crystallization
of amorphous silicon nitride powder.
several separate peaks. The split peaks centered at 853 cmꢀ1
,
892 cmꢀ1, and 981 cmꢀ1 correspond to the stretching mode
of the Si–N bonds in a-Si3N4 and Si2N2O. The other two
split peaks centered at 461 cmꢀ1 and 495 cmꢀ1 correspond
to the bending mode of the Si–N bonds in a-Si3N4. Compari-
son between results reveals that the phase transformation
process from amorphous powder to a-Si3N4 and Si2N2O
occurred in sample J.
The XRD patterns of the products obtained after anneal-
ing amorphous silicon nitride powder added with 5% Si for
15 min at 1400°C (sample H), 1450°C (sample J), and 1500°
C (sample L) are shown in Fig. 5. It can be seen that evident
crystallization occurs in the amorphous powder over the
entire experimental temperature range. The major phase of
the crystallization product is a-Si3N4 and Si2N2O. The peaks
of residual Si can also be observed in the diffractograms. The
crystalline phase contents of the three samples are 53%,
77%, and 65%, and the residual Si contents are 1.3%, 0.5%,
and 1.1%, respectively. It can be inferred that the crystalliza-
tion rate of amorphous powder as well as the nitridation rate
of added Si at an annealing temerpature of 1450°C is faster
than at 1400°C and 1500°C.
(3) Effect of Duration
The products obtained after annealing amorphous silicon
nitride powder added with 5% Si at 1450°C for different
durations have been labeled as samples I (8 min), J (15 min)
and K (30 min). The XRD results are compared in Fig. 6.
Evident phase transformation from amorphous to crystalline
can be observed even after a short annealing time of 8 min.
The residual Si is found after annealing for 8 and 15 min,
but disappears after annealing for 30 min. The crystalline
phase content of sample K is nearly 100%. It can be inferred
that the crystallization process is almost complete after
annealing for 30 min which is much shorter than the 6 h
reported in the earlier study without the addition of Si pow-
der.16 Once more, a comparison between results reveals that
the added Si powder accelerates the crystallization process
considerably. The residual Si content decreases with anneal-
Fig. 4. FTIR spectra of amorphous silicon nitride powder added
with 5% Si and the product obtained after annealing at 1450°C for
15 min (sample G and J).

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Fig. 5. The XRD patterns of the products obtained after annealing amorphous silicon nitride powder added with 5% Si for 15 min at 1400°C
(sample H), 1450°C (sample J), and 1500°C (sample L).
Fig. 6. The XRD patterns of the products obtained after annealing amorphous silicon nitride powder added with 5% Si at 1450°C for 8 min
(sample I), 15 min (sample J) and 30 min (sample K).
ing time until completely consumed. This reveals that the
nitridation of Si occurs simultaneously with the crystalliza-
tion of amorphous silicon nitride powder. Thus, the added Si
powder would transform to a-Si3N4 and the final powder
would be in a multiphase of a-Si3N4 and Si2N2O without Si
impurity.
by adding 10% Si and annealing at 1450°C for 15 min or by
adding 5% Si and annealing at 1450°C for 30 min.
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