Floating combustion synthesis of spherical vitreous silica nano-powder

Article abstract of DOI:10.1016/j.materresbull.2008.03.025

Vitreous silica nano-powder, with spherical particles of 50-300 nm in size, was successfully produced via floating combustion synthesis. Mechano-chemically activated and spray-dried Si-powder was oxidized in fluidized bed between 900 °C and 1550 °C in air

Full text of DOI:10.1016/j.materresbull.2008.03.025

Materials Research Bulletin 44 (2009) 130–133
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Materials Research Bulletin
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Floating combustion synthesis of spherical vitreous silica nano-powder
a
b
a
c
a,
Yuan Zhu , Hai Bo Jin , Ke Gang Ren , Simeon Agathopoulos , Ke Xin Chen *
a
State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, PR China
b
School of Materials Science and Engineering, Beijing Institute of Technology, Beijing 100081, PR China
c
Department of Materials Science and Engineering, University of Ioannina, Greece
A R T I C L E I N F O
A B S T R A C T
Article history:
Vitreous silica nano-powder, with spherical particles of 50–300 nm in size, was successfully produced via
floating combustion synthesis. Mechano-chemically activated and spray-dried Si-powder was oxidized
in fluidized bed between 900 8C and 1550 8C in air. After 10 min, rapid removal of powder from the
furnace and quenching maintained silica’s vitreous nature. The process comprises sequential cycles of the
following stages until nano-sized particles of silica are obtained. Superficial melting of Si-particles
initially occurs and the particles obtain a spherical shape. The surface of the particles is oxidized.
The stress at silica/silicon interface causes collapse of particles to smaller ones.
Received 5 December 2007
Received in revised form 22 February 2008
Accepted 15 March 2008
Available online 8 April 2008
Keywords:
A. Amorphous materials
A. Oxides
ß 2008 Elsevier Ltd. All rights reserved.
C. Electron microscopy
1
. Introduction
Floating combustion synthesis is a modern low-cost method for
of spherical vitreous silica nano-powder with floating combustion
synthesis in an industrial-scale plant. The results are discussed in
order to shed light in the stages of the process.
. Materials and experimental procedure
The silicon powder (>45 m, Dadizelin Co. Ltd., Beijing) was
industrial production of high quality ceramic powders because of
the simple equipment and installation, low energy consumption,
and short processing time. It actually combines combustion
synthesis and fluidized bed [1,2].
2
m
In the production of silicon nitride with floating combustion
synthesis in fluidized bed, where silicon particles react with
nitrogen, the process is largely based on the progressive collapse of
bigger particles to smaller ones during the process due to the
stresses developed at the interface between the surface-crystal-
first subjected to pretreatment of mechano-chemical activation [7]
and then spray-dry [8]. In particular, silicon powder was mixed
4
with 1 wt% NH Cl (dispersant) and then activated by dry grinding
in a vibration-mill for 1.5 h. The activated silicon powder was
spray-dried with 1 wt% poly-ethylen-oxide (binder).
The experimental setup is sketched in Fig. 1. The pretreated
silicon powder enters the furnace with a flow rate of 20 l/min at
atmospheric pressure. The temperature profile inside the furnace
is 900 8C at the entrance and gradually rises up to 1550 8C
line-Si
combustion synthesis of Ta
out [4]. Our research team has applied this technique to produce
high quality Si and TiC hyper-fine powders [5].
3
N
4
and the bulk-Si of particles’ core [3]. Microwave-assisted
2
N in fluidized bed has been also carried
3
N
4
x y
N
Nowadays, nano-powders are highly demanded for producing
high quality materials for many advanced applications. In the
particular case of silica, nano-powders of spherical vitreous silica
have high potential in electric and electronic applications because
of the high dielectric constant and thermal moisture resistance, as
well as the low thermal expansion, residual stresses, and friction
(
maximum) in the centre of the furnace. After 10 min, the powder
is taken out from the furnace drifted from a strong air-flow in the
cyclone, and then rapidly cooled (quenching) in the cooler.
The following methods and equipments were employed to
characterize the materials. X-ray diffraction (XRD, Automated
D/Max B, Rigaku) was used to determine the crystalline regime
of the powder. Chemical analysis was carried out with the aid of
X-ray fluorescence (XRF-1800, SHIMADZU). Observations of the
microstructure were made with scanning electron microscopy
[
6]. Other applications, such as biomedical, electrical, magnetic,
and structural ones, should be also added in the list. Accordingly,
this paper presents experimental results of successful production
(
SEM, GSM-6460, JEOL, and FESEM, S-4300, Hitachi E1030) and
transmission electron microscopy (TEM, 200CX, JEOL), equipped
with energy dispersive X-ray spectroscopy (EDS) and selected area
electron diffraction (SAED).
*
Corresponding author. Tel.: +86 10 6277 2548; fax: +86 10 6277 2548.
E-mail address: kxchen@mail.tsinghua.edu.cn (K.X. Chen).
0025-5408/$ – see front matter ß 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2008.03.025

Y. Zhu et al. / Materials Research Bulletin 44 (2009) 130–133
131
chemical activation (Fig. 2a) is expected to favor development of
cohesive forces among the particles. The big spherical Si-particles
(
ꢀ60
mm in diameter, Fig. 2b), obtained after spray-dry, anticipate
good fluidity of powder at the entrance of furnace [9].
After the course of the process of Fig. 1, the resultant powder
had white color. Its features are summarized in Fig. 3. The powder
consisted of well-shaped spherical particles with a diameter of c.a.
50–300 nm (Fig. 3a). X-ray fluorescence analysis showed that the
produced powder was slightly depleted of oxygen, i.e. mol ratio of
Si/O = 1/1.95. The diffractogram of Fig. 3b confirms the presence of
Fig. 1. Outline of the floating combustion synthesis plant.
3
. Results
Fig. 2 shows the influence of pretreatment on morphology and
particle size of Si-powder. Mechano-chemical activation was
actually applied to lower the reaction temperature of the process.
Nevertheless, the small average size of ꢀ5
mm after mechano-
Fig. 2. Morphology of Si-powders after (a) mechano-chemical activation and (b)
Fig. 3. (a) Morphology (FESEM) of the produced vitreous silica nano-particles, (b)
spray-dry.
their diffractogram, and (c) HRTEM image with SAED pattern (inset).

1
32
Y. Zhu et al. / Materials Research Bulletin 44 (2009) 130–133
travel towards the centre of the furnace (maximum temperature
550 8C), that is further intensified because of the strong
1
exothermic effect of the oxidation–combustion reaction of Si.
The superficially molten particles spontaneously obtain spherical
shape (stage ii) due to the effect of surface tension under a micro-
gravity field (the fluidized bed conditions [10] balance the effect of
gravity). Bigger particles (stage ii) are expected to be less spherical
than smaller ones (stage v) because the shorter the radius the
stronger the cohesive force (
place at the molten surface layer of Si-particles (stage iii). The
inevitably developed stresses at the SiO /Si interface, due to the
Dp = 2g/r) [11]. Oxidation easily takes
2
lattice misfit and the mismatch of thermal expansion coefficients
of the contacting phases, result in crack formation and propagation
in the surface but also in the bulk of the particles (Fig. 4a).
Therefore, the particles collapse in such a way as shown in the
stage (iv) of Fig. 4b.
Accordingly, the collapse of the particles is a stage of high
importance. The interface between the oxide layer and the core of
silicon at three different temperature ranges is sketched in Fig. 4c.
Qualitative earlier studies have suggested that a thin film of
crystalline SiO
2
forms at the interface between Si and vitreous SiO
2
[
12–14]. There is pronounced difference among the coefficients of
ꢂ6 ꢂ1
thermal expansion (CTE) of crystalline SiO
vitreous SiO
2
(15.6 ꢁ 10
K
K
) [15],
) [17].
ꢂ6 ꢂ1
ꢂ6 ꢂ1
2
(0.58 ꢁ 10
K
) [16], and Si (3.5 ꢁ 10
However, it has been postulated [18] that this particular interface
is stress-free between 975 8C and 1000 8C. This means that the
stress due to CTE mismatch balances the stress due to lattice misfit
within that narrow temperature range (i.e. 975–1000 8C). In other
words, that stress-free regime suggests a transition stage, with
regard to the type of the stresses dominating at the interface (i.e.
due to lattice misfit or CTE mismatch). In the light of the big
difference among the CTEs of the three phases (mentioned above),
it would be assumed that the stress at the interface at
temperatures higher than 1000 8C is due to CTE mismatch. The
lower value of the CTE of vitreous silica (than the other two phases
mentioned above) implies development of tensile stress at
temperatures higher than 1000 8C. The maximum misfit strain,
e, between 1000 8C (i.e. the upper limit of the stress-free regime)
and 1500 8C (i.e. the maximum temperature of the furnace) is
calculated as
Fig. 4. (a) SEM image of a superficially melted Si-sphere and a similar sphere locally
cracked at the surface (inset). (b) Schematic representation of the stages of the
process. (c) Regimes of stresses at the Si/SiO
2
interface for different temperature
2
is also anticipated to form at the
ranges (note that a thin film of crystalline SiO
interface).
non-reacted Si in the produced powder, suggesting imperfect
oxidation. However, it is worthy to note that the investigated
processing completely suppresses formation of crystallized
silica. The HRTEM image of Fig. 3c confirms the amorphous nature
of the produced silica since the observed spots, presumably
attributed to atomic level units, are randomly arranged. Moreover,
the SAED pattern (inset of Fig. 3c) indicates an amorphous nature
¼ ð15:6 ꢁ 10ꢂ6
ꢂ1
ꢂ 0:58 ꢁ 10ꢂ6
ꢂ1Þ500 K ¼ 0:751%
e
K
K
Assuming the stress-free regime (975–1000 8C) at the interface
(
since the patterns of crystallized nano-particles are broad but
of Fig. 4c as a transition stage and the development of tensile stress
at temperatures higher than 1000 8C, compressive stresses must be
developed at temperatures lower than 975 8C, predominantly
resulted from lattice misfit at the interface. Experimental results of
thermal oxidation of silicon substrates, used in MEMS and VLSI
processes, where stoichiometric dry oxidation took place at
temperatures above 900 8C [19], support fairly well our hypothesis
(the silica developed was also non-crystalline, similar to the silica
produced in the present study). Compressive stresses of ꢀ300 MPa
were measured at room temperature [19].
distinct rings).
To shed light in the stages of the process, we collected
intermediate products after short reaction (oxidation) time, i.e.
ꢀ
30 s. Fig. 4a shows a typical superficially oxidized Si micro-
sphere with size of about 10 m. We also observed many
m
spheres whose surface was extensively but locally cracked (inset
of Fig. 4a).
4
. Discussion
The stages (ii)–(iv) of Fig. 4b are repeated (i.e. stages v and vi)
until particle size is reduced down to nano-level. In our plant and
with the given (in this study) materials’ initial stage and
pretreatment as well as processing conditions, this process lasts
10 min. Then, rapid cooling (which is feasible because of the whole
experimental set up, based on fluidized bed concept) maintains the
vitreous nature of the resultant silica powder (stage vii).
According to the experimental results, the stages of the process
can be depicted in Fig. 4b. When the silicon particles (Fig. 2b) enter
the furnace, the organic binder immediately burns out (i.e. at
9
00 8C) and hence, the morphology of Si-particles must be close to
Fig. 2a (stage i of Fig. 4b). Bulk Si melts at 1412 8C. However, earlier
independent experiments (unpublished data, whose detail
description is beyond the scope of the present paper) have shown
that mechano-chemical activation causes reduction of superficial
melting temperature of Si-particles at 900 8C. Therefore, the
temperature at the entrance of the furnace was set at 900 8C. The
melting at particles’ surface layer expands because the particles
5. Conclusions
Floating combustion synthesis was successfully employed to
produce vitreous silica powder with spherical nano-particles. The

Y. Zhu et al. / Materials Research Bulletin 44 (2009) 130–133
133
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Acknowledgements
[
[
The participation of S. Agathopoulos in this research and paper
was done in the framework of the Project ENTER 04EP26, co-
financed by E.U. (European Social Fund, 75%) and the Greek
Ministry of Development (GSRT, 25%).
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