QUALITATIVE ANALYSIS OF SILICON NITRIDE SYNTHESIZED BY AMMONOLYSIS
OF SILICON TETRACHLORIDE
Richetto, K.C. S.1 ; Silva, C R. M.2; Baldacin, S. A.3
1
UNITAU – Universidade de Taubaté
CTA/IAE/AMR
2,3
Keywords: Silicon Nitride, Synthesis, Qualitative Analysis.
ABSTRACT:
Silicon nitride powder synthesis, performed by the ammonolysis of silicon tetrachloride, is
described in this work. Gaseous ammonia reacts with liquid SiCl4 , at low temperature and inert
atmosphere, producing an insoluble ammonolysis product. At a second step, a powder pyrolysis is
performed between 1300 and 15000C, for ammonium chloride removal and Si3N4 formation. Phase
formation is evaluated by X-ray diffraction. Particle medium size and form is determined by
Scanning Electron Microscopy.
INTRODUCTION
Silicon nitride ceramic (Si3N4), is strongly considered a important material for use in
structural applications, and its performance is severely influenced by modern synthesis processes
for high quality powders[1].
The advantages of using this ceramic for several structural applications is related to its high
thermal stability up to 18000C, oxidation resistance up to15000C[2], low electrical conductivity, low
coefficient of thermal expansion, excellent thermal shock and creep and corrosion resistance in
hostile environment[2,3]. Such properties are highly dependent upon microstructural characteristics,
originated on fabrication processes[4].
There are three major routes for the preparation of silicon nitride[2]: high temperature
reaction of gaseous nitrogen with elemental silicon; gas-phase reaction of ammonia with a
chlorosilane (SiCl4, HSiCl3, H2SiCl2) at high temperatures; reaction of ammonia with SiCl4 or
HSiCl3 in solution, followed by pyrolysis of the insoluble ammonolysis product and ammonium
chloride removal.
The higher free energy formation of NH3 makes reactions involving this compound more
favorable than similar reactions with N2, considering the higher free energy of formation of NH3[3].
In the present work, synthesis of Si3N4 powder by ammonolysis of SiCl4, at low temperature
in the presence of an organic solvent (hexane) is described. The chemistry reaction has been
examined in some detail by Billy[5], et al., Glemser, et al. [6], Mazdiyasni, et al. [3], and Morgan[7, 8, 9],
as described.
The Hexane/Silicon Tetrachloride solution reacts with gaseous ammonia, yielding a
precipitate of silicon diimide and ammonium chloride, according to the reaction:
SiCl4 (l) + 6 NH3 (g)
n-C6H14
00 C
Si (NH)2 + 4NH4Cl
(1)
The kinetic of reaction is more effective at low temperatures
product causes NH4Cl sublimation [3,11], as following:
[10]
. Pyrolysis of obtained
3600C
nSi(NH)2 (l) + 4 NH4Cl
vac.
4 NH4Cl + [Si (NH)2]n
(2)
The Si(NH)2 polymerizes readily and spontaneously with increasing temperature; its
pyrolysis results in two intermediate species before it yields α-Si3N4, according to the reaction (3):
4000C
6[Si(NH)2]n (l)
- 2NH3
6500C
2 [Si3(NH)3N2]n
- NH3
3 [Si2(NH) N2]n
12000C
2 α-Si3N4
- NH3
(3)
EXPERIMENTAL
Diimide synthesis was performed in a two litters capacity glass Kettle reactor (figure1):
Fig. 1: Kettle Reactor.
A high purity solution of silicon tetrachloride/hexane was introduced in the reactor, in Argon
or Nitrogen atmosphere. A controlled flow of gaseous ammonium is introduced in the above
mentioned reactor, starting a very exothermic reaction.
Better results are obtained in a temperature range of 0 and 50C, during approximately four
hours. The homogenization of reaction media was achieved using a magnetic agitator.
In a second step, the obtained silicon diimide was transferred from the reactor to the
alumine and silicon carbide crucibles. This step was performed inside a glove box (figure 2), to
avoid diimide contamination by oxygen .
Fig. 2: Glove box
The formed white precipitate was then isolated by the n-hexane distillation under reduced
pressure at 250C. Last step for silicon nitride powder synthesis was the diimide pyrolysis, carried
out in high purity Argon or Nitrogen atmosphere, at 14000C, for two hours. During such heat
treatments, ammonium chloride evolution was always observed, at approximately 900 0C. Table 1
shows the conditions of diimide silicon pyrolysis, during 2 hours:
Table 1: Pyrolysis conditions
Speciment
a
b
c
d
Temperature
14500C
14000C
14000C
14000C
Atmosphere
Argon
Argon
Argon
Nitrogen
Crucible
Silicon carbide
Silicon carbide
Alumina
Alumina
Preliminary powder characterization was performed by X-Ray diffraction an scanning
electron microscopy. For accurate morphologic examination, powder samples were previously
prepared by ultrasonic dispersion, during five minutes, using distilled water and “Disperlan”, as
dispersion media.
RESULTS AND CONCLUSIONS
Two intermediary compounds has been produced by the ammonolysis of silicon
tetrachloride: silicon diimide and ammonium chloride. The compounds were subsequently heat
treated in Ar or N2, obtained previously, are heat treated, and are transformed during heating.
Ammonium chloride starts its transformations from solid to gaseous state at 3600C. The
subsequent heating causes decomposition of the remanent product, followed by liberation of gas
ammonia and hydrogen chloride, aiming crystalline silicon nitride powder production. The possible
reaction, in this case, is described as follows:
NH4Cl (s)
NH3 (g) + HCl (g)
(4)
During the pyrolysis, vapor evolution was observed only after 4000C.
Very fine grey particles of silicon nitride powder were obtained, occupying a small crucible
volume, compared to the starting silicon diimide powder. They were subsequently characterized by
scanning electron microscopy and X-ray diffraction.
Silicon Nitride powder α+ β morphology is shown in the figure 3. High percentage of
cilinder particles are observed.
Fig. 3: Morphological analysis of Si3N4 α+ β to MEV.
Figure 4 shows a X-Ray diffraction to different speciment. The speciment –a peaks present a
β phase presence, while speciments b, c/d, shows α and α+ β phases, respectively.
Speciment - a
Si3N4 β
1500
β
1250
β
α
β
1250
α
α α
1000
Intensity (cps)
1000
Intensity (cps)
Speciment - b
Si3N4 α
1500
750
β
500
750
α
α
500
250
250
0
0
20
25
30
20
35
25
30
2θ Angle
Speciment - c
Si3N4 α / β
α
75
35
2θ Angle
α
α α
α
Speciment - d
Si3N4α / β
750
α
α
50
α
25
α
β
β
α
β
β
Intensity (cps)
Intensity (cps)
500
α
α
α
250
β
α
β
β
0
0
20
25
30
35
20
2θ Angle
25
30
35
2θ Angle
Fig. 4: X-Ray Diffracton of Si3N4 : (a) β ; (b) α ; (c) and (d) α+ β.
The β peaks appears more frequently above 14000C (figure 4-a), while α phase is
predominant below the same temperature. Very fine α - silicon nitride powders were obtained,
using silicon carbide crucible, in Argon atmosphere (Fig 4-b). Apparently, the β-silicon nitride
phase is not nucleated, for the same process conditions.
For alumina crucibles, both phases were observed (figure –c/d), using either Argon or N2, as
furnace atmosphere.
Results suggest that oxygen is provided from alumina crucible, during pyrolysis, generating
appropriate conditions for α plus β nucleation, therefore, this study is not concluded.
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