CP620, Shock Compression of Condensed Matter - 2001
edited by M. D. Furnish, N. N. Thadhani, and Y. Horie
© 2002 American Institute of Physics 0-7354-0068-7/02/$ 19.00
SHOCK-INDUCED CUBIC SILICON NITRIDE AND ITS PROPERTIES
Toshimori Sekine
Advanced Materials Laboratory, National Institute for Materials Science,
Namlki 1-1, Tsukuba 305-0044, Japan
Abstract. The shock-induced phase transition of Si3N4 has been investigated through recovery
technique and Hugoniot measurements. Recovered samples indicates the formation of a cubic spinel (cSi3N4) and the yield of c-Si3N4 increases with increasing the shock pressure and reaches 100% at 63
GPa. Hugoniot measurements have revealed a phase transition above 36 GPa and the associated volume
change of about 25%. The shock synthesized c-Si3N4 is nano crystalline and displays a hightemperature metastability up to about 1620 K, c-Si3N4 is one of hard materials based on the measured
equation of state. Shock-synthesized c-Si3N4 has been characterized by electron microscopy and 29Si
magic angle spinning NMR spectroscopy.
INTRODUCTION
Silicon nitride ceramics exhibit high strength at
high temperature, good thermal stress resistance and
good resistance to oxidation. Previous studies [1-4]
on shock-loaded Si3N4 have centered on processing
techniques, Hugoniot measurements on sintered
Si3N4 with considerable amounts of additives have
been carried out and they indicated no phase
transition up to about 40 GPa [5, 6], It has recently
found that the low-pressure phases transform into a
high-pressure phase of Si3N4, c-Si3N4 [7], There
are three methods for high-pressure synthesis of cSi3N4. The first one is a reaction of Si and N2 fluid
at 15-30 GPa and 2000 K in diamond anvil cells
coupled with laser heating [7], The second one is
a shock transformation from the low-pressure
phases, a-Si3N4 and p-Si3N4? above 20 GPa [8],
The third one is a solid-solid transformation from
the low-pressure phases, at pressures of 18-20 GPa
in multi-anvil high-pressure 'cells [9], c-Si3N4 is
the first known nitride spinel in which one third and
two thirds of the Si atoms are tetrahedrally and
octahedrally coordinated to nitrogens, respectively.
It has been generally accepted that high-pressure
phases display better performances in strength and
resistance, because the chemical bond is stronger in
the high-pressure phases than in the low-pressure
ones. Therefore we need to know properties of cSi3N4 for industrial applications.
We have already announced a massive
production method of c-Si3N4 from p-Si3N4 [8], In
this paper, shock synthesis of c-Si3N4 is reviewed as
well as a method for purifying and separating cSi3N4, Some properties of shock synthesized cSi3N4 powders and the Hugoniot of sintered P~Si3N4
with less amount of additive are summarized. The
shock-induced transformation of p-sialon to a cubic
spinel also is investigated in recovery experiments.
The results indicate that the oxynitride spinel makes
a solid solution with c-Si3N4.
EXPERIMENTAL METHODS
For the shock synthesis, we used a propellant
gun to generate shock wave by hypervelocity
impact. A projectile with a flyer metal plate is
accelerated up to a velocity of - 2 km/sec and
impacts a container target. Samples are stored in
copper containers to protect them from the
destruction by the rarefaction wave after the shock
compression. A detailed description has been
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in the heated and quenched samples were identified
by the XRD method. 29Si magic angle spinning
NMR spectroscopy was investigated to characterize
the local structures around Si atoms in c-Si3N4.
published elsewhere [10].
Preliminary shock
recovery experiments [11] indicated the chemical
reactions between the steel containers and the
silicon nitride powder. Flyers are copper and
tungsten, dependent on the required pressure.
Pressure is estimated by the impedance match
method and temperature is calculated on the
thermodynamic ground. After successful recovery
of the container, a sample was taken out from the
container and immersed into a nitric acid solution to
remove copper pressure media. Thus obtained
powders were investigated by X-ray diffraction
(XRD), electron microscopy, thermal analysis, and
NMR spectroscopy.
For Hugoniot measurements, we used a twostage light-gas gun to increase the impact velocity
up to about 6 km/sec. Inclined mirror method
coupled with an electrical streak camera has been
employed to measure shock velocity and particle
velocity simultaneously.
The details of the
techniques have been reported elsewhere [12].
Pressure and density at the compressed state have
been calculated with aid of the Rankine-Hugoniot
equations.
Two kinds of Si3N4 starting powders were used
for recovery experiments: a pure p-Si3N4 and a
mixture of 96% a-Si3N4 and 4% p-Si3N4 powders.
The powders contain 0.5 wt% and 1.3 wt% oxygen,
respectively. These Si3N4 powders were mixed
with a large amount of copper powders (9 times by
weight) to increase the shock pressure and
temperature. Two kinds of sintered p-Si3N4 were
used for the Hugoniot measurements. One is black
and has about 2wt% Nd2O3 and Y2O3 as additions
and the other contains no additive and is light gray.
These sintered bodies were cut into the pieces of
about 10 mm x 12 mm x 2.5 mm.
We have developed a method of purifying and
separating shock-synthesized c-Si3N4. By applying
a heated concentrated hydrofluoric acid only lowpressure Si3N4 phase as well as oxygen-rich,
amorphous parts could be dissolved out. c-Si3N4
powder survived for a limited duration. This
treatment allowed us to purify and separate as the
residual powders of c-Si3N4.
Thermal properties were investigated by
thermogravimetry and differential thermal analysis
up to about 1800 K in Ar flow with a range of
heating rates of 5 to 20 K/min. The phases present
RESULTS AND DISCUSSION
A. Formation of c-Si3N4 from p-Si3N4 and aSi3N4
Figure 1 summarizes a series of XRD patterns of
initial P~Si3N4 and post-shock samples quenched at
pressures of 33 GPa to 63 GPa. The yield of cSi3N4 increases with increasing peak shock pressure,
and it reaches almost 100% at 63 GPa [11]. The
shock temperature increases with increasing peak
shock pressure when the density of initial mixtures
is kept nearly constant. We compare the XRD
patterns of post-shock samples quenched from PSi3N4 at 49 GPa and from 96% a-Si3N4 and 4% pSi3N4 at 43 GPa. The shock temperatures were not
much different. According to a comparison of the
relative yield of c-Si3N4, the starting material of aSi3N4 is preferred in the shock transformation. A
theoretical consideration [13] also indicates a lower
pressure required for the phase transition from aSi3N4 than p-Si3N4 because p-Si3N4 is slightly
denser than a-Si3N4.
Shock-synthesized c-Si3N4 powders were
subjected to transmission electron microscopy
40
50
20(deg)
FIGURE 1. XRD patterns of starting material and its postshock samples, (a) Starting p-Si.iN,*, (b) post-shock sample
quenched at 33 GPa and 1770 K, (c) post-shock sample
quenched at 49 GPa and 2400 K, and (d) post-shock sample
quenched at 63 GPa and 3300 K
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about 2.5 km/sec, The shock velocity for the highpressure-induced phase appears above Up =2.5
km/sec. The Us-Up relation for the high-pressure
phase can be approximated by a linear equation of
Us = 4.59+1.49 Up.
The pressure-density relation also indicates a
phase transition starting at about 36 GPa. The
high-pressure region can be fitted to derive an
isentropic compression curve of the high pressure
phase through the third-order Birch-Murnaghan
equation of state [12]. The results indicate a zeropressure bulk modulus of 300 ± 10 GPa and its
pressure derivative of 3.0 ± 0,1, as seen in Fig.3.
We assumed that the high-pressure phase is a cubic
spinel with a zero-pressure density of 4.01 g/cm3,
based on the recovery experimental results.
There seems to be a slight difference in the onset
pressure for the phase transition between Hugoniot
measurements and the recovery experiments. It
should be noted that the temperature in the
Hugoniot experiments is significantly lower than
that in recovery experiments because of the
presence of pores in the powder mixtures. The
estimated bulk modulus of c-Si3N4 is in good
agreement with the estimations by the theoretical
calculations [7, 11, 17], and is greater by about
30 % than that of p-Si3N4. Vogelgesang et al. [18]
(TEM) observations [8, 14]. The grain sizes range
between 7 and 35 nm. The electron diffraction
pattern and high-resolution TEM image also were
taken and analyzed. They are consistent with a
cubic spinel structure. Some small c-Si3N4 grains
have been observed to be embedded in the
amorphous oxygen-rich phase. The composition
of c-Si3N4 has been checked by EELS and EDX
analyses during the TEM observations, and
indicates no significant change from the initial
Si3N4.
The electron energy loss near edge
spectroscopy method has been applied for structural
identification and the measured results have been
compared with the pattern predicted by theoretical
simulation.
The results has been reported
separately [15].
B. Hugoniot measurements
We have measured Hugoniot of p-Si3N4 up to
pressures of 150 GPa [12], It is illustrated in the
planes of shock velocity (Us)-particie velocity (Up)
(Fig. 2) and pressure-density (Fig. 3). According
to the Us-Up relation, elastic and elasto-plastic
regions are seen in the low pressure phase. The
HEL value is about 16 GPa, which is close to the
previous data [5], A phase transition has been
detected at about Up=l.l km/sec and last up to
-
HEL
PT
0
1
2
3
4
5
3,5
Up (Km/s)
4.0
4.5
5.0
5,5
6.0
Density (g/cm)
FIGURE 2. A relationship between shock velocity (Us, Km/s)
and particle velocity (Up, Km/s) of p-Si^, measured by the
inclined mirror method, HEL = Hugoniot elastic limit, and
PT = phase transition.
FIGURE 3. Hugoniot pressure-density relation of p-Si3N4
and a calculated isentrope of c- Si3N4 with a bulk modulus
of 300 GPa and its pressure derivative of 3.0,
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oxygen-rich amorphous phase in shock-synthesized
c-Si3N4 sample. The pressure required for the
complete transition to c-Si3N4 is too high to be
achieved by conventional explosive methods used
in industries. Shock recovered samples subjected
to pressures of 30-50 GPa contain c-Si3N4 and lowpressure phases of Si3N4. Therefore it is necessary
to develop a method to separate c-Si3N4 from the
samples recovered below the complete transition
pressure. We have developed a chemical method
to purify and to separate c-Si3N4 from a mixture
containing c-Si3N4, low-pressure phase Si3N4 and
oxygen-rich amorphous phase. After using this
method to purify shock-synthesized c-Si3N4
powders, we have obtained DTA data again and
compared with that for c-Si3N4 sample without
purification. The results have indicated a greater
metastability. XRD data of heated and quenched
samples are shown in Figure 5. c-Si3N4 survived
partially in a heat treatment up to 1793 K. The
DTA result indicate that the transition from c-Si3N4
to p-Si3N4 occurs at about 1670 K with an enthalpy
change of 29.2±3.5 kJ/mol.
measured the elastic constants of single crystal PSi3N4 using Brillouin scattering. He et al. [12]
determined the elastic constants of sintered p-Si3N4
with no additives by the resonance sphere technique.
These two data are significantly smaller than that
based on the P-V data of p-Si3N4 in DAC by Li et al
[19].
C. Metastability of c-Sia^ at high temperature
In order to check the metastability of shocksynthesized c-Si3N4 powders, DTA analysis up to
1770 K has been investigated in Ar atmosphere.
The DTA curve obtained for c-Si3N4, quenched
from 63 GPa indicate a broad, exothermic reaction
at temperatures over 1370 K and a relatively sharp
exothermic reaction at temperatures 1620 K - 1770
K when temperature increases at a rate of 20 K /mm,
XRD patterns for quenched samples from
temperatures of 1073 K, 1473 K and 1773 K are
shown in Fig. 4. Oxidation starts at about 1470 K
and a significant amount of SiO2 cristobalite is
identified in the sample quenched from 1773 K.
The weigh gain indicate a formation of about 7 wt%
SiO2. c-Si3N4 is stable at 1473 K and transforms
mostly into P«Si3N4 as well as a small amount of <xSi3N4 at 1773 K. These DTA and XRD data
indicate that c-Si3N4 is thermally metastable at least
up to 1620 K at one atmosphere.
High-resolution TEM image has indicated
1200
1000
(2)
A
800
..J. I -AL.A
^-A
600
(D
1 1*-
400
200
0
10
^ (b)
10
c(3 1)
c<22
30
40
50
60
40
50
60
70
80
FIGURE 5. XRD patterns of c-Si3N4 powders shock-synthesized
at about 40 OPa and purified by high-temperature HF solution
and its heated samples. (1) sample heated at a rate of 5 K/min
and quenched from 1753 K, (2) sample heated at a rate of 5
K/min and quenched from 1793 K. (3) sample heated at a rate
of 10 K/min and quenched from 1793 K, (4) sample heated at a
rate of 20 K/min and quenched from 1 793 K. a, P, c and SiO2
indicate peaks from phases a-Si3N4, p-Si,iN4, c-SisN,!, and
cristabalite, respectively.
i,-i.fr in nA^KJ^
20
30
26 (degree)
c(440)
«"» 2' IT
20
70
FIGURE 4. XRD patterns of c- Si3N4 powders (a) quenched
from 63 GPa and its heated samples at 1073 K (b), 1473 K (c),
and 1773 K (d). a and P are Si3N4 polymorphs. SiO2 is
cristobalite.
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D.29Si MAS NMR data of c-Si3N4
29
Si MAS NMR spectroscopy for c-Si3N4
powders has been performed to characterize the
local structure of Si atoms in the spinel. The
details have been published elsewhere [20]. The
chemical shift has been reported to be -46.1 and
-48.2 ppm for a-Si3N4 and -48.8 ppm for p-Si3N4
[21, 22]. The measured values for c-Si3N4 have two
sharp peaks of -50.0 ± 0.2 ppm and -225.0 ± 0.2
ppm. These correspond to SfN4 and SiN6 units
and an integration of the spectrum of SiN4/SiN6 is
about 1/2, that of the spinel structure. The
chemical shift for SiN6 is more negative than that
for SiOg and provides a useful information to
determine the units in oxynitrides.
spinel formula A13O3N (A12O3/A1N =1) which is not
known yet. 7-Al2O3 exists as a spinel with defects. It
is generally considered that Al may prefer the
octahedral site to the tetrahedral site in spinel, and it
is quite interesting to investigate whether sialon
spinel can form like in Si3N4 spinel. We have
carried out some recovery experiments using
crystalline sialon powders with z = 1.8 and 2.8.
XRD data of recovered sialon from 30 to 50 GPa
indicate the formation of sialon spinels [23].
However the sample with z =2.8 quenched from 60
GPa becomes amorphous.
The amorphization
occurs in a wide range of the shock conditions, and
a detailed investigation of shock condition reveals
that the amorphization is due to a sluggish
transformation of P-sialon into spinel but not due to
melting. We are carrying out further study to
obtain more information.
E. Formation of sialon spinel
P-sialon, Si6-zAlzOzNg,z (0<z<4.2), is known as
oxynitride with a similar structure to p-Si3N4.
Compositionally the sialon extends toward a
CONCLUDING REMARKS
Shock synthesis of c-Si3N4 was summarized
from ct-Si3N4 and p-Si3N4 in the presence of copper
powders. The yield of c-Si3N4 increases with
increasing shock pressure, and a higher yield was
indicated from a-Si3N4 than P~Si3N4. The shocksynthesized c-Si3N4 powders are nano crystals and
displays a high-temperature metastability up to
about 1620 K. c-Si3N4 was characterized by 29Si
MAS NMR spectroscopy.
The determined
equation of state indicates that c-Si3N4 is one of
hard materials. Our recent experimental results
indicate that sialon spinel exists as well.
(a)
-100
-200
-300
ppm
ACKNOWLEDGEMENTS
(b)
The author thanks H. He, F. Mitsuhashi, M.
Tansho, I. Tanaka, K. Kimoto, Y. Yajima, T.
Kobayashi, M. Kanzaki, and M. Mitomo for
collaboration research and discussion.
-100
-200
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1117
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