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Journal of Alloys and Compounds 348 (2003) 325–331
L
www.elsevier.com / locate / jallcom
Synthesis and characterization of IT-electrolyte with perovskite structure
La 0.8 Sr 0.2 Ga 0.85 Mg 0.15 O 32d by glycine–nitrate combustion method
a
a,
a
a
a
a,b,c
Ligong Cong , Tianmin He *, Yuan Ji , Pengfei Guan , Yinglong Huang , Wenhui Su
a
Department of Physics, and State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, PR China
b
Center for Condensed Matter Science and Technology, Harbin Institute of Technology, Harbin 150001, PR China
c
International Center of Materials Physics, Academia Sinica, Shenyang 110015, PR China
Received 8 April 2002; received in revised form 11 June 2002; accepted 11 June 2002
Abstract
The intermediate-temperature electrolyte with perovskite-type La 0.8 Sr 0.2 Ga 0.85 Mg 0.15 O 32d (LSGM) was synthesized using glycine–
nitrate combustion method. The formation process of the LSGM phase was investigated using DTA–TGA and XRD, and the thermal and
electrical properties of the sintered samples were studied by thermal expansion and ac impedance technology. The research results show
that the formation process of the LSGM phase suffered several different reaction stages. The perovskite phase was formed essentially at a
sintering temperature of 1200 8C. The stable perovskite phase was formed completely after a sintering temperature of 1400 8C. The
sinterability shows that the sintering temperature of the sample, which was synthesized by glycine–nitrate combustion method, is at least
100 8C lower than that of the conventional solid state method that was reported previously. The thermal expansion coefficient of the
LSGM is somewhat higher than that of the YSZ. Impedance spectra indicate that the grain boundary resistance of the LSGM synthesized
by glycine–nitrate combustion method is smaller. The conductivity of sample sintered at 1500 8C is higher than that at 1550 8C. The
conductivity of a sample is 7.8310 22 S cm 21 at 850 8C. The thermal analysis shows that the LSGM is relatively stable over an
intermediate temperature range of 500–800 8C.
 2002 Elsevier Science B.V. All rights reserved.
Keywords: Glycine–nitrate combustion method; Solid electrolyte; Lanthanum gallate; Oxide ion conductivity; Thermal stability
1. Introduction
Solid oxide fuel cell (SOFC) as an energy conversion
device has attracted much attention because of its higher
conversion efficiency and environmental friendship. It is
becoming a new clean energy developed in the world [1,2].
A typical high-temperature SOFC used 8 mol% yttriastabilized zirconia (YSZ), with a fluorite structure as an
electrolyte. SOFC used YSZ as an electrolyte required to
operate at high temperatures of 800–1000 8C. At high
operating temperatures, some harsh terms are proposed for
required materials, such as the sealed problem of high
temperature, thermal matches between materials and interface reaction between electrolytes and electrodes, etc.,
leading to higher manufacturing costs and extending
applications are limited for SOFC [3]. In order to reduce
the operating temperature of SOFC, designing and de*Corresponding author. Tel.: 186-431-892-2331x2697; fax: 186-431892-1479.
E-mail address: [email protected] (T. He).
veloping novel intermediate temperature (IT) electrolytes
for IT-SOFC operating temperature over 500–800 8C has
now become one of the major responsibilities for SOFC
developers [4]. Doped-CeO 2 and doped-Bi 2 O 3 with fluorite structure are the traditional IT-electrolyte materials.
Bi 2 O 3 shows high ionic conductivity at intermediate
temperatures. However, it is reduced easily and changed
into metal bismuth at an oxygen partial pressure of about
10 28 Pa at 600 8C, while CeO 2 can depart from ideal
stoichiometry, accompanied by electronic conductivity at
higher temperatures and in a reducing atmosphere [1].
Therefore, the practical application of CeO 2 and Bi 2 O 3 is
still improving. In recent years, development of electrolyte
with unfluorite structure has gained much interest. Ishihara
and Matsuda [5] and Feng and Goodenough [6] has
reported a new LaGaO 3 -based IT-electrolyte with perovskite structure (ABO 3 ). The electrolyte LaGaO 3 doped
with Sr for La site and Mg for Ga site exhibited a high
oxygen ionic conductivity. The ionic conductivity of
La 0.9 Sr 0.1 Ga 0.9 Mg 0.1 O 3 was higher than that of Sc-doped
ZrO 2 and somewhat lower than that of doped Bi 2 O 3 . Petric
0925-8388 / 02 / $ – see front matter  2002 Elsevier Science B.V. All rights reserved.
PII: S0925-8388( 02 )00859-9
326
L. Cong et al. / Journal of Alloys and Compounds 348 (2003) 325–331
et al. [7] has studied NdGaO 3 -based electrolyte by means
of doping with Ca for A site and Mg for B site; the
corresponding maximum conductivity was 0.035 S cm 21 at
800 8C. Trofimenko et al. [8] has obtained the electrolyte
La 0.85 Sr 0.1 (Ga 0.9 Co 0.1 )Mg 0.2 O 32x 1d using transition metal
double doping for B site, and the good conductivity is 0.12
S cm 21 at 800 8C. We have studied the conductivity of
PrGa 0.9 Mg 0.1 O 3 perovskite-type electrolyte doping Mg for
B site, the conductivity reaches 0.05 S cm 21 at 800 8C and
than that of the YSZ at the same temperature [9,10].
Recently, Huang et al. [11] has reported an electrolyte of
La 0.8 Sr 0.2 Ga 0.83 Mg 0.17 O 2.815, for which the conductivity is
0.17 S cm 21 . A maximum power density of 700 mW cm 22
in a single cell that used this material as an electrolyte has
been achieved at 800 8C [12]. Consequently, new LaGaO 3 based electrolyte with perovskite structure is an IT-electrolyte material with vast vistas for application, and has
great developing potential and applied value.
In general, the synthesis of the IT-electrolyte used
usually conventional solid state method and the sol–gel
method [13,14]. The conventional solid state method
requires grinding different oxide mixtures for a lengthy
period of time and suffers much sintering; the synthesized
component distributions are not homogeneous and particle
sizes are larger, while sol–gel method requires expensive
metal alkoxide precursors and great care in mixing the
precursors to achieve the desired stoichiometry. The
glyine–nitrate combustion method, which was used as a
new technology for materials synthesis, has been developed by Chlik et al. [15] in recent years. It has become
an attractive synthetic method for the preparation of
multiple component inorganic oxides. This method offers
several distinct advantages, which the homogeneous mixtures of several components at molecular or atomic levels
can obtain in solution, gaining ultra-fine oxide materials.
Furthermore, this method does not require special igniting
equipment, and the igniting temperature is lower as well.
Therefore, it is important to study the synthesis and
properties of new IT-electrolyte materials using the
glycine–nitrate combustion method. In this paper, we
report the preparation and characterization of the IT-electrolyte of La 0.8 Sr 0.2 Ga 0.85 Mg 0.15 O 32d with perovskite
structure by glycine–nitrate combustion method.
2. Experimental
2.1. Materials and preparation
Ga (99.999%), La 2 O 3 (99.99%), MgO (98%) and
SrCO 3 (99%) were used as starting materials, and La 2 O 3
was precalcinated at 900 8C for 2 h before use. The powder
of composition La 0.8 Sr 0.2 Ga 0.85 Mg 0.15 O 32d (denoted as
LSGM in this paper) was prepared by glycine–nitrate
combustion method, as above. According to the formula of
LSGM, stoichiometric amounts of Ga, La 2 O 3 , MgO and
SrCO 3 were first dissolved into strong HNO 3 to get a
corresponding nitrate solution. Then, these nitrate solutions
were mixed together with water in a glass beaker, and the
glycine (as a fuel and complexant) was added into the
mixed nitrate solution by use of a 1:1 molar ratio for
glycine–oxidant. The glass beaker with the above mixed
glycine–nitrate solutions was heated on a hot plate, and
sufficient water was evaporated until the solution boiled,
began to froth and spontaneously burn at some instant. In
this way, the homogeneous white powder products were
obtained. According to propellant chemistry [16], the
combustion gases are generally CO 2 , H 2 O and N 2 , a
possible reaction equation of combustion can be written as
follows:
80La(NO 3 ) 3 (aq) 1 20Sr(NO 3 ) 2 (aq) 1 85Ga(NO 3 ) 3 (aq)
1 15Mg(NO 3 ) 2 (aq ) 1 314C 2 H 5 NO 2 (aq)
→ 100La 0.8 Sr 0.2 Ga 0.85 Mg 0.15 O 32d
1 785H 2 O (g ) 1 628CO 2 ( g)
(s)
1 440N 2 ( g)
(1)
Here, (aq), (s) and (g) means liquid, solid and gas state,
respectively.
2.2. Characterization
The differential thermal analysis (DTA) and thermogravimetric analysis (TGA) of synthesized powders were
carried out using a thermal analyzer (Netgsch, STA 499C)
in air, the powders were heated from 30 to 1200 8C at a
heating rate of 20 8C min 21 .
The phase analysis was determined by use of an X-ray
diffractometer (Rigaku, D/ Max-rA) with Cu Ka ( l 5
0.15418 nm), a step width of 0.028 and a scanning range of
20–808.
The synthesized powders were pressed uni-axially into a
cylinder sample of [ 636.5 mm at 200 MPa. The
sintering shrinkage was measured with a dilatometer
(Netgsch, DIL 402C), from room temperature to 1500 8C.
The thermal expansion coefficient was measured on the
cylinder sample, which sintered at 1500 8C for 6 h. The
dilatometer was calibrated by the Netgsch Al 2 O 3 standard.
For the above measurements, a heating rate of 5 8C min 21
was applied, argon was used as a purge gas, and the
applied flow rate was 60 ml min 21 .
Impedance spectra were measured using an impedance
analyzer (Solartron SI 1260) for the samples sintered at the
varying temperatures of 1400, 1450, 1500 and 1550 8C for
6 h, which were pressed uni-axially into pellets of 6 mm in
diameter and about 0.7–1.0 mm in thickness at 200 MPa.
Silver paste (DAD-87) was used as an electrode to be
painted on the two faces of the electrolyte pellet. The
measuring frequency range was from 0.5 Hz to 3.1 MHz,
and the temperature ranged from 250 to 850 8C in air. A
Z-view 2.0 software was used to analyze the impedance
data and to calculate the conductivity of the samples.
L. Cong et al. / Journal of Alloys and Compounds 348 (2003) 325–331
Fig. 1. DTA and TGA curves of the synthesized powders.
3. Results and discussion
3.1. Analysis of DTA–TGA
Fig. 1 shows the DTA–TGA curves of the synthesized
powders. It can be seen from Fig. 1 that three pronounced
endothermic peaks near 600, 740 and 1150 8C are indicated in the DTA curve, and two weak peaks accompanied
by them are near 400 and 790 8C. Two obvious exothermal
peaks appear near 650 and 855 8C. It is clear that the
decomposition of synthesized products and perovskite
phase forming undergo several stages, and different stages
may certainly be crossed and overlapped. By combining
reaction Eq. (1) and a phase analysis of XRD (see Table
1), three different stages may be roughly described as
follows: The first stage occurred before 600 8C, which can
be attributed mainly to the decomposition process of the
residual organic matter, that is the elimination of the
incomplete glycine combustion. The weak endothermic
peak corresponded to 400 8C in the DTA curve, and the
thermal weight loss of 5.1% is found in the TGA curve.
The second stage occurred over a temperature range of
600–800 8C; this stage is related to the decomposition of
carbonate and nitrate. Two endothermic peaks of 600 and
740 8C correspond in the DTA curve, and there is a
thermal weight loss of 11.6 and 12.8% in the TGA curve,
respectively. The third stage, which corresponds to a large
portion of unperovskite phase to perovskite phase transformation, occurred over a temperature range of 800–
327
Fig. 2. XRD of the synthesized powders and the samples sintered at
different temperatures: (s) LaGaO 3 ; (1) SrLaGa 3 O 7 ; (n) unknown
phase.
1200 8C. An extraordinary evident endothermic peak appears at 1150 8C in the DTA curve, and the corresponding
thermal weight loss is 16.5% in the TGA curve. It can be
seen from the TGA curve that the changing rate of the
thermal weight loss is smooth after 1150 8C. This shows
that the perovskite phase is formed essentially. Two clear
exothermic peaks appear at 650 and 850 8C in the DTA
curve, and thermal weight losses of 11.9 and 16.0%
correspond in the TGA curve. This may have been the
result coming from the escaped gases of CH 4 , N 2 , H 2 and
CO through different reaction processes [15,17].
3.2. Phase analysis of XRD
The XRD of synthesized powders and samples sintered
at different temperatures are shown in Fig. 2. It can be
seen from Fig. 2, the perovskite phase has existed in the
resulting powders, but the impurity phases exist clearly as
well. Note that the XRD of the sample sintered at 1000 8C
presents many intermediate phases. This further shows that
the forming process of the perovskite phase requires the
undergoing of different intermediate stages. This is in
keeping with the results made by DTA–TGA. The relatively complete diffractive peaks of the perovskite phase
has been observed in XRD of the sample sintered at
1200 8C, indicating the perovskite phase has been essentially formed, but the impurity phase still exists as well.
Table 1
Phase analysis under different sintering temperatures
Sintering
temperatures
Phase compositions
Unsintered powders
1000 8C36 h
1200 8C36 h
1400 8C36 h
Perovskite
Perovskite
Perovskite
Perovskite
SrLaGa 3 O 7
SrLaGa 3 O 7
SrLaGa 3 O 7
SrLaGa 3 O 7
Sr(NO 3 ) 2
La 4 Ga 2 O 9
Unknown phase
Unknown phase
Unknown phase
La 2 O 2 CO 3
Unknown phase
328
L. Cong et al. / Journal of Alloys and Compounds 348 (2003) 325–331
Only the trace impurity phase of SrLaGa 3 O 7 is detected in
the sample sintered at 1400 8C. This shows that a stable
perovskite phase has been formed completely sintered
above 1400 8C. In the synthesis of LSGM, the impurity
phases are rather difficult to eliminate, the impurity phases
depend strongly on the synthesized method and technology, and different impurity phases can be detected in the
used methods [18–20]. As for the intrinsic nature of
impurity presence, according to the phase diagram of the
quaternary system La 2 O 3 –SrO–MgO–Ga 2 O 3 [21], the
maximum solid solution of Sr and Mg in LaGaO 3 compositions is 6 and 7.5 mol%, respectively. Consequently,
when Sr and Mg contents exceed their solid solutions, the
presence of impurities, for example SrLaGa 3 O 7 in this
paper, must be inescapable. It can be seen from Table 1
that the resulting products contain mainly the impurity
phases of SrLaGa 3 O 7 , Sr(NO 3 ) 2 , La 4 Ga 2 O 9 , La 2 O 2 CO 3
and an unknown phase. In contrast, the products prepared
by conventional solid state method contain many impurity
phases, such as La 2 O 3 , MgO, SrLaGa 3 O 7 , Sr 3 Ga 2 O 6 ,
La 4 Ga 2 O 9 , La 3 Ga 5 O 12 , MgGa 2 O 4 , La 2 SrO x and
LaSrGaO 4 [20]. This means that the purity of the LSGM
synthesized by glycine–nitrate combustion method can be
enhanced and the impurity phases can be reduced or
eliminated so that it is favorable to improving the electrical
properties of the LSGM.
3.3. Determination of sintering temperature
temperature of 1435 8C is the sintering one for the LSGM
sample. In comparison with the sintering temperature of
LaGaO 3 -based electrolyte prepared by the conventional
solid state method, the sintering temperature of LSGM
synthesized by glycine–nitrate combustion method is at
least 100 8C lower than that of conventional solid state
method [17]. This is mainly due to the fact that ultra-fine
powders with a higher specific surface area can be
obtained using glycine–nitrate combustion method, resulting in the diffused paths that shorten and in which
sintering drive force increases, further reducing the sintering temperature of the LSGM as well. With further
increasing of the temperature, the sintering curve begins to
rise and shrinkage reduces. A point of inflexion appears at
approaching 1500 8C, indicating that the range of the
sintering temperatures have been reached. Therefore, the
sintering temperature range of the LSGM prepared by a
glycine–nitrate combustion method is around 1435–
1500 8C. In order to further identify the sintering temperatures and the sintering temperature range of the LSGM, a
real density experiment was performed for the sintered
samples using a pycnomter with deionized water as the
medium. In this study, the real density data is slightly
higher than that reported in literature [22]. However, the
results made by the real density experiment are in agreement with the results of the sintering shrinkage. That is,
the sintering temperature is near 1435 8C and the sintering
temperature range is over 1435–1500 8C.
Fig. 3 shows the sintering shrinkage curve of the
compact powder sample. It can be seen that the linear
shrinkage begins to descend sharply after a sintering
temperature of 1200 8C is reached, indicating the sintering
of the sample begun from 1200 8C. This is in agreement
with the analyzed results carried out by XRD. A maximum
shrinkage of the sample occurred at 1435 8C, showing the
3.4. Thermal expansion coefficients of the sample
Fig. 3. Sintering shrinkage curve for the synthesized powder compact
sample.
Fig. 4. Thermal expansion curves for the LSGM and YSZ samples
sintered at 1500 8C for 6 h.
Fig. 4 shows the linear thermal expansion plots of the
LSGM and the YSZ samples sintered at 1500 8C for 6 h. It
can be seen that two thermal expansion coefficients are
close before reaching 500 8C. As the temperature is
increased, however, the thermal expansion coefficient of
the LSGM sample is gradually higher than that of the YSZ
L. Cong et al. / Journal of Alloys and Compounds 348 (2003) 325–331
sample. The average thermal expansion coefficients of the
LSGM and the YSZ samples are 12.2310 26 and 10.33
10 26 per 8C over temperature ranges of 30–1000 8C,
respectively.
3.5. Electrical properties of sample
3.5.1. Impedance spectra
In general, impedance spectroscopy consists of bulk,
grain boundary conduction and interface processes between electrolyte and electrodes from high to low frequencies. The interceptions obtained from impedance
spectroscopy and real axle correspond to grain, grain
boundary or impurity and electrolyte / electrode interface
resistances [17], respectively. A series of resistances and
capacitances, or CPE (constant phase element), in series
and in parallel are used to represent an equivalent circuit,
which has been reported in previous papers by different
authors [23]. The impedance spectra measured from 300 to
800 8C for the LSGM samples sintered at 1500 8C for 6 h
are shown in Fig. 5. It can be seen from Fig. 5 that the
329
impedance spectroscopy measured at 300 8C is composed
of two depressed semicircles and a line, which corresponds
to bulk, grain boundary and electrode processes in the
samples. The impedance spectroscopy measured at 450 8C
consists of a depressed semicircle and an arc. The semicircle and arc is relative to grain conduction and interface
conduction between electrolytes / electrodes, respectively.
With the increasing of temperature, the semicircle becomes
small until it fully disappears; only one arc, which is due to
ion and electron transference at the interface between
electrolyte and electrodes, remains. In contrast with the
sample synthesized by conventional solid-state method
[17], the grain boundary resistance is obviously small in
the sample synthesized by glycine–nitrate combustion
method. This indicates that the LSGM synthesized by
glycine–nitrate combustion method is favorable for the
reduction of impurity phases, especially a glass phase
among grain boundary, and for improving the electrical
properties of the samples. In general, the impurities in
electrolyte do not conduct, for example, the impurities of
SrLaGa 3 O 7 and glass phase are isolators, the presence of
these impurities may lower the conductivity of LSGM. In
this study, because only a small amount of impurities exist
in LSGM, the effect of the impurities on the conductivity
of samples is relatively small.
3.5.2. Conductivity of samples
Fig. 6 shows Arrhenius plots of conductivity for LSGM
samples sintered at different temperatures. It can be seen
from Fig. 6 that the conductivity of the samples increases
gradually with increasing of the sintering temperatures.
The conductivity of the sample sintered at 1500 8C for 6 h
is higher than that of others, but the conductivity of the
sample sintered at 1550 8C for 6 h is somewhat lower than
that of 1500 8C. As aforementioned, it can be seen from
Fig. 3 that the sintering temperature range of the LSGM
sample is around 1435–1500 8C. An excessive sintering
Fig. 5. Impedance spectra for the LSGM sample sintered at 1500 8C for 6
h, measured at 300–800 8C.
Fig. 6. Arrhenius plots for the LSGM samples sintered at different
temperatures.
L. Cong et al. / Journal of Alloys and Compounds 348 (2003) 325–331
330
phenomenon will occur when the sintering temperature
exceeds a temperature of 1500 8C. In this case, the grains
grow excessively, and the pores are trapped among the
grains or grain boundaries, blocking oxygen ion migration,
leading to the decrease in the conductivity of the sample
[23]. Furthermore, a similar phenomenon was reported by
Stevenson et al. [24]. The volatilization of gaseous Ga 2 O
and O 2 from the LSGM at excessively high sintering
temperatures were detected by mass spectroscopy, resulting in the sample volumes bloating and density reduction, so that the conductivity decreases. Table 2 shows the
conductivity of samples sintered at different temperatures
and measured temperatures from 500 to 850 8C. In order to
conveniently compare it with YSZ, the conductivity of the
YSZ sample sintered at 1500 8C for 6 h are listed in Table
2. It can be seen that the conductivity of the LSGM
samples sintered above 1450 8C are higher than that of the
YSZ sample, showing that LSGM is a promising ITelectrolyte material.
3.6. Thermal stability of sample
In order to examine the thermal stability of the LSGM
synthesized by glycine–nitrate combustion method, DTA–
TGA was used to detect the thermal stability of the LSGM
sample sintered at 1500 8C for 6 h. Fig. 7 is DTA–TGA
curve of the LSGM sample in air. It can be seen that a
small endothermic peak appears at near 130 8C in the DTA
curve, and a thermal weight increases by 0.02% in TGA
curve. This is mainly due to the fact that LSGM surfaces
absorb oxygen from air during equilibrium at low temperatures. The evidence of the endothermal and exothermal
phenomena are insignificant over temperatures from 300 to
800 8C in the DTA curve, and the maximum thermal
weight loss is 0.32% near 650 8C in TGA curve. The other
small endothermic peak presents after 1150 8C, which is
mainly due to the loss of volatilization of Ga 2 O and
latticed oxygen at high temperatures [18,25]. It can be
deduced that the LSGM is essentially stable over temperatures from 500 to 800 8C, in which IT-SOFC operates one.
The thermal and chemical stability of the LSGM remains
to be extensively studied in reduced atmospheres over long
periods of time.
Fig. 7. Thermal stability of the LSGM sample by DTA–TGA.
4. Conclusion
The IT-electrolyte LSGM with perovskite structure was
synthesized by glycine–nitrate combustion method. The
results show that the decomposition of the synthesized
products and the forming of the perovskite phase underwent several stages. The decomposition of the residual
organic matter occurred before 600 8C. The decomposition
of the carbonates and nitrates were mainly corresponding
to the temperature range of 600–800 8C. A large portion of
the transformation from unperovskite phase into perovskite
phase occurred over a temperature range of 800–1200 8C.
The perovskite phase is essentially formed in the sample
sintered at 1200 8C for 6 h. The stable perovskite phase is
completely formed in the sintered sample above 1400 8C
for 6 h. The sintering temperature of the LSGM synthesized by glycine–nitrate combustion method was
around 1435 8C, which was lower than that of LSGM
prepared by conventional solid state method. The thermal
expansion coefficient of the LSGM is slightly higher than
that of the YSZ. The contribution of grain boundary to the
conductivity of the LSGM synthesized by this method is
smaller, indicating that the glycine–nitrate combustion
method is favorable in enhancing the purity of the LSGM.
The conductivity of the sample sintered at 1500 8C is
higher than that sintered at 1550 8C. The conductivity of
Table 2
Conductivities in the range of 500–850 8C for the samples sintered at different temperatures
Sintering
temperatures
1400 8C
1450 8C
1500 8C
1550 8C
YSZ
Conductivity (S cm 21 )
500 8C
600 8C
24
3.38310
1.04310 23
1.46310 23
1.37310 23
2.34310 24
700 8C
23
1.74310
5.35310 23
8.09310 23
6.91310 23
1.76310 23
800 8C
23
5.55310
1.66310 22
2.63310 22
2.13310 22
8.12310 23
850 8C
22
1.79310
3.28310 22
6.06310 22
4.33310 22
2.59310 22
3.36310 22
5.05310 22
7.82310 22
5.05310 22
4.47310 22
L. Cong et al. / Journal of Alloys and Compounds 348 (2003) 325–331
the LSGM is 7.8310 22 S cm 21 at 850 8C, which is
obviously higher than that of the YSZ at the same
temperature. As an IT-electrolyte, the LSGM is relatively
stable over intermediate temperature ranges.
Acknowledgements
This work is supported by Jilin Province Department of
Science and Technology. The authors wish to thank Dr
Zhong-hua Yan (Texas Biotechnology Corporation, Houston, TX) for helpful discussion.
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