J. Am. Ceram. Soc., 90 [9] 2759–2765 (2007)
DOI: 10.1111/j.1551-2916.2007.01831.x
r 2007 The American Ceramic Society
Journal
Phase Evolution During Formation of SrAl2O4 from SrCO3 and
a-Al2O3/AlOOH
Yu-Lun Chang and Hsing-I Hsiangw
Department of Resources Engineering, Particulate Materials Research Center, National Cheng Kung University, Tainan
701, Taiwan, Republic of China
Ming-Tsai Liang
Department of Chemical Engineering, I-Shou University, Kaohsiung 840, Taiwan, Republic of China
cess,5 precipitation method,6 Pechini process,7 and combustion
method.8 Among them, solid-state reaction has the highest potential to satisfy industrial applications and is the major process
in SrAl2O4 preparation. Many studies usually conducted the
solid-state reaction to prepare SrAl2O4 using SrCO3 and Al2O3
as the raw materials.4,9,10 However, the reaction mechanism between SrCO3 and Al2O3 has not been fully elucidated.
Considering the formation of BaTiO3 from the solid-state reactions involving BaCO3 and TiO2, different sequences have
been proposed.11,12
Through the execution of experimental investigation, thermogravimetry, X-ray diffractometry, Fourier transforminfrared spectrometry, transmission electron microscopy, and
energy-dispersive spectrometry, a variant reaction mechanism
model was proposed for the solid-state reaction between SrCO3
and Al2O3/AlOOH for formation of SrAl2O4 material. The
solid-state reaction is observed to be dependent on the calcination temperature. At temperatures lower than the transformation temperature of SrCO3 from orthorhombic to hexagonal
(9201C), the reaction is attributed to the interfacial reaction between SrCO3 and alumina. Conversely, at temperatures higher
than that, the solid-state reaction is dominated by the diffusion
of Al31 ions into the SrCO3 lattice. In this mechanism, two
metastable species, hexagonal SrCO3 and hexagonal SrAl2O4,
were observed. The activation energies of SrCO3 decomposition
in the solid-state reaction also support these results. The interfacial reaction at low temperatures is characterized by a high
activation energy of B130 kJ/mol; whereas, in the reaction at
higher temperatures, the activation energy of SrCO3 decomposition decreases to 34 kJ/mol.
(1) First Scheme
(a) Decomposition of BaCO3 according to
BaCO3 ! BaO þ CO2
(1-1)
(b) Formation of Ba2TiO4 by reaction between the two oxides:
2BaO þ TiO2 ! Ba2 TiO4
(1-2)
I. Introduction
T
(c) Synthesis of BaTiO3 according to
SrAl2O4 is well known as a host material for long
persistent phosphors.1 It is a stable compound in the
SrO–Al2O3 system. The stable crystalline phase at ambient temperature is monoclinic, which would transform to hexagonal via
an endothermic process at temperatures of about 6501C. Conversely, the reverse hexagonal–monoclinic transformation occurs exothermically at those temperatures.2 SrAl2O4 has a
tridymite structure constructed by corner-sharing AlO4 tetrahedra. The charge deficiency introduced by the occupancy of Al31
ions in the tetrahedra is compensated for by the incorporation of
Sr21 ions within the interstitial sites. Therefore, it is called the
stuffed tridymite structure.3 For monoclinic SrAl2O4, there are
two interstitial sites for cations to occupy, i.e. Sr(I) and Sr(II).
These sites provide different spatial symmetry and orientation
direction that result in various emission properties of substituted
optical center, like Eu21 ions.4 The typically broad band spectrum centered at 520 nm of Eu-doped SrAl2O4 phosphor is attributed to the emission of Eu21 ions at Sr(II).
Recently, many processes have been proposed to prepare
SrAl2O4 phosphors, such as solid-state reaction,4 sol–gel proHE
Ba2 TiO4 þ TiO2 ! 2BaTiO3
(1-3)
(2) Second Scheme
(a) Decomposition of BaCO3 according to reaction (1);
(b) Formation of BaTiO3 by direct reaction between the
oxides:
BaO þ TiO2 ! BaTiO3
(1-4)
(c) Formation of Ba2TiO4 at the expense of BaTiO3 according
to
BaO þ BaTiO3 ! Ba2 TiO4
(1-5)
(d) Finally, Ba2TiO4 reacts with TiO2 to form BaTiO3 according to reaction (3).
Although both schemes are plausible, identification of the reaction mechanism requires a detailed microscopic investigation
of the growth process at the interface between the two reactants
and identification of the prevailing diffusing species. Recently,
Lotnyk et al.13 investigated the phase-formation sequence during BaTiO3 formation in thin-film diffusion couples and observed that the decomposition of BaCO3 before its participation
M. Rigaud—contributing editor
Manuscript No. 22352. Received October 12, 2006; approved April 4, 2007.
This work was financially co-sponsored by the Ministry of Economic Affairs of the
Republic of China through contract (92-EC-17-A-08-S1-023) and National Science Council
of the Republic of China (NSC94-2216-E-006-026).
w
Author to whom correspondence should be addressed. e-mail: n4893115@
ccmail.ncku.edu.tw
2759
2760
Journal of the American Ceramic Society—Chang et al.
in the reaction is rather unlikely and that the intermediate
Ba2TiO4 formed by direct reaction between BaCO3 and rutile
always precedes the formation of BaTiO3.
Furthermore, some investigators obtained novel results while
the solid-state formation of BaTiO3 was conducted at certain
specific temperatures. Buscaglia et al.14 discovered that the isothermal kinetic behavior of the solid-state reaction between
BaCO3 and TiO2 at 7001C is reasonably comparable to the diminishing-core reaction using the Valensi–Cater equation. However, the data at higher temperatures, like 8001C, cannot be
satisfactorily described using any diminishing-core models. The
authors regarded the results as the complexity in the reaction
mechanism, which cannot be reduced to the simple case of
phase-boundary-controlled growth or diffusion-controlled
growth over the whole temperature range. Bera and Sarkar15
synthesized BaTiO3 using barium oxalate and TiO2 and showed
that the formation of BaCO3 resulted from decomposition of
barium oxalate, decreased slowly below 8001C, and then rapidly
decreased above 8001C. The rapid decomposition of BaCO3 at
above 8001C is considered to be caused by the Hedvall effect,
which is due to the critical polymorphic transformation of
BaCO3.
The alkaline earth carbonates, MCO3 (M 5 Ca, Sr, and Ba),
have several interesting properties. At certain temperatures, the
MCO3 would transform the structure polymorphically by the
rotational disorder transition of the anion group at a certain
temperature, such as the transformation of CaCO3 from orthorhombic to hexagonal at 4701C, SrCO3 from orthorhombic to
hexagonal at 9201C, and BaCO3 from orthorhombic to hexagonal at 8001C (further to cubic at 9701C).16 The polymorphic
transformation results in an increase of crystal symmetry and
volumetric expansion. As these phase transformations are often
reversible, the high-temperature phase of MCO3 cannot be easily observed at room temperature. Nishino investigated the stabilization of the high-temperature modification of MCO3 at
room temperature by quenching from the phase transformation
temperature in the presence of a small amount of BaSO4.
The stabilization mechanism was attributed to a partial replace2
17
ment of the CO2
3 ions in the host with SO4 ions. In addition,
thermal decomposition of carbonates often occurs after polymorphic phase transformation at higher temperatures. Nevertheless, Sweeney18 observed that the thermal decomposition
temperature of the carbonates can be lowered drastically by
adding certain oxides. For instance, the thermal decomposition
temperature of SrCO3 was lowered from 11501 to 11021C by
adding a specific amount of Al2O3.
The reaction mechanism of solid-state formation of SrAl2O4
from SrCO3 and Al2O3 may be studied by the similar concepts
about the solid-state preparation of BaTiO3 as well. It should be
pointed out that the importance of the polymorphic transformation on the solid-state reactivity of alkaline earth carbonates
and the effect of SrCO3 polymorphic transformation on the
formation of SrAl2O4 has not been reported yet. Therefore,
SrCO3 was separately reacted with a-Al2O3 and AlOOH in this
study. The formation of SrAl2O4 in the solid-state reaction was
investigated by thermogravimetry (TG), X-ray diffractometry
(XRD), Fourier transform-infrared spectrometry (FT-IR),
transmission electron microscopy (TEM), and energy-dispersive
spectrometry (EDS). TG was used to investigate the kinetics of
decomposition of SrCO3 in the solid-state reaction between
SrCO3 and a-Al2O3/AlOOH.
II. Experimental Procedure
(1) Materials Preparation
The powders used in this study were high-purity orthorhombic
SrCO3 (Alfa Aesar, Ward Hill, MA; 99%, SBET 5 4.18 m2/g,
d50 5 630 nm), a-Al2O3 (Alfa Aesar, 99.9%, SBET 5 7.82 m2/g,
d50 5 220 nm), and AlOOH (Alfa Aesar, 20% in H2O/HNO3,
after drying, SBET 5 76.34 m2/g, d50 5 80 nm). A suspension
containing 1:1 molar ratio of SrCO3 and a-Al2O3 with a small
Vol. 90, No. 9
amount of ethanol was mixed using ball milling for 6 h. The
ratio was 1:2 when using AlOOH. Subsequently, the well-mixed
powder was dried at 1201C for 12 h and ground slightly using an
agate mortar.
(2) Thermal Treatment
To observe the phase transformation effect on the reaction between SrCO3 and a-Al2O3, thermal treatment was carried out at
8501 and 9501C with a heating rate of 101C/min. Calcination
was held for 0, 1, 3, and 6 h, respectively. All thermal treatments
were conducted in air and under furnace cooling.
(3) Characterization
(A) XRD: Crystalline phase identification was performed using XRD (Siemens, D5000, Karlsruhe, Germany)
with CuKa radiation. XRD scanning conditions were set at
0.041 per step and the counting time at 1 s for each step.
(B) FT-IR: The FT-IR spectrometer (Bruker, EQUINOX 55, Billerica, MA) was used to investigate the molecular
and coordination structure of the specimens. The specimen was
dispersed in KBr powder at a 1:9 ratio and determined using the
DRIFT apparatus.
(C) TEM/EDS Analysis: The TEM (JEOL, JEM-3010,
Tokyo, Japan) was used to observe the size and morphology of
the specimens. The electron diffraction patterns of the crystalline
species were also obtained using the TEM with the camera constant at 80 cm. Semiquantitative determination of the element
content was detected using the EDS (Noran, Voyager 1000,
Waltham, MA) attached to the TEM.
(D) TG and Kinetic Study: Isothermal TG was performed using the thermal analysis instrument (Netzsch STA,
409 PC, Burlington, MA) under an airflow rate of 40 mL/min.
Thermal treatment was carried out at a heating rate of 101C/min
and held at the desired temperatures of around 8501 and 9501C
for 1 h. The kinetics of the thermal decomposition of SrCO3 was
approximated by the Avrami equation, which describes the variation in conversion (a) with time (t) as19,20
aðtÞ ¼ 1 expðkðTÞtn Þ
(2-1)
where k(T) is the reaction constant.
The temperature dependence of k(T) can be expressed as an
Arrhenius equation:
kðTÞ ¼ A expðEa =RTÞ
(2-2)
where A is the temperature-independent frequency factor; R, the
gas constant; and T, the absolute temperature.
III. Results and Discussion
(1) Reaction Between SrCO3 and a-Al2O3 at 8501C
Figure 1 shows the XRD patterns of the specimen calcined at
8501C for different durations. These patterns reveal that the
formation of monoclinic SrAl2O4 (34-0379) and cubic Sr3Al2O6
(24-1187) increases slowly with the increase in the holding time,
accompanied by a gradual reduction in the quantities of SrCO3
(05-0418) and a-Al2O3 (46-1212). Holding for 3 h, considerable
quantities of SrCO3 and a-Al2O3 are still noted. The diffraction
peaks of SrCO3 nearly disappeared by further holding for 6 h.
The TEM micrograph and EDS results for specimens calcined at 8501C for 1 h are shown in Fig. 2. At the Interface 1 and
Interface 2 sites, small amounts of monoclinic SrAl2O4 are observed at the interfacial region between SrCO3 and a-Al2O3.
Moreover, the cubic Sr3Al2O6 phase can be further observed at
the interfacial region between SrAl2O4 and SrCO3 (Interface 3 in
Fig. 2). These results are similar to the microstructural observations reported by Beauger et al.12 Therefore, the solid-state
formation of SrAl2O4 and Sr3Al2O6 at 8501C can be attributed
to the interfacial reaction between SrCO3 and a-Al2O3. The
September 2007
Phase Evolution During Formation of SrAl2O4
Fig. 1. X-Ray diffraction patterns of specimens of SrCO3–a-Al2O3
system calcined at 8501C held for different duration, (A) 0 h, (B) 1 h,
(C) 3 h, and (D) 6 h.
formation of Sr3Al2O6, although expected from the SrO–Al2O3
phase diagram, is likely to be enhanced by the local excess of
SrCO3 because of the much larger particle size of the carbonate
in comparison with alumina. Besides, the SrO decomposed from
SrCO3 was not observed, so that the decomposition of SrCO3
before its participation in the reaction is unlikely as well and the
interfacial reactions between SrCO3 and a-Al2O3 may be demonstrated as follows:
SrCO3 þ Al2 O3 ! SrAl2 O4 þ CO2
(3-1)
2SrCO3 þ SrAl2 O4 ! Sr3 Al2 O6 þ 2CO2
(3-2)
Sr3 Al2 O6 þ 2Al2 O3 ! 3SrAl2 O4
(3-3)
(2) Reaction Between SrCO3 and AlOOH at 8501C
Figure 3 shows the XRD patterns of the specimen containing
SrCO3 and AlOOH calcined at 8501C for different durations.
For the specimen calcined at 8501C without holding time, only
orthorhombic SrCO3 and the amorphous species attributed to gAl2O3 derived from AlOOH were observed. The amount of
SrCO3 has been shown to decrease drastically with the concomitant appearance of the monoclinic SrAl2O4 for the specimen
calcined for 1 h. As the holding time was increased to 3 h, SrCO3
nearly disappeared and monoclinic SrAl2O4 and cubic Sr3Al2O6
increased gradually. The formation mechanism of SrAl2O4 for
the specimen containing SrCO3 and AlOOH calcined at 8501C is
shown to be similar to that of the specimen containing SrCO3
and a-Al2O3 calcined at the same temperature.
Figure 4 represents TG curves between SrCO3 and a-Al2O3
and between SrCO3 and AlOOH, revealing the thermal behavior
differences of the two systems during the calcination process.
2761
Fig. 2. Transmission electron micrograph and energy-dispersive spectrometry results of the specimen of the SrCO3–a-Al2O3 system calcined at
8501C for 1 h.
The specimen containing SrCO3 and a-Al2O3 did not lose weight
until the temperature reached a value close to 9001C. Nevertheless, the total weight loss of the specimen containing SrCO3 and
AlOOH, which further includes the weight loss of B5001C related to the dehydration of AlOOH, nearly completed at 9001C.
It indicates that the decomposition of SrCO3 involved in the
above-mentioned interfacial reaction, whether in the formation
of SrAl2O4 or Sr3Al2O6, is more rapid for the specimen containing AlOOH. This is probably due to the nanocrystalline
g-Al2O3 derived from AIOOH dehydration, providing a large
surface area to facilitate the reaction with SrCO3.
(3) Reaction Between SrCO3 and a-Al2O3 at 9501C
Figures 5 shows the XRD patterns of the calcined specimens at
9501C. Small quantities of hexagonal SrCO3 and hexagonal
SrAl2O4 (31-1336) started to appear at the beginning. By increasing the holding time to 10 min, the quantities of both hexagonal SrCO3 and hexagonal SrAl2O4 increased rapidly and
emerged to become the major phases in the specimen, while the
amount of orthorhombic SrCO3 decreased significantly. As the
holding time was raised to 20 min, the cubic Sr3Al2O6 and
monoclinic SrAl2O4 became the major phases instead of the
hexagonal SrCO3 and hexagonal SrAl2O4. A small amount of aAl2O3 remained. By increasing the holding time at 9501C for a
period above 1 h and longer, the quantities of cubic Sr3Al2O6
and a-Al2O3 both decreased gradually to form the pure monoclinic SrAl2O4. Therefore, the phase evolution at 9501C is quite
different from that observed at 8501C and characterized by the
appearance of metastable phases, hexagonal SrCO3 and hexagonal SrAl2O4.
The FT-IR results provide further evidence to support the
phase evolution observed in the XRD patterns. According to the
FT-IR spectra (Fig. 6), the absorbance of Al–O stretching in
the AlO4 tetrahedron (gs, 700 cm1) appeared in the specimen at
2762
Journal of the American Ceramic Society—Chang et al.
Fig. 3. X-ray diffraction patterns of the specimens of the SrCO3–
AlOOH system calcined at 8501C held for different duration, (A) 0 h,
(B) 1 h, (C) 3 h, and (D) 6 h.
9501C without being held. As the holding time increased, the
absorbance of Al–O stretching (gs) became more obvious. By
increasing the holding time to 20 min, the peak of Al–O stretching (gs) is noted to split and other absorbance like O–Al–O
(ss, 447–420 cm1), Al–O (gas, 780–900 cm1), and O–Al–O
(sd, 550–650 cm1) may be observed, attributable to the decrease in the symmetry of the AlO4 tetrahedron.21,22 Therefore,
the FT-IR result readily reveals that the formation of the targeted SrAl2O4, made up of the AlO4 tetrahedron, started to occur at 9501C. Subsequently, SrAl2O4 formation was increased
by prolonging the holding time. As the holding duration was
Fig. 4. Thermogravimetry curves of the reactions, (A) SrCO3 and
AlOOH, and (B) SrCO3 and a-Al2O3.
Vol. 90, No. 9
Fig. 5. X-ray diffraction patterns of specimens of SrCO3–a-Al2O3 system calcined at 9501C held for different duration, (A) 0 h, (B) 10 min, (C)
20 min, (D) 1 h, (E) 3 h, and (F) 6 h.
increased to above 20 min, the decrease in the symmetry of the
structure illustrates a critical phase transformation of SrAl2O4
from hexagonal (P6322) to monoclinic (P21). The Sr–CO3 (band
1
combination, 1768–1774 cm1)23 and CO2
3 (nd, 1320–1530 cm
1 24
and nS, 1020–1100 cm ) peaks represent the absorbance of
the SrCO3 compound. These absorbance peaks are rather high
for the specimen at 9501C for 10 min, indicating the occurrence
of hexagonal SrCO3. Additionally, the small amount of the
CO2
3 group still represented in the specimen at 9501C for 1 h
may be attributed to the absorption of CO2 from the air.25
The TEM micrograph and EDS results of the specimen treated at 9501C are shown in Fig. 7. Both Interface 4 and Interface 5
spots represent the interfaces between SrCO3 and a-Al2O3. A
sequence of various species can be observed at Interface 5. According to the EDS results and the electron diffraction patterns,
the areas at E, F, and G represent the phases of a-Al2O3, SrCO3,
and hexagonal SrAl2O4, respectively. Furthermore, the electron
diffraction pattern could not be collected on area H because
phase transformation occurred under the influence of the electron beam. Area H is primarily composed of Sr atoms and a
small amount of Al atoms and, consequently, it can be attributed to hexagonal SrCO3, in agreement with XRD (Fig. 5) and
FT-IR (Fig. 6) results. A small amount of Al31 ions is supposed
to enter the SrCO3 structure, thus stabilizing the hexagonal
phase at lower temperatures.
Douy and Capron2 reported that the hexagonal SrAl2O4 can
be stabilized at room temperature by the presence of excess Al31
ions. Shi and co-workers also suggested that oxygen defects play
an important role in stabilizing the high temperature phase,
hexagonal SrAl2O4, at room temperature.26,27 The defect chemistry of SrAl2O4 with excess Al31 ions can be represented as
September 2007
Phase Evolution During Formation of SrAl2O4
Fig. 6. Fourier transmission-infrared spectra of specimens of SrCO3–aAl2O3 system calcined at different temperatures, (A) 8501C, 0 h, (B)
9501C, 0 h, (C) 9501C, 10 min, (D) 9501C, 20 min, and (E) 9501C, 1 h.
x
Al2 O3 ! 2AlAl x þ V 00Sr þ V O þ 3OO
(3-4)
Therefore, the incorporation of excess Al2O3 in the SrAl2O4
and the corresponding nonstoichiometry results in the increase in
oxygen vacancy concentration, providing some clues to the stabilization of the hexagonal SrAl2O4 at room temperature. [Correction added after online publication 17 July 2007: in equation
3-4, AlAlx was corrected to 2AlAl x ; Oox was corrected to 3Oox].
From the results of the TEM observation and the previous
considerations, it is proposed that the reaction between SrCO3
and a-Al2O3 at 9501C, which involves the formation of hexagonal SrCO3 and hexagonal SrAl2O4 as transient phases, is dominated by diffusion of Al31 ions in the SrCO3 lattice. Al
diffusion can be enhanced by the orthorhombic to hexagonal
transformation because the high-temperature modification has a
lower density.
(4) Kinetic Study of the Reaction Between SrCO3 and Al2O3
The decomposition of SrCO3 was studied by separately mixing
equal moles of a-Al2O3 and AlOOH feed materials, employing
different techniques, as the isothermal setting. Figure 8 shows
the decomposition fraction of SrCO3 as a function of time at
different calcination temperatures. It indicates that the decomposition of SrCO3 with a-Al2O3 at temperatures above 9301C
and with AlOOH at temperatures below 8501C both finished
within 60 min, while those with a-Al2O3 below 8901C did not.
Figure 9 shows a plot of ln(ln(1a)) vs ln t with a regression
line fitted, and the slope provides the exponent n. Moreover, the
intercept of the linear fitting represents the kinetic constant,
k(T). According to the Arrehenius equation (Eq. (2-2)), the
activation energy (Ea) for the decomposition of SrCO3 can be
determined from the slope of the ln K vs 1/T plot in Fig. 10.
Table I shows the activation energies, exponent n and which
2763
Fig. 7. Transmission electron micrograph and energy-dispersive spectrometry results of the specimen of the SrCO3–a-Al2O3 system calcined at
9501C without hold.
standard deviations of SrCO3 with the addition of AlOOH
calcined at 8301–8501C, separately, and a-Al2O3 calcined at
temperature intervals of 8401–8901C and 9301–9801C, respectively. The activation energy for the SrCO3–AlOOH system
specimens at temperatures around 8301–8501C is shown to
have a mean value of 130.21 kJ/mol, which is close to that of
the SrCO3–aAl2O3 system specimens at temperatures around
8401–8901C, having a mean value of 125.94 kJ/mol, is almost
four times larger than those found for the SrCO3–a-Al2O3 specimens at temperatures around 9301–9801C (34.01 kJ/mol). In
addition, the average time exponent n is also shown to be analogous to the pattern for activation energy derivations. The average time exponent n is noted to be relatively close to unity
(0.89) for the specimen with a-Al2O3 at temperatures around
9301–9801C; however, other two specimens calcined at lower
temperatures show exponent n, having values much lower than
0.5 (0.45 for a-Al2O3 and 0.39 for AlOOH).
The activation energy of SrCO3 decomposition was influenced
significantly by the calcination temperature rather than
by the nature of alumina precursor, i.e. a-Al2O3 versus AlOOH
(or g-Al2O3). Based on the above discussion, the formation mechanism of SrAl2O4 can be formulated as follows: (1) when the
calcination temperature is lower than the SrCO3 phase transformation temperature (9201C), such as 8501C, the formation mechanism of SrAl2O4 involves the interfacial reaction between SrCO3
and Al2O3 (likewise the formation of BaTiO3 from BaCO3 and
TiO2) and is characterized by a higher activation energy (B130
kJ/mol) for SrCO3 decomposition; (2) at calcination temperatures
higher than the SrCO3 phase transformation temperature, such as
9501C, the SrAl2O4 formation mechanism is dominated by the
diffusion of small Al31 ions, which drastically lower the difficulty
of the reaction. Therefore, the activation energy for SrCO3
decomposition decreases significantly.
2764
Journal of the American Ceramic Society—Chang et al.
Fig. 8. Decomposition fraction of SrCO3 as a function of time at different calcination temperatures, (A) SrCO3 and a-Al2O3, and (B) SrCO3
and AlOOH.
In addition, the exponent n values can be categorized into two
different reaction types28:
n ¼ b þ g ðinterface-controlledÞ
n ¼ b þ g=2 ðdiffusion-controlledÞ
(3-5)
(3-6)
where b is the number of steps involved in nucleation (b 5 1 for
constant nucleation rate, b 5 0 for instantaneous nucleation,
and 0obo1 for decelerating nucleation) and l the number of
dimensions in which the nuclei grow (l 5 3 for spheres or hemispheres, l 5 2 for disks or cylinders, and l 5 1 for linear
development).
In this study, the exponent n of the specimens calcined at
lower temperatures are B0.5. This should be the diffusion-controlled reaction involved with instantaneous nucleation (b 5 0)
and linear development (l 5 1). On the other hand, the decomposition at higher temperatures has exponent nB1, so that it can
be of two possible types, i.e. the diffusion-controlled reaction
involved with instantaneous nucleation (b 5 0) and disks or cylinders development (l 5 2), and another interface-controlled reaction involved with instantaneous nucleation (b 5 0) and linear
development (l 5 1). Hancock and Sharp29 reported that the
decomposition of BaCO3 is a diffusion-controlled mechanism
(nB0.5) at low temperatures, and a boundary-controlled mechanism (nB1.07) at high temperatures. In Tani et al’s.30 study
of formation of BaPbO3 in the BaCO3 and PbO system, the
exponent n close to 1 can be attributed to the phase-boundarycontrolled reaction mechanism. Thus, the phase-boundarycontrolled reaction involved with instantaneous nucleation and
Vol. 90, No. 9
Fig. 9. Plots of ln(ln (1a)) vs ln t for specimens calcined at different
temperatures, (A) SrCO3 and a-Al2O3, and (B) SrCO3 and AlOOH.
linear development is more favored for the decomposition of
SrCO3 in the high-temperature reaction.
IV. Conclusions
The solid-state reaction between SrCO3 and Al2O3/AlOOH is
observed to be dependent on the calcination temperature. While
the calcinations are conducted at temperature below the transformation temperature of SrCO3 from orthorhombic to hexagonal (9201C), such as 8501C, it performs as an interfacial
reaction like the solid-state formation of BaTiO3. In this temperature interval, the reaction between SrCO3 and AlOOH is
facilitated probably due to the large surface area of nanocrystalline g-Al2O3 derived from AlOOH dehydration. Conversely, at temperature higher than the SrCO3 transformation
temperature, 9501C, the solid-state reaction is dominated by
diffusion of Al31 ions in the SrCO3 lattice. Al diffusion can be
enhanced by the orthorhombic to hexagonal transformation of
SrCO3 because the high-temperature modification has a lower
density.
The kinetic study of SrCO3 decomposition in the solid-state
reaction reveals that the activation energy of SrCO3 decomposition was influenced by the calcination temperature rather than
the nature of alumina precursor. The interfacial reaction at low
temperatures is characterized by a high activation energy of
B130 kJ/mol; whereas, in the reactions at higher temperatures
the activation energy of SrCO3 decomposition decreases drastically to 34 kJ/mol. On the other hand, the exponent n shows that
the decomposition of SrCO3 in the reaction at low temperature
is the diffusion-controlled reaction involved with instantaneous
nucleation and linear development (nB0.5). While in the decomposition of SrCO3 at the high temperatures, the reaction is
September 2007
Phase Evolution During Formation of SrAl2O4
2
Fig. 10. Arrhenius plots for various specimens, (A) SrCO3 and
a-Al2O3, at T 49201C; (B) SrCO3 and a-Al2O3, at To9201C; and
(C) SrCO3 and AlOOH, at To9201C.
Table I. Calculated Exponent (n) and Activation Energies
(Ea) for the Thermal Decomposition of SrCO3 in the Solid-Sate
Reaction
Kinetic
parameters
Ea
s
n
s
SrCO31a-Al2O3
840–8901C
SrCO31a-Al2O3
930–9801C
SrCO31AlOOH
830–8501C
125.94 KJ/mol
0.02031
0.45
0.00761
34.01 KJ/mol
0.00115
0.89
0.00922
130.21 KJ/mol
0.00984
0.39
0.00578
supposed to be diffusion-controlled and involved with instantaneous nucleation and linear development (nB1).
Acknowledgments
The authors would like to thank Miss L. Z. Wang and Mr. M. Z. Lin of National Sun Yat-sen University for assistance in TEM photography.
References
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