Preparation, sintering, and ferroelectric properties of
layer-structured strontium bismuth titanium oxide ceramics
Chung-Hsin Lu*, Chung-Han Wu
Department of Chemical Engineering, National Taiwan University, Taipei, Taiwan, ROC
Received 11 January 2001; received in revised form 22 May 2001; accepted 29 May 2001
Abstract
The preparation and ferroelectric properties of layer-structured SrBi4Ti4O15ceramics were investigated in this study. During the
solid-state reaction, the formation of SrBi4Ti4O15started at 700C, and the pure layered structure was completely formed at 900C.
The obtained SrBi4Ti4O15powder was found to be hardly sintered at low temperatures. Raising the sintering temperature higher
than 1100C led to an increase in the density of SrBi
4Ti4O15ceramics; however, a secondary phase having a pyrochlore structure
was also produced on the surface of the ceramics due to the thermal decomposition reaction. This thermal decomposition was caused by the vaporization of bismuth species outward from the SrBi4Ti4O15ceramics at elevated temperatures. In order to
sup-press the decomposition reaction, a process of covering SrBi4Ti4O15compacts on the sintered ceramics was developed. This process
successfully resulted in highly densified SrBi4Ti4O15 ceramics having the pure layer-structured phase. These well-sintered
SrBi4Ti4O15ceramics exhibited good ferroelectric properties with high remnant polarization (2Pr=13.52 mC/cm2) and low coercive
field (2Ec=52.31 kV/cm). # 2002 Published by Elsevier Science Ltd.
Keywords:Ferroelectric properties; Phase composition; Reaction; Sintering; SrBi4Ti4O15
1. Introduction
Recently the families of ferroelectrics receive great attention for their use in the nonvolatile ferroelectric random access memory (FeRAM).1 Among these
fer-roelectrics, perovskite Pb(Zr, Ti)O3 (PZT) had been
extensively investigated for the characteristics of large remnant polarization (Pr), moderately low coercive field
(Ec), and high Curie temperature.2,3However, there is a
serious polarization fatigue problem when using plati-num as the electrodes of PZT thin films.4,5 The other
class of ferroelectric materials of recent interest is the bismuth layer-perovskite ferroelectrics.6 This type of
material exhibits good ferroelectric properties including moderate remnant polarization, low coercive field, long retention, and low tendency to imprint. Most important of all, layer-perovskite materials exhibit excellent fatigue endurance in comparison with PZT and its family.7,8
The layer-perovskite materials described by Aurivillius are named after him as Aurivillius compounds.911The
chemical formula of Aurivillius compounds is
(Bi2O2)2+(Ax1BxO3x+1)2, where A represents the
12-fold coordinated cation with low valences in the per-ovskite sublattice; B denotes the octahedral site with high valences; x is the number of octahedral layers in the perovskite block between the rock-salt type (Bi2O2)2+ layers along the c axis. Strontium bismuth
titanate is one of the family of layer-perovskite ferro-electrics (SrBi4Ti4O15, x=4). The SrBi4Ti4O15thin films
are demonstrated to possess promising ferroelectric properties and low fatigue of the remnant polarization after 1010cycling with platinum electrodes.12In order to
apply ferroelectric materials to IC industry, several processes have been adopted for depositing ferroelectric thin films. In physical vapor deposition (PVD) pro-cesses, such as radio-frequency (rf) sputtering and laser deposition, the preparation of dense ceramic targets is an important issue. However, the sinterability and the behavior of SrBi4Ti4O15 ceramics under high
tempera-tures have not been investigated in detail. In this study, the reaction mechanism of SrBi4Ti4O15in the solid-state
reaction was examined for synthesizing the pure com-pound. The sintering behavior and the microstructural development of SrBi4Ti4O15 in three different sintering
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* Corresponding author. Tel.: 3635230; fax: +886-2-3623040.
processes were studied. The ferroelectric properties of the obtained SrBi4Ti4O15ceramics were also investigated.
2. Experimental
The starting materials included high-purity SrCO3
(Aldrich, 99% pure), Bi2O3(Aldrich, 99.9% pure), and
TiO2 (Aldrich, >99% pure). These materials were
weighed according to the stoichiometric composition of SrBi4Ti4O15, followed by ball-milling in ethanol for 48
h, using zirconia balls in a polyethylene jar. After mil-ling, the slurry was dried in a rotary evaporator under a reduced pressure. Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were per-formed to investigate the thermal behavior of the dried mixtures. The dried powder was heated in air from 500 to 1000C without soaking and then quenched to room
temperature. The other 2 h-calcination was also per-formed from 800 to 1200C. The phases present in the
quenched and calcined samples were identified by X-ray powder diffraction (XRD) analysis using CuKa
radia-tion. The calcined powder was first treated by ultrasonic vibration using ethyl alcohol as a dispersion agent, and then a laser diffraction method was performed to measure the particle size.
PVA (3 wt.%) was added to the obtained SrBi4Ti4O15
powder to study the sintering behavior. The powder mixtures were pressed into disks under a pressure of about 200 MPa and sintered by three different processes at 1000–1200 C for 2 h. The schematic diagrams of
these three processes are shown in Fig. 1. Process I was to sinter the pellets without any covering in air. In pro-cess II, a powder-bed consisting of SrBi4Ti4O15powder
was employed to cover the pressed pellets during sin-tering. In process III, monophasic SrBi4Ti4O15 powder
was first pressed at 30 MPa, then laminated on the pressed pellets containing the calcined powder, and the
sintering process was performed in the powder bed as described in process II.
The phase variation and composition of the sintered ceramic surface were analyzed by XRD and energy dis-persive X-ray spectroscopy (EDS), respectively. The microstructure of the sintered ceramics was observed by a scanning electron microscope (SEM). To measure the ferroelectric properties, silver paste was painted on the surface of the sintered SrBi4Ti4O15ceramics and heated
at 500 C. The hysteresis behavior was analyzed by a
Sawyer-Tower circuit and recorded on a digital oscillo-scope.
3. Results and discussion
3.1. Formation of SrBi4Ti4O15in the solid-state reaction
The starting materials of SrBi4Ti4O15 were heated
with a heating rate of 10 C/min at elevated
tempera-tures and quenched to room temperature. Fig. 2 shows the XRD patterns of the quenched specimens, and the variation of the relative intensity of each phase is illu-strated in Fig. 3 based on the results of Fig. 2. The relative intensity of each compound was calculated from the ratio of the intensity of the specific diffraction peak of each compound to the sum of the intensity of the specific diffraction peak of all compounds. A small amount of Bi2O3 and an unidentified compound were
present at 400 C. Bi
12TiO20 was formed and the
amount of unidentified phase was reduced at 500C. At
600 C, another intermediate phase-Bi
4Ti3O12was also
produced. When the temperature was raised to 700C,
SrBi4Ti4O15 started to form with the presence of a
minor amount of SrTiO3 and residual Bi12TiO20. The
amount of SrBi4Ti4O15 increased rapidly with a rise in
heating temperatures. The single phase SrBi4Ti4O15was
obtained at 1000C as all SrTiO
3disappeared.
In the DSC/TGA thermal analysis, a significant weight loss of the starting materials was detected at temperatures ranging from 500 to 600 C, and an
endothermic peak was observed in the same tempera-ture range in DSC. This weight loss was ascribed to the release of carbon dioxide from SrCO3 in the starting
materials. There was another sharp exotherm detected just below 700C in DSC. The exothermal reaction was
deduced to be the formation of SrBi4Ti4O15. From the
XRD analysis, the formation of SrBi4Ti4O15 can be
expressed by the following equation:
Bi4Ti3O12þSrTiO3 ! SrBi4TiO15 ð1Þ
In order to lower the temperature for synthesizing SrBi4Ti4O15, the starting materials were calcined at
var-ious temperatures for 2 h. It was found that SrTiO3still
existed in the sample after calcination at 800C. Once
the calcination temperature reached 900C, the single
phase SrBi4Ti4O15 was obtained. The XRD pattern of
synthesized SrBi4Ti4O15is consistent with that reported
in literature.13The phase of SrBi
4Ti4O15had no change
between 1000 and 1100 C. According to the particle
size analysis, the 900C-calcined powder had a narrow
size distribution, with the average particle size around 2.2 mm.
3.2. Sintering of SrBi4Ti4O15ceramics
The obtained SrBi4Ti4O15 powder was pressed and
sintered at various temperatures for 2 h by using process I. As shown in Fig. 4, after sintering at 1050 C, the
density of the sintered ceramics remained nearly unchanged just like the green density of the compacts. Sintering at higher temperatures led to an increase in
Fig. 2. X-ray diffraction patterns of the starting materials of SrBi4Ti4O15quenched at various temperatures.
Fig. 3. Relative intensity of resultant phases in the starting materials
density. The density of the 1200 C-sintered pellets
reached a maximum of 92.7% relative density (the the-oretic density of SrBi4Ti4O15=7.448 g/cm3).
Never-theless, there existed a large amount of glassy scraps on the surface of this specimen, and a weight loss around 1.5 wt.% was found. The surfaces of the sintered pellets were examined by XRD. Fig. 5 indicates that after 1050C-sintering, only SrBi
4Ti4O15 existed in the
sam-ple. However, at temperature higher than 1100 C, a
secondary phase was observed. The amount of
SrBi4Ti4O15gradually decreased with increasing
tempera-ture. The secondary phase became the major compound when the sintering temperature reached 1200 C. The
above phenomena are attributed to high-temperature decomposition reaction. The powder ground from the 1200 C-sintered sample was also examined by XRD
and only a small amount of secondary phase was detected. According to the XRD pattern, the secondary phase formed on the surface of decomposed samples was found to have a crystal structure of pyrochlore phase. This pyrochlore phase was also observed by Nibou et al. when they prepared SrBi4Ti4O15 thin
films.14The microstructure of decomposed SrBi 4Ti4O15
ceramics is shown in Fig. 6. The pyrochlore phase pos-sessed a plate-like morphology with an exceptionally large dimension greater than 10 mm.
The composition of the surface of sintered pellets was analyzed by the energy dispersive X-ray spectroscopy (EDS). The surface composition of the ceramics sintered at 1050 C was very close to stoichiometry. However,
for the 1200 C-sintered samples, the compositional
ratio of Sr: Bi: Ti was 1: 1.2: 4.1. This indicates that the
amount of bismuth species had a 70 mol% reduction while other components remained almost unchanged. According to the EDS analysis, decomposition reaction was due to the volatilization of bismuth species at ele-vated temperatures. Similar deficiency behavior of bis-muth species was also found during the sintering of another bismuth-layered compound-SrBi2Ta2O9.15
The 1150 C-sintered samples were polished at
var-ious depths from the surface and examined by XRD. Fig. 7 indicates that as the depth from the surface
Fig. 5. X-ray diffraction patterns for the surface of SrBi4Ti4O15
cera-mics sintered at (a) 1050C, (b) 1100C, (c) 1150C, and (d) 1200C
for 2 h.
Fig. 7. X-ray diffraction patterns of the 1150C-sintered SrBi
4Ti4O15
ceramics at various depth from sample surface.
Fig. 6. Scanning electron micrograph of the SrBi4Ti4O15 ceramics
increased, the intensity of pyrochlore phase gradually decreased while SrBi4Ti4O15 became the major phase.
The amount of pyrochlore phase present in the ceramics was semi-quantified by calculating the ratio of pyro-chlore intensity to the sum of the intensity of pyropyro-chlore (py) and SrBi4Ti4O15(SBTi) [Ipy(444)/(Ipy(444)+ISBTi(119))].
Fig. 8 illustrates the relationship between the quantity of pyrochlore phase and the distance from bulk surface. The amount of pyrochlore phase decreased steeply near the pellet surface while the tendency of reduction became moderate as the depth of ceramic increased to 20 mm. It was found from this diagram that the decom-position reaction also extended to the interior part of the pellets with considerable depth. Since the thermal decomposition was attributed to the vaporization of bismuth species from the surface, the formation of pyr-ochlore phase inside the samples is considered to result from the diffusion of bismuth species from interior to the surface, and the concentration gradient of bismuth species was attributed to the driving force of diffusion. 3.3. Modified sintering processes of SrBi4Ti4O15
ceramics
Since the sintering of SrBi4Ti4O15 is accompanied
with thermal decomposition at high temperatures, sup-pressing the formation of pyrochlore phase is necessary for producing high quality ferroelectric ceramics. The results in Section 3.2 indicate that the decomposition reaction started from the surface of ceramics; therefore, process II that heated SrBi4Ti4O15 compacts in a
pow-der-bed was designed to prevent the exposure of bulk surface to air. The obtained pure-phase SrBi4Ti4O15
powder was used in this process. Fig. 9 reveals the sintering behavior of SrBi4Ti4O15ceramics in process II.
The density of the ceramics was elevated to 96% of the theoretical density after sintering at 1200C. The XRD
pattern of the surface of the sintered pellets in process II is shown in Fig. 10. Although the major phase of the ceramic surface was perovskite SrBi4Ti4O15, a small
amount of pyrochlore phase was also formed on the
Fig. 8. The relationship between the relative amount of pyrochlore and the depth from the sample surface.
Fig. 9. Density of SrBi4Ti4O15 ceramics sintered in process II and
process III.
Fig. 10. X-ray diffraction patterns of the surface of SrBi4Ti4O15
cera-mics sintered in (a) process I, (b) process II, and (c) process III at 1200C.
surface of the pellets. This result indicates that the decomposition reaction cannot be completely inhibited by the powder bed process.
For obtaining pure-phase and well-sintered
SrBi4Ti4O15ceramics, a pellet-laminated process (process
III) was designed to suppress the diffusion of bismuth species by laminating other ceramic compacts. In order to avoid unnecessary reaction at high temperatures, the pellets adopted in this process were the compacts that pressed slightly from monophasic SrBi4Ti4O15 powder.
As shown in Fig. 9, the density of ceramics reached a maximum of about 97.2% relative density after sinter-ing at 1200 C. Not only the dense ceramics were
obtained, this process also produced pure SrBi4Ti4O15
ceramics without decomposition as shown in Fig. 10(c). Fig. 11 shows the micrographs of SrBi4Ti4O15ceramics
obtained by process III. The microstructure of 1100
C-sintered SrBi4Ti4O15 was very porous, fine grains with
grain size around 0.1–0.8 mm were observed in the spe-cimen [shown in Fig. 11(a)]. Apparent grain growth occurred as the sintering temperature raising to 1150C
[Fig. 11(b)]. Although the structure became denser with the growing grains, a number of voids still existed in the specimen. As shown in Fig. 11(c), raising the sintering temperature to 1200 C significantly reduced the
num-ber of pores and densified the SrBi4Ti4O15ceramics. The
grains were further coarsened to 1.4–1.7 mm. The varia-tion of microstructures confirms the results of sintering density obtained previously.
3.4. Ferroelectric properties of SrBi4Ti4O15ceramics
The electric properties of the 1200 C-sintered
samples prepared by three different sintering processes were measured. For the sample obtained by process I, no hysteresis behavior was detected because of the occurrence of a serious leakage. From the micro-structure observed previously, the large leakage current is attributed to the insufficient connection of the inter-face between ceramics and silver electrodes. The ferro-electric properties of the 1200 C-sintered SrBi
4Ti4O15
obtained by the powder bed process (process II) are shown in Fig. 12(a). A well-developed P–E loop was obtained, and the remnant polarization (2Pr) and
coer-cive field (2Ec) were 12.4 mC/cm2 and 82.2 kV/cm,
respectively. The decomposition-free SrBi4Ti4O15
cera-mics obtained by the pellet-laminated process (process III) also exhibited a well saturated hysteresis loop as shown in Fig. 12(b). The ferroelectric properties of the ceramics prepared by process III (2Pr=13.5 mC/cm2,
2Ec=52.3 kV/cm) are superior to those of the specimens
sintered by process II. The differences in the ferro-electric properties observed between these two processes are ascribed to the existence of pyrochlore phase which will reduce the spontaneous polarization and increase the coercive field. The above ferroelectric properties of
Fig. 11. Scanning electron micrographs of the SrBi4Ti4O15ceramics
sintered in process III at (a) 1100C, (b) 1150C, and (c) 1200C for
SrBi4Ti4O15 are better than those previously reported16
because the decomposition of SrBi4Ti4O15 at elevated
temperatures was suppressed in this study. Moreover, the highly densified SrBi4Ti4O15ceramics possess larger
remnant polarization than SrBi2Ta2O9 ceramics.17 The
densified SrBi4Ti4O15 ceramics obtained in this study
not only offer more information on the intrinsic prop-erties of bismuth-layer structured materials, but can also be applied to the PVD processes for preparing SrBi4Ti4O15thin films.
4. Conclusions
The formation and sintering processes of SrBi4Ti4O15
were investigated in this study. SrBi4Ti4O15 started to
form at 700 C and the reaction was completed after
calcination at 900C. SrBi
4Ti4O15ceramics could not be
densified at low temperatures. Sintering at temperature higher than 1050C led to an increase in the density of
SrBi4Ti4O15 ceramics; however, it also caused the
decomposition reaction of SrBi4Ti4O15. The thermal
decomposition of SrBi4Ti4O15 was attributed to the
vaporization of bismuth species. Moreover, the diffu-sion of bismuth species caused the decomposition reac-tion to extend to the interior of ceramics. Highly densified SrBi4Ti4O15 ceramics with pure layered
struc-ture were obtained by the pellet-laminated process after sintering at 1200C. Well-sintered SrBi
4Ti4O15ceramics
exhibited relatively good ferroelectric properties com-pared with other layer-perovskite compounds. The remnant polarization (2Pr) and coercive field (2Ec) for
the decomposition-free SrBi4Ti4O15 ceramics were 13.5
mC/cm2and 52.3 kV/cm, respectively.
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