Thermal Stability of (1�x)BiScO3�xPbTiO3 Piezoelectric Ceramics forHigh-Temperature Sensor Applications

Si Chen, Xianlin Dong,w Chaoliang Mao, and Fei Cao

Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 Dingxi Road, Shanghai 200050, China

High-temperature piezoelectric (1�x)BiScO3�xPbTiO3 ce-ramics with composition (x5 0.62, 0.63, 0.64, and 0.65) nearthe morphotropic phase boundary were fabricated. A thermaldepoling experiment was used to investigate the stability of po-larization at the temperature range from 2001 to 4601C. Theresults indicated that samples with rhombohedral phase began todepole when the temperature was higher than 3001C, while sam-ples with a tetragonal phase showed good resistance to thermaldepoling up to 4401C, close to their Curie temperature.

I. Introduction

PIEZOELECTRIC sensors are being developed for extreme-tem-perature applications. These devices may find applications in

space exploration, electric aircraft, oil and geothermal well-drill-ing tools, and automotive smart brakes. Many of these appli-cations require an operation temperature higher than 3001C.1

The traditional piezoelectric materials based on Pb(ZrxTi1�x)O3

(PZT) exhibit a TC around 3601C (PZT5A), with a piezoelectriccoefficient d33 of about 370 pC/N. However, these materials canonly be used below 2001C due to the loss of polarization.2 Re-cently, bismuth-based perovskite systems Bi(Me)O3–PbTiO3

(Me5Sc31, In31, Yb31, etc.) were found to possess much high-er Curie temperatures.3 Specifically, the (1�x)BiScO3�xPbTiO3

system was found to have a Curie temperature around 4501Cand excellent piezoelectric properties near the MPB composi-tion, which make this material a promising candidate for high-temperature sensor applications.3–9

The piezoelectricity in ferroelectric ceramics is conditionedby the existence of a microscopic polarization induced by astrong external electrical field. However, the poled piezoelectricceramics often begin to lose their piezoelectric properties at atemperature somewhat below the Curie temperature due to thethermal instability of polarization. Thermal depoling of ferro-electrics determines the upper temperature limit of their appli-cation as piezoelectrics.10–11 However, there is very limitedinformation on the thermal depoling behavior of (1�x)BiScO3�xPbTiO3 materials. It is the intent of this work to in-vestigate systematically the thermal stability of polarization of(1�x)BiScO3�xPbTiO3 with compositions near the MPB. Ma-terials with different phase structures are fabricated, and theirthermal depoling behaviors are studied. The final goal is to finda best composition for a high-temperature piezoelectric sensorapplication.

II. Experimental Procedure

Ceramic powders of (1�x)BiScO3�xPbTiO3 (x5 0.62, 0.63,0.64, and 0.65) were prepared using a conventional solid-state

synthesis. The raw starting materials were Bi2O3 (99.9%), TiO2

(99.38%), Sc2O3 (99.9%), and Pb3O4 (99.72%). They weremixed stoichiometrically, considering the volatilization ofBi2O3 and PbO. Aqueous suspensions of raw materials wereball milled with stabilized zirconia media for 24 h, and dried at1201C. The dried powders were calcined at 7501C for 6 h, andthen ball milled again for 24 h to crush the agglomerates. Afterdrying, 4 wt% PVA was mixed into the powders. The mixturewas dried, and crushed to pass through a 40-mesh sieve. Thepowders were then pressed into disks. Following an 8001C bind-er burnout, pellets were then sintered in sealed crucibles at11001C for 90 min.

X-ray diffraction (XRD) was performed using an automateddiffractometer (Model Rigaku RAX-10 D/max 2550V, RigakuCo., Tokyo, Japan) with CuKa1 radiation operated at roomtemperature to determine phase assemblage and purity withindetection limits. For electrical measurements, the sintered sam-ples were electroded by a silver paste and poled in an oil bathunder a field of 40 kV/cm at 1001C for 15 min. The piezoelectricconstant was measured using a d33 meter (Model ZJ-3D, Inst-itute of Acoustics, Beijing, China), electromechanical couplingcoefficient kp was measured using Agilent HP4294A (40 Hz–110MHz, Hewlett-Packard, Palo Alto, CA), and dielectric constantsand loss were measured using a precision LCRmeter (HP4284A,Hewlett-Packard, Palo Alto, CA) connected to a furnace.

The thermal depoling experiments were conducted by holdingthe poled samples with Ag electrodes for 10 h at various hightemperatures, cooling to room temperature, measuring their d33and kp value, and repeating the procedure up to 4601C, abovetheir Curie temperature.

III. Results and Discussion

Compositions in the (1�x)BiScO3�xPbTiO3 system near theMPB are fabricated by solid-state method and characterizedwith XRD. As presented in Fig. 1, a typical rhombohedral sym-metry is observed at room temperature while x5 62 mol%PbTiO3. The presence of a rhombohedral to tetragonal MPBis indicated from XRD patterns when x5 0.63 mol% PbTiO3.The tetragonal phase is observed while x5 0.64 and 0.65 mol%PbTiO3. The lower symmetry phases, being either rhombohedralor tetragonal, are identified by splitting of pseudocubic perovs-kite peaks {hkl}, either {111} splitting for rhombohedral, or{110} and {100} splitting for tetragonal symmetry. This resultis in agreement with the result reported by Eitel et al.4

The temperature dependences of the dielectric constant at afrequency of 1 kHz for (1�x)BiScO3�xPbTiO3 ceramics areshown in Fig. 2. Transition temperatures TC of 4321C and4351C were found when x5 62 and 63 mol% PbTiO3, whilesamples with composition x5 64 and 65 mol% PbTiO3 wereabout 101C higher. The detailed dielectric and piezoelectricproperties of (1�x)BiScO3�xPbTiO3 ceramics are summarizedin Table I. Similar to PZT, the properties show a strong com-positional dependence near the MPB region. Poled samplesshow peak values of electromechanical planar couplingkp5 0.56 and d335 460 pC/N all for 0.37BiScO3–0.63PbTiO3.

Journal

J. Am. Ceram. Soc., 89 [10] 3270–3272 (2006)

DOI: 10.1111/j.1551-2916.2006.01201.x

r 2006 The American Ceramic Society

3270

R. Eitel—contributing editor

wAuthor to whom correspondence should be addressed. e-mail: xldong@sunm.shcnc.ac.cn

Manuscript No. 21651. Received March 31, 2006; approved May 15, 2006.

It is observed that samples with a tetragonal phase show slightlyworse piezoelectric properties, but their dielectric losses aremuch lower than the material with a mixed phase (x5 0.63).

Figure 3 shows the effect of thermal depoling on the piezo-electric properties of (1�x)BiScO3�xPbTiO3 ceramics. The pi-ezoelectric coefficient d33 and planar coupling factor kp aremeasured at room temperature after annealing for 10 h at thex-axis temperature. The d33 and kp values of all compounds de-crease slightly with temperature up to 2001C and are then stableup to 3001C.When the annealing temperature is near TC, the d33and kp values of all samples decrease rapidly, and tend to zerowhen the temperature is increased above TC. The thermal de-poling behavior of materials differs when the annealing temper-ature is higher than 3001C. Samples with a tetragonal phase(x5 0.64 and 0.65) can resist from depoling even when anneal-ing at 4401C, showing excellent thermal stability. Samples withrhombohedral phase (x5 0.62) or mixed phase (x5 0.63) be-

came thermally unstable when the temperature is higher than3001C. Their piezoelectric properties showed a significant de-cline when annealed at 4001C, and their piezoelectric activitywas almost lost when the temperature was higher than 4201C.

It is well known that the MPB composition exhibits excellentpiezoelectric properties. The existence of two thermodynamical-ly equivalent phases leads to the production of a highly domain-oriented material during the required poling process.12–14 It isobserved from the experimental results that the MPB composi-tion becomes thermally unstable and their piezoelectric proper-ties begin to degrade far below the Curie point. Althoughmaterials with a tetragonal phase have worse piezoelectric prop-erties than the MPB composition at room temperature, they canresist from depoling even approaching the Curie temperature,showing excellent thermal stability.

IV. Conclusions

(1�x)BiScO3�xPbTiO3 ceramics with the composition near theMPB were fabricated. Thermal depoling experiments were car-ried out to investigate the thermal stability of the polarization

Fig. 1. X-ray diffraction patterns of (1�x)BiScO3�xPbTiO3 with com-positions near the MPB.

Table I. Detailed Piezoelectric and Dielectric Properties of (1�x)BiScO3�xPbTiO3 Ceramics

Composition Phase structure

Curie

temperature (1C)

Dielectric

constant, er (1 kHz)

Dielectric

loss (1 kHz)

Piezoelectric

coefficient, d33 (pC/N)

Coupling

factor, kp

x5 0.62 Rhombohedral 432 850 0.025 320 0.53x5 0.63 Mixed 435 1210 0.042 460 0.56x5 0.64 Tetragonal 442 1420 0.020 305 0.47x5 0.65 Tetragonal 443 1550 0.019 345 0.49

Fig. 2. Temperature dependence of dielectric constant at a frequencyof 1 kHz for (1�x)BiScO3�xPbTiO3.

Fig. 3. Effect of thermal depoling on piezoelectric properties: (a) pie-zoelectric coefficient d33; (b) electromechanical planar coupling factor kp.

October 2006 Communications of the American Ceramic Society 3271

for samples with different phase structures. The tetragonal-structured materials showed good resistance to thermal depo-ling up to the temperature close to their Curie point. However,samples with a rhombohedral or mixed phase began to depolewhen the temperature was higher than 3001C, far below theirCurie point. The results suggested that the polarization of thesample with a tetragonal phase was thermally more stable thanthat of the sample with a rhombohedral or mixed phase.

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3272 Communications of the American Ceramic Society Vol. 89, No. 10