Growth and Di-/Piezoelectric Properties of Al-Doped PMN-30PT Single Crystals Xifa Long,*,† Jibei Ling,† Xiuzhi Li,† Zujian Wang,† and Zuo-Guang Ye*,‡ Key Laboratory of Optoelectric Materials Chemistry and Physics, Fujian Institute of Research on Structure of Matter, Chinese Academy of Sciences, Fujian, Fuzhou 350002, China, and Department of Chemistry and 4DLABS, Simon Fraser UniVersity, 8888 UniVersity DriVe, Burnaby, British Columbia, V5A 1S6, Canada
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 2 657–659
ReceiVed August 9, 2008; ReVised Manuscript ReceiVed NoVember 26, 2008
ABSTRACT: Aluminum (Al3+)-doped Pb(Mg1/3Nb2/3)O3-30PbTiO3 (PMN-30PT) solid solution piezo-crystals were grown by a topseeded solution growth method. The dielectric, piezo-, and ferro-electric properties were characterized. A dispersive maximum of dielectric permittivity was found around TC ) 135 °C, which does not depend on frequency. Interestingly, the rhombohedral-tetragonal phase transition, which appears at 60 °C in pure PMN-PT and thereby causes the depoling of the crystals at a relatively low temperature, has been increased to 105 °C by means of the Al3+-doping. This extends the upper limit of application temperature for electromechanical transducers up to the TC. The piezoelectric coefficient (d33) was found to be 2000 pC/N at room temperature. The longitudinal electromechanical coupling factor k33 reaches 85-90%, which shows a good thermal stability upon heating up to 135 °C. The peak to peak bipolar strain was measured to be 0.25% at an electric field of up to 17 kV/cm. Single crystals of lead magnesium niobate-lead titanate solid solution, (1 - x)Pb(Mg1/3Nb2/3)O3-xPbTiO3 (PMN-xPT), has been paid considerable attention in the past several years due to its very large electromechanical coupling factors, high piezoelectric coefficients, high dielectric constants and low dielectric losses, which result in improved bandwidth, sensitivity, and source level in electromechanical sensing and actuating applications.1-14 Despite recent progress in the growth of these complex perovskite piezocrystals, some major issues remain to be solved. The main drawback is that PMN-PT single crystals of morphotropic phase boundary (MPB) compositions exhibit a rather low depoling temperature (TMPB ) 60-80 °C) due to a curved morphotropic phase boundary,9 allowing the materials to be depoled easily. This inherent drawback decreases the thermal stability of piezoelectric properties and reduces the acoustic power and the operation temperature range of the devices, thus limiting the use of PMNPT crystals with MPB composition in many applications. Therefore, there is urgent demand to develop new materials with high Curie temperature and high piezoelectric performance, or to increase the depoling temperature of the PMN-PT crystals by suppressing the rhombohedral-tetragonal phase transition by approximate chemical doping so as to extend the temperature range. Another drawback of the PMN-PT crystals is a low coercive field (2-3 kV/cm), making them unsuitable for high-field applications. Doping of an appropriate amount of M3+ ions on the B-site of the perovskite structure is expected to create cationic vacancies that help stabilize ferroelectric domains, and to generate an internal electric field that offsets the external applied electric field, resulting in an increase in the morphotropic phase transition temperature (TMPB or TRT) and also an increase in the coercive field Ec. This technique had been demonstrated to be effective in some Fe3+- and Mn3+-doped PMN-PT single crystals.15,16 It was based on this consideration that we have carried the doping of Al3+ ions on the B-site of the PMN-PT single crystals with a view to increasing the coercive field Ec and the depoling temperature TRT of the piezocrystals. Crystal Growth. Recently, we developed a top-seeded solution growth (TSSG) method which has allowed us to grow PMN-PT * To whom correspondence should be addressed. (X.L.) Phone: (86)-59183710369; fax: (86)-591-83710369; e-mail:
[email protected]; (Z.-G.Y.) phone: 1-778-782-8064; fax: 1-778-782-3765; e-mail:
[email protected]. † Fujian Institute of Research on Structure of Matter, Chinese Academy of Sciences. ‡ Simon Fraser University.
single crystals of large size and high quality.17 Compared with the other growth methods, the TSSG technique offers some advantages in growing single crystals of good quality and high compositional homogeneity. Therefore, the TSSG technique was used to grow the Al-doped PMN-PT single crystals in this work. A mixture of PbO and B2O3 (with a molar ratio of PbO/B2O3 ) 95:5) was used as a high-temperature solution. The starting chemicals, PbO (99.99%), TiO2 (99.99%), MgO (99.9%), Nb2O5 (99.9%), Al2O3 (99.9%), and B2O3 (99.9%), were weighed according to the stoichiometric composition of 0.68Pb[(Mg0.95/3Al0.1/9Nb2/ 3]O3-0.32PbTiO3 (solute) and the flux to solute molar ratio of 60: 40. The detailed growth process is similar to the one described in ref 15. At the end of TSSG growth, Al3+-doped PMN-PT single crystals with typical dimensions of 30 × 30 × 15 mm3 were obtained (as shown in Figure 1). The concentrations of Al3+, Mg2+, Nb5+, and Ti4+ ions in the as-grown crystal were determined to be 0.1334 atom %, 6.993 atom %, 14.7777 atom %, and 9.344 atom %, respectively, based on the data from ion coupled plasma (ICP) spectrometry (Ultima 2). Therefore, the actual composition of the grown crystal should be close to 70PMN-30PT. The X-ray powder diffraction pattern (Figure 2) shows a complex profile for the (200) peak (marked by arrow), indicating a mixture of rhombohedral and tetragonal/monoclinic phases,9 confirming that the composition of the grown crystals indeed falls into the MPB region. Characterization Procedures. A (001)-oriented crystal plate with dimensions of 6 × 6 × 0.8 mm3 cut from an as-grown crystal was polished and sputtered with gold layers as electrodes for electric measurements. The dielectric constant (ε′) and dielectric loss (ε′′) at various frequencies were measured as a function of temperature upon heating from -50 to 300 °C using a Novocontrol broadband dielectric spectrometer, with an AC signal of 1 V (peak-to-peak) applied. The same setup was used to measure the resonance (fa) and antiresonance (fr) frequencies of a bar sample. The electric fieldinduced strain was measured using an MTI 2000 Fotonic sensor (Range 1). The electric field applied and the strain measured were in the same direction, perpendicular to the electrode faces, that is, the (001) plane of the sample, giving rise to piezoelectric coefficient d33. The piezoelectric coefficient was measured using a ZJ-6B d33/ d31 Meter (Institute of Acoustics, Chinese Academy of Sciences). The polarization-electric (P-E) field hysteresis loops were displayed on the (001) platelet by means of a RT-66A standard ferroelectric test system (Radiant Technology).
10.1021/cg800878x CCC: $40.75 2009 American Chemical Society Published on Web 12/22/2008
658 Crystal Growth & Design, Vol. 9, No. 2, 2009
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Figure 1. As-grown PMN-30PT single crystal and a polished (001) platelet (scale in mm).
Figure 2. X-ray diffraction pattern of the as-grown PMN-30PT single crystal. The split of the (200) peak suggests the coexistence of rhombohedral and tetragonal phases, typical of MPB nature.
Properties and Discussion. (a) Dielectric Properties. The variations of ε′ as a function of temperature at the frequencies of 100 Hz, 1 kHz, 10 kHz, and 1 MHz are shown in Figure 3a. It can be seen that the dielectric constant shows a maximum at 135 °C, which does not depend on frequency, indicating no relaxor behavior. In order to determine the depolarization temperature, the sample was poled at an electric field of 10 kV/cm which was applied at 100 °C and kept on upon cooling down to -50 °C. The dielectric measurement was then performed upon zero field heating from -50 to 300 °C. The variations of ε′ as a function of temperature at different frequencies is shown in Figure 3b. A dielectric anomaly was observed at TR-T ) 105 °C. Compared with the pure PMN32PT,9 the morphotropic rhombohedral-tetragonal phase transition temperature (TR-T) of Al-doped PMN-30PT increases significantly from about 60 °C in undoped PMN-32PT to 105 °C. Therefore, TR-T have been increased by means of the doping of Al3+ ions,
which extends the upper limit of application temperature for electromechanical transducers up to the TC. (b) Piezoelectricity. Figure 4 shows the strain-electric field relations for the same (001)-oriented Al-doped PMN-30PT sample under an unipolar drive (a) and a bipolar drive (b), respectively. A peak-to-peak strain value of 0.25% is reached at E ≈ ( 17 kV/cm. The typical butterfly-like curve reflects the ferroelectric domain switching during the bipolar drive. The piezoelectric coefficient d33 was calculated to be 2000 pC/N based on the slope of the unipolar strain curve, which is consistent with the value of 1950 pC/N measured by the d33 meter. For the resonance measurement, a -oriented crystal bar with the dimensions of 0.7 × 0.7 × 7 mm3 was cut from the asgrown crystal, polished, and sputtered with gold layers on the (001) faces as electrodes. It was then poled with an electric field of 10 kV/cm applied at room temperature for 5 min. The resonance (fr) and antiresonance (fa) frequencies for the sample at various temperatures were measured, and the longitudinal electromechanical coupling factor k33 was derived by the following equations:18,19
k233 )
(
π fa - fr π fr cot 2 fa 2 fa
)
(1)
Figure 5 shows the variation of k33 as a function of temperature for the Al-doped PMN-30PT crystal. It can be seen that k33 reaches a value of 85% which is maintained in the temperature range of 25 °C up to about 100 °C, before dropping down at the temperature TC ) 135 °C. This upper limit of temperature is higher than that of the PMN-xPT crystal with MPB compositions. (c) Ferroelectricity. The ferroelectric properties of the Al-doped PMN-30PT crystals were developed by the well-developed polarization-electric field hysteresis loops measured at various temperatures, as shown in Figure 6. The saturation of polarization is achieved at an electric field of (12.5 kV/cm at room temperature. Such a hysteresis loop with almost vertical lines indicates the sharp
Figure 3. Variations of dielectric constant (ε′) and dielectric loss (ε′′) of the (001)-oriented Al-doped PMN-30PT single crystal, measured upon heating: (a) unpoled; (b) poled (zero field heating after poling).
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Figure 4. Variation of strain as a function of unipolar (a) and bipolar (b) electric field drives for the Al-doped PMN-30PT single crystal.
phase transition was detected by a dielectric anomaly at TR-T ) 105 °C in the poled-sample. The value of TR-T is much higher than that of the pure PMN-32PT crystals of MPB compositions (∼60 °C), thus extending the upper limit of the depolarization temperature. The (001)-oriented single crystal shows a maximum value of ε′max ) 16 800 at 1 kHz at the Curie temperature TC (Tmax) ) 135 °C. The piezoelectric coefficient d33 was found to be 2000 pC/N, with a strain level reaching 0.25% at 17 kV/cm. The longitudinal electromechanical coupling factor k33 reaches 85%, which is maintained upon heating up to 100 °C. Compared with the PMN-PT crystal of MPB compositions so far developed, the Al-doped PMN-30PT single crystals exhibit a higher upper limit of depolarization temperature, thus offering a wider temperature range for applications in electromechanical transducers. Figure 5. Longitudinal electromechanical coupling factor k33 of a (001)oriented Al-doped PMN-30PT crystal as a function of temperature.
Acknowledgment. This work were supported by the Key Project from FJIRSM (SZD08002) and the U.S. Office of Naval Research (Grant No. 00014-06-1-0166).
References
Figure 6. Polarization vs electric field (P-E) hysteresis loops displayed on the (001)-oriented Al-doped PMN-30PT crystal.
switching of macroscopic domains in the crystal, which is facilitated by the defects introduced by the doping of Al3+ ions in the crystal lattice, as expected from our initial concept. The remnant polarization reaches Pr ≈ 25 µC/cm2, with a coercive electric field of EC ≈ 3.2 kV/cm. Conclusions. The Al-doped piezoelectric single crystals of PMN30PT were grown in the perovskite structure by the top-seeded solution growth method. The temperature and frequency dependences of dielectric permittivity (ε′) indicate maxima at a Curie temperature of 135 °C, which does not depend on frequency, indicatingnorelaxorbehavior.Themorphotropicrhombohedral-tetragonal
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