Top-seeded solution growth and characterization of rhombohedral PMN–30PT piezoelectric single crystals

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Acta Materialia 55 (2007) 6507–6512 www.elsevier.com/locate/actamat

Top-seeded solution growth and characterization of rhombohedral PMN–30PT piezoelectric single crystals Xifa Long, Zuo-Guang Ye

*

Department of Chemistry, Simon Fraser University, 8888 University Drive, Burnaby, BC, Canada V5A 1S6 Received 20 January 2007; received in revised form 17 June 2007; accepted 5 August 2007 Available online 24 September 2007

Abstract Piezoelectric single crystals from 70Pb(Mg1/3Nb2/3)O3–30PbTiO3 solid solution were grown by a top-seeded solution growth method that prevented phase segregation and promoted (0 0 1) growth. Rhombohedral symmetry was confirmed by polarized light microscopy and the dielectric, piezo- and ferro-electric properties were characterized. A dispersive maximum of dielectric permittivity appears around TC = 125 C, which slightly shifts towards higher temperatures with increasing frequency, indicating weak relaxor behaviour. Rhombohedral–tetragonal phase transition was detected at 105 C. This temperature is higher than the depoling temperature of the morphotropic phase boundary compositions (50–80 C), thus extending the upper limit of the application temperature for electromechanical transducers. The piezoelectric coefficient (d33) was found to be 1240 pC N1, with a strain level reaching 0.12% at an electric field of 12 kV cm1. The longitudinal electromechanical coupling factor k33 reaches 85%, which persists upon heating up to 100 C.  2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: PMN–PT solid solution; Crystal growth; Piezoelectric properties

1. Introduction Single crystals of lead magnesium niobate-lead titanate solid solution, (1  x)Pb(Mg1/3Nb2/3)O3  xPbTiO3 (PMN– 100xPT), constitute a new generation of piezoelectric materials because of their 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]. Their excellent piezoelectric properties occur in compositions within the morphotropic phase boundary (MPB) region, especially close to the boundaries between the rhombohedral and monoclinic phases and the monoclinic and tetragonal phases. This is because the materials with MPB compositions possess multiple polarization states (dipole orienta-

*

Corresponding author. Tel.: +1 604 291 3351; fax: +1 604 291 3765. E-mail address: [email protected] (Z.-G. Ye).

tions) which can be switched more susceptibly by an electric field drive, making the materials more electrically active, thereby enhancing the piezoelectric response [15]. Despite recent progress in the growth of these complex perovskite piezocrystals, some major issues remain to be solved. Compositional variations usually occur along the boule of the Bridgman-grown PMN–PT crystals because of the intrinsic liquid/solid phase segregation of the solid solution system and also because of typical power and concomitant temperature fluctuations during growth and consequently, the property varies within wafers and from wafer to wafer cut from the grown crystals [16–18]. This has hampered the use of relaxor single crystals in medical and other industrial imaging devices and undersea transducers, for which high consistency in composition and properties of the material is essential. In addition, PMN– PT crystals tend to grow at the fastest rate along the Æ1 1 1æcub direction in the Bridgman technique as a result of the close-packed ionic structure in the (1 1 1) planes. This results in significant wastage of material when the crystal is

1359-6454/$30.00  2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.08.009

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X. Long, Z.-G. Ye / Acta Materialia 55 (2007) 6507–6512

compared to compositions within the MPB region. Moreover, the composition is close enough to the MPB for a good piezoelectric performance to be preserved to a large extent. Based on this consideration, we have investigated the synthesis and characterization of the PMN–30PT single crystals. This paper reports the synthesis by the top-seeded solution growth method and the properties of these rhombohedral piezoelectric crystals. 2. Crystal growth

Fig. 1. Phase diagram of the PMN–PT solid solution (after Noheda et al. [9]).

sliced into (0 0 1)-oriented wafers and unavoidable variations in composition and properties within wafers. Furthermore, PMN–PT crystals exhibit the highest piezoelectric response in the Æ0 0 1æ direction. Therefore, one of the challenges is to develop alternative techniques to grow piezocrystals of improved uniformity and consistency in composition and thereby the properties. On the other hand, the PMN–PT single crystals with MPB compositions exhibit a rather low depoling temperature (TMPB = 50–80 C) owing to a curved morphotropic phase boundary [9], making the materials depole 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 PMN–PT crystals with MPB composition in many applications. Examination of the phase diagram of the PMN–PT solid solution [9,10] (as shown in Fig. 1) indicates that the MPB composition range is within 30.5–38% of the PT-content. The composition of PMN–30PT solid solution falls in the rhombohedral phase region and should possess a relatively high rhombohedral to tetragonal phase transition temperature, TMPB. Therefore, the PMN–30PT crystals should exhibit a higher depoling temperature limit, resulting in a wider temperature range for applications

So far a number of experiments have been carried out on the growth of PMN–PT single crystals by various methods, including flux growth and modified Bridgman growth [5,6,16–22]. However, no reports have been available on the growth of PMN–PT crystals by the top-seeded solution growth (TSSG) technique. Compared with other methods, the TSSG technique offers some advantages in growing single crystals of good quality, high compositional homogeneity and controlled morphology thanks to its unique temperature field design and slow growth process. Therefore, the TSSG technique was applied to the growth of PMN–30PT 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%) and B2O3 (99.9%), were weighed according to the stoichiometric composition of PMN–30PT (solute) and a flux to solute molar ratio of 60:40. The weighed chemicals were thoroughly mixed and loaded into a platinum crucible 50 mm3 in volume, which were then placed into a vertical tubular furnace to melt. A small PMN single crystal was used as seed and the saturation temperature of the solution was determined accurately by repeated seeding trials. The crystal growth took place upon cooling from 1180 C to 1100 C at a rate of 0.2 C h1. At the end of a slow cooling process, the grown crystal was pulled out of the melt surface and then cooled down to room temperature at a rate of 15 C h1. A brown quadrate single crystal with dimensions of 17 · 17 · 15 mm3 was obtained, as show in Fig. 2. The morphology of the grown crystal suggests a twodimensional growth mechanism in the (0 0 1) plane, leading

Fig. 2. As-grown PMN–30PT single crystal and a polished (0 0 1) platelet (scale in mm).

X. Long, Z.-G. Ye / Acta Materialia 55 (2007) 6507–6512

to a quadrate shape with large (0 0 1) facets. In order to confirm the face direction, an X-ray diffraction analysis was employed to determine the crystal facet direction as shown is Fig. 3. Only two clear (0 0 l) peaks appear in the pattern, confirming the (0 0 1) orientation for the as-grown facet. In order to determine the composition homogeneity of the PMN–30PT crystal, the compositions of 12 points on a crystal plate (as shown in Fig. 4) were analysed by means of electron probe microanalysis (EPMA). Table 1 summarizes the measured results. It can be seen that the average value of Ti/(Mg + Nb + Ti) is 0.292 with a fluctuation range of ±0.014 despite the large area probed. These results indicate that the relative composition of the TSSG-grown PMN–PT single crystals is more homogenous than crystals grown by other methods, such as Bridgman growth. A (0 0 1)-oriented thin plate sample with dimensions of 3 · 3 · 0.09 mm3 was cut from the as-grown PMN–30PT crystal and polished for crystal optical studies by polarized light microscopy. The polishing process took place as follows: the sample was adhered to a holder with wax and polished using diamond paper (of particle size of 25–3 lm) to make the surface of the sample flat and mirror-polished. The as-grown cubic face (0 0 1) was used as a reference. The crystal shows a multiple domain state with birefrin-

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Table 1 Electron probe microanalysis results for a PMN–30PT single crystal Measuring point

Mg (at%)

Nb (at%)

Ti (at%)

Ti/(Mg + Nb + Ti)

1 2 3 4 5 6 7 8 9 10 11 12

0.0985 0.09498 0.08443 0.10202 0.08443 0.08794 0.08794 0.09146 0.10202 0.09498 0.09146 0.08443

0.1872 0.18444 0.18996 0.19272 0.19365 0.18536 0.1918 0.18812 0.19088 0.19272 0.19365 0.1872

0.11256 0.1197 0.1197 0.10899 0.1197 0.1197 0.11791 0.11256 0.11256 0.11791 0.11434 0.12007

0.28263 0.29991 0.30374 0.26996 0.30092 0.30458 0.29652 0.28704 0.27761 0.2907 0.28624 0.30654

gence. Total extinction was observed under crossed polarizers, with the axes of the optical indicatrix sections of the domains being oriented along the Æ1 1 0æcub directions. This type of domain structure and orientation indicates a rhombohedral symmetry [23]. Therefore, the as-grown PMN–30PT crystal belongs to the ferroelectric rhombohedral phase at room temperature, consistent with the phase diagram in Fig. 1. 3. Characterization procedure

(001)

Intensity (a.u.)

15000

(002)

10000

5000

0 10

20

30

40

50

60

70

2-Theta Fig. 3. X-ray diffraction pattern indicating the (0 0 1) naturally grown facet of the PMN–PT crystal.

A (0 0 1)-oriented crystal plate with dimensions of 5.3 · 4.3 · 0.83 mm3 was cut from the as-grown PMN– 30PT crystal was polished by the above polishing process and sputtered with gold layers as electrodes for electric measurements. The dielectric constant (e 0 ) and dielectric loss (tan d) at various frequencies were measured as a function of temperature upon heating from 50 C to 300 C using 5 ± 0.5 C increments using a Solartron 1260 impedance analyser in conjunction with a 1296 dielectric interface, with an AC peak signal of voltage 0.3 V applied. The same setup was used to measure the resonance (fa) and anti-resonance (fr) frequencies of a bar sample. The electric field-induced strain was measured using a MTI 2000 Fotonic sensor (Range 1). The electric field and the strain were in the same direction, perpendicular to the electrode faces, i.e. the (0 0 1) plane of the sample. The piezoelectric coefficient was measured using a ZJ-6B d33/d31 Meter (Institute of Acoustics, Chinese Academy of Sciences). The polarization–electric field (P–E) hysteresis loops were displayed on the (0 0 1) platelet using a RT-66A standard ferroelectric test system (Radiant Technology). 4. Properties and discussion 4.1. Dielectric properties

Fig. 4. Selected points on an as-grown crystal plate for electron probe microanalysis.

The variations in e 0 and tan d as a function of temperature at frequencies of 10 Hz, 100 Hz, 1 kHz, 10 kHz and 100 kHz are shown in Fig. 5a. It can be seen that the dielectric constant shows a maximum with weak frequency

X. Long, Z.-G. Ye / Acta Materialia 55 (2007) 6507–6512 50000

1.0 100kHz 10kHz 1kHz 100Hz 10Hz

40000

(a) Unpoled

0.9 0.8 0.7 0.6 0.5

20000

tanδ

ε'

30000

0.4 0.3 10000 0.2 0.1

0

0.0 -50

0

50

100

150

200

250

300

4.2. Piezoelectricity

o

Temperature ( C)

0.5 100kHz 10kHz 1kHz 100Hz 10Hz

50000

(b) Poled

0.4

40000 o

0.3

T MPB = 105 C

30000

ε'

tanδ

20000

0.2

10000

0.1

0 -50

0

50

100

150

200

to 50 C. The dielectric measurement was then performed upon zero field heating from 50 C to 300 C. The variations in e 0 and tan d as a function of temperature at different frequencies are shown in Fig. 5b. A dielectric anomaly was observed at TMPB = 105 C, corresponding to the rhombohedral–tetragonal phase transition owing to the curved MPB (Fig. 1). Below TMPB, the dielectric dispersion was significantly attenuated. Above TMPB, it reappeared, followed by relaxor maxima at Tmax. The (0 0 1)-oriented PMN–30PT single crystal shows a maximum value of relative permittivity e 0 = 38,000 at 1 kHz at Tmax. The e 0 and tan d values at 1 kHz at room temperature for unpoled and poled sample are 2400 and 0.03 and 3100 and 0.006, respectively.

250

0.0 300

o

Temperature ( C) Fig. 5. Variations of dielectric constant (e 0 ) and loss (tan d) for the (0 0 1)oriented PMN–30PT single crystal, measured upon heating: (a) unpoled and (b) poled.

dispersion, the temperature of which (Tmax) shifts slightly to higher temperatures with increasing frequency, i.e. from Tmax = 124 C at 10 Hz to 127 C at 100 kHz, indicating weak relaxor behaviour which is retained in PMN–30PT despite the fact that macroscopic a polar order was developed in this solid solution with 30% PT. This result is consistent with the observation and analysis in Ref. [24]. According to the phase diagram in Fig. 1, the measured TC  125 C indicates that the composition of the grown crystal indeed corresponds to PMN–30PT, i.e. the same as the nominal loaded composition. Therefore, the TSSG method was able to prevent the phase segregation phenomenon from happening and thereby minimize the composition variation, leading to the formation of more homogeneous single crystals with the desired composition. In order to determine the depolarization temperature, the sample was poled at an electric field of 10 kV cm1 which was applied at 100 C and kept upon cooling down

Fig. 6 shows the strain–electric field relation for the same (0 0 1)-oriented PMN–30PT sample under a bipolar drive. A peak-to-peak strain value of 0.12% is reached at E  ±12 kV cm1. The typical butterfly-like curve reflects the ferroelectric domain switching during the bipolar drive. The piezoelectric coefficient d33 was measured to be 1240 pC N1. For the resonance measurement, a Æ0 0 1æ-oriented crystal bar with dimensions of 1.0 · 1.0 · 5 mm3 was cut from an as-grown crystal with a diamond wheel saw. Then the two end cross-sections of the (0 0 1) faces were polished using diamond paper (25–3 lm particles size) and sputtered with gold layers as electrodes. It was then poled with an electric field of 10 kV cm1 applied at room temperature for 5 min. The resonance (fr) and antiresonance (fa) frequencies for the PMN–30PT sample at various temperatures were measured and the longitudinal electromechanical coupling factor k33 was derived by the following equation [25,26]   p fa  fr p fa  fr k 233 ¼ cot ð1Þ 2 fa 2 fa

0.12 0.10 0.08

Strain (%)

6510

0.06 0.04 0.02 0.00 -0.02 -15.0

-10.0

-5.0

0.0

5.0

10.0

15.0

E (KV/cm) Fig. 6. Bipolar strain Vs. electric field curve of the PMN–30PT single crystal.

X. Long, Z.-G. Ye / Acta Materialia 55 (2007) 6507–6512

Fig. 7 shows the variation of the longitudinal electromechanical coupling factor k33 as a function of temperature for the (0 0 1)-oriented PMN–30PT crystal. It can be seen that k33 reaches a value of 85% which is maintained in the temperature range of 0 C up to about 100 C, before dropping down at the depolarization temperature TMPB = 105 C. This upper limit of temperature is higher than that of the PMN–xPT crystal with MPB compositions.

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Table 2 Comparisons of some properties for PMN–30PT crystals grown by different methods a

PMN–30PT PMN–30PTb a b

e 0 At RT

Loss

TC

TMPB

Pr

EC

d33

k33 (%)

3000 5000

0.005 0.005

125 138

105 98

20 26

1.9 2.3

1240 1600

86 89

This work. Ref. [28].

4.3. Ferroelectricity The ferroelectric properties of the PMN–30PT crystal were displayed by well-developed polarization-electric field hysteresis loops measured at various temperatures, as shown in Fig. 8a. Saturation of polarization is achieved at an electric field of ±7 kV cm1 at room temperature. Such a hysteresis loop with almost vertical lines indicates the sharp switching of macroscopic domains in the crystal. The remnant polarization reaches Pr  21 lC cm2 under a

1.0 0.9 0.8

bipolar drive of E = ±7 kV cm2, with a coercive electric field of EC  2 kV cm2. Fig. 8b shows that the remnant polarization Pr decreases gradually with temperature increasing from 0 to 105 C and very sharply at the depoling temperature TMPB = 105 C. Above TMPB, Pr persists and gradually decreases to zero at T = 160 C. This kind of variation in Pr is quite different from the normal ferroelectrics, whose remnant polarization falls to zero at TC or in its vicinity, but is consistent with the characteristic relaxor ferroelectric behaviour, as observed in PMN [27]. The comparisons of some properties of PMN–30PT crystals grown by different methods are given in Table 2. It can be seen that the longitudinal coupling factor k33 is comparable with, but the depoling temperature TMPB (or Trt) is higher than, the data reported in Ref. [28].

0.7

K33

0.6

5. Conclusions

0.5 0.4 0.3 0.2 0.1 0.0 0

10 20 30 40 50 60 70 80 90 100 110 120 o

Temperature ( C) Fig. 7. Longitudinal electromechanical coupling factor k33 as a function of temperature.

Perovskite piezoelectric single crystals of PMN–30PT (nominal) were grown by the top-seeded solution growth method which effectively prevented phase segregation in the solid solution system and promoted (0 0 1) growth, giving rise to crystals of quadrate morphology with large (0 0 1) faces. This result has significant meaning since the composition variation arising from the phase segregation has been a major hurdle in the development of PMN–PT single crystals in commercial applications. The TSSG

30

15

Pr (μC/cm2)

10

P (μC/cm2)

(b)

20

(a)

20

0

10

-10 5 -20

0

-30 -8

-6

-4

-2

0

2

E (KV/cm)

4

6

8

0

20

40

60

80

100

120

140

160

o

Temperature ( C)

Fig. 8. (a) Polarization Vs. electric field (P–E) hysteresis loops displayed on the (0 0 1)-oriented PMN–30PT crystal and (b) remnant polarization as a function of temperature measured from the P(E) hysteresis loops with E = ±6 kV cm1.

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X. Long, Z.-G. Ye / Acta Materialia 55 (2007) 6507–6512

method developed in this work has proved to be a viable alternative for the synthesis of medium-size piezocrystals with homogeneous composition and controlled (0 0 1)faced morphology. The rhombohedral symmetry of the grown crystals was revealed by crystal optical examination of the domain structure using polarized light microscopy. The frequency-dependence of the dielectric permittivity (e 0 ) maxima indicates that relaxor behaviour is preserved in the PMN–30PT crystal. The rhombohedral–tetragonal phase transition was detected by a dielectric anomaly at TMPB = 105 C in the poled-sample. The value of TMPB is higher than that of the PMN–PT crystals with MPB compositions (50–80 C), thus extending the upper limit of the depolarization temperature. The (0 0 1)-oriented single crystal shows a maximum value of e0max ¼ 38; 000 at 1 kHz at a Curie temperature TC (Tmax) = 125 C. The piezoelectric coefficient d33 was found to be 1240 pC N1, with a strain level reaching 0.12% at 10 kV cm1. The longitudinal electromechanical coupling factor k33 reaches 85%, which is maintained upon heating up to 100 C. The remnant polarization gradually decreases with increasing temperature and persists above TC, consistent with the relaxor behaviour. Compared with the PMN–PT crystals with MPB compositions developed so far, rhombohedral PMN–30PT single crystals exhibit a higher upper limit of depolarization temperature, and thus a wider temperature range for applications in electromechanical transducers.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

[17] [18] [19] [20] [21] [22] [23] [24]

Acknowledgements [25]

This work was supported by the US Office of Naval Research (Grant N00014-06-1-0166) and the Natural Science and Engineering Research Council of Canada (NSERC).

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