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J. Korean Ceram. Soc. > Volume 53(2); 2016 > Article
Lee, Oh, and Lee: Dielectric and Piezoelectric Properties of “Lead-free” Piezoelectric Rhombohedral Ba(Ti0.92Zr0.08)O3 Single Crystals

Abstract

Rhombohedral Ba(Ti0.92Zr0.08)O3 single crystals are fabricated using the cost-effective solid-state single crystal growth (SSCG) method; their dielectric and piezoelectric properties are also characterized. Measurements show that (001) Ba(Ti0.92Zr0.08)O3 single crystals have an electromechanical coupling factor (k33) higher than 0.85, piezoelectric charge constant (d33) of about 950 [pC/N], and piezoelectric voltage constant (g33) higher than 40 [×10−3 Vm/N]. Especially the d33 of (001) Ba(Ti0.92Zr0.08)O3 single crystals was by about six times higher than that of their ceramics. Because their electromechanical coupling factor (k33) and piezoelectric voltage constant (d33, g33) are higher than those of soft PZT ceramics, it is expected that rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals can be used as “lead-free” piezoelectric materials in many piezoelectric applications such as actuator, sensor, and transducer.

1. Introduction

Pb(Zr,Ti)O3 (PZT)-based polycrystalline ceramics have been used mainly for diversified piezoelectric applications up to now.1) They have been used in various ways for not only civilian application areas such as piezoelectric ignition device, ultrasonic motor, piezoelectric speaker, piezoelectric sensor, piezoelectric actuator, piezoelectric transformer and medical ultrasonic diagnostic instruments, etc. but also military application areas such as SONAR. However, as interests on environmental problems due to the hazards of lead(Pb) contained in piezoelectric PZT ceramics are increased recently, development of lead-free piezoelectric materials capable of replacing the PZT ceramics has been actively implemented by Japan, Europe and Korea at the center.2-4)
The lead-free piezoelectric ceramics developed thus far to replace PZT ceramics include tungsten-bronze structure, bismuth layer structure and perovskite structure, etc. However, the lead-free piezoelectric ceramics developed are known to have a difficulty in replacing the PZT ceramics yet due to the lower dielectric and piezoelectric properties compared with those of general PZT ceramics or a difficulty in mass production processes. Recently, to overcome the low dielectric and piezoelectric properties of lead-free piezoelectric ceramics, attempts have been made to replace PZT ceramics by development of high-performance textured ceramics or single crystals. As revealed by many previous study results on linked PMN-PT single crystals,5-7) lead-free textured ceramics or single crystals are expected to show very high piezoelectric constants and electromechanical coupling factors compared with general polycrystalline piezoelectric ceramics composed of randomly oriented grains. And, recently-developed lead-free KNN-based single crystals were reported to show a high electromechanical coupling factor (k33) higher than 0.8 as had been expected.8-10) Since, however, single-crystal piezoelectric materials such as KNN series, etc. have prices which are a few to a few ten times as high as those of general polycrystalline PZT ceramics due to a difficulty in single crystal manufacturing, replacement of polycrystalline piezoelectric materials remains to be difficult. Therefore, to replace the PZT ceramics, development of a technology capable of mass-producing high-performance lead-fee piezoelectric single crystals at a low manufacturing cost is required.
In the present study, Ba(Ti0.92Zr0.08)O3 single crystals were selected as a lead-free piezoelectric single crystal, and development of high-efficiency lead-free piezoelectric single crystals was attempted by using solid-state single crystal growth [SSCG] method.7,11) Since Ba(Ti0.92Zr0.08)O3 compositions showing incongruent melting behavior are difficult to manufacture by general single crystal growth methods such as flux method or Bridgman method, etc. solid-state single crystal growth method was employed without including a melting process upon growth of single crystals. And, the solid-state single-crystal growth method is known to have a low cost for single-crystal manufacturing, and to allow mass production. Dielectric and piezoelectric properties of the grown Ba(Ti0.92Zr0.08)O3 single crystals were measured and evaluated,6,7,12) and compared with those for the existing piezoelectric PZT polycrystalline ceramics. Through the present study, excellent dielectric and piezoelectric properties of Ba(Ti0.92Zr0.08)O3 single crystal have been affirmed, and the future possibility of its use as a high-performance lead-free piezoelectric material has been demonstrated.

2. Experimental Procedure

Lead-free piezoelectric Ba(Ti0.92Zr0.08)O3 single crystals used in the present study were prepared by using solid-state single-crystal growth (SSCG) method. First, by using raw material powders of BaCO3, TiO2 and ZrO2, polycrystalline Ba(Ti0.92Zr0.08)O3 ceramics were obtained through a general sintering process. After weighing each raw material powder for Ba(Ti0.92Zr0.08)O3 composition, they were mixed for 24 h using ball-milling process, and calcined at 1,000°C following drying. The calcined powders were subjected to the second ball-milling, forming under uniaxial compression following drying, and then sintered. After the first sintering, a BaTiO3 seed single crystal was placed on the sintered body of Ba(Ti0.92Zr0.08)O3, and the second heat treatment for solid-state single-crystal growth was carried out for 100 hours.
The BaTiO3 seed single crystal was continuously grown into the Ba(Ti0.92Zr0.08)O3 polycrystalline body during the heat treatment process for single-crystal growth, and Ba(Ti0.92Zr0.08)O3 single crystals of a size larger than 25 × 25 × 5 mm3 were prepared as a result. Since no melting in Ba(Ti0.92Zr0.08)O3 occurred during single-crystal manufacturing process in the single-crystal growth method, composition gradients were not observed within the grown single crystal, giving rise to a chemical homogeneity. Thus prepared single crystals larger than 25 × 25 × 5 mm3 in size were cut into a plate shape(specimen of [3 × 3 × 0.5(t) m3] for thickness mode measurement) and a square-column shape(specimen of [1 × 1 ×5(t) m3] for 33 mode measurement) single-crystal specimens, and an Au electrode was formed on the cut single-crystal plate by using vacuum deposition method. Dielectric and piezoelectric properties of the manufactured Ba(Ti0.92Zr0.08)O3 single crystals and polycrystalline ceramics were measured in accordance with the IEEE standard.12) And (001) single crystals and (011) single crystals were produced respectively for observation of property changes in the single crystals as a function of crystal orientation.

3. Results and Discussion

3.1 Ba(Ti0.92Zr0.08)O3 single crystals prepared by solid-state single-crystal growth method

Figure 1 shows a Ba(Ti0.92Zr0.08)O3 single crystal prepared by the solid-state single-crystal growth method. A BaTiO3 seed single crystal is positioned at the center part of the specimen, and the BaTiO3 seed single crystal was continuously grown into the Ba(Ti0.92Zr0.08)O3 polycrystalline body during the heat treatment process for single-crystal growth. After heat treatment process, Ba(Ti0.92Zr0.08)O3 single crystals larger than 25 × 25 × 5 mm3 in size were manufactured inside the Ba(Ti0.92Zr0.08)O3 polycrystalline body. Since single-crystal growth in the solid-state single crystal growth method occurs at a temperature below the melting point for the Ba(Ti0.92Zr0.08)O3 phase, the shape of polycrystalline Ba(Ti0.92Zr0.08)O3 ceramics around the grown Ba(Ti0.92Zr0.08)O3 single crystal was maintained. And, no composition gradients were observed in the manufactured Ba(Ti0.92Zr0.08)O3 single crystals and polycrystalline bodies with a chemical homogeneity being obtained even after the heat treatment process for single crystal growth.

3.2 Polarization properties of rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal

In Fig. 2, variations in polarization properties as a function of electric field variation were observed by using a plate-shaped (001) Ba(Ti0.92Zr0.08)O3 single crystal[4 × 4 × 0.5(t) mm3]. In Fig. 2, the value of saturated polarization (PS) was shown to be about 28.8 [μC/cm2], and remanent polarization (Pr) was observed to be about 12.5 [μC/cm2]. In addition, coercive electric field (EC) was measured to be about 1.5 kV/cm.

3.3. Dielectric and piezoelectric properties of rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal

In Table 1, dielectric and piezoelectric property values of plate-shaped rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals and polycrystalline ceramics are arranged. According to Table 1(a) for rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals, dielectric constant(K3T) was measured to be higher than 2,000, dielectric loss (tan δ, %) about 0.4, and piezoelectric charge constant (d33) about 950 [pC/N]. In Table 1(b) for Ba(Ti0.92Zr0.08)O3 polycrystalline ceramics, dielectric constant (K3T) was measured to be higher than 1,230, dielectric loss (tan δ, %) about 3.7, and piezoelectric charge constant (d33) about 165 [pC/N]. As compared with the polycrystalline ceramics, the (001) Ba(Ti0.92Zr0.08)O3 single crystal had a piezoelectric charge constant higher by more than 6 times and a dielectric loss smaller by more than 9 times.
In Table 2, dielectric and piezoelectric properties in 33 mode were measured by using rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals of a square column form [1 × 1 × 5(t) mm3]. According to Table 2 for (001) Ba(Ti0.92 Zr0.08)O3 single crystals, electromechanical coupling factor (k33) was measured to be 0.85, piezoelectric charge constant (d33) about 950 [pC/N], piezoelectric voltage constant (g33) about 41 [×10−3 Vm/N], and elastic compliance (s33E) about 39.2 [× 10−12 m2N]. Such high electromechanical coupling factor (k33) and piezoelectric constants (d33, g33) of such rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals showed higher values than those of general polycrystalline PZT ceramics. Therefore, rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals exhibited the possibility for replacement of the linked piezoelectric PZT ceramics in diversified piezoelectric applications.

3.4 Variations in dielectric and piezoelectric properties as a function of temperatures of rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal

Figure 3 shows variations in dielectric constant for a rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal as a function of temperature variation. As shown in Fig. 3(a) for an unpoled (001) Ba(Ti0.92Zr0.08)O3 single crystal, the phase transition temperature between rhombohedral and orthorhombic phases (TRO), that between orthorhombic and tetragonal phases (TOT), and that between tetragonal and cubic phases (TC) were observed to be 49, 72, and 100°C, respectively. And, TRO, TOT and TC for a poled (001) Ba(Ti0.92Zr0.08)O3 single crystal [Fig. 3(b)] were observed to be 48, 71, and 109°C, respectively.
As compared with the unpoled single crystal[Fig. 3(a)], the poled (001) Ba(Ti0.92Zr0.08)O3 single crystal [Fig. 3(b)] had similar TRO and TOT but higher TC by about 9°C. And, the (001) Ba(Ti0.92Zr0.08)O3 single crystal [Fig. 3(b)] had a dielectric constant higher by more than twice than the unpoled single crystal [Fig. 3(a)] at each phase transition temperature.
Figure 4 shows variations in piezoelectric charge constant (d33) of a rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal as a function of temperature variation. When the rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal was heated from room temperature, piezoelectric charge constant (d33) was continuously increased to reach a maximum near each phase transition temperature [TRO, TOT and TC]. And, when the phase transition temperature was exceeded on the basis of each phase transition temperature [TRO, TOT and TC] as shown in Fig. 4, piezoelectric charge constant (d33) was sharply reduced and then continuously increased as the temperature was raised. And, above TC, the piezoelectric charge constant (d33) was drastically reduced. Thus, as the piezoelectric charge constant (d33) became a maximum near each phase transition temperature [TRO, TOT and TC] with an increase in temperatures, the spontaneous polarization direction is discontinuously changed with a drastic reduction in the piezoelectric charge constant. Such phenomenon is well known in the previous studies on BaTiO3 and PMNPT piezoelectric single crystals.5-7,11) Also, with the phase transition from tetragonal phase to non-ferroelectric cubic phase near 109°C, the piezoelectric charge constant (d33) was drastically reduced to less than 100 [pC/N]. Variations in piezoelectric charge constant (d33) for a rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal as a function of temperature in Fig. 4 show the results in agreement with behavior of dielectric property variations for a rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal as a function of temperature variation in Fig. 3.
Figure 5 shows variations in electromechanical coupling factor (k33) for a rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal as a function of temperature variation. When a rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal was heated from room temperature, the electromechanical coupling factor (k33) exhibited a uniform value within the range of ± 10% below TC as shown in Fig. 5. Namely, unlike dielectric constant (Fig. 3) and piezoelectric constant (Fig. 4), the phenomenon was not observed where a maximum value occurred near each phase transition temperature [TRO, TOT and TC] with a drastic reduction again when the phase transition temperature was exceeded. Such behavior differed from the previous study results on BaTiO3 and PMN-PT piezoelectric single crystals where the electromechanical coupling factor(k33) was reported to show a maximum value at each phase transition temperature.5-7,11) The electromechanical coupling factor (k33) shows a tendency of being proportional to the piezoelectric constant squared and inversely proportional to the dielectric constant [k33 = d33 2/s33 Eɛr]. Therefore, the fact that the electromechanical coupling factor (k33) for a rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal did not show a drastic change near each phase transition temperature [TRO, TOT and TC] seems to suggest that an increase in the piezoelectric constant and an increase in the product between elastic compliance[s33E] and dielectric constant[ɛr] were shown to be offset. Due to such offset phenomenon, relatively uniform values are considered to have been observed without showing a drastic change in the electromechanical coupling factor (k33) near each phase transition temperature.

3.4 Dielectric and piezoelectric properties of rhombohedral (011) Ba(Ti0.92Zr0.08)O3 single crystal [d32 mode]l

As well-known in the previous study results on PMN-PT piezoelectric single crystals,5-7,11) piezoelectric charge constant (d32) and electromechanical coupling factor (k32) in the largest 31 mode were observed for a plate shape in d32 mode [length direction (100); width direction(0/11); thickness direction (011)]] in the case of a rhombohedral piezoelectric single crystal. In the present study, (011) Ba(Ti0.92 Zr0.08)O3 single crystal plates of 5×1.5×0.5(t) mm size in d32 mode were also produced for measurement of dielectric and piezoelectric properties. According to Table 3 for the (011) Ba(Ti0.92Zr0.08)O3 single crystal, the electromechanical coupling factor (k32) was measured to be 0.77, piezoelectric charge constant(d32) about 630 [pC/N], piezoelectric voltage constant(g32) about 45 [×10−3 Vm/N], and elastic compliance(s22E) about 48 [×10−12 m2N]. Such high electromechanical coupling factor (k32) and piezoelectric voltage constant (d32) for a rhombohedral (011) Ba(Ti0.92Zr0.08)O3 single crystal showed higher values by more than twice compared with the values for general PZT polycrystalline ceramics. Consequently, the rhombohedral (011) Ba(Ti0.92Zr0.08)O3 single crystals showed the possibility for replacement of the linked piezoelectric PZT ceramics in diversified piezoelectric application areas using 31 or 32 mode.

4. Conclusions

Rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals prepared by solid-state single crystal growth method showed excellent piezoelectric properties such as electromechanical coupling factor (k33) of higher than 0.85, piezoelectric charge constant (d33) of about 950 [pC/N], and piezoelectric voltage constant (g33) of about 41 [× 10−3 Vm/N] in the case of 33 mode. And, the rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals in 32 [or 31] mode showed values such as electromechanical coupling factor (k32) of 0.77, piezoelectric charge constant (d32) of about 630 [pC/N], and piezoelectric voltage constant (g32) of about 45 [× 10−3 Vm/N]. For poled (001) Ba(Ti0.92Zr0.08)O3 single crystals, phase transition temperature between rhombohedral phase and orthorhombic phase(TRO), that between orthorhombic phase and tetragonal phase (TOT) and that between tetragonal phase and cubic phase (TC) were observed to be 48, 71, and 109°C, respectively. The rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals showed higher values for electromechanical coupling factors (k33, k32) and piezoelectric constants (d33, d32, g33, g32) than general PZT polycrystalline ceramics, although limitations in use range exist due to low phase transition temperatures, and hence are expected to be capable of replacing the linked piezoelectric PZT ceramics in diversified piezoelectric application areas. In particular, the rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystals will allow application as an excellent lead-free piezoelectric material in the application areas where use of lead(Pb) harmful to human body and environments is difficult.

Fig. 1
A (001) Ba(Ti0.92Zr0.08)O3 single crystal grown by the SSCG method.
jkcs-53-2-171f1.gif
Fig. 2
Polarization of a rhombohedral (001) Ba(Ti0.92Zr0.08)O3 single crystal plate [4 × 4 × 0.5(t) mm3] as a function of electric field.
jkcs-53-2-171f2.gif
Fig. 3
Dielectric constants of (001) Ba(Ti0.92Zr0.08)O3 single crystal plates [4 × 4 × 0.5(t) mm3] as a function of temperature: (a) unpoled and (b) poled.
jkcs-53-2-171f3.gif
Fig. 4
Piezoelectric constant (d33 [pC/N]) of a (001) Ba(Ti0.92 Zr0.08)O3 single crystal bar [1 × 1 × 5(t) mm3] as a function of temperature.
jkcs-53-2-171f4.gif
Fig. 5
Electromechanical coupling factor (k33) of a (001) Ba(Ti0.92Zr0.08)O3 single crystal bar [1 × 1 × 5(t) mm3] as a function of temperature.
jkcs-53-2-171f5.gif
Table 1
Dielectric and Piezoelectric Properties of (a) (001) Single Crystals and (b) Rhombohedral Ba(Ti0.92Zr0.08)O3 Polycrystalline Ceramics at Room Temperature
(a) (001) Ba(Ti0.92Zr0.08)O3 single crystal plates [3 × 3 × 0.5(t) m3]

Thickness Mode [001] Ba(Ti0.92Zr0.08)O3 Single Crystal Plates-Poled along the [001] axis

11 12 13 Average STDEV.
Thickness mm 0.50 0.50 0.50 0.50 0.00
Width mm 3.00 3.00 3.00 3.0 0.00
Length mm 3.00 3.00 3.00 3.0 0.00
Density g/cm3 5.60 5.60 5.60 5.6 0.00
Capacitance nF 0.330 0.323 0.331 0.328 0.004
tan δ / 0.003 0.003 0.005 0.004 0.001
fr kHz 4,404.6 4,374.0 4,374.0 4,384.2 17.7
fa kHz 5,170.0 5,466.3 5,562.4 5,399.6 204.5
Zr kOhm 0.02 0.02 0.02 0.02 0.00
K3T / 2,072 2,028 2,079 2,060 27
kt / 0.563 0.639 0.656 0.62 0.05
d33 pC/N 944 926 961 943.7 17.5

(b) Polycrystalline Ba(Ti0.92Zr0.08)O3 ceramic plates [3 × 3 × 0.5(t) m3]

Thickness Mode Ba(Ti0.92Zr0.08)O3 Ceramic Plates

11 12 13 Average STDEV.

Thickness mm 0.50 0.50 0.50 0.50 0.00
Width mm 3.00 3.00 3.00 3.0 0.00
Length mm 3.00 3.00 3.00 3.0 0.00
Density g/cm3 5.60 5.60 5.60 5.6 0.00
Capacitance nF 0.190 0.200 0.198 0.196 0.005
tan δ / 0.034 0.041 0.036 0.037 0.004
fr kHz 4,872.8 4,872.8 4,722.4 4,822.6 86.8
fa kHz 5,260.8 5,242.6 5,224.3 5,242.6 18.3
Zr kOhm 0.06 0.06 0.07 0.06 0.01
K3T / 1,193 1,257 1,241 1,230 33
kt / 0.412 0.403 0.465 0.43 0.03
d33 pC/N 154 162 177 164.3 11.7
Table 2
Dielectric and Piezoelectric Properties of Rhombohedral (001) Ba(Ti0.92Zr0.08)O3 Single Crystal Bars [1 × 1 × 5(t) mm3] at Room Temperature
k33 Mode (LE) [001] Ba(Ti0.92Zr0.08)O3 Single Crystal Bars - Poled along the [001] axis

11 12 13 Average STDEV.
Thickness mm 5.00 5.00 5.00 5.00 0.00
Width mm 1.00 1.00 1.00 1.0 0.00
Length mm 1.00 1.00 1.00 1.0 0.00
Density g/cm3 5.60 5.60 5.60 5.6 0.00
Capacitance nF 0.003 0.004 0.003 0.003 0.000
tan d / 0.008 0.001 0.004 0.004 0.004
fr kHz 229.55 226.69 230.99 229.08 2.19
fa kHz 405.91 418.82 400.86 408.53 9.27
Zr kOhm 2.89 2.51 3.19 2.86 0.34
K3T / 1,852 2,005 1,804 1,887 105
k33 / 0.850 0.864 0.843 0.85 0.01
g33 ×10−3 V m/N 41 41 41 41 0
s33D ×10−12 m2/N 10.8 10.2 11.1 10.7 0.5
s33E ×10−12 m2/N 39.0 40.2 38.4 39.2 0.9
Y33 GPa 25.7 24.9 26.0 25.5 0.6
N33 Hz · m 1148 1133 1155 1145 10.9
d33 [d33 meter] pC/N 1,010 951 902 954 54
Table 3
Dielectric and Piezoelectric Properties of Rhombohedral (011) Ba(Ti0.92Zr0.08)O3 single Crystal Plates [5 × 1.5 × 0.5(t) mm3] at Room Temperature
k32 Mode (LTEXY2) [011] Ba(Ti0.92Zr0.08)O3 Single Crystal Plates-Poled along the [011] axis

11 12 13 14 Average STDEV.
Thickness mm 0.50 0.50 0.50 0.50 0.50 0.00
Width mm 1.50 1.50 1.50 1.50 1.5 0.00
Length mm 5.00 5.00 5.00 5.00 5.0 0.00
Density g/cm3 5.60 5.60 5.60 5.60 5.6 0.00
Capacitance nF 0.211 0.209 0.208 0.212 0.210 0.002
tan d / 0.008 0.007 0.009 0.008 0.008 0.000
fr kHz 192.32 194.05 192.32 192.32 192.75 0.87
fa kHz 265.52 265.52 267.91 267.91 266.71 1.38
Zr kOhm 0.14 0.15 0.15 0.13 0.14 0.01
K3T / 1,586 1,575 1,563 1,593 1,579 13
k32 / 0.772 0.764 0.780 0.780 0.77 0.01
d32 pC/N 636 621 637 644 634 9
g32 × 10−3 V m/N 45 45 46 46 45 1
s22E × 10−12 m2/N 48.3 47.4 48.3 48.3 48.1 0.4
s22D × 10−12 m2/N 19.5 19.7 18.9 18.9 19.3 0.4

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