Growth and properties of Li, Ta modified (K,Na)NbO₃ lead-free piezoelectric single crystals

Abstract

Li, Ta modified (K,Na)NbO₃ single crystals with the size of 18 mm × 18 mm × 10 mm were successfully grown by top-seeded solution growth method, with orthorhombic–tetragonal phase transition temperature ~79 °C and Curie temperature ~276 °C. The electromechanical coupling factors k₃₃ and k_t were found to be ~88% and ~65%, respectively. The piezoelectric coefficient d₃₃ for the [001]_c poled crystals reached 255 pC/N. In addition, the electromechanical coupling factor exhibited high stability over the temperature range of −50 °C to 70 °C, making these lead free crystals good candidates for electromechanical applications.

1 Introduction

(K,Na)NbO₃ (KNN) based lead free ferroelectric materials, such as Li, Sb and Ta modified KNN, have attracted a lot of attention owing to their good piezoelectric properties [1-8]. Noticeable improvement of piezoelectric properties has been achieved through shifting the orthorhombic-tetragonal phase transition temperature (T_O–T) from 200 °C to near room temperature [9-12].However, the strong temperature dependence of properties limited their applications. Therefore, research on KNN based lead-free piezoelectric materials has been focused on improving properties with better temperature stability [13, 14]. Recently, KNN based single crystals have been grown successfully by different methods and some electrical properties have been reported [15-18]. KNN single crystals with the thickness of 160 μm and (K,Na,Li)(Nb, Ta)O₃ single crystals with the thickness of 20–50 μm were grown using solid state crystal growth process in 2007 [15, 16], but limited properties were reported due to the small size of the as-grown crystal. Li-modified KNN based single crystals with high piezoelectric properties and Curie temperature were grown by the Bridgman method, but the hysteresis loop exhibited high leakage current characteristic[17], which was thought to be related to the ionic conductivity induced by oxygen vacancies and can be reduced by annealing or modified by dopants, such as Mn^2+,3+ [19, 20].

Most previous studies on KNN based crystals focused on improving the piezoelectric coefficients d₃₃, and it has been proven that the piezoelectric coefficients can be improved by Li, Ta modification [1, 2, 4, 6]. To date, however, limited studies on electromechanical coupling factors have been conducted on these crystals. Electromechanical coupling factor is one of the most important parameters controlling the performance of ultrasonic transducers. High coupling piezoelectric materials, such as relaxor-based piezoelectric single crystals, including Pb(Mg_1/3Nb_2/3)–PbTiO₃ and Pb(Zn_1/3Nb_2/3)–PbTiO₃ single crystals, have attracted a lot of attention in recent years [21-23]. For the lead-free piezoelectric materials, high thickness electromechanical coupling factor k_t along certain orientations was predicted in KNbO₃ and Li modified (K,Na)NbO₃ single crystals, which was experimentally verified [24-26]. Recently, longitudinal electromechanical coupling factor k₃₃ ~ 64% for Mn modified KNN single crystals was reported, which is much lower than that of the relaxor-PT crystals but reasonably good compared to PZT ceramics [27]. It is expected that high coupling factors can be achieved by optimizing the composition and improving the quality of the KNN based single crystals.

In this work, Li, Ta modified (K,Na)NbO₃ (KNN-LT) single crystals were grown by top-seeded solution growth (TSSG) method, which is in orthorhombic phase at room temperature. The longitudinal and the thickness electromechanical coupling factors were measured according to the IEEE Standard on Piezoelectricity [28]. The piezoelectric coefficients d₃₃ and the electromechanical properties were also evaluated as a function of temperature.

2 Experimental

2.1 Crystal growth

High purity powders of Na₂CO₃ (99.99%), K₂CO₃ (99.99%), Li₂CO₃ (99.99%), Ta₂O₅ (99.99%) and Nb₂O₅ (99.99%) were used as raw materials and weighed according to the composition Li_x(K_zNa_1–z)_1–xNb_1–yTa_yO₃, x = 0.03–0.09, y = 0.05–0.15, z = 0.7–0.9. These raw materials were mixed and ball-milled in ethanol for 12 h, then calcined at 850 °C for 4 h to synthesize KNN compound. The obtained compound was put into a platinum (Pt) crucible with 20 mm in height and 65 mm in diameter, and then put in a resistant heating furnace. The compound and flux were melted at 1200 °C, and kept a few hours to stabilize the melt. An [001] oriented KNN crystals seed was dipped to the melt. Then the temperature was slowly cooling down at a constant rate with the crystal seed slowly pulling up at 0.1–1 mm/h. The first KNN based single crystal was grown using Pt line as the seed. Then part of the crystal was cut along [001] direction and used as the seed for next crystal until the good enough seeds were got to grow high quality crystals. In order to verify the compositional homogeneity of the as-grown crystals, the radios for the ions except Li of all the used samples were checked by EDS. The content of Li cannot be determined by EDS analysis due to the small atomic weight. The radios x for K/(K + Na) were about 0.59–0.61 and the radios y for Ta/(Ta + Nb) were 0.16–0.17. According to the data by Zheng et al. [29], the slight compositional variation can be ignored. And the concentration of Li was determined to be 0.01–0.03 [26, 30]. The density ρ of the as-grown crystals was measured using the Archimedes method to be 5.20 g/m³. Powder X-ray diffraction (XRD) with Cu K_α radiation was used to confirm the perovskite structure.

2.2 Characterization procedure

In order to investigate the electrical properties, the crystals were oriented by the Laue X-ray machine with an accuracy of 0.5° and the samples were prepared according to the IEEE standards. The longitudinal k₃₃ bar was prepared with the size of 0.8 × 0.8 × 3.0 mm³ with the end face along [001]_c. The samples were sputtered with gold electrodes and poled in silicon oil under a DC field of 30 kV/cm at room temperature for 3 minutes, then aged for 24 hours prior to electrical measurements. The temperature dependence of the dielectric constant was determined by the measured capacitance of k_t lates using an HP4284A LCR meter. The resonance and antiresonance frequencies of different samples were measured by an HP4194A impedance-phase gain analyzer, from which the coupling factors and elastic constants were calculated. Silver wires were attached to the end face of the k₃₃ bar for electric connections to avoid the clamping effect. The electric-field-induced strain was measured using a linear variable differential transducer (LVDT) driven by a lock-in amplifier at 1 Hz.

3 Results and discussion

The as-grown KNN-LT single crystal with the size of 18 mm × 18 mm × 10 mm is shown in the small inset of Fig. 1. The single crystals are transparent above the Curie temperature, during cooling, the crystals change from paraelectric cubic phase to ferroelectric tetragonal phase with domains being formed. Then, the single crystals become opaque with milky white color due the light scattering of the domain walls. The crystals were grown along the [001]_c direction and the large habitual faces are along {100} due to their slower growth rate. Fine powder of KNN-LT single crystals were prepared for XRD analysis. Figure 1 shows XRD pattern of the crystals, exhibiting pure perovskite structure at room temperature. The lattice parameters with orthorhombic phase (space group mm2) were calculated to be: a = 5.6431 Å, b = 3.9603 Å and c = 5.6656 Å. The lattice parameters of orthorhombic symmetry are further converted to the lattice parameters of pseudo-monoclinic cell (a′ = c′, b, and β) using the formula as follows: a = 2a′ sin1/2β, c = 2a′ cos1/2 β [31]. The lattice parameters of pseudo-monoclinic cell were found to be a = c = 3.9982 Å, b = 3.9603 Å, β = 90.23°, where a/b was 1.0096.

Figure 1.

KNN-LT single crystal grown by TSSG method and its X-ray powder diffraction patterns.

Figure 2 shows the dielectric constant as a function of temperature at different frequencies. The tetragonal–cubic phase transition temperature (Curie temperature) T_C for the single crystals was found to be 276 °C, while the orthorhombic–tetragonal phase transition occurred at 79 °C, as shown Fig. 2. The dielectric constant ${\epsilon }_{33}^T⁄{\epsilon }_0$ was found to be 500 at room temperature, determined using the capacitance of the samples measured at 1 kHz by an HP4284A precision LCR meter. Correspondingly, the dielectric loss was found to be on the order of 1% at room temperature and increased with temperature. It showed a peak at T_C, above which the dielectric loss went up quickly due to the ionic conductivity at elevated temperature.

Figure 2.

Temperature dependence of dielectric permittivity for KNN-LT single crystals at different frequencies.

Plate samples with the thickness of 0.41 mm were prepared to determine k_t. The resonance and antiresonance frequencies were found to be 6.50 MHz and 8.25 MHz, respectively. The maximum phase angle in the curve was in the order of 75°. In order to check the accuracy of the result, second resonance frequency was measured to be 24.3 MHz. The frequency radio f₂/f₁ was 3.74, and the corresponding k_t was about 65%, which is consistent with the result got from the resonance method [32].

Figure 3 shows the resonance and antiresonance characteristics of the impedance spectra of the longitudinal vibration bar of KNN-LT single crystals at 20 °C and 60 °C (inset), respectively, measured using an HP4194A impedance-phase gain analyzer. The maximum phase angle was about 89° and maintained similar value over a wide temperature range. The temperature dependence of the longitudinal coupling factor k₃₃, piezoelectric coefficient d₃₃, elastic compliances $S_{33}^E$and $S_{33}^D$, were calculated and given in Fig. 4. The elastic compliances $S_{33}^E$ and $S_{33}^D$ were found to be 19.0 pm²/N and 4.4 pm²/N, respectively, at room temperature. The elastic compliance constant $S_{33}^E$ increased with temperature, but the elastic compliance $S_{33}^D$ maintained the same value in the temperature range of −50 °C to 70 °C, as shown in the inset of Fig. 4. Correspondingly, the coupling factor k₃₃ was calculated to be 88% at room temperature, with variation being on the order of 5% in the temperature range of −50 °C to 70 °C, showing excellent temperature stability. The piezoelectric coefficient d₃₃ was found to be 255 pC/N at room temperature, and increased to 520 pC/N at 70 °C, above which the value sharply decreased, due to partially depolarization near the O–T phase transition.

Figure 3.

Measured impedance amplitude and phase of k₃₃-bar samples for KNN-LT single crystals at 20 °C and 60 °C (insert).

Figure 4.

(a) Temperature dependence of coupling factor k₃₃ and piezoelectric coefficient d₃₃; (b) temperature dependence of elastic compliances $S_{33}^E$ and $S_{33}^D$.

Figure 5 shows the unipolar electric-field-induced strains for KNN-LT single crystals. The normalized strain S_33max/E_max was calculated to be 470 pm/V at 10 kV/cm. The total obtainable strain over a range of applied electrical field is affected by the existence of domain wall motions. The S_33max/E_max measured at low frequency (1 Hz) is higher than the small signal d₃₃ value calculated based on IEEE standard, indicating the extrinsic contribution from domain wall motion at high field, analogous to other lead free materials [1, 7, 34]. The detailed electrical properties of KNN-LT lead free single crystals are listed in Table 1 and compared to KNN-Mn single crystals, KNbO₃ single crystals, KNN ceramics, KNN-LT ceramics and KNN-LS (Li, Sb) ceramics. As shown in the table, the dielectric loss for the [001] poled KNN-LT crystal was the lowest, being ~1%. The piezoelectric coefficient d₃₃ was improved in the KNN based single crystals owing to the Li and Ta doping. Compared to the KNN-LS ceramics, the T_O–T of KNN-LT single crystals was higher while maintaining the comparable piezoelectric coefficient d₃₃. Of particular importance is that the coupling factor k₃₃ was much higher than other KNN-based materials, making the KNN-LT crystals good candidates for practical ultrasonic transducer applications.

Figure 5.

Unipolar electric-field-induced strain for KNN-LT single crystals measured at 1 Hz: longitudinal vibration mode.

Table 1.

Comparison of KNN based materials.

	TO–T (°C)	TC (°C)	loss (%)	k33 (%)	kt (%)	d33 (pC/N)
KNN-LT crystalsa	79	276	1	88	65	255
KNN-Mn crystalsb	195	407	–	64	–	161
KN crystalsc	225	435	–	61	–	30
KNN ceramicsd	195	420	1.5	61	46	127
KNN-LT ceramicse	65	320	<10	57	34	174
KNN-LS ceramicsd	35	368	2.0	62	46	265

^aThis work.

^bSee [25].

^cSee [31, 33].

^dSee [8].

^eSee [34].

4 Conclusions

In summary, lead-free KNN-LT piezoelectric single crystals have been successfully grown by the TSSG method. The T_O–T and T_C for the crystals were found to be 79 °C and 276 °C, respectively. The large longitudinal electromechanical coupling factor k₃₃ ~ 88% and thickness electromechanical coupling k_t ~ 65% were achieved in the [001]_c poled KNN-LT single crystals. Furthermore, coupling factor k₃₃ exhibited good thermal stability up to the O–T phase transition temperature, with variation being only 5% in the temperature range of −50 °C to 70 °C. The large and stable coupling factors make the Li, Ta modified KNN single crystals promising for a range of piezoelectric applications with good thermal stability.