Influences of ScTa co-substitution on the properties of Ultra-high temperature Bi₃TiNbO₉-based piezoelectric ceramics

Abstract

The effect of (Sc,Ta) doping on the properties of Bi₃TiNbO₉-based ceramics was investigated. The (Sc,Ta) modification greatly improves the piezoelectric activity of Bi₃TiNbO₉-based ceramics and significantly decreases the dielectric dissipation. The d₃₃ of Bi₃(Ti_0.96Sc_0.02Ta_0.02)NbO₉ was found to be 12 pC/N, the highest value among the Bi₃TiNbO₉-based ceramics and almost 2 times as much as the reported d₃₃ values of the pure BTNO ceramics (~6pC/N). The high T_C (higher than 900 °C) and stable piezoelectric and dielectric properties, demonstrating that the (Sc,Ta) modified Bi₃Ti_1−xSc_x/2Ta_x/2NbO₉-based material a candidate for ultrahigh temperature applications. The new (ScTa) modification has an important typical significance, the way should be used for reference in constructing the new high performance materials.

1 Introduction

Bismuth layer-structured ferroelectrics (BLSFs) ceramics are potential candidate lead-free materials in piezoelectric device application, especially at high temperatures and high frequencies application. In addition to their high Curie temperatures, they exhibit correspondingly low temperature coefficients of dielectric and piezoelectric properties, low aging rate, strong anisotropic electromechanical coupling factors and low temperature coefficient of resonant frequency, making them suitable for pressure sensors, trapped energy filters, etc [1, 2].

In recent years, BLSFs have been given more attention. BLSFs, such as SrBi₂Na₂O₉ (SBN), SrBi₄Ti₄O₁₅ (SBTi), SrBi₂Ta₂O₉ (SBT), La_0.75Bi_3.25Ti₃O₁₂ (BLT) etc. have been found to be excellent materials for nonvolatile ferroelectric random access memory (FRAM), owing to their fatigue free polarization behavior [3–7]. The general formula of BLSF is (Bi₂O₂)²⁺(A_m−1B_mO_3m+1)²⁻, where A is a mono-, di- or trivalent element (or a combination of them) allowing dodecahedral coordination and B is a transition element with octahedral coordination, e.g., Ti⁴⁺, Nb⁵⁺, Fe³⁺, W⁶⁺ or Ta⁵⁺; m is the number of octahedral layers in the perovskite slab, which varies from 1 to 6 [4, 8]. The poling of the BLSF ceramics requires relatively high electric field because of their high coercive fields and the two-dimensional orientation restriction of rotation of the spontaneous polarization.

Bismuth titanate niobate (Bi₃TiNbO₉, hereinafter called BTNO) (m = 2), which is made up of (Bi₂O₂)²⁺ layers between which (BiTiNbO₇)²⁻ layers are inserted [9], is promising for high temperature piezoelectric sensors because of its very high Tc (914 °C) [8]. However, the piezoactivity of pure BTNO ceramics is quite low (d ₃₃ < 7 pC/N) [10] for high temperature applications. The A-site substitution or/and B-site substitution have been shown to be effective in modifying the structure and polarization process [11–23]. Only a few works have addressed the properties of cation-modified BTNO-based ceramics. For La- and Ti/W-substituted BTNO[24] and Bi₂K_1/6Bi_5/6TiNb_2/3W_1/3O₉ [25] compounds, only a few properties (e.g., lattice parameters and Tc) have been mentioned, and no detailed structural and electrical properties have been studied. To modify BTNO structure for improving its piezoelectricity, the ScTa co-substitution into B-site of BTNO was conducted.

The only valence of Sc is 3 and the only valence of Ta is 5, so the average valence of (ScTa) is 4 which is equal to the only valence of Ti⁴⁺. The radius of Sc³⁺ is 0.073 nm, and the radius of Ta⁵⁺ is 0.068 nm. They are almost the same as the radius of Ti⁴⁺ (r _Ti ⁴⁺ =0.068 nm). More than that, Sc, Ta, Ti and Nb in the BLSF structure have the same favorable coordinate number which is 6. All of those can assure the (Sc³⁺Ta⁵⁺) co-substitute Ti⁴⁺ into B-site of BTNO. Because the valence of Nb is 5 which is different from the average valence of (ScTa), and the law of conservation of electric, the ScTa will not co-substitute Nb. In addition, the samples will be prepared according the formula Bi₃Ti_1−x(ScTa)_x/2NbO₉, so there is not lack of Nb which will lead Nb to stay at the original position without being substituted .

2 Experiment

The starting raw materials were high purity Bi₂O₃ (99.8 %), TiO₂ (99.8 %), Nb₂O₅ (99.5 %), Sc₂O₃ (99.27 %), Ta₂O₅(99.5 %). The samples were prepared according the formula Bi₃Ti_1−x(ScTa)_x/2NbO₉ (x = 0.00, 0.02, 0.04, 0.06). The chemicals were weighed according to the composition, and then mixed using ball milling, dried and calcined at 808 °C for 3 h. After calcination, the ball-milled ground powders were pressed into disks with 13 mm in diameter and 2 mm in thickness. Densification was achieved by sintering the disks at 1080 °C for 2 h in a sealed crucible to prevent volatilization.

The X-ray diffraction (XRD) patterns for the ceramic powers were obtained with an X-ray diffractometer (PGeneral XD-3) patterns using Cu Kα radiation. For room temperature electrical and dielectric properties measurement, platinum electrodes (1 cm²) are fixed on both surfaces of the sintered pellets and fired at 800 °C for 20 min in air. Samples were poled in silicone oil at 200 °C under a dc electric field from 100 kV to 150 kV/cm for 60–120 min. The piezoelectric coefficient d ₃₃ was measured using a quasi-static d ₃₃ meter (Institute of Acoustics, Academia Sinica, ZJ-2). The temperature dependence of the resistivity ρ was determined using a high resistance meter (Shanghai ZC43) with an applied voltage of 20 Volts. The planar coupling k _p and the thickness coupling k _t were determined by the resonance-antiresonance method by using an Impedance Analyzer (Agilent 4294A). The dielectric behavior was also measured using an Impedance Analyzer (Agilent 4294A) at 1000 kHz as a function of temperature.

3 Results and discussion

Figure 1 presents the SEM micrographs of the BTNO-based ceramics. As shown, the BTNO (x = 0.00) ceramic exhibits the smallest grain size when compared to the (ScTa) modified counterparts, indicating that the addition of (ScTa) enhanced the grain growth of the ceramics.

Fig. 1

Micrographs of the surfaces of the Bi₃Ti_1−x(ScTa)_x/2NbO₉ materials. (a)x = 0.00, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06

Figure 2 shows the XRD patterns of Bi₃Ti_1−x(ScTa)_x/2NbO₉ (x = 0.00, 0.02, 0.04, 0.06) ceramics scanned in the range 2θ of 20–60 degree. Compared with the pattern of pure Bi₃TiNbO₉ ceramics, the Bi₃Ti_1−x(ScTa)_x/2NbO₉ (0.02, 0.04, 0.06) ceramics are also bismuth layer-structured ferroelectrics with m = 2. From Fig. 2, the ceramics possess a pure phase of layer-structured structure and no second phases were found, which is believed that Sc³⁺ and Ta⁵⁺ diffuse into the lattices to form solid solutions.

Fig. 2

X-ray diffraction patterns of the Bi₃Ti_1−x(ScTa)_x/2NbO₉ ceramics. (a)x = 0.00, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06

Figure 3 shows the dielectric permittivity (ε/ε ₀) and dielectric loss (tanδ) measured at 1000 kHz as a function of temperature for the Bi₃Ti_1−x(ScTa)_x/2NbO₉-based ceramics. As expected, the dielectric maxima occurred over 900 °C, which corresponding to the Curie temperatures (T _C ). T _C of the ceramics in this work gradually decreased from 913 °C to 901 °C with the diffusion of (ScTa)_1/2 instead of Ti (r _Ti ⁴⁺ =0.068 nm, r _Sc ³⁺ =0.073 nm, r _Ta ⁵⁺ =0.068 nm) into the lattices. The temperature dependence of dielectric behavior in the temperature range of 30–600 °C was found to be very low $\left(\frac{\partial \epsilon }{\partial T}=0.09/{}^{\circ} C\right)$ and the dielectric loss at 600 °C and 1000 kHz was found to be less than 7 %, exhibiting the Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ ceramics possess high stability of the dielectric properties.

Fig. 3

Dielectric permittivity and dielectric loss as a function of temperature for the Bi₃Ti_1−x(ScTa)_x/2NbO₉ ceramics, measured at 1000 kHz

Figure 4 presents piezoelectric constant d ₃₃ as a function of temperature for Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ ceramic, in which one can see the two-layer BTNO orthorhombic structured materials is very stable to thermal annealing. The d ₃₃ of samples are measured after annealing for 1 h, and all the samples are short-circuit during the annealing. After the d ₃₃ measurement, The same samples are put in the oven to be annealed again for the next d ₃₃ measurement. Among the ScTa co-substituted BTNO, one composition, Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ with a quite high piezoelectric constant d ₃₃ of 12 pC/N and an ultrahigh T _C of 905 °C have been obtained recently by using ordinary sintering process. The value of piezoelectric constant d ₃₃ of the piezoelectric ceramics was found to be relative temperature independent to 800 °C and drop to zero when the temperature over 905 °C, related to the de-poling temperature, which is useful for high temperature applications.

Fig. 4

Effect of thermal depoling on d ₃₃ after annealing for 1 h of the Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ ceramic

Figure 5 shows the temperature dependence of the resistivity ρ for the Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ material. The high temperature resisitivity is important for high temperature piezoelectric applications and the ability to achieve high electric field poling at high temperature. The resistivity of the Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ sample is still higher than 10⁵Ω · cm at 550 °C. That is important for its use in high-temperature piezoelectric devices. The activation energy E _a was calculated according to Arrhenius law:

Fig. 5

Temperature dependence of resistivity ρ for the Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ material

$$ \rho ={\rho}_0 \exp \left(\frac{-{E}_a}{ kT}\right). $$

which was found to be 1.02 eV for Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉. This prove the main charge carriers of the material are oxygen vacancies.

Frequency constants N _p (planar frequency constant) and N _t (thickness frequency constant) for BTNO-x = 0.04 material were found to decrease with increasing temperature, indicating that the materials become softer, as shown in Fig. 6. The reduction is very small, where N _p decreases by 5.0 % of its room temperature value when at 620 °C, while the change for N _t was found to be only 1.6 %, exhibiting very low temperature coefficient of resonant frequency.

Fig. 6

Frequency constant as a function of temperature for Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ material

Figure 7 presents electromechanical coupling factors as a function of temperature for Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ material, in which one can see the planar coupling k _p is about 5 % at room temperature, much lower than the value of thickness coupling k _t (~ 19 %), exhibiting a strong anisotropic behavior. Both the coupling factors k _p and k _t were found to be relative temperature independent to 840 °C and drop to zero when the temperature over 890 °C, related to the de-poling temperature, which is useful for high temperature applications.

Fig. 7

Electromechanical coupling factors as a function of temperature for Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ material

The detailed properties of the Bi₃TiNbO₉-based materials were characterized at room temperature and listed in Table 1. The T _C was found to be 913 °C for Bi₃TiNbO₉ (x = 0.00) sample and gradually decrease with the modification of ScTa. The mechanical quality factor Q of the Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ ceramic were found to be higher than that of the other ceramics. The dielectric loss (tanδ) of Bi₃TiNbO₉-based materials increase with modification, where the tanδ of Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ (x = 0.04) was found to be only 0.21 %. The piezoelectricity of the pure BTNO was improved due to the lattice distortion caused by the ScTa co-substitution into B-site, the d ₃₃ of Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉ was found to be 12 pC/N, the highest value among the Bi₃TiNbO₉-based ceramics, also almost 2 times as much as the d ₃₃ values of the pure BTNO ceramics (~6pC/N).

Table 1 Characteristics of the Bi₃Ti_1−xSc_x/2Ta_x/2NbO₉-based ceramics

4 Conclusion

In summary, Bi₃TiNbO₉-based materials were synthesized using conventional solid state processing. The dielectric and piezoelectric properties of Bi₃Ti_0.96Sc_0.02Ta_0.02NbO₉-based ceramics exhibiting a very stable temperature behavior, together with its high T _C ~905 °C, excellent piezoelectric coefficient ~12 pC/N and very low temperature coefficient of resonant frequency, making the (ScTa) modified Bi₃TiNbO₉-based ceramics a candidate for ultra-high temperature applications. The new (ScTa) modification of Bi₃TiNbO₉-based materials resulted in the obvious improvement of the piezoelectric activity, having an important typical significance. The way should be used for reference in constructing the new high performance materials.