Effects of Ta₂O₅ addition on relaxation behavior and electric properties of PMS–PNN–PZT ceramics

Abstract

Pb(Mn_1/3Sb_2/3)_0.01(Ni_1/3Nb_2/3)_0.495(Zr_0.3Ti_0.7)_0.495O₃+x wt% Ta₂O₅ (PMS–PNN–PZT, x = 0, 0.2, 0.4, 0.6, 0.8) lead piezoelectric ceramics were prepared by a traditional two-step solid-state reaction method. The effect of Ta₂O₅ content on the phase structure, microstructure, electrical properties and dielectric relaxation of PMS–PNN–PZT ceramics was investigated. The XRD patterns show that all ceramics have pure perovskite structure. Ta₂O₅ doping can promote the grain growth and improve electrical properties. And the ceramics have high relaxation behavior. When x = 0.4, it exhibits optimum electrical performance: d₃₃ = 805 pC/N, k_p = 66%, ε_r = 6838, tanδ = 1.4%, T_c = 118.5 °C, γ = 1.9618, E_c = 3.652 kV/cm, P_r = 21.91 µC/cm². This indicates that Ta₂O₅ can be used as an effective dopant in PMS–PNN–PZT ceramics and the ceramics can be used as the main material for multilayer ceramic capacitors and electro-strictive actuators.

1 Introduction

Lead based piezoelectric ceramics are widely used in national defense, information and communication, aerospace, navigation, medicine, bioengineering, agriculture and other fields [1,2,3]. In recent years, lead nickel niobate (PNN) based and lead zirconate titanate (PZT) based piezoelectric ceramics have been widely researched. Pb(Ni_x, Nb_y)O₃ are relaxor ferroelectrics with high dielectric constant and piezoelectric constant [4]. Pb(Zr_x, Ti_y)O₃ have high piezoelectric properties, especially at the morphotropic phase boundary (MPB). It has the advantages of convenient preparation and low cost [5]. At present, many scholars have studied how to improve the performance of piezoelectric ceramics for meeting the needs of social development. The performance of piezoelectric ceramics is improved by introducing elements and doping trace elements [6]. So, there are appearing some composite piezoelectric ceramic systems now. The main examples are as follows: Pb(Ni_1/3Nb_2/3)_x(Zr,Ti)_yO₃ (PNN–PZT) [7], Pb(Mg_1/3Nb_1/3)O₃–Pb(Zr,Ti)O₃ (PMN–PZT) [8], Pb(Mn_1/3Sb_2/3)O₃–PbZrO₃–PbTiO₃ (PMS–PZT) [9], Pb_1−xBa_x(Zn_1/2Nb_1/2)_y(Ni_1/3Nb_2/3)_z(Zr,Ti)_1−y−zO₃ (PZN–PNN–PZT–BZT) [10], yPb(In_1/2Nb_1/2)O₃–(1–x–y)Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ (PIN–PMN–PT) [11], PbHfO₃–PbTiO₃–Pb(Mg_1/3Nb_2/3)O₃ (PMN–PST) [12], PbHfO₃–PbTiO₃–Pb(Mg_1/3Nb_2/3)O₃ (PMN–PHT) [13]. Meanwhile, PNN–PZT based piezoelectric ceramics are represented by high piezoelectric properties, but dielectric loss of the system is relatively high. PMS based ceramics are typical relaxor ferroelectrics with good piezoelectric properties, especially low dielectric loss. PMS–PNN–PZT piezoelectric ceramics were selected in this experiment. On the other way, in order to satisfy the requirement of different electrical products, many researchers try to achieve the desired requirements by doping trace elements.

The results show that appropriate doping of trace elements in lead based piezoelectric ceramics will improve their properties [14,15,16]. Ta₂O₅ is a kind of high-valence oxide. Some researchers doped it into the lead-free piezoelectric ceramics and studied the improvement of lead-free piezoelectric ceramics [17, 18]. Lin et al. [19] found that piezoelectric constant and electromechanical coupling coefficient are effectively improved by adding Ta⁵⁺ into the K_0.5Na_0.5Nb_1−xTa_xO₃ system. Du et al. [20] reported that BiFe_0.5Ta_0.5O₃ doping into KNN system can availably increase the grain size and improve piezoelectric performance.

But few researchers studied the effect of Ta⁵⁺ on the electrical properties of lead based piezoelectric ceramics. Ta⁵⁺ and Nb⁵⁺ have similar chemical properties. Doping Nb⁵⁺ can improve electrical properties [21]. So, Ta₂O₅ was introduced into 0.01PMS–0.495PNN–0.495PZT ceramics to study effects of Ta⁵⁺ additive on the phase structure, micro-structure, electric properties and relaxation behavior in this paper. The results show that addition of Ta₂O₅ effectively improves the piezoelectric performance. Moreover, the ceramics exhibit obvious dielectric relaxation behavior. It is indicated that the PMS–PNN–PZT + xTa⁵⁺ piezoelectric ceramics can be applied at multilayer ceramic capacitors and electro-strictive actuators.

2 Experimental procedure

Pb(Mn_1/3Sb_2/3)_0.01(Ni_1/3Nb_2/3)_0.495(Zr_0.3Ti_0.7)_0.495O₃+x wt% Ta₂O₅ (x = 0, 0.2, 0.4, 0.6, 0.8) piezoelectric ceramics were synthesized by traditional solid sintering method. The raw materials are analytical-grade metal oxides powders: Pb₃O₄(99.0%), ZrO₂(99.5%), TiO₂(99.6%), Nb₂O₅(99.5%), Ni₂O₃(99.0%), MnO₂(99.0%), Sb₂O₃ (99.0%) and Ta₂O₅(99.99%). Firstly, each raw material needed to be calculated according to the chemical formula and measured with a balance. In order to compensate for the volatilization of Pb during sintering process, the Pb₃O₄ weighed with excess 1 wt%. Then the mixture was placed a ball mill with alcohol. The purpose of adding alcohol was to make the powder more refined. The mixtures were ball-milled in alcohol for 10 h. The ball milled materials were dried and then burned for 4 h at 920 °C. The calcined powders were ball-milled again for 10 h. After drying, the powders were pressed into tablets with 6 wt% paraffin waxes under 5 MPa with the size of ϕ12 mm × 1 mm. Subsequently, samples were placed in a sintering furnace and sintered for 2 h at 1260 °C under closed conditions. The heating rate was 2 °C per minute. Then samples were cooled down to room temperature with the furnace.

The crystal phase composition of samples was measured by using an X-ray diffractometer (XRD, Model Panalytical/X’pert-PRO, Holland). The cross-sectional microstructure of specimens was observed by scanning electron-microscopy (SEM, Model SUPRA40, Germany). Samples were applied silver paste on both sides and heated for 20 min at 700 °C. Then they were poled in silicon oil at 50 °C under DC electric field of 2 kV/mm for 30 min. The piezoelectric constants of ceramics were determined by using a quasi-static d₃₃ meter (ZJ-3A; Institute of Acoustics Academic Science, China). The electromechanical coupling coefficients (k_p) were calculated by resonance and anti-resonance techniques using an electric oscillator (Model UX21) under 1 kHz. The relative permittivity and dielectric loss as functions of temperature were evaluated with an LCR meter (TZDM system), which connects to a computer-controlled temperature chamber. The test frequencies range from 1 kHz to 1 MHz. According to the following formula [22, 23]:

$${{\text{k}}_{\text{p}}}=\sqrt {2.54 \times \frac{{({{\text{f}}_{\text{p}}} - {{\text{f}}_{\text{s}}})}}{{{{\text{f}}_{\text{s}}}}}} {\text{~}}$$

(1)

$${\varepsilon _r}=\frac{\varepsilon }{{{\varepsilon _0}}}=\frac{{{\text{Cd}}}}{{{\varepsilon _0}{\text{S}}}}=\frac{{4{\text{Cd}}}}{{{\varepsilon _0}\pi {\phi ^2}}} \approx \frac{{144{\text{Cd}}}}{{{\phi ^2}}}$$

(2)

The free space dielectric constant value ε₀ is 8.85 × 10^− 12 F/m, C denotes capacitance (units 10^− 12 F), d denotes the sample thickness (mm), S denotes the area (mm²), ϕ denotes the sample diameter.

$${Q_m}={f_a}^{2}{[2\pi {R_f}C{f_r}({f_a}^{2} - {f_r}^{2})]^{ - 1}}$$

(3)

The f_r is resonance frequency (Hz), f_a is antiresonance frequency (Hz). Q_m is the minimum impedance at f_r (ohm) resonance impedance, C is capacitance mechanically free at 1 kHz.

3 Results and discussion

After samples preparation, the conclusions of X-ray diffraction, scan electron-microscopy, piezoelectric, dielectric and ferroelectric properties have been done for evaluating the quality of PMS–PNN–PZT based piezoelectric ceramics.

3.1 Phase structure of ceramic

Figure 1a shows the X-ray diffraction (XRD) patterns of 0.01PMS–0.495PNN–0.495PZT ceramics with different Ta₂O₅ content sintered at 1260 °C. In the Fig. 1a, no detectable secondary phase peak can be found, which indicates that all the samples are pure perovskite structure. It proves that Ta⁵⁺ diffused into the lattice and fused with the substrate to form a solid solution. Figure 1b shows the magnified XRD patterns of ceramics in the range of 44–46°. It can be clearly observed that location of peaks changes with increase of Ta₂O₅ amount, the diffraction peak near the 45° shifts to lower angles. With Ta₂O₅ doping, elements substitution occurs in the system. According to the principle of ionic radius similarity, when Ta⁵⁺ doped into the main lattice of the system, it will give priority to the substitution of B-site ion. So, it makes the lattice constant of crystal larger and the diffraction peak offsets to the lower angle. This phenomenon indicates that doping Ta⁵⁺ not only causes lattice distortion, but also changes the phase composition of samples. All samples exhibit the tetragonal phase structure without phase structure transformation.

Fig. 1

X-ray diffraction patterns of 0.01PMS–0.495PNN–0.495PZT+x wt% Ta₂O₅ ceramics

3.2 Sectional microstructure of ceramics

Figure 2 shows the SEM images of the PMS–PNN–PZT+x wt% Ta₂O₅ (x = 0, 0.2, 0.4, 0.6, 0.8) ceramics sintered at 1260 °C for 2 h. It can be seen from Fig. 2 that when moderate amount of Ta₂O₅ was doped, the grains inside the ceramics become full, and the grain size distribution is more uniform. At x = 0.4, the structure is the most compact. At this point, it exhibits optimum electrical performance value. However, there are many fine grains when x > 0.4. This is due to excessive Ta⁵⁺ concentrate on the grain boundary without entering the lattice and forming a solid solution. The specific surface area between the fine grains is increased. The dislocation will tangle at the grain boundary, which hinders the motion of the domain. And it has the clamping effect on the domain [24]. So, polarization of the domain is difficult and electrical properties become worse.

Fig. 2

Sectional microstructure of 0.01PMS–0.495PNN–0.495PZT+x wt% Ta₂O₅ ceramics. a x = 0, b x = 0.2, c x = 0.4, d x = 0.6, e x = 0.8

3.3 Electrical properties of ceramics

Figure 3 shows the electrical properties of PMS-PNN-PZT+x wt% Ta₂O₅ ceramics. In the Fig. 3a, it can be seen that the Ta⁵⁺ doping can effectively improve performance of the piezoelectric ceramics. With the increase of Ta⁵⁺ doping, the piezoelectric constant (d₃₃) and the electromechanical coupling coefficient (k_p) have different degrees of improvement. When the amount of doping is 0.4 wt%, d₃₃ and k_p show the maximum values. It is due to the increase of the lattice constant caused by doping, resulting in an increase in the electrical dipole moment. B-site ions are further away from the center. So, the piezoelectric performance is improved. In the other hand, due to the occurrence of high-valent ion substitution, cation vacancies are formed, which improves the mobility of the crystal structure. Thereby promoting the deflection of the domain, the ceramic sample is more polarized. The d₃₃ and k_p values are effectively improved.

Fig. 3

a d₃₃ and k_p, b ε_r and tanδ properties of the 0.01PMS–0.495PNN–0.495PZT + x wt% Ta₂O₅ ceramics

In the Fig. 3b, the ε_r value increases with increase of Ta⁵⁺ content. It may be that the incorporated Ta⁵⁺ replaces Mn²⁺. Mn²⁺ ions can effectively reduce ε_r values of system [25]. The dielectric constant gradually increases when content of Mn²⁺ ions decreases in this system. For the change of tanδ value, it is probable that substitution of high-valent ions makes the electrovalency unbalance. This system forms the cation vacancy, which is beneficial to the motion of domain boundary. The tanδ value begins to increase gradually. However, when the content of Ta⁵⁺ ions is more than 0.4 wt%, change of tanδ is not obvious. Excessive Ta⁵⁺ ions stockpile at the grain boundary, make the domain boundary deflection more difficult. Thus, the tanδ value does not increase further.

3.4 Dielectric properties of ceramics

Figure 4a shows temperature dependence of the dielectric constant for PMS–PNN–PZT + x wt% Ta₂O₅ ceramics at the frequency of 10 kHz. The test temperature ranges from room temperature to 280 °C. Figure 4b is the Curie temperature at 10 kHz. It can be seen from the Fig. 4 that the Curie temperature (T_c) decreases with increase of Ta⁵⁺ content. T_c value of piezo-ceramic materials reflects the difficulty of B-bite ions deviating from the center position in BO₆ oxide octahedral. Because of increase of lattice constant caused by ion-doping, the distance between B-site ions and oxygen ions will increase. As the distance increases, the interaction between the B-site ions and oxygen ions weakens and transition energy of phase structure will be reduced. So, T_c decreases with increase of Ta⁵⁺ content. It is shown from Fig. 4a that the maximum permittivity of the dielectric constant increases with increase of Ta⁵⁺ content and no other peaks are found from room temperature to 280 °C. The results also show that there is no transition from rhombohedral to tetragonal phase in this system. It is in accordance with the results of the XRD graph analysis.

Fig. 4

Temperature dependence of a dielectric constant ceramics at 10 kHz, b Curie temperature

3.5 Relaxation behavior of ceramics

Figure 5 shows the inverse dielectric constant as a function of temperature for 0.01PMS–0.495PNN–0.495PZT + x wt% Ta₂O₅ ceramics at 10 kHz. For normal ferroelectrics, when temperature is higher than the Curie temperature, dielectric permittivity should follow the Curie–Weiss law described by Ref. [26]:

Fig. 5

The inverse dielectric constant at 10 kHz as a function of temperature for the 0.01PMS–0.495PNN–0.495PZT + x wt% Ta₂O₅ ceramics

$$~\frac{1}{\varepsilon }=\frac{{T - {T_{CW}}}}{C}~$$

(4)

where C is the Curie–Weiss constant and T_cw is the Curie–Weiss temperature. The T_cw can be obtained by formula (4). Through the Fig. 5 can observe the relationship between dielectric constant and temperature, it does not follow the Curie–Weiss formula. The degree of the deviation from the Curies–Weiss law is defined by ΔT_m as formula (5) [27]:

$${\text{~}}\Delta {T_m}={T_B} - {T_m}$$

(5)

T_B is the initial temperature where the dielectric constant conforms to Curie’s law and T_m is corresponding temperature of the maximum dielectric constant. Table 1 shows some values about dielectric properties. From the Table 1, there is no obvious change about ΔT_m with increase of Ta⁵⁺ content. It indicates that the dielectric relaxation of all samples is relatively close.

Table 1 Summary of some parameters extracted from ceramics dielectric measurements at 10 kHz

For the relaxation ferroelectric, it can be studied according to the modified Curie–Weiss law [28,29,30]:

$$~\frac{1}{\varepsilon } - \frac{1}{{\varepsilon _{m} }} = \frac{{\left( {T - T_{m} } \right)^{\gamma } }}{{C_{1} }},\;~\left( {{\text{T}}> {\text{T}}_{m} ,\;1 \le \gamma \le 2} \right)~$$

(6)

where C₁ is the Curie constant, ε_m and T_m are the maximum dielectric constant and temperature correspondingly, γ is called a diffusion coefficient ranging from 1 (normal ferroelectric) to 2 (complete relaxor ferroelectric). Figure 6 shows that plot of ln(1/ε_r − 1/ε_m)/ln(T − T_m) at 10 kHz for ceramics. Figure 6f is the variation of dispersion coefficient of Ta⁵⁺ ceramics doped with different contents and the dispersion coefficient (γ) is fitted by formula (6). The dispersion values of all samples are > 1.90, indicating that they have obvious dielectric relaxation behavior. With increase of Ta⁵⁺ content, the dispersion coefficient has hardly changed, indicating that doping in this range will not affect dielectric relaxation degree of this system.

Fig. 6

Plot of ln(1/ε − 1/ε_m)/ln(T − T_m) at 10 kHz for 0.01PMS–0.495PNN–0.495PZT + x wt% Ta₂O₅ ceramic samples

3.6 Ferroelectric behavior of ceramics

Figure 7a shows P–E hysteresis loops of samples with different Ta₂O₅ content. Figure 7b shows remnant polarization (P_r) and coercive field (E_c) of samples. From Fig. 7, all samples show typical ferroelectric hysteresis loops. With increase of Ta₂O₅ content, the remnant polarization (P_r) and the coercive field (E_c) increase first and then decrease. But the changes are not obvious. When x = 0.4, P_r reaches the maximum value (21.91 µC/cm²) and E_c value is smaller, which is 3.652 kV/cm. In addition, the hysteresis loop is the fullest and the polarization is the most sufficient, which further verifies the change of electrical properties. To sum up, it is suggested that Ta₂O₅ doping can strengthen the ferroelectric properties of the ceramics.

Fig. 7

a P–E hysteresis loops and b P_r and E_c for 0.01PMS–0.495PNN–0.495PZT + x wt% Ta₂O₅ samples

4 Conclusions

In conclusion, Pb(Mn_1/3Sb_2/3)_0.01(Ni_1/3Nb_2/3)_0.495(Zr_0.3Ti_0.7)_0.495O₃+x wt% Ta₂O₅ (PMS–PNN–PZT, x = 0, 0.2, 0.4, 0.6, 0.8) lead piezoelectric ceramics were prepared by a traditional two-step solid-state reaction method. The effects of Ta₂O₅ on the phase structure, micro-structure, electrical properties and relaxation behavior of the ceramics were systematically studied. The result of XRD diagram shows that all samples are pure perovskite structure. They all show obvious dielectric relaxation behavior. The dispersion coefficient value almost does not change, indicating that incorporation of Ta⁵⁺ does not affect the degree of dielectric relaxation in the system. When x = 0.4, it exhibits optimum electrical performance: d₃₃ = 805 pC/N, k_p = 66%, ε_r = 6838, tanδ = 1.4%, T_c = 118.5 °C, γ = 1.9618, E_c = 3.652 kV/cm, P_r = 21.91 µC/cm². These results confirm that the ceramic is with the potentialities to be applied in multilayer ceramic capacitors and electro-strictive actuators.