Piezoelectric Characteristics of 0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr,Ti)O₃ Ceramics with Different MnO₂ Concentrations for Ultrasound Transducer Applications

Abstract

In this study, we investigate the piezoelectric characteristics of 0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr,Ti)O₃ (PNN-PZT) with MnO₂ additive (0, 0.25, 0.5, 1, 2, and 3 mol%). We focus on the fabrication of a piezoelectric ceramic for use as both actuator and sensor for ultrasound transducers. The actuator and sensor properties of a piezoelectric ceramic depend on the piezoelectric strain coefficient d and piezoelectric voltage coefficient g, related as g = d/ε^T. To increase g, the dielectric constant ε^T must be decreased. PNN-PZT with MnO₂ doping is synthesized using the conventional solid-state reaction method. The electrical properties are determined based on the resonant frequencies and vibration modes measured by using an impedance analyzer. The MnO₂ addition initially improves the tetragonality of the PNN-PZT ceramic, which then saturates at a MnO₂ content of 1 mol%. Therefore, the dielectric constant and piezoelectric coefficient d₃₃ steadily decrease, while the mechanical properties (Q_m, Young’s modulus), tanδ, electromechanical coupling coefficient k, and piezoelectric voltage coefficient g were improved at 0.5–1 mol% MnO₂ content.

1. Introduction

Piezoelectric materials have attracted considerable interest for various applications such as multi-layer ceramic actuator, transducer, sensor and actuator applications, and for analyses on fundamental science. Lead-based piezoelectric ceramics such as Pb(Zr,Ti)O₃ (PZT) have been extensively used in electrical devices because of their excellent piezoelectric properties [1]. Recently, the policies suggesting lead elimination have triggered studies on alternative compounds such as (K _0.44Na_0.52Li_0.04)-(Nb_0.86Ta_0.10Sb_0.04)O₃ (KNL-NTS) [2]. However, the lead-based compositions have exhibited higher piezoelectric performances than those of the lead-free compositions such as KNL-NTS.

Recently, many piezoelectric ceramic compositions of the ternary system have been reported as the ternary system has a larger morphotropic phase boundary (MPB) area than that of a secondary system such as PZT. Lead-based relaxor ferroelectrics have a general formula of Pb(B’ B”)O₃ (where B’ is Mg²⁺, Ni²⁺, Zn²⁺, Fe³⁺, Sc³⁺, In³⁺, Mn⁴⁺, and Sn⁴⁺ and B” is Nb⁵⁺, Ta⁵⁺, Sb⁵⁺, and W⁶⁺) [3,4]. In this ternary system, 0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr,Ti)O₃ (PNN-PZT) has attracted increasing attention owing to its excellent piezoelectric properties. Modifications of lead-based relaxor ferroelectrics have been extensively investigated to improve the piezoelectric properties. In 1974, Luff et al. [5] investigated a 0.5Pb(Ni_1/3Nb_2/3)O₃-0.35PbTiO₃-0.15PbZrO₃ solid solution in the PNN-PZT ternary system and observed excellent piezoelectric properties. Vittayakorn et al. [6] reported the phase diagram between PNN and PZT ((1–x)Pb(Ni_1/3Nb_2/3)O₃-xPb(Zr_1/2Ti_1/2)O₃ (x = 0.4–0.9)), where the Zr/Ti composition was fixed close to the MPB of PZT, and new MPB within this system. Cao et al. [7] investigated 0.3Pb(Ni_1/3Nb_2/3)O₃-xPbTiO₃-(0.7−x)PbZrO₃ (x = 0.33–0.43) as a function of the content of PbTiO₃. Nam et al. [8] reported the 0.35Pb(Ni_1/3Nb_2/3)O₃-0.65Pb(Zr_1−xTi_x)O₃ (x = 0.56–0.63) ceramic composition sintered at 1200 °C with good electrical properties including d₃₃ = 605 pC/N, K_p = 0.61, and ε_r = 3600. Du et al. [3] have reported the 0.55Pb(Ni_1/3Nb_2/3)O₃-0.135PbZrO₃-0.315PbTiO₃ ceramic with good electrical properties including d₃₃ = 1070 pC/N, K_p = 0.69, ε_r = 8710, and tanδ = 26 × 10⁻³. Figure 1 shows the reported compositions of the PNN-PZT ceramics.

The doping of proper elements is an effective approach to enhance their properties for targeted applications. Yu et al. [9] investigated the effects of MnO₂ additive on the 0.12Pb(Ni_1/3Sb_2/3)-0.48PbTiO₃-0.40PbZrO₃ ceramic. A 0.15 wt% MnO₂-doped sample exhibited enhanced piezoelectric properties, including K_p = 0.68, ε_r = 3069, Q_m = 181, and tanδ = 5.4 × 10⁻³. Bamiere et al. [10] reported that a Sr-doped 0.674Pb,Nd(Zr,Ti)O₃-Pb(Ni_1/3Nb_2/3)O₃ exhibited good ferroelectric properties at low sintering temperatures. Du et al. [11] investigated the effects of small additions of xFe₂O₃ (x = 0~1.6 mol%) on the microstructures and electrical properties of 0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr_0.3Ti_0.7)O₃ ceramics sintered at 1200 °C for 2 h and reported high piezoelectric performances at 1.2 mol% of Fe₂O₃ (ρ = 7.97 g/cm³, d₃₃ = 956 pC/N, K_p = 0.74, ε_r = 6095, and tanδ = 26 × 10⁻³). Yoo et al. [12] reported high values of piezoelectric properties (ρ = 7.816 g/cm³, d₃₃ = 356 pC/N, k_p = 0.597, ε_r = 920, Q_m = 1186) for the 0.2 wt% MnO₂ 0.02Pb(Mn_1/3Nb_2/3)O₃-0.12Pb(Ni_1/3Nb_2/3)O₃-0.86Pb(Zr_0.5Ti_0.5)O₃ ceramic. Liao et al. [13] reported that Fe doping largely reduced tanδ and d₃₃, but improved Q_m and tetragonality of the 0.35BiScO₃-0.6PbTiO₃-0.05Pb(Zn_1/3Nb_2/3)O₃ ceramic. Liu et al. [4] investigated 0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr_0.3Ti_0.7)O₃ (PNN-PZT) with varying MnO₂ additive content and reported high performance (d₃₃ = 710 pC/N, k_p = 0.595, ε_r = 3092.25, tanδ = 14.9 × 10⁻³, Q_m = 176) at a MnO₂ content of 1 mol%.

Table 1. Literature table of piezoelectric ceramic and properties according to doping materials.

Authors	Piezoelectric Ceramic	Doping	Properties
Yu et al. [9]	0.12Pb(Ni1/3Sb2/3)-0.48PbTiO3-0.40PbZrO3	0.15 wt% MnO2	Kp = 0.68, εr = 3069, Qm = 181, tanδ = 5.4 × 10−3
Bamiere et al. [10]	0.674Pb,Nd(Zr, Ti)O3-Pb(Ni1/3Nb2/3)O3	SrCO3 (0~4 mol%)	Lower the sintering temperature
Du et al. [11]	0.55Pb(Ni1/3Nb2/3)O3-0.45Pb(Zr0.3Ti0.7)O3	Fe2O3 (0~1.6 mol%)	ρ = 7.97 g/cm3, d33 = 956 pC/N, Kp = 0.74, εr = 6095, tanδ = 26 × 10−3
Yoo et al. [12]	0.02Pb(Mn1/3Nb2/3)O3-0.12Pb(Ni1/3Nb2/3)O3-0.86Pb(Zr0.5Ti0.5)O3	0.2 wt% MnO2	ρ = 7.816 g/cm3, d33 = 356 pC/N, kp = 0.597, εr = 920, Qm = 1186
Liao et al. [13]	0.35BiScO3-0.6PbTiO3-0.05Pb(Zn1/3Nb2/3)O3	Fe2O3 (0~1.6 mol%)	Largely reduced tanδ and d33, but improved Qm
Liu et al. [4]	0.55Pb(Ni1/3Nb2/3)O3-0.45Pb(Zr0.3Ti0.7)O3	1 mol% MnO2	d33 = 710 pC/N, kp = 0.595, εr = 3092.25, tanδ = 14.9 × 10−3, Qm = 176

shows piezoelectric ceramic and properties according to doping from references.

Most lead-based relaxor ferroelectrics such as PNN-PZT have high dielectric constants and d₃₃ values, but low g₃₃ values and poor mechanical properties (Q_m, Young’s modulus) for applications as sensors [13]. Modifications of the properties of the ternary complex systems have not been extensively investigated. The doping of materials such as FeO₃ and MnO₂ can be used to easily change the properties of ternary complex systems. To improve the g₃₃ values and mechanical properties of the PNN-PZT systems, Mn can be added to selectively improve sensor characteristics. The MnO₂ doping can improve the sinterabilities of lead-based relaxor ceramics owing to the increase in number of oxygen vacancies, generated by the substitutions of the high-valence Ti⁴⁺ and Zr⁴⁺ in the perovskite lattice by the low-valence Mn²⁺ and/or Mn³⁺ [4]. The oxygen vacancies in the perovskite lattice can promote lattice diffusion, thus assisting the sintering and grain growth [12], as the grain boundary movement is dragged by these defects. The pores in the lead-based relaxor ceramics easily diffuse through the movement of oxygen vacancies and are eliminated at the grain boundaries. Therefore, the densification of the ceramic originated from the introduction of MnO₂ improving the piezoelectric properties [13]. It is possible to adjust the properties of the PNN-PZT ceramic according to the MnO₂ content.

The aim of this study was to better understand the effects of doping on PZT-based complex ceramics and improve the sensing performances and mechanical properties by using MnO₂ as an additive in the 0.55Pb(Ni_1/3Nb_2/3)O₃-0.135PbZrO₃-0.315PbTiO₃ ternary ceramic. The effects of the MnO₂ content on the piezoelectric, dielectric, and mechanical properties of the ceramic were investigated.

2. Synthesis of the MnO₂-Doped PNN-PZT

The ceramics of PNN-PZT + MnO₂ (0, 0.25, 0.5, 1, 2, and 3 mol%) were synthesized by using the conventional solid-state method. Raw material powders (PbO, NiO, Nb₂O₅, ZrO₂, and TiO₂) (Sigma Aldrich, 99.99%, Gillingham, UK) and MnO₂ additive (Sigma Aldrich, 99.99%, Gillingham, UK) were weighted in chemically stoichiometric proportions and ball-milled with distilled water for 24 h. After the ball milling, the slurry was dried at 80 °C, and then calcined at 900 °C (heating rate: 100 °C/h) for 2 h. The calcined powders were ball-milled again for 24 h with a 5 wt% polyvinyl alcohol (PVA) (Sigma Aldrich, 99+%, Gillingham, UK, M_w = 89000–98000) solution as a binder for ceramic formation. The mixture was dried and crushed by a high-energy ball mill. The powder was sieved to control the particle size below 5 μm. The sieved powder was pressed into a mold (Φ = 30 mm) under a pressure of 100 MPa. The PVA in the ceramic disk was burnt out at 600 °C (heating rate: 100 °C/h) for 2 h. Subsequently, the ceramic disk was sintered at 1200 °C (heating rate: 100 °C/h) for 2 h with a spacer powder in an alumina crucible [14]. The sintered PNN-PZT ceramic disk was polished and coated with silver paste to obtain electrodes on both surfaces. The samples were polarized by applying a direct-current (DC) electric field of 2.5 kV/mm for 30 min at 50 °C in silicon oil.

The crystal structures of the sintered samples were characterized by X-ray diffraction (XRD) (X’pert Pro Powder, PANalytical, Netherlands). The surface microstructures of the as-sintered ceramics were observed by using field-emission scanning electron microscopy (SEM) (JSM-5900, JEOL, Akishima City, Japan). Three PNN-PZT specimens were fabricated for each composition. Their properties were measured, and then the average values were calculated. The piezoelectric coefficients (d₃₃) of the piezoelectric ceramics were measured by using a quasi-static piezoelectric d₃₃ meter (HY2730, Yangzhou, China). The planar electromechanical coupling coefficients (k_p, k₃₁), piezoelectric coefficient (d₃₁), mechanical factor (Q_m), dielectric constant (ε_r = ε^T₃₃/ε₀, ε₀ = 8.854 × 10⁻¹² F/m), piezoelectric voltage coefficients (g₃₃, g₃₁), and Young’s modulus (Y^E₁₁) were determined according to the method of resonance and antiresonance frequencies by using an impedance analyzer (HP 4194A, Agilent, Santa Clara, CA, USA) based on the Institute of Electrical and Electronics Engineers (IEEE) standards. Length-mode specimens (22 by 4 by 0.8 mm³) were used to calculate Y^E₁₁, k₃₁, d₃₁, and g₃₁.

3. Results and Discussion

3.1. Phase and Microstructure

Figure 2 shows XRD patterns of the 0.55PNN-0.45PZT ceramics doped with different MnO₂ contents (0, 0.25, 0.5, 1, 2, and 3 mol%). All samples exhibited typical ABO₃ perovskite structures without the pyrochlore phase. Rhombohedral and tetragonal phases were found to coexist in the PNN-PZT ceramics.

As shown in Figure 3, the XRD patterns were fitted by Gaussian functions in the 2θ range of 44.5–45.5°. The (200) peak consisted of three peaks, tetragonal (200) and (002) diffractions peaks presented in green and blue, respectively, and rhombohedral (200) diffraction peak presented in magenta. The red peak represents the overlap intensities of T(200), T(002), and R(200). Figure 2 shows the apparent changes in diffraction peaks, which indicate a gradual rhombohedral-to-tetragonal phase transition. Tetragonality is a crucial structural parameter of the perovskite lattice because it may affect the material properties. The tetragonality was calculated as I_T(200)/I_R(200), where I_T(200) is the T(200) intensity and I_R(200) is the R(200) intensity of the XRD pattern in Figure 3. As shown in Figure 4, with the increase in MnO₂ content, the tetragonality of the PNN-PZT ceramic initially largely increases, and then saturates at MnO₂ contents higher than 1 mol% [13].

Figure 5 shows SEM images of the PNN-PZT ceramics with MnO₂ contents of 0–3 mol%. The pure PNN-PZT ceramic exhibited small grain sizes (microstructures) of almost 1.5 μm. When a small amount of MnO₂ was added, the grain size of the microstructure was increased. The maximum average grain size of 2.8 μm was observed in PNN-PZT with the MnO₂ content of 3 mol%.

3.2. Dielectric Properties

Figure 6 shows the dielectric constants ε_r and dielectric losses tanδ (%) of the PNN-PZT ceramics with different MnO₂ concentrations measured at 1 kHz at room temperature. The dielectric loss largely decreased with the MnO₂ content of 1 mol%, and then steadily increased in the MnO₂ range of 1 to 3 mol%. The dielectric loss reached the maximum (5.6%) for the undoped PNN-PZT ceramic and minimum (1.6%) for the 1 mol% MnO₂ sample. Therefore, the dielectric loss decreased by almost five times upon slight MnO₂ doping. This is consistent with the change rate of the tetragonality. The dielectric loss rapidly decreased with the increase in tetragonality from 0 to 1 mol%. The dielectric loss was improved at MnO₂ content above 1 mol%, because the tetragonality was saturated at 1 mol%. Figure 6 shows the negative and positive effects on the dielectric constant ε_r and dielectric loss tanδ (%). A negative effect, decrease in dielectric constant, was observed with the increase in tetragonality with the MnO₂ content. In terms of the dielectric loss, the minimum value could be explained by the competition between the positive effect (increase in tetragonality) and negative effect of the Mn ions on the motion of the wall domains.

With the increase in MnO₂ content, the dielectric constant decreased because the hardener Mn ions affected the domain movement. The decrease in dielectric constant was caused by the oxygen vacancies generated by the substitutions of the high-valence Ti⁴⁺ and Zr⁴⁺ in the perovskite lattice by the low-valence Mn²⁺ and/or Mn³⁺. The dielectric constant rapidly decreased up to the MnO₂ content of 1 mol%. However, the decrease rate of the dielectric constant was smaller at MnO₂ contents of 1 to 3 mol%.

3.3. Mechanical Properties

Figure 7 shows the mechanical properties of the PNN-PZT ceramics with varying MnO₂ content (0–3 mol%). The mechanical quality factor Q_m reflects the steepness of the resonance of the mechanical vibration around the resonance frequency. Therefore, Q_m has been considered the main parameter of an ultrasonic actuator. Q_m was improved by approximately five times (from 42.70 to 202.26) with the increase in MnO₂ content. In addition, the Young’s modulus Y^E₁₁ was improved from 7.14 to 10.56 with the increase in MnO₂ content (a similar curve shape was observed). The changes in mechanical properties with the MnO₂ content can be attributed to the oxygen vacancies generated by the accepter doping effect. Therefore, the densifications of the ceramics resulted from the introduction of MnO₂, which improved the piezoelectric properties [13]. The density was improved from 7771.09 to 7938.41 kg/m³ with the increase in MnO₂ content (

Table 2. Piezoelectric coefficients d₃₃ and −d₃₁ of the PNN-PZT ceramics with different MnO₂ concentrations in the range of 0–3 mol%.

MnO2 Content	Density (kg/m3)	εr at 1 kHz	tanδ (%) at 1 kHz	Qm	YE11 (× 1010 N/m2)	K31	kp	−d31 (pC/N)	d33 (pC/N)	−g31 (× 10−3 V∙m/N)	g33 (× 10−3 V∙m/N)
0 mol%	7771.09	4925.51	5.6	42.70	7.14	0.35	0.36	243.40	746	6.40	17.10
0.25 mol%	7782.96	3772.38	2.85	50.99	7.70	0.26	0.27	167.07	592	5.40	17.71
0.5 mol%	7864.18	3177.74	1.6	63.49	8.70	0.27	0.32	140.40	538	6.06	19.13
1 mol%	7936.33	2965.10	1.6	139.70	9.56	0.26	0.41	115.01	533	5.99	20.31
2 mol%	7938.13	2408.93	2.4	173.50	10.11	0.21	0.34	78.51	396	5.33	18.55
3 mol%	7938.41	1826.92	4.5	202.26	10.56	0.18	0.28	68.98	342	5.24	21.13

). The oxygen vacancies improved the mechanical properties.

3.4. Piezoelectric Properties

The electromechanical coupling coefficient (k_p, −k₃₁) is a constant representing the piezoelectric efficiency of a piezoelectric ceramic, i.e., it represents the efficiency of conversion of electrical energy into mechanical energy. As shown in Figure 8, k_p decreased with the increase in MnO₂ content to 0.25 mol%. However, it was largely increased at MnO₂ contents of 0.25 and 1 mol%, where it reached the maximum of 0.41. Above this concentration, it steadily decreased in the MnO₂ content range of 1 to 3 mol%.

The curve shape of −k₃₁ was different. It had the maximum value for the undoped PNN-PZT ceramic, and then steadily decreased with the increase in MnO₂ content. The −k₃₁ values at 0.25–1 mol% were similar. At MnO₂ concentrations less than 1 mol%, the changes may be attributed to the relatively large increase in tetragonality. Above this concentration, k_p and −k₃₁ decreased owing to the hardener doping effect (acceptor substitution inducing point defects) [12,13]. The piezoelectric coefficients (−d₃₁, d₃₃) reflect the distortion originating from the application of an electric field having a uniform strength without stress. Therefore, the piezoelectric coefficients (−d₃₁, d₃₃) have been considered the primary parameters of actuators. The piezoelectric coefficient was calculated by using −k₃₁, Y^E₁₁, and ε^T₃₃:

$$d_{3x}=k_{3x}\sqrt{\frac{{\epsilon }_{33}^T}{Y_{xx}^E}}$$

(1)

Figure 9 shows the piezoelectric coefficients (−d₃₁, d₃₃) of the PNN-PZT ceramics with varying MnO₂ content (0–3 mol%). −d₃₁ and d₃₃ exhibited similar curve shapes; they decreased with the increase in MnO₂ content because the decrease rate of ε^T₃₃ was considerably higher than the increase rates of k₃₁ and Y^E₁₁. The piezoelectric coefficient exhibited the minimum decrease rate between 0.5 and 1 mol% MnO₂, because of the relatively significant increase in tetragonality at 0.5–1 mol%.

The piezoelectric voltage coefficients (−g₃₁, g₃₃) reflect the field strength, originating from a uniform applied stress without electrical displacement, and thus they represent the sensor properties. The values of −g₃₁ and g₃₃ were calculated by using the relationship between the piezoelectric coefficient d and dielectric constant, g = d/ε^T. −g₃₁ exhibited a similar tendency to that of k₃₁ (Figure 10). g₃₃ steadily increased to a high value of 20.31 × 10⁻³ Vm/N at 1 mol%. However, it was decreased at 2 mol%, and then largely increased to the maximum of 21.13 × 10⁻³ Vm/N at 3 mol%, as the decrease rate of d₃₃ was smaller than the rate of decrease in ε^T₃₃. Table 2 shows the densities and dielectric, mechanical, and piezoelectric properties of the PNN-PZT samples with different MnO₂ contents (0, 0.25, 0.5, 1, 2, and 3 mol%).

The MnO₂ doping can affect the PNN-PZT ceramic properties owing to the increase in number of oxygen vacancies generated by the substitutions of the high-valence Ti⁴⁺ and Zr⁴⁺ in the perovskite lattice by the low-valence Mn²⁺ and/or Mn³⁺, as mentioned above [4]. The oxygen vacancies improved the mechanical properties (Young’s modulus, Q_m). The Young’s modulus and Q_m exhibited similar curve shapes. Initially, they were rapidly improved, but their increase rates were reduced at MnO₂ contents above 1 mol%. The phase of PNN-PZT transformed from rhombohedral to tetragonal with the increase in MnO₂ content. The phase transition is attributed to the enhancements in electrical properties, which led to the piezoelectric property changes. Therefore, the dielectric constant, Young’s modulus, electromechanical coupling coefficients (k_p,−k₃₁), and piezoelectric properties (−d₃₁, d₃₃, −g₃₁, and g₃₃) of PNN-PZT changed with the density of MnO₂ [4].

4. Conclusions

In this study, we investigated the piezoelectric characteristics of 0.55Pb(Ni_1/3Nb_2/3)O₃-0.45Pb(Zr,Ti)O₃ according to the MnO₂ additive content (0, 0.25, 0.5, 1, 2, and 3 mol%). We measured ε_r, tanδ, Q_m, Y^E₁₁, d₃₁, d₃₃, g₃₁, and g₃₃ as functions of the MnO₂ concentration. The MnO₂ addition initially improved the tetragonality of the PNN-PZT ceramic, which then saturated at a MnO₂ content of 1 mol%.

Q_m and Young’s modulus (Y^E₁₁) also increased with the MnO₂ content owing to the oxygen vacancies generated by the MnO₂ doping. The dielectric properties and electromechanical coupling coefficient k were optimal at MnO₂ contents of 0.5–1 mol%, where tanδ and k_p had the maximum values. The changes in electrical properties were attributed to the increased tetragonality. PNN-PZT is a soft piezoelectric material more suitable for actuator applications. The addition of MnO₂ to PNN-PZT showed its potentials for use in sensory actuators. In following studies, we aim to investigate vibration control and nondestructive testing applications based on the enhanced PNN-PZT ceramics.