1. Introduction

take extreme values when Zr/Ti ratio corresponds to the composition of the MPB which separates the tetragonal (T) and rhombohedral (R) phases towards Ti-rich and Zr-rich sides, respectively[3,4]. Rangson Muanghlua et al.[5]. studied the effect of Zr/Ti ratio on the structure and ferroelectric properties of0.07Pb(Mn_1/3Nb_2/3)O₃-0.6Pb(Ni_1/3Nb_2/3)O₃-0.87Pb(Zr_xTi_1-x)O₃ ceramics. From XRD analysis and ferroelectric properties measurements indicated the existence of the MPB composition between x = 0.50. At this composition, the ceramics exhibits the highest ferroelectric properties. Yoo and Lee[6] also reported that the PZT-PNN-PMnN ceramics were strongly influence by the Zr/Ti ratio. They found that the electromechanical coupling factor k_p, the piezoelectric constant d₃₃ and the mechanical quality factor Q_m increased with the increase of Zr/Ti ratio and then decreased after the ratio exceeded 50/50. Recently, Fares Kahoul et al.[7] studied the structure and electrical properties of Pb_0.98Sm₀.₀₂[(Zr_x,Ti_1-x)_0.98 (Fe_1/2Nb_1/2)_0.02]O₃ ceramics with the composition near the MPB. They found that the k_p, d₃₁ and Q_m of the ceramics are enhanced with the increase of Zr/Ti ratio. At Zr/Ti ratio of 55/45, the ceramics has the optimal electromechanical properties, k_p = 0.631, d₃₁ = 120.10^-12C/N, Q_m = 462.

The research results of the above authors clearly showed the significance of Zr/Ti ratio in controlling the electromechanical properties of the PZT-based ceramics. In this paper, we investigated the effect of Zr/Ti ratio content on some physical properties of the low temperature sintering PZT-PZN-PMnN ceramics.

2. Experimental Procedure

The general formula of the material was 0.8Pb(Zr_xTi_1-x)O₃ – 0.125Pb(Zn_1/3Nb_2/3)O₃ – 0.075Pb(Mn_1/3Nb_2/3)O₃ + 0.7 wt% Li₂CO₃, where x is 0.46, 0.47, 0.48, 0.49, 0.50 and 0.51. They are denoted by M46, M47, M48, M49, M50 and M51, respectively.

The raw materials including powders (purity of 99%) of ZrO₂, TiO₂, Nb₂O₅, ZnO and MnO₂ for the given composition were weighted by mole ratio and the powders were mixed and milled (the PM 400/2 milling machine) for 8 h using ziconia balls and ethanol as the medium, then were calcined. Thereafter, PbO was added and milled again. Powders were calcined at 850^oC for 2 h, then the sintering aid, Li₂CO₃ was added[8] and milled again for 16h. The ground materials were pressed into disk 12mm in diameter and 1.5mm in thickness under 100 MPa. The samples were sintered in a sealed alumina crucible with PbZrO₃ coated powder at temperature 950^oC for 2h.

The crystalline phase was analyzed using an X-ray diffactometer (XRD, D8 ADVANCE). The microstructure of the samples was examined by using a scanning electron microscope (SEM). The density of samples was measured by Archimedes method. The samples were poled in a silicone oil bath at 120^oC by applying 30 kVcm⁻¹ for 20 min, then cooled at this electric field. They were aged for 24h prior to testing. The piezoelectric properties were determined by the resonance and antiresonance technique using an impedance analyzer (Agilent 4396B and RLC HIOKI 3532). The calculation equations are as follows:

where f_s and f_p are resonant and antiresonant frequencies (Hz), and Z_min and C_s are resonant impedance (ohms) and electrical capacitance (farads). The dielectric properties were measured by RLC HIOKI 3532.

3. Results and Discussion

3.1. Phase Analysis and Microstructure

Figure. 1 shows X-ray diffraction patterns (XRD) of the PZT–PZN–PMnN ceramics with the variation of Zr/Ti ratio content. All the samples showed a tetragonal perovskite phase without any secondary phase. The lattice parameters for various compositions were calculated using the least square method from the double (002) and (200) peaks of tetragonal structure and results for the tetragonality c/a of perovskite phase are shown in Figure 2. The c/a ratio decreases with increasing Zr/Ti ratio content, indicating that the tetragonality of PZT-PZN-PMnN ceramics decreased when Zr increased.

Figure 1. X-ray diffraction patterns of the ceramics with different Zr/Ti ratio contents

Figure 2. The tetragonality c/a of the ceramics as a function of Zr/Ti ratio

Figure 3. Microstructures of samples with the different Zr/Ti ratio contents

Figure 3 shows the SEM image of the fractured surface of PZT–PZN–PMnN ceramics at different Zr/Ti ratio contents. It is observed from the micrographs that the average grain size of samples are increased with the increasing amount of Zr/Ti ratio (Table 1). It is explained that the initial and intermediate stages of sintering, densification and grain growth occur through liquid-phase diffusion (Li₂CO₃)[8], and then the liquid phase at the intergranular boundaries gradually dissolves into the grains with grain growth[9,10]. However, figure 3 also shows that further increasing the Zr/Ti content to 51/49 gives rise to an abnormal grain boundary, and the average grain size is reduced.

Table 1. The average grain size of ceramic samples

3.2. The Ceramic Density

The densities of the samples containing different amounts of Zr/Ti ratio were measured, and the results are shown in Figure 4. As seen, the densities of the ceramics are in the range from 7.80g/cm³ to 7.86g/cm³ (94-97 % theoretical density) and dependent on Zr/Ti ratio content. When Zr/Ti ratio content increases, the density of samples increases and achieves the highest value (7.86g/cm³) at the ratio of Zr/Ti = 48/52, and then decreases. This may be explained from microstructures of ceramic samples. When the amount of Zr/Ti ratio increased, the ceramic samples became more dense, and at Zr/Ti = 48/52, the ceramic sample was almost fully dense (fig. 3). When the further increasing the Zr/Ti ratio content to 49/51 and above, a large number of pores were present, giving rise to an abnormal grain boundary. Hence, the densities of the ceramics are decreased. The variation in density of the ceramic samples is in good accordance with the microstructure.

Figure 4. The density of PZT-PZN-PMnN samples as a function of the Zr/Ti content

3.3. Dielectric Properties

The change of Zr/Ti content also significantly affects the piezoelectric and dielectric properties of PZT–PZN–PMnN ceramics.

Figure 5 shows the dependence of dielectric constant ε and dielectric loss tanδ of the ceramic on temperature at 1 kHz. As observed, the dielectric properties exhibited characteristics of a relaxor material in which the phase transition temperature occurs within a broad temperature range. This is one of the characteristics of ferroelectrics with disordered perovskite structure. The maximum dielectric constant ε_max increases with increasing Zr/Ti ratio content, and at Zr/Ti = 48/52, the highest dielectric constant ε_max of about 19500 and then sharply decreases beyond this point. This can be explained by the increase of grain size effect[10]. The dielectric loss tanδ shows the reversed trend, this appropriate to the characteristics of dielectrics[11].

Figure 5. Temperature dependence of the dielectric constant and dielectric loss tanδ at 1 kHz of ceramic

In Figure 6, the change of Curie temperature T_C with the Zr/Ti ratio is shown. With the increase in Zr amount, the Curie temperature decreases because the Curie temperature of PbZrO₃ is about 232^oC and it is lower than that of PbTiO₃, 490^oC[11,12].

Figure 6. The Curie temperature TC of PZT-PZN- PMnN ceramics with different amounts of Zr/Ti ratio

Figure 7 shows the temperature dependence of the dielectric constant ε and dielectric loss tanδ of the M48 sample measured at frequency of 1kHz, 10kHz, 100kHz and 1MHz, respectively. It show that the shape of the ε peaks was broad, which is typical of a case diffuse transition with frequency dispersion. When the measured frequency increased, the maximum of ε_max was decreased and shifted to higher temperature while dielectric loss increased near the Curie point, which is typical of a relaxor material[13].

Figure 7. Temperature dependence of relative dielectric constant ε and dielectric loss tanδ of M48 sample at different frequencies

3.4. Piezoelectric Properties

To determine piezoelectric property of ceramics, resonant vibration spectrum of samples were measured at room temperature in Figure 8.

Figure 8. Spectrum of radial resonance of M48 sample

From these resonant spectra, electromechanical coefficients k_p, piezoelectric coefficients d₃₁, mechanical quality factor Q_m and dielectric loss tanδ were determined. Figure 9 shows the changing in the electromechanical coupling factor (k_p), the piezoelectric constant (d₃₁), mechanical quality factor Q_m and dielectric loss tanδ as a function of the amount of Zr/Ti ratio. As can be seen, both k_p and d₃₁ show a similar variation with increasing Zr/Ti ratio content. When the amount of Zr/Ti ratio is lower than 48/52, the k_p and the d₃₁ are rapidly increased with increasing Zr/Ti ratio content, while the mechanical quality factor Q_m and the dielectric loss tanδ are desreased. The optimized values for k_p of 0.62, d₃₁ of 140.10^-12C/N, Q_m of 1112 and tanδ of 0.005 were obtained at Zr/Ti = 48/52. This is probably related to characteristics of the increasing grain size. As well known, the increased grain size makes domain reorientation easier and severely promotes domain wall motion, which could increase the piezoelectric properties[13]. However, the further increasing of Zr/Ti ratio content gives rise to an abnormal grain boundary and the average grain size is also reduced as shown in Fig. 3. Therefore, electrical properties of PZT-PZN-PMnN ceramics are reduced.

Figure 9. The values of kp, d31, Qm, and tanδ of the PZT-PZN-PMnN ceramic samples

4. Conclusions

We have investigated some physical properties of the low temperature sintering 0.8Pb(Zr_xTi_1-x)O₃ – 0.125Pb(Zn_1/3Nb_2/3)O₃ – 0.075(Mn_1/3Nb_2/3)O₃ + 0.7 wt% Li₂CO₃ ceramics. Results of this study are summarized as follows:

- All the specimens displayed a tetragonal perovskite structure without secondary phase. With increasing the Zr/Ti ratio content, the tetragonality c/a was decreased.

- At the Zr/Ti ratio content of 48/52 and sintering temperature (T_s) of 950^oC, the density, the electromechanical coupling factor k_p, the dielectric constant ε_max, the piezoelectric constant d₃₁, the dielectric loss tanδ and the mechanical quality factor Q_m showed the optimum values of 7.86g/cm³, 0.62, 19500, 140 pC/N, 0.005 and 1112, respectively. Therefore, they are a promising candidate material used in high power piezoelectric devices.