INTRODUCTION:

Lead zirconate titanate Pb(Zr_x,Ti_1−x)O₃(PZT) are ceramics based on a continuous solid solution system of perovskite ferroelectric PbTiO₃ and antiferroelectric PbZrO₃, are known as an important piezoelectric materials^1,2. These materials have been studied extensively since the discovery of the miscibility of lead titanate and lead zirconate early in the 1950's¹.

PZT ceramics have been used in several technological applications such as ultrasonic sensors, high energy capacitors, piezoelectric actuators, non-volatile random access memories (NVRAMs), and photoelectric devices ^3-5 order to enhance its piezoelectric properties, both of the effects of different processing conditions^6,7 and substitutions of various dopants^8,9 of different ionic sizes and valences have been studied. Doping with different element changes the physical and chemical properties of PZT ceramics. Due the broad range of possible isomorphism in the perovskite structure of ABO₃, PZT ceramics can be accept dopants with different valences into both A-site (Pb-site) and B-site (Zr/Ti –site) of the lattice^10,11. Based on aliovalent substituents in the compound, dopants can be classified into two types: donors (higher valence ions) and acceptors (lower valence ions. PZT ceramics can be form a “soft” PZTs, when doped with donor such as La³⁺,¹² for A-site, and Nb⁵⁺,^12,13 for B-site which lead to creation of site A vacancies, or a “hard” PZTs, when doped with acceptor like Na⁺ at A-site and Ni²⁺, Fe³⁺ at B-site to create some oxygen vacancies in the lattice¹⁴. Soft PZTs have high piezoelectric characteristics and are easy to pole. Conversely, hard PZTs are difficult to pole and have low piezoelectric characteristics¹⁵. Doping PZT with Nb⁵⁺ increases the electric permittivity and piezoelectric coefficients¹⁶, but PZT doped with iron presents lower dielectric constant and loss constant¹⁷.

Furthermore, the morphotropic phase boundary (MPB)¹⁸ is an essential parameter to be considered because in this region, tetragonal and rhombohedral phases coexist, and consequently the properties of PZT are improved (dielectric and piezoelectric properties).

The aim of the present work is to study the dielectric and piezoelectric properties for PFN-PNN-PZT quaternary ceramics. An effort has been made to determine the MPB phase contents with variations in the Zr/Ti ratio. The effects of Zr/Ti ratio on the properties of sintered PFN-PNN-PZT quaternary system ceramics were investigated systematically.

MATERIAL AND METHODS:

Chemicals and reagents:

The starting materials were analytical reagents and commercially purchased [Pb₃O₄(99.90%), ZrO₂(99.90%), TiO₂(99.90%), Fe₂O₃(98%), NiO (99.90%) and Nb₂O₅(99.6%)].

Instrumentation:

Diffraction powder experiments were performed at room temperature on a Siemens D500 diffractometer and X pert Graphics). Cu K_α radiation with a nickel filter (λK_α1=1,54056 Å and λK_α2=1,54439Å), operated at 40 kV and 30 mA, with a step of 0.02° and 2θ range 5-60° was used. The bulk densities of sintered ceramics were determined by the Archimedes method. The electromechanical coupling factor (kp) and the piezoelectric constant (d₃₁) were calculated by using the resonance-antiresonance method, using quasi-static method. The capacitance at 1 kHz can be measured directly (agilent 4249 A Precision impedance Analyzer). The Dielectric properties (dielectric constant, ε_r and loss tangent, tan δ) were measured using an automated set up as a function of temperature and frequency. The set up consists of LCR meter 800 series and a programmable temperature chamber (D 2804) interfaced to PC. From the plots of dielectric constant vs. temperature, phase transition temperature, T_c, was determined.

Preparation of 0.05Pb (Fe_1/2Nb_1/2) O₃-0.05Pb (Ni_1/3Nb_2/3) O₃-0.90Pb (Zr_xTi_(1-x)) O₃

The general formula of the materials was 0.05Pb (Fe_1/2Nb_1/2) O₃-0.05Pb (Ni_1/3Nb_2/3) O₃-0.90Pb (Zr_xTi_(1-x)) O₃, where (0.49≤x≤0.55). mixed oxides were milled and calcined at 800 °C for 2 h at heating and cooling rates of 2°C/min. After calcinations, the ground and milled powders were pressed into discs 12 mm in diameter and about 1 mm in thickness at 150 MPa. The samples were sintered at 1180 °C for 2 h in a covered alumina crucible. To prevent PbO evaporating from the pellets, a powder of PbZrTiO₃ was used as the embedding powder. The sintered PZT specimens were polished and electroded with silver paste and subsequently cured at 700 ◦C for 30 min and poled by applying a dc electric field of 2.5 kVcm⁻¹ at temperatures of around 120 ◦C for 60 min in a silicone oil bath. As- sintered samples were ground and polished to remove the surface layer for X-ray diffraction

RESULTATS AND DISCUSSION:

Bulk density of piezo-ceramics is an important factor influencing the properties of ceramics. Commonly, the dielectric and piezoelectric properties are positively correlated with the density of the specimens. ^19-20. The density of sintered PFNN-PZT as a function of Zr is plotted in Fig. 1 With increasing of Zr, the density increase. Maximum density is observed for Zr =51composition with a few grains exhibiting grain growth. This is attributed to a relatively high ratio of grain growth to the densification rate.

Fig. 1. Change in bulk density as functions of Zr in the composition and sintering temperature

The XRD patterns of PFNN-PZT ceramics with various Zr/Ti ratio at 49/51, 51/49, 53/47 and 55/45 sintered at 1180°C are shown in Fig. 2. All peaks are well matched with the Perovskite structure. The secondary phase was not observed at all. It is reported that tetragonal (T) and rhombohedral (R) and T–R phases were identified by an analysis of the peaks of tetragonal (002), tetragonal (200), rhombohedral (200) that are in the 2q range of 43–47◦. The splitting of (002) and (200) peaks indicates that they are the ferroelectric tetragonal phase, while the single (200) peak indicates the ferroelectric rhombohedral phase. Triplet peak indicates that the samples are consists of a mixture of tetragonal and rhombohedral phases.

Some researchers have reported the possible co-existence of an extended region of morphotropic phase boundary composition, where tetragonal and rhombohedral phases are both stable. The tetragonal form is stable over the composition range from x = 0 to x = x_T, and the rhombohedral form from x = x_R to x = 1, where x_R <x_T. The width, Δx = x_T - x_R, of the ‘coexistence region’ is close to that obtained in different studies. X-ray diffraction patterns indicate that the MPB of the tetragonal and rhombohedral phase lies in the range 0.49<x<0.53.

Fig. 2. XRD patterns for composition at (a) 49/51; (b) 51/49; (c) 53/47; (d) 55/45

The changes in the dielectric and piezoelectric characteristics of the 0.05Pb (Fe_1/2Nb_1/2) O₃-0.05Pb (Ni_1/3Nb_2/3) O₃-0.90Pb (Zr_xTi_(1-x)) O₃systems is mentioned in Figures 3 to 6. PFNN-PZT exhibits high piezoelectric coefficient and electromechanical coupling factor around the PMB. Fig. 3 shows the variation in (ε) of PFNN-PZT with temperature at 10 kHz of frequency. Here, the dielectric permittivity increases gradually with an increase in temperature up to transition temperature (Tc, K) and then decreases. The region around the dielectric peak is broadened. The broadening or diffuseness of peak occurs mainly due to compositional fluctuation and/or substitution disordering in the arrangement of cations in one or more crystallographic sites of the PFNN-PZT structure. With increasing of Zr/Ti ratio, the Curie temperature of PFNN-PZT becomes lower and consequently the peak of dielectric spectrum corresponding to the Curie temperature moves toward low temperature as shown in Figure 3. PbTiO₃ has a high Curie temperature (T_c=490 °C). The variation of dielectric loss (tan d) with composition (at room temperature, 1 kHz) sintered at various temperatures (1100, 1150, and 1180 °C) is shown in Figure 4. The dielectric loss tangent is of the same order at room temperature for all the samples, while near the transition temperature region the dielectric loss tangent has a lower value for lower Zr compositions.

Piezoelectric coefficient (d₃₁) is one of the most important parameters to characterize the piezoactivity of ceramic specimens, which are a key property for applications, such as actuators and transducers. The piezoelectric coefficients of specimens with various Zr/Ti ratios are shown in Figure 5. From the trend of the variation of piezoelectricity, it reaches the maximum values of d₃₁= 141. 10^-12 C/N. Electromechanical coupling factor is another critical property of piezoceramics for applications in transducers.

Fig. 3. Variation of the dielectric constant as a function of Zr composition

Fig. 4. Dielectric loss tangent (tg d) as a function of Zr composition.

Fig. 5: Variation of the Piezoelectric coefficient as a function of Zr composition

Fig. 6: Variation of the Electromechanical coupling factor as a function of Zr composition

Figure 6 plots the changing tendencies of the Electromechanical coupling factors (kp) specimens with different Zr/Ti ratio. Similar to the values of d₃₁, the kp values of the ceramic specimens form a parabola shape, and reach the peak value (0.64) at the same Zr/Ti ratio of 51/49.

The maximum of g₃₁ as shown in Figure 7 coincides with the maxima of the electromechanical coupling factor and piezoelectric constant (K_p,d₃₁). Reciprocal elastic compliance (Young’s modulus) of the system is shown in Figure 8 with an increasing Zr/Ti ratio. It can be seen that the curve is characterized by a minimum near the phase transition. The value of Y decreases as the Zr increases, and (Y) attains a minimum value at Zr = 51. This could be explained by the gradual decrease of tetragonality and a pronounced drop of the curve near the phase transition (51/49: Zr/Ti).

Fig .7. Variation of the Tension factor (g31) as a function of Zr composition

Fig.8. Variation of the Young’s modulus as a function of Zr composition

High values of piezoelectric coefficient d₃₁ and electromechanical coupling factor kp in poled PZT-based ceramics are believed to arise from the motion of domain walls under the action of applied field or stress ^21-23. Additionally, a small amount of doping Nb⁵⁺ will increase the densification and reduce the grain sizes of ceramics during the sintering procedure^24-26. These phenomena can all be observed in our results. Certainly, the increase of bulk density and small grain size is mainly attributed to the hot-pressing procedure under oxygen atmosphere because the application of pressure during sintering process helps to expel pores and to suppress grain growth.

CONCLUSION:

In this study, ceramics within the 0.05Pb (Fe_1/2Nb_1/2)O₃-0.05Pb(Ni_1/3Nb_2/3)O₃-0.90Pb (Zr_xTi_(1-x))O₃, solid solution system (where (0.49≤x≤0.55) were successfully prepared using a solid-state mixed oxide technique. From the present results, it can be revealed that the MPB coexisting the tetragonal and rhombohedral phases in the present system is a broad composition region of 0.49<x<0.53.

PFNN-PZT exhibits high piezoelectric coefficient and electromechanical coupling factor around the MPB and it reaches the maximum of d₃₁ = 141. 10^-12 C/N, kp= 0.64 at Zr/Ti = 51/49 at the same time, the dielectric constant also reaches the maximum value (e _r=13108). PFNN-PZT composition x = 0.51 is found to optimum piezoelectric characteristics suitable for transducer applications.

ACKNOWLEDGEMENT:

The authors are grateful to the authorities of Amar Telidji University Laghouat, for the facilities.

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Received on 11.05.2019 Modified on 23.06.2019

Asian J. Research Chem. 2019; 12(3):182-186.

DOI: 10.5958/0974-4150.2019.00035.X