Effect of lanthanum, neodymium on piezoelectric, dielectric and ferroelectric properties of PZT

The effect of La3+ and Nd3+ on piezoelectric, dielectric and ferroelectric properties of PZT (Pb(Zr0.53Ti0.47)O3) was studied. The powders were prepared by “wet-chemical” route followed by calcination at 800 °C for 4 h. XRD analysis of the calcined powders confirms the tetragonal phase in the undoped PZT, which gradually decreases with the dopant concentration and the appearance of rhombohedral phase simultaneously. SEM study of sintered pellets reveals the decrease in grain size with the increase in dopant concentration beyond 0.02 mol Nd3+ and 0.03 mol La3+ respectively. The piezoelectric constant (d33) and dielectric properties were maximum for 0.02 mol Nd3+ and 0.03 mol La3+ respectively. The remnant polarization of La3+-doped sample was higher than Nd3+-doped sample, and the sample with combined dopants shows intermediate remnant polarization (Pr). Based on the above study, it is concluded that La3+ is more effective than Nd3+, including in a mixture of La3+ and Nd3+ dopants.


Introduction
Lead zirconate titanate (PZT) is considered as one of the most widely studied piezoelectric materials for various applications. Currently, these materials are in the fore front for the application in aerospace vibration control, precision flow control as a replacement of solenoid-based flow control, fuel injection system in automobile industry, energy harvesting from different vibration sources, and so on [1][2][3][4][5]. These applications of PZT materials require the development of materials with very good properties. In the case of PZT, the properties are enhanced or tailor made by the addition of different dopants in "A" or "B" sites [6][7][8][9][10][11]. These dopants are mainly of two types: "donor" dopants, such as La 3+ , Nb 5+ , Ce 3+ and Ta 5+ , which produce "soft" PZT; "acceptor" dopants, such as K + , Na + , Sc 3+ and Fe 3+ , which produce "hard" PZT. The soft dopants facilitate the domain wall motion; thereby, the electronic properties are much better compared to undoped PZT. La is one of the most widely studied dopants in PZT system [12][13][14][15]. Haertling [12] reported that La 3+ increases the squareness of the hysteresis loop and decreases the coercive field (E c ), improves the dielectric constant, electromechanical coupling coefficient, mechanical compliance (S) and enhances the optical transparency. Xiang et al. [13] reported that lanthanum substitution increases the level of diffuseness of the phase transition and 7.6 mol%  lanthanum-modified PZT exhibits relaxor behavior. They also reported the tetragonality of PZT decreases with the increase in La 3+ concentration. Mohidden et al. [16] studied the effect of Nd-doped PZT and reported that the grain size decreases with the increase of Nd concentration and the phase transition is of diffused nature without relaxor behavior. They also reported that the dielectric constant and dissipation factor have shown strong lower frequency dispersion. Thamjaree et al. [17] studied the development of phase formation of Nd on PZT. However, there is hardly any study about the effect of combination of both La and Nd on properties of PZT. Therefore, in this study, an attempt had been made to study the effect of both the dopants on microstructure, piezoelectric, dielectric and ferroelectric properties of PZT.

2 Preparation of powders
PZT powders corresponding to compositions Pb 1x (La x or Nd x )(Zr 0.53 Ti 0.47 )O 3 , where x = 0, 0.01, 0.02, 0.03, 0.04, 0.06 or 0.08 mol, were prepared by wet-chemical method using salt solution as precursor [18,19]. To compensate PbO loss during calcination and sintering, 3 wt% excess lead solution was added. The combined salt solution was converted to a mixed hydroxide in the pH range of 8.0-8.5 by adding dilute ammonia solution. The precipitate was filtered and repeatedly washed till it was free of nitrate ions. The precipitate was dried at 110 ℃ overnight and calcined at 800 ℃ for 4 h. A typical flow sheet for the preparation of PZT powder by wet-chemical route is presented in Fig. 1. After calcination, the powders were de-agglomerated, granulated, uniaxially pressed into pellets and sintered at 1250 ℃ for 2 h in a closed lead-rich atmosphere to minimize lead oxide loss. Sintered pellets were leveled, polished and electroded with silver paste. The samples were then poled in a DC field of 2 kV/mm in a silicone oil bath for 30 min, and the piezoelectric, dielectric and ferroelectric properties were measured.

Characterization
PZT phase formation was confirmed by X-ray diffraction (XRD) technique (M/s. Phillips, Holand). The morphology of the PZT sintered pellets was ascertained by scanning electron microscope (LEO 440i). The linear piezo-electric strain coefficient (d 33 ) and room temperature dielectric properties were measured using a piezometer (Model PM-35, M/s. Take Control, UK). The P-E hystersis loop (Model HLT-2, M/s. Digital System, Mumbai) was studied by The results are similar to that observed by Xiang et al. [13] and are attributed to the change in lattice parameters, i.e. the lattice parameters "c" decreases while "a" increases with lanthanum content; therefore, the tetragonality (c/a) of lanthanum-modified PZT ceramics decreases.

2 Microstructural analysis
Typical SEM pictures of chemically etched sintered pellets are presented in Fig. 3

3 Piezoelectric properties
The piezoelectric properties of the samples are presented in Fig. 4. The d 33 of doped samples are much higher than undoped PZT samples due to "soft" nature of the dopants which facilitates the easy reorientation of dipoles in the presence of a DC electric field. In general, La 3+ and Nd 3+ ions replace Pb 2+ ions from "A" site and extra positive charges creates lattice vacancy to maintain electrical neutrality [6]. These Pb vacancies facilitate domain wall movement by application of an electric field [20,21]. It was also observed that d 33 increases with the increase in Nd 3+ concentration up to 0.02 mol and then decreases. Similarly, it increases up to 0.03 mol for La 3+ . Higher   dopant concentration leads to precipitation of the dopant on grain boundaries, as observed in case of ceria doped PZTs [8]. Due to this, the grain size as well as domain size decreases (also observed by Hoffmann et al. [21]); therefore, hinders domain wall orientation and subsequently decreases d 33 .

4 Dielectric properties
The dielectric constant and loss factor of PZT samples are presented in Fig. 5. The dielectric constant was maximum for samples with 0.02 mol of Nd 3+ and also for samples with 0.03 mol of La 3+ . As per micro structural study, the grain size was maximum at these compositions. This could be based on a similar effect observed by Kong et al. [22], i.e., the decrease in grain size increases the volume fractions of grain boundaries. Therefore, decreasing the domain wall mobility thus, decrease the dielectric constant. As expected, the loss factor trend was opposite to that of dielectric constant.

5 Ferroelectric properties
P-E hysteresis loops of the samples are presented in Fig. 6. The undoped sample possesses very low remnant and saturation polarization compared to the doped samples. Sample with La 3+ (0.03 mol) measured maximum remnant polarization (P r ) of 18.03 C/cm 2 . Nd 3+ -doped samples have less remnant and saturation polarization than La 3+ counterpart. The combined dopants produce intermediate P r value compared to individual dopants. Due to better uniform microstructure, achievable domain alignment occurs with higher value of saturation and remnant polarization in La 3+ -doped samples than the undoped and Nd 3+ -doped samples.

Conclusions
PZT samples were prepared by wet-chemical route doped with Nd 3+ , La 3+ and their combinations. Based on the study of piezoelectric, dielectric and ferroelectric properties, it is concluded that La 3+ is more effective than Nd 3+ , including in a mixture of La 3+ and Nd 3+ dopants.