International Journal of Educational Planning & Administration. ISSN 2249-3093 Volume 2, Number 2 (2012), pp. 97-108 © Research India Publications http://www.ripublication.com/ijepa.htm

EFFECT OF Sr ON STRUCTURAL AND DIELECTRIC PROPERTIES OF LEAD

LANTHANUM ZIRCONATE TITANATE PEROVSKITE CERAMICS

Pratiksha Devi1, R. K. Sharma1, Alok Srivastava1, O P Kulshreshtha1, Arun Chamola2 and Amar Pal3

1Department of Physics, DAV (PG) College, Dehradun, Uttarakhand, India 2Department of Physics, Hemwati Nandan Bahuguna Garhwal University, Srinagar

(Garhwal)-246001 Pauri Campus, Uttarakhand, India 3Shree Ganpati Institute of Technology, Ghaziabad-201302,Uttar Pradesh, India

Corresponding Author: Dr Ramesh Kumar Sharma Department of Physics, DAV P.G. College, Dehradun 248001, Uttarakhand, India

Email: davrks@gmail.com

Abstract:

The oxygen and lead vacancies and the charge compensation play an important role in the compositional and microstructural homogenization of strontium-modified lead lanthanum zirconium titanate ceramics. The incorporation of isovalent Sr2+ in the PLZT lattice enhanced the tetragonality and significantly increased grain growth thereby promoted density of the ceramics. Introduction of Sr in PLZT lattice remarkably decreased the dielectric constant (εRT) which could be attributed to the process of incorporation of Sr2+ cations in the perovskite while the decreasing trend of Curie transition temperature may be due to the Sr2+ replacing Pb2+ in the perovskite. Microstructure studies revealed that increased grain growth could enhance polarization mechanism resulting in increased ferroelectric properties (Pr) and (Ps). It revealed that Sr modification into PLZT lattice plays a dominant role in influencing electrical performance and grain growth also contributed to this effect, which may be suitable for possible device applications.

1 Introduction Lead zirconate titanate (PZT), near morphotrophic phase boundary (MPB) region,

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normally shows a high electromechanical response, which make them suitable especially for sensor and actuator, and non-volatile memory applications [1, 2]. PZT ceramics can be classified into two groups: “hard” and “soft”. Hard PZT ceramics can withstand high levels of electrical excitation which are well suited for high voltage or high power generators and transducers, while the important feature of soft PZT is the high sensitivity and permittivity which are well suited for various sensors and actuators, transducers, etc. [3]. The properties of PZT are known to be effected by microstructure, and the nature and concentrations of dopants. The various dopants both on A-site (Pb) and B-site (Zr/Ti) with pentavalent, isovalent and/or aliovalent elements or donor/acceptor ions can influence the microstructural development, electrical and mechanical properties of the ceramics. Particularly, donor elements like La at Pb or Nb at (Zr/Ti)-sites have been reported to positively influence the reliability of PZT thin films [4], although the dielectric and ferroelectric properties seem to be rather decreased in contrast to what should be expected from the effects on bulk ceramics [5]. The microstructure and ferroelectric properties of Nb-modified PZT (PNZT) thin films were investigated and reported that higher concentrations more than 2 mol% were harmful for ferroelectric properties [6]. Lead may be substituted by isovalent cations including Ca2+, Ba2+, Sr2+

and Cd2+as well as off-valent substitutions such as rare-earth ions Sm3+ and Y3+ while maintaining the perovskite crystal structure responsible for the strong ferroelectric behavior [7,8]. PZT modified with different elements have been extensively studied [9, 10]. Lead lanthanum zirconium titanate (PLZT) is formed by doping lanthanum in PZT. The influence of various dopants like Sm[11], Ba [12], Sr [13], Nb[14], Na [15], Ag[16], etc. have been extensively investigated. Dielectric properties of ferroelectric ceramics substituting alkaline-earth elements such as Ca, Sr, and Ba for La in PLZT were investigated for MLCs applications with thin dielectric layers [13]. The MPB shifts from 53/47 (Zr/Ti) to 56/44 (Zr/Ti) with isovalent substitutions have been found to have optimum ferroelectric and piezoelectric nature. Substitutions influence the microstructure, density, dielectric and ferroelectric properties depending on the different synthesis routes applied as well as the sintering temperatures. Dielectric, ferroelectric and piezoelectric properties of ceramics can be improved or tailored by substituting Pb with several acceptor and/or donor cations. We are systematically investigating the influence of both donor and/or acceptor doped ceramic systems at A-site with Zr:Ti ratios near MPB. In relation to the systematic study of A-site and/or B-site modification with donor and/or acceptor dopants in PLZT system, people have been investigating the effect of dopant content on different properties. (i) Ba-doped PZT (65/35) [17], they have taken Zr:Ti as 53.2:46.8 to find out the effect of isovalent Ba in PZT (53.2:46.8) system. Isovalent Ba influence on PZT near 53.2:46.8 had been reported. Whereas, (ii) Ba-doped PLZT (1.2/55/45): in one of the previous study, Ba-doped PLZT (1.2/55/45) system [18], they had taken Zr:Ti as 55:45 to find out the effect of isovalent Ba in PLZT (1.2/55/45) system. In our present study, Sr-doped PLSZT isovalent Sr has been doped in a PLZT system, and the effect of both isovalent Sr and donor tetravalent La influence on different properties have been investigated and reported. Thus, PZT ceramic

EFFECT OF Sr ON STRUCTURAL 99

compositions modified with different dopants have shown different tendencies on various properties. The isovalent dopant content (Sr) has been chosen after thorough literature survey. It has been observed that Sr at 3 mol% decreased the piezoelectric properties [19]. Considering [19], we have chosen smaller amounts of Sr along with La-modified PZT to investigate its effect on these properties. The isovalent substitution (Sr) influences the properties in PZT ceramic owing to the fluxing effect of doping ions during the period of sintering. Since our main concentration is to find out optimum composition for device application, we have taken Sr in smaller amounts and studied its effect on electrical properties of PLSZT. The influence of isovalent dopant strontium in lead lanthanum zirconium niobium titanate (PLSZT) for phase formation, microstructure, apparent density, dielectric and ferroelectric have been studied. 2 Experimental procedure 2.1. Synthesis of ceramic compositions The ceramic compositions were prepared by the solid-state reaction method, starting with analytical reagent grade powders (purity 99.99%) of PbO, La2O3, SrCO3, ZrO2 and TiO2. The weighed starting reagents were mixed in appropriate stoichiometric ratios with the addition of excess 3 wt% PbO (which is standardized among 2–6 wt% PbO) to compensate lead loss during high temperature sintering to form the final ceramic compositions with formula: Pb0.9(La1_zSrz)0.1Zr0.65Ti0.35)0.975O3 where z=0, 0.2, 0.04 and 0.06 abbreviated hereafter as PLSZT. The batch powders were grinded using Agate-Mortar in acetone for 4 h. The dried powders were calcined at 900 °C for 2 h in a high-purity alumina crucible by maintaining air atmosphere. Calcined powders were again grinded in ethanol for 2 h to crush agglomerates and to get homogenized ceramic. The calcined fine powders were mixed with 2 wt% polyvinyl alcohol (PVA) as a binder and were pressed into pellets of 8 mm diameter and 1–2 mm thickness using a steel die and hydraulic press with uniaxial pressure of 800–900 kg/cm2. The pressed compacts were first heated at a slow rate of heating to 500 °C in air to burn off the organic binder (PVA) and sintered at 1225–1250 °C for 4 h in a closed alumina crucible by maintaining a PbO atmosphere around the sample. After the sintering process, the samples were cooled to room temperature in the furnace. 2.2. Structural characterization The phase formation in the sintered specimens were analyzed by applying X-ray diffraction (XRD) technique (Philips X-ray diffractometer PW-1710) using Cu K radiation with Ni filter at room temperature and a step scan from 2θ =20o to 60o. As sintered ceramic surfaces were polished, thermally etched at 1075 oC for 1 h and gold coated using a sputtering technique to analyze microstructure. Microstructural studies were observed through scanning electron microscopy (SEM) of JEOL Model JSM 840A. Fractured surfaces of the ceramics were also coated with gold for SEM studies. The grain sizes were estimated by the linear interception method with scanning electron micrographs [20]. The apparent densities of sintered ceramics were measured using the Archimedes method.

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2.3. Electrical characterization The lapped pellet surfaces were painted with silver paste and cured at 300 oC for 1 h to form perfect electrodes on the sample surfaces and these specimens were characterized for room temperature dielectric constant (εRT), dielectric maximum (εTc), Curie temperature (Tc), dissipation factor (tan δ) and temperature dependent dielectric response (ε) at 1 kHz using 4192A HP Impedance Analyzer. In this study, the temperature change was provided by an automatic temperature chamber (Delta 2300) controlled by a HP computer. The electroded specimens were poled in silicon oil bath at 100 oC by applying a dc field of 20 kV/cm. After 24 h ageing, the poled specimens were cleaned with acetone and characterized at room temperature for ferroelectric studies (remnant (Pr), spontaneous (Ps) polarization and coercive field (Ec)) with modified sawyer-tower circuit. 3 Results and discussion 3.1. XRD studies of strontium-modified PLSZT ceramics XRD patterns of undoped and strontium-modified PLSZT ceramics are depicted in Fig. 1. Incorporation of dopant into the perovskite ABO3 structure is assumed to occur through partial substitution at Pb-site ions by La3+ and Sr2+ thereby charge compensation by the creation of Pb2+ vacancies (VPb). In the present work, direct priority is given to the cationic charge vacancy compensation mechanism, as the raw materials were purposely mixed in quantities so as to satisfy reaction. XRD studies revealed that Sr2+ doping at A-site on MPB composition of PLZTN resulted in intensified ferroelectric tetragonality (FETET). The ionic radii of Sr2+ and La3+ are slightly smaller than that of Pb2+, hence partially substitute Pb2+ and the oxygen vacancies are being compensated by the distribution of both A-site and B-site vacancies (Pb2+, La3+, Sr2+, Zr4+ and Ti4+) as per our PLSZT equation. We have discussed the oxygen vacancy compensation in relation to the A-site and B-site dopants with respect to charge balance. The distribution of A-site and B-site vacancies in PLZT has been extensively investigated by Hardtl and Hennings [23]. In principle, at MPB in PZT, both tetragonal and rhombohedral phases coexist. The Zr-rich region in the phase diagram is rhombohedral while the Ti-rich region is tetragonal. Addition of dopants can alter the phase stability. In comparison to the literature study [22–24], our results indicate that Sr and La contribute to the alteration of phase formation from coexistence to tetragonal and additional Sr resulted in intensified tetragonality as can be observed from the compared literature.

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Figure1: XRD patterns for PLSZT compositions with various Sr content

Microstructure studies of strontium-modified PLZTN ceramics Scanning electron micrographs of strontium-modified PLSZT ceramic surfaces are represented in Fig. 2(a-d), respectively. The average grain size in ceramics was determined directly from the SEM micrographs by using the linear interception method [25]. Microstructure controls the properties of the ceramic materials being this a consequence of the processing route. The strong extrinsic mechanisms help to tailor the specific properties of the ferroelectric ceramic materials. Extensive investigations have demonstrated that A-site and B-site cation substitutions have significant influence on domains and properties of ABO3 structured PZTs. In general, the effect of oxygen vacancies is most crucial, which can be generated via several possible causes: (i) lower valent “acceptor” modification which requires charge compensation, (ii) higher valent “donor” modification which results in anti-site occupation, and (iii) synthesis route and technique, etc. Oxygen vacancies can be eliminated by using high purity raw materials, donor dopants and appropriate synthesis techniques as well as sintering conditions. As the isovalent Sr2+ increased, the average grain size increased up to 2 m due to smaller grains nucleation as evidenced by the SEM pictures and this considerable grain growth would release the internal stress field from the grain boundaries in order to enhance grains bonding. The properties of PZT are known to be effected by microstructure and the nature and amount of doping elements [1, 25,

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26]. In our study, the elements modified, trivalent donor (La3+), isovalent (Sr2+) at A-site or Pb-site have effected the microstructure development and electrical properties of the ceramics. Our results also indicate that for 0.6 mol% Sr addition to the PLZT ceramic system leads to grain growth increment. It is believed that elements modified contribute to high domain wall movement thereby resulting in increase of grain growth. Apparent density of undoped and strontium-modified PLZTN ceramics is depicted in Fig. 2. The apparent density of strontium and lanthanum modified PZT ceramics increased with the introduction of isovalent Sr2+ content in the perovskite. If the distribution of grain sizes is considered, as often found, the ideal densification of the system should be reached through an optimal mixture of coarse and fine grains, the fine grains filling the interstices formed by the larger grains. The strontium-modified PLZT ceramics show a dense surface of fine grains tightly bound which considerably promoted densification with an increasing trend. In this Sr-induced PLZT ceramics, Sr concentration considerably effected the final behavior of density, with the particularity of showing an apparent maximum (7.73 g/cm3) at z = 0.06 mol% Sr. The increase of Sr additive content substitutes the Pb ions on the A-site of the perovskite structure to achieve electroneutrality. Significant amounts of Pb vacancies are created which enhance the volume diffusion and the main densification mechanism being volume diffusion, which is controlled by the number of vacancies [27], thereby promoting density of the ceramics. The ceramic systems have shown increasing tendency in density profile with respect to donor and/or acceptor modification in PLZT systems [28–30].

(a) Z=0 (b) Z=0.04

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(c) Z=0.06 (d) Z=0.20

Fig. 2 Shows the SEM micrographs of Sr doped PLZT a) z=0.00, b) z=0.02, c) z=0.06 and (d) z=0.20 3.3. Dielectric behavior of strontium-modified PLSZT ceramics Fig. 3 and Fig. 4 shows dielectric constant and dielectric loss as function of temperature of undoped and strontium-modified PLZTN ceramics at 1 kHz, respectively. In the soft PZT ceramic materials, as polarization increases within the domains, the domain wall movement influences dielectric behavior. The microstructure could modify the dynamics of the domain walls, several mechanisms related with their influence on the dielectric loss, dielectric constant, transition temperature, etc. The decrease in the dielectric constant could be attributed partly to the increase of the compositional heterogeneity produced by a greater atomic diffusion, optimized for the presence of lead vacancies, during the sintering process. Due to the existence of multiple cations at A-site (isovalent Sr2+ and trivalent donor La3+) in the perovskite, the charge mobility within the lattice will be supported by neighboring cations and thus room temperature dielectric loss (tan δRT) will be minimized. Moreover, acceptor dopants will be very effective in reducing the dielectric loss of PZT [31, 32]. The increase of compositional hetrogeneity and porosity reduction also contribute to the decrease of dielectric loss of the ceramics. The increment of the domain size contributes to the increase of dielectric constant, as the grain boundary increases which facilitates the movement in the ferroelectric domain walls. As shown in Fig. 3, the room temperature dielectric constant decreased with increasing isovalent Sr2+ content and exhibited a maximum value of (εRT = 1523) at a composition of z = 0.6 mol% Sr, which can be attributed to the effect of donor La3+, and isovalent Sr2+ multiple ions with a relatively high dielectric constant with decreasing transition temperature. As can be observed from Fig. 3, Sr2+ replacing Pb2+ diminishes the transition temperature and could also be attributed to the process of incorporation of Sr2+ cations in the perovskite. As per literature [4, 18, 33–36], the donor and/or acceptor modified PLZT compositions have influenced dielectric

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properties as room temperature dielectric constant had improved moderately while having decreasing trend of transition temperature and dielectric permittivity at transition temperature. In our study, Sr influenced εRT, while showing decreasing trend of εTc and Tc. The multiple cations in PLSZT perovskite minimized tan δRT in contrast with tan δTc.

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Fig. 3 Variation of dielectric constant as a function of temperature of strontium-modified PLZT ceramics (PLSZT) compositions.

EFFECT OF Sr ON STRUCTURAL 105

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Fig. 4 Variation of dielectric loss as a function of temperature of strontium-modified PLZT ceramics (PLSZT) compositions. Ferroelectric properties of remanent (Pr), spontaneous (Ps) polarization and coercive field (Ec) of undoped and strontium modified PLZT ceramics are depicted in Fig. 5. The effect of increased density results in the ferroelectric domain orientation thereby influencing the polarization mechanism in the poled ceramics. The remnant polarization increases from undoped to z=0.06 Sr-modified PLZT ceramic. The spontaneous polarization increases from undoped to z=0.06 Sr-modified PLZT ceramic. It was revealed that penetration of oxygen into PLZT plays dominant role in controlling ferroelectric performance, while increased grain growth could enhance polarization and ferroelectric properties. Nasar et al. [37] reported that increase in Sr content increases remnant polarization and coercive field of PLZT (53/47) system

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where La3+ ion in PZT system acts as a donor dopant. Effect of La3+ doping on microstructural and electrical characterization in PZT was studied by Laurent et al. [38] and reported that coercive field of 2 mol% La3+ modified PZT increased because of smaller grain size. In our investigation, an addition of small quantities of isovalent Sr2+ produces Pb vacancies (A-vacancy in ABO3 perovskite) in PLZT and enhances domain reorientation, resulting in square hysteresis loops, low coercive field, high remnant and spontaneous polarization.

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Fig. 6.5 Ferroelectric properties of remnant (Pr), spontaneous (Ps) polarization and coercive field (Ec) of undoped and strontium modified PLZT ceramics Conclusion The incorporation of isovalent Sr2+ in the PLZT lattice enhanced the tetragonality and significantly increased grain growth thereby promoted density of the ceramics. Introduction of Sr2+ in PLZT lattice remarkably decreased the dielectric constant

EFFECT OF Sr ON STRUCTURAL 107

(εRT) which could be attributed to the process of incorporation of Sr2+ cations in the perovskite while the decreasing trend of Curie transition temperature may be due to the Sr2+ replacing Pb2+ in the perovskite. Microstructure studies revealed that increased grain growth could enhance polarization mechanism resulting in increased ferroelectric properties (Pr) and (Ps). It revealed that Sr modification into PLZT lattice plays a dominant role in influencing electrical performance and grain growth also contributed to this effect, which may be suitable for possible device applications. References [1] B. Jaffe, W.R. Cook, H. Jaffe, Piezoelectric Ceramics, Academic Press,

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