Synergetic effect of rare-earths doping on the microstructural and electrical properties of Sr and Ca co-doped BaTiO3 nanoparticles

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Synergetic effect of rare-earths doping on the microstructural and electrical properties of Sr and Ca co-doped BaTiO3 nanoparticles Aditya Jain, Amrish K. Panwar∗ Lithium Ion Battery Technology Lab, Delhi Technological University, Delhi, 110042, India

A R T I C LE I N FO

A B S T R A C T

Keywords: Microstructure Dielectric Ferroelectric Piezoelectric Energy storage

Single-phase BST-BCT ceramic doped with different rare-earth materials (La, Gd, Dy, and Yb) have been prepared using a modified mechano-chemical activation technique. The modified BaTiO3 doped with different rareearths elements have manifested improved characteristics and desirable application in the field of high-power pulse capacitors. In this work, a comparative study has been performed to analyze the effect of different rareearths on the dielectric, ferroelectric, piezoelectric and electrical breakdown strength of already optimized Sr and Ca co-doped BaTiO3 ceramic. As predicted from finite element simulation, the doping of rare-earth may lead to a change in microstructure and surface characteristics of Sr and Ca co-doped BaTiO3 ceramic and therefore, an enhancement in electrical characteristics of the material. The addition of various rare-earths with the same cationic charge but different ionic radii has resulted in a significant change in morphology of BST-BST ceramic. The XRD results indicate that all the samples retain the parent perovskite tetragonal crystal symmetry, and there is no substantial change in the crystal structure compared to the base material. The addition of rare-earths has resulted in a decrease in transition temperature and significant changes in the electrical properties of modified BaTiO3 ceramic. Lanthanum and Gadolinium doping have resulted in an overall improvement in electrical characteristics; however, Dysprosium and Ytterbium doping deteriorated the same characteristics, which is mainly attributed to a significant change in the microstructure of the materials.

1. Introduction Perovskite-based oxide materials, such as Pb(Zr,Ti)O3 (PZT), (Pb,Sr)TiO3 and PbMg1/3Nb1/3O3 have attracted much attention in previous few years because of their versatile applications, particularly in multilayer ceramic capacitors, piezoelectric actuators, pyroelectric detectors, ferroelectric random access memory devices, electro-mechanical and electro-optical, etc. [1,2]. However, these ceramic materials suffer from their ineffectuality to conserve the human health and therefore cause serious environmental issues. Recently, a high amount of research is focused on the development of novel lead-free, environment-friendly ceramic materials to meet the commercial requirements. Since the discovery of barium titanate, numerous A and B site isovalent and aliovalent substituted/doped compositions have been studied. Among the various available isovalent substituents, most of the investigations are carried out on Sr2+ and Ca2+ at A-site while Zr4+ and Hf4+ at B-site of the BaTiO3 [3–8]. Sr2+ (1.44 Å) and Ca2+ (1.34 Å) possess comparable ionic radii to that of Ba2+ (1.61 Å) in 12 coordination and thus, BaTiO3 easily forms complete solid solution on Sr2+ and Ca2+ substitution [9,10]. While aliovalent rare-earth

∗

substitutions such as La3+, Pr3+, Sm3+, Gd3+, Dy3+, Ho3+ etc. shows limited solubility on A and B site of BaTiO3 [11,12]. However, rareearths are crucial dopants in the designing of BaTiO3-based ceramic capacitors [13]. They are intermittently substituted/doped to govern the grain growth, electrical resistance, temperature coefficient of resistance and reliability of BaTiO3 based ceramics [12,14]. As the ionic radius of most 3 + valence rare-earths ranges from 0.81 Å to 1.32 Å and hence these substituents could occupy either Barium (1.61 Å) or Titanium (0.605 Å) sites which depend on Ba/Ti ratio and ionic radius [15,16]. The microstructural development, limit of solubility, and electrical characteristics are firmly reliant on the site selection of rareearth dopants in the BaTiO3 unit cell. In general, as the ionic radii become higher, the nature becomes more and more donor-type, indicating an increase in partial occupation at the Ba2+ site [13]. With higher ionic radii ions, namely, Nd3+ and Sm3+, the donor nature is more noticeable in the materials with a lower Ba/Ti ratio. Moreover, the cations having intermediate ionic radii are more prone to occupy both Ba2+ and Ti4+ sites [17,18]. Barium Strontium titanate, Ba1-xSrxTiO3 (BST) with high relative permittivity coupled with a low dielectric loss, makes BST as one of the

Corresponding author. E-mail address: [email protected] (A.K. Panwar).

https://doi.org/10.1016/j.ceramint.2020.01.020 Received 10 December 2019; Received in revised form 2 January 2020; Accepted 3 January 2020 0272-8842/ © 2020 Published by Elsevier Ltd.

Please cite this article as: Aditya Jain and Amrish K. Panwar, Ceramics International, https://doi.org/10.1016/j.ceramint.2020.01.020

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eliminate the binder. After the low-temperature annealing, the pellets were finally sintered at 1225 °C with a heating rate of 4 °C/min for 3 h in an ambient environment. Henceforth, these ceramic samples were named as BSC, BSCLa, BSCGd, BSCDy, and BSCYb for undoped and rare-earth (La, Gd, Dy, and Yb) doped samples, respectively. Here in the nomenclature BSC indicates the combination of BST and BCT, and the following term designated the doped rare-earth.

most favorable materials for ceramic capacitor fabrication [19]. The Sr substituted BaTiO3 based systems are popular for their better response to the externally applied electric field. This property of BST systems is quite important for many applications and has been utilized to fabricate several kinds of electronic devices functioning in the micrometer and millimeter range, such as tunable capacitors, phase-shifting devices, filters, etc. [20,21]. Furthermore, the substitution of Ba2+ by Sr2+ in BaTiO3 can reduce the dielectric Curie temperature (ferroelectric to paraelectric transition temperature), a substantial increase in permittivity, reduction in tangent loss and improvement in pyroelectric constant [22–24]. In the past, Mseddi et al. investigated the effect of Sr in BaTiO3 and reported a substantial improvement in electromechanical properties [25]. Emadi et al. studied the Ni/C doped BST composition and observed a significant enhancement in microstructural and optical properties compared to pure BT [26]. Kurniawan et al. fabricated a Ba0.5Sr0.5TiO3 Thin Film and demonstrated its possible application in temperature sensors for satellite technology [27]. Chavan et al. investigated a Ba0.9Sr0.1TiO3–Co0.90Ni0.10Fe2O4 composite system and observed a slight improvement in magnetoelectric coupling in comparison to pure BT. Further, Barium calcium titanate (BCT) is extensively used for the fabrication of ceramic capacitors due to its superior dielectric and ferroelectric properties. BCT can be utilized in applications, such as energy storage devices, optical connectors, photorefractive, and advanced laser systems [28–30]. Moreover, the substitution of Ca2+ at A-site in BaTiO3 decreases the probability for the formation of undesirable non-ferroelectric hexagonal phase [31]. It is known that the mechano-chemical activation technique has been widely used for synthesizing nanocrystalline ceramic materials. This method has many advantages, such as ease of production, suitable for any class of materials, and comparatively low-cost synthesis [19,32–34]. The modified mechano-chemical activation method has another benefit that it can trigger solid-state reactions at a significantly lower temperature in comparison to that used in the conventional solidstate technique. In our continuing exploration in the field of BT based ceramic materials with improved electrical properties for MLCCs and high energy density capacitors, here in the present work, we report the thorough investigation of rare-earths doped 0.65(Ba0.9Sr0.1TiO3)–0.35(Ba0.7Ca0.3TiO3) (abbreviated as BST-BCT) ceramic. In the present study, rare-earths, La3+, Gd3+, Dy3+, and Yb3+ have been doped in BST-BCT. The composition 0.65(Ba0.9Sr0.1TiO3)–0.35(Ba0.7Ca0.3TiO3) has been considered as it shows the overall better properties among the other combinations of BST and BCT.

2.2. Characterization Malvern Panalytical Aeris Benchtop X-ray diffractometer is used for investigating the phase structure of all the five synthesized powder samples. The results obtained from the XRD pattern are analyzed using Fullprof software to obtain the theoretical density and lattice parameters of the samples via Rietveld analysis. The chemical composition and occupancy of different rare-earth dopants are also confirmed using Rietveld analysis. The surface morphology of the pellet samples was investigated using the Hitachi S–3700 N scanning electron microscope. Further, ImageJ software has been used to estimate the average grain size of all the synthesized ceramic samples. In order to make electrodes, the highly-dense disk-shaped sintered pellets were carefully polished to nearly 0.9 mm thickness, and thick silver paste was coated on each side of the pellet to obtain a prototype of capacitor circuit. To dry-off the silver coating, the pellets were heated at 175 °C for 20 min. Afterward, the dielectric measurement of the pellet samples has been performed using Keysight E4990A impedance analyzer at several frequencies. Polyk PE loop tester has been used to analyze the temperature-dependent ferroelectric properties of synthesized pellet samples. 3. Results & discussion 3. Crystallographic study Fig. 1(a–f) shows the combined X-ray diffraction results and structural refinement for the BSC-RE samples. XRD results indicate that despite the rare-earth doping, the main phase of all the samples was a perovskite phase. All the BSC-RE samples synthesized through the ball milling technique show tetragonal phase. The sharp and intense diffraction peaks of the BSC-RE samples indicate the highly crystalline nature of the synthesized samples. The crystallographic refinement of the samples has been carried out using the Rietveld method with the help of Fullprof software. The diffraction peaks were refined using Gaussian function by taking BaTiO3 (JCPDS no. 005–0626) as the benchmark pattern for BSC, BSCLa, and BSCGd samples. It can be observed that BSCLa and BSCGd samples do not show any significant extra peak other than the BSC sample, which suggests that La and Gd have successfully incorporated into the unit cell of BSC and does not result in unit cell distortion. Moreover, the XRD pattern of BSCDy and BSCYb samples shows few weak diffraction peaks which belong to Dy2Ti2O7 and Yb2Ti2O7, respectively and therefore dysprosium titanium oxide (JCPDS no. 017–0453) and ytterbium titanium oxide (JCPDS no. 017–0454) were taken as the additional benchmark pattern for the refinement. The calculated profile of BSC-RE samples matches well with the experimentally observed profile, which indicates the efficacy of the refined model and various Rietveld factors. The Rietveld analysis also indicates that the basic crystal structure (i.e., tetragonal) does not change with rare-earth doping. However, as the rare-earths are doped, a slight change in c/a ratio has been observed, which consequently affects the tetragonality of the materials. Table 1 and Table 2 lists the lattice parameters and detailed Rietveld refined parameters obtained from the structural refinement. An intense peak (111) of crystal plane has been chosen and fitted using the Gaussian function. The fitted Gaussian peaks were then used to obtain FWHM, and by using the Scherrer formula, the average crystallite size has been calculated and shown in Table 1. Fig. 2(a and b) depicts the crystal structure of BSC and BSCLa

2. Experimental details 2.1. Synthesis procedure A series of compounds with 3 mol% rare-earths (RE) doping in 0.65(Ba0.9Sr0.1TiO3)–0.35(Ba0.7Ca0.3TiO3) were synthesized by mechanochemical activation technique, where, RE = La, Gd, Dy, and Yb. The raw oxide materials BaCO3 (99.0%), SrCO3 (99.8%), TiO2 (99.8%), CaCO3 (99.0%), La2O3 (99.95%), Gd2O3 (99.9%), Dy2O3 (99.99%) and Yb2O3 (99.9%) of Sigma-Aldrich make were used to form powder mixture. Afterward, the mixture powder was ball milled for a duration of 30 h at 450 rpm. Retsch PM200 ball mill is used to perform the milling of raw mixtures. A 50 ml capacity stainless steel container and 180 Zirconia balls were used as the milling environment. After the completion of milling process, the powders were dried for 15 min to eliminate the liquid content that was added before the start of milling process. The calcination of the obtained powder mixture is carried out at 1075 °C for 4 h. After the gradual cooling of the samples, the powder is blended with 4 wt% binder (polyvinyl alcohol) followed by uniaxial pressing of powder via hydraulic press to convert it into cylindrical shape disks of nearly 0.9 mm thickness and 8 mm diameter. These cylindrical pellets were then allowed to anneal at 300 °C for 15 min to 2

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Fig. 1. Comparative and Rietveld refined X-ray diffraction patterns of BSC-RE samples: (a) combined XRD pattern of the five prepared specimens, (b) BSC, (c) BSCLa, (d) BSCGd, (e) BSCDy and (f) BSCYb. Table 1 Comparison of lattice parameters, cell volume, type of structure and average crystallite size for BSC-RE specimens. Sample

a,b (Å)

c (Å)

c/a

Cell volume (Å3)

Structure

Average crystallite size (nm)

BSC BSCLa BSCGd BSCDy BSCYb

3.9721 3.9816 3.9893 3.9833 3.9782

4.0088 4.0121 4.0841 4.0092 4.0114

1.0092 1.0076 1.0237 1.0049 1.0083

63.24 63.60 64.99 63.61 62.48

Tetragonal Tetragonal Tetragonal Tetragonal Tetragonal

45 59 63 47 66

Table 2 Comparison of Rietveld refined parameters, theoretical density and experimental density for BSC-RE specimens. Sample

χ2

Rexp

Rf

RBragg

Theoretical density (gm/cm3)

Experimental density (gm/cm3)

BSC BSCLa BSCGd BSCDy BSCYb

1.78 2.05 2.54 2.14 3.21

6.19 4.66 5.01 6.09 4.53

8.54 7.13 7.93 9.51 6.27

5.62 6.85 6.17 4.93 6.40

5.711 5.839 5.472 5.011 5.749

5.652 5.731 5.343 4.891 5.582

well as Ti site, which indicates the amphoteric nature of La. The black stripes in Ba and Ti site demonstrate the occupancy of La. Further, the addition of La in BSC lattice also resulted in a slight modification of unit cell by the downward movement of Ti atom; the black arrow in Fig. 2(b) indicates the movement of Ti atom from its initial position shown in Fig. 2(a) for the BSC sample.

samples. The atomic positions estimated from the Rietveld analysis for the corresponding samples are further used to draw the crystal structure by using VESTA software. The lattice parameters, as well as the atomic positions, confirm that the BSC and BSCLa exhibit a tetragonal phase. Orange and white-colored stripes in the crystal structure of BSC shows the occupancy of calcium and strontium ions at Ba site, respectively. The Rietveld analysis confirms the occupancy of Lanthanum in Ba as 3

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Fig. 2. Crystal structure of BSC sample.

morphological characteristics, as well as the density of doped materials, are driven by the interplay between the processes of dopant melting and movement through the accessible pores during the sintering process. Typically, a low-temperature sintering results in poor densification with porous structure and inhomogeneous grain morphology, whereas in case of high-temperature sintering, the grains are well attached at their grain boundaries, and much better density is obtained, however, high temperature may sometimes result in exaggerated grain growth. Therefore, to obtain high-density morphology with homogeneous grain size, it is necessary to optimize the sintering temperature as well as sintering duration. In the present case, after many attempts, the

3.2. Microstructural study The cross-sectional SEM micrograph of undoped and rare-earth doped BSC-RE samples are shown in Fig. 3(a–e). The average grain of the samples calculated from ImageJ software is found to be nearly 195, 210, 240, 155 and 255 nm with an accuracy of ± 5 nm for BSC, BSCLa, BSCGd, BSCDy, and BSCYb, respectively. The theoretical density obtained from Rietveld analysis and experimental density calculated from the Archimedes Principle for all the five prepared samples is presented in Table 2. Both the theoretical as well as experimental density are in good agreement with the obtained SEM micrograph. The final

Fig. 3. SEM micrograph of sintered pellets of BST-RE samples: (a) BSC, (b) BSCLa, (c) BSCGd, (d) BSCDy and (e) BSCYb. 4

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3.3. Dielectric studies

sintering conditions are kept at 1225 °C for 3 h duration with a gradual cooling rate of 4 °C per minute. Further, The La and Gd-doped samples have improved densification and a slight increase in grain size compared to BSC sample; however, at lower magnification, the microstructure of BSCGd sample found to be slightly porous. The much higher homogeneity of BSCLa and BSCGd samples may be attributed to the amphoteric nature of La and Gd. The La and Gd ions are occupying both the A-site and B-site of perovskite structure, which depends predominantly on Ba/Ti ratio. The results clearly indicate that an appropriate amount of acceptor/donor doping is the prime requirement for the development of optimum surface characteristics and morphology of the material. The obtained results are in good agreement with the crystal structure analysis performed through the Rietveld method. The BSCDy sample has demonstrated poor densification coupled with small grains. The addition of dysprosium has resulted in inhibition of grain growth. The BSCYb sample shows the presence of larger grains combined with few smaller grains suggesting the existence of the secondary phase. BSCYb sample shows the segregation of second phase at different points of the microstructure indicates the lower miscibility of ytterbium ions into the BSC lattice.

Fig. 4(a–j) shows the dielectric properties for bare and doped BSCRE samples over a wide temperature range of 25 °C–200 °C. The dielectric constant and dielectric loss of the materials have been measured in an extensive frequency range, starting from 50 Hz to up to 500 kHz. In all the five samples, the typical nature of dielectric constant variation that exists for normal ferroelectrics is observed. The doped BT based ceramics are particularly influenced by three factors, namely resistance at the interfacial grain boundaries, presence of charge defects and electrical properties of dopant and the host materials [35,36]. BSCLa sample has shown significant improvement in dielectric constant coupled with broadening of transition peak indicating an increase in diffuseness compared to BSC sample. This increase in dielectric constant may be attributed to better surface characteristics, homogeneity and improved morphology of BSCLa sample compared to BSC sample. The increase in dielectric constant above room temperature mainly ascribed to transition temperature is due to higher influence of interfacial polarization over dipolar polarization [37]. For typical ferroelectric ceramics, the dielectric constant of the material decreases above the Curie temperature, and this anomaly can be explained with the help of Curie-Weiss law:

Fig. 4. Dielectric constant and dielectric loss as a function of temperature; inset shows the Plot of ln(1/ε – 1/εm) as a function of ln(T-Tc) for the BST-RE samples: (a,b) BSC, (c,d) BSCLa, (e,f) BSCGd, (g,h) BSCDy and (i,j) BSCYb. 5

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Fig. 5. Ferroelectric hysteresis loops of BSC-RE samples at different temperatures: (a) at room temperature, (b) at 60 °C, (c) at 125 °C (inset shows the variation of remnant polarization, 2Pr and ratio of Pr/ Pm) and (d) variation of charge and discharge energy density with addition of different rare-earth materials and inset shows the energy efficiency.

ε=

C ; Where , T > T0 T − T0

1 1 (T − T0)γ − = ε ε0 C

(1)

(2)

Where, ε and ε0 represent the dielectric constant value at Temperature T and T0, respectively, and γ indicates the diffuseness parameter. For a material to follow ideal Curie-Weiss law, the value of γ should be exactly equal to 1, whereas γ = 2 indicates a complete diffuse phase transition. The inset of the dielectric loss curve represents the diffuseness factor calculated for all the five samples. It has been observed that the diffuseness of the samples has been increased with the addition of rare-earth dopants and found to be maximum for BSCDy sample. Fig. 4 (b, d, f, h, and j) demonstrate the variation of dielectric loss with change in temperature for the five synthesized samples. BSCLa sample has shown a slight reduction in the dielectric loss at high frequency of operation. The decrease in dielectric loss for BSCLa sample may be ascribed to the reduction in leakage current through the grain boundaries, which is typically affected by crystal structure impurities, such as vacancies and dislocations. BSC, BSCDy, and BSCYb samples are nearly temperature independent at lower temperature of operation which may be due to no or little change in the electrical conductivity of the samples [38]. Moreover, the significant increase in dielectric loss of BSCGd and BSCYb samples is attributed to ionic charge imbalance inside the material. Further, the (i) insignificant contribution of thermally excited charge carriers, (ii) higher movement of ions and (iii) the defects as well imperfection in the material results in lower contribution of dipolar polarization and consequently an increased dielectric loss at higher temperatures [42].

Where, T0 represents the Curie-Weiss temperature or transition temperature, and C is the Curie constant. Among other constraints, those are responsible for the deterioration of dielectric properties, fine grain ceramics also tend to reduce the dielectric constant of ceramic materials, which may be the main cause for the decrease in the dielectric constant of BSCGd sample. The dielectric constant of the material has been shifted slightly towards the lower temperature side with the addition of rare-earth dopants, and BSCGd shows the highest shift of 21 °C compared to pure BSC sample. This shift in transition temperature may be attributed to modification in the unit cell volume of rare-earth doped samples compared to BSC sample [13]. Further, a rather significant change in BSCGd transition temperature is due to the highest increase in unit cell volume (see Table 1) among the four other doped samples. BSCLa sample shows a small change in transition temperature in comparison to BSC sample, which may be caused by the splitting of La3+ ion in both Ba2+ and Ti4+ sites, giving rise to self-compensation and relatively better occupancy compared to other doped samples. The shift in transition temperature of BSCDy and BSCYb may be due to the occupancy of Dy3+ and Yb3+ into Ti4+ site as an acceptor ion. Further, it has been clear from XRD analysis that Dy and Yb have limited solubility in BaTiO3 lattice and therefore higher lattice distortion has resulted in a decrease of dielectric constant compared to BSC sample. Further, All the samples have shown higher dielectric constant at low operating frequency, which may be attributed to easy hopping between the ions in the ferroelectric phase and better orientation of dipoles in the direction of an applied electric field [38]. Another reason for an increase in dielectric constant at lower frequency is due to the presence of a high resistance grain boundary layer, whereas the relatively low dielectric constant at high-frequency region is attributed to low resistance of grain boundary layer [39]. BSCLa, BSCDy, and BSCYb samples have shown deviation from the linear characteristics of CurieWeiss law, which is due to diffused phase transition (DPT) of these materials. The primary reason for DPT is the overlapping of the ferroelectric and non-ferroelectric regions around the grain boundary region. The diffuse nature of transition peak can be described by modified Curie-Weiss law [40,41]:

3.4. Ferroelectric studies The ferroelectric loop of five synthesized ceramic samples was recorded and shown in Fig. 5(a–c). The ferroelectric loops were measured at three different temperatures of 27, 60 and 125 °C at a maximum electric field of 30 kV/cm with 20 Hz test frequency. The remnant polarization (Pr), saturation polarization (Pm), and coercive field (Ec) values are listed in Table 3. The inset of Fig. 5(a–c) shows the variation of remnant polarization and Pr/Pm ratio for different rare-earth dopants. All the samples possess a typical ferroelectric character. Interestingly, the remnant and saturation polarization, as well as the coercive field of BSCDy and BSCYb sample decreased significantly while BSCLa and 6

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Table 3 Room temperature saturation polarization (2Ps), remnant polarization (2Pr), coercive field (2Ec), Discharged energy density (J), dielectric breakdown strength (Ebd) and piezoelectric constant (d33) for BSC, BSCLa, BSCGd, BSCDy and BSCYb. Sample BSC BSCLa BSCGd BSCDy BSCYb

2Ps (μC/ cm2) 24.46 38.22 28.76 20.58 23.18

2Pr (μC/ cm2) 9.30 23.04 11.76 0.82 3.42

2Ec (kV/ cm) 17.02 15.74 15.68 1.66 6.54

J (mJ/ cm3) 95 73 101 136 124

Ebd (kV/ cm) 223 278 245 156 238

d33 (pC/ N) 293 389 362 265 304

BSCGd samples have shown a remarkable increase in ferroelectric properties. It can be observed that Dy3+ and Yb3+ doping in BSC has a drastic effect on its ferroelectric properties. As the hysteresis loops become slimmer and more inclined, the value of remnant polarization and coercive field decreases, and a transformation from normal ferroelectric to relaxor ferroelectric takes place. This is because the Dy3+ and Yb3+ doping results in the weakening of long-range polar interaction of TiO6 octahedra in ABO3 perovskite structure [43]. The composition AA΄A″BO3 (Ba1-x-ySr xCayTiO3) can be considered as a network of BO6 octahedra, where Ti4+ ions are located as B-site cations inside the octahedral whereas, A-site cations are situated in the interstitial sites of octahedra. In the case of BSC sample, the movement of TiO6 octahedrons produces ferroelectric activity, and it couple with each other through A-site cations (Ba2+/Sr2+/Ca2+) and this coupling is relatively strong to offer a classical ferroelectric response. At all the three temperatures, BSCLa sample shows the highest saturation and remnant polarization and comparatively lower coercive field. The superior ferroelectric properties of BSCLa sample may be due to larger grain size and relatively higher density of the material. The larger grain size of the material results in lesser pinning of the domain walls leading to high remnant polarization. Further, larger grains contain a higher number of individual ferroelectric domains and a lesser number of insulating grain boundaries which are regarded as the non-ferroelectric areas [44]. In addition to this, the surface morphology, type of composition, crystal phase, lattice defect like oxygen vacancies, etc. also affect the ferroelectricity of the ceramic materials. At room temperature, the BSC, BSCLa and BSCGd show almost comparable coercive field. However, at 60 °C, BSC sample shows a large decrease in the coercive field. Further, at 125 °C, all the samples show a slimmer loop combined with small remnant polarization and coercive field which may be attributed to the presence of paraelectric phase of the materials. Charge and discharge energy density, as well as the energy efficiency of undoped and rare-earth doped BSC samples has been evaluated and is shown in Fig. 5(d). The inset shows the variation of energy efficiency with different rare-earth doping. The energy storage density, J of linear and non-linear ferroelectric materials can be obtained from discharge energy density under the application of an externally applied electric field. In the first quadrant of the P-E hysteresis loop, the area confined by the discharge curve represents the discharge energy density and it can be expressed as follows [45]:

Fig. 6. Dielectric breakdown strength (Ebd) of BST-RE samples and the inset shows the variation Dielectric breakdown strength of BSCLa with different pellet thickness.

3.5. Breakdown strength measurement Fig. 6 shows the dielectric breakdown strength of BSC samples as a function of rare earth content. There are mainly three forms of solid dielectric breakdown, namely electrical breakdown, electrochemical breakdown and electrothermal breakdown. Electrical breakdown occurs in a relatively short time of 100 nsec or even less in solid dielectric; it occurs due to impact ionization by electrons. Further, the electrical breakdown is an electronic process, but it may also begin with mechanical disruption as well. Whereas the other two forms of breakdown generally develop slowly over time. In the present case, the breakdown belongs to electrical breakdown which may also occur due to fracture surfaces in the specimen. These regions are the shortest pathway between the two electrodes for electron flow leading to an electron avalanche and consequently dielectric breakdown [46]. Therefore, it can be inferred that electrical breakdown commonly occurs at voids and pores where the current can pierce through the inner region of abnormally large grain. In addition to this, internal parameters namely porosity, grain size and some measurement factors, such as electrode configuration, sample thickness, sample area, etc. and external parameters viz. ramp rate and frequency of the applied electric field also plays a vital role in determining the dielectric breakdown strength of a material [47,48]. In order to have an accurate and reliable measurement, the breakdown strength has been tested in 8 identical pellets for all the five samples. Again, BSCLa and BSCGd samples show the superior dielectric breakdown strength among the five samples. BSCLa sample shows a 24% increase in Ebd value over the undoped BSC sample. Microcracks, interstitial defects, amount of porosity and size of voids in the ceramic material are some of the major parameters that act as the initiation point for the dielectric breakdown [49]. The inset of Fig. 6 shows the thickness-dependent dielectric breakdown strength of BSCLa sample. A significant drop in Ebd value with an increase in pellet thickness is observed. This behavior can be directly linked to the increase in the number of micropores and microcracks as well as a rise in leakage current with increasing pellet thickness [50]. These defective regions are more severe to dielectric breakdown, which results in a subsequent increase in voltage per unit thickness. This rise in voltage leads to an avalanche of electron at the surface and consequently give rise to the failure of the material [51]. This study suggests that for achieving the highest dielectric breakdown strength, a number of thin dielectrics should be stacked, instead of manufacturing a thick dielectric material.

E

J=

∫ EdP 0

(3)

Where, P represents the polarization, and E is the externally applied electric field. Further, energy efficiency is the ratio of discharge energy density to charge energy density of the material. A slimmer ferroelectric loop combined with low value of remnant polarization and coercive field is needed to achieve high energy efficiency and large discharge energy density. Therefore, highest energy efficiency of 87% at 30 kV/ cm is obtained for BSCDy sample whereas BSCLa sample shows the lowest energy efficiency among the five synthesized samples.

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improvement in microstructure and consequently in the electrical characteristics of the material. 4. Conclusion The doping of rare-earths (La, Gd, Dy, Yb) in Sr and Ca co-doped BaTiO3 have been performed by using a modified mechano-chemical activation technique. La and Gd doping have prompted the grain growth of the ceramic, whereas, Dy doping has resulted in inhibition of grain growth. In addition, Yb doped sample has shown di-phasic behavior with the presence of larger and small grains. Further, the La addition has resulted in a small shift in body-centered Ti atom resulting in a slight modification in the parent unit cell. The increase in density, as well as grain morphology for the La-doped sample, has resulted in a significant increase of 9% increase in the dielectric constant and 40% reduction in the dielectric loss with a slight decrease of Curie temperature towards low-temperature side compared to pure BSC sample. The ferroelectric characteristics of the material are also found to be improved with La doping at all three temperatures. The piezoelectric constant and electrical breakdown strength of the La-doped sample were also found to be improved, which is attributed to increased density as well as the morphology of the BSCLa sample compared to BSC. The optimum grain size combined with homogeneous morphology, enhanced dielectric constant, low dielectric loss, improved ferroelectric characteristics and high electrical breakdown strength for BSCLa sample can provide a potential application in the fabrication of multilayer ceramic capacitor and energy storage devices.

Fig. 7. Variation of piezoelectric constant (D33) with the addition of different rare-earth materials; inset shows the variation of piezoelectric voltage coefficient for BSC-RE samples.

3.6. Piezoelectric studies The effect of rare earth addition on the electromechanical characteristics of Sr and Ca co-doped BaTiO3 has been observed. Fig. 7 shows the variation of piezoelectric constant (d33) and piezoelectric voltage coefficient (g33) with La, Gd, Dy, and Yb addition in optimized BSC ceramic. Initially, in the raw sintered sample, the dipoles inside the material are non-uniformly distributed in random direction; therefore, before the piezoelectric measurement, it is required to pole the cylindrical disk by applying an appropriate electric field to align the dipoles in the same direction as that of the electric field. For any ferroelectric crystalline material, poling plays a crucial role in determining the optimum piezoelectric properties. Moreover, few key constraints, namely poling temperature, applied poling voltage and poling duration also sincerely affect the piezoelectric characteristics and therefore, optimization of all these parameters is admissible before the measurement. In the present work, after optimizing all these parameters, the samples were poled at E ~12 kV/mm for 90 min. For each sample, the temperature has been kept 5 °C less than their Curie temperature to obtain the highest piezoelectric constant. The main purpose of keeping an optimized temperature is to accumulate charge carriers in the vicinity of grain boundaries which will introduce an internal electric field that can improve the piezoelectric constant value of the sample under study. Again, BSCLa sample has shown a significant improvement in piezoelectric constant compared to BSC sample. This increase in d33 value BSCLa sample is attributed to the optimum combination of microstructure homogeneity, grain size, interfacial density, poling temperature and poling electric field of the material. In addition to these, when phase structure transforms (as in the case of BSCLa), there is a reduction in energy barrier and therefore, the dipoles movement becomes rather effortless and as a result, enhancement in the electromechanical response of the material [52]. Moreover, it has been observed that as the poling temperature is decreased towards room temperature, the piezoelectricity of the material tends to decrease, which suggests that the ferroelectric domains are switching back to their initial state [53]. Further, the inset shows the piezoelectric voltage coefficient (g33), which has been calculated by using the expression shown below [54]:

g33 =

d33 ε ∗ TP

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(4)

Where, ε represent the permittivity of the material at the corresponding poling temperature TP. Further, the values of d33 and g33 obtained in the present investigation are comparable to previous reports on BaTiO3 based materials. Finally, it can be concluded that the addition lanthanum in Sr and Ca co-doped BaTiO3 has resulted in significant 8

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