Scholarly article on topic 'Tuning of dielectric, pyroelectric and ferroelectric properties of 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 ceramic by internal clamping'

Tuning of dielectric, pyroelectric and ferroelectric properties of 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 ceramic by internal clamping Academic research paper on "Materials engineering"

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Academic research paper on topic "Tuning of dielectric, pyroelectric and ferroelectric properties of 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 ceramic by internal clamping"

Tuning of dielectric, pyroelectric and ferroelectric properties 0.065BaTiO3-0.22SrTiO3 ceramic by internal clamping

Satyanarayan Patel, Aditya Chauhan, Swarup Kundu, Niyaz Ahamad Madhar, and K. B. R. Varma

Citation: AIP Advances 5, 087145 (2015); doi: 10.1063/1.4929328 View online: http://dx.doi.Org/10.1063/1.4929328 View Table of Contents: http://scitation.aip.org/content/aip/journal/adva/5/8?ver=pdfcov Published by the AIP Publishing

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of 0.715Bi0.5Na0.5Tto3-

Bouraoui Ilahi, Rahul Vaish,

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Tuning of dielectric, pyroelectric and ferroelectric properties of 0.715Bi05Na05TiO3-0.065BaTiO3-0.22SrTiO3 ceramic by internal clamping

Satyanarayan Patel,1,a Aditya Chauhan,1,a Swarup Kundu,2

Niyaz Ahamad Madhar,3 Bouraoui Ilahi,3 Rahul Vaish,1,b and K. B. R. Varma2

1School of Engineering, Indian Institute of Technology Mandi, 175 001, Himachal Pradesh, India

2Materials Research Center, Indian Institute of Science Bangalore, 560 012, India 3Department of Physics and Astronomy, P.O. Box no. 2455, College of Sciences, King Saud University, Riyadh 11451, Kingdom of Saudi Arabia

(Received 7 June 2015; accepted 5 August 2015; published online 17 August 2015)

This study systematically investigates the phenomenon of internal clamping in ferroelectric materials through the formation of glass-ceramic composites. Lead-free 0.715Bi0.5Na0.5Ti03-0.065BaTi03-0.22SrTi03 (BNT-BT-ST) bulk ferroelectric ceramic was selected for the course of investigation. 3Ba0 - 3Ti02 - B203 (BTBO) glass was then incorporated systematically to create sintered samples containing 0%, 2%, 4% and 6% glass (by weight). Upon glass induction features like remnant polarization, saturation polarization, hysteresis losses and coercive field could be varied as a function of glass content. Such effects were observed to benefit derived applications like enhanced energy storage density -174 kJ/m3 to -203 kJ/m3 and pyroelectric coefficient 5.7x10-4 Cm-2K-1 to 6.8x10-4 Cm-2K-1 by incorporation of 4% glass. Additionally, BNT-BT-ST depolarization temperature decreased from 457K to 431K by addition of 4% glass content. Glass incorporation could systematically increases diffuse phase transition and relaxor behavior temperature range from 70 K to 81K and 20K to 34 K, respectively when 6% and 4% glass content is added which indicates addition of glass provides better temperature stability. The most promising feature was observed to be that of dielectric response tuning. It can be also used to control (to an extent) the dielectric behavior of the host ceramic. Dielectric permittivity and losses decreased from 1278 to 705 and 0.109 to 0.107 for 6% glass, at room temperature. However this reduction in dielectric constant and loss increases pyroelectric figures of merit (F0Ms) for high voltage responsivity (Fv) high detectivity (Fd) and energy harvesting (Fe) from 0.018 to 0.037 m2C-1, 5.89 to 8.85 ¡j,Pa-1/2 and 28.71 to 61.55 Jm-3K-2, respectively for 4% added ceramic-glass at room temperature. Such findings can have huge implications in the field of tailoring ferroelectric response for application specific requirements.© 2015 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License. [http://dx.doi.org/10.1063/L4929328]

I. INTRODUCTION

Ferroelectric materials are widely used in a variety of applications such as sensor, actuator, transducer, pulse power circuit, transportation, spacecraft, medical devices, weapons and X-ray equipment.1-3 Most of these applications are based on the either one of the piezoelectric or pyro-electric response in a direct or indirect manner.4 This is due to fact that all the ferroelectric

aThese authors have contributed equally to this work.

bEmail: rahul@iitmandi.ac.in, Phone: +91-1905-237921, Fax: +91-1905-237945

2158-3226/2015/5(8)/087145/17

5, 087145-1

© Author(s) 2015

materials are necessarily pyroelectric and piezoelectric in nature which make them a good candidate for multiphysical applications.5 Further, ferroelectric materials are also promising candidates for efficient energy storage and conversion.1,3,4,6 Enhanced energy storage density is manifested in the ferroelectrics due to large electrical polarization and low losses. It has been established in the literature that such properties are a function of domain structure and lattice symmetry of the material concerned.1,3,7,8 Thus, domain engineering has been proposed as an alternate route to chemical modification in order to tune the ferroelectric response.810 In this direction different academic/research groups have focused on tweaking the chemistry of existing materials or investigate novel compositions to get desired results. Literature present copious instances of design and fabrication of morphotrophic phase boundary (MPB)-based ferroelectric solid solutions.1118 The MPB compositions have pronounced pyroelectric and piezoelectric constants as opposed to single phase ferroelectric materials.15,16,18 Moreover, these materials can also exhibit large strain variation due to structural transformation. This strain response makes them a good candidate for actuator applications. Often, upon exposure to suitable intensity of stress, electric field and temperature these materials undergo structural (phase) transformation.9,11,16,19 These phase transitions are accompanied by drastic changes in their ferroelectric response.9,11,16,19

Furthermore, application of compressive pre-stress (mechanical confinement) has been proven to alter piezoelectric, pyroelectric and ferroelectric properties.3,4,20-22 These properties are tailored by altering the domain structure by inducing ferroelastic domain switching.3,4,6,22 In real time applications actuators and transducer are subjected to both pre-stress and electric loading. Therefore analysis of stress influence on the functional properties of active ferroelectrics becomes essential.1,3,4,6 Directional confinement has been extensively reported to tailor properties like remnant polarization, coercive field, saturation polarization, dielectric and piezoelectric properties.4,23-26 Additionally, a number of researchers have shown enhanced energy storage density in ferroelec-tric/antiferroelectric materials through mechanical clamping.6,25-27 The energy storage density has also been tuned by glass addition (chemical route) in polycrystalline ferroelectric materials.2,28-30 It has been reported that the breakdown strength of ferroelectric materials can be affected by addition of glass.28,30-32 Moreover, it can also decrease hysteresis loss which results in improved energy storage.28,30-32 These advantages are obtained in addition to reduction of sintering temperature (viscous sintering) through glass addition.2,33,34 However, there is a scarcity of studies which shows the effect of glass addition pyroelectric properties. Further, quantitative analysis of domain dynamics by glass addition in ferroelectrics is also unexplored.

In this direction, lead-based ferroelectric ceramics have been extensively studied and used due to their excellent piezoelectric, pyroelectric and ferroelectric properties.16-19 Therefore, a number of lead-based ferroelectric are fabricated such as PbZrxTi1-xO3 (PZT), PbMgV3№>2/3O3 (PMN), PbMg1/3Nb2/3O3-PbTiO3 (PMN-PT), PbIn1/2Nb1/2O3-PbMg1/3Nb2/3O3-PbTiO3 (PIN-PMN-PT), PbZn1/3Nb2/3O3-PbTiO3 (PZN-PT), PbMg1/3Nb2/3O3-PbZn1/3Nb2/3O3 (PMN-PZN) and many more.1,15-18,35 Nevertheless, lead has carcinogen nature which is main cause of cancer.36 Therefore, due to environmental concern demands has been increase to elimination of toxic lead for these materials systems. In this direction, Bi0 5Na0 5TiO3 (BNT) and K0 5Na0 5NbO3 (KNN) based ferroelectric materials has been extensively studied as a substitution of lead-based materials due to their large piezoelectric/pyroelectric constants.37-40 Further, to improve the electromechanical, piezoelectric and pyroelectric responses, MPB-based compositions are fabricated such as the Bi0 5Nao.5TiO3-BaTiO3 (BNT-BT), Bi0.5Na0.5TiO3-Bi0.5Ki5TiO3 (BNT-BKT), Ki.5Na0.5NbO3 - LiSbO3 (KNN-LS), K0.5Na0.5NbO3 - LiSbO3-CaTiO3 (KNN-LS-CT), Bi0.5Na0.5TiO3-BaTiO3-KNa0.5Nb0.5O3 (BNT-BT-KNN).37-43 Moreover, Bi0.5Na0.5TiO3-BaTiO3-SrTiO3 has been also extensively studied by a number of researchers.7,8,44 It possesses a MPB of (1-x)Bi05Na05TiO3-0.065BaTiO3-xSrTiO3 solid solution, where pure 0.935Bi0 5Na0 5TiO3-0.065BaTiO3 have almost square shaped hysteresis loops which shrinks with addition of SrTiO3 (ST).7,8,36,44

This study aims to explore 0.715Bi0 5Na0 5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) bulk ferroelectric ceramic for tuning its properties by addition of 3BaO-3TiO2-B2O3 (BTBO) glass. We have evaluated the electrical energy storage density dependence on glass content. Additionally, pyroelectric coefficient and pyroelectric figure of merits (FOMs) were also estimated. Moreover,

temperature and frequency dependent dielectric constant were measured. Furthermore, diffuse-phase transition (relaxor) was described using the modified Curie-Weiss law. Finally, domain dynamics behavior, in terms of activation energy, was also evaluated.

II. EXPERIMENTAL METHOD

A. Sample Preparation

Glass composition of 3BaO-3TiO2-B2O3 (BTBO) was used in present work. Reagent grade powders of BaCO3, TiO2 and H3BO3 (> 99.8% pure) were used as starting agents for glass preparation. The raw powders were weighed and mixed thoroughly according to their stoichiometric ratio (in moles). The mixture was melted in a platinum crucible at 1300°C for 1hr.45 The melt was quenched between stainless plates at room temperature to obtain transparent glass samples.6

Strontium titanate-modified 0.715Bio.5Nao.5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) ceramics was prepared by solid state synthesis technique. For this purpose Bi(OH)3, Na2CO3, BaCO3, TiO2, and SrCO3 were used as starting powders (> 99.8% pure). The powders were mixed together in stoichiometric ratio using mortar and pestle. The mixture was calcined at 900°C for 2.5 h and ground again to good compositional homogeneity. Further, glass was also ground in fine powder form and added in calcined BNT-BT-ST powder in fixed 2, 4, 6% (by weight) batches. These mixture was again thoroughly mixed with addition of 2 (wt.%) polyvinyl alcohol (PVA) binder. The powders were subsequently pressed into 12 mm*1 mm (diameter*thickness) pellets. It has been reported that the addition of glass additive can reduce the sintering temperature because presence of glass can induce a liquid phase (viscous) greatly aiding material diffusion during sintering. Pure 0.715Bi0 5Na0 5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) ceramic's sintering temperature is reported to be at 1200°C. However, glass added BNT-BT-ST samples (2, 4 and 6% by weight) were sintered at 1150°C. To minimize evaporation of volatile elements, samples were embedded within powder of the same composition. The sintered samples were ground for surface finish and silver painted onto both parallel surfaces of the disks. The density of all samples was measured using the Archimedes principles.

B. Characterization

The amorphous nature of as-quenched glass was confirmed by X-ray powder diffraction (XRD) technique (Rigaku Smart Lab, Japan) using Cu (K-a) radiation in the (28) range of 20°-80°. BNT-BT-ST compositional homogeneity was confirmed by the same XRD in the (20 ) range of 20°-85° at room temperature. The polarization-electric field (P-E) measurements were carried out using a modified Sawyer-Tower circuit (Marine India, New Delhi) at different magnitude of electric field and temperatures. The samples were immersed in silicone oil bath during measurement to prevent air discharge. The electric field-strain behavior was confirmed by using Radiant Precision workstation at room temperature. The electrical measurements were carried out as a function of frequency at various temperatures using an impedance analyzer (Agilent 4294 A; Agilent Technologies Inc., Santa Clara, CA). The measurements were performed at 3K temperature interval at a heating rate of 3K/min.

III. RESULTS AND DISCUSSION

The XRD pattern of pure and glass added 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) compositions are shown in figure 1. It is observed that all compositions have formed a proper single phase perovskite structure (indicated by lack of extra peaks). Further, it indicates that addition of 3BaO-3TiO2-B2O glass upto 2% (by weight) has no effect on the crystal structure. However, upon addition of glass in 4 and 6% (by weight) leads to the formation of some unidentified phase as indicated by the presence of two small unidentified peaks in the XRD patterns. However, owing to the small peak intensity it can be assumed that these phases are present in

FIG. 1. X-Ray diffraction (XRD) pattern of sintered BNT-BT-ST samples containing 0%, 2%, 4% and 6% (by weight) BTBO glass. Inset shows XRD of as-quench glass samples.

small quantity only. Hence, it can be said that their contribution to the ferroelectric behavior of the sintered ceramics will be negligible. Finally, it can be stated that the present glass is amorphous in nature and present only outside the grain structure. This can be used to alter pyroelectric, ferroelectric and dielectric properties. Inset of figure 1 shows XRD pattern of as-quenched glass sample at room temperature. It indicates that no diffraction peak is observed which confirm the amorphous nature of glass when added to the ceramic. In the present study 3BaO-3TiO2-B2O3 (BTBO) glass composition was selected for two reasons. First, the chemical composition of BTBO is almost similar to that of the BNT-BT-ST compositions. Second, it has been reported for high dielectric constant and low losses.45,46 Furthermore, recently our group published an article on BTBO glass added enhanced energy storage in BaTiO3-V2O5 ferroelectric ceramics.6 Therefore, it is selected as a suitable glass composition for incorporation into BNT-BT-ST ceramic. Moreover, density of the pure sample and glass added samples are found to be higher than 94% of the theoretical density.

In order to further analyze, the polarization versus electric field hysteresis (P-E) loops of pure and glass-added BNT-BT-ST compositions are depicted in figure 2(a), at room temperature. The P-E loops are measured at a constant frequency of 50 Hz. It is observed (from figure 2(a)) that the hysteresis loop parameters (coercive field, remnant and maximum polarization) decrease with increasing glass content. Further, pure BNT-BT-ST has a remnant polarization (Pr) of 0.082 C.m-2 which reduces significantly with addition of 2, 4, 6% glass to 0.07, 0.056 and 0.054 C.m-2, respectively. This reduction in remnant polarization is also accompanied with the decrease in maximum polarization (Pmax) and coercive field. This is due to the fact that presence of glass hinders the domain wall growth and movement which ultimately reduces the saturation and remnant polarizations. Furthermore, it can also be speculated that glass addition can induce domain pinning or domain wall clamping under the electric field. This phenomenon is normally known as internal physical clamping of the domains. The details of domain growth and pinning have been discussed in the next section. Thus, the hysteresis loop parameters are expected to decrease with increasing glass content. Hence, to achieve an optimum polarization an appropriate amount of glass addition is the key factor. Furthermore, figure 2(b) shows electric field-strain behaviors of pure BNT-BT-ST and glass-added BNT-BT-ST samples at room temperature. It also shows that as the glass content increases electric strain decreases.

Generally, glass remains in amorphous phase during the fabrication process. During sintering the amorphous phase is apparently retained indicated by lack of extra peaks, and expected to be present between the grains boundary.6 In this context, a number of researchers have confirmed this phenomenon in glass added ferroelectric ceramics using scanning electron microscope (SEM)

FIG. 2. (a): Polarization versus Electric field (P-E) hysteresis loops of BNT-BT-ST sintered samples. (b): Field induced strain (elastic compliance) plots as a function of applied electric field for BNT-BT-ST sintered samples.

micrograph.29,31-33,47-49 Our recently published article also discussed SEM analysis of BaTiO3-V2O5 ferroelectric with addition of BTBO glass with clear depiction of glass between grain boundaries.6 For the present work we have provided a typical schematic of glass addition in ceramic is shown in figures 3(a) and 3(b) which can help to understand effect of glass addition on domain dynamics of ferroelectric ceramics. Figure 3(a) represents pure ferroelectric ceramic which has domain boundary with randomly oriented domains. However, addition of glass content is depicted in figure 3(b) which shows that glass is present within the domain boundary (thick domain boundary indicates glass filling). Therefore, ferroelectric properties of the ceramic composite decreases upon induction of glass within the boundary region. In this context a number of studies report that the addition of glass can alter ferroelectric behavior.28,30-32 Moreover, electric field alter polarization in ferroelectric materials in three distinct regions. (a) Initially when the magnitude of applied field is below the coercive field (Ec), domain growth takes place in the preferentially aligned domains.1,3,22 This is a result of reversible domain wall motion. (ii) When field strength increases beyond (Ec), it causes domain rotation and the strength of the polar vector increases exponentially with the applied electric field. This behaviour is observed till the point of saturation polarization is reached. This phenomenon is ferroelectric switching and is associated with 180° domain rotations.1,3 The rotation is measured with respect to the direction of applied electric field as shown in the figure 3(c). (iii) Beyond the saturation polarization, any further increase in the magnitude of electric field results in electrical straining of the material, ultimately leading to dielectric breakdown. Conversely, the effect of applied strain is to depolarize the ferroelectric material by increasing the symmetry of the

FIG. 3. The figure depicts the (ferroelectric) domain switching behavior of pure ceramic and ceramic-glass composites in the absence and presence of electric field. (a): Pure ceramic in the absence of electric field, (b): Ceramic composite in the absence of electric field, (c): Pure ceramic under the influence of external electric field and (d): Ceramic composite under the influence of external electric field. Inset describes the mechanism of internal clamping experienced by the individual grains of the ceramic composite.

crystal lattice. The is known as ferroelastic switching and induces only non-180° domain rotation unlike ferroelectric switching (180°).1,3 The switching is achieved by the motion of central atom to an energetically preferred site. This motion tries to compensate for the increased energy of the strained material under the influence of applied stress. The dipolar rotation and thus, collapse of polar vector is dependent on the lattice geometry of concerned material.1,3,4 In a tetragonal structure, the switching occurs at 90° to the direction of applied stress and there are four available side sites for movement of central atom. For a rhombohedral lattice, two possible switching directions of 70.5° and 109.5° are available.1,3,4 In a single crystal, the effect may differ based on the mode of confinement (x31 or x33) and the direction of stress applied with respect to poling vector.1,3 This effect has been demonstrated in detail for tuning of ferroelectric properties by directional confinement of PMN-PT single crystals.4,50 Therefore, upon successive application of suitable mechanical confinement polarization and ferroelectric response can be altered.

Similarly, addition of glass can also produce the required mechanical confinement. Figure 3(d) depicts polarization behavior of glass-added ferroelectric ceramic under electric field. In this situation domains are also orientated in the direction of applied field. However, they possess some degree of independent orientation which depends on glass content and crystal structure (such as rhombo-hedral, tetragonal) of the ceramic composite. The inset of schematic (Figure 3(d)) clearly represents internal clamping phenomenon of domains due to glass content. This is owing to the fact that under applied electric field, ferroelectric ceramics are strained due to ferroelectric switching. However, glass being non-ferroelectric in nature remains ideally seated. Therefore, an internal stress is generated between glass and ceramics phases, which reduce net polarization. Hence, it can be concluded that glass addition leads to ferroelastic domain switching away from the applied electric field. The orientation is in the energetically preferred direction(s). This, self-confinement or internal clamping can also help tune other properties of ferroelectric ceramics as discussed below. Further, it has been reported that glass addition increases the breakdown strength of ceramics which in-turn increases the energy storage capacity.28,30-32 The energy storage can be further improved by minimizing hysteresis losses. In this regards the reader is referred to numerous studies previously reported in the literature as the scope of the current work is limited. This study is aimed to analyze effect of glass content on pyroelectric, ferroelectric and dielectric properties of BNT-BT-ST ceramics.

0 2 4 6

Glass content (%)

FIG. 4. Electrical energy storage density (per unit volume) plotted as a function of glass content (wt. %), for BNT-BT-ST sintered samples.

The energy storage performance of pure and glass added BNT-BT-ST compositions have been estimated by the P-E loop as presented in figure 4. Electrical energy storage density can be calculated by integrating the area between the discharge curve of P-E loop and the polarization axis, within the interval of Pr to Pmax. The numerical relationship is given as follows:2,6,30

W = & E .dP (1)

where, W, E, Pr and Pmax represent electrical energy storage density, applied external electric field, remnant polarization and maximum polarization, respectively. Further, equation (1) represents the recoverable energy density because integration is considered with respect to discharging curve. Moreover, figure 4 shows that glass addition increases electrical energy storage capacity in compositions under study. This can be credited to the reduction in hysteresis upon glass addition. Internal clamping reduces polarization reversibility which results in reduction of hysteresis loop area (as evident from figure 2) and increase in energy storage.32 Furthermore, it is noted that energy storage density is improved up-to 4(wt.%) glass content after which it decreases. It can be said that addition of 6(wt.%) glass content greatly influence reduction of Pmax as opposed to Pr, which ultimately reduces the energy storage capacity. The polarization reduction is again credited to domain clamping as previously discussed.

Further, ferroelectric materials exhibit remnant polarization in the absence of electric field. This leads to a layer of charge being built-up on each surface of the material. Thus, upon creating a surface charge imbalance, poled ferroelectric material display pyroelectric response under temporal thermal fluctuations (polarization variation). The pyroelectric coefficient (p) of poled ferroelectric material is defined by the expression p = dPr/dT where dPr/dT is the polarization gradient with respect to temperature. When the material is under short circuit conditions, an electric current can be made to flow between opposite surfaces of the material. This is known as pyroelectric current (ip) which can be expressed as ip = dQ/dT. = p. A. dT/dt. where dQ/dt is the rate of charge flow, A is the electrode area of the materials and dT/dt is the rate of temperature change. This phenomenon has been widely employed for a host of applications including infra-red detectors, thermal imaging, and energy harvesting.51-54 Such applications in particular require pyroelectric materials with low dielectric constant and loss, low specific heat capacity, low thermal conductivity and high pyroelec-tric coefficient.51-54 Since most of this can be achieved through internal clamping, hence, one of the focus of this work is to analyze the effect of glass addition on pyroelectricity. The pyroelectric coefficient for compositions under study is estimated as shown in figure 5 at room temperature. Further,

FIG. 5. Pyroelectric coefficient for BNT-BT-ST sintered samples plotted as a function of glass content (wt. %). Inset shows the remnant polarization gradient with respect to temperature for all four samples.

the inset of figure 5 shows variation of the remnant polarization (Pr) as a function of temperature (T) for all compositions. A good fit can be obtained by employing linear least square-fitting method (with Adj. R2 ~ 0.98 - 0.99) indicating a very good linearity of the Pr - T curves. The slopes of Pr - T curves are used to estimate the pyroelectric coefficient (p). Figure 5 shows that (p) increase with the glass content (up-to 4%) and then decreases. This is due to clamping enhanced pyroelectric coefficient, the mechanism of which has been already discussed in detail.55-57

Additionally, room temperature dielectric constant (e) and loss (tan 6) as a function of frequency have been measured. Figures 6(a) and 6(b), represent dielectric constant (e) and loss (tan 6) respectively for pure and glass added BNT-BT-ST ceramics. Dielectric constant has a decreasing trend with increasing glass content. It is established that physical-confinement (compressive stress) reduces dielectric constant due to domain pining.58-60 A number of researchers have discussed similar behavior in various ferroelectric and antiferroelectric materials using domain dy-namics.61-64 In a similar fashion, glass also induces domain clamping or domain pining by means of internal-confinement. Hence, the dielectric constant is reduced with increasing glass content, as shown in figure 6(a).

The Curie temperature and dielectric permittivity decreases with increasing glass content, and the ferroelectric-paraelectric phase transition becomes progressively diffuse (figures 7(a) and 7(b)). These properties can be understood in terms of the effects of the grain size and the creation of oxygen vacancies due to incorporation of glass forming elements within the perovskite structure.65-67 In our case, the effect of substitution of Ba2+ with Bi2+ results in changing the bonding force between the A-site ion and the oxygen ion of ABO3 perovskite structure. The ionic radius of Ba2+ is larger than that of the Bi2+ ion. The bonding force Ti-O(Bi), therefore, becomes weaker than the Ti-O(Ba) bond. The weakening of the Ti-O bond leads to a weaker distortion of the octahedron and brings about a decrease in the c/a ratio, thus inducing a drop in the Curie temperature, dielectric permittivity and higher dielectric loss.65-67 Further, it is to be noted that 4% ceramic-glass shows highest dielectric loss, which could be the effect of grain size as similar results are also observed by many researcher.29,67-69 Incorporation of glass into ceramic matrix has been reported to inhibit grain growth. Further, the grains so formed are separated from adjacent grains through thicker grain boundaries with glass in-between.6 Grain boundaries may have a much lower resistance because of glassy region as compared to grains itself; hence such structures are expected to yield higher dielectric loss. It can be due to easy the hopping between oxygen vacancies and residual cations in the grain boundary of the BNT-BT-ST ceramics.70 Glass having a much lower density than ceramic acquires comparatively larger volume in the composite than it's associated contribution by wt. %. It is expected that upto 4% glass addition, the glass progressively fills up the voids. However, upon further addition, higher glass content might result into a homogenized structure where the glass

FIG. 6. (a): Relative dielectric permittivity plots of BNT-BT-ST samples for applied a/c frequencies of 10 kHz, 100 kHz and 1 MHz, represented as a function of glass content (wt. %). (b): Dielectric losses in BNT-BT-ST samples as observed for applied a/c frequencies of 10 kHz, 100 kHz and 1 MHz, represented as a function of glass content (wt. %).

overflows into the grains, thereby producing a molten state.6 However extensive microstructural characterizations are needed in order to confirm this effect. Furthermore, dielectric constant also varies with frequency due to space-charge polarization contributions. The effect of frequency has been extensively studied by a number or researchers.7,29,61,71-74

Pyroelectric coefficient (p) and dielectric constant (e) are used to estimate the pyroelectric figures of merit (FOMs). A number of FOMs have been derived for pyroelectric materials based on application specific requirements such as thermal/infrared imaging.51-54 The most common FOMs are based on consideration of thermal and electrical circuits employed or maximum current/voltage for a given input. In this direction, for maximizing the pyroelectric voltage for a given heat input, high voltage responsivity (Fv) FOM can be expressed as Fv = p/Cvee0 where Ce or Cv is volume specific heat (constant electric field) and e0 is permittivity of free space.51-53 Here, Cv is expressed as Cv = Cp .p where Cp is the specific heat capacity and p is the density. For infrared detection devices based on the current responsivity, the relevant FOM (F,) is given by F, = p/Cv.51,75 Moreover, high detectivity (Fd) based FOM has been proposed as Fd = p/CvVeeot and.48,49 These FOMs are generally used for materials selection of heat and infrared sensors/transducers. On the other hand thermal to electrical energy conversion is also an important application of pyroelectric materials. For energy harvesting FOM (Fe) is proposed as Fe = p2/e.51,52,54 This Fe FOM has been widely used by a number of researchers for pyroelectric energy harvesting material selection and design

FIG. 7. (a): Relative dielectric permittivity plots of BNT-BT-ST samples for all samples, represented as a function of operating temperatures (K). (b): Dielectric losses in BNT-BT-ST samples, represented as a function of operating temperatures (K).

problems.51'52'54 However, for energy harvesting a new FOM (Fe*) has been recently proposed as F* = p2/eC^. It can be further expressed in terms of Fe* = F x Fv.51,52,54

Table I shows various FOMs (Fv F, Fd, Fe and F*) for pure and glass added BNT-BT-ST ceramics. It is observed that the FOMs increase with increasing glass content. This is due to the fact that pyroelectric coefficient increases and dielectric constant decreases with increasing glass as compared to pure ceramic. However, it is evident from the nature of FOMs that for greater performance high pyroelectric coefficient (p) and low dielectric constant (e) is essential. Hence, addition of glass serves to increase FOMs in BNT-BT-ST composite ceramics. This helps to augment the pyroelectric response of the material thereby increasing its effectiveness for use in sensing, transducing, and energy conversion systems. The results are particularly favorable as it indicates that a significantly smaller sample can be used to deliver the same performance when suitably clamped. This would not only prove to be economical as lesser volume of material is required but also enable on-board or embedded installation due to the significant reduction in the size of the sample required. This will break ground for a plethora of novel applications, which were earlier deemed impossible.

Further results include figures 7(a) and 7(b) which display temperature dependence of dielectric constant (e) and loss (tan 6), respectively in the temperature range of 298-575K for all samples. To eliminate space-charge polarization contributions, 1 MHz frequency data has been chosen. For pure BNT-BT-ST ceramics, broad dielectric peaks can be observed at 457K, which correspond to permittivity-maximum temperature Tm. At Tm, phase transition can occur from rhombohedral-tetragonal to cubic which is accompanied with ferroelectric to paralectric phase shift.

TABLE I. Pyroelectric figure of merits (FOMs) for BNT-BT-ST ceramics with different glass content (wt. %).

Physical parameter Glass content

0% 2% 4% 6%

tan 5 0.109 0.12 0.143 0.107

e 1278 972 734 705

p (10-4Cm-2K-1) 5.70 6.20 6.80 6.20

Fv (m2C-1) 0.018 0.026 0.038 0.037

Ft (10-10mV-1) 2.08 2.27 2.50 2.29

Fd (jiPa-1/2) 5.89 7.05 8.19 8.85

Fe (Jm-3K-2) 28.71 44.66 71.08 61.55

F'e (10-12m3J-1) 3.81 5.98 9.61 8.40

It can also be observed from figure 7 that Tm has been shifted to lower temperature with increasing glass content, as given in table II. This result is conclusive that the phase transition temperature range (around Tm) become lower and broader with increasing of glass content which can be attributed to diffuse phase transition. The diffuse phase transition is typical of relaxor ferroelectrics. This relaxor behavior indicates that material chemistry has been altered among the ceramic and glass phases. A small secondary phase was detected as shown in XRD Figure 1. However, due to physical limitation of the XRD accuracy and experimental setup, we are unable to establish the exact nature of change as yet. However, similar dielectric behavior due to glass addition has also been reported by a number of researchers.31,47-49,76 The detail discussion on relaxor-like behavior has been dealt with subsequently. It is an interesting phenomenon and similar response has been recently reported by a number of researchers.12,17,77-79 Additionally, low dielectric loss is observed in the phase transition range. It indicates that this material can be used in wide range of high temperature applications such as multilayer ceramic capacitors in automobiles. The result indicates that this material has great potential for high temperature applications.

Dielectric permittivity of normal ferroelectrics is expected to obey the Curie-Weiss law under the influence of high temperatures. The law is defined as e = C/(T - T0) where C is the Curie-Weiss constant, T the temperature and T0 the Curie-Weiss temperature.18,51-53 It is expected that onset of T0 is accompanied by second-order phase transition. However, relaxor ferroelectric materials exhibit diffuse phase transition of dielectric permittivity. Under this circumstance, modified Curie-Weiss law is used which can be expressed as:77,80

1 1 _ (T - Tm)Y ...

---= —c— (2)

e em C1

where y is the diffusion coefficient and C1 is assumed to be constant, Tm is the temperature corresponding to maximum dielectric permittivity (em). Further, y = 1 render the equation for conventional Curie-Weiss law while y = 2 gives an equation applicable for diffusi phase transition, for relaxor ferroelectric materials. However, a value of y between 1 and 2 corresponds to incomplete

TABLE II. Parameters for obtaining the modified Curie-Weiss law fit for BNT-BT-ST ceramics with different glass content (wt. %) at 1 MHz.

Glass content

Tm(K )

ATrelaxor (K) 100 kHz-100 Hz

AT diffuse (K ) y

431 1593 34 81 1.97

_16 _i_I_i_I_i_I_i_I_i_I_i_I_i_I_

1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0

In (T-TJ (K)

FIG. 8. The graph represents the linear fit for modified Curie-Weiss law in order to determine slope (y) for different samples of BNT-BT-ST. The value of y can be used to identify the onset of diffuse phase transition in various samples.

diffuse phase transition. In order to analyze the effect of glass content on diffusion phase transition behavior in BNT-BT-ST ceramics, the plots of In ((1/e) - (1/em)) a function of In (T - Tm) at 1 MHz are depicted in figure 8. It can be said that linear relation is observed for under study composition. Further, linear least square-fitting method (with Adj. R2 ~ 0.98 - 0.99) is used to obtained degree of diffuse transition (y) from the slope of the fitting. The parameters obtained by the modified Curie-Weiss law are listed in the table II. Further, at 1 MHz, it is observed that y takes on values 2.03, 2.02, 1.64 and 1.97 for BNT-BT-ST ceramics with glass content of 0, 2, 4 and 6% (by weight), respectively. It clearly depicts that BNT-BT-ST ceramics with different glass content exhibit different features of diffuse phase transition. The observation has some major implications as it indicates that, to a degree, the relaxor behavior in ferroelectric materials can be tuned by glass addition. This could prove to be extremely valuable for a large number of applications involving ferroelectric materials. Furthermore, the degree of diffusivity of phase transition can be expressed in terms of an empirical parameter ATdiffuse. It can be represented as:81-83

AT diffuse = T0.9em (1MHz) - T6m(1MHz) (3)

where T0.9em(1MHz) is the temperature corresponding to 90% of the maximum of dielectric constant (em) in the high-temperature side and Tem(1MHz) is the temperature corresponding to em. The observed value ATdiffuse is listed in the table II. It clearly indicates that diffusivity increases with glass content (except for 4% glass). This is due to fact that addition of 4% glass can be sufficient to produce the domain pining which hinders domain activity. Furthermore, the degree of relaxation behavior ATrelaxor (in the frequency range 100 Hz to 100 kHz) is described as follows:82

ATrelaxor = Tm (100 kHz) - Tm (100 Hz) (4)

Here ATrelaxor indicates relaxation strength, Tm (100 kHz) and Tm (100 Hz) are Tm at 100 KHz and 100 Hz, respectively. In this work, ATrelaxor is observed to take on values of 20, 31, 31 and 34K for 0, 2, 4, and 6% (by weight) glass, respectively as given in table II. Moreover ATrelaxor also increases with addition of glass this could be a result of lesser domain wall interaction.

The large values of Tm are clear indications of relaxor behavior in BNT-BT-ST. The increase in Tm verifies the enhancement of relaxation strength with increasing glass content. The relaxation behavior in ferroelectrics can be explained by many model theories such as composition fluctuation theory, superpara electricity theory, merging of micropolar regions into macropolar regions, and random-field model.77,81-83 The common point of these models is based on the local distortion of the crystal structure, giving rise to polar nano-regions (PNRs).81 In solid solution of BNT-BT-ST,

Na results in the formation of the local electric fields owing to the local charge imbalance and local elastic fields due to local structure distortions. A similar phenomenon also occurs at the B site of ABO3 perovskite structure, which also possess different valencies and ionic radii.77 The presence of random fields, including the local electric elastic fields, hinders long-range dipole alignment, and gives rise to PNRs.77 These PNRs are isolated in BNT-BT-ST and are embedded in the disordered matrix, which results in relaxor behavior. Consequently, the relaxor behavior of BNT-BT-ST ceramics is believed to result from the complex response of the originating PNRs and matrices.

It is observed that BNT-BT-ST ceramics with different glass content present characteristics typical of relaxor ferroelectrics and different maximum temperatures for dielectric constant. The plot of ln u versus Tm is shown in figure 9 for all samples. It shows an obvious non-linear behavior. Therefore, Debye equation cannot be employed to obtain a best fit. In order to analyze the relaxation features, the relaxation rate (u) corresponding to freezing of relaxation time spectrum is described by the Volgel-Fulcher relation as:78,80

u = uo exp(Ea/k(Tm - Tvf)) (5)

Here u0 is the pre-exponential factor, Ea is the hindering barrier, TVF is the Vogel-Fulcher temperature (freezing temperature) and k is the Boltzmann constant. Figure 9 depicts that ln u versus Tm plots can be fitted very well using Volgel-Fulcher relationship with good R2. Further, TVF decreases from 406 to 321 K with increasing glass content, as listed in table III. Moreover, difference in u0 and Ea are consistent with thermally activated polarization fluctuations which could result due to the large difference in grain size as an effect of glass addition, given in table III. In addition, a parameter AT1 (AT1 = Tm - TVF) is also listed in table Table III, to compare the relative freezing temperatures for all samples.80 For pure and glass added BNT-BT-ST ceramics, AT1 also increases. It suggests that the relaxor behaviors in these systems are analogous to that of a dipolar glass with polarization fluctuations above a freezing temperature. The relaxor behavior can be induced by several means such as microscopic composition fluctuation, the merging of micropolar regions into macropolar regions, or a coupling of order parameter and local disorder mode through the local strain.78,80 Additionally, it was observed that temperature also affects domain growth and domain wall motion in ferroelectric ceramics. It is reported in literature that as the temperature increases coercive field, Ps and Pr decreases.68,71 A higher temperature provides higher lattice vibrations which reduce interaction among the dipoles (domain switching) and oxygen trapping. Furthermore, increase in temperature can also reduce the stability of the polarization in each domain which o decreases the hysteresis loop area. This temperature induced domain switching phenomenon and hysteresis parameter reduction is valid for all conventional ferroelectric materials. However, it can

FIG. 9. The graph represents Volgel-Fulcher relationship fit for all samples of BNT-BT-ST. The plots can be used to identify the relaxation frequency of the ferroelectric as a function of temperature.

TABLE III. Parameters (<y0, freezing temperature TVF, AT = Tm - TVp and activation energy EA) from Volgel-Fulcher relation for BNT-BT-ST ceramics with different glass content (wt. %).

Parameter Glass content

0% 2% 4% 6%

^0 (Hz ) 7.76x106 1.35x107 1.09x109 3.05x109

Tvf (K ) 406.18 381.74 334.87 321.77

AT1 (K ) 30.82 50.26 72.13 85.23

E A(eV ) 0.0132 0.0133 0.0135 0.0168

be easily tuned by chemical and physical methods such as fabrication at MPB, doping, defect induction, self and mechanical confinement. Additionally, temperature dependent polarization behavior in glass added compositions is also an important aspect. Hence, temperature induced polarization behavior is explained in terms of domain back-switching.

In this context, temperature dependence of Pr is an important aspect to study the back-switching in domains because it comprises a part of switched domains. However, it has no contribution in back switching at zero electric field. In general, the switchable polarization in domains is known as Ps and the back-switching polarization in domain is proportional to Ps - Pr .36 It is well known that when electric field is applied polarization reaches to saturation/switching polarization Ps.36 However, when electric field is removed polarization moves to Pr. This process is again valid for all ferroelectric materials. The switchable polarization/back switching (Pb) phenomenon can be expressed as:84,85

Pb = Ps - Pr

Figure 10 depicts plots for back-switching polarization (Pb) as a function of temperature for understudy compositions at constant electric fields. Further, it is observed that as temperature increases Pb also increases. This is due to the fact that higher temperature induces more back-switching in domains owing to enhanced lattice vibrations and relaxation. Further it is noted that Pb decreases abruptly above 380K. This could be credited to the start of diffuse phase transition as marked by square symbol. Additionally, according to the classical theory of nucleation by random fluctuations, it is observed that rate of nucleation ($) increases with increase in T. The temperature dependence of dynamic relaxation response (t) for domain nucleation and growth can be determined by:54,55

FIG. 10. The plots represent magnitude of temperature dependence back-switching polarization in all samples of BNT-BT-ST as a function of operating temperature (lines are only guiding eye).

i -2.00

▼ 6% glass ▼

— Linear fit

2.6 2.7

2.9 3.0 3.1 3.2 3.3 3.4

1000/T (K1)

FIG. 11. Plots of In (Pm - Pr) versus 1 /T for all the understudy compositions. Solid lines are linear fit of the data.

where U0 is the energy barrier for domain nucleation and growth and T is the temperature. It is evident from equation that as temperature increases, switching time decreases. Further, when the applied electric field is removed Pb relaxation process is in accordance with dynamics for domain nucleation and growth. Therefore, it can be concluded that below phase transition temperature and high-field ranges, the relationship between Ps - Pr and T obeys the Arrhenius law:54,55

where EA is the average activation energy of trapped charge defects such as oxygen vacancy and P0 is a constant. In order to further analyze Arrhenius plots (In (Ps - Pr) versus 1/T) are shown in figure 11 for all samples. It is used to determine EA for domain switching for under study compositions as given in table III for a constant electric field of 3.5 MV.m-1. This activation energy also depends on the applied electric field. However, in current work focus is on the effect of temperature and glass content on EA. Table III indicates activation energy of ~0.013 eV for pure BNT-BT-ST ceramic. However average activation energy increases to ~0.017 eV upon addition of 6% glass. It has been suggested that the lower value of activation energy is associated mainly with the creation of a large domain switching phenomenon. However, glass addition increases clamping of the domains as described in above sections therefore EA increases.

IV. CONCLUSION

Ferroelectric response in bulk samples can be tailored by the application of suitable directional-confinement. A number of studies have investigated and reported the phenomenon for a host of ferroelectric compositions. However, bulk materials require mechanical confinement through external means. This limits the number of applications where such tuning can be performed. In this regards, this article aims to report the phenomenon of internal clamping in ferroelectric materials through the formation of glass-ceramic composites. Lead-free 0.715Bi0.5Na0.5TiO3-0.065BaTiO3-0.22SrTiO3 (BNT-BT-ST) bulk ferroelectric ceramic was incorporated with 3BaO-3TiO2-B2O3 (BTBO) glass. Sintered samples containing 0%, 2%, 4% and 6% glass (by weight) were created for the purpose of investigation. Characterization and materials parameters were then investigated as a function of glass content. It was observed that the sintered pellets contained single phase BNT-BT-ST with BTBO glassy phase dispersed homogeneously in between the grain boundaries.

Glass incorporation induced changes in the material parameters of the composite samples when compared to the pure ceramic. Features like remnant polarization, saturation polarization, hysteresis losses and coercive field could be varied as a function of glass content. BNT-BT-ST energy storage density and pyroelectric coefficient were enhanced from ~ 174 kJ/m3 to ~203 kJ/m3 and 5.7x10-4 Cm-2K-1 to 6.8x10-4 Cm-2K-1, respectively upon addition of 4% glass content. Additionally, depolarization temperature decreased from 457K to 431K upon incorporation of 4% glass. Moreover, diffuse phase transition and relaxor behavior temperature range increases from 70 K to 81K and 20K to 34 K, respectively when 6% and 4% glass content is added. The most promising feature was observed to be that of dielectric response tuning. It can be also used to control (to an extent) the dielectric behavior of the host ceramic. Further, dielectric permittivity and losses reduced from 1278 to 705 and 0.109 to 0.107, respectively when 6% glass is added at room temperature. However, pyroelectric figures of merit (FOMs) for high voltage responsivity (Fv) high detectivity (Fd) and energy harvesting (Fe) improved by almost twice from 0.018 to 0.037 m2C-1, 5.89 to 8.85 ^Pa-1/2 and 28.71 to 61.55 Jm-3K-2, respectively for 4% added ceramic-glass at room temperature. Such findings can have huge implications in the field of tailoring ferroelectric response for application specific requirements and trigger extensive research in the field of internal clamping through glass addition.

ACKNOWLEDGMENTS

The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at Kind Saud University for funding this research group no. RG-1436-014.

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