- Email: [email protected]
Effect of excess of bismuth doping on dielectric and ferroelectric properties of BaBi4Ti4O15 ceramics
Author's Accepted Manuscript
Effect of excess of bismuth doping on dielectric and ferroelectric properties of BaBi4Ti4O15 ceramics Anita Khokhar, Parveen K. Goyal, O.P. Thakur, K. Sreenivas
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(14)02024-0 http://dx.doi.org/10.1016/j.ceramint.2014.12.103 CERI9704
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
13 November 2014 12 December 2014 17 December 2014
Cite this article as: Anita Khokhar, Parveen K. Goyal, O.P. Thakur, K. Sreenivas, Effect of excess of bismuth doping on dielectric and ferroelectric properties of BaBi4Ti4O15 ceramics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.12.103 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of excess of bismuth doping on dielectric and ferroelectric properties of BaBi4Ti4O15 ceramics Anita Khokhara, Parveen K. Goyalb,*, O. P. Thakurc and K. Sreenivasa,†
a
b
Department of Physics & Astrophysics, University of Delhi, Delhi-110 007, India
Department of Physics, ARSD College, University of Delhi, New Delhi-110 021, India
c
Electroceramics Group, Solid State Physics Laboratory, Lucknow Road, Delhi 110 054, India
Abstract
The effect of excess bismuth oxide Bi2O3 (2-10 wt.%) for processing BaBi4Ti4O15 (BBT) ceramics by solid state reaction has been investigated. The formation of a single phase and a change in the orthorhombic distortion are confirmed with varying excess of bismuth content. Changes in the density are marginal, and use of excess bismuth is seen to promote enhanced grain growth. Dielectric response is improved markedly exhibiting reduced dielectric losses and dispersion over a wide frequency range (10-3 – 106 Hz). A high dielectric constant (ε’~ 226), low loss factor (tan δ~ 0.01) and low dc conductivity (σdc ~ 10-14 Ω-1cm-1) are achieved with an optimum content 6 – 8 wt.% of excess of bismuth oxide. Temperature dependent dielectric data fits well to the modified Curie – Weiss law and the frequency dependent maximum temperatures (Tm and Tm1) corresponding to real and imaginary parts of dielectric permittivity (ε’ and ε”) show a good fit to the non-linear Vogel-Fulcher (V-F) relationship. A clear relaxor behavior is observed with a degree of diffuseness, γ ~ 1.97. Saturated hysteresis loops with high remnant polarization (Pr ~ 12.5 μC/cm2), low coercive fields (Ec ~26 kV/cm) are measured and a high piezoelectric coefficient (d33 ~ 29 pC/N) is achieved in poled BaBi4Ti4O15 ceramics prepared with up to 8 wt.% of excess bismuth oxide. Such a relaxor ferroelectric material with high Curie temperature is useful for high temperature piezoelectric transducer applications.
1
Keywords: Relaxor ferroelectrics; Dielectric behavior; V-F fit; P-E measurements.
Corresponding authors. Tel.: +91-1127667834 E-mail address:
† [email protected] (K.Sreenivas) * [email protected] (P.K. Goyal)
1. Introduction The high dielectric permittivity, diffuse phase transition and strong electrostriction exhibited by relaxor ferroelectric materials have attracted a lot of scientific interest during the last few decades. They have potential use in a wide range of device applications, including sensors, actuators, transducers and memory elements [1-4]. Relaxor materials belonging to the lead-based perovskite compounds, such as lead magnesium niobate (PMN) and (PMN-0.3PbTiO3) solid solutions are well known for their superior properties and have contributed to significant understanding of ferroelectric -relaxor materials. However, in recent years the obvious environmental and health concerns due to the volatile and toxic nature of lead have directed research efforts on alternate lead-free based eco-friendly material compositions. In recent years, bismuth layer-structured ferroelectrics (BLSFs) have attracted a lot of attention for impending applications as high-temperature piezoelectric resonators [5-7] and ferroelectric random access memory devices [8-9], because of their relatively high Curie temperature (Tc) and excellent fatigue endurance property. BLSFs are generally formulated as (Bi2O2)2+(Am−1BmO3m+1)2−, where A is mono-, di-, or trivalent large cation with 12-fold coordination, B is tetra-, penta-, or hexavalent small cation with octahedral coordination, and m varying from 2 to 5 corresponds to the number of BO6 octahedra layers in the perovskite blocks (Am−1BmO3m+1)2−. The crystal structure is described as a sequence of alternating (Bi2O2)2+ and (Am−1BmO3m+1)2− layers stacked along c-axis. [10-11]. Polycrystalline barium bismuth titanate BaBi4Ti4O15 (BBT) with m = 4 is a ferroelectric material belonging to the sub group of BLSFs having Ba2+ and Bi3+ ions at A-site and Ti4+ ions at the B-sites of its structure, and is reported to exhibit a relaxor-like dielectric behavior with reproducible ferroelectric and piezoelectric properties [12].
2
Much of the earlier reported work on BBT ceramics [12-15], synthesized by conventional solidstate reaction route is seen to be overwhelmed with problems associated with high conductivity, increased dielectric losses, and increased dielectric dispersion in ε’ and tanδ with frequency, arising due to oxygen vacancies as a result of bismuth vaporization during high temperature sintering process. The reported ferroelectric and piezoelectric properties show wide variations, and are found to be affected significantly due to compositional variations. Therefore, further improvements in its electrical properties are needed for a better understanding of its true dielectric behavior. A number of suitable substituents/dopants for Bi3+ are available to suppress the effects of bismuth vacancies and oxygen vacancies in BLSFs to improve their electrical properties. There are a few reports wherein improvements in the ferroelectric and/or dielectric properties of BBT ceramics have been described with the substitution of La3+, Er3+, Nd3+, etc. for Bi3+ in BaBi4Ti4O15 [16-21]. The observed improvements in the basic properties with compositional modification due to substitution at the A and B-sites have been always accompanied with a drastic decrease in the phase transition temperature (Tc). Since the high phase transition temperature is the most attractive feature of BLSFs for their potential use in high temperature piezoelectric transducers, a mere improvement in the electrical properties at the cost of lowering the phase transition temperature is undesirable. Thus, there is a need for developing suitable methods to control the bismuth loss and to improve the dielectric and ferroelectric response of BaBi4Ti4O15 without affecting its phase transition temperature. Processing of BaBi4Ti4O15 by the conventional solid-state reaction method demands for a careful optimization of the calcination and sintering characteristics because loss of Bi2O3 during sintering is a potential problem and the resulting non-stoichiometry leads to undesirable conductivity effects which tend to degrade its properties [19, 22]. Addition of an appropriate amount of excess of bismuth oxide in the starting powder mixture is one of the convenient approaches for compensating the loss of bismuth and is helpful to realize a stoichiometric composition [23]. These developments have motivated us to further investigate the electrical properties of BaBi4Ti4O15 when prepared with slight excess of bismuth. The excess of bismuth doping in various BLSFs with improved electrical properties is reported earlier [24-27] but, to the best of our knowledge, no report is available on excess of bismuth doping in BBT ceramics. In this paper, improvements in the dielectric and ferroelectric properties of BBT are shown to be significant with an optimum content of excess of bismuth oxide in the starting powders during ceramic processing.
3
2. Experimental Polycrystalline barium bismuth titanate BaBi4Ti4O15 powders with 0, 2, 4, 6, 8 and 10 wt.% of excess of bismuth oxide, abbreviated as BBT, Bi-2, Bi-4, Bi-6, Bi-8 and Bi-10, respectively) were synthesized by solid-state reaction method. BaCO3, Bi2O3 and TiO2 (purity > 99.5%) were weighed for the required amount and thoroughly mixed and ball milled for 24 h in de-ionized water using Al2O3 balls. The mixture was placed in an alumina crucible and calcined at 900 oC for 5 h in a programmable muffle furnace. After calcination the ball-milled powder was mixed with 2 wt.% polyvinyl alcohol (PVA) as binder. After a thorough mixing, the dry powders were pressed into disks of 10 mm in diameter and ~1 mm thickness at a pressure of about 500 MPa using a hydraulic press and sintered in a closed crucible for 5 h at 1050 oC. The density was measured using the Archimedes method and dense ceramics > 97% of theoretical density were obtained. The crystal structure and phase formation was estimated by X-ray diffraction using a Bruker D-8 Advance X-ray diffractometer with Cu-Kα radiation. The microstructure and grain size distribution on fractured ceramic surfaces was examined by scanning electron microscopy (JEOL JSM – 6510). Sintered pellets were electroded with a fired-on silver paste and were cured at 600 oC for 30 min. The dc conductivity was measured using a 614 Keithley electrometer, and the dielectric measurements were carried out as a function of temperature (30– 600 oC) in the frequency (10-3 – 106 Hz) using a Nova Control impedance spectrometer and Agilent 4284A precision LCR meter. Ceramic samples were poled at 150 °C for 30 min in silicon oil under an applied DC electric field of 35 KV/cm and the piezoelectric charge coefficient, d33 was measured using a Piezotest PM 300 piezometer at 110 Hz under a dynamic force of 0.25 N. The ferroelectric hysteresis loops were measured at 50 Hz frequency and 150 °C temperature using an automated PE loop tracer (Marine India Electricals).
4
3. Results and Discussion
3.1 Structural Properties
Fig. 1, shows the room temperature X-ray diffraction (XRD) patterns of undoped and 2-10 wt.% of excess of bismuth doped BBT ceramics. Single phase formation is confirmed, and the observed XRD peaks are identified to be in good agreement with the standard powder diffraction data (JCPDS No. 350757) belonging to the pure orthorhombic perovskite phase with A21am space group [11, 28-29]. These observations clearly suggest that Bi3+ ions are basically occupied in the pseudo perovskite layer of BBT lattice indicating, and complete solid solubility for the added excess bismuth up to 10 wt. % in the lattice of BaBi4Ti4O15 is in agreement with the earlier reported observations [24-26]. The peak associated with the (119) plane shows the highest intensity, indicating the prepared BBT composition possess a bismuth layered structure with m = 4, and the results are in agreement with earlier observations reporting the strongest diffraction corresponding to (112m+1) reflection in the Aurivillius phase for BLSF compositions [12,15,16,30]. Lattice parameters are determined by least square refinement of powder diffraction data using the Powd-X diffraction data analysis software and their values with associated orthorhombic distortion are listed in table 1. A marginal increase in parameter ‘a’ is noted with increasing excess of bismuth content which may be due to the occupancy of Bi3+ ions in the pseudo perovskite layers. Fig. 2 shows the scanning electron micrographs of the undoped and excess of Bi doped BBT ceramics as seen on the unpolished ceramic surfaces. The microstructural details reveal the formation of randomly oriented plate-like grains of varying size with a plate-like morphology commonly reported in BLSFs. Such a characteristic feature is attributed to the anisotropic nature of the crystal structure in these compounds. The low surface energy along the (00Ɩ) planes promotes grain growth preferentially in the a-b plane leading to a plate-like morphology [12,16,31-32]. Excess bismuth oxide in the starting compositions is known to increase the density and grain size [33]. In the present case, there is an enhancement in the grain size and the density is marginally increased when compared within the limits of measuring error. The addition of excess amount of Bi content is found to be very critical and needs careful optimization, because too much excess Bi2O3 (>10 wt. %) can lead to undesirable effects. Adequate addition of excess Bi2O3 leads to liquid phase formation and benefits in the development of a
5
dense structure and accelerates the rearrangement of grains and mass transfer during the sintering process [34]. Whereas excess Bi beyond an optimum limit creates a superabundant liquid phase at the liquid-solid interface and defeats the densification process, leading to excess Bi aggregation at grain boundaries and promotes the formation of secondary phases in the final reacted product [35]. 3.2 Dielectric Properties
3.2.1 Room Temperature Dielectric Response
Fig. 3(a)-(c) shows the variation in the dielectric constant (ε’), dissipation factor (tanδ) and the dielectric loss (ε”) of undoped and excess of bismuth doped BBT measured over a broad frequency range (10-3 Hz to 106 Hz) at room temperature (300 K). In the case of undoped BBT the room temperature dielectric permittivity (ε’) is a decreasing function of frequency and shows a logarithmic dispersion and a similar trend is observed in all the other samples prepared with increasing Bi content. The observed decrease in the dielectric constant with increasing frequency is a characteristic feature observed in highly disordered dielectrics including relaxor ferroelectrics. It indicates that BBT ceramics exhibit relaxor behavior in their dielectric characteristics, and is related to the existence of polar nanoregions [36]. As the Bi content increases, the room temperature dielectric permittivity when compared at 1MHz increases from 204 to 226, and is maximum for Bi-8 (inset of Fig. 3(a)), and then decreases slightly for the Bi-10 sample. It is interesting to note that the dielectric loss factor (tanδ ~ 0.01) is considerably reduced with increase in Bi content (inset of Fig. 3(c)) and is found to be minimum for Bi-8 sample. In the case of undoped BBT, a strong dispersion in the dielectric loss (< 10 Hz) clearly indicates the influence of space charge polarization effects, and is due to excess oxygen vacancy concentration arising because of bismuth loss. Similar to the highly volatile nature of lead oxide (PbO) in lead zirconate titanate (PZT) ceramic processing, bismuth (or bismuth oxide) particularly in BBT and generally in all BLSFs has relatively low melting and boiling points compared to other elements. Bismuth is vaporized easily during the high temperature sintering step and the loss of bismuth causes many defects and leads to non-stoichiometry. Therefore, addition of an excess amount of bismuth can be helpful to realize stoichiometric composition [23, 37]. In the present case, the oxygen vacancy concentration due to bismuth losses is seen to be restrained when an increasing amount of excess Bi content is added to the system and results in reduced dielectric loss and dispersion as shown in Fig. 3(b).
6
The reduced dispersion in ε” (f) and tanδ (f) in the low frequency region up to Bi-8 corroborates closely with the decreasing values of dc conductivity measured at room temperature (Table 3).
3.2.2 Temperature dependent dielectric response
Fig. 4(a)-(b) show the temperature dependence of the dielectric constant (ε’) and loss tangent (tanδ) of the BBT ceramic measured at 1 MHz. In some of the earlier reported studies an excess of bismuth doping in various BLSFs, has been shown to decrease the ferroelectric phase transition temperature Tc with the increase in bismuth content [24-26]. Normally a change in Tc is related to substitution of two or more cations having different ionic radii [38-39]. However, in the present case we do not observe any change in Tc. Moreover, all the BBT compositions prepared with varying excess Bi content exhibit an increase in the maximum dielectric constant (εm’) at Tc. This can be attributed to the increase in the lattice parameter ‘a’ and associated relaxation in the structural distortion. Larger the lattice parameter ‘a’, higher will be the polarization as ferroelectricity in BLSFs arises by the A-type cation displacement along ‘a’ direction [40-41]. The change in maxima of dielectric permittivity can also be explained on the basis of grain-size effects. The decrease in the maximum dielectric constant with a decrease in grain size can be qualitatively attributed to a low permittivity grain-boundary region (dead layer) possibly composed of partially amorphous oxide or pyrochlore second phase. It is assumed that in the dead layer the polar nanoregions (PNRs) are locked-in by the grain boundaries, and do not contribute to the permittivity [42-43]. The dielectric dispersion is more pronounced in loss factor (tanδ) versus temperature plots shown in figure 4(b), and with increasing bismuth concentration, the loss tangent decreases significantly in the high temperature range (above 350 oC). As discussed above, A-site (bismuth) vacancies and oxygen vacancies, responsible for the dielectric losses in BLSFs [12] are suppressed with excess of Bi3+ doping and hence the dielectric losses are reduced with increase in bismuth concentration. Fig. 5 shows the temperature dependence of the dielectric permittivity, ε’(T) of BBT ceramics prepared with excess of bismuth content (0, 2, 4, 6, 8 and 10 wt. %) at different frequencies. The dielectric constant exhibits a broad diffused transition around the phase transition temperature (Tm) and a strong frequency dispersion of the dielectric permittivity in all the BBT samples is in agreement with
7
earlier observations [15,30,44]. It is believed that the diffused phase transition and the frequency dispersion of dielectric permittivity at Tm are typical characteristics associated with relaxor ferroelectrics. In BBT ceramics without any excess Bi, a large dielectric dispersion in ε’(T) below and above Tm is observed with a decrease in ε’m with increasing frequency, and a shift in Tm is noted with increasing frequency from 1 kHz to 1 MHz. It has been suggested that there is a possibility for Ba2+ ions to substitute not only on the A-site of the perovskite blocks but also enter into the (Bi2O2)2+ layers and can create a local charge imbalance [45]. An inhomogeneous distribution of Ba leads to the formation of defects and results in thermally activated conduction of mobile ions and/or other defects [12] contributing to the observed dielectric dispersion. These observations indicate a relaxor-like behavior in the BBT ceramics. In comparison to the classical relaxor behavior where a complete merging of the dielectric spectra is often observed at T > Tm over a wide frequency range, the undoped BBT ceramics show a merging of ε’(T) above Tm only for a specific range of frequencies (50 kHz to 1 MHz) in a specific temperature range [Tm to (Tm+70) °C] with a large dispersion at low frequencies (<10 kHz). Such a nonmerging of ε’(T) at higher temperatures (T > Tm) may be due to the presence of defects resulting in dielectric losses and higher conductivity at high temperatures. As the excess bismuth concentration increases in BBT ceramics, the dispersion in the temperature region above Tm continuously decreases and for the Bi-8 sample a reasonably good merging of the dielectric spectra above Tm is observed. Dispersion in the high temperature region is again observed in Bi-10 sample. This may be due to the aggregation of additional bismuth on grain boundaries which again disturb the stoichiometry of the system [33]. From the frequency dispersion of the dielectric constant at Tm indicating relaxor behavior, the degree of relaxation (ΔTm) for all the BBT compositions may be described as [46]: ΔTm = Tε’m(1 MHz) − Tε’m(1 kHz)
(1)
Where Tε’m(1 MHz) and Tε’m(1 kHz) are the temperatures where the permittivity shows a maximum at 1 MHz and 1 kHz, respectively. ΔTm of different compositions are listed in Table 2. It can be noted that ΔTm increases from 20 °C (for undoped BBT) to 23 °C (for Bi-10) for a change in the bismuth content from 0 to 10 wt.%. This indicates that degree of relaxation or the frequency dispersion of the ferroelectric – paraelectric phase transition increases with increase in excess bismuth concentration.
8
The dielectric loss (ε” = ε’. tanδ) shown in Fig.6 exhibits diffused peaks and the shifts in the maximum temperature (Tm1) with frequency in ε”(T) could be observed clearly and the sense of frequency shift is similar to that observed for ε’(T). The maximum temperatures (Tm1) of ε”(T) spectra are lower than the maximum temperatures (Tm) of the ε’(T) spectra. Such a behavior in the ε”(T) characteristics has been predicted by Tagantsev [47] for the relaxor materials. The significant increase in ε”(T) at low frequency and high temperature is attributed to the appearance of dc conductivity. For nearly half a century since the discovery of relaxors, several theoretical models, including the super para-electricity, dipolar glass, and random-field models, have been proposed to explain relaxor behaviour. A common point in these models relates to the local order–disorder of the crystal structure that gives rise to polar clusters embedded in the matrix. The relaxor behavior may exist irrespective of whether the matrix is ferroelectric or paraelectric, which is the dynamic response of the polar clusters induced in the system [48]. It is well accepted that the broadness in relaxor originates from the compositional fluctuation and disorder in crystallographic sites when one or more cations occupy the same site in the structure [49]. The origin for the observed relaxor-like behavior has been attributed to the positional disorder of cations on A or B sites of the perovskite blocks [50]. The compositional fluctuations and the structural disorder can result in microscopic heterogeneity with different Curie points and cause the observed diffuseness [12,16]. For a relaxor ferroelectric a modified Curie–Weiss law is used for defining the variation in the reciprocal of the dielectric constant peak at the high temperature side, and the diffuseness of the phase transition is expressed in terms of γ as suggested by Uchino and Nomura [51],
1
ε'
−
1 1 γ = (T − Tm ) ε 'm C
(2)
Where, ε’m is the maximum value of the dielectric constant, C is the Curie-like constant and the value of γ (1< γ <2) represents the degree of diffuseness. For normal ferroelectrics γ = 1, while γ = 2 for relaxors. A plot of ln(1/ε’ −1/ε’m) as a function of ln(T−Tm) at 1 MHz for doped BBT ceramics shows a linear relationship (Fig. 7). The experimental data in these plots show some systematic deviations from the best fit line. However, the coefficient of determination R2 for these plots are found to in the range from 0.96 - 0.99 which show that the experimental data are close to the fitted line within permissible limit of deviations. The diffuseness parameter (γ) is obtained from the slopes of these linear plots and the
9
obtained values of γ are listed in table 2. It is noted that for undoped BBT, γ is found to be ~1.79 and it increases with increase in excess of bismuth concentration and approaches ~1.97 for Bi-8 sample. This can be attributed to the reduction in the number of oxygen vacancies with excess of bismuth doping. Such a transition from relaxor-like behavior to relaxor-ferroelectric behavior has been observed in other BLSFs with reduced oxygen vacancies [16,17]. For further increase in bismuth content beyond 8 wt.%, the bismuth oxide collected near grain boundaries reduces γ to ~1.92 as seen for Bi-10 sample. The rounded peaks in the ε’(T) characteristics accompanied with a strong frequency dispersion on the low-temperature side are characteristic features relating to relaxor ferroelectric. It is known that the frequency dependence of Tm in the relaxor ferroelectrics can be described by a nonlinear Vogel– Fulcher (V-F) relation [52]:
Ea k B (Tm − T f )
ω = ωo exp −
(3)
where, ω = 2πf is the measurement frequency, ωo is the pre-exponential factor (Debye frequency – the attempt frequency of dipole reorientation), Ea is the activation energy describing the relaxation process (i.e. the energy barrier between two equivalent polarization states), kB is the Boltzmann constant and Tf is the static freezing temperature of the polarization fluctuations below which the dynamic reorientation of the dipolar cluster polarization is no longer thermally activated. All the doped BBT compositions fit well to the V-F model. A typical V-F plot of Tm – ln(1/ω) for Bi-8 sample is shown in figure 8(a) which confirms that Bi-8 has a typical relaxor behavior. The V-F fitting parameters Ea, ω0, and Tf for undoped and excess of bismuth doped BBT ceramics shown in Table 3 and are in good agreement with earlier reports [12,14,16,30]. The increase in the value of Ea with increasing Bi3+ concentration confirms the strengthening of the relaxation behavior [8,53]. As stated above, Ea represents the energy barrier between two equivalent polarization states under ac field; therefore, an increase in Ea suggests that polarization becomes more frequency dependent, which is one of the major characteristics of relaxor materials. An increase in Ea from 0.032 eV to 0.057 eV, and a corresponding increase in ΔTm from 20 to 23 K suggest an increase in the relaxation strength with increasing bismuth concentration. The static freezing temperature Tf is found to decrease with increasing Bi content.
10
In the case of relaxor ferroelectrics, Tagantsev [47] emphasizes that: (i) frequency dependent maximum temperatures (Tm and Tm1) noted from ε’(T) and ε”(T) data must follow the V-F relationship, (ii) Tm1 < Tm and (iii) Tf (ε”) < Tf (ε’). Although in some of the BLSF compositions, relaxor-like behavior has been commonly reported but the frequency dependent maximum temperatures Tm1(ω) observed from dielectric loss ε”(T) data has not been fitted to the V-F relationship. In the present study, amongst all the BBT compositions that were prepared, only the Bi-6 and Bi-8 compositions containing the optimum amount of excess of bismuth (6 and 8 wt.%), permit the measured temperatures Tm1(ω) obtained from their respective ε”(T) characteristics for a close fit to the V-F relationship. Fig. 8(b) shows a typical V-F fit of Tm1 – ln(1/ω) for Bi-8 sample. It is interesting to note that in these two compositions the dielectric losses, ε”(T) are seen to be relatively low at higher temperatures in comparison to the other samples and correspondingly their ε’(T) characteristics (Fig. 5) exhibit a good merging of the dielectric constant (ε’) spectra for temperatures T > Tm, and therefore match the requirements expected of a typical relaxor behavior.
3.3 Ferroelectric and Piezoelectric Properties Fig. 9 shows the polarization vs. electric field (P-E) hysteresis loops for undoped and excess bismuth doped BBT ceramics. It is observed that bismuth doped compositions result in the formation of welldefined P-E loops with high remnant polarization (Pr). It is reported earlier that ferroelectric properties are known to be affected by compositional modification, processing conditions and inherent lattice defects such as cation vacancies and oxygen vacancies [54-55]. These inherent defects, accumulated at domain grain boundaries leading to strong pinning of the domain walls, are the major cause for degradation in the ferroelectric properties [56-57] and hence, affect the measured remnant polarization, Pr and coercive field, Ec values. In the BBT-based system improvements in the remnant polarization have also been observed with increasing grain size [58]. In the present study, a significant increase in the grain size is noted with increasing bismuth content, and besides this beneficial change in the microstructure, the oxygen vacancy concentration in the system is also found to be decreased as evidenced from the decreasing values of dc conductivity (Table 3). The Pr values continue to increase with increasing bismuth content and a maximum value of ~12.5 μC/cm2 at a low coercive field Ec ~26 kV/cm are measured. A high piezoelectric coefficient (d33 ~29 pC/N) is achieved in poled BaBi4Ti4O15 ceramics prepared with up to 8 wt.% of excess bismuth oxide, and with further increase in bismuth content (10 wt.%), both the ferroelectric and piezoelectric properties are diminished besides the increase of grain
11
size and reduction of oxygen vacancies. Such a decrease in the properties at the highest bismuth content (10 wt.%) may be related to undesirable segregation effects at the grain boundaries, or its incorporation in (Bi2O2)2+ layers [33,57] and necessitates further investigation.
Conclusions An optimum amount of excess of bismuth oxide content (6 - 8 wt. %) in the starting powders before calcination is found to be beneficial during BaBi4Ti4O15 ceramic processing by solid state reaction. It helps to compensate the loss of bismuth during the high temperature sintering stage. Changes in the density are marginal while grain size, microstructure, and electrical properties show significant improvements. A low dielectric loss (tan δ ~ 0.01) and reduced dispersion over a wide frequency range (10-3 to106 Hz) are measured, and there is considerable decrease in the room temperature dc conductivity. The transition temperatures Tm and Tm1 for ε’(T) and ε”(T) from the frequency and temperature dependent dielectric characteristics fit well to the Vogel-Fulcher relation indicating a clear relaxor behavior. A high remnant polarization (Pr) and low coercive fields (Ec) are measured, and polycrystalline BBT ceramics poled at 35 KV/cm exhibit a high piezoelectric coefficient (d33). Use of excess of bismuth oxide during barium bismuth titanate processing is found to be beneficial and convenient for improving the properties. The high phase transition temperature is maintained, and the improved material properties are useful for potential applications as high temperature piezoelectric devices.
Acknowledgments Authors thank the University of Delhi for the R&D grant and for the use of facilities at the University Science Instrumentation Center (USIC). One of the authors (AK) is also thankful to the Council of Scientific and Industrial Research (CSIR), India for the award of Senior Research Fellowship (SRF).
12
References •
1. F. Bahri and H. Khemakhem, Dielectric properties of Bi-doped Ba0.8Sr0.2TiO3 ceramic solid solutions, Ceramics International, 39 (2013) 7571–7575.
•
2. B. Jaffe, W. Cook and H. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971.
•
3. G. H. Haertling, Ferroelectricceramics: history and technology, Journal of American Ceramic Society 82 (1999) 797–818.
•
4. S.E. Park and T.R. Shrout, Ultra high strain and piezoelectric behaviour in relaxor based ferroelectric single crystals, Journal of Applied Phys., 82 (1997) 1804–1811.
•
5. L. Bellaiche and D. Vanderbilt, Intrinsic piezoelectric response in perovskite alloys: PMN-PT versus PZT, Physical Review Letters 83 (1999) 1347–1350.
•
6.
L.
Pardo,
A.
Castro,
P.
Millan,
C.
Alemany,
R.
Jimenez
and
B.
Jimenez,
(Bi3TiNbO9)x(SrBi2Nb2O9)1-x Aurivillius type structure piezoelectric ceramics obtained from mechanochemically activated oxides, Acta Mater. 48 (2000) 2421. •
7. S.H. Hong, S. Trolier-Mckinstry and G.L. Messing, Dielectric and Electromechanical Properties of Textured Niobium-Doped Bismuth Titanate Ceramics, J. Am. Ceram. Soc. 83 (2000) 113.
•
8. B.H. Park, B.S. Kang, S.D. Bu, T.W. Noh, J. Lee and W. Jo, Lanthanum substituted bismuth titanate for use in non-volatile memories, Nature (London), 401 (1999) 682-684.
•
9. C.A. Paz de Araujo, J.D. Cuchiaro, L.D. McMillan, M.C. Scott and J.F. Scott, Fatigue-free ferroelectric capacitors with platinum electrodes, Nature (London), 374 (1995) 627-629
•
10. B. Aurivillius, Mixed bismuth oxides with layer lattices, III structure of BaBi4Ti4O15, Arkiv för Kemi 1 (1950) 519–527.
•
11. E.C. Subbarao, A family of frrroelectric Bismuth compounds, J. Phys. Chem. Solids, 23 (1962) 665-667.
•
12. A. Khokkar, M. L. V. Mahesh, A. R. James, P. K. Goyal and K. Sreenivas, Sintering characteristics and electrical properties of BaBi4Ti4O15 ferroelectric ceramics, J. Alloys Compd. 581 (2013)150–159.
•
13. S. Kumar and K.B.R. Varma, Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics, J. Phys. D: Appl. Phys. 42 (2009) 075405.
13
•
14. J. D. Bobic, M. M. Vijatovic, S. Greicius, J. Banys and B. D. Stojanovic, Dielectric and relaxor behavior of BaBi4Ti4O15 ceramics, J. Alloys and Comp., 499 (2010) 221-226.
•
15. S. K. Rout, E. Sinha, A. Hussain, J. S. Lee, C. W. Ahn, I. W. Kim, and S. I. Woo, Phase transition in ABi4Ti4O15 (A=Ca, Sr, Ba) Aurivillius oxides prepared through a soft chemical route, J. Appl. Phys., 105 (2009) 024105.
•
16. Anita Khokhar, Parveen K. Goyal, O. P. Thakur , A.K. Shukla and K. Sreenivas, Influence of lanthanum distribution on dielectric and ferroelectric properties of BaBi4-xLaxTi4O15 ceramics, Mater. Chem. and Phys. 2014, DOI: 10.1016/j.matchemphys.2014.11.074.
•
17. D. Peng, H. Zou, C. Xu, X. Wang and X. Yao, Er doped BaBi4Ti4O15 multifunctional ferroelectrics: Up-conversion photoluminescence, dielectric and ferroelectric properties, J. Alloys and Comp., 552 (2013) 463–468.
•
18. J. D. Bobic, M. M. V. Petrovic, B. Banys, B. D. Stojanovic, Effect of La substitution on the structural and electrical properties of BaBi4−xLaxTi4O15, Ceram. Intl. 39 (2013) 8049-8057.
•
19. A. Chakrabarti and J. Bera, Effect of La-substitution on the structure and dielectric properties of BaBi4Ti4O15 ceramics, J. Alloys Comp., 505 (2010) 668-674.
•
20. P. Fang, H. Fan, J. Li, L. Chen and F. Liang, The microstructure and dielectric relaxor behavior of BaBi4−xLaxTi4O15 ferroelectric ceramics J. Alloys Comp., 497 (2010) 416-419.
•
21. C.L. Diao, H.W. Zheng, Y.Z. Gu, W.F. Zhang, L. Fang, Structural and electrical properties of four-layers Aurivillius phase BaBi3.5Nd0.5Ti4O15 ceramics, Ceramics International, 40 (2014) 5765– 5769.
•
22. Z.Z. Lazarevic, N. Z. Romcevic and J. D. Bobic, Study of ferroelectric BaBi4Ti4O15 obtained via mechanochemical synthesis, Optoelectronics and Adv. Mater., 3 (2009) 700-703.
•
23. H. Watanabe, T. Mihara, H. Yoshimori, C. A. Paz de Araujo, Preparation of Ferroelectric Thin Films of Bismuth Layer Structured Compounds, Jpn. J. Appl. Phys., 34 (1995) 5240-5244.
•
24. J. Lee, B. Park and K. Hong, Effect of excess Bi2O3 on the ferroelectric properties of SrBi2Ta2O9 ceramics, J. Appl. Phys., 88 (2000) 2825-2829.
•
25. T. Chen, T. Li, X. Zhang and S. B. Desu, The effect of excess bismuth on the ferroelectric properties of SrBi2Ta2O9 thin films, J. Mater. Res., 12 (1997) 1569-1575.
14
•
26. A.Z. Simoez, G.C.C. da Costa, M.A. Ramirez, J.A. Varela, E. Longo, Effect of the excess of bismuth on the morphology and properties of the BaBi2Ta2O9 ceramics, Mater. Lett. 59 (2005) 656–661.
•
27. A.K. Nath and N. Medhi, Piezoelectric properties of environmental friendly bismuth doped barium titanate ceramics, Mater. Lett., 73 (2012) 75-77.
•
28. J. Tellier, P. Boullay, M. Manier and D. Mercurio, J. Solid State Chem., A comparative study of the Aurivillius phase ferroelectrics CaBi4Ti4O15 and BaBi4Ti4O15, 177 (2004) 1829-1837.
•
•
29. B.J. Kennedy, Y. Kubota, B. A. Hunter, Ismunandar, K. Kato, Structural phase transitions in the layered bismuth oxide BaBi4Ti4O15, Solid State Commun. 126 (2003) 653–658. 30. P. Fang, H. Fan, Z. Xi and W. Chen, Studies of structural and electrical properties on four-layers Aurivillius phase BaBi4Ti4O15, Solid State Commun., 152 (2012) 979-983.
•
31. J. D. Bobic, M.M.V. Petrovic, J. Banys, B.D. Stojanovic, Electrical properties of
niobium doped barium bismuth-titanate ceramics, Mater. Res. Bull., 47 (2012) 1874-1880. •
32. J.A. Horn, S.C. Zhang, U. Selvaraj, G.L. Messing, and S. T.-McKinstry, Templated Grain Growth of Textured Bismuth Titanate, J. Am. Ceram. Soc., 82 (1999) 921-926.
•
33. H. Liu, Q. Li, J. Ma, and X. Chu, Effects
of Bi3+ content and grain size on electrical
properties of SrBi2Ta2O9 ceramic, Mater. Lett. 76 (2012) 21-24 •
34. H.Q. Wang , X. W. Zhang , Y.J. Dai, Sintering
behavior and electrical properties of
the (Li, Ta, Sb) modified (K, Na)NbO3 lead-free ceramics with KNbO3 as sintering aid, Mater. Lett., 67 (2012) 145–147.
•
35. R. Jain, V. Gupta, K. Sreenivas, Sintering characteristics and properties of sol gel derived Sr0.8Bi2.4Ta2.0O9 ceramics, Mater. Sci. Engg. B, 78 (2000) 63-69.
•
36. I. Rychetsky, S. Kamba, V. Porokhonskyy, A. Pashkin, M. Savinov, V. Bovtun, J. Petzelt, M. Kosec and M. Dressel, Frequency-independent dielectric losses (1/f noise) in PLZT relaxors at low temperatures, J. Phys.: Cond. Mater., 15 (2003) 6017-6030.
•
37. H. Irie and M. Miyayama, Dielectric and ferroelectric properties of SrBi4Ti4O15 single crystals, Appl. Phys. Lett., 79 (2001) 251-253.
15
•
38. V. K. Seth and W. A. Schulze, Grain-Oriented Fabrication of Bismuth Titanate Ceramics and Its Electrical Properties, IEEE Trans. Ultrasonics, Ferroelectrics, Freq. Con. Soc. 36 (1989) 41–49.
•
39. D.Y.Saurez, I.M. Reaney and W.E. Lee, Relation between tolerance factor and T-c in Aurivillius compounds, J. Mater. Res., 16(11) (2001) 3139-3149.
•
40. R.Z. Hou, X.-M. Chen, Dielectric properties of La3+ substituted BaBi8Ti7O27 ceramics, Mater. Res. Bull., 38 (2003) 63-68.
•
41. Y. Shimakawa, Crystal and electronic structures of Bi4-xLaxTi3O12 ferroelectric materials, Appl. Phys. Lett., 79 (2001) 2791-2793.
•
42. P. Papet, J. P. Dougherty, and T. R. Shrout, Particle and grain size effects on the dielectric behavior of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3, J. Mater. Res., 5 (1990) 2902–2909.
•
43.
J.
Petzelt,
Dielectric
Grain-Size Effect
in
High-Permittivity Ceramics,
Ferroelectrics, 400 (2010) 117–134. •
44. D.B. Jannet, M.E. Maaoui and J.P. Mercurio, Ferroelectric
versus Relaxor
Behaviour in Na0.5Bi4.5Ti4O15 -BaBi4Ti4O15 Solid Solutions, J. Electroceram., 11 (2003) 101–106. •
45. C. Miranda, M. E. V. Costa, M. Avdeev, A. L. Kholkin, and J. L. Baptista, Relaxor properties of Ba-based layered Perovskites, J. Eur. Ceram. Soc., 21 (2001) 1303-1306.
•
46. J. Arreguin-Zavala, Cation distribution in the Bi4 − xRExTi3O12 (RE = La, Nd) solid solution and Curie temperature dependence, Mater. Charact., 60, 2009, 219-224.
•
47. A.K. Tagantsev, Vogel-Fulcher relationship for the dielectric permittivity of relaxor ferroelectrics, Phys. Rev. Lett., 72 (1994) 1100-1103.
•
48. C. Ang, Z. Yu and Z. Jing, Impurity-induced ferroelectric relaxor behavior in quantum paraelectric SrTiO3 and ferroelectric BaTiO3, Phys. Rev. B, 61(2000) 957–961.
•
49. M.J. Forbess, S. Seraji, Y. Wu, C. P. Nguyen and G. Z. Cao, Dielectric properties of layered perovskite Sr1-xAxBi2Nb2O9 ferroelectrics (A=La, Ca and x=0,0.1), Appl. Phys. Lett., 76 (2000) 2934-2936.
•
50. A.L. Kholkin, M. Avdeev, M.E.V. Costa, J.L. Baptista and S. N. Dorogovtsev, Dielectric relaxation in Ba-based layered perovskites, Appl. Phys. Lett., 79 (2001) 662-664.
16
•
51. K. Uchino, S. Nomura, Critical exponents of the dielectric constants in diffused-phasetransition crystals, Ferroelectrics, 44 (1982) 55-61.
•
52. A. A. Bokov and Z. G. Ye, Recent progress in relaxor ferroelectrics with perovskite structure, J. Mater. Sci., 41 (2006) 31-52.
•
53. Z.G. Yi, Y.X. Li, J.T. Zeng, Q.B. Yang, D. Wang, Y.Q. Lu and Q.R. Yin, Lanthanum distribution and dielectric properties of intergrowth Bi5−xLaxTiNbWO15 ferroelectrics, Appl. Phys. Lett., 87 (2005) 202901.
•
54. Y. Naguchi, I. Miva and M. Miyayama, Defect Control for Large Remanent Polarization in Bismuth Titanate Ferroelectrics —Doping Effect of Higher-Valent Cations, Jpn. J. Appl. Phys., 39, (2000) L1259-L1262.
•
55. M. Yamaguchi, T. Nagamoto and O. Omoto, Preparation of highly c-axis-oriented Bi4Ti3012 thin films and their crystallographic, dielectric and optical properties, Thin Solid Films, 300 (1997) 299.
•
56. W. T. Lin, T. W. Chiu, H.H. Yu, and S. Lin, Effects of W doping and annealing parameters on the ferroelectricity and fatigue properties of sputtered Bi3.25La0.75Ti3O12 films, J. Vac. Sci. Tech., A, 21 (2007) 787.
•
57. S. B. Desu, P. C. Joshi, X. Zhang, and S. O. Ryu, Thin films of layered-structure (1-x)SrBi2Ta2O9xBi3Ti(Ta1-yNby)O9 solid solution for ferroelectric random access memory devices, Appl. Phys. Lett., 71 (1997) 1041.
•
58. T. Sakai, T. Watanabe, M. Osada, M. Kakihana, Y. Naguchi, and H. Funakubo, Crystal Structure and Ferroelectric Property of Tungsten-substituted Bi4Ti3O12 Thin Films Prepared by Metal-Organic Chemical Vapor Deposition, Jpn. J. Appl.Phys., 42 (2003) 2850.
17
Table 1: Variation in the lattice parameters of BaBi4+δTi4O15 ceramics with varying bismuth content
Sample
a
b
c
(Å)
(Å)
(Å)
BBT
5.452
5.491
41.98
Bi-2
5.461
5.486
Bi-4
5.467
Bi-6
b/a
Measured density
Percentage of
(g/cc)
theoretical density
1.007
7.192
96.24
41.92
1.005
7.235
96.82
5.489
41.92
1.004
7.266
97.23
5.471
5.484
41.92
1.002
7.309
97.80
Bi-8
5.478
5.486
41.92
1.001
7.315
97.89
Bi-10
5.485
5.486
41.92
1.000
7.306
97.76
18
Table 2: Variation of room temperature dielectric constant (ε’RT ), dielectric loss factor (tanδ), maximum value of dielectric constant (ε’m), temperature of dielectric maximum (Tm), the degree of diffuseness (γ) from modified Curie-Wess law, the degree of relaxation behavior (ΔTm)
Sample code BBT Bi-2 Bi-4 Bi-6 Bi-8 Bi-10
ε’RT
tanδ
ε’m
γ
@1MHz
@1MHz
@1MHz
@1MHz
204 208 216 220 226 222
1.65e-2 1.51e-2 1.31e-2 1.10e-2 1.07e-2 1.12e-2
2113 2464 2730 2815 2821 2725
1.79 1.84 1.88 1.93 1.97 1.92
Tm @1MHz (K) 693 683 688 688 691 693
ΔTm (K) 20 20 22 23 23 23
Table 3: Variation of V-F fitting parameters (ωο, Ea and Tf), room temperature dc conductivity (σdc), remnant polarization (Pr ), coercive field (Ec) and piezoelectric charge coefficient (d33) for BaBi4+δTi4O15 ceramics with varying Bi content
ωο
Ea
Tf
RT σdc
Sample code
Ec Pr (µC/cm2)
d33 (pC/N)
(Hz)
(eV)
(K)
(Ω-1cm-1)
BBT
4.09e+09
0.031
642
2.18e-13
6.00
32.9
24
Bi-2
2.08e+08
0.038
632
1.13e-13
7.1
27.5
24
Bi-4
1.92e+09
0.043
626
6.87e-14
8.1
30.0
25
Bi-6
1.09e+09
0.045
626
3.41e-14
9.5
25.6
26
Bi-8
1.51e+08
0.057
617
1.06e-14
12.5
26.2
29
Bi-10
1.88e+08
0.042
632
3.19e-14
9.9
24.9
27
19
(kV/cm)
Fig. 1: X-Ray Diffraction patterns of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
20
BBT
Bi-2
Bi-4
Bi-6
Bi-8
Bi-10
Fig. 2: Scanning Electron Micrographs of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
21
22
Fig. 3: Variation of room temperature dielectric permittivity and loss of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
Fig. 4: Temperature dependent dielectric behavior of BaBi4+δTi4O15 ceramics with varying excess of bismuth content measured at 1 MHz frequency.
23
Fig. 5: Temperature and frequency dependence of dielectric permittivity (ε’) of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
24
Fig. 6: Temperature and frequency dependence of dielectric loss (ε”) of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
25
Fig. 7: Modified Curie-Weiss Law fitted curves for BaBi4+δTi4O15 ceramics with varying excess of bismuth content
26
Fig. 8: Non linear V-F fitting of Tm – ln(1/ω) and Tm1 – ln(1/ω) for real and imaginary parts dielectric permittivity of BaBi4+δTi4O15 ceramics with δ = 8 wt.% (Bi-8 sample).
27
Fig. 9: P-E hysteresis loops of BaBi4+δTi4O15 ceramics with varying excess of bismuth content at 150 C temperature and 50 Hz frequency. o
28
Effect of excess of bismuth doping on dielectric and ferroelectric properties of BaBi4Ti4O15 ceramics Anita Khokhar, Parveen K. Goyal, O.P. Thakur, K. Sreenivas
www.elsevier.com/locate/ceramint
PII: DOI: Reference:
S0272-8842(14)02024-0 http://dx.doi.org/10.1016/j.ceramint.2014.12.103 CERI9704
To appear in:
Ceramics International
Received date: Revised date: Accepted date:
13 November 2014 12 December 2014 17 December 2014
Cite this article as: Anita Khokhar, Parveen K. Goyal, O.P. Thakur, K. Sreenivas, Effect of excess of bismuth doping on dielectric and ferroelectric properties of BaBi4Ti4O15 ceramics, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2014.12.103 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Effect of excess of bismuth doping on dielectric and ferroelectric properties of BaBi4Ti4O15 ceramics Anita Khokhara, Parveen K. Goyalb,*, O. P. Thakurc and K. Sreenivasa,†
a
b
Department of Physics & Astrophysics, University of Delhi, Delhi-110 007, India
Department of Physics, ARSD College, University of Delhi, New Delhi-110 021, India
c
Electroceramics Group, Solid State Physics Laboratory, Lucknow Road, Delhi 110 054, India
Abstract
The effect of excess bismuth oxide Bi2O3 (2-10 wt.%) for processing BaBi4Ti4O15 (BBT) ceramics by solid state reaction has been investigated. The formation of a single phase and a change in the orthorhombic distortion are confirmed with varying excess of bismuth content. Changes in the density are marginal, and use of excess bismuth is seen to promote enhanced grain growth. Dielectric response is improved markedly exhibiting reduced dielectric losses and dispersion over a wide frequency range (10-3 – 106 Hz). A high dielectric constant (ε’~ 226), low loss factor (tan δ~ 0.01) and low dc conductivity (σdc ~ 10-14 Ω-1cm-1) are achieved with an optimum content 6 – 8 wt.% of excess of bismuth oxide. Temperature dependent dielectric data fits well to the modified Curie – Weiss law and the frequency dependent maximum temperatures (Tm and Tm1) corresponding to real and imaginary parts of dielectric permittivity (ε’ and ε”) show a good fit to the non-linear Vogel-Fulcher (V-F) relationship. A clear relaxor behavior is observed with a degree of diffuseness, γ ~ 1.97. Saturated hysteresis loops with high remnant polarization (Pr ~ 12.5 μC/cm2), low coercive fields (Ec ~26 kV/cm) are measured and a high piezoelectric coefficient (d33 ~ 29 pC/N) is achieved in poled BaBi4Ti4O15 ceramics prepared with up to 8 wt.% of excess bismuth oxide. Such a relaxor ferroelectric material with high Curie temperature is useful for high temperature piezoelectric transducer applications.
1
Keywords: Relaxor ferroelectrics; Dielectric behavior; V-F fit; P-E measurements.
Corresponding authors. Tel.: +91-1127667834 E-mail address:
† [email protected] (K.Sreenivas) * [email protected] (P.K. Goyal)
1. Introduction The high dielectric permittivity, diffuse phase transition and strong electrostriction exhibited by relaxor ferroelectric materials have attracted a lot of scientific interest during the last few decades. They have potential use in a wide range of device applications, including sensors, actuators, transducers and memory elements [1-4]. Relaxor materials belonging to the lead-based perovskite compounds, such as lead magnesium niobate (PMN) and (PMN-0.3PbTiO3) solid solutions are well known for their superior properties and have contributed to significant understanding of ferroelectric -relaxor materials. However, in recent years the obvious environmental and health concerns due to the volatile and toxic nature of lead have directed research efforts on alternate lead-free based eco-friendly material compositions. In recent years, bismuth layer-structured ferroelectrics (BLSFs) have attracted a lot of attention for impending applications as high-temperature piezoelectric resonators [5-7] and ferroelectric random access memory devices [8-9], because of their relatively high Curie temperature (Tc) and excellent fatigue endurance property. BLSFs are generally formulated as (Bi2O2)2+(Am−1BmO3m+1)2−, where A is mono-, di-, or trivalent large cation with 12-fold coordination, B is tetra-, penta-, or hexavalent small cation with octahedral coordination, and m varying from 2 to 5 corresponds to the number of BO6 octahedra layers in the perovskite blocks (Am−1BmO3m+1)2−. The crystal structure is described as a sequence of alternating (Bi2O2)2+ and (Am−1BmO3m+1)2− layers stacked along c-axis. [10-11]. Polycrystalline barium bismuth titanate BaBi4Ti4O15 (BBT) with m = 4 is a ferroelectric material belonging to the sub group of BLSFs having Ba2+ and Bi3+ ions at A-site and Ti4+ ions at the B-sites of its structure, and is reported to exhibit a relaxor-like dielectric behavior with reproducible ferroelectric and piezoelectric properties [12].
2
Much of the earlier reported work on BBT ceramics [12-15], synthesized by conventional solidstate reaction route is seen to be overwhelmed with problems associated with high conductivity, increased dielectric losses, and increased dielectric dispersion in ε’ and tanδ with frequency, arising due to oxygen vacancies as a result of bismuth vaporization during high temperature sintering process. The reported ferroelectric and piezoelectric properties show wide variations, and are found to be affected significantly due to compositional variations. Therefore, further improvements in its electrical properties are needed for a better understanding of its true dielectric behavior. A number of suitable substituents/dopants for Bi3+ are available to suppress the effects of bismuth vacancies and oxygen vacancies in BLSFs to improve their electrical properties. There are a few reports wherein improvements in the ferroelectric and/or dielectric properties of BBT ceramics have been described with the substitution of La3+, Er3+, Nd3+, etc. for Bi3+ in BaBi4Ti4O15 [16-21]. The observed improvements in the basic properties with compositional modification due to substitution at the A and B-sites have been always accompanied with a drastic decrease in the phase transition temperature (Tc). Since the high phase transition temperature is the most attractive feature of BLSFs for their potential use in high temperature piezoelectric transducers, a mere improvement in the electrical properties at the cost of lowering the phase transition temperature is undesirable. Thus, there is a need for developing suitable methods to control the bismuth loss and to improve the dielectric and ferroelectric response of BaBi4Ti4O15 without affecting its phase transition temperature. Processing of BaBi4Ti4O15 by the conventional solid-state reaction method demands for a careful optimization of the calcination and sintering characteristics because loss of Bi2O3 during sintering is a potential problem and the resulting non-stoichiometry leads to undesirable conductivity effects which tend to degrade its properties [19, 22]. Addition of an appropriate amount of excess of bismuth oxide in the starting powder mixture is one of the convenient approaches for compensating the loss of bismuth and is helpful to realize a stoichiometric composition [23]. These developments have motivated us to further investigate the electrical properties of BaBi4Ti4O15 when prepared with slight excess of bismuth. The excess of bismuth doping in various BLSFs with improved electrical properties is reported earlier [24-27] but, to the best of our knowledge, no report is available on excess of bismuth doping in BBT ceramics. In this paper, improvements in the dielectric and ferroelectric properties of BBT are shown to be significant with an optimum content of excess of bismuth oxide in the starting powders during ceramic processing.
3
2. Experimental Polycrystalline barium bismuth titanate BaBi4Ti4O15 powders with 0, 2, 4, 6, 8 and 10 wt.% of excess of bismuth oxide, abbreviated as BBT, Bi-2, Bi-4, Bi-6, Bi-8 and Bi-10, respectively) were synthesized by solid-state reaction method. BaCO3, Bi2O3 and TiO2 (purity > 99.5%) were weighed for the required amount and thoroughly mixed and ball milled for 24 h in de-ionized water using Al2O3 balls. The mixture was placed in an alumina crucible and calcined at 900 oC for 5 h in a programmable muffle furnace. After calcination the ball-milled powder was mixed with 2 wt.% polyvinyl alcohol (PVA) as binder. After a thorough mixing, the dry powders were pressed into disks of 10 mm in diameter and ~1 mm thickness at a pressure of about 500 MPa using a hydraulic press and sintered in a closed crucible for 5 h at 1050 oC. The density was measured using the Archimedes method and dense ceramics > 97% of theoretical density were obtained. The crystal structure and phase formation was estimated by X-ray diffraction using a Bruker D-8 Advance X-ray diffractometer with Cu-Kα radiation. The microstructure and grain size distribution on fractured ceramic surfaces was examined by scanning electron microscopy (JEOL JSM – 6510). Sintered pellets were electroded with a fired-on silver paste and were cured at 600 oC for 30 min. The dc conductivity was measured using a 614 Keithley electrometer, and the dielectric measurements were carried out as a function of temperature (30– 600 oC) in the frequency (10-3 – 106 Hz) using a Nova Control impedance spectrometer and Agilent 4284A precision LCR meter. Ceramic samples were poled at 150 °C for 30 min in silicon oil under an applied DC electric field of 35 KV/cm and the piezoelectric charge coefficient, d33 was measured using a Piezotest PM 300 piezometer at 110 Hz under a dynamic force of 0.25 N. The ferroelectric hysteresis loops were measured at 50 Hz frequency and 150 °C temperature using an automated PE loop tracer (Marine India Electricals).
4
3. Results and Discussion
3.1 Structural Properties
Fig. 1, shows the room temperature X-ray diffraction (XRD) patterns of undoped and 2-10 wt.% of excess of bismuth doped BBT ceramics. Single phase formation is confirmed, and the observed XRD peaks are identified to be in good agreement with the standard powder diffraction data (JCPDS No. 350757) belonging to the pure orthorhombic perovskite phase with A21am space group [11, 28-29]. These observations clearly suggest that Bi3+ ions are basically occupied in the pseudo perovskite layer of BBT lattice indicating, and complete solid solubility for the added excess bismuth up to 10 wt. % in the lattice of BaBi4Ti4O15 is in agreement with the earlier reported observations [24-26]. The peak associated with the (119) plane shows the highest intensity, indicating the prepared BBT composition possess a bismuth layered structure with m = 4, and the results are in agreement with earlier observations reporting the strongest diffraction corresponding to (112m+1) reflection in the Aurivillius phase for BLSF compositions [12,15,16,30]. Lattice parameters are determined by least square refinement of powder diffraction data using the Powd-X diffraction data analysis software and their values with associated orthorhombic distortion are listed in table 1. A marginal increase in parameter ‘a’ is noted with increasing excess of bismuth content which may be due to the occupancy of Bi3+ ions in the pseudo perovskite layers. Fig. 2 shows the scanning electron micrographs of the undoped and excess of Bi doped BBT ceramics as seen on the unpolished ceramic surfaces. The microstructural details reveal the formation of randomly oriented plate-like grains of varying size with a plate-like morphology commonly reported in BLSFs. Such a characteristic feature is attributed to the anisotropic nature of the crystal structure in these compounds. The low surface energy along the (00Ɩ) planes promotes grain growth preferentially in the a-b plane leading to a plate-like morphology [12,16,31-32]. Excess bismuth oxide in the starting compositions is known to increase the density and grain size [33]. In the present case, there is an enhancement in the grain size and the density is marginally increased when compared within the limits of measuring error. The addition of excess amount of Bi content is found to be very critical and needs careful optimization, because too much excess Bi2O3 (>10 wt. %) can lead to undesirable effects. Adequate addition of excess Bi2O3 leads to liquid phase formation and benefits in the development of a
5
dense structure and accelerates the rearrangement of grains and mass transfer during the sintering process [34]. Whereas excess Bi beyond an optimum limit creates a superabundant liquid phase at the liquid-solid interface and defeats the densification process, leading to excess Bi aggregation at grain boundaries and promotes the formation of secondary phases in the final reacted product [35]. 3.2 Dielectric Properties
3.2.1 Room Temperature Dielectric Response
Fig. 3(a)-(c) shows the variation in the dielectric constant (ε’), dissipation factor (tanδ) and the dielectric loss (ε”) of undoped and excess of bismuth doped BBT measured over a broad frequency range (10-3 Hz to 106 Hz) at room temperature (300 K). In the case of undoped BBT the room temperature dielectric permittivity (ε’) is a decreasing function of frequency and shows a logarithmic dispersion and a similar trend is observed in all the other samples prepared with increasing Bi content. The observed decrease in the dielectric constant with increasing frequency is a characteristic feature observed in highly disordered dielectrics including relaxor ferroelectrics. It indicates that BBT ceramics exhibit relaxor behavior in their dielectric characteristics, and is related to the existence of polar nanoregions [36]. As the Bi content increases, the room temperature dielectric permittivity when compared at 1MHz increases from 204 to 226, and is maximum for Bi-8 (inset of Fig. 3(a)), and then decreases slightly for the Bi-10 sample. It is interesting to note that the dielectric loss factor (tanδ ~ 0.01) is considerably reduced with increase in Bi content (inset of Fig. 3(c)) and is found to be minimum for Bi-8 sample. In the case of undoped BBT, a strong dispersion in the dielectric loss (< 10 Hz) clearly indicates the influence of space charge polarization effects, and is due to excess oxygen vacancy concentration arising because of bismuth loss. Similar to the highly volatile nature of lead oxide (PbO) in lead zirconate titanate (PZT) ceramic processing, bismuth (or bismuth oxide) particularly in BBT and generally in all BLSFs has relatively low melting and boiling points compared to other elements. Bismuth is vaporized easily during the high temperature sintering step and the loss of bismuth causes many defects and leads to non-stoichiometry. Therefore, addition of an excess amount of bismuth can be helpful to realize stoichiometric composition [23, 37]. In the present case, the oxygen vacancy concentration due to bismuth losses is seen to be restrained when an increasing amount of excess Bi content is added to the system and results in reduced dielectric loss and dispersion as shown in Fig. 3(b).
6
The reduced dispersion in ε” (f) and tanδ (f) in the low frequency region up to Bi-8 corroborates closely with the decreasing values of dc conductivity measured at room temperature (Table 3).
3.2.2 Temperature dependent dielectric response
Fig. 4(a)-(b) show the temperature dependence of the dielectric constant (ε’) and loss tangent (tanδ) of the BBT ceramic measured at 1 MHz. In some of the earlier reported studies an excess of bismuth doping in various BLSFs, has been shown to decrease the ferroelectric phase transition temperature Tc with the increase in bismuth content [24-26]. Normally a change in Tc is related to substitution of two or more cations having different ionic radii [38-39]. However, in the present case we do not observe any change in Tc. Moreover, all the BBT compositions prepared with varying excess Bi content exhibit an increase in the maximum dielectric constant (εm’) at Tc. This can be attributed to the increase in the lattice parameter ‘a’ and associated relaxation in the structural distortion. Larger the lattice parameter ‘a’, higher will be the polarization as ferroelectricity in BLSFs arises by the A-type cation displacement along ‘a’ direction [40-41]. The change in maxima of dielectric permittivity can also be explained on the basis of grain-size effects. The decrease in the maximum dielectric constant with a decrease in grain size can be qualitatively attributed to a low permittivity grain-boundary region (dead layer) possibly composed of partially amorphous oxide or pyrochlore second phase. It is assumed that in the dead layer the polar nanoregions (PNRs) are locked-in by the grain boundaries, and do not contribute to the permittivity [42-43]. The dielectric dispersion is more pronounced in loss factor (tanδ) versus temperature plots shown in figure 4(b), and with increasing bismuth concentration, the loss tangent decreases significantly in the high temperature range (above 350 oC). As discussed above, A-site (bismuth) vacancies and oxygen vacancies, responsible for the dielectric losses in BLSFs [12] are suppressed with excess of Bi3+ doping and hence the dielectric losses are reduced with increase in bismuth concentration. Fig. 5 shows the temperature dependence of the dielectric permittivity, ε’(T) of BBT ceramics prepared with excess of bismuth content (0, 2, 4, 6, 8 and 10 wt. %) at different frequencies. The dielectric constant exhibits a broad diffused transition around the phase transition temperature (Tm) and a strong frequency dispersion of the dielectric permittivity in all the BBT samples is in agreement with
7
earlier observations [15,30,44]. It is believed that the diffused phase transition and the frequency dispersion of dielectric permittivity at Tm are typical characteristics associated with relaxor ferroelectrics. In BBT ceramics without any excess Bi, a large dielectric dispersion in ε’(T) below and above Tm is observed with a decrease in ε’m with increasing frequency, and a shift in Tm is noted with increasing frequency from 1 kHz to 1 MHz. It has been suggested that there is a possibility for Ba2+ ions to substitute not only on the A-site of the perovskite blocks but also enter into the (Bi2O2)2+ layers and can create a local charge imbalance [45]. An inhomogeneous distribution of Ba leads to the formation of defects and results in thermally activated conduction of mobile ions and/or other defects [12] contributing to the observed dielectric dispersion. These observations indicate a relaxor-like behavior in the BBT ceramics. In comparison to the classical relaxor behavior where a complete merging of the dielectric spectra is often observed at T > Tm over a wide frequency range, the undoped BBT ceramics show a merging of ε’(T) above Tm only for a specific range of frequencies (50 kHz to 1 MHz) in a specific temperature range [Tm to (Tm+70) °C] with a large dispersion at low frequencies (<10 kHz). Such a nonmerging of ε’(T) at higher temperatures (T > Tm) may be due to the presence of defects resulting in dielectric losses and higher conductivity at high temperatures. As the excess bismuth concentration increases in BBT ceramics, the dispersion in the temperature region above Tm continuously decreases and for the Bi-8 sample a reasonably good merging of the dielectric spectra above Tm is observed. Dispersion in the high temperature region is again observed in Bi-10 sample. This may be due to the aggregation of additional bismuth on grain boundaries which again disturb the stoichiometry of the system [33]. From the frequency dispersion of the dielectric constant at Tm indicating relaxor behavior, the degree of relaxation (ΔTm) for all the BBT compositions may be described as [46]: ΔTm = Tε’m(1 MHz) − Tε’m(1 kHz)
(1)
Where Tε’m(1 MHz) and Tε’m(1 kHz) are the temperatures where the permittivity shows a maximum at 1 MHz and 1 kHz, respectively. ΔTm of different compositions are listed in Table 2. It can be noted that ΔTm increases from 20 °C (for undoped BBT) to 23 °C (for Bi-10) for a change in the bismuth content from 0 to 10 wt.%. This indicates that degree of relaxation or the frequency dispersion of the ferroelectric – paraelectric phase transition increases with increase in excess bismuth concentration.
8
The dielectric loss (ε” = ε’. tanδ) shown in Fig.6 exhibits diffused peaks and the shifts in the maximum temperature (Tm1) with frequency in ε”(T) could be observed clearly and the sense of frequency shift is similar to that observed for ε’(T). The maximum temperatures (Tm1) of ε”(T) spectra are lower than the maximum temperatures (Tm) of the ε’(T) spectra. Such a behavior in the ε”(T) characteristics has been predicted by Tagantsev [47] for the relaxor materials. The significant increase in ε”(T) at low frequency and high temperature is attributed to the appearance of dc conductivity. For nearly half a century since the discovery of relaxors, several theoretical models, including the super para-electricity, dipolar glass, and random-field models, have been proposed to explain relaxor behaviour. A common point in these models relates to the local order–disorder of the crystal structure that gives rise to polar clusters embedded in the matrix. The relaxor behavior may exist irrespective of whether the matrix is ferroelectric or paraelectric, which is the dynamic response of the polar clusters induced in the system [48]. It is well accepted that the broadness in relaxor originates from the compositional fluctuation and disorder in crystallographic sites when one or more cations occupy the same site in the structure [49]. The origin for the observed relaxor-like behavior has been attributed to the positional disorder of cations on A or B sites of the perovskite blocks [50]. The compositional fluctuations and the structural disorder can result in microscopic heterogeneity with different Curie points and cause the observed diffuseness [12,16]. For a relaxor ferroelectric a modified Curie–Weiss law is used for defining the variation in the reciprocal of the dielectric constant peak at the high temperature side, and the diffuseness of the phase transition is expressed in terms of γ as suggested by Uchino and Nomura [51],
1
ε'
−
1 1 γ = (T − Tm ) ε 'm C
(2)
Where, ε’m is the maximum value of the dielectric constant, C is the Curie-like constant and the value of γ (1< γ <2) represents the degree of diffuseness. For normal ferroelectrics γ = 1, while γ = 2 for relaxors. A plot of ln(1/ε’ −1/ε’m) as a function of ln(T−Tm) at 1 MHz for doped BBT ceramics shows a linear relationship (Fig. 7). The experimental data in these plots show some systematic deviations from the best fit line. However, the coefficient of determination R2 for these plots are found to in the range from 0.96 - 0.99 which show that the experimental data are close to the fitted line within permissible limit of deviations. The diffuseness parameter (γ) is obtained from the slopes of these linear plots and the
9
obtained values of γ are listed in table 2. It is noted that for undoped BBT, γ is found to be ~1.79 and it increases with increase in excess of bismuth concentration and approaches ~1.97 for Bi-8 sample. This can be attributed to the reduction in the number of oxygen vacancies with excess of bismuth doping. Such a transition from relaxor-like behavior to relaxor-ferroelectric behavior has been observed in other BLSFs with reduced oxygen vacancies [16,17]. For further increase in bismuth content beyond 8 wt.%, the bismuth oxide collected near grain boundaries reduces γ to ~1.92 as seen for Bi-10 sample. The rounded peaks in the ε’(T) characteristics accompanied with a strong frequency dispersion on the low-temperature side are characteristic features relating to relaxor ferroelectric. It is known that the frequency dependence of Tm in the relaxor ferroelectrics can be described by a nonlinear Vogel– Fulcher (V-F) relation [52]:
Ea k B (Tm − T f )
ω = ωo exp −
(3)
where, ω = 2πf is the measurement frequency, ωo is the pre-exponential factor (Debye frequency – the attempt frequency of dipole reorientation), Ea is the activation energy describing the relaxation process (i.e. the energy barrier between two equivalent polarization states), kB is the Boltzmann constant and Tf is the static freezing temperature of the polarization fluctuations below which the dynamic reorientation of the dipolar cluster polarization is no longer thermally activated. All the doped BBT compositions fit well to the V-F model. A typical V-F plot of Tm – ln(1/ω) for Bi-8 sample is shown in figure 8(a) which confirms that Bi-8 has a typical relaxor behavior. The V-F fitting parameters Ea, ω0, and Tf for undoped and excess of bismuth doped BBT ceramics shown in Table 3 and are in good agreement with earlier reports [12,14,16,30]. The increase in the value of Ea with increasing Bi3+ concentration confirms the strengthening of the relaxation behavior [8,53]. As stated above, Ea represents the energy barrier between two equivalent polarization states under ac field; therefore, an increase in Ea suggests that polarization becomes more frequency dependent, which is one of the major characteristics of relaxor materials. An increase in Ea from 0.032 eV to 0.057 eV, and a corresponding increase in ΔTm from 20 to 23 K suggest an increase in the relaxation strength with increasing bismuth concentration. The static freezing temperature Tf is found to decrease with increasing Bi content.
10
In the case of relaxor ferroelectrics, Tagantsev [47] emphasizes that: (i) frequency dependent maximum temperatures (Tm and Tm1) noted from ε’(T) and ε”(T) data must follow the V-F relationship, (ii) Tm1 < Tm and (iii) Tf (ε”) < Tf (ε’). Although in some of the BLSF compositions, relaxor-like behavior has been commonly reported but the frequency dependent maximum temperatures Tm1(ω) observed from dielectric loss ε”(T) data has not been fitted to the V-F relationship. In the present study, amongst all the BBT compositions that were prepared, only the Bi-6 and Bi-8 compositions containing the optimum amount of excess of bismuth (6 and 8 wt.%), permit the measured temperatures Tm1(ω) obtained from their respective ε”(T) characteristics for a close fit to the V-F relationship. Fig. 8(b) shows a typical V-F fit of Tm1 – ln(1/ω) for Bi-8 sample. It is interesting to note that in these two compositions the dielectric losses, ε”(T) are seen to be relatively low at higher temperatures in comparison to the other samples and correspondingly their ε’(T) characteristics (Fig. 5) exhibit a good merging of the dielectric constant (ε’) spectra for temperatures T > Tm, and therefore match the requirements expected of a typical relaxor behavior.
3.3 Ferroelectric and Piezoelectric Properties Fig. 9 shows the polarization vs. electric field (P-E) hysteresis loops for undoped and excess bismuth doped BBT ceramics. It is observed that bismuth doped compositions result in the formation of welldefined P-E loops with high remnant polarization (Pr). It is reported earlier that ferroelectric properties are known to be affected by compositional modification, processing conditions and inherent lattice defects such as cation vacancies and oxygen vacancies [54-55]. These inherent defects, accumulated at domain grain boundaries leading to strong pinning of the domain walls, are the major cause for degradation in the ferroelectric properties [56-57] and hence, affect the measured remnant polarization, Pr and coercive field, Ec values. In the BBT-based system improvements in the remnant polarization have also been observed with increasing grain size [58]. In the present study, a significant increase in the grain size is noted with increasing bismuth content, and besides this beneficial change in the microstructure, the oxygen vacancy concentration in the system is also found to be decreased as evidenced from the decreasing values of dc conductivity (Table 3). The Pr values continue to increase with increasing bismuth content and a maximum value of ~12.5 μC/cm2 at a low coercive field Ec ~26 kV/cm are measured. A high piezoelectric coefficient (d33 ~29 pC/N) is achieved in poled BaBi4Ti4O15 ceramics prepared with up to 8 wt.% of excess bismuth oxide, and with further increase in bismuth content (10 wt.%), both the ferroelectric and piezoelectric properties are diminished besides the increase of grain
11
size and reduction of oxygen vacancies. Such a decrease in the properties at the highest bismuth content (10 wt.%) may be related to undesirable segregation effects at the grain boundaries, or its incorporation in (Bi2O2)2+ layers [33,57] and necessitates further investigation.
Conclusions An optimum amount of excess of bismuth oxide content (6 - 8 wt. %) in the starting powders before calcination is found to be beneficial during BaBi4Ti4O15 ceramic processing by solid state reaction. It helps to compensate the loss of bismuth during the high temperature sintering stage. Changes in the density are marginal while grain size, microstructure, and electrical properties show significant improvements. A low dielectric loss (tan δ ~ 0.01) and reduced dispersion over a wide frequency range (10-3 to106 Hz) are measured, and there is considerable decrease in the room temperature dc conductivity. The transition temperatures Tm and Tm1 for ε’(T) and ε”(T) from the frequency and temperature dependent dielectric characteristics fit well to the Vogel-Fulcher relation indicating a clear relaxor behavior. A high remnant polarization (Pr) and low coercive fields (Ec) are measured, and polycrystalline BBT ceramics poled at 35 KV/cm exhibit a high piezoelectric coefficient (d33). Use of excess of bismuth oxide during barium bismuth titanate processing is found to be beneficial and convenient for improving the properties. The high phase transition temperature is maintained, and the improved material properties are useful for potential applications as high temperature piezoelectric devices.
Acknowledgments Authors thank the University of Delhi for the R&D grant and for the use of facilities at the University Science Instrumentation Center (USIC). One of the authors (AK) is also thankful to the Council of Scientific and Industrial Research (CSIR), India for the award of Senior Research Fellowship (SRF).
12
References •
1. F. Bahri and H. Khemakhem, Dielectric properties of Bi-doped Ba0.8Sr0.2TiO3 ceramic solid solutions, Ceramics International, 39 (2013) 7571–7575.
•
2. B. Jaffe, W. Cook and H. Jaffe, Piezoelectric Ceramics, Academic Press, London, 1971.
•
3. G. H. Haertling, Ferroelectricceramics: history and technology, Journal of American Ceramic Society 82 (1999) 797–818.
•
4. S.E. Park and T.R. Shrout, Ultra high strain and piezoelectric behaviour in relaxor based ferroelectric single crystals, Journal of Applied Phys., 82 (1997) 1804–1811.
•
5. L. Bellaiche and D. Vanderbilt, Intrinsic piezoelectric response in perovskite alloys: PMN-PT versus PZT, Physical Review Letters 83 (1999) 1347–1350.
•
6.
L.
Pardo,
A.
Castro,
P.
Millan,
C.
Alemany,
R.
Jimenez
and
B.
Jimenez,
(Bi3TiNbO9)x(SrBi2Nb2O9)1-x Aurivillius type structure piezoelectric ceramics obtained from mechanochemically activated oxides, Acta Mater. 48 (2000) 2421. •
7. S.H. Hong, S. Trolier-Mckinstry and G.L. Messing, Dielectric and Electromechanical Properties of Textured Niobium-Doped Bismuth Titanate Ceramics, J. Am. Ceram. Soc. 83 (2000) 113.
•
8. B.H. Park, B.S. Kang, S.D. Bu, T.W. Noh, J. Lee and W. Jo, Lanthanum substituted bismuth titanate for use in non-volatile memories, Nature (London), 401 (1999) 682-684.
•
9. C.A. Paz de Araujo, J.D. Cuchiaro, L.D. McMillan, M.C. Scott and J.F. Scott, Fatigue-free ferroelectric capacitors with platinum electrodes, Nature (London), 374 (1995) 627-629
•
10. B. Aurivillius, Mixed bismuth oxides with layer lattices, III structure of BaBi4Ti4O15, Arkiv för Kemi 1 (1950) 519–527.
•
11. E.C. Subbarao, A family of frrroelectric Bismuth compounds, J. Phys. Chem. Solids, 23 (1962) 665-667.
•
12. A. Khokkar, M. L. V. Mahesh, A. R. James, P. K. Goyal and K. Sreenivas, Sintering characteristics and electrical properties of BaBi4Ti4O15 ferroelectric ceramics, J. Alloys Compd. 581 (2013)150–159.
•
13. S. Kumar and K.B.R. Varma, Influence of lanthanum doping on the dielectric, ferroelectric and relaxor behaviour of barium bismuth titanate ceramics, J. Phys. D: Appl. Phys. 42 (2009) 075405.
13
•
14. J. D. Bobic, M. M. Vijatovic, S. Greicius, J. Banys and B. D. Stojanovic, Dielectric and relaxor behavior of BaBi4Ti4O15 ceramics, J. Alloys and Comp., 499 (2010) 221-226.
•
15. S. K. Rout, E. Sinha, A. Hussain, J. S. Lee, C. W. Ahn, I. W. Kim, and S. I. Woo, Phase transition in ABi4Ti4O15 (A=Ca, Sr, Ba) Aurivillius oxides prepared through a soft chemical route, J. Appl. Phys., 105 (2009) 024105.
•
16. Anita Khokhar, Parveen K. Goyal, O. P. Thakur , A.K. Shukla and K. Sreenivas, Influence of lanthanum distribution on dielectric and ferroelectric properties of BaBi4-xLaxTi4O15 ceramics, Mater. Chem. and Phys. 2014, DOI: 10.1016/j.matchemphys.2014.11.074.
•
17. D. Peng, H. Zou, C. Xu, X. Wang and X. Yao, Er doped BaBi4Ti4O15 multifunctional ferroelectrics: Up-conversion photoluminescence, dielectric and ferroelectric properties, J. Alloys and Comp., 552 (2013) 463–468.
•
18. J. D. Bobic, M. M. V. Petrovic, B. Banys, B. D. Stojanovic, Effect of La substitution on the structural and electrical properties of BaBi4−xLaxTi4O15, Ceram. Intl. 39 (2013) 8049-8057.
•
19. A. Chakrabarti and J. Bera, Effect of La-substitution on the structure and dielectric properties of BaBi4Ti4O15 ceramics, J. Alloys Comp., 505 (2010) 668-674.
•
20. P. Fang, H. Fan, J. Li, L. Chen and F. Liang, The microstructure and dielectric relaxor behavior of BaBi4−xLaxTi4O15 ferroelectric ceramics J. Alloys Comp., 497 (2010) 416-419.
•
21. C.L. Diao, H.W. Zheng, Y.Z. Gu, W.F. Zhang, L. Fang, Structural and electrical properties of four-layers Aurivillius phase BaBi3.5Nd0.5Ti4O15 ceramics, Ceramics International, 40 (2014) 5765– 5769.
•
22. Z.Z. Lazarevic, N. Z. Romcevic and J. D. Bobic, Study of ferroelectric BaBi4Ti4O15 obtained via mechanochemical synthesis, Optoelectronics and Adv. Mater., 3 (2009) 700-703.
•
23. H. Watanabe, T. Mihara, H. Yoshimori, C. A. Paz de Araujo, Preparation of Ferroelectric Thin Films of Bismuth Layer Structured Compounds, Jpn. J. Appl. Phys., 34 (1995) 5240-5244.
•
24. J. Lee, B. Park and K. Hong, Effect of excess Bi2O3 on the ferroelectric properties of SrBi2Ta2O9 ceramics, J. Appl. Phys., 88 (2000) 2825-2829.
•
25. T. Chen, T. Li, X. Zhang and S. B. Desu, The effect of excess bismuth on the ferroelectric properties of SrBi2Ta2O9 thin films, J. Mater. Res., 12 (1997) 1569-1575.
14
•
26. A.Z. Simoez, G.C.C. da Costa, M.A. Ramirez, J.A. Varela, E. Longo, Effect of the excess of bismuth on the morphology and properties of the BaBi2Ta2O9 ceramics, Mater. Lett. 59 (2005) 656–661.
•
27. A.K. Nath and N. Medhi, Piezoelectric properties of environmental friendly bismuth doped barium titanate ceramics, Mater. Lett., 73 (2012) 75-77.
•
28. J. Tellier, P. Boullay, M. Manier and D. Mercurio, J. Solid State Chem., A comparative study of the Aurivillius phase ferroelectrics CaBi4Ti4O15 and BaBi4Ti4O15, 177 (2004) 1829-1837.
•
•
29. B.J. Kennedy, Y. Kubota, B. A. Hunter, Ismunandar, K. Kato, Structural phase transitions in the layered bismuth oxide BaBi4Ti4O15, Solid State Commun. 126 (2003) 653–658. 30. P. Fang, H. Fan, Z. Xi and W. Chen, Studies of structural and electrical properties on four-layers Aurivillius phase BaBi4Ti4O15, Solid State Commun., 152 (2012) 979-983.
•
31. J. D. Bobic, M.M.V. Petrovic, J. Banys, B.D. Stojanovic, Electrical properties of
niobium doped barium bismuth-titanate ceramics, Mater. Res. Bull., 47 (2012) 1874-1880. •
32. J.A. Horn, S.C. Zhang, U. Selvaraj, G.L. Messing, and S. T.-McKinstry, Templated Grain Growth of Textured Bismuth Titanate, J. Am. Ceram. Soc., 82 (1999) 921-926.
•
33. H. Liu, Q. Li, J. Ma, and X. Chu, Effects
of Bi3+ content and grain size on electrical
properties of SrBi2Ta2O9 ceramic, Mater. Lett. 76 (2012) 21-24 •
34. H.Q. Wang , X. W. Zhang , Y.J. Dai, Sintering
behavior and electrical properties of
the (Li, Ta, Sb) modified (K, Na)NbO3 lead-free ceramics with KNbO3 as sintering aid, Mater. Lett., 67 (2012) 145–147.
•
35. R. Jain, V. Gupta, K. Sreenivas, Sintering characteristics and properties of sol gel derived Sr0.8Bi2.4Ta2.0O9 ceramics, Mater. Sci. Engg. B, 78 (2000) 63-69.
•
36. I. Rychetsky, S. Kamba, V. Porokhonskyy, A. Pashkin, M. Savinov, V. Bovtun, J. Petzelt, M. Kosec and M. Dressel, Frequency-independent dielectric losses (1/f noise) in PLZT relaxors at low temperatures, J. Phys.: Cond. Mater., 15 (2003) 6017-6030.
•
37. H. Irie and M. Miyayama, Dielectric and ferroelectric properties of SrBi4Ti4O15 single crystals, Appl. Phys. Lett., 79 (2001) 251-253.
15
•
38. V. K. Seth and W. A. Schulze, Grain-Oriented Fabrication of Bismuth Titanate Ceramics and Its Electrical Properties, IEEE Trans. Ultrasonics, Ferroelectrics, Freq. Con. Soc. 36 (1989) 41–49.
•
39. D.Y.Saurez, I.M. Reaney and W.E. Lee, Relation between tolerance factor and T-c in Aurivillius compounds, J. Mater. Res., 16(11) (2001) 3139-3149.
•
40. R.Z. Hou, X.-M. Chen, Dielectric properties of La3+ substituted BaBi8Ti7O27 ceramics, Mater. Res. Bull., 38 (2003) 63-68.
•
41. Y. Shimakawa, Crystal and electronic structures of Bi4-xLaxTi3O12 ferroelectric materials, Appl. Phys. Lett., 79 (2001) 2791-2793.
•
42. P. Papet, J. P. Dougherty, and T. R. Shrout, Particle and grain size effects on the dielectric behavior of the relaxor ferroelectric Pb(Mg1/3Nb2/3)O3, J. Mater. Res., 5 (1990) 2902–2909.
•
43.
J.
Petzelt,
Dielectric
Grain-Size Effect
in
High-Permittivity Ceramics,
Ferroelectrics, 400 (2010) 117–134. •
44. D.B. Jannet, M.E. Maaoui and J.P. Mercurio, Ferroelectric
versus Relaxor
Behaviour in Na0.5Bi4.5Ti4O15 -BaBi4Ti4O15 Solid Solutions, J. Electroceram., 11 (2003) 101–106. •
45. C. Miranda, M. E. V. Costa, M. Avdeev, A. L. Kholkin, and J. L. Baptista, Relaxor properties of Ba-based layered Perovskites, J. Eur. Ceram. Soc., 21 (2001) 1303-1306.
•
46. J. Arreguin-Zavala, Cation distribution in the Bi4 − xRExTi3O12 (RE = La, Nd) solid solution and Curie temperature dependence, Mater. Charact., 60, 2009, 219-224.
•
47. A.K. Tagantsev, Vogel-Fulcher relationship for the dielectric permittivity of relaxor ferroelectrics, Phys. Rev. Lett., 72 (1994) 1100-1103.
•
48. C. Ang, Z. Yu and Z. Jing, Impurity-induced ferroelectric relaxor behavior in quantum paraelectric SrTiO3 and ferroelectric BaTiO3, Phys. Rev. B, 61(2000) 957–961.
•
49. M.J. Forbess, S. Seraji, Y. Wu, C. P. Nguyen and G. Z. Cao, Dielectric properties of layered perovskite Sr1-xAxBi2Nb2O9 ferroelectrics (A=La, Ca and x=0,0.1), Appl. Phys. Lett., 76 (2000) 2934-2936.
•
50. A.L. Kholkin, M. Avdeev, M.E.V. Costa, J.L. Baptista and S. N. Dorogovtsev, Dielectric relaxation in Ba-based layered perovskites, Appl. Phys. Lett., 79 (2001) 662-664.
16
•
51. K. Uchino, S. Nomura, Critical exponents of the dielectric constants in diffused-phasetransition crystals, Ferroelectrics, 44 (1982) 55-61.
•
52. A. A. Bokov and Z. G. Ye, Recent progress in relaxor ferroelectrics with perovskite structure, J. Mater. Sci., 41 (2006) 31-52.
•
53. Z.G. Yi, Y.X. Li, J.T. Zeng, Q.B. Yang, D. Wang, Y.Q. Lu and Q.R. Yin, Lanthanum distribution and dielectric properties of intergrowth Bi5−xLaxTiNbWO15 ferroelectrics, Appl. Phys. Lett., 87 (2005) 202901.
•
54. Y. Naguchi, I. Miva and M. Miyayama, Defect Control for Large Remanent Polarization in Bismuth Titanate Ferroelectrics —Doping Effect of Higher-Valent Cations, Jpn. J. Appl. Phys., 39, (2000) L1259-L1262.
•
55. M. Yamaguchi, T. Nagamoto and O. Omoto, Preparation of highly c-axis-oriented Bi4Ti3012 thin films and their crystallographic, dielectric and optical properties, Thin Solid Films, 300 (1997) 299.
•
56. W. T. Lin, T. W. Chiu, H.H. Yu, and S. Lin, Effects of W doping and annealing parameters on the ferroelectricity and fatigue properties of sputtered Bi3.25La0.75Ti3O12 films, J. Vac. Sci. Tech., A, 21 (2007) 787.
•
57. S. B. Desu, P. C. Joshi, X. Zhang, and S. O. Ryu, Thin films of layered-structure (1-x)SrBi2Ta2O9xBi3Ti(Ta1-yNby)O9 solid solution for ferroelectric random access memory devices, Appl. Phys. Lett., 71 (1997) 1041.
•
58. T. Sakai, T. Watanabe, M. Osada, M. Kakihana, Y. Naguchi, and H. Funakubo, Crystal Structure and Ferroelectric Property of Tungsten-substituted Bi4Ti3O12 Thin Films Prepared by Metal-Organic Chemical Vapor Deposition, Jpn. J. Appl.Phys., 42 (2003) 2850.
17
Table 1: Variation in the lattice parameters of BaBi4+δTi4O15 ceramics with varying bismuth content
Sample
a
b
c
(Å)
(Å)
(Å)
BBT
5.452
5.491
41.98
Bi-2
5.461
5.486
Bi-4
5.467
Bi-6
b/a
Measured density
Percentage of
(g/cc)
theoretical density
1.007
7.192
96.24
41.92
1.005
7.235
96.82
5.489
41.92
1.004
7.266
97.23
5.471
5.484
41.92
1.002
7.309
97.80
Bi-8
5.478
5.486
41.92
1.001
7.315
97.89
Bi-10
5.485
5.486
41.92
1.000
7.306
97.76
18
Table 2: Variation of room temperature dielectric constant (ε’RT ), dielectric loss factor (tanδ), maximum value of dielectric constant (ε’m), temperature of dielectric maximum (Tm), the degree of diffuseness (γ) from modified Curie-Wess law, the degree of relaxation behavior (ΔTm)
Sample code BBT Bi-2 Bi-4 Bi-6 Bi-8 Bi-10
ε’RT
tanδ
ε’m
γ
@1MHz
@1MHz
@1MHz
@1MHz
204 208 216 220 226 222
1.65e-2 1.51e-2 1.31e-2 1.10e-2 1.07e-2 1.12e-2
2113 2464 2730 2815 2821 2725
1.79 1.84 1.88 1.93 1.97 1.92
Tm @1MHz (K) 693 683 688 688 691 693
ΔTm (K) 20 20 22 23 23 23
Table 3: Variation of V-F fitting parameters (ωο, Ea and Tf), room temperature dc conductivity (σdc), remnant polarization (Pr ), coercive field (Ec) and piezoelectric charge coefficient (d33) for BaBi4+δTi4O15 ceramics with varying Bi content
ωο
Ea
Tf
RT σdc
Sample code
Ec Pr (µC/cm2)
d33 (pC/N)
(Hz)
(eV)
(K)
(Ω-1cm-1)
BBT
4.09e+09
0.031
642
2.18e-13
6.00
32.9
24
Bi-2
2.08e+08
0.038
632
1.13e-13
7.1
27.5
24
Bi-4
1.92e+09
0.043
626
6.87e-14
8.1
30.0
25
Bi-6
1.09e+09
0.045
626
3.41e-14
9.5
25.6
26
Bi-8
1.51e+08
0.057
617
1.06e-14
12.5
26.2
29
Bi-10
1.88e+08
0.042
632
3.19e-14
9.9
24.9
27
19
(kV/cm)
Fig. 1: X-Ray Diffraction patterns of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
20
BBT
Bi-2
Bi-4
Bi-6
Bi-8
Bi-10
Fig. 2: Scanning Electron Micrographs of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
21
22
Fig. 3: Variation of room temperature dielectric permittivity and loss of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
Fig. 4: Temperature dependent dielectric behavior of BaBi4+δTi4O15 ceramics with varying excess of bismuth content measured at 1 MHz frequency.
23
Fig. 5: Temperature and frequency dependence of dielectric permittivity (ε’) of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
24
Fig. 6: Temperature and frequency dependence of dielectric loss (ε”) of BaBi4+δTi4O15 ceramics with varying excess of bismuth content
25
Fig. 7: Modified Curie-Weiss Law fitted curves for BaBi4+δTi4O15 ceramics with varying excess of bismuth content
26
Fig. 8: Non linear V-F fitting of Tm – ln(1/ω) and Tm1 – ln(1/ω) for real and imaginary parts dielectric permittivity of BaBi4+δTi4O15 ceramics with δ = 8 wt.% (Bi-8 sample).
27
Fig. 9: P-E hysteresis loops of BaBi4+δTi4O15 ceramics with varying excess of bismuth content at 150 C temperature and 50 Hz frequency. o
28