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ISSN : 1225-0562(Print)
ISSN : 2287-7258(Online)
Korean Journal of Materials Research Vol.29 No.8 pp.469-476
DOI : https://doi.org/10.3740/MRSK.2019.29.8.469

Dielectric and Electrical Characteristics of Lead-Free Complex Electronic Material: Ba0.8Ca0.2(Ti0.8Zr0.1Ce0.1)O3

Manisha Sahu1, Sugato Hajra1, Ram Naresh Prasad Choudhary2
1Department of Electronics and Instrumentation
2Department of Physics, Siksha ‘O’ Anusandhan University, Bhubaneswar-751030, India
Corresponding author E-Mail : manishafl@outlook.com (M. Sahu, Siksha ‘O’ Anusandhan Univ.)
April 14, 2019 May 21, 2019 August 6, 2019

Abstract


A lead-free bulk ceramic having a chemical formula Ba0.8Ca0.2(Ti0.8Zr0.1Ce0.1)O3 (further termed as BCTZCO) is synthesized using mixed oxide route. The structural, dielectric, impedance, and conductivity properties, as well as the modulus of the synthesized sample are discussed in the present work. Analysis of X-ray diffraction data obtained at room temperature reveals the existence of some impurity phases. The natural surface morphology shows close packing of grains with few voids. Attempts have been made to study the (a) effect of microstructures containing grains, grain boundaries, and electrodes on impedance and capacitive characteristics, (b) relationship between properties and crystal structure, and (c) nature of the relaxation mechanism of the prepared samples. The relationship between the structure and physical properties is established. The frequency and temperature dependence of the dielectric properties reveal that this complex system has a high dielectric constant and low tangent loss. An analysis of impedance and related parameters illuminates the contributions of grains. The activation energy is determined for only the high temperature region in the temperature dependent AC conductivity graph. Deviation from the Debye behavior is seen in the Nyquist plot at different temperatures. The relaxation mechanism and the electrical transport properties in the sample are investigated with the help of various spectroscopic (i.e., dielectric, modulus, and impedance) techniques. This lead free sample will serve as a base for device engineering.



초록


    © Materials Research Society of Korea. All rights reserved.

    This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

    1. Introduction

    During recent years, there is an increasing demand for the development of ferroelectric and piezoelectric based functional materials in the electronic industries.1) The lead zirconate titanate(PZT) are considered to be the most superior device material due to its excellent electromechanical, piezoelectric and ferroelectric properties.2) However, in view of the toxic nature, lead based materials during processing and handling sintering may cause environmental hazards. Therefore, the use of lead based materials for the fabrication of devices in the future has been put on hold. In view of this, the researchers raised their interest in the formulation and fabrication of leadfree the electro-ceramic materials which is eco-friendly with high humidity sensitive electrical properties comparable to those of Pb-based ones. Such type of lead-free ceramics including BaTiO3(BTO), Na0.5Bi0.5TiO3(NBT), K0.5Na0.5NbO3 (KNN) and their solid solutions are well known.3-5) Wang et al.6) have reported a new promising lead-free ceramic (Ba1-xCax)(ZryTi1-y)O3(BCZT) system with high piezoelectric properties of 650 pC/N(1540 oC sintered sample6)). Sun et al.7) have prepared Ba0.85Ca0.15Ti0.90Zr0.10O3 ceramics at high temperatures (1,320 to 1,560 oC for 2 hours) and showed its phase transition and piezoelectric coefficient.7) Xiong reported the enhanced piezoelectric and dielectric properties of BCZT system prepared by gel-casting route.8) It is well known that BaTiO3(BTO) is a ferroelectric with low loss factor, reduced leakage current and high dielectric constant(2,000 with Tc at 120 ºC) CaTiO3 is an incipient ferroelectric or quantum paraelectric in which permittivity rises with decrease in temperature and exhibits its maxima at very low temperatures. The reason of using CeO2 as an additive because it provides structural stability as well as reduction of the sintering temperature.

    Detailed studies of the reported literatures show that only the dielectric and piezoelectric properties near the morphological phase boundary(MPB) of Ce doped BCZT system has been reported so far. The detailed electrical properties(impedance, modulus, transport properties) are still unclear. Therefore, in this communication, we have planned study a correlation between the structure and physical properties, and understanding of the electrical parameters using combined complex modulus/impedance spectroscopy tools in prepared BCTZCO.

    2. Material Preparation and Characterization Techniques

    The lead-free complex electronic material BCTZCO was prepared through high temperature mixed oxide route. This ceramic processing route has wide advantages like cost effective, environment friendly and no unwanted waste is produced after the process is complete. The solid reactants in this process react chemically in the absence of solvent at high temperatures, yielding a product which is stable. The highly pure (>99.5 %) carbonates and oxides, such as BaCO3(M/S CDH Limited), CaCO3 (M/S Loba Chemie) Co Limited), TiO2(M/S Loba Chemie Co Ltd), ZrO2(M/S Himedia), CeO2(M/S Loba Chemie Co Ltd) were brought. To fabricate the material, the required amount of starting compounds (in powder form) was weighted carefully as per the stoichiometry of the BCTZCO system. The starting powders were mixed with dry grinding followed by wet grinding using agate motor and pestle. The uniformly mixed powder was then calcined at an optimized temperature of 1,100 ºC for 4 h (heating rate 5 ºC/min, alternative heating and cooling methodology). The powder was then put in a labeled sample holder and have a smooth surface using a glass slide. To affirm the phase and formation of required system, the calcined powder was analyzed through XRD(X-ray diffraction) technique. The homogenized mixture of calcined powder was undergone for more grounding to prepare fine powder. The organic binder PVA(polyvinyl alcohol, 5 weight %) was mixed and then the powder was pressed into disc shape (pellet) by using a hydraulic press. The green pellets were sintered at 1,120 ºC by placing them in a boat for 4 h in a furnace. The bulk density of the pressed pellet is noted to be 7.82 gm/cm3 and the theoretical density is 8.05 gm/cm3. The XRD data were used to determine the sample density by comparing the theoretical density with the measured bulk density of the sample pellet obtained after sintering. The polished sintered pellet specimen was obtained which is painted with highly conducting silver paint to derive its electrical properties. The phase formation of BCTZCO was determined using diffraction data obtained from Rigaku Smartlab Xray diffractometer at a scanning rate of 2º/min over Braggs angle (20 ≤ 2θ ≤ 80, radiation = Cukα). The surface structure was obtained employing the FESEM (M/S Quanta-200 FEG, Netherland). For the SEM analysis one of the pellet was sputtered with gold to remove the charging effect. For measuring the various electrical parameters (impedance, loss tangent, capacitance) and other dielectric properties a computer-controlled impedance or LCR meter (UK made N4L PSM) has been used over a range of frequency (103 Hz-106 Hz) and temperature (25 ºC to 450 ºC).

    3. Results and Discussion

    3.1 Structural and surface morphology

    Fig. 1 represents XRD pattern of the calcined powder of BCTZCO. In this pattern the a black line corresponds to the experimental diffraction data at room temperature of the sample. The pattern and phase analysis of the material are similar to those of reported one.9) The experimental data is found to be matched with the previous reference pattern of BaTiO3(red stick) and CeO2(blue stick) bearing JCPDS card no. 01-079-226510) and 00-043-100211) respectively. The marks(#, *) represent the presence of BaTiO3 and CeO2 phases in the pattern. The crystallographic information of both the phases is compared, and shown in Table 1. The surface morphology and densification is illustrated in Fig. 2. It suggests that the pattern of grains is irregular and non-uniform having varying sizes between 580 nm-2,200 nm. It is clearly seen from the micro-graph that the maximum grains are densely packed throughout the surface with some voids. The difference in the size and shape of the grains suggests the formation of a good quality sample of BCTZCO.

    3.2 Dielectric analysis

    Study of dielectric properties offers the relevant data regarding the permittivity and the tangent loss factor of a specimen under definite orientation of the electric field and operated frequency relying on the kinds of polarization accountable for the relaxation. Fig. 3(a, b) discusses the frequency dependent relative dielectric permittivity(ɛr) and tangent loss factor(tan δ) of the BCTZCO at selected temperatures. The plot illustrates that with frequency rise, both ɛr and tan δ, falls and this occurs due to the various polarization mechanisms which is the well-known trend of dielectrics.12) The polarization is created in the materials when the field is applied leading to the electronic exchange of ions by locally displacing the electrons in the direction of the field. At low frequencies, dielectric permittivity and tangent loss depend on temperature that may be due to the presence of an ion core type of polarization in the sample. However, at high temperatures, they are temperature dependent which lead to rise with an increase in temperature and decreases with an increase in frequency, which occurs due to the inter-facial polarization mechanism. The contribution of space charge and dipolar polarization is significant towards dielectric permiitivity at higher temperature and low frequency. The high value of dielectric permittivity is developed due to blocking of charge carriers at the electrodes. At high frequency, curves of almost all the temperatures the permittivity merges/coincides because the electronic polarization is significantly active ceasing all other polarizations(space charge, dipolar, etc.).13) The tangent loss(tan δ) increases slowly at lower temperatures, but is relatively higher at higher temperatures [Fig. 3(b)]. The tangent loss behavior at high temperatures may be related to some vacancies and defects in the sample, which enhance the conductivity/ scattering of charge carriers.

    Fig. 3(c) presents temperature dependent permittivity at selected frequencies. The Curie temperature(Tc) at 1 kHz is found at 320 ºC, and then it shifts backward with diffused phase transition for all higher frequencies. The space charge polarization or the electron phonon interaction rises the dielectric permittivity with an increment of temperature.14)

    3.3 Complex electrical impedance

    Impedance spectroscopy is a powerful technique to examine of dielectric and electrical characteristics of materials.15) This technique has extensively been used to study the influence of interface in the polycrystalline ionic and dielectric conductors, grains and grain boundaries.16) The dynamics of the ionic movement in various solids can also be easily understandable through this tool. The response of electrode material on the application of an AC electric field was measured and illustrated in several formats (frequency versus Z′/Z′′ and Nyquist plot) in Fig. 4(a, b, c). Fig. 4(a) shows the frequency dependence of the real component of impedance. The real component of impedance can be expressed with the help of equation:

    Z ' = R 1 + ( ω τ ) 2 .

    It is observed that there is a decreasing trend in the value of Z' with the rise in both temperature and frequency. It can be seen that at low frequency, the Z' value is higher, and on increasing in frequency, it declines and merges at the higher frequency. The Z' value coincides with each other for all temperatures at high frequencies due to the release of space charge.17)

    Fig. 4(b) illustrates the frequency dependence of the imaginary component of impedance. The imaginary component of impedance can be expressed with the help of equation:

    Z '' = ω R τ 1 + ( ω τ ) 2 .

    The Z'' versus frequency graph shows the presence of immobile defects at the low temperatures and the defects, anomalies/ oxygen vacancies in the high-temperature region, which is accountable for relaxation process generally.18) The electrical conductivity is higher due to the hopping of oxygen vacancies among the localized site.19) The complex impedance (Z'') plot suggests that at low-frequency slopes are strongly dependent on temperature while at high-frequencies, slopes do not depend on temperature.

    Fig. 4(c) illustrates the Nyquist Plot (Z′ vs Z′′) of the prepared sample. The impedance data obtained from the experimental analysis contains both real (Z') and imaginary (Z") components of impedance, which is validated by seeing a good agreement between calculated and experimental values of complex impedance of non linear fitting using the Zsimpwin software package. It depicts the presence of single conduction process in the sample. Consequently, the grain boundary outcome does not contribute to their electrical as well as impedance components. However the semicircle displays some degree of depression trend on the real axis showing the presence of non-Debye type relaxation processes in the prepared sample.20) The values of grain resistance(R) and bulk capacitance(C) are calculated at various temperatures by using the fitted curves along with the equivalent electrical circuit (as shown in Table 2). A corresponding circuit involves parallel association of both R, Q and C where the term Q denotes CPE(constant phase element). The CPE element is introduced in the circuit instead of C because it deviates from the ideal Debye type of relaxation. In this case, we obtained a non-Debye type of relaxation process, and with an increase in temperature, the grain resistance reduces showing semiconductor behavior(based on negative temperature coefficient resistance(NCTR) nature) in the prepared sample. The αdc (DC conductivity) also sheds light on the arbitrary steps of ion diffusion throughout by accomplishing repeated hops between charge- compensating sites.21) The obtained thickness and area of the pellet as well as the grain resistance helps to fetch the DC conductivity.

    3.4 Conductivity

    The ionic or electronic conductor types of dielectric material can be categorized on the basis of the type of ions (cations/anions) or charge carriers (holes/electrons) that dominate during the conduction process. The effect of electric field causes movement of loosely bound ions leading to the conduction phenomena.

    Fig. 5(a) presents frequency dependence of AC conductivity. It is helpful to understand the hopping dynamics of ions. The AC conductivity of the materials can be calculated using the well-known expression:

    σ ( ω ) = σ a c + σ d c σ a c = σ ( ω ) σ d c = ω ε ε 0 tan δ σ d c

    It is clearly seen that the trend of conductivity has dispersion in the range of low-frequency, while with enhancement of frequency, the conductivity increases constantly. The well known universal law of Jonscher22) σAC(ω) = σDC + Aω n, symbols have their same mean was applied to investigate the existence of hopping mechanism in the compound. The NTCR behavior can be depicted from the analysis, as the value of AC conductivity rises with a rise in both temperature and frequency. It is seen that the equation σAC = Aω n fits well with the AC conductivity plot in the high-frequency region, but a small portion towards low frequency are not fitted. The reason persisting behind it is the relaxation process is associated with grain at higher frequency side while at the low- frequency side, there is the presence of some blocking effect of grain boundary. The equation σAC = Aω n; where the constant dependent temperature is represented by A, ω shows the angular frequency and the power law exponent is given by n, where n is between o < n < 1. Here from (inset) it is seen that the value of n first decreases and thereby increases with rise in temperature depicting an overlapping large Polaron tunnel mechanism (OLPT).23)

    Fig. 5(b) presents temperature dependent AC conductivity. In this plot the activation energy is calculated using the relation established by Arrhenius: σac = σoexp(-Ea/KT), where symbols have similar meaning. The linear fit is applied in the high temperature region for all selected frequencies to obtain the activation energies. The values at 1 kHz, 10 kHz, 100 kHz, 500 kHz are obtained as 0.025, 0.017, 0.007, 0.006 eV respectively. The decrease in activation energy may be due to the enhancement of the electron movement between localized states with the increase in frequency of applied electric field.24,25)

    3.5 Complex electrical modulus

    The electric modulus analysis of a compound is a very precise technique to identify grain boundary effects, electrode polarization, bulk features and electrical conductivity appliance under the effect of operating frequency and temperature.26) The following general formula of electrical modulus could be treated as to evaluate both real (M′) and imaginary (M′′) component,

    M' = η [ ( ωRC ) 2 1 + ( ωRC ) 2 ] = η [ ω 2 τ 2 1 + ω 2 τ 2 ] M'' = η [ ωRC 1 + ( ωRC ) 2 ] = η [ ωτ 1 + ω 2 τ 2 ]

    Fig. 6(a) shows the frequency dependent real part of modulus at selected temperature. The value of M’ approaches to a smaller value (almost zero) at lower frequency, and with increasing temperature, the curve of M’ tends to increase and coincides with each other at the high frequency side illustrating the nonexistence of electrode polarization effect in the prepared sample. This phenomenon might arise because of the presence of short mobility (charge carriers), ionic and electrode polarization.27)

    Fig. 6(b) shows the frequency dependent imaginary part of modulus at selected temperature. The value of M'' rises at lower frequency and at higher frequency it coincides/ merges. The temperature dependent asymmetrical M'' peak broadening, shows the existence of non-Debye type of relaxation behavior, which suggest the distribution of relaxation with different time constants.28)

    4. Conclusion

    The lead-free complex electronic system BCTZCO is fabricated using cost effective mixed oxide technique. The XRD (X-ray diffraction) pattern illustrated that the fabricated material has both BaTiO3 and CeO2 phases. The SEM analysis of sintered pellet shows close packing of grains with few voids. The prepared sample is both frequency and temperature dependent that can be signified from the electrical properties. It obeys Jonscher’s universal power law which can be depicted observing the nature of frequency dependent AC conductivity spectra. An equivalent circuit has been used to illuminate the electrical phenomenon occurrence inside the synthesized sample. Comprehensive analysis of impedance characteristics along with the related parameters of the sample illustrated the existence of grain effects, non-Debye type of relaxation phenomena, the subsistence of semiconductor nature (NTCR -type behavior) in the prepared sample. Modulus and impedance spectroscopic analysis tells about the conductivity mechanism and relaxation phenomenon.

    Acknowledgement

    The authors like to sincerely thanks Professor KL Yadav, Department of Physics, IIT Roorke for carrying out the SEM experiment in his laboratory.

    Figure

    MRSK-29-8-469_F1.gif

    X-Ray diffraction pattern for BCZTCO sample at room temperature.

    MRSK-29-8-469_F2.gif

    SEM micrograph of natural surface of sintered BCZCTO pellet.

    MRSK-29-8-469_F3.gif

    Frequency dependence of (a) dielectric permittivity, (b) tangent loss and (c) temperature dependence of the dielectric permittivity.

    MRSK-29-8-469_F4.gif

    Frequency dependence of (a) real part, (b) inset the imaginary part of impedance and (c) Nyquist plot of BCZTCO sample at selected temperatures and (inset) equivalent circuit model.

    MRSK-29-8-469_F5.gif

    (a) Variation of AC conductivity with frequency at selected temperatures, (inset) variation of frequency exponent (n) with temperature and (b) temperature dependence of AC conductivity.

    MRSK-29-8-469_F6.gif

    Frequency dependence of (a) real part and (b) imaginary part of modulus.

    Table

    Crystallographic data of the standard reference pattern (i.e. BaTiO3 and CeO2)

    The fitted parameters of grain capacitance (Cg), and grain resistance (Rg) for BCZTCO at selected temperatures (from Nyquist plot)

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