Exploring the conduction mechanism of multiferroic SrM–BCZT composite

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Exploring the conduction mechanism of multiferroic SrM–BCZT composite Muhammad Asif Rafiqa,∗, Tanveer Uz Zamana, Hafiz Ahmad Ishfaqa,b, Adnan Maqboola,∗∗, Moaz Waqara,c, Qaisar Khushi Muhammada,d, Aneeq Anjuma, Abdul Warisa a

Department of Metallurgical and Materials Engineering, University of Engineering and Technology (UET), Lahore, 54890, Pakistan Fuel Cell Research Center, Korea Institute of Energy Research, Daejeon, 34129, Republic of Korea Department of Materials Science and Engineering, National University of Singapore, 117574, Singapore d Institute of Materials Science, Technische Universität Darmstadt, Darmstadt, 64287, Germany b c

A R T I C LE I N FO

A B S T R A C T

Keywords: Multiferroic composite AC conductivity Impedance spectroscopy NTCR

Multiferroic composite of strontium hexaferrite (SrM, SrFe12O19) and barium calcium zirconium titanate (BCZT, Ba0·85Ca0·15Zr0·1Ti0·9O3) have been investigated and the conventional solid-state reaction (SRR) process is utilized to prepare the SrM–xBCZT composite (where x = 0.0, 0.10, 0.20 and 0.30). Diphase stoichiometry was confirmed by the phase analysis, for undoped composition presence of ferromagnetic SrM with complex magnetoplumbite structural phase and hexagonal structural phase with P63/mmc space group was confirmed. Further, doping of BCZT suppressed the secondary phase appeared (Fe2O3) and caused increase in lattice constants. Fourier Transform Infra–Red study revealed the presence of absorption bands of phases. Microstructure analysis suggests the formation of dense composite and confirmed the effect of BCZT as sintering aid. Impedance spectroscopy study suggested two electrical contributions in the microstructure (grain and grain boundary) of charge carriers with short range mobility, while high temperature AC conductivity analysis confirmed double ionized oxygen vacancies conduction. Further, NTCR the negative value of temperature coefficient of resistance and non–Debye type phenomenon is observed, which suggest suitability of these compositions for the heat sensor applications.

1. Introduction Multiferroics have drawn the significant attention of present era researchers because of the scientific importance and technological advances. Potential applications of multiferroics include electronic devices e.g. sensors, generators, phase shifter, tunable filters etc. Multiferroics possess at least two types of ferroic orders simultaneously which include ferroelectric, ferroelastic and/or ferromagneti [1,2]. This coupling interaction results in magnetoelectric “ME” effect, which can be of two types including the direct effect “also known as MEH: P = αH”, the phenomenon consists of evolution of electric polarization “P” in the presence of magnetic field “H”. While converse ME effect “MEE: M = αE” is the emergence of magnetization “M” by using electric field “E” and “α” being the linear magnetoelectric coefficient [3,4]. In recent past, intensive research has been done on single phase compounds possessing ME effect at low temperatures [4-10]. However, a very few single phase materials have been found yet to have ME coupling above room temperature such as the archetypal multiferroic BiFeO3 but this inherent coupling is usually too low to be used for practical applications

∗

[10,11]. As an alternative, multiferroics composites have been proposed due to their multi-functionality & greater design flexibility, by combining ferromagnetic and ferroelectric compounds together [4]. In multiferroic composites, the coupling interaction of ferromagnetic and ferroelectric structural phases yields a significant increase in ME response, which is appropriate for specific applications e.g. magnetic-electric transducers, actuators, sensors and multifunctional devices [2]. Owing to their viability, new multiferroic composite systems have been reported in recent years such as thin films [12-15] and bulk ceramic composites [16-19]. Multiferroic composites can be developed via two different methods i.e. by combining a ferroelectric and ferromagnetic materials or doping of the magnetic impurity (transition metals 3d or rare earth metals 4f) with the ferroelectric ceramics [20,21]. Few reported multiferroic composite systems (by combining ferroelectric and magnetic phases) include BaTiO3–CoFe2O4 [22], LaFeO3–PbTiO3 [23], Pb(ZrTi) O3–CoFe2O3 [24], BiFeO3–PbTiO3 [25-27], BiFeO3–BaTiO3 [11,28], BaTiO3–LaFeO3 [29] and BaTiO3–MgFe2O4 [30]. Some other systems developed via doping of magnetic impurity into ferroelectric ceramics

Corresponding author. Corresponding author. E-mail addresses: asifrafi[email protected] (M.A. Rafiq), [email protected] (A. Maqbool).

∗∗

https://doi.org/10.1016/j.ceramint.2019.09.243 Received 23 May 2019; Received in revised form 15 September 2019; Accepted 24 September 2019 0272-8842/ © 2019 Published by Elsevier Ltd.

Please cite this article as: Muhammad Asif Rafiq, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.09.243

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for 12 h and granulated by sieving before calcining at 1200 °C for 5 h. Following the calcination process, the obtained powder was again ball milled for 4 h, heated over-night in the oven and finally the powder was granulated by sieving.

such as BaTiO3. xMn [31], BiFeO3. xCo [32] and Pb(ZrTi)O3. xNb [33]. Most of these composites take advantage from the combination of perovskite ferroelectric and spinel ferrite structure. By considering such approach, in current work authors focused on developing multiferroic composite of perovskite structured ferroelectric ceramic barium calcium zirconium titanate (BCZT, Ba0·85Ca0·15Zr0·1Ti0·9O3) with hexagonal M–type strontium hexaferrite (SrM, SrFe12O19). Furthermore, a few recent studies have explored the gradient compositional behavior and microwave assisted binding of BaTiO3. SrM multiferroic which also include an investigation of dielectric behavior [34,35]. Ferroelectric BCZT is engineered form of BaTiO3 ceramics; a renowed ferroelectric perovskite and widely studied material for the electronic industry which include capacitors, piezo/pyroelectric detectors etc. [36]. It is well acknowledged that electrical properties of perovskites can be modified via substitution or doping of either A– or B–site cations or combination of both [37-40]. In this study, BCZT with lattice constants a = 4.009 Å, c = 4.036 Å and c/a = 1.007 has been used as the ferroelectric component of the composite based on its exceptional properties suggested by the recent literature [41-46]. As the ferromagnetic component, M-type strontium hexaferrite SrM is used which belongs to the hexagonal ferrites family having P63/mmc based complex hexagonal crystal structure. The large unit cell with lattice constants a = 5.8758 Å, c = 22.958 Å involves two formula units (64 ions in total), while per unit cell volume contains 20 Bohr magneton (μB) saturation magnetization [47-49]. One of the attractive property of SrM is its high magnetic crystalline anisotropy along c-axis, which gives it enhanced magnetic properties which include large coercivity and saturation magnetization [50]. It is expected that these two components with considerably different crystal structures and functionalities will complement each other to generate a new function in the form of ME coupling. This study particularly emphasizes on finding the best composition for optimized electrical properties, furthermore relationship of electrical properties is established with alternating current (AC) frequency (f) and temperature (T). According to best of authors knowledge, a comprehensive study on SrM based multiferroic composite with BCZT has not been reported so far, so authors intends to cover the structure, infra-red, dielectric, impedance and ac conductivity analysis of stoichiometric controlled doping of BCZT in SrM. Other studies including magnetic and magnetoelectric measurements will be reported elsewhere.

0.85BaCO3 + 0.15CaCO3 + 0.1ZrO2 + 0.9TiO2 → (Ba0.85 Ca0.15)(Zr0.1 Ti 0.9) O3 + CO2(↑)

(2)

2.3. For composites SrM–xBCZT (where x = 0.0, 0.10, 0.20 and 0.30) composites were produced using conventional SRR process. The various compositions of SrM–xBCZT were measured and milled for 4 h in ethanol. The obtained mixture of powders was heated at 75 °C for 12 h in oven and granulated by sieving and then the powder was shaped in the form of solid cylindrical pellets via uniaxial pressing using 6000 Psi pressure. These pellet samples were fired at 1230 °C for 3 h in the air atmosphere of a box furnace. The resultant phase structure was analyzed via PANalytical X–ray diffractometer (XRD), while microstructural examination was carried out via field emission scanning electron microscope “Nova Nano FESEM 450”. Fourier transform infrared (FTIR) study was conducted by using Fourier transform infrared spectrometer “Agilent Cary 630”. High temperature (300 °C-500 °C) AC impedance spectroscopy in frequency range of (100–1 MHz) was performed by the impedance analyzer (Tonghui LCR–TH–2829C). Both surfaces of obtained pellets were painted with paint and baked in oven at 100 °C for 30 min for electrical measurements. 3. Results & discussion 3.1. Phase analysis Fig. 1 shows room temperature XRD spectra of 1–x (SrM)–x (BCZT) (x = 0.0, 0.1, 0.2 and 0.3) sintered compositions developed via solid state mixed oxide method, with two theta (2θ) in the range of 20–70°. All diffraction peaks are indexed according to Bragg's Law; phases present, crystal structure, lattice constants & cell volume of studied compositions are identified/calculated. Peak Indexing and other calculations are done by using Jade 5.0® software (Materials Data Inc.) after refinement. SrM possess complex magnetoplumbite type structural

2. Experimental procedure Multiferroic composite of strontium hexaferrite (SrM, SrFe12O19) and barium calcium zirconium titanate (BCZT, Ba0·85Ca0·15Zr0·1Ti0·9O3) have been investigated and the conventional solid-state reaction (SRR) process is utilized to prepare the SrM–xBCZT composite (where x = 0.0, 0.10, 0.20 and 0.30) as explained in the following procedure; 2.1. For SrM Stoichiometric amounts of SrCO3 and Fe2O3 were weighed on the basis of Eq. (1) and milled in ethanol for 4 h using a planetary ball milling (Fritsch Planetary Mill Pulverisette 6). The prepared powder mixture was heated in oven at 75 °C for 12 h and granulated by sieving before calcining at 1200 °C for 4 h. Following the calcination, the obtained mixture was again ball milled for 4 h, heated over-night in the oven and finally the powder was granulated by sieving.

SrCO3 + 6Fe2 O3 → SrFe12 O19 + CO2(↑)

(1)

2.2. For BCZT Fig. 1. XRD patterns of SrM–xBCZT multiferroic composite ceramics. Clear peak shift (with BCZT substitution) towards lower angle is observed. Impurity phase is highlighted with symbol (*).

Stoichiometric composition of BaCO3, CaCO3, TiO2 and ZrO2 was measured according to Eq. (2) and milled in ethanol for 4 h using ball milling. The produced mixture of powders was heated in oven at 75 °C 2

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Table 1 Lattice parameters of 1–x (SrM)x (BCZT) compositions, “a” and “'c”, “c/a” ratio and “Vcell”. Parameters

“a” (Å) “c” (Å) “c/a” ratio Cell Volume ‘Vcell’ (Å3)

1–x (SrM).x (BCZT) X=0

X = 0.1

X = 0.20

X = 0.30

5.87 23.06 3.928 688.10

5.88 23.16 3.938 693 .44

5.88 23.17 3.940 693.74

5.88 23.20 3.945 694.64

phase and hexagonal structural phase with P63/mmc space group, which matches very well with the literature [51]. Lattice parameters for all studied compositions are calculated according to the following equation and presented in Table–1

4(h2 + hk + k 2) 1 l2 = + 2 2 2 3a c dhkl

(3)

Fig. 2. FTIR spectra of all studied multiferroic compositions; labelling has been done inside the figure.

where “d” is interplanar spacing given by the Bragg's Law “nλ = 2dsin θ” [52]. Unit cell volume “Vcell” was measured by employing equation [53].

Vcell =

3 2 ac 2

(0.0–0.3) sintered multiferroic composite recorded by FE–SEM. Polycrystalline nature of the studied compositions is confirmed from parameters of microstructure i.e. size, shape & grain distribution. It is found that with increase in dopant (BCZT) content, dense microstructure is developed. This increment in density of SrM and formation of voids free sample shows the effect of BCZT as sintering aid. Similar results were reported for SrM–BT multiferroic composite by R.C. Pullar [62].

(4)

XRD data of undoped composition reveals the formation of SrM as major phase and presence of a secondary/impurity phase. It is well established from literature that synthesis of precise SrM is difficult via wet & solid state reaction method [54,55]. However, peak of the secondary phase was submerged with rise in dopant (BCZT) concentration. There could be found several reports in which addition of substitution enhances the sinterability and confines the development of secondary/ impurity phase [53,54,56,57]. Further, the diffraction data of doped compositions reveals the occurrence of a diphase structure suggesting the stoichiometry distinct phases maintain in the composites. It can also be seen from Fig. 1 that with addition of dopant peak shift towards lower ‘2θ’ is observed which shows increase in unit cell volume, Table-1 confirms the gradual rise in lattice constant parameters and unit cell volume which strengthens the argument.

3.4. Impedance spectroscopy analysis Impedance spectroscopy (IS) is well established method to investigate the electric behavior of microstructure i.e. grain, grain boundaries and their interface. Electronic features of prepared multiferroic composite system is studied in a frequency band of 100 Hz–1MHz at various temperatures ranging from 300–500 °C. Alternating current (AC) is used in IS as the input of sinusoidal perturbation and material's response is recorded. Obtained electrical parameters includes, “complex admittance (Y*), impedance (Z*), electrical modulus (M*) and permittivity (ε*)”, related dielectric quantities i.e. dielectric loss (DF) and dielectric constant (K) can also be measured. These parameters consist of real & imaginary components e.g. “complex dielectric constant (ε∗ = ε′− i ε″), complex admittance (Y∗ = Y′ − i Y″), complex impedance spectroscopy, CIS (Z∗ = Z′ − i Z″) electric modulus (M∗ = M′ − i M″) etc. (where i = √−1)”. In present study authors focused on complex impedance and modulus analysis, since these two parameters are important in elaborating the grain/grain boundary conductivity, “NTCR” the negative value of temperature coefficient of resistance performance, relaxation behavior, mobility of charge carriers and conduction mechanism. Following relations are associated to real (Z′) and imaginary (Z″) factors of complex impedance [63]:

3.2. Infra–red spectrometry FTIR analysis is key technique which provides useful information associated with of existence of distinctive functional groups (FGR) and evidence of molecular fingerprint (FPR) which could be used to identify the nature of samples. IR spectrum consists of percent transmittance/ absorbance (%) and wave number (ύ) while wave number represents frequency (ν) of IR absorption. Usually, the spectrum is divided into two regions, identification of groups/FGR (4000–1500 cm−1) and spectral matching/molecular finger print/FPR (1500–400 cm−1) [58]. Fig. 2 shows the IR spectra recorded for SrM (inset) and SrM–BCZT. Two strong absorption peaks observed in molecular fingerprint (FPR) region confirms the characteristic vibration band of hexagonal ferrites [54]. Bands in 600–800 cm−1 range could be attributed to Fe–O stretching by Fe–O6 and bending of Fe–O by Fe–O4, while Sr–O bond stretching has been proposed at 936 cm−1, these results are in accordance with previous reports [59,60]. Absorption bands for BCZT doped compositions are very similar (no peak shift as per vertical lines), while minor variation occurred in the relative intensities. Small absorption peaks of 600–700 cm−1 range are found in all doped compositions which could be associated with the stretching vibrations of Fe2O3 of metal–oxygen [61].

Z′ =

R 1 + (ωτ )2

(5)

Z" =

ωRτ 1 + (ωτ )2

(6)

Here, “(ω) is angular frequency (ω = 2πf), while (τ) is relaxation time, τ = RC, C and R stands for capacitance (F) & resistance (Ω) respectively”. Fig. 4 (a–d) shows Nyquist plots which relates Z′ and Z″ elements of CIS at various frequencies (100 Hz–1MHz) and temperatures (300 °C500 °C). Nyquist plots consists of semicircles which could be sketched in the particular frequency range (f); i.e. first (at the lower frequency) is associated to grain or bulk, while second (at higher frequency)

3.3. Microstructure analysis Fig. 3 (a–d) shows fractured surface micrographs of SrM–xBCZT 3

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Fig. 3. FESEM representative micrographs of 1–x (SrM)–x (BCZT) (a) x = 0.0 (b) x = 0.1 (c) x = 0.2 & (d) 0.3. The results are showing the different microstructure and gradually improvement of densification with the addition of BCZT ceramics.

relaxation behavior which are temperature dependent. In studied compositions some depression in semicircle is observed which points towards non–Debye type relaxation [64,65]. Based on the observed data, an equivalent circuit representing the

corresponds to grain boundary impedance. In some cases, there is third semi–circle which corresponds to the impedance of electrode. Intercept of these semicircular arcs on (abscissa) Z′ axis could be utilized to compute the grain “R1” and grain boundary resistance “R2” and

Fig. 4. Nyquist plots (100 Hz–1 MHz) of real (Z′) & imaginary (Zʺ) part complex impedance for 1–x (SrM). x (BCZT) (a) x = 0.0 (b) x = 0.1 (c) x = 0.2 & (d) 0.3. 4

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Fig. 6 (a–d) insets]. Fitting has been done by using impedance software Z–view® “Ver. 3.2 Scribner Associates, Inc.“. Temperature dependent values of R1, R2, C1 and C2 has been extracted while time constant for τ1 and τ2 are also reported in Table 2. 3.4.1. Grain/grain boundary conductivity analysis In order to study electrical response of ceramic/multiferroic microstructure i.e. grain and/or grain boundaries activation energy (Ea) investigation is important since this is the energy required for the charge carriers to break the energy barrier. For this, AC impedance spectroscopy technique has been employed successfully on several ceramic/composite systems in literature (Refer to Table-3). Where “τo is Pre-exponential factor, Ea is activation energy (energy barrier for the charge carriers) while Kb stands for Boltzmann constant with value 8.6173 × 10−5 eV.K−1.” Time constant evaluated (from Table 1) is utilized to calculate activation energies (Ea) for gain (bulk) and grain boundary (ranging from 300 °C-500 °C) for all the sintered compositions are and presented in Table-4 (Plot for x = 0.3 is presented in Fig. 5). The obtained values of Ea are quite matching with previous reported results for Ba0·85Ca0.15 Zr0·10Ti0·90O3 (BCZT) and La0·67Ca0·33MnO3 (LCMO) [(1–x) BCZT–xLCMO] multiferroic composite by Li, S·B., et al. [73].

Fig. 5. Arrhenius behavior of time constant (τ) for grain and grain boundary, Ea values calculated are reported in Table-4. For reference purpose plot for 1–x (SrM). x (BCZT) at 0.3 is presented.

two resistances (R) connected in series, a capacitor (C) and constant phase element (CPE) in parallel is proposed (presented in Fig. 7). The resistances embodies the path of conduction, while capacitor exhibits space charge polarization [66]. It is quite clear that experimentally measured data is supporting the data calculated theoretically [Refer to

3.5. Negative temperature coefficient of resistance (NTCR) behavior Bode plots of real part impedance (Z′) provides information of NTCR behavior which refers to decrease in electrical resistance with increase

Fig. 6. Bode plots (100 Hz–1 MHz) of real part impedance (Zʹ) for 1–x (SrM). x (BCZT) (a) x = 0.0 (b) x = 0.1 (c) x = 0.2 & (d) 0.3. Studied compositions showing NTCR behavior i.e. decline in magnitude of Zʹ with rise in temperature. Inset shows the fitting results agreement with actual data. 5

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Fig. 7. Bode plots (100 Hz–1 MHz) of imaginary part impedance (Zʺ) for 1–x (SrM). x (BCZT) (a) x = 0.0 (b) x = 0.1 (c) x = 0.2 & (d) 0.3. Curves displays temperature dependent relaxation phenomenon while inset shows equivalent circuit used for fitting.

It is quite clear from Table-1 that studied compositions possess large resistance (R) & comparatively small capacitance (C) values, this effect could be elaborated via bode plots of imaginary part impedance and electric modulus (Z″ & M″). Both complex quantities are plotted against frequency at different temperatures for all studied compositions and presented in Fig. 8 (a–d). This comparison of Z″ and M″ is of utmost importance since these plots differentiate mobility of charge carriers in relaxation process. Maxima's of imaginary part impedance (Z″max) and complex electric modulus (M″max) mismatches at same temperature (T) and frequency (f) which is associated to localized or short range mobility of charge carriers. On the other hand, complete match (merging) of these two maxima's i.e. Z″max & M″max indicates long–range type mobility of charge carriers. Moreover, peaks of Z″max & M″max are separated from each other which is probably due to non–Debye type relaxation phenomenon with short range mobility of charge carriers in studied compositions [63-65].

in temperature. Fig. 6 (a–d) shows the bode plots in frequency band of 100 Hz–1MHz for real part impedance (Z′) at different temperatures from 300 °C to 500 °C. Spectrum shows sigmoidal variation in low frequency region while saturation (amalgamation) is studied at high frequency region which is associated to the existence of mixed nature (orientation, dipolar & electronic) polarization. It can also be observed that (in low–frequency region) with rise in temperature (T), real part impedance (Z′) decreases which shows negative temperature coefficient of resistance (NTCR) behavior in studied compositions. Further, In high frequency region (~10 KHz) merging of at all curves could be associated to release of space charge which indicates the dependence of the ac conductivity (σac) on studied frequency (f) and temperature (T) [74,75].

3.6. Relaxation phenomenon Bode plots of imaginary part impedance (Zʺ) are shown in Fig. 7 (a–d), inset shows the equivalent circuit which has been employed for fitting purpose. These plots are of much importance since relaxation frequency could be effectively studied from these curves. It was found that an imaginary peak was evident for all temperature ranges (300 °C500 °C) which is associated with the single dielectric relaxation phenomenon in studied compositions. This type of relaxation is associated to mobility of charges at low temperature while defects and vacancies and elevated temperatures [63,76].

3.7. High temperature A.C. Conductivity analysis High temperature A.C. conductivity (σac) variation with temperature ranging from 300 °C-500 °C at frequency of 10 KHz is presented in Fig. 9 (a–d). σac can be calculated by employing following relation;

σac = ωεr εo ω tanδ

(8)

Where ω is a. c. source angular frequency (2πf), tan–δ is dielectric loss while εo is dielectric permittivity in vacuum [77]. Collective σac of 6

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Table 2 Resistance, capacitance, relaxation time values determined for grain (R1, C1, τ1) and grain boundary (R2, C2, τ2) for all studied compositions. T (°C)

1–x (SrM).x (BCZT) X = 0.0 R1 (Ω)

C1 (F)

τ1 (s)

n1

R2 (Ω)

C2 (F)

τ2 (s)

300 325 350 375 400 425 450 475 500

2.44E+9 1.13E+9 1.11E+9 6.59E+8 4.11E+8 2.14E+8 1.35E+8 5.46E+7 3.91E+7

2.23E-12 3.45E-12 3.30E-12 6.67E-12 1.71E-11 7.28E-12 2.01E-11 3.28E-11 5.54E-11

5.47E-03 3.90E-03 3.68E-03 4.40E-03 7.06E-03 1.56E-03 2.71E-03 1.80E-03 2.17E-03

0.97 0.95 0.95 0.90 0.83 0.91 0.86 0.81 0.78

62391 276600 1.67E+7 2.47E+7 1.80E+7 2.07E+7 1.61E+7 1.15E+7 7.14E+6

3.84E-12 3.04E-12 6.48E-12 7.00E-12 1.31E-11 2.98E-12 3.79E-12 4.59E-12 5.96E-12

2.40E-07 8.42E-07 1.09E-04 1.74E-01 2.37E-04 6.18E-05 6.10E-05 5.31E-05 4.26E-05

T (°C)

1–x (SrM).x (BCZT) X = 0.1 R1 (Ω) C1 (F)

τ1 (s)

R2 (Ω)

C2 (F)

τ2 (s)

n2

300 325 350 375 400 425 450 475 500

4.35E+7 4.99E+7 4.47E+7 3.12E+7 1.93E+7 9.81E+6 5.29E+6 2.46E+6 1.44E+6

7.75E-05 7.14E-05 5.71E-05 3.46E-05 2.04E-05 1.02E-05 6.16E-06 3.20E-06 2.07E-06

1.38E+9 1.07E+9 3.81E+8 1.15E+8 4.98E+7 1.99E+7 9.48E+6 4.69E+6 2.63E+6

8.00E-13 8.37E-13 1.82E-12 3.68E-12 1.08E-11 3.53E-11 1.03E-10 3.06E-10 5.85E-10

1.11E-03 9.04E-04 6.96E-04 4.23E-04 5.42E-04 7.04E-04 9.84E-04 1.44E-03 1.54E-03

0.98 0.95 0.98 0.95 0.89 0.82 0.75 0.67 0.62

T (°C)

1–x (SrM).x (BCZT) X = 0.2 R1 (Ω) C1 (F)

τ1 (s)

R2 (Ω)

C2 (F)

τ2 (s)

n2

300 325 350 375 400 425 450 475 500

5.53E+7 4.39E+7 3.03E+7 1.79E+7 8.87E+6 4.61E+6 2.37E+6 1.26E+6 0.96E+6

1.20E-04 8.11E-05 5.09E-05 2.92E-05 1.46E-05 7.81E-06 4.35E-06 2.62E-06 2.16E-06

5.72E+8 2.99E+8 1.48E+8 6.41E+7 2.62E+7 1.09E+7 4.78E+6 2.46E+6 2.09E+6

2.11E-12 3.36E-12 6.08E-12 1.85E-11 3.42E-11 8.46E-11 2.14E-10 5.19E-10 6.34E-10

1.21E-03 1.01E-03 9.03E-04 1.19E-03 8.98E-04 9.28E-04 1.03E-03 1.28E-03 1.33E-03

0.99 0.98 0.95 0.89 0.85 0.79 0.73 0.66 0.65

T (°C)

1-x (SrM).x (BCZT) X = 0.3 R1 (Ω) C1 (F)

τ1 (s)

R2 (Ω)

C2 (F)

τ2 (s)

n2

2.17E+7 2.48E+7 1.56E+7 7.96E+6 4.30E+6 2.39E+6 1.34E+6 0.74E+6 0.46E+6

4.54E-05 4.46E-05 2.67E-05 1.51E-05 8.32E-06 5.10E-06 3.14E-06 2.05E-06 1.44E-06

1.57E+8 1.10E+8 6.42E+7 3.92E+7 2.27E+7 1.42E+7 8.22E+6 5.04E+6 2.98E+6

2.88E-12 1.01E-11 2.37E-11 8.82E-11 1.77E-10 4.29E-10 9.69E-10 1.90E-09 3.55E-09

4.56E-04 1.12E-03 1.53E-03 3.46E-03 4.03E-03 6.12E-03 7.98E-03 9.58E-03 1.06E-02

0.98 0.91 0.86 0.76 0.73 0.66 0.61 0.56 0.52

300 325 350 375 400 425 450 475 500

1.78E-12 1.42E-12 1.27E-12 1.11E-12 1.05E-12 1.03E-12 1.16E-12 1.29E-12 1.43E-12

2.16E-12 1.84E-12 1.67E-12 1.63E-12 1.64E-12 1.69E-12 1.83E-12 2.07E-12 2.23E-12

2.08E-12 1.79E-12 1.70E-12 1.89E-12 1.93E-12 2.12E-12 2.33E-12 2.76E-12 3.07E-12

Table 3 A brief review of conduction mechanisms proposed and corresponding the activation energy values (Ea) reported in the previous result. ⎛− Ea ⎞ Kb T ⎠

(7)

τ = τo e ⎝

Activation Energy (eV)

Compound

Technique Employed

Proposed Conduction Mechanism

Reference

0.41 eV 1.07–1.31 eV 0.27–0.32 eV 0.24–0.26 eV 0.74–0.81 eV 0.26–0.48 eV

1–x (Ba0·7Sr0·3TiO3).x (Ni0·5Zn0·5Fe2O4) BiFeO3–NaTaO3 Nd–0.5BiFeO3–0.5PbTiO3 1–xBaTiO3–xBiScO3 0.5(BiLaxFe1–xO3)–0.5(PbTiO3) 0.5BaFe12O19–0.5 Na0·5Bi0·5TiO3

AC Impedance AC Impedance AC Impedance

Hopping conduction Mobility of the oxygen ions or vacancies Hopping of charge

[67] [68] [69,70]

AC Impedance AC Impedance

Singly ionized oxygen vacancies Hopping of charge

[71] [72]

Ea for both grain and grain boundary is calculated by studying Arrhenius behavior according to following relation [53].

σac = σdc + Kωn

the studied compositions could be calculated by using Jonscher's power law:

(9)

where “σdc is the d. c. conductivity, σ(ω) is frequency dependent conductivity, K is intrinsic material and temperature dependent parameter 7

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SrM–BCZT multiferroic solid solution the existence of OVs can be ascribed to the high volatilization of TiO2 along with the co–existence of Fe3+ and Fe2+ ions during sintering. Conduction phenomenon proposed is pretty much in accordance with (1–x)Na0·5Bi0·5TiO3–(x)BaTiO3 (NBT–BT) [81] and BiFeO3–BaTiO3 (BFO–BT) multiferroic solid solutions [82].

Table 4 Activation energy (Ea) values of grain and grain boundary for all studied samples. 1–x (SrM).x (BCZT)

X = 0.0 X = 0.1 X = 0.2 X = 0.3

Activation Energy Ea (eV) Grain (Bulk)

Grain Boundary

0.79 0.74 0.82 0.72

0.73 0.82 0.62 0.59

4. Conclusion Multiferroic composite system of strontium hexaferrite (SrFe12O19/ SrM) and barium calcium zirconium titanate (BCZT, Ba0·85Ca0·15Zr0·1Ti0·9O3) have been investigated and the conventional solid-state reaction (SRR) process is utilized to prepare the SrM–xBCZT composite (where x = 0.0, 0.10, 0.20 and 0.30). XRD studies revealed both the parental phases in doped compositions; traces of a minor secondary phase are observed in SrM with complex magnetoplumbite structural phase and hexagonal structural phase with P63/mmc space group was confirmed. FTIR study employed to study the absorption bands of present phases. FE–SEM study revealed the disappearance of porosity and dense structure is developed which suggests the role of BCZT as sintering aid. A comprehensive electrical characterization via IS study suggested two electrical contributions in the multiferroic composite in the microstructure (grain and grain boundary) of charge carriers with short range mobility. Temperature dependent non-Debye relaxation and NTCR behavior is observed for studied compositions. Bode plot study of complex impedance and modulus proposed short range mobility of charge carrier. High temperature AC conductivity analysis carried out in temperature range of 300 °C to 500 °C, subjected

while n (exponent) is constant having no dimension with values 0 < n < 1” [78]. It is quite clear that all compositions exhibits increasing trend with rise in temperature, this is might be because of higher mobility of charge carriers at elevated temperatures. To elaborate the conduction mechanism; activation energies (Ea) have been calculated for all sintered samples by means of Arrhenius equation [53] and presented in Table-5.

ln(σac ) = ln(σo) − Ea/ kT

(10)

Temperature-dependent a. c. conductivity activation energy values are found to be in the range of 0.58–1.16 eV. It is well established from literature that conduction process in perovskite materials is controlled by ionic species such as singly and/or double ionized oxygen vacancies (OVs) [79]. The “Ea” values for single ionized oxygen OVs ranges between 0.3–0.5 eV, while for double ionized OVs the “Ea” is up to 1.2 eV [79,80]. In present case, Ea values lies close to double ionized OVs. In

Fig. 8. Variation of Imaginary part Impedance (Zʺ) & Electric Modulus (Mʺ) as function of frequency (100 Hz–1MHz) at different temperatures for 1–x (SrM). x (BCZT) (a) x = 0.0 (b) x = 0.1 (c) x = 0.2 & (d) 0.3. 8

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Fig. 9. Plot of natural log of A.C. conductivity (ln σac) versus temperature from 300 to 500 °C for 1–x (SrM). x (BCZT) (a) x = 0.0 (b) x = 0.1 (c) x = 0.2 & (d) 0.3 at 10 KHz frequency.

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Table 5 Activation Energies (Ea) values at different frequencies in temperature range of (300 °C-500 °C) for studied compositions. 1–x (SrM).x (BCZT)

X = 0.0 X = 0.1 X = 0.2 X = 0.3

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Acknowledgment “Authors would like to pay their special thanks to Department of Metallurgical and Materials Engineering, University of Engineering and Technology (UET), Lahore for providing the support and facilities for experimentation. This work was supported by the National Research Program for Universities (NRPU) through the Research and Development Division, as funded by the Higher Education Commission, Pakistan of Pakistan (9231/Punjab/NRPU/R&D/HEC/2017).” 9

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