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CuTl-1223 nanoparticles-superconductor composites
Journal of Alloys and Compounds 712 (2017) 696e703
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Conduction mechanism and impedance spectroscopy of (MnFe2O4)x/ CuTl-1223 nanoparticles-superconductor composites M. Naveed a, M. Mumtaz a, *, Rashid Khan a, Abrar A. Khan a, M. Nasir Khan b a b
Materials Research Laboratory, Department of Physics, FBAS, International Islamic University (IIU), Islamabad, 44000, Pakistan Central Diagnostic Laboratory, Physics Division PINSTECH, Islamabad, 45500, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 February 2017 Received in revised form 2 April 2017 Accepted 4 April 2017
Manganese ferrite (MnFe2O4) nanoparticles and Cu0.5Tl0.5Ba2Ca2Cu3O10-d (CuTl-1223) superconducting phase were prepared by sol-gel and solid-state reaction methods, respectively. MnFe2O4 nanoparticles were added in CuTl-1223 matrix to get (MnFe2O4)x/CuTl-1223 (x ¼ 0 ~ 2.0 wt.%) nanoparticles esuperconductor composites. Different experimental techniques like XRD, SEM, R-T measurements and Impedance spectroscopy were used to characterize these composites. It was observed that crystal structure of host CuTl-1223 phase remained unaltered after addition of MnFe2O4 nanoparticles, which indicated about the occupancy of these nanoparticles at grain-boundaries. Over all decreasing trend in superconducting properties may be attributed to spin-charge reflection and trapping of charge carriers across these magnetic MnFe2O4 nanoparticles at grain-boundaries of CuTl-1223 phase. In complex impedance spectroscopy (CIS), role of MnFe2O4 nanoparticles at the grain-boundaries of host CuTl-1223 phase was also investigated. The decrease in impedance (Z) with increasing temperature witnessed the occurring of thermally activated processes in the system. Higher value of activation energy at grainboundaries showed that grain-boundaries are more resistive than grains due to non-stoichiometric distribution of oxygen and dangling bonds at grain-boundaries. The impedance master curves indicated that the distribution of relaxation time (dynamic process) is nearly temperature independent. The decrease in ac-conductivity with increasing content of these nanoparticles indicated the enhancement of space charges at grain-boundaries. © 2017 Elsevier B.V. All rights reserved.
Keywords: (MnFe2O4)x/CuTl-1223 composites Impedance spectroscopy Grain-boundaries Thermally activated processes
1. Introduction Complex impedance spectroscopy (CIS) is most widely used technique based on electric impedance measurement of a material in wide range of frequency (impedance spectrum) [1e3]. This technique is useful to investigate the relaxation processes in a complex inhomogeneous system over a wide range of characteristic times. This method enables to make a link between the characterization of bulk properties and the individual constituents of complex material [4e7]. In present time, impedance spectroscopy is an important characterization technique, which provides information regarding the movement of carriers in superconductors at various temperatures, which may be useful in understanding the superconducting phenomenon [8]. The bulk cuprates superconductors are complex materials, which consist of grains separated by
* Corresponding author. E-mail address: [email protected] (M. Mumtaz). http://dx.doi.org/10.1016/j.jallcom.2017.04.034 0925-8388/© 2017 Elsevier B.V. All rights reserved.
grain-boundaries. Cuprates superconductors have vast applications in technical field such as power transmission lines, Josephson junction and electronic devices. This method helps to make proper separation among the bulk grains, grain-boundaries with electrode-interface properties between grains and grainboundaries [9e12]. Therefore, cuprates superconductors have received the great deal of attention due to their potential technological aspects. In impedance measurement of Ba2CoNbO6 and Sr2CoNbO6 ceramics in frequency range of 1 Hze10 MHz, both showed separate relaxation processes with distinguishable relaxation times, which was most probably due to grains and grain-boundaries contributions [13]. Impedance study of Ba2BiTaO6 exhibited inter-grains and intra-grains contributions. The hopping of oxygen vacancies gave rise to conduction mechanism, while impurities was not influenced the overall conduction mechanism [14]. Fitting of Cole-Cole data showed two different conduction processes in p-CuIn3Se5 semiconductor. When temperature was increased, the calculated values of grains and grain-boundaries resistance were decreased, which
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was in agreement with the Arrhenius law used for calculation of activation energies. The estimated activation energies for relaxation mechanism were compared with those values, which were found during electric modulus analysis [15]. Ba(Zr0.25Ti0.75)O3 showed a non-Debye relaxation phenomenon, when relaxation frequency peak shifted toward positive side with increase in temperature. Frequency and temperature dependent ac-conductivity exhibited that the conduction process was thermally activated and was shown negative temperature coefficient of resistance (NTCR) behaviour [16]. At low frequency and for T < 175 K in Fe2TiO5 compound, the grains contribution was dominated but for T 175 K, the grain-boundaries contribution was activated at low frequencies [17]. ZnO nanoparticles substitution in polycrystalline lead-free BaZr0.15Ti0.85O3 ceramics has increased grain-boundaries resistance, while intra-grains resistance has been decreased with the increase in temperature. Relaxation time was decreased in both the cases [18]. Substitution of Nd in Y2/3Cu3Ti4O12 ceramics has increased the activation energies of grain-boundaries, and slightly influenced the dielectric relaxation process [19]. The polycrystalline KBa2V5O15 ceramics showed NTCR behaviour with grains ‘G’ and grain-boundaries ‘Gb’ contributions for separate frequency values [20]. Impedance study of Ba5GdTi3V7O30 suggested that bulk resistance ‘Rb’ was decreased with the rise in temperature, which showed NTCR behaviour [21]. To improve the conductivity of YBaCuO superconducting particles in a polypropylene matrix, different amounts of carbon black and copper were incorporated into the binary based system and investigated the effects of these conductor fillers on superconductor polymer composites by using complex impedance spectroscopy. The conduction mechanism of materials consisting of polymers and conductor particles significantly depends on affinity and adhesion between particles and polymers [22]. The temperature dependent impedance of barium cuprates BaCuO2þd with the frequency in the ranges from 5 Hz to 105 Hz proved the semiconducting nature of barium cuprates. The increase in conductivity with increasing temperature due to release of space charge as a result of the reduction in the barrier properties of barium cuprates was observed [23]. Detailed literature review on CuTl-1223 superconducting phase revealed that there is still no research work has been reported on frequency and temperature dependent impedance properties to make proper separation among the bulk grains, grain-boundaries with electrode-interface properties between grains and grainboundaries. Therefore, we have investigated the role of MnFe2O4 nanoparticles with different concentrations on conduction mechanism and electrical response of Cu0.5Tl0.5Ba2Ca2Cu3O10-d (CuTl1223) superconductor.
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2.2. Preparation of (MnFe2O4)x/(CuTl-1223) nanoparticlessuperconductor composites Cu0.5Tl0.5Ba2Ca2Cu3O10-d (CuTl-1223) phase was synthesized by solid-state reaction method. Cu2(CN)2, Ca(NO3)2 and Ba(NO3)2 compounds were mixed in appropriate ratios and then ground for 2 h. The ground material was loaded in quartz boats and heated in chamber furnace at 860 C for 24 h and allowed to cool down up to room temperature. Repeat the same heating process with intermediate grinding for 1 h at least. MnFe2O4 nanoparticles with different wt% and Tl2O3 were mixed with precursor Cu0.5Ba2Ca2Cu3O10-d material and grounded each sample for 1 h. The ground powder was pelletized in gold capsules and placed in pre-heated furnace for sintering at 860 C for 10 min and got (MnFe2O4)x/ (CuTl-1223) with (x ¼ 0, 0.5, 1.0, 1.5 and 2.0 wt%). Structural and phase purity of (MnFe2O4)x/(CuTl-1223) nanoparticles-superconductor composites were determined by using XRD. Morphology was examined by SEM images. Impedance properties and conduction mechanism in frequency ranges from 40 Hz to 10 MHz at various temperatures from normal state to superconducting (i.e. 253 K to 78 K) was investigated by complex impedance spectroscopy. 3. Results and discussion 3.1. XRD analysis XRD pattern of MnFe2O4 nanoparticles is shown in the inset of Fig. 1. Various planes like (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) are indexed according to the cubic structure corresponding to different 2q values such as 29.66 , 34.90 , 42.51, 56.21 and 61.71, respectively. Average crystallite size is calculated by DebyeScherer's formula, which is found to be 20 nm. Almost all the diffraction peaks are well indexed according to cubic crystal structure of MnFe2O4 nanoparticles and no prominent peaks of other impurity were detected. Representative XRD spectra of (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composites with x ¼ 0 and 2.0 wt% are shows in Fig. 1. The well indexed diffraction peaks in these XRD spectra show the dominance of host CuTl-1223 phase. XRD spectrum clearly shows that the tetragonal structure of host CuTl-1223 superconducting phase remains unaltered after addition of these MnFe2O4 nanoparticles. In these XRD spectra, some un-indexed peaks of very low intensities are also observed, which shows the presence of unknown impurities and some other superconducting phases [24]. 3.2. SEM analysis
2. Experimental details 2.1. Preparation of MnFe2O4 nanoparticles Sol-gel method was used to synthesize MnFe2O4 nanoparticles. Initially, two solutions were prepared separately. First solution was prepared by mixing iron nitrate and manganese nitrate in ethanol with appropriate ratios and second solution was prepared by mixing citric acid and distilled water. Now second solution was mixed slowly in first solution. To maintain pH level up to 5, ammonia was added drop by drop with continuous stirring process. Solution was stirred constantly at 70 C till the formation of gel. The gel was placed in oven for drying at 110 C for overnight. The dried material was ground in agate mortar and pestle to get powder. Then annealed powder was placed in a furnace at 900 C for 2 h. The resultant material was again ground to get fine powder of MnFe2O4 nanoparticles as end product.
Scanning electron microscopy (SEM) is a characterization technique, which is widely used to study surface features and morphological structure analysis. SEM images of (MnFe2O4)x/CuTl1223 nanoparticles-composites with x ¼ 0, 2.0 wt% are shown in the inset of Fig. 2. Pure sample with x ¼ 0 shows higher density of inter-grain weak-links, voids and pores. The empty spaces healed up and inter-grain weak-links are improved and grain size was also increased after addition of MnFe2O4 nanoparticles. SEM images confirm the decrease in density of voids and pores of host bulk CuTl-1223 superconductor after addition of (MnFe2O4) up to 2.0 wt %. The addition of these nanoparticles changes the thermodynamics of reaction, which have changed the grains morphology of the material. 3.3. Resistivity measurement Resistivity
versus
temperature
(R-T)
measurements
of
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Fig. 1. XRD spectra of (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composites for x ¼ 0 and 2.0 wt%. In the inset there are shown XRD spectrum of MnFe2O4 nanoparticles.
(MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composites with x ¼ 0 and 2 wt% is shown in Fig. 2. The value of Tc onset (K) was found to be 94 K for un-added pure CuTl-1223 sample, which was decreased to 92 K for the sample with x ¼ 2.0 wt% concentration of MnFe2O4 nanoparticles. This decrease in Tconset (K) with MnFe2O4 nanoparticles addition in CuTl-1223 phase is most probably due to presence of these magnetic natured nanoparticles at grainboundaries. The enhanced the charge accumulation at grainboundaries has reduced the mobility of charge carriers in the material. Normal state resistivity showed non-monotonic variation with addition of MnFe2O4 nanoparticles inclusion in host CuTl1223 phase, which is most probably due to due to inhomogeneous and non-uniform distribution of these nanoparticles at grain-boundaries [25].
3.4. Impedance analysis Complex impedance analysis has been used to correlate the ac-
Fig. 2. Resistivity vs temperature measurements of (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composite samples for x ¼ 0 and 2.0 wt%. In the inset, there are shown SEM images of (MnFe2O4)x/CuTl-1223 composites with (a) x ¼ 0 and (b) 2.0 wt%.
electrical properties with the microstructure of material. We have studied the impedance properties of (MnFe2O4)x/CuTl-1223 (x ¼ 0, 0.5, 1.0, 1.5 and 2.0 wt%) nanoparticles-superconductor composites. The electrical response of ferrites and cuprates can be understood in detail with the help of some theoretical models. Koop's model is considered as a two-layer model (i) grains and (ii) grainboundaries. The grains are considered as conducting layers and grain-boundaries are considered as resistive layers. The flow of charge carriers have to face hurdle from conducting region (grain) to non-conducting region (grain-boundaries) [26]. To understand this, we can modelled an equivalent circuit, which contains a grain resistance ‘Rg’ connected in series with grain-boundaries resistance ‘Rgb’ parallel with grain-boundaries capacitance ‘Cgb’ as shown in Fig. 3. The representative Nyquist plots of (MnFe2O4)x/CuTl-1223 (x ¼ 0, 1.0 and 2.0 wt%) nanoparticles-superconductor composites at various temperatures from 78 K to 253 K, in frequency ranges from 40 Hz to 10 MHz are shown in Fig. 4 (aec). We observed that the diameters of semicircles get depressed with the increase of temperature, which indicated the non-Debye type relaxation phenomenon in the material. The decrease in diameter of semicircle behaviour proves that conduction mechanism is thermally activated [23]. From impedance spectrum, we can easily deduce ‘Rg’ and ‘Rgb’ values. The left intercept of the semicircle with Z/-axis gives ‘Rg’ value and the right one gives the total resistance R ¼ Rgb þ Rg. From Nyquist plot, it was found that ‘Rgb’ values are greater than ‘Rg’, which proves that the grain-boundaries are more resistive than grains. This is probably due to the non-stoichiometric distribution of oxygen, spin-charge reflection and dangling bonds on the grain-boundaries [27,28]. At normal state the electrons can
Fig. 3. The electrical equivalent circuit for impedance.
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result enhance the polarization and dielectric response of the material. The enhanced charge accumulation at grain-boundaries causes the reduction of flow of charge carriers. This band spread of Z/ (U) versus Z// (U) with the addition of MnFe2O4 nanoparticles clearly indicate that the dielectric response of the material has been improved. We found that by increasing temperature from 78 K to 253 K, the value of Z/ (U) were decreased from 2.0 103 to 1.6 103, 5.4 104 to 3.7 104 and 9.7 105 to 6.0 104 for x ¼ 0, 1.0, and 2.0 wt% respectively, which is probably due to release of space charges at grain-boundaries by increasing temperatures. The activation energy of grains and grain-boundaries were calculated by applying small polaron hopping model (SPH);
R Ec ¼ A0 ekT T
(1)
where Ao is a pre-exponential factor, R is resistance, T is the absolute temperature, Ec is activation energy of conduction and k is the Boltzmann constant. From the slopes of fitted straight lines, the calculated activation energies of grains and grain-boundaries for (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composites with x ¼ 0 and 2 wt% are shown in Fig. 5 (a, b). The calculated activation energy for conduction on grains was found to be 0.215 meV, 0.2158 meV and 0.216 meV, and for grains-boundaries was 0.207 eV, 5.18 eV and 10.6 eV for x ¼ 0, 1.0 and 2.0 wt%, respectively. The higher value of activation energy at grain-
Fig. 4. (aec). The Nyquist plot of impedance between real part (Z/) and imaginary part (Z//) for (MnFe2O4)x/CuTl-1223 with (a) x ¼ 0 at T ¼ 78 Ke253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K.
be captured or localized across these magnetic nanoparticles at the grain-boundaries of the bulk CuTl-1223 superconducting matrix. In superconducting state, cooper pairs can easily be broken in to normal electrons across these magnetic nature nanoparticles settled at grain-boundaries acting as efficient scattering centers. These electrons can also be accumulated at the grain-boundaries due to magnetic interaction across these magnetic nanoparticles. In this way these electrons pile up at the grain-boundaries and in
Fig. 5. (a, b). (a) The plots of Rg/T versus 1000/T for x ¼ 0, 1.0 and 2.0 wt%, (b) The plots of Rgb/T versus 1000/T for x ¼ 1.0 wt% and x ¼ 2.0 wt% and in the inset of (b) for x ¼ 0%.
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boundaries clearly shows that the resistive property is more prominent at grain-boundaries than grains [29,30]. In superconducting materials, the number of carriers is much larger as compared to semiconductor, which piles up across these magnetic MnFe2O4 nanoparticles at grain-boundaries causing larger potential barrier for the mobile charge carriers. We found the activation energy of grain-boundaries for (MnFe2O4)x/CuTl-1223; x ¼ 0, 1.0 and 2.0 wt% nanoparticle-superconductor composites is about 0.207 eV, 5.18 eV and 10.6 eV, respectively, which is a linear trend in increase of activation energy against added nanoparticles. The increase in activation energy can be explained in term of cooper pairs' breaking due to spin-charge reflection and elections localization across these magnetic nanoparticles at the grain-boundaries of CuTl-1223 phase. These magnetic impurities suppress the tunneling of carriers between grain and grain-boundaries, which result in insulating behavior. High tendency of oxygen vacancies may also cause such increase in activation energy [31e33]. To understand the conduction mechanism and dynamic of mobility, we plotted frequency dependent real part of impedance Z/ at various temperatures from 78 K to 253 K as shown in Fig. 6 (aec). The variation of Z/ versus operating temperatures T (K) at 40 Hz frequency indicating the gradual decrease in Z/ with increasing the value of temperature are shown in the insets of Fig. 6 (aec). The maximum value of Z/ at frequency of 40 Hz and at low temperature (78 K) were found to be 2.07 103, 5.4 104, 9.8 104, for the samples with x ¼ 0, 1.0 and 2.0 wt%, respectively. At low frequencies, the value of Z/ (U) jumps to high value, this is because at low frequency, the charges at interface trap can easily follow the applied ac-electric field and yield an excess capacitance, which depends on the frequency and time constant of interface states [34]. It is observed that the magnitude of Z/ is more pronounced after the inclusion of MnFe2O4 nanoparticles in CuTl-1223 phase, which is due to the magnetic nature of these nanoparticles. The nanoparticles were settled at grain-boundaries and enhance the interface polarization mechanism, which result the decrease in acconductivity of composite. The maximum value of Z/ at frequency of 40 Hz varied from 2.07 103 to 1.6 103, 5.4 104 to 3.8 104, 9.8 104 to 6.0 104 at 78e253 K for the samples with x ¼ 0, 1.0 and 2.0 wt%, respectively. Initially, the value of Z/ decreases with frequency and temperature for all the samples, this may be due to the slow dynamics of mobility. At higher frequency, Z/ merge regardless of temperature, this is due to the release of space charges at grain-boundaries as a result of reduction in the barrier properties of material with the increase of temperature [23]. To understand the relaxation process within crystallite grains and grain-boundaries, we plotted graphs between imaginary part of impedance Z// and test frequency from 40 Hz to 10 MHz over a wide range of temperatures from 78 K to 253 K as shown in Fig. 7(aec). The maximum values of Z//max were found to be 9.1 102, 2.5 104,-4.3 104 at low temperature (78 K) for the sample with x ¼ 0, 1.0 and 2.0 wt%, respectively. In the inset of Fig. 7(aec), the variation of Z//max versus operating temperatures is shown, which indicates the gradual decrease in Z//max with increasing value of operating temperature. This is due to the release of space charges at grain-boundaries. This decrease in Z//max with temperature may be responsible factor for the increase in conductivity of the material. By increasing concentration of added MnFe2O4 nanoparticles in CuTl-1223 matrix, the magnitude of Z//max were increased, which is probably due to the magnetic interaction of these nanoparticles with mobile carriers at grainboundaries [35]. The maximum value of Z//max varied from 9.1 102 to 7.3 102, 2.5 104 to 1.8 104, 4.3 104 to 2.5 104 at temperatures from 78 to 253 K for the samples with x ¼ 0, 1.0 and 2.0 wt%, respectively. The curves indicate that by adding these MnFe2O4 nanoparticles in bulk CuTl-1223 matrix, the
Fig. 6. (aec). Variation in real part (Z/) of impedance versus frequency (40 Hz- 10 MHz) for (MnFe2O4)x/CuTl-1223 with (a) x ¼ 0 at T ¼ 78 Ke253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K and in the insets there are shown the variation of Z/ versus operating temperature (T) at frequency of 40 Hz.
peak positions of Z//max shifted towards lower frequency. This is because the nanoparticles are settled at grain-boundaries, which enhance the charge accumulation at grain-boundaries and suppress the tunneling of electrons between grain and grainboundaries. Now the grain-boundaries offered more resistance
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u ¼ 2pf ¼
1
¼
t
701
1 Cgb Rgb
(2)
By increasing ‘Rgb’ and ‘Cgb’, the relaxation time (t) increases, this causes the shifting of peaks to lower frequency. The shifting of these peaks towards the lower frequency arises due to increasing losses in the materials, which is witnessed from the decrease in acconductivity by increasing concentration of MnFe2O4 nanoparticles. The dielectric relaxation time is an important parameter to understand the conduction mechanism in materials. In Debye relaxation process all dipoles in the system relax with the same relaxation time (single relaxation time approximation). Ideal Debye relaxation has no practical application except in liquids and perfect crystal. But real materials usually exhibit broad relaxation time distribution and can be characterized by a pre-exponential factor. The non-Debye type relaxation phenomenon can be understood by following relation;
∅ðtÞ ¼ e
Fig. 7. (aec). Variation in imaginary part (Z//) of impedance versus frequency (40Hz10 MHz) for (MnFe2O4)x/CuTl-1223 with (a) x ¼ 0 at T ¼ 78 K to 253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K and in the insets there are shown the variation of Z//max versus operating temperature (T).
t
t
b
with
0
(3)
where ∅ðtÞ is the time evolution of an electric field within the composites, t is the relaxation time and b is Kohlrausch parameter, which can be calculated by the relation b ¼ 1.14/FWHM. We plotted an impedance master curves at various temperatures for (MnFe2O4)x/CuTl-1223 composites with x ¼ 0, 1.0 and 2.0 wt% between normalized parameters Z///Z//max and f/fmax to confirm that the distribution of relaxation time is temperature dependent or not, as shown in Fig. 8(aec). The coincidence of all curves into one master curves at operating temperatures indicates that the distribution of relaxation time (dynamics process) is nearly temperature independent with non-exponential type of conduction process. The value of full width half maximum (FWHM) for x ¼ 0, 1.0 and 2.0 wt% are found to be greater than 1.14 decades (ideal Debye characteristics b ¼ 1), causes the smaller value of b, which is found to be less than one for all the samples. This is the confirmation of deviation from Debye-type relaxation. These outcomes suggest that the ions migration takes place through hopping mechanism [36]. The ac-conductivity (sac) of (MnFe2O4)x/CuTl-1223 (x ¼ 0, 1, and 2 wt%) nanoparticles-superconductor composites at operating temperatures from 78 K to 253 K with frequency range from 40 Hz to 10 MHz are shown in Fig. 9 (aec). The value of sac at lower frequency of 40 Hz were found around 6.6 106, 2.04 107, 1.05 107 at 78 K temperature for the samples with x ¼ 0, 1.0, and 2.0 wt%, respectively. This decrease in sac with increasing concentration of added these MnFe2O4 nanoparticles, clearly the result of formation of space charges by magnetic interaction of these MnFe2O4 nanoparticles with charge carriers at grain-boundaries. In the insets of Fig. 9 (aec), the variation of sac versus operating temperatures at low frequency of 40 Hz is shown. The value of sac at lower frequency of 40 Hz varied from 6.7 106 to 8.67 106, 2.04 107 to 3.3 107 and 1.05 107 to 1.7 107 at temperatures 78 Ke253 K for the samples with x ¼ 0, 1.0 and 2.0 wt%, respectively. This is attributed to the gradual increase in sac with increasing value of operating temperatures for all the samples, which enhances hopping rate of mobile charge carriers [37]. 4. Conclusion
Z//max
‘Rgb’ and capacitance ‘Cgb’. The shifting of peak positions of towards lower frequency can be explained by this increase of the grain-boundaries resistance ‘Rgb’ and capacitance ‘Cgb’ according to relation;
(MnFe2O4)x/CuTl-1223 (x ¼ 0, 1.0 and 2.0 wt%) nanoparticlessuperconductor composites were synthesized and their structural, morphological, superconducting and electrical properties were investigated. XRD spectra indicated that tetragonal structure
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Fig. 8. . (aec). The master curves of normalized parameters Z///Z//max verses f/fmax for (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composite with (a) x ¼ 0 at T ¼ 78 Ke253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K.
of CuTl-1223 matrix remained unaltered after addition of MnFe2O4 nanoparticles. SEM images confirmed the decrease in density of voids and pores of the host bulk CuTl-1223 superconductor after addition of MnFe2O4 nanoparticles. Superconducting properties
Fig. 9. (aec). Variation in ac-conductivity (sac) versus frequency (40 Hz- 10 MHz) for (MnFe2O4)x/CuTl-1223 with (a) x ¼ 0 at T ¼ 78 Ke253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K and in the insets there are shown the variation of sac versus operating temperature (T) at frequency of 40 Hz.
were decreased with the increase of MnFe2O4 nanoparticles content in CuTl-1223 matrix, which was attributed to spin-charge reflection and trapping of charge carriers across these nanoparticles at grain-boundaries of CuTl-1223 phase. Temperature and frequency dependence of complex impedance has shown the non-
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Debye type relaxation in this material. The broadness of Nyquist plot of Z(U)/ versus Z(U)// with the addition of MnFe2O4 nanoparticles is due to the enhancement of the interface polarization mechanism. Complex impedance plots revealed that the grainboundaries are more resistive than grains. MnFe2O4 nanoparticles suppressed the tunneling of electrons between grain and grainboundaries which resulted in increasing activation energy. The impedance master curves indicated that the distribution of relaxation time (dynamic process) was nearly temperature independent. The decrease in ac-conductivity with increasing contents of these nanoparticles indicated the enhancement of space charges at grainboundaries. In conclusion, the superconducting properties were over all suppressed and dielectric response was improved after addition of these magnetic MnFe2O4 nanoparticles in CuTl-1223 superconducting phase.
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Conduction mechanism and impedance spectroscopy of (MnFe2O4)x/ CuTl-1223 nanoparticles-superconductor composites M. Naveed a, M. Mumtaz a, *, Rashid Khan a, Abrar A. Khan a, M. Nasir Khan b a b
Materials Research Laboratory, Department of Physics, FBAS, International Islamic University (IIU), Islamabad, 44000, Pakistan Central Diagnostic Laboratory, Physics Division PINSTECH, Islamabad, 45500, Pakistan
a r t i c l e i n f o
a b s t r a c t
Article history: Received 27 February 2017 Received in revised form 2 April 2017 Accepted 4 April 2017
Manganese ferrite (MnFe2O4) nanoparticles and Cu0.5Tl0.5Ba2Ca2Cu3O10-d (CuTl-1223) superconducting phase were prepared by sol-gel and solid-state reaction methods, respectively. MnFe2O4 nanoparticles were added in CuTl-1223 matrix to get (MnFe2O4)x/CuTl-1223 (x ¼ 0 ~ 2.0 wt.%) nanoparticles esuperconductor composites. Different experimental techniques like XRD, SEM, R-T measurements and Impedance spectroscopy were used to characterize these composites. It was observed that crystal structure of host CuTl-1223 phase remained unaltered after addition of MnFe2O4 nanoparticles, which indicated about the occupancy of these nanoparticles at grain-boundaries. Over all decreasing trend in superconducting properties may be attributed to spin-charge reflection and trapping of charge carriers across these magnetic MnFe2O4 nanoparticles at grain-boundaries of CuTl-1223 phase. In complex impedance spectroscopy (CIS), role of MnFe2O4 nanoparticles at the grain-boundaries of host CuTl-1223 phase was also investigated. The decrease in impedance (Z) with increasing temperature witnessed the occurring of thermally activated processes in the system. Higher value of activation energy at grainboundaries showed that grain-boundaries are more resistive than grains due to non-stoichiometric distribution of oxygen and dangling bonds at grain-boundaries. The impedance master curves indicated that the distribution of relaxation time (dynamic process) is nearly temperature independent. The decrease in ac-conductivity with increasing content of these nanoparticles indicated the enhancement of space charges at grain-boundaries. © 2017 Elsevier B.V. All rights reserved.
Keywords: (MnFe2O4)x/CuTl-1223 composites Impedance spectroscopy Grain-boundaries Thermally activated processes
1. Introduction Complex impedance spectroscopy (CIS) is most widely used technique based on electric impedance measurement of a material in wide range of frequency (impedance spectrum) [1e3]. This technique is useful to investigate the relaxation processes in a complex inhomogeneous system over a wide range of characteristic times. This method enables to make a link between the characterization of bulk properties and the individual constituents of complex material [4e7]. In present time, impedance spectroscopy is an important characterization technique, which provides information regarding the movement of carriers in superconductors at various temperatures, which may be useful in understanding the superconducting phenomenon [8]. The bulk cuprates superconductors are complex materials, which consist of grains separated by
* Corresponding author. E-mail address: [email protected] (M. Mumtaz). http://dx.doi.org/10.1016/j.jallcom.2017.04.034 0925-8388/© 2017 Elsevier B.V. All rights reserved.
grain-boundaries. Cuprates superconductors have vast applications in technical field such as power transmission lines, Josephson junction and electronic devices. This method helps to make proper separation among the bulk grains, grain-boundaries with electrode-interface properties between grains and grainboundaries [9e12]. Therefore, cuprates superconductors have received the great deal of attention due to their potential technological aspects. In impedance measurement of Ba2CoNbO6 and Sr2CoNbO6 ceramics in frequency range of 1 Hze10 MHz, both showed separate relaxation processes with distinguishable relaxation times, which was most probably due to grains and grain-boundaries contributions [13]. Impedance study of Ba2BiTaO6 exhibited inter-grains and intra-grains contributions. The hopping of oxygen vacancies gave rise to conduction mechanism, while impurities was not influenced the overall conduction mechanism [14]. Fitting of Cole-Cole data showed two different conduction processes in p-CuIn3Se5 semiconductor. When temperature was increased, the calculated values of grains and grain-boundaries resistance were decreased, which
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was in agreement with the Arrhenius law used for calculation of activation energies. The estimated activation energies for relaxation mechanism were compared with those values, which were found during electric modulus analysis [15]. Ba(Zr0.25Ti0.75)O3 showed a non-Debye relaxation phenomenon, when relaxation frequency peak shifted toward positive side with increase in temperature. Frequency and temperature dependent ac-conductivity exhibited that the conduction process was thermally activated and was shown negative temperature coefficient of resistance (NTCR) behaviour [16]. At low frequency and for T < 175 K in Fe2TiO5 compound, the grains contribution was dominated but for T 175 K, the grain-boundaries contribution was activated at low frequencies [17]. ZnO nanoparticles substitution in polycrystalline lead-free BaZr0.15Ti0.85O3 ceramics has increased grain-boundaries resistance, while intra-grains resistance has been decreased with the increase in temperature. Relaxation time was decreased in both the cases [18]. Substitution of Nd in Y2/3Cu3Ti4O12 ceramics has increased the activation energies of grain-boundaries, and slightly influenced the dielectric relaxation process [19]. The polycrystalline KBa2V5O15 ceramics showed NTCR behaviour with grains ‘G’ and grain-boundaries ‘Gb’ contributions for separate frequency values [20]. Impedance study of Ba5GdTi3V7O30 suggested that bulk resistance ‘Rb’ was decreased with the rise in temperature, which showed NTCR behaviour [21]. To improve the conductivity of YBaCuO superconducting particles in a polypropylene matrix, different amounts of carbon black and copper were incorporated into the binary based system and investigated the effects of these conductor fillers on superconductor polymer composites by using complex impedance spectroscopy. The conduction mechanism of materials consisting of polymers and conductor particles significantly depends on affinity and adhesion between particles and polymers [22]. The temperature dependent impedance of barium cuprates BaCuO2þd with the frequency in the ranges from 5 Hz to 105 Hz proved the semiconducting nature of barium cuprates. The increase in conductivity with increasing temperature due to release of space charge as a result of the reduction in the barrier properties of barium cuprates was observed [23]. Detailed literature review on CuTl-1223 superconducting phase revealed that there is still no research work has been reported on frequency and temperature dependent impedance properties to make proper separation among the bulk grains, grain-boundaries with electrode-interface properties between grains and grainboundaries. Therefore, we have investigated the role of MnFe2O4 nanoparticles with different concentrations on conduction mechanism and electrical response of Cu0.5Tl0.5Ba2Ca2Cu3O10-d (CuTl1223) superconductor.
697
2.2. Preparation of (MnFe2O4)x/(CuTl-1223) nanoparticlessuperconductor composites Cu0.5Tl0.5Ba2Ca2Cu3O10-d (CuTl-1223) phase was synthesized by solid-state reaction method. Cu2(CN)2, Ca(NO3)2 and Ba(NO3)2 compounds were mixed in appropriate ratios and then ground for 2 h. The ground material was loaded in quartz boats and heated in chamber furnace at 860 C for 24 h and allowed to cool down up to room temperature. Repeat the same heating process with intermediate grinding for 1 h at least. MnFe2O4 nanoparticles with different wt% and Tl2O3 were mixed with precursor Cu0.5Ba2Ca2Cu3O10-d material and grounded each sample for 1 h. The ground powder was pelletized in gold capsules and placed in pre-heated furnace for sintering at 860 C for 10 min and got (MnFe2O4)x/ (CuTl-1223) with (x ¼ 0, 0.5, 1.0, 1.5 and 2.0 wt%). Structural and phase purity of (MnFe2O4)x/(CuTl-1223) nanoparticles-superconductor composites were determined by using XRD. Morphology was examined by SEM images. Impedance properties and conduction mechanism in frequency ranges from 40 Hz to 10 MHz at various temperatures from normal state to superconducting (i.e. 253 K to 78 K) was investigated by complex impedance spectroscopy. 3. Results and discussion 3.1. XRD analysis XRD pattern of MnFe2O4 nanoparticles is shown in the inset of Fig. 1. Various planes like (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) are indexed according to the cubic structure corresponding to different 2q values such as 29.66 , 34.90 , 42.51, 56.21 and 61.71, respectively. Average crystallite size is calculated by DebyeScherer's formula, which is found to be 20 nm. Almost all the diffraction peaks are well indexed according to cubic crystal structure of MnFe2O4 nanoparticles and no prominent peaks of other impurity were detected. Representative XRD spectra of (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composites with x ¼ 0 and 2.0 wt% are shows in Fig. 1. The well indexed diffraction peaks in these XRD spectra show the dominance of host CuTl-1223 phase. XRD spectrum clearly shows that the tetragonal structure of host CuTl-1223 superconducting phase remains unaltered after addition of these MnFe2O4 nanoparticles. In these XRD spectra, some un-indexed peaks of very low intensities are also observed, which shows the presence of unknown impurities and some other superconducting phases [24]. 3.2. SEM analysis
2. Experimental details 2.1. Preparation of MnFe2O4 nanoparticles Sol-gel method was used to synthesize MnFe2O4 nanoparticles. Initially, two solutions were prepared separately. First solution was prepared by mixing iron nitrate and manganese nitrate in ethanol with appropriate ratios and second solution was prepared by mixing citric acid and distilled water. Now second solution was mixed slowly in first solution. To maintain pH level up to 5, ammonia was added drop by drop with continuous stirring process. Solution was stirred constantly at 70 C till the formation of gel. The gel was placed in oven for drying at 110 C for overnight. The dried material was ground in agate mortar and pestle to get powder. Then annealed powder was placed in a furnace at 900 C for 2 h. The resultant material was again ground to get fine powder of MnFe2O4 nanoparticles as end product.
Scanning electron microscopy (SEM) is a characterization technique, which is widely used to study surface features and morphological structure analysis. SEM images of (MnFe2O4)x/CuTl1223 nanoparticles-composites with x ¼ 0, 2.0 wt% are shown in the inset of Fig. 2. Pure sample with x ¼ 0 shows higher density of inter-grain weak-links, voids and pores. The empty spaces healed up and inter-grain weak-links are improved and grain size was also increased after addition of MnFe2O4 nanoparticles. SEM images confirm the decrease in density of voids and pores of host bulk CuTl-1223 superconductor after addition of (MnFe2O4) up to 2.0 wt %. The addition of these nanoparticles changes the thermodynamics of reaction, which have changed the grains morphology of the material. 3.3. Resistivity measurement Resistivity
versus
temperature
(R-T)
measurements
of
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Fig. 1. XRD spectra of (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composites for x ¼ 0 and 2.0 wt%. In the inset there are shown XRD spectrum of MnFe2O4 nanoparticles.
(MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composites with x ¼ 0 and 2 wt% is shown in Fig. 2. The value of Tc onset (K) was found to be 94 K for un-added pure CuTl-1223 sample, which was decreased to 92 K for the sample with x ¼ 2.0 wt% concentration of MnFe2O4 nanoparticles. This decrease in Tconset (K) with MnFe2O4 nanoparticles addition in CuTl-1223 phase is most probably due to presence of these magnetic natured nanoparticles at grainboundaries. The enhanced the charge accumulation at grainboundaries has reduced the mobility of charge carriers in the material. Normal state resistivity showed non-monotonic variation with addition of MnFe2O4 nanoparticles inclusion in host CuTl1223 phase, which is most probably due to due to inhomogeneous and non-uniform distribution of these nanoparticles at grain-boundaries [25].
3.4. Impedance analysis Complex impedance analysis has been used to correlate the ac-
Fig. 2. Resistivity vs temperature measurements of (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composite samples for x ¼ 0 and 2.0 wt%. In the inset, there are shown SEM images of (MnFe2O4)x/CuTl-1223 composites with (a) x ¼ 0 and (b) 2.0 wt%.
electrical properties with the microstructure of material. We have studied the impedance properties of (MnFe2O4)x/CuTl-1223 (x ¼ 0, 0.5, 1.0, 1.5 and 2.0 wt%) nanoparticles-superconductor composites. The electrical response of ferrites and cuprates can be understood in detail with the help of some theoretical models. Koop's model is considered as a two-layer model (i) grains and (ii) grainboundaries. The grains are considered as conducting layers and grain-boundaries are considered as resistive layers. The flow of charge carriers have to face hurdle from conducting region (grain) to non-conducting region (grain-boundaries) [26]. To understand this, we can modelled an equivalent circuit, which contains a grain resistance ‘Rg’ connected in series with grain-boundaries resistance ‘Rgb’ parallel with grain-boundaries capacitance ‘Cgb’ as shown in Fig. 3. The representative Nyquist plots of (MnFe2O4)x/CuTl-1223 (x ¼ 0, 1.0 and 2.0 wt%) nanoparticles-superconductor composites at various temperatures from 78 K to 253 K, in frequency ranges from 40 Hz to 10 MHz are shown in Fig. 4 (aec). We observed that the diameters of semicircles get depressed with the increase of temperature, which indicated the non-Debye type relaxation phenomenon in the material. The decrease in diameter of semicircle behaviour proves that conduction mechanism is thermally activated [23]. From impedance spectrum, we can easily deduce ‘Rg’ and ‘Rgb’ values. The left intercept of the semicircle with Z/-axis gives ‘Rg’ value and the right one gives the total resistance R ¼ Rgb þ Rg. From Nyquist plot, it was found that ‘Rgb’ values are greater than ‘Rg’, which proves that the grain-boundaries are more resistive than grains. This is probably due to the non-stoichiometric distribution of oxygen, spin-charge reflection and dangling bonds on the grain-boundaries [27,28]. At normal state the electrons can
Fig. 3. The electrical equivalent circuit for impedance.
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result enhance the polarization and dielectric response of the material. The enhanced charge accumulation at grain-boundaries causes the reduction of flow of charge carriers. This band spread of Z/ (U) versus Z// (U) with the addition of MnFe2O4 nanoparticles clearly indicate that the dielectric response of the material has been improved. We found that by increasing temperature from 78 K to 253 K, the value of Z/ (U) were decreased from 2.0 103 to 1.6 103, 5.4 104 to 3.7 104 and 9.7 105 to 6.0 104 for x ¼ 0, 1.0, and 2.0 wt% respectively, which is probably due to release of space charges at grain-boundaries by increasing temperatures. The activation energy of grains and grain-boundaries were calculated by applying small polaron hopping model (SPH);
R Ec ¼ A0 ekT T
(1)
where Ao is a pre-exponential factor, R is resistance, T is the absolute temperature, Ec is activation energy of conduction and k is the Boltzmann constant. From the slopes of fitted straight lines, the calculated activation energies of grains and grain-boundaries for (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composites with x ¼ 0 and 2 wt% are shown in Fig. 5 (a, b). The calculated activation energy for conduction on grains was found to be 0.215 meV, 0.2158 meV and 0.216 meV, and for grains-boundaries was 0.207 eV, 5.18 eV and 10.6 eV for x ¼ 0, 1.0 and 2.0 wt%, respectively. The higher value of activation energy at grain-
Fig. 4. (aec). The Nyquist plot of impedance between real part (Z/) and imaginary part (Z//) for (MnFe2O4)x/CuTl-1223 with (a) x ¼ 0 at T ¼ 78 Ke253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K.
be captured or localized across these magnetic nanoparticles at the grain-boundaries of the bulk CuTl-1223 superconducting matrix. In superconducting state, cooper pairs can easily be broken in to normal electrons across these magnetic nature nanoparticles settled at grain-boundaries acting as efficient scattering centers. These electrons can also be accumulated at the grain-boundaries due to magnetic interaction across these magnetic nanoparticles. In this way these electrons pile up at the grain-boundaries and in
Fig. 5. (a, b). (a) The plots of Rg/T versus 1000/T for x ¼ 0, 1.0 and 2.0 wt%, (b) The plots of Rgb/T versus 1000/T for x ¼ 1.0 wt% and x ¼ 2.0 wt% and in the inset of (b) for x ¼ 0%.
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boundaries clearly shows that the resistive property is more prominent at grain-boundaries than grains [29,30]. In superconducting materials, the number of carriers is much larger as compared to semiconductor, which piles up across these magnetic MnFe2O4 nanoparticles at grain-boundaries causing larger potential barrier for the mobile charge carriers. We found the activation energy of grain-boundaries for (MnFe2O4)x/CuTl-1223; x ¼ 0, 1.0 and 2.0 wt% nanoparticle-superconductor composites is about 0.207 eV, 5.18 eV and 10.6 eV, respectively, which is a linear trend in increase of activation energy against added nanoparticles. The increase in activation energy can be explained in term of cooper pairs' breaking due to spin-charge reflection and elections localization across these magnetic nanoparticles at the grain-boundaries of CuTl-1223 phase. These magnetic impurities suppress the tunneling of carriers between grain and grain-boundaries, which result in insulating behavior. High tendency of oxygen vacancies may also cause such increase in activation energy [31e33]. To understand the conduction mechanism and dynamic of mobility, we plotted frequency dependent real part of impedance Z/ at various temperatures from 78 K to 253 K as shown in Fig. 6 (aec). The variation of Z/ versus operating temperatures T (K) at 40 Hz frequency indicating the gradual decrease in Z/ with increasing the value of temperature are shown in the insets of Fig. 6 (aec). The maximum value of Z/ at frequency of 40 Hz and at low temperature (78 K) were found to be 2.07 103, 5.4 104, 9.8 104, for the samples with x ¼ 0, 1.0 and 2.0 wt%, respectively. At low frequencies, the value of Z/ (U) jumps to high value, this is because at low frequency, the charges at interface trap can easily follow the applied ac-electric field and yield an excess capacitance, which depends on the frequency and time constant of interface states [34]. It is observed that the magnitude of Z/ is more pronounced after the inclusion of MnFe2O4 nanoparticles in CuTl-1223 phase, which is due to the magnetic nature of these nanoparticles. The nanoparticles were settled at grain-boundaries and enhance the interface polarization mechanism, which result the decrease in acconductivity of composite. The maximum value of Z/ at frequency of 40 Hz varied from 2.07 103 to 1.6 103, 5.4 104 to 3.8 104, 9.8 104 to 6.0 104 at 78e253 K for the samples with x ¼ 0, 1.0 and 2.0 wt%, respectively. Initially, the value of Z/ decreases with frequency and temperature for all the samples, this may be due to the slow dynamics of mobility. At higher frequency, Z/ merge regardless of temperature, this is due to the release of space charges at grain-boundaries as a result of reduction in the barrier properties of material with the increase of temperature [23]. To understand the relaxation process within crystallite grains and grain-boundaries, we plotted graphs between imaginary part of impedance Z// and test frequency from 40 Hz to 10 MHz over a wide range of temperatures from 78 K to 253 K as shown in Fig. 7(aec). The maximum values of Z//max were found to be 9.1 102, 2.5 104,-4.3 104 at low temperature (78 K) for the sample with x ¼ 0, 1.0 and 2.0 wt%, respectively. In the inset of Fig. 7(aec), the variation of Z//max versus operating temperatures is shown, which indicates the gradual decrease in Z//max with increasing value of operating temperature. This is due to the release of space charges at grain-boundaries. This decrease in Z//max with temperature may be responsible factor for the increase in conductivity of the material. By increasing concentration of added MnFe2O4 nanoparticles in CuTl-1223 matrix, the magnitude of Z//max were increased, which is probably due to the magnetic interaction of these nanoparticles with mobile carriers at grainboundaries [35]. The maximum value of Z//max varied from 9.1 102 to 7.3 102, 2.5 104 to 1.8 104, 4.3 104 to 2.5 104 at temperatures from 78 to 253 K for the samples with x ¼ 0, 1.0 and 2.0 wt%, respectively. The curves indicate that by adding these MnFe2O4 nanoparticles in bulk CuTl-1223 matrix, the
Fig. 6. (aec). Variation in real part (Z/) of impedance versus frequency (40 Hz- 10 MHz) for (MnFe2O4)x/CuTl-1223 with (a) x ¼ 0 at T ¼ 78 Ke253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K and in the insets there are shown the variation of Z/ versus operating temperature (T) at frequency of 40 Hz.
peak positions of Z//max shifted towards lower frequency. This is because the nanoparticles are settled at grain-boundaries, which enhance the charge accumulation at grain-boundaries and suppress the tunneling of electrons between grain and grainboundaries. Now the grain-boundaries offered more resistance
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u ¼ 2pf ¼
1
¼
t
701
1 Cgb Rgb
(2)
By increasing ‘Rgb’ and ‘Cgb’, the relaxation time (t) increases, this causes the shifting of peaks to lower frequency. The shifting of these peaks towards the lower frequency arises due to increasing losses in the materials, which is witnessed from the decrease in acconductivity by increasing concentration of MnFe2O4 nanoparticles. The dielectric relaxation time is an important parameter to understand the conduction mechanism in materials. In Debye relaxation process all dipoles in the system relax with the same relaxation time (single relaxation time approximation). Ideal Debye relaxation has no practical application except in liquids and perfect crystal. But real materials usually exhibit broad relaxation time distribution and can be characterized by a pre-exponential factor. The non-Debye type relaxation phenomenon can be understood by following relation;
∅ðtÞ ¼ e
Fig. 7. (aec). Variation in imaginary part (Z//) of impedance versus frequency (40Hz10 MHz) for (MnFe2O4)x/CuTl-1223 with (a) x ¼ 0 at T ¼ 78 K to 253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K and in the insets there are shown the variation of Z//max versus operating temperature (T).
t
t
b
with
0
(3)
where ∅ðtÞ is the time evolution of an electric field within the composites, t is the relaxation time and b is Kohlrausch parameter, which can be calculated by the relation b ¼ 1.14/FWHM. We plotted an impedance master curves at various temperatures for (MnFe2O4)x/CuTl-1223 composites with x ¼ 0, 1.0 and 2.0 wt% between normalized parameters Z///Z//max and f/fmax to confirm that the distribution of relaxation time is temperature dependent or not, as shown in Fig. 8(aec). The coincidence of all curves into one master curves at operating temperatures indicates that the distribution of relaxation time (dynamics process) is nearly temperature independent with non-exponential type of conduction process. The value of full width half maximum (FWHM) for x ¼ 0, 1.0 and 2.0 wt% are found to be greater than 1.14 decades (ideal Debye characteristics b ¼ 1), causes the smaller value of b, which is found to be less than one for all the samples. This is the confirmation of deviation from Debye-type relaxation. These outcomes suggest that the ions migration takes place through hopping mechanism [36]. The ac-conductivity (sac) of (MnFe2O4)x/CuTl-1223 (x ¼ 0, 1, and 2 wt%) nanoparticles-superconductor composites at operating temperatures from 78 K to 253 K with frequency range from 40 Hz to 10 MHz are shown in Fig. 9 (aec). The value of sac at lower frequency of 40 Hz were found around 6.6 106, 2.04 107, 1.05 107 at 78 K temperature for the samples with x ¼ 0, 1.0, and 2.0 wt%, respectively. This decrease in sac with increasing concentration of added these MnFe2O4 nanoparticles, clearly the result of formation of space charges by magnetic interaction of these MnFe2O4 nanoparticles with charge carriers at grain-boundaries. In the insets of Fig. 9 (aec), the variation of sac versus operating temperatures at low frequency of 40 Hz is shown. The value of sac at lower frequency of 40 Hz varied from 6.7 106 to 8.67 106, 2.04 107 to 3.3 107 and 1.05 107 to 1.7 107 at temperatures 78 Ke253 K for the samples with x ¼ 0, 1.0 and 2.0 wt%, respectively. This is attributed to the gradual increase in sac with increasing value of operating temperatures for all the samples, which enhances hopping rate of mobile charge carriers [37]. 4. Conclusion
Z//max
‘Rgb’ and capacitance ‘Cgb’. The shifting of peak positions of towards lower frequency can be explained by this increase of the grain-boundaries resistance ‘Rgb’ and capacitance ‘Cgb’ according to relation;
(MnFe2O4)x/CuTl-1223 (x ¼ 0, 1.0 and 2.0 wt%) nanoparticlessuperconductor composites were synthesized and their structural, morphological, superconducting and electrical properties were investigated. XRD spectra indicated that tetragonal structure
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Fig. 8. . (aec). The master curves of normalized parameters Z///Z//max verses f/fmax for (MnFe2O4)x/CuTl-1223 nanoparticles-superconductor composite with (a) x ¼ 0 at T ¼ 78 Ke253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K.
of CuTl-1223 matrix remained unaltered after addition of MnFe2O4 nanoparticles. SEM images confirmed the decrease in density of voids and pores of the host bulk CuTl-1223 superconductor after addition of MnFe2O4 nanoparticles. Superconducting properties
Fig. 9. (aec). Variation in ac-conductivity (sac) versus frequency (40 Hz- 10 MHz) for (MnFe2O4)x/CuTl-1223 with (a) x ¼ 0 at T ¼ 78 Ke253 K, (b) x ¼ 1.0 wt% at T ¼ 78 Ke253 K, (c) x ¼ 2.0 wt% at T ¼ 78 Ke253 K and in the insets there are shown the variation of sac versus operating temperature (T) at frequency of 40 Hz.
were decreased with the increase of MnFe2O4 nanoparticles content in CuTl-1223 matrix, which was attributed to spin-charge reflection and trapping of charge carriers across these nanoparticles at grain-boundaries of CuTl-1223 phase. Temperature and frequency dependence of complex impedance has shown the non-
M. Naveed et al. / Journal of Alloys and Compounds 712 (2017) 696e703
Debye type relaxation in this material. The broadness of Nyquist plot of Z(U)/ versus Z(U)// with the addition of MnFe2O4 nanoparticles is due to the enhancement of the interface polarization mechanism. Complex impedance plots revealed that the grainboundaries are more resistive than grains. MnFe2O4 nanoparticles suppressed the tunneling of electrons between grain and grainboundaries which resulted in increasing activation energy. The impedance master curves indicated that the distribution of relaxation time (dynamic process) was nearly temperature independent. The decrease in ac-conductivity with increasing contents of these nanoparticles indicated the enhancement of space charges at grainboundaries. In conclusion, the superconducting properties were over all suppressed and dielectric response was improved after addition of these magnetic MnFe2O4 nanoparticles in CuTl-1223 superconducting phase.
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