CuTl-1223

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Available online at www.sciencedirect.com Progress in Natural Science Materials International Progress in Natural Science: Materials International 25 (2015) 204–208 www.elsevier.com/locate/pnsmi www.sciencedirect.com

Original Research

Synthesis and superconductivity of (Ag)x/CuTl-1223 composites Abdul Jabbar, Irfan Qasim, M. Mumtaz1, K. Nadeem Materials Research Laboratory, Department of Physics, Faculty of Basic and Applied Sciences (FBAS), International Islamic University (IIU), Islamabad 44000, Pakistan Received 27 August 2014; accepted 21 December 2014 Available online 9 July 2015

Abstract Series of (Ag)x/(Cu0.5Tl0.5Ba2Ca2Cu3O10-δ) {(Ag)x/CuTl-1223} nano-superconductor composites were synthesized with different concentrations (i.e. x ¼0 4.0 wt%) of silver (Ag) nanoparticles. Low anisotropic CuTl-1223 superconducting matrix was prepared by solid-state reaction and Ag nanoparticles were prepared by a sol–gel method separately. The required (Ag)x/CuTl-1223 composition was obtained by the inclusion of Ag nanoparticles in CuTl-1223 superconducting matrix. Structural, morphological, compositional and superconducting transport properties of these composites were investigated in detail by x-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive x-rays (EDX) spectroscopy and four-point probe electrical resistivity (ρ) measurements. The inclusion of Ag nanoparticles enhanced the superconducting properties without affecting the tetragonal structure of the host CuTl-1223 matrix. The improvement in superconducting properties of (Ag)x/CuTl1223 composites is most likely due to enhanced inter-grains coupling and increased superconducting volume fraction after the addition of metallic Ag nanoparticles at the inter-crystallite sites in the samples. The presence of Ag nanoparticles at the grain-boundaries may increase the number of flux pinning centers, which were present in the form of weak-links in the pure CuTl-1223 superconducting matrix. & 2015 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: (Ag)x/CuTl-1223 composites; Ag nanoparticles; weak-links; superconducting properties

1. Introduction The selection of superconducting materials for their practical applications demands higher values of critical transition temperature (Tc), critical current density (Jc) and critical magnetic field (Hc). It is obvious from literature that in spite of granular and porous nature of bulk (Cu0.5Tl0.5) Ba2Ca2Cu3O10 δ (CuTl-1223) phase of (Cu0.5Tl0.5) Ba2CanCun þ 1O2n þ 4 δ high Tc superconducting family has relatively higher values of Jc, Tc, Hc and lower superconducting anisotropy (γ ¼ ξab =ξc ) [1–3]. Therefore, it is necessary to study in the selected CuTl-1223 superconducting phase. The presence of inter-grain voids, impurity phases, oxygen vacancies, inhomogeneous microdefects, etc., in its bulk form due to granular nature affects the performance of this compound. This Tel.: þ92 51 9019926; fax: þ 92 51 9210256. E-mail address: [email protected] (M. Mumtaz). Peer review under responsibility of Chinese Materials Research Society. 1

issue can, however, be resolved up to some extent by filling the pores with some suitable nanostructures (nanoparticles, nanorods, etc.) at inter-granular sites in the bulk superconductor [4]. The effects of nanostructures addition on physical and structural properties of different high temperature superconducting (HTSC) families in their bulk as well as thin films were studied by different research groups [5–9]. Generally, it is observed from the literature reviews that the addition of nanostructures in HTSCs improves the inter-grain connectivity and superconductivity parameters up to certain extent without affecting the crystal structures of superconducting phases [10– 19]. The addition of nanoparticles also enhances the additional pinning effects in the bulk polycrystalline samples for the improvement of infield superconducting properties [20–22]. It was observed that low concentration of nano-ZnO addition in (Cu0.5Tl0.25Pb0.25)-1223 matrix enhanced superconducting properties, while high concentration of nano-ZnO enhanced the secondary phases and grain-boundaries resistance due to agglomeration of nano-ZnO particles [23]. The addition of

http://dx.doi.org/10.1016/j.pnsc.2015.06.001 1002-0071/& 2015 The Authors. Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

A. Jabbar et al. / Progress in Natural Science: Materials International 25 (2015) 204–208

MgO nanoparticles in Bi-2212 superconducting matrix improved Jc by enhancing the grains morphology due to improved inter-grains weak-links without altering the crystal structure [16]. It was observed that superconducting properties were improved by nano-Fe2O3 particles addition in CuTl-1223 superconductor up to certain concentration level and then started to decrease with higher concentration [24]. There was also no effect on the structural symmetry of YBa2Cu3Oy superconducting phase after the addition of Al2O3 nanoparticles. In applied magnetic field, Jc of the superconducting material was enhanced significantly by nano-Al2O3 particles addition, which can be rendered to the existence of enhanced flux pinning centers in the superconducting matrix [25]. The values of pinning force density, onset temperature, activation energy and Jc in the applied magnetic field were improved by the addition of nano-Al2O3 particles in polycrystalline (Bi, Pb)-2223 superconductor [26]. The addition of Ag nanoparticles in YBCO superconducting thin films reveals greater Jc values and improved crystal structure in the form of higher crystallinity [27]. Bulk superconductor YBCO doped by Ag nanoparticles of different sizes and concentrations showed monotonic increase in superconducting properties especially Jc, which is attributed to the improvement of crystallites connectivity and superconducting volume fraction [10]. Optimal addition of Ag and SiC nano-powder of size 30–130 nm in MgB2 superconductor has increased the artificial pinning centers along with the increase of infield superconducting properties. A further increase of SiC and Ag nano-powder above 16 wt% decreases the superconductivity of MgB2 [28]. The improvement of mechanical properties of (Bi Pb)–Sr–Ca– Cu–O superconducting matrix was also observed after nanoAg particles addition that may be due to cementing effects of these additives at the grains boundaries [29–31]. In literature, the optimal concentration of nano-Ag particles in different superconducting families that can enhance the superconducting properties without affecting the crystal chemistry was not yet clearly determined. In this paper, we reported that effects of the nano-Ag particles addition on structural and superconducting properties of CuTl-1223 matrix. We also tried to find out the optimal concentration of Ag nanoparticles to obtain the maximum improvement in superconducting transport properties without affecting the crystal structure of CuTl1223 phase. The appropriate concentration of Ag nanoparticles' addition heals up inter-grains voids to improve the weaklinks and act as facilitators for the carrier transport in bulk superconductors.

quartz boats and calcinated in chamber furnace at 860 1C for 24 h followed by furnace cooling to room temperature. The calcination steps were repeated twice following 1 h intermediate grinding each time to get the fine Cu0.5Ba2Ca2Cu3O10–δ precursor material. The nano-Ag particles were prepared by a sol– gel method separately. The precursor material was mixed with Tl2O3 and different wt% of Ag nanoparticles of 35 nm in average size at second stage and then ground again for 1 h to get (Ag)y/CuTl-1223 (y=0.0, 0.5, 1.0, 2.0, 4.0) wt% superconductor composites. The material was then pelletized under 3.8 tons/cm2 pressure and the pellets were enclosed in gold capsules for sintering at 860 1C for 10 min followed by quenching to room temperature. The structure and phase purity of material was determined by XRD (D/Max IIIC Rigaku with a CuKα source of wavelength 1.54056 Å). The morphology and composition of material were determined by SEM and EDX, respectively. A conventional four-point probe method was used for dcresistivity measurements. The phonon modes related to the vibrations of various oxygen atoms were observed by Fourier Transform Infrared (FTIR) spectroscopy in the wavenumber range of 400–700 cm 1. 3. Results and discussion The XRD pattern of Ag nanoparticles is shown in Fig. 1. The prominent peaks are indexed according to facecentered cubic (FCC) structure of Ag and the average size of Ag nanoparticles calculated by Sherrer's formula is about 35 nm. The XRD analysis shows exquisitely indexed (111), (200), (220), and (311) planes of FCC structure. The lattice parameter calculated for FCC pattern of Ag nanoparticles is about a ¼ 4.09 Å. No peak of impurity crystalline phases was observed. The typical XRD patterns of (Ag)x/CuTl-1223 composites with x ¼ 0, 1.0 and 2.0 wt%, are shown in Fig. 2. Most of the diffraction peaks are well indexed according to the tetragonal structure of CuTl-1223 phase following the P4/mmm space group and the characteristic (001) diffraction peak of this phase appeared at 2θ ¼ 5.791. The stoichiometry and crystal structure of CuTl-1223 phase remained preserved even after the addition

2. Experimental details of samples' synthesis and characterization The nano-Ag particles added bulk ceramic CuTl-1223 superconductor composites were synthesized by two-cycle solid-state reaction method. Initially, Ba(NO3)2, Ca(NO3)2 and Cu(CN) compounds were used as starting compounds to prepare Cu0.5Ba2Ca2Cu3O10–δ precursor. These starting compounds were mixed in appropriate ratios and ground in an agate mortar and pestle for 2 h. The mixed and ground material was loaded in

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Fig. 1. XRD pattern of silver (Ag) nanoparticles.

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of Ag nanoparticles. This provides evidence about the occupancy of these nanoparticles at the inter-crystallite sites and the improvement of inter-grain weak-links by filling the pores and voids. Besides the dominance of CuTl-1223 phase, few un-indexed diffraction peaks appearing in XRD patterns are possibly due to the presence of some impurities and other superconducting phases mentioned in the inset of Fig. 2. The calculated cell parameters are (a ¼ 4.45 Å, c¼ 14.45 Å), (a¼ 4.46 Å, c¼ 14.96 Å), and (a¼ 4.46 Å, c¼ 14.97 Å) for x ¼ 0, 1.0, and 2.0 wt%, respectively. Slight variation in c-axis length may be due to some strains and variation of Oδ oxygen contents after Ag nanoparticles addition in CuTl-1223 superconducting matrix. The typical SEM images and EDX spectra of (Ag)x/CuTl1223 samples for x ¼ 0, and 2 wt% are shown in Fig. 3. The improvement of inter-grains weak-links as well as the grains

Fig. 2. XRD patterns of (Ag)x/CuTl-1223 composites with (a) x¼ 0, (b) x¼ 1.0 wt%, and (c) x¼ 2.0 wt%.

size is obvious from these SEM images after the addition of Ag nanoparticles in the host CuTl-1223 superconducting phase. The improvement of inter-grains weak-links and the grains size may be due to cementing effects of Ag nanoparticles occupying the inter-grains boundaries by filling the voids and pores present in the bulk form of CuTl-1223 matrix. The improved grains size enhances the superconducting volume fraction and superconducting properties of the host material by the addition of Ag nanoparticles. The EDX spectra show the presence of different elements in the composition. The mass percentages of different elements determined from the EDX analysis are listed in Table 1. The FTIR absorption spectra of (Ag)x/CuTl-1223 composites with x¼ 0, 0.5, 1.0, 2.0 and 4.0 wt%, in the range from 400–700 cm 1 are shown in Fig. 4. The absorption bands in the range from 400 to 540 cm 1 are associated with the apical oxygen atoms and in the range around from 541 to 600 cm 1 are associated with CuO2 planar oxygen atoms [32,33]. The absorption bands in the range from 670 to 700 cm 1 are associated with Oδ atoms in the charge reservoir layer [34–38]. The apical oxygen modes of type Tl–OA–Cu(2) and Cu(1)– OA–Cu(2) are observed around 430 cm 1 and 460–499 cm 1 and CuO2 planner oxygen mode is around 536 cm 1 in the pure Cu0.5Tl0.5Ba2Ca2Cu3O10 δ samples. The nominal variation in the position of the apical oxygen modes may be due to the presence of stresses and strains in the bond lengths of the unit cells caused by the nano-Ag particles in the composites. The CuO2 planner oxygen modes and Oδ modes remained almost unaffected after the addition of nano-Ag particles in the host CuTl-1223 matrix. The FTIR study also supports our claim and objective that the Ag nanoparticles do not substitute the constituent atoms of CuTl-1223 unit cell but they occupy the inter-granular spaces, fill up the pores and heal up the cracks.

Fig. 3. Typical SEM images and EDX spectra of (Ag)x/CuTl-1223 composites with (a) x¼ 0 and (b) x¼ 2.0 wt%.

A. Jabbar et al. / Progress in Natural Science: Materials International 25 (2015) 204–208 Table 1 Elemental analysis by EDX of (Ag)x/CuTl-1223 composites with (a) 0 and (b) 2.0 wt%. Elements

OK Ca K Cu L Ba L Tl M Ag K Total

(a): x ¼0

(b): x ¼2.0 wt%

KeV

Mass%

Atom%

KeV

Mass%

Atom%

0.535 3.695 0.952 4.484 2.325 –

20.68 8.87 26.93 31.83 11.70 – 100.00

58.05 9.94 19.03 10.41 2.57 – 100.00

0.540 3.702 0.960 4.492 2.315 1.525

20.22 8.78 27.48 30.63 11.12 1.77 100.00

55.96 9.70 19.15 9.88 2.41 2.90 100.00

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room temperature down to onset of superconductivity with zero resistivity critical temperature {Tc (R ¼ 0)} around 73 K, 95 K, 98 K, 102 K and 91 K for x¼ 0, 0.5, 1.0, 2.0 and 4.0 wt %, respectively. These measurements show that the value of Tc (R ¼ 0) is increased after the addition of nano-Ag particles matrix till x ¼ 2.0 wt% concentration and then is suppressed on further increase of nanoparticles concentration into CuTl-1223. The increase in Tc is an evidence of improved superconducting volume fraction after nano-Ag particles addition in the host CuTl-1223 superconducting matrix. The improved inter-grain weak-links facilitate the charge carriers transport processes and reduce the energy losses across the grain-boundaries. But the superconducting volume fraction starts to decrease after certain optimum inclusion level of Ag nanoparticles, which causes the suppression of superconductivity parameters. The nonsuperconducting metallic Ag nanoparticles reduce the superconducting volume fraction beyond certain optimum level of Ag nanoparticles inclusion in superconducting state of CuTl1223 matrix. After a certain optimum level of Ag nanoparticles addition, the agglomeration and segregation of these nanoparticles result in the reduction of Tc by various mechanisms like scattering, pair-breaking, etc. [39,40]. Still, there are many issues to be addressed in future studies like homogeneous and uniniform distribution of Ag nanoparticles at the inter-grain boundaries of the host CuTl-1223 superconducting matrix. The effects on the superconductivity of the host (Cu0.5Tl0.5) Ba2CanCun þ 1O2n þ 4 δ superconducting family with addition of different sizes and concentrations of nanoparticles and their response in applied magnetic field will also be addressed in our future research work in this area.

4. Conclusion Fig. 4. FTIR absorption spectra of (Ag)x/CuTl-1223 composites with (a) x ¼0, (b) x¼ 0.5 wt%, (c) x ¼1.0 wt%, (d) x¼ 2.0 wt% and (e) x¼ 4.0 wt%.

Fig. 5. Resistivity versus temperature measurements of (Ag)x/CuTl-1223 composites with x ¼0, 0.5, 1.0, 2.0 and 4.0 wt%.

The resistivity versus temperature measurements of (Ag)x/ CuTl-1223 composites with different contents of nano-Ag particles from 0 rx r 4.0 wt% are shown in Fig. 5. All these samples have shown a metallic variation in the resistivity from

Ag nanoparticles are metallic in nature and their influence on structural and superconducting properties of CuTl-1223 matrix has been investigated thoroughly to locate the optimal conditions for the enhanced superconducting parameters. The addition of nano-Ag particles in the host CuTl-1223 superconducting matrix shows no change in the stoichiometry and crystal structure. The added Ag nanoparticles occupy intercrystallite sites and improve the weak links. This improvement leads to the enhancement of superconducting properties of (Ag)x/CuTl-1223 composite matrix, increased inter-grains coupling and superconducting volume fraction. The systematic increase in Tc and decrease in ρ300 K (Ω-cm) of (Ag)x/CuTl1223 composites after addition of Ag nanoparticles has been observed up to some optimum level of concentration which is x¼ 2.0 wt%. Suppression of superconducting properties after the addition of the nanoparticles beyond this optimum level has been observed, which is probably due to degradation of samples quality, non homogeneous distribution of Ag nanoparticles causing agglomeration and segregation at the grainboundaries of host CuTl-1223 matrix. Electrically Conductive nature of metallic Ag nanoparticles is responsible for the reduction in normal state resistivity and improvement in superconducting volume fraction due to their facilitating

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