J Supercond Nov Magn (2013) 26:3209–3214 DOI 10.1007/s10948-013-2177-5
O R I G I N A L PA P E R
X-Ray and High-Resolution Neutron Diffraction Studies on Ndx Y1−x Ba2 Cu3 O7−δ Superconductors W.G. Suharta · H. Mugirahardjo · S. Pratapa · D. Darminto · S. Suasmoro
Received: 15 January 2013 / Accepted: 15 March 2013 / Published online: 5 April 2013 © Springer Science+Business Media New York 2013
Abstract A synthesis of neodymium-substituted YBCO superconductor Ndx Y1−x Ba2 Cu3 O7−δ (x = 0, 0.25, 0.5, 0.75, 1) has been done using a dissolved method in order to obtain homogeneous crystals and higher critical current density. The effects of the substitutions on the structural and magnetic properties of the superconductors after sintering at 970 ◦ C have been examined. Crystallinity of the synthesized powders was confirmed using X-ray and high-resolution neutron diffraction (XRD and HRPD) techniques. Rietveld analyses for both diffraction data sets gave increasing lattice parameters with addition of Nd content and decreasing orthorhombicity. Such addition also caused a decrease in occupancy of the oxygen in the O(4) site. Further investigation using SQUID showed critical temperature of the superconductors between 90.9 and 92.0 K. The critical current density (Jc ) was calculated from the magnetic hysteretic loop at 5 K as 40 kA cm−2 for Nd0.25 Y0.75 Ba2 Cu3 O7−δ sample and 100 kA cm−2 for Nd0.5 Y0.5 Ba2 Cu3 O7−δ sample. We also found that increasing Nd content on the Ndx Y1−x Ba2 Cu3 O7−δ superconductor samples can improve their resilience of superconductivity and critical current density. W.G. Suharta () · S. Pratapa · D. Darminto · S. Suasmoro Department of Physics, Faculty of Mathematics and Natural Sciences, Institute of Technology Sepuluh Nopember, Kampus ITS Sukolilo, Surabaya 60111, Indonesia e-mail:
[email protected] W.G. Suharta Department of Physics, Faculty of Mathematics and Natural Sciences, Udayana University, Jimbaran, Bali, Indonesia H. Mugirahardjo Center for Technology of Nuclear Industrial Materials, Puspiptek Serpong, Indonesia
Keywords YBCO superconductor · Dissolved method · X-ray and high-resolution neutron powder diffraction · Structure · dc-susceptibility
1 Introduction YBa2 Cu3 O7−δ (YBCO) is one of high-Tc superconductors expected for various practical applications. Several studies related to the application of superconductors have been carried out mainly for transmission-line cable system [1–3] and superconducting power devices [4]. For this purposes, the material has to have high-critical current density (Jc ). To yield the superconductor with high Jc , some researchers [5–7] have conducted a variety of methods of syntheses, such as solid state reaction and melt process as previously reported [5–7] with the maximum Jc exceeding 1.7 × 104 A/cm2 at 77 K. In terms of Nd substitution, some other researchers have prepared NdBa2 Cu3 O7−δ (NBCO) superconductors by melt process and obtained the maximum of Jc is 4.6 × 104 A/cm2 at 77 K for various applied fields [8–10]. It was found that some physical and chemical factors such as grain boundaries, oxygen content, porosity, and microcracks may influence the critical current density. Regarding the grain boundaries, the sub-grain nonsuperconducting particles can exist in a small (sub-micron or nanometric) size and are homogeneously distributed in the crystal [5, 11]. Such characteristics, however, have not been observed in Nd-substituted YBCO systems since there is a very limited literature on the related topics. Meanwhile, the dissolved method is a well-known technique for producing oxide ceramic materials where one can easily control grain or particle size. The method is therefore of potential for the synthesis of Nd-substituted YBCO.
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Our study is devoted to the effect of Nd substitution on YBCO to form Ndx Y1−x Ba2 Cu3 O7−δ (NYBCO) powders synthesized using the dissolved method. The research concerns with microstructural characteristics, homogeneity of grains, and magnetic properties of NYBCO. In order to achieve the purposes, we employed X-ray diffraction (XRD), high-resolution neutron powder diffraction (HRPD), scanning electron microscopy (SEM) and dc-SQUID magnetometry techniques.
2 Experimental Samples were prepared using dissolved method from the following high-purity (≈99.9 %) raw materials: Nd2 O3 , Y2 O3 , BaCO3 , and CuO. These powders were mixed to starting compositions of the designated NYBCO with variations of x from 0 to 1 (x = 0, 0.25, 0.50, 0.75, 1.00). Each starting material was dissolved with HNO3 solution accompanied by stirring for 30 minutes to form the designated homogeneous solutions. All solutions were mixed and then further stirred until crusted and dried in oven at 100 ◦ C for 1 hour. The results were subsequently heated in a furnace at 600 ◦ C for 3 hours, calcined at 900 ◦ C for 10 hours and sintered at 970 ◦ C for 10 hours. The structures of the samples were examined using room temperature powder X-ray (CuKα radiation, 10–90◦ 2θ -range with a step size of 0.02◦ ) and high-resolution neutron (wavelength of 1.819479 Å, 10– 154◦ 2θ -range with a step size of 0.05◦ ) diffraction techniques (XRD and HRPD). Rietveld refinements to acquire the crystal data of phases for each of the synthesized materials were performed using FullProf [12] by employing Thompson-Cox-Hasting pseudo-Voigt peak shape function. SEM images of the samples were collected to investigate the morphology and distribution of the grains and determine their size. The magnetic susceptibility (χ ) and magnetization (M) of the samples, depending on magnetic field (H ) and temperature (T ), were measured using a QuantumDesign MPMS-5 SQUID magnetometer, covering the temperature range down to 5 K and the magnetic field up to 50 kOe.
3 Results and Discussion Figure 1 gives the XRD patterns for all Nd-added YBCO samples. Generally, each sample shows sharp peaks indicating that the samples are well crystallized. Diffraction peaks of YBCO (PDF No. 39-0486) and NBCO (PDF No. 460229) structures are obviously dominating. There are no Nd-related phase detected, which implies the dissolution of Nd in the YBCO system to form NYBCO. BaCuO2 compound (PDF No. 46-0324) as impurity is observed, as shown
Fig. 1 XRD patterns (CuKα radiation) of the Ndx Y1−x Ba2 Cu3 O7−δ (x = 0, 0.25, 0.5, 0.75, 1.0) superconductor samples. The phase indicators: (o) Ndx Y1−x -123, and () BaCuO2
Fig. 2 Peak shifts in the (013), (110) and (103) reflections of the XRD patterns associated with Fig. 1
by some minor peaks at around 27–32◦ which is probably caused by imperfect initial mixing of the raw materials hence incomplete reaction between them. Addition of Nd results in peak shift as well as reduction— as shown by Fig. 2—which relates to ionic substitutions, namely of Y3+ by Nd3+ . In particular, the peaks are shifting to the lower 2θ with increasing Nd content which can be associated with difference in ionic radii between the ions, the ionic radii of Nd3+ (112.3 pm) being larger than Y3+ (104 pm) [13]. The dissolved method have been successfully employed to produce Ndx Y1−x Ba2 Cu3 O7−δ powders.
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Fig. 5 Crystal structure of Nd1 Ba2 Cu3 O7−δ Fig. 3 HRPD patterns (λ = 1.819479 Å) of Ndx Y1−x Ba2 Cu3 O7−δ (x = 0, 0.25, 0.5, 0.75, 1.0) superconductors
Fig. 4 Peak shifts in the (013), (110) and (103) reflections of the HRPD patterns associated with Fig. 3
HRPD patterns of NYBCO samples for all compositions are presented in Fig. 3. Generally, these patterns notify similar identification results from those of XRD. A peak of Y2 BaCuO5 (Y-211) at around 36.29◦ (PDF No. 38-1434) can be observed in addition to BaCuO2 as impurities. Scrutinized patterns at 2θ between 53 and 60◦ for rather overlapping (013), (110) and (103) reflections are shown in Fig. 4. Similar to XRD results, those of HRPD also indicate substitution of Y3+ by Nd3+ particularly as shown by peak shifts to the lower angle as Nd content increases originated from the difference of ionic radii. It is obvious, therefore, that the results from HRPD are in accordance with those from XRD.
Fig. 6 Orthorhombic strain of Ndx Y1−x Ba2 Cu3 O7−δ superconductor samples
The crystal data for NYBCO in the samples were derived from HRPD Rietveld refinement. Isotropic thermal parameters of all sites were refined while all atomic site occupancy parameters were fixed except O(4) site. The oxygen O(4) site in the blocking layer as shown in Fig. 5 is slightly changing with Nd content. This indicates the existence of oxygen vacancies on O(4) site of some unit cells. The vacancies of oxygen (O4), in this situation, may lead to an effective flux pinning center, as reported for Bi-based superconductors [14], which will be described later. Rietveld-derived lattice parameter values from the XRD and HRPD patterns are depicted in Table 1. These values
3212 Table 1 Lattice parameters (a, b, c) of Ndx Y1−x Ba2 Cu3 O7−δ after Rietveld refinement using FullProf program [12]
J Supercond Nov Magn (2013) 26:3209–3214 Sample
Starting compounds
Cell volume (Å3 )
Lattice parameters a-Axis (Å)
b-Axis (Å)
c-Axis (Å)
1
N-123 (XRD)
3.8666(2)
3.9198(3)
11.759(3)
178.223
2
N-123 (HRPD)
3.8665(4)
3.9193(5)
11.757(5)
178.165
3
N075 Y025 -123 (XRD)
3.8544(2)
3.9104(2)
11.729(1)
176.782
4
N075 Y025 -123 (HRPD)
3.8542(6)
3.9098(7)
11.723(5)
176.656
5
N05 Y05 -123 (XRD)
3.8462(2)
3.9033(2)
11.713(9)
175.846
6
N05 Y05 -123 (HRPD)
3.8454(3)
3.9018(4)
11.709(6)
175.682
7
N025 Y025 -123 (XRD)
3.8408(7)
3.9005(7)
11.698(3)
175.248
8
N025 Y025 -123 (HRPD)
3.8339(5)
3.8942(4)
11.697(1)
174.636
9
Y-123 (XRD)
3.8278(3)
3.8907(3)
11.677(2)
173.903
10
Y-123 (HRPD)
3.8211(2)
3.8842(6)
11.674(5)
173.265
increase with Nd content, while the orthorhombic strain decreases. This result is understandable from the fact that there is a significant difference of ionic radius of Nd3+ resulting in peak shifts as previously discussed. The orthorhombic strain was calculated following [15]: η = 2(b − a)/(b + a), where a and b are lattice parameters, which their values were taken out from the Rietveld analyses of XRD and HRPD data. Figure 6 shows that the strains derived from XRD and HRPD spectra are in the same order of magnitude and the orthorhombicity decreases with increasing x. The latter is due to the fact that the presence of Nd3+ in the Y3+ site has led to oxygen migration, where O(4) site is lack of oxygen for each sample in range of around 1–10 % with different content of Nd3+ . Decreasing the occupancy of the oxygen in O(4) site with increasing Nd content on Ndx Y1−x Ba2 Cu3 O7−δ samples showed in Fig. 7. Figure 8 shows the typical SEM microstructures of NYBCO samples. The grain size of samples is around 150– 250 nm, which agrees quite well with the result of refinement by using the Scherrer equation. In general, relatively homogeneous morphology and grain size are observed, in spite of somewhat smaller grains. The YBCO (x = 0) sample exhibits smaller grain size than the NYBCO samples. The SEM images also ascertain that the dissolved method is generally very effective to yield relatively homogeneous grains. The magnetic susceptibility (χ ) vs. temperature (T ) data of Nd0.25 Y0.75 Ba2 Cu3 O7−δ and Nd0.5 Y0.5 Ba2 Cu3 O7−δ superconductor samples measured in the zero field cooling (ZFC) and field cooling (FC) modes in a magnetic field of 10 Oe showed that the onset superconducting transition occurs at Tc−on ≈ 90.9 K–92.0 K as demonstrated in Fig. 9. Relative susceptibility [χ(T )/χ(10 K)] shows significantly difference characteristics at the ZFC and FC measurements. This indicates that the resilience of superconduc-
Fig. 7 Oxygen occupancy of O(4) with increase x in the Ndx Y1−x Ba2 Cu3 O7−δ samples after Rietveld refinement for the HRPD data
tivity in applied magnetic field of Nd0.5 Y0.5 Ba2 Cu3 O7−δ sample which is larger than that of Nd0.25 Y0.75 Ba2 Cu3 O7−δ sample. The magnetization versus field (M–H ) curve measured at 5 K is shown in Fig. 10. The critical current density (Jc ) is proportional to the magnetization difference (M) according to M = M + (H ) − M − (H ), referring to the magnetization curves measured for increasing and decreasing applied magnetic field. The superconducting volume fraction at 5 K is about 35 % for Nd0.25 Y0.75 Ba2 Cu3 O7−δ sample and 26 % for Nd0.5 Y0.5 Ba2 Cu3 O7−δ sample, which was calculated from ZFC with χ = −M/H V and compared with the perfect diamagnetic χ = −1/4π [16, 17]. By using Bean’s model [18], the critical current density at 5 K was es-
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Fig. 8 SEM images of Ndx Y1−x Ba2 Cu3 O7−δ (x = 0, 0.25, 0.5, 0.75, 1.0) superconductor samples: (a) Y-123, (b) Nd0.25 Y0.75 -123, (c) Nd0.5 Y0.5 -123, (d) Nd0.75 Y0.25 -123, (e) Nd-123
timated to be 40 kA cm−2 for Nd0.25 Y0.75 Ba2 Cu3 O7−δ and 100 kA cm−2 for Nd0.5 Y0.5 Ba2 Cu3 O7−δ .
4 Conclusion The synthesis of Ndx Y1−x Ba2 Cu3 O7−δ (x = 0, 0.25, 0.5, 0.75, 1) superconductors by using dissolved method has been successfully performed. Both analyses on XRD and HRPD spectra of all samples have confirmed the presence of the dominantly desired NYBCO phase and also the minor Y-211 phase as impurity. The increased content of Nd in the NYBCO crystals has led to monotonic increasing
lattice parameters, decreasing orthorhombic strain and reducing occupancy of oxygen in O(4) site. The oxygen deficiency has in turn induced a more effective flux pinning as reflected by the enhanced magnetic properties of the samples obtained. Acknowledgements This work was partially supported by “Hibah Riset Pascasarjana” provided by DP2M, Directorate General of Higher Education (DGHE) of Indonesia, under the contract No. 762/I2.7/PM/2011. One of us (WGS) would like to thank DGHE for the scholarship through BPPS Program and funding of Sandwich Program to Japan in 2011. The use of SQUID magnetometer at RIKEN Nishina Center, Japan, is gratefully appreciated.
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Fig. 9 M–T curve of Ndx Y1−x Ba2 Cu3 O7−δ (x = 0.25, 0.5) superconductor samples
Fig. 10 M–H curve of Ndx Y1−x Ba2 Cu3 O7−δ (x = 0.25, 0.5) superconductor samples
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