The effect of Gd diffusion-doped on structural and superconducting properties of YBa2Cu3O7−x superconductors

Journal of Alloys and Compounds 433 (2007) 46–52

The effect of Gd diffusion-doped on structural and superconducting properties of YBa2Cu3O7−x superconductors ¨ urk a , S¸. C¸elik b , U. C K. Ozt¨ ¸ evik a , E. Yanmaz a,∗ a

Karadeniz Technical University, Faculty of Arts and Sciences, Department of Physics, Trabzon, Turkey b Karadeniz Technical University, Faculty of Arts and Sciences, Department of Physics, Rize, Turkey Received 25 May 2006; received in revised form 20 June 2006; accepted 22 June 2006 Available online 25 July 2006

Abstract The effect of Gd diffusion on structural evolution and superconducting properties of YBa2 Cu3 O7−x (Y123) prepared in the bulk polycrystalline form were studied. The gadolinium (Gd) diffusion in superconducting Y123 ceramic has been studied over the temperature range of 600–900 ◦ C by energy dispersive X-ray fluorescence technique (EDXRF). The temperature dependence of Gd diffusion coefficient in grains (D1 ) and over the grain boundaries (D2 ) are described by the relations: D1 = 8.9 × 10−6 exp(−1.25/kT) and D2 = 2.3 × 10−5 exp(−1.09/kT). For the Gd diffused-doped samples, magnetization and resistivity measurements show that the critical transition temperature, Tc , increased from 88 to 91 K and the critical current density, Jc , which was calculated from M-H loops taken at 77 K, increased from 55 to 122 A cm−2 in comparison with those of undoped Y123. The possible reasons for the observed increases in Tc and Jc due to Gd diffusion were discussed. In addition, magnetic hysteresis curves of the Gd900, Gd800, Gd700 and undoped Y123 are presented for temperature of 5 K. It was found that the hysteresis is enhanced by the Gd diffusion. Such enhancement, which is considered to represent a characteristic strength of intergrain coupling, is more clearly recognized when critical current densities are compared. © 2006 Elsevier B.V. All rights reserved. Keywords: Gd-diffusion; YBCO superconductor; M-H loops; Critical current density

1. Introduction Many researches on RE123 (RE = Nd, Sm, Eu, Gd) superconductor were reported and revealed the promising characteristics that these materials show higher Tc , higher Jc in the magnetic field and larger irreversible field as compared with Y123 [1–3]. The rare earth elements (RE) substitute for both Y and Ba sites in Y123 compounds. For trivalent rare earth ion substitution of barium a correlation of solution energies (of Lu, Ho, Gd, Eu and La) with ion size is observed. Substitution of large rare earth ions is energetically more favorable at Ba site [4]. The recent study on RE123 superconductor in bulk form have shown that RE/Ba substitution was highly attractive since it may work as pinning centers in the magnetic field [5]. Also it is known that the Gd site can be replaced by other rare earth elements such as R = Y, Nd, Sm, Er, Eu, etc. with the identical crystal structure. However,

∗

Corresponding author. Tel.: +90 462 325 31 97; fax: +90 462 325 31 95. ¨ urk), [email protected] E-mail addresses: [email protected] (K. Ozt¨ (E. Yanmaz). 0925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.06.082

the difference in the ionic radii of R, ions possibly introduces difference in the electronic state in the CuO2 plane and affects the transition temperature Tc [6]. It is well known that the properties of grain boundaries in Y123 differ drastically from those of bulk material. It has been shown both experimentally and theoretically that the grain boundaries in Y123 superconductors are depleted of carriers compared to the bulk. This depletion leads to weak intergrain links and limits critical currents in superconductor [7]. It was reported [8,9] that the doping of Y123 with Gd and Ca resulted in substantial enhancement of the grain boundary critical current Jc . Partial cation substitution in the parent Y123 structure has been investigated intensively in an attempt to clarify or improve the superconducting behavior of this type superconductor [10,11]. The diffusion method of impurity doping of ready Y123 opens new possibilities for the controlled doping of samples at relatively low concentrations (1018 –1021 cm−3 ). Therefore the diffusion studies of Gd may be useful for determination of location sites of gadolinium in lattice and for the understanding of the migration mechanism, changes of microstructure and superconducting properties of Y123 under low level doping.

¨ urk et al. / Journal of Alloys and Compounds 433 (2007) 46–52 K. Ozt¨

47

Although numerous studies have been undertaken on oxygen [12] and cation diffusion [13–16] in Y123, no studies on Gd diffusion have been reported. Here we report the effect of Gd diffusion on the properties of Y123, in order to obtain information about the diffusion parameters of Gd and the improvement of the crystalline structure and the superconducting properties of bulk samples. 2. Experimental Superconducting Y123 samples were prepared by the solid state reaction method from high-purity starting powders Y2 O3 , BaCO3 and CuO. The powders were thoroughly mixed in the desired proportions and then heated in air for 24 h at 900 ◦ C in a square muffle furnace. The heating and cooling rates of the temperature were chosen to be 5 and 1 ◦ C min−1 , respectively. After cooling to room temperature, the powder was ground and pressed under 300 MPa into pellets 13 mm in diameter. Then, these pellets were sintered at 930 ◦ C in air for 24 h, followed by oxygenation at 500 ◦ C. Gd diffusion in Y123 pellets was carried out using an ultra high vacuum electron-beam deposition system to get layer of Gd (thickness ∼4 ␮m) on the one side surface of the sample. The diffusion annealing of samples with the deposited layer of Gd were executed in air at temperatures 900, 850, 800, 700 and 600 ◦ C for 12 h. For comparison, a pure Y123 (uncoated with Gd layer) was also annealed at 900 ◦ C for 12 h in air. The samples will be here after denoted as Gd900, Gd850, Gd800, Gd700, Gd600 and pureY123, respectively. The magnetization properties were measured using a vibrating sample magnetometer (VSM) made by the Quantum Design PPMS system for the respective constant temperatures such as 5, 25, 50, and 77 K with the magnetic field along the perpendicular to the diffusion-doped surface. All the measurements were performed by the sweep rate of 5 mT s−1 after application of magnetic field to avoid from trapped magnetic flux inside of sample at the specified constant temperature above Tc for a few minutes. The critical current density Jc has been estimated by the extended Bean model [17]. Resistance measurements in PPMS system after ZFC regime, from 50 to 100 K by the step of 0.25 K while the heating rate of 3 K min−1 were carried out under a constant magnetic field of 0 and 0.5 T. Typical dimension of the sample for electrical resistance study was 1 mm × 2 mm × 9 mm. The surface morphology of the Gd-diffused and pure samples was studied by using a JEOL JST-6400 scanning electron microscope (SEM). The X-ray diffraction (XRD) data were collected using a Rigaku D/Max-IIIC diffractometer with Cu K␣ radiation over the range of 20◦ –60◦ with a scan speed of 0.2◦ min−1 at room temperature. The orthorhombic lattice parameters (a, b and c) were calculated from (0 0 6), (0 2 0), (2 0 0), (0 1 3), (1 0 3) and (1 1 6) peaks using least square methods. The energy dispersive X-ray fluorescence technique (EDXRF) was used for the determination of the concentration of Gd atoms in the diffusion regions of Y123 samples [18]. For the excitation of Gd atoms, an annular Co-57 radioactive source (25 mCi) emitting 122 keV photon was used. An ultra LEGe detector was used for intensity measurements of Gd K␣ peaks at 42.9 keV. Determination of the Gd concentration distribution was performed by the sequential removal of thin layer (about 10–20 ␮m) from the sample and measuring the EDXRF intensity. In our runs, the sensitivity of the EDXRF technique for the determination of Gd concentration was estimated as N ≥ 2 × 1018 cm−3 . The diffusion coefficient of Gd in Y123 was determined by differentiation of the measured distribution curve of the residual EDXRF intensity with respect to the thickness of the sample [19].

Fig. 1. XRD patterns from the surface of the samples after removal of a layer of thickness 20 ␮m (a) pure Y123 and (b) Gd-diffusion-doped Y123 at 900 ◦ C for 12 h (Gd900).

ture. No secondary phase including Gd ions is observed at diffusion stage. It may think that the impurity peaks of Gd should appear in XRD profile if there is free Gd at grain boundaries. But, it was indicated that the XRD data was collected at the 20–35 ␮m deep of the sample surface. This point indicates the diffusion part into grain (Fig. 5, curve 1), so, the free Gd may not present at this level and the impurity peaks of Gd may not appear in XRD profile. The diffusion doping of the Y123 sample by Gd, which applied different diffusion annealing temperature, exhibited by an increase of the lattice parameter c as 0.23% by comparing the pure and Gd900 at 20 ␮m depth from the surface of both samples. In addition, the increasing of the intensity of the peaks can be compared for the Gd900 and pure Y123 (Figs. 1 and 2). The change in the c lattice parameter implied that Y atoms may be partly substituted by Gd atoms, which is in agreement with the other works [4,6]. This shows that the Gd3+ ions may be entering the crystal lattice of Y123. The observed increases in the intensities of the peaks for the Gd-doped bulk and film samples may testify to the enhanced grain growth and ori-

3. Result and discussion Fig. 1 shows the XRD patterns from the surface of the both samples after removal of a layer of thickness 20 ␮m (a) pure Y123 and (b) Gd-diffusion-doped Y123 at 900 ◦ C for 12 h and slowly cooled with a rate of 5 ◦ C min−1 to room temperature. The peaks were well matched to the orthorhombic Y123 struc-

Fig. 2. Variation of c-lattice parameter with diffusion annealing temperature for samples Gd600–Gd900.

48

¨ urk et al. / Journal of Alloys and Compounds 433 (2007) 46–52 K. Ozt¨

Fig. 3. Variation of c-lattice parameter with removed surface depth (x) of sample diffused at 900 ◦ C for 12 h (Gd900).

entation of grains with Gd diffusion as reported by [2,20]. Fig. 3 shows the variation of c-lattice parameter with removed surface depth of sample Gd900. The c-parameter for the pure Y123 was ˚ from XRD patterns taken at 20 ␮m determined to be 11.6605 A depth of the surface. This value is approximately corresponds the value of c-parameter of Gd900 at 100 ␮m depth. The decreasing of c-parameter with increasing depth can be explained as the transformation from the intragrain diffusion to the intergrain diffusion character. Fig. 4 shows the SEM micrographs of (a) pure Y123 and (b) Gd diffusion-doped Y123 which were slowly cooled after diffusion at 900 ◦ C with a rate of 5 ◦ C min−1 . The granular nature and the porosity of the samples are clearly seen, in particular, in Fig. 4a. The pure sample Y123 exhibits non-uniform structure and a relatively large number of pores. The surface of the Gd-diffused sample is much smoother, denser and the edges of the grains more rounded than that of the undoped one. These results indicate that the surface morphology of the Y123 sample is improved by Gd diffusion-doping. Fig. 5 illustrates the concentration profile of Gd over the thickness of the sample, exposed to Gd diffusion at 900 ◦ C for 12 h. The solid curves 1 and 2 represent the calculated concentration profiles of the impurity diffusion from a constant source into a semi-infinitive solid [19]:   x (1) N(x, t) = N0 1 − erf 2(Dt)1/2 Here, N(x, t) is the impurity concentration at depth x, N0 = N0 (0, t) the constant concentration on surface of the sample, D the diffusion coefficient, t the duration of diffusion, and erf x/2(Dt)1/2 the error function. The experimental data in Fig. 5 are sufficiently approximated by two theoretical concentration distributions: curve 1 for the near-surface region (x = 0–35 ␮m) and curve 2 for the inner region (x = 35–100 ␮m) of the sample. The diffusion coefficients in these regions are D1 = 3.8 × 10−11 cm2 s−1 and D2 = 4.9 × 10−10 cm2 s−1 , respectively. Similar two-region concentration profiles of Gd in Y123 were also observed at other temperatures of the diffusion process. The temperature depen-

Fig. 4. SEM micrographs of (a) pure Y123 and (b) Gd diffusion-doped Y123.

dences of the Gd diffusion coefficient D1 and D2 at 800, 850 and 900 ◦ C (Fig. 6) are described by the following relations:   1.25 ± 0.10 D1 = 8.9 × 10−6 exp − , (2) kT   1.09 ± 0.10 D2 = 2.3 × 10−5 exp − . (3) kT

Fig. 5. Concentration profile of Gd over the thickness of the sample, exposed to Gd diffusion at 900 ◦ C for 12 h.

¨ urk et al. / Journal of Alloys and Compounds 433 (2007) 46–52 K. Ozt¨

49

Fig. 6. Temperature dependences of the diffusion coefficients of Gd in the (D1 ) near-surface and (D2 ) inner region of Y123.

It should be noted that the impurity diffusion in polycrystalline sample of Y123 ceramic takes place simultaneously over the grain boundaries and into grains [13,19]. Therefore the fast Gd diffusion (D2 ) may be related with migration over grain boundaries, pores and other defects. This situation is seen in some ratio in Fig. 4b SEM micrographs. The slow diffusion of gadolinium in Y123 (D1 ) may be caused by migration into grains. Fig. 7a shows the variation of the normalized resistivity (that each sample’s resistivity normalized at 100 K with no magnetic field) as a function of temperature for samples with different Gd diffusion-doping annealing temperatures. As can be seen from the curves, the pure Y123 sample showed a broadening transition, which indicates presence of impurities and weak-links between superconducting grains. As the diffusing annealing temperature increases the transition becomes much sharper and the sharpest transition was observed for sample Gd900. This improvement can be attributed to the decrease of the impurity phase and porosity because of extra annealing and increase of Gd content inside samples. The variation of the transition temperature Tc,offset and the corresponding values Tc with respect to the diffusion-doping annealing temperature is shown in Fig. 7b. It is interesting to note that all the Gd-diffused samples show higher Tc values compared to the pure sample (Tc,offset = 88 K) and the Gd900 sample shows Tc,offset of 91 K. Data of increasing Tc,offset by about 3 K in the Gd-diffused samples over that for undoped Y123 are reproducible using different batches of the sample. Fig. 7a shows that the onset of transition is almost unchanged with Gd-doping. The increase of offset Tc accompanied with sharp transition might be caused by an improved homogeneity of the hole-concentration. It was also thought that the doping of Gd in Y123 replaces the Y3+ ions with Gd3+ ions as reported in reference [6]. This replacement of difference ionic radius ions possibly introduces difference in the electronic state in the CuO2 plane and affects the transition temperature Tc . We may suppose that the effect of increasing the critical temperature of the Gddiffused samples is caused by optimization of the hole density in Y123 as in cases of doping in sintering Y123 ceramic by Au [21], and (Bi, Pb)-2212 bulk compound by Gd [20]. Another

Fig. 7. (a) Variation of the normalized resistivity (that each sample’s resistivity normalized at 100 K) as a function of temperature for samples with different Gd diffusion-doping annealing temperatures and (b) variation of the transition temperature Tc,offset and the corresponding values Tc with respect to the diffusion-doping annealing temperature.

possible cause of the increase of critical temperature Tc may be related with changes of lattice vibrations of Y123 ceramic under substitution of Y ions by more heavy Gd ions. In addition, as seen in Fig. 7a, sharper Tc for the Gd900 and Gd800 samples can be attributed to the improving of connection of between grains because of Gd-diffusion. The duration of the diffusion dependence of the normalized resistivity of the sample diffused at 800 ◦ C for 12, 24 and 48 h is shown in Fig. 8. It can be seen that the superconducting transition becomes slightly sharper with increasing diffusion annealing time. Fig. 9 shows the resistivity curves of Gd-diffused at different temperatures for Y123 samples measured in magnetic field of 0.5 T. The figure indicates that when an external field is applied to the sample, the transition temperature of the sample can be lowered. It is expected that further increasing the magnetic field will cause a further decrease in transition temperature. In addition, all samples including pure Y123 showed the tail effect which indicates these samples contained weak-links between superconducting grains because of granular structure. It can be assumed that the field will penetrate into the sample from

50

¨ urk et al. / Journal of Alloys and Compounds 433 (2007) 46–52 K. Ozt¨

Fig. 8. Duration of the diffusion dependence of the normalized resistivity of the sample diffused at 800 ◦ C for 12, 24 and 48 h.

the grain boundaries containing the weak link. Then, the superconductivity in the grain boundary will diminish and cause the decrease in zero resistive transition temperature. As seen from curves, the pure Y123 and sample Gd600 showed zero resistance transition temperature around 62 K under 0.5 T magnetic field. The zero resistance transition temperatures for samples Gd800 and Gd900 are determined to be around 82 K. This result indicates that samples Gd800 and Gd900 are much stronger against the external magnetic field comparing that of samples pure Y123 and Gd600. In order to study Gd diffusion-doping effects on the superconducting magnetic properties, we next examine the effects of Gd diffusion annealing temperature on the superconducting magnetic hysteresis and critical current density. Fig. 10a shows magnetization hysteresis loops measured at 5 K for samples Gd900, Gd800, Gd700 and pure Y123 with 12 h diffusion time. The curves clearly indicate that the magnetization values

Fig. 9. Resistivity curves of Gd-diffused at different temperatures for Y123 samples measured in magnetic field of 0.5 T.

Fig. 10. (a) Magnetization hysteresis loops measured at 5 K for samples Gd900, Gd800, Gd700 and pure Y123 with 12 h diffusion time and (b) Jc values estimated from M-H loops against the magnetic field for samples of Gd900, Gd800, Gd700 and pure Y123.

systematically increase with increase of diffusion annealing temperature. The critical current density Jc (in A cm−2 ) of above indicated samples has been estimated by the extended Bean model [17] with the equation:   L1 −1 20 M 1− Jc = (4) L1 3L2 where M (in A cm−1 ) is the width of the hysteresis, L1 and L2 (both in cm) are sample dimensions perpendicular to the magnetic field with L2 > L1 . As for M, we used an average of the width for positive and negative magnetic fields. The Jc values in zero fields at 5 K were estimated to be 2.1 × 104 , 1.7 × 104 , 1.55 × 104 , and 1.64 × 104 A cm−2 , respectively, for Gd900, Gd800, Gd700 and pure Y123 as shown in Fig. 10b. By considering the polycrystalline structure and weak-links between the grains, the small magnetic fields caused to increase the interaction of the grains and results the increase in critical current density as seen in Fig. 10b. These results indicate the improvement of superconducting properties by Gd diffusion at near annealing temperature. Additionally, the M-H loops were deter-

¨ urk et al. / Journal of Alloys and Compounds 433 (2007) 46–52 K. Ozt¨

51

Fig. 11. The M-H loops vs. magnetic field measured at 5, 25, 50 and 77 K under 3 T magnetic fields in ZFC regime for the sample Gd900.

mined at 5, 25, 50 and 77 K under 3 T magnetic fields in ZFC regime for the sample Gd900 as shown in Fig. 11. The curves indicate that the magnetization value increases with decrease of measurement temperature and that the shape of loops changes. The magnetization value at 77 K is fully became zero at 3 T field. This result shows that the magnetic field easily penetrates into the superconducting grains at 77 K. In Fig. 12a, magnetic hysteresis curves of the Gd900, Gd800 and pure Y123 are presented for temperature of 77 K. It was found that the hysteresis is enhanced by the Gd diffusion. Such enhancement is more clearly recognized when critical current densities are compared. Fig. 12b shows the results obtained in such a way [17], which is considered to represent a characteristic strength of intergrain coupling. The critical current densities for the pure Y123, Gd800 and Gd900 samples were determined to be 55, 101 and 122 A cm−2 , respectively. A similar increase of critical current density was observed in Gd-added (Bi, Pb)-2212 and Ag-doped Y123 superconductors [20,22]. The increased value of Jc in the Gd diffusion-doped samples (compared with the pure sample) in our runs can be interpreted as a result of dominantly Gd diffusion in intergrain boundaries. Thus, unsubstituted Gd elements stay around just outside of the grain, and might act like catalysis to improve the structural quality at the grain boundaries. This in turn causes the increase of intergrain contact surfaces (decreasing the intergrain resistance) and the increase number of flux pinning centers due to presence of Gd in the intergrain regions. In conclusion, in the temperature range of 800–900 ◦ C, diffusion of Gd in the Y123 sample takes place with two diffusion coefficient, D1 and D2 (activation energies 1.25 and 1.09 eV, respectively), which are attributed to the relatively slow migration in the grains and fast Gd migration over the grain boundaries, respectively. The lattice parameter c of the Gd diffusion-doped sample increases by about 0.23% while the parameters a and b do not change significantly. The diffusion doping of Y123 by Gd increases the critical current density from 55 to 122 A cm−2 and the critical transition temperature by about 3 K compared with the pure Y123.

Fig. 12. (a) Magnetization hysteresis loops measured at 77 K for samples Gd900, Gd800, and pure Y123 with 12 h diffusion time and (b) Jc values estimated from M-H loops against the magnetic field for samples of Gd900, Gd800 and pure Y123.

References [1] M. Murakami, N. Sakai, T. Higuchi, S.I. Yoo, Supercond. Sci. Technol. 9 (1996) 1015. [2] K. Miyachi, K. Sudoh, Y. Ichino, Y. Yoshida, Y. Taka, Physica C 392–396 (2003) 1261–1264. [3] A. Nishida, S. Teshima, C. Taka, Physica C 378–381 (2002) 349– 353. [4] M.S. Islam, R.C. Baetzold, Phys. Rev. B 40 (1989) 10926. [5] N. Koshizuka, A.K. Pradhan, S. Shibata, Y. Feng, T. Machi, K. Nakano, Physica C 364–365 (2001) 320. [6] B.W. Veal, A.P. Paulikas, J.W. Downey, H. Claus, K. Vandervoort, G. Tomlins, H. Shi, M. Jensen, L. Morss, Physica C 162–164 (1989) 97. [7] H. Hilgenkamp, C.W. Scheider, R.R. Schulz, B. Goetz, A. Schmehl, H. Bielefeldt, J. Mannhart, Physica C 326–327 (1999) 7. [8] A. Nishida, S. Teshima, C. Taka, I. Shigeta, Physica C 388–389 (2003) 419–420. [9] A.V. Berenov, S.R. Foltyn, C.W. Schneider, P.A. Warburton, J.L. MacManus-Driscoll, Solids State Ionics 164 (2003) 149–158. [10] A. Tachikava, T. Inoune, K. Zama, Y. Hikichi, Supercond. Sci. Technol. 5 (1992) 386. [11] E. Mendoza, T. Puig, E. Varesi, A.E. Carrillo, J. Plain, X. Obradors, Physica C 334 (2000) 7. [12] K. Conder, Mater. Sci. Eng. R 32 (2001) 41.

52

¨ urk et al. / Journal of Alloys and Compounds 433 (2007) 46–52 K. Ozt¨

[13] T.D. Dzhafarov, M. Altunbas¸, A. Varilci, U. C ¸ evik, A.˙I. Kopya, Mater. Lett. 26 (1996) 305–311. [14] O. G¨or¨ur, C. Terzio˘glu, A. Varilci, M. Altunbas¸, Supercond. Sci. Technol. 18 (2005) 1233–1237. [15] N. Chen, S.J. Rothman, J.L. Routhbort, J. Mater. Res. 7 (9) (1992) 2308. [16] T.D. Dzhafarov, U. C ¸ evik, J. Mater. Sci.: Mater. Electron. 12 (2001) 193. [17] E.M. Gyorgy, R.B. van Dover, K.A. Jackson, L.F. Schneemeyer, J.V. Waszezak, Appl. Phys. Lett. 55 (1989) 283.

[18] E.P. Bertin, Principles and Practise of X-ray Spectrometric Analysis, Plenum Press, New York, 1975. [19] G.B. Abdullaev, T.D. Dzhafarov, Atomic Diffusion in Semiconductor Structures, Harwood, New York, 1987. [20] A. Biju, R.B. Aloysius, U. Syamaprasad, Supercond. Sci. Technol. 19 (2005) 1454. [21] M.Z. Cieplak, G. Xiao, C. Xiao, C.L. Chien, et al., Phys. Rev. B 42 (1990) 6200. [22] Y. Zhao, C.H. Cheng, Physica C 386 (2003) 286.