High temperature phase relationships in (Y,Nd)123 superconducting oxides

Physica C 357±360 (2001) 649±653

www.elsevier.com/locate/physc

High temperature phase relationships in (Y,Nd)123 superconducting oxides M. Kambara *, N. Hari Babu, Y.H. Shi, D.A. Cardwell IRC in Superconductivity and Department of Engineering, University of Cambridge, Madingley Road, Cambridge CB3 0HE, UK Received 16 October 2000; accepted 20 November 2000

Abstract High temperature phase relationships in the quasi-ternary section of the YBa2 Cu3 O6‡d (Y-123), Nd1 Ba2 Cu3 O6‡d and Nd2 Ba1 Cu3 O6‡d phase diagram have been investigated around the peritectic temperature of Y-123. It has been found that the Yx Nd1 x‡y Ba2 y Cu3 O6‡d ((Y,Nd)123ss) solid solution phase is stable over a wide composition and temperature range in this system. The stability ®eld of the (Y,Nd)123ss single phase reduces with increasing temperature and tends towards the Nd-enriched region. Simultaneously, the ``(Y,Nd)211ss ‡ liquid'' two phase ®eld emerges from the Y-123 corner of the phase diagram via a three phase ®eld consisting of ``(Y,Nd)123ss ‡ (Y,Nd)211ss ‡ liquid''. No signi®cant in¯uence of Y/Nd substitution on Tc of (Y,Nd)123ss is observed in this study, in contrast to the abrupt decrease in Tc associated with Nd/Ba substitution. Ó 2001 Elsevier Science B.V. All rights reserved. PACS: 74.70; 74.25.D Keywords: Phase diagram; Solid solution; Yx Nd1

x‡ y Ba2 y Cu3 O6 ‡ d

1. Introduction Various combinations of rare earth elements on the RE site yield enhanced critical current density, Jc , compared with YBa2 Cu3 O6‡d (Y-123) [1]. Yx Nd1 x‡y Ba2 y Cu3 O6‡d ((Y,Nd)123ss) is particularly signi®cant in that the current carrying properties of this material can be improved at low magnetic ®elds compared to other (RE)123 compounds by processing under air [2±4]. In contrast, Nd1‡x Ba2 x Cu3 O6‡d (Nd-123ss) processed under these conditions is characterized by a low Jc , due mainly to the formation of low Tc Nd-123ss with a

*

Corresponding author. Fax: +44-1223-337074. E-mail address: [email protected] (M. Kambara).

signi®cant amount of Nd/Ba substitution. It is anticipated therefore that the formation of a mixed (Y,Nd)123ss boundary layer could suppress a decrease in Jc at the interface between joined Nd1 Ba2 Cu3 O6‡d (Nd-123) bulk crystals using Y123 as a solder, which has clear implications for high ®eld engineering applications. In general, however, the values of Jc in (Y,Nd)123ss, the presence of Nd/Ba substitution vary signi®cantly, despite similar growth conditions [2,3,5]. This is probably due to the increased number of elements involved. For example, the number of degrees of freedom for phase formation increases to 3 for (Y,Nd)123ss ‡ liquid (L), compared with 2 for the two phase region of Y-123 ‡ L. In other words, adding the Nd element requires another variable to be ®xed in order to determine

0921-4534/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 4 5 3 4 ( 0 1 ) 0 0 3 5 1 - 3

650

M. Kambara et al. / Physica C 357±360 (2001) 649±653

the liquid composition in the (Y,Nd)123ss system. Therefore, it is important to clarify the phase relations within this system in order to glean a better understanding of solidi®cation and microstructural control of (Y,Nd)123ss. In this work, the phase relationships in the Y-123, Nd-123 and Nd2 Ba1 Cu3 O6‡d (Nd-213) system were investigated in air at elevated temperatures as the ®rst step in developing an understanding of the (Y,Nd)123ss solidi®cation process.

2. Experimental Precursor powders of (Y,Nd)123ss were prepared via conventional solid state processing from Nd2 O3 , Y2 O3 , BaCO3 and CuO oxides with various combinations of x and y in Yx Nd1 x‡y Ba2 y Cu3 O6‡d (x ˆ 0:1; . . . ; 1; y ˆ 0; 0:2; . . . ; 1). Mixtures of raw powders were ground thoroughly and calcined at 950°C for 24 h under air, which was repeated four times. X-ray di€raction analysis (XRD) of the fully calcined powders con®rmed them to contain only the 123 phase. Di€erential thermal analysis (DTA) was performed under air at a heating rate of 2°C/ min. In parallel, the precursor powders were pressed uniaxially into pellets and they were heated slowly up to 1000°C, 1030°C, 1050°C and 1070°C in Al2 O3 crucibles, neglecting the relatively small effects of over-heating. After holding the samples at temperature for over 300 h, they were quenched rapidly, together with their crucibles, into a water bath. One part of the quenched pellet was pulverized for X-ray powder di€raction analysis. The chemical composition of the rest part of the specimen was investigated by electron probe microanalysis (EPMA). Finally, the superconducting critical temperature, Tc , of the powders annealed at 400°C for 150 h in O2 atmosphere, was measured by AC susceptometry.

3. Results and discussion Initially, the phase relationships in the quasibinary system of Y-123 and Nd-123 (Yx Nd1 x‡y Ba2 y Cu3 O6‡d ; x ˆ 0±1; y ˆ 0) were investigated.

Fig. 1. DTA curves measured on heating for Yx Nd1 x Ba2 Cu3 Oz compositions.

Fig. 1 shows the DTA results on heating for samples containing di€erent amounts of Y (i.e. x). In each case, it can be seen that the endothermic peaks appear above 1000°C and that no major peaks occur around the Y-123 peritectic temperature of 1010°C. It is also clear, particularly for x  0:4, that each endotherm consists of two different peaks. In general, the transformation temperature of the samples, as indicated in ®gure, decreases gradually with an increase in Y content from the peritectic temperature of Nd-123 (1085°C) to that of Y-123 (1010°C). This suggests that a (Y,Nd)123ss form at the processing temperature, rather than a two-phase coexistence of Nd-123 and Y-123 over a range of Y concentrations. In order to identify the phase transformations present in the individual compositions, each pellet was heated to, and held at, a temperature around the peak position and quenched to freeze-in the phases present. Fig. 2 shows typical XRD spectra of the samples with x ˆ 0:2, 0.5 and 0.8 quenched from 1000°C, 1050°C and 1070°C. Only the peaks corresponding to the 123 crystal structure were detected up to 1070°C for the sample with x ˆ 0:2, which is consistent with the absence of a signi®cant

M. Kambara et al. / Physica C 357±360 (2001) 649±653

Fig. 2. XRD patterns at di€erent temperatures for (a) x ˆ 0:2, (b) 0.5 and (c) 0.8 for Yx Nd1 x Ba2 Cu3 Oz .

endothermic peak for this composition below 1080°C in the DTA data. As the Y content increases to 0.5, new peaks from the Y2 BaCuO5 (Y211) crystal structure emerge at 1050°C, together with the BaCuO2 (0 1 1) and BaCu2 O2 (0 1 2) phases, indicating the presence of a liquid phase. Only the di€raction pattern of the 123 phase structure exists at 1000°C, on the other hand, which subsequently disappears completely at 1070°C. XRD peaks corresponding to the 211 phase and the so-

651

lidi®ed liquid are observed above 1050°C for a further increase in Y content, with no evidence of the 123 phase. Another important feature is that the Nd4 Ba2 Cu2 O10 (Nd-422) phase structure is not present in the XRD data for any of the specimens investigated. Compositional analysis was performed on the samples to con®rm the presence of the solid solution by a spot scan EPMA measurement. Fig. 3 shows typical back scattered electron images for the samples with (a) x ˆ 0:4 heated to 1000°C and (b) x ˆ 0:6 heated to 1050°C. It can be seen from Fig. 3(a) that no secondary phase particles are observed in the matrix. In addition, the average composition of the matrix was con®rmed to be almost the same as the initial composition, which indicates the presence of a single phase of the relevant (Y,Nd)123ss solid solution. On the other hand, two di€erent regions of contrast are apparent in Fig. 3(b) within the gray colored matrix of the 123 phase, which di€ers slightly from the initial composition. Compositional analysis revealed that the darker gray region corresponds to the solidi®ed liquid of the Ba±Cu±O phase with the lighter color corresponding to the high temperature stable phase, (Y,Nd)211ss, in which Nd substitutes partially for Y and Ba. As a result, it is clear that (Y,Nd)123ss is a stable phase at lower temperature which transforms with increasing temperature into the ``(Y,Nd)211ss ‡ L'' two-phase ®eld via the threephase ®eld of ``(Y,Nd)123ss ‡ (Y,Nd)211ss ‡ L''. Furthermore, the compositions of the phases in the three-phase ®eld are related through the tietriangle, within which the phase variation is similar to that in the Nd-123 and Nd-213 quasi-binary section [6].

Fig. 3. Back scattered electron images for (a) x ˆ 0:4 at 1000°C and (b) x ˆ 0:6 at 1050°C for Yx Nd1 x Ba2 Cu3 Oz .

652

M. Kambara et al. / Physica C 357±360 (2001) 649±653

Similarly, the phase content and transformations were investigated for the samples with different compositions in the Y-123, Nd-123 and Nd-213 system. The variation of the phase relationship with temperature was determined, as shown in Fig. 4. It can be seen that the (Y,Nd)123ss solid solution is stable over a wide range of Y-123, Nd-123 and Nd-213 concentrations at 1000°C. As temperature increases, the three-phase ®eld emerges from the Y-enriched corner and the (Y,Nd)123ss stability ®eld reduces towards the Nd-enriched region. According to the Nd123ss stability ®eld in the Nd-123 and Nd-213 section [7], the solubility limit

of Nd123ss increases with temperature and deviates slightly from the stoichiometric Nd-123 …x ˆ 0; y ˆ 0† compound to marginally enhanced Nd/ Ba substitution …x ˆ 0; y ˆ 0:04†. This suggests that the three-phase ®eld of the ``(Y,Nd)123ss ‡ (Y,Nd)211ss ‡ L'' composition, rather than the (Y,Nd)123ss, could be stable in the vicinity of the Nd-enriched corner above 1030°C in the ``Y123Nd123-Nd213'' quasi-ternary system. For the sample with x ˆ 0:1 and y ˆ 0, however, no signi®cant evidence of the (Y,Nd)211ss and liquid can be detected either by XRD or DTA analysis. It is therefore considered that the region of stability of the three-phase ®eld is suppressed generally and

Fig. 4. Quasi-ternary phase diagram of the Y123-Nd-123-Nd213 system in air at (a) 1000°C, (b) 1030°C, (c) 1050°C and (d) 1070°C. Values of Tc are shown in (a).

M. Kambara et al. / Physica C 357±360 (2001) 649±653

exists only near the Nd-123 corner of the quasiternary system with the (Y,Nd)123ss phase extending to less than x ˆ 0:1. Fig. 4(a) includes the variation of Tc measured for the samples annealed at 950°C for 300 h under air followed by an oxygenation at 400°C for 150 h. It can be seen that the samples in the ``Y123-Nd123'' quasi-binary section exhibit Tc 's of around 92 K and that no signi®cant di€erence in Tc due to Y/ Nd substitution is observed. With an increase of Nd/Ba substitution, y, on the other hand, Tc was found to decrease dramatically with the samples becoming insulating at y ˆ 0:2. These results, therefore, suggest that an e€ective melt-process based on (Y,Nd)123ss can be achieved in which Nd/Y substitution occurs but Nd/Ba substitution is suppressed to yield high Tc and Jc values of the optimally processed crystal. 4. Conclusion The high temperature phase relationship in the (Y,Nd)123ss system has been investigated at elevated temperatures. The solubility limit of the (Y,Nd)123ss was found to cover almost entirely the quasi-ternary section of the Y-123, Nd-123 and

653

Nd-213 at 1000°C. As temperature increases, the stability ®eld of this solid solution reduces towards the Nd-rich corner and transforms into the(Y,Nd)211ss ‡ L two-phase ®eld via the three phase (Y,Nd)123ss ‡ …Y,Nd†211ss ‡ L system. The variation of Tc for sintered (Y,Nd)123ss specimens containing various amounts of solid solution revealed that this parameter remains relatively high in spite of Y/Nd substitution, compared to its abrupt decrease with increasing Nd/Ba substitution.

References [1] M. Muralidhar, H.S. Chauhan, T. Saitoh, K. Kamada, K. Segawa, M. Murakami, Supercond. Sci. Technol. 10 (1997) 663. [2] D.N. Matthews, J.W. Cochrane, G.J. Russell, Physica C 249 (1995) 255. [3] P. Schatzle, W. Bieger, U. Wiesner, P. Verges, G. Krabbes, Supercond. Sci. Technol. 9 (1996) 869. [4] X. Yao, E.A. Goodlin, Y. Yamada, H. Sato, Y. Shiohara, Appl. Supercond. 6 (1998) 175. [5] A.S. Mahmould, G.J. Russell, Physica C 332 (1999) 193. [6] E. Goodilin, M. Kambara, T. Umeda, Y. Shiohara, Physica C 289 (1997) 251. [7] M. Kambara, T. Umeda, M. Tagami, X. Yao, E.A. Goodilin, Y. Shiohara, J. Am. Ceram. Soc. 81 (1998) 2116.