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Physica C 445–448 (2006) 295–298

The influence of Nd–Ba–Cu–Mg–O generic seed crystal compositionon Tc of seeded, bulk (RE)–Ba–Cu–O grains

Y. Shi *, N. Hari Babu, K. Iida, D.A. Cardwell

IRC in Superconductivity and Department of Engineering, University of Cambridge, Madingley Road, Cambridge, CB3 OHE, UK

Received 21 March 2006Available online 14 June 2006

Abstract

A Mg-doped Nd–Ba–Cu–O generic seed crystal has been developed for the fabrication of any type of rare earth (RE) based (RE)–Ba–Cu–O single grain superconductor by the top seed melt growth (TSMG) technique. Melt processed crystals of Nd–Ba–Cu–O enrichedwith various amounts of MgO exhibit increased melting points compared to other (RE)–Ba–Cu–O systems, whilst retaining a similarcrystal structure. The Mg-doped Nd–Ba–Cu–O phase is stable with Ba–Cu–O liquid at elevated temperature and, as a result, can be usedto seed the growth of any (RE)–Ba–Cu–O single grain. Key seed properties have been investigated as a function of MgO content, includ-ing Tc, the distribution of MgO inclusions within the YBa2Cu3O7�d (Y-123) matrix, melting point and composition. MgO doping resultsusually in a decrease in Tc of the parent (RE)BCO compound. It is particularly important, therefore, to investigate fully any variation ofTc associated with the diffusion of Mg into a grain grown from a Mg-containing seed. We report the superconducting and microstruc-tural properties of YBCO, SmBCO and GdBCO single grains fabricated using various Nd–Ba–Cu–Mg–O generic seed crystals with awide range of MgO content. Finally, we report the optimum composition of Nd–Ba–Cu–O generic seed crystal from a study of the var-iation of Nd–Ba–Cu–O melting temperature with concentration of Mg in the precursor seed composition.� 2006 Elsevier B.V. All rights reserved.

PACS: 74.26.Ha; 74.72.BK; 74.76.Bd

Keywords: Superconductors; Seed crystals

1. Introduction

Melt processed (LRE)–Ba–Cu–O superconductors((LRE)BCO), where LRE is a light rare earth element suchas Nd, Sm, Eu and Gd, are known to exhibit higher criticalcurrent densities, Jc’s, and higher irreversibility fields thanY–Ba–Cu–O (YBCO) [1–3]. In the absence of a suitableseed, however, it has been difficult to fabricate large grainbulk (LRE)BCO superconductors using a practical coldseeding process. We have reported recently that Mg-dopedNdBCO exhibits generally a similar crystal structure tothat of the (RE)Ba2Cu3O7�d (RE-123) family of com-pounds but with a significantly higher melting temperature.

0921-4534/$ - see front matter � 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.physc.2006.04.070

* Corresponding author. Tel.: +44 1223 337443; fax: +44 1223 337074.E-mail address: ys206@cam.ac.uk (Y. Shi).

As a result, we have demonstrated successfully that Mg-doped NdBCO melt-processed crystals can be used as gen-eric seeds for the growth of large grain, bulk (LRE)BCOsuperconductors [4], and that the new seed crystal simpli-fies significantly the fabrication process of light rare earth(LRE)–Ba–Cu–O single grains grown by top seeded meltgrowth (TSMG) [5].

During their development, we observed the melting pointof MgO–NdBCO seeds containing greater than 0.6 wt%MgO to be about 15–20 �C higher than that of undopedNdBCO. This increase in melting temperature remains con-stant with further increase in MgO doping level, althoughthis is accompanied by an undesirable, and significant,decrease in Tc of the melt processed NdBCO grain [6]. Inthe earlier study it was found that MgO substitutes com-pletely into the YBa2Cu3O7�d (Y-123) phase matrix for a

Fig. 1. Cross sections of an YBCO single grain seeded by a genericNdBCO seed containing 20 wt% MgO (a) in the vicinity of seed crystaland (b) 1.5 mm away from the seeded grain/seed boundary.

296 Y. Shi et al. / Physica C 445–448 (2006) 295–298

maximum doping level of 4 at.% [7], and that higher levelsof doping leads to the segregation of MgO during the meltprocess. It is important to investigate fully any variation ofTc caused by Mg diffusion into a grain grown from the gen-eric seed. YBCO was selected as a key material to investi-gate the sensitivity of Tc to NdBCO seeds with differentMgO doping levels due mainly to the absence of RE/Basolid solution formation in the stochiometric YBa2Cu3O7�d

compound, which consequently exhibits constant Tc whengrown under air. In this paper, we report the superconduc-ting and microstructural properties of YBCO and GdBCO,SmBCO single grains fabricated using Nd–Ba–Cu–Mg–Ogeneric seed crystals containing a wide range of MgO addi-tion. Finally, we report the optimum composition of Nd–Ba–Cu–O generic seed crystal based on the results of adetailed investigation of the variation of melting tempera-tures of seed crystals fabricated from Nd–Ba–Cu–O dopedwith various amounts of Mg.

2. Experimental

Mg-doped Nd–Ba–Cu–O generic seed crystals withvarying MgO content were fabricated by conven-tional TSMG processing. NdBa2Cu3O7�d (Nd-123) andNd4Ba2Cu2O10 (Nd-422) phase powders of purity 99.9%were mixed thoroughly with starting compositions Nd-123 + 12 mol% Nd-422 + z wt% MgO, where z = 1, 2, 5,10 and 20. YBCO precursor powders were prepared withthe composition of Y-123 + 30 wt% Y-211 + 0.1 wt% Pt.The powders were then pressed uniaxially into pellets andmelt processed under air using the thermal profile describedin [4]. Nd–Ba–Cu–Mg–O seeds containing differentamounts of MgO were placed on the top surface of eachpellet prior to melt processing. Initially the pellets wereheated rapidly at 300 �C/h to their melting temperature,Tm, and held for 0.6 h to ensure complete decomposition.The partially molten sample was cooled at 145 �C/h to itscrystallization temperature, Tg1, and then cooled slowlyto Tg2 at a rate of 0.5–0.8 �C/h prior to cooling to roomtemperature at 300 �C/h. The appropriate values of Tm,Tg1 and Tg2 were identified from DTA measurements oneach precursor powders.

GdBCO and SmBCO (LRE)–Ba–Cu–O single grainsuperconductors were also fabricated in air by TSMG fromprecursor powders with compositions of Gd-123 + 30 wt%Gd-211 + 1 wt% BaO2 + 0.1 wt% Pt and Sm-123 + 30 wt%Sm-211 + 2 wt% BaO2 + 0.1 wt% Pt, respectively. A gen-eric seed of composition of Nd-123 + 12 mol% Nd-422 + 1 wt% MgO was used to cold seed the grain in eachcase, in a process similar to that described above.

Oxygenated single grains sized about 13 mm in diameterwere cut into 1 mm · 1 mm · 1 mm pieces along a- and c-axis, respectively. Individual sample cross-sections in thevicinity of the seed were polished and their microstructuresexamined using an optical microscope. The smaller speci-mens cut from the parent grain were used to measure Tc

using a MPMS XL SQUID magnetometer.

3. Results and discussion

Fig. 1(a) and (b) show the microstructures of a YBCOgrain fabricated using a generic seed containing 20 wt%MgO in the vicinity of seed along the c-axis growth sectorand of the seed at the position 1.5 mm away from the seed/seeded grain boundary, respectively. The microstructuralfeatures evident in Fig. 1(a) are very similar to thoseobserved in a YBCO grain seeded by undoped NdBCO.Specifically, there are no MgO inclusions present in theYBCO grain grown from the Nd–Ba–Cu–Mg–O genericseed in the vicinity of the seed. Area fraction of MgO inclu-sions in the seed, close to the seeded YBCO grain, andaway from the seed/seeded grain boundary is observed tounaltered, suggesting that there is no apparent MgO lossfrom the seed during melt processing. Optical microscopyof other single YBCO grains grown from Mg–NdBCOseeds but containing different concentrations of MgO indi-cate that Mg doping does not influence the sample micro-structure. It is known that the level of Mg substitution inthe Y-123 matrix saturates at 4 at.%, and that furtherMg addition to YBCO leads to the formation of MgOinclusions in Y-123 matrix [7]. The YBCO grain grownfrom a generic seed containing 20 wt% containing generic

0 4 8 12 16 2086

87

88

89

90

91

92

Tc(

K)

Wt% MgO in precursors

0

1

2

3

4

5

ΔΤc (Κ

)

Fig. 3. Tc and DTc of a seeded grain just beneath the seed position grownwith Nd–Ba–Cu–Mg–O seeds containing 0–20 wt% MgO.

Y. Shi et al. / Physica C 445–448 (2006) 295–298 297

seed shown in Fig. 1(a) has no visible MgO inclusions inthe Y-123 matrix, providing further evidence that Mg sub-stitution in Y-123 matrix is negligible. The transition tem-peratures of (RE)BCO grains grown with generic seedscontaining various amounts of MgO were measured toinvestigate further the degree of Mg diffusion into theRE-123 matrix during melt processing, given that MgO isknown to suppress significantly Tc of NdBCO and YBCO.

Fig. 2 shows the transition temperature, Tc, and the tran-sition width, DTc, measured using the SQUID magnetome-ter for YBCO grains grown with seed crystals containingvarious amounts of MgO (1 < z < 5) as a function of dis-tance from the seed crystal position along the a- and c-axisof the seeded grain. It can be seen from the figure that Tc

along both a- and c-axis is almost constant (90 ± 1 K) forYBCO grains grown with generic seed crystals containingwide range of MgO. In addition, the transition width DTc

is also observed to be constant (1–2 K) for the same sam-ples. The Tc and DTc data measured for the specimens cutfrom immediately beneath the seed crystals, circled inFig. 2, are re-plotted with MgO content reaching to20 wt% in Fig. 3 as a function of MgO content of the genericseed. No significant change in either Tc or DTc of the YBCOseeded grains are observed with increasing MgO concentra-tion in the seed crystal up to 20 wt%. The observed constantTc for YBCO grains grown from the generic seeds suggestsstrongly that negligible Mg diffusion from the seed to theYBCO grain occurs during the grain nucleation process.

89

90

91

92

Tc(K

) al

ong

a-ax

is

1wt2wt3wt4wt5wt

0 1 2 3 4 5 60

1

2

3

4

5

ΔTc(K

) al

ong

a-ax

is

Distance(mm) from the seed

1wt2wt3wt4wt5wt

0

Fig. 2. Tc as a function of distance from the seed crystal along both a- and c-ax0–5 wt% MgO.

Fig. 4 shows the variation of Tc with distance from theseed crystal position along the a- and c-axis for GdBCOand SmBCO grains grown with the generic seed. Tc inthe vicinity of the seed is observed to be as high as Tc moredistant from the seed position. In addition, DTc is less than1.0 K and 1.5 K measured along a- and c-axis, respectively,for the GdBCO grain. Similar sharp superconducting tran-sitions are observed for the SmBCO grain [5].

It is known from our previous research that the meltingtemperature of the MgO doped NdBCO system is greatest

1 2 3 4 5 6

ΔTc(K

) al

ong

c-ax

is

Distance(mm) from the seed

1wt2wt3wt4wt5wt

0

1

2

3

4

5

Tc(K

) al

ong

c-ax

is

1wt2wt3wt4wt5wt

89

90

91

92

es of YBCO single grains grown from Nd–Ba–Cu–Mg–O seeds containing

0 1 2 3 4 5 60.0

0.5

1.0

1.5

ΔTc

(K)

Distance(mm) from the seed

GdBCO in c-axis

GdBCO in a-axis

SmBCO in a-axis

90

91

92

93

94

95

GdBCO in c-axis

GdBCO in a-axis

SmBCO in a-axis

Tc

(K)

Fig. 4. Spatial variation of Tc for GdBCO and SmBCO single grainsgrown with Nd–Ba–Cu–Mg–O seeds containing 1 wt% MgO.

298 Y. Shi et al. / Physica C 445–448 (2006) 295–298

for z = 0.6 wt%, and is then constant for up to z = 20 [6].With the present results, therefore, it may be concludedthat seed crystals of starting composition Nd-123 +12 mol% Nd-422 + 0.6–20.0 wt% MgO can be used aseffective seed crystals for YBCO. We confirm further thatseed crystals of starting composition Nd-123 + 0.6–1.0 wt% MgO do not affect significantly the superconduc-

ting properties of GdBCO and SmBCO systems. We haveyet to measure the influence of seeds with higher MgO con-centrations on Tc of these (LRE)BCO systems.

4. Conclusions

Mg doped NdBCO single crystals can be used as effec-tive seeds for top seeded melt growth of any (RE)BCO sys-tem due to their higher melting point and similar latticeparameters compared to the target bulk material and totheir chemical stability in the peritectically decomposedstate. The transition temperature of YBCO grains grownfrom Nd–Ba–Cu–Mg–O generic seed crystals is observedto be insensitive to Mg content for seeds containingbetween 1 and 20 wt% MgO. Negligible Mg diffusion fromthe seed crystal into the bulk YBCO grain has beenobserved in this study. The generic seed containing 1 wt%MgO does not affect Tc of the SmBCO and GdBCO sys-tems, although further study is required to confirm theinfluence of higher MgO concentrations on the Tc of(LRE)BCO single grains, in general.

Acknowledgements

The authors would like to thank EPSRC for providingfinancial support.

References

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