FABRICATION OF MELT-TEXTURED Nd123

SUPERCONDUCTORS WITH Nd2BaO4 ADDITION

A. HU*$, X. OBRADORS$, V. GOMIS$, T. PUIG$, A. CARRILLO$,E. CARDELLACH$, E. MENDOZA$, Z. X. ZHAO% and J. W. XIONG%$Institut de Ciencia de Materials, C.S.I.C., Campus de la UAB, 08193 Bellaterra, Barcelona, Spain%National Lab for Superconductivity, Institute of Physics, Chinese Academy of Sciences, Beijing

100080, People's Republic of China

(Received 30 December 1997; accepted 15 March 1998)

AbstractÐLarge melt-textured NdBa2Cu3Oy crystals with a sharp superconducting transition have beenfabricated in air with Nd2BaO4 addition. Experiments show that NdBa2Cu3Oy and Nd4Ba2Cu2Oy arepresent in melt-textured crystals which grow in the centre of cylindrical samples while NdBa4Cu3Oy, asa stable phase, instead of the NdBa6Cu3Oy phase, which has been reported previously, has beenobserved in the residual liquid phase located in the edge of samples. In comparison with the e�ects ofNd4Ba2Cu2Oy addition, the re®ned Nd422 particles can be obtained in the present composition. The in-¯uence of the short-time thermal treatment in Ar before oxygenation has been discussed. Fabricatedbulk samples exhibit onset Tc values of 95 K and the secondary peak e�ect in the M±H loop. The peakvalues of Jc are 2.4�104 A/cm2 in 0 T at 77 K and 2� 104 A/cm2 in 1.3 T at 77 K. # 1998 ElsevierScience Ltd. All rights reserved

INTRODUCTION

Large, melt-textured crystals have been proposed for practical applications such as levitation

and permanent magnets with high trapped ®elds [1±3]. The NdBa2Cu3Oy superconductor is the

promising choice since it has demonstrated the highest Tc of all rare earth element 123 s [4]; it

has enhanced Jc values in higher magnetic and irreversibility ®elds [5,6]. Recent studies show

that NdBa2Cu3Oy has a faster crystal growth rate than YBCO [7] and high Tc NdBa2Cu3Oy

bulks can be prepared in air [8±10]. These characteristics promise further bene®ts for commer-

cial application.

However, large melt-textured Nd123 crystals with high Tc have not been fabricated success-

fully. It is well known that Nd1 + xBa2 ÿ xCu3Oy solid solution is easily formed because the ionic

radius of Nd+3 is close to that of Ba+2. Such a Nd/Ba substitution will result in a serious sup-

pression of Tc [11±13]. Therefore, NdBa2Cu3Oy samples prepared in air exhibit a broad super-

conducting transition [14, 15]. The processing in low oxygen partial pressure has been found

e�ective in depressing the substition between Nd and Ba and leads to a high Tc [6, 16].

Unfortunately, the solubility of Nd in the liquid phase is also depressed and a low crystal

growth rate is inevitable [17, 18]. This indicates that it is not easy to grow large NdBa2Cu3Oy

crystals in the reduced oxygen partial pressure [18]. Moreover, owing to the lattice mismatching

of the seed and Nd123 crystal, the orientation of NdBa2Cu3Oy crystal is very di�cult to control

[19]. Because of the much higher peritectic temperature of NdBa2Cu3Oy, a better seed than

MgO has not been found. These problems become challenges for the preparation of high quality

NdBa2Cu3Oy bulks.

We have developed a novel process to prepare melt-textured NdBCO in air [20]. Large melt-

textured NdBCO crystals can be prepared by a top-seeded melt processing technique, and a

high Tc phase is obtained by the post thermal treatments in Ar and O2. W. Bieger et al. [21]

investigated the in¯uence of the initial composition on the properties of melt-textured

NdBaCuO in reduced oxygen partial pressure . The re®nement of Nd4Ba2Cu2Oy particles has

been obtained by adding Nd2BaO4. In this paper, high quality NdBa2Cu3Oy samples, melt- tex-

Applied Superconductivity Vol. 6, Nos. 2±5, pp. 129±137, 1998# 1998 Elsevier Science Ltd. All rights reserved

Printed in Great Britain0964-1807/98 $ - see front matterPII: S0964-1807(98)00093-3

*Corresponding author.

129

tured in air with Nd2BaO4 addition, have been investigated. A new phase relation is reported inthe reaction between NdBa2Cu3Oy and Nd2BaO4 in air. The re®nement of Nd4Ba2Cu2Oy par-ticles is also observed. Many promising superconductive properties such as the Tc value of 95 Kand the remarkable enhancement of Jc in a higher ®eld at 77 K are presented.

EXPERIMENT

Various stoichiomitric powders were prepared by the conventional solid state reaction in air.The fabrication process for NdBa2Cu3Oy (marked as Nd123) has been reported elsewhere [8].The Nd2BaO4 ( marked as Nd210) powder was calcined at 9008C, 9508C, 10508C and 11008Cfor 16 h, respectively, with intermediate milling. Then, Nd123 and Nd210 powders were mixedaccording to the nominal composition of Nd123 + 0.5 Nd210. The mixed precursor was pressedinto a pellet after annealing at 9808C for 16 h. The top-seeded melt texturing processes havebeen reported in Ref. [20].

In order to investigate the phase relationship, stoichiometric NdBa6Cu3Oy (marked as Nd163)and NdBa4Cu3Oy ( marked as Nd143) have also been prepared in air. The synthesis process forNd163 was to anneal the mixed powder at 9008C, 9208C and 9608C for 16 h, respectively, withintermediate milling. The synthesis process for Nd143 was to anneal the mixed powder at9008C, 9408C, 9608C, 10008C, 10158C and 10308C for 16 h, respectively, with intermediatemilling. The composition of the powder was examined by the X-ray di�raction method aftereach annealing steps.

The morphology of the sample was observed by a polarized optical microscope. The micro-structure was studied by scanning electron microscopy (SEM). The local composition of di�er-ent phases was measured by energy dispersive X-ray spectroscopy (EDX).

DC magnetization measurements were performed with a quantum design superconductingquantum interference device (SQUID) magnetometer. The zero-®eld-cooling is used for the Tc

measurement with the applied ®eld of 12 Oe. The magnetization hysteresis loops were alsoobtained with the SQUID magnetometer for the applied ®eld perpendicular to the a±b plane. Acuboid crystal with a size of 1.73� 0.96� 0.6 mm was cleaved from large grains for measure-ment. The a±b plane was con®rmed by observing an angle of 908 in twin boundaries underpolarized light. The critical induced current densities were estimated by the extended Beanmodel [22].

RESULTS AND DISCUSSION

Top-seeding crystal growth

Figure 1(a) shows a typically polished surface of melt-textured Nd123 samples with Nd2BaO4

addition. Some large grains grew in the centre of the sample, and some residual liquid phasesexist in the edge of the sample. Figure 1(b) presents a local area (near the seed) of a large singledomain with size of 16� 18 mm. It is worth noting that a much better crystal growth controlledby the MgO seed is demonstrated here than that in Nd123 samples with Nd422 addition in pre-vious studies [23, 24]. In the case of Nd123 samples with Nd422 addition, more grains growfrom the MgO seed in comparison with that in the present studies. The further investigation ofthe crystal growth in Nd123 samples with Nd210 addition will be reported later [25].

Phase relations

In order to clarify the composition of the sample and phase relations, the obtained crystaland the unreactive part which are located in the centre and edge of the sample, respectively, [inFig. 1(a)] were crushed and examined by X-ray di�ractometer. The X-ray di�raction patternsare shown in Fig. 2(b), (c). Nd123 and Nd422 can be detected in the melt-textured crystals.Nd422, Nd123, minor BaCuO2 and minor NdBa4Cu3Oy are detected in the residual liquidphase. The small Nd123 phase obtained in the residual liquid phase can be attributed to afurther reaction during the fast cooling after the melt-textured growth. The composition of theprecursor with a nominal composition of Nd123 + 0.5 Nd210 after annealing at 9808C for 16 his also shown in Fig. 2(a). Nd123 and Nd422 are the main phases through the solid state reac-tion. The residual BaCO3 is also observed in the obtained precursor. After the melt texturing

A. HU et al.130

Fig. 1. (a) An image of the polished surface of melt-textured NdBa2Cu3Oy samples with Nd2BaO4 ad-dition. Some unreactive parts exist in the edge of the sample. The diameter of the sample is 12 mm; (b)a local image of a single domain near the seed. Note that macrocracks originated from the seed. The

diameter of this sample is 20 mm. The bar presents 2 mm.

Fabrication of melt-tectured Nd123 superconductors 131

growth, Nd123 and Nd422 are the obtained phases in the melt-textured crystals. Nd422 is themain phase in the unreacted part in the edge of the samples. The minor Nd123, BaCuO2 andNd143 are also in the unreacted part in the edge of the samples. The obtained Nd422, as solidprecipitate, keeps the shape of the sample and prevents the loss of the liquid phase during thepartial molten state. The residual Nd422 particles, due to the incomplete peritectic reaction, willbe trapped in the Nd123 matrix and lead to the enhancement of the superconducting propertiesof the samples [32]. These results show that Nd123, with Nd210 addition, can be an alternativecomposition for the proper melt texturing processes. However, the Nd163 phase reported inRef. [21] is not observed in the present samples. But this is not contradictory to the results inRef. [21]. It has been reported that the Nd163 phase must be synthezised in CO2-reduced atmos-phere [26]. Figure 3 shows the XRD patterns of the powders with a stoichiometric Nd163 com-position after serieal annealing treatments. There results show that the Nd163 phase cannot beprepared in air. The further treatment for the longer annealing time gives support to this idea.The Nd163 powder which is annealed at 9608C up to 60 h gives the same results shown inFig. 3(c). Figure 4 shows the XRD patterns of the powders with the stoichiometric Nd143 com-position. It is shown that, by annealing at higher temperature, more than 10008C, with thelonger annealing time, the amount of Nd143 phase remarkably increases. These treatment con-ditions are also comparible with the melt processing process [20]. This also indicates that theNd143 phase can be synthezised during melt processing. Therefore, on the basis of the above ex-perimental results, a simple formula (1) can express the obtained phases after the melt-texturedprocessing.

NdBa2Cu3Oy � a Nd2BaO4 ÿ4 b Nd4Ba2Cu2Oy � c BaCuO2 � d NdBa4Cu3Oy �1�

Since the intermediate melt texturing is complicated and the Nd422 distribution is not homo-geneous, an exact relationship in Equation (1) is not available yet. On the base of XRD patternsshown in Fig. 2, the obtianed amount of BaCuO2 or Nd143 is less than the obtained amount ofNd123 or Nd422.

Fig. 2. X-ray di�raction patterns for (a) the precursors with a nominal composition of Nd123 + 0.5Nd2BaO4 after annealing at 9808C for 16 h and di�erent parts of the samples, (b) the melt-textured

crystals, (c) the unreactive phases in the edge of the samples.

A. HU et al.132

Fig. 3. XRD patterns of the powder with a stoichomitric composition of NdBa6Cu3Oy after a series ofannealing steps: (a) at 9008C for 16 h (b) at 9208C for 16 h (c) at 9608C for 16 h. Note that powders

after each annealing step have undergone the previous annealing at low temperatures.

Fig. 4. XRD patterns of the powder with a stoichomitric composition of NdBa4Cu3Oy after variousannealing steps: (a) 9008C for 16 h + 9408C for 16 h + 9808C for 16 h (3rd step ), (b) 10008C for 16 h,(c)10158C for 16 h, (d) 10308C for 16 h. Note that powders in each step have undergone the previous

annealing at low temperatures.

Fabrication of melt-tectured Nd123 superconductors 133

Microstructure

It is generally believed that Y211/Y123 interfaces [27, 28] or microstructure defects [29, 30] as-

sociated with these Y211/Y123 interfaces act as e�ective pinning centers for the critical current

density in the Y123 matrix. Obviously, the re®nement of Y211 particles will result in the furtherenhancement of the ¯ux pinning. Pt and PtO2 have been used to re®ne Y211 particles. However,

the e�ect of these dopants on the Nd422 re®nement is not remarkable [31]. Recent transmission

electron microscope (TEM) studies [32] have observed rich edge dislocations near the Nd422/

Fig. 5. SEM images of melt-textured NdBa2Cu3Oy with (a) Nd2BaO4 addition; and (b) Nd4Ba2Cu2Oy

addition. The bar corresponds to 10 mm.

A. HU et al.134

Nd123 interfaces. The further re®nement of Nd422 is expected to produce many dislocationswhich attribute to improving the ¯ux pinning in low ®elds [33].

Figure 5 (a), (b) shows the microstructures of NdBa2Cu3Oy with Nd210 and Nd422 additions,respectively. A pronouncedly re®ned e�ect of Nd210 addition is easily observed. The averagesizes of Nd422 particle are 1±12 mm for Nd210 addition and 6±16 mm for Nd422 addition. Thephenomenon is similar to that in Bieger et al [21]. These results strongly indicate that Nd123samples with Nd210 addition is a better method of processing to strengthen the ¯ux pinning.

Superconducting properties

Figure 6 presents the temperature dependence of the magnetization for the melt-texturedsamples. The samples oxygenated at 3008C for 200 h after an intermediate thermal treatment inAr at 9808C for 16 h exhibit a high onset Tc value of 95 K with the relatively sharp transitionwidth of less than 5 K. These results exhibit that a higher and sharper superconducting tran-sition can be obtained in melt-textured Nd123 samples with Nd210 addition than in melt-tex-tured Nd123 samples with Nd422 addition if the melt-textured process is carried on in air [23].Shiohara et al. [34] reported that higher Tc Nd123 crystals with a sharp transition can beobtained by using the ¯ux with a higher Ba/Cu ratio. Recently, Kojo et al. [9] have also fabri-cated high Tc Nd123 samples with sharp transitions by using the Ba-rich Nd4+2zBa2+2zCu2ÿzOy

as the precursor. This indicates that a high Ba/Cu ratio can depress the occurrence of the substi-tution between Nd and Ba. In the present study, Nd2BaO4 addition will lead to a higher Ba/Curatio than Nd422 addition. These studies exhibit that high quality melt-textured Nd123 bulkscan be prepared in air by the improvement of the Ba/Cu ratio in the initial composition.

Fig. 6. The temperature dependence of the magnetization for the melt-textured NdBaCuO samples. Thesample has been treated in ¯owing O2 at 3008C for 200 h after annealing in Ar at 9808C for 16 h.

Fig. 7. The critical current densities of melt-textured NdBaCuO as functions of magnetic ®eld at severaltemperatures. The applied ®eld is parallel to the c-axis of the sample.

Fabrication of melt-tectured Nd123 superconductors 135

Figure 7 presents the critical current densities as a function of magnetic ®eld at severaltemperatures. The remarkable ®shtail e�ect can be attributed to the ¯uctuation of Tc due to aslight Ba substitution by Nd in the samples [5, 6]. It is interesting to note that the much moreobvious ®shtail e�ect has been presented here than that in Nd123 samples with Nd422 addition[20]. The enhancement of the ®shtail e�ect with higher and sharper superconducting transitionshas been discussed in the framework of a spinodal decomposition mechanism [9, 35]. This mech-anism has been supported by the direct observation of a nano-size modulated structure [35] andNd/Ba composition ¯uctuation [36]. However, these defect structures have been interpreted as aresult of a solid state decomposition reaction of Nd123 due to the long thermal treatment at5008C in O2 [35, 36]. In our melt processing technology, an intermediate annealing in Ar at9808C for 15 h has been employed to optimize Tc [8]. The present studies suggest that such ashort-time thermal treatment in Ar, at high temperature, speeds the decomposition of Nd123and results in an ideal pinning structure with ¯uctuated Tc for high Jc superconductors.

CONCLUSIONS

High quality Nd123 bulks can be fabricated in air by using Nd2BaO4 addition. A new phaserelation of this mixed composition has been found in the melt processing in air. Better seededcrystal growth, further re®nement of Nd422 and Tc of 95 K with sharper transition have beendemonstrated in the present composition. The e�ect of the short-time thermal treatment in Arbefore oxygenation is thought to speed the decomposition of Nd123 and result in an ideal pin-ning structure. These results strongly indicate that Nd123 with Nd210 addition is a promisingcomposition for preparation of large single domain Nd123 monoliths for cryogenic applications.

AcknowledgementsÐThis work was supported by Institute de Ciencia de Material de Barcelona, C. S. I. C, the NationalResearch Organisation in Spain. The author (A. Hu) wants to thank J. Gil for the XRD analysis, W. Bieger in Dresden,Germany for stimulating discussion, Y. Yang and J. Mora for preparation of the manuscript.

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