Microstructure and superconducting properties of Bridgman-melt-textured Y1Ba2Cu3O7−δ

PhysicaC 198 (1992) North-Holland

181-184

Microstructure and superconducting properties of Bridgman-melttextured Y 1Ba2Cu307_-6 M. Ullrich,

D. Miiller,

Institut fir Metallphysik, Received

K. Heinemann,

Hospitalstr.

L. Niel and H.C. Freyhardt

3/7, 3400 GBttingen, Germany

22 May 1992

The X-ray diffraction pattern (XRD) of Y,BazCu,O,_d (YBCO), grown by a modified Bridgman method, exhibits a strong texture. The c-axis is found to be almost parallel to the temperature gradient. From measurements of the magnetization hysteresis loop and from p( T, B) measurements, the critical current density, j,, and the activation energy for thermally activated flux flow (TAFF), U,, were found to be about four times higher than in sintered bulk material. This is not only due to the strong texture which leads to better coupling between the grains, but also due to the presence of lo-60 nm tine precipitates with partially associated dislocations in the 123 matrix.

1. Introduction

2. Experimental

Since the discovery of YBCO [ 1 ] with a superconducting transition temperature, T,, above the boiling point of liquid nitrogen, there have been extensive efforts to increase the critical current density, j,, in particular for practical use at 77 K. Because of the weak links between the grains, j, in polycrystalline samples is very low and decreases sharply in weak magnetic fields at 77 K [2,3]. Especially a misorientation of 10 degrees results in a sharp dropoff in j, [ 41. Thus, specimens without large-angle grain boundaries should have much higher j, values. For bulk material melt-texturing enables the growth of well oriented grains with preferred orientations causing high j, values [5-g 1. In this melt textured material Y2BalCu105 (2 11) precipitates with typical dimensions of 1 urn are found to increase j, [ 9, lo]. These precipitates are large compared to the small coherence length of YBCO which is in the order of a few A. Thus, their interface with the YBCO matrix is considered to be responsible for the stronger pinning [ 11,121. In the present investigation TEM observations are reported of very fine precipitates with dimensions of lo-60 nm, which are thought to represent even stronger additional pinning centers.

The precursor stoichiometric material was made by the conventional solid-state reaction of BaC03, CuO and Y203 and pressed to pellets with 10 mm in diameter and up to 4 mm in height. Some pellets with larger dimensions were produced by Solvay Barium Strontium GmbH. The pellets were put into Y-stabilized ZrO, crucibles, which were inserted in quartz tubes. After adjusting an oxygen partial pressure of 200-400 mbar, the tubes were sealed and mounted in a three-zone Bridgman furnace, with a fixed vertical temperature gradient of 25 K/cm. The lateral temperature gradient was much smaller. The samples were slowly lowered through the temperature gradient from 1050°C to 900°C with a velocity of 0.4 mm/h resulting in a cooling rate of 1 K/h. A slow cooling rate is required because of the peritectic reaction of YzBalCulO,+L(Ba, CU)-+Y~B~~CU@-~ [ 71. Subsequently they were cooled to room temperature with a rate of 0.1-l K/min. From the resulting bulk cylinder of N 1 cm3 small bars were cut with a diamond saw. The structure of the samples were investigated by X-ray diffraction analysis (XRD) using Co Ku radiation and by analytical scanning electron microscopy (SEM) with EDX. After grinding, polishing and ion milling the microstructure was evaluated by

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and SCproperties

transmission electron microscopy (TEM). The p( T, B) values were measured by the standard four-probe method using a DC current of 10 mA flowing perpendicular to the external magnetic fields. The critical current density, j,, was estimated from the hysteresis loop of the magnetization by applying the Bean critical-state model [ 13 1. To avoid strong demagnetization effects, the magnetic field was applied parallel to the cylinder axis of the samples with rectangular cross-section. Magnetization measurements in external magnetic fields up to 13 T were performed by using a Faraday balance with a fixed magnetic field gradient of 1 T/m.

of MT YBCO

a)

20

10

30

40

50

60

20 CoKc(

b)

3. Results and discussion x

x

To obtain information on the orientation distribution of the crystallites in the directionally soliditied YBCO, XRD measurements from the different faces of the samples were carried out. The diffractogram taken from a face perpendicular to the temperature gradient exhibits a c texture (fig. 1 (a) ), whereas taken from a face parallel to the temperature gradient, exhibits an a, b texture (fig. 1 (b) ). From this it becomes obvious that the c-axis is almost parallel to the temperature gradient. The c-axis lattice parameter, which amounts to 11.6985 ( 10) A, reveals that these specimens are not oxygen deficient. The S of YIBa2Cu307_-6 turns out, by using this lattice parameter, to be of the order of 0.04 (2) [ 14 1. In spite of an ideal stoichiometry and the slow cooling rate of 1 K/h, a small amount of 211 could be detected (fig. 1 (b ) ) . This is probably due to an uncomplete peritectic reaction and the leakage of the (Ba, Cu)-rich liquid phase, which wets the crucible walls. Additional small amounts of reflections from random oriented grains could be detected (fig. 1 (a) >. A rocking curve of the (005 ) reflection, having a full width at half maximun of less than 0.2 degree, confirms the strong c texture (fig. 2). However, the existence of small-angle grain boundaries can be concluded from the appearance of smaller peaks near to the main peak. Our suspension in the Bridgman furnace vibrates markedly, however with only small amplitudes. TOgether with the very small growth rate this is probably the reason for the breaking off of the melting

E t % N +

+

4

Fig. 1. X-ray diffraction pattern taken from a face perpendicular (a) and parallel (b) to the temperature gradient. The star marks the reflections from random oriented grains and the crosses from the 2 11 phase.

Fig. 2. Rocking curve of the (005 ) peak from a face perpendicular to the temperature gradient.

front. Therefore XRD measurements reveal reflections from random oriented grains and small-angle grain boundaries. The fact, that the samples are not oxygen deficient is also demonstrated by the high superconducting

M. Ullrich et al. /Microstructure

transition temperature of 92.5 K and the transition width which is smaller than 1 K. The ratio of ~(300 K)/p( 100K) is about 3 in the case of transport currents parallel to the a, b planes (fig. 3). The specific resistance for orientations of the transport current parallel to the c-axis is about a hundred times higher. M(B) measurements of these samples show a large magnetic hysteresis at all measured temperatures (e.g. fig. 4). Using the relation j,=2A&I/d [ 151, where AII~ denotes the width of the hysteresis and d the sample diameter, we calculate a critical current density at 77 K and 1 T j,2 5 x lo3 A/cm*. p( T, B) measurements show only a small broadening in magnetic fields. From an Arrhenius plot of lnp versus T-I, the activation energy for TAFF is estimated to be about 3.5 eV for an external magnetic field of B=O.l T [ 161. 1

,

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140

and SCproperties of MT YBCO

183

TEM observations reveal small precipitates with diameters between 10 and 60 nm (fig. 5 ) . Their average size is about 35 nm. Up to now their nature is still unknown. The moire pattern [ 171 indicates that dislocations are associated with the precipitates (fig. 6). Thus, these melt textured samples are not only well oriented, but they contain also very fine precipitates with associated dislocations which could represent additional attractive pinning centers, being responsible for the high critical current density and the high activation energy for TAFF.

’

1000 800 z s II 01

600 400 200 0

,I,,,),” 20

60

180

T [Kl

Fig. 5. Transmission electron micrograph of Bridgman-melt-textured Y,Ba2Cu307_6. Extremely small precipitates with an average size of 35 nm and a small-angle grain boundary are visible.

Fig. 3. Temperature dependence of the resistivity for currents flowing Ila, b.

0

I 1

I 2

/

3

I, 4

I 5

B eff

[T1

I, 6

I, 7

1 8

Fig. 4. Part of the magnetization loop at 6.7 K.

Fig. 6. TEM micrograph of a moire pattern. Note that the dislocations are in the immediate vicinity of the fine precipitates of fig. 5.

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4. Conclusions 1 cm x 1 cm x 1 cm melt textured YBCO samples have been prepared, using a Bridgman method. The specimen shows a texture with VTllc. The critical current density and the activation energy for TAFF are much higher than in polycrystalline samples. The texture and small precipitates with an average diameter of 35 nm are thought to be responsible for the high j, and U,, value. Further investigations are necessary to identify the precipitates and to determine the density of the dislocations and precipitates in order to estimate their pinning strength. In future experiments the temperature gradient will be moved instead of the sample, using the vertical gradient freeze method (VGF) instead of the Bridgman method. Thus, there will be no more vibration effects, causing a breaking off of the melting front.

Acknowledgements The authors would like to thank C. Mewes for performing the directional solidification experiments and the Solvay Barium Strontium GmbH for the which was used in these YBCO powder, investigations. This work is supported by the Bundesministerium fur Forschung und Technologie (BMFT) under the grant number 13N5493A.

ofMT YBCO

References

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