INTERFACIAL MICROSTRUCTURE IN Y1 Ba2 Cu3 O7-δ HIGH Tc SUPERCONDUCTORS

(1)

HAL Id: jpa-00230061

https://hal.archives-ouvertes.fr/jpa-00230061

Submitted on 1 Jan 1990

HAL

is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire

HAL, est

destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTERFACIAL MICROSTRUCTURE IN Y1 Ba2 Cu3 O7-δ HIGH Tc SUPERCONDUCTORS

J. Alarco, G. Dunlop

To cite this version:

J. Alarco, G. Dunlop. INTERFACIAL MICROSTRUCTURE IN Y1 Ba2 Cu3 O7-δ HIGH Tc SUPERCONDUCTORS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-959-C1-964.

�10.1051/jphyscol:19901149�. �jpa-00230061�

(2)

COLLOQUE DE PHYSIQUE

Colloque C l , supplbment au n o l , Tome 51, j a n v i e r 1990

INTERFACIAL MICROSTRUCTURE I N Y , B ~ , C U , O , _ ~ H I G H T, SUPERCONDUCTORS

J . ALARCO and G . L . D U N L O P ( ' )

Department o f P h y s i c s , Chalmers U n i v e r s i t y o f Technology, 5-412 96 G o t h e n b u r g , Sweden

Abstract

-

The microstructures of YBa2Cu307-g ceramics prepared from freeze dried powders and containing an excess of CuO have been studied by analytical electron microscopy. Special attention has been paid to the interfacial microstructure. It was found that a liquid phase formed during sintering between 890°C and 920°C and this promoted grain growth and densification. Both clean grain boundaries and boundaries containing an amorphous intergranular film, which was rich in Cu, have been observed. Both CuO and BaCu02 were present as secondary phases.

1

-

INTRODUCTION

Most of the possible applications of the new high T, superconductors in bulk polycrystalline form still require improvement of the transport critical current density / l / . This property, which is sensitive to the microstructure and processing, is found to be hindered by different kinds of grain boundaries. These act as poor or non-superconducting barriers

-

'weak links'

-

to the

supercurrent conduction, and exert their deleterious effect as a consequence of the short coherence length in the new materials and the anisotropy of their properties /2/.

Solution routes for the preparation of powders of the new ceramics allow for better control of the preparation chemistry and subsequently of the microstructure / 3 / . This may lead to the

establishment of a better understanding of the microstructure dependent properties, and perhaps, to their improvement. Up to now, most microstructural characterization has been carried out on solid state reacted material.

The objective of this work is to elucidate the effect of varying the sintering temperature on the microstructure and properties of materials prepared from freeze dried powders. Emphasis is placed upon the interfacial microstructure.

0 ) Present address: Department of Mining and Metallurgical Engineering, University of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901149

(3)

COLLOQUE DE PHYSIQUE

The materials were prepared from powders obtained through a solution freeze dry route.

They were sintered in air (open furnace) for about 14 hours and furnace cooled to room temperature. Then they were then annealed in 1 atm. of pure oxygen at 540°C for about 12 houi A detailed description of the preparation of these materials has been published elsewhere /4/.

This work is concerned with two materials prepared from the same starting powder containing an excess of CuO. This corresponds to 4.7% extra Cu relative to the stoichiometric value, determined in the solution by ion plasma spectroscopy. One of the materials was sintered at 890°C and the other at 920°C. Densities determined from pellet dimensions and weights were about 8588% and 88-90% of the theoretical value, respectively. Materials prepared under other conditions are currently being investigated.

The analysis of the microstructures was performed with optical microscopy, x-ray diffractometr SEM and TEM (STEM)-EDX.

Specimens for investigation by transmission electron miaoscopy were prepared by grinding th slices on Sic paper followed by polishing to a final thickness of ca 40pm. They were then thinn with argon ion beams which were incident at both surfaces at a sputtering angle of 12-15' using gun voltage of 2.5-3kV and gun current of 0.5mA. A cold stage was used.

Thin foils were examined either at a lOOkV in a Philips EM300 transmission electron microscol or at 200kV in a JEOL 2000FX scanning transmission electron microscope equipped with a Link Systems AN10000 energy-dispersive x-ray spectrometer.

3

-

RESULTS and DISCUSSION

Optical microscopy showed that different average grain sizes and size distributions developed during the two different sintering temperatures (Fig. 1). The sample sintered at 890°C was entirely in the solid state and showed a smaller average grain size with more uniform size distribution compared to the sample that had been sintered at 920°C. The length of the grains v about 5-10pm and their width 1-2pm in the material sintered at 890°C, and 20-60pm and 8-20p respectively, in that sintered at 920°C. The presence of characteristic twins, which results in an indirect way of identifying the orthorhombic phase, was evident in the sample sintered at 920'1 In addition, secondary phases could be observed in this sample. They appeared in the form of particles that were adjacent to some of the twinned grains and also in some intergranular regic suggesting that a liquid solidified between the grains.

X-ray diffractometry of polished slices showed that the sample that had been sintered at 890°C was tetragonal with cell parameters a=3.86A and c=11.~2A, while the sample sintered at 920°C was orthorhombic with cell parameters a=3.82& b=3.88A and c=11.69A. The presence of BaCuC in the sample sintered at 920°C could be detected in the diffraction pattern.

TEM showed that the twin width in the sample sintered at 920°C was in the range 600-2000A. I general, twins appeared quite uniformly spaced within each grain.

Both clean boundaries and boundaries containing an intergranular film were observed by TEIv in the liquid sintered material. Clean boundaries were identified by the presence of regular arr:

of grain boundary dislocations (Fig. 2). Dark field images of the second type of boundaries, usin

(4)

diffuse scattered electrons, showed that the intergranular films were amorphous (Fig. 3). The widths of these films was about 40-120A. EDX of the intergranular films showed that they were copper rich relative to the (123) grains.

The presence of BaCu02 was found in both samples in the form of isolated particles within (123) grains. It was also found as larger grains in the sample sintered at 920°C. The latter also contained CuO typically connecting (123) grains which were rounded at the corners. This phase had a high density of dislocations and appeared with bright contrast.

The liquid phase in the sample sintered at 920°C must have promoted grain growth and densification. The observed secondary phases are also consistent with an initial composition on the CuO

-

YBa2Cu307a principal join or within the C U O - B ~ C U O ~ - Y B ~ ~ C U ~ O ~ compatibility triangle of the phase diagram, where eutecting melting is expected /6/.

Probably, the formation of the oxygen richer orthorhombic phase was facilitated by a higher diffusivity of oxygen through the intergranular phases that formed in the liquid phase sintered material. This may explain the oxygenation for the tetragonal to orthorhombic transformation during cooling which was not achieved in the solid phase sintered sample.

Many of the grain boundaries were parallel to the (001) basal plane of one of the adjacent grains.

Such boundaries were usually clean. In particular, the long boundary of elongated grains almost invariably corresponded to the basal plane. These (001) basal plane boundaries have been reported for the tetragonal phase /7/ and the orthorhombic phase /8/ of the Y-Ba-Cu-0 system.

Similar results have also been obtained for the Bi-Ca-Sr-Cu-0 compounds /g/. As pointed out by Ishida and co-workers /10/, the preferential development of these boundaries during sintering could be an intrinsic characteristic of the layered perovskite structure.

Cracks running perpendicular to the c axis were o c c a s i o n a l l y observed inside grains of the sample that contained orthorhombic phase (Fig. 4). If they formed during sintering and were not produced during the specimen preparation, such cracks would be result of tensile stresses generated by the contraction of the c-axis when the material undergoes the tetragonal to orthorhombic transformation /8,11/. In such a case, their existence would affect the transport properties in the material. Preferential argon ion beam thinning seems to occur at many grain boundaries where there was an intergranular phase, and a partial gap was generated between the adjacent grains. This suggests that some intergranular gaps, which could also be attributed to cracks, were produced during the specimen preparation.

The superconducting characteristics of the orthorhombic material were confirmed by resistivity and AC susceptibility measurements. Resistivity measurements showed an onset at 92.6K, a midpoint at 91.5K and zero resistance at 79.4K (FigSa). A tail, corresponding approximately to the separation between the midpoint and zero resistance temperatures, was present in the transition.

As suggested by Sawano and co-workers /12/, this tail could be associated with the presence of weak links at the grain boundaries. As shown in figure 2b, the AC susceptibility of this sample showed a sharp onset at 92.2K, a midpoint at 90K, and the rest of the transition was asymptotic to the perfect diamagnetic behaviour. The tetragonal sample, tested by AC susceptibility, showed a broad transition starting at 88.4K which was still incomplete down to 45K (Fig5b). This may indicate the presence of a very small fraction of the superconducting phase in this sample.

(5)

Cl-962 CoLLOQUE DE PHYSIQUE

1.- Considerable differences in microstructure were caused by the 30°C difference in sintering temperature in the CuO rich region due to the formation of a liquid phase between 890°C a.

920°C. This promoted both grain growth and densification.

2.- The off-stoichiometry toward the CuO rich region in the starting solution led to the presenc of CuO and BaCu02 as secondary phases in the final sintered products.

3.- Solid phase sintering at 890°C did not allow for the oxygenation of the tetragonal phase to produce the orthorhombic superconducting phase.

4.- Many boundaries were found to be parallel to the (001) basal plane of one of the adjacent grains.

Fig. 1

-

Optical micrographs showing the miaostructures of different YBa2Cu307-6 high T, superconductors. The samples were polished and imaged using polarized light. It can be seen I

the sintering temperature affects the grain size and the twin formation: (a) sample with CuO excess, sintered at 890°C; (b) sample with CuO excess, sintered at 920°C.

Fig. 2

-

Transmission electron micrograph of a clean boundary identified by the presence of a regular array of grain boundary dislocations when imaged inclined to the incident beam.

(6)

Fig. 3

-

Transmission electron micrographs of a typical grain boundary containing an amorphous intergranular phase: (a) b r i g h t field image; (b) dark field image using diffuse scattered electrons.

The width of the intergranular phase was about 40-120A. EDX analysis showed an enrichment of Cu relative to the (123) grains.

Fig. 4

-

Transmission electron micrograph of a crack running perpendicular to the c axis.

(7)

C 1-964 COLLOQUE DE PHYSIQUE

Temperature [K] Temperature [K]

resistive transition b AC susceptibility

Fig. 5

-

Superconducting characteristics of samples with CuO excess: (a) resistive transition of sample sintered at 920°C; (b) real parts of AC susceptibility measurements for the samples sintered at 920°C ^{( 0 )}and sintered at 890°C (+). The shapes of the samples for these AC susceptibility measurements were different, so the two respective curves should only be compared qualitatively.

ACKNOWLEDGMENTS

++-***********

+ + ++

-

+ + I

+ . m m

. . . m g '

- 5 - . u . o . v . a . m . n . l

J.A. Alarco is grateful to The Swedish Institute for the award of a scholarship during the perioc this research and to the Swedish Board for Technical Development for financial support.

Collaboration with H. Medelius and D.J. Rowcliffe at the Royal Institute of Technology, Stockholm is gratefully acknowledged as are fruitful discussions with Dr. E. Olsson.

20 40 60 80 100 I20 140 160 W 40 60 80 LOO 120 140 16C

1 - 3 0 -

-

.E'

^c ^-1

-

S -2

-

j,

g

^{- 3 -}

4

^{4 -}

5

-

4

-

3 -

2

-

1

-

0 7

REFERENCES

( m " ' " " " '

i

t

: . S

-

~ - 7⁸ ~. ^U .. I .

/ l / Murphy, D.W., Johnson, D.W., Jin, S. and Howard, R.E., Science 241 (1988) 922 /2/ Ekin, J.W., Braginski, A.I., Panson, A.J., Janocko, M.A., Capone 11, D.W., Zaluzec, N.J.,

Flandermeyer, B., d e Lima, O.F.,Hong, M., Kwo, J. and Liou, S.H., J. Appl. Phys. 6_2 (12), (198 4821

/3/ Johnson, S.M., Gusman, M.I. and Rowcliffe, D.J., Adv. Ceram. Mat. 2 (3B), (1987) 337 /4/ Medelius, H. and Rowcliffe, D.J., MRS series High T, Superconductors, part 11, summer 19E

Strasbourg, France.

/5/ Cook, R.F.,Shaw, T.M. and Duncombe, P.R., Adv. Ceram. Mat. 2 (3B), (1987) 606

/ 6 / Oka, K., Nakane, K., Ito, M., Saito, M. and Unoki, H., Jpn. J. Appl. Phys. 27 (6), (1988) L1065 /7/ Zandbergen, H.W. and Thomas, G., Acta Cryst. (1988) 772

/8/ Nakahara, S., Fisanick, G.J., Yan, M.F., Van Dover, R.B. and Boone, T., J. Cryst.Growth 85 (1987) 639

/9/ Liu, P., Knutson, M., Liu, Z.Y., Olsson, E. and Dunlop, G.L., Supercond. Sci. Technol. i ( 1 9 8 254

/10/ Ishida, Y., Hagege, S., Ichinose, H. and Takahashi, Y., J. Electron Microsc.Tech. 12 (1989) 24 /11/ Shaw, T.M., Shinde, S.L., Dimos, D., Cook, R.F., Duncombe, P.R. and Kroll, C., J. Mater. Rc

4 (2), (1989) 248

-

/12/ Sawano, K., Hayashi, A., Ando, T., Inuzuka, T. and Kubo, H., Research Update, Ceram.

Supercond. 11 (Editor: Man F. Yan) (Ohio: The Amer. Ceram. Soc.) (1988) 282