Physica C 196 ( 1992 ) 105-110 North-Holland PHVSICA

Large, flux-free Y1Ba2Cu307_ single crystals by improved CuO- BaO flux growth

Peter Fischer a,b a UniversiRit Bayreuth, Experimentalphysik V,, Postfach 101251, W-8580 Bayreuth, Germany b Siemens AG, Zentralabteilung Forschung und Entwicklung, W-8520 Erlangen, Germany

Received 12 March 1992

Y1BaCu3OT_ ~ single crystals were grown using a modified CuO-BaO self flux method. The nominal composition and the tem- perature program were optimized to decrease soaking time and to obtain crystals of a cubic habit. The use of BaO instead of BaCO3 enhances the melting of the powder; so did the use of an Ar atmosphere. Melting, growth and cooling was done with different vertical temperature gradients in a specially designed chamber furnace, allowing strong convection during melting and nucleation at the bottom of the crucible, respectively. After in situ decanting of the residual flux, the free standing crystals were pre-oxygenized while slowly cooling to room temperature in oxygen. The crystals had a nearly cubic shape with a size of up to 4 × 3 × 1.5 mm 3 and reached a Tc of 91.7 K with a transition width of less than 0.5 K after a two month oxygen anneal.

1. Introduction the basis for the improvements presented in this paper.

A challenging problem of the high tempera ture su-

perconductors is the correlat ion between the super-

conduct ing propert ies and the crystal lographic struc-

ture of these highly anisotropic materials. To investigate anisot ropic propert ies, large and high

quali ty single crystals are needed. Invest igat ions on

the phase diagram of YtBa2Cu307_a [ 1-3] revealed the problems of dealing with a sequence of peri tect ic

reactions of a mixture o f oxides with very high melt-

ing points. Fortunately, the existence of an eutectic

in the b inary system of B a O - C u O at about 900°C

[4-6 ] helps to solve the problem by flux growth us-

ing a flux with two components present in excess. There have been a mul t i tude of growth exper iments

using different composi t ions, crucible materials,

tempera ture programs and a tmospheres [ 6 -24 ] .

Some authors tr ied different fluxes that have been

known to work with other oxides [25 -30] . The use of pedestals or gold foils to separate the crystals from the residual flux was invest igated in refs. [ 7-9, 31 ]. Large crystals with nearly cubic shape were grown by Wolf et al. [ 32 ]. Wolf ' s experiences helped to form

2. Experimental

Due to the high temperatures needed, which result from the high melt ing points of the constituents, pol- lution from e.g. the crucible or stirring tools cannot be avoided. In order to reduce this pol lut ion to a min imum, fastest melting at the lowest possible tem- perature must be achieved by opt imizing the stoi- chiometry. Therefore several nominal composi t ions

were tested: Y3Ba3tCu66Ox, Y3Ba3oCu67Ox, YaBa3oCu660 x (opt imal according to ref. [32 ] ) , YsBa29Cu66Ox, YsBa3oCu65Ox, mixed from Y203, BaCO3 or BaO resp. and CuO. The amount of start- ing materials was i00 g for survey, 450 g for final growth experiments. BaCO3 is more stable against moisture and CO2 of the a tmosphere but must be calcined to BaO at 850°C before melting. The mix- ture enriched by 50 ml of ethanol or cyclohexane was mil led in a ball mill for one hour. During calcination the powder was sintering, thus another mill ing was necessary. Crucibles were of A1203 or ZrO2 with vol- umes of 50 ml for the first experiments, and 200 ml

0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

106 P. Fischer / Large, lluxz[J'ee YB('O single crvstal~

subsequently. The powder was compacted in the cy- l indrical crucibles with a pestle.

A complete and homogeneous melt was obta ined within 12 to 96 h at 1000 to 1020°C. This was ver- ified by observing the d isappearance of solid part i - cles through a quartz window above the crucible po- sition. The melt became slushy when stirring. Bottom heating led to large convect ion which facil i tated fast and complete melting. An Ar a tmosphere protected BaO against CO2. After changing the vertical tem- perature gradient to top heating, which transfers the crystal l ization zone to the bo t tom of the crucible, the melt was cooled to 960 or 930°C. Cooling with a rate between 1 and O. 1 K / h was per formed in air since for this system the phase diagram is best known. Then the crucible was inver ted in order to decant the re- sidual flux into a porcelaine capsule. It was not nec- essary to open the furnace door and thus the tem- perature remained stable. Subsequently the tempera ture was decreased at a rate of ini t ial ly 5 K /

h, between 700 and 300°C at 1 K / h while flushing with pure oxygen to d iminish thermal strains during the t e t ragona l -o r thorhombic transi t ion of the cwstal structure.

Compared to the efforts to remove the crystals from residual flux not proper ly decanted, it was very easy to separate the flux free crystals from the bot- tom of the crucible with a razor blade (fig. 1 ). Sub- sequently they were annealed in pure oxygen for six weeks, with the furnace temperature decreasing slowly from 600 to 300°C. As the tempera ture de- creased, the cooling rate was decreased as well to deal with the decreasing coefficient of oxygen diffusion.

3. Results and discussion

3.1. S to ich iometry

The composi t ion with least Y and most Ba (Y3Ba31Cu660,) melted most rapidly and left the

Fig. 1. Flux free YiBa2CU3OT_a single crystals grown at the bottom of the crucible. The wall of the crucible was cut off to allow easy removal of the crystals. The crucible was 60 mm in diameter and 120 mm in height.

P. Fischer / Large, flux-free YBCO single crystals 107

smallest amount of residual flux. Y203 has a very high melting point of 2410 °C and therefore dissolves very slowly in the flux. A high content of Y prevents drops of already melted flux from flowing together [32]. It is very important to thoroughly mix the powders, minimize the grain size and find the correct grade of compression. In loose powder the melted drops are too far from each other, in pressed pellets the mo- bility of the drops is severely reduced which hinders the drops from flowing together, too. By using more BaO in the flux (32% BAO-68% CuO) than the eu- tectic ratio (28% BAO-72% CuO) nearly cubic shaped crystals were obtained. This was already no- ticed in an early work [ 10 ]. BaO instead of BaCO3 accelerated the melting process significantly. The small lots (25 g) of BaO packed under inert atmo- sphere (by Alfa Products Inc. ) are sufficiently stable against CO2 and moisture of the atmosphere for the short time of weighing. Using BaO, the laborious procedure of calcination and remilling can be omit- ted. The easy melting of the composition with BaO compared to the difficult melting of the calcinate of BaCO3, showed that the calcination was not complete.

3.2. Crucibles

Crucibles of A1203 proved to be more stable against the aggressive melt than those of ZrO2. Especially for longer runs or sudden temperature changes, the cru- cibles of ZrO2 cracked and the melt leaked. ZrO2 was therefore of no use for the growth experiments de- scribed here. The melt had a higher viscosity which led to more residual flux remaining in the crucible. A1203 yielded a low viscosity, guaranteeing complete and homogeneous melts. The solubility of A1 in the melt as well as in the crystal is higher than for Zr. A1 is substituting on Cu places and reduces the oxygen diffusion coefficient [ 10 ]. Consequently,a much longer annealing time is required to minimize the oxygen deficiency 6 and to obtain a high homoge- neity of 6. The cylindrical shape of the crucibles with their flat bottom is responsible for a well defined crystallization zone at the bottom resulting from a vertical temperature gradient.

3.3. Melting and crystallization

Using BaO instead of BaCO3 in an inert Ar at- mosphere allowed one to reduce the maximum fur- nace temperature from 1020 to 1000°C, which al- ready helps very much to reduce pollution from the crucible wall. Despite of this reduction, complete melting was achieved within less than 12 h without stirring. The nearly exclusive heating from the bot- tom obviously led to a strong convection optimizing the melting process.

With the inverted strong vertical temperature gra- dient, crystals grew exclusively from the bottom of the crucible. Microprobe analysis and X-ray diffrac- tion of several parts of an undecanted regulus proved the majority of Y to be in the lower part of the cru-

---9

277 a m

(-...

[ I " ~ : ] 0 nm height 9.7 nm

50 100 150 200 250 x(nm)

Fig. 2. Scanning tunneling microscope scan across a crystal sur- face [33]. (a) Scan of 277 nm×277 nm, showing a growth step of one lattice cell height. (b) Profile of the cross- section indi- cated in (a). The image of the growth step is very sharp resulting from the high resolution and the very small drift.

108 P. Ftscher/ Large. flux-free YBCO single crystals

cible, that is close to the crystallization zone. The slower the cooling rate, the bigger and more cubic the crystals grew. Rates under 1 K / h are necessary to yield crystals with a lateral dimension of more than 1 mm. For larger and thicker crystals, rates must de- crease to 0.1 K/h. A high temperature stability, of 0.1 K, is needed for a smooth and undisturbed layer by layer growth. Under such conditions the free sur- face was mirror-like and atomically flat as shown by scanning tunneling microscope investigations [33]. In fig. 2, a surface scan is shown. The only feature is a growth step of one lattice cell height. Hot in situ decanting finally removes the residual flux, lets the crystals oxygenize in situ and allows easy removal of the crystals.

3.4. Annealing

The as grown crystals already showed supercon- ductivity, because they were preoxygenized while cooling through the tetragonal-orthorhombic tran- sition. Although it was necessary to anneal them for

a long time after they were removed from the cru- cible. It was found by microprobe analysis that only after several weeks in pure oxygen and slow expo- nential decrease of the temperature the oxygen de- ficiency is small and homogeneous enough to yield a high and sharp T,,. The annealing time increases with increasing A1 concentration in the crystal [ 32 ].

3.5. ('O,stal characterization

Characterization was done using AC-suscepto- metry, DC-SQU 1D-magnetometry, microprobe analysis, X-ray diffraction, Laue photography and scanning tunneling microscopy. The latter revealed smooth surfaces as mentioned above. To succeed in growing thick crystals, the small excess of BaO and the very' slow cooling rates of 0.1 K /h mentioned are essential. The slow cooling rate and high tempera- ture stability are also responsible for the smooth sur- faces. The crystals reached sizes of up to 4 x 3 × 1.5 mm 3 (fig. 3). Tc was 91-92 K with a width of less than 0.5 K measured by AC-susceptibility (fig. 4), indicating that the crystals are well oxygenized and

I I 1 m m

Fig. 3. Photograph of a typical crystal. The left and bottom edge are natural growth edges, the upper and right edges result from breaking the crystal. The thickness is about 1.5 ram.

P. Fischer / Large, flux-free YBCO single crystals 109

- 100 < " -

- 6 0 0

- 7 0 0

8 0 0 - J i J

9 0 9 1 9 2

T e m p e r a t u r e T [ K ]

Fig. 4. Superconducting transition of a single crystal as measured by AC-suseeptibility. The left scale shows the screening signal Z', the right scale shows the loss signal Z", both vs. temperature. T¢ is 91.7 K with a transition width of less than 0.5 K.

, ~ - 2 0 0

- 3 0 0

bo 400

- 5 0 0

- 1 0

- 2 0

& -30 --~

- 4 0 .~o

o

- 5 0 ~

- 6 0

' - 70 93

the oxygen vacancies are distributed very homoge- neously. This was verified by microprobe analysis. However, a slight increase in oxygen concentration of 5 + 1% at the edges was determined by microprobe

scans across a section of a crystal. Slices of 100 gm in thickness were cut with a dia-

mond wire saw for X-ray diffraction. An oxygen con-

centration of 6.90-6.95 was calculated [34] from the c-lattice constants of 11.665 A-1 1.660 A for slices taken form the inside and from the surface of the crystal, respectively. Oxygen loading is not only a surface transit ion problem but also a diffusion prob- lem inside the crystal, as can be seen from the shape of the oxygen distr ibution across the crystal. Laue pictures showed an orientat ion distr ibution of 1.2 ° and a mosaic distr ibution of less than 0.3 ° [ 3 5 ]. An increase of one magnitude in the magnetic critical current by neutron irradiation indicates a lack of ef- fective pinning centers in the unirradiated crystal due to its high purity [36]. No efforts were undertaken

to obtain twin-free crystals.

4. Conclusion

High quality YiBa2Cu307_5 single crystals of nearly cubic shape and several millimeters in size were produced by a modified flux growth using an excess of BaO and CuO. Stoichiometry as well as the

temperature program were optimized including the use of different temperature gradients and atmo-

spheres. Residual flux was decanted with a mecha- nism integrated in the furnace without the need of

opening the door. Superconducting and crystallo- graphic properties were determined using AC-sus- ceptibility, DC-SQUID-magnetometry, microprobe analysis, X-ray diffraction, Laue camera and a scan- ning tunnel ing microscope.

Acknowledgements

For characterizing the crystals the work of Th. Berthoid, J. Burger, A. Urstadt and Y. Uzel is grate- fully acknowledged. I am indebted to M. Wilhelm and T. Wolff for valuable advice in the initial stages of this work and to H.-W. Neummiiler (Siemens AG, Erlangen) and H.F. Braun (Universit~it Bayreuth) for their cont inued interest and support.

References

[ 1 ] R.S. Roth, K.L. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1987) 303.

[2] K. Oka, K. Nakane, M. Saito, M. lto and H. Unoki, Jpn. J. Appl. Phys. 27 (1988) L1065.

[ 3 ] K. Oka and H. Unoki, J. Cryst. Growth 99 ( 1990 ) 922. [4] M. Nevfiva, E. Pollert, L. Matejkov~t and A.J. Tfiska, Cryst.

Growth 91 (1988) 434. [5] F. Greuter, P. Kluge-Weiss, H. Zimmermann and C

Schiiller, Physica C 153-155 (1988) 361. [6] G. Balestrino, S. Barbanera and P. Paroli, J. Cryst. Growtll

85 (1987) 585. [ 7 ] J. Kowalewski, D. Nikl and W. Assmus, Physica C 153-155

(1988) 429; J. Kowalewski and W. Assmus, Eur. J. Solid State Inorg Chem. 27 (1990) 135.

[ 8 ] F. Holtzberg and C. Feild, J. Cryst. Growth 99 (1990) 915 [9]D.L. Kaiser, F. Holtzberg, M.F. Chrisholm and T.K

Worthington, J. Cryst. Growth 85 ( 1987 ) 593. H.J. Scheel and F. Licci, MRS Bulletin 13 (1988) 56. L.F. Schneemeyer, J.V. Waszczak, T. Siegrist, R.B. vat Dover, L.W. Rupp, B. Batlogg, R.J. Cava and D.W. Murphy Nature 328 (1987) 601. K.L. Keester, R.M. Housley and D.B. Marshall, J. Cryst Growth 91 (1988) 295. D.L. Kaiser, F. Holtzberg, B.A. Scott and T.R. McGuire Appl. Phys. Lett. 51 (1987) 1040; D.L. Kaiser, F. Holtzberg, M.F. Chisholm and T.K Worthington, J. Cryst. Growth 85 (1987) 593.

[ 1 0 ]

[ I l l

[12]

[13]

110 P. Fischer / Large. Jluxqree YBCO single crystals

[ 14] T.F. Ciszek and C.D. Evans, J. Cryst. Growth 109 ( 199t ) 418, and references therein.

[ 15 ] A.M. Azad and O.M. Sreedhaven, Supercond. Sci. Technol. 3 (1990) 159.

[ 16] U. Anselmi-Tamburini, P. Guigna, G. Spinolo and G. Flor, J. Phys. Chem. Solids 52 ( 1991 ) 715.

[ 17 ] P.G. Wahlbeck, D.L. Myers and R.R. Richards, Mater. Res. Soc. Symp. Proc. 169 (1990) 377.

[ 18 ] S. Shamoto, Solid State Commun. 66 ( 1988 ) l 151. [19]C.N.W. Darlington, D.A. O'Conner and C.A. Hollin, J.

Cryst. Growth 91 (1988) 308. [20]Y. Hidaka, Y. Enomoto, M. Suzuki, M. Oda and T.

Murakami, J. Cry'st. Growth 85 (1987) 581. [21 ] H.J. Scheel, W. Sadowski and L. Schellenberg, Supercond.

Sci. Technol. 2 (1989) 17. [22] Y. Laligant, G. Ferey, M. Hervieu and B. Raveau, Europhys.

Lett. 4 (1987) 1023. [23] H.J. Scheel and F. Licci, J. Cryst. Growth 85 (1987) 607. [24] K. Dembinsky, M. Gervais, P. Odier, J. Coutures and J.P.

Coutures, Mater. Sci. Eng. B 5 (1990) 345. [25jS. Bosi, B. Hunter, S. Town, T. Puzzer, A. Nouruzi-

Khorasani, J. Cochrane, A. Bailey, G.J. Russell and K.N.R. TayLor, Physica C 162-164 (1989) 895.

[26] K.N.R. Taylor, G.J. Russell, S. Bosi, D.N. Matthew, J. Chochrane, S. Town, B. Hunter, T. Puzzer, A. Bayley and R.A. Vaile, in: Physics and Materials Science of High Temperature Superconductors ( Kluwer, Dordrecht/NL 245, 1990).

[27] J.S. Abell, C.N.W. Darllington, A. Drake, C.A. Hollin, E.M. Forgan, D.A. O'Conner and S.D. Sutton, Physica C 162- 164 (1989) 909.

[28 ] A. Drake, J.S. Abell and S.D. Sutton, J. Less-Common Met. 164-165 (1990) 187.

[29] K. Watanabe, J. Cryst. Growth 100 (1990) 293. [30] Y.K. Tao, H.C. Chen, J.G. Lin, P.H. Hot and C.W. Chu. J.

Mater. Sci. Lett. 10 (1991) 583. [31]Y. Wang, P. Bennema and P. van der Linden, J. Cry'st.

Growth 106 (1990} 483. [32] T. Wolf, W. Goldacker, B. Obst, G. Roth and R. FliJkiger,

J. Cryst. Growth 96 (1989) 1010. [33] J. Burger, P. Bauer, M. Veith and G. Saemann-lschenko.

Ultramicroscopy ( 1992 ), to be published. [34] R.J. Cava, A.W. Hewat, E.A. Hewat, B. Batlogg, M. Marezio,

K.M. Rabe, J.J. Krajewski, W.F. Peck Jr. and L.W, Rupp Jr., Physica C 165 (1990) 419.

[35] Th. Bertholt, private communication. [36] P. Fischer, R. Busch, H.-W. Neumuller and H.F. Braun,

Supercond. Sci. Technol. 5 ( 1992 ) $440.