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Growth of large and untwinned single crystals of YBCO
PHYSICAG
Physica C 195 (1992) 291-300 North-Holland
Growth of large and untwinned single crystal of YBCO C.T. Lin, W. Zhou and W.Y. Liang IRC in Superconductivity, Universityof Cambridge, Cambridge CB3 0HE, UK
E. Sch0nherr and H. Bender Max-Planck-lnstitutJ~r Festk6rperforschung, Heisenbergstr. 1, 7000Stuttgart 80, Germany
Received 27 February 1992
Large (up to 5 X 4 X 1.5 mm3) single crystals of YBa2Cu307_awith perfect orthorhombic morphology were successfullygrown from favourable flux composition in alumina crucibles. The number of nucleation centres was reduced by using smooth inner wall crucibles and a temperature oscillation method. The crystals could be isolated from the CuO flux by tipping the crucible over while remaining in the furnace at 970°C. By applying a uniaxial stress at the annealing temperature, both macro- and micro-twin domains in the thick crystals could be removed and a single untwinned domain was obtained. The structures of both twin- and single-domain crystals were characterised by means of high resolution electron microscopy. We also report the magnetic property of the as-grown single crystals.
1. Introduction Many important questions regarding the nature and mechanism of superconductivity in YBCO cannot be answered without high quality single crystals. In some experiments, large single crystals are needed, for example, in the measurements o f conductivity anisotropy and neutron diffraction experiments. Twins are generally present in single crystals or polycrystalline specimens of orthorhombic YBCO. This has hindered the resolution o f the anisotropy in physical properties in the a - b plane. Furthermore, the twinned structure complicates the crystal structure determination as this often requires a deconvolution procedure. Many publications on the growth o f superconductor YBCO crystals have appeared recently [ 1-7]. These are generally concerned with using the crystals in various physical and chemical studies. Attempts have been made to enlarge the size o f the crystals and to improve their superconducting characteristics. However, there remain considerable difficulties in growing large single crystals and the subsequent separation o f the as-grown crystals from the flux, because the c o m p o u n d has a low thermal and chemical
stability, melts incongruously in the temperature range of 8 8 0 - 1 0 0 0 ° C and decomposes at low oxygen partial pressure. These are important problems which must be solved before large YBCO crystals of good quality can be obtained. We concentrate on the choice o f melt composition and the method o f separating the as-grown crystals from residual flux. Techniques such as decanting or tilting and liquid suction have been used [6,9-11 ], and they have significantly improved the ease with which to separate the as-grown crystals from the flux. These methods usually involve removing the hot crucible from the furnace to pour out the flux. This process can introduce thermal shock to the as-grown crystals with an evidently harmful effect and at the same time the crystal surface tends to be contaminated by droplets o f the liquid flux. The suction technique can also damage or destroy the crystals due to the use o f porous pieces. In this work we report a method which employs a favourable flux composition and a new technique of separating crystals from CuO flux. These crystals contain twinning domains as revealed by high resolution electron microscopy. The n u m b e r o f domains can then be reduced to one by the technique o f thermomechanical treatment.
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
292
C.T. Linet al. / Growth of large and untwinned single crystal
2. Apparatus and procedures for crystal growth The crystal growth a p p a r a t u s consists o f a furnace designed in such a way that the crucible in which crystals are grown m a y be t i p p e d over while rem a i n i n g inside the furnace. The t i p p i n g over takes place when the growth t e m p e r a t u r e has just terminated at 970°C. This is schematically shown in fig. 1 ( a ) . Two holes were drilled through the top wall o f a muffle furnace, one for the t h e r m o c o u p l e a n d the other for a Pt wire. T h e t h e r m o c o u p l e was located above the melt a n d is free to m o v e up a n d d o w n to allow m e a s u r e m e n t s o f the t e m p e r a t u r e d i s t r i b u t i o n to be made. It is also lifted during the t i p p i n g o f the crucible. There are two crucibles. The one for growing crystals is p o s i t i o n e d a b o v e the other used to receive the m o l t e n flux at a later stage o f the procedure. The top crucible is s u p p o r t e d by a horizontal a l u m i n a rod about which it can rotate. The tipping action is furnished by m e a n s o f the p l a t i n u m wire,
one end o f which is tied to the crucible and the other end is t h r e a d e d through a hole at the furnace top for pulling by the operator. Finally the whole o f the crucible assembly is enclosed within a vertical tube in o r d e r to p r o v i d e the support a n d to a v o i d heat convection. This allows a relatively u n i f o r m temperature field inside the vertical tube to be achieved. The c o m p o s i t i o n o f starting mixtures investigated in this work is summarised in table 1. N o r m a l l y 100300 g o f powders o f 3N YO,.5, BaO a n d CuO m a d e up to the c o m p o s i t i o n were ground and m i x e d in a ball mill overnight and then transferred into an alu m i n a crucible o f 5 or 10 cm diameter. The crucible was heated in air to 1025°C in the furnace at a rate o f 100°C/h. It was then allowed to soak for 2 - 4 days, followed by a growth p r o c e d u r e which was carried out in five steps as follows: ( 1 ) cool from 1025°C to 1000°C at l ° C / h ; ( 2 ) cool from 1000°C to 970°C at a rate within the range o f 0 . 1 - 5 ° C / h for growth and the a p p r o p r i a t e
8 7 lal 6
5
10
4 3 2
I
Fig. 1. Apparatus for growing single crystals ofYBCO. (a) Crucible in normal position, (b) crucible being tipped over to pour out excess flux into lower crucible. Captions are ( 1) furnace, (2) collecting crucible, ( 3 ) crucible (alumina), (4) flux, ( 5 ) support rod (alumina), ( 6 ) support tube (alumina), ( 7 ) suspension wire (Pt), ( 8 ) thermocouple (S), ( 9 ) flux, (10) YBCO single crystals.
C.T. Lin et aL /Growth of large and untwinnedsingle crystal Table 1 Conditions of flux composition for growth of single crystal Run no. 1 2 3 4 5 6 7 8 9 l0
Molar fraction of melt Y
Ba
Cu
Cooling rate of step 2 (°C/h)
11 9 6 5 4 3 3 3 2.5 2
22 27 27 30 24 21 31 33 30 28
67 64 67 65 72 76 66 64 67.5 70
4 1 1 0.1 0. l 0.4 0.2 0.2 4 5
Melt
Incomplete Incomplete Equilibrium Equilibrium Complete Complete Complete Complete Complete Complete
293
ously while the mean temperature is being lowered slowly to 970°C. Another feature is that the rising path is fast and the cooling rate is much slower at 0.1 to 0.4°C/h.
3. Results and discussion
3.1. Favourable flux composition and cooling rate
rate is given in table 1; (3) at 970 ° C the liquid flux is decanted into the collecting crucible; (4) further cool to 880°C at 1 0 ° C / h and during this time all the molten flux should be completely rem o v e d from the surfaces o f the crystals; (5) quench from 880°C to room temperature in order to avoid the tetragonal to orthorhombic phase transformation between 6 7 5 - 4 0 0 ° C [12], i.e. to minimize the formation o f twin domains. Whenever possible, we employ a technique which involves a repeated cycling o f temperature when the temperature reaches about 980°C in step 2 above. As shown in fig. 2, the span of these temperature oscillations starts at about 10°C, decreasing continu-
It was found that the habit o f crystal growth in terms of size and morphology was determined mainly by the melt composition and the cooling rate. The flux composition was carefully varied but kept within the region o f partial melting from the unusual phase diagram [ 13 ]. As shown in table 1 when the cooling rate is > = 1 °C/h, crystals obtained from the slightly copper-rich ( R u n no. 9) melts were thin with large surface area but from the very copper-rich ( R u n no. 10) melts were heavily contaminated by impurities. On the other hand for the copper-poor ( R u n no. 2) melts the crystals were very small. Run no. 1 gave an incomplete poly-ceramic result. Runs no. 4-8 which were cooled much more slowly ( 0 . 1 - 0 . 4 ° C / h ) were found to be satisfactory giving favourable flux compositions for large crystals.
3.2. Melting The kinetics o f melting is a complicated problem at the beginning o f the crystal growth procedure. In the system C u O - B a O the eutectic occurs at 60-72%
990 ~w (D
980
.=a t~
2 hrs
f=====--==~ Q / // ~Q7
"
978 "C ~ - -
9 7 6 "C
~> over 24hrs
970
9 7 0 "C
F--
960 Time Fig. 2. A typical temperature-time profile showing temperature oscillations.
294
c. T. Lin et al. / Growth of large and untwinned single crystal
CuO, 870-900 ° C [ 13 ] and subsequently dispersed droplets with composition close to the eutectic are formed. This results in excess Y in the remaining powder. Due to the limited solubility of Y in the melt, therefore, it is not necessary to use more Y in the melt composition, as this would otherwise give rise to an inhomogeneous melt. In the case of a complete melt stage a long soaking time or high soaking temperature is required. During this time the crucible material, such as alumina, can react with the molten mixture and aluminium may be incorporated into the YBCO crystals or other aluminium containing compounds may be formed. Therefore growth parameters such as high soaking temperature, long soaking time, low cooling rate and density of alumina crucible have to be taken into account, so that the crystal growth can be optimised while keeping a low alumina dissolution rate. Our EDS measurement confirmed that there was less than 1% (sensitivities of the detection) aluminium contamination in the crystals. This reduced aluminium is probably due to the use of a specially treated alumina crucible (described in section 3.3). 3.3. Nucleation control
One of the critical problems for growing large YBCO single crystals is how to limit the number of nucleation centres. Because incomplete and incongruent melts contain various dispersed droplets, inclusions and impurities together with the 123 phase, a large number of nucleation centres are always present during the cooling procedure. Moreover, a rough inner wall of the crucible also causes undesirable nucleations to start on the surface of the wall. Previously a large number of small crystallites of less than 0.3 m m were often found on the wall of a rough crucible. We believe the use of the temperature oscillation method (fig. 2) together with a smooth inner wall crucible which is preheated to 1200°C has reduced considerably the number of nucleation centres. The temperature oscillations, in particular, allow small crystallites to remelt and give the larger crystals more space and nutrient supply for their own growth. This has resulted in large crystals being successfully obtained from the solidified melt surface, as shown in fig. 3(a) and (b).
3.4. Growth
Inside the chamber of a muffle furnace the temperature is not uniform and the gradient is also not stable, since the heat source is mounted on the wall with heat convection between the wall and the centre. For this reason the cold region is often assumed to be at the centre of the furnace. In fact the centre is not necessarily the region of lowest temperature because of the lack of cooling source while heat convection is random and changeable. An unsteady temperature field would cause an unstable state of crystal growth, leading to problems of multiple nucleation centres, mass transportation and defects in crystals. The introduction of the large support tube, part (6) in fig. 1 (a), to the centre of the furnace serves to prevent a random heat convection and produces a relatively uniform temperature field inside the tube. Nevertheless, the temperature at the bottom centre is 10 °C lower than that at the wall of a 10 cm diameter crucible. In the case of an incomplete melt, crystals start to grow on the solidified floating residues, because the incomplete melt contains residual powder acting as innumerable nucleation centres. This leads to the formation of many crystals intergrowing on one another with an overall area of several cm 2 in size. By our arrangement and a steady temperature gradient in the crucible, crystals preferentially grow at the bottom of the crucible where the temperature is lowest. To minimize the number of nucleation centres, the method of temperature oscillation was applied to remelt small crystallites and a smooth inner wall crucible with high density (type 001 which is denser than type 004) was used. It is important, therefore, to have a complete melt in order to lower the number of nucleation centres for large crystals to be grown. Our optimum conditions are: (1) melt composition Y : B a : C u = 1:6: 18; (2) mill for 1 day to obtain fine mixtures; (3) soaking at 1025°C for 4 days; (4) cooling rate of 0.1-0.4 ° C/h. Under these conditions some large, thick and isolated crystals could often be obtained. Among the 10 runs of crystal growth experiments listed in table 1, Run no. 5 and 6 have the most favourable conditions for obtaining large single crystals (up to 5 × 4 × 1.5
C.T. Lin et al. /Growth of large and untwinned single crystal
295
Fig. 3. (a) Typical YBCO single crystal isolated from the residual flux; (b) twin crystals with striated ( 101 ) face. mm3). Figure 4 shows the temperature dependence of the magnetisation o f an as-grown crystal ( R u n no. 5 ) with thickness 1 m m and oxygen content 6.8. The latter was determined indirectly by means o f R a m a n mode frequency characterisation.
3.5. Separation
The most suitable temperature for crystal growth was found to be 970°C. When cooling terminated at this temperature, the residual flux was poured into the collecting crucible by tipping the crucible over as
C.T. Lin et al. / Growth of large and untwinned single crystal
296
+
0.000E+00-
~
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+
-I.000E-04
-
-2.000E-04 -3.000E-04 -4.000E-04 -
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-9.000E-04 .....
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[K]
Fig. 4. The temperature dependence of the magnetisation of the as-grown single crystal YBCO of approximately 1 mm thick.
shown in fig. 1 (b). The subsequent cooling rate of 10 ° C / h was applied to avoid contaminating the surface of the crystal due to quick flux freezing. Compared with other liquid separation techniques the present method showed no thermal shock in the crystals, safe and complete removal of the flux from the surface of the as-grown crystals and we also have the option of in-situ annealing of the crystals in an oxygen atmosphere.
3.6. Twinning and detwinning The crystals were studied by means of optical polarizing microscopy with crossed polarisers to reveal twin domains. For an orthorhombic crystal, twin boundaries show up as straight, dark lines along (110) direction. It is difficult to avoid twinning in YBCO crystals because different thermal contraction occurs between the solidified flux and the crystals as well as non-uniform stress fields set up by impurity inclusion in the crystals. We found that if the as-grown tetragonal crystals was quenched from above the tetragonal to orthorhombic phase transformation temperature of 675-400°C, the number of twins was reduced, in agreement with Jorgeusen et al. [12].
Frequently crystals with a thin and shiny surface are twin-free under a polarizing microscopy. Thicker crystals tend to contain considerable number of twins. This is probably caused by the thermal stress induced as heat diffusion is more difficult in thicker crystals during quenching. The twin domains can be removed by annealing the twinned crystal under a uniaxial stress in a-b plane where the compression is directed along the a/b axis. Since YBCO is characterised by the ferroelastic behaviour, 4 / m m m F m m m , detwinning can be achieved by means of the ferroelastic poling to obtain an orthorhombic single domain. A particular condition [ 17 ] for such thermomechanical treatment ws l07 N / m 2 at 450°C in a flowing oxygen overnight. After treatment the annealed crystal was quenched to room temperature. Figure 5(a) and (b) show the same crystal of thickness 0.8 m m before and after the treatment. Figure 5 (a) exhibites { 110} type bands in the (001 ) planes. In this case the whole crystal is in the state of twinning and at the edge the twinning bands oriented parallel to the (110) planes. After the removal of twins, fig. 5 (b), the crystal shows a single domain, i.e. it has become twin-free.
C.T. Lin et al. / Growth of large and untwinned single crystal
297
Fig. 5. (a) (001 ) face of a large single crystal showing ( 11O) and ( 1TO) twin lines as observed under a polarizing optical microscope, the spacing between the wide dark stripes is 50 lim; (b) the same face after the detwinning treatment. 3. 7. Microstructural characterization
T w i n n i n g s in YBCO occur in a range of spatial scales and we need to e x a m i n e the extend to which our d e t w i n n i n g procedure has removed them. We
identified at least three different t w i n n i n g in an ordinary untreated crystal. The relatively large scale t w i n n i n g ( ~ 5 0 ~tm) observed in a polarizing microscope is due to formation of d o m a i n s which is the result of the retention of strain on cooling. Figure
298
C. T. Lin et al. / Growth of large and untwinned single crystal
Fig. 6. Microtwin defects as observed in an electron microscope before the application of detwinning treatment. (a) TEM image of medium scale defects; (b) periodic twin defects with steps. 6 (a) and (b) show the transmission electron microscopic ( T E M ) images from two pieces o f Y B C O microcrystals obtained by grounding a large untreated single crystal. Twinning associated with impurities
or oxygen defects within a range of about 1000 A are observed. The fine twinning o f fig. 6 (b) is believed to be associated with oxygen defects in the chain sites and is periodic [ 14]. High resolution electron mi-
C. T. Lin et al. / Growth o f large and untwinned single crystal
299
Fig. 6. (c) HREM image of microtwin domains.
croscopy ( H R E M ) using a Jeol J E M - 2 0 0 C X microscope reveals the t h i r d type o f twinning as shown in fig. 6 ( c ) o f a Y B C O crystal viewed down the [001 ] direction. Some m i c r o t w i n d o m a i n s within the size o f a b o u t 100 A r a d i u s a n d the zig-zag shape are ind i c a t e d by arrows. The zig-zag d o m a i n chains are likely to be caused by the O ( 4 ) a t o m s away from the chain axis [ 15 ]. By comparison, when the same study was carried out on the d e - t w i n n e d single d o m a i n crystals, n o n e o f the a b o v e three types o f twin defects could be observed.
4. Conclusion By adjusting growth p a r a m e t e r s such as flux composition, temperature field, soaking time, cooling rate a n d nucleation control, etc., we have o b t a i n e d large Y B C O single crystals which also show good superconducting characteristics. It is suggested that a cold centre, seeding technique a n d a rotating crucible w o u l d further restrict nucleation. The crystal separation technique has been i m p r o v e d by t i p p i n g the crucible over while inside the furnace without d a m aging the as-grown crystals. U n t w i n n e d crystals have
been o b t a i n e d by means o f a t h e r m o m e c h a n i c a l treatment or quenching as-grown crystal from 880 ° C. The detwinning p r o c e d u r e removes all three twinning structures as revealed by polarizing optical a n d high resolution electron microscopies.
References [ 1] D.L. Kaiser, F. Haltzberg, B.A. Scott and T.R. Macquire, Appl. Phys. Lett. 51 (1987) 1040. [2] L.F. Schneemeyer, Z.V. Waszczak, T. Siegrist, R.B. van Dover, L.W. Rupp, B. Batlogg,R.J. Cava and D.W. Murphy, Nature 328 (1987) 601. [3] Y. Hikada, Y. Enomoto, M. Oda, A. Katsui and T. Murahami, Jpn. J. Appl. Phys. 26 (1987) L726. [4] R.A. Laudise, L.F. Schneemeyer and R.Y. Barns, J. Cryst. Growth 85 (1988) 569. [5] E. Walker and V. Sadowski, Helv. Phys. Acta 61 (1988) 470. [6] H.J. Scheel, Physica C 153-155 (1988) 44. [ 7 ] H.J. Scheel and F. Licci, J. Cryst. Growth 85 ( 1987 ) 607. [8] Y.S. Wang, P. Bennema and P. van der Linden, J. Cryst. Growth 106 (1990) 483. [9] Y.S. Wang, L.W.M. Schreurs, P. van der Linden, Y. Li and P. Bennema, J. Cryst. Growth 106 (1990) 487. [ 10 ] F. Licci, H.J. Scheel and T. Besagni, Physica C 153-155 (1988) 431.
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C. T. Lin et al. / Growth o f large and untwinned single crystal
[ 11 ] R. Boutellier, B.N. Sun, H.J. Scheel and H. Schmid, J. Cryst. Growth 96 (1989) 465. [ 12] J.D. Jorgeusen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace, I.K. Schuller, C.V. Segre, K. Zhang and M.S. Kleefish, Phys. Rev. B 36 (1987) 3608. [ 13 ] R.S. Roth, K.L. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1987) 303.
[ 14 ] A. Nouruzi-Khorasani, K.N.R. Taylor and S. Bosi, J. Cryst. Growth 98 (1989) 461. [ 15 ] H. Schmid, E. Burkhardt, B.N. Sun and J.P. Rivera, Physica C 157 (1989) 555. [ 16 ] D.L. Kaiser, F.W. Gayle, R.S. Roth and L.J. Swartzendruber, J. Mater. Res. 4 (1989) 745. [ 17 ] U. Welp, M. Grimsditch, H. You, W.K. Kwok, M.M. Fang, G.W. Grabtree and J.Z. Liu, Physica C 161 (1989) 1.
Physica C 195 (1992) 291-300 North-Holland
Growth of large and untwinned single crystal of YBCO C.T. Lin, W. Zhou and W.Y. Liang IRC in Superconductivity, Universityof Cambridge, Cambridge CB3 0HE, UK
E. Sch0nherr and H. Bender Max-Planck-lnstitutJ~r Festk6rperforschung, Heisenbergstr. 1, 7000Stuttgart 80, Germany
Received 27 February 1992
Large (up to 5 X 4 X 1.5 mm3) single crystals of YBa2Cu307_awith perfect orthorhombic morphology were successfullygrown from favourable flux composition in alumina crucibles. The number of nucleation centres was reduced by using smooth inner wall crucibles and a temperature oscillation method. The crystals could be isolated from the CuO flux by tipping the crucible over while remaining in the furnace at 970°C. By applying a uniaxial stress at the annealing temperature, both macro- and micro-twin domains in the thick crystals could be removed and a single untwinned domain was obtained. The structures of both twin- and single-domain crystals were characterised by means of high resolution electron microscopy. We also report the magnetic property of the as-grown single crystals.
1. Introduction Many important questions regarding the nature and mechanism of superconductivity in YBCO cannot be answered without high quality single crystals. In some experiments, large single crystals are needed, for example, in the measurements o f conductivity anisotropy and neutron diffraction experiments. Twins are generally present in single crystals or polycrystalline specimens of orthorhombic YBCO. This has hindered the resolution o f the anisotropy in physical properties in the a - b plane. Furthermore, the twinned structure complicates the crystal structure determination as this often requires a deconvolution procedure. Many publications on the growth o f superconductor YBCO crystals have appeared recently [ 1-7]. These are generally concerned with using the crystals in various physical and chemical studies. Attempts have been made to enlarge the size o f the crystals and to improve their superconducting characteristics. However, there remain considerable difficulties in growing large single crystals and the subsequent separation o f the as-grown crystals from the flux, because the c o m p o u n d has a low thermal and chemical
stability, melts incongruously in the temperature range of 8 8 0 - 1 0 0 0 ° C and decomposes at low oxygen partial pressure. These are important problems which must be solved before large YBCO crystals of good quality can be obtained. We concentrate on the choice o f melt composition and the method o f separating the as-grown crystals from residual flux. Techniques such as decanting or tilting and liquid suction have been used [6,9-11 ], and they have significantly improved the ease with which to separate the as-grown crystals from the flux. These methods usually involve removing the hot crucible from the furnace to pour out the flux. This process can introduce thermal shock to the as-grown crystals with an evidently harmful effect and at the same time the crystal surface tends to be contaminated by droplets o f the liquid flux. The suction technique can also damage or destroy the crystals due to the use o f porous pieces. In this work we report a method which employs a favourable flux composition and a new technique of separating crystals from CuO flux. These crystals contain twinning domains as revealed by high resolution electron microscopy. The n u m b e r o f domains can then be reduced to one by the technique o f thermomechanical treatment.
0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.
292
C.T. Linet al. / Growth of large and untwinned single crystal
2. Apparatus and procedures for crystal growth The crystal growth a p p a r a t u s consists o f a furnace designed in such a way that the crucible in which crystals are grown m a y be t i p p e d over while rem a i n i n g inside the furnace. The t i p p i n g over takes place when the growth t e m p e r a t u r e has just terminated at 970°C. This is schematically shown in fig. 1 ( a ) . Two holes were drilled through the top wall o f a muffle furnace, one for the t h e r m o c o u p l e a n d the other for a Pt wire. T h e t h e r m o c o u p l e was located above the melt a n d is free to m o v e up a n d d o w n to allow m e a s u r e m e n t s o f the t e m p e r a t u r e d i s t r i b u t i o n to be made. It is also lifted during the t i p p i n g o f the crucible. There are two crucibles. The one for growing crystals is p o s i t i o n e d a b o v e the other used to receive the m o l t e n flux at a later stage o f the procedure. The top crucible is s u p p o r t e d by a horizontal a l u m i n a rod about which it can rotate. The tipping action is furnished by m e a n s o f the p l a t i n u m wire,
one end o f which is tied to the crucible and the other end is t h r e a d e d through a hole at the furnace top for pulling by the operator. Finally the whole o f the crucible assembly is enclosed within a vertical tube in o r d e r to p r o v i d e the support a n d to a v o i d heat convection. This allows a relatively u n i f o r m temperature field inside the vertical tube to be achieved. The c o m p o s i t i o n o f starting mixtures investigated in this work is summarised in table 1. N o r m a l l y 100300 g o f powders o f 3N YO,.5, BaO a n d CuO m a d e up to the c o m p o s i t i o n were ground and m i x e d in a ball mill overnight and then transferred into an alu m i n a crucible o f 5 or 10 cm diameter. The crucible was heated in air to 1025°C in the furnace at a rate o f 100°C/h. It was then allowed to soak for 2 - 4 days, followed by a growth p r o c e d u r e which was carried out in five steps as follows: ( 1 ) cool from 1025°C to 1000°C at l ° C / h ; ( 2 ) cool from 1000°C to 970°C at a rate within the range o f 0 . 1 - 5 ° C / h for growth and the a p p r o p r i a t e
8 7 lal 6
5
10
4 3 2
I
Fig. 1. Apparatus for growing single crystals ofYBCO. (a) Crucible in normal position, (b) crucible being tipped over to pour out excess flux into lower crucible. Captions are ( 1) furnace, (2) collecting crucible, ( 3 ) crucible (alumina), (4) flux, ( 5 ) support rod (alumina), ( 6 ) support tube (alumina), ( 7 ) suspension wire (Pt), ( 8 ) thermocouple (S), ( 9 ) flux, (10) YBCO single crystals.
C.T. Lin et aL /Growth of large and untwinnedsingle crystal Table 1 Conditions of flux composition for growth of single crystal Run no. 1 2 3 4 5 6 7 8 9 l0
Molar fraction of melt Y
Ba
Cu
Cooling rate of step 2 (°C/h)
11 9 6 5 4 3 3 3 2.5 2
22 27 27 30 24 21 31 33 30 28
67 64 67 65 72 76 66 64 67.5 70
4 1 1 0.1 0. l 0.4 0.2 0.2 4 5
Melt
Incomplete Incomplete Equilibrium Equilibrium Complete Complete Complete Complete Complete Complete
293
ously while the mean temperature is being lowered slowly to 970°C. Another feature is that the rising path is fast and the cooling rate is much slower at 0.1 to 0.4°C/h.
3. Results and discussion
3.1. Favourable flux composition and cooling rate
rate is given in table 1; (3) at 970 ° C the liquid flux is decanted into the collecting crucible; (4) further cool to 880°C at 1 0 ° C / h and during this time all the molten flux should be completely rem o v e d from the surfaces o f the crystals; (5) quench from 880°C to room temperature in order to avoid the tetragonal to orthorhombic phase transformation between 6 7 5 - 4 0 0 ° C [12], i.e. to minimize the formation o f twin domains. Whenever possible, we employ a technique which involves a repeated cycling o f temperature when the temperature reaches about 980°C in step 2 above. As shown in fig. 2, the span of these temperature oscillations starts at about 10°C, decreasing continu-
It was found that the habit o f crystal growth in terms of size and morphology was determined mainly by the melt composition and the cooling rate. The flux composition was carefully varied but kept within the region o f partial melting from the unusual phase diagram [ 13 ]. As shown in table 1 when the cooling rate is > = 1 °C/h, crystals obtained from the slightly copper-rich ( R u n no. 9) melts were thin with large surface area but from the very copper-rich ( R u n no. 10) melts were heavily contaminated by impurities. On the other hand for the copper-poor ( R u n no. 2) melts the crystals were very small. Run no. 1 gave an incomplete poly-ceramic result. Runs no. 4-8 which were cooled much more slowly ( 0 . 1 - 0 . 4 ° C / h ) were found to be satisfactory giving favourable flux compositions for large crystals.
3.2. Melting The kinetics o f melting is a complicated problem at the beginning o f the crystal growth procedure. In the system C u O - B a O the eutectic occurs at 60-72%
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c. T. Lin et al. / Growth of large and untwinned single crystal
CuO, 870-900 ° C [ 13 ] and subsequently dispersed droplets with composition close to the eutectic are formed. This results in excess Y in the remaining powder. Due to the limited solubility of Y in the melt, therefore, it is not necessary to use more Y in the melt composition, as this would otherwise give rise to an inhomogeneous melt. In the case of a complete melt stage a long soaking time or high soaking temperature is required. During this time the crucible material, such as alumina, can react with the molten mixture and aluminium may be incorporated into the YBCO crystals or other aluminium containing compounds may be formed. Therefore growth parameters such as high soaking temperature, long soaking time, low cooling rate and density of alumina crucible have to be taken into account, so that the crystal growth can be optimised while keeping a low alumina dissolution rate. Our EDS measurement confirmed that there was less than 1% (sensitivities of the detection) aluminium contamination in the crystals. This reduced aluminium is probably due to the use of a specially treated alumina crucible (described in section 3.3). 3.3. Nucleation control
One of the critical problems for growing large YBCO single crystals is how to limit the number of nucleation centres. Because incomplete and incongruent melts contain various dispersed droplets, inclusions and impurities together with the 123 phase, a large number of nucleation centres are always present during the cooling procedure. Moreover, a rough inner wall of the crucible also causes undesirable nucleations to start on the surface of the wall. Previously a large number of small crystallites of less than 0.3 m m were often found on the wall of a rough crucible. We believe the use of the temperature oscillation method (fig. 2) together with a smooth inner wall crucible which is preheated to 1200°C has reduced considerably the number of nucleation centres. The temperature oscillations, in particular, allow small crystallites to remelt and give the larger crystals more space and nutrient supply for their own growth. This has resulted in large crystals being successfully obtained from the solidified melt surface, as shown in fig. 3(a) and (b).
3.4. Growth
Inside the chamber of a muffle furnace the temperature is not uniform and the gradient is also not stable, since the heat source is mounted on the wall with heat convection between the wall and the centre. For this reason the cold region is often assumed to be at the centre of the furnace. In fact the centre is not necessarily the region of lowest temperature because of the lack of cooling source while heat convection is random and changeable. An unsteady temperature field would cause an unstable state of crystal growth, leading to problems of multiple nucleation centres, mass transportation and defects in crystals. The introduction of the large support tube, part (6) in fig. 1 (a), to the centre of the furnace serves to prevent a random heat convection and produces a relatively uniform temperature field inside the tube. Nevertheless, the temperature at the bottom centre is 10 °C lower than that at the wall of a 10 cm diameter crucible. In the case of an incomplete melt, crystals start to grow on the solidified floating residues, because the incomplete melt contains residual powder acting as innumerable nucleation centres. This leads to the formation of many crystals intergrowing on one another with an overall area of several cm 2 in size. By our arrangement and a steady temperature gradient in the crucible, crystals preferentially grow at the bottom of the crucible where the temperature is lowest. To minimize the number of nucleation centres, the method of temperature oscillation was applied to remelt small crystallites and a smooth inner wall crucible with high density (type 001 which is denser than type 004) was used. It is important, therefore, to have a complete melt in order to lower the number of nucleation centres for large crystals to be grown. Our optimum conditions are: (1) melt composition Y : B a : C u = 1:6: 18; (2) mill for 1 day to obtain fine mixtures; (3) soaking at 1025°C for 4 days; (4) cooling rate of 0.1-0.4 ° C/h. Under these conditions some large, thick and isolated crystals could often be obtained. Among the 10 runs of crystal growth experiments listed in table 1, Run no. 5 and 6 have the most favourable conditions for obtaining large single crystals (up to 5 × 4 × 1.5
C.T. Lin et al. /Growth of large and untwinned single crystal
295
Fig. 3. (a) Typical YBCO single crystal isolated from the residual flux; (b) twin crystals with striated ( 101 ) face. mm3). Figure 4 shows the temperature dependence of the magnetisation o f an as-grown crystal ( R u n no. 5 ) with thickness 1 m m and oxygen content 6.8. The latter was determined indirectly by means o f R a m a n mode frequency characterisation.
3.5. Separation
The most suitable temperature for crystal growth was found to be 970°C. When cooling terminated at this temperature, the residual flux was poured into the collecting crucible by tipping the crucible over as
C.T. Lin et al. / Growth of large and untwinned single crystal
296
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shown in fig. 1 (b). The subsequent cooling rate of 10 ° C / h was applied to avoid contaminating the surface of the crystal due to quick flux freezing. Compared with other liquid separation techniques the present method showed no thermal shock in the crystals, safe and complete removal of the flux from the surface of the as-grown crystals and we also have the option of in-situ annealing of the crystals in an oxygen atmosphere.
3.6. Twinning and detwinning The crystals were studied by means of optical polarizing microscopy with crossed polarisers to reveal twin domains. For an orthorhombic crystal, twin boundaries show up as straight, dark lines along (110) direction. It is difficult to avoid twinning in YBCO crystals because different thermal contraction occurs between the solidified flux and the crystals as well as non-uniform stress fields set up by impurity inclusion in the crystals. We found that if the as-grown tetragonal crystals was quenched from above the tetragonal to orthorhombic phase transformation temperature of 675-400°C, the number of twins was reduced, in agreement with Jorgeusen et al. [12].
Frequently crystals with a thin and shiny surface are twin-free under a polarizing microscopy. Thicker crystals tend to contain considerable number of twins. This is probably caused by the thermal stress induced as heat diffusion is more difficult in thicker crystals during quenching. The twin domains can be removed by annealing the twinned crystal under a uniaxial stress in a-b plane where the compression is directed along the a/b axis. Since YBCO is characterised by the ferroelastic behaviour, 4 / m m m F m m m , detwinning can be achieved by means of the ferroelastic poling to obtain an orthorhombic single domain. A particular condition [ 17 ] for such thermomechanical treatment ws l07 N / m 2 at 450°C in a flowing oxygen overnight. After treatment the annealed crystal was quenched to room temperature. Figure 5(a) and (b) show the same crystal of thickness 0.8 m m before and after the treatment. Figure 5 (a) exhibites { 110} type bands in the (001 ) planes. In this case the whole crystal is in the state of twinning and at the edge the twinning bands oriented parallel to the (110) planes. After the removal of twins, fig. 5 (b), the crystal shows a single domain, i.e. it has become twin-free.
C.T. Lin et al. / Growth of large and untwinned single crystal
297
Fig. 5. (a) (001 ) face of a large single crystal showing ( 11O) and ( 1TO) twin lines as observed under a polarizing optical microscope, the spacing between the wide dark stripes is 50 lim; (b) the same face after the detwinning treatment. 3. 7. Microstructural characterization
T w i n n i n g s in YBCO occur in a range of spatial scales and we need to e x a m i n e the extend to which our d e t w i n n i n g procedure has removed them. We
identified at least three different t w i n n i n g in an ordinary untreated crystal. The relatively large scale t w i n n i n g ( ~ 5 0 ~tm) observed in a polarizing microscope is due to formation of d o m a i n s which is the result of the retention of strain on cooling. Figure
298
C. T. Lin et al. / Growth of large and untwinned single crystal
Fig. 6. Microtwin defects as observed in an electron microscope before the application of detwinning treatment. (a) TEM image of medium scale defects; (b) periodic twin defects with steps. 6 (a) and (b) show the transmission electron microscopic ( T E M ) images from two pieces o f Y B C O microcrystals obtained by grounding a large untreated single crystal. Twinning associated with impurities
or oxygen defects within a range of about 1000 A are observed. The fine twinning o f fig. 6 (b) is believed to be associated with oxygen defects in the chain sites and is periodic [ 14]. High resolution electron mi-
C. T. Lin et al. / Growth o f large and untwinned single crystal
299
Fig. 6. (c) HREM image of microtwin domains.
croscopy ( H R E M ) using a Jeol J E M - 2 0 0 C X microscope reveals the t h i r d type o f twinning as shown in fig. 6 ( c ) o f a Y B C O crystal viewed down the [001 ] direction. Some m i c r o t w i n d o m a i n s within the size o f a b o u t 100 A r a d i u s a n d the zig-zag shape are ind i c a t e d by arrows. The zig-zag d o m a i n chains are likely to be caused by the O ( 4 ) a t o m s away from the chain axis [ 15 ]. By comparison, when the same study was carried out on the d e - t w i n n e d single d o m a i n crystals, n o n e o f the a b o v e three types o f twin defects could be observed.
4. Conclusion By adjusting growth p a r a m e t e r s such as flux composition, temperature field, soaking time, cooling rate a n d nucleation control, etc., we have o b t a i n e d large Y B C O single crystals which also show good superconducting characteristics. It is suggested that a cold centre, seeding technique a n d a rotating crucible w o u l d further restrict nucleation. The crystal separation technique has been i m p r o v e d by t i p p i n g the crucible over while inside the furnace without d a m aging the as-grown crystals. U n t w i n n e d crystals have
been o b t a i n e d by means o f a t h e r m o m e c h a n i c a l treatment or quenching as-grown crystal from 880 ° C. The detwinning p r o c e d u r e removes all three twinning structures as revealed by polarizing optical a n d high resolution electron microscopies.
References [ 1] D.L. Kaiser, F. Haltzberg, B.A. Scott and T.R. Macquire, Appl. Phys. Lett. 51 (1987) 1040. [2] L.F. Schneemeyer, Z.V. Waszczak, T. Siegrist, R.B. van Dover, L.W. Rupp, B. Batlogg,R.J. Cava and D.W. Murphy, Nature 328 (1987) 601. [3] Y. Hikada, Y. Enomoto, M. Oda, A. Katsui and T. Murahami, Jpn. J. Appl. Phys. 26 (1987) L726. [4] R.A. Laudise, L.F. Schneemeyer and R.Y. Barns, J. Cryst. Growth 85 (1988) 569. [5] E. Walker and V. Sadowski, Helv. Phys. Acta 61 (1988) 470. [6] H.J. Scheel, Physica C 153-155 (1988) 44. [ 7 ] H.J. Scheel and F. Licci, J. Cryst. Growth 85 ( 1987 ) 607. [8] Y.S. Wang, P. Bennema and P. van der Linden, J. Cryst. Growth 106 (1990) 483. [9] Y.S. Wang, L.W.M. Schreurs, P. van der Linden, Y. Li and P. Bennema, J. Cryst. Growth 106 (1990) 487. [ 10 ] F. Licci, H.J. Scheel and T. Besagni, Physica C 153-155 (1988) 431.
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[ 11 ] R. Boutellier, B.N. Sun, H.J. Scheel and H. Schmid, J. Cryst. Growth 96 (1989) 465. [ 12] J.D. Jorgeusen, M.A. Beno, D.G. Hinks, L. Soderholm, K.J. Volin, R.L. Hitterman, J.D. Grace, I.K. Schuller, C.V. Segre, K. Zhang and M.S. Kleefish, Phys. Rev. B 36 (1987) 3608. [ 13 ] R.S. Roth, K.L. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1987) 303.
[ 14 ] A. Nouruzi-Khorasani, K.N.R. Taylor and S. Bosi, J. Cryst. Growth 98 (1989) 461. [ 15 ] H. Schmid, E. Burkhardt, B.N. Sun and J.P. Rivera, Physica C 157 (1989) 555. [ 16 ] D.L. Kaiser, F.W. Gayle, R.S. Roth and L.J. Swartzendruber, J. Mater. Res. 4 (1989) 745. [ 17 ] U. Welp, M. Grimsditch, H. You, W.K. Kwok, M.M. Fang, G.W. Grabtree and J.Z. Liu, Physica C 161 (1989) 1.