Effect of BaSnO3 additions in MPMG-processed YBCO

PHYSICA PhysicaC 233 (1994) 155-164

ELSEVIER

Effect of BaSnO3 additions in MPMG-processed YBCO I. Monot *, T. Higuchi, N. Sakai, M. Murakami Superconductivity Research Laboratory, Division VII, International Superconductivity and Technology Center, 1-i6-25 Shibaura, Minato-ku, Tokyo 105, Japan

Received 16 August 1994

Abstract

Many tentatives have been made these last years to increase the pinning properties of melt-textured YBaCuO by secondary phases additions. The results are more or less positive depending on the processing conditions and the reactivity of the dopant, especially its action on the size and shape of 211 particles. In this paper, the effect of BaSnO3 additions up to 30 mol% on the microstructure and the superconducting properties of MPMG-processed YBCO is investigated. At high temperatures, BaSnO3 reacts rapidly with the liquid phases to form few microns size inclusions of a stable Y-Ba-Sn-Cu-Pt-O phase. This phase is found to consume platinum and to favor 211 coarsening at high temperature. For 30% doping, the entire microstructure is strongly affected and for low levels of doping (2% and 5%), we could not evidence any improvement in the superconducting properties, compared to undoped samples. But when the doping level is increased up to 10%, one can locally find a high density of submicron size 211 inclusions and Tc enhancement is observed. In this case, we could obtain improved Jc(B) properties up to 110 000 A/cm 2, at 77 K in zero field.

1. Introduction

The obtention o f sufficiently high values o f critical current density and very large grains are indispensable conditions for engineering applications o f the high-temperature YBazCu307_a ( Y B C O ) superconductors, such as magnetic bearings, levitation or energy storage. Melt processings have been demonstrated to be very effective in achieving big oriented grains and increasing the Jr values by eliminating the weak links present in the system [ I - 3 ] ; M T G and now M P M G [4] processes can nearly reach the required Jc values for applications. In these processes, the fine dispersion o f Y2BaCuO5 inclusions (211 phase) above the peritectic temperature o f ' 1 2 3 ' de* Corresponding author; new address from September 1st: CRIMAT/ISMRA, 6 Bd. du Marechal Juin, 14050 Caen cedex, France.

composition is believed to be crucial in obtaining the desired microstructure. This is attributed to the ability of 211 particles of shortening the diffusion length between each constituent required for maintaining proper stoichiometry as well as promoting the directional growth o f the 123 grains. Experimentally, structural imperfections such as twin boundaries, structural defects, stacking faults or secondary phases have been reported to increase the flux pinning and the critical current densities [5-8 ]. Many secondphase additions (Pt, BaZrO3, SiC, CeO2, ZrO2 etc.) have been employed in this effort [ 9-13 ] and they were often observed to modify the volume, the morphology, the size and the distribution o f 211 inclusions, leading to microstructure and pinning improvement. Several groups [ 14-16 ] have reported on the beneficial effect o f BaSnO3 addition in the Y - B a - C u - O system. BaSnO3 has been found to be effective in re-

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L Monot et al. / Physica C 233 (1994) 149-154

ducing the 211 particle size in MTG-processed samples but the mechanism for this reduction is not clear [17,18]. Moreover, some groups have reported on the interaction between BaSnO3 and the melt [ 10] and some others have not noticed any reaction between these phases [ 19,20 ]. Recently, Vanarasi et al. [21 ] reported on the comparison between 20% BaSnO3 and 0.5% PtOz additions in MTG-processed YBCO and their effect on 211 refinement. They observed that both reduce 211 coarsening but PtO2 is much more effective. The platinum is found to lower the liquid/211 interracial energy and/or change the yttrium diffusivity and BaSnO3 seems to reduce coarsening by acting as yttrium sink to form a Y - B a - S n - O phase. Magnetization measurements have shown enhanced pinning properties in samples MTG-processed with PtO2 addition, but in the BaSnO3-doped sample, the small enhancement observed at 60 K disappeared with decreasing temperature. Thus, further investigations are required to elucidate the exact reactivity and action of BaSnO3 and the relations between process, microstructure and the resulting superconducting properties. We report here on the effect of BaSnO3 addition in the MPMG-processed samples. In this process, the size and distribution of the 211 inclusions are already very well controlled, so it can be interesting to observe how BaSnO3 will act on 211 size and morphology, or if it can act itself as a new kind of pinning center and contribute to Jc improvement.

2. Experimental The different samples were prepared from Y203, BaCuO2 and CuO powder oxides with the nominal composition YLsBa2.4Cu3.4Ox. The mixed components are melted at 1400*C, rapidly quenched in air and reground; the powder is then mixed with submicronic commercial BaSnO3 from 0 to 30 mol% for several hours. The samples are then two tons isostatically pressed (CIP) in 30 m m diameter pellets. For the observation of the high-temperature microstructure, the pellets were simply quenched in air by pulling them quickly out of the furnace. Observations have been provided on fracture surfaces, chemically etched with bromoethanol in order to dissolve

the 123 phase and directly observe the green phase morphology. The second part of the M P M G process was performed in a muffle furnace, without any imposed gradient; the pellets were placed on small ZrO2 bars in order to minimize the interactions between the support and the liquid phase coming from the sample. After a high-temperature plateau consisting in 30 min at 1100°C, the samples were rapidly cooled to 1020°C and then cooled from 1020°C to 900°C at 1°C/h, then 50°C/h to 400°C and finally, 100°C/h to room temperature. After annealing between 600°C and 400°C for 100 h the samples were characterized by X-ray diffraction, polarized optical microscopy, scanning electron microscopy (SEM) equipped with EDS analysis systems. The critical temperatures (Tc) were determined by DC SQUID measurements and the hysteresis curves were recorded at 77 K, with a vibrating sample magnetometer (VSM); for this measurements we extracted from the bulk small cleaved samples in order to clearly find the a - b plane, and the external field was applied parallel to c-axis. J¢ values are deduced from the hysteresis curves using the modified Bean model [22 ].

3. Results and discussion In order to understand the reactivity of BaSnO3 with the melt-quenched components, its further action on the 211 inclusions and the new parameters involved in this system, the X-ray patterns and the microstructure of melt-quenched samples have been investigated for different times and temperatures. 3.1. High-temperature observations

In the diffraction pattern of 30% doped pellets quenched after 5 min at 1100°C, no evidence of remaining BaSnO3 in the microstructure can be detected. This means that the reactivity of BaSnO3 with the melt-quenched phases seems to be very high and very different from other melt processes. The microstructure of the polished surface of a 30% doped sample completely processed (Fig. 1 ) exhibits many square inclusions between 1 and 5 ~tm in size. Such small square inclusions have already been ob-

L Monot et al. IPhysica C 233 (1994) 149-154

157

clusions, realized on a polished surface; it suggests a Y - B a - S n - C u - O phase with a small amount of copper. The XRD patterns in Fig. 3 indicate that this YBaSnCuO phase, present after 5 min at 1100°C and until the end of the process, is isomorphous to the quadratic YBaESnOs.5 phase reported by Paulose et al. [23] which can be synthesized at 1500°C from Y 2 0 3 , BaCO3 and S n O 2 precursors. The results from quantitative analysis, performed on 30% doped samples, give the following average composition for the 'tin-phase' inclusions: YBa2.6SnCu0.6Pt0.]Ox. The ratio between copper and tin increases from 1 to 1.6 with increasing doping level (from 2 to 30 mol%), so we cannot exclude the existence of a wide domain of solid solution in the YBa- ( Sn, Cu ) - P t - O system. To further investigate the interaction of these new inclusions with the other components of the MPMG process, we have observed the microstructure of the samples with different levels of doping, quenched after different times at 1100°C. Moreover, during the texturing process, the green phase particles spend a

Fig. 1. SEM micrograph of the polished surface of a 30% doped sample after a complete MPMG process. The microstructure includes many square inclusions which are between l and 5 ~tm in size.

served in MTG-processed doped samples [ 10] but their identification was not clear and the formation of an unidentified Y - B a - S n - O phase was only suggested. Fig. 2 shows a qualitative EDS analysis of these in-

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Fig. 3. XRD patterns of a 30% doped sample (a) quenchedfrom 1100°Cand (b) after a completethermal treatment. One can find in the doped samplea phase isotypeof YBa2CuOs.5reportedby Pauloseet al. [ 19], shownin pattern (c). long time at a high temperature in the liquid phase and coarsening may occur. Figs. 4-6 show the etched fracture surfaces of the quenched pellets corresponding to undoped (Fig. 4), 10% doped (Fig. 5 ) and 30% doped (Fig. 6) samples kept 5 min (a), 2 h (b) and I0 h (c) at 1100°C. The chemical etching allows us to observe clearly the shape and the size of '21 t' inclusions. In the case of undoped samples (Fig. 4(a), (b), (c)), no 211 coarsening occurs, even after 10 h and the grains remain small and needle-shaped mainly due to the small amount of platinum included in the powders during the process [ 24 ]. As in any coarsening process, the driving force is the reduction of the total

surface energy; it has been demonstrated that the effect of platinum is to lower the 211/liquid interracial energy so that 211 coarsening is not favored [25 ]. For the 10% doped samples (Fig. 5 ), the 211 inclusions still remain small after l0 h at 1100°C (Fig. 5(c) ) but the shape is slightly different, more isotropic and less needle-like. In 30% doped samples after 5 min (Fig. 6 (a) ), one can observe many small needle-shaped 211 grains, mixed with few microns size small cubic grains which are the tin-phase inclusions. After 2 h, the inclusions are much bigger than in 0% or 10% doped samples and after 10 h, the morphology of the green phase grains has completely changed and the whole sample

I. Monot et al. / Physica C 233 (1994) 149-154

Fig. 4. Etched fracture surfaces of undoped samples quenched from 1100*C after (a) 5 min, (b) 2 h and (c) 10 h.

159

Fig. 5. Etched fracture surfaces of 10% BaSnO3 doped samples quenched from 1100°C after (a) 5 min, (b) 2 h and (c) 10 h.

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I. Monot et al. /Physica C 233 (1994) 149-154

Fig. 6. Etched fracture surfaces of 30% doped samplesquenched from 1100*Cafter (a) 5 min, (b) 2 h and (c) l0 h.

exhibits now very large 211 grains between 10 and 30 ~tm in size, as illustrated in Fig. 6(c). During that time, the cubic grains have remained small and unchanged, which means that no further reactivity neither coarsening has occurred concerning the 'tinphase'. These high-temperature observations and analyses allow us to conclude that during the high-temperature plateau of the M P M G process, at I 100°C, BaSnO3 reacts rapidly and completely with the meltquenched components to form a Y - B a - S n - C u - P t O phase. Such a rapid formation of this phase suggests a high solubility and/or a high diffusion rate of yttrium due to the presence of BaSnO3 in the liquid phases. The early presence of Y 3+ in the liquid around the Y203 grains might favor the rapid peritectic formation of many very small 211 grains. On the other hand, we should also underline the fact that this new phase contains platinum. Considering the quantitative analysis, over 5% BaSnO3 doping, all the platinum present in the melt-quenched powders may be consumed, so this latter is no more effective to positively act on further 211 formation, size and morphology. Thus, in the stability domain of the 211 phase, a stage in this temperature range may favor 211 coarsening, depending on the doping level and the length of the stage. That is why for 10% doping the 211 particles size is still small but their shapes are different from the undoped sample, due to platinum consumption, and for 30% doping, large 211 coarsening and morphology modification has occurred after only 2 h. So the action of the tin-phase on 211 inclusions is very different from that of platinum. If we consider the study of Izumi et al. [26 ] who have shown that the limited factor of 123 growth is the yttrium diffusion in the liquid phase from 211 to the 123 growth front, the only beneficial effect of BaSnO3 addition might come from the change in yttrium diffusion mechanisms which might help 123 recrystallization to proceed faster or with a lower degree of undercooling. DTA measurements, performed with a cooling rate of I ° C / m i n from 1100°C, have shown that 123 formation occurs at the same temperature for doped and undoped samples ( ~ 935°C in air and with this fast cooling rate). This result shows that, after its formation, the tin-phase is very stable, so the yttrium solu-

L Monot et al./Physica C 233 (1994) 149-154

161

bility and diffusivity recover the values of undoped samples, and 123 formation proceeds in the usual way; so, from that point of view, the 123 growth conditions are not improved by BaSnO3 addition. 3. 2. Microstructure after complete M P M G process

After the complete MPMG process, the microstructural observations of the samples lead to the following features: In the doped samples, no BaSnO3 seems to remain in the limit of detection of the apparatus. The Y - B a S n - C u - P t - O phase observed at high temperature is still present in the same form of white square-shaped inclusions which size is below 5 ~tm, but its distribution is rather inhomogeneous whatever the doping level is. For doping levels below 15%, the tin-phase inclusions often remain in the intergrain areas or are bundled together in the 123 matrix and associated with a small amount of solidified liquid phases (CuO and BaCuO2), as shown in Fig. 7 (b); besides the 211 particle size and distribution seem to remain similar to undoped samples (Fig. 7 (a)). One can only notice a little bit less 211 inclusions around the tin-phase; two possible interpretations can be given for this behavior. The first one is that the new phase needs yttrium for its formation and thus might act as an yttrium sink, so the concentration of 211 particles in these areas is lowered. The second possibility might be that in the vicinity of the tin-phase inclusions, many submicronic 211 panicles have nucleated due to the presence of a high concentration of y3+ in the liquid phase during the tin-phase formation. Then, during the peritectic formation of 123 grains, many of these small inclusions are consumed, leading to a lower concentration of 211 inclusions in these areas. Some remaining ones could be observed in the 10% doped sample where their former concentration was certainly optimal. Considering the macrostructural point of view, we observed that the domain size tends to decrease with increasing doping levels, certainly due to the extra liquid phase resulting from the yttrium consumption due to the tin-phase formation. For 30% BaSnO3 addition, the sample is composed of very small 123 grains (200 to 500 ~tm) with large 211 inclusions ( l0 to 30 ~tm), separated by large in-

Fig. 7. Optical micrographof the general microstructureof (a) the undoped and (b) the BaSnO3doped samples (below 15%) after a completeMPMGprocess. homogeneous multiphase areas. The large 211 inclusions can be correlated with the coarsening we have previously observed in samples quenched from 1100°C. For this high level of doping, the phase distribution is entirely affected by the formation of the tin-phase and the 123 stoichiometry is difficult to recover, leading to a large excess of liquid phase. SEM microstructural observations of cleaved surfaces give complementary informations concerning the size and the distribution of the green phase inclusions. We have observed that in the doped samples, the size of 211 particles does not seem to be greatly reduced compared to undoped samples, excepted in some areas of 10% doped sample which contains many submicron size inclusions. These different microstructural observations show that the tin-phase does not seem to help so much the 123 grain growth and can even favor 211 coarsening,

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reveals a high-quality material. Unfortunately, this high Tc value associated with a sharp transition is not reproducible and some other parts of the same sample exhibit slightly lower To. A m o r e detailed study o f the case o f 10% d o p e d samples has been reported elsewhere [27]. These inhomogeneities m a y reflect inhomogeneities in the material and might be correlated with the tin-phase inclusions distribution which is not uniform as previously observed in the first part. The Tc e n h a n c e m e n t might be due to partial substitution o f tin in the 123 structure as previously suggested [27,28 ], but further investigations are needed to confirm which site o f the structure is substituted. If we consider now the critical current densities of these samples at 77 K and under various magnetic fields (Fig. 9), one can figure out that the worst Jc(B) properties correspond to the 30% d o p e d sample, exhibiting a greatly disturbed microstructure. F o r 2% a n d 5% d o p e d samples, any i m p r o v e m e n t in Jc(B) properties can be found, c o m p a r e d to u n d o p e d sampies. The values are even lower a n d this confirms the fact that we did not observe any microstructural imp r o v e m e n t in these samples; they just contain an " i m p u r i t y phase" which cannot act as pinning center. In a 10% d o p e d sample, the local i m p r o v e m e n t s underlined in the microstructural observations and

but we observed that for doping levels below 15% molar ratio, the tin-phase f o r m a t i o n does not affect the general microstructure a n d we could even find m a n y submicron-size 211 inclusions. At that point, it is difficult to conclude on the beneficial or d e t r i m e n t a l effect o f BaSnO3 doping in the M P M G - p r o c e s s e d Y B C O samples. Therefore the superconducting properties o f the different d o p e d samples were investigated a n d c o m p a r e d to the properties o f an u n d o p e d sample synthesized in the same conditions.

3.3. Superconducting properties Fig. 8 shows the n o r m a l i z e d susceptibility vs. temperature for 2%, 5%, 10% a n d 30% d o p e d samples, m e a s u r e d with a D C S Q U I D magnetometer, with an excitation field o f 10 G. The critical t e m p e r a t u r e o f the 30% d o p e d sample is very low; the T~ ..... is located at 90 K a n d the transition is b r o a d ( > 2 K ) . On the other h a n d one can observe T~ e n h a n c e m e n t from 2% to 10% d o p e d samples. In the case o f the 5% sample, the T¢ onset is at 92.8 K but the transition is quite b r o a d and the T¢ze~o is located below 90 K. In 10% d o p e d samples we could find Tc .... up to 93 K with a rather sharp transition (T~ .... = 91 K ) , which

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L Monot et al. I Physica C 233 (1994) 149-154

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4. Conclusions The effect of BaSnO3 additions up to 30% has been investigated in MPMG-processed YBCO. We could evidence the high reactivity of BaSnO3 in the pres-

163

ence of liquid phases with the formation of a complex Y - B a - S n - C u - P t - O phase. The cubic particles of this new phase are difficult to homogeneously distribute in the microstructure and they do not seem to help the 123 nucleation nor grain growth; moreover for 30% doping, the entire microstructure is strongly affected and the 123 grains are very small. The rapid formation of this phase which consumes yttrium and platinum seems to influence the nucleation and the shape of the 211 particles. Above 15% addition and after several hours above the peritectic temperature of 123 decomposition, large 211 coarsening occurs but on the other hand, in 10% doped sample and after a complete M P M G process, we could locally observe high densities of submicron-size green phase inclusions in the final microstructure. These very small particles have certainly nucleated right after the tin-phase formation, due to the high diffusivity and/or solubility of yttrium in the liquid phase at that time. Tc improvement by 1 K has also been observed in the doped samples with the sharpest transition for the 10% doped sample. So, the good J c ( B ) properties observed in the 10% doped sample are believed to come from local 211 refinement in the vicinity of the tin-phase inclusions and from partial tin substitution in the 123 matrix structure, suggested by the T~ enhancement. Considering the size and the distribution of the tin-phase inclusions, any evidence of their direct contribution to pinning enhancement could be found. In conclusion, to obtain a positive effect with BaSnO3 addition in MPMG-processed YBCO, the process and the doping level should be accurately controlled in order to obtain a structural and microstructural effect which result in improved superconducting properties.

Acknowledgements IM wants to thank the Science and Technology Agency of Japan for its postdoctoral support. The authors want to thank P. Laffez, R. Itti, F. Frangi and S.I. Yoo for their helpful discussions. We also thank K. Takada, T. Oka, K. Yamada and T. Akahori for their help in sample preparation.

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