Properties of large-scale melt-processed YBCO samples

Properties of large-scale melt-processed YBCO samples* S. Gauss, S. Elschner and H. Bestgen Corporate Research II, Hoechst AG, W-6230 Frankfurt, Germany Magnetic bearings and superconducting permanent magnets are some of the first possible applications of bulk high Tc superconductors. Large samples were prepared by a new melt process starting from reacted YBCO 123 and 211 powders. The addition of PtO2 to the mixture led to reduced 211 inclusion size and better homogeneity. Simultaneously the density of microcracks dividing the a - b basal plane was reduced. For testing the overall magnetic properties of these samples magnetization and levitation force measurements were performed. In comparison to samples without PtO2 addition a strong increase in the magnetization M and the repulsion force from a magnet were observed. The maximum in the field dependence of M increased to more than 1000 G. According to the time dependence of the trapped field after a field cooling experiment an acceptable flux creep at 77 K for a long-term application was achieved.

Keywords: YBCO; microstructure; magnetization

Weak links between different grains strongly determine the electric properties of high Tc superconductors. They particularly limit the critical current density j¢ in bulk Y - B a - C u - O (YBCO) samples. However, fairly high critical current densities have been observed in material prepared by different melt processes. This preparation route leads to very large grains and a better connectivity between the grains. Several preparation methods have been proposed, such as melt textured growth (MTG) t-4, quench-melt-growth (QMG) 5-7 and melt - powder - melt - growth (MPMG) 8- J0. Because of the large grains, very strong levitation forces and high magnetizations have been obtained. Possible uses of these materials could be in magnetic bearings of in a new kind of permanent magnet using the trapped flux after field cooling. In this study large samples were prepared by a different preparation process without the difficult quenching of the Y - B a - C u - O melt from 1400 °C. The microstructurai features of two kinds of sample with different sizes of YBCO 211 phase precipitates in the YBCO 123 matrix will be discussed. The magnetic properties were analysed by magnetization and by the levitation force on a permanent magnet. The hysteresis of the levitation force between approaching and removing the superconductor from the magnet hinders the definition of a fixed levitation distance. For application in a magnetic bearing the hysteresis had to be optimized

*Paper presented at the conference 'Critical Currents in High Tc Superconductors', 2 2 - 24 April 1992, Vienna, Austria 0011 - 2 2 7 5 / 9 2 / 1 1 0 9 6 4 - 0 5 ,~ 1992 Butterworth - Heinernann Ltd

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and simultaneously the flux creep in and out of the sample has to be small to give a long operation time.

Sample preparation The sample preparation started from powders of YBCO 123 and YBCO 211 reacted by a solid-state route. The grains had various sizes in the range of ds0 = 3 - 3 0 tzm (123) and 1 . 4 # m (211). Different mixtures were prepared with 0, 17, 26 wt.% 211 and with 1 0 - 2 0 wt.% Ag20 as a crack-reducing additive. Recently a positive influence of Pt addition--by the use of Pt crucibles--on the final 211 inclusion after the melt process size has been reported ~. Therefore a small amount of PRO2, 0.6 wt.%, was added to some samples. To obtain a good homogeneity of the different powders they were mechanically mixed for 6 h. Cylinders with diameters of up to 32 mm and 20 mm thickness were made by cold isostatic pressing at 2300 bar. In the following heat treatment the samples were partially melted at 1050-1100 °C and then cooled from the peritecticum at a rate of 1 ° h-~. Further details of the preparation are described in previous papers ~2'~3. For further enlargement of the YBCO grains, techniques such as seeding the partial melt or cooling in a temperature gradient are known from the literature t2''4 but these processes were not used in this study. Finally oxygenation was performed in the temperature range 6 3 0 - 4 0 0 °C for 1 6 - 4 0 h. All samples kept their shape during the melting process owing to the formation of solid 211. The densities of the cylinders before the melt process were in the range of 92% and increased up to 97% during the heat treatment.

Properties of large-scale melt-processed YBCO samples: S. Gauss et al. Microstructure The microstructural features of the samples, such as grain morphology, precipitates, microcracks, twins, dislocations and strain fields, were analysed by optical microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The TEM investigations were performed at 300 kV using a scanning transmission electron microscope (STEM, Philips CM30) fitted with an EDAX energy~dispersive X-ray system. Thin foils for TEM were mechanically ground, dimpled to a thickness of about 60 ~m and finally ionbeam-thinned with effective nitrogen cooling. In similar fashion to the results of other melt processes on YBCO, the samples exhibit large grains with lengths ranging up to 18 mm. Some pores in the centre of the cylinders were observed, owing to the low green density of 62% before the heat treatment and the loss of BaCuO2 flux during the slow cooling. The influence of Pt addition on the size of the 211 inclusions has recently been reported by Ogawa et al. ,t. The addition of 0.6 wt. % PtO2 in our study also led to a reduction in size from 4 - 6 / x m without PtO2 addition to 1 - 3 / x m with PtO2. Figure 1 is a SEM micrograph of a fracture surface and Figure 2 a TEM bright field image showing the 211 precipitates embedded in the 123 matrix. Furthermore the reduced size led to a more homogeneous distribution of the 211 phase and so to an increased specific surface of the normal conducting inclusions in the superconducting matrix ,5. As shown earlier there is no correlation between the starting 211 powder size and the final 211 inclusion in the melt-processed samples ,2,,3. The grains of all samples exhibit a distinct lamellar microstructure, with microcracks lying parallel to the a - b basal plane. Cracking is a well-known problem for melt-processed YBCO specimens 2"3''2, especially for large mm-sized grains, and is a result of the strain associated with the transformation from tetragonal to orthorhombic phase. The twin structure in the grains is hardly influenced by these cracks '2. In quenched tetragonal samples only a few microcracks were observed which were also aligned parallel to the a - b plane and are probably due to the anisotropy of the thermal expansion. However, compared to samples prepared without the PtO2 and Ag20 additives, the

Figure 1 SEM micrograph of a fracture surface of a meltprocessed sample with 0.6 w t % PtO 2 addition showing fine YBCO 211 inclusions

Figure 2 TEM bright field image showing the YBCO 211 precipitates in the 123 matrix. Additionally the contrast of strain fields (S) and very small Zr-containing particles can be seen

amount of cracking could be reduced and the mean crack distance increased from about 2 ~m to about 4 #m. While silver particles absorb strain and especially reduce the nucleation rate of the cracks, the mechanically harder 211 particles absorb the energy of the propagating cracks by crack deflection or bowing. Thus in addition a fine dispersion of the 211 precipitates has the advantage of increasing the fracture toughness which was measured for MPMG-processed samples by Fujimoto et al. 16 by indentation experiments. The larger hardness of the 211 particles with respect to the 123-matrix is indicated by the higher melting point and becomes obvious in TEM observations because hardly any lattice defects can be imaged inside those inclusions. As well as the 211 precipitates, silver particles of dimensions between about 0.2 t~m and 3/zm were easily detected by TEM, but Pt could not be localized. Instead, very small (about 1 0 - 5 0 nm) particles containing Zr could be observed embedded both in the 211 precipitates (see Figure 2) and in the 123 matrix. They are obviously residues of the ZrO2 grinding balls, by which the starting powder was ground and may, perhaps, have a similar effect on the nucleation rate of the 211 particles as the PtO2 additive. Another typical feature of the melt-processed samples is a high density of dislocations which is shown in Figure 3. Such a density is not observed in sintered YBCO ceramics and is probably due to plastic deformation during the solidification of the melt and stress fields generated by the 211 inclusions. Also, contrasts of strain fields around 211 particles can be seen in Figure 2. In Figure 3 additionally the distance between the twin

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Properties of large-scale melt-processed YBCO samples: S. Gauss et al. that the intragrain current is therefore mainly determined by the intrinsic pinning effect and additional pinning by artificial 211 inclusions is low. Comparing the field dependence of the magnetization for powdered samples and bulk a different characteristic could be observed. For the powdered samples a maximum was obtained in the field range of 2 0 0 - 4 0 0 G independent of the amount of 211 addition or PtO2. For the cylinder shaped samples a higher maximum in the range of 4 0 0 - 1 2 0 0 G was observed. Increased grain size is believed to be the reason for this. Therefore, samples with PtO2 addition and a defined oxygen annealing with carefully performed tetragonalorthorhombic phase transition having a low microcrack density exhibit the largest external field values for the maximum in the magnetization. Figure 4 gives a comparison of the dependence of the magnetization for a standard YBCO sample and an optimized cylinder with PtO2 addition; both samples consist of powder mixtures with 17 wt. % 211 and 10 wt. % AgO2 added to YBCO 123. The absolute value of the magnetization increased by a factor of 6 in the optimized samples.

Levitation force measurements Figure 3 TEM bright field image exhibiting dislocations and t w i n boundaries in melt-processed YBCO material

boundaries is exhibited which fluctuates between about 50 and 150 nm. All these different kinds of defect within the grains, such as normal conducting precipitates, twin boundaries, dislocations and strain fields, may act as pinning centres ~5.

Magnetic properties The resistively determined critical temperature for all samples was Tc(R = 0) > 90 K. The magnetic properties of powder samples were obtained by d.c. SQUID measurements (Quantum Design, MPMS) of the magnetization in fields of 0 - 5 T in the temperature range 5 - 9 0 K. For testing the overall magnetization properties of large bulk samples at 77 K their shielding effect was measured. Therefore a Hall probe was placed between a two-layer sample consisting of a pair of cylinders and the field reduction of an externally applied field ( 0 - 0 . 5 T) was measured. The same experimental setup was used to measure the trapped magnetic flux density after field cooling of the samples. By measuring the time dependence of the residual field after switching off the magnet the flux creep properties can be observed. From the magnetization the intragrain critical current density jc can be calculated by using Bean's model ~7. But for a quantitative interpretation of hysteresis curves different averages, such as grain orientation and demagnetization for example, had to be taken into account. The influence of the 211 inclusions on the absolute value ofjc and its field dependence was low. For powdered melt-processed samples with 40 #m grain sizes critical current densities at 77 K and 0.2 T were calculated as 0.9 x 104 A cm -2 and 2.2 x 104 A cm -2 for 0 wt. % 211 and 26 wt. % 211 respectively. It seems

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A further characterization of the bulk samples was performed by levitation force measurements. This method gives an easy and fast characterization of the magnetic properties of large bulk samples. The YBCO cylinders were mounted in a sample holder and cooled with liquid N2. They were then mechanically moved slowly towards a magnet ( N d - F e - B , 20 mm dia. x 5 mm) and removed while the repulsion or attraction force was measured. The absolute values of the forces depend not only on the sample size and weight but also on the shape of the magnetic field distribution. Since in the experimental set-up we always used the same geometries a comparison of the overall quality of different samples is possible. For thermal isolation reasons the nearest

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Properties of large-scale melt-processed YBCO samples: S. Gauss et al. distance between magnet and sample was 3 mm. At that point a maximum field of 1600 G was obtained. In Figure 5 the levitation force vs. distance for two samples with and without Pt addition is compared. Additionally the sample with 0.6 wt.% Pt was oxygenated more carefully. The sample size for this measurement was 20 mm dia. and 15 mm thickness. A more than fourfold increase of the maximum levitation force was measured. The slope is much steeper for the optimized sample leading to a higher axial magnetic stiffness Ca = 6F/SX, where 6X is the change in the distance from magnet to sample. For the given experimental set-up (i.e. magnetic field, field distribution and geometry) a factor of 20 could be calculated between the levitation force and the mass of the sample for 1 mm levitation spacing. For application in magnetic bearings the hysteresis behaviour between approaching and removing from the magnet is a problem. For a given load no fixed levitation spacing is defined when the axis is vibrating. The hysteresis is a consequence of the magnetization history and the linearity of the magnetization versus field characteristic. For samples with a maximum in the magnetization at low fields (i.e. 300 G) this value is already exceeded at larger distances from a given magnet. Approaching the magnet further causes trapped flux in the sample after Bean's model ]7. Depending on the grain size more or less magnetic flux is trapped in the cylinders. In samples with a large number of grain boundaries the amount of penetrated flux increases, leading to a deviation of the magnetization from the theoretical maximum shielding by the Meissner limit.

For optimized samples this appears at fields larger than 5 0 0 G , while for standard samples small fields ( < 100 G) prevent the full diamagnetism (Figure 4). Flux creep observations The most important advantage of high Tc superconductors in comparison to conventional ones is the possibility of operating them at higher temperatures. For economic reasons 77 K, the temperature of boiling N2, seems to be the most promising. But at this temperature level a distinct flux creep is to be expected limiting long-term applications. For a first characterization of the meltprocessed samples the time dependence of the trapped field after field cooling was observed. Figure 6 shows the nearly logarithmic time dependence of the trapped flux density for a pair of 20 mm dia. samples with 17 wt.% 211, 10 wt.% AgEO and 0.6 wt.% PtO2. In this case the samples were cooled in a field of 2000 G and after switching off the magnet 675 G were trapped. The time dependence was observed in the range of 5 s up to 8 h and only a very small upward curvature was observed. This small effect, also seen in other experiments ~8"~9, can be explained by a field dependence of the pinning potential. As can be calculated from the measurement shown in Figure 6 the drop in the trapped flux within one year would be in the range of 22 %. So after a relaxation time of one day, corresponding to a field reduction of 18%, the magnetic properties of the material remain good enough for a long-term application. Conclusions

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Large samples were prepared by melt-processing of mixtures of YBCO 123, YBCO 211 and Ag20 powders with properties comparable to material prepared by other routes ~- ~0. The addition of small amounts of PtO2 reduced the size of the 211 precipitates to 1 - 3 / ~ m leading simultaneously to a more homogeneous distribution in the 123 matrix. Microcracks dividing the a - b basal plane led to a lamellar microstructure of all meltprocessed samples. The density of these cracks could be reduced by the addition of PtO 2 and Ag20. In these

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Properties of large-scale melt-processed YBCO samples: S. Gauss et al. samples a small twin spacing and many dislocations were also observed. For testing the overall quality of the cylinders' magnetization and levitation, force measurements were performed. Samples with PtO2 addition exhibit higher magnetizations, and the maximum in the field dependence was increased to more than 1000 G. Simultaneously large repulsion forces from a magnet were obtained and the hysteresis between approaching and removing the sample from the magnet was reduced. The flux creep properties of material at 77 K were tested by measuring the relaxation of trapped flux after field cooling. The observed rates are acceptable for a long-term application.

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