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Improvement of critical current density in Y-Ba-Cu-F-O compounds by addition of platinum Tsugio Hamada"'*, Yoshitaka Ikeda ", Tadahiro Akune b, Nobuyoshi

Sakamoto b

"Department of Electrical Engineering, Miyakonojyo College of Technology, 473-1 Yoshio, Miyakonojyo 885, Japan bDepartment ~ Electrical Engineedng, gyushu Sangyo University, 2-3-1 Matsugadai, Higashi-ku, Fukuoka 813, Japan Received 27 January 1997

Abstract Platinum added Y-Ba-Cu-F-O comp ~mds with nominal composition YBa2Cu3Fo 4Oxl:h:y {y=0, 0.3, 0.5} were prepared by a partial melt process performed at lower temperature than the conventional melt process. Optical micrograph observations showed that Y2BaCuO5 (211 ) particles increase in density and are dispersed finely when increasing the doping of Pt up to 0.3 wt%. For the 0.5 wt% Pt sample, however, the size of the 211 particles became larger than in the samples containing less than 0.3 wt% Pt. The magnetization was measured using a SQUID magnetometer in the temperature range 20-77 K under magnetic fields Be up to I T. The magnetization M also increased with increasing quantity of platinum up to 0.3 wt%. The J~ values estimated from the width of the magnetization curves were 9.8×10 ~ Alto z for the 0.3 wt% Pt sample and 5.4×107 Aim 2 for the undoped sample at 77 K and B~=O.8T. In addition, the whereabouts of the platinum in the quenched samples was investigated by using electron spectroscopy for chemical analysis (ESCA). © 1997 Elsevier Science S.A. Keywords: Y-Ba-Cu-F-O compound; Y,BaCuO~ particles; Critical current density; Platinum; Magnetic properly

L Introduction Applications of high temperature superconductors have been considered in various fields such as power application. However, there are many controversial issues regarding the application of the results. For instance, the high temperature superconductors are very inferior to metallic superconductors as regards the critical current density. A number of studies have been carried out in order to obtain a higher critical current density, J,, in high temperature superconductors. It is well known that the QMG (Quench and Melt Growth) processed YBCO superconductor including fine Y~_BaCuO5 (211) particles have large J~ values, losnloa et al. [ 1. .l. .nave . . . . . .repo~teu-I .. a tnuunm~uu~n u~ QMG using quenchless processing and a small Pt addition less than l wt%. They obtained YBCO bulk superconductors with a critical current density exceeding 1.8×108 A / m 2 at 77 K and 1 T. Platinum metals act as effective additives for achieving hnely dispersed 211 particles. Recently KJkuchi et at, [2! prepared Tl-based high temperature superconductors by the diffusion process. They showed that the critical current density at 77 K is *Corresponding author. 0925-8388/971517.00 © 1997 Elsevier Science S.A. All rights reserved. PII S0925-8388(97 )09128-X

significantly improved by fluorine addition to the coating layer and that fluorine addition promotes the phase transformation from 2223 to 1223. it is also reported that in YBCO compounds doped with fluorine the growth of grains was activated by fluorine addition and large magnetizations were observed [3,4]. In this paper, the effect of Pt addition on partial melt processed Y - B a - C u - F - O bulk superconductors is studied in conjunction with SEM observation and X-ray powder diffraction characteristics. In addition, the behavior and influence of platinum in the bulk superconductor during synthesis are examined by using electron spectroscopy for chemical analysis (ESCA).

2. Experknen~l procedure A sample with nominal composition YBa2Cu3F040~ ~ s prepared from two master compositions, YBa2Cu30 x and YBa2Cu3F404. 5. Starting powders used to prepare the oxide were Y203, BaCO 3 arid CuO. In the fluoroxide sample BaF 2 replaced BaCO 3. The powder mixture of each master composition was calcined in air at 900 °C for 8 h. After powdering and pressing a pellet (10 ram~× 10

218

T. Hamada etal. I Jom~al of Alloys and Compounds 260 (1997) 217-221

ram), it was heated at 950 °C for 1 h for partial melting, cooled dowrJ to 800 °C at a rate of 2 °C/h, and then cooled down to room temperature in a furnace. Annealing in oxygen was employed at 600 °C for 12 h after mixing the calcined powders of YBazCu3Ox and YBa2Cu3F404.5. A SQUID magnetometer was used to measure magnetic moments. The magnetization measurement was done in the temperature range T=20~I00 K, and in the magnetic field range B~=0-1 T. The microstructure was investigated by an optical microscope and by X-ray diffraction measurements. The behavior and influence of platinum in the Y - B a - C u - F - O bulk superconductor during synthesis are examined by using electron spectroscopy for chemical analysis (ESCA) for the two platinum (0.5 wt%) doped samples and the undoped samples.

3. Results and discussion

platinum. The 211 particles acting as pinning centers are distributed in the YBa2Cu3Ox (!23) matrix. This microstructure is similar to that observed in the melt processed sample. It is found that the 211 particles increase in density and are dispersed finely when increasing the doping with Pt up to 0.3 wt%. The size of the 211 particles in the 0.3 wt%Pt sample is smaller than that in the undoped sample. Cracks are also observed in Fig. l(b). The 0.5 wt% Pt sample contains voids and relatively large 211 particles, the length of which exceeds Ixm. This is considered to be much too large to achieve effective pinning. This increase in the size of the 211 phase is ascribed to the excess nucleation of the 211 phase. X-ray diffraction patterns of the undoped and the 0.3 wt% Pt sample are showlt in Fig. 2. Solid and open symbols denote the peaks from the 123

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219

T. Hamada et al. I .lournal of Alloys and Compounds 260 (1997) 217-221

and the 211 phase, respectively. It can be seen that the peaks observed correspond to a well cD'stallized 123 phase. The lattice parameters calculated from the 123 peaks are a=3.8242, b=3.8895 and c= 11.6872 ,~ for the undoped sample and a =3.8256, b=3.8898 and c = ! 1.6893 for the 0.3 wt% Pt sample. These values are nearly equal to those of an 123 single crystal. The 211 phase is the main secondary phase and the diffraction peaks are observed near 20=30 °. As can be seen from Fig. 2, the 0.3 wt% Pt sample shows sharp peaks and the crystaiiinity is higher than that of the undoped sample. The 20 values of the peaks from the 211 phase in both staples are nearly equal. The magnetization was measured using a SQUID magnetometer in the temperature range 20-77 K under magnetic fields Be up to 1 T. Fig. 3 shows the temperature dependence of magnetization in an applied field of 1 mT for the undoped, 0.3 and 0.5 wt% Pt samples in zero field cooled (zfc) a_n_d field c ~ ! e d (fc) condition. The critical temperature T¢ is 93 K for the Pt undoped sample, which means that YBCO structure is unchanged and fluorine has not substituted for oxygen. This corresponds to the absence of fluorine atoms in the Pt-doned sample observed by EPMA analysis [5]. It is also found that zfc behavior of the 0.3 wt% Pt sample is very sharp in the vicinity of T¢ compared to that of the undoped and the 0.5 wt% Pt samples. Magnetization curves for the undoFed, 0.3 and 0.5 wt% Pt samples at 77 K are shown in Fig. 4. The magnetization M increases with increasing platinum content up to 0.3 wt%. For the 0.3 wt% Pt sample, the peak effect is observed, the origin of which has not yet been clarified completely. The values of ,/¢ were estimated from the width of the magnetization curves by using Bean's critical state model. Fig. 5 shows the magnetic field dependence of the critical current density J~ at 77 K for

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undov:.d and doped samples. The Jc values for both the undoped and the 0.5 wt% Pt samples decrease at low magnetic fields Be, which may be caused by the cutting of the weak links. The ,/c values of the 0.3 wt% Pt sample are nearly constant at Be up to 0.8 % which may indicate a fortified effect of the grain boundary caused by the appropriate addition of Pt. The estimated -/c values were 5.4x 10 ? A i m 2 at 77 K and 0.1 T for Yt.2Bal.aCu3 oFo.+O~ [4i. The Jc value of 9.8×107 Aim 2 for the 0,3 wt% Pt-doped sample is comparable with that of QMG processed samples. Here we shall investigate the role of the doped platinum during synthesis of tae superconductor and

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T. Hamada et al. I Journal of Alloys and Compounds 260 (1997) 217-221

220

why the platinum has an effect on the distribution of the fine 211 particles in the mau'ix. It has Ken proposed that the platinum doping leads to a Ba4CuPt20 9 compound during synthesis, as revealed in the X-ray diffraction pattern by peaks at 20 angles of 27.4, 30.6 and 41.8 (degs.) [5]. Therefore quenched samples with 0.5wt% Pt were prepared at several temperatures and used for an analysis of the B a 4 C u P t 2 0 9 phase by X-ray diffraction. These samples are listed in Table 1. The marks of "up" and "dn" in Table 1 pertain to the quenching temperature that increases or decreases to the maximum temperarare of 950°C. Peaks of the B a a C u P t 2 0 9 compound were not detected in these samples. The result is shown in Fig. 6 (see arrows;. These samples were also analyzed for the presence of platinum on the sample surface using EPMA, the result being the same as that of the X-ray diffraction measurement. In the present condition, it is not easy to explain exactly why Pt has not been detected in the form of the Ba4CuPt20 9 compound. However, because Pt cannot be detected in these samples, it is possible that the platinum is located at a deep position from the surface. The behavior of fluorine for the generation of Y2BaCuO5 particle was reported before [6]. It was mentioned that the fluorine evaporated into the air from the sample surface during the cooling process. However, the fluorine existed slightly inside the sample, but there was no fluorine in that part of the sample surface where the Y:.BaCuO 5 particles became distributed finely [7,8]. Thus if the platinum exists iside the sample, the ESCA depth profile is expected to change for Pt-doped samples as compared to undoped samples. Thus the element signals of yttrium, barium, copper, and platinum were investigated for the samples with Pt doping and without Pt doping. However, signals of platinum could not be detected in the samples A and B, and it was found that the profiles of barium, copper, and platinum were hardly changed by Pt doping. However, there was just a change of the yttrium profile when doping with platinum. The result is shown in Fig. 7. Therefore, it can be believed that platinum affects the chemical binding of yttrium during the synthesis process of these superconductors, but it is necessary to investigate in much more detail the ro~e of platinum in the fluorine-doped superconductors. Table 1 Survey of samples quenched at various temperatures Samples

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4. Conclusion YBa2Cu3Fo40 x compounds doped up to 0.5 wt% with platinum were prepared by a partial melt process performed at temperatures lower than in the conventional melt process. Optical micrograph observations showed that the Y2BaCuO5 (211) particles increase in density and are dispersed finely when increasing the doping content of Pt up to 0.3 wt%. For the 0.5 wt% Pt sample, however, the 211 particles segregated further and the size became much too large to achieve effective pinning. The magnetization M also increased with increa,,'~.ng the platinum content up to 0.3 wt%. The value of J~=9.8× 107 Alto 2, estimated from the width of the magnetization curves, was obtained for the 0.3 wt% Pt doped sample, which compares with that of an QMG processed sample. The role of the doped platinum in the samples could not be clarified in this system. However, it is believed that platinum affects the chemical binding of the yttrium atoms in Y-based superconductors.

221

T. Hamada et al. I Journal of Alloys and Compounds 260 (1997) 217-221

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Fig. 7. Depth profile of y:trium for samples A and C by using ESCA. The numbers inserted in both depth profiles show a etching time (rain) using Ar + ions. For instance, sample surface is !ndicated as 0.00. As etching times increase, the number increases, and information from inside the sample is obtained. Acknowledgemen~ The authors want to thank Dr. T. Hashiguchi for preparation of the samples. We are also indebted to Drs. Y. Hamada, K. Mitsuda, K. T a n a k a and S. Satoh for their measurements.

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[2] A. Kikuchi, T. Kinoshita, N. Nishikawa, S. Komiya, K. Tachikawa, Jpn. LApp]. Phys. 34 (1995) L167. [3] F. Sumiyoshi, T. Hamada, S. Kawabata, Cryogenics 28 (1988) 3. {4] Y. Hakuraku, F. Sumiyoshi, K. KJttaka, T. Hamada, S. Kawabata, T. Ogushi, Cryogenics 29 (1989) 415. [5] N. Ogawa, I. Hirahayashi, S. Tanaka, Physica C 177 (1991) 10|. [6] T. Hamada, R. Morimo, Phys. Star. Sol (a) 151 (1995) 199. [7] T Ramada, R. Morimo, R. Ogura, J. Mater. Sei. 31 (1996) 2579. [8] ~. Hamada, R. Morimo, Phys. St,at. Sol. (a) 145 (1994) 61.