Surface texturing of bulk YBa2Cu3O7−δ samples

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Journal of Crystal Growth 91(1988)410—413 North-Holland, Amsterdam

SURFACE TEXTURING OF BULK YBa2Cu3O7..8 SAMPLES D.N. MA’TTHEWS, T. PUZZER, A. BAILEY, N. MONDINOS, G. ALVAREZ, G.J. RUSSELL and K.N.R. TAYLOR Advanced Electronic Materials Group, School of Physics, The University of New South Wales, P.O. Box 1, Kensington. NS W 2033, Au.stralia Received 19 April 1988; manuscript received in final form 26 April 1988

A process technique is described which results in significant surface texturing of bulk YBa2Cu3O78 samples. Using standard calcining and sintering cycles, surface textured material with grain size ~ 50 ~smhas 3 A cm2 beeninproduced. zero applied The field T,~ofatthe 77material K. is 93 K and J~(transport), for predominantly textured material, is in excess of 1.5 x i0

Magnetization measurements on single crystals of YBa 2Cu3O7~(123) at low temperatures (~5 K) by Dinger et al. [1] andand Crabtree et al. [2] show anisotropic flux pinning a critical magnetization current density (Jo) of 2 x 106 A cm2 in the plane perpendicular to the c-direction. This current density is reduced by at least a factor of 10 in the planes perpendicular to the a and b-directions. The J~for the field in the c-direction varies little for applied fields up to 40 kG, while the ~Jcfor the a- and b-directions is strongly field dependent [1,2]. Further, there is a dramatic decrease in .i~,for current in the a—b plane [2], with increasing temperature [2]. At 77 K, the J~appears to be only a factor of 35 above that for polycrys2, talline material value of field, 3.5being x i0~ A cm extrapolated to [2]; zeroa applied indicated by the work of Kumakura et al. [3] for polycrystalline material. Therefore, texturing of polycrystalline samples, so that the c-direction is perpendicular to the transport current flow direction, should provide a significant increase in jT (transport) over2 the commonly quoted of 5—100 A (at 77 K), provided thererange is strong coupling cm between the textured grains. In fact, Jin et al. [4] have announced a “melt-textured growth” process for (123) material which has raised jT to 1.7 x i0~ A cm2 in zero field at 77 K. In this brief report a processing technique is described for the surface texturing of bulk (123)

samples, which does not involve melting of the material, leads J~TK. in excess of 1.5 x i0~ 2 inand zero fieldtoata 77 A cm All samples were prepared by thoroughly mixing powders of Y 2O3, CuO and BaCO3 in the proper ratios. The mixture was then pressed uniaxially to 3.9 MPa in a case hardened steel die to give a sample 30 mm in a diameter and 2 mm in thickness. All samples then underwent a calcination cycle in air which consisted of heating, at 150 °C/h, to 930°C, holding the temperature at 930°Cfor 12 h, and then cooling to room temperature at 60°C/h. Figs. la and lb show X-ray diffraction spectra for the surface and powder of one of the after thisthat cycle. The powdered material is samples consistent with reported by Noto et al. [5] and has line intensities representative of powdered (123) samples with the orthorhomic unit cell having values a 0 = 3.821 A, b0 = 3.880 A and c0 = 11.663 A. The surface spectrum shows no surface texturing but does however, clearly mdicate a greater orthorhombic splitting of the (013)/(103) line compared with that found in the powder spectrum. After the calcination cycle all samples were then reground until the particle size was <38 ~tm. The powder was again pressed into pellets using a uniaxial pressure of 463 MPa and “semiconductor grade” ethanol as a binding agent. Fig. ic shows an X-ray diffraction spectrum for the surface of a

0022-0248/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

D.N. Matthews et a!. / Surface texturing of bulk YBa

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parallel to the surface of the sample, occurs at this step in the processing cycle. The next processing step is the first sintering cycle which involved heating of the samples at 150°C/h to 930°C where they were held for 12 h and then cooled in an oxygen atmosphere to room temperature at 60°C/h. X-ray powder diffraction spectra for these sintered samples showed all the samples to still consist of single phase material with the, observed lines accounted for by the =

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standard orthorhombic unit cell [5]; a0 3.824 A, b0 3.876 A and c0 11.658 A. However, X-ray diffraction spectra obtained from either of the

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texturing still beofpresent but with no further large area to surfaces each sample showed surface enhancement of the (001) peak intensities. Further, energy dispersive X-ray analysis performed on a scanning electron microscope (SEM) showed that both surface and bulk were uniformly (123) material, with SEM studies clearly revealing surface and bulk (exposed by fracturing) growth habits. The surface structure (fig. 2) consists of 10 ~tm “rectangular” grains which penetrate to a depth of 15 j.sm from either surface. Close inspection of the surface rmcrographs indicates that this region has negligible porosity. However, the bulk growth habit shows a number of grains of recognizable crystal habit and the overall material appearance is similar to that observed for flux

21 (deg) Fig. 1. (a) X-ray diffraction spectrum from the large area surface of a (123) sample calcmed at 9300 C for 12 h. Note that there is no surface texturing of the sample but significant splitting of the (013)/(103) line compared with that found for the powder spectrum shown in (b). (b) X-ray powder diffraction spectrum for a (123) sample calcined at 9300 C for 12 h. (c) X-ray diffraction spectrum from the large area surface of a (123) sample that has been calcined, reground and the powder pressed uniaxially to 30 ton with ethanol as the binding agent. Note the surface texturing of this sample as indicated by the enhanced intensity of the (001) peaks.

repelletised sample before sintering. The (00!) lines have all significantly increased in intensity, the enhancement being a factor of 6 with respect to those of statistically oriented powder. Thus, appreciable grain orientation, with the a—b plane

Fig. 2. SEM micrograph of the structured surtace 01 a YBa2Cu3O~sample showing = 10 ~tm “rectangular” grains which penetrate to a depth of 15 ~sm.

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D.N. Matthews eta!.

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Fig. 3. SEM micrograph of the bulk growth habit showing the formation of a single grain of recognizable crystal habit.

grown single crystals (fig. 3). Careful removal of the surface and part of the bulk region, by polishing with 1.0 ~em SiC paper, exposed the bulk region further sis. Thefor spectra so surface obtainedX-ray were diffraction identical toanalythose of a (123) powder specimen and indicated that the crystal habit of the bulk had no preferred orientalion, The critical temperature, measured by the standard four point probe technique, using either silver paste or silver epoxy contacts, gave a value of 93 K (~T< 1 K) for all the textured samples. Transport critical current measurements were

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Fig. 5. X-ray diffraction spectrum from the large area surface of a (123) sample that had been calcined and processed for six sintering cycles, each of 12 h at 930°C. The enhancement of the (00/) peak intensities is 17.

made in conjunction with Josephson supercurrent measurements which used the ceramic bridge method [6]. For this technique, a small bridge of2 triangular areaby asymmetrical 1 x 1O~cm and lengthcross-section 1 mm was with formed cutting of the bulk sample. This bridge was cut so that the surface textured layer was undisturbed and formed the base of the triangle. After the two processing cycles detailed above, the J~T value for the material was measured by passing a current through the bridge, parallel to the textured surface material. jT was found in this way to be 150 A cm 2 (at 77 K and no

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Fig. 4. (a) SEM micrograph of the hulk growth habit after cakination and one sinter at 93(1°C for 12 h. Grain s17e same sample but after a further five sinters, each for 12 h at 930°C. Grain size ~ 50 urn.

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D.N. Matthews et a!. / Surface texturing of bulk YBa ,Cu307._

applied field), a value greater than those commonly measured for sintered material, A number of the samples were then further heat treated without re-grinding, using only the sintering cycle described above for each one. SEM analysis of the surface and bulk regions during this cycling showed gradual merging of the surface grains with further penetration into the bulk region and a consequent reduction of the number of visible grain boundaries. The bulk also showed further growth of the larger crystallites and the merging of grains. In fact, after a total of 84 h at 930°C (five further sintering cycles), both the surface and the bulk grains had grown from an initial size of 10 ~em to ~ 50 ~.tm(see fig. 4). Fig. 5 shows the X-ray diffraction spectrum for the surface of this sample and the enhancement of the (001) peak intensities is now approximately 17, a significant increase in the surface texturing. This highly surface textured material still had a 7~of 93 K, but jT was directly measured as being well in 3 —2 excess of 1.5 x 10 A cm A sample that had undergone the first calcination cycle (at 930°C) and then heated for a total of 72 h at 930°C during the sintering cycle was found to be surface textured with the bulk consisting of highly crystalline material. Both the surface texturing and grain growth were observed to have occurred to a much lesser degree than for material which had undergone six separate sintering cycles. These results show that surface texturing that is, the alignment of crystalhne grains with their c-axis perpendicular to large area surfaces of a sample, had been obtained for (123) supercon-

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ducting material. The simple processing route outlined in this letter shows that material melting is not required and that the degree of texturing would be enhanced if the initial grain size, formed during the calcination cycle, was significantly increased. Optimization of the texturing parameters, pelletization pressure, calcination temperature and soak time, oxygenation, sintering cycle conditions and binding agents are now being actively investigated. Preliminary results indicate that an increase in calcination temperature and pelletization pressure significantly increases the surface texturing and its depth of penetration into the bulk.

References [1] T.R. Dinger, T.K. Worthington, W.J. Gallagher and R.L. Sandstrom, Phys. Rev. Letters 58 (1987) 2687. [2] Sowers, G.W. Crabtree, J.Z. Liu, Umezawa, W.K. Kwok, C.H. S.K. Malik, B.W.A.Veal, D.J. Lam, M.B. Brodsky, and J.W. Downey, Phys. Rev. B36 (1987) 4021. Kurnakura, M. Uehara, Y. Yoshida and K. Togano, Phys. Letters A124 (1987) 367. [41S. Jin, R.C. Sherwood, T.H. Tiefel, RB. van Dover, R.A. Fastnacht and ME. Davis, in: Extended Abstracts Materials Research Society Meeting on High Temperature Superconductors II, Reno, NV, 1988, Eds. D.W. Capone II, W.H. Butler, B. Batlogg and C.W. Chu, p. 153. [5] K. Noto, K. Watanabe, H. Morita, Y. Murakami, I. Yoshii, I. Sato, H. Sugawara, N. Kobayashi, H. Fujimori and Y. Muto, in: Novel Superconductors, Eds. S.A. Wolf and V.Z. Kresin (Plenum, New York, 1987) p. 801. [6] PH. Wu, Q.H. Cheng, S.Z. Yang, J. Chen, Y. Li, M. Ji, J.M. Song, H.X. Lu, X.K. Gao, J. Wu and X.Y. Zhang, Japan. J. AppI. Phys. 26 (1987) L1579.

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