INFLUENCE OF POWDER STRUCTURE ON TEXTURE FORMATION IN SUPERCONDUCTING Bi ( Pb )-SrCa-CuO CERAMICS IN MAGNETIC FIELD

Starting powder of Bi(Pb)-Sr-Ca-Cu-O superconductor consisting mainly of 2223phase was separated into fractions by precipitation in toluene. The high-oriented stacking of grains of the monocrystalline fractions with different dispersity has been obtained in magnetic field of 2 T at room temperature. The size of the powder grains in the range of 2-20 lm has no strong influence on the texture. The best result of grain alignment has been obtained for the powder grains of 10 tm in size.

It is known that the electric charge transport in high-Tc super- conducting ceramics takes place mainly in the ab-plane.That is why it is necessary to make strong texture, in the material, to attain the high critical current density in these ceramics.The most evident way of grain alignment is deformation.However, when deforming the material, destruction of crystallites takes place besides their alignment, and the ceramics change gradually into amorphous state (Kanai et al., 1990).
That prevents a large deformation, and hence, one cannot obtain a very sharp texture in bulk samples.Consequently, it is important to discover other methods of grain alignment without considerable destruction of crystallites.It is desirable to orientate the powder grains during the sample formation.In this case, a well-textured sample with insignifi- cant deformation of crystallites can be obtained after pressing.
In several works, strong magnetic fields were used successfully to align the YBa2Cu307 ceramic crystallites.For example, Farrell et al.
(1987) and Nakagawa et al. (1989) obtained well-textured samples by use of magnetic fields of 9 and 7 T, respectively.The present work was undertaken to clear up the possibility of oriented stacking of the powder grains of Bi(Pb)-Sr-Ca-Cu-O ceramics with the aid of a magnetic field.
The starting powder of Bil.6Pb0.aSr2Ca2Cu3010+6consisting of 2223- and 2212-phases was separated into five fractions as follows.The powder was stirred up in a large amount of toluene and precipitated on the bottom of a vessel for various thne periods.The most coarse- grained precipitate (first fraction) was obtained after 3 s.The remaining suspension was poured off into another vessel, stirred up again, and the second fraction was precipitated on the bottom of the vessel after 15 s.Quite analogously the third, fourth and fifth fractions were obtained after 30 s, and 10min, correspondingly.
Textured samples were prepared from the separated fractions using a magnetic field in the following way.A quartz cuvette of mm in depth was filled with the powder mixed with toluene.The cuvette was placed in magnetic field of 2 T for 20min till the complete evaporation of toluene.A sample was a precipitate of about 0.3 mm in thickness on the bottom of the cuvette.
The X-ray diffraction patte,rns of the dry precipitates were taken.
The texture was estimated from the ratio of the diffraction lines intensities of the investigated precipitate to the ones of the untextured sample.The cuvette was set always horizontally in the electromagnet.
The magnetic field was directed vertically (HIIg) or horizontally, (H+/-g). .Additional investigations show that this texture takes place on the surface of the precipitate and, evidently, arises because of the surface tension of the fluid during drying.When the samples were prepared in the vertical magnetic field, this texture became stronger (Fig. (a)).In the case of the magnetic field parallel to the bottom of the cuvette, a new texture with the preferred c-axis orientation parallel to the bottom appears.It results in a strengthening of the H00 and HKO lines and a weakening of the 00L ones (Fig. (d)).
Figure 2 shows the partial (200) pole figure of the sample prepared in horizontal magnetic field.It is obvious that the c-axis of the 2 Thota FIGURE X,ray diffraction patterns of the samples prepared from the fraction No. 4: (a) in vertical magnetic field of 2 T, (b) in zero magnetic field, (c) untextured sample, (d) in horizontal magnetic field of 2 T. (o) 2223-phase, (,) 2212-phase.crystallites orientates along the field in the case of the vertical mag- netic field as well.
The magnetic field was varied from 0 to 2.5 T to define its effect on the texture formation intensity which was estimated from strengthening of the o0010 and .,200diffraction lines.As shown in Fig. 3, the dependence of the texture on magnetic field becomes weak in mag- netic fields of more than 2 T, especially, in the case of vertical magnetic field.A field of 2 T was used in the present work.
To estimate the influence of the powder dispersity on the intensity of texture formation, all the fractions were investigated.The results are summarised in Table I.
One can see from Table I that the strongest texture is observed for the sample obtained from the fraction No. 4. Microscopic examination of powders shows that the fraction No. 4 consists mainly of mono- crystalline plates of size from 5 to 10 Ixm in the ab-plane.Besides, there is a small quantity of clots of the same dimensions.Apparently, the latter are polycrystalline conglomerates.The amount of these con- glomerates increases in the more coarse-grained fraction No. 3. The fraction No. consists of the most coarse-grained conglomerates as a whole.From the microscopic and texture examinations, one can   0010) is the intensity of the 0010 diffraction line obtained from the 2223-phase of the precipitates, and J0(*0010) is the intensity of this line from the untextured sample.
conclude that weakening of the texture with increasing grain size of fraction is the result of increasing the amount of polycrystalline particles and decreasing the amount of monocrystalline ones.Apparently, such a powder structure is explained by chemical and struc- tural heterogeneity of the starting material sintered before grinding.
Figure shows X-ray diffraction patterns of the fraction No. 4. All the patterns were taken under the same conditions, they are represented on the same scale.As shown in Fig. l(b), the precipitate texture in which the preferred c-axis orientation is perpendicular to the bottom of the cuvette arises in zero field.This is obvious from the strengthening of the 00L lines in comparison with the corresponding lines of the untextured sample (Fig. (c)) FIGURE 2 (200) pole figure of the sample obtained from the fraction No. 4 in horizontal magnetic field of 2T.(H) direction of magnetic field, (g) direction of gravitation.

FIGURE 3
FIGURE 3 Dependence of the X-ray diffraction line intensities on grain alignment in the magnetic field: (a) Hllg, the 0010 line of 2223-phase; (b) H_l_g, the 200 line of 2223-and 2212-phases.

TABLE Degree
*J(