Improvement of the magnetic shielding and trapping properties of BiPbSrCaCuO superconducting tubes by the use of multiple thermomechanical processing

PhysicaC225 (1994) 361-368

ELSEVIER

Improvement of the magnetic shielding and trapping properties of BiPbSrCaCuO superconducting tubes by the use of multiple thermomechanical processing V. P l e c h d ~ e k *, E. P o l l e r t , J. H e j t m d n e k , D . S e d m i d u b s l o ) , K . K n i ~ e k Institute of Physics, Academy of Sciences of Czech Republic, Cukrovarnickd 10, 162 O0Prague 6, Czech Republic

Received 22 February 1994

Abstract A series of (Bio.9Pbo.i)2Sr2Ca2Cu3Ozo+xsuperconducting tubular shields was prepared by repeated thermomechanical processing, cold isostatic pressing combined with a thermal treatment. The applied procedure provided the desirably textured material and enhanced the apparent pinning potential U~o.Thus the shielded (B,~) and trapped (Btr) magnetic-fiux densities of more than 20 mT at the temperature of 77.3 K could be routinely reached in superconducting tubes with a wall thickness of about 2 ram. Based on the measurement of the time decay of Bt~ in a medium region of the magnetic field B, the U~o~ B-°'59 dependence, close to the expected U~ ~ B-o.5 one, confirming the two-dimensional character of the BiPbSrCaCuO superconductor, was experimentally found.

1. Introduction

Superconducting magnetic shields for neuromagnetic measurements in biology and medicine, geomagnetic measurements, as well as a number of uses as e.g. associated with the requirements of microelectronic and computer circuit components, and for Johnson noise thermometry, belong to the most important and promising applications of high-temperature superconductors [ 1-4 ]. In this context increasing attention is paid to the B i ( P b ) S r C a C u O s u p e r c o n d u c t o r [ 3 - 1 0 ] , which in contrast to YBaCuO superconducting material [ 2,4,11-15 ] exhibits some advantages; above all a higher T¢, resistance against corrosion and an appreciable tendency to texturing. The magnetic shielding and trapping properties * Correspondingauthor.

depend significantly on the critical current density Jc and its improvement leads to an improvement of these properties, too. One way to increase the critical current density already successfully applied on Bi based bulks and silver-sheathed tapes is repeated thermomechanical processing [8,9 ]. Furthermore, recently it has been shown that in repeatedly processed silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Olo+x tapes the critical current is controlled by flux pinning in the region of the liquid-nitrogen temperature while the weak links limit the critical current at low temperatures [ 16,17 ]. Therefore attention was now devoted to the possibility to influence the flux pinning and limit flux creep in the prepared BiPbSrCaCuO superconducting tubular shields. The main goal was to study the influence of the multiple thermomechanical processing on the apparent pinning potential and consequently the magnetic shielding and trapping properties.

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2. Experimental The studied samples were prepared as follows: ( 1 ) The prereacted superconducting powder of the nominal composition (Bi0.gPb0.1)2Sr2Ca2Cu2Olo+x, containing predominantly the 2223 phase, see ref. [18 ], was isostaticly compacted, using a pressing mandrel, into the form of tubes, (2) the tubes were subjected to a multiple thermomechanical intermediate process consisting of repeated cold isostatic pressing and thermal treatment under lowered oxygen partial pressure [9], up to a m a x i m u m of six steps. With respect to the aim of the study particular attention was paid to the attempt to prepare a series of the BPSCCO superconducting tubes with dimensions allowing for their coaxial arrangement, see Table 1, samples No's. 23, 24, and 25. In this case, in order to ensure the same resulting properties of the tube material, the number of thermomechanical steps with equal conditions was chosen to be five. X-ray diffraction analysis employing Cu Kct radiation was used to reveal the purity of the 2223 samples and to estimate roughly the degree of the grains preferred orientation. The transport and magnetic properties were studied at 77.3 K. The dependence of the critical current density Jc on the magnetic field applied perpendicularly to the direction of the flowing current was measured using the criterion of 1 ~tV cm -1. The dependence of the internal magnetic-flux density Bin in the

center of the tube on the external magnetic-flux density Bex and the time decay of Btr were measured using a Hall probe. Bex was applied parallel to the tube axis; the sweep of cyclic magnetization was 1 m T s - 1. For the determination of the shielded magnetic flux density Be, the criterion of 0.1 m T was used. Using the Bean model of the critical state [ 19 ] and the B i o t - S a v a r t law, the following f o r m u l a was derived: Jcm =~0 -IBtr W-1 ( ( d + w)2+/2) 1/2/- 1 ,

( 1)

where ~.0 is the magnetic permeability of the vacuum and d, l and w are inner diameter, length and wall thickness, respectively. From this the overall critical current density J~m in a relatively thin wall of the superconducting tube can be estimated.

3. Results and discussion 3. I. Influence of the superconducting grains preferred orientation The X-ray analysis performed on samples cut from the prepared tubes gave evidence that the used technological route gives rise to a nearly single 2223 phase textured material with (ab) planes of the grains oriented along the tube axis. Thus e.g. the intensity ratio loolo/I115 measured perpendicularly to the tube axis on sample No. 3 was found to be approximately five

Table I Dimensions and properties of various arrangements of BiPbSrCaCuO superconducting tubes No's. 23, 24 and 25, together with the dimensions and properties of the other BiPbSrCaCuO superconductingtubes. The temperature was 77.3 K Arrangement No. of tube (s)

23 24 25 23+24 23 + 25 24+25 23+24+25 3 33

Dimensions (ram) Inner diameter

WaU thickness

Length

5.8 7.8 9.9 5.8 5.8 7.4 5.8 15.0 14.2

0.96 1.01 1.15 1.97 2.11 2.16 3.12 0.65 2.16

26 24 26 30 33

B,h (mT)

Bt~ (roT) (at a time of 300 s)

Jm (Acm - 2)

14.1 12.7 13.0 20.1 20.5 19.4 25.0 10.1 20.9

14.5 12.4 13.6 21.3 22.2 20.2 27.6 11.2 22.1

1240 1040 1020 1550 910

I4. Plech~ek et al. / Physica C 225 (1994) 361-368

364

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I4. Plech~ek et al. / Physica C 225 (1994) 361-368

measured for the various arrangements of superconducting tubes could be used to verify the validity of the U~ ~B-1/2 dependence in a medium region of magnetic-flux density (realized by the trapped magnetic-flux density). Let us note that for higher values of B the two-dimensional character of the Bi superconductor has been already experimentally confirmed, e.g. by the resistance measurement of BiSrCaCuO wires [25 ]. In Fig. 3 the results of the Btr time-relaxation measurements at 77.3 K up to a time of more than 106 s for seven various arrangements of three superconducting tubes are shown. In the first approximation the measured dependences could be well fitted by a logarithmic function and the respective values of Up at the time of 1000 s calculated. Their plot in Fig. 4 gives the fitted Bff °'59 dependence with the exponent close to the value of -0.5, expected for a two-dimensional superconductor. In this context let us note that the U~ ~ B - ~dependence is suggested for the threedimensional YBaCuO superconductor [ 26 ].

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3.3. Influence of multiple thermomechanical processing Recent investigations showed that Jc, B~hand Btr of the BiPbSrCaCuO tubes can be gradually improved by increasing the number of thermomechanieal steps. However, if a certain number of steps is exceeded a deterioration of the properties occurs, probably due to a microcracking [9 ]. Thus the present study was limited to six steps. The superconducting tube No. 33 six times gradually processed is an example of such a study. The multiple processing of the tube, with the dimensions after the last treatment step given in Table 1, leads to a gradual improvement of the shielding-trapping properties as it demonstrates the Bm-Bex dependences in Fig. 5. The time decay of Btr at T=77.3 K measured after each step is plotted in Fig. 6 and the values of Up determined for the time of 200 s are given in Fig. 7. As can be seen from Fig. 7, the apparent pinning potential does not fall with Btr, like in the previous case of the uniformly processed superconducting ma-

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I4. Plech~ek et al. / Physica C 225 (1994) 361-368

366

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V. Plech~ek et aL / Physica C 225 (1994) 361-368

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Trapped Magnetic Flux Density [mT] Fig. 7. Effectofthe stepnumbern on the magnetic-fluxdensitydependencesofthe apparentpinningpotentialoftubeNo. 33. terial, but in a contrast, starting from n = 2 it increases reaching the value of U~ = 0.42 eV for the last step, n = 6 (Bt,=22.3 mT). In the light of the U~ decrease with increasing Bt~ for the uniformly processed superconducting material it seems to be rightful to conclude that besides the improvement of the grain interconnections and preferred grain orientation [ 9 ], the multiple thermomcchanical process influences favorably the flux pinning and limits the flux creep in the range of higher temperatures about 77 K.

4. Conclusions The study carried out on BiPbSrCaCuO superconducting tubes has shown that the special isostatic pressing led to the formation of textured material with a high critical current density and consequently to enhanced magnetic-flux shielding and trapping properties. Besides a high degree of preferred orientation and an improvement of grain interconnections, multiple thermomechanical processing leads to a gradual im-

provement of the flux-pinning potential and in this way improves Jc, B,h, Btr and suppresses the thermally activated flux creep. The U~ ~ B-°'59 dependence, close to the expected U~ ~B -°'~, experimentally found in a medium region of B for the system of three uniformly processed coaxial tubes is typical for the two-dimensional superconductor and confirms the layered character of the Bi based superconductors even at the temperature of 77.3 K. In addition, let us note a fact important from the point of view of the applicability of the oxidic superconducting materials, i.e. that the superconducting tubular shields with wall thickness of about 2 mm and B,h and Btr higher than 20 mT at 77.3 K with very small long time degradation of magnetic shielding and trapping properties can be routinely produced.

Acknowledgements The authors would like to acknowledge the support of this work by the Grant Agency of Academy of Sciences of the Czech Republic - Grant No. 29079 as

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V. PlechMek et aL /Physica C 225 (1994) 361-368

well as b y the G r a n t A g e n c y o f the Czech R e p u b l i c G r a n t s No. 1 0 9 / 9 3 / 1 152 a n d No. 2 0 2 / 9 3 / 1 1 4 9 .

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