Growth rate effects on thin Bi2Sr2CaCu2O8+δ textured rods

Ž .Physica C 302 1998 39–50

Growth rate effects on thin Bi Sr CaCu O textured rods2 2 2 8qd

L.A. Angurel ), J.C. Dıez, E. Martınez, J.I. Pena, G.F. de la Fuente, R. Navarro´ ´ ˜Instituto de Ciencia de Materiales de Aragon, CSIC-UniÕersidad de Zaragoza, Departamento de Ciencia y Tecnologıa de Materiales y´ ´

Fluidos, Maria de Luna 3, 50015 Zaragoza, Spain

Received 20 December 1997; revised 28 February 1998; accepted 26 March 1998

Abstract

Cylindrical polycrystalline textured Bi-2212 samples of lengths up to 10 cm and 1 mm of diameter have been grownŽ . Ž .using a Laser Float Zone LFZ melting technique. In order to improve their transport critical current density J at 77 K,c

the influence of growth rate on their microstructure has been analysed. The final transport properties of the texturedmaterials are determined by the balance between better grain alignment and larger grain size, that takes place at the lowestgrowth rates, and the simultaneous appearance of cracks. The potential of LFZ grown BSCCO rods for the development ofcurrent leads at low fields is suggested. Optimum transport properties correspond to thin rods obtained using intermediate

Ž . 2growth rates of 15 to 30 mmrh, with J 77 K values up to 5500 Arcm in the self field. These also exhibit minimumc

resistivity values above T and irreversibility lines shifted towards higher temperatures and fields. q 1998 Elsevier Sciencec

B.V. All rights reserved.

PACS: 7460J; 7470M

Keywords: BSCCO; Superconductivity; Laser processing

1. Introduction

Intergranular anisotropy and short coherencelength are important limitations to improvements of

Ž .the critical current density J values in High Tem-cŽ .perature Superconductors HTS in useful polycrys-

talline form. To avoid these restrictions on HTS bulkmaterials, improvements of the intergranular linkstrength and appropriate relative orientation of thegrains are induced by texturing methods. Float zonemelting techniques have successfully been used toperform directional solidification on bulk HTS

w xmonoliths 1 . In these processes, a small molten

) Corresponding author.

volume moves through the sample with a controlledŽ .speed R growth speed in such a way that the

composition is self adjusted while the temperaturegradient G at the solidification interface determinesthe grain growth direction, while the final mi-

w xcrostructure is determined by the ratio GrR 2 .The use of a laser beam, in the so called Laser

Ž .Floating Zone LFZ method, reduces the volume ofŽ .the molten zone and increases G )1000 Krmm ,

w xallowing an increase in growth rates 3 . In particu-lar, it has been applied to texture systems withincongruent melting, such as Bi–Sr–Ca–Cu–OŽ . w xBSCCO based HTS materials 3–7 . Several pa-rameters, as the growth and feeding rates, the laserpower and wavelength, the thermal fluctuations orthe precursor stoichiometry, can influence the final

0921-4534r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved.Ž .PII S0921-4534 98 00188-9

( )L.A. Angurel et al.rPhysica C 302 1998 39–5040

microstructure, and hence the properties of thesew xHTS products 7,8 . Presently, there is enough exper-

imental knowledge to produce bulk BSCCO materi-als of technological interest. Most previous LFZstudies have used CO laser beams to produce tex-2

tured cylindrical monoliths with diameters below 1w xmm 4,5,9 . Important challenges of these studies

were to determine conditions for single crystalŽ .Bi Sr CaCu O Bi-2212 growth, concluding2 2 2 8qd

that, in as-grown samples, the superconducting phaseŽonly develops for thinner diameters in the order of

.100–250 mm and low growth rates. These corre-spond to when the system is close to equilibriumconditions. When R is increased, growth is domi-nated by constitutional supercooling. Under suchconditions, the first solid phases formed during solid-ification are Sr–Ca–Cu oxides, while the melt be-comes simultaneously enriched in Bi O .2 3

Within the BSCCO family of superconductors,Ž .the Bi Sr Ca Cu O Bi-2223 phase would be2 2 2 3 10qd

desirable for applications at 77 K, because of itsw xhigher critical temperature 10,11 . Although early

w xresults were promising 12 , this stoichiometry hasbeen recently suggested as inappropriate for LFZ

w xtexturing 13 . Due to slow phase formation kinetics,2223 is formed only after prolonged annealing times.Furthermore, it forms without any acceptable degreeof texture, via liquid phase sintering in regions be-tween Bi-2212 grains, where Sr–Ca–Cu oxides arelocated just after solidification. The transport proper-ties of these samples consequently correspond to thetextured Bi-2212 grains present and, since they onlymake up a fraction of the total rod cross section, themeasured J values appear lower than in the corre-c

w xsponding pure Bi-2212 monoliths 13 . The latterforms with fast rates of reaction between 2201 andSr–Ca–Cu oxides, thus implying lower annealingtimes to develop the 2212 phase, as well as reducedamounts of secondary phases. For the above reasons2212 is the simplest BSCCO superconducting phaseto be developed via LFZ processing for applicationswhere high J values are needed for use below 77c

K, on HTS systems cooled with helium vapour orcryocoolers.

As part of the above development studies carriedout in our laboratory, the present work is focused onthe influence of the growth rate on the electric andmagnetic properties of textured Bi–2212 thin bars.

Previous studies have suggested these as potentialw xcandidates for current leads 14 . The LFZ processed

2212 thin rods presented here have diameters of 1mm and lengths of 100 mm. The resulting mi-crostructures have been investigated in order to un-derstand the differences and evolution of their physi-cal properties with the growth rate as well as withthe post growth annealing process. An importantissue associated with the measurement of the trans-port properties at high currents, as well as to thefeasibility of the current leads, is the need of lowresistance contacts between the BSCCO bars and themetal current feeding wires. This problem has been

w xsolved 15 , reaching contact values of few mV,enabling the study of the electric transport propertiesat 77 K. Furthermore, this characterisation has beencompleted with magnetic susceptibility studies per-formed in a wide range of fields and temperatures.

2. LFZ fabrication and experimental characteri-sation

The fabrication procedures used here have beenselected on the grounds of reproducibility and easeof industrial scale-up. Precursor cylinders with diam-eters of 2 mm, lengths of 100 mm and densities up to85% of the theoretical, were fabricated from Bi-2212

w xpowders prepared with a polymer matrix method 16Ž .and compacted using a cold isostatic press CIP .

The layered structure of Bi-2212 grains, with adja-cent Bi–O sublayers weakly bonded by van derWaal’s forces, is proposed as the reason why highdensity cylinders are obtained by CIP of appropriateBSCCO powders. These precursor cylinders weredirectly melted in the growth chamber by the inci-

Ž .dence of a Nd:YAG laser ls1.06 mm beam splitinto three, equally powered branches and spatiallydistributed around the precursor rod to ensure asymmetric heat distribution. From the power at theoutput of the laser and considering the losses in thesplitting and focusing mirrors, it has been calculatedthat the required power to melt these samples isbelow 10 W. This complete fabrication procedure is

w xunder patent 17 .An initial process of melting of the precursors in a

synthetic air atmosphere at fast growth rates of 50mmrh was performed in order to reduce the diame-ter and to increase the density of the precursors.

( )L.A. Angurel et al.rPhysica C 302 1998 39–50 41

Secondly, a complete LFZ texturing process wasperformed in the same atmosphere at five differentgrowth rates: 5, 15, 30, 50 and 70 mmrh. Hereafter,these samples will be distinguished by their growthrate value. In order to homogenise the molten vol-

Ž .ume, counter rotations 24 rpm between seed andprecursors were applied in both processes simultane-

w xously while pulling 7 . After these two steps, thetextured materials had a thin rod like appearance,diameters of 1.0"0.05 mm, lengths up to 100 mmand densities of 6.0"0.1 grcm3. In the range ofgrowth speeds and with these range of diameters, thesamples do not present any faceting phenomenon.As-grown samples, although textured, do not showsuperconducting behaviour at liquid nitrogen, possi-bly due to inadequate oxygen content and inappro-priate grain stoichiometry. These need further an-nealing to fully develop the 2212 phase. For thepurpose of monitoring the annealing process at itsdifferent stages, the macroscopic properties and theevolution of the microstructure have been studied in2.5 cm long rods.

Silver contacts were painted onto the samplesand, in order to decrease the electrical resistancebetween the metallic wires and the textured Bi-2212material, annealing processes were performed fol-

w xlowing different time and temperature schedules 15 .Current feeding contacts at least 5 mm in lengthwere placed at the extremes of each rod, while twoshorter length voltage taps, spaced 10 mm apart,were placed in between them.

To analyse the phase evolution of the thin rodsgrown at various rates and their influence on J ,c

isothermal annealing experiments were performed inair at 8458C for different time lengths in a tubularfurnace where the temperature was kept to withinone degree along the central 15 cm length. In addi-tion, other thermal treatments at different tempera-tures have also been performed. In particular, theannealing temperature has specifically been in-creased up to 8558C. Consequently, after each suc-cessive annealing step, characterisation measure-ments have been performed on each sample studiedhere.

Electric transport measurements have also beenperformed on each sample at different temperatures,using the four probe technique with pulsed and DCcurrents in a laboratory built apparatus. Above the

critical temperature, measurements of the evolutionŽ Ž ..of the resistivity with temperature r T were per-Ž .formed at low DC current values 25 mA . Below

Ž .T , voltage vs. current characteristics V–I werec

performed at liquid nitrogen. In order to support thehigh currents involved in these type of experimentsŽ .up to 60 A , metallic end capsules were soldered tothe silver contacts with commercial tin alloy. Thisminimises the contact resistance, improving heattransfer between the contact and the coolant. More-over, local heating was reduced using a pulsed cur-rent source which makes a sinusoidal rip up to itsmaximum value within 1 s. Values of J werec

derived from these V–I curves using the 1 mVrcmcriterium. After each transport measurement, andbefore the following annealing step, the remainingtin was mechanically removed and new contactspainted onto the sample. Although this process dete-riorates the surface of the sample and, in conse-quence, the quality of the contact, it has to beperformed in order to avoid the formation of aAg–Sn low temperature eutectic which destroys thesuperconductor rod during annealing.

Ž . Ž .Irreversibility lines T H or H T were de-irr irr

rived from magnetic AC susceptibility measurementsŽ Ž ..x T obtained with a Quantum Design SQUIDac

magnetometer under parallel DC fields up to 5 T forboth orientations, parallel and perpendicular to therod growth axis. Frequencies of 120 Hz and excita-tions m h of 10y5 T were used for these experi-0 0

ments. The temperature position of the x out-of-acYŽ .phase component maximum, x T , for each polaris-

ing field H yields a point on this line.Microstructural studies have been performed us-

ing a JEOL 2400 SEM microscope provided withEDS microanalysis, using polished longitudinal andtransversal sections of as-grown and annealed sam-

Ž .ples 12 and 48 h . Due to the lack of imagingcontrast in the annealed samples, after an initial EDSmicroanalysis, the polished section has been chemi-cally etched for 30 s at 08C with one part of 60%perchloric acid mixed with 99 parts of 2-butoxy-

w xethanol 18 .

3. Microstructure analyses

As-grown thin rods are polycrystalline, with themicrostructure shown in Figs. 1 and 2 being repre-


Fig. 1. Backscattered electron SEM images of polished transversalcross-sections of as-grown Bi-2212 thin rods for three different

Ž . Ž . Ž .growth rates: a F5, b F30 and c F70. The enlarged micro-graph corresponds to the centre of the rods.

sentative. The backscattered electron SEM micro-graphs of the transversal and longitudinal sections ofthe rods reveal the presence of three main phases, inaddition to some CaO grains that have been detected

in the samples. As it is expected in the LFZ process,this microstructure indicates that the system is farfrom equilibrium conditions in the range of growthspeeds that have been analysed in this work. Theratios of the Bi:Sr:Ca:Cu cations in these majorityphases, determined with EDS microanalysis, are pre-sented in Table 1. Since these ratios have beendetermined without carrying out an optimum calibra-tion procedure, i.e., using samples of similar compo-sition, only approximate comparative analyses can beperformed without extracting exact information onthe particular cation stoichiometries. The primarysolidification phase is the Bi-free Ca Sr CuO1yx x 2Ž .black contrast in the micrographs , independently ofthe growth rate used. This phase appears to bealigned with the rod axis, as expected from the flatsolidification interface observed with Nd:YAG laser

Fig. 2. Backscattered electron SEM images of polished longitudi-nal cross-sections of as-grown Bi-2212 thin rods for two different

Ž . Ž .growth rates: a F15, b F50.


Table 1Ratios of the Bi:Sr:Ca:Cu cations deduced from EDS elemental microanalysis of the three dominant phases observed in the Bi-2212as-grown and annealed thin rods during 48 h at 8458C

Samples F5 F15 F30 F50 F70

As-grown Bi:Sr:Ca:Cu Bi:Sr:Ca:Cu Bi:Sr:Ca:Cu Bi:Sr:Ca:Cu Bi:Sr:Ca:CuDark grey 32:21:13:33 38:22:11:30 38:20:12:30 35:16:15:34 39:15:15:31Light grey 42:20:12:26 44:25:6:26 47:22:7:24 47:22:8:22 47:22:7:24Black 0:15:30:53 0:18:27:54 0:23:22:54 0:23:20:54 0:21:24:53Annealed 48 h Bi:Sr:Ca:Cu Bi:Sr:Ca:Cu Bi:Sr:Ca:Cu Bi:Sr:Ca:Cu Bi:Sr:Ca:CuGrey 32:21:14:33 36:19:14:31 35:21:13:31 36:20:13:31 34:20:13:31Black 0:16:32:52 0:19:26:53 0:22:23:54 0:23:21:53 0:23:22:54Black 0:15:17:66 0:17:18:63 0:17:15:66Black 0:6:59:34 0:6:58:35 0:6:60:33 0:5:60:33

In the as-grown samples, bold characters have been used to indicate the major phase.

w xheating 8,19 . However, although the amount anddistribution of this phase depends largely on thegrowth rate, in no case is it the most abundant

Žmajority phase. In lower growth rate samples F5.and F15 , where growth approaches equilibrium con-

ditions, the overall amount of Ca Sr CuO is1yx x 2

lower than in the rest of the samples, and it tends tobe concentrated almost exclusively in the outer part

Ž .of the rod Fig. 1a . In higher growth rate samples,this phase appears radially distributed in a morehomogeneous fashion.

Grains which appear with two different grey con-trasts correspond to Bi containing phases, as ob-served in Table 1. These agree with values found on

w xsimilar materials reported in the literature 5,9 . Pre-w xvious TEM studies on as-grown Bi-2223 rods 20

have concluded that these grains are intergrowths ofBi-2212 and Bi-2201 phases. The composition of thedarker grey phase is very similar to Bi-2212, as it isthe phase that appears after performing the anneal-ing. A maximum amount of this phase is thus de-sired, although, as may be deduced from Table 1, amajority of this phase is only present in as-grownsamples obtained at low rates. The lighter grey con-trast regions correspond to a majority of Bi-2201.This phase’s higher abundance at faster growth ratesis promoted by faster solidification kinetics from theBi-enriched liquid, a process which follows initial

Ž .nucleation of SrCa CuO . Moreover, from the trends2

observed in Figs. 1 and 2, it can be deduced that thegrain growth of one of these Bi rich phases follows

Ž .the alignment of the SrCa CuO , except for very2

Ž .high growth rates )50 mmrh , while the otherphase fills the holes left between them.

The microstructural differences observed betweenŽ .the rod grown at the lowest rate sample F5 and

those grown at higher rates are assigned to the aboveŽmentioned kinetics. In the former case, grains dark

.grey with the Bi-2212 composition grow directlyform the melt, while some small traces of the Bi-2201are observed between these grains. These grainsgrow with their c-axis in the plane perpendicular tothe rod axis with a misorientation angle lower than

w x108 21 . The opposite happens in all of the othersamples, where the grains contain mainly Bi-2201and the holes are filled with Bi-2212. No degree oftexture is observable in the latter. These differencesbetween the F5 rod and the rest of as grown samplesis also reflected in the x measurements, whereac

sample F5 is the only one that shows diamagnetismat low temperatures, before thermal annealing.

Upon annealing during 48 h, as may be observedin Fig. 3a, and in the EDS data of Table 1, most ofthe non-superconducting phases have disappeared.This trend is also observed after the initial annealingof 12 h. The different grey contrasts seen herecorrespond to dissimilar grain orientations, as con-firmed by EDS microanalysis performed before at-tacking the surface, which yields the same composi-tion for all Bi-rich phases. The stoichiometry of thissuperconducting phase coincides with the light greyphase observed in the as-grown samples, assigned toBi-2212. The remaining non-superconducting phasesare concentrated in the periphery of the thin rods.


Fig. 3. Secondary electron SEM micrographs of a longitudinalŽ .cross-section of sample F50 after a 48 h anneal a and detail of

Ž .the microstructure b .

Some of these grains maintain the as-grownCa Sr CuO proportion, although three additional1yx x 2

Ž .compositions have also been observed: Ca Sr1yx x 14Ž .Cu O with more than 65% of Cu in Table 1 ,24 41

Ž . ŽCa Sr CuO non-superconducting phase with1yx x 2 3.33% of Cu , and finally CaO, as in the as-grown

samples.It is well known that the growth speed has a great

w xinfluence on the size and orientation of the grains 5 .As was previously mentioned, the Ca Sr CuO1yx x 2

grains are very well aligned along the growth direc-tion of the rod. The Bi containing phases follow the

Ž .same habits for moderate rates as well see Fig. 2a ,but this is not the case when the value of R increasesŽ .see Fig. 2b . In the latter situation, the low degreeof texture is observable as the average misorientation

w xof the grains increases 21 . The adjacent plateletgrains, which stack one upon another with the a and

b directions tilted, keep their c-axis perfectly alignedforming colonies. As the growth rate is lowered, thegrains become larger, although it should be notedthat, as the rod edge is approached, their size alwaysdecreases. The average sizes of these colonies in thecentre of the different rods studied here are given inTable 2. Each individual grain has a thickness of0.2–0.3 mm, as observed in the micrographs of theannealed samples. Observations indicate that laserbeam focusing modifications affect the grain sizesfor fixed R values, although the relative trendsremain as discussed above.

For rods grown at low rates, annealing at tempera-tures below melting induces additional growth of thelarger Bi-2212 grains. Their initial orientation ismaintained resembling that of Bi-2201 grains, whiletheir stoichiometry is adjusted by reaction withCa Sr CuO . In this manner the most stable 22121yx x 2

phase becomes the major component within thissystem. At faster growth rates a similar processoccurs and the annealed rods exhibit improved tex-

w xture 21 , with the almost complete disappearance ofthe disordered 2212 filled holes.

Sample F5, and to a lesser degree F15, exhibitŽ .important cracks between grains Fig. 4 . In sample

F5, the size of these cracks is of the order of the sizeof the sample and even small grains may be cleavedwhen the rod is cut. These cracks are very importantbecause they reduce the intergranular critical currentdensity and deteriorate the mechanical resistance ofthe rods. These cracks could have been developedduring growth, and probably as a consequence of theextremely large thermal gradients present at the so-lidification interface, combined with the anisotropic

Table 2Evolution of the grain size within Bi-2212 as-grown thin rodswith the growth rate, as deduced from backscattered electron SEMimages obtained on polished rod cross-sections

3Ž .Sample Size a= b=c mm

F5 2000=200=25Ž . Ž .F15 100–200 =15= 1–10Ž . Ž .F30 100–150 =15= 1–8

Ž .F50 80=15= 1–8Ž .F70 20=15= 1–3


Fig. 4. SEM micrograph of a polished transversal section of sample F5 showing some examples of the cracks that are induced in thissample.

thermal expansion and growth kinetics characteristicof these superconducting phases.

4. Electric transport measurements

4.1. Metal–superconducting contacts

The attainment of high currents during transportmeasurements may be hindered when the electricalcontacts between the HTS and the feeding wires arenot of sufficient quality. This is most important intextured BSCCO due to its low thermal conductivity,which is one order of magnitude lower than forclassical metallic superconductors, and its higherheat capacity, which is two orders of magnitude

w xhigher at 77 K 22 . In particular, for the Bi-2212textured materials, this is the reason for the scarceamount of transport I results available in the litera-c

w xture 7 . Initially, the contacts were annealed afterpainting during 3–4 h at temperatures between 400–5008C, reaching contact resistances of about 1 mV.With this value, the maximum current was limited toabout 15 A. This is because at higher current valuesthe BSCCO samples, immersed in liquid nitrogen,melted in the proximity of the contacts due to theirlow thermal conductivity. This process has been

w ximproved 15 to obtain reproducible, low resistancecontacts below 1 mV at 77 K on Bi-2212 texturedsamples, allowing the flow of currents as high as 400

A without degradation of the critical current. Theelectrical contact is established through the lateralsurface of the textured BSCCO rods, in such a waythat the limiting value of the contact resistance for agiven sample, about 0.3 mV at 77 K, is defined by

w xits geometry 15 .

4.2. Critical current behaÕiour

Ž .The influence of the growth rate on I 77 K inc

the self field is shown in the semi logarithmic scaleof Fig. 5. However, these determinations are per-formed at temperatures very close to T and arec

Fig. 5. Evolution of the self field critical current, I at 77 K,c

during annealing at 8458C for samples grown at different rates.The straight lines are eye guides to differentiate two regimes; alogarithmic dependence and saturation.


extremely sensitive to their value. An annealing timeof 12 h has been considered necessary in order toobtain a major content of the Bi-2212 phase in allthin rods. Comparisons between rods grown at dif-ferent rates are thus meaningful if this minimumannealing procedure is performed. The maximumcritical current reached 48 A for an intermediate rate

Ž .Rs15 mmrh, which corresponds to J 77 K val-c

ues up to 5500 Arcm2. The optimum growth ratevalues depend on the other growth conditions, inparticular on those that influence the thermal gradi-ents in the melt, i.e., the laser focusing system and

w xthe laser wavelength 8 . However, in all the cases itis possible to reach a compromise between the posi-tive factors associated to low growth rates and thenegative ones due to the production and propagationof cracks, which are common in ceramics with largegrains. As a consequence, the maximum J is ob-c

tained on thin rods grown at intermediate R values.The effects of different annealing times on the Ic

values can also be observed in Fig. 5. For lowerannealing times, I increases following a logarithmicc

Ž .dependence continuous straight lines before a satu-ration value is reached, in such way that higher Ic

values correlate with faster attainment of saturation.Most of the non-superconducting phases have disap-

Žpeared in the initial stages of the annealing first 12.h and it is expected that further annealing would

complete the grain growth process and stoichiometryadjustment of the Bi-2212 phase until equilibriumsaturation values are reached, as observed in other

w xsuperconducting materials 23 . This saturation valueof I does not depend strongly on the sinteringc

temperature, as has been observed on samples an-nealed between 840 and 8558C. It should be notedthat, although during V–I experiments the sample isimmersed in liquid nitrogen, the I values recordedc

on different days for the same sample and withoutdeterioration may have a random dispersion of 10%.This may be understood on the basis of liquid nitro-gen temperature changes related to the atmosphericpressure between measurements, combined with thegreat influence of temperature on I . A temperaturec

decrease of 1 K may increase I by more than 20%c

near 77 K. At 65 K I increases by a factor of threecw x14,21 .

The temperature dependence of the resistivity andthe corresponding derivative on two samples grown

at different rates are displayed in Fig. 6, togetherwith their evolution with the annealing time. As in

w x Ž .other superconducting materials 24 , r T at highŽ .temperatures T)150 K may be fitted to a linear

dependence

r T sr 0 qAT . 1Ž . Ž . Ž .Ž . Ž .The fitting parameters r 0 and A rrT areTs300 K

very sensitive to both, growth rates and annealingtimes. They correlate with the superconducting prop-

Ž .erties in such a way that lower r T results yieldŽ .higher I 77 K values. For a fixed growth rate, ac

longer annealing improves the metallic behaviour,Ž .i.e., rrT decreases and extends the range of con-

Ž .stant values, but also shifts the rrT maxima to-Ž .wards higher temperatures ameliorate T and re-c

duces their width, reflecting in a more homogeneoussuperconducting material. The lower resistivity valuefound for a given annealing procedure also corre-

Ž .sponds to optimum I 77 K growth rates. ValuescŽ . Ž .down to r 300 K 14 mV m, r 0 1.7 mV m and

Ž . y1rrT 0.04 mV m K have been measuredTs300 K

which are roughly one order of magnitude higherthan analogue results on the a–b plane of Bi-2212

w xsingle crystals 25 . These great differences point outthat the intergranular electric contacts determine the

Ž .actual resistivity values. Certainly, the above r Timprovements have often been observed on HTSmaterials, and have been associated with a moreuniform oxygen distribution. The observed mi-

Ž .Fig. 6. Temperature dependence of the resistivity r T and of theresistivity temperature derivative, rrT , for two different growth

Ž .rates F30 and F70 after different annealing times at 8458C.


Fig. 7. E – I characteristics at 77 K, in the self field, for samplesgrown at different rates after a common annealing of 24 h at

Ž .8458C. The lines correspond to the best fit using Eq. 2 with theparameters collected in Table 3.

crostructure evolution with annealing, however, couldalso be assigned to stoichiometry changes within theBi rich phases. In both cases, the evolution towardsequilibrium would produce the observed saturationeffects.

The E–I characteristics at 77 K in the self field,for different samples after a heat treatment of 24 hare showed in the double logarithmic plot of Fig. 7.It has been found that the V–I characteristics of agiven sample, after different annealings, scale if thecurrent is presented in units of I . This fact indicatesc

that the pinning mechanisms are not modified in theannealing process, they just become more effectivebecause the pinning barriers become higher. Theshape of these curves has been understood consider-ing that the I limiting mechanism is the movementc

of the flux lines in the self-field generated by thew xtransport current 26 , i.e., a flux flow regime. As-

Fig. 8. Magnetic field dependence of the current transport densityJ at 77 K, in fields perpendicular to the current flow and onc

samples grown at different rates, all after annealing for 12 h at8458C.

suming that the dominant pinning mechanism, athigh temperatures and low currents, correspond tothe pinning in a network of low angle tilt grainboundaries with planar contacts between platelet-like

w xgrains M. Mora, private communication , thesecurves can be fitted to the following expression:

21r2 3r2 3r2Esb I I ya 2Ž . Ž .where a can be considered as the real critical cur-rent, i.e., the current at which dissipation starts. Theparameters a and b used to fit these curves arepresented in Table 3.

The influence of an external perpendicular mag-netic field in the transport J value at 77 K has alsoc

been analysed on the samples F5, F15, F30 and F50,and the results are presented in the Fig. 8. Measure-ments have been limited to samples annealed 12 hbecause the maximum current attainable in the ex-perimental set-up only may surpass the values ob-

Table 3Ž . w Ž 2 .x w xParameters used in the best fit to Eq. 2 of the E–I characteristics at 77 K in the self field; b mVr cm A and a A , on Bi-2212 rods

grown at different rates and annealed during 24 h at 8458C

Parameters F5 F15 F30 F50 F70y4 y5 y5 y4 y3b 2.8=10 3.0=10 7.8=10 3.3=10 4.1=10

a 14.8 29.0 20.4 10.3 11.4Ž .I 0 25 48 40 28c

H 57 170 124 1110

Ž . w x w xThe fitting parameters I 0 A and H Oe have been derived from the exponential field dependence of the critical current at 77 K, Eq.c 0Ž .3 , on the same materials but after 12 h of annealing.


tained in materials with these initial annealing stages.The field was applied perpendicular to the sampleaxis. An initial drop of the I value has been foundc

in all cases, corresponding to an exponential fielddependence of the form

I H s I 0 exp yHrH 3Ž . Ž . Ž . Ž .c c 0

Ž .with the I 0 and H values summarised in Table 3.c 0Ž .There is an important correlation between I 0 andc

H , showing that the samples with a better behaviour0

under applied magnetic fields correspond to the oneswith higher critical current densities. In the range offields used in the experiments, this field dependenceshows a near negligible hysteresis. Only at very lowfields are these effects important, reducing the criti-cal current less than 10%.

This exponential dependence, which is in agree-ment with the inductive current results deduced frommagnetisation measurements on similar materials and

w xtemperatures 21 , does not change with annealing,Ž .but instead the values of I 0 and H increase up toc 0

saturation. Consequently, it may be considered ascharacteristic of these textured materials at liquidnitrogen temperatures.

5. Irreversibility line

The irreversibility line, which has been deter-YŽ .mined from the position of the x T maximum

recorded at different DC fields, marks the opera-Ž .tional limits of the rods. The evolution of H Tirr

through the fabrication process will be correlatedwith the changes of J already described. An exam-c

ple of its variation may be seen in the semi-logarith-mic scale of Fig. 9, where the values for samples F5and F70 for different annealing times have beenrepresented. For a given growth rate, longer thermaltreatments shift the irreversibility line towards highertemperatures and fields, but reach saturation afterprolonged times. During the first annealing steep of12 h, values of T above 90 K are reached, whilec

longer thermal annealing processes mainly extendthe field range. Moreover, for comparisons sake,

w xresults on a single crystal 27 have also been in-w xcluded. In agreement with previous studies 28 the

rods exhibit more effective pinning barriers than

Ž .Fig. 9. Evolution of the irreversibility lines B T on perpendicu-irr

lar fields for the F5 and F70 rods with two different annealingŽ .times at 8458C lines are eye guides . Results on a Bi-2212 single

w xcrystal with fields parallel to the c-axis, taken from 27 , havebeen depicted for comparison.

single crystals, although differences are observedbetween them. Optimum results are also found forintermediate growth rates, and only small differencesare observed at high rates. In the best case Birr

reaches values of 0.04 T at liquid nitrogen, becom-ing very sensitive to the temperature in this regionnear T . This is exemplified by its increase of onec

order of magnitude at 60 K. A parallel evolution ofŽ .the J 77 K values in the self field, a less sensitivec

field dependence and an enlargement of the irre-versibility region indicate the improvement of inter-granular coupling, including more efficient flux pin-

w xning centres 28 .The field dependence of J , which follows anc

exponential law at high temperatures, as has beenpreviously pointed out, follows a potential law at lowtemperatures, and also under perpendicular fieldsw x Ž .29 . Along the H T line there is thus a crossoverirr

Žbehaviour taking place at about 60 K depending on.the sample to a low temperature regime which is

less sensitive to the field.

6. Conclusions

The effect of growth rate on the LFZ processingof Bi-2212 textured polycrystalline superconductorshas been studied. The production of 10 cm long,1.0"0.05 mm diameter thin rods with critical tem-


peratures above 90 K and critical self field currentŽ .I 77 K values up to 45 A has been demonstrated.c

Analyses of their microstructure, electric transportproperties above and below T and their irreversibil-c

ity line, has been used to define optimum processingparameters. For the present geometry, the highest

Ž . y2self field J 77 K value obtained of 5500 A cmc

corresponds to intermediate growth rates. These cor-respond to a good compromise between the excellenttexture and large grain size observed at lower growthrates, with the minimum appearance of cracks athigher rates.

Ž .The finding of an optimum I 77 K for a givenc

growth rate and its increase with the annealing treat-ment up to saturation values are also correlated withlower resistivity above T and the shift of the irre-c

versibility line towards higher temperatures andfields. In as-grown and shortly annealed samples, the

Ž .performances and initial improvements on I 77 KcŽ .and r T are associated to the actual value and

enhancements of T . This is determined by the pres-c

ence and full development of the Bi-2212 phase atthe expense of other non superconducting phases.Further annealing develops stronger conductive links

Ž .between grains, decreasing r T and simultaneouslyincreasing the grain boundary critical current at 77 Kwithout changing T .c

The above results encourage the use of the LFZprocessing technique for the fabrication of texturedBi-2212 polycrystalline materials suitable for thetechnological development of current leads operating

Ž .below 77 K. Moreover, from the observed H Tirr

lines and considering the flux flow origin of thepower losses, I values may be improved by a factorc

of three to five times just by lowering the tempera-ture down to 65 K.

Acknowledgements

The authors are indebted to the Spanish MinistryŽof Education and Science CICYT, MAT 95-0921-

.C02-01 and 02, MAT 96-1057-C02-01 , the pro-Žgrama de Acciones Integradas Accion hispano–´

.alemana 1996, n8192A , the MIDAS program and´ ˜ ŽRED ELECTRICA DE ESPANA Project REE-ID-

.97017 for financial support of this research. E.Martınez thanks the Spanish Ministerio de Educacion´ ´

y Ciencia for her scholarship. Technical support byCarlos Estepa is also acknowledged.

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