The influence of elevated magnetic fields on the texture formation of melt-processed Bi-2212

Transcription

1 Ž. Physia C The influene of elevated magneti fields on the texture formation of melt-proessed Bi-2212 E. Cehetti, P.J. Ferreira ), J.B. Vander Sande Department of Materials Siene and Engineering, Massahusetts Institute of Tehnology, Room , Cambridge, MA 02139, USA Reeived 12 Otober 1999; reeived in revised form 21 Marh 2000; aepted 30 Marh 2000 Abstrat Melt-proessing of BSCCO high-t superondutors Ž HTS. under an elevated magneti field is an effetive tehnique for produing superondutors with enhaned ritial urrent. This result is a onsequene of the high degree of rystallographi texture ahieved in the polyrystalline superondutor proessed under a high magneti field. Possible mehanisms for the orientation of Bi-2212 plate-like rystals under the influene of a magneti field are analyzed, in partiular, the orientation of Bi-2212 superondutor rystals during nuleation, rystal growth and grain growth. In order to understand the relevane of eah of these mehanisms, the effet of an applied magneti field during the different stages of the partial-melt proess is studied. Experimental results onfirm that most of the alignment is ahieved in the early stages of rystal growth. q 2000 Elsevier Siene B.V. All rights reserved. Keywords: Bi-2212; Magneti field; Texture 1. Introdution The use of high-t superondutor oxides Ž HTS. with high transport ritial urrent density Ž J. re- quires the presene of highly textured superondutor grains in order to minimize grain boundary weak links wx 1. One possible route to fabriate HTS polyrystalline materials with a preferred orientation of the superondutor rystals is to melt-proess samples under a high magneti field. In fat, if a HTS grain exhibits an anisotropi paramagneti suseptibility in its normal state, then when plaed in a magneti field, it minimizes its magneti energy by ) Corresponding author. Tel.: q ; fax: q address: ferreira@mit.edu Ž P.J. Ferreira.. aligning its axis of maximum suseptibility parallel to the magneti field. Among the different tehniques for produing superondutor grain alignment, the appliation of an elevated magneti field during the proessing of the HTS is appealing beause its influene on texture development ould be ombined with other tehwx 2 have employed a high niques. Noudem et al. magneti field in assoiation with a hot pressing tehnique to enhane texture in Bi-2223 samples. Despite some previous enouraging results w3 6 x, the prospet of using high magneti fields for extended time portions during superondutor fabriation remains unattrative for large-sale implementation beause of limited throughput. Therefore, an important issue for the use of high magneti fields in proessing HTS is to redue the time of exposure to the r00r$ - see front matter q 2000 Elsevier Siene B.V. All rights reserved. Ž. PII: S

2 ( ) E. Cehetti et al.rphysia C magneti field without ompromising the texture enhanement. In order to optimize the proessing of HTS under a magneti field, it is important to understand the mehanism by whih grain-alignment is ahieved. Magneti field indued grain alignment has been reported both during the solidifiation from the molten state w5 7x or during the annealing proess w8,9 x, suggesting that the alignment may our in two ways: Ž. 1 rotation of paramagneti superondutor rystals within a liquid phase, and Ž. 2 seletive grain growth where grains aligned with their -axis parallel to the magneti field preferentially grow at the expense of grains with their -axis oriented at an angle u with respet to the magneti field diretion. In a reent paper, Ferreira et al. w10x proposed a model to explain the degree of texture ahieved in HTS during melt-proessing under an elevated magneti field. The model suggests that the enhanement in texture is primarily obtained through grain rotation during the early stages of rystal growth from the liquid. During the nuleation of rystals from the melt, when the rystals ahieve a ritial volume and are still surrounded by a large liquid volume fration, the magneti driving fore is apable of introduing grain rotation. However, at the later stages of growth, the presene of nearby grains indues grain-to-grain interation whih hinders the magneti field indued alignment. If the results of this model are valid, a short time exposure to the magneti field during the initial stages of rystal growth ould be suffiient to enhane the texture of HTS materials. Consequently, the opportunity of employing a magneti field for proessing HTS ould be reonsidered for large-sale industrial appliations. In order to test the model above w10x and estimate the relevane of the various mehanism of alignment, several experiments were arried out in whih a high magneti field Ž 7 T. was applied during different stages of the Bi-2212rAg thik film meltproess. aording to the nominal omposition Bi 2.15Sr2.05- Ca 0.95Cu 2O x. The mixed powders were alinated at 7808C, 8008C and 8208C in air for 12, 12 and 70 h, respetively, with intermediate grinding in an agate mortar and pressed into pellets using an uniaxial and isostati press Ž 40,000 psi.. Subsequently, the samples were ground into fine partiles and the partiles were dispersed in ethanol. The solution was plaed in an ultrasoni bath for 5 min and subsequently poured onto silver ribbons of retangular shape Ž2 = 12 mm.. Following the evaporation of ethanol at room temperature, the samples were melt-proessed in a vertial furnae plaed inside a toroidal 52 mm bore superonduting magnet. The final thikness of the films proessed was 30"5 mm. The heat treatment seleted was designed to verify the possibility of enhaning the texture formation after a short time appliation of a magneti field and to test the relevane of rystal rotation versus seletive grain growth to align the superondutor grains. Hene, the heat treatment for the thik films onsisted of a partial melting step at 8808C, a first stage of ooling to 8608C at a rate of 18Crmin, a seond stage of ooling to 8408C at a rate of 0.18Crmin and an isothermal annealing at 8408C for 40 h Ž Fig. 1.. The temperature of 8608C was hosen beause it is below the solidus temperature w11x and onsequently rystal rotation is inhibited. In this fashion, we may determine whether the presene of a magneti field 2. Experimental proedure 2.1. Sample preparation Bi-2212rAg thik films were prepared with highly pure Ž 99%. Bi O, SrCO, CaCO and CuO weighed Fig. 1. Melt-proessing onditions of Bi-2212rAg thik films in the absene and presene of an elevated magneti field.

3 194 ( ) E. Cehetti et al.rphysia C ould enhane texture formation by the seletive grain growth mehanism. A magneti field of 7 T oriented parallel to the long axis of the furnae was applied during different stages of the heat treatment Ž Fig. 1.. The films plaed inside the furnae had their surfae orthogonal to the magneti field. The sweep rate for the ativationrdeativation of the magneti field was Trmin. Films labeled A were melt-proessed under an applied magneti field during both stages of ooling Ž C and C.. Films labeled B were melt-proessed under an applied magneti field during the first stage of ooling Ž C., so that the magneti field ould play a role during the initial stages of rystal growth. Films labeled C were melt-proessed under an applied magneti field during the slow ooling Ž C.. In this ase, the influene of the magneti field is expeted to be limited to the grain growth regime. Finally, films labeled D were melt-proessed in the absene of a magneti field. Table 1 and Fig. 1 summarize the use of the magneti field during the different stages of melt-proessing. For eah proess type Ž A D. three samples were generated; eah speimen was proessed independently Sample haraterization Ž. Surfae X-ray diffration XRD an be used to determine the degree of texture in a polyrystalline material. A quantitative measure of -axis orientation an be obtained from the XRD spetra by using Table 1 The use of a magneti field Ž 7 T. in the melt-proessing of Bi-2212 films Sample C C 8408C Ž 40 h. Ž 20 min. Ž 200 min. Crystal Seletive rotation grain growth Magneti Magneti Magneti field field field A on on off B on off off C off on off D off off off the Lotgering fator Ž F. w12 x, whih an be alulated as follows Fs Ž PyP0. rž 1yP 0., Ž 1. where PsSŽ00l. IŽ00l. rsžhkl. Ihkl is the sum of XRD peak height intensities for all Ž 00 l. refletions divided by the sum of all intensities Ž hkl. in the textured speimen, P0 is the XRD peak height intensity ratio of a randomly oriented speimen. The fator F varies from 0 Ž random. to 1 Žompletely- oriented.. The XRD work was performed in a Rigaku 300 X-Ray diffratometer, using CuKa X-ray radiation at 60 KV and 300 ma. Mirostrutural observations were performed in a JEOL 6320 FEGSEM using an aelerating voltage of 10 kev. Baksattered eletron images were used to produe ontrast from the different phases onsisting of different elements. The J was evaluated using a transport ur- rent tehnique with the four-probe DC method. A riterion of 1 mvrm was used to determine J. 3. Results and disussion Surfae XRD patterns of film A Ž Fig. 2a., B ŽFig. 2b., C Ž Fig. 2., and D Ž Fig. 2d. show strong Ž 00l. peaks, very weak Ž 2210., Ž peaks and a few peaks orresponding to impurity phases. This implies that all the melt-proessed Bi-2212 thik films exhibit a high degree of texture sine the intense Ž 00l. peaks shown by the surfae XRD pattern are a harateristi of films exhibiting a strong -axis grain alignment orthogonal to the film surfae. A differene in the position and intensity of X-ray peaks orresponding to the presene of impurities is evident. This variation is likely due to small flutuations in temperature and humidity Žone degree temperature differene and minor hanges in the atmospheri pressure are apable of introduing visible differenes in the X-ray spetra.. However, these minor hanges in impurity ontent do not lead to hanges in the ritial urrent density observed as onfirmed by omparing samples A and C. In the latter speimen, the X-ray spetrum shows a small impurity peak whereas sample A exhibits a larger number of these peaks. Notwithstanding, sample A exhibits a larger ritial urrent density than sample C.

4 ( ) E. Cehetti et al.rphysia C value of F was alulated on the basis of three samples. All the samples exhibit a high level of texture Ž F) However, the highest value for F was obtained for film type A, in whih both rystal rotation and seletive grain growth was expeted to our. The appliation of a magneti field solely during the first stage of ooling Ž film type B., where rystal rotation an our, seems to be signifiantly more effetive in enhaning the texture than the appliation of a field during grain growth Žfilm type C.. SEM baksattered images of polished ross-setions of films A, B, C and D are shown in Fig. 4. The films onsist of Bi-2212 with small amounts of impurity phases, mainly Bi-2201 Ž white phase. and the Bi-free 014 = 24-phase Ž blak phase. w13 x. The grains appear to be aligned with the rystallographi -axis Ž the diretion of lowest growth rate. orthogonal to the surfae through the entire thikness of the films. In Fig. 5, the transport ritial urrent density of the films, measured at liquid helium temperature, are plotted as a funtion of the proessing onditions. The maximum value in J ours in films where the magneti field was present during the entire ooling proess Ž film type A.. The appliation of a magneti field during the solidifiation from the molten state Ž film type B. results in higher J s than film type C, Fig. 2. X-Ray diffration of films A Ž. a, B Ž. b, C Ž. and D Ž. d. The presene of impurity phases is indiated by the symbol Ž ).. In order to differentiate the degree of texture among films A, B, C and D, the Lotgering fator Ž F. was alulated, as shown in Fig. 3. The average Fig. 3. Average Lotgering fators of films A, B, C and D based on alulated values from three samples.

5 196 ( ) E. Cehetti et al.rphysia C Fig. 4. Baksattered eletron images of Bi-2212rAg ross-setions melt-proessed under a 7 T magneti field during both ooling stages Ž C, Film A. Ž A., during the first stage of ooling Ž C, Film B. Ž B., during the seond stage of ooling Ž C, Film C.Ž C.,and in the absene of a magneti field Ž Film D.Ž D.. where the magneti field is present during grain growth. The proess in the absene of a magneti field Ž film type D. exhibits the lowest J, although Ž. Fig. 5. Average ritial urrent density 4.2 K, self field of films A, B, C and D, alulated on the basis of three samples. Measurements were performed with the four-probe DC method. A riterion of 1 mvrm was used to determine J. the value is very lose Žwithin the standard deviation. to the ritial urrent density obtained for films C. The improvement in J for thik films A and B is a onsequene of the enhanement in the degree of texture, whih is onfirmed by the Lotgering fators shown in Fig. 3. The differene in texture improvement is diffiult to distinguish from the SEM mirographs Ž Fig. 4., due to the high degree of alignment obtained in all samples. These results onfirm that a magneti field is more effetive in enhaning the texture development of Bi-2212 thik films during the solidifiation regime than during the grain growth proess. In this ase, although the appliation of a magneti field was limited to 20 min, a J enhanement of approxi- mately 50% was ahieved in omparison with samples proessed in the absene of a magneti field. On the other hand, the appliation of a 7 T magneti field during the grain growth proess only improved the ritial urrent density by less than 10%. Hene, it an be argued that the modest improvement in ritial urrent density shown by samples C Žmag- neti field applied during grain growth. may be

6 ( ) E. Cehetti et al.rphysia C explained by a statistial flutuation, sine their J value is within one standard deviation of the J value for samples D Ž absene of a magneti field.. However, the variane between the ritial urrent density of films A and B, for whih the only differene in the proessing proedure was the absene of a magneti field during grain growth in film type B, orroborates the hypothesis that texture formation an only be weakly enhaned during grain growth. This result ould have been expeted, sine several researh works have reported a J improvement in samples sintered for long times Ž ) 10 h. under a high magneti field, i.e. in the grain growth regime w8,9 x. This onlusion is in agreement with the analyw10 x. After deriving sis developed by Ferreira et al. the driving fore for the grain growth of a grain aligned with the field, a semi-empirial analysis showed that high intensities and long time exposures are required for the magneti field to be effetive in enhaning texture during grain growth. The urrent experiments have demonstrated that a magneti field an be effetive in improving J during the early stages of solidifiation from the molten state. Two alignment mehanisms seem to be possible during this stage, namely: preferred nuleation and rystal rotation. Preferred nuleation may our sine the presene of a magneti field lowers the energy for nuleation for nulei aligned with their -axis parallel to the magneti field. However, it has been shown w10x that the effet of an elevated magneti field on the nuleation rate is inonsequential. On the other hand, rystal rotation may our beause a superondutor rystal immersed in a liquid, under a magneti field, experienes a torque, in the form ts1r2ž DxH 2 Vosu sinu., Ž 2. where Dx is the differene in the volume suseptibility of a rystal, H is the magneti field, V is the volume of the rystal, and u is the angle between the magneti field and the -axis of the rystal. The torque tends to ause an angular rotation of the -axis of the rystal in the diretion of the applied field. Consequently, the torque will tend to inrease the texture of the superondutor. Sine preferred nuleation is negligible, rystal rotation under a magneti field must be the mehanism leading to the rystal alignment. It an be notied Ž Eq. 2. that the torque t is proportional to the volume of the rystal, thus it is expeted that larger grains are more easily oriented. However other fators may hinder the orientation indued by the torque t, suh as thermal agitation, impingement with other rystals, the presene of impurity phases and the visosity of the surrounding liquid. Assuming that a high magneti field enhanes the texture through rystal rotation in the early stage of solidifiation, a question regarding the exposure time limit to a high magneti field for improving effiiently the texture in Bi-2212 arises. Thus, it is important to keep in mind that the ooling rate annot be effiiently dereased below a ertain value Ž lose to 18Crmin. sine with a faster ooling, the amount of seondary phases, detrimental for J, inreases drastially w14 x. Consequently, sine rystal rotation ours in a ertain interval of time depending on the ooling rate, it may follow that the exposure time annot be effiiently redued below min. 4. Conlusions The degree of texture and transport ritial urrent density in Bi-2212 thik films an be enhaned by a short time appliation Ž 20 min. of a magneti field during the early stages of solidifiation. The mehanism behind this effet onsists of a grain rotation during the early stages of rystal growth. A mehanism of seletive grain growth under a high magneti field may enhane the texture formation and transport ritial urrent density in later stages, but to a muh lesser extent. Aknowledgements This work was supported by the DOE, under Grant No. DE-FG02-85ER Referenes wx 1 A. Bourdillon, N.X. Tan Bourdillon, High Temperature Superondutors: Proessing and Siene, Aademi Press, 1993.

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