Influences of Y2BaCuO5 particle size and content on the transport critical current density of YBa2Cu3Ox superconductor

Physica C 202 (1992) 83-96 North-Holland

Influences of Y2BaCuO5 particle size and content on the transport critical current density of YBaECu3Ox superconductor D. F. Lee, V. S e l v a m a n i c k a m a n d IC S a l a m a Mechanical Engineering Department and Texas Centerfor Superconductivity, Universityof Houston, Houston, TX 77204-4792, USA

Received 30 June 1992 Revised manuscript received 17 August 1992

Transport critical current density anisotropy measurementshave been conducted on liquid-phase processedYBa2Cu3Oxsuperconductors ( 123 ) with differingY2BaCuOs (211 ) content and particle size. It is found that for specimenswith micron-size 211 inclusions and low 211 content, the critical current density, Jc, increases with 211. In contrast, Jc is determined to decrease with 211 content when the specimenscontain large 211 particles and high volume fraction of211. Moreover, the ability of211 to act as pinning sites appears to diminish at moderate magnetic fields, whereasthe crystal defects that are associatedwith the 123-211 interfaces appear to be effectivepinning centers at high fields. It is proposed that the variation in Jc with 211 content is not unique, but depends on the 211 content and the particle size, as wellas the strength of the applied magneticfield.

1. Introduction In order to utilize the YBa2Cu3Ox (123) superconductor in practical high energy applications such as superconducting magnets and generators, the improvement in the current carrying capability of the material remains a paramount issue. There are three main factors that limit the critical current density, J~, of these high temperature superconductors, namely, the grain boundary weak links, the degree of alignment of the strongly superconducting C u - O ( a b) planes, and the amount of effective flux pinning centers in the material. The first two deficiencies have, to a large extent, been overcome by the melttexturing of 123, and transport Jc values on the order of 104 A / c m 2 under the influence of an applied field of a few tesla can now be routinely obtained [ 1 ]. These depinning J~s, however, are still two orders of magnitude lower than those of the 123 thin films. Consequently, any further improvement in the J~ of melt-textured 123 will have to be achieved through the introduction of additional flux pinning centers. Although the relatively large Jc values indicate the existence of effective flux pinning centers in these melt-textured 123, the exact pinning mechanisms are

Still unclear. The intrinsic weakly-superconducting layers of 123 have been suggested to be natural pinning sites [2 ]. In addition, defects such as twin planes [ 3 ], stacking faults [ 4 ], dislocations [ 5 ], oxygendeficient regions [6], and Y2BaCuO5 (211) particles [ 7 ] have been proposed to act as pinning centers. Even though experimental evidences suggest that these defects may serve as pinning sites, their effectiveness in pinning and the proportion of their contribution to the measured Jc values are as yet not established. I n particular, whether the relatively large 211 particles that are typically found in melt-textured 123 can act as effective pinning centers remains controversial. Numerous studies have shown that the 211 particles are effective in flux pinning, and Jc increases with 211 content [ 8-13 ]. Yet, other investigators have determined that Jc is either independent of the amount of 211 inclusions [ 14 ] or even decreases with 211 content [ 15 ]. The resolution of this issue is important since the size and the amount of 211 particles in textured 123 can now be controlled, to a significant degree, through processing techniques [ 16,17 ]. This means that if the 211 particles are indeed effective pinning sites, the J~ can

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D.F. Lee et al. / Influences of211 particle size and content on transport Jc

then be tailored through variations in both the size and the amount of 211 inclusions. One possible reason for the discrepancy in the reported effects of 211 may be due to variations in the size of the particles, and the amount of their associated defects in specimens fabricated by different techniques. It has been shown in conventional low temperature superconductors that large nonsuperconducting particles can provide flux pinning at the superconductor-particle interface; the Jc has been determined to increase with the surface area of the particles. However, J~ has also been found to decrease with the second-phase content for a fixed second-phase surface area per unit volume of superconductor [ 18 ]. Therefore, whether J~ is enhanced or not, depends on the relative difference between these two influences. This interplay between particle size and content is also expected to be in effect in the 123 superconductor, and the influence of 211 on Jc cannot be established unless these factors are taken into consideration. Moreover, a majority of the studies on the effect of 211 inclusions is conducted using J¢ data obtained from magnetization measurements where the fields are directed along the c-axis (BIIc) only. Magnetization J¢ results are highly dependent on factors such as the assumed diameter of the shielding current and the calculated demagnetization factor. In addition, transport measurements under the influence of an applied field have shown that the Jc behavior in these layered superconductors is highly anisotropic [ 19 ]. In particular, it has been found that for current flowing in the a - b planes with the applied field directed along the c-axis (conditions of the magnetization J~ measurements), the J¢ value is also influenced by extrinsic defects such as twin boundaries [ 20 ], stacking faults and dislocations [ 21 ], whose density may be altered by the processing technique. Due to this added influence of the crystal defects, the effect of 211 cannot be obtained directly by comparing the magnetization J~ results in the BIIc configuration alone. It appears that in order to be able to understand the effect of the 211 inclusions, comparison needs to be made between J~ values obtained under various magnetic field orientations on specimens containing different 211 particle size and content. In this paper, melt-textured 123 specimens with differing 211 par-

ticle size and content are fabricated using the liquidphase processing technique [22], by varying the cooling rate and by the addition of 211 particles. Transport Jc measurements in which the direction of the applied field is varied incrementally from Bllc t o BUa-b will be presented. From these results, the effect of 211 on the transport Jc for all the field angles can be examined.

2. Experimental Melt-textured 123 specimens with relatively high content of 211 inclusions of 2 to 20 Itm size are manufactured by the liquid-phase processing technique. The details of the processing method can be found elsewhere [22]. Essentially, presintered stoichiometric 123 specimens are vertically introduced into a furnace, and are rapidly heated to 1100°C in air. After being held at this temperature for 15 to 20 min, the specimens are rapidly cooled to 1025°C where slow cooling commences. The slow cooling rate is varied from 1 to 4 ° C / h in order to obtain specimens with different 211 content; the amount of 211 particles increases with cooling rate due to the sluggish peritectic recombination process. The slow cooling step is terminated when the specimens have reached a temperature of 925°C. Textured 123 are subsequently cooled to room temperature at a rate of 60oC/h. In order to obtain specimens containing micronsize 211 particles with a minimum amount of microcracks, 123/15 wt.% Ag composites with 0, 10 and 15 wt.% 211 additions of 3 to 8 Ixm initial particle size are fabricated by the liquid-phase processing technique. Silver particles of 3.5 to 8 lxm size are included in the starting material because the combination of Ag and 211 inclusions can almost completely eliminate the microcracks along the a - b basalplane grain boundaries that are typical in melt-textured 123 with low 211 content [23]. A Detail description of the melt processing can be found elsewhere [16]. Presintered composites with varying amount of 211 additions are rapidly heated to 1075 °C in air and held at this temperature for 15 to 20 min. After this period of time, the specimens are rapidly cooled to 995°C where slow cooling at a rate of 1 oC/h commences. Both the holding temperature

D.F. Lee et al. / Influencesof211 particlesize and contenton transportJc and the temperature at which slow cooling commences are slightly lower for these composites due to the presence of the low temperature Ag phase. Slow cooling is terminated when the temperature reaches 925°C, and the specimens are subsequently cooled at a rate of 60°C/h to room temperature. It is important to point out that the processing parameters of specimens with different starting materials need to be adjusted accordingly in order to obtain the desired microstrueture. The samples used in the transport J¢ measurements are single oriented grains cut from the textured superconductors such that the effect of domain boundaries can be excluded. Specimens of suitable dimensions are first cleaved along their a-b planes. Subsequent cutting and polishing with the aid of a precision grinder and an optical microscope equipped with cross-polarized light resulted in specimens of desired orientation where the length of the specimen is parallel to the a-b plane to within 5 ° as shown in fig. 1. The cross-sectional area of the transport Jc specimens are between 0.2X0.2 to 0.3X0.5 mm 2. Specimens of such small cross-sectional areas are used in order to minimize contact heating generated by the passage of large currents necessary to drive the superconductors into their normal state. Silver leads are attached to the specimens using epoxy silver paste in a four probe configuration. The specimens, together with the silver leads, are annealed at 500°C for 48 h and at 400°C for 24 h in flowing oxygen for oxygen uptake and minimization of contact resistance. In order to determine the influence of applied

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85

magnetic fields on the transport Jc at 77 K, the specimen is mounted on a holder situated between the two coils of an electromagnet capable of producing a field up to 1.5 T (Walker Scientific HF-9H). The specimen-current-field configuration is selected in such a way that the current flows along the a-b planes, and the direction of current transport is always perpendicular to the magnetic field as seen in fig. 1. The specimen holder is connected to a rotary stage such that the angle 0 between the, c-axis of the specimen and the direction of the applied field can be adjusted to within an angular resolution of 0.5. Continuous DC current is generated by a DC power supply of 100 A rating (H.P. Model 6062B) and is increased at a rate of 0.1 A/s. The voltage drop along the specimen is measured with a nanovoltmeter (Keithley 181 ), and the transport Jc is determined using a 1 pV/cm electric field criterion.

3. Results

3.1 Melt-textured YBaeCu~Ox without YzBaCu05 addition In melt-textured 123 superconductors fabricated by the liquid-phase processing technique, 211 particles are typically found to remain in the microstructure. These particles are the result of incomplete peritectic recombination during the nucleation and growth of the 123 phase. By carefully adjusting the rate of slow cooling, textured 123 with a wide range of 211 content can be obtained. If the cooling rate is extremely slow such that the peritectic reaction can almost go to completion, certain areas of the bulk material are found to contain nearly zero percentage 211 as shown in fig. 2. It can be seen from this figure that textured 123 superconductors with a minute amount of 211 inclusions contain a large amount of cracks both parallel and perpendicular to the grain orientation. Because of the abundance of these cracks, it is not possible to perform transport measurements on these samples. A low Jc value is expected in these specimens since the cracks will severely restrict the size of the current flow path as shown by Murakami et al. [7]. When the 211 content is in excess of a few volume percentages, the amount of microcracks in these tex-

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D.F. Lee et al. / Influences o f 211 particle size and content on transport Jc

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tured 123 is found to decrease, and the remaining cracks are mostly oriented parallel to the a-b planes. In order to determine the influence of these peritectic 211 particles on the transport Jc, the slow cooling rate is varied between 1 and 4 ° / h such that specimens with 17, 30 and 50 vol.% 211 are obtained. Figures 3 and 4 show the size and distribution of the particles in the a-b plane of the 17 and 50 vol.% 211 specimens, respectively. It can be seen from these figures that the rod-shaped 211 particles are randomly distributed throughout the superconducting matrix, and the particle size ranges between 2 and 20 Ixm with a typical rod diameter o f approximately 3 ~tm and a length of 10 Ism. Variations in the transport Jc at 77 K and 1.5 T with the angle 0 between the direction of applied field and the c-axis for the 17, 30 and 50 vol.% 211 specimens are shown in fig. 5. These data represent the typical J~ values of the respective specimens, with a scatter in J¢ of up to 10 % between specimens that are processed in an identical fashion. This scatter in Jc points out the importance of microstructural fea-

tures on the properties of melt-textured materials even if the processing conditions are kept constant. Even with the scatter it can be seen from this figure that the transport Jc at all field orientations decreases with increasing amount of 211 particles. Also from fig. 5, it is seen that two Jc peaks, one centered at Blla-b and the other at Bllc, are found. The narrow and intense Jc peak located at BI]a-b can be qualitatively explained by the intrinsic pinning mechanism of weakly-superconducting planes [24,25 ], and contributions from extrinsic pinning centers such as stacking faults and dislocations are also expected. The broader and weaker peak centered at BIIc is believed to be due to extrinsic defects such as twin planes, stacking faults and dislocations [25 ]. On the other hand, the randomly distributed 211 inclusions should provide isotropic bulk pinning and contribute to the background J~ values. Since the J~ at the two characteristic field orientations, Bllc and Blla-b, are likely to be associated with intrinsic and extrinsic anisotropic pinning centers, the minimum J~ values, Jc rain, in the J~ anisotropy curves are also analyzed in order

D.F. Lee et al. / Influences of211 particle size and content on transport Jc

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to understand the effect of the 211 panicles in the absence of favorably oriented extrinsic crystal defects. The variations in averaged Jcmin, Jcnllc and J~ma-b with 211 content for the specimens studied are shown in fig. 6, where the error bars represent the amount of scatter. It can be seen from this figure that, for the specimens with 17 to 50% volume fraction of relatively large 211 particles, all three characteristic J¢ values decrease steadily as the amount of 211 particles is increased. For example, the averaged Jcmin value decreases from 12 800 A/cm 2 for the specimens containing 17 vol.% 211 to 8000 A/cm 2 for the specimens containing 50 vol.% 211 panicles. This represents approximately a 40% decrease in the transport J~, and implies that within the high volume content range, these relatively large 211 particles have a mild but noticeable effect on the current carrying capability of melt-textured 123.

3.2. Melt-textured YBa2CusOx with Y2BaCu05 additions Similar to textured 123 fabricated by the liquidphase processing technique, melt-textured 123/Ag with 211 additions are found to be comprised of oriented domains where the size of the domains ranges from 5 to 12 ram. A crossed-polarized micrograph of one such domain in a textured specimen with 15 wt.% 211 addition is shown in fig. 7. Two main features are immediately evident from the figure. Firstly, the microcracks along the a-b plane grain boundaries typically associated with melt-textured 123 are almost completely eliminated through Ag and 211 additions. Secondly, there are two distinct types of 211 inclusions. The first type is the typical rod-shape peritectic 211 that are 2 to 20 ~tm in size. The second type is spherical panicles of 0.2 to 3 lim in size which are arranged occasionally in the form of clusters within the superconducting matrix. A fracture surface SEM micrograph of the specimen is shown in fig. 8 where these micron-size spherical particles can

D.F. Lee et al. / Influences of211 particle size and content on transport Jc

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Fig. 6. Variations in Jemia, Jcau~, and Jca|=-b with 211 content for s p e c i m e n s with large 211 inclusions (curves are drawn as visual guides).

clearly be seen. The stoichiometry of these spherical particles is determined to be 211 by selected area diffraction from TEM studies. Using an image=analysis system, the volume fraction of the rod=shaped 211

particles is found to be approximately 8% in all the samples, whereas the fraction of the spherical 211 particles are 8% and 13% for the specimens with 10 and 15 wt.% 211 additions, respectively. In addition

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D.F. Lee et aL / Influences of211 particle size and content on transportJc

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lO~um Fig. 7. Cross-polarizedoptical mierographof melt-textured 123/15 wt.%Ag with 15 wt.% 211 addition. to these microstructural features, the average thickness of the 123 grains is found to decrease from 15 Ixm for specimens without 211 addition to 6 ~tm for 123 with 211 additions. The large Ag inclusions in melt-textured 123 have been shown to be ineffective pinning sites, and transport J~ decreases with Ag in specimens with high Ag contents [25 ]. However, when the Ag content is low as in the present case, the effect of Ag inclusions on the'iransport J¢ has been found to be negligible, and the main function of these Ag inclusions is in the reduction of microcracks. The variations in transport J¢ with field angle 0 for the specimens with 0, 10 and 15 wt.% 211 additions at 0.5 and 1.5 T are shown in figs. 9 and 10, respectively. Due to insufficient amount of Ag lead wires used for the specimens with 10 wt.% 211 addition, a continuous DC current of larger than 30 A could not be used without lead wire burn out. Therefore in actuality, the transport J~ values of these specimens for the Blla-b configuration are larger than the 30 000 A / c m 2 measured values. Scattering in the measured J~ values of up to 10% is

found between specimens with the same amount of 211 addition that are processed under identical conditions. This variation is believed to be due to subtle differences in specimen quality, and accentuates the importance of microstructure on the properties of the superconductors. Even with the scatter in the transport Jc values, it is evident from these figures that there is a consistent improvement in the current carrying capability with increasing micron-size 211 particles throughout the whole range of 0. For example, the averaged transport Jc for the Blla-b configuration at 0.5 T increases from 28 000 A / c m 2 to 65 000 A / c m 2 with 15 wt.% 211 addition, which represents an increase of more than 130%. Variations in averaged Jcmc and JcB,a-b with 211 content at 0.5, 1.0 and 1.5 T are shown in figs. 11 and 12, respectively. It can be seen from these figures that the transport Jc for these two field-specimen configurations increase steadily with 211 content, and indicate that the anisotropic pinning mechanisms associated with these two orientations are enhanced by the 211 addition. Moreover, the de-

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D.F. Lee et al. / Influences of211 particle size and content on transport J~

Fig. 8. Fracture surface SEM micrographof melt-textured 123/15 wt.%Ag with 15 wt.%211 addition showing the micron-size spherical 211 inclusions. 50000

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Fig. 10. Variations in transport Jc anisotropy with angle 0 between B and the c-axis for specimens with 0,10, and 15 wt.% 21 1 additions at 1.5 T.

gree o f Jc enhancement remains substantial even at a field o f 1.5 T. In order to exclude the flux pinning contributions from the anisotropic pinning centers, variations in the averaged Jc rain values o f the specimens are shown as functions o f 211 content at 0.5,

1.0 and 1.5 T in fig. 13. It is seen that even though the Jcr, i, increases consistently with 211 content, the degree o f enhancement appears to d i m i n i s h as the field strength is increased. For instance, the averaged Jcmin increases by more than 130% from a value o f

D.F Lee et al. / Influences of211 particle size and content on transport J¢

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11 500 A / c m 2 to 27 000 A / c m 2 with 15 wt.% 211 addition at 0.5 T, whereas the increase is only from 10 150 A / c m 2 to 14000 A / c m 2 at 1.5 T, which represents an improvement of less than 40%.

4. Discussion Due to the extremely small coherence length of 123, there is a general agreement that the relatively large 211 particles are much too big to be effective pinning centers. It has, however, been argued that the 123211 interfaces and the crystal defects associated with these interfaces should be able to operate as pinning

Fig. 13. J~ mr*of specimens with 211 additions as functions of 211 content at 0.5, 1.0 and 1.5 T (curves are drawn as visual guides).

centers [7,25,26]. If the 123-211 interfaces or the interface associated defects are effective pinning sites, then Jc should increase with the surface area of the inclusions. That is, Jc should increase with the 211 content, as in the case of conventional low temperature superconductors [ 18 ]. However, this increase in J¢ will not be monotonic because as the amount of second-phase inclusions is increased, the superconducting volume and consequently the percolation paths are reduced. This decrease in J~ with second-phase content has been shown in certain low temperature superconductors [ 18 ] where, for a fLxed second-phase surface area per unit volume of superconductor, J¢ decreases with second-phase volume fraction, Vf, as [ 1 - 1.5 Vf]. A balance between these two influences will necessitate an optimal secondphase content for a given particle size, where further increases in the volume fraction of second phase will result in a decrease in the current carrying capability of the material. Moreover, this optimal second-phase content should vary with the particle size, shifting to a higher percentage when the inclusions are small. Therefore, for a certain amount of second-phase material, different J~ may result if the specimens under comparison contain inclusions of different average particle sizes. With the competing influences of surface area and volume fraction in mind, averaged Jc rain both meltprocessed 123 with and without 211 additions are normalized by their respective 123 volume fraction. Since this field configuration is not favorable for di-

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D.F. Lee et al. / Influences of211 particle size and content on transport J~

rectionally-oriented crystal defects, any enhancement in Jc should be due mainly to increased interfacial pinning. The normalized transport critical current density values, J~°i~ of the two sets of specimens are shown together as functions of 211 content in fig. 14. Under such normalization, an increase in J ¢ ~ , will signify an enhancement in pinning, whereas a decreasing value means that there is degradation in the amount of flux pinning. In the case that the decrease in measured Jc m~, is due to a reduction in the superconducting volume, the normalized value will remain constant indicating that the material is percolation-limited. It can be seen from fig. 14 that for the specimens containing micron-size 211 inclusions, J~°i~ increases with 211 content for relatively low 211 volume percentage of 8 to 21%. This increase indicates that flux pinning is enhanced with increasing amount of micron-size 211 particles. On the other hand, j ~ o ~ remains relatively constant with changing 211 for specimens with large 211 inclusions and relatively high 211 content. This signifies that the reduction in measured J¢ m~nin specimens without 211 addition is due to the reduction in superconducting volume. The contrasting behavior of these two sets of specimens indicate that the variation in critical current density

with 211 is also likely to depend on the 211 particle size. From fig. 14, it is interesting to note that the av,,raged Jcmin values of specimens with 16 vol.% 211 (with 211 addition) and 17 vol.% 211 (without 211 addition) are nearly the same. The similar J~¢%~ values imply that the 211 surface areas of these specimens are approximately the same if the efficiency of interfacial pinning mechanism is equal to unity. For an averaged spherical 211 diameter of 2 pm and a typical 211 rod diameter of 3 pm with length of 10 Ixm, the 211 surface areas of the two specimens are found to differ by roughly 38 %. The j~or~ of these specimens, however, differ only by 5.5 % (difference is 23% when scattering in Jc~n is taken into consideration). This smaller than expected difference in Jn°i~n may mean that the efficiency ofinterfacial pinning is much less than unity. In agreement with this while the increase in 211 surface area is approximately 300% when the particle content is increased from 8 to 21 vol.% for specimens with 211 additions, the enhancement in Jn°mirn~ is only about 40 %. Based on these results and the probable existence of an optimal 211 content which varies with the size of the inclusions, it is proposed that the dependence of J¢ on 211 content in melt-textured 123 is not unique, norlTl

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D.F. Lee et al. / Influences of211 particle size and content on transport Jc

Increasingparticlesize or magneticfield

Volume percentage 2t 1 (%) Fig. 15. Schematic of proposed variation in normalized Jc with 211 content.

93

come the reduction in current carrying capability due to decreased percolation path. The j~O~c mm value will now either remain constant or decrease with 211 content depending on whether the net pinning force is reduced due to the decreasing amount of superconducting phase. rn1 In addition to the variations in Jch omin due to 211 content and panicle size, it has been shown in Figs. 9, 10 and 13 that the effectiveness of 211 in flux pinning appears to diminish with increasing field strength. It is therefore expected that at higher fields, rl~ the Jcn omin behavior shown in fig. 15 will be affected in such a way that the optimal 211 content will be lowered. Also, the enhancement in j~orm ¢ t ~ l n within the first and second regimes will become more gradual, and any decrease in ~°=~n in regime three will be more severe.

but changes according to the regime of the 211 content, the size of the 211 inclusions, and the strength of the applied magnetic field. This proposed j ~ om~ ~ behavior is illustrated in the schematic shown in fig. 15. As seen in the figure, three regimes of the 211 volume content are envisioned, with the ~°m~ (or Jc ~in) behavior being different in each regime. In the first regime of low 211 content, j ~ o ~ is expected to increase rapidly with 211 due to a combination of favorable factors. These include the decrease in cracking, the increase in pinning by 123211 interfaces, and other microstructural refinements. In the second regime, the improvement in J~°m~ is expected to be more gradual because of the disappearance of cracks transverse to the 123 plates. The degree of enhancement within this regime will depend on the size of the 211 inclusions and the effectiveness of pinning. That is, if the 211 inclusions are small and pinning is strong, j~or~ should increase appreciably until the optimal 211 content is reached. On the other hand, if the 211 inclusions are large and the pinning is weak, the increase in j n o ~ due to increased pinning may be offset by the reduction in the superconducting volume, and the improvement in critical current density will be small. Moreover, the optimal 211 content for specimens with large 211 inclusions should be somewhat smaller than that of the 123 with small 211 panicles. Once the optimal 211 content is exceeded, the superconductor will enter into the third regime where the contribution from pinning will no longer be able to over-

Following this approach, the improvement in ~°ir~ observed in the present investigation for specimens with 211 additions is suggested to lie within the first two regimes as shown by the dashed line in fig. 14. The enhancement m " ~mi~ orm is then due to microstructural refinement and increasing 211 pinning. The specimens with high volume fraction of large 211 particles are believed to lie within regime three, where the optimal 211 content has been reduced and JcitlOrlYl mi~ values remain roughly constant due to the reduction in superconducting percolation paths. Even though the Jcr~n values are found to be improved by micron-size 211 particles, the enhancement appears to diminish with increasing magnetic field strength. If flux pinning for J~ rain is due solely to the 123-211 interfaces, this diminishing improvement may mean that the effectiveness of this particular pinning mechanism decreases with increasing field. In such circumstances, other pinning sites may have to be introduced into the superconductor in order to obtain enhanced J¢ at high magnetic fields. In contrast, when the field is directed along either the c-axis or the a-b planes, the enhancement in Jc with 211 content for specimens with micron-size 211 panicles remains substantial even at high fields. For these two field orientations, anisotropic extrinsic pinning centers such as dislocations and stacking faults, which lie on the a-b planes, are suitably oriented with respect to the field and can provide a high degree of flux pinning. While these defects may be naturally occurring features of the melt-texturing

94

D.F. Lee et al. /Influences of211 particle size and content on transport J~

process, their density is likely to depend on the processing technique. Furthermore, these crystal defects have been found to be closely associated with the 211 inclusions. For example, from a high-temperature deformation study, Selvamanickam et al. [ 21 ] have determined that 211 inclusions act as dislocation sources, and the transport J~ increases with dislocation density throughout the complete revolution of field angle 0. Moreover, the J~ improvement is found to be especially pronounced for the Bile configuration where the Jomc value can even surpass that of the usually dominant J~ma_b. In another study, Wang et al. [26] have shown that there are numerous stacking faults associated with the 123-211 interfaces, and stacking fault densities as high as 10 t5 / cm 2 have been observed. Furthermore, the stacking fault density appears to increase with the curvature of the interface. If this is the case, the stacking fault density will be higher for specimens with small 211 inclusions when compared with specimens containing typical large rod-shape 211 particles, and can lead to higherJc values. In order to examine the effectiveness of these 211 associated crystal defects in flux pinning and to interpret the various conflicting magnetization J¢ resuits, variations in JcB,~ at 1.0 T for specimens with 211 additions are normalized by their respective 123 volume fraction. These normalized values are shown together with the magnetization data of Murakami et al. [7] and Jin et al. [14] as functions of211 content in fig. 16. It can be seen that this figure resembles the proposed J~°i~ behavior shown in fig. 15, and indicates that the competing influences of increasing J~ with pinning and decreasing J~ with reduction in superconducting volume are still in effect. In the present field configuration, however, there will be a substantial amount of pinning contributions from suitably-oriented extrinsic pinning centers in addition to the possible 123-211 interracial pinning. For specimens with relatively low volume fraction of micron-size 211 particles examined in this investigation, TEM studies have revealed that there is a high density of stacking faults and dislocations in the form of pileups and tangles. The J~$N~ of these specimens is believed to fall within the first two regimes, and its value increases by approximately 200% when the 211 content is raised from 8 to 21 vol.%. In comparison, the corresponding enhancement in J¢ rain at

1.0 T is estimated to be only about 70%. Furthermore, these J~$,~ values are in close agreement with the data obtained by Murakami et al. [ 7 ] on MPMG specimens with micron-size 211 inclusions prepared from Y-riched starting materials. The similar behavior"I n JcB~c normbetween these two sets of specimens implies that these samples contain similar crystal defects. The significant improvement in these JcBllc together with the exaggerated Jc peaks in this field orientation (see for example fig. 10) points to the effectiveness of the crystal defects in acting as pinning centers even at high fields. On the other hand, McGinn et al. [15] have reported that melt-textured 123 with 211 additions resulted in a decrease in the magnetization Jc. Unfortunately, the 211 content was not given in that study. However, since these specimens are fabricated by zone-melting, the final 211 content can be high and the particle size may be large due to prolonged resident time the specimens spent in their semi-solid state. If this is the case, the J~mc values will be percolation-limited and their resultsmay fallwithin the third regime, which can explain their observation of a monotonic decrease in Jcmc with 211 content. In addition to the variation in 211 content, the particle size of the MTG-processed 123 examined by Jin et al. [14 ] is also varied. The lack of J¢ improvement in these specimens may be interpreted in the following way. The specimens with 19 and 25 vol.% 211 contain particles ranging from 5 to I0 tim in size, and is believed to fallwithin the second regime. Duc to the relativelylarge 211 inclusions, the amount of crystaldefectsassociatedwith these 211 particlesmay not be sufficientin overcoming the reduction in percolation path such that no Jc improvement is seem. For the specimens with 34 and 35 vol.% 211 inclusions, the particlesize is now roughly I to 2 ~tm. Due to the high 211 content, these specimens are believed to lic within regime three. In this case, even if the density of crystal defects associated with the smaller 211 particle is high, pinning will not bc able to overcome the reduction in superconducting volume and ~ $ ~ should remain constant or even decrease. These interpretations of the magnetization data are represented by the dashed lines shown in fig. 16. Another feature of the JcB~, data obtained by Jin et al. is their lower magnitude when compared to the J~m~ determined in this investigation and by Murakami

D.F. Lee et aL / Influences of211 particle size and content on transport J~

95

50000

40000

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With 211 addition

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et al. The value of the JosHc in the former case is similar to that of Jc min and may result when there is insufficient crystal defects to provide pinning in the BJlc field orientation. Unfortunately, the type and the amount of crystal defects have not been reported for these specimens. Therefore, whether the smaller Jomc values in MTG-processed 123 is due to a lower crystal defect density or whether M T G processing may result in a lower defect density cannot be established. From the J~mc values of liquid-phase processed 123 with 211 additions, it appears that the crystal defects associated with 211 inclusions are effective pinning centers even at high fields. Consequently, the J~ may be improved with 211 content provided that the particles are sufficiently small such that the defect density associated with these particles is high and the concomitant reduction in percolation path is low. In addition, the defect density has to be taken into consideration when the J~mc behavior with respect to 211 content is examined. Although the role of 211 is still not clearly understood, the Jc of melt-textured 123 is found to improve with 211 content within the limit of low 211 volume fraction. Since the presence of 211 particles is inevitable in melt-textured 123, means should be found to take advantage of these particles such that Jc can be further improved through the optimization of the particle size and content and

the maximization of 211 associated defects.

5. Conclusions

Transport Jc anisotropy measurements on melttextured 123 superconductors show that for specimens with micron-size 211 inclusions and relatively low 211 content, Jc increases with the amount of 211. In contrast, J~ is found to decrease with 211 content if the volume fraction of 211 is high and the particles are large. It is proposed that the dependence of J~ on 211 content in melt-textured 123 is not unique, but changes with the amount of 21 l, the particle size, and the strength of the magnetic field. According to the observed J~ behavior, the influence of the 211 content on J~ can be divided into three regimes. The lc within the first regime, where the 211 content is low, is proposed to be microstructure-limited, and J~ increases with 211 content. For an intermediate amount of 21 l, the material falls within the second regime where the Jc is pinning-limited, and the measured Jc is the consequence of competing influences between increasing J~ with pinning and decreasing J~ with reduction in the superconducting volume. The J~ within this regime can then slightly increase or remain relatively constant with 211 content depending

96

D.F. Lee et al. / Influences of211 particle size and content on transport Jc

on the 211 particle size a n d the strength o f the field. F o r high 211 content, the s u p e r c o n d u c t o r will be within the p e r c o l a t i o n - l i m i t e d t h i r d regime, where flux pinning cannot o v e r c o m e the reduction in the percolation path, and J~ decreases monotonically with 211. In a d d i t i o n to possible p i n n i n g by the 123-211 interfaces, t r a n s p o r t a n i s o t r o p y m e a s u r e m e n t s indicated that J~ is sensitive to interface associated crystal defects such as stacking faults a n d dislocations when the a p p l i e d field is directed along the c-axis. W h i l e interfacial pinning by 211 particles appears to d i m i n i s h with increasing field, crystal defects a p p e a r to be effective p i n n i n g centers even at high magnetic fields. However, in o r d e r to obtain i m p r o v e d J¢ through pinning b y these crystal defects, both the size o f the 211 particles a n d the 211 content should rem a i n low such that the 123-211 interface associated defect density can be substantial while limiting the r e d u c t i o n in the percolation path.

Acknowledgements T h e authors would like to t h a n k H. Ktipfer at the Institut fiir Technische Physik, K a d s r u h e , G e r m a n y for valuable discussions. T h e y would also like to thank. J. Davis, N. Nguyen a n d S. Son o f the Mechanical Engineering D e p a r t m e n t at the U n i v e r s i t y o f H o u s t o n for their assistance in specimen preparation. This work is s u p p o r t e d by the Texas Center for S u p e r c o n d u c t i v i t y at the U n i v e r s i t y o f H o u s t o n u n d e r P r i m e G r a n t M D A 972-88-G-0002 from D A R P A a n d the State o f Texas.

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