Shock consolidation of crystallographically aligned Bi2Sr2CaCu2O8 powders

PHYSICA

Physica C 184 ( ! 991 ) 1-12 North-Holland

Shock consolidation of crystallographically aligned BiESrECaCu208 powders S.T. Weir and W.J. Nellis Lawrence Livermore National Laboratory, University of California, PO Box 808, Livermore, CA 94550, USA

C.L. Seaman, E.A. Early and M.B. Maple Department of Physics and Institute for Pure and Applied Physical Sciences, University of California, San Diego, La Jolla, CA 92093, USA

M. Kikuchi and Y. Syono institute for Materials Research, Tohoku University, Katahira 2-1-1, Sendal 980, Japan Received 1 July 1991

Bulk samples of crystailographically aligned Bi2Sr2CaCu2Os were fabricated by shock-consolidation of tapped micaceous powders. Basal plane alignment was verified by means of optical microscopy, X-ray diffractior, and magnetic measurements. We have explored the dependence of this crystallographic alignment as a function of both shock pressure (from 35 kbar to 135 kbar) and powder panicle size (from < 5 lam to 30-37 lam ). The degree of crystallographic alignment as determined by SQUID magnetometer measurements and X-ray diffraction measurements was found to be insensitive to the shock consolidation pressure, but sensitive to powder particle size, with a decrease in the powder panicle size resulting in a decrease in the amount of crystallographic alignment.

1. lntioduction Dense high-T¢ bulk samples can be fabricated from ceramic powders by shock-consolidation. This technique has several interesting features which address important aspects of the fabrication of bulk, high-T~ superconductors having high critical current densities [ 1,4 ]. It has been shown [ 5 ], for example, that shock-induced dislocations significantly enhance fluxpinning energies, and can raise intragranular critical current densities in YBaECU307 by at least a factor of 10. Shock-consolidation also addresses the widespread problem of low intergranular current densities in bulk high-T¢ materials. For instance, highly textured compacts of BiESr2CaCu208 with very good basal-plane crystallographic alignment can be fabricated by shock-consolidation of pre-aligned powders [6 ]. In the shock-consolidation experiments reported here, starting samples of Bi2SrECaCu208 powder

which had been pre-packed to about 65% of full crystal deusity were subjected to shock-waves with pressures ranging from 35 kbar to 135 kbar. Interpanicle bonding is accomplished by means of the high applied shock pressure together with heterogeneous heating generated at the panicle surfaces by interpanicle friction and surface deformation as the shockwave propagates through the powders. Shock-heating of powders is extremely heterogeneous; although the particle surfaces attain high temperatures, the interiors of the powders particles, which undergo relatively little deformation and heating in the shock pressure regime discussed here, remain at a much lower temperature. Thus, heterogeneous shock heating is a very useful property for the consolidation of high-T¢ powders since the interiors of the powder panicles are not subjected to high temperatures which might drive out oxygen and reduce the critical temperature To. Indeed, no measurable decrease in the magnetically measured onset Tc of shock-consoli-

0921-4534/9 !/$03.50 © 1991 Elsevier Science Publishers B.V. All rights reserved.

2

S.T. Weir et ai. / Shock consolidation ofcrystallographically aligned Bi=,SrzCaCu20s powders

dated YBa,Cu307 was observed for pressures up to at least 167 kbar [5 ]. Similar results have been obtained for shock-consolidated Bi2Sr2CaCu2Os, as will be shown here. We present here the results of an extensive study of shock-consolidated Bi2Sr2CaCu2Os over a wide range of shock-pressures and powder particle sizes. The shock compacted samples were examined by a variety of methods including optical microscopy, Xray diffraction, TEM, electrical resistance and magnetization measurements. A portion of this study was reported previously [ 6 ].

sifted to smaller sizes in order to examine the effects of varying powder size. These three capsules were loaded with powders sifted to the following sizes: 1015 gm, 5-10 gm and < 5 gm. These three capsules were all shocked to the same pressure of 100 kbar. All powder specimens were 10 mm in diameter and about 0.5 mm thick. Specimen masses ,sere measured using a microbalance, which measured the masses of the capsules and of the capsules plus powder to 10 gg resolution. Powder specimen thicknesses were measured with a laser ruler having a 1 gm resolution. 2.1. Shock wave generation

2. Sample preparation The starting Bi2Sr2CaCu208 powder for the compaction experiments was synthesized by solid state reaction by the DuPont company [6 ]. To ensure powder size uniformity, this micaceous powder was sifted to various size ranges prior to shock compaction. Proper control of powder size and packing density are important because the amount of heterogeneous shock heating is sensitive to these parameters [ 7,8 ]. Powder sifted to the 30-37 gm size was examined by electron microscopy and found to consist of platelet-shaped particles about l 0 gm thick, 3040 lam wide, and 40-100 gm long. Micaceous or platelet-shaped powder particles of controlled size facilitate achieving crystallographic orientation of grains, as shown below. Five copper shock-recovery capsules were loaded with the 30-37 ~tm sized powder, and the capsules were shocked to 35, 50, 70, 100 and 135 kbar, respectively (see table 1 ). Additionally, three other capsules were loaded with powder

The shock waves in our experiments were generated by high velocity impacts of lexan projectiles onto copper shock-recovery capsules containing the sample powuers. The 20 mm diameter projectiles weighing about 5 g were accelerated up to speeds of several km/s by means of the 6.5 m long two-stage light-gas gun at Lawrence Livermore National Laboratory. These experiments are characterized by small quantity samples shocked under carefully controlled conditions. The starting powder density, the powder particle size and the shock pressure were all precisely measured. The shock recovery technique is described in detail elsewhere [ 2 ]. A schematic of the sample target fixture is shown in fig. I. The starting powder is loaded into the copper shock recovery capsule and then tapped with an electromechanical device to about 65% of crystal density. Due to the platelet-like morphology of the starting powder, this tapping process tends to align the BiESrECaCu208 grains so that the c-axes are par-

Table 1 Summary of samples with sifted particle sizes and shock pressures Shock Pressure

Projectile velocity

Particle size

Initial density

Sample mass

Sample name

35 kbar 50 kbar 70 kbar 100 kbar i 35 kbar 100 kbar 100 kbar 100 kbar

0.86 km/s 1.19 km/s 1.50 km/s 1.9"/km/s 2.38 km/s 1.94 km/s 1.98 km/s 1.99 km/s

30-37 am 30-37 am 30-37 am 30-37 am 30-37 am 10-15 am 5-10 am <5gin

4.37 4.39 4.27 4.40 4.19 4.38 4.39 4.01

167.9 141.8 170.7 162.4 159.5 162.4 160.7 158.6

BT03 BT05 BT01 BT02 BT04 BT06 BT07 BT08

g/cm ~ g/cm 3 g/cm 3 g/cm 3 g/cm 3 g/cm 3 g/crn 3 g/cm 3

mg mg mg mg mg mg mg mg

S.T. Weir et ai. I Shock consolidation of crystallographically aligned Bi=,Sr2CaCu20s powders

3

Copper Shock-Recovery Capsule Pre-Tapped Bi2Sr2CaCu 2Oa Powder Lexan Projectile

Steel Block

Fig. I. Schematic diagram of the recovery fixture for shock-consolidating high-T¢ powders. A planar shock wavc is generated by the impact ofa lexan projectile onto a copper capsule containing the Bi2Sr2CaCuEOspowder.

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Fig. 2. Schematic diagrams of the fabrication of a textured Bi2Sr2CaCu2Os compact by shock-consolidation. (a) Starting platelet-shaped powder. (b) Starting powder after tapping. Due to the platelet-like nature of the grains, uniaxial tapping results in a high degree of c-axis alignment. (c) After shock-consolidation with the shock direction parallel to the tapping direction (or, lbl ti~ equivalently, parallel to the c-axis I~l-':i 1.~ i. l.l ~.l i ., I i i ), thl l.. , . .i.#l. .~ ' l ~ l i l;~,,,n particles are bonded into a dense, crystallographically aligned compact. A shock-direction parallel to the tapping direction minimizes kinkingof the a-b planes.

allel to the tapping direction, as illustrated in fig. 2. The alignment of an as-tapped sample was verified by X-ray diffraction. Consolidation of the tapped powder was achieved by means of a planar shock wave generated by the impact of a high-speed projectile. The shock direction for all compactions was parallel to the tapping direction or, equivalently, parallel to the c-axes of the aligned, tapped powder. Table 1 summarizes our samples. The shock pressures reported here were calculated through the use of the Rankine-Hugoniot equations. These equations relate the particle velocity up and shock-front velocity u~ of a shock wave to the pressure, volume and energy of the material behind the shock front. For mout materials the relationship between the kinematic parameters us and u~ can be approximated to very good accoracy by a linear Hugoniot equation us = C + S u n ,

( 1)

w h e ~ is the bulk sound velocity and S is a matel~-.l-dependent parameter. Furthermore, it can be shown through a conservation of momentum argument that the shock pressure P of a material initially at rest is related to u, and up by P=pousup ,

(2)

where Po is the starting density of thc material. Combining this equation with eq. ( 1 ) yields the following

4

S.T. Weir et al. / Shock consolidation ofcrystailographically aligned Bi:,Sr2CaCu20s powders

equation expressing P as a function of the particle velocity up:

P=po(C+Sup)up.

(3)

For an impact involving an impactor of some material striking a target of the same material (a "symmetric" impact), the particle velocity u, is equal to half of the impact velocity u~. For "non-symmetric" impacts one must make use of the shock impedance matching principle, which is a postulate that the shock pressure and the particle velocity are continuous across the impact interface. If the us-Up relationships (eq. ( 1 ) ) of both the impactor and the target are known, then up can be determined (see fig. 3 ). A precise value for the shock pressure P of the copper capsule is thus obtained by the m?asurement 400

Cu

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Fig. 3. Pressure vs. particle velocity (t,,p) of impact-interface. The construction shown is used to determine the shock pressure generated by the impact of dissimilar materials if the pressure-vs-up dependence of each material is known. The construction is for an impact of a lexan projectile with a velocity of 1.97 km/s onto a copper target. Note that since the projectile decelaratcs upon impact with the copper target, lower-interface particle velocities correspond to higher shock pressure in lexan. On the other hand, since the copper target is initially at rest, higher impact-interface particle velocities correspond to higher shock pressures in copper. The shock-impedance matching principle states that the particle velocity and shock pressure must be continuous across an impact interface, i.e. that the shock pressures and particle velocities of the copper target and the lexan impactor must be equal. By the construction shown, the shock pressure and panicle velocity of the shock wave must be 100 kbar and 0.26 km/s, respectively.

of the impact velocity u, of the lexan projectile through the use of eq. (3), with C and S being the Hugoniot parameters for copper. Table 2 lists the C and S parameters for lexan and copper. Due to the large shock-impedance mismatch between the copper capsule and the Bi2Sr2CaCuzOs powder, the copper-Bi2Sr2CaCu2Os interfaces strongly reflect shock waves° As a result, the Bi2Sr2CaCu208 samples achieve the shock pressures listed in table l by means of a rapid succession of several pressure steps caused by the reverberation of the shock wave between the two copperBi2Sr2CaCu2Os interfaces.

3. Optical microscopy Optical micrographs of the four samples shockcompacted at 100 kbar are shown in fig. 4. The particle ranges are 30-37 pm, 10-15 ~tm, 5-10 ~tm, and < 5 ~tm. These micrographs are all cross-sectional views of the samples with the shock direction shown by the arrows. These micrographs illustrate the large average aspect ratio of the Bi2Sr2CaCu208 powder particles and reveal the high degree of crystallographic alignment possible by shock-consolidating pre-aligned micaceous powder. Moreover, the shocked samples exhibit a high density ( > 95% crystal density) as determ.~ned by the absence of visible voids and cracks. No texturing is evident in the optical micrograph of the < 5 lam particle size sample (fig. 4(d)). Since the aspect ratio of the plateletshaped Bi2Sr2CaCu2Os particles tends to decrease with decreasing particle size and since the process of the mechanical alignment of the Bi2Sr2CaCu2Os platelet particles by tapping is dependent on the large aspect ratios of the powder particles, a decrease in texturing with decreasing particle size is to b,, expected. Note, also, that the smaller particles are also heated much more uniformly. Table 2 Hugoniot parameters C and S for lexan and copper. Material

C

S

Lexa• Copper

2.20 km/s 3.94 km/s

1.67 1.49

S.T. Weir et ai. / Shock consolidation of crystallographically aligned BizSr2CaCuzOs powders

5

Fig. 4. Photomicrographsof the shockconsolidated Bi2Sr2CaCu2Ossamples. The arrows point in the direction of pre-shocktapping and of shock-loading. (a) Sample shocked at 100 kbar, with 30-37 l~m powder (BT02). (b) 100 kbar, 10-15 ~m powder (BT06), (c) 100 kbar, 5-10 pm powder (BT07). (d) 100 kbar, < 5 I~mpowder (BT08).

The optical micrographs also reveal that the mechanical stresses due to the shocks are accommodated mostly by plastic deformation of the powder particles, with very little shock-induced fracturing. This is in contrast to the behavior of shock-consolidated YBaeCu307 powder, which showed a considerable amount of shock-induced fracturing in the same pressure range [2 ].

4. X-ray diffraction For those shocked samples with particle sizes of 30-37 ~tm (BT01 to BT05), X-ray diffraction measurements were performed to determine the degree of crystallographic alignment at each stage of processing. The results for an as-tapped specimen and specimens shock-compacted to various pressures are shown in fig. 5. Figure 5(b) for the as-tapped specimen shows that crystallographic alignment is essentially achieved by tapping. Figures 5 (c-e) show that alignment is maintained by shock compaction. The

6

S.T. Weir et al. / Shock consolidation ofcrystallographically aligned Bi~Sr2CaCuzOs powders

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5 (d) ). This annealing was done in order to improve the transport properties of the samples, as detailed below. Upon annealing at 850°C in 02, the (001) peaks are significantly reduced and peaks not associated with Bi2Sr2CaCu2Os are evident (fig. 5 (e)), indicating that at this temperature the preferential alignment is decreasing and impurity phases are forming. Thus, Bi2Sr2CaCu2Os powder maintains its preferential alignment due to tapping upon shocking at pressures of up to 135 kbar and annealing at temperatures of up to 800 ° C. The effects of reducing the Bi2Sr2CaCu2Oa particle size are shown in fig. 6. Figure 6 (a) is the X-ray diffraction for powder sifted to 10-15 pm and shockconsolidated with a 100 kbar shock-wave; figure 6 (b) shows the pattern for 5-10 pm powder shocked to 100 kbar; and fig. 6 (c) shows the pattern for < 5 pm

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Fig. 5. X-ray ¢i"ffraction spectra of the starting Bi2Sr2CaCu2Oa 30-37 pm powder and of a crystaliographically aligned Bi2Sr2CaCu~O8 compact obtained by shock-consolidation with a 100 kbar shock wave. (a) The starting randomly aligned Bi2Sr:CaCu2Oa powder. (b) Bi2SrECaCu2Os powder after uniaxial tapping to about 65% of crystal density. (c) Bi2Sr2CaCu2Os tapped powder after shock-consolidation ( 100 kbar). Note that this dense compact retains the high degree of c-axis texture that was present in the tapped powder sample. (d) Shock-consolidated Bi2Sr2CaCu2Os after annealing at 800°C in oxygen. (e) Shock-consolidated Bi2Sr:CaCu2Os after annealing at 850°C in oxygen.

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results for all shock pressures are similar, so detailed discussion will be limited to the sample shocked at 100 kbar. A random powder of Bi2Sr2CaCu2Os is shown in fig. 5 (a), with the peaks indexed. Note that the three m o s t intense peaks are ( ! 15 ), ( I 17) and (200). However, after tapping the powder into the sample target fixture prior to shocking, the powder orients preferentially with the c-axes of the grains parallel to the tapping direction. This is shown in fig. 5(b), where the (00/) peaks have the greatest intensity. The alignment is maintained after shocking (fig. 5 (c)) and after annealing at 800 ° C in 02 (fig.

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S.'T. Weir et aL / Shock consolidation of crystallographically aligned Bi:,Sr2CaCu20s powders

7

ments of',he samples we, e made in two different orientations: ( 1 ) with the c-axis texture of the sample parallel to the applied magnetic field, and ~2) with the c-axis texture of the sample perpendicular to the applied magnetic field. Note that since there is a loss of superconductivity at the particle surfaces of a shock-consolidated sample, the grains in the asshocked specimens are effectively decoupled from one another by the nonsuperconducting, oxygen-depleted surface layers. Thus, low-field magnetization measurements probe the geometries and demagnetization factors of the individual grains of the sampies. If the individual grains comprising a sample are, say, spherical or have small aspect ratios, then these low-field magnetization measurements should be independent of sample orientation. If, however, the sample consists of platelet-shaped grains having large aspect ratios which are aligned with one another, the magnetization measured in orientation (1) should be appreciably larger than that measured for the sample in orientation (2), assuming that the c-axes are normal to the planes of the platelO particles. Indeed, fig. 8 shows that the magnetization ratios M,,,/M,±,. for the shocked samples are all considerably larger than 1, as expected for samples consisting of aligned, decoupled, platelet-shaped particles. For comparison, the magnetization ratios for a

powder shocked to 1O0 kbar. Note that the relative intensities of non- (OOl) lines progressively increase with decreasing particle size, indicating a decrease in c-axis alignment as the particle size decreases.

5. Magnetic measurements Magnetic screening measurements of the shocked compacts revealed no decrease in the onset Tc for shock pressures up to 135 kbar, the highest pressure used in our experiments (see fig. 7 ). The magnitude of the diamagnetic screening signal, however, decreased considerably with increasing shock pressure. This decrease in screening signal is apparently due to the effects of heterogeneous shock-heating at particle surfaces during consolidation, resulting in loss of oxygen and superconductivity at the particle surfaces but not from within the interiors of the particles. Thus, although the magnitude of the diamagnetic screening signal decreases with shocking, the onset Tc is not affected. Since magnetic screening measurements are sensitive to sample geometry through demagnetization factor effects [ 9 ], low-field screening measurements were used to determine the aspect ratios of the grains in our shocked samples. Magnetization measure0.5

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Temperature (K) Fig. 7. Magnetic susceptibility after zero-field-cooling of the samples. The 20 Oe magnetic field was applied parallel to the c-axis textures of the samples. Note that the onset T¢ is unaffected by shocking up to a pressure of at least 135 kbar, the highest pressure used in our experiments.

8

S. 7. Weir et al. / Shock consolidation ofcrystallographically aligned Bi,,SrzCaCu20s powders

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Temperature (K) Fig. 8. Temperature dependence of zero-field-cooled magnetic anisotropy, defined as the ratio of the zfc susceptibility measured parallel and perpendicular to the c-axis texture (x(HIIc)/)c(H.I-c)) for the as-shocked samples. (a) Magnetic anisotropy vs. temperature for several pressures ( particle size = 30-37 ~tm ), (b) magnetic anisotropy as a function of particle size ( pressure = 100 kbar).

tapped and pressed pellet sintered at 800°C is also shown. Note the enhancement of the magnetic anisotropy for the shocked samples over that for the sintered pellet. The large anisotropy ratios for the shocked specimens indicate extremely large average aspect ratios for the shocked particles, presumably

due to plastic flow of the grains in directions perpendicular to the applied shock stresses. This enhancement in the aspect ratio is beneficial for superconducting transport since it results in increased grain overlap areas, which leads to enhanced intergranular coupling [ 10,11 ].

S.T. Weir et al. / Shock consolidation of crystallographicall.v aligned Bi:Sr2CaCu208 powders

9

Estimates of the aspect ratios of the various samples can be easily obtained from the magnetic anisotropy measurements of fig. 8. If one assumes that the demagnetization factors of the particles are equal to zero in the HA_c-texture orientation, then the magnetization ratio Mm,./Mul,. is related to the average demagnetization factor D of the particles in the Hllc-texture orientation by the equation [9].

Mm,./M,j.,.= 1] ( 1 - D ) .

(4)

To the approximation that the particle shapes are ellipsoids, the demagnetization factor D can be related to the aspect ratio of the particles through the equation [9]

1 D= l - ( a / b ) 2

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s i n - t ( x / l - ( a / b ) 2)) ,

(5)

where b is the length of the equatorial axis of the ellipsoid, and a is the length of the polar axis. The ratio b/a, then, is the aspect ratio of the ellipsoid. For example, the anisotropy ratio Mm,./M,±,. of approximately 4 for the pressed and annealed pellet corresponds to a demagnetization factor 1!) of 0.75 and, by eq. (5), to an average particle aspect ratio of 5; in contrast, for the as-shocked samples the anisotropy ratio Mm,./Mnj.,. averages about 6, which translates into an average particle aspect ratio of 8.

6. Transmission electron microscopy (TEM) To determine the defect structure of shocked Bi2Sr2CaCu2Os, the sample consisting of 30-37 ~tm powder shocked to 100 kbar (BT02) was studied by transmission electron microscopy (TEM). The TEM work was performed with a JEM4000 Ex microscope at Tohoku University. A lattice image of the 100 kbar shocked specimen is shown in fig. 9. Note the many shock-induced kink boundaries in the material, although the amount of kinking in this sample, in which the shock direction was parallel to the c-axis texture, was found to be less than that found in a shocked BiESrECaCu208 sample which was not crystallographically oriented [12]. Apparently, the density of kink boundaries can be

Fig. 9. TEM micrograph of a specimen shocked to 100 kbar (BT02, 30-37 ~tm powder ). The Cu-O planes are imaged in this micrograph. Note the large number of shock-induced kink boundaries.

varied by choice of the orientation of shock propagation relative to the c-axis texture. Unfortunately, magnetic relaxation and magnetic hysteresis experiments on these specimens do not seem to reveal any enhanced flux pinning due to these kink boundaries, in contrast to the case of YBaECU307 in which shockinduced dislocations were found to significantly enhance flux pinning and intragranular Jc's. Weak bonding between the (00/) basal planes of Bi2SrzCaCu20~ i~ apparent by the readiness with which slip occurs on these planes. This material exh~blts much more ductility than YBa2Cu307 as evidenced by the largc amount of bending and plastic deforma~.ion seen in the lower part of the photograph, a~d oy the absence of v~slo~c ~:Lu~,:~.

7. Transport measurements Electrical resistivity p measurements were r.aade

S. 7". Weir et aL/ Shock consolidation of crystallographically aligned Bi:,Sr2CaCu20s powders

10

using an AC self-balancing four-wire bridge operating at 16 Hz. Figure 10 shows the temperature dependence ofp for a sample shocked to 100 kbar and after various oxygen anneals. As exemplified by this sample, the resistivity of all as-shocked samples increased as the temperature was lowered below room temperature, but reached a maximum at a low temperature Tma,, below which p decreased rapidly, indicating the onset of superconductivity along the curren: path. Since the current is forced to flow through the boundaries of the particles (as well as the particle interiors with lower resistivity) which are bonded by shock compaction, the resistivity is dominated by these high resistance regions of the sample. Indeed, our resistivity data look similar to that of oxygen-deficient crystals of Bi2Sr2CaCu2Os, which have a reduced T~ and semiconducting-like behavior above T~ [ 13 ], which suggest that the oxygen content of the particle boundaries of the shocked samples is reduced, probably due to the heat generated in these regions during the shock process. Because of the fast I~Stime scale of this process, the oxygen might not have time to diffuse out of the sample, but may be trapped in regions of the specimen. Figure I l shows how Tma, and the room temperature resistivity p(RT) of the as-shocked samples

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Fig. 10. Normalized resistance vs. temperature of the 100 kbar shocked sample after various heat treatments, along with data from an unshocked, aligned, sintered pellet. With the exception of the as-shocked data, the resistance curves have been normalized so that R~T=295 K ) = 1.

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varied with shock pressure P. The monotonic decrease of Tma x with P is consistent with a greater amount of oxygen deficiency at particle boundaries with increased P. The monotonic decrease o f p ( R T ) with P suggests that higher shock pressures yield stronger bonding between particles. Indeed, the samples shocked at higher pressures were mechanically more robust. In order to induce a higher resistive To, a series of anneals was performed. Annealing is expected to homogenize the oxygen content of the sample, improve bonding between particles, heal cracks, and reduce defects, depending on the tempe~'ature and duration of the anneal. We first tried annealing in 100% flowing He at 600°C since this was the last step in the preparation of the starting powders. A 6 h anneal of BT01 (70 kbar) greally improved the resistivity, increasing the resistive Tc onset to 90 K and inducing metallic behavior above To. However, the transition was broad. Upon longer anneals, the zero-resistance temperature T¢(R = 0 ) never got higher than ~ 60 K, which was observed after annealing for a total of 18

S.T. Weir et al. / Shock consolidation of crystallographically aligned Bi.,Sr2CaCu20s powders

h; longer anneals gave worse results. Even after annealing for 36 h, the resistivity of BT02 ( 100 kbar) remained semiconductor-like, although T¢(R=0) ~25 K. To get higher transport T¢'s, further oxygenation of the samples was necessary. After an anneal of these same two samples in 100% flowing oxygen at a low temperature of 450°C for 6 h, the resistive behaviors of both samples were found to be similar and as good as the best results of the above described He anneals. Clearly, these samples had a deficiency of oxygen at the particle boundaries due to shock compaction, especially BT02, which was replenished by the low temperature soak in oxygen. Further oxygen anneals of these samples at successively higher temperatures caused successively higher values of Tc (R = 0 ). A 12 h anneal at 800 ° C gave the best resistivity results which nearly duplicated that of a sintered pellet of the unshocked, original powder (see fig. 10). X-ray diffraction revealed that the preferential alignment of the grains was maintained. All oxygen anneals were followed by rapid cooling in flowing He to quench in the oxygen content. This type of anneal at 850°C resulted in a poor resistive result for both samples. Furthermore, X-ray diffraction revealed destruction of alignment at this temperature and the formation of impurity phases. We subsequently simultaneously annealed the other shock-compacted specimens in oxygen at 800 °C for 12 h and measured the resistivities. The room temperature resistivity is seen to be a monotonically increasing function of P, while T¢ and the temperature coefficient of resistivity decrease monotonically. This seems to indicate that the effects of shock are not completely erased after annealing, with lower shock pressures giving the better results. The transport critical current densities J~ of the 800°C oxygen annealed samples were measured at 4.2 K. While the unshocked pellet had a Jc of 40 A/ cm 2, values for the shocked samples varied from 100 A/cm 2 ( P = 7 0 kbar) to 250 A/cm 2 ( P = 5 0 kbar), a sixfold increase.

8. Conclusions Textured bulk compacts of Bi2Sr2CaCu208 having very good c-axis alignment have been fabricated by shock consolidation of pre-aligned platelet-shaped

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particles. Such compacts are valuable for transport studies since a high degree of texturing is essential for forming high-J¢ bulk samples having their highJc a - b planes aligned along the direction of bulk Jc transport. Furthermore, since the grain morphology of the BiESr2CaCu208 grains is platelet-like, crystallographic alignment ensures large grain-to-grain overlap areas, which results in enhanced intergranular coupling. Indeed, the magnetic anisotropy measurements presented here indicate that shock-consolidation significantly increases the aspect ratios of the individual grains, which should result in larger grain-to-grain overlap areas and enhanced intergranular coupling. Electrical resistivity measurements on the asshocked samples show that the bulk behavior is semiconducting, presumably due to oxygen-deficient semiconducting particle boundaries resulting from localized shock-heating. These measurements further indicate that the amount of oxygen deficiency increases with increasing shock pressure. Superconducting transport can be improved, however, by subjecting the as-shocked specimens to 800°C oxygen anneals. Critical current measurements of the shocked + annealed samples reveal an enhancement of the transport Jc's by a factor of 2 to 6 over that for a statically pressed+annealed pellet. However, the characteristically low transport Jc's of all these samples ( ~ 100 A/cm 2) indicates that further processing work is still needed with higher quality BizSr2CaCu208 powders to determine the maximum transport Jc achievable in bulk Bi2Sr2CaCu208 samples fabricated by shock-consolidation.

Acknowledgements We thank H.W. Jacobson and K.J. Leary of E.I. DuPont de Nemours and Company for providing the Bi2Sr2CaCu208 powder. Also, we thank P.C. McCandless and W.F. Brocious for fabricating the specimen holders, P. Bowen for firing the two-stage gas gun, and E. Aoyagi of the High Voltage Electron Microscope Laboratory, Tohoku University, for his technical contribution in TEM observation. This work was performed under the auspices of the U.S. Department of Energy by the LLNL under contract No. W-7405-ENG-48, with support from thc SDI

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S. T. Weir et ai. / Shock consolidation of cr.vstallographicallv aligned Bi=,Sr2CaCuzOs powders

Office of Innovative Science and Technology (SDI OIST). Work at UCSD was supported by SDI OIST and by the LLNL Branch of the University of California Institute of Geophysics and Planetary Physics. References [ ! ] W.J. Nellis and L.D. Woolf, Novel Preparation Methods for High-To Superconductors, MRS Bull. 14 (1989) 63. [ 2 ] W.J. Nellis, C.L. Seaman, M.B. Maple, E.A. Early, J.B. Holt, M. Kamegai, G.S. Smith, D.G. Hinks and D. Dabrowski, in: High Temperature Superconducting Compounds: Processing and Related Properties, eds. S. Whang and A. DasGupta, (TMS Publications, Warrendale, ! 989) p. 249. [ 3 ] L.E. Mutt, A.W. Hare and N.G. Eror, Nature 329 (1987) 37. [4] E.L. Venturini, R.A. Graham, D.S. Ginsley and B. Morosin, in: Proc. of the American Physical Society Topical Conference on the Shock Compression of Condensed Matter, Albuquerque, NM, 14--!7 August 1989, p. 583-586.

[51 S.T. Weir, W.J. Nellis, M.J. Kramer, C.L. Seaman, E.A. Early and M.B. Maple, Appl. Phys. Left. 56 (1990) 2042.

I61 C.L. Seaman, S.T. Weir, E.A. Early, M.B. Maple, W.J. Nellis, P.C. McCandless and W.F. Brocious, Appi. Phys. Lett. 57 (1990) 93. [71 W.J. Gourdin, J. Appl. Phys. 55 (1984) 172. [81 R.L. Williamson, J. Appl. Phys. 68 (1990) 1287. [91 E.C. Stoner, Philos, Mag. 36 (1945) 803. [101 J. Mannhart and C.C. Tsuei, Z. Physik B 77 (1989) 53. [Ill A.P. Malozemoff, to be published in: High Temperature Superconducting Compounds II, eds. S.H. Whang, A. DasGupta and R.B. Laibowitz (TMS Publications, Warrendale PA, 1990). [12] Y. Syono, M. Nagoshi, M. Kikuchi, A. Tokiwa, E. Aoyagi, T. Suzuki, K. Kusuba and K. Fukuoka, in: Proc. of the 1989 APS Topical Conference on the Shock Compression of Condensed Matter, Albuquerque, NM 14-17 August 1989, p. 579. G. Briceno and A. Zettl, Phys. Rev. B 40 (1989) 11352.