fabrication and electrical properties of ceramic superconductor/polymer composites

Fabrication and Electrical Properties of Ceramic Superconductor I Polymer Composites

ASOK K. SARKAR"

University of Dayton Research Institute Dayton, Ohio 45469-01 70

and

T . L. PETERSON

Wright Laboratory, Materials Directorate, WL / MLPO Wright-Patterson Air Force Base, Ohio 45433-6533

Superconductor/polymer composites were prepared by mixing powders representing the Y-Ba-Cu-0 and Bi(Pb)-Sr-Ca-Cu-0 systems with high-density polyethylene. Their electrical resistivities were measured as a function of temperature. By controlling the powder preparation techniques, it was possible to fabricate composites with superconductive transition to zero resistivity in the Bi(Pb)-Sr- Ca-Cu-0 system. These composites can be

INTRODUCTION

ith the advent of high-T, ceramic supercon- W ductors, many investigators prepared superconductor/polymer (SIP) composites because of their expected beneficial properties (1 -5). But these materials in most cases exhibited high electrical resistivities at room temperature, with values as high as l O I 4 to 10l6 ohm-cm, and did not show a superconductive transition to zero resistivity down to liquid helium temperature. Alford, et al. (5). however, prepared such composites using YBa,Cu 307.x powders and thermoset resins with very limited suc- cess. It is now generally agreed that the magnetic properties of the superconductors, such as shielding and levitation, can be preserved in these composites. However, the bulk zero resistivity transition temperature, T,, can be transferred to these composites only when they are fabricated via novel infiltration techniques (6, 7). Unfortunately, one very im- portant attribute of a polymer composite, namely, the ease of forming a desired shape, is lost when this technique is used.

In a renewed interest to modify the electrical properties of S/P composites, we carried out further ex- ploratory investigations. The aim of these experi- ments was to explore the feasibility of designing moldable S/P composites capable of undergoing a zero resistivity transition. A special processing method was used to enhance particle-to-particle contact. Here, we present our findings regarding the

*WL/POOX-3. Wright-Patterson Air Force Base. OH 45433-6563.

fabricated in certain desired shapes.

electrical properties of various composite samples fabricated by mixing a solid polymer with superconducting powders from two different material systems.

EXPERIMENTAL

High-T, superconducting powders representing both the Y-Ba-Cu-0 (123) and Bi(Pb)-Sr-Ca-Cu-0 (BSCCO) system were synthesized in the laboratory using a standard solid state reaction technique. The powder prepared from the Y-Ba-Cu-0 system was identified as single phase superconducting YBa,Cu307., (T, = 90 K) by powder X-ray diffrac- tion (XRD). On the other hand, since the BSCCO system has two superconducting phases, several different powders were prepared for this system by varying the nominal compositions and the processing conditions. These powders contained various amounts of both the 2212 (T, = 65 K) and the 2223 (T,= 108 K) phases along with some impurity phases, such as Ca,Pb04, Ca,CuO,, and CuO. Trace amounts of the semiconducting 2201 phase were also detected in these powders by XRD. Typical XRD patterns of these powders are shown in Fig. 1. The density values used for calculating volume compositions of the composites were 6.37 g/cc and 6.02 g/cc for the 123 and the BSCCO powders, respectively. Handling of the powders was carried out in regular laboratory atmosphere.

To prepare the S/P composites, the superconducting powders were sieved through a 200 mesh screen and then mixed with different proportions of Marlex

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Asok K. Sarkar and T. L. Peterson

6006 powder, a commercial high-density polyethylene (density = 0.962 g/cc), which had been sieved through a 100 mesh screen. Two kinds of superconducting powders were used. The first kind was ei- ther the straight precursor powder (for 123 powder) or that obtained (for BSCCO powder) by grinding and sieving the sintered pellet. However, as described later, improved properties were obtained when the sieved powder was reannealed at - 870°C for a period of 40 to 120 h before mixing with the polymer. The blending of the superconducting and polymer powders was accomplished by gentle grinding in an agate mortar until a uniform colored mixture was produced. The mixture ( - 2 g) was care-

fully poured into a mold, heated in an oven to - 180°C (the softening temperature of the polymer), and finally pressed in a hydraulic press to fabricate the composite pellet disks. The dc electrical resistivity of various composite pellets was measured by the four-probe technique, and their microstructural properties were examined via scanning electron mi- croscopy (SEMI. The compositions of all pellets, along with their room temperature resistivity values, are presented in Table 1.

RESULTS AND DISCUSSION

At the beginning of our work, freshly prepared 123 powder was used to fabricate the composites.

c m Z W I- Z -

4 14 24 34 44 4 14 24 34 44

DEGREE (2 e)

z m Z W I- Z -

DEGREE (2 0)

I I I

I (D) I

4 14 24 34 44 4 14 24 34 44

DEGREE (2 0) DEGREE (2 e) Fig. 1. XRD pat terns of BSCCO powders used in preparing A) sample B; B) sample C; CI sample D: and Dl samples E and F.

Table 1. Physical Properties of S IP Composites.

Room Temperature Sample Superconductor Composition Used Vol % Resistivity, Ohm-cm

A YBa2Cu30, Precursor 25 0 15 50

C "1 8aPb0 3aSrl 91Ca2 03cu3 06Oy Annealed 17 0 27 90 D "1 8SPbO 35sr1 91Ca2 lCu3 lo, Annealed 9 5 39 40 E B'l asPb, 35'll 91Ca2 lCu3 1 0 , Sieved 9 5 177 80

Polymer Nature of Powder

B 91, ,Pbo ,Sr2Ca2Cu30, Annealed 0 0 0 2 0

F BIl 85Pb0 35sr1 91Ca2 lcu3 10, Annealed 8 0 4 2 2

306 POLYMER ENGINEERING AND SCIENCE, MID-MARCH 1992, Vol. 32, NO. 5

Fabrication a n d Electrical Properties

Despite using fresh powder with a well-oxygenated orthorhombic structure, the S/P composites made with this powder showed semiconducting resistance vs. temperature behavior. A typical resistivity vs. temperature plot for a 123/polyethylene composite (sample A) is shown in Fig. 2. The room temperature resistivities of these composites could be changed by varying the volume fraction of the polymer. However, a resistivity drop near 90 K was never observed in these specimens because of very poor grain-to-grain connectivity.

The electrical characteristics of these composite pellets were also affected by the degree of handling of the superconducting powders. The 123 powder is known to be extremely sensitive to moisture and carbon dioxide present in the environment (8). Even when a pellet was produced without any polymer binder by simply compacting the freshly prepared oxygen annealed powder, the off-stoichiometric nature of the degraded powder surfaces dominated the electrical behavior. The resistivity of this pellet con- tinued to show semiconducting behavior as it was cooled to 4 K. It was thus concluded that unless the surfaces of individual powder particles were pro- tected by handling them in an inert atmosphere, making superconducting 123/polymer composites by the present technique would be very difficult.

The BSCCO system, on the other hand, has been reported to be environmentally more stable than the 123 system (9). To improve our chances of fabricat- ing a S/P composite with a zero resistivity Tc, most of our efforts therefore focused on the BSCCO system. Sample B was made from well-annealed BSCCO powder pressed into a pellet in the absence of any polymer. No further treatment occurred after pressing. Note in Fig. 3 that unlike 123 samples prepared similarly, this sample undergoes a superconducting transition to zero resistivity at about 25 K. Using the XRD pattern, it was estimated that about two thirds of this powder was the 2212 phase and one third

E c 400 Y

W

= 100

0 5 0 100 1 5 0 2 0 0 2 5 0 300 TEMPERATURE (K)

Fig. 2. Resistivity us. temperature for sample A, containing 25 uol. % polymer.

was the 2223 phase (by volume). The presence of the 2223 phase in sample B is indicated in Fig. 3 by the drop in resistivity at about 110 K.

Resistivity vs. temperature plots for various BSCCO powder/polymer composite pellets are shown in Figs. 4 through 6. The composites (samples C and D) in Fig. 4 were made with BSCCO powders, which contained a lesser amount of the 2223 phase (one fourth by volume) than the powder used in sample B. These samples were found to be semiconducting down to a temperature of about 20 to 30 K, and then their resistivities dropped signifi- cantly upon further cooling to 4 K. These samples did not show any drop in resistivity near 110 K because of the presence of the 2223 phase or at 65 K because of the 2212 phase. The reason for the resistivity drop near 25 K is not clearly understood.

The final attempts to fabricate a superconducting S/P composite were made using a BSCCO powder

0.30 .

TEMPERATURE (K) Fig. 3. Resistivity us. temperature for sample B, containing no polymer.

180 I

SAMPLE C

" ~ " ' ~ " ' ~ ~ ~ " ~ ~ " ' ~ ' " " ~ . 0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0

TEMPERATURE (K) Fig. 4. Resistivity us. temperaturefor samples C and D, containing 1 7 vol% and 9.5 vol% polymer, respectively.

307 POLYMER ENGINEERING AND SCIENCE, MID-MARCH 1992, VOl. 32, NO. 5

Asok K. Sarkar and T. L. Peterson

0.4

9 E - 0.3 W 0 a 0.2

that had the highest volume fraction (> 95%) of the high-T, 2223 phase in it. Resistivity as a function of temperature of a composite (sample E) made from this powder is shown in Fig. 5. Unfortunately, sample E behaved in a semiconducting manner very much like sample A (123/polymer), described ear- lier. We believe that the surfaces of the individual superconducting particles in sample E were highly contaminated as a result of the grinding and sieving actions. Even though the powder had a large volume fraction of the high T, phase, the surface condition of the superconducting particles was very detri- mental and dominated the electrical behavior of the composite produced with them.

The surface condition of the particles also affects the room temperature resistivity values of the composites, and these values can be used to judge the quality of the composite. A low room temperature resistivity (less than 40 ohm-cm) seems to enhance the chances that these composites will have a large drop in resistivity at low temperatures. This is con- sistent with the hypothesis that the resistivity of these specimens is determined by the quality of the superconductor as well as the proximity of the superconducting particles to each other.

The powder used for sample E was annealed in air for 40 h at 870°C and then used to prepare composite sample F. The resistivity of sample F as a function of temperature is shown in Fig. 6. It is noted that the room temperature resistivity is quite low (about 4 ohm-cm), and although its resistivity ini- tially rises as the temperature is lowered, its resistivity eventually reaches zero at 15 K. The presence of the high-T, phase is indicated by a leveling off of the resistivity near 100 K, and the resistivity begins to fall below about 80 K. Although this sample has not been optimized for its superconductive properties, we have shown that it is possible to produce a SIP composite sample that becomes resistively superconducting using a technique other than infiltration of a superconductor with polymer. The resistivity data for this sample was somewhat noisy near the zero resistivity transition. A current of only 1 pA (J = 5.6 X A/cm2) was used, but this gave us sufficient voltage resolution on our Keithly 181 nanovoltmeter. We also measured the critical current at 4.2 K, and the voltage vs. current data, taken as the current was first increased and then de- creased, are shown in Fig. 7. A critical current of 0.3 mA at 4.2 K was measured. By use of the cross- sectional area of the composite sample, the critical density of the composite was calculated to be 17 A/cm2.

The electrical properties of S/P composites are expected to depend on the degree of filling and the proximity of individual superconducting particles. The intergranular region between the superconducting particles is critical to the performance of the composite. Some attempts to understand the behavior of the composites prepared in this study were performed by studying their microstructure with

-

-

-

2400 I 2ooo t \ h

E Y c 1600 w

1200 -

I- v, 800 -

400 -

0 5 0 100 150 200 250 300 TEMPERATURE (K)

Fig. 5. Resistivity us. temperature for sample E, containing 9.5 ~ 0 1 % polymer.

" 0 5 0 100 150 200 250 300

TEMPERATURE (K) Fig. 6. Resistivity us. temperature for sample F , containing 8 uol% polymer.

0.5 1 I t * I

0

0

0

0

1 0

0 w

* o @ ' 0.0 , . I . # . , .

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 CURRENT (mA)

Fig. 7. Voltage us. current a t 4.2 K for sample F, containing 8 vol% polymer.

308 POLYMER ENGINEERING AND SCIENCE, MID-MARCH 1992, Vol. 32, NO. 5

Fabrication and Electrical Properties

SEM. Typical micrographs showing the morphologi- cal characteristics of the superconducting powders in three of the composites are shown in Fig. 8. The 123 particles are angular, whereas the BSCCO particles are fibrous and agglomerated. The micrograph for the 123/polymer composite (sample A) shows that the 123 particles are quite separated by the polymer. The BSCCO/polymer composites, on the other hand, contain much smaller amounts of polymer between the particles, thus facilitating improved intergranular contact. The particles are more uniformly distributed in sample E than in sample F, which may be one of the reasons for the poorer electrical performance of sample E.

CONCLUSION

We have been able to show that an SIP composite prepared by a technique other than infiltration can be made to be resistively superconducting. Al- though no moldability test for this composite, which contained about 8 vol% polymer, was performed, we believe that this technique can be applied to produce SIP composites of certain desired shapes. For example, simple shapes very similar to our pellets and hollow rings can be fabricated by uniaxial or isostatic pressing operations. It is necessary, of course, to keep the percentage of the polymer to a minimum in order to make these composites resistively superconducting. The surface characteristics of the superconducting powders also play an impor- tant role in the room-temperature resistivity and the temperature dependence of resistivity of these composites.

REFERENCES

1. J . M. Ferreira, M. B. Maple, H. Zhou, R. R. Hake, B. W. Lee, C. L. Seaman, M. V. Kuric, and R. P. Guertin, Appl. Phys. A , 47, 105 (1988).

2. P. A. Fuierer, T. T. Srinivasan, and R. E. Newnham, in Ceramic Superconductors I1 - Research Update, p. 438, M. F. Yan, ed., American Ceramic Society, Westerville, Ohio (1988).

3. H. C. Van Woert, J r . , and G. C. Brumlik, Adv. Polym. Technol., 8 , 63 (1988).

4 . A. S. De Reggi, C. K. Chiang, L. Swartzendruber, and G. T. Davis, Proc. High-T, Superconductors: Magnetic Interactions, p. 340, L. H. Bennet, Y . Flow, and G. C. Vezzoli, eds., World Scientific, Singapore (1989).

5. M. McN. Alford, J. D. Birchall, W. J . Clegg, M. A. Harmer, and K. Kendall, in Ceramic Superconductors 11-Research Update, p. 428, M. F. Yan. ed., American Ceramic Society, Westerville, Ohio (1988).

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7. C. Vipulanadan, S. Salib, and K. Ravi-Chandar, Mater. Res. Bull., 24, 1059 (1989).

8 . M. F. Yan, R. L. Barns, H. M. O’Bryan, P. K. Gallagher. R. C. Sherwood, a n d S . J in , Appl. Phys. Lett., 51, 572 (1987).

9. H. Maeda, Y. Tanaka, M. Fukutomi, and T. Asano, Jpn. J. Appl. Phys., 2 7 , L209 (1988).

(Cl

Fig. 8. SEM photographs (polished section) of various superconductor /po lymer composites: a) sample A containing 25 uol% polymer; b) Sample E containing 9.5 001% polymer; and c) Sample F containing 8 uol% polymer.

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fabrication and electrical properties of ceramic superconductor/polymer composites

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