Pergamon Materials Research Bulletin, Vol. 29, No. 5, pp. 529-536, 1994

Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved

0025-5408/94 $6.00 + .00

FABRICATION AND CHARACTERISATIONS OF HIGH.To SUPERCONDUCTING CERAMIC/POLYMER 0-3 COMPOSITES

Jia Du* and Joe Unsworth Centre for Materials Technology, Faculty of Science

University of Technology, Sydney N.S.W. 2007, Australia

(Received J anua ry 26, 1994; Refereed)

ABSTRACT High-Tc superconducting ceramic YBa2Cu307_x/thermosetting plastic 0-3 composites were fabricated. The structure, physical property, magnetic susceptibility, levitation, and mechanical strength of the composites were accessed. The influence of filler content on these properties was also studied. Although the 0-3 composites lack an electrical superconducting path through materials, the intrinsic diamagnetic properties were preserved. The magnetic superconducting transition temperature was not degraded. The values of magnetic susceptibility and levitation force for the composites were basically proportional to the actual volume fraction of superconducting filler. These new composite materials are most suitable for the applications in levitating vehicles and mechanical beatings.

MATERIALS INDEX: Superconductor, thermosetting polymer, composites

Introduction

The discovery of high-Tc oxide ceramic superconductors with a transition temperature well above the liquid nitrogen temperature (77K) has opened up a wide variety of applications for superconductivity. However, there are a number of problems associated with the oxide ceramics, which have seriously impeded their practical applications. Like many other ceramics, high-Tc superconducting ceramics are inherently brittle and very difficult to be fabricated into any usable forms. Their mechanical properties like flexural strength, ductility and fracture toughness are generally poor. YBa2Cu307-x are also susceptible to environmental deterioration. The degradation is catalysed by water and carbon dioxide through the formation of barium hydroxide (Ba(OH)2) and carbonate (BaCO3) (1, 2).

Current address: Dept. of Organic Materials, National Inst. of Materials and Chemical Research, 1-1, Higashi, Tsukuba, Ibaraki 305, Japan.

529

530 J. DU et al. Vol. 29, No. 5

One effective approach to solve the above problems is to incorporate high-Tc ceramics into composites with a flexible polymer as the matrix. The polymer matrix provides mechanical integrity, chemical stability and processing flexibility. Versatile polymer processing techniques can be used to form composites into various useful shapes. There have been a few reports in the investigations of high-Tc superconductor/polymer Composites, which can be divided into the 0-3 type (3-7) and 3-3 type (8-11). The 0-3 composites consist of superconducting powder embedded in polymer matrices and 3-3 composites refer to the porous superconducting ceramics filled with polymer. In general, composites show some improvement in certain physical and mechanical properties such as flexural strength, Young's modulus, and hardness (5, 8, 10). In 3-3 composites, filling of polymer into the pores of ceramic superconductors has little effect on their electrical and magnetic superconducting properties (9, 11). In contrast, the characterisations of 0-3 composites (3, 4, 6) showed that the electrical resistance usually did not go to zero below Tc because of the lack of electrical connectivity between superconducting particles. However, the diamagnetic properties of superconductors were found to be preserved in composites (3, 6, 12). In particular, the previous work (12) showed that the 0-3 composites had very similar levitation properties including the hysteretic behaviour to that of the sintered ceramic superconductors. It suggests that the first potential applications for 0-3 composites would be in levitation areas, such as high speed vehicles and bearings in rotating machinery. Compared with pure ceramics and 3-3 composites, 0-3 composites can be more easily fabricated into the desirable forms with polymer processing techniques.

In this study, we fabricated a new class of 0-3 type superconducting composites with epoxy and phenolic thermosetting polymers as the matrix materials, which have not been previously reported. Work on 0-3 composites before has mainly concentrated on thermoplastic or rubber matrices. Thermosetting plastics have advantages of high toughness, superior abrasion, dimensional stability, and heat, water and chemical resistive. These features are of important in the applications of levitated vehicles and mechanical bearings. In particular, phenolic is a cheap and most commonly used plastic and can be processed into a wide range of items. The composites with phenolic matrix were very easy to fabricate by hot compression technique and high loading of filler could be achieved. The phenolic composites made in this investigation had the superconducting filler ranging from 0 to 80 vol.%. The epoxy composites ranged from 0 to 36 vol.% of filler. The structural and physical properties, magnetic susceptibility, levitation capability, tensile strength and the influence of filler content on these properties were studied and the results are presented here.

Fobrication of Composites

Sintered bulk YBa2Cu307.x ("123") ceramics, supplied by ANSTO, Australia, were milled into fine powder. Some ceramic pellets with 22-25mm diameter and 3-5 mm thickness were kept for characterisations and comparison. Phenol-formaldehyde resin (phenolic) (Union Carbide Corp.) used in this investigation was a partly cured powdered resin which allows fusion and crosslinking to a final continuous phase by application of heat and pressure. The "123"/phenolic composites were made by mixing the "123" and phenolic powders according to desired volume fraction and then processing into circular discs by hot compression technique on a hydra-press moulding machine. Larger samples with 30mm diameter and 4-5 mm thickness were used in levitation force measurement. Smaller samples with 25 mm diameter and 3.5 -4 mm thickness were used in magnetic susceptibility and mechanical strength measurements. The nominal composition of "123" filler ranged from 0 to 80 vol.%. The samples with 60 vol.% or below "123" filler had very good physical properties; they were tough and had smooth and shiny surfaces. The 70 and 80% composites had rough and crumbly surfaces which indicated a significant porosity introduced. The samples with different compositions were lapped down to the same thickness for measurements.

The "123"/epoxy composites were made by following steps. Araldite GY 250 epoxy resin and HY 956 hardener (CIBA-GIGY) were pre-degassed in a vacuum desiccator to remove

Vol. 29, No. 5 CERAMIC/POLYMER COMPOSITES 531

the trapped air bubbles. The resin was then mixed with "123" powder and degassed under vacuum. For the filler exceeding 20 vol.%, mixing was carried out at a slightly higher temperature (40-70 °C) to reduce the viscosity. HY 956 hardener was then added in, mixed, and degassed again for about 25 minutes. The final mixture was poured into a mould which was made of a Teflon slab with 30 mm diameter and 4 mm thick holes on it. The composite in mould was then allowed to cure and post-cure at room temperature over night. The filler ranged from 0 to 36 vol.%. Higher content was difficult to obtain due to high viscosity. In comparison, it was more convenient to process "123"/phenolic composites and much higher loading could be achieved. The characterisations were mainly carried out on the phenolic composites. The epoxy composites were evaluated with magnetic susceptibility and levitation force.

Measurement Results and Discussion

Fig. 1 shows the results of x-ray diffraction (XRD) analysis for "123" powder, phenolic disc and a 60% "123"/phenolic composite disc. The standard YBa2Cu3OT.x peaks (13) are also given at bottom for comparison. Both powder and composite show only the peaks of "123" phase and there are no additional peaks in composites which may indicate a third phase introduced during the processing. The pure phenolic polymer is basically an amorphous structure. The change of relative intensity in XRD peaks for the composite suggests that the crystals were aligned in composites during the hot compression processing.

The measured density for "123"/phenolic composites is shown in Fig.2. The straight line is the calculated theoretical density given by the formula:

D= v Ds + (l-v) Dp (1)

where D, Ds and Dp are the densities of composite, superconductor powder and polymer matrix respectively, v is the volume fraction of "123" filler in composites. From the graph, the composite density starts to deviate from the calculated value above 50 vol.% which indicates the increase of air porosity. There is a large drop in the density at 80%. It is probably due to the fact

(a) "123" powder I

I

(b) "123"/phen. composite 1 ,

(c) phenolic disc

(d) JCPDS-tCDD "123" peaks

[ i, i _ _ l ~

FIG. 1

X-ray diffraction patterns for (a) "123" powder, (b) 60% "123"/phenolic composite, (c) phenolic, and (d) standard "123" peaks (13).

3. DU et al. Vol . 29, No. 5 532

6.0

5.~

4.0

3.0

2.0

1.0

O0 20 40 60

VQI.% of '123' f i l ler

FIG. 2

Density vs vol.% of "123" filler in "123" phenolic composites.

that insufficient polymer could not wet all particles and the particle segregation might occur at this high level loading. SEM photographs of the cross-section of high loading (60 - 80%) composites are shown in Fig.3. From these photographs, "123" particles are very closely packed and a small amount of polymer can be identified in 60 and 70% samples. The 80% sample hardly shows any polymer matrix (black colour). It was difficult for the scarce polymer to wet all the particles and the bindings between the particles were probably weak. This would introduce significant voids. For this system, greater than 20 vol.% phenolic resin is needed to obtain a homogeneous dispersion.

FIG. 3

SEM photographs of microstructures of "123"/phenolic composites with (a) 60, (b) 70, and (c) 80 vol.% filler (the scale bar is 10~tm).

Electrical resistance measurements did not show any superconducting transitions near the critical temperature (Tc) even for very high loading composites. Diamagnetic susceptibility was measured for both epoxy and phenolic matrix composites. The values at 77K versus the vol.% of filler are shown in Figs.4 and 5 respectively. The measurement was carried out with a susceptometer having a balanced-coil system as detection coil (14) and the voltage induced in the secondary detection coil was recorded. The system was not calibrated against another standard system and, therefore, the measured susceptibility was given in arbitrary unit. It was adequate for the relative comparison between different samples. All composite samples were under same measurement conditions. From the figures, the susceptibility value increases linearly with the volume fraction of "123" filler in the composites. Since only superconductor powder contributed to diamagnetism, the composite susceptibility could be expressed as follows:

Z = v Zs + ( l -v) Zp = v Z~ (2)

I I I

Vol. 29, No. 5 CERAMIC/POLYMER COMPOSITES 533

where Z , Zs, Zp are the susceptibilities for composite, superconductor powder and polymer

matrix, and Zp ---~0. It must be emphasized that Zs in formula (2) is the susceptibility for superconducting powder which is different from the susceptibility of sintered superconductor. The diamagnetic susceptibility of bulk ceramic is usually composed of intrinsic (intragrain) and coupling (intergrain) components. In powder and 0-3 composites, the coupling component was suppressed and only intrinsic component was preserved (14). The derivation from the linear relationship above 50% for "123"/phenolic composites (Fig.4) was due to the increase in porosity. The existence of voids reduces the effective volume fraction of superconducting material and therefore decreases the susceptibility value. It is consistent with the density measurement. The measurement of susceptibility versus temperature for composites showed that the diamagnetic transition occurred at around 91K which was the same with that of pure sintered "123" ceramic. Fig.6 gives an example of diamagnetic transition curve, measured for a 32%

190

160

140

120

100

B0

60

40

20

0

O

[]

10 20

O

O

30 4O 50

Vol% o1123 filler

FIG. 4

D

70 80

120

100

B0

60 []

40 0

20

0 0 10 20 30

Vol % of 123 filler

FIG. 5

Susceptibility at 77K vs vol.% of "123" filler for "123"/phenolic composites.

Susceptibility at 77K vs vol.% of "123" filler for "123"/epoxy composites.

i t

f

/

. / ,

. . . . . . . . . . . . . . ~ , ; ' " ' ; ' ; " ~ ' ~ , . . . . I , , I , 30 60 90

Temperature (K)

N o i n t e r g r a i n o r

c o u p l i n g l o s s

H a ( O e r .m.s . )

- 0 . 0 0 6 5

- 0 . 0 6 5

- 0 . 6 5

T I , i 120

FIG. 6

Susceptibility vs temperature for a TI 3 ~, 32% 12. /epoxycomposlte.

i50

534 J . DU e t a l . V o l . 29 , No . 5

"123"/epoxy composite. From the graph, there are no intergrain coupling loss peaks (X") and a

sharp transition of real susceptibility (X') occurs at around 91K. The x-T curve was insensitive to the applied fields at low field range. The magnetic transition curves for other composites showed same features except for different magnitudes of susceptibility.

All composite samples demonstrated magnetic levitation capability. A simple electronic balance technique (6) was used to measure the levitation force versus volume fraction of "123" filler for the composites. A small permanent magnet was placed on the balance pan and the composite sample in a liquid nitrogen container was put above the magnet. The weight change detected by the sensitive balance was due to the repulsive force generated by the magnetic field on the superconducting sample. The value of force is given by

F(mN) = AW(g) x 9.8 (m/s 2) (3)

where AW is the apparent mass change of the magnet. The measurement conditions were kept the same for all the samples. The results are shown in Figs. 7 and 8. The nominal 100 vol.% data in Fig.7 was for a bulk "123" ceramic sample. The data for the sintered bulk ceramics was found to vary from sample to sample, depending on the bulk density of each disc. From the graphs, the values of levitation force are approximately proportional to the volume fraction of "123" filler in composites. The force exerted by the magnet on the sample is given by (15)

F = m(dI-l/dx) (4)

m is the magnetic moment of the superconducting sample, dH/dx is the field gradient produced

by the magnet, m= M(H) V* = M(H) vV, where M is the magnetisation and it is field dependent, V* is the volume of superconductor phase, and V is the total volume of composite sample. Equation (4) can then be expressed as

F = vVM(H) dH/dx (5)

A

o~

5

[]

[ ] I o o 2 0

o

[] [ ]

[ ]

[ ]

[ ]

I I I

4 0 6 0 8 0

V o l . % o f " 1 2 3 " F i l l e r

FIG. 7

Levitation force vs vol.% of "123" filler for "123"/phenolic composites. The data at 100% (nominal composition) was for a bulk "123" ceramic disc.

0 ~ I I I I I I

5 1 0 15 2 0 2 5 3 0

V O I . % o f " 1 2 3 " F i l l e r

3 5

FIG. 8

Levitation force vs vol.% of "123" filler for "123"/epoxy composites.

Vol. 29, No. 5 CERAMIC/POLYER COMPOSITES 535

-ff 0 . .

v e "

e "

I t l ¢ . .

b "

In this experiment, the distance between samples and the magnet was fixed and, therefore, the field strength H, field gradient dH/dx and magnetisation M were the same for all samples. The forces are merely proportional to v, the volume fraction of "123" filler. The experimental results agreed with the above formula. The deviation at high loading in "123"/phenolic composites (Fig.7) is due to the increase of voids, which reduces the actual volume fraction of "123" superconductor phase. Even though the superconducting particles are basically separated from each other in 0-3 composites, the magnetic levitation forces generated by superconductors are essentially not diluted in composites and the values depend only on the real mass of superconducting fillers. This suggests that the superconducting loops responsible for levitation are localised in individual grains and the intergrain coupling currents have little effect. The 0-3 composite materials, therefore, can be used to replace the hard and brittle superconducting ceramics in levitation applications.

The tensile strength of "123"/phenolic composites was measured with a diametral- compression technique(l 6). A disc specimen was compressed along a diameter between two flat platens. The tensile stresses caused the specimen to fracture along the diameter. The maximum tensile stress which acts across the loaded diameter is given by (16)

= 2P/(n dt) (MPa) (6)

where P(N) is the applied load at fracture, d(mm) is the specimen diameter and t(mm) is the specimen thickness. Test was performed with an Instron machine (mould 6022) at a cross-head speed of 0.5 mm/min. The result of tensile strength versus the volume fraction of "123" filler for phenolic composites is given in Fig.9. The composites with 50% or less filler have values of strength between 10.3 and 14.2 MPa, which is comparable to the measured value of 12.5 MPa for the pure phenolic disc. There is a large decrease in strength when the filler concentration exceeds 60 vol.%. This is attributed to the increase of defects such as voids and cracks in high loading composites. Also, due to the scarcity of phenolic resin in the high loading composites, the polymer might not wet the particles thoroughly and the adhesive bonding between the particles became relatively weak. The cracks would initiate and propagate under tension at the voids and the interfaces of particles and polymer, thus reduce the strength of the materials. Reliable data was not able to obtained for "123" ceramic due to the incorrect fracture mode.

16

14

12

10

8

6

4

2

0 0

i

10

T (3 E]

1 1

i

20 30

i i i i

40 50 60 70

i

8o 90

FIG. 9

Tensile strength vs vol.% of "123" filler for "123"/phenolic composites by diametral-compression test.

Vol,% of 123 filler

536 J. DU et al . Vol . 29, No. 5

Low and Lim (8) reported the tensile strength of 6.5 MPa for "123" ceramic and 19.3 to 26 MPa for the epoxy impregnated "123" samples (3-3 composites) by diametral compression testing. The data obtained here for the "123"/phenolic composites with below 60% "123" filler falls within their values for "123" ceramic and the epoxy impregnated "123" 3-3 composites.

Conclusions

High-Tc superconducting ceramic/thermosetting polymer composite materials with a wide range of compositions were fabricated. Composites in general showed good physical properties and improved mechanical strength provided that the filler not exceeds 60 vol.%. Magnetic susceptibility and levitation force measurements showed that the intrinsic diamagnetic properties of superconductor were preserved in composites and the magnetic superconducting transition temperature was not degraded. The values of both susceptibility and levitation force were basically proportional to the volume fraction of superconducting filler. Particularly, levitation force measurement showed that a continuous superconducting path throughout the material was unnecessary for magnetic levitation. The composites performance as well as ceramic superconductor. Therefore, the first applications for these new composite materials would be in the levitation areas. In comparison with ceramics, composites offer a better chance to construct material into complex shapes with better mechanical strength and lower weight.

Acknowledgment

The authors would like to thank Mr. B.J. Crosby for helping with susceptibility measurement, Dr. M. Stevens and G. Heness for useful suggestions.

This work was supported by the GIRD New Materials Grant, Department of Industry, Technology and Commerce of Australian Government.

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