(BI,PB)2SR2CA2CU3O10+DELTA SUPERCONDUCTOR COMPOSITES - CERAMICS VS FIBERS

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Physica C 185-189 (1991) 2401-2402 North-Holland

(Bi, Pb)2Sr2Ca2Cu3Ol0+8 SUPERCONDUCTOR COMPOSITES: CERAM[CS VS. FIBERS Y. HUANG, G. F. DE LA FUENTE, A. SOTELO, A. BADIA, F. LERA, R. NAVARRO, C. RILLO, R. IBAI~EZ*, D. BELTRAN*, F. SAPIiqA* and A. BELTRAN. ICMA, CSIC-Universidad de Zaragoza, 50009 Zaragoza, Spain *UICBM, Dpt. Qufmica Inorg~inica, Universitat de Valencia, C/Dr. Moliner 50, 46100 Burjassot (Valencia), Spain..

Well characterized (Bi, Pb)2Sr2Ca2Cu3Ol0+l 5 superconductor powder has been used to prepare superconductor-glass, -metal and -alloy composites through solid state reaction method. A recently developed Laser Noating Zone (LFZ) apparatus has been used to transform the ceramic precursors into oriented fibers. The diamagnetic properties have been studied by a.c. susceptibility. The microstmcture of fibers has been studied by SEM and compared with that of the original ceramic precursors. XRD has been used to study phase composition on representative composite samples and fibers. The ~sults indicate some potential for the 2223-Ag composite, which displays improved diamagnetic properties and grain orientation (fiber). 1. INTRODUCTION Ceramic composites offer many advantages for potential applications based on high T c superconductors. These may include improved grain orientation, interconnectivity, and mechanical properties. In addition, a metallic composite matrix may provide an alternate conducting pathway for current and heat dissipation in case of device failure. This report presents results obtained in a study of glass, alloy and metal-superconductor matrix composites prepared by solid state reaction, and later transformed into fibers by the LFZ growth method.l 2. EXPERIMENTAL Nearly single

phase

ceramic

powder

of

mm3). The Laser Floating Zone growth method, used to transform the ceramic composite bars imo oriented fibers, has been described elsewhere. 3 3. RESULTS AND DISCUSSION The %'(T) results obtained for the stm-fingmateriaJ, two of the composites studied and a fiber are compared in Fig. 1. The glass composite gives no diamagnetic si~al even at 4.2 K This is {:ortsis~.e~ with a considerable degree of reaction taking piace between the glass and the superconductor phases during sintering. Such a reaction seems to destroy the 2223 and 2212 phases present iniliaUy in the superconductor. For the CuSn alloy composite, the 2223 phase almost disappear. The Ag composite, oa Ne other hand, still contains a significant amount of 2223

(Bi,Pb)2Sr2Ca2Cu3010+t ~, previously prepared following published procedures 2, was mixed thoroughly in an agate mortar with 10 wt% CaB407 (glass) (1), CuSn (alloy) (tlD and Ag (metal) (II/), respectively. The solid mixtures were compacted into square cross-section bars and sintered in air under the conditions described in Table I. Phase composition within the bulk of the resulting composite pellets was performed using standard powder XRD. Microstructure was determined using a JEOL 258 SEM. Superconducting phases were characterised by a.c. susceptibility.

The in-phase component, z'(T), was

recorded a~ a frequency v = 333 Hz and an a.c. field amplitude ho = 1 0 e for bar shaped samples (10x2x2

TABLE g. Composite eom~:x3si~on and processing cond~ons Stoichiometry

Sinlering Conditions

Powder XRD Results

2223 + CaB407 (10 wt% gnass)

830 °C / 50 h

No SC phases four~d Other phases uniden~fied

2223 + Cu94Sn6 (I0 wl% alloy)

830 °C / 50 h

22'{ 2 {Majob'}; 2223 (Mino;',, "Ca2#bO 4, CuO C u 2 0 Sr~o2

2223 + Ag (10 wt% metal)

850 ~ C / 5 0 h

2212 (Major); 2223 "~ Ag20*

LFZ Processing + 800 °C/50 h

2212 (Major); 2223 (Minor) Ag20"

(2223 + Ag) Fiber

" Impurity phases (9ow cone ); *° Significaql amounl present

0921-4534/91/$03.50 © 199] - Elsevier Science Publishers B.V. All fights ~scr~,ed.

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0,000

"0,010

munllmm

s

3

,a

a& & 2 ~ 4

T (K) -0,020

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I

40

|

I

80

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120

|

160

FIGURE 1: Ac susceptibility of several 2223 samples:

1) 2223 Powder obtained by polymer mute. 2) 2223+Cu/Sn (10% wt) composite (pellet). 3) 2223 +Ag (10% wt) composite (pellet). 4) 2223 +Ag (10% wt) composite (LFZ grown fiber). phase, combined with an increased amount of 2212. While these are not the ideal characteristics of a good HTSC composite, the deterioration of the original 2223 properties may be due to the sintering and cooling conditions imposed in this particular procedure, and not neccessarily to the nature of the metal-superconductor interaction at high temperature. The fiber does not seem to have poorer properties than the corresponding ceramic composite from which it was grown, and, since it required melt processing, it indk:atcs that the interaction between the Ag metal and the oxide superconductor in the molten state does not lead to destruction of the original superconductor characteristics. SEM micrographs obtained for an as-grown 2223-Ag composite fiber, and for the corresponding ceramic bar used as precursor in LFZ growth, are compared in Fig. 2. In the ceramic, grains are oriented at random and show a particle size of about 10-20 gm along the elongated dimensions. For the fiber, however, the distinct orientation of the mm-sized grains along the growth direction can be clearly observed. Nevertheless, from transport measurements, there is evidence of poor intergranular connectivity ,,;:ran ,h~.e~ ~ ; ~ . . . . A ¢.,~u. . . . . . .~;. . . . . . . . . underway to characterise and opfimise transport properties. For the superconducting phases present in all the composites developed (see table I), susceptibility and XRD results seem to agree rather well. The presence of nonsuperconducting phases has been confh-rned by XRD, as shown in the Table. These insulating phases do not appear in the case of the Ag composite, indicating a greater potential for this material.

FIGURE 2: SEM pictures of samples in fig, 1. Left: 2223+Ag

pellet.Marker=10 lain. Right: 2223+Ag (fiber).

4. CONCLUSION Superconductor glass, alloy and metal composites have been developed as ceramic bars and fibers using conventional solid state sintering and LFZ growth. These composites have properties which tend to be poorer than those of the original 2223 superconductor material. The Ag composite ceramic and fiber, however, seems to offer much better properties than its counterparts, as well as some potential for future improvements.

ACKNOWLEDGEMENTS This work has been supported by programme MIDAS (CICYT, REDESA, UNESA) project nos. 90/642, 89/3799, 89/3797 and ECC projects n-° SCI-0389-C and SCI-0036-F. Y. Huang acknowledges an FPI grant from the Spanish Ministry of Education. A. Badfa acknowledges an FPI grant from the CICYT. REFERENCES 1. R.S. Feigelson, Mater. Res. Bull. XIK (1988) 47. 2. A. Maeda et al Japn. J. Appl. Phys. 28 (1989) L 576. 3. G . F . de la Fuente et al J. Less-Common Met. 150 (1989) 253. 4. G . F . de la Fuente et al, Submitted to Boletfn de la OU~lCUtltl ~?~5~3~Jd|U-~UU L~I~IHIUUt y Yit.[I'll~.