B O L E T Í N DE LA S O C I E D A D E S P A Ñ O L A DE

Cerámica y Vidrio A R T I C U L O

• • •

Textured superconductors: progress ¡n processing and development of applications

X.OBRADORS, B.MARTÍNEZ, F.SANDIUMENGE, S.PIÑOL, J.FONTCUBERTA, V.GOMIS, X.GRANADOS, A. GOU, R.YU, N.VÍLALTA AND J.MORA

Institut de Ciencia de Materials de Barcelona , C.S.I.C. Campus Universität Autónoma de Barcelona. 08193 Bellaterra, Catalunya, Spain

The present review stresses the potentiaHty of directional soHdification techniques for the preparation of bulk YBa2Cu307/Y2BaCu05 composites with high critical currents useful for several applications . We analyse the growth mechanisms and we show how the size of 211 precipitates can be refined . We also present detailed TEM investigations of the defect formation mechanisms and a complete investigation of the influence of 211 phase concentration on the critical currents . The pinning mecha­nisms have been identified from the field (up to 20 T) and temperature dependence of the critical currents . This allow us to propo­se the first complete magnetic phase diagram of these type II superconducting materials . Finally , we stress the potential of these melt textured ceramics in several technological applications such as current leads, fault current limiters, flywheel batteries and hys­teresis motors.

key words: Ceramic superconductors, directional solidification, microstructure, critical currents, flux pinning centres, applications of YBa2Cu^Oy.

Superconductores texturados: avances de procesamiento y desarrollo de aplicación

La presente revisón enfatiza la capacidad de las técnicas de solidificación direccional para la preparación de materiales compuestos masivos de YBa2Cu307/Y2BaCu05 (123/211) con elevada corriente crítica útiles para varias aplicaciones. Analizamos los meca­nismos de crecimiento y mostramos como se puede reducir el tamaño de las inclusiones de 211. También presentamos una investi­gación detallada de los mecanismos de formación de defectos mediante microscopía electrónica de transmisión (TEM), así como de la influencia de las inclusiones de 211 sobre la corriente crítica. Los mecanismos de anclaje de flujo se han identificado a partir de las dependencias de la corriente crítica con el campo magnético (hasta 20 T) y la temperatura. Ello nos permite proponer el primer dia­grama de fases magnético de estos materiales superconductores de tipo II. Finalmente, enfatizamos la potencialidad de estas cerá­micas texturadas mediante fusión en varias aplicaciones tales como barras de alimentación, limitadores de corriente, sistemas de almacenamiento de energía inerciales y motores de histéresis.

Palabras clave: Superconductores cerámicos, solidificación direccional, microestructura, corrientes críticas, anclaje de flujo magnético, aplicacio­nes del YBa2CußOy.

1. INTRODUCTION

Directional solidification is one of the most promising pro­cessing methodology which allows to prepare bulk supercon­ducting YBaCuO ceramics having high critical currents. Among all the different versions of melt processing of YBCO now being investigated (see ref. 1 - 4 for recent reviews) the use of external parameters such as temperature gradients (5), mag­netic fields or seeds (6), generate a driving force for the growth of YBCO with well defined crystallographic orientations.

Grain boundaries are not, however, the unique crystallo­graphic defect having some influence on the superconducting performance of bulk YBCO monoliths. There are many defects which have been identified in 123/211 ceramic composites and their concentration and mutual interaction strongly depends on the processing methodology. It seems clear then that a full investigation of the defect formation mechanisms in relations­

hip with the procesing methodology and with the critical currents of these materials needs to be carried out.

It's also straighforward to note that the relevance of the defects as pinning centers will strongly depend on their con­centration, magnetic field orientation and the position chosen in the H-T magnetic phase diagram, where vortex interaction and fluctuation effects may strongly differ. It's then very important to relate any given defect in melt textured 123/211 composites to their general behavior under different physical situations.

In this paper we review some recent progress in the unders­tanding and improvement of directional soHdification proces­sing techniques which allow to prepare long single domain textured ceramics and we describe new evidences for the sub­tle relationship between microstructure and pinning mecha­nisms in these materials.

Finally, we briefly describe some applications which can be

Bol. Soc. Esp. Cerám. Vidrio, 36 [2-3] 123-131 (1997) 123

X.OBRADORS, B.MARTÍNEZ, F.SANDIUMENGE, S.PINOL, J.FONTCUBERTA, V.GOMIS, X.GRANADOS, A. GOU, R.YU, N.VILALTA, J.MORA

developped based on directionally solidified 123/211 compo­site ceramics.

2. DIRECTIONAL SOLIDIFICATION

All the melt texturing techniques of YBCO are based on the peritectic reaction occuring around 1.020 °C in air, when a semisolid melt composed of solid 211 particles and a liquid with typical composition 3 BaCu02 + 2 CuO react to generate 123 crystals.

This reaction involves the dissolution of some Y into the liquid and their transport to the growth interface. Because of the low solubility of Y into the liquid it seems that this Y diffu­sion from 211 particles within the liquid to the growth front is the mechanism limiting the growth rate of 123. This reaction, however, is sluggish and hence is rarely completed resulting in ceramic composites where 211 particles remain embedded in the 123 matrix. The non-completeness of the peritectic reaction has as undesired consequence that some liquid should remain in the final ceramic monolith. This factor introduces the first advantage of the directional solidification techniques. While typical melt processed YBCO contain some trapped particles of BaCu02 and CuO, properly directionally solidified materials are free of these inclusions because the excess of liquid phase is pushed by the growing front until the end of the monolith where these impurities can be easily eliminated.

The directionally solidified 123/211 composites described in the present paper have been grown under a temperature gra­dient G~20 °C/cm and with growth rates R==lmm/h (7). Alternative procedures such as top-seeding with Ndl23 seeds are presently being developped to grow large ceramic tiles with diameters up to 30 mm. In the Bridgman growth we have maintained the G/R ratio above 710^ °Cscm"^ to remain in the conditions of planar growth (8) (see Fig.l). The ceramic pre­cursors (123 and 211) have been either from commercial origin (9) or prepared by solid state reaction. Additionally, small amounts of Ce02 (~1 % wt) have been included in the starting mixtures due to their benefitial effects in different aspects of the growth process and the final microstructure (10), as it will be discussed later.

Preforming of ceramic monoliths has been achieved through different techniques, including uniaxial pressing, sintering in alumina moulds (11), plastic extrussion of a polymer-ceramic composite (12) and cold isostatic pressing. Small amounts of Ce02 additives (up to 1.5 wt% ) were added to the initial sin­tered bars. The more evident advantage of using Ce02 additi­ves is that at high temperature, in the semisolid state, the capi­llarity of the liquid is augmented thus allowing to increase the maximum temperature where the ceramic bars can be suspen­ded without dropping. The use of Ce02 imposes an optimi­sation of the processing conditions.

High temperature X-ray diffraction experiments using 211-Ce02 and 123-Ce02 samples was performed in order to inves­tigate the reaction mechanisms associated to the Ce02 additives (13,14) (Figure 2). Both, 211 and 123 phases react with Ce02 below the 123 peritectic temperature according to the reactions:

211+ Ce02-^BaCe03 + Y2O3 + CuO 211231 + 3 Ce02 -^SBaCeOß + 5 CuO + 211

[1] [2]

Fig. 1. Example of a flat growth front achieved in the directional solidification process.

-Y Y:Y203 CiBaCeOg T Y2BaCu05

A 950X

Aw/J t A A V V 900^C

V A M / ^

SOO' C

30 35 40 two-theta

45

It's clear then that through the inclusion of Ce02 oxide in the starting mixture we can generate Y2O3 particles even at tem-

Fig. 2. High temperature XRD patterns of the reaction between 211 and Ce02 below the peritectic temperature to give BaCeO^ , ^2^?) V^^^ ^ ^ ^ •

peratures much lower than the peritectic temperature of the 211 phase, i.e. 1.200°C (10).

We can get evidence of the fact that these Y2O3 inclusions act as nucleation centres in the semisolid state through a TEM study of the microstructure of the 211 precipitates remaining in the textured ceramic. Figure 3 shows a Y2O3 inclusion within a 211 precipitate. The interface between one Y2O3 inclusion and the 211 precipitate is typically clean at the atomic scale as expected for the proposed nucleation process. The above observations thus give strong confidence to the idea that the addition of Ce02 to the ceramic precursor results in the forma-

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Y203

20 nm

Fig. 3. TEM image of an Y20^ inclusion within a 211 precipitate

tion of very small dispersed Y2O3 particles at temperatures much lower than those required for instance in the Melt Powder-Melt-Growth method (1).

The anisotropy of YBCO has a strong influence on the final orientation of the ab planes in the monolith ceramics. The growth process is initiated at the interface with the non-melted ceramic where a multinucleation process occurs. After some growth competition a single domain remains which can extend all along the ceramic.

A question which has been largely debated is that concer­ning the final orientation of the domains respect the external temperature gradient. Transverse temperature gradients or the growth rate R have been proposed to be responsible of this effect (15,16).

Actually, we have demonstrated that the diameter of the bar influences the crystallographic orientation of the domains. When D=l mm, the single domain fibers are oriented with the ab planes along the vertical axis (12), when D=6 mm the ab pla­nes lie at an angle of about 60° from the gradient axis (17), and finally when D>20 mm the ab planes are oriented perpendicu­lar to the vertical axis (18).

A simple explanation for these observations can be formula­ted if we consider the anisotropic thermal conductivity of YBCO crystals. The solidification at the interface leads to a local generation of heat which needs to be evacuated. The poor thermal conductivity of YBCO ceramics makes the conduction process highly unefficient and hence a second alternative needs to be considered : the dissipation by convection through the lateral surfaces of the ceramic tile which is strongly depen­dent on the diameter of the ceramics.

3. MICROSTRUCTURE DEVELOPMENT

The 211 precipitate size distributions were determined using an automated image analysis system of digitalized SEM pictu­res. A drastic reduction was observed from the non added sample, upon addition of properitectic 211 (1-3,14).

The influence of Ce02 addition has also been investigated and it has been found, for instance in a sample with 20 % 211, that minute additions of Ce02 result in a decrease of the ave­rage particle size from 1.6]jm in the Ce02 free sample down to l.l]jim, as well as a narrowing of the distribution in the 1% Ce02 added sample (13,14) (Figure 4a and 4b). Further refine­ment down to 0.3|Lim has recently been achieved when the size of properitectic 211 particles is reduced by ball milling (17).

The refinement process has been found independent of the processing temperature, so that the most likely explanation for the 211 refinement is that which takes into account the increa-

1.0 1.5 2.0 2.5 3.0 3.5 4.0

Mean diamater D (¡xm) Fig. 4 . (fl) A representative SEM image of the 123-211 ceramic composites , (h) Size distribution of a typical 123-211 composite as determined from image analy-

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X.OBRADORS, B.MARTÍNEZ, F.SANDIUMENGE, S.PINOL, J.FONTCUBERTA, V.GOMIS, X.GRANADOS, A. GOU, R.YU, N.VILALTA, J.MORA

Fig. 5. HRTEM image of a 123-211 interface displaying contrast variations in the matrix.

se of nucleation sites when 211 is added. Supporting this assumption, we can mention the experimental evidence for the heterogeneous nucleation of the peritectic 211 on previously added properitectic particles. The most direct evidence was given by McGuinn who showed Er-rich cores in all 211 preci­pitates contained in a zone-melted (Y)-123 wire with Er2BaCu05 additions (3). We can conclude, therefore, that the observed decrease of precipitate size in 211 added samples results from an enhanced heterogeneous nucleation on the added particles.

Now, considering the Ce02+211 added samples, the obser­vation of nanometric Y2O3 inclusions within some 211 precipi­tates, provides direct evidence for the heterogeneous nuclea­tion of the peritectic 211 phase in the Y2O3 particles previously generated, below Tp. This mechanism supports the idea that the additional refinement observed upon addition of Ce02 results from an increased population of nucleation centres for the peritectic 211 phase. We have not observed, on the other hand, appreciable variations of the 211 precipitate size with position in the bar, thus indicating that no coarsening effects occur upon addition of Ce02.

The important defects present in 123/211 composites are microcracks, twin boundaries, dislocations, stacking faults, grain boundaries, and 123/211 interfaces. The nature of these defects in YBa2Cu307_§ has been investigated by several aut­hors (see (19) for a recent review), and precise knowledge on their generation and interaction during the fabrication process is essential for further progress.

TEM observations of non-oxygenated 123/211 samples (20) have revealed interesting features. Such samples present twin­ned and untwinned areas, and twinned areas typically display (1/2 0 0) superlattice spots indicating the Oil structure, which correspond to oxygen stoichiometries in the range O^ 4-O5 5. Figure 5 shows a HRTEM image of the 123-211 interface which indicates that they are clean. The variations in contrast indica-

Fig. 6. (a) Microcrack near a 123-211 interface, (b) Typical stacking faults lying parallel to the (001) planes. In both cases a strong degradation of the matrix is observed associated to the aging effect.

te local strains in the matrix. Interestingly, observations along the [001] direction did not reveal stacking fault contrast. In addition, very few dislocations, typically associated to the 211 precipitates, are present. Another important feature is that the same samples, observed along the [100] axis, do not display

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TEXTURED SUPERCONDUCTORS: PROGRESS IN PROCESSING AND DEVELOPMENT OF APPLICATIONS

microcracking, which in oxygenated samples constitutes one prominent defect. From this simple observation v^e can con­clude that all these defects are introduced during the low tem­perature (450 °C) oxygen uptake process. In particular, the absence of microcracks contrasts with previous models which attribute the origin of microcracks to growth discontinuities along the c-axis induced by the interaction of fast growing a-b platelets with unreacted 211 particles present in the melt (21). The fact that these models are supported on observations made on ceramic blocks processed in low temperature gradients in the presence of 211 particles with sizes well above 2 ]im, indi­cates that these large cracks can be avoided if steady growth conditions are achieved in sufficiently large thermal gradient.

Therefore careful control of the oxygenation conditions is essential in controlling the resulting microstructure : the effects of insufficient oxygenation (22) or over-exposure to an oxygen flow (23), translate into well characterised frigerprints of the superconducting properties, which nicely correlate with the microstructures observed in either case. The oxygenation con­ditions must then be adapted to each 211 precipitate density.

As oxygenation proceeds in the orthorhombic phase, the crystal lattice shrinks along [001] while expands in the (001) plane. Thermal contraction acts in the same way, resulting in the preferential (001) cleaving of the matrix. The length and spacing of the resulting microcracks roughly scale with the mean precipitate separation and diameter, respectively (14,18). On the other hand a shortenning of the length of the cracks occurs when the concentration of 211 increases because these precipitates are very effective in stopping the crack propaga­tion (Figure 6a). This is further corroborated by the observation of severe plastic deformation within the precipitates, in parti­cular associated to the intercepts with the microcracks, which is an indication of the absorption of the energy associated to the crack (18). As a final consequence the mechanical though-ness of the composite increases.

As a result of the chemical instability of YBa2Cu307 in the presence of oxygen at elevated temperatures (24), degraded bands are often observed associated to the microcracks (Figure 6a and 6b). The degradation even extends along the 211-123 interfaces, being intesified around the intercepts of microcracks with the precipitates (23). The stacking fault density (Figure 7) is increased also at these places and this mechanism would explain the preferential location of faults at 123/211 interfaces, in parti­cular where the interface is perpendicular to the (001) planes.

Small angle grain boundaries constitute another defect pre­sent in single domain samples which maniphest as a polygoni-zation contrast in the polarized light microscope in well oxy­genated samples. It is observed that the polygonization density increases when decreasing the size and increasing the concen­tration of the 211 precipitates. Dislocations in YBa2Cu307 pre­sent the [1001(001) and [110](001) glide systems (25) and fre­quently they become pinned at twin boundaries. The observa­tion of dislocation-twin boundary interactions (25) give further support to the idea that dislocations are introduced in the orthorhombic phase, i.e. during the low temperature oxygena­tion process.

Oxygenation induced aging of the composites shows up as a shift of the irreversibility line to lower fields and a strong reduction of the magnetic shielding capability when the mag­netic field is applied parallel to the (001) planes (23). Such a change of the irreversible superconducting properties is accompanied by characteristic microstructural features. First, the degradation bands associated to the microcracks are wide-

Fig. 7. Short stacking faults and twin boundaries as observed in a matrix region free from 211 particles.

ned and a strong matrix deterioration appears along the 211/123 interfaces. This process is accompamied by the growth of long stacking faults, which preferentially originate from easy diffusion paths, namely, 211/123 interfaces, grain boun­daries and dislocation tangles. These stacking faults are elon­gated in the [100] and [010] directions, displaying a finger-like shape and their massive presence can be regarded as an indi­cation of oxygenation-induced aging of the microstructure. In addition the aging process induces a severe plastic deforma­tion of the matrix, with extensive formation of dislocations and disociation. This process is accompanied by the nucleation of small faults between dissociated segments. The dislocation density in such areas may be increased by an order of magni­tude relative to the non-aged samples.

In conclusion, it has been shown in this section that the final microstructure of 123/211 composites results from the inter­play between several effects. Namely, the ferroelastic transition of YBa2Cu307 (twins), local plastic deformation of the matrix due to the mechanical mismatch between 211 precipitates and 123 matrix (dislocations), embrittlement of the material due to (001) cleavage in the orthorhombic phase (microcracks), strain accomodation by polygonization (small angle grain bounda­ries) and phase instability in the process of oxygen flow at low temperatures (matrix degradation and stacking faults).

4. CRITICAL CURRENTS AND PINNING MECHANISMS

The critical currents in single domain samples have been investigated by means of magnetization measurements carried

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X.OBRADORS, B.MARTÍNEZ, F.SANDIUMENGE, S.PIÑOL, J.FONTCUBERTA, V.GOMIS, X.GRANADOS, A. GOU, R.YU, N.VILALTA, J.MORA

out either with a SQUID magnetometer or by extraction mag-netometry at Service National des Champs Intenses (Grenoble) up to 20 T. We have focused our investigation up to now in the analysis of the active role played by 211 inclusions. Previously to these investigations a careful optimization of the oxygena­tion process has been needed to avoid either oxygen defi­ciencies (22) or the aging effects recently described (23).

We have studied the thermal dependence of the critical currents, determined by using the anisotropic Bean model j^ab^20AM/[a(l-a/3b)], in a series of samples having different content of 211 phase ranging from 4% to approximately 38% (26). In Fig. 8 we show the thermal dependence of J . ^ (Fl II c) in self field for several textured samples having different con­tents of 211 precipitates and a single crystal. First of all it is worth mentioning that a net increase of the critical currents with the increase of the 211 precipitates is clearly observed and what is even more important: the thermal dependence of the critical current in the single crystal is much higher than in the textured samples. This effect is probably a consequence of the existing 211 precipitates which also induce the observed softe­ning of Jç^^(T) at intermediate temperatures. In the region of low fields, the low temperature (T<40K) dependence of J, (T) is consistent with the single vortex behavior predicted by the vor­tex glass-collective pinning theory.

At higher temperatures (40K<T<80K) we have found that ]Q{T) may be properly described by the following expression:

JC (T) - Jc exp[-3(T/T^)2] [1]

as it is also shown in Fig.8 (dashed Unes). This expression has been proposed to account for the temperature dependence of the critical current in the case of linear correlated disorder (27). In Fig. 9 we show the evolution of the parameters J^^ and T' , obtained from the fitting of the experimental data by using Eq.[l], as a function of the composition of the samples given by the parameter V/d, that represents the ratio of the volume per­centage of 211 phase precipitates and their mean diameter, and measures the interface area between 211 particles and 123 superconducting matrix. We observe that a linear relation does exist between the parameters J^^ and T% and V / d thus sug­gesting that 123/211 interfacial pinning is the dominant pin­ning mechanism in this high temperature-low field region of the magnetic phase diagram while at low temperatures addi­tional pinning centres may also become active as thermal acti­vation decreases. The differences between the single crystal and 123/211 composites become more evident in the high tem­perature regime (40K<T<80K) i.e. where 123/211 interfacial pinning plays the dominant role. The occurrence of the interfa­cial pinning mechanism in our samples has also been checked by fittine our experimental J^ÍH) curves to the expression J^^^ßB"^/^ typical for single vortex pinning regime where a simple summation of the individual microscopic pinning for­ces may be performed. Very good agreement is obtained in the range of fields below 1.5 T and temperatures above 45K (see Fig. 10) with a linear dependence of the parameter ß on V/d, corroborating the active role of 211-123 interfacial pinning.

Nevertheless, on lowering temperature a progressive depar­ture from the above simple picture, i.e. J^ocB"^ with a = l / 2 , is observed. After a fast decrease of the critical currents J^ß^iH) in which a=^l/2, a regimen in which the field dependence of M(H), or L^b(H), is very smooth (a~0.1) is reached in a wide region of fields above 1.5-2 T. On lowering temperature the values of the a exponent in the low and high field regimes

C5

T ( K )

Fig. 8. Temperature dependence of]¿^^ in a single crystal and textured cera­mic samples with different content of 211 phase in self field, a) Solid lines correspond to vortex glass model while dashed lines correspond to correlated disorder (see text).

0.8

5 0.6

O 0.4

0.2

0

-EB ffl..-^---- -----s

- ffi.--- ""m - - - - - - - "^ ' " ffl

ffi

Q

- O -'-' --

, 'O' ' ' -1 <^ - o VO) ,'-'' o -- ^ . - ' O Q -- ,o ' ' T* s — -- o ,,

1 1 ) 1 1 1 < i 1 < 1 , 1 1 1 1 1 1 , 1 1 1 .1 1 1 1 1 -

100

90

70 ?5

60

50 0 500 1000 1500 2000 2500 3000 3500

V/d(cm-l)

Fig. 9. Dependence of JQ and T"^ obtained from the fitting by using Eq. [1] on the 123-211 interface specific area in the H\ \c geometry and in self field conditions.

14

13 h

S 11

2 10 CS

9 h

• • • • •••

-1 0 1

Ln[H(T)]

Fig. 10. Log-Log plot ofjj^'^ vs. H making evident the existence of a potential relation j/^ocH-^

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TEXTURED SUPERCONDUCTORS: PROGRESS IN PROCESSING AND DEVELOPMENT OF APPLICATIONS

Fig. 11. Magnetic phase diagram of the mixed state of the 211/123 composites obtained by using the sample having 38% of 211 phase. Region I: Correlated disorder single vortex pinning. Region II: Single vortex with strong and weak pinning centres. Region III: Small bundle regime. Region IV: Large bundle regime.

2- b

1

0

> - 1 •

X! - 2 "

-3-

^

1 ^ ^ ^ ^

1

0

> - 1 •

X! - 2 "

-3-

^

1 ^ ^ ^ i -4 ^ S ™ SS^^^^

-2 •0 -20 - 10 0 10 20 30 X (mm)

Fig. 12. a) Vertical force measured in a zero field cooling (ZFC) process, b) Measurement of the transversal force with transversal displacement in a field cooling (FC) process.

slowly approach each other and below about 20 K all the Jç(H) curve may be described with only one value of a~0.3.

The crossover field between these two regions depends on temperature but stabilizes around 1.5 - 2 T at high temperatu­re. Above 1.5 - 2 T we observe that J^ is almost independent of the parameter V/d, thus indicating that interfacial pinning only plays a secondary role in the high field region and other pinning centers are domiinant. Finally approaching the IL a much faster decrease of the critical currents is observed. In this part of the J CH) curves it is possible to identify, for some tem­peratures, a region in which J^o^H"^ before reaching the irre­versibility line. This field dependence of the critical current has been theoretically predicted for the so-called large bundle regi­me in the collective pinning theory.

Collecting all these data we can propose the phase diagram of the mixed state depicted in Fig. 11. Region I is single vortex pinning in nature and 123-211 interfacial pinning dominates, on lowering temperature other weaker pinning centres become active and the system enters in region II that is also a regime of single vortex pinning where strong and weak pinning centres coexist. In region III, above 1-1.5T the field dependence of J is weaker and pinning could be understood as due to small bundles. Further increasing the field a region where Jç(H)ocH" ^ is reached. This region, labelled as region IV in Fig. 11, has been associated to large bundles of vortices and is upper-boun­ded by the IL.

5. APPLICATIONS

The directional solidification process has been shown to be a very valuable technique to fabricate high critical current bulk superconductors. Several technological opportunities arise now based on the use of these materials.

The fabrication of long single domain bars of YBCO allows to use them as high capacity current leads. This application requires low specific resistance contacts and high critical currents (17). In samples with D=6 mm, having the ab planes at about 60° from the long axis of the bar we measured J . = 2 10^ A/cm^, This means that the critical current of our bars having D=6mm should be around 6700 A, which is much higher than the values we can generate with our experimental facihty. We have, however, indeed verified that no dissipation is observed up to I ^ 3000 A which is already within the hmits required for most current lead applications such as those required for the superconducting magnets included in the LHC accelerator at CERN.

Another very appeahng application of directionally solidi­fied YBCO ceramic tiles is in magnetic lévitation systems such as bearings, hnear transportation devices or advanced flywhe­el-type batteries. All these devices make use of the very special interaction between permanent magnets and bulk supercon­ducting tiles. The levitating flywheel prototype built at ICMAB has a diameter of 10 cm and it has been accelerated up to 25.000 rpm with very low energy losses.

The integral vertical lévitation force and the transversal force were measured by using a triaxial force sensor based on strain gauges and having a computer controlled movement along two perpendicular axis (28). All the materials were tested with NdFeB or SmCo permanent magnets having (|)=25 mm, h=20 mm and Bj.=0.3T. A typical lévitation force loop obtained, after a zero field cooling procedure, is displayed in Figure 12a.

Transverse force measurements were also obtained when a

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4 Cl

X.OBRADORS, B.MARTÍNEZ, F.SANDIUMENGE, S.PINOL, J.FONTCUBERTA, V.GOMIS, X.GRANADOS, A. GOU, R.YU, N.VILALTA, J.MORA

p M 1 VA

m

HTc Ring

Ip Rp Lcrp I ^\AAr

->

1ÍM A

Primary winding

O Magnetic Core o

i R

Rs (Is J 3 )

Vr lim

H Im L (Im)

T' I Las oj

oc

Fig. 13. Schematic diagram of an inductive fault current limiter and its equivalent circuit.

lateral displacement was introduced after a field cooling pro­cess. A typical hysteresis loop is displayed in Figure 12b, where the magnet stays at 1.5 mm above the superconductor. The stiffness constant corresponding to the linear recuperation force observed near the origin was found to be 300 N / m , which is similar to that observed by other authors in similar geometries (29).

Bulk superconductors are also being actively investigated as magnetic shields in inductive fault current limiters. The con­cept of these inductive fault current limiters is represented in Figure 13, where it may be appreciated that superconductor rings or hollow cylinders are used as a secondary in a transfor­mer. The working principle of these devices has been analyzed recently (30) and it may be described as a system with short response times (~lms), low impedance in the normal operation state and large impedance during the fault condition. In Figure 14 we can observe the limiting effect obtained with a YBCO superconducting ring prepared by melt texturing (31).

Finally, a very stimulating application of bulk MTG super­conductors is their use as a rotor in motors. The prototype developed at ICMAB has a diameter of about 15 mm and it's magnetically suspended by the field generated at the stator. The motor has a synchronous behavior and display enhanced characteristics such as high starting torque and improved power density.

ACKNOWLEDGEMENTS

We are greatful to : CICYT (MAT91-0742), Programa MIDAS (93-2331), EC-EURAM (BRE2CT-1011) and Generalität de Catalunya. The development of the fault current limiter has been carried out in collaboration with the Laboratory of Superconductivity at CEDEX. •

REFERENCES

1. M. Murakami "Melt processing", in Melt Processed High-Temperature Superconductors, edited by M. Murakami (World Scientific Ltd., Singapore, 1992), chapter 3, pp. 21-44.

30 40 50 Time (ms)

60 70

Fig. 14. Response curve of the current limiter. When a faidt occurs, the current flowing through the primary winding increases hut, the sudden incre­ment of its impedance limits the over current stabilizing the system.

2. K. Salama, V. Selvamanickam and D. F. Lee "Melt processing and properties of Y-Ba-Cu-O", in Processing and properties of high T . superconductors, edi­ted by Sungho Jin (World Scientific Ltd., Singapore, 1993), vol. 1, chapter 5, pp. 155-211.

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TEXTURED SUPERCONDUCTORS: PROGRESS IN PROCESSING AND DEVELOPMENT OF APPLICATIONS

10. S. Pinol, F. Sandiumenge, B. Martinez, V. Gomis, J. Fontcuberta, X. Obradors, E. Snoeck and Ch. Roucau "Enhanced critical currents by Ce02 additions in directionally solidified YBa2Cu307", Appl. Phys. Lett. 65,1448-1450 (1994)

11. S.Pinol, V.Gomis, F.Sandiumenge, N.Vilalta, B.Martinez, X.Obradors, A.J.Salinas, M.Vallet-Regi, J.San Román and J.M.González Calbet "Shaping of YBa2Cu307-Y2BaCu05 bulk superconductor composites", presented at Int. Conf. on Shaping of Advanced Ceramics, Mol, Belgium (April 25-27,1995). J. European Ceram. Soc. 17, 393-396 (1997).

12. A.J.Salinas, M.Vallet-Regi, J.San-Román, J.M.Gonzealez-Calbet, S.Piñol,V.Gomis, B.Martínez, X.Obradors "Ceramic-polymer precursor for YBa2Cu307 superconductor fibers". Fourth Euro-Ceramics, edited by A.Barone, D.Fiorani and A.Tampieri ( Faenza Editrice, 1995), vol.6, p.271-274

13. S. Pinol, F. Sandiumenge, B. Martinez, N. Vilalta, X. Granados, V. Gomis, F. Galante, J. Fontcuberta and X. Obradors "Modified growth mechanism in directionally solidified YBa2Cu307", IEEE Trans, on Appl. Supercond. 5, 1549-1552 (1995).

14. N.Vilalta, F.Sandiumenge, S.Pinol and X.Obradors "Precipitate size refine­ment by Ce02 and Y2BaCu05 additions in directionally solidified YBa2Cu307.", J.Mater.Res. 12, 38-46 (1997)

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17. X.Granados, V.Gomis, S.Pinol, M.Carrera, B.Martínez, F.Sandiumenge, N.Vilalta, J.Fontcuberta, X.Obradors,J.lglesias and S.Portillo "Transport criti­cal currents in (Y,Gd)Ba2Cu307 long bars for current lead applications", in Applied Superconductivity 1995, edited by D.Devz-Hughes ( Inst.of Physics 1995) vol.148, p.207-210.

18. F. Sandiumenge, S. Pinol, X. Obradors, E. Snoeck and Ch. Roucau "Microstructure of high critical current directionally solidified Y2BaCu05-YBa2Cu307 composites",. Phys. Rev. B 50 7032-7045 (1994)

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22. B. Martinez, V. Gomis, S. Pinol, I. Catalán, J. Fontcuberta, and X. Obradors "Field induced decoupling of superconducting bands in oxygen deficient melt textured YBa2Cu307", Appl. Phys. Lett. 63, 3081-3083 (1993)

23. F. Sandiumenge, N. Vilalta, S. Pinol, B. Martinez and X. Obradors "Aging of the microstructure of melt textured Y2BaCu05-YBa2Cu307 composites and impHcations on their superconducting properties", Phys. Rev. B 51, 6645-6654 (1995); B. Martinez, F. Sandiumenge, S. Pinol, N. Vilalta, J. Fontcuberta and X. Obradors "Aging of the critical currents and irreversibility line in melt textu­red YBa2Cu307 " Appl. Phys. Lett. 66 772-774 (1995); B. Martinez, S. Pinol, V. Gomis, F. Sandiumenge, N . Vilalta, J. Fontcuberta and X. Obradors "Influence of the oxygenation process in the critical currents of bulk melt tex­tured YBa2Cu307" IEEE Trans, on Appl. Supercond. 5,1611-1614 (1995).

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28. J.Mora, X.Granados, V.Gomis, M.Carrera, F.Sandiumenge, S.Pinol, J.Fontcuberta and X.Obradors "Directional sohdification of YBa2Cu307 cylinders for magnetic lévitation appHcations", Applied Superconductivity 1995, edited by D.Dew-Hughes, (Inst.of Physics 1995), vol.148, p.679-682.

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