Reduced twinning efficiency and tri-dimensional crack structure in melt-textured

NdBa2Cu3O7−δ bulk samples fragmentation process

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Supercond. Sci. Technol. 25 (2012) 125017 (10pp) doi:10.1088/0953-2048/25/12/125017

Reduced twinning efficiency andtri-dimensional crack structure inmelt-textured NdBa2Cu3O7−δ bulksamples fragmentation process

M Gombos1,2,3,8, V Gomis4, R Ciancio5, D Zola6, A E Carrillo7,G Carapella1,3, A Vecchione1,3, M Polichetti1,3, S Pace1,3 and X Obradors7

1 CNR-SPIN UOS Salerno, Fisciano (Salerno), Italy2 CNR-SPIN UOS Napoli, Napoli, Italy3 Universita di Salerno, Dipartimento di Fisica ‘E R Caianiello’, Fisciano (Salerno), Italy4 Universitat Politecnica de Catalunya, Barcelona, Catalunya, Spain5 CNR-IOM TASC Trieste, Basovizza (Trieste), Italy6 ENEA CR Casaccia, Santa Maria di Galeria (Roma), Italy7 ICMAB-CSIC, Bellaterra (Barcelona), Catalunya, Spain

E-mail: gombos@spin.cnr.it and gombos@sa.infn.it

Received 24 July 2012, in final form 3 October 2012Published 6 November 2012Online at stacks.iop.org/SUST/25/125017

AbstractHere we report on a detailed investigation on NdBa2Cu3O7−δ (Nd123) crack generation achieved by an extensivemorphological and structural study on 1 cm diameter top-seeded melt-textured grown pellets, where crack developmentstops at an earlier stage, and on the parallelepiped fragments resulting from the spontaneous breaking of larger (3 and5 cm diameter) pellets. In this way complementary information on the process is obtained.

Sample morphology was investigated by polarized light optical microscopy, scanning electron microscopy, andatomic force microscopy, confirming that microscopic fissures and macroscopic cracks act as preferential oxygenatingchannels. This fact suggests that mechanical stress, generated locally by an oxygenation inducedtetragonal–orthorhombic transition, is the driving mechanism for fissure evolution into cracks and their further growth.Our observations reveal that Nd123 fissure bundles have a tri-dimensional character, lying not only along the a–b plane,but also along two a–b out-of-plane perpendicular directions. Twin and fissure bundle orientations appear to differ by45◦, thus suggesting that fissures lie along a–c and/or b–c crystallographic planes.

This observation was confirmed by θ–2θ x-ray diffraction and x-ray pole figure analysis (XPFA) performed on theparallelepiped fragments, which revealed that samples are single domain, with edges oriented along the principalcrystallographic planes. XPFA detected a–b axes twinning in the domain on the global scale.

A remarkable result achieved by electron backscattered diffraction (EBSD) is the evidence that cracking along b–cplane is preferred with respect to a–c, acting in this way as a second preferential cleavage direction. Indeed EBSDmeasurements show long-range ordering of a and b axes, suggesting that twinning is less effective in releasing in-planemechanical stress in Nd123 than in parent compounds. Results are discussed on the basis of the identification of Nd–Bacationic substitutions as the main competing relaxation mechanism.

The superconducting properties of the analysed samples were studied by measuring the fragments superconductingparameters by vibrating sample magnetometry.

(Some figures may appear in colour only in the online journal)

8 Address for correspondence: CNR-SPIN Salerno c/o Universita di Salerno,Dipartimento di Fisica ‘E R Caianiello’, Via Ponte Don Melillo, I-84084

Fisciano (SA), Italy. Temporarily with CNR-SPIN Napoli, Via Cinthia,I-80126 Napoli, Italy.

10953-2048/12/125017+10$33.00 c© 2012 IOP Publishing Ltd Printed in the UK & the USA

Supercond. Sci. Technol. 25 (2012) 125017 M Gombos et al

Introduction

RE123 (REBa2Cu3O7−δ RE = Y or rare earth) cupratesuperconductors are largely studied materials for applications.However, due to the strong anisotropy of their superconduct-ing properties, the fabrication of large crystallographicallyoriented samples is necessary for technical purposes.

A successful technique to obtain bulk oriented RE123samples is top-seeded melt-textured growth (TSG) [1–3]. Thedifficulty of scaling experimental parameters for large grainsample growth is an important limitation of this technique [4],but a more serious limitation for applications is representedby the mechanical fragility of the samples [5–8], for whichvarious post-growth treatments have been proposed [9, 10].

Earlier studies show that, in Y123, cracks occur asa consequence of the accumulation of mechanical stressduring sample preparation [6, 7]. The presence of inclusionswith a different dilatation temperature coefficient, as it isfor the Y211 phase (Y2BaCuO5), induces mechanical stressat the cooling stage during the melt growth process [11].Cracking is highly enhanced by the post-annealing processin oxygen atmosphere required to yield the orthorhombicsuperconducting phase. The volume reduction of the unitcell associated with this structural modification produces aconsistent mechanical stress, which usually leads to cracking.Due to the anisotropy of the oxygen diffusion, the oxygenationmainly occurs along the crystallographic a–b planes [12,13]. The typical twin planes structure, observed in theorthorhombic phase, acts as an effective stress releasingmechanism along the a–b plane, so preventing the formationof fissures [14]. In contrast, the large reduction of thec axis due to the transition from the tetragonal to theorthorhombic structure and the anisotropic fracture toughnessinduce cracking parallel to the a–b plane, so that (0 0 1) isthe first cleavage plane in the orthorhombic phase [6]. Asthe overall values of the oxygen diffusion coefficients arerather low, micro-cracks act as an effective oxygen path forthe oxygenation process [13]. An orthorhombic band structuretherefore arises surrounding the fissures, further driving theirdevelopment by the progressive shrinking of nearby cells’ caxes. As a result, a rather regular parallel crack distribution isformed along a–b planes. Pellet oxygenation in high-pressurefurnaces, which allows one to thwart c axis tensile stresses bycompression, has been proposed in earlier works as a possiblea–b cracks preventing mechanism [15]. In Y123, some cracksare also observed along the c direction: they present a moreirregular shape than a–b ones, and are normally associatedwith precipitates, pores or other crystal defects [16, 17].

The RE123 material displaying the highest supercon-ducting transition temperature and critical fields is Nd123.It belongs to Nd–Ba–Cu–O chemical system, which ischaracterized by an extreme chemical complexity [18–21].Moreover, severe cracking occurs in Nd123 textured samples.For these reasons the fabrication of Nd123 bulk sampleswith large oriented domains [22, 23] is a very hard task. Inparticular, the reported fragility of Nd123 represents to dateone of the major limitations for the development of devicesbased on this superconductor.

In this work, Nd123 pellets with different diameters(d = 1, 3 and 5 cm: d1, d3 and d5 pellets respectively)were prepared using the TSG technique. The d1 pellets aregenerally stiffer than larger ones, so crack generation isgenerally less destructive in them because crack developmentstops at an earlier stage. Therefore, the d1 pellets are mainlyfabricated as a reference for the study of larger pellets andto observe the first stages of crack growth, whereas d3 andd5 pellets and fragments provide information on the furtherevolution of the process.

Quite a large number of fissures have been observed inour pellets by means of polarized light optical microscopy(PLOM) and scanning electron microscopy (SEM). Inparticular, two kinds of fissures are found: the usual a–bones, due to c axis shrinking as in Y123, and other a–b out-of-plane ones, lying in different crystallographic directions,perpendicular to (0 0 1), and forming a regular 3d-crackingstructure. The regular distribution of the out-of-plane fissuresis significantly different with respect to that observed inY123. The cracking structure displays a pattern which canbe associated with the crystallographic a–c and/or b–cplanes, as suggested by atomic force microscopy (AFM)and electron backscattered diffraction (EBSD) analysis onthe resulting fragments. We believe that the occurrence ofthese fissures is correlated with the oxygenating processesas well. We hypothesize that, in Nd123, twin-boundariesgeneration lacks efficiency in releasing the stress due to a localtetragonal–orthorhombic transition induced by oxygenation.For this reason, alternative relaxation mechanisms, such as theNd–Ba cationic substitution phenomenon, assume remarkablerelevance in this material.

Finally, the superconducting properties of sampleswere studied by vibrating sample magnetometer (VSM)measurements. Implications on the sample oxygenationprocess are also discussed.

1. Samples fabrication

Precursor powders are a mixture of separately prepared (bysolid state reaction [24, 25]) Nd123 and Nd2BaO4 (Nd210) inthe molar ratio Nd210:Nd123 = 1:4 (25%). The Nd210 phaseprevents Nd–Ba substitutions in Nd123 [4, 26] and reacts withNd123, forming Nd422 precipitates in the final samples [4,20]. It has to be remarked that a minority product of theNd123–Nd210 reaction, the NdBa6Cu3O10.5 (Nd163) phase,is also formed. This phase rapidly decomposes into Nd123plus oxides [24]. The very small quantity and the instabilityof produced Nd163 are such that we can assume it does notplay any role in the crack formation process.

Single-domain pellets were obtained by the TSGtechnique in air. The precursor powders were uniaxiallypressed into 8 mm thick pellets with 1, 3 and 5 cmdiameters (d1, d3 and d5 pellets respectively) and mounted onsintered CeO pedestals, with a thin protective layer of Y2O3powder interposed [26]. MgO 1× 1 mm2 single-crystal seedswere placed at the centre of their top surface. Pellets werecompletely melted in a muffle furnace by holding for 5 h atTm = 1135 ◦C. To obtain pre-sintered high-density pellets and

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Figure 1. (a) Upper view of an Nd123+ 0.25 Nd210single-domain d1 pellet showing the central domain and thequenched liquid deposits on the periphery. (b) PLOM view of apellet region after polishing: huge perpendicular cracks are visible,twin plane cross-entangled traces pattern correspond to the oneexpected for the a–b plane. Pores are visible as round black spots.(c) PLOM observations of a pellets a–b plane region: micro-fissuresare directed along two perpendicular directions (yellow arrows),parallel and perpendicular to a macroscopic crack and identified as(100) and (010), lying at 45◦ from the twin directions (light bluedouble headed arrows).

prevent crack formation before melting, Tm was reached by atwo-step ramp: 300 ◦C h−1 up to 800 ◦C and then further upat 60 ◦C h−1. The furnace was cooled to 1075◦C and then,at a growth cooling rate of 0.6 ◦C h−1 (d1 and d3 pellets)or 0.4 ◦C h−1 (d5 ones), to 1060◦C. Finally, the furnace wascooled to room temperature.

2. Experimental results

2.1. d1 pellet characterization

Naked-eye observation of as-grown d1 pellets show a singleNd123 domain with sharp flat edges, surrounded by sandylooking ‘quenched liquid’ deposits, mainly constituted byNd422 particles loosely bound by unreacted BaCuO2 andCuO (figure 1(a)). These deposits detach spontaneously fromNd123 domains, leaving clean domain edges [8].

To allow polarized light optical microscopy (PLOM)observation, as-grown d1 pellets were annealed at 420 ◦C inpure flowing oxygen during 12 h, and polished by cloth downto a 1 µm grain. Macroscopic fissures (cracks) extendingacross the whole pellet surface were observed after polishing:figure 1(b) shows a crack intercepted perpendicularly on both

sides by two slightly thinner ones. All fissures appear to beparallel to the flat growing fronts. The structure of alternatingdark and bright stripes on the surface is the twin-boundariesfingerprint, the uniformity of these stripe fissures indicates thepresence of only one single domain.

Fissures observed in Nd123 run along two well-defined,mutually perpendicular, directions. PLOM observation ofNd123 TSG samples confirms that microscopic fissures groupin families of several parallel fissures, such as the oneshown in figure 1(c), forming 45◦ with twin-boundariesstripes. Twin-boundaries form along a plane individuated bycrystallographic c axis and either the (1 1 0) or (1 1 0)direction. The cross-entangled tissue formed by perpendiculartwin-boundaries families indicates that the plane visualizedin figure 1(c), in which the micro-fissures are observed,corresponds to the a–b plane. Two perpendicular crackingpropagation directions can be identified for a–b out-of-planecracks and micro-fissures (along the yellow arrows). Theselie along the bisector of the angle formed by the twotwin-boundaries directions, which correspond to a–c and b–cplanes (blue arrows). The observed macroscopic cracks arethen simply the largest members of these fissure families,as shown in the figure where the two micro-fissure familiesappear close to a crack: it is easy to verify that all fissuresare only directed either parallel or perpendicular to thecrack. This well-defined pattern shown by the a–b planecracks observed in the bulk of oxygenated orthorhombic TSGNd123 samples, is completely different from the irregularstructure characterizing the similarly oriented cracks reportedin analogous Y123 samples [16, 17, 27]. These are apparentlydue to a high precipitate density, but the only analogousstructures found in Nd123 samples are the irregular cracksthat can be observed in the peripheral growth bands ofsamples obtained by high initial Nd210 content precursorpowders [25], which are clearly due to a very high localdensity of 422 particles (figure 2(a)) as well.

As for Y123 [13, 28], in Nd123 fissures act as effectivechannels for oxygen diffusion. Twin-boundaries bandsoccurring inside the tetragonal non-superconducting phasematrix indicate a local, oxygenation-induced, transition to theorthorhombic superconducting phase. PLOM observation ofpartially oxygenated Nd123 samples shows, indeed, that theseoxygenation band structures form along fissures (figure 2(b)).Oxygenation processes can be then further assisted bythe particular three-dimensional character for Nd123 fissuredevelopment, again in concordance with the well-known 3Dcharacter for dislocations found in this system by TEM [29,30].

More information can be obtained by the observationof pellet sections along planes different from the a–bplane. In figure 3(a), the angle formed by twin boundaries(78◦) indicates that in this image the view plane is about36◦ rotated from the a–b plane orientation. In this orientationmicro-fissures lie along three directions parallel to the threeprincipal crystallographic planes, giving a clear evidence ofthe 3D character of cracks. Moreover, a transverse sectionof a d1 sample was performed by cutting it perpendicularlyto the a–b plane along a previously identified crack. In

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Figure 2. (a) PLOM view of a–b plane micro-fissures in aperipheral high precipitate density band, where crack paths appeardistorted by the presence of many 422 particles. (b) PLOM image ofoxygenation bands (the darker bands) surrounding twomicro-fissures, giving a clear indication that fissures act as effectivechannels for oxygenation.

this configuration the same very regular structure commonlyreported for Y123 [31], several micro-fissures all parallel toa–b plane, is observed (figure 3(b)). As in Y123, we canindeed recognize a single family of twin boundaries along thec axis direction, and only one fissure family, perpendicular totwin boundaries, lying parallel to the a–b plane [32].

Atomic force microscopy (AFM) a–b plane observations(figure 4) have been performed on a d1 single-domainsample after polishing its upper surface with a colloidal silicapolishing suspension (Buelher). Images were acquired by aVeeco-Digital Instruments Multimode AFM in contact mode,using silicon nitride (NP-20) tips. A very flat surface isachieved after polishing, with a mean square roughness Rms ∼

0.5 nm. Regularly spaced tracks, attributed to twin-boundariestraces lying at 45◦ from a micro-fissure were detected(figure 4(a)). The distance between traces is around 0.5 µm. Itis noteworthy that the step depth of the traces is as low as 1 nm(of the order of a single unit cell). Traces of the two families ofthe twin-boundaries planes were also observed in figure 4(b).In figure 4(c) we show an a–b out-of-plane micro-fissure nearNd422 particles. The micro-fissure is straight, going from oneparticle to another, no radial or curved micro-fissures weredetected.

Figure 3. (a) PLOM image of a d1 pellet surface polished along aplane with an inclination of 36◦ from the a–b plane (as can becalculated from the shown 78◦ angle formed by twin traces), clearlyshowing the 3D character of fissure development: the three observedcrack directions (light yellow arrows) correspond to the principalcrystallographic planes. (b) PLOM image of a d1 pellet surfacepolished along a plane perpendicular to the a–b plane: only fissuresparallel to the a–b plane are visible here. Twin traces are all parallel,indicating the (001) direction.

2.2. d3 and d5 pellet fragments characterization

Whereas d1 pellet analysis provides us insights on thefirst stages of the fragmentation processes, more detailson the further development of cracks may be obtainedby studying bigger pellets (d3 and d5), and by analysingthe fragments resulting from their fragmentation processes.Indeed, in d3 and d5 pellets cracks are generated in largenumbers, and their progressive development leads to thealmost complete destruction of the samples [8]. This processoccurs spontaneously, even in non-oxygenated, non-polishedand most carefully stored samples, and completes within a fewweeks. Fragments present a parallelepiped (facetted) shape,which suggests that Nd123 cleaves following not only the a–bplane but also the b–c and/or a–c planes.

A relatively large fragment (5 × 5 mm2, 2 mm in depth)was taken from a completely destroyed d5 pellet. By severaldays observation of the fragmentation process, the originalpellet upper side can be located on one of the 5 × 2 mm2

sides of the fragment, whereas the fragment larger side (5 ×

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Figure 4. Atomic force microscopy performed near micro-fissures ((a) and (b)) and 422 precipitates (c). The parallel traces are associatedwith twin-plane traces. A single twin trace family is observed in (a) with a 45◦ inclination from the micro-fissure. Two perpendicularfamilies of twin traces are also observed in (b). (c) Image of a micro-fissure between two 422 precipitates.

Figure 5. θ–2θ XRD diffractogram, performed on the largersurface of the fragment in figure 6, displaying only Nd123 (00l)reflections (very small Nd422 peaks are also visible).

5 mm2) is parallel to the growing front. An XRD θ–2θdiffractogram, performed by a Philips PW1700 on the largerfacet of the fragment, shows the presence of (0 0 l) reflectionsalone (figure 5). This finding indicates that our fragment isa single domain and that it is oriented along a c axis that isperpendicular to the growing front.

The fragment was slightly polished on the larger sideto allow PLOM observation. An a–b plane mosaic image ofa large region of the fragment is displayed in figure 6. Inthis single-domain fragment the darker regions correspond tothe oxygenated areas located near either the sample surface(upper side) or the macro-crack surface (left edge). Thedifference in the oxygenation band thickness is due to thefact that the sample surface was in contact with an unlimitedoxygen source (the atmosphere) during the whole growth,whereas oxygen dispersion in the crack was limited by the

crack volume itself. Bands appear to form also around somepores; this is probably due to these pores being produced byoxygen losses during melting, so they could act as an effectiveoxygen reservoir for the later oxygenation process.

Fragment observations confirm that cracking is clearlyassociated with the oxygenated regions. Indeed, only a fewfissures can be observed inside the tetragonal matrix, whereasthe oxygenated areas are finely fractured, thus indicatingthat fissures were mainly produced by the mechanical stressinduced by oxygenation.

As in d1 samples, in the orthorhombic areas, fissuresgroup in parallel bundles that mainly follow two perpendiculardirections (figure 6, indicated by the arrows) that visiblycoincide with sample and macro-crack edges.

XRD pole figure analysis (XPFA), was performed onthe polished fragment by a Philips X’Pert four-circlesdiffractometer. Measurements were performed around (0 0 6)(figure 7a) and (1 1 3) (figure 7(b)) reflections, and confirm theorientation of the single domain along the c axis. In particular,the presence of all four (1 1 3) peaks highlights the occurrenceof twinning in a single-domain sample. Moreover, the slightdeviation from the origin of (0 0 6) barycentre gives evidenceof a small misalignment of the c axis and surface normal,likely due to the polishing procedure.

Electron backscattered diffraction (EBSD) analysis wasperformed with an Oxford Instruments INCA CRYSTALdetector mounted on a LEO EVO 50 HV scanning electronmicroscope (SEM). In figure 8(b), a SEM micrograph of thea–b plane of the NdBaCuO crystal is shown, where an orderedarray of parallel micro-fissures can be discerned. EBSD isa powerful technique to elucidate the local crystallographicorientation relationships on a micrometric scale. For crystalorientation determination, a sample coordinate system hasto be defined with respect to the microscope reference

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Figure 6. PLOM mosaic image of a 1.4× 2.4 mm2 region of the larger surface of a 5× 5× 2 mm3 single-domain fragment; measurementsshow the surface to be parallel to the a–b plane. The darker regions visible at the fragment periphery and surrounding some porescorrespond to the oxygenated orthorhombic phase, fissures are only observed inside these regions. Micro-cracks, grouped in a set of parallelmicro-fissures, mainly follow the two perpendicular directions indicated by the arrows to the crack edge (slight mis-orientation can beassociated with low-angle grain boundaries or the influence of defects).

Figure 7. X-ray pole figure analyses performed on the larger surface of the fragment in figure 6 along the (006) (a) and the (113) (b)reflections, showing that the fragment is single domain with the c axis perpendicular to the image.

system. In our case, the NdBaCuO samples were mountedon the microscope stage so that the fissures were orientedparallel to the transverse direction (the y axis of themicroscope stage, figure 8(a)). To perform the EBSD analysis,the crystallographic data of the NdBaCuO phase wereentered into the customized EBSD software database forthe orientation identification. During the analysis, the beamwas stepped across the area of interest, indicated by the redrectangle, with a spot size of 1 µm. At each step electronswere diffracted from the atomic planes and the correspondingKikuchi pattern automatically recorded by the software. Theresults are given in orientation maps, where a colour code isassociated to the specific crystallographic direction (rolling,

transverse and normal), shown in figure 8(b). The rollingdirection is found to correspond to the (1 0 0) orientation ofthe NdBaCuO cell, while the transverse direction is parallelto (0 1 0) direction of the crystal. By taking into account thepreviously mentioned reference system, these findings give anindication that the favoured fissure orientation is along the b–cplane of the crystal.

Finally, it is worth stressing that such a uniformorientation is again in contrast with EBSD studies performedon Y123 [33], which showed clearly a band structure dueto a–b axes twinning. Nd123 long-range orientation suggestsinstead that twinning may be less favoured, so that its

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Figure 8. (a) Sketch of chosen sample configuration for EBSD analysis; with the normal direction z axis perpendicular to the image,transverse direction y axis parallel to the fissures and rolling direction x axis perpendicular to the fissures. (b) SEM image (left) of a regionaround a crack and EBSD analysis (the three smaller images on the right) of the selected region of the image. It is apparent that the maincrystallographic orientation has its c axis perpendicular to the image (normal direction), b axis parallel to the image plane and to the fissures(transverse direction) and a axis perpendicular to the fissures (rolling direction).

efficiency as a mechanical stress releasing mechanism maybe negligible for this material.

The best superconducting parameters for Nd123 areobtained by optimizing its oxygen content; in this perspectiveour samples underwent several annealing processes in flowingoxygen. In particular, the highest Tc temperature was obtainedby a 160 h 360 ◦C thermal treatment in PO2 = 1 atm (flowingrate 20 l h−1). Magnetic measurements were performed by avibrating sample magnetometer (VSM) inserted in a QuantumDesign Inc. PPMS 6000 operating in the temperature rangefrom 1.9 up to 400 K and in the magnetic field rangeup to 90 kOe. The following presents only a sketch ofmeasurement results, which are better reported and discussedelsewhere [34].

Zero-field cooling temperature-dependent magnetizationperformed on an annealed 2.9 × 1.8 × 0.35 mm3 fragmentfrom a d5 pellet shows a transition onset around 95.7 K.Isothermal magnetization measurements have been performedboth in the parallel orientation (H parallel to c axis) andin the perpendicular orientation (H parallel to a–b plane),showing the anisotropic sample behaviour. Critical currentswere obtained by the Bean model reaching values JC ∼ 1.4×105 A cm−2 for H ‖ c at 77 K at self-field (figure 9). Theirreversibility line for this orientation is located at fields higherthan 5 T at 77 K and 4 T at 80 K. The results are comparableto those of high-quality samples obtained by other groupsworldwide.

3. Discussion

The tetragonal-to-orthorhombic transition in Y123 causes adistortion of the unit cell due to the notable reduction of thec and a axes and the corresponding increase of the b axis.Such a structural transition is strongly local, its occurrencebeing necessarily correlated to the oxygen diffusion insidethe sample. For this reason the c axis contraction causes aninternal stress (strain) that leads to sample cracking parallel

Figure 9. Temperature dependence of Jc of a 2.9× 1.8× 0.35 mm3

single-domain fragment at H = 1 T evaluated from VSMmeasurements performed in parallel (H ‖ c) and in perpendicular(H ⊥ c) orientations.

to the a–b plane. Analogously a and b axes variations canlead to stress generation liable to cause in-plane cracking. InY123, this induced internal strain is released by the twinningmechanism [15], the alternating of a and b axis indeed reducesthe strain due to the transformation, so that cracks mainlyoccur parallel to the a–b planes, and the few perpendicularcracks present an erratic shape, without any preferentialorientation [11, 17, 35]. On the other hand, EBSD analysisseems to indicate that the stress relaxation mechanism bytwinning is not very effective in Nd123; figure 8(b) indicates,in fact, that a and b axes show a preferential alignmentinside quite large areas. The internal strain can then propagatethrough the a–b plane, causing micro-crack generation. Thisaxes-preferential orientation is not due to any effectivein-plane axis texturing but, by preventing twinning, appearsto be important enough to reduce or remove its mechanicalstress releasing effect.

Micro-cracks can be grouped in families of parallelfissures, which can indicate preferential a axis orientation

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sub-domains inside the matrix. It is important to notethe coincidence between the a axis preferential directionand the micro-cracks orientation, which points to thetetragonal-to-orthorhombic transition as the main mechanismfor fissure formation. This last assumption is also reinforcedby the observed lack of fissures in the tetragonal matrix(figure 6).

One of the most remarkable differences between Nd123and Y123 is that Nd123 allows the formation of solidsolutions and antisite defects, which can largely modify theunit cell volume and oxygen stoichiometry. We can thenwonder if the preferential a and b axis orientation observedin Nd123 may be correlated to this feature. It is knownthat Nd123 admits a wide range of solid solutions Nd123(x)(= Nd1+xBa2−xCu3Oz) where Ba2+ is partially substituted byNd3+ [18–21, 36]. The disordering of Nd and Ba antisitesin Nd123(x) depresses the superconductivity and also hasimportant repercussions on its crystallographic structure [37].Antisite defects in Nd123(x) are indeed associated withoxygen disorder, so causing the tetragonal-to-orthorhombictransition suppression for x > 0.3 [38]. Non-oxygenatedhighly substituted Nd123 does not present a tetragonalphase but only an orthorhombic one, stable at hightemperatures, in which similar a–b plane micro-fissures canbe recognized [31]. Although the rather high Tc and criticalcurrents found in our samples seems to indicate less antisitedisordering, the presence of such defects can greatly influencethe sample micro-structure. We can speculate that thepresence of an antisite defect can induce local crystallographicrearrangement due to the strong axis distortion associatedwith the cationic substitution. So, a small nanocluster ofhighly substituted orthorhombic phase, present inside thenot-substituted matrix, could induce a local preferentialorientation of a and b axes.

Whatever the formation mechanism, this 3D crackstructure makes Nd123 very brittle against any mechanicalstress, especially in large samples, where Nd123 domains tendto spontaneously break along the identified cleavage planes(the edges of the facetted grains), in exact correspondence tothe observed fissure families

Oxygen diffusion is enhanced by the 3D fissurestructure. This can increase the mechanical fragility inlarge pellets for a system such as Nd123 with a very lowoptimal oxygenation temperature (300 ◦C) with respect toY123. Room-temperature oxygen diffusion found in poorlyoxygenated Y123 samples [39, 40] is likely to be farmore important in Nd123, with implications in the samplemechanical stability. The process is, in fact, self-sustaining:the more the cracks extend, the more easily oxygen isabsorbed, causing further tetragonal–orthorhombic transitionsand crack propagation until the destruction of the sample.This evolution of cracks is favoured in large samples by themechanical stress coming from liquid–solid interface. Indeed,for large domains the growing fronts usually have a concaveshape, imprisoning quenched liquid. Compression stressesdue to the different thermal expansions of the Nd422-richliquid and the Nd123 crystal matrix lead to cracking alongthe preferential cleavage planes.

For a flat growing front, as in our d1 pellets, dilatationstresses are not sufficient to crack the grown domain. Theinsurgence of a non-flat growing front, due to push-and-trapdynamics effects [25, 41, 42], may be induced in d1 samplesby increasing the Nd210 initial content. Anyway, d1 pelletspresent a diameter versus height ratio Rdh = d/h very closeto 1, which improves their stiffness, whereas for d5 pelletsit results that Rdh > 10. Reduction of the fragility of largersamples can be achieved by using a low Rdh. A possiblesolution could be multi-seeded MTG [43, 44], by whichthe same thermal treatment is used to obtain, in principle,indefinitely long bars, and show an increased mechanicalstability with respect to similar size pellets [8]. Finally, severalother different procedures, already used for other RE123compounds, might be considered and adapted to Nd123 TSGpellets, such as sample impregnation by epoxy resin [45] oroxygenation under mechanical pressure [46].

4. Conclusions

For implementing manufacturing technologies based onhigh-temperature superconductor devices obtained by melt-textured growth techniques, a better understanding of thecauses of crack formation and development is needed. In thisperspective NdBa2Cu3O7−δ (Nd123), a HTSC of the Re123family particularly prone to cracking, is a good model materialto investigate.

For this purpose, large (1, 3 and 5 cm diameter)single-domain Nd123 pellets have been prepared by theTSG technique. These pellets show a tendency to developmicro-fissures not only parallel to a–b crystallographic plane,as in Y123, but also out-of-plane. In particular, bundles ofparallel a–b out-of-plane fissures are observed by PLOM,lying along two perpendicular directions. SEM observationsshow analogous out-of-plane fissure generation along paralleldirections in the parallelepiped fragments coming from thespontaneous breaking of Nd123 pellets. The main key ofNd123 brittleness is, then, in the spontaneous evolutionprocess of these micro-fissures into macroscopic cracks.

Fissure shapes appear, by AFM, not to be influencedby Nd422 precipitates. On the other hand, an oxygenationband structure is revealed by PLOM, running all alongfissures. Bands are constituted by a–b axes twin boundaries,due to a local tetragonal–orthorhombic transition. The meanseparation of twin-boundaries, measured on AFM images, isaround 0.5 µm. AFM allows also one to measure that thetwin and fissure orientations differ by 45◦, suggesting thatout-of-plane fissures are parallel to the a and/or b axes, sothat the fracture planes correspond to the a–c and/or b–ccrystallographic planes.

These observations confirm that fissures play animportant role in the oxygenation process, acting aspreferential oxygenation pathways, and suggest that themechanical stress caused locally by the oxygenation-inducedtetragonal–orthorhombic transition is the driving mechanismfor fissure growth.

By XRD θ–2θ analysis, fragments resulted to be singledomain, with edges lying along the principal crystallographic

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Supercond. Sci. Technol. 25 (2012) 125017 M Gombos et al

planes. XPFA confirmed the occurrence of global a–baxes twinning, whereas EBSD measurements show locallong-range orientation for a and b axes correlated withcracking along the b–c planes. These planes then providea second preferential cleavage direction, besides the a–bplanes, that was not previously detected, broadening our basicknowledge on the causes of Nd123 sample fragility.

Moreover, the observation of long-range a and b axesordering suggests that twinning may result as being lesseffective in releasing in-plane mechanical stress in Nd123with respect to other RE123 compounds. The correlationbetween this reduced efficiency and competing relaxationmechanism provided by Nd–Ba cationic substitutions isdiscussed in the text, giving new insights into the relevanceof this last phenomenon in Nd123, rather than in other Re123compounds.

VSM measurements have been performed to check thesuperconducting parameters of the analysed samples. Theyalso have allowed the observation of the intrinsic anisotropyof Nd123 superconducting properties.

Acknowledgments

We thank Dr Immacolata Angelica Grimaldi, now withUniversita di Napoli ‘Federico II’—Dipartimento di ScienzeFisiche, for assistance during VSM measurements.

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