Mechanical Properties of Melt Textured Y

This work reports the mechanical properties of ab and a(b)c-plane of melt textured Y 1-x Pr x Ba 2 Cu 3 O 7-δ ( x = 0.00 and 0.05) superconductor. Y 1-x Pr x Ba 2 Cu 3 O 7-δ have been prepared by top seeding technique. Hardness and elastic modulus were obtained by instrumented indentation. Independent of the plane, the hardness values were around 5.0-7.5 GPa. Young’s modulus was approximately the same for both samples for ab-plane, around 132 GPa at the deepest tip penetration. However, for a(b)c-plane, the value of Young’s modulus for x = 0.05 is higher than observed for x = 0.00 sample.


Introduction
Since the beginning of studies in high critical temperature superconductors, one undesirable property has been the presence of granularity in sintered YBa 2 Cu 3 O 7-δ (Y -123) material.To overcome this problem, a top-seeding melt texture growth (TSMTG) process was introduced 1,2 .With this process, it was possible to fabricate large single grains without high-angle grain boundaries in the YBa 2 Cu 3 O 7-δ matrix 3 , which also contains small dispersed particles Y 2 BaCuO 5 (Y -211) 4 and usually a small addition of CeO 2 5, 6   .In melt textured samples, the c-axis direction is well defined, while a-and b-axis directions are highly interchanged because of twinning.
Most of the orthorhombic REBa 2 Cu 3 O 7-δ (RE = Rare Earth elements) compounds exhibit superconductivity with critical temperature T C above 90 K.The exceptions to this behavior are the elements Ce, Pr, and Tb.The suppression of superconductivity for Y 1-x Pr x Ba 2 Cu 3 O 7-δ occurs with increasing x, and superconductivity disappears with concomitant loss of metallic conductivity at x > 0.6 7 .For PrBa 2 Cu 3 O 7 (Pr -123), the compound is non-metallic and non-superconducting, an antiferromagnetic insulator.Many models have been proposed to explain the disappearance of superconductivity in Pr, such as hole filling, magnetic pair breaking, hole localization, percolation, and so on [8][9][10][11] , but the question of non-superconductivity in Pr -123 is still open.
The interest in the praseodymium-containing YBCO system dated from the 1990s, when studies had already demonstrated the dependence of elastic properties with temperature and the Pr amount in polycrystals 12 .Ivanov et al. 13 studied elastic properties of granular Y 1−x Pr x Ba 2 Cu 3 O 7-δ with 0 > x > 0.45 (superconducting) and x = 0.65 (nonsuperconducting), employing an ultrasonic pulse-echo technique during heating from cryogenic temperatures to the room temperature.The shear velocity , where G is the shear modulus and ρ the density, decreases with the increasing temperature for the same x, while, for the same temperature, it also decreases with increasing x.They observed anomalies in the shear modulus concerning stoichiometry and temperature.The authors indicated that the study of single crystals could bring insights to explain the observed variations in elastic properties with the Pr concentration in the material 13 .The melt-texturing of REBCO arose as a possible technique for producing largegrain materials on an industrial scale, aiming at applications in engineering devices such as energy-storage systems and motors.However, the mechanical performance was always a concern due to the intrinsic brittleness and high density of crystal defects of the RE-123 system 14 .
In this scenario, single crystal study can help understand the basic mechanical behavior of melt textured yttrium-based materials.Variables such as hardness and elastic modulus of insulating/superconducting Y 1-x Pr x Ba 2 Cu 3 O 7-δ are of interest aiming at applications in electronic devices such as superconductor-insulator-superconductor junctions.In this study, we produced and characterized Y 1-x Pr x Ba 2 Cu 3 O 7-δ with different contents of Y and Pr.As a transition occurs from the superconductor state to an antiferromagnetic insulator one at x ≅ 0.56, then we decided to produce a sample with a low Pr concentration (around 5%) to avoid undesired effects.The mechanical properties were measured by an instrumented indentation on ab-and a(b)c-planes.All analyzes are based on single samples.*e-mail: jfhlmonteiro@hotmail.com For the melt textured samples, the starting composition was 75 wt % Y(Pr)Ba 2 Cu 3 O 7-δ + 25 wt % Y 2 BaCuO 5 + 1 wt % CeO 2 .Commercial powder Y 2 BaCuO 5 was employed in the synthesis.Also, it was used CeO 2 addition to avoid the Y 2 BaCuO 5 coarsening 5,6 .The mixture, in appropriate quantities, was pressed into cylindrical pellets, and a seed crystal of NdBa 2 Cu 3 O 7-δ was placed on the top of the pellet.Then, the temperature was increased to 920 0 C at a rate of 40 0 C/h and maintained in this condition for 24 h.After that, the temperature increased above the peritectic temperature, at around 1040 o C for x = 0.00 and 1045 o C for x = 0.05, and maintained for 3 h.Then, the temperature was quickly decreased in 0.1 h (6 minutes) within a temperature window of approximately 40 o C (45 o C) for the initiation of sample growth.The next step was the temperature decreasing at 0.5 o C/h up 980 o C.Then, the temperature was slowly cooled to 700 o C up to the ambient temperature.Finally, the melt textured samples were heated in flowing oxygen at 420 o C for 120 h. Figure 1 illustrates the temperature ramps, and Figure 2 shows a real photograph of the samples.

Experimental Details
The single-phase formation of Y 1-x Pr x Ba 2 Cu 3 O 7-δ was confirmed by X-ray analysis, using a Shimadzu Ultima IV diffractometer with CuK α radiation in θ-2θ configuration.The X-ray diffraction patterns were collected from 3º < 2θ < 120º with 0.02º steps and 3.5 s counting time.The microstructure and the growth process were analyzed by scanning electron microscopy (SEM) with energy-dispersive spectroscopy (EDS).Before the surface mechanical tests, samples were previously grounded and polished up to 1 μm diamond paste.Nanoindentation tests on Y 1-x Pr x Ba 2 Cu 3 O 7-δ samples were performed using a Nanoindenter XP TM and a diamond Berkovich-type indenter, with applied loads ranging from 10 to 300 mN in a 5 x 4 indentation matrix parallel and perpendicular to the c-axis.The loading-unloading curves consisted of 10 cycles with a loading time of 10 s and a holding time of 15 s.The distance between each indentation was 50 μm.The indentations were performed in pieces taken from the central region of the sample, thus avoiding regions with excess green phase (Y-211) and other impurities.Care was taken to perform the indentations in a region free of pores and cracks, and the mechanical properties near the surface and inside the grains could be measured.The H and E were determined following the Oliver and Pharr method 15 .

Results and Discussion
The structure of the samples was investigated using X-ray diffraction.It is known that the solubility of rare earth (RE) ions in the superconducting matrix increase with increasing RE ionic size, while the 211 particles embedded in the matrix will be poor in Pr 17,18 .A random distribution of BaCeO 3 particles between 1 and 5 μm in diameter are homogeneously dispersed within the Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ .However, the granular nature and the porosity are clearly shown.Defects such as pores are created during the heating and cooling steps or formed during the oxygenation process.Also, as shown in panels (b) and (d), it was revealed microcracks parallel to (001) planes.The bulk critical temperatures for these samples are 91.7 K and 88.7 K for YBa 2 Cu 3 O 7-δ and Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ superconductors, respectively 19 .Further details on the sample preparation and characterization may be found in Reference 19,20 .
Figure 5 shows a SEM image of indentions produced with ten loading cycles, from 10 to 300 mN on the melt textured YBa 2 Cu 3 O 7-δ superconductor and the corresponding loading curves.At the highest applied load of 300 mN, the maximum depth reached by the tip was around 1500-1700 nm in ab-and a(b)c-planes.For this x = 0.00 sample, curves were continuous, presenting no pop-ins caused by cracks nucleation or pores crushing.No radial cracks emanating from the corners of the indentation were observed.However, in both planes, notably in the ab-plane, it was possible to observe more material displaced around the tip imprint due to the nucleation and propagation of lateral cracks.The material displacement in successive indentation tests is a phenomenon typically observed in lamellar-like materials 21,22 .Generally, when these lateral cracks are nucleated, radial cracks are suppressed.However, the involved mechanisms seem to depend on specific features of the compound.
The analyses of the imprints morphologies shown in Figure 5 suggest that the plastic deformation mechanism is similar to that proposed by Pimentel et al. 21.They studied the mechanical properties of the highly oriented FeSe 0.5 Te 0.5 superconductor compound.The samples' morphology also consisted of plate-like crystals with a lamellar-like structure and the same behavior of cracking for the ab-plane as observed in this material.According to the authors, as the indenter penetrates in the ab-plane, cracks parallel to the surface are nucleated at the edges of the indenter, lifting material at the tip edges and eventually leading to the surface´s partial detachment (as seen in Figure 5a).In this mechanism, the crack pattern is assumed as perpendicular to the c-axis.
Figure 6 shows SEM images of indentions produced with ten loading cycles, from 10 to 300 mN on the melt textured Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ superconductor in the ab-and a(b) c-planes and the corresponding loading curves.At the highest applied load of 300 mN, the maximum depth reached by the tip was around 1450-1750 nm.For the ab-plane, the curve is not continuous, presenting tip incursions in the 1300 nm depth range.During the tests, the indenter is submitted to a triaxial stress distribution, and delamination can occur around the indenter, or lateral cracks can be nucleated, with the material being ejected by these tangential forces parallel to the surface.When this effect occurs, the indenter loss sustentation and penetration depth suddenly increase, reflecting in the loading curves as such discontinuities.In this material, as seen for the YBa 2 Cu 3 O 7-δ , the material displacement around the tip imprint was also observed due to the nucleation and propagation of these cracks, again with a higher degree for ab-plane.No radial cracks emanating from the corners of the indentation were observed.To sum up, the surfaces morphologies after localized mechanical tests were alike between the x = 0.00 and x = 0.05 samples, although the latter disclosed intensified brittleness under normal loading.
Hardness profiles as a function of penetration depth for ab-and a(b)c-planes are presented in Figure 7 for  x = 0.00 and x = 0.05 samples.All curves presented higher hardness values at low penetration depths and then decreased continuously in the analyzed penetration depth.Independent of the investigated plane or sample, the profiles tend to reach a plateau at deeper regions, around 5-7.5 GPa at approximately 1500-1700 nm.The x = 0.05 profile at the ab-plane was the lower one, explained by the fracture-promoted increase in the contact area during indentation, evidenced by tipincursions as shown in Figure 6b.Note that the profiles are deeper than 200 nm (the tip rounding radius) to avoid errors in the contact area measurement, as well as the indentation size effect 23 .Nevertheless, others factors may originate the variation observed in hardness with penetration depth, as outermost plastic deformation layers produced by polishing, microcracking in adjacent lamellae, or the crushing of inner pores.Actually, such decrease was small compared to those observed in other lamellar-like single crystals 21,22 (it is in the order of the error bars), having little influence in the measurements.
Studying the indentation hardness H of REBa 2 Cu 3 O 7-δ , researchers obtained values of 5 + 2 GPa for polycrystalline, 8.0 + 0.5 for single crystals and 9.0 + 0.5 GPa for both ab-and a(b)c-plane in melt-textured superconductor bulks 24 .The RE corresponds to a natural mixture of rare earth elements (Y -Yb -Dy -Ho -Tm -Lu -Gd -Tb -Sm -Ce -Eu -Nd) extracted from the xenotime mineral.In accordance with the authors, the different values for the hardness are due to the different microstructures present in these materials.The hardness of the polycrystalline REBa 2 Cu 3 O 7-δ is the lowest because pores collapse under indentation, leading to the tip incursion and the subsequent overestimation of the contact area (since H ∝ A -1 ).On the other hand, the highest value observed for the melt textured sample is due to the presence of the 211 precipitates, which are harder and stiffer than the 123 phase.The hardness of the 211 phase is 14 + 2 GPa, while it is about 10-11 GPa for the pure 123 phase.Values of the single crystal with lamellar-like structure, with a small size of approximately 1 mm and 20 μm thick (observed at the lamellar edges), were lower than those of the melt textured samples, explained by the specific deformation features of lamellar-like structures, as discussed previously in this report and in 21,22 .
Figure 8 shows elastic modulus profiles as a function of penetration depth for ab-and a(b)c-planes of melt textured YBa 2 Cu 3 O 7-δ and Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ superconductor.Open and closed symbols, respectively, represent pure and doped samples.The elastic modulus was higher near the surface and decreased with the increasing penetration depth due to the aforementioned artifacts affecting indentation measurements.It is conspicuous that, differently from hardness, the planes present significant variations in elastic modulus.For ab-plane, the elastic modulus is approximately the same for x = 0.00 and x = 0.05 samples, around 132 GPa at the deepest tip penetration.However, for a(b)c-plane, the elastic modulus value for x = 0.05 is higher than that observed for the x = 0.00 sample, i.e., around 183 GPa and 147 GPa, respectively, both at the deepest tip penetration.
The ~24% increase in elastic modulus, attained by varying the stoichiometry from YBa .The potential energy among adjacent atoms also changes locally since the electron screening of the nucleus is stronger in Pr than it is in Ba.Such effect is more noticeable in the c direction, where the atomic density is higher from a RE atom´s point of view.The understanding of the mechanism through which the electron balance affects mechanical properties of the REBa 2 Cu 3 O 7-δ material is beyond the scope of this report.Nevertheless, variations in the potential energy U through the c-axis do result in different atomic bonding forces (F = -∇U) and equilibrium separation r 0 in that direction.As a result, the elastic modulus varies as well, as measured here for the a(b)c-plane, since it strongly depends on bonding.From a simplified approach 25 ,

Y 1 -
x Pr x Ba 2 Cu 3 O 7-δ superconductor pellets (x = 0.00 and x = 0.05) have been prepared by top seeding technique using melt-textured NdBa 2 Cu 3 O 7 seeds.Firstly, a ceramic sample was prepared by the conventional solid-state reaction method.Appropriate amounts of Y 2 O 3 , Pr 6 O 11 , BaCO 3, and CuO were mixed and calcinated in the air at 880 o C, 900 o C, and 920 o C for 24 hours and then slowly cooled up to 700 o C.

Figure 6 .
Figure 6.(a) and (c) Secondary-electrons scanning electron microscopy images of imprints produced with a Berkovich indenter under 300 mN load on the melt textured Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ superconductor in the ab-and a(b)c-planes, respectively; in (b) and (d), the corresponding loading-unloading curves related with the imprints shown in (a) and (c).

2
Cu 3 O 7-δ to Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ , is possibly a consequence of different interactions experienced by neighboring atoms at the orthorhombic structure.The electrons distributions are Ba = [Xe] 6s 2 and Pr = [Xe] 4f 3 6s 2 .Hence, substituting barium for praseodymium in the position of the rare earth atom introduces more electrons in the band structure.The consequences to the lattice parameters were minimal, as inferred from XRD results, shown in Figure 3. On the other hand, T C varies from 91.7 K for YBa 2 Cu 3 O 7-δ to 88.7 K for Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ 19,20

Figure 7 .
Figure 7. Hardness profiles as a function of penetration depth for ab-and a(b)c-planes of melt textured YBa 2 Cu 3 O 7-δ and Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ superconductor.

Figure 8 .
Figure 8. Elastic modulus profiles as a function of penetration depth for ab-and a(b)c-planes of melt textured YBa 2 Cu 3 O 7-δ and Y 0.95 Pr 0.05 Ba 2 Cu 3 O 7-δ superconductors.