Y2BaCuO5 composites by micro-Raman spectrometry

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Physica C 468 (2008) 388–393 www.elsevier.com/locate/physc

Evidence of oxygen content heterogeneity in TSMTG YBa2Cu3O7 d/Y2BaCuO5 composites by micro-Raman spectrometry F. Delorme a,*, J.-F. Bardeau b, C. Harnois a, I. Monot-Laffez c b

a Laboratoire CRISMAT, UMR CNRS 6508, ISMRA, Boulevard du Mare´chal Juin, 14050 Caen Cedex, France Laboratoire de Physique de l’Etat Condense´, Faculte´ des Sciences, Universite´ du Maine, Avenue Olivier Messiaen, 72085 Le Mans Cedex 09, France c Laboratoire LEMA, CNRS FRE-2077– CEA-LRC M01 – IUT de Blois, 3 Place Jean Jaure`s, CS2903, 41029 Blois, France

Received 19 July 2007; received in revised form 11 December 2007; accepted 18 December 2007 Available online 28 December 2007

Abstract The homogeneity of the oxygen content of TSMTG-YBa2Cu3O7 d/Y2BaCuO5 composites has been investigated by micro-Raman spectrometry. The Y123 compound has been found to be very sensitive to the laser power, but a laser power of 0.04 mW has been shown to not to be harmful for the samples, even after 2 h of irradiation. Raman spectra have shown that the oxygen content of YBa2Cu3O7 d/ Y2BaCuO5 ceramics is not homogeneous at the micrometer scale. In addition, no monotonic decrease of the oxygen content is observed from the periphery to the core of the sample, confirming that the oxygen uptake is not strictly controlled by a diffusion process. Ó 2007 Elsevier B.V. All rights reserved. PACS: 74.62.Bf; 74.62.Dh; 74.72.Bk; 78.30. j Keywords: YBaCuO; Raman spectrometry; Oxygen content; TSMTG

1. Introduction Since its discovery in 1987 by Wu et al. [1], the compound YBa2Cu3O7 d (Y123) has been the subject of numerous studies due to its superconducting properties beyond the nitrogen boiling point. Most of these works were devoted to the improvement of the superconducting properties of these materials [2–6]. Nowadays the best properties are obtained, for bulk materials, in textured ceramics [7]. Oxygen content is now well known to have the most important effect on superconducting properties of YBa2Cu3O7 d compound. Indeed, it was shown that the tetragonal phase (7 d < 6.4) is non-superconducting, whereas the orthorhombic phase is superconducting with optimum properties exhibited for 7 d  6.92 [8]. Usually, * Corresponding author. Present address: BRGM, MMA/MIN, 3 Avenue Claude Guillemin, BP 6009, 45060 Orleans Cedex 2, France. Tel.: +33 2 38 64 35 22; fax: +33 2 38 64 37 11. E-mail address: [email protected] (F. Delorme).

0921-4534/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2007.12.006

titration or gravimetric methods are used to determine the oxygen content in powders or single crystals [9,10]. Top-seeding melt-textured grown (TSMTG)-ceramics have now reached superconducting properties high enough to be planned for applications [11–13]. TSMTG-ceramics are constituted by homogeneously dispersed micron-size Y2Ba1Cu1O5 (Y211) precipitates in an Y123 matrix. Most of the authors believe that defects (such as Y211 inclusions, oxygen vacancies, dislocations, stacking faults, twin boundaries, . . .) act as pinning centers for vortices, and then increase the critical current densities. The exact amount of Y211 in the sample after processing is not exactly known due to the incomplete peritectic reaction: Y123 ? Y211 + liquid. Thus, the presence of the Y211 phase, which contains CuII too, does not allow the use of the chemical titration method to determine the oxygen content of the TSMTG samples. Moreover, gravimetric measurements cannot be used as it also includes contributions from impurities such as CO2 or water [14]. So techniques able to give indirectly the oxygen content

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have to be used to study the distribution of oxygen in these ceramics. Raman spectrometry is a well known non-destructive technique which was used very early (since the discovery of the Y123 in 1987) for characterizing the high-temperature superconductors (HTS) [1]. The first Raman measurements were performed to study the crystallographic structure of HTS compounds [15–17], and some authors [15,16] observed that the position of some peaks could vary according to the oxygen content. All these investigations (see Cardona [18] for a recent overview) show that the superconducting YBa2Cu3O7 d compound is orthorhombic (space group Pmmm) and belongs to the point group D2h. The 15 Raman active modes are 5Ag(xx,yy,zz) + 5B2g(xz) + 5B3g(yz), with the Ag modes being the most intense ones. So Raman spectra of Y123 with good superconducting properties (i.e. an oxygen content between 6.9 and 7) usually exhibit only the five peaks due to Ag modes at about (the exact position of some peaks depends on the oxygen content – see below): 115 cm 1, 150 cm 1, 340 cm 1, 440 cm 1 and 500 cm 1. The peak at 115 cm 1 involves mainly vibrations of Ba atoms and does not evolve with the oxygen content. The peak at 150 cm 1 involves vibrations of Cu2 atoms of the CuO2 planes and shifts to lower wave numbers as the oxygen content decreases. The peaks at 340 cm 1 and 440 cm 1 are related to vibrations of the O2–O3 plane oxygen atoms, but the peak at 340 cm 1 does not depend on the oxygen content whereas the peak at 440 cm 1 shifts to higher wavenumbers as the oxygen content decreases. The peak at 500 cm 1 is related to vibrations of the apical O4 oxygen atoms and presents the most important shift to lower wavenumbers as the oxygen content is lowered. As many defects, and especially oxygen vacancies, are believed to act as pinning centers for vortices, and thus lead to increase critical current densities, a careful investigation of the homogeneity of the oxygen content of YBa2Cu3O7 d/Y2BaCuO5 composites have been performed in a non-destructive way by micro-Raman spectrometry through the recording of the position of the 500 cm 1 peak.

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tic decomposition temperature of the sample and similar crystallographic parameters, on the top of the pellet to initiate the growth of an oriented domain. A presintering step at 1000 °C is applied to the samples in order to decarbonate the material and homogenise their composition. The samples are subsequently decomposed at 1045 °C during 5 h and cooled down at a rate of 1 °C/h between 1035 and 950 °C to allow the growth of the domain initiated from the seed. After the domain formation, a quite slow cooling rate of 35 °C/h is used to avoid microcracks formation. Finally, an annealing under oxygen flow at an optimum temperature for oxygen diffusion for a duration depending on the size of the sample, changes the tetragonal YBa2Cu3O6 phase into the orthorhombic superconducting YBa2Cu3O7 d phase. Pellets exhibiting a diameter of 3 cm and a thickness of 1.5 cm have been made (Fig. 1) by following the full process described by Leblond et al. [19]. Their critical temperature is around 91 K (cerium additions lead to a slight decrease of the critical temperature) and their critical current density Jc is in the range of 50,000 A
cm 2 under self-field, near 20,000 A
cm 2 for H = 1 T, and Jc = 0 A
cm 2 for H  3.5 T, after the optimal annealing treatment. The microstructure of the sample was observed on polished surfaces along the c-axis with a SEM (Philips XL30 FEG) at 30 kV. Raman experiments were performed using a T64000 Raman spectrometer (from Jobin–Yvon–Horiba) in a single monochromator mode, using a Notch Rayleigh rejection filter, a 1800 lines/mm diffraction grating, and a liquid-nitrogen-cooled CCD detector. The excitation source was an argon–krypton ion laser (from Coherent Innova-70) operating at 514.5 nm with a laser power intentionally limited to 0.04 mW on a sample surface to prevent possible sample deterioration. The spectra were recorded

2. Experimental The YBa2Cu3O7 d/Y2BaCuO5 composites have been synthesised from raw powders of commercial YBa2Cu3O7 d (Solvay Barium Strontium), commercial spray dried Y2Ba1Cu1O5 (Seattle Speciality Ceramics) and CeO2 (Aldrich), with a 99.9% purity. The starting composition is Y123 + 25 mol% Y211 + 0.5 wt% CeO2. The mixture of powders is uniaxially pressed at 103 kg cm 2 to obtain 30 mm diameter pellets. The pellets are placed on an Y2O3 powder layer on an alumina support. Then the TSMTG-process is used to grow the samples. Briefly it consists in putting a seed (here a SmBa2Cu3O7 d single domain with 40 mol% of Sm2Ba1Cu1O5), with a melting temperature higher than the peritec-

Fig. 1. Photograph of a 3 cm-diameter pellet of YBa2Cu3O7 Y2BaCuO5 composites obtained by the TSMTG process.

d/

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under a confocal microscope equipped with a 100 objective (0.95 numerical aperture), which yields a spot diameter less than 2 lm on the sample. The typical spectral resolution is better than 0.7 cm 1. 3. Results and discussion As already mentioned, the conditions of acquisition of Raman spectra have to be carefully chosen because the laser beam could bring with the 100 objective a high energy flow on small areas. In the case of absorption, this energy could lead to an important increase of the sample temperature or a deterioration of the sample during the experiment. In the case of Y123, it is well known that the compound undergoes a structural transition as temperature increases from the orthorhombic YBa2Cu3O7 superconducting phase to the tetragonal YBa2Cu3O6 nonsuperconducting phase. Consequently, the energy coming from the laser is susceptible to modify the oxygen content of the material, and therefore the value that can be determined from the quantitative analysis of the Raman spectra. A shift of the 500 cm 1 peak to lower wavenumbers had been observed for apical oxygen atoms vibration with a laser power of only 1 mW on the sample; such an evolution reveals structural changes. So it has been decided to work at a laser power of 0.04 mW on the sample to avoid any structural modification or partial degradation during the measurements. The effects of long time irradiation have been checked. Fig. 2 presents the Raman spectra obtained on the same point after irradiation from 20 to 120 min. No change in the position of the 500 cm 1 peak is observed even after 2 h of irradiation, at this laser power, the laser irradiation leaves the sample unchanged. In the following, all the spectra have been recorded under the same conditions of laser power for 20 min. Raman spectrometry does not give directly the oxygen content present in the sample, but the position of peaks that can be correlated to the oxygen content. Thus, it requires a reference technique and a calibration curve. Different methods, such as titration or gravimetric methods, have already been proposed in the past. Fig. 3 shows for

520 510

Wavenumber (cm-1)

390

500

Feile [20] Huong [21]

490

Huong [22] Long [23]

480 470 460 6

6.2

6.4

6.6

6.8

7

Oxygen Content (7-δ)

Fig. 3. Calibration curves oxygen content-wavenumber from literature.

example, the calibration curves proposed by Feile in a review article [20], Huong et al. [21], Huong [22], and Long et al. [23]. These four curves are following the same trend: a higher wavenumber for a higher oxygen content. However, they present various slopes and origin (from 463 [22] to 484 cm 1 [23] for YBa2Cu3O6). Recently, Liarokapis [24] has presented variations from 478 to 502 cm 1, with a softening for overdoped samples (7 d > 6.9). At this point two assumptions can be made to explain this apparent discrepancy. First, some Raman measurements might be wrong due to a too energetic incident laser beam as mentioned above. The second hypothesis is that mistakes lie at the level of the reference technique, due for example to heterogeneities of the reference material. Some experiments realised to obtain a new calibration curve tend to confirm this hypothesis. Indeed, the 99.9% purity commercial Y123 powder used to synthesise the composites that present excellent superconducting properties, has shown at the scale reached with the 100 objective of the Raman spectrometer the presence of impurities, and especially several mineral phases containing copper other than Y123, such as Y211, BaCuO2 or CuO (Fig. 4). However, the curve issued from the data of Long et al. [23] seems to be more consistent with the Raman measurements on the YBa2Cu3O7 d/Y2BaCuO5 composites presented below. Indeed, Cava et al. [1] have shown that the best superconducting properties are reached for an oxygen content of 7 d  6.92. The oxygen annealing treatment of

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5000 4000 120 min irradiation 100 min irradiation

3000

80 min irradiation

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60 min irradiation

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40 min irradiation 20 min irradiation

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Intensity (arbitrary unit)

Intensity (arbitrary unit)

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7000 6000 5000 4000 BaCuO2

3000 2000

Y211

1000

CuO

Y123

300

400

500

600

700

800

Wavenumber (cm-1)

Fig. 2. Raman spectra of the same area after irradiation from 20 to 120 min.

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Wavenumber (cm-1)

Fig. 4. Raman spectra of Y123, Y211, BaCuO2 and CuO.

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F. Delorme et al. / Physica C 468 (2008) 388–393 20000

Y211 18000

16000

H 14000

G Intensity (arbitrary unit)

the 3 cm-diameter pellets has been optimised to obtain the best superconducting properties. So the mean oxygen content of the studied YBa2Cu3O7 d/Y2BaCuO5 composites should be close to this value of 6.92. The lowest wavenumber measured for the 500 cm 1 peak in the optimally oxygenated YBa2Cu3O7 d/Y2BaCuO5 composites is 505 cm 1. This value gives for the curves based from the data of Feile [20], Huong et al. [21], Huong [22] an oxygen content above 7 (the corresponding wavenumber for an oxygen content of 7 is respectively 501, 502.961, and 502.8 cm 1). This value is also higher than the value proposed by Liarokapis [24] for YBa2Cu3O7. So, as the superconducting properties of the 3 cm-diameter pellets are optimal, the calibration curve used in this study to estimate the oxygen content of the YBa2Cu3O7 d/Y2BaCuO5 composites from the Raman spectra will be the calibration curve obtained by Long et al. [23]. In order to proceed to the Raman spectra measurements, the 3 cm-diameter pellet had been sawn in two equal parts by a wire-saw using alcohol as a cooling liquid. Indeed, water contact has been shown to be very detrimental to the Y123 compound and its superconducting properties [25]. The Raman measurements have been done on the section of the pellet as shown in Fig. 5. Analysed points were taken from top (A) and bottom (B) and from periphery to the core of the pellet (C to H). The Raman spectra obtained for these points are presented in Fig. 6. All these spectra are similar except spectra F and H. The spectra A, B, C, D, E and G correspond to the spectrum of the Y123 compound, whereas spectra F and H correspond to the spectrum of the Y123 compound added with the spectrum of CuO and Y211 respectively (see Fig. 4). The presence of the Y211 phase is easy to explain. Indeed, 25 mol% of this compound were added to Y123 at the beginning of the growth process. Moreover, the peritectic decomposition of Y123 in air leads to the formation of solid Y211 and a liquid phase rich in barium and copper [26]. This leads to the typical microstructure of YBa2Cu3O7 d/Y2BaCuO5 composites obtained by the TSMTG process: micron-size Y211 particles, homogeneously dispersed, embedded in a Y123 matrix (Fig. 7). This kind of microstructure shows

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CuO

F 10000

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B

A 0 200

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500

600

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800

-1

Wavenum b er (cm )

Fig. 6. Raman spectra of the area localised in Fig. 5.

clearly why it is highly probable to observe contributions of the Y211 spectrum while using a Raman probe of about 1 lm3. For point H, the contribution of the Y211 particle is weak and does not seem to present a peak that interferes with the peak of the Y123 compound at 500 cm 1. So it is assumed that in this case, the presence of a contribution of the Y211 spectrum will not change the estimation of the oxygen content. The presence in the spectrum F of a peak attributed to CuO requires a different explanation. Indeed, microstruc-

Fig. 5. Section of the pellet and localisation of the analysed areas (A–H).

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Fig. 7. Typical microstructure of an YBa2Cu3O7 d/Y2BaCuO5 composite obtained by the TSMTG process: micron-size Y211 particles, homogeneously dispersed, embedded in a Y123 matrix (secondary electron image).

ture studies by SEM of YBa2Cu3O7 d/Y2BaCuO5 composites have never shown the presence of CuO particles embedded in the Y123 matrix as in the case of the Y211 compound. However TEM studies have shown that the Y123 compound presents stacking faults and that these stacking faults correspond to intercalated CuO planes in the Y123 structure [27,28]. Some areas rich in stacking faults do exist in Y123/Y211 composites. Some of these areas are perpendicular to the c-axis and represents cleavage planes, very easy to recognize once the sample is cleaved to the metallic luster of the two cleaved surfaces. The spectrum F, presenting a peak assignable to CuO, is surely related to such an area. The positions of the 500 cm 1 peak for points A to H are summarized in Table 1. The position of the peak varies from 505 cm 1 for points B and D, to 510 cm 1 for point A. These values seem to be heterogeneously distributed. In peculiar, no monotonic decrease in the oxygen content is observed from the periphery to the core of the sample. This is in agreement with the usual belief that the oxygen uptake is not strictly controlled by a diffusion process from periphery to the core. The presence of cracks, as shown in Fig. 5, can explain why the oxygen uptake is not a pure diffusion mechanism. Peak positions from 505 to 510 cm 1 correspond to oxygen contents of 6.76 to 6.93, respectively (Table 1), accordTable 1 Positions of the 500 cm 1 peak and corresponding oxygen contents for the areas localised in Fig. 5 Point localisation

Wavenumber (cm 1)

Estimated oxygen content according to [13]

A B C D E F G H

510 505 509 505 509 509 509 506

6.93 6.76 6.90 6.76 6.90 6.90 6.90 6.80

ing to the model of Long et al. [23]. These results are in good agreement with those of Akase et al. [29,30]. Indeed, Akase et al. [29] have shown by selected area electron diffraction (SAED) that the oxygen content of an Y123 sintered sample varies from grain to grain and even in one grain. They have also shown, using intensities of convergent beam electron diffraction (CBED) patterns [30], that in a QMG single crystal [31], the oxygen content of two points separated by 1 lm could be as different as 6.80 and 6.92. The results presented in this paper, on TSMTG-YBa2Cu3O7 d/Y2BaCuO5 composites demonstrate a similar range of variation of the oxygen content from 6.76 to 6.93. However, it is difficult to determine the origin of these variations. They surely have to be related to the microstructure of the samples. Akase et al. [30] have attributed the variation of oxygen content that they observed in the QMG single crystal to lattice strain around an Y211 particle. However, other microstructural features very frequent in the Y123 compound, such as dislocations, twin boundaries or point defects (vacancies or substitutions) could be responsible for these variations of the oxygen content. New facilities coupling scanning electron microscopy to Raman spectrometer will be a powerful tool for precising the relations of some microstructural features and the oxygen content as some of them can be precisely studied by SEM.

4. Conclusions First of all, this study has demonstrated that the Y123 compound is very sensitive to the laser power. Indeed, it has been shown that even for laser power of only 1 mW (with a 100 objective), the 500 cm 1 peak shifts to lower wavenumbers, indicating thus a loss of oxygen. We determined that a laser power of 0.04 mW on sample is perfect to investigate the vibrational properties without altering the sample, even after 2 h of irradiation. Several previous calibration curves have been compared. The data obtained are in accordance with those of Long et al. [23]. Finally, it has been demonstrated that the oxygen content of YBa2Cu3O7 d/Y2BaCuO5 composites obtained by TSMTG is not homogeneous. In peculiar, no monotonic decrease of the oxygen content is observed from the periphery to the core of the sample, confirming that the oxygen uptake is not strictly controlled by a diffusion process. Moreover this heterogeneity could have an important influence on the superconducting properties of these composites as some part of the samples can become non-superconducting before the whole composite and thus serve as pinning for vortices and lead to increased superconducting properties. The impact on the superconducting properties has to be further quantified since it can provide a future way to improve the superconducting properties of Y123/Y211 composites for bulk or thin films.

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