Structural and magnetic properties of YBa2Cu3O7/BaZrO3 composites

Structural and magnetic properties of YBa₂Cu₃O₇/BaZrO₃ composites

David A. Landínez-Téllez^I; Jairo Roa-Rojas^I; Gabriel Peña-RodríguezII,^*

^IPhysics of New Materials Group, Department of Physics, Universidad Nacional de Colombia – UNAL, Bogotá-Colombia

^IICeramic Technology Research Group, Universidad Francisco de Paula Santander – UFPS, Av. Gran Colombia, 12E-96 Cúcuta-Colombia

ABSTRACT

We reported a study of the structural and magnetic properties of BaZrO₃ (BZO) as possible substrate material for the production of YBa₂Cu₃O_7-δ (YBCO) superconducting films. Rietveld analyses of the X-ray diffraction pattern show that BZO crystalizes as a cubic perovskite, space group Pm3m (#221). Chemical stability and crystallographic coupling between BZO and YBCO were examined by characterizing YBCO/BZO (10, 30 and 50 YBCO vol%) polycrystalline composites. Morphological and compositional analyses of composites were performed through scanning electron microscopy and energy dispersive X-ray experiments, respectively. Response of magnetization measurements revealed that the proximity of BZO does not affect the superconducting transition temperature (T_c=90,2 K) of YBCO material. Our results evidenced that the BZO is an excellent candidate to be substrate for the fabrication of YBCO superconducting thin films.

Keywords: complex perovskite, magnetic properties, superconducting film substrates

1. Introduction

The complex cubic perovskite oxides have been investigated for their use as potential new substrates for the production of cuprate superconducting films^1,2. For good quality substrates, the material candidate must satisfy the following requirements: chemical stability, e. g., absence of chemical reaction at the interface substrate-film; good lattice parameter coupling at the crystallographic plane substrate-film to guarantee the epitaxial growth; and the presence of the substrate material must not affect the superconducting properties of the film³. The high chemical reactivity of YBa₂Cu₃O_7-δ (YBCO) with most conventional substrate materials at the processing temperature imposes severe restrictions on the availability of substrates for the YBCO superconducting films⁴. Moreover, the divergence of the crystallographic properties at the interface between YBCO and the commonly available substrates, as well as the difficulty in obtaining chemical and structural properties compatible with the YBCO superconductor, constitute a great motivation to produce new optimal materials for this application. For example, MgO, the widely utilized substrate for YBCO thin films, produces an interlayer of barium salt at the YBCO/MgO interface when the processing temperature is above 700 ºC⁵. Another extensively used substrate for YBCO, LaAlO₃, exhibits an excellent lattice coupling but it has the disadvantage of being available only as twinned single-crystal⁶. Studies on the effect of BaZrO₃ on YBa₂Cu₃O₇ superconducting thin films as pinning centers in order to improve the critical current density have been reported⁷. A microstructural analysis of the BaZrO₃ as potential substrate for YBa₂Cu₃O₇ films was performed without consider it effect on the superconducting properties⁸. In this paper, we reported the systematical steps followed for the production and characterization of BaZrO₃ (BZO) as eventual substrate for the fabrication of YBCO high temperature thin films. Pure BZO and YBCO/BZO (0 to 100 vol%) were produced and characterized to study the viability of utilizing BZO as substrate to produce YBCO superconducting thin films.

2. Experimental

BZO polycrystalline samples were prepared by a solid-state reaction process. Stoichiometric ratios of the precursor powders BaCO₃ (purity 99,98%) and ZrO (99.99%) were finely ground and thoroughly mixed. The precursor powder was pressed into a disc and the material was calcined at 850 ºC for 148 h in an ambient atmosphere. By using a Nickel-filtered CuKα radiation (λ=1.5406 Å) of a SIEMENS D5000 diffractometer, X-ray diffraction (XRD) spectra were taken of the calcined powder of samples. The material was again crushed, finely ground and pressed at 6 ton/cm² to form a disc (10 mm diameter, 2 mm thickness). This disc was sintered at 900 ºC for 17 h in a vacuum atmosphere and furnace-cooled to room temperature. The XRD pattern of the sintered material was analyzed by a Rietveld refinement through the GSAS code⁹. For chemical stability studies, a single-phase YBCO superconductor was prepared by solid-state synthesis¹⁰. 50%, 70% and 90% vol% of BZO were mixed in YBCO superconductor powder (50/50, 30/70 and 10/90) and the mixture was pressed into circular discs (10 mm diameter, 1 mm thickness) at a pressure of 5 ton/cm² and heat treated at 960 ºC in oxygen atmosphere for 12 h. After the heat treatment, YBCO-BZO samples were slowly cooled down to room temperature for proper oxygenation. XRD spectra of these samples were recordered from 20º to 90º for crystallographic phase characterization, the chemical stability and the crystallographic parameter coupling of the YBCO/BZO composites. The morphological characterization of samples was systematically effectuated from scanning electron microscopy (SEM), by using Quanta 200 SEI Electron-Optics equipment. Compositional analysis of films was performed by energy dispersive X-ray (EDX) experiments by means of a microprobe coupled to the SEM microscopy. The influence of BZO on the superconducting properties of YBCO was studied in the composite samples through DC magnetization measurements by using a Quantum Design model 2000 MPMS SQUID system.

3. Results and Discussion

The XRD pattern of Figure 1 consists of strong peaks which are characteristics of a primitive cubic perovskite ABO₃, where A=Ba is the large ion (149 Å) suitable to the 12-coordinated cube-octahedral sites and B=Zr is the smaller ion (86 Å) suitable to the 6-coordinated octahedral site. No evidence for a distortion from the cubic symmetry is observed in the XRD pattern.

The lattice parameters of BZO, calculated from the Rietveld refinement of the XRD data is a=4,196(9) Å. This result is 99% in agreement with the theoretical value given by the Structure Prediction Diagnostic Software,¹¹ which predicts a cell parameter a=4,156(0) Å. The Rietveld parameters were χ²=1,497 and R_F=6%, which are between the considered acceptable values χ²<2 and R_F<10%. For the YBCO, the obtained XRD pattern is close to the standard reported in the JCPDS crystallographic database¹².

In the perovskite like materials the distortion of the octahedral from the ideal ABO₃ cubic structure is determined by the tolerance factor

where r_A, r_B and r_O are the ionic radii of the A, B and O ions¹³. The calculated value of the tolerance factor for the BZO compound is τ=1.004, which is close to the ideal τ=1.000 of the perfect cubic perovskite.

The atomic positions of Ba, Zr and O ions into the unit cell are shown in Table 1.

In order to verify the possibility of using BZO as a substrate material for thin films of YBCO superconductor, we have studied the chemical reactivity of YBCO with BZO (50/50, 30/70 and 10/90). XRD patterns of these composites are shown in Figure 2. As seen in this figure, all the peaks in the XRD could be indexed either for YBCO or for BZO and there is no extra XRD peak is found. Within the accuracy of XRD technique, these results show that YBCO and BZO remain as two distinct separate phases in the YBCO/BZO composites and BZO is chemically stable with YBCO superconductor for all vol% addition of BZO. Regarding the crystallographic coupling, we notice that there is a matching of 1.3% between the BZO lattice parameter and the crystallographic constant of the most used substrate MgO (a=4,212 Å). Figure 2 shows the XRD pattern analyzed for the YBCO/BZO 50/50 composite.

It is observed that for all vol% of YBCO/BZO concentrations, only the crystalline phases of YBCO (orthorhombic) and BZO (cubic) were obtained.

Energy dispersive X-ray (EDX) analysis was performed on the single phase BZO for the quantitative elemental chemistry analysis. The result revealed the expected percentages for the calculated stoichiometric composition (up to 98%), without evidences of impurity traces in the samples (Table 2).

The surface morphology of sintered YBCO/BZO composites was investigated by scanning electron microscopy (SEM). The result for the 50/50 YBCO/BZO composite is shown in the micrograph of Figure 3. This indicates that the surface of the samples presents a crystalline character, which is typical of a polycrystalline ceramic material.

Additionally, Figure 3 shows a homogeneous surface morphology and particle size distribution with the grain average size estimated to be 2 µm for the YBCO material and sub-micrometric for the BZO compound. We notice that there is no detectable interface interaction between the BZO and the YBCO grains. It is observed that the BZO particles are clearly distributed in the YBCO matrix. This result was corroborated from the EDX experiments. From these results, it can be confirmed, that there is chemical stability between the YBCO and BZO compounds in the material.

The superconductivity of YBCO/BZO composite samples was studied by measuring the DC magnetization with an applied field H=100 Oe and in the temperature range 10 to 120 K. Figure 4 shows curves of the DC magnetization as a function of temperature for the YBCO/BZO composites with 50, 70 and 90 vol% of BZO addition in YBCO superconductor.

As shown on this picture, all the YBCO/BZO composites have the same superconducting transition temperature T_c=90,2 K as expected in a pure YBCO superconductor¹⁵. This shows that for all vol% of an insulating ceramic oxide (BZO) addition in YBCO there was not any deteriorating effect on the transition critical temperature of YBCO superconductor.

The Meissner effect, characterized by the negative magnetization response, is also observed in all composites. Thus as discussed earlier BZO is chemically stable with YBCO superconductor and at the same time it did not have any deteriorating effect on the superconducting property characterized by the transition temperature.

4. Conclusions

It is found by the structural characterization that the cell parameter of BZO is close to the commercial MgO lattice parameter and is chemically non-reacting with the YBCO superconducting even under extreme processing conditions. It is observed that there is no chemical reaction between these compounds into the YBCO/BZO composites. Were carried out morphological characterization to evaluate the chemical reaction between the insulating BZO and the metallic YBCO materials. In the YBCO/BZO composites experimental analysis, the formation of separate single-phase grains of YBCO and BZO was observed by SEM images and XRD characterization. Energy dispersive X-ray (EDX) analysis shows that there is no evidence of impurity traces in the samples. DC magnetization measurements reveal that the presence of BZO does not affect the superconducting properties of YBCO. This systematic work corroborated our hypothesis that the BZO cubic perovskite can be utilized as substrate material for the fabrication of YBCO thin films.

Acknowledgments

This work was partially financed by Division of Investigations of the National University of Colombia (DIB – Hermes Code 13893) and "El Patrimonio Autónomo Fondo Nacional de Financiamiento para la Ciencia, la Tecnología y la Innovación Francisco José de Caldas" Contract RC -No. 0850-2012". Authors wish to thank Karen Hallt and Vanessa Rangel, for careful review of the paper.

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Received: December 20, 2012

Revised: February 5, 2013

Publication Dates

• Publication in this collection
  18 June 2013
• Date of issue
  Oct 2013

History

• Received
  20 Dec 2012
• Reviewed
  05 Feb 2013