Corrosion induced degradation of textured YBCO under operation in high humidity conditions

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Abstract

The early stages of aggressive degradation at 85% of humidity for 24 h of single crystals of YBa2Cu3O7 − δ textured by Bridgman technique have been analyzed by means of atomic force microscopy, X-ray photoelectron spectroscopy, nanoindentation and focused ion beam. The results show that the initial formation of barium hydroxide and the non-superconducting green phase, Y2BaCuO5, induces the surface degradation and the loss of the mechanical properties as a consequence of the mechanical transformation after the degradation process due to an embrittlement of the superficial layer.

Highlights

► We analyze the degradation and mechanical properties of YBCO samples. ► The nanoindentation technique acquired the hardness and elastic modulus. ► XPS, FIB and AFM helps to understand the degradation process. ► Results revealed that Y-211 phase is formed on the surface.

Introduction

One of the partial problems affecting adversely the technical exploitation of superconductors is that of surface reactivity and the resultant propensity for rapid and irreversible degradation due to the exposure to ambient atmospheric conditions, and other environments which High Temperature Superconductor (HTS) materials and devices may encounter in service, or during fabrication, handling, transport and shelf storage [1], [2]. It is well known that all families of HTS superconductors are susceptible to degradation by environmental agents, such as atmospheric moisture or with a high amount of humidity [1], [3], [4], [5], [6]. In particular, the well-known HTS superconductor, YBa2Cu3O7 − δ (YBCO or Y-123) tends to be readily degraded when exposed to a humid atmosphere, especially when the relative humidity exceeds about 30–40% [7], [8], due to different reactions with CO, CO2 and H2O. These observations have motivated the investigation of moisture effects to highly humid air for better understanding of the mechanisms of the degradation process. These studies are necessary for the development of effective protection methods for YBCO materials against the effects of exposure to ambient atmospheric conditions during their whole life cycle, including fabrication, handling, transport and application.

Several authors studied the stability of YBCO by X-ray diffraction [9], AC magnetic susceptibility measurements and thermogravimetrical analysis to calculate the lattice parameters through Cohen's method [10], [11] for degraded samples with water; yielding that the new material presents an orthorhombic phase with higher lattice parameters than the conventional structure.

In this study, the degradation behavior of YBCO in wet atmosphere will be analyzed for melt-textured samples prepared by the Bridgman technique. Phase identification was performed by X-ray diffraction analysis (XRD), while compositional detection of the degraded surface was carried out by X-ray Photoelectron Spectroscopy (XPS). In order to gain knowledge into the different activated mechanisms, atomic force microscopy (AFM) has also been used to further detail and characterize the surface topography, which is crucial to the correct understanding and prediction of the mechanism of degradation. This technique allows us to clearly examine the sample topography [12] and to provide micro and nanoscale information [13]. On the other hand, further data about the mechanical properties of the degraded layer have been obtained by nanoindentation and Focused Ion Beam (FIB).

Section snippets

Sample preparation

The YBCO powders were prepared by the PVA method [14]. The composition was 69% (w/w) Y-123, 30% (w/w) Y-211 and 1% (w/w) CeO2, which was chosen to maximize critical current density [15]. CeO2 was added to improve the size and distribution of Y-211 particles. The calcinated powder was gently deagglomerated in an agate mortar.

Green bulk rods were obtained by cold isostatic pressing (CIP) at 1200 bar followed by presintering at 945 °C for 10 h and finally melt-textured using the Bridgman technique

Degradation of YBCO observed by AFM

Fig. 1 exhibits an AFM topographic image at the initial state (Fig. 1a) and after 360 min under 85% humidity conditions (Fig. 1b). Fig. 1a shows a heterogeneous distribution of Y-211 embedded inside of the matrix, Y-123. During the degradation process, some degraded particles grow in the matrix, Y-123, see Fig. 1b. This image clearly shows the degradation process on the c-axis carried out in these ceramic samples. Moreover, we can observe that the degradation of this sample is heterogeneous, due

Conclusions

The YBCO samples after the degradation process present a high amount of Y-211 phases in the degradated region due to the reaction of decomposition of YBCO with water. The degradation layer presents an exponential growing due to the increasing of the amount of Ba and Y elements into the surface determined by XPS.

Nanoindentation and AFM have been used to characterize the mechanical properties and observe the topography respectively, of the YBCO samples at different degradation times, (from 0 up

Acknowledgments

The corresponding authors would like to thank the Scientific and Technical Centers of the University of Barcelona (CCiTUB) and the “Centre of Nanoenginyeria, Polytechnical University of Catalonia” for the AFM and FIB data, respectively. A.C. wishes to thank MICINN for the Spanish Ramon y Cajal program.

References (21)

  • S. Rekhi et al.

    Physica C

    (1994)
  • J.G. Thompson et al.

    Mater. Res. Bull

    (1987)
  • J.J. Roa et al.

    J. Eur. Ceram. Soc.

    (2011)
  • S. Piñol et al.

    J. Alloys Compd.

    (1993)
  • X. Granados et al.

    Crygenics

    (1994)
  • A. Barkatt et al.

    MRS Bull.

    (1993)
  • S. Myhra et al.
  • W. Günther et al.

    Philos. Mag. A

    (1999)
  • P. Henzi et al.

    Appl. Phys. A

    (2001)
  • M. Regier et al.

    IEEE Trans. Appl. Supercond.

    (1999)
There are more references available in the full text version of this article.

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