Laboratory for Energy Materials and Nano-Biomedicine

   DEPARTMENT OF CHEMICAL AND MATERIALS ENGINEERING

Superconducting Materials Processing and Characterization

(1) YBCO Coated Conductors

In large-scale applications of superconductors, the conductors are required not only to carry high critical current density at respectable magnetic fields, but also to possess high ductility, mechanical strength, and stability.    To improve the transport properties of high-temperature superconductors, extensive experimental investigations have been carried out in developing highly textured thin films on a variety of substrates. The fabrication of YBa₂Cu₃O_x (YBCO) films by epitaxial deposition on rolling assisted biaxially textured substrates (RABiTS) has proved to be successful in the conductor development for large-scale applications.  The RABiTS technique uses well-established, industrially scaleable, thermomechanical processes to impart a high degree of grain texture to a base metal.  Buffer layers are then deposited to yield chemically and structurally compatible surfaces.  Epitaxial YBCO films are grown on such a surface, resulting in a critical current density at 77 K on the order of 106 A/cm2. This approach has shown a promise for developing long conductors for industrial applications.

            We have developed the so-called non-fluorine sol gel synthesis of YBCO on single crystal substrates including YSZ and LAO.  The YBCO sol-gel solutions were deposited on both YSZ and LAO substrates via spin coating.  The x-ray diffraction (XRD) analysis of the film on YSZ and LAO showed a well-textured, c-axis oriented grain structure. A similar XRD pattern indicated similar structures for the YBCO film on LAO.  The oxygenated films on all substrates were characterized by four-probe measurement.  Both films on YSZ and LAO exhibited sharp T_c values near 90K.  Preliminary measurements on the transport critical current density were performed using the four-probe method using a 10^-6 V/cm voltage criterion. The sample dimensions were 15 x 3 x 0.0004 mm³. The transport J_c has been determined as a function of temperature in zero-field for YBCO on LAO.  At 77 K, the transport J_c of YBCO on LAO has reached a value above 10⁶A/cm² at 77K and zero magnetic fields.  However, this value is expected to improve as the processing parameters are optimized. For details, please see our papers from “The Latest.”

 (2) Development of Single Domain YBCO RF Cavities for Wireless Telecommunications

Microwave Cavity Filters are widely used in RF systems for high Q signal filtering. However, their characteristically bulky size at lower operating frequencies make them quite inconvenient to be used especially in the frequency range 1-4 GHz, which is the range of most of the present day commercial wireless applications. This study aims to enhance the usability of cavity filters in these lower frequency ranges by exploring a unique re-entrant structure for the cavity filter design. The study further explores the use of high temperature superconductors in this particular design for improving the performance of the filters by reducing the associated losses. After verifying the basic workability of the concept, a coupled re-entrant cavity filter design is designed with an equivalent high Q.

 (3) Suppression of ab-Plane Cracks by a c-axis uniaxial Pressure

A uniaxial c-axis pressure has been applied to YBa₂Cu₃O_x (YBCO) single domain during a prolonged oxygenation experiment up to 72 h. It has been found that, due to this c-axis pressure, the local tensile stress responsible for ab-plane crack formation can be compensated effectively.  The optical microscopy experimental results indicate that there is a sharp contrast in microstructure between the samples with and without c-axis pressure.  The ab-plane cracks in the sample with c-axis pressure appear to be suppressed.  X-ray diffraction results show that, for a 72h-oxygen anneal, both samples with and without pressure are well oxygenated with equal orthorhombicity on the polished surfaces where the microstructures are studied.  Also discussed is the mechanism of ab-plane crack suppression by the c-axis pressure.

　

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