T. L. Francavilla

Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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Publications (69)79.42 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: A 7.25 T laboratory magnet utilizing Bi 2223 conductor has been designed, built, and installed at the Naval Research Laboratory. Operating at 21 K at full field, the coil provides field homogeneity of ±1% in a 2-inch warm-bore. The system is conduction cooled with a pair of Leybold single stage cryocoolers that allow cooldown in less than 36 hours and allow extended fast ramp operation. Operation at a total refrigerator input power of 6 kW is facilitated by the use of ASC Cryosaver<sup>TM</sup> HTS current leads. The fully integrated system consists of the magnet, cryogenic system, control and protection system and power supply. This paper presents information on the magnet design, construction and subsequent testing
    IEEE Transactions on Applied Superconductivity 07/1999; · 1.20 Impact Factor
  • T.L. Francavilla, H.R. Khan, V.M. Browning
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    ABSTRACT: Mercury-based ceramic superconductors have been prepared by powder metallurgy, utilizing a technique of direct synthesis from the oxide powders. The tetragonal Hg-1223 phase was prepared and stabilized over a range of Pb and BiPb compositions. The most interesting material had an onset superconducting transition temperature of approximately 133 K. Critical current densities were obtained directly from transport measurements and indirectly from magnetization measurements, The transport critical current densities were orders of magnitude smaller than the intra-grain critical current densities. The data suggest that the transport critical current is limited by weak coupling between the grains, and that the intra-grain critical current densities deteriorate rapidly above 20 K
    IEEE Transactions on Applied Superconductivity 07/1999; · 1.20 Impact Factor
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    ABSTRACT: We have measured the current-voltage (I-V) characteristics of thin film composites made from YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-δ </sub> (YBCO) and Ag. When deposited upon a substrate, this material appears to form arrays of superconducting-normal-superconducting (SNS) junctions. To make these measurements, the films were patterned into microbridges and immersed directly into the cryogen to provide a constant temperature environment and to avoid problems of local heating. The data were fitted by several phenomenological models in an effort to understand the dissipation mechanism appropriate to these materials
    IEEE Transactions on Applied Superconductivity 07/1997; · 1.20 Impact Factor
  • Engineering Fracture Mechanics 10/1996; 55(3). · 1.41 Impact Factor
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    ABSTRACT: We have measured a number of tapes, coils, and magnets produced by commercial vendors and determined their properties as functions of magnetic field and temperature. The tapes were measured at the National High Magnetic Field Laboratory in magnetic fields to 20 tesla and at temperatures of 4.2 K, 27 K, 65 K, and 77 K. For the tapes we report critical currents and current-voltage characteristics. Six inch diameter coils were measured at NRL in zero magnetic field. Critical currents, current-voltage characteristics, and reliability studies are reported for the coils. Larger 10 inch diameter coils, which are to be used in a 200 hp superconducting motor, were also measured and results will be presented. The talk will also review the status of the most recent tests of the superconducting motor.
    03/1996;
  • 03/1996;
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    ABSTRACT: Magnets made from high-temperature superconducting materials can benefit from the judicious use of iron. Iron used in a cylindrical geometry was found to reduce the radial component of the magnetic field seen by the superconductor, and so results in greater critical currents than those attainable without the iron cladding. When properly used, iron can affect the magnetic field seen by the superconductor, allowing the use of higher currents with a consequent increase in the maximum attainable fields. The advantages noted at low currents and at a temperature of 77 K in this paper can be realized at greater currents and at other temperatures.
    Cryogenics 01/1996; · 1.17 Impact Factor
  • Desalination 01/1996; 100(1). · 3.04 Impact Factor
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    ABSTRACT: The occurrence of a transverse voltage peak has been observed in YBCO thin films. This phenomenon occurs under conditions of temperature and current in the earth's magnetic field such that the films are in the mixed state. The magnitude of the peak was found to scale with both temperature and current, and exhibited a Sin(θ) orientational dependence measured between the film-substrate and the position of the measurement electrodes on the film surface. These observations are similar to observations made in conventional superconducting thin films such as Nb, NbN, NbCN, and PbBi.
    IEEE Transactions on Applied Superconductivity 07/1995; · 1.20 Impact Factor
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    ABSTRACT: Measurements of the resistance versus temperature R(T) were made on samples of Pb0.2Hg0.8Ba2Ca1.75Cu3Ox above their superconductive transition temperatures. Current-voltage (I-V) characteristics were also studied at 77, 27, and 4.2 K in applied magnetic fields varying from 0 to 4.5 T. The R(T) data confirmed the visual observation that the samples were granular. The I-V curves were generally fit best by a modified Ambegaokar-Halperin model from which were obtained the resistance of the samples, the critical current density, and the pinning potential. The first two properties were dominated by the granular nature of the samples, whereas the pinning potential represented an intrinsic property of the material.
    Physical review. B, Condensed matter 02/1995; 51(2):1393-1396. · 3.77 Impact Factor
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    ABSTRACT: We have measured and interpreted the current-voltage characteristics of several thin YBaâCuâO{sub 6.9} films. They were prepared by metal-organic deposition and microscopy revealed that they had numerous stacking faults. The measurements were taken at temperatures of 77 and 4.2 K in applied magnetic fields varying from 0 to 6 T. Values of {ital J}{sub {ital c}} as large as 2 MA/cm² were found at 77 K for films with {ital T}{sub {ital c}}=92 K, whereas values of {ital J}{sub {ital c}} as large as 80 MA/cm² were found at 4.2 K for the same films. The data were compared with the predictions of a modified Ambegaokar-Halperin model, a conventional power-law model, and a vortex-glass model. The data were generally fit best by the Ambegaokar-Halperin model from which were obtained the resistance of the film, the critical current density, and the pinning potential as functions of the magnetic field and temperature. These parameters varied widely but systematically among the films and were interpreted in terms of differences in their imperfections. The critical current density and pinning potential obtained from this analysis were compared to the same quantities inferred from a more traditional method. The dependencies on the magnetic field were very similar, although the magnitudes were different. The advantage of defining the critical current density in terms of a physical model is emphasized.
    Physical review. B, Condensed matter 01/1995; 51(17):11838-11847. · 3.77 Impact Factor
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    ABSTRACT: Measurements of the resistance versus temperature R(T) were made on samples of Pb0.2Hg0.8Ba2Ca1.75Cu3Ox above their superconductive transition temperatures. Current-voltage (I-V) characteristics were also studied at 77, 27, and 4.2 K in applied magnetic fields varying from 0 to 4.5 T. The R(T) data confirmed the visual observation that the samples were granular. The I-V curves were generally fit best by a modified Ambegaokar-Halperin model from which were obtained the resistance of the samples, the critical current density, and the pinning potential. The first two properties were dominated by the granular nature of the samples, whereas the pinning potential represented an intrinsic property of the material.
    Phys. Rev. B. 01/1995; 51(2).
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    ABSTRACT: We have measured the current-voltage (I-V) characteristics of several high-temperature-superconducting materials with widely different morphologies (bulk Ag/Pb-Bi-Sr-Ca-Cu-O tapes, thin films of Y-Ba-Cu-O, and melt-textured, bulk Y-Ba-Cu-O samples). The I-V curves were taken at several magnetic fields ranging from 0 to 8 T. The measurements were carried out at three temperatures (4.2, 27, and 77 K) where the samples were immersed in liquid cryogens to ensure good thermal equilibrium. We compared our experimental results to the predictions of dissipation in superconductors made by the following physical models: modified Ambegaokar-Halperin, flux creep, vortex glass, collective flux creep, and a power law. The fits were extremely good for the first model and were not nearly as good for the others. Using the modified Ambegaokar-Halperin model, the critical current Ic, the normal-state resistance Rn, and gamma, which is proportional to the pinning potential U(H,T), were obtained for each material. Since the Ambegaokar-Halperin model is the only one which uniquely defines Ic, we conclude that its use puts this parameter on a solid physical basis.
    Physical review. B, Condensed matter 08/1994; 50(1):478-487. · 3.77 Impact Factor
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    ABSTRACT: Values of the transport current have been obtained as a function of angle of the magnetic field to the plane of the thin film YBa2Cu3O7–d (YBCO) prepared by metalorganic deposition. Current flowed in thea, b planes at various angles to the applied magnetic fields to 4.2 T. Films with thicknesses near 350 nm were studied. For these thin films andHc a secondary maximum is observed whenJ c> 1 MA/cm2. NearHa, b a large increase inI c is observed, and the sharpness, field dependence, and field directional dependence is discussed. The response to currents aboveI c is determined and the fit to a power lawVI n shows a strong correlation betweenI c andn.
    Journal of Superconductivity 03/1994; 7(2):303-308.
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    ABSTRACT: We report measurements of the current-voltage (I–V) characteristics of a sample of Pb0.2Hg0.8Ba2Ca1.25Cu3Ox at 77 K, 27 K and 4.2 K in applied magnetic fields varying from 0 to 6 T. These data are fitted very well by a modified Ambegaokar-Halperin model. Useful information about the material, including its critical current and pinning potential, may be obtained from the fits.
    Cryogenics 01/1994; 34:821-828. · 1.17 Impact Factor
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    ABSTRACT: In order to utilize melt-textured YBa2Cu3OX superconductors in conductor applications, both the size of the oriented domains and the alignment of the grains have to be improved. A seeded directional solidification method has been developed to accomplish this goal. Due to the presence of the seed, single-domained bulk samples with preferential grain alignment have been obtained. Since the plate-shaped grains are aligned along the sample axis, the current capacity of these seeded superconductors are found to be insensitive to thermal cycling. In addition, the magnetic field dependency of critical current density in these long seeded superconductors is found to be in excellent agreement with that of well-textured short specimens.
    Physica C Superconductivity 01/1994; · 0.72 Impact Factor
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    ABSTRACT: Critical current measurements were made using films of YBa2Cu3O7-d prepared by a metalorganic process. The film thickness was purposely reduced to about 15 nm so that measurements of the critical current could be made at temperatures of 4.2 K with reasonable width. Magnetic fields to 5.53 T were applied parallel to the film plane and as a function of angle from that plane. The current and applied field were always orthogonal. Comparison with measurements at 77 K show that at low fields, 0.5 T, the peak in critical near H//a,b is broadened at the lower temperature. Increasing fields tend to sharpen the peak more rapidly at 4 K. Measurements with H//c show a decrease by a factor of 4 at 5.5 T and did not change during a second thermal cycle between room temperature and 4.2 K. The power law exponents were determined from the I-V data ranging from 1-150muV.
    Physica C Superconductivity 01/1994; 235:3069-3070. · 0.72 Impact Factor
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    ABSTRACT: We report on a new way to determine Ic for low temperature superconductors. We have analyzed a series of I-V curves taken at 4.2 K in magnetic fields from 0 to 10 Tesla. Our results on multifilamentary NbTi wire in an Al matrix are used to illustrate this technique. The I-V curves are analyzed using a model based on the Ambegaokar-Halperin equation for a single Josephson junction. The critical current defined in this fashion is larger than that defined from the traditional E-field criterion.
    Physica C Superconductivity 01/1994; 235:3083-3084. · 0.72 Impact Factor
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    ABSTRACT: The Navy has had a long standing interest in power applications of superconductivity. Recent advances in high temperature superconducting (HTS) materials have caused the Navy science and technology community to assess the potential of HTS magnets for Naval applications. A program has begun to test HTS conductors and magnets in order to determine realistic performance expectations from which to determine proper systems integration. This paper presents some early results of industrially produced HTS magnets and discusses possible Naval applications. Speculative application toward magneto-hydrodynamic (MHD) propulsion is also discussed-specifically with respect to HTS materials.
    Journal of Electronic Materials 09/1993; 22(10):1305-1309. · 1.64 Impact Factor
  • T. L. Francavilla, R. J. Soulen, J. H. Claassen
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    ABSTRACT: A simple apparatus for liquefying neon is described for use in experiments that require a stable thermal environment at 27 K. This apparatus makes use of a closed‐cycle refrigerator attached to an insulated cold finger inserted into a conventional stainless steel Dewar to liquefy a quantity of neon gas stored previously in a tank. The system is inexpensive and easy to use. Furthermore, the neon is recovered for subsequent reuse upon completion of the experiment. Starting with a Dewar holding a small coil that had not been precooled, and using a refrigerator with rated cooling capacity of 15 W at 26 K, approximately 0.5 l of liquid neon was condensed into the Dewar in ∼4 h. The apparatus has been used to measure the critical currents of high T c oxide tape coils both with and without the application of an external magnetic field.
    Review of Scientific Instruments 08/1993; · 1.60 Impact Factor