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"There are many hypotheses in science which are wrong.
That's perfectly all right; they're the aperture to finding out what's right.
Science is a self-correcting process.
To be accepted, new ideas must survive the most rigorous standards of evidence and scrutiny."

--- Carl Sagan (1934-1996), COSMOS television series -[Saganism # 2] ---
 
 

JLG´s Research Page
 

In Progress  /  Submitted

IN PROGRESS 

TRANSPORT CURRENT EFFECTS ON MAGNETIZATION IN HIGH-Tc SUPERCONDUCTORS
J. L. Giordano FONDECyT Project No. 1040668 (See below)

Collaboration with the Bariloche Atomic Center
FONDECyT Project No. 7040043 and 7050089

NON-LINEAR RESPONSE OF THE CONDUCTIVITY IN HIGH-TEMPERATURE
SUPERCONDUCTING FILMS DUE TO FLUCTUATIONS INDUCED PHENOMENA
J. Ossandón and J. L. Giordano FONDECyT Project No. 1040666

Collaboration with the UNIV.OF LEIPZIG, FAKULTAT FUR PHYSIK AND
GEOWISSENSCHAFTE FONDECyT Project No. 7040046 and 7050058

FONDECyT PROJECTS
 
 

Transport Current Effects on Magnetization in High-Tc Superconductors

J. L. Giordano

Chilean Funds for Scientific and Technological Research Projects
Regular 2004 FONDECyT Contest; Submitted in June 2003

Status: In progress 1040668 + 7040043 + 7050089

ABSTRACT

A two year-long experimental project to study the effects of variable transport currents within a high-Tc superconductor (HTS) is proposed by the Principal Investigator (P.I.) Dr. José Luis Giordano. The P.I. is Associate Professor at the University of Talca, in charge of the Materials Research Lab (MRL), and responsible of the Electromagnetism and Materials Engineering courses, at the Engineering Faculty in Curicó.
Due to the high cost involved in low temperature experimental Physics research, the project will be carry out in collaboration with scientists of the Bariloche Atomic Center (CAB), Argentina.

It has been shown that electric transport currents reduce the shielding capacity in superconducting wires. The detailed mechanism by which this phenomenon occurs is a subject of present research and technological interest. (see, for instance, Leblanc and Çelebi, 2003) Many important experimental facts in strongly pinned type II superconductors (such as magnetization DC-hysteresis and AC-losses) can be explained by means of the phenomenological Critical State Model (CSM). (Bean, 1962 and 1964; London, 1963) In order to account for the observation of the flux-flow voltage observed in conventional type II superconductors carring a current in stationary parallel magnetic fields, the flux-line cutting concept was introduced. (Walmsley, 1972; Campbell and Evetts, 1972) The superposition of an applied axial magnetic field and the magnetic field generated by the transport current cause helical flux lines to nucleate and enter the surface of a wire. However, a continuous creation and inward migration of helical flux lines will lead to an increasing accumulation of longitudinal magnetic flux at the core of the wire, in disagreement with observations and the requirements of a steady-state condition. The idea of flux line cutting was put forward by several workers to resolve this paradox, and the debate on the energy barrier for this process continues (see, for instance, Fisher et al., 1997, and references 7-15 in Leblanc and Çelebi, 2003)
To generalize the CSM, several researches have studied two-component configurations (Boyer et al., 1980; Clem and Pérez-González, 1984; Taylor, 1967; Sugahara and Kato, 1971; Campbell and Evetts, 1972; Lachaine and LeBlanc, 1975; Gauthier and LeBlanc, 1977; LeBlanc and Çelebi, 2003; Park et al., 1993) and the flux cutting and cross-flux appear to be intrinsic to the vortex dynamics. However, several aspects remains controversial and not universally recognized (see, for instance, Obukhov and Rubinstein, 1991). Consequently, it is desirable to provide additional experimental information.
The current distribution within a hard superconductor in bulk HTS samples was studied by the P.I. (Giordano et al., 1994; Giordano, 1998), and the problem was also approached applying Optimal Control (OC) Theory to superconductivity for the first time (Badía, López and Giordano, 1998). The P.I. have also presented observations when cycling transport current variations are superimposed by a magnetic field within a superconductor (Giordano and Angurel, 2001). Their preliminary results are very useful for the current distribution and the vortex dynamics understanding in hard superconductors. (see, for instance, the citation of the P.I.´s paper, in LeBlanc and Çelebi, 2003) Then, the project will be focus in the study of the effects of a varying transport current on the shielding capacity of HTS samples, in the framework of a generalized CSM and the flux-line cutting process.

The project also intends to develop appropriate experience in experimental Applied Magnetism and Superconductivity in Chile, which has become an increasingly relevant field in today's race for technological progress, and additionally, to introduce advanced engineering students to Research and Development (R&D), contributing to the engineering thesis of undergraduate students of the host Faculty.
 

I - INTRODUCTION

A two year-long experimental project in the field of Applied Superconductivity is proposed hereby. The Principal Investigator (P.I.) Dr. José Luis Giordano, is Associate Professor at the University of Talca, with experience in Experimental Applied Magnetism, Magnetometry and Superconductivity.
The research work will be partially carried out at the Materials Research Laboratory (MRL) of the Faculty of Engineering, University of Talca, located in the city of Curicó, near Santiago, Chile. The Faculty have near 1000 students among four careers: Mechanics, Industrial, Computing and Biocomputing Engineering. Particularly, the MRL has been developed since 1999, and it is strongly devoted to Materials Science research, mainly sensors technology, detectors systems, applied magnetism and high-Tc superconductivity, all of them are the fields of present high scientific and technological relevance.
Moreover, beyond the intrinsic interest of understanding the phenomenon of shielding and conduction within hard superconductors, the project intends to develop appropriate technical and scientific experience in experimental superconductivity in Chile, which has become an increasingly relevant field in today's race for technological progress. Particularly, the project intends to introduce advanced engineering students to Research and Development (R&D), and contribute to the engineering thesis of undergraduate students of the host Faculty.

The high cost involved in low temperature experimental materials research requires to work on a collaborative basis with already available experimental facilities abroad. Particularly, the experimental work will be carried out under collaboration agreement with scientists of the Superconductivity Group of the Low Temperatures Division, at the Bariloche Atomic Center (CAB), Atomic Energy National Commission, San Carlos de Bariloche, Argentina. Thus, in order to include the participation of two co-investigators of the CAB, we plan to apply for an International Cooperation Fondecyt 2004 Program.
The coordination of the project, the analysis of data and interpretation of results will be performed mainly at the MRL, University of Talca, in Curicó.

In this project, a systematic study of the reduction of the shielding capacity of high critical temperature superconductors (HTS) measuring the DC-magnetization under varying transport currents is proposed.
 

II - THEORETICAL FRAMEWORK

It has been shown that electric transport currents reduce the shielding capacity in superconducting wires. The detailed mechanism by which this phenomenon occurs is a subject of present research and technological interest. (see, for instance, Leblanc and Çelebi, 2003)
A superconductor develops nondissipative “shielding” currents which try to expel any applied magnetic field from the interior of the material. This phenomenon, so-called Meissner-Ochsenfeld effect, can be observed in the magnetization curve M(H) by measuring the macroscopic induced magnetic moment of the sample per unit volume (i.e., the magnetization) with increasing as well as with decreasing applied magnetic field H. Even very small magnetization changes can be detected by using a sensitive Superconducting Quantum Interference Device (briefly, SQUID magnetometer).
Traditional superconductors can be divided in two types. The type I superconducting materials (or “soft superconductors”) can carry only small direct currents without losses, because slight magnetic DC field strengths will destroy their superconducting state; hence, these materials are not useful in energy transport or high power applications (see for instance, de Gennes, 1989). These have very small critical currents above which a noticeable dissipation occurs. In type II superconductors however, a more complex superconducting state (the “vortex state”) is observed, even under high fields and currents. These materials allow the penetration of discrete and quantized flux lines associated with microscopic current loops, so-called “vortices” (due to fluid dynamics analogy).
In the vortex state, transport currents through the material will drive the vortices across the specimen and energy will be dissipated by the flux flow. Therefore, to avoid the generation of heat, some pinning centers for the vortices (such as impurities) must be introduced. In this kind of type II materials (called “hard superconductors”), high critical current densities are reached due to strong pinning mechanisms (see for instance, Campbell and Evetts, 1972). One important and special class of technologically useful materials which exhibit a type II behaviour are the HTS cuprates discovered in 1986. In these granular superconductors, the magnetic flux is trapped by the multiple defects of the ceramic structure, and high intergranular supercurrents can flow throught the material.
Many important experimental facts in strongly pinned type II superconductors (such as magnetization DC-hysteresis and AC-losses) can be explained by means of the phenomenological Critical State Model (CSM), without specifying the detailed mechanism that controls the vortex pinning. The CSM approach postulates i) the identification of the current density strength J with the critical (maximum) value Jc throughout the region of the sample reached by the magnetic field, and ii) strong vortex pinning such that the shielding currents persist even when the external field is changed (Bean, 1962 and 1964; London, 1963).

Since the original CSM formulation in 1962 by Charles P. Bean and H. London, this phenomenological model has been successfully applied for almost four decades. However, the CSM is readily applicable when current has only one component. That is, when there is a unique, known in advance, direction for the transport current. The situation is mostly restricted to idealized geometries; moreover, it is not clear how to derive the constitutive equations necessary to solve multicomponent problems. (Prigozhin, 1996) Even in apparently simple configurations, more constitutive equations are necessary.
The flux-line cutting concept within hard superconductor was introduced in order to account for the observation of the flux-flow voltage observed in conventional type II superconductors carring a current in stationary parallel magnetic fields. (Walmsley, 1972; Campbell and Evetts, 1972)
The vector superposition of an externally applied axial magnetic field and the azimuthal magnetic field generated by the conduction current cause helical flux lines to nucleate and enter the surface of the wire. A radially inward migration of these helical flux lines is required to account for the generation of a longitudinal electric field, hence a flux flow voltage when the static current exceeds the critical limit. A continuous creation and inward migration of helical flux lines will, however, lead to an increasing accumulation of longitudinal magnetic flux at the core of the wire, in disagreement with observations and the requirements of a steady-state condition. The idea of flux line cutting was put forward by several workers to resolve this paradox, and the debate on the energy barrier for this process continues (see, for instance, Fisher et al., 1997, and references 7-15 in Leblanc and Çelebi, 2003)
To generalize the CSM, several researches have studied with high sensitivity experiments, two-component configurations by using rotating samples or rotating magnetic fields. (Boyer et al., 1980; Clem and Pérez-González, 1984) Other workers have investigated the AC-losses in superconducting ribbons and wires under stationary parallel magnetic fields. (Taylor, 1967; Sugahara and Kato, 1971; Campbell and Evetts, 1972; Lachaine and LeBlanc, 1975; Gauthier and LeBlanc, 1977) The similarities in the response among different geometries and samples, were explained as a consequence of similar flux cutting processes. (Boyer et al., 1980) Moreover, the flux-line cutting and cross-flux effects on the vortex dynamics were reported and discussed for different types of HTS samples (LeBlanc and Çelebi, 2003; Park et al., 1993), and the flux cutting and cross-flux appear to be intrinsic to the vortex dynamics. However, although an appreciable body of experimental evidence has been accumulated over the last three decades showing that flux-line cutting is a real phenomenon which occurs whenever type II superconductors are subjected to magnetic fields (which are changing in direction and perhaps also in magnitude), the occurrence of this process remains controversial and not universally recognized (see, for instance, Obukhov and Rubinstein, 1991). Consequently, it is desirable to provide additional observations which support this idea. Further, various important ramifications of this concept remain to be explored.
In contrast, some workers have studied the problem of the current distribution in a transport current carrying hard superconductor under parallel fields. (see Refs. 14-22 in Giordano and Angurel, 2001) In particular, Badía, López and the PI of this proposal, have applied the Optimal Control (OC) Theory to superconductivity for the first time (Badía, López and Giordano, 1998), showing that this variational calculus generalization provides the mathematical framework for multicomponent critical state problems.
Variational calculus is a basic mathematical tool not only successfully applied in fundamental physics but also in engineering applications. It solves the problem of finding the paths which are stationary for an integral functional depending on position and velocity under arbitrary variations of the variables and given restrictive conditions. In 1962, a generalization of variational calculus, the OC Theory, was developed by L. S. Pontryagin (Pontryagin et al., 1962). In this formulation, some control variables drive the evolution of the state variables through a differential equation, while some “cost” or objective functional must be minimized. Moreover, the Pontryagin maximum principle allows to consider spaces with boundary constraints, so that closed as well as open sets can be managed. This powerful generalization of variational calculus has been successfully applied in a broad range of fields, such as geometry, engineering, agriculture and economics.
Since the magnetostatic free energy of a current carrying superconductor is a functional of the current density which is restricted to have a magnitude not greater than the critical value, the general problem of hard superconductors can be treated within the framework of OC theory, where the current density components are the control variables. (Badía, López and Giordano, 1998; Badía and López, 2001, 2002, 2003)
 

III - WORKING HYPOTHESES

In stationary applied fields and currents, the phenomenological single region current distribution model will be used, according to which transport and shielding currents inside a hard superconductor share the same region. (Giordano et al., 1994; and Giordano, 1998) An important characteristic of this current distribution model, is that the currents are linearly superposed and it predicts less electromagnetic free energy in the sample than alternative models found in the literature (see for example, Baltaga et al., 1990 and Voloshin et al., 1991). Moreover, the numerical simulation predictions on magnetization and susceptibility agree with the observations on granular YBCO superconducting cylinders (Giordano, 1998).
Also, an extension of the minimum energy model based on the following working hypothesis will be used: if the external conditions change (such as an increasing, decreasing or inverting field or current), the variations in the field profile correspond to minimum energy changes. Particularly, this least action hypothesis does not assume any a priori critical state constitutive equation. Consequently, it is necessary to minimize an energy-functional to provide the necessary constitutive equations that describe the hysteretic magnetization cycle.
It should be remarked that both the phenomenological (single region) and the theoretical (minimum energy change) models complement each other. The first one is easy to implement and simulate, while the second one gives the theoretical bases on which the former rests.
 

IV - AIMS

It is proposed to investigate the reduction of the shielding capacity of superconducting samples under transport current variations, test minimum energy models, provide useful information to test the flux cutting models, and additionally, to develop a Magnetism and Superconductivity Lab in Chile, with the participation of graduate engineering students interested in R&D activities, working in collaboration with research centers abroad. Specifically, the aims of the project are the following:

(i) to study the magnetic response of current carrying technological high-Tc superconductors under magnetic fields when current is cycled, providing useful information to test the vortex dynamics models, and develop a general approach to establish the penetration current and field profiles. This knowledge is relevant to understand how to minimize losses in energy transport and to predict the behavior of HTS in the design of superconducting devices, such as nanotesla sensors, current limiters and current leads.

(ii) to develop in Chile (in particular at the Faculty of Engineering in Curicó) a growing experience in experimental magnetism and superconductivity. With this aim, the Chemical Engineer Igor Ruíz-Tagle, who has experience in the preparation of HTS samples in the University of Talca, will be contracted for two months to collaborate in the samples preparation at the CAB. The project also intends to engage undergraduate engineering students interested in doing their thesis in R&D.

(iii) to strengthen the scientific collaboration between the University of Talca and international research centers in Applied Physics, particularly, Magnetism and Superconductivity.

I commit myself to submit each year at least one abstract to an international conference or symposium; and at least one full length manuscript to an international journal before or at the end of the second year of the project.
 

V - METHODOLOGY

The theoretical models will be tested on 30-60 mm3 volume Polycrystalline yttrium-barium copper oxide (YBa2Cu3O7-x or YBCO) samples of different average grain size. The modeling of the behaviour of hard superconductors in superconducting devices of nearly 30 mm3 volume requires both careful design of decisive experiments and accurate determination of very weak changes in magnetic moment at low temperatures (The sensitivity must be better than one nA m2 or, equivalently, one  erg/G).
The design and development of a dedicated SQUID-probe for parallel magnetometric measurements under transport currents, the preparation and characterization of the samples, and the preparation of sintered electrical contacts will be made in collaboration with the Physicists Dr. Gladys Nieva and Dr. Javier Luzuriaga from CAB (in Bariloche, Argentina), and the Chemical Engineer Igor Ruíz-Tagle, from University of Talca (in Curicó). The microstructure of these samples will be analyzed by scanning electron microscopy (SEM) at CAB.
The experiments with low frequency (below 100 Hz) AC-transport currents from ?5 A to +5 A under applied stationary magnetic fields (below 1T near the 50-90 K temperature range), will be carried-out by using SQUID magnetometers and sensitive susceptometers at the Low Temperatures Division in the CAB. Numerical simulations based on phenomenological models will be used to fit experimental data in order to establish the value of relevant parameters.
The critical current value for different temperatures will be obtained from resistive experiments under DC-applied field. The effect of transport currents on pinning and the transition from the intergranular to the intragranular regimes will be studied in AC-susceptibility and DC-magnetization experiments, as a function of temperature, AC-magnetic field amplitude, DC-magnetic field and transport current intensity. The ceramic permeability will be estimated from susceptibility vs. magnetic field experiments, and the critical temperature will be determined from the susceptibility vs. temperature plots.
The analysis of data will be conducted in the host institution in Chile by the P.I. and his colleagues from CAB, using the mainframe computer facilities available. The results will be compared with tentative, phenomelogical models developed for this configuration, and compared with the results of other workers.
It is also expected, that one or two graduating engineering student and two undergraduate engineering students from University of Talca (in Curicó), make inductive measurements in our MRL (on HTS samples under liquid nitrogen with lock-in amplifier and FFT spectrum analyzer), perform calculations and participate in the probe design, discussion and analysis processes.
 

VI - WORK PLAN

(1) The main experimental work of this proposal will be done during the first year of the project (2004).

The first two-week visit of the P.I. to the CAB is planned for the beginning of the project (May, 2004) in order to carry out the development of the dedicated SQUID-probe for simultaneous measurements, and also the preparation and characterization of two sets of cylindrical samples. Magnetization M as a function of temperature T and static, external field Hdc, will be recorded for the samples. AC-susceptometry and microstructural characterization will be performed by using SQUID magnetometers, susceptometers, and the SEM of the CAB.

A second two-week visit of the P.I. and the research assistant to the CAB, is scheduled for September 2004 in order to conduct the main, long run transport current experiments and magnetometric measurements under various experimental conditions.
 

(2) The second year of the project (2005) will be devoted eventually to perform additional measurements and mainly to test the hypothesis, data analysis, comparison with other works, discussion of results, and modeling of the phenomena involved. The results will be interpreted in the framework of generalized critical state models.

If necessary, a third two-week visit of the P.I. to the CAB would be scheduled for May 2005 in order to perform complementary measurements.

A fourth two-week visit of the P.I. to the CAB is scheduled for September 2005, in order to discuss the conclusions with the collaborators, prepare a manuscript for publication, and eventually, to perform final measurements.
 

In order to present the results of this research project, it is also expected to attend to a scientific meeting each year, and submit at least one manuscript for publication in an international journal.
 

VII - ADVANCED WORK BY THE PROJECT´S AUTHOR

The current distribution within a hard superconductor in bulk HTS samples was studied by the P.I. of this project (Giordano et al., 1994; Giordano, 1998). Three kinds of experiments were performed on polycrystalline YBCO: i) Resistive measurements with continuous and pulsed DC current under steady applied magnetic fields, ii) SQUID-magnetometry and iii) AC-susceptibility. The last two techniques were used with superimposed transport currents. A thermal treatment for metal-ceramic surface diffusion was developed in order to obtain low resistivity sintered contacts. Also, a probe to perform the simultaneous magnetic and resistive measurements was designed and constructed.

It was shown that the effect of the transport currents on the magnetic response can be neglected when the current value is less than ~ 25% of the critical value. This effect qualitatively agrees with the predictions of a single region current distribution model. It should be also pointed out that a scaling of the experimental susceptibility with the penetration field and the ceramic permeability of the material was established. A similar behaviour for a given sample was observed at different temperatures and transport currents.

As an application of transport current effects to the magnetic properties, a liquid nitrogen cooled YBCO sensor-based magnetometer was developed (Giordano, 1998). This instrument was not only used as a nanotesla sensitive magnetometer, but also as a second-harmonic susceptometer. It allowed to study different aspects of granular superconducting materials, such as thermal-cycle aging, ceramic sensitivity to magnetic fields and superimposed currents, density of microcracks and global critical current density.

On the other hand, in order to develop a more general approach, the P.I. of this project, in collaboration with a group of scientists at the University of Zaragoza, proposed a minimum energy model based on an OC method (Badía, López and Giordano, 1998). Using a variational approach, the authors were able to derive the condition J = Jc, which actually is the Critical State Bean-London hypothesis postulated by Bean in the 60´s, and solve two multicomponent situations (see also the paper´s “Report of the Referee” ES6373B, and the citation in High-Tc Update 12 (17) Sept. 1, 1998, Nota Bene). Apparently, this work was the first application of OC theory in Superconductivity research. It also showed that OC approach is a useful tool to derive the necessary constitutive equations for modeling the current distribution in technological superconductors. Particularly, the predictions of the minimum energy model for cylindrical specimens under parallel fields agree well with the phenomenological model used to explain previous measurements and numerical simulations. Nevertheless, an extension of the OC model accounting for irreversible (hysteresis) phenomena remains to be completed; also, the application of the minimum energy model to hard low temperature superconductors (LTS) remains to be tested.

The P.I. have also presented observations when cycling transport current variations are superimposed by a magnetic field within a superconductor (Giordano and Angurel, 2001). He has observed that the 'butterfly' loops in the magnetization occur when the current is cycled after the full penetration regime was reached. These preliminary results are very useful for the current distribution and the vortex dynamics understanding in hard superconductors. (see, for instance, the citation of the P.I.´s paper, Giordano and Angurel, 2001, in LeBlanc and Çelebi, 2003)
 

VIII - AVAILABLE RESOURCES AND FACILITIES

The University of Talca, host institution of the P.I., will provide office space, computing and communication facilities, scientific and technical library services and secretarial paperwork. High speed computing and printing capabilities are already available at the host institution. Library references can be obtained through electronic mail communications. The entire University (including the Engineering Campus in Curicó) is linked by a digital network to world wide web, electronic mail, video-conferences, fax and telephone communications. The researchers have access to the Central Library and to the Chemistry and other laboratories in the main Campus of the University in Talca.

The Faculty of Engineering, located at Curicó, is still acquiring additional equipment and materials for its laboratories and library, thanks to a MECESUP project (No. TAL 9901) for USD 1,400,000 during a 3 year-period. It implements the Materials Research Lab (MRL), General Physics Lab, Control and Automatization Lab, Thermal Engines Lab, Chemistry Lab and Computing laboratories in the new Curicó Campus. Particularly, the MRL has

a Lectures Room (40 students capacity) with:
    PC Computers and Workstation
    Digital Regulated Power Supplies
    Digital Bench Frequency Counter and Function Generators
    Analog Oscilloscopes
    Data-Show, Slides and Transparencies Proyector facilities
    Dedicated Technical and Scientific Library (mostly Electronics, Materials and Physics)

a Sample Preparation Room with:
    Digital Programmable Power Supply
    Iron Core Electromagnet for Magnetic Characterization
    Controlled Temperature Soldering Station
    High Temperature Soldering Facility
    Optical Stereoscopic Microscope
    Digital Programmable J/K type 2-Thermocouple Thermometers
    Vacuum System
    20 L Dewar for Liquid Nitrogen
    Automatic Controlled Furnace with different Atmospheres
    Stirrer with heater
    Two sinks

and an Electronic Instrumentation Room, in which the following new equipment for calibration, testing and experimental works is available:
    Digital Precision Bench Function Generator
    Digital Milliohm Bench Meter
    Digital Precision Bench LC Meter
    Digital Precision Bench Multimeter
    Digital Oscilloscope
    Digital Teslameter with axial and transversal Hall probes
    Analog X-Y and Y-t Recorder
    Dynamic Curve Tracer for electronic components characterization
    Digital Lock-In Amplifier
    FFT Spectrum Analyzer
    Sink
 

IX - RESEARCH ASSISTANTS

As a research assistant for the present proposal, it will be requested to contract for 2 months on a part-time basis (20 h/week), a Chemical Engineer with experience in HTS sample preparation. The research assistant duties include participation in the sample preparation at CAB (S. C. de Bariloche, Argentina), with low resistivity sintered electrical contacts.
 

X - UNDERGRADUATE THESIS STUDENTS

We intend to engage on a part-time basis (20 h/week) some undergraduate engineering students interested in doing their thesis in the following projects, respectively for the first and the second years:

1 - Design and construction of an electronic system for an electric furnace for automatically controlled heat treatments of HTS materials under oxigen atmosphere

2 - Design and construction of an automatic capacitive-discharge magnetizer to study HTS response under high applied currents
It has been estimated a period of 10 month/year for each project.

Additionaly, during each of the two years of the project, two undergraduate advanced engineering students will be engaged as 8 h/week assistanship during the academic period (10 month/year), and 44 h/week during non-academic period (1.5 month/year).
 

XI - CAPITAL GOODS

No capital goods is requested for the present proposal.
 

XII - COLLABORATORS ABROAD

Luzuriaga, Javier (luzuriaga@cab.cnea.gov.ar; Ph. +54-2944-445171)

Nieva, Gladys (gnieva@cab.cnea.gov.ar; Ph. +54-2944-445113 y 445160)

Low Temperatures División; Bariloche Atomic Center (CAB)
Av. Bustillo km 9.5; RA-8400 San Carlos de Bariloche; Argentina / Fax +54-2944-445299
 

REFERENCES

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Phys. Rev. B 58 (14) 9440-9449

Badía A and López C 2001 Critical State Theory for Nonparallel Flux Line Lattices in Type-II Superconductors
Phys. Rev. Lett. 87 (12) 127004-7

Badía A and López C 2002 Vector magnetic hysteresis of hard superconductors
Phys. Rev. B 65 104514-27

Badía A and López C 2002 Magnetic flux bifurcation and frequency doubling in rotated superconductors
J. Appl. Phys. 92 (10) 6110-8

Badía A and López C 2003 The Critical State in Type-II Superconductors with Cross-Flow Effects
J. Low Temp. Phys. 130 (3-4) 129-153

Baltaga IV, Makarov N M, Yampol´skii V A, Fischer L M, Il´in N V and Voloshin I F 1990
Collapse of superconducting current in high-Tc ceramics in alternating magnetic field
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Boyer R, Fillion G and LeBlanc M A R 1980
Hysteresis losses and magnetic phenomena in rotating disks of type-II superconductors
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Brandt E H 2001
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Brojeny A A B, Mawatari Y, Benkraouda M, Clem J R 2002
Magnetic fields and currents for two current-carrying parallel coplanar superconducting strips in a perpendicular magnetic field
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Giordano J L 1998 On the shielding and transport distribution currents in granular superconductors and the application in the design of a superconductor-based sensor magnetometer; PhD Thesis, University of Zaragoza

Giordano J L, Angurel L A, Lera F and Navarro R 1994
Interaction between parallel magnetic fields and transport currents in YBCO superconductors
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Giordano J L and Angurel L A 2001
Flux pinning in high-Tc superconductors under transport current cycles Superconductor
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LeBlanc MAR, Çelebi S 2003 Flux line cutting and cross-flow in a high-Tc superconducting YBCO tube
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London H 1963 Alternating current losses in superconductors of the second kind Phys. Lett. 6 (2) 162-165

López C and Badía A 2000 Private communication (June 16, 2000 fax)

Obukhov S P and Rubinstein M 1991 Phys. Rev. Lett. 66 2279

Park S J, Kouvel J S, Radousky H B and Liu J Z 1993
Cross-flux effect as a vortex pinning process in YBa2Cu3O7 and Y0.8Pr0.2Ba2Cu3O7 crystals
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Pontryagin L S, Boltyanskii V, Gramkrelidze R, Mischenko E 1962 The Mathematical Theory of Optimal Processes (Wiley Interscience, New York)

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SUBMITTED
 

Magnetization Measurements of High-Tc Superconductors under Dissipative Transport

J. L. Giordano, J. Luzuriaga, I. Ruíz-Tagle, A. Badía-Majós, G. Nieva

7th. European Conference on Applied Superconductivity (EUCAS´05)
Vienna, Sept. 11-15, 2005

Chilean Funds for Scientific and Technological Research Projects
Regular 2004 FONDECyT Contest

Status: In progress 1040668 + 7040043 + 7050089

Non-Linear Response of ac Conductivity in Narrow YBCO films strips at the Superconducting Transition

J. G. Ossandon, J.L. Giordano, U. Schaufuss, S. Sergeenkov, Pablo Esquinazi, K. Kempa

7th. European Conference on Applied Superconductivity (EUCAS´05)
Vienna, Sept. 11-15, 2005

Chilean Funds for Scientific and Technological Research Projects
Regular 2004 FONDECyT Contest

Status: In progress 1040666 + 7040046 + 7050058