Research on MgB2 bulk superconductors exposed to Ag nanoparticles diffusion

Studies of microstructural, mechanical and superconducting properties of Ag diffused MgB₂ superconductors by way of scanning electron microscopy, X-ray diffraction and Vickers microhardness measurements. A systemic examination of the magnetoresistance measurements for our prepared samples is exerted under applied magnetic fields started from 0.0 up to 7 T, and their results are described according to thermally activated flux creep. Further, this study reveals explanations of the diffusion coefficient (diffusivity) and activation energy of Ag impurities in the MgB₂ materials using the resistivity measurements for the first time. Additionally, all the samples display the indentation size effect nature under the applied indentation test load. Regarding the theoretical modeling of the microhardness evidences, the calculations performed by HK approach are quite closer to the values of the load dependent microhardness.     

Hysteresis losses of multifilament superconductors in superimposed dc and ac magnetic fields

Theoretical investigations, magnetization and calorimetric ac loss measurements have been performed on the problem, how the superposition of dc- and ac-magnetic fields influences the hysteresis losses of filamentary superconductors. For a given ac-field amplitude the maximum loss value as a function of the dc field can easily be predicted without knowledge of the superconducting properties.     

Internal fields in magnetic materials and superconductors

This Paper reviews some of the concepts needed for the correct analysis of magnetization data, both for magnetic materials and superconductors. Demagnetization factors, initial susceptibilities and hysteresis losses are discussed.     

Properties of MgB2 superconductors with regard to space applications

Thin MgB₂ conductors with diameters down to 50 μm have been developed for application as current leads on a satellite and as LH₂ level sensor. The high transition temperature of 39 K makes applications of MgB₂ at temperatures around 20 K possible, significantly above the temperature margin of low temperature superconductors like NbTi or Nb₃Sn. The absence of weak link behaviour and the fact that there is no need to texture MgB₂ to obtain high critical current densities is a crucial advantage compared to high temperature superconductors. We give an overview of the current status of MgB₂ conductor development and show the potential of these wires for space applications.     

AC loss characteristics of YBCO conductors carrying transport currents in external AC magnetic fields

In this work we present dependence of AC losses in a YBCO tape conductor carrying an AC transport current in an external AC magnetic field on the angle between the direction of the external field and the tape face of the conductor. The losses were measured by a calorimetric method whose validity was proven by simultaneous electric and calorimetric measurements of the losses in the same part of the conductor. The experiment was conducted at 77 K. The measured data show that the AC losses are dominated by the normal component of the external magnetic field to the tape surface. It is also shown that the AC losses sharply drop when the external magnetic field is parallel to the tape surface, which is due to extremely high aspect ratio of the YBCO conductor.     

Meissner shielding currents and magnetic flux penetration in thin-film superconductors

We have observed the magnetic flux penetration at 4.2 K into superconducting lead films of about 4 µm thickness using a high-resolution magnetooptical technique. The specimens had the shape of small squares or long strips. In increasing magnetic field it was found that magnetic flux starts to penetrate into the bulk of the specimens in the form of flux tubes containing about 60 flux quanta. For the samples with the strip geometry the magnetic fieldH*, at which flux penetration into the bulk sets in, was investigated in the presence of an external electrical currentI along the strip superimposed on the Meissner shielding currents. The fieldH* was found to decrease linearly with increasing transport currentI. The magnetic field near a long superconducting cylinder in the presence of both a transverse applied magnetic field and an applied electrical current parallel to the cylinder axis has been calculated. The calculation is valid for a cylinder of elliptical cross section, which approximates a long, flat strip. The experimental observations for the slope H*/I are in good agreement with the calculations.Work performed in part in the Argonne National Laboratory and in part in the Ames Laboratory of the U.S. Atomic Energy Commission.     

Multiferroic composite for combined detection of static and alternating magnetic fields

Magneto-electric (ME) effects in bi-layer multiferroics consisting of single crystal hexagonal SrAl_xFe_12 − xO₁₉ ferrites (hexaferrites, where x = 0, 0.8, 1.2) and PbZrTiO₃ (PZT) have been studied. The main objective of the study was to determine the effect of the Al doping concentration on the ME coupling. We experimentally demonstrate that the Al doping improves dramatically the magnetically induced ME voltage. Moreover, our data indicate that the ME response is a linear function of both ac and dc applied magnetic fields. This is in contrast to previously reported multiferroic laminates that show a non-linear magneto-electric induced voltage as a function of the applied dc magnetic field. The multiferroic structures reported in this work have therefore enhanced functionality because they can linearly operate as a dual ac/dc magnetic field sensors.     

The angular dependence of AC losses in BSCCO/Ag tapes under AC magnetic fields and AC transport currents

The measurement results of AC transport current losses in BSCCO/Ag tapes caused by AC transport currents and the simultaneously presented AC magnetic fields were reported. The measurements were carried out at 77 K and power frequency (50 Hz) for AC applied magnetic fields of 48, 80 and 120 mT, respectively. The angle between the direction of the magnetic field and the wide side of the tape varied from 0° to 90° with a step of 10°. Analyses on the angular dependency of the AC losses for BSCCO/Ag tapes were given. A theoretical model to describe the angular dependency of AC losses was proposed and compared with the experimental results.     

ac losses of multifilament superconductors reduced below the level of filament hysteresis losses due to field screening by filament coupling currents

In measurements on multifilament superconductors, it was found that overall ac losses fall below the level of filament hysteresis losses at sufficiently high frequencies, due to field screening by filament coupling currents. At low frequencies, overall loss should also fall below the hysteresis loss level, provided f?>_m a critical frequency defined in the next text). This may be arranged by the choice of large values of the twist pitch, or small values of the mean transverse resistivity p_e.     

Numerical calculation of superheating magnetic fields and currents for superconducting slabs

Numerical calculations of superheating magnetic fields and superheating currents for superconducting slabs for a wide range of the sample thickness are presented. The calculations were made for low values of Ginzburg-Landau parameter , i.e., for type-1 superconductors. We propose also experimental procedures to measure superheating fields and currents in films and bulk samples.On leave from Kapitza Institute for Physical Problems, 117334 Moscow, Russia     

FRP Dewar for measurements in high pulsed magnetic fields

We describe a liquid helium glass-fibre reinforced plastic (G-FRP) Dewar which we designed, fabricated and tested for excitation spectrum measurements in high pulsed magnetic fields of up to 50 T. The sensitivity of high-resolution magnetic measurements carried out at low temperatures in such high fields is limited inevitably by magnetic and electric properties of the structural Dewar materials involved. Magnetic properties of various G-FRP Dewar-purpose materials are explored with a χ-meter furnished with a RF SQUID magnetometer. The Dewar materials and multilayer insulation effects contributing to the magnetic response signal are analyzed. It has also been discovered that field noise caused by the magnetization of the Dewar materials can be suppressed substantially by using special glass-epoxy technologies. The liquid helium evaporation rate is 3.8 l/day while the hold time is 27 h, the influencing factors are discussed.     

Specific heat of type-II superconductors in magnetic fields nearT c

The GLAG theory predicts that the value of the specific heatC _M in the mixed state just below the transition to the normal state, is a function of the Ginzburg—Landau (GL) parameter , even for vanishingly small magnetic fields(TT _c ).C _M is thus different from the specific heatC _S in the pure superconducting state(H=0) for all except 1.9. Experimental results of the authors generally confirm this paradoxical prediction which is not in contradiction with the thermodynamics of phase transitions. To experimentally clarify this behavior close toT _c , Nb₈₀Mo₂₀(4) and Pb₉₈In₂(0.8) specimens are studied in fields much smaller than those usually applied (down to17 Oe). A new method for measuring specific heat in decreasing as well as increasing temperature is introduced. Very good agreement with the theory is found for fields higher than 120 Oe while a deviation is observed close toT _c for lower fields where values of the Abrikosov lattice parameter would become larger than 1 µ. The sense and the magnitude of the deviation are such thatC _M andC _S would be equal for fields of 20 Oe and below. Possible explanations are discussed. It is suggested that a fundamental modification of the vortex state structure occurs in small fields close toT _c .     

Transformation of alternating magnetic and electric fields in a ferroelectric-conductor ring structure

We have studied a new phenomenon according to which an alternating magnetic field is transformed into electric voltage and vice versa in a structure comprising a ferroelectric ring with metal electrodes deposited onto its surface. The ferroelectric is polarized in the radial direction and placed into a constant magnetic field, which is perpendicular to the ring plane. The transformation results from a combination of the electromagnetic induction in the conducting circuit and the piezoelectric effect in the ferroelectric ring. This phenomenon can be used for creating magnetic sensors with linear characteristics.     

Instrumentation and methods for low temperature measurements in high magnetic fields

There are several difficulties associated with the accurate measurement of low temperature in the presence of an intense magnetic field B. Most of the problems stem from the direct effect of the field on the thermometric properties of almost all of the comcommonly used sensors. Because the magnitude of the field effect, eg magnetoresistance, varies widely as a function of B, T, and the thermometer itself, a careful selection process is necessary to minimize the error. As an aid to such a process, a detailed comparison is presented of the field-dependent errors, Δ/T, as a function of T, of carbon, carbon-glass, germanium, and platinum resistance thermometers, thermistors, Si and GaAs diodes, thermocouples, capacitance thermometers, and several other less popular devices. Specific recommendations are made on the basis of the comparison. The related problem of magnetic field measurement is also examined, with emphasis on the recent characterizations of commercially available InAs, InSb, and GaAs Hall effect probes. From the results of measurements over the 1.5–300 K range and to fields as high as 23 T, several encouraging conclusions may be drawn concerning the performance of the sensors as magnetometers in the 1% accuracy range.