Irreversibility Field Research Articles

Enhanced higher temperature irreversibility field and critical current density in MgB2 wires with Dy2O3 additions

Bulk samples of magnesium diboride (MgB2) doped with 0.5 wt% of the rare earth oxides (REOs) Nd2O3 and Dy2O3 (named B-ND and B-DY) prepared by standard powder processing, and wires of MgB2 doped with 0.5 wt% Dy2O3 (named W-DY) prepared by a commercial powder-in-tube processing were studied. Investigations included x-ray diffractometry, scanning- and transmission electron microscopy, magnetic measurement of superconducting transition temperature (T c), magnetic and resistive measurements of upper critical field (B c2) and irreversibility field (B irr), as well as magnetic and transport measurements of critical current densities versus applied field (J cm(B) and J c(B), respectively). It was found that although the products of REO doping did not substitute into the MgB2 lattice, REO-based inclusions resided within grains and at grain boundaries. Curves of bulk pinning force density (F p) versus reduced field (b = B/B irr) showed that flux pinning was by predominantly by grain boundaries, not point defects. At all temperatures the F p(b) of W-DY experienced enhancement by inclusion-induced grain boundary refinement but at higher temperatures F p(b) was still further increased by a Dy2O3 additive-induced increase in B irr of about 1 T at all temperatures up to 20 K (and beyond). It is noted that Dy2O3 increases B irr and that it does so, not just at 4 K, but in the higher temperature regime. This important property, shared by a number of REOs and other oxides promises to extend the applications range of MgB2 conductors.

Pupillary Complexity for the Screening of Glaucoma

Glaucoma is a leading cause of irreversible visual field loss. The early detection and diagnosis of the disease are therefore necessary to prevent blindness. Pupillary light responses are an interesting new technique for the detection of glaucoma. However, the analysis of pupillary signals has been associated with manual supervision or involved high computational costs. The present paper is to propose an analysis framework to automatically investigate changes in the complexity of pupillary signals under ambient light conditions for the screening of glaucoma. In this work, pupillary data of 13 glaucoma patients, 13 age-matched controls, and 11 young controls were recorded at the light intensity of 100 cd/m2 using a commercial eye tracker. The pupillary complexity of the participants was analysed using Higuchi’s fractal dimension, permutation entropy, and conditional entropy. We found that there was a statistically significant difference in the pupillary complexity between glaucoma patients and control groups (P < 0.0001). Specifically, the difference was more pronounced when using the fractal dimension measure. These results confirm the potential of using pupillary complexity for the screening of glaucoma using commercial devices.

High temperature superconducting material based energy storage for solar-wind hybrid generating systems for fluctuating power management

The green energy infrastructure plays an important part in existing world of power scarcity. Owing to their dependence on environmental factors, renewable energy technologies continue to fluctuate. Using the energy storage device will solve the problem. The technical and economical requirements of the energy storage process can be met with the use of hybrid combinations. This paper proposes a hybrid energy management system based on the Super conductive magnetic material and the lead acid battery to provide power stabilization in the grid-connected unstable microgrid. Here, second-generation High Temperature Superconducting (HTS) material is used as Super Conducting Magnet Energy Storage (HTSMES) which exhibits a high irreversibility field and critical current density within an active magnetic field. The proposed system may be a good solution to minimize the impact of the Point of Common Coupling (PCC) power variability during fault condition. To improve the efficiency and flexibility of the proposed system, the two hybrid power sources are connected via dual input single output zeta converter. The HTSMES is used to minimize the difference in active power during fault and to give the reactive current to handle the fault. Grid connected microgrid with HTSMES-battery was simulated using the MATLAB Simulink platform and tested using an energy management algorithm with and without the presence of fault. The system proposed is demonstrated with detailed simulation results.

Irreversibility field and anisotropic δl-pinning in type II superconductors

The article presents a brief generalization of our recent experiments confirming that “irreversibility field” in Nb-Ti superconducting tapes is determined by the inhomogeneity of superconducting properties. It is shown that the effect is a direct consequence of the so-called δl-pinning, i.e. pinning caused by local reduction of the electrons mean free path. Based on the understanding of the irreversibility field nature, an assessment of the individual pinning force is performed. Good agreement is obtained with the estimations performed in the framework of the Larkin-Ovchinnikov pinning model (LO), which is valid for magnetic field less than the irreversibility field. The competing model is considered which is also valid for field range below the irreversibility field - the anisotropic pinning model (APM). The comparison of the fundamental length scales introduced in the LO and APM models is given.

Magnetic nanowires for quantitative detection of biopolymers

Magnetic nanowires (MNWs) rank among the most promising multifunctional magnetic nanomaterials for nanobarcoding applications, especially biolabeling, owing to their nontoxicity and remote excitation using a single magnetic source. Until recently, the first-order reversal curve (FORC) technique has been broadly used to study the MNWs for biolabeling applications. However, since FORC measurements require many data points, this technique is very slow which makes it inapplicable for clinical use. For this reason, we recently developed a fast new framework, named the projection method, to measure the irreversible switching field (ISF) distributions of MNWs as the magnetic signature for the demultiplexing of magnetic biopolymers. Here, we illustrate the ISF distributions of several MNWs types in terms of their coercivity and interaction fields, which are characterized using both FORC and projection methods. Then, we explain how to tailor the ISF distributions to generate distinct signature to reliably and quantitatively demultiplex the magnetically enriched biopolymers.

Effects of surface damage on critical current density in MgB2 thin films

We investigated the influence of surface damage on the critical current density (Jc) of MgB2 thin films via 140-keV Co-ion irradiation. The Jc(H) of the surface-damaged MgB2 films was remarkably improved in comparison with that of pristine films. The strong enhancement of Jc(H) caused by a surface damage in MgB2 films can be ascribed to additional point defects along with an atomic lattice displacement introduced through low-energy Co-ion irradiation, which is consistent with the change in the pinning mechanism, from weak collective pinning to strong plastic pinning. The irreversible magnetic field (Hirr) at 5 K for surface-damaged MgB2 films with a thickness of 850 and 1300 nm was increased by a factor of approximately 2 compared with that of a pristine film. These results show that the surface damage produced by low energy ion irradiation can serve as an effective pinning source to improve Jc(H) in a MgB2 superconductor.

Repeatability and agreement of intraocular pressure measurement among three tonometers

BackgroundElevated intraocular pressure (IOP) is one of the causes of irreversible optic nerve head damage and visual field loss. It is often measured with applanation tonometers but the use of rebound tonometry devices has been proposed as an alternative to assess IOP. Rebound tonometers have also been proposed as a method for patients to measure their own intraocular pressure (that is, self‐tonometry). The purpose of this study was to determine the intrasession repeatability and the agreement of the IOP measurement with two rebound measuring principle tonometers, ICare ic100 and ICare Home with Perkins tonometer.MethodsThis study involved 27 healthy volunteers (18 to 30-years old). We performed three consecutive IOP measurements with ICare Home, ICare ic100, and Perkins. The means of the three measurements from each device were calculated. Repeatability and agreement were defined according to the British Standards Institute and the International Organization for Standardization. The agreement was assessed using the method described by Bland and Altman, where 95 per cent of the differences or limits of agreement were between 1.96 standard deviations of the mean difference.ResultsAll tonometers showed close measurements (Perkins 15.34 ± 3.45-mmHg, range 10.00–24.00; ICare ic100 15.40 ± 4.06-mmHg, range 9.67–23.33; and ICare Home 14.22 ± 4.72-mmHg, range 7.33–24.00). The co‐efficient of variation (CV) and within‐subject standard deviation (Sw) was low for ICare ic100 and Perkins (close to 6.30 per cent and one) with higher values for ICare Home (CV = 9.55% and Sw = 1.33). The intraclass correlation co‐efficient showed values higher than 0.96 for all tonometers. The difference between both rebound tonometers was statistically significant (p = 0.03).ConclusionThe ICare ic100 tonometer provides repeatable IOP measurements close to the measurements of the Perkins IOP (good agreement); however, ICare Home provides less repeatable values, showing worse agreement with the Perkins tonometer in healthy subjects.

The Effect of Contact Lens-spectacle Reversed Galilean Telescope on the Visual Field of Patients with Open-angle Glaucoma.

PurposeGlaucoma causes irreversible visual field defects. This study aims to evaluate the effect of a reversed Galilean telescope on the visual field of patients with open-angle glaucoma.Methods Fifty-two glaucoma patients with a restricted visual field were recruited for this study. Central 30° visual field measurements were performed using a Humphrey visual field analyzer before and after applying the reversed Galilean telescope. To be more cosmetically acceptable, a combination of contact lens–spectacle was used as the reversed Galilean telescope.ResultsOur data analysis showed that the reversed Galilean telescope had a significant effect on all measured perimetric indices. Visual field index (VFI) improved from a basic value of 44.38 26.96 to 49.30 29.83 percent by using the reversed telescope (P 0.001). Moreover, the mean deviation (MD) was significantly improved from the initial value of –19.91 7.19 dB to a value of –18.69 7.73 dB (P 0.001). However, our results showed a significant reduction in the pattern standard deviation (PSD) comparing before (9.83 2.82) and after (8.51 3.30) values using the reversed Galilean telescope (P 0.001).ConclusionThe contact lens–spectacle combination reversed Galilean telescope significantly improved the central 30° visual field of glaucoma patients with the restricted visual field.

Partially-oriented MgB2 superconducting bulks with addition of B4C and cubic BN obtained by slip casting under high magnetic field and spark plasma sintering

Partially-oriented MgB2 bulk discs (13 and 9 %) with the starting compositions of (MgB2)0.99(B4C)0.01 and (MgB2)0.99(c-BN)0.01 were fabricated by slip casting under an H0 = 12 T magnetic field (perpendicular to the disc surface) and subsequent spark plasma sintering. The maximum critical current density and irreversibility field are for H//H0 (H=applied field). These values are higher or similar to the randomly-oriented samples with the same composition. The maximum volume pinning force (Fp) is lower in the partially-oriented ones than in the randomly-oriented samples. The pinning-force-related parameters depend on the additive and orientation. Assessment of the major pinning mechanism within the scaling and percolation models considering these parameters shows significant limitations. A method to scale Fp is proposed; for the randomly and partially-oriented samples (that show an extra peak in Fp), the single and double Gaussian functions fit well. The results suggest an anisotropic influence of carbon substituting for boron in the MgB2.

Automated segmentation and classifcation of retinal features for glaucoma diagnosis

Glaucoma is a progressive optic neuropathy, which damages the optic nerve head and causes irreversible visual field loss. It is considered as one of the major cause of blindness. Clinical diagnosis of glaucoma involves fundus photography for examining the changes occurring due to glaucoma. Fundus images are capable of being processed by computational algorithms. Thus, development of an automated diagnostic system using image processing techniques is of great importance for detection of glaucoma during mass screening. The proposed method aims to develop a computer aided diagnostic (CAD) system for glaucoma detection. First, automated segmentation algorithms for optic disc and optic cup are developed which overcomes the reduced variability present between the region of interest and the background. Second, the segmented regions are used to obtain the clinical and textural features. Finally an efficient classification model is developed by considering the dynamic classifier selection methods. The proposed method is tested on a hospital dataset and publically available Drishti dataset. The quantitative results proves the efficiency of the adopted methodology and thus, can be incorporated in CAD of glaucoma.

Influence of eutectic microstructure on the critical current density of V1−xZrx superconductors

We present here the studies on the effect of annealing on the structural and magnetic properties of V1−xZrx (x = 0.33 and 0.40) superconducting alloys. We show that the γ-ZrV2 phase grows at the expense of (α-Zr)-(γ-ZrV2) eutectic microstructure when V1−xZrx alloys are annealed at 1473 K for one week. The annealed samples predominantly have a γ-ZrV2 phase with small amounts of β-V and α-Zr phases. The irreversibility field, critical current density, and pinning force density of the as-cast samples are found to be significantly higher than those of annealed alloys. These results indicate that eutectic microstructure is important in utilizing these materials for high field applications.

Atopic dermatitis as a risk factor for severe visual field loss in youth-a retrospective cohort study of glaucoma under steroid treatment.

To assess the characteristics of glaucoma patients who received systemic or topical steroid treatment. Patients who received steroid treatment were selected from a total of 4256 patients at our tertiary referral center of glaucoma management between August 2011 and October 2017. Clinical characteristics of the subjects were extracted from clinical records. To evaluate the factors influencing highest intraocular pressure during observation (max-IOP) or mean deviations (MDs) of visual field at the first and last visits, univariate and multivariate regression analyses were performed using a generalized estimating equation. Three hundred and eighty-two eyes of 196 patients were included in this study. The most frequent disease as the reason for steroid treatment was atopic dermatitis (58 eyes) followed by autoimmune diseases. The patients with atopic dermatitis were significantly younger (38.0 ± 11.2 years old, p < 0.001) and had lower MD (- 9.3 ± 9.1 dB at first visit, p = 0.01; - 10.6 ± 9.2 dB at last visit, p = 0.004) than those with other diseases. In multivariate regression analysis, age and MD at first visit, max-IOP, and atopic dermatitis were correlated with MD at last visit. The results of the present study suggest that patients with atopic dermatitis carry the risk of irreversible visual field loss even in youth. For earlier detection of high intraocular pressure, reinforcement of ophthalmological screening in management of atopic dermatitis should be recommended.

Transport and Point Contact Measurements on Pr1-xCexPt4Ge12 Superconducting Polycrystals.

We performed a detailed investigation of the superconducting properties of polycrystalline Pr1−xCexPt4Ge12 pellets. We report the effect of Ce substitution, for x = 0.07, on magnetic field phase diagram H-T. We demonstrate that the upper critical field is well described by the Ginzburg–Landau model and that the irreversibility field line has a scaling behaviour similar to cuprates. We also show that for magnetic fields lower than 0.4 T, the activation energy follows a power law of the type 𝐻−1/2, suggesting a collective pinning regime with a quasi-2D character for the Ce-doped compound with x = 0.07. Furthermore, by means of a point contact Andreev reflection spectroscopy setup, we formed metal/superconductor nano-junctions as small as tens of nanometers on the PrPt4Ge12 parent compound (x = 0). Experimental results showed a wide variety of conductance features appearing in the dI/dV vs. V spectra, all explained in terms of a modified Blonder–Tinkham–Klapwijk model considering a superconducting order parameter with nodal directions as well as sign change in the momentum space for the sample with x = 0. The numerical simulations of the conductance spectra also demonstrate that s-wave pairing and anisotropic s-waves are unsuitable for reproducing experimental data obtained at low temperature on the un-doped compound. Interestingly, we show that the polycrystalline nature of the superconducting PrPt4Ge12 sample can favour the formation of an inter-grain Josephson junction in series with the point contact junction in this kind of experiments.

Structural and magnetic properties of superconductor NiBi3/α-Bi2O3 synthesized by mechanical alloying

A nanostructured NiBi3/α-Bi2O3 superconductor composite has been synthesized by the process of mechanical alloying through 19 h of high energy ball milling. The kinetics of formation is analyzed in detail by x-ray diffraction and the Rietveld method. A simple method is proposed to determine the crystallinity of the resulting sample by using the background Rietveld fitting that permits a quantitative analysis of the diffuse x-ray profile taking into account the air and Compton scattering. The diffuse scattering is attributed to the formation of a nanocluster of α-Bi2O3 which is confirmed by high-resolution transmission electron microscopy and Raman measurements. The magnetic measurements show superconducting transition at around 4 K and the existence of NiBi3/α-Bi2O3 nanophases with a low irreversibility field at around 1.01 T at 2 K. Analysis of magnetic susceptibility and isothermal magnetic hysteresis of the NiBi3/α-Bi2O3 nanocomposite indicates the simultaneous existence of bulk superconductivity and ferromagnetism below 4 K.

Enhancement of Bc2 and Birr in bulk MgB2 superconductors with SnO2 Additions

Three (MgB2)1-x(SnO2)x samples with x ranging from 0 to 5 wt% were prepared by the in situ route to study the effect of tin dioxide additions on the superconducting properties of MgB2 bulk materials. All of the reacted samples were slightly Mg deficient although the starting Mg:B precursor powder ratio was 1:2. A heat treatment (HT) temperature of 700 °C with a dwell time of 30 min was used. XRD results showed evidence of peak shifts for MgB2 phases with SnO2 addition. The magnitude of the a-axis lattice constant change (0.361 ± 0.075 %) calculated for the 3 wt% doped samples is comparable in magnitude to that seen previously for the C-doped MgB2 bulks which exhibited enhanced BC2. The upper critical fields (BC2) and the irreversibility fields (Birr) were measured resistively in fields up to 14 T at 5 K to Tc. The best BC2 value at 20 K (15.2 T based on extrapolation) was seen for sample IS3 (x =3 wt%), and was comparable to the best BC2 values (≈ 15 T at 20 K) seen for C-doped MgB2 bulks. IS3 had a corresponding Birr = 10.8 T (20 K). The superconducting transition temperature (Tc) appeared to increase slightly with doping, although within the range of error bars (37.4 K to 37.6 K for 1.6 T BC2 increase at 20 K), in contrast to C doping which is accompanied by a significant decrease in Tc (39 K to 36 K for 3.8 % C doped MgB2 bulk). We attribute the observed increase in both BC2 and Birr for SnO2-additions to lattice strain caused by the introduction of precipitates within the grains.

Magnetic field sweep rate influence on the critical current capabilities of a Fe(Se,Te) crystal

The relaxation phenomena of a FeSe0.5Te0.5 single crystal sample, with twinned morphology, exhibiting a second magnetization peak effect have been analyzed by means of dc magnetization measurements as a function of magnetic field (H). Using different magnetic field sweep rates for performing the superconducting hysteresis loops m(H), a change in the width of these curves has been observed together with a shift of the second peak field position. A consequent and significant variation of critical current density Jc has been extracted from the superconducting m(H) curves within the Bean critical state model. In this framework, the ratio between the critical current densities Jc calculated at 200 Oe/s and 5 Oe/s has been calculated showing interesting results in terms of transport capabilities. From the Jc(H) curves at T = 10 K for different sweep rates, the irreversibility field has been obtained for T = 10 K and fitted with a power law. The increasing trend of the irreversibility field as a function of sweep rate could suggest the use of this material for power applications where overheating due to vortices movement inside the superconductor has to be limited.

The Influence of Wire Bending and Wire Diameter on Transport Critical Current Density in Small MgB2 Superconducting Coils for Applications in Multi-Section Coils

This article presents the impact of MgB2 wire bending and diameter on transport critical current density and irreversible magnetic field of a resultant coil. Unreacted MgB2 wires 500 mm in length and 0.63 or 0.83 mm in diameter have been used in the fabrication of small diameter (14 mm) superconducting coils. The coils were subsequently annealed under isostatic pressure of 1 GPa for 15 min at 700 °C and 725 °C. Our results indicate that larger wire diameter, higher annealing temperature, and bending lead to slight reduction of critical current density and irreversible magnetic field in the coil.

NdFeAs(O,H) epitaxial thin films with high critical current density

We report on the growth of NdFeAs(O,H) epitaxial thin films on MgO (001) substrates and their electrical transport properties. The maximum carrier density of the NdFeAs(O,H) films was more than twice as large as that of our NdFeAs(O,F) films. This enabled us to investigate the physical properties of heavily electron doped NdFeAsO, which could not be achieved with NdFeAs(O,F). The irreversibility field Hirr of NdFeAs(O,H) was larger than that of NdFeAs(O,F) for H || c due to the decrease in the anisotropy of the upper critical field. A very high critical current density of 17 MA cm−2 was recorded at 4 K, which corresponds to about 13% of the depairing current density of NdFeAs(O,F).

Flux pinning and microstructure of a bulk MgB2 doped with diverse additives

In bulk high-temperature superconductors, Ag is usually used as an additive thus improving mechanical performance. In MgB2 doped with Ag, the Ag reacts with Mg, forming Mg–Ag phases acting as a vortex pinning medium. In this work, we analyze the electromagnetic and pinning properties of bulk MgB2 doped with 1 wt% MgB4, 4 wt% Ag, and 1 wt% Dy2O3, prepared at the Shibaura Institute of Technology (SIT), Tokyo. In three compounds of MgxB2 + 4 wt% Ag with x = 1, 1.075, and 1.1, the effect of Mg excess was studied. The magnetic moment was measured by a vibrating sample magnetometer (VSM). Pinning was studied in terms of a pinning diagram, i.e. the field dependence of the normalized pinning force density, F n = F/F max. In all studied samples, the peak of the F n (b) dependence (b= B/B irr, B irr being the irreversibility field) was observed at around b= 0.2, indicating a prevailing flux pinning at grain boundaries. A slight shift of the peak with decreasing temperature indicated a defect size distribution in the pinning landscape. Transmission electron microscopy (TEM) showed a granular structure of all samples composed of MgB2 grains of about 230 nm (average size), with ensembles of small grains (22 nm in average) of Ag3Mg, AgMg, Ag, Dy2O3, and MgB4. While the large MgB2 grains control the main pinning mechanism, the small precipitates seem to determine details of the current flow through the grain boundaries.

Mixed state properties analysis in AC magnetic field of strong pinning Fe(Se,Te) single crystal

The fundamental harmonic of the AC magnetic susceptibility of a Fe(Se,Te) single crystal iron based superconductor has been studied at different AC field amplitudes and frequencies with and without a superimposed DC magnetic field with the aim of analyzing its superconducting properties that can be also suitable for power applications such as the critical current density and the pinning energy in AC regime. From the analysis of the fundamental Cole-Cole plots a finite influence of flux creep phenomena has been detected. Its study, by means of the Arrhenius plots, evidences thermal activation energy U values as high as 103 K, whose dependence on the DC and AC fields, and of the temperature, has been also obtained. The magnetic field dependence shows the existence of the strong pinning regime characterized by plastic deformations of the vortex lattice, whereas the temperature dependence of the critical current density Jc(T) is consistent with the δl pinning model. Finally, the irreversibility field values Hirr(T) have been extracted from the Jc(T) at the different AC frequencies showing very high values if compared with the literature.