Dependence Of Current Density Research Articles

Chemical CeO2-Based Buffer Layers for Fe(Se,Te) Films

Among other Fe-based superconductors, Fe(Se,Te) is particularly interesting because of the low structural anisotropy, large upper critical fields, low field dependence of the critical current density and low toxicity. It can also be grown as an epitaxial film on a metallic oriented substrate, making the fabrication of a Fe-based coated conductor (CC) possible. Less strict requirements on the template microstructure allow for the design of a simplified design compared to REBCO CCs. This design requires a buffer layer to promote the oriented growth of the superconducting film and avoid diffusion from the metallic template. In this work, CeO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> based buffer layers are prepared on single crystals via two chemical deposition techniques, metal organic decomposition (MOD) and polymer assisted deposition (PAD). With the design of a suitable thermal treatment, it is possible to obtain oriented buffers with large flat grains and low values of surface roughness. Fe(Se,Te) films are deposited on these templates via laser deposition, and excellent samples are obtained when a Fe(Se,Te) seed layer is used to favour chemical matching with the buffer: sharp superconducting transitions around 16 K and critical current densities exceeding 1 MA cm <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">⁻</i> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 4.2 K in self-field are observed. These results are the demonstration of the feasibility of a Fe-based CC architecture, with all the relative advantages concerning process simplification and cost reduction.

Impact on on-resistance of p-n junction diodes by using heavily Ge-doped GaN substrate

The characteristics of p-n junction diodes on heavily Ge-doped substrates were compared with those on conventional Si-doped substrates. A large decrease in on-resistance was observed which greatly exceeded the component due to the lower resistance of the Ge-doped substrate. This result implied an occurrence of a strong conductivity modulation by the heavily doped substrate. Current-density dependence on the anode-electrode diameter could be minimized by the use of the substrate. No significant difference was observed in the breakdown voltage even when the doping concentration in the substrate was increased. It has been clarified that the heavily Ge-doped GaN substrate is essential for low on-resistance and large size GaN power devices.

Effects of TiO2 nanoparticle addition on the flux pinning properties of the Bi1.6Pb0.4Sr2Ca2Cu3O10+δ ceramics

The effects of adding TiO2 nanoparticles on the flux pinning properties of the Bi–Pb–Sr–Ca–Cu–O (BPSCCO) system were investigated. Polycrystalline samples of (Bi1.6Pb0.4Sr2Ca2Cu3O10+δ)1-x(TiO2)x, where x = 0.000, 0.002, 0.004, 0.006, 0.008, and 0.010, were fabricated by the conventional solid-state reaction method. The X-ray diffraction (XRD) results showed that all samples consisted of Bi-2223 and Bi-2212 phases. The Bi-2223 volume fraction decreased with the addition of TiO2. Scanning electron microscopy (SEM) examination presented a degradation in texture structure with increasing misoriented grain and porosity with high doping contents. The critical temperature was decreased by TiO2 appearance in relation to Bi-2223 deceleration. Critical current density dependence on the magnetic field Jc(B) was enhanced with x = 0.002, 0.004 and reached maximal values on the x = 0.002 sample. In the collective pinning theory framework, the small bundle field (Bsb) and the large bundle field (Blb) were estimated, revealing the extension of the small and large bundle regimes with proper amounts of dopant. The temperature-dependent normalized critical current density (j) indicated that δl pinning is the dominant flux pinning mechanism in all samples. The Dew-Hughes model analyzed the pinning center properties and confirmed that TiO2 nanoparticles create normal core point pinning centers in the matrix.

Investigation of the flux dynamics in KCa2Fe4As4F2 single crystal by ac susceptibility measurements

Systematic ac susceptibility measurements have been performed to study the flux dynamics in a high quality KCa2Fe4As4F2 single crystal. The irreversible line in the vortex diagram is closed to the (T) line suggesting the strong intrinsic pinning in this material. The critical field = 0.4 T between single vortex pinning state and collective pinning state is obtained according to thermally activated flux creep model. The abnormal behavior in the single vortex pinning region that the activation energy U follows a dependence might arise from the planar defects of submicron scale. The strong field dependences of the activation energy in both regions suggest that artificial pinning centers are needed to enhance the pinning for practical applications. Moreover, the field and current density dependences of the activation energy at relatively high fields are determined as U(J, ).

Magneto-optical studies of flux pinning in high-temperature superconductors

Abstract Quantitative magneto-optical imaging of magnetic flux distributions has developed in a powerful tool for the analysis of the local transport properties of superconductors. It allows a model-independent determination of the current density distribution of thin films and, thus, the local current density through individual defects. Also, local metastable properties are detectable, such as the local electric field distribution E with a high sensitivity down to 10–12 V/m caused by thermally activated flux creep. Based on these tools, in this paper we present a systematic comparison of vortex pinning, vortex movement and current transfer of two kinds of planar defects which are typically present in high-temperature superconducting thin films: low-angle grain boundaries and antiphase boundaries. Special attention is drawn to the local magnetic field dependence of the critical current density and to the spatial distribution of E, giving insight into the collective behavior of vortices at planar defects.

Optical, structural and thermal performances of black nickel selective coatings for solar thermal collectors

In the present study, the possibility of devising efficient and durable black nickel selective coating based solar thermal collectors (STCs) at ambient temperature was explored. For this, NASA electrochemical bath initially formulated by Peterson and Lin [Final Report: NASA-CR-149928, 1976] was employed. Quality of the STCs were optimized by evaluating pH, temperature, electric current density (Id) and deposition time (Td) dependence of optical, structural and thermal performance of the black nickel coating on the copper sheets and tubes. The optical measurements in the UV–Vis–NIR and NIR-MIR ranges inferred us that at pH ≈ 5.8, temperature in the range 20–25 °C, 2 mA/cm2 ≤ Id ≤ 4 mA/cm2 and 5 min. ≤ Td ≤ 7 min. result in reasonably good black nickel coatings with optimal selectivity factor. Moreover, from the analysis of XRD spectra, for the first time we observed the significantly larger crystallites size of coatings on Cu sheets than that of the coating on tubes. Observed thermal performances (evaluated by measuring the collector efficiencies) indicate that the electro-deposited black nickel-based STCs were superior by more than 20% over black paint based STCs.

Design and modelling tools for DC HTS cables for the future railway network in France

The use of high-temperature superconducting (HTS) cables in power systems increases the transmission capacity, whereas reducing the volume of the installation. In addition, in high current applications, HTS cables considerably reduce power losses, right-of-ways and total system mass. This paper presents different multi-physics studies to be performed to accurately design the direct current (DC) HTS cables for the future railway network planned by the French company SNCF. The process used to design DC cables for high operating current (between 5 kA and 20 kA at 1750 V) made of commercial (RE)BaCuO tapes is presented. In this design process, the critical current density dependence J c(B, θ, T) of the superconducting tapes, the thermal properties of the materials used as well as different cable cooling configurations are considered. Finally, we discuss the selection of the appropriate cooling configuration to ensure adequate cooling of the cable.

Energy yield of perovskite solar cells: Influence of location, orientation, and external light management

Accurate forecasting of yearly energy yield of photovoltaic devices under realistic operating conditions is key for numerous practical applications related to further evolution of photovoltaics. In this work we present development and validation of an energy yield model applied to planar perovskite solar cells. The model is based on the combination of advanced optical modelling and extensive opto-electrical characterization and takes the geographical location and the orientation of the device into account. Using the model, we first perform a detailed sensitivity analysis and indicate the key parameters and their interdependencies that most significantly affect the calculated energy yield values. We show that the results are dominated (within 4% of the final value) by the solar irradiance dependency of the short-circuit current density, and further by the temperature dependency of the open-circuit voltage and the fill factor, whereas the irradiance dependency of the open-circuit voltage and the fill factor are mutually compensated. In the second part of the paper, we investigate how the energy yield of the device is affected by the application of external light management foils. Different texture morphologies, geographical locations, and device orientations are included in the investigation. The results show that even under realistic operating conditions, the overall trends of the light management efficiency of textured foils are not that different from those observed under standard test conditions; the highest energy yield gains in the order of 7–11% are obtained by the tetrahedral texture which is indicated as optimal in all studied cases. • EY forecasting crucial for LCOE estimation, degradation monitoring, and device optimization. • J SC under realistic conditions can be simulatend using advanced optical modelling. • V OC and FF under realistic conditions can be obtained through extensive electrical characterization. • External light management foils provide significant EY boosts under realistic outdoor conditions; tetrahedral micro texture indicated as the optimal in all studied cases. • Similar trends observed as under STC conditions, with similar achievable relative gains.

Моделирование особенностей магнитных свойств осесимметричных гранул жестких сверхпроводников II рода

Based on the equations of electrodynamics and the concept of a critical state for hard superconductors of the 2nd kind, numerical simulation of the magnetic properties of axisymmetric superconducting samples, in particular, granules, is performed for a number of models of the dependence of the critical current density on the magnetic field induction. The magnetic moment loops are calculated directly by integrating the integral equation for the current density over time. The phenomena of the peak effect and the asymmetry of the magnetization hysteresis loop are also considered using the indicated equation. Various versions of the functions used in the literature were used as peak functions. In addition to the hysteresis loop of the magnetic moment, the magnetic field induction at the center of axisymmetric samples, and the total penetration field, the profiles of the critical current density Jc(B) and the equilibrium magnetic moment for spherical granules were obtained. The method used for calculating the magnetic moment of superconductors makes it possible to take into account the equilibrium and nonequilibrium regions of the magnetization of the samples independently.

Simulation of the features of the magnetic properties of axisummetric granules of hard type II superconductors

Based on the equations of electrodynamics and the concept of a critical state for hard superconductors of the 2nd kind, numerical simulation of the magnetic properties of axisymmetric superconducting samples, in particular, granules, is performed for a number of models of the dependence of the critical current density on the magnetic field induction. The magnetic moment loops are calculated directly by integrating the integral equation for the current density over time. The phenomena of the peak effect and the asymmetry of the magnetization hysteresis loop are also considered using the indicated equation. Various versions of the functions used in the literature were used as peak functions. In addition to the hysteresis loop of the magnetic moment, the magnetic field induction at the center of axisymmetric samples, and the total penetration field, the profiles of the critical current density Jc(B) and the equilibrium magnetic moment for spherical granules were obtained. The method used for calculating the magnetic moment of superconductors makes it possible to take into account the equilibrium and nonequilibrium regions of the magnetization of the samples independently Keywords: magnetic moment, axisymmetric granules, type II superconductor, critical state, peak effect, equilibrium magnetic moment.

Electrical properties of NbN/NbN x /NbN Josephson junctions

In this work, we report the electrical properties of NbN internally shunted Josephson junctions with NbN x barriers. Cross-sectional scanning transmission electron microscopy analysis shows that all layers have the same cubic structure; NbN/NbN x /NbN trilayers were epitaxially grown on MgO substrates. The resistivity of the NbN x films could be varied in the range of 1–104 mΩ cm by controlling both the N2 partial pressure and the deposition time during reactive sputtering. The temperature dependence of the critical current density (J c) and characteristic voltage (I c R n) of the junctions with different barrier resistivities were measured for various barrier thicknesses. For the 10 nm-thick NbN x layer with resistivities of 73.44, 385.72, and 711 mΩ cm, the coherence length of the barrier was determined to be 5.55 ± 0.07, 4.88 ± 0.06, and 1.40 ± 0.13 nm, respectively, corresponding to carrier diffusion rates of 2.741 ± 0.004, 1.211 ± 0.002, and 0.008 ± 0.001 cm2 s−1, respectively. Thus, the reduction in barrier resistivity leads to a larger coherence length and a faster diffusion rate, which will further increase the J c and I c R n of the junction. By adjusting the barrier resistivity and thickness, the J c of the junction can be easily tuned over more than four orders of magnitude, and an I c R n value of 0.97 ± 0.07 mV was obtained at 10 K. The results indicate that the all-NbN self-shunt junction is a promising candidate in high-speed and high-temperature applications.

Electrooxidation of Iohexol in Its Commercial Formulation Omnipaque on Boron Doped Diamond Electrode

This study investigated the electrochemical behavior of iohexol in its commercial formulation omnipaque on a boron-doped diamond electrode using cyclic voltammetry and chronoamperometry. The dependence of the anodic peak current density vs. iohexol concentration is linear and can be applied to the determination of the substrate concentration in environmental samples and pharmaceuticals. The iohexol electrooxidation on boron-doped diamond electrode is diffusion-controlled process and proceed via two ways: a direct electron transfers at the surface of boron-doped diamond electrode and an indirect oxidation mediated by in situ oxidative species. The iohexol electrooxidation in pH range from 2 to 6 includes exchange of 4 electrons and 1 proton, at pH superior to 6 it includes an exchanged of 2 electrons and 1 proton. The values of activation energy, anodic transfer coefficient, heterogenous rate constant, diffusion coefficient and the catalytic rate constant were 14.164 kJ mol-1, 0.428, 1.06 s-1, 4.47 cm2 s-1 and 3.61 M-1 s-1 respectively. It appears from those results that, on our electrode, for the high potential scan rates, few actives sites mainly those located at the electrode surface are involved in the iohexol oxidation process. As the potential scan rate decreases, more actives sites are involved in the process.

Synthesis of Dense MgB2 Superconductor via In Situ and Ex Situ Spark Plasma Sintering Method.

In this study, high-density magnesium diboride (MgB2) bulk superconductors were synthesized by spark plasma sintering (SPS) under pressure to improve the field dependence of the critical current density (Jc-B) in MgB2 bulk superconductors. We investigated the relationship between sintering conditions (temperature and time) and Jc-B using two methods, ex situ (sintering MgB2 synthesized powder) and in situ (reaction sintering of Mg and B powder), respectively. As a result, we found that higher density with suppressed particle growth and suppression of the formation of coarse particles of MgB4 and MgO were found to be effective in improving the Jc-B characteristics. In the ex situ method, the degradation of MgB2 due to pyrolysis was more severe at temperatures higher than 850 °C. The sample that underwent SPS treatment for a short time at 850 °C showed higher density and less impurity phase in the bulk, which improved the Jc-B properties. In addition, the in situ method showed very minimal impurity with a corresponding improvement in density and Jc-B characteristics for the sample optimized at 750 °C. Microstructural characterization and flux pinning (fP) analysis revealed the possibility of refined MgO inclusions and MgB4 phase as new pinning centers, which greatly contributed to the Jc-B properties. The contributions of the sintering conditions on fP for both synthesis methods were analyzed.

Compensation of Nonlinearity in Superconducting Transmission Line by Geometrical Restructuring

Superconductors are popular for microwave engineers because they are high performance, high selectivity, and low volume. However, the use of superconductors is limited due to their inherent nonlinearity. Intrinsic nonlinearity in superconductors is due to the dependence of surface current density on the depth of London penetration in them. This leads to adverse effects such as intermodulation distortion (IMD) and the production of a third harmonic in a superconductor. In this article, we intend to improve the effects of nonlinearity by modifying the superconducting geometric structure, which we call the method of geometric reconstruction. To achieve this, we first designed a superconducting transmission line (TL), and then created slots on the superconducting TL to improve the IMD and the third harmonic. Using this method, we managed to reduce the level of the third harmonic from –6 to –31 dBm. To verify the results, after simulating the slotted superconducting TL in ADS software, simulation and solving of London equations using three-dimensional-FEM in COMSOL software has been done again.

Two-terminal vertical thyristor using Schottky contact emitter to improve thermal instability

In a two-terminal-electrode vertical thyristor, the latch-up and latch-down voltages are decreased when the memory operation temperature of the memory cells increases, resulting in a severe reliability issue (i.e. thermal instability). This study fundamentally solves the thermal instability of a vertical-thyristor by achieving a cross-point memory-cell array using a vertical-thyristor with a structure of vertical n++-emitter, p+-base, n+-base, and p++-emitter. The vertical-thyristor using a Schottky contact metal emitter instead of an n++-Si emitter significantly improves the thermal stability between 293 K and 373 K. Particularly, the improvement degree of the thermal stability is increased significantly with the use of the Schottky contact metal work function. Because the thermal instability (i.e. degree of latch-up voltage decrement vs. memory operation temperature) decreases with an increase in the Schottky contact metal work function, the dependency of the forward current density between the Schottky contact metal and p+-Si based on the memory operation temperature reduces with increase in the Schottky contact metal work function. Consequently, a higher Schottky contact metal work function produces a higher degree of improvement in the thermal stability, i.e. W (4.50 eV), Ti (4.33 eV), Ta (4.25 eV), and Al (4.12 eV). Further research on the fabrication process of a Schottky contact metal emitter vertical-thyristor is essential for the fabrication of a 3D cross-point memory-cell.

DC modeling and characterization of HTS coils with non uniform current density distribution

In this paper, a fast and realistic approach is proposed for the DC modeling of superconducting coils made of first-generation (1G) high temperature superconducting (HTS) tapes, taking into consideration the non-uniformity of the electric current density distribution in the HTS tapes cross-section, by using a power minimization criterion with the dependence of the critical current density on the magnetic flux density (MFD). Integral equations are used to evaluate the MFD, enabling to discretize only the active parts of the system. The model is then combined with an inverse procedure for the characterization of non-inductive HTS coils using measurements.

Pressure Dependence of Superconducting Properties, Pinning Mechanism, and Crystal Structure of the Fe0.99Mn0.01Se0.5Te0.5 Superconductor.

We have investigated the pressure (P) effect on structural (up to 10 GPa), transport [R(T): up to 10 GPa], and magnetic [(M(T): up to 1 GPa)] properties and analyzed the flux pinning mechanism of the Fe0.99Mn0.01Se0.5Te0.5 superconductor. The maximum superconducting transition temperature (Tc) of 22 K with the P coefficient of Tc dTc/dP = +2.6 K/GPa up to 3 GPa (dTc/dP = −3.6 K/GPa, 3 ≤ P ≥ 9 GPa) was evidenced from R(T) measurements. The high-pressure diffraction and density functional theory (DFT) calculations reveal structural phase transformation from tetragonal to hexagonal at 5.9 GPa, and a remarkable change in the unit cell volume is observed at ∼3 GPa where the Tc starts to decrease, which may be due to the reduction of charge carriers, as evidenced by a reduction in the density of states (DOS) close to the Fermi level. At higher pressures of 7.7 GPa ≤ P ≥ 10.2 GPa, a mixed phase (tetragonal + hexagonal phase) is observed, and the Tc completely vanishes at 9 GPa. A significant enhancement in the critical current density (JC) is observed due to the increase of pinning centers induced by external pressure. The field dependence of the critical current density under pressure shows a crossover from the δl pinning mechanism (at 0 GPa) to the δTc pinning mechanism (at 1.2 GPa). The field dependence of the pinning force at ambient condition and under pressure reveals the dense point pinning mechanism of Fe0.99Mn0.01Se0.5Te0.5. Moreover, both upper critical field (HC2) and JC are enhanced significantly by the application of an external P and change over to a high P phase (hexagonal ∼5.9 GPa) faster than a Fe0.99Ni0.01Se0.5Te0.5 (7.7 GPa) superconductor.

Critical switching current of a perpendicular magnetic tunnel junction owing to the interplay of spin-transfer torque and spin-orbit torque

Using the linearized Landau–Lifshitz-Gilbert (LLG) equation in the rotation coordinate, we calculate the critical switching current density of a perpendicular magnetic tunnel junction (MTJ), where magnetization switching is achieved by the interplay of spin-transfer and spin-orbit torques. In terms of the critical current density, we find that as the current density inducing the spin-orbit torque increases, the current density inducing the spin-transfer torque decreases nonlinearly. In the presence of the spin-orbit field-like torque (β), the critical switching current density by the spin-transfer torque is proportional to the damping constant (α) as the conventional spin-transfer torque switching, while the critical switching current density by the spin-orbit torque increases with when . We also investigated the β dependence of the critical switching current density. For a given , the critical switching current density for the spin-transfer torque, decreases linearly with increasing β and nonlinearly decreases with increasing for a given β. Further, we discuss the β dependence of the critical switching current density on energy.

Thickness dependence of structural and superconducting properties of Co-doped BaFe2As2 coated conductors

SummaryHigh-quality Co-doped BaFe2As2 thin films with thickness up to 2 μm were realized on flexible metal tapes with LaMnO3 as buffer layers fabricated by an ion beam-assisted deposition technique. Structural analysis indicates that increasing thickness does not compromise the film crystallinity, except for a small amount of impurities. Two types of thickness dependence of critical current density (Jc) were found: one is almost thickness independent in the range of 0.6–1.5 μm and the other is highly thickness dependent. In addition, the maximum value for crucial current Ic at 9 T and 4.2 K is about 55 A/12 mm-W for the 1.5-μm-thick film. Anisotropic Ginzburg–Landau scaling demonstrates that dominant pinning centers develop from correlated to uncorrelated with increasing film thickness. The further theoretical analysis shows that with film thickness increasing the pinning mechanism evolves progressively from a δl pinning to the δTc pinning mechanism.

Exchange Current Density of Solid Oxide Electrolysis Cell Electrodes

Introduction Numerical simulation is a powerful technique in evaluating solid oxide fuel cell (SOFC) performance. Such method enables to visualize SOFC performance e.g. in using new materials. It is also desirable to develop such method that visualizes internal phenomena in solid oxide electrolysis cell (SOEC). While various parameters are important in simulating phenomena in SOEC stacks, we focus on exchange current density (1,2). Here, exchange current density is experimentally measured and evaluated using single SOEC / SOFC reversible cells. Furthermore, the exchange current density for different electrode materials in both SOEC and SOFC operation modes is examined. Exchange current density is defined as the current exchanged at the electrode/electrolyte interfaces in both directions in equilibrium and thus , so that an identical value for a given electrode has to be obtained in both SOFC and SOEC modes. However, as such values are practically measured by applying certain current in opposite directions, apparent exchange current density measured could be somehow different due to various irreversible processes at the electrode/electrolyte interfaces. In this study, such apparent exchange current density values are obtained and discussed. Experimental In this study, we evaluated the exchange current densities of solid oxide cells with three types of fuel electrodes (A-1, 2, 3) and two types of air electrodes (B-1, 2). The exchange current densities were measured in the SOFC and SOEC modes using following materials: (A-1) Ni-ScSZ (NiO-ScSZ (ScSZ: 10mol%Sc2O3-1mol%CeO2-89mol%ZrO2)), (A-2) Rh-GDC (La0.1Sr0.9TiO3, Gd0.1Ce0.9O2, volume ratio 50:50 (LST-GDC), Rh 0.178 mg/cm2 supported on LST-GDC) co-impregnated material, and (A-3) Ni-GDC (Ni 0.167 mg/cm2 supported on LST-GDC) co-impregnated material for the fuel electrode; (B-1) LSCF/GDC ((La0.6 Sr0.4)(Co0.2 Fe0.8)O3/GDC), and (B-2) LSM ((La0.8Sr0.2)0.98MnO3) for the air electrode. The exchange current density was derived using the Butler-Volmer-type equation. The exchange current density obtained was fitted by the commonly used phenomenological formula of the exchange current density. We compared the obtained parameters and the dependencies on gas partial pressure. Results and Discussion Here, the differences in exchange current density at the fuel electrode are examined. Figure 1 shows the exchange current density as a function of humidity in the fuel, and fitted curves. The fitted curves of exchange current densities are almost similar in the SOFC and SOEC modes. However, the maximum value measured in the SOFC mode shown in Fig. 1 (b) was somehow higher than that measured in the SOEC mode shown in Fig. 1 (a). The difference in using the Ni-ScSZ fuel electrode was more remarkable than that using the Rh-GDC co-impregnated fuel electrode shown in Figs. 1 (c) and (d). It may be possible that the apparent exchange current density at the fuel electrode could include influences of irreversible processes, associated with steam electrolysis. The dependence of exchange current density on gas partial pressure (3) will be examined to discuss electrode processes in steam electrolysis. The exchange current density obtained in this study enables various simulations for SOECs, besides SOFCs. References T. Yonekura, Y. Tachikawa, T. Yoshizumi, Y. Shiratori, K. Ito, and K. Sasaki, ECS Trans., 35 (1), 1007-1014 (2011).K. Takino, Y. Tachikawa, K. Mori, S. M. Lyth, Y. Shiratori, S. Taniguchi, and K. Sasaki, Int. J. Hydrogen Energy, 45 (11), 6912-6925 (2020).T. Hosoi, T. Yonekura, K. Sunada, and K. Sasaki, J Electrochem Soc., 162 (1), F136-F152 (2015). Figure 1