Feasibility study on large-scale, high current density superconductor by dynamic stabilization Part 2: stability margin

The magnetic stability of a dynamically stabilized Nb----Ti or Nb₃Sn strip conductor has been analysed theoretically when the external field is applied perpendicular to the wide plane of the conductor. Under the condition that the conductor is face-cooled, an expression of the limiting current density of stability J_c has been derived as a function of the pertinent parameters of the conductor and the heat transfer of the coolant. It should be noted that J_c is independent of the width of the strip conductor. An average critical current density as high as 300 A mm^-2 can be attained without magnetic instability. Even if the structural member is introduced in the winding, overall current density in excess of 100 A mm^-2 is expected to be achieved in large-scale magnets.     

The recovery current density of hard superconductors

In order to quantify stability in superconductors, the power supplied as external heat to drive the superconductor normal was measured. At constant current and temperature, it was found that the conductor could not be driven normal below a certain critical field, and likewise at constant field there was a threshold critical current I_t. The threshold current density J_t was found to correspond to the critical current density for the recovery of superconductivity after a disturbance, J_r.     

Failure of brittle polymers by slow crack growth

The mechanical properties of a silica particle-filled epoxy resin composite system have been investigated in air as a function of volume fraction of particles for volume fractions ranging from 0 to 0.52. The Young's modulus and the compressive yield stress both increase as the volume fraction of silica particles is increased and various models of particle strengthening have been used to explain this behaviour. Slow crack growth in the various particulate composites has been studied using a fracture mechanics approach. The variation of crack velocity (V) with stress intensity factor (K _I) has been measured for each of the compositions investigated. In each case, a unique relationship between V and K _I has been found with K _I increasing with volume fraction of particles at a given value of V. The failure mechanisms and the variation of other fracture mechanics parameters, for example, crack opening displacement and plastic zone size with increasing particle volume fraction have been discussed.     

Feasibility study on large-scale,high current density superconductor by dynamic stabilization: Part 1: magnetic stability

The magnetic stability of a dynamically stabilized Nb-Ti or Nb₃Sn strip conductor has been analysed theoretically when the external field is applied perpendicular to the wide plane of the conductor. Under the condition that the conductor is face-cooled, an expression of the limiting current density of stability J_c has been derived as a function of the pertinent parameters of the conductor and the heat transfer of the coolant. It should be noted that J_c is independent of the width of the strip conductor. An average critical current density as high as 300 A mm^-2 can be attained without magnetic instability. Even if the structural member is introduced in the winding, overall current density in excess of 100 A mm^-2 is expected to be achieved in large-scale magnets.     

Breakdown characteristics of Z continuous winding in HTS transformer

For the development of electrical insulation design of a HTS transformer with Z continuous winding, we have discussed insulation composition and investigated breakdown characteristics such as breakdown of LN₂, polymer and surface flashover on FRP and breakdown-surface combination in LN₂. Also we have designed and manufactured a bobbin that has spiral slot for the Z continuous winding. A Z continuous winding mini-model from Kapton film insulated Cu tape for simulated 22.9 kV class HTS transformer has been constructed using 0.1% breakdown strength obtained by Weibull distribution. The winding model was measured with respect to its insulation characteristics such as ac (50 kV, 1 min) and impulse (154 kV, 1.2 × 50 μs full wave, 3 times) withstand test and its excellent performance was confirmed.     

Magnetic studies of spin wave excitations in Fe/Pt multilayers

The magnetization of Fe/Pt multilayers has been measured as a function of temperature. For Fe/Pt multilayers with fixed iron layer thickness of 2.5 Å, the magnetization decreases faster with temperature as the platinum layers are made thicker. A simple theoretical model has been used to explain the temperature dependence of the magnetization. From the model the approximate values for the bulk exchange interaction J_b and the interlayer coupling strength J_I for various Fe/Pt multilayers have been obtained.     

Synthesis of Thick Tl-Based High-Tc Oxide Layer Through a Diffusion Process

Tl-based high-T _c oxide layers have been synthesized through the reaction between Tl-free substrate and coating layer containing Tl. The coating layer with the eutectic composition in V₂O₅-Tl₂O₃ system much enhances the formation of Tl-2223 phase without degrading its superconducting properties. The TIF substitution for T1₂O₃ in the coating layer promotes the formation of Tl-1223 phase, which significantly improves the I _c-B performance at higher temperatures. The behavior of F during the reaction has been studied. Thick Tl-1223 layer is formed on Ni tape with good bonding through the diffusion reaction.     

Polymer coating for filamentary YBa2Cu3Ox superconductors

Polymer coating for filamentary YBa₂Cu₃O _x superconductors was examined. The precursor filaments were prepared by solution spinning through aqueous poly(vinyl alcohol) solution containing mixed acetates of Y, Ba and Cu. The as-drawn filaments were heated to remove volatile components and to generate a superconducting phase. The synthesis of polyamide acid for the polymer coating was made. The filamentary superconductors were coated with the polyamide acid and cured to convert to the polyimide. The surface of the sample was an insulator. The critical current (I _c) value of the filamentary sample at 77 K was preserved by the polyimide coating. Moreover, the I _c for the polyimide-coated sample was maintained after holding at room temperature for more than 6 months. After thermal cycling between 77 K and room temperature 10 times, the I _c for the coated sample did not deteriorate, whereas the I _c for the sample without polymer coating disappeared. Polymer coating using epoxy resin applied to the filamentary superconductors was also examined.     

Solid state ionics: a Japan perspective

Abstract

The 70-year history of scientific endeavor of solid state ionics research in Japan is reviewed to show the contribution of Japanese scientists to the basic science of solid state ionics and its applications. The term ‘solid state ionics’ was defined by Takehiko Takahashi of Nagoya University, Japan: it refers to ions in solids, especially solids that exhibit high ionic conductivity at a fairly low temperature below their melting points. During the last few decades of exploration, many ion conducting solids have been discovered in Japan such as the copper-ion conductor Rb₄Cu₁₆I₇Cl₁₃, proton conductor SrCe_1–xY_xO₃, oxide-ion conductor La_0.9Sr_0.9Ga_0.9Mg_0.1O₃, and lithium-ion conductor Li₁₀GeP₂S₁₂. Rb₄Cu₁₆I₇Cl₁₃ has a conductivity of 0.33 S cm^–1 at 25 °C, which is the highest of all room temperature ion conductive solid electrolytes reported to date, and Li₁₀GeP₂S₁₂ has a conductivity of 0.012 S cm^–1 at 25 °C, which is the highest among lithium-ion conductors reported to date. Research on high-temperature proton conducting ceramics began in Japan. The history, the discovery of novel ionic conductors and the story behind them are summarized along with basic science and technology.     

Mechanical and transport properties of IBAD/EDDC–SmBCO coated conductor tapes during fatigue loading

In electrical devices like superconducting motor, generator and SMES, HTS coated conductor (CC) tapes will be subjected to alternating stress or strain during manufacturing and operation. The repeated loading will affect the mechanical integrity and eventually the electrical transport property of CC tapes. Therefore in such applications, electro-mechanical property of CC tapes should be evaluated. In this study, the endurance of an IBAD/EDDC–SmBCO CC tape under high-cycle fatigue loading has been evaluated. Applied maximum stress and fatigue life (S–N) relation was obtained at 77 K. The mechanical properties and the critical current, I_c, of the sample under fatigue loading were investigated at 77 K. Considering the practical operating environment, the effect of the stress ratio R, on the degradation behavior of I_c under fatigue loading was also examined.     

Influence of high heat capacity substances doping on quench currents of fast ramped superconducting oval windings

The effect of high heat capacity substances doping on superconducting magnets ramp rate induced quench currents has been investigated for three oval windings. The windings were wound from Rutherford type cable (1.44 × 4.64 mm²), made of 10 multifilamentary NbTi strands 0.85 mm dia. Before application of electrical insulation and winding the cable was covered with epoxy resin with three different fillers: BN (boron nitride—a standard filler for preventing of epoxy cracking at low temperatures), and two rare-earth intermetallic compounds (HoCu₂ and CeCu₆). The specific heat of these compounds at liquid helium temperatures is extremely high. The volumetric fraction of these compounds was 2.9% of the total winding volume, corresponding to 4.5 times increase of averaged winding heat capacity for HoCu₂ and to 1.5 times increase for CeCu₆. At high ramp rates (∼4 kA/s, or ∼6.5 T/s) quench current of HoCu₂ doped winding was 35% higher than that for the BN doped one, while for CeCu₆ doped winding the quench current increase was 12%. Measured quench currents match values calculated with our theoretical model, in which whole doping substances enthalphy was taking into account. It indicates that the enhanced enthalphy is fully utilized in the used range of dB/dt.     

Fabrication of Lamellar Nanosphere Structure for Effective Stress‐Management in Large‐Volume‐Variation Anodes of High‐Energy Lithium‐Ion Batteries

The use of high‐capacity anode materials to overcome the energy density limits imposed by the utilization of low‐theoretical‐capacity conventional graphite has recently drawn increased attention. Until now, stress management (including strategies relying on size, surface coating, and free volume control) has been achieved by addressing the critical problems originating from significant anode volume expansion upon lithiation. However, commercially viable alternatives to graphite have not yet been found. A new stress‐management strategy relying on the use of a lamellar nanosphere Si anode is proposed. Specifically, nanospheres comprising ≈50 nm Si nanoparticles encapsulated by SiO_x /Si/SiO_x /C layers with thicknesses of <20 nm per layer are synthesized via one‐pot chemical vapor deposition in various atmospheres. SiO_x is found to act as a stress management interlayer when it is located between Si and mitigates stress intensification on the surface layer, allowing nanospheres to maintain their morphological integrity and promoting the formation of a stable solid electrolyte interphase layer during cycling. When tested using an industrial protocol, a full cell comprising a nanosphere/graphite blended anode and a lithium cobalt oxide cathode achieve an average energy density of 2440.2 Wh L^?1 (1.72 times higher than that of conventional graphite) with a capacity retention ratio of 80% after 101 cycles.     

A Statistical Model of <Emphasis Type="Italic">I</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Changes for Bending Bi-2223 Tapes

Bi-2223 tape was one of the high-temperature superconductors with commercial applications. One of the applications was bending and winding Bi-2223 tape into solenoids to produce high magnetic fields. To study bending properties, three multifilamentary of Bi-2223 tapes sheathed with silver alloys were manufactured. Bending experiments for the tapes were performed, and critical currents I _c of tapes with definite bending radius were measured. And, current transferring mechanisms in filaments were analyzed, as well. Experimental results showed that silver alloy sheathed tapes had better bending properties than pure silver-sheathed one. On the contrary, to describe bending radius dependence of I _c , a statistical model was suggested. The model expected that bending radius dependence of I _c was following an exponential law that was quantitatively expressed by mathematic expressions. Bending dependence of I _c could be calculated from the expression and calculated results agreed with experimental data very well. Therefore, the suggested model has successfully explained the experimental results.     

Optical and electrical properties of electrodeposited silver based thin films

The electrodeposited superionic conductor Ag₆I₄WO₄ was doped with various concentrations of [CrO₄]^2– to form the quarternary compound Ag₆I₄WO_4(1–xCrO_4(x). The doping level,x, was varied from 0 to 0.6 and the optimum compound was used for further analysis. X-ray diffraction (XRD) analysis indicated major peaks occurring atd values of 3.75,2.29,1.96 and 3.96 in the order of decreasing intensity. The energy dispersive analysis of X-rays (EDAX) technique verified quantitatively the ratio of the components in the solid electrolyte. From the fringes seen in the interference pattern of the transmission spectrum, the refractive index and thickness of the film was calculated. The absorption spectrum indicated the characteristic chromate peak at 310 nm when the dopant was present. An open circuit voltage (OCV) of 670 mV was observed for the fabricated cells with optimum performance at a doping level ofx=0.1, where the best discharge characteristics were observed. The subsequent conductivity was calculated to be of the order 10^–3–1 cm^–1 from the Cole-Cole plot.     

Notices

This paper presents the small sample optimum choice of the k(≤ r ₂) order statistics for the best linear unbiased estimate (BLUE) of the parameters μ and σ or σ alone (μ known) when the sample is Type II censored on the right. For n = 2(1)10, k = 1(1)r ₂ and Tr ₂ = ([.504n]+1}(1)n, the optimum ranks, the coefficients of the BLUES have been presented in Table I.     

Spin-Wave Excitations in Evaporated Co/Pt Multilayers

The magnetic properties of evaporated Co/Pt multilayers have been studied by magnetic measurements and ferromagnetic resonance (FMR). The spin-wave resonances were observed in some multilayers in FMR experiments, which implied that spin waves were sustained by the whole and propagated through Pt layers. The relation of the resonance field H _res with the mode number n obeys the so-called n ² law and the interlayer coupling strength J _I has been determined. The temperature dependence of the spontaneous magnetization can be well described by Bloch’s law, in all multilayers. The increase of the spin-wave parameter B with decreasing cobalt thickness has been discussed. A spin-wave theory has been used to explain the temperature dependence of the magnetization and the approximate values for the bulk exchange interaction J _b and surface exchange interaction J _S for various Co/Pt multilayers have been obtained.     

Continuous Deposition of YBCO Films for Coated Conductor Using PLD Method

A reel-to-reel PLD system was set up for studying YBCO coated conductor. YBCO films could be continuously grown on the CeO₂/YSZ/Y₂O₃ buffered Ni-5W tape. Some deposition parameters were investigated. XRD θ–2θ scans were employed to characterize the c-axis orientation and in-plane texture of YBCO films deposited with different laser repetition rates and tape moving speeds. We investigated the dependence of critical current I _c on laser repetition rates and tape moving speeds for YBCO films. It had been found that a-axis oriented grains appeared as YBCO layer thickness increased, which prevented the values of I _c improved.     

Influence of Additions of Water on the Quality of Filled Anodic Oxide Coatings

We investigate the influence of anion-cation composition of an aqueous medium on the etching rate in an acid electrolyte and the resistance of the insulation of a filled oxide coating. We reveal the cations and anions of the aqueous medium that increase the etching rate of the coating and decrease its insulation resistance. We establish that Al³⁺ and Mg²⁺ cations and SiO ₃ ^2- anions have a considerable effect on the quality of the filled coating. A high-quality coating is obtained if their concentrations in the aqueous medium are at most (mg/liter) 100 for Al³⁺, 700 for Mg²⁺, 800 for Cl^–, and 13 for SiO ₃ ^2- . The optimum pH of the environment is equal to 6 and the time of treatment of a coating in distilled water is equal to 60 ± 5 min.     

Characteristics of sol-gel derived PZT thin films with lead oxide cover layers and lead titanate interlayers

The sol-gel derived PbZr_0.53Ti_0.47O₃ (PZT) films were fabricated on the bare Pt/Ti/SiO₂/Si substrates or the same substrates coated by the PbTiO₃ (PT) interlayers. The post-deposition annealing temperature and time were optimized when the PbO cover layers and PbO vapour-containing atmosphere were compared with each other and adopted as the method to diminish the lead-loss problem during the high-temperature post-deposition annealing. The X-ray diffraction patterns, microstructures, and electrical properties such as relative permittivity, _r, remanent polarization, P _r, and coercive electrical field, E _c, were investigated in relation to the annealing conditions. The PZT films deposited on the bare Pt/Ti/SiO₂/Si substrates under the PbO vapour-containing atmosphere showed better electrical properties. This indicates that the PbO vapour-containing atmosphere may be the better method of lead-loss-prevention to process the lead-containing films rather than the PbO cover layer method. The electrical characteristics of the PZT films, _r=1150, a dissipation factor of 0.039, P _r=26 C cm^–2, and E _c=40.5 kV cm^–1 were measured at 1 kHz. When PZT films were deposited on substrates coated by the PT layers, PZT-PT films with single perovskite phase were derived by post-deposition annealing at 500 °C for 1 h. However, the relative electrical properties are very poor, i.e. E _r=160, P _r=2.0 C cm^–2 and E _c=75 kVcm^–1. The optimum combination for preparing PZT-PT films is a 40 nm PbTiO₃ interlayer and annealing conditions of 6 h at 550 °C in a PbO vapour-containing atmosphere; the derived films exhibit electrical properties of E _r=885, P _r=21.5 C cm^–2 and E _c=64 kV cm^–1. The combination of inserting a PT interlayer and annealing in a PbO vapour-containing atmosphere can prevent the formation of electrical short paths. In this case, nearly pin-hole-free PZT films can be grown on the PT (interlayer) /Pt/Ti/SiO₂/Si substrates. It is believed that it is possible to prepare the PZT films with nano-scale uniformity, reproducible quality, which may be worth considering for commercial applications.     

On one selected problem in mathematical modeling of superconducting cylindrical coils in case of using the Ic(B) characteristic linearized in parts

The task to design the dimensions of cylindrical superconducting coil, which generates the maximum central field within a constant overall conductor length, represents mathematically the problem to find for a solution of the constrained extremum of a function. This problem can be transformed into the task of solving a non-linear equation because the value of gradient in the position of extremum (maximum) must be zero. The optimization procedure requires inter alia to evaluate the derivative of the I_c(B) characteristic of the conductor concerned. We show that a certain problem may arise if the I_c(B) characteristic is expressed by the function linear in parts. In this particular case, the derivative of the I_c(B) is a discontinuous function consisting of constants. As a consequence, the results of optimization needn’t be correct. The aim of this study is to analyze the nature of the above problem and to propose a way to minimize it. We show that the computational error in determining the dimensions of optimum coil reduces with decreasing a distance between the I_c(B) nodal points. Moreover, the error is completely eliminated if the I_c(B) is expressed in analytical form. This effect is typical for superconducting conductors with a strong non-linearity in I_c(B), such as e.g. MgB₂. On the other hand, the effect is not applied in case of e.g. NbTi superconducting conductors, the I_c(B) of which is linear in a broad field range. In this work we only study the case of isotropic I_c(B) characteristic.