Jc Degradation Research Articles

Methods of introduction of MgO nanoparticles into Bi-2212/Ag tapes

The effect of addition of ultra-fine MgO particles, obtained by Mg combustion, in Bi-2212/Ag tapes was investigated. Bi-2212/Ag tapes were prepared by coating and partial melt processing. Four different methods of MgO particles embedment were applied in green tape preparation; settlement of MgO in Bi-2212, pre-mixing of MgO in the solvent, deposition of the MgO into the solvent, and deposition of MgO onto the surface of Bi-2212. Microstructural studies of processed tapes show that uniformity of MgO particles distribution in Bi-2212 matrix depends on the methods of tape preparation.MgO particles, introduced in Bi-2212 matrix by all applied methods, significantly enhanced transport properties of the tapes. The Jc degradation with the increase of magnetic field and/or temperature in all doped samples was significantly lower compared to un-doped Bi-2212 samples. The best improvement was achieved when MgO particles were introduced into Bi-2212 matrix by settlement method of the tape coating. The improvement also depends on a magnitude of the magnetic field and operating temperatures.

Thermal resistance degradation of surface mounted power devices during thermal cycling

Backside scanning acoustic tomography (SCAT) images have been correlated to alloy morphology (cross-section) and composition data (stoichiometry) to model the Θ JC degradation for surface mounted device packaged power ICs as a function of the temperature cycling range. We find that an appropriate setting of the die attach process can suppress needle-shaped Cu 3Sn in favor of roughly spheroid Cu 6Sn 5. We derived, from the degradation of the Θ JC during thermal cycling stress tests with different temperature swings, an acceleration factor with the use of the Coffin–Manson law. The fit parameter q in this formula is 9.3 for the new improved setting of the die attach process when the high SO power (HSOP) package is used. The moisture sensitivity level has no significant influence. Finally, a maximum Θ JC degradation of 0.33 K/W based on the normal distribution approach results in a minimum lifetime of 12 years. When a customer requires a maximum Θ JC of 2.0 K/W at the end of life, 50 years can be guaranteed.

Characterization of Incoloy 908 and Avoidance of Stress Accelerated Grain Boundary Oxidation (SAGBO) during ITER Model Coil Fabrication

Incoloy 908, an iron-nickel base superalloy that was developed as a Nb3Sn jacket material for Cable-In-Conduit Conductors, has been selected as the jacket material for the International Thermonuclear Experimental Reactor (ITER) Toroidal Field (TF) and Central Solenoid (CS) coils. It has a coefficient of expansion matching Nb3Sn (to minimise Jc and Tc degradation due to differential contraction after the reaction heat treatment). The alloy exhibits a characteristic of iron-nickel base superalloys: oxygen embrittlement along grain boundaries as a result of heating in an oxygen atmosphere when tensile surface stresses are present. For applications using Incoloy 908, techniques are required to control levels of either oxygen or tensile surface stresses during heat treatment. R & D results performed to develop and qualify such techniques for industrial applications are presented. The work has concentrated on establishing the lowest achievable oxygen levels inside the cable space during the reaction heat treatment and determining the conditions that can be tolerated inside and outside the jacket before SAGBO occurs. The results were applied in the ITER Model Coil programmes, in which about 5.5 km of conductor have been successfully heat treated.

The effect of deformation regimes on superconducting properties and microstructure of Bi-2223 tapes

The rolling process of Bi-2223 tape conductors was optimized through the regressive analysis of experimental data. This procedure ensures the effective densification of the powder precursor in the core and homogeneous distribution of powder density in conductor cross section, thus shortening the rolling process. The Jc values were realized for the monofilament and 19-filament conductors processed by "powder in tube" method with one step intermediate rolling such as 3.1/spl times/10/sup 4/ A/cm/sup 2/ (77 K, OT) and 2.6/spl times/10/sup 4/ A/cm/sup 2/ (77 K, OT) respectively. Characteristics of single pancake coils are presented fabricated from 19-filament conductor: no Jc degradation was observed after 10 steps of thermocycling in the range between 77 and 293 K.

Measurement of HbA 1c by pharmacia FPLC: An inter-hospital comparison

For most applications of high-temperature superconducting tapes, a high critical current (Ic) and critical current density (Jc) under a magnetic field are both required. The simplest way to achieve high Ic is to increase the thickness of the superconducting layer. However, doing so also reduces Jc, which is called the thickness effect. In this work, to inhibit the thickness effect, a series of Y0.5Gd0.5Ba2Cu3O7-σ (YGBCO) films with different thicknesses (240–1200 nm) were deposited by multi-step pulsed laser deposition on cerium oxide (CeO2) buffered substrates. The thickness dependence of the in-field Jc was systematically investigated. The in-field Ic at 77 K and 0–0.5 T was measured using the four-probe method, the surface morphology was observed using scanning electron microscopy, the structure and texture were measured using X-ray diffraction, and the internal residual stress was evaluated using the Williamson–Hall method. It was found that Jc first increased and then decreased with increasing YGBCO film thickness. The 480-nm YGBCO film had the highest Jc of 4.33 MA/cm2 at 77 K and a self-field>1.5 MA/cm2 under a magnetic field of 0.5 T. A 720-nm YGBCO film showed the slowest rate of Jc degradation under a magnetic field. The experimental results are discussed by considering the deposition process and growth model.

Influence of Ag-Au and Ag-Cu alloys on Bi2Sr2CaCu2Ox superconductor

The influence of Ag-Au and Ag-Cu alloy substrates on the microstructure and superconducting properties of Bi2Sr2CaCu2Ox (Bi-2212) was studied. The critical current density (Jc) of Bi-2212/(Ag-Au) was not degraded by increasing the Au content of the Ag-Au alloy, while the Jc of Bi-2212/(Ag-Cu) was drastically degraded by increasing the Cu content of the Ag-Cu alloy. The microstructure of Bi-2212 on the Ag-Au alloy was uniform and no precipitates were observed. The Jc degradation of Bi-2212 in contact with the Ag-Cu alloy was understood by the fact that Cu absorption of Bi-2212 from the Ag-Cu alloys changed the composition of the oxide layer. Ag-Au alloys are superior substrate materials for Bi-2212 for power current leads for superconducting magnets, taking into account their low thermal conductivity.