Jozef Janovec

AC Loss in Pancake Coil Made From 12 mm Wide REBCO Tape

The design of a superconducting coil from high-performance REBCO coated conductors is often complicated because of complex anisotropy of the critical current density Jc . It is important to understand how much detail of this feature must be taken into consideration in the prediction of maximum achievable current and the expected ac loss. We present the results of investigation performed with a small (ten turns, 60 mm inner diameter) coil made from SuperPower tape of 12 mm width. The knowledge of Jc(B,θ) determined on short sample allowed prediction of the maximum achievable current of the coil and the ac loss behavior. We have also investigated the effect of the tape nonuniformity. Our results confirm that the lateral nonuniformity when Jc at tape edges is lower than in its center leads to significant increase of ac loss. A longitudinal nonuniformity, in particular a reduction of critical current in some portion along the tape length, is hardly observable in the ac loss result. On the other hand, using a piece of tape with lower Jc in the innermost coil turn would significantly reduce the maximum current. We also present calculations showing the change in current-voltage curve and redistribution of ac dissipation in the case of nonuniform tape quality.

Non-uniformity of coated conductor tapes

Non-uniformity of superconductor properties, e.g. a critical current reduction close to the edge of a coated conductor (CC) tape could degrade its performance in some power applications. Reliable characterization of such non-uniformity and understanding of its mechanism requires investigation of the character and causes of degradation. In this paper spatial distribution of critical current density across the width of a CC tape is studied. Three different experimental methods allowing estimation of the local current density were utilized for this purpose: (i) magnetic field mapping above the tape through which a DC current is flowing, (ii) measurement of the critical current of separate strips prepared by patterning of the CC tape, and (iii) magnetization measurements of the pieces cut from various positions within the tape width. Very good agreement between the results obtained by these methods was found, showing a reduction of the critical current density at the tape edges with respect to its centre. Moreover, structural investigation by scanning electron microscopy revealed a correlation between the morphology and the critical current density across the tape width. Insertion of such real non-uniform distribution of critical current density into AC loss calculation resulted in a dramatic improvement in the agreement with experimental results. (Some figures may appear in colour only in the online journal)

Precipitation related anomalies in kinetics of phosphorus grain boundary segregation in low alloy steels

Abstract It has been shown that particles of M 6 C in Cr–Mo–V low alloy steels can influence phosphorus grain boundary segregation in a similar manner to Laves phase particles in 12CrMoV steel. The influence of these particles arises from their ability to dissolve non- metallic elements such as phosphorus and silicon. As phosphorus is progressively segregated to the grain boundaries during prolonged ageing, the precipitation of M 6 C particles leads to an anomalous decrease in the phosphorus grain boundary concentration as the ageing time is extended. This phenomenon originally observed for the Laves phase in long-term aged 12CrMoV steel at temperatures of 753, 773, and 803 K has now been observed for M 6 C carbide in a 2.5Cr–0.4Mo–0.25V low alloy steel aged at 853 K.

Effect of Grain Boundary Segregation on Mechanical Properties of P-Doped Fe-Si Base Alloys

Abstract The grain boundary segregation in two Fe-Si base alloys with the different bulk phosphorus contents was investigated. The phosphorus grain boundary concentrations determined with the use of Auger electron spectroscopy were related to the microstructure, fracture mode, and mechanical properties of the alloys. Transgranular cleavage was found to be the dominant fracture mode of the samples tested by impact at room temperature. The correlations between the phosphorus grain boundary concentration, the portion of intergranular fracture, and the dynamic fracture toughness were presented, which fulfil the theoretical expectation. Two possibilities were proposed to explain a discrepancy in the temperature dependence of the phosphorus grain boundary concentration for the alloy with the higher bulk phosphorus content. Introduction Impurity segregation and precipitation of secondary phases at the grain boundaries are the fundamental diffusion-controlled processes contributing to the intergranular embrittlement of commercial steels [1]. With intention to quantify the particular effects of the above processes, simple alloys have been investigated. In this respect the P-doped Fe-Si alloys, mostly free of precipitates, are suitable for the segregation studies [2,3]. As was described by McLean [4], the equilibrium grain boundary concentration of the impurity depends directly on its bulk concentration and the annealing temperature. The segregation kinetics can also be influenced by other factors as are grain size, type of the microstructure etc. [5]. It is generally known that small changes in the grain boundary chemistry can provide significant changes in mechanical properties of the alloy steels. This process was excellently characterised on the phenomenological level [6]. However, the precise quantification of the relationship between the grain boundary composition and the energy of intergranular fracture is still missing [7].

A Study of Phase Equilibria in the Al–Pd–Co System at 700 ∘C

Al68Pd14.6Co17.4, Al69.8Pd13.8Co16.4, Al72Pd12.8Co15.2, Al73.8Pd11.9Co14.3, and Al76Pd11Co13 alloys annealed at 700 ∘C for 2000 h were studied. In the investigation, scanning electron microscopy including energy dispersive X-ray spectroscopy and electron backscatter diffraction, X-ray diffraction, and transmission electron microscopy were used. Altogether five near-equilibrium phases (β, U, Al5Co2, e, Al9Co2) were identified. Transitions between β, U, and e phases were also determined dependent on the alloy bulk metal composition. The experimental results were used to propose the partial isothermal section of the Al–Pd–Co phase diagram at 700 ∘C. The maximum solubilities at 700 ∘C of Pd in Al9Co2 and Al5Co2 were determined as 1.7 and 2.69 at.%, respectively.

Contribution to thermodynamic description of Al-Pd system

The homogeneity range around the Al3Pd stoichiometry, representing the family of quasicrystalline approximants denoted as n and/or (Al3Pd), was involved in the calculated Al-Pd phase diagram. The calculations were performed by means of the CALPHAD method using the Thermo-Calc software. A novel thermodynamic description of the n/(Al3Pd) phase was proposed based on the (Al%,Pd)3(Al,Pd%)1 two-sublattice model. Existing thermodynamic parameters of stoichiometric phases Al4Pd and Al21Pd8 were slightly adjusted. The transition between phases n/(Al3Pd) and γ-Al21Pd8 was predicted thermodynamically at 619°C.

Microstructure Similarities between Phases F and U in Al-Pd-Co Alloys

The work is aimed at searching for compositionally variant isostructural mutations of the cubic F-phase in both as-annealed (850°C/500 h) and DTA-cooled conditions of the Al72Pd9Co19 alloy. Recently, the mutations were reported for the U-phase in the Al69.8Pd13.8Co16.4 alloy annealed at 700°C for 2000 h. In the investigation, scanning electron microscopy including energy dispersive X-ray spectroscopy, powder X-ray diffraction, and differential thermal analysis (DTA) were used. Two isostructural mutations of the F-phase, FL (the product of L-parent) and FV (the product of V-parent), were identified (symbol L stands for liquid). On DTA-cooling, the bivariant heterogeneous transition (Lpart→V), the monovariant heterogeneous transformation (Lrest→V+FL), and the monovariant quasi-eutectoid transformation (Vrest→Al5Co2+FV) were recorded.

Structural Study of Commercially Produced (RE)BCO Films

Various deposition techniques used in the coated conductor industry result in the formation of different structural inhomogeneities within the superconducting film, having an impact on its electromagnetic properties. In this paper, an attempt for complex characterization of the Y(Gd)Ba2Cu3O7-x [(RE)BCO] structure in tapes from two major producers was performed. Scanning and transmission electron microscopy techniques were used for the characterization of the fine structure of (RE)BCO thin films in the cross-sectional and planar views. The examination was focused on structural inhomogeneities, e.g., outgrowths, precipitates, stacking faults, nanocolumns, and their mutual correlations. The structural study was correlated to critical current dependence on magnetic field and temperature, where the effect of particular structural inhomogeneities on functional properties was evaluated.

Joining of CC Tapes With Lead-Free Solders

Overlap joints of the SuperPower 2G HTS tape SCS12050 with surrounding copper stabilizer were prepared, in controlled conditions, using a lead-free solder and two types of fluxes in air atmosphere. A clamping device was developed, in order to control the evolution of temperature during the process. The reflow time (during which the solder melting is expected) and the pressure exerted on the joint area were utilized as parameters in the optimization of the soldering procedure. The melting point of the used Sn96.5Ag3Cu0.5 alloy was 217 °C; soldering temperatures slightly exceeded the latter temperature. Quality of the joint was evaluated by measurement of the voltage-current curve, from which the joint resistance, critical current, and n-value were determined. Resilience against temperature cycling was tested on one of the samples. Electrical characterization is complemented by structural investigation with scanning electron microscopy, which was focused on the microstructure of the soldered joint layer. Surprisingly, the best joints were obtained at the lowest pressures used in these experiments. With regard to the reflow times and used fluxes, a slight reduction in the joint resistance was observed at longer duration of melting conditions and using rosin flux.

INHOMOGENEITIES IN THE STRUCTURE OF HIGH TEMPERATURE SUPERCONDUCTING LAYER

Abstract Superconducting layer in coated conductor was investigated to assess the inhomogeneities present in its structure and to get better insight into the effect of inhomogeneities on degradation of electric properties. In the investigation scanning electron microscopy, energy-dispersive X-ray spectroscopy, electron backscattered diffraction, and laser scanning confocal microscopy were used. The results obtained showed good correlation between the density of inhomogeneities across the tape width and the degradation of current transport properties determined by measurements of the current density