High-speed fluorescent thermal imaging of quench propagation in high temperature superconductor tapes

Fluorescent Microthermographic Imaging, a method using rare-earth fluorescent coatings with temperature-dependent light emission, was used for quench investigation in high temperature superconductors (HTS). A fluorophore was embedded in a polymer matrix and used as a coating on top of an HTS tape, while being excited with UV light and recorded with a high-speed camera. Simultaneously, the tape was pulsed with high amplitude, short duration DC current, and brought to quench with the help of a localized defect. The joule heating during a quench influences the fluorescent light intensity emitted from the coating, and by recording the local variations in this intensity over time, the heating of the tape can be visualized and the developed temperatures can be calculated. In this paper, the fluorophore Europium tris[3-(trifluoromethylhydroxymethylene)- (+)-camphorate] (EuTFC) provided sufficient temperature sensitivity and a usable temperature range from 77 K to 260 K. With the help of high-speed recordings, the normal zone development was imaged in a 20 \mu m copper stabilized HTS tape held in a liquid nitrogen bath, and using a calibration curve, the temperatures reached during the quench have been calculated

Efficient modeling of high temperature superconductors surrounded by magnetic components using a reduced H-ϕ\phi formulation

Although the H-formulation has proven to be one of the most versatile formulations used to accurately model superconductors in the finite element method, the use of vector dependent variables in non-conducting regions leads to unnecessarily long computation times. Additionally, in some applications of interest, the combination of multiple magnetic components interacting with superconducting bulks and/or tapes leads to large domains of simulation. In this work, we separate the magnetic field into a source and reaction field and use the H-$\phi$ formulation to efficiently simulate a superconductor surrounded by magnetic bodies. We model a superconducting cube between a pair of Helmholtz coils and a permanent magnet levitating above a superconducting pellet. In both cases, we find excellent agreement with the H-formulation, while the computation times are reduced by factors of nearly three and four in 2-D and 3-D, respectively. Finally, we show that the H-$\phi$ formulation is more accurate and efficient than the H-A formulation in 2-D

Concepts of Static vs. Dynamic Current Transfer Length in 2G HTS coated conductors with a Current Flow Diverter Architecture

This paper uses both experimental and numerical approaches to revisit the concept of current transfer length (CTL) in second-generation high-temperature superconductor coated conductors with a current flow diverter (CFD) architecture. The CFD architecture has been implemented on eight commercial coated conductors samples from THEVA. In order to measure the 2-D current distribution in the silver stabilizer layer of the samples, we first used a custom-made array of 120 voltage taps to measure the surface potential distribution. Then, the so-called "static" CTL ($\lambda_s$) was extracted using a semi-analytical model that fitted well the experimental data. As defined in this paper, the static CTL on a 2-D domain is a generalization of the definition commonly used in literature. In addition, we used a 3-D finite element model to simulate the normal zone propagation in our CFD samples, in order to quantify their "dynamic" CTL ($\lambda_d$), a new concept introduced in this paper and defined as the CTL observed during the propagation of a quenched region. The results show that, for a CFD architecture, $\lambda_d$ is always larger than $\lambda_s$, whereas $\lambda_d = \lambda_s$ when the interfacial resistance between the stabilizer and the superconductor layers is the same everywhere. We proved that the cause of these different behaviors is related to the shape of the normal zone, which is curved for the CFD architecture, and rectangular otherwise. Finally, we showed that the NZPV is proportional to $\lambda_d$, not with $\lambda_s$, which suggests that the dynamic CTL $\lambda_d$ is the most general definition of the CTL and should always be used when current crowding and non-uniform heat generation occurs around a normal zone.Comment: 11 pages, 10 figure

Electric field induced alignment of multiwalled carbon nanotubes in polymers and multiscale composites

Carbon fiber reinforced polymers (CFRPs) have a highly anisotropic electrical resistivity, which limits their use in electrical applications. In this contribution, an electric field was used to align multiwalled carbon nanotubes (MWCNTs) to create preferential conductive pathways within a nanocomposite and a multiscale composite in order to reduce their resistivity. Investigation on epoxy containing MWCNTs have shown that an electric field of 40 V mm21 or higher applied for 2 h can lead to a reduction of the resistivity parallel to the field up to four orders of magnitude with only 0.01 wt-% loading. In the case of CFRPs reinforced with 0.01 and 0.1 wt-% of MWCNTs, we observed reductions of the through the thickness resistivity of 36 and 99% respectively, when an electric field of 60 V mm21 was applied for 2 h during the fabrication of the samples

Improved Method for Determining the n-Value of HTS Bulks

International audienceThe complete penetration magnetic field Bp is the main feature of a superconducting pellet submitted to an axial applied magnetic field. The electric E-J characteristics of HTS bulk is generally described by a power law E(J) = Ec(J/Jc)^n. The influence of the n-value and applied magnetic field rise rate Vb on the Bp of a HTS cylindrical pellet has been presented in a previous paper. The numerical results presented come from numerical resolution of a non linear diffusion problem. With the help of these simulations a linear relationship between Bp, ln Vb and n-value has been deduced. This comparison allows determining the critical current density Jc and the n-value of the power law based on direct measurement of Bp in the gap between two bulk HTS pellets. In this paper, an improvement of this method is presented. The influence of geometric parameters R and L is studied to give generality to the relationship between Bp, Vb and n-value. Previous Bp formula is confirmed by these new simulations. To correctly connect simulation and experimental results, the influence of spacing e between bulks is studied and presented. A relationship between Bp and measured complete penetration magnetic field Bpm is determined

Magneto-thermal finite element modeling of 2nd generation HTS for FCL design purposes

Coated conductors are very promising for the design of novel and e±cient Fault Current Limiter (FCL). However, before considering using them in a power grid, their thermal and electromagnetic behaviors in the presence of over-critical currents need to be investigated in details. In this context, we performed ¯nite element magneto-thermal modeling of coated conductors under over-critical current on several geometries. Accordingly, we have investigated the substrate electrical connectivity and thermal properties on the HTS-FCL behavior. All simulations were performed using in COMSOL Multiphysics, a commercial finite element package, which has a built-in coupling between the thermal and electrical equations, allowing us to compute both quantities simultaneously during the solving process. Our simulations allowed us to formulate thresholds for the current density usable in coated HTS as well as limitation capability of a device made of these new conductors

2D Magneto-Thermal Modeling of Coated High-Temperature Superconductors

Thin films are very promising for the design of novel and efficient Fault Current Limiter (FCL) made of high-temperature superconductors (HTS). However, their thermal and highly non-linear electromagnetic behavior in the presence of over-critical currents is crucial to their use in future electrical grid. In this paper, we propose a numerical approach to solve magneto-thermal models for geometries of high aspect ratio. The used electromagnetic formulation, inspired in good part from the work of Brambilla et al., were performed in 2D using COMSOL Multiphysics. Also, the thermal part of our model has been implemented to consider the thermal exchange with the nitrogen bath needed for HTS-FCL operation. Our simulations allow us to observe design limitations of FCL made of these new shaped conductors

Etching the oxide barrier of micrometer-scale self-organized porous anodic alumina membranes

We develop a quantitative model to calculate the optimal experimental conditions for the etching of the oxide barrier of porous anodic alumina (PAA) membranes. The method is applied to a membrane fabricated at 370 V in a solution of 2% citric acid. The process creates a network of small pores at the bottom of the larger pores, which accelerates the oxide barrier etching relatively to the pore walls of the PAA membranes, when etched in a solution of phosphoric acid. The oxide barrier etching is confirmed by observation of PAA membranes using scanning electron microscopy, revealing the formation of the small pores and the preferential etching of the bottom of the pores rather than the pore walls. The proposed method, which leads to a better control over the fabrication of nanoporous templates, can be adapted to oxide barriers of different PAA membranes formed at different voltages and in different acids