High-field thermal transport properties of REBCO coated conductors

The use of REBCO coated conductors is envisaged for many applications, extending from power cables to high-field magnets. Whatever the case, thermal properties of REBCO tapes play a key role for the stability of superconducting devices. In this work, we present the first study on the longitudinal thermal conductivity ($\kappa$) of REBCO coated conductors in magnetic fields up to 19 T applied both parallelly and perpendicularly to the thermal-current direction. Copper-stabilized tapes from six industrial manufacturers have been investigated. We show that zero-field $\kappa$ of coated conductors can be calculated with an accuracy of $\pm 15$ from the residual resistivity ratio of the stabilizer and the Cu/non-Cu ratio. Measurements performed at high fields have allowed us to evaluate the consistency of the procedures generally used for estimating in-field $\kappa$ in the framework of the Wiedemann-Franz law from an electrical characterization of the materials. In-field data are intended to provide primary ingredients for the thermal stability analysis of high-temperature superconductor-based magnets.Comment: Accepted for publication in Superconductor Science and Technolog

Thermal Conductivity of Industrial Nb3Sn Wires Fabricated by Various Techniques

We have developed a new experimental setup specifically designed for measuring thermal conductivity on technical superconductors in the range of temperatures from 3 to 330 K in magnetic fields up to 21 T. Three Nb3Sn wires, produced by the powder in tube technique, the bronze route and the internal tin restacked rod process, respectively, have been investigated. We show that, due to the complexity of the architecture of these wires, direct measurement of thermal conductivity is required for a correct estimation of thermal stability in superconducting magnets.Comment: Accepted for publication in IEEE ASC 2012 Special Issu

An equation for the quench propagation velocity valid for high field magnet use of REBCO coated conductors

Based on a study of the thermophysical properties, we derived a practical formula for the normal zone propagation velocity appropriate for REBa$_2$Cu$_3$O$_{7-x}$ coated conductors in high magnetic fields. An analytical expression to evaluate the current sharing temperature as a function of the operating conditions is also proposed. The presented study has allowed us to account for experimental results not fully understood in the framework of the models widely used in the literature. In particular, we provided a fundamental understanding of the experimental evidence that the normal zone propagation velocity in REBa$_2$Cu$_3$O$_{7-x}$ coated conductors can be mainly determined by the operating current, regardless of the applied field and temperature

Thermal conductivity and stability of commercial MgB2_2 conductors

This paper presents a study of the thermal transport properties of MgB$_2$ tapes differing in architecture, stabilization and constituent materials. The temperature and field dependence of thermal conductivity, $\kappa(T,B)$, was investigated both along the conductor and in the direction perpendicular to the tape. These data provide fundamental input parameters to describe the 3D heat diffusion process in a winding. Thermal transport properties - even in field - are typically deduced using semi-empirical formulas based on the residual resistivity ratio of the stabilizer measured in absence of magnetic field. The accuracy of these procedures was evaluated comparing the calculated $\kappa$ values with the measured ones. Based on the experimental thermal conduction properties $\kappa(T,B)$ and critical current surface $J_C(T,B)$ we determined the dependence of minimum quench energy and normal zone propagation velocity on the operating parameters of the conductor. The correlation between thermal properties and tape layout allowed us to provide information on how to optimize the thermal stability of MgB$_2$ conductors.Comment: Accepted for publication in Superconductor Science and Technolog

Transverse Thermal Conductivity of REBCO Coated Conductors

REBCO coated conductors (CCs) have the potential to widen considerably the application areas of superconductivity. Quench protection of REBCO-based devices represents one of the major obstacles to this expansion. Thermal conductivity data are mandatory ingredients for quench simulation studies. In particular, the transverse thermal conductivity ($\kappa_T$) of the CC plays a key role in case of quenches in superconducting magnets. In the present work, we report $\kappa_T$ of thermally stabilized REBCO CCs produced by different manufacturers. We have measured $\kappa_T$ on single CCs rather than on stacks of soldered tapes, this excludes spurious contributions in the measurements. We have found that the presence of the stabilizer at the tape edges in Cu-electroplated CCs increases the overall $\kappa_T$. In the absence of this effect, the overall thermal resistance is dominated by the substrate.Comment: ASC Charlotte 2014, IEEE Transactions on Applied Superconductivity Vol. 25 Issue 3 JUNE 201

Temperature and time scaling of the peak-effect vortex configuration in FeTe0.7_{0.7}Se0.3_{0.3}

An extensive study of the magnetic properties of FeTe$_{0.7}$Se$_{0.3}$ crystals in the superconducting state is presented. We show that weak collective pinning, originating from spatial variations of the charge carrier mean free path ($\delta l$ pinning), rules in this superconductor. Our results are compatible with the nanoscale phase separation observed on this compound and indicate that in spite of the chemical inhomogeneity spatial fluctuations of the critical temperature are not important for pinning. A power law dependence of the magnetization vs time, generally interpreted as signature of single vortex creep regime, is observed in magnetic fields up to $8 ~ T$. For magnetic fields applied along the c axis of the crystal the magnetization curves exhibit a clear peak effect whose position shifts when varying the temperature, following the same dependence as observed in YBa$_2$Cu$_3$O$_{7-\delta}$. The time and temperature dependence of the peak position has been investigated. We observe that the occurrence of the peak at a given magnetic field determines a specific vortex configuration that is independent on the temperature. This result indicates that the influence of the temperature on the vortex-vortex and vortex-defect interactions leading to the peak effect in FeTe$_{0.7}$Se$_{0.3}$ is negligible in the explored range of temperatures.Comment: Accepted for publication in Phys. Rev.