Quench propagation in commercial REBCO composite tapes

Highlights

• Measurement of quench propagation velocity in commercial tapes.

• Comparison to analytical model keeping into account minimum propagation current.

• Quench velocity behaviour is dependent on applied magnetic field and temperature.

Abstract

The Normal Zone Propagation Velocity, NZPV, is a widely studied feature in superconducting composite tapes for its technological implications. The stability features and the NZPV values, in different operative conditions, have been investigated in different second generation high critical temperature superconductor tapes. In particular, we focused our studies on tapes, produced by the manufactures SuNAM© and Superpower©. Experimental procedures on the determination of the NZPV, related problems and peculiarities, have been presented and discussed. Data obtained with our experimental configuration, have been analyzed and compared with a phenomenological model.

Introduction

After more than three decades of research spent on the realization of reliable high critical temperature superconductor (HTS) wires and composite tapes, the availability of kilometer long tapes has focused attention on power devices based on this superconducting material. The main technological potential applications of HTS range from high field magnets, motors, generators, energy storage, fault current limiters as well as transmission cables [1], [2], [3], [4].

Whereas superconducting properties of low critical temperature superconductor (LTS) are well known and the technological application of superconducting devices based on LTS are widely used [5], [6], a better understanding of HTS composite tapes/cables properties is still needed.

Since 1990, reliable first generation (1G) HTS wires, based on Bismuth Strontium Calcium Copper Oxide (BSCCO) compound, have been commercially available. Nowadays, most manufacturers are also producing Rare-Earth Barium Copper Oxide (REBCO) based on rare earths such as Gadolinium or Yttrium, known as second generation (2G) HTS, or coated conductors. Wires and tapes made by these HTS materials, especially REBCO composites, are recognized as superior superconductors for their attracting potential at higher cryogenic temperatures (liquid Nitrogen - LN2 - operation) and better performances than LTS at low temperature (liquid Helium - LHe - operation) when in presence of magnetic field.

The production process of 1G-HTS tapes has some similarities with the production processes of LTS wires. The powdered superconducting material fills silver alloy pipes and undergoes several swaging cycles in order to realize thin extruded pipes. These pipes are then bundled together into metallic sheaths and squeezed into tapes or wires [7]. The 2G-HTS composite tapes, on the other side, require a totally different fabrication approach. In fact, in REBCO compounds the high crystalline alignment, necessary to achieve high performances, can only be obtained by epitaxial growth process. The solution adopted by manufacturers (and in particular by SuperPower© and SuNAM©, the manufacturers of composites under our investigation) is the creation of a composite tape consisting on the bi-axially oriented thin layer of the ceramic superconductor, epitaxially grown on a flexible substrate. For both manufacturers, the choice for substrate fell on the C276 Hastelloy, a steel alloy with suitable mechanical properties [8]. Unfortunately, REBCO film cannot be grown directly on Hastelloy but one or more matching buffer layers is required. The role of the buffer layer architecture is to avoid detrimental contaminations of the REBCO layer and to provide a better template for film epitaxial growth [9], [10]. Several kinds of compounds may be adopted as buffer layers. In investigated tapes Al₂O₃, Y₂O₃ and MgO have been adopted. Typically, the superconducting film is capped by a thin protective silver layer and then finished by copper layers surrounding the whole tape. The role of copper is to compensate the poor thermal and normal state electrical properties of the REBCO, acting as a stabilizing layer and reducing the possibility of a tape burn-out. From the thermal point of view, the copper acts as a thermal reservoir and, thanks to its high thermal conductivity, it also protects the tape from temperature spikes. From the electrical point of view, the copper layer protects the composite tape by offering an alternative path to the current flow [1], [2]. A sketch of the layered tape is reported in Fig. 1, while further details on the composite architecture, materials, and manufacturing properties are summarized in Table I as reported in manufacturers web pages [9], [10].

We focused our attention on the behavior of 2G-HTS tapes when a rapid transition from the superconducting to the normal state, known as the quench phenomenon, arises. Quench in HTS composite must be fully understood for tape and wire applications since the high current densities flowing into superconductors, after transition to normal state, can lead to permanent damages of the device. Thermal, mechanical, electromagnetic stresses can release energy in a certain area of the superconductor, leading to a local transition to the normal state. Under certain conditions, discussed later in the paper, this normal spot can enlarge with well-defined velocity called normal zone propagation velocity (NZPV). The knowledge of the normal zone propagation (NZP) dynamic along the superconducting wire is mandatory to proper design quench protection systems. This dynamic is well known in LTS wires operating at LHe temperatures, however for 2G HTS tapes, due to their complex architecture and anisotropy, the quench phenomenon exhibits a different behavior compared to LTS and it is still under investigation [11], [12]. Generally, the NZPV has been found to be much lower in HTS tapes with respect to LTS wires, thus the dissipations in case of quench are much more localized. This behavior exposes HTS based devices to a higher risk of a destructive quench since heat does not distributes along the superconductor.

This paper is devoted to the measurement of the NZPV aiming at describing NZP dynamic for tape application and protection system design. In particular, we investigate the NPZV in HTS tapes at temperatures higher than 37 K and applied magnetic field up to 12 T. Special care will be devoted to point out the experimental problems that are generally not considered in the analytical models and that can affect measurements, leading to non-correct calculation for applications.

The content of the paper is arranged as follows: an introduction on quench modeling is briefly presented in section 2; experimental setup, measurement procedures as well as technical aspects concerning NZPV measurements are treated in Section 3; in Section 4, preliminary measurements and characterization of tape samples are reported, useful for NZPV calculations; in Section 5, obtained NZPV values as a function of the temperature and bias current for various values of the applied magnetic field are presented and discussed; finally, conclusions are summarized in section 6.