Superconducting Materials for Electric Machines

Classes of Materials. Superconductors divide themselves into two major groups: the low-Tc and the high-Tc materials. The low-Tc superconductors (LTSs), the earlier ones, are industrial materials with high performance in terms of current capacity under high fields (1 T—5 T, Fig. 2). However, they operate in general only at temperatures near 4.2 K, which require a complex but well-controlled cryogenic system. The most common low-Tc superconducting multifilamentary composites use niobium titanium (NbTi) with typical cost of about 1 $/kAm to 2 $/kAm. This figure of merit, the cost of 1 m of wire carrying 1 kA, enables comparisons between materials. The compound niobium tin (Nb3Sn) is only used for very high-field applications (> 8 T) and is less used for electric machines. Its use is more complicated than that of NbTi, due to the long thermal treatments required after winding it, and its cost is higher (5 $/kAm to 10 $/kA m).

High-Tc superconductors (HTSs), assuming similar costs and performance to those of NbTi, will lead to a reduction in cryogenic costs (capital and especially operating costs), but the main advantage is the improvement in the stability of the superconducting state, which leads to higher reliability. At 20 K the specific heats are 200 times higher than at 4 K. The specific heat, being the amount of heat input necessary to raise the temperature, represents the materials inherent brake on temperature rise. HTSs are thus less sensitive to thermal disturbances.

Even with the large research effort focused on HTSs, these materials have not yet achieved the state of development stage of NbTi. HTSs are very complex anisotropic ceramic materials, difficult to fabricate in a conventional wire or cable. Intrinsically brittle, they are sensitive to mechanical stresses, and their transport properties under fields are still much poorer than those of NbTi (Fig. 2), except for highly oriented, essentially epitaxial films. They are also very expensive materials, and the current price is the main barrier to their
economic development. Their cost must be lowered to 10 $/kA m to be competitive (4). At present it is nearly 50 times higher.

There are two main routes to fabricate HTS wires. The more advanced one is the (powder-in-tube) (PIT) technique (5,6) based on bismuth-compound filaments embedded in a silver or silver alloy matrix (Fig. 2). Lengths of Bi-PIT tapes as long as 1 km are produced routinely by several companies throughout the world, and their typical critical current densities are shown in Fig. 2. Still higher critical current densities are obtained on small samples (Jc = 760 MA/m2 at 77 K, 0 T; Je & 250 MA/m2). Some specialists think nevertheless that the limits have almost been reached. The pure silver matrix unfortunately is not suitable for ac applications, due to the high ac coupling losses, and new PIT wires are under development for ac applications (5) (silver alloys, resistive barriers, etc.).

The second route consists of so-called coated conductors (7) and has much potential. Yttrium compounds are deposited in thick films (a few micrometers) on industrial flexible textured metallic substrates through a buffer layer. Very good performance has been obtained with these coated conductors, but only for short lengths. The engineering current density (overall current density including substrate) is large in liquid nitrogen (on the order of 200 megamperes per square meter at 77 K at present), and its decrease under field is small. A lot of difficulties must be overcome to fabricate long, high-performance coated conductors, and there is now no low-cost industrial deposition technique. High quality Y superconductor bulk pellets, up to 100 mm in diameter (8), have been processed, and they can be used in some special machines (hysteresis, reluctance, trapped-field, etc.).

(Fig. 3). As shown in Fig. 3, this minimum work increases rapidly at low temperatures. In order to take into account the real cycle and the imperfections of the thermodynamic transformations, this ratio should be divided by the efficiency factor of the refrigeration system:

Ac losses. One of the most spectacular properties of a superconductor is its absence of resistive losses. This is true, however, only for non-time-varying electromagnetic quantities (dc conditions). As soon as the magnetic induction varies with respect to time, ac losses appear in superconducting wires. The magnetic induction can be external or due to the current in the wire (self-field). The ac losses have two main consequences. On the one hand, they induce a temperature rise in the superconductor. Since the temperature margin is very small (< 1 K) for low-Tc materials (NbTi for example) such a rise can easily quench the superconducting coil, that is, destroy its superconductivity. On the other hand, ac losses are very expensive energetically, since they are dissipated at low temperatures. They therefore greatly reduce the advantage of using superconductors. From the second law of thermodynamics, the removal of energy at a cold temperature (Tc), requires work at a high temperature (T0), usually room temperature. For an ideal closed cycle the ratio of the minimum required work (Wmin) at T0 to the energy (Q) to be removed at Tc is given by Carnot’s expression

This depends mainly on the cold power and little on the cold temperature (Fig. 3, Ref. 9).

The ratio Wmin/Q in real conditions [Eq. (3)] is called the specific work, and its reciprocal the coefficient of performance. To calculate the cost of refrigeration, the losses at low temperature must be multiplied by the specific work. For an efficiency factor of 10%, it amounts to 740 W/W and 29 W/W for cold temperatures of 4

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K and 77 K, respectively. These two figures illustrate the advantage of operating at high temperatures from a cryogenic point of view and again underlines the interest in using HTSs. The ac loss cost is especially high for LTSs, and it must be reduced to an ultralow level for the system efficiency to be acceptable.

A simple way to understand the ac losses is to consider the Maxwell-Faraday law (curl E = — dB/dt). This shows that an electric field appears as soon as the magnetic induction varies with time. The induced electric field associated with a current density (transport current or persistent currents) results in losses. The losses per unit volume are the scalar product of these two vectors.

If it is not possible to suppress the ac losses, it is possible to reduce them by a suitable multifilament structure. This will depend on the field configuration (self-field, transverse or axial field), but ultralow-ac-loss superconducting strands are generally achieved with very fine twisted filaments embedded in a high-resistance matrix or with resistive barriers between filaments. The strand diameter should be low as well. Ac NbTi wires have small (< 0.2 mm) elementary strands with hundreds of thousands of filaments (< 0.2 ^m) in a CuNi resistive (0.4 /xQ-m) matrix (Fig. 2). The first NbTi low-ac-loss composites were developed only in the eighties when the technology for fine filament fabrication was sufficiently developed (10). Those strands have greatly extended the potential range for superconductivity (11). For high-Tc materials the requirements are less severe, since the cost of removing the ac-loss heat is reduced (29 W/W at 77 K compared to 740 W/W at 4 K). Nevertheless, no oxide superconducting tape actually fulfils them with present HTS wire technology.

The ac losses explain why superconducting devices are confined to applications with dc current and without or with time-varying fields, but in the latter case, protected from them.

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