Snapshot of 3D temperature distribution in YBCO tape during quench show a very high local temperature gradient. A multiscale modeling approach developed at North Carolina State University helps optimize superconductor geometry and properties. Credit: Justin Schwartz, NCSU.

Twenty-five years after their discovery, some high-temperature superconductors are starting to be used in applications. Since the beginning, attention has focused on the ytrrium barium copper oxide family of superconductors, and they are starting to be used in some systems. But, basic physics presents a few obstacles yet to overcome.

One of them is the issue of “quenching,” which is the sudden loss of the superconductive property. Quenching can be catastrophic because the sudden release of stored electricity can destroy systems. Quench protection is designed into most superconducting magnet systems, but quench onset must be detected early enough for the protection mechanism to react effectively. A difficulty yet to overcome with YBCO materials is that the “normal-zone propagation velocity” is slow, which means that the property changes that signal the start of quench, like voltage and temperature, are too small to detect.

Complicating the obstacles provided by physics alone, is the challenge facing system designers to design products based on the properties of existing materials. Device configuration can influence the quench properties, but to what extent has not been clear.

A recent paper by Wan Kan Chan and Justin Schwartz at North Carolina State University introduces a way of looking at how device geometry and material selection affect the properties of a YBCO superconductor. In the press release, Schwartz says, “This approach moves us closer to the ideal of having materials engineering become part of the product design process.”

In an earlier paper (actually “Part I” of a two-part series, with this new paper serving as Part II), the authors introduced a model to simulate quench propagation in micrometer-scale, mixed dimension commercial tapes. (“Mixed dimension” refers to the varying dimensions of the individual constituent layers in a commercial superconductor tape. A typical tape is comprised of a bottom layer, substrate, buffer layer, YBCO and lastly a silver layer, all of which is encased in a stabilizer. Each constituent contributes to the device performance and needs to be modeled accordingly.) In the first paper, the authors were able to demonstrate the validity of the model based on previously published results.

With a valid model in hand, the authors set about using it to study how device configuration and material selection affect quench behavior, that is, to understand the trade-offs in HTS magnet design.

The authors used their model to evaluate systematically seven variations of magnet design. Parameters varied include the thicknesses of the YBCO and copper stabilizer, the material used for the substrate and stabilizer and the conductivities of the silver and buffer layers.

They say in the paper that the simulations show “quench behavior is very sensitive to changes in the thickness and the electrical conductivity of the stabilizer.” Also, the normal-zone propagation velocity—a key property for quench detection—is very sensitive to YBCO thickness. The substrate material also exerts considerable influence as there is a tradeoff between quench behavior and mechanical properties.

In the conclusion the authors state, “The benefit of the approach developed here is that one can identify parameters for which relative impacts on these three key quench parameters can be optimized. … Ultimately, however, this can be incorporated into a multivariable, multiobjective design optimization process that can vary more than one parameter and also consider other aspects of magnet optimization based on the mission of the magnet in question.”

This paper practically screams “Materials Genome Initiative,” even though it was deep into the editorial process long before the MGI was announced in June. It has all the hallmark features: computer modeling and simulation, materials selection and property optimization leading to product design. Schwartz agrees, stating in the press release, “The focus of [the MGI] initiative is to expedite the process that translates new discoveries in materials science into commercial products—and I think our process is an important step in that direction.”

See: “Three-Dimensional Micrometer-Scale Modeling of Quenching in High-Aspect-Ratio YBa2Cu3O7-δ Coated Conductor Tapes-Part II: Influence of Geometric and Material Properties and Implications for Conductor Engineering and Magnet Design,” IEEE Transactions on Applied Superconductivity (doi: 10.1109/TASC.2011.2169670)

The first paper describing the model is: “Three-dimensional micrometer-scale modeling of quenching in high aspect ratio YBa2Cu3O7-δ coated conductor tapes. Part I: Model development and validation,” IEEE Transactions on Applied Superconductivity (doi: 10.1109/TASC.2010.2072956)