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November 26th, 2012

Harnessing strain to self-assemble nanocomposite superconductor films

Published on November 26th, 2012 | By: Eileen De Guire

Spontaneous self-assembly of a multication nanophase in another multication matrix phase is a promising bottom-up approach to fabricate novel, nanocomposite structures for a range of applications. An ORNL team reports on experimental and theoretical studies on the self-assembly of insulating BaZrO3 nanodots within rare earth barium cuprate superconducting films. Credit: Goyal; Wiley.

It is not easy to keep up with the rapid pace of discovery on the nanofrontier, and that certainly includes the fast pace with which the science of interfaces is advancing.

For example, in a recent new paper, an Oak Ridge National Laboratory team led by ACerS Fellow Amit Goyal, reported on a self-assembly approach to synthesizing multication, nanophase superconducting film composites.

Nanocomposite films, like all composites, offer a palette of properties that the constituent materials cannot. In this case, Goyal’s team was investigating nanocomposite films based on rare earth barium cuprates with the general composition of REBa2Cu3O7-δ (REBCO) for high temperature superconducting applications. Based on previous work, they knew that incorporating self-assembled stacks of BaZrO3 (BZO) nanodots or nanorods, which are insulating, could enhance vortex pinning and increase the critical current density, Jc.

What was missing was an understanding of how the thermodynamics and kinetics of the self-assembly process control the strain-driven self-assembly process. In a press release, Goyal says, “We found that a strain field that develops around the embedded nanodots and nanorods is a key driving force in the self-assembly. By tuning the strain field, the nanodefects self-assembled within the superconducting film and included defects aligned in both vertical and horizontal directions.”

Using combined experimental and modeling techniques, the group found that self-assembly of BZO nanodots/nanorods in a REBCO matrix occurs by simultaneous phase separation and strain ordering during the pulsed laser deposition process. Under the right conditions, BZO self-assembles into an oriented structure aligned with REBCO’s c-axis. The group found that strains could be controlled, or tuned, which meant they could control the spatial ordering of the nanophase. These engineered, nanophase “defects” are effective vortex-pinning sites, which improves the critical current density when a high magnetic field is applied parallel to the orientation of the BZO phase.

The self-assembly process is similar to the heteroepitaxial ordering observed in semiconductor systems, where columnar nanostructured features form in response to local lattice mismatch strain fields. However, kinetics drives several important differences in the REBCO-BZO system. The phase separation and vertical alignment of the nanodots/nanorods during self-assembly occur simultaneously because of the materials are ablated simultaneously, and the system achieves an equilibrium feature size, even though “the equilibrium state in sequential deposition in semiconductor quantum dots is an infinitely long wavelength.” Also, relatively more strain is needed to provide the kinetic energy that drives self-assembly, but within a specific range. If the lattice mismatch is too small to create enough strain, randomly oriented epitaxial nanoparticles form. If the strain is too much, randomly oriented nonepitaxial nanoparticles form. The ideal lattice mismatch appears to be 5-12 percent.

According to the paper, other possible applications of self-assembled nanocomposite films include multiferroics, magnetoelectrics, thermoelectrics and ultrahigh density data storage.

The paper is “Self-assembly of nanostructured, complex, multication films via spontaneous phase separation and strain-driven ordering,” S.H. Wee, et al., Advanced Functional Materials, doi:10-2002/adfm.201202101.

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