Understanding Why Grain Boundaries Limit the Critical Current Density of Fe-Based Superconductors and Exploring Ways to Increase Current Density

• Other Authors: Collantes Goicochea, Yesusa Kimberlim (author)
• Format: Others
• Language: English; English
• Published: Florida State University
• Subjects: Materials science
• Online Access: http://purl.flvc.org/fsu/fd/2019_Spring_CollantesGoicochea_fsu_0071E_14910

The main application of superconducting materials is to generate very high magnetic fields in reduced spaces i.e. built strong magnets (16 T – 100 T) for diverse applications such as nuclear magnetic resonance (NMR), magnetic resonance imaging (MRI), and particle accelerators. Since their discovery in 2008, Fe-based superconductors (FBS) have drawn attention from the technological point of view due to the interesting combination of properties that these materials possess for potential high field magnet applications. Also from the scientific community because superconductivity is a property of Fe-As layers in these compounds, yet magnetism in Fe has long been assumed to destroy superconductivity. Although, FBS have been extensively studied, it was not until 2012 that Weiss et al. demonstrated the potential of FBS for practical applications, reporting a surprisingly high critical current density (Jc) of 104 Acm-2 at 10T in untextured polycrystals. This result is considered a breakthrough because previous studies in cobalt-doped (Co-doped) BaFe2As2 bicrystals suggested that, similarly to YBa2Cu3O7-δ (YBCO), high-angle grain boundaries block supercurrent. That fact indicated that FBS would need to be textured like YBCO coated conductors in order to carry significant Jc for practical applications. YBCO coated conductors are state of the art materials for high field magnet applications. However, due to texturing the manufacturing of these materials is still very expensive ($100/Km of flat wire) reducing their usage to a small niche of applications. The unexpected high intergrain Jc was measured in potassium-doped BaFe2As2 (K-doped Ba-122) untextured round wires; the round geometry is preferred by far by magnet builders and eliminates the costs of expensive substrates needed for texturing. The high Jc in K-doped Ba-122 was associated with its having a fine grain size. However, even with the surprisingly high Jc, current transport across grain boundaries is still about a factor of 10 too low for practical applications. The main goals of this research were to understand what blocks current at grain boundaries of Ba-122, and to develop methods to increase current transport across grain boundaries to obtain a polycrystalline conductor that is closer to the application limit 105 Acm-2 at 10 T. This was done: 1) By investigating what type of impurities and other extrinsic factors are blocking Jc of Ba-122 samples; 2) By developing new protocols for cleaner synthesis process to continue raising Jc in Ba-122 compounds; 3) By studying effects of grain size on Jc; 4) By studying how different doping schemes change the electromagnetic properties of Ba-122 polycrystals. The significance of this research was to explore new ways to increase Jc in untextured Ba-122 polycrystals. I studied the impact that careful processing and chemical doping have on the microstructural, nano-structural, and superconducting properties of untextured polycrystals of Ba-122. The aim was to produce materials with clean and well connected grain boundaries that allow effective current flow. One of my contributions was to synthesize samples using the low temperature processing developed in Weiss’s study, but focusing on the elimination of oxygen and moisture absorption during synthesis to avoid oxides and hydroxides formation along the grain boundaries that blocks supercurrent. Also, I optimized the previous processing to produce even finer grain samples to raise Jc at low fields by modifying the milling process and heat treatment of the samples. Another contribution was the study of novel dopant combinations such as double doping different sites to investigate how doping alters Jc within grains and across grain boundaries in Ba-122. K-doped Ba-122 combines a very high upper critical field (> 100 T), a low anisotropy, and high intragranular Jc. And because we have shown that intergranular Jc is high in an untextured polycrystal that can be formed into a round wire, this technology could potentially displace the highly-textured YBCO coated conductors for high-field NMR magnets at 4.2 K. Coated conductors are currently considered the state of the art technology for these applications. === A Dissertation submitted to the Program in Material Science and Engineering in partial fulfillment of the requirements for the degree of Doctor of Philosophy. === Fall Semester 2018. === November 9, 2018. === Ba-122, Bulk superconductors, critical current density, grain boundaries, Superconductivity, TEM === Includes bibliographical references. === Eric Hellstrom, Professor Directing Dissertation; Munir Humayun, University Representative; David Larbalestier, Committee Member; Theo Siegrist, Committee Member; Kenneth G. Hanson, Committee Member.