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Michael SumptionAdjunct ProfessorPh.D., Ohio University, 1992 Tel. (614) 688-3684 Office: 394 Watts Hall
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Mike Sumption joined the MSE department at the Ohio State University in 1995. Previously he had worked in the Advanced Materials Department and then later the Engineering Mechanics Department of Battelle Memorial Institute. Dr. Sumption’s Research Focuses on the Materials Science and Solid State Physics of Superconducting and Magnetic Materials.
Research interests include (i) MgB2 materials, with a focus on dopant-induced enhancements of the upper critical fields and the related structural and transport properties, (ii) flux pinning and upper critical fields in Nb3Sn and MgB2, (iii) the study of energy loss due to flux motion in superconducting materials and the influence of composite structure, (iv) flux pinning in multilayer YBCO, (v) the current limiting mechanisms in superconducting strands based on materials with the A15 structure (as well as others), (vi) the study of the diffusion length of Cooper pairs from one filament to another in very fine filament composite strands, (vii) phase formation and diffusion in the Nb-Cu-Sn-Ti-Ta system, (viii) conductor fabrication and large scale application.
Dr. Sumption’s research has been funded by various organizations, including the US Department of Energy, Division of High Energy Physics, NASA, the Navy, AFOSR, NIH, NRC, and private industry. Dr. Sumption’s research includes both experimental and theoretical aspects. Within the Laboratories for Applied Superconductivity and Magnetism itself, numerous transport and magnetic techniques are in use. Resistivity ratios and critical currents have been used to study superconducting transport properties for many materials. Various magnetic techniques have been used including susceptibility (χAC, χDC), vibrating sample magnetometry, SQUID, and magnetically measured contact resistance. Energy loss for large scale samples has been measured using calorimetric measurements of AC loss in superconducting strands and cables and electrical, magnetic, and bolometric techniques for YBCO conductors. Structural and compositional studies using the department’s SEM and TEM facilities are a key part of much of this work, which typically focuses on phase and structure-to-properties correlations.
Materials investigated include low Tc superconductors; NbTi, Nb3Sn, Nb3Al, NbTiTa, as well as high Tc superconductors; YBCO, Bi, Tl-, and MgB2 compounds. This has includes work with thin films, bulk, single crystals, OPIT wires, and melt grown samples. Dr. Sumption’s studies of magnetic materials includes work with magnetic tape as well as spin glass alloys. Previous programs have also included the mechanical alloying of intermetallic compounds and its influence on mechanical and superconductive properties. Metastable phase formation in Nb-Al has also been a significant topic of research, along with deformation studies of composite metallic and intermetallic wires.
Theoretical work has had a strong concentration in the area of anisotropic continuum modelling, including; bridging loss and magnetic creep in HTSC and LTSC, and proximity and eddy currents in LTSC. Additionally, a model for synergistic pinning in heterogeneous superconducting structures was developed. Theoretical work has also included FEM studies of cyclic field induced losses in macroscopic superconductor/ferromagnetic element composites. In addition, current path effects in high and low Tc strands, tapes, and cables have been investigated.
One example of recent work is research into increasing the upper critical field and irreversibility fields of MgB2. Chemical doping studies have successfully increased the thermodynamic upper critical field of MgB2 by 10 T as shown below.
Enhanced Bc2 in MgB2
Crystallographic and Electronic Structure of MgB2
The presence of Mg in the compound MgB2 stabilizes the boron sub-lattice in the form of a honeycomb-like stack of hexagonal networks, Figure 1. Although MgB2 has the appearance of an intercalation compound with planes of small B atoms sandwiched between planes of the somewhat larger (ratio 1:1.6) Mg atoms, it functions more like a 3-D B lattice moderated or stabilized by layers of Mg atoms which serve as electron donors. Thus the B honeycomb dominates the electronic structure which can be thought of as deriving from s bonding within the B planes and π bonding orbitals out of the plane [5]. In the MgB2 crystal the in-plane σ orbitals lead to a corresponding 2-D σ band while the π space charge extends both out-of-plane and in-plane to form the 3-D π band. This partially covalent structure of the MgB2 crystal gives rise to a Fermi surface with two conduction bands, designated π and σ, each one associated with its own partial Fermi density-of-states, NF, impurity-scattering relation time, τ, and superconductive energy gap, Δ.
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Figure 1: Crystal structure of MgB2 [Preuss, in LBNL Research News, Aug 14, 2002] |
The Upper Critical Field, Hc2, and its Enhancement through Impurity Scattering
Doping to Enhance Bc2 and Irreversibility Field
Based on the detailed analysis of Gurevich, and further approximation
and
It is then possible to add resistivity enhancing dopants to the MgB2. Two kinds have been seen to be successful, those appearing to substitute on B-sties, and those substituting on Mg sites. Below the Metal Diboride substitutions are seen to be quite different from the C-bearing substitutions.
