“I venture to define science as a series of interconnected concepts and conceptual schemes arising from experiment and observation and fruitful of further experiments and observations. The test of a scientific theory is, I suggest, its fruitfulness.” James Bryant Conant (1893-1978) U. S. Chemist and Educator. MgB2 SUPERCONDUCTORS: PROCESSING, CHARACTERIZATION AND ENHANCEMENT OF CRITICAL FIELDS DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy in the Graduate School of The Ohio State University By Mohit Bhatia, M.S. *** The Ohio State University 2007 Dissertation Committee: Approved By: Professor Suliman A. Dregia, Adviser _____________________________________ Adviser Professor Michael D. Sumption, Adviser _____________________________________ Professor John Morral Adviser Graduate Program in Materials Science and Engineering Professor Sheikh Akbar ABSTRACT In this work, the basic formation of in-situ MgB2, and how variations in the formation process influence the electrical and magnetic properties of this material was studied. Bulk MgB2 samples were prepared by stoichiometric, elemental powder mixing and compaction followed by heat-treatment. Strand samples were prepared by a modified powder-in-tube technique with subsequent heat-treatment. The influence of various heat- treatment schedules on the formation reaction was studied. Two different optimum heat- treatment windows were indentified, namely, low-temperature heat-treatment (below the melting point of Mg i.e. between 620 - 650oC) and high-temperature heat-treatment o (>650 C) for the preparation of MgB2 with good transport properties. XRD was used to confirm phase formation and microstructural variations were studied with the help of SEM. Following a study of the reaction temperature regimes, the focus turned to critical field enhancement via doping with various compounds targeting either the Mg or the B sites. The effects of these dopants on the superconducting properties, in particular the critical fields, were studied. Large increases in irreversibility field, oHirr, and upper critical field, Bc2, of bulk and strand superconducting MgB2 were achieved by separately adding SiC, amorphous C, and selected metal diborides (NaB2, ZrB2, TiB2) in bulk samples and three different sizes of SiC (~200 nm, 30 nm and 15 nm) in strand samples. Lattice spacing shifts and resistivity measurements (on some samples) were consistent with dopant introduction to the lattice. It was also found that both oHirr and Bc2 depend ii on the sensing current level which may be an indication of current path percolations. These increases in the Bc2 were also complimented by an increase in the transport Jcs, especially for the SiC doped samples. It was important to differentiate between the effects on the transport properties arising from possible particulate enhanced flux pinning from that due to Bc2 enhancements, associated with smaller length scale disorder. Flux pinning analysis performed on SiC doped samples showed that while some small level of particulate-enhanced pinning was present, the majority of the pinning was associated with a grain boundary mechanism, suggesting that transport Jc increases were predominantly Bc2 related. Lastly, since the residual resistivity of a material is directly related to the electron scattering and hence Bc2, it can therefore be used as a measure to confirm the dopant introduction into the lattice. Normal-state resistivities were measured for various binary and doped MgB2 samples as a function of temperature. These resistivities were modeled based on the Bloch-Gruneissen equations. This allowed extraction of the residual resistivities, Debye temperatures and current carrying volume fractions for these samples, as well as providing information on the electron-phonon coupling constant. The residual resistivity was found to increase by a factor of three, Debye temperature decreased and the electron-phonon coupling constant increased marginally for the SiC doped samples as compared to the binary sample. This change in 0 and D confirmed the XRD evidence that the dopants were increasing oHirr and Bc2 by substituting on the B and Mg sites of the crystalline lattice. iii ACKNOWLEDGMENTS It is a pleasure to thank the many people who made this thesis possible. First of all, I would like to express my deep and sincere gratitude to my advisor, Professor M.D. Sumption. His wide knowledge of the subject and his logical way of thinking has been of great value for me. His understanding, encouraging and personal guidance have provided a good basis for the present thesis. I am also deeply grateful to my advisor, Professor S.A. Dregia for his detailed and constructive comments, and for his important support throughout this work. I wish to express my warm and sincere thanks to Professor E.W. Collings. With his enthusiasm, inspiration and great efforts to explain things clearly and simply, he helped to make the subject fun for me. I would like to thank Professor S.X. Dou, of University of Wollongong, Australia for their invaluable guidance on various issues and for providing some valuable samples for the study. Many thanks to Michael Tomsic, Mathew Reindfleisch and the entire team at The HyperTech Research Inc., Columbus, Ohio for their help with strand sample processing. I would also like to thank Dr. Bruce Brant, Dr. Scott Hannahs and Dr. Alexey Suslov at the NHMFL, Tallahassee for providing their support for the critical field measurements during my numerous trips to the national lab. I am indebted to my many past and present student colleagues for providing a stimulating and fun environment in which I learnt and grew. My special thanks to Dr. iv Alexander Vasiliev for his help with TEM characterization. I also wish to thank the technical staff in the Materials Science and Engineering department at The Ohio State University especially Henk Colijn, Cameron Begg, Gary Dodge, Ken Kushner and Steve Bright who have helped me at all stages in the research. My gratitude also goes towards the U.S. Dept. of Energy - Division of High Energy Physics and NIH for funding this research under Grant Nos. DE-FG02- 95ER40900, DE-FG02-05ER84363, DE-FG02-07ER84914, 2R44EB003752-02, 1R44EB006652-01 and 4R44EB006652-02. I also wish to thank my friends Srikant, Syadwad, Alex, Zaina, Ekta, Bijula, Vivek and all others who have stood by me in good and bad times and have been like a family to me far away from the home. Lastly, and most importantly, I wish to thank my parents, Girish and Ruchi Bhatia and my brother Madhur. I would not be where I am, without their foresight, constant support and love. To them I dedicate this thesis. v VITA August 3, 1979 ……………………………….Born, Bharatpur, India 2002. ………………………………………….B.Tech (Honrs.) Ceramic Engineering Institute of Technology, BHU Varanasi, India 2005 ………………………………………….M.S., Materials Science and Engineering The Ohio State University, USA 2002-2007…………………………………….Graduate Research Associate The Ohio State University, USA PUBLICATIONS 1) “Superconducting Properties of SiC Doped MgB2 Formed Below and Above Mg‟s Melting Point” Bhatia, M; Sumption, M. D.; Bohnenstiehl, S.; Collings, E. W; Dregia, S.A.; Tomsic, M.; Rindflisch, M.; submitted to IEEE Transactions on Applied Superconductivity (2006). 2) “Increases in the irreversibility field and the upper critical field of bulk MgB2 by ZrB2 addition.” Bhatia, M.; Sumption, M. D.; Collings, E. W.; Dregia, S.A.; Applied Physics Letters (2005), 87(4), 042505/1-042505/3. 3) “Effect of various additions on upper critical field and irreversibility field of in- situ MgB2 superconducting bulk material.” Bhatia, M.; Sumption, M. D.; Collings, E. W.; IEEE Transactions on Applied Superconductivity (2005), 15(2, Pt. 3), 3204-3206. 4) “Influence of heat-treatment schedules on magnetic critical current density and phase formation in bulk superconducting MgB2.” Bhatia, M; Sumption, M. D.; Tomsic, M.; Collings, E. W.; Physica C: Superconductivity and Its Applications (2004), 415, 158-162. vi 5) “Influence of heat-treatment schedules on the transport current densities of long and short segments of superconducting MgB2 wire.” Bhatia, M; Sumption, M. D.; Tomsic, M.; Collings, E. W.; Physica C: Superconductivity and Its Applications (2004), 407, 153-159. 6) “High Critical Current Density in Multifilamentary MgB2 Strands” Sumption, M.D.; Susner, M.; Bhatia, M.; Rindflisch, M.; Tomsic, M.;McFadden, K.; Collings, E.W.; submitted to IEEE Transactions on Applied Superconductivity (2006). 7) “Transport properties of multifilamentary in-situ route, Cu-stabilized MgB2 strands: one meter segments and the Jc(B,T) dependence of short samples.” Sumption, M.D.; Bhatia, M.; Rindfleisch, M.; Tomsic, M.; Collings, E.W.; Superconductor Science and Technology (2006), 19(1), 155-160. 8) “Magnesium diboride superconducting strand for accelerator and light source applications.” Collings, E.W.; Kawabata, S.; Bhatia, M.; Tomsic, M.; Sumption, M.D.; Proceedings MT19 (Genoa, 2005) – (submitted) 9) “Solenoidal coils made from monofilamentary and multifilamentary MgB2 strands.“ Sumption, M. D.; Bhatia, M.; Buta, F.; Bohnenstiehl, S.; Tomsic, M.; Rindfleisch, M.; Yue, J.; Phillips, J.; Kawabata, S.; Collings, E. W. Superconductor Science and Technology (2005), 18(7), 961-965. 10) “MgB2/Cu racetrack coil winding, insulating, and testing.” Sumption, M. D.; Bhatia, M.; Rindfleisch, M.; Phillips, J.; Tomsic, M.; Collings, E. W.; IEEE Transactions on Applied Superconductivity (2005), 15(2, Pt. 2), 1457-1460. 11)