Thesis (M.S.)--Florida State University, 2006. / Advisor Juan C. Ordonez, Florida State University, College of Engineering, Dept. of Mechanical Engineering. Title and description from dissertation home page (viewed Sept. 19, 2006). Document formatted into pages; contains x, 267 pages. Includes bibliographical references.

The thesis work presented here emphasizes important aspects of crystal growth and the influence of chemical substitution in Fe-As superconductors. High temperature solution growth technique is one of most powerful and widely used technique to grow single crystals of various materials. The biggest advantage of high temperature solution growth technique is the, possibility of growing single crystals from both congruently and incongruently melting materials. Solution growth technique has the potential to control high vapour pressures, given the fact that, in Fe-based superconductors elements with high vapour pressure like As, K, Li and Na have to be handled during the crystal growth procedure. In this scenario high temperature solution growth is the best suitable growth technique to synthesize sizable homogeneous single crystals. Using self-flux high temperature solution growth technique, large centimeter-sized high quality single crystals of BaFe2As2 were grown. This pristine compound BaFe2As2 undergoes structural and magnetic transition at TS/N=137 K. By suppressing this magnetic transition and stabilizing tetragonal phase with chemical substitution, like Co-doping and Na-doping, bulk superconductivity is achieved. Superconducting transitions of as high as Tc = 34 K with Na substitution and Tc = 25 K with Co-doping were obtained. A combined electronic phase diagram has been achieved for both electron doping with Co and hole doping with Na in BaFe2As2. Single crystals of LiFe1−xCoxAs with x = 0, 0.025, 0.05 and 0.075 were grown by a self-flux high temperature solution growth technique. The charge doping in LiFeAs is achieved with the Co-doping in Fe atoms. The superconducting properties investigated by means of temperature dependent magnetization and resistivity revealed that superconductivity is shifted to lower temperatures and with higher amount of charge carriers superconductivity is killed. Single crystals of KFe2As2 were grown with two different fluxes, namely, FeAs-flux and KAs-flux. The superconducting transition is found to be at 3.8K in both the crystals. The influence of doping with selected elements like Na, Rh, Co and Cr has been investigated systematically in KFe2As2 single crystals. With Na-doping at the K-site, yield (K1−xNax)Fe2As2; superconductivity is suppressed to lower temperatures. Substitution of Co and Cr at Fe site, yield K(Fe0.95Co0.05)2As2, K(Fe0.95Cr0.05)2As2 superconductivity is rapidly killed. Single crystals of (Ba0.6Eu0.4)(Fe1−xCox)2As2 with x = 0, 0.05, 0.1, 0.15 and 0.2 were grown with solution growth technique using Fe-As flux and investigated with several physical measurements. The growth conditions are highly optimized to grow flux free large single crystals especially in case of BaFe2As2 family. The high quality of the crystals were revealed by several physical properties, for e.g. single crystals of Ba(Fe1−xCox)2As2 are of the highest quality which was confirmed by the magnetic ac susceptibility which showed a very sharp superconducting transition.

This thesis describes a portfolio of work aimed at the high field applications of superconductors and can be split into four main topics: The thermal stability of technical superconductors. This section investigates the effects of thermal perturbations on technical superconducting wire used in MRI scanner construction. The ultimate aim of this section is to predict how the architecture of the wire may affect its thermal stability. To this end a detailed finite element analysis model was constructed, verified by detailed experimental data, which could then be used to quickly and easily vary the wire’s parameters. Design of a high field pulsed electromagnetic coil for flux trapping in superconductors. This section details the design, construction and testing of a novel pulsed high field magnet. The design uses finite element analysis to predict the electromagnetic, thermal and structural properties of the coil. Explosive testing of high tensile fibres used in the construction of the high field coil. This section describes the refinement and use of a novel method for testing the mechanical properties of high tensile fibres in cylindrical geometries by using highly pressurized copper vessels. Pulsed field magnetization of bulk high temperature superconductors. This section discusses the process of magnetizing bulks of high temperature superconductors by using pulsed magnetic fields. It investigates how the trapped field varies with the magnitude and rise-time of the magnetizing field, sample temperature and time after magnetization.