The Effect of Dopant Additions on the Microstructure of Boron Fibers Before and After Reaction to MgB 2
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The Effect of Dopant Additions on the Microstructure of Boron Fibers Before and After Reaction to MgB2 James V. Marzik1, Raymond J. Suplinskas1, William J. Croft2, Warren J. MoberlyChan2, John D. DeFouw3, and David C. Dunand3 1 Specialty Materials, Inc., Lowell, MA, U.S.A. 2 Harvard University, Cambridge MA, U.S.A. 3 Northwestern University, Evanston, IL, U.S.A. ABSTRACT Boron fibers made by a commercial chemical vapor deposition (CVD) process have been used as precursors for the formation of magnesium diboride (MgB2) superconducting wires. Prior to a reaction with magnesium, the addition of dopants such as carbon and titanium to the boron fiber has been shown to enhance the superconducting properties of MgB2. These dopants also influence the kinetics of the reaction with magnesium. In this study, the effect of carbon dopant additions on the microstructure of boron fibers was investigated using powder x-ray diffraction, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Additionally, bundles of boron fibers were pressure infiltrated with molten magnesium and reacted at elevated temperatures. The microstructure and microchemistry of the fiber-metal interfaces were investigated by TEM and energy dispersive x-ray analysis (EDS). INTRODUCTION Chemically vapor deposited (CVD) boron fibers [1] have been used for over three decades as a composite reinforcement in a number of structural aerospace applications. Of more recent interest is the use of CVD boron (B) fibers as precursors for the synthesis of MgB2 superconductors. The discovery of superconductivity in MgB2 at temperatures near 40K [2] has provided a technical opportunity to achieve high performance superconductors with lower fabrication and operating costs. B fibers can be converted to MgB2 via reaction with Mg vapor [3], in principle eliminating a separate wire-forming manufacturing step. It has been shown [4-7] that doping the B starting material by co-deposition of the dopant and boron through CVD yields improved superconductor properties when the material is converted to MgB2. Significant increases in both the critical current density, (Jc ~ 5 x 106 A cm-2 at 5K) and the upper critical magnetic field (Hc2(0)>32T) have been demonstrated. Further, CVD B fibers have been infiltrated with liquid magnesium, and subsequently reacted to continuous MgB2 fibers resulting in Mg/MgB2 composites after solidification of the excess metal [8]. Hence, the potential exists for a process, based on existing commercial technology, to produce continuous, optimally doped MgB2 superconducting wires embedded within a ductile, conductive metallic matrix (Figure 1). The introduction of carbon impurities into B fibers by CVD methods results in homogeneous dopant distribution and predictable dopant concentrations, based on the reported superconducting properties [6] of doped B fibers that were converted to Mg(B1-xC)2. However, it was observed that increases in the dopant concentration in B fibers resulted in significant decreases in the reaction rates
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