Year

2008

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials - Faculty of Engineering

Abstract

Discovered in 2001, magnesium diboride (MgB2) is the latest superconductor suitable for large scale applications (magnetic resonance imaging systems (MRI), fault current limiters (FCL), high-field magnets). Compared to other metallic superconductors like NbTi (Tc = 9 K) and Nb3Sn (Tc = 18 K), it has the advantage of a higher critical temperature (Tc = 39 K), which enables its application at the temperature of 20 K and, hence, significantly reduces the cost of the cooling system. This, together with the abundance of the magnesium and boron raw materials, as well as the relatively simple fabrication of MgB2 wires and tapes, has motivated the active investigation and study of this superconductor by many research groups all over the world. As a result, a significant breakthrough for application of MgB2 conductors at high fields has been made with introduction of carbon into the crystal lattice of MgB2 via chemical doping. The main focus of this work was on the study of the microstructural and superconducting properties of MgB2-xCx superconductors and establishing correlations between them. To obtain MgB2-x Cx compounds with different characteristics, various C-based doping materials and processing parameters were employed. The systematic study of the microstructural and superconducting properties of MgB2-x Cx samples conducted in this work allowed us: (i) to predict suitable dopants for MgB2; (ii) to improve chemical doping by carbon; (iii) to identify the relevant negative microstructural factors and estimate their effects on limitation of the current-carrying ability in MgB2-xCx samples. The results described in this work can be used as a guide for the achievement of the characteristics required for practical applications of MgB2 superconductors. Investigation of the properties of MgB2-xCx superconductors as a function of the processing parameters showed that the doping level, sintering temperature, and cooling time control the density of pinning centers in MgB2-xCx, and affect the connectivity of grains and transparency of grain boundaries to current flow. Analysis of the pinning mechanism in the samples studied has led to establishing that the dominant pinning is on grain boundaries in the pure MgB2 samples, and on grain boundaries and crystal lattice defects in the MgB2-xCx samples. To demonstrate the effect of the pinning environment on the current-carrying ability in MgB2-xCx superconductors, a comparative study of the microstructural and superconducting properties for pure, nano SiC-, and C-doped MgB2 wires was carried out. In both SiC- and C-doped samples carbon substitution into the MgB2 crystal lattice results in the enhancement of the upper critical field, Bc2. However, it was revealed that the presence of SiC dopant allowed carbon substitution and MgB2 formation to take place simultaneously at low temperatures. Therefore, the microstructure of this SiC-doped sample assures maximal density of pinning centers (large number of grain boundaries, i.e. small grain sizes, and crystal lattice defects) and enhances pinning. These factors (higher Bc2 value and stronger pinning) are responsible for the superior enhancement in critical current density at relatively high fields in the SiC-doped sample. In contrast, for C-doping, higher processing temperatures are required for generation of a dense network of crystal lattice defects. In this case, the microstructure consists of larger grains, and the pinning on smaller number of grain boundaries becomes weaker, reducing the total pinning force and critical current density. An important outcome of this study was the establishment of the dual reaction model (simultaneous formation of MgB2 compound and C substitution into the lattice), which enables us to predict desirable dopants for enhancing the properties of MgB2. These should be C-based compounds which decompose, producing highly reactive C at temperatures below the temperature of MgB2 formation. Ideally, dopants should be homogeneously distributed within host the Mg and B powders and not contaminate grain boundaries in formed MgB2. The liquid mixing approach, a new advanced and at the same time simplified approach to chemical doping of MgB2 superconductor with carbon, was found to partially fulfill these requirements. Carbohydrates (sugar and malic acid) and polycarbosilane (a polymer analog to nano SiC-doping) were employed as doping materials. Liquid mixing has been shown to coat each individual nano sized boron powder particle with a nano-layer of amorphous carbon. Fresh unpassivated carbon extracted from carbohydrates or polycarbosilane easily incorporates itself into the MgB2 crystal lattice. This enhanced incorporation promoted by the maximal reaction surface assured by coating generates microstructure with a dense network of pinning sites and results in significant improvement of the superconducting properties in MgB2-xCx material. The results observed suggest that sugar as a dopant exhibits a stronger potential for practical application of MgB2 superconductor in the high field region than nano carbon doping. Similarly, stronger enhancement of superconducting properties was observed in polycarbosilane-doped MgB2 compared to nano SiC doping. The latter was ascribed to the formation of a microstructure with Mg2Si impurity phases mainly distributed within superconducting MgB2 grains. In this case, transparency of grain boundaries was likely improved, which resulted in the observed enhancement of critical current density over the entire field range. Systematic analysis of the microstructures and superconducting properties of sugar-, malic acid-, and polycarbosilane-doped MgB2 samples demonstrated that the critical current density was also significantly affected by the microstructural properties in the low field region. The results observed led to the development of a model, which allowed us to estimate the level of critical current density (Jc) limitation due to the microstructural features of pure and C-doped MgB2 samples. This model is based on the identification of individual contributions by various defects to critical current density limitation. These defects in the MgB2 microstructure include porosity and non-superconducting phase inclusions (so-called “geometrical” defects), as well as the connectivity and transparency of grain boundaries. The results observed showed that a higher level of “geometrical” defects results in stronger limitation of critical current flow through the sample and lower measured Jc values. The elimination of the “geometrical” defects would result in critical current densities that are a factor of 1.5 - 2 higher than currently measured values. The role of grain boundaries connectivity was found to be even more dramatic. For samples with fully connected grains, the estimated critical current densities were about one order of magnitude higher than the measured values. Moreover, the results of analysis showed that the low field Jc values are mainly determined by the connectivity and transparency of grain boundaries, while in field Jc(Ba) performance is affected by these defects in the microstructure to a lesser extent, and its behaviour is mainly determined by the pinning environment in the samples. It also was observed that while a denser pinning network favors in field Jc(Ba) behaviour, this results in reduction of grain boundary transparency and more pronounced critical current density limitation in the low field region.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.