Year

2011

Degree Name

Doctor of Philosophy

Department

Institute for Superconducting and Electronic Materials

Abstract

The aim of this thesis was to study polycrystalline magnesium diboride (MgB2) materials for improving the critical current density (Jc). The next-generation superconducting devices require low-cost operation, inexpensive manufacturing processes, and reduced size and weight. MgB2 has been judged potentially capable of meeting these needs because of its high critical transition temperature (Tc), low priced raw materials, and light constituent elements. It is also well known that MgB2 thin films have high superconducting performance, which is crucial for superconducting applications. Specifically, the upper critical field (Bc2) at absolute zero temperature reaches ~ 60 T, and the Jc at around 0 T can reach 107-108 A/cm2. However, in the case of polycrystalline MgB2 materials such as bulks, wires, and tapes, the Jc values are approximately two orders of magnitude less than for the thin films. This is due to poor connectivity between grains, and it mainly arises from the large fraction of voids and secondary impurity phase caused by particular manufacturing processes. In order to address this issue, it is necessary to control the microstructural properties of those materials.

Densification is especially important to eliminate voids and increase effective areas for supercurrent flow. The Mg diffusion method is known to be one of the best methods for obtaining highly dense MgB2 bulks and wires, and it yields higher self-field Jc compared with conventional in situ processed materials. However, the sintering reaction has been usually performed within the high temperature range of 700 - 950 ºC, which has resulted in good crystallinity and large grain size, but could significantly decrease the in-field Jc, owing to the lack of flux pinning centers such as crystal defects and large amounts of grain boundary. Therefore, it is obviously important to control not only the mass density, but also the crystallinity and grain size for enhancing the in-field Jc of polycrystalline MgB2 materials.

In this thesis, an in situ heat-treatment technique was employed to provide very strong in-field pinning for use within an Mg diffusion process for the preparation of near-fully-dense un-doped MgB2 bulks. The heat treatment of compacted boron sealed in Ta tube with Mg pellets employs an initial short high-temperature sintering at 1100 °C, followed by a low-temperature annealing below 660 °C. A high mass density of 2.5 g/cm3 (95% of the theoretical density) was achieved in the bulks treated by the two-step process. The in-field Jc is nearly one order of magnitude higher than for the samples prepared by single-step sintering at high or low temperature. Microstructural analysis suggested the unique feature of well-connected small grains with a high level of disorder in the un-doped MgB2 samples created by the two-step process.

Further, an oxygen-free pyrene (C16H10) gas diffusion method was developed to improve the in-field Jc of highly dense MgB2 materials. Carbon (C) is known to be a doping element for effectively improving the high-field Jc of MgB2. However, carbon doping strongly decreases the low-field Jc. This is most likely due to agglomeration of un-reacted C at grain boundaries, which arises from the difficulty in homogeneous mixing of Mg, B, and C powders in sample preparation. In the proposed method, oxygen-free pyrene gas as a C dopant was delinked and incorporated into the highly dense MgB2 structure via gas phase diffusion. The technique offers the advantages that molecular C is homogeneously distributed into the MgB2 and substituted at the boron sites without any severe deterioration of structural integrity. The C substitution causes a significant shrinkage of the a-axis lattice parameter and an increase in the lattice strain, resulting in high disorder. The introduction of structural disorder as a result of C doping leads to a considerable enhancement of the in-field Jc and Bc2.

In order to evaluate the effects of densification on superconducting properties, especially on transport Jc, further studies of polycrystalline MgB2 materials were conducted. The effects of wire and tape shapes of un-doped MgB2 conductors on the transport Jc, flux pinning strength, and microstructure were examined. All samples were prepared by an in situ powder-in-tube process and sintered in the temperature range of 650 to 700oC for 30 min. It was observed that the two types of samples sintered at lower temperature, ~ 650oC, had a relatively good high-field Jc at 4.2 K and 12 T. This can be attributed to the strong grain boundary pinning due to smaller grain size. Specifically, flat tape samples are expected to further improve the self-field Jc properties and grain connectivity due to core densification and reduction of porosity.

The hot-pressing effects on un-doped and carbon doped MgB2/Nb/Monel wires were also investigated. Hot-pressing at 100 MPa resulted in the improvement of the mass density, the Jc, and the grain connectivity of the wires. However, this also caused additional Jc anisotropy, which is associated with the texture of a- and b-axis oriented grains. In particular, the anisotropy factors of Jc for the un-doped and the carbon doped wires were estimated to be 2.6 and 1.7, respectively, under an external magnetic field of 14 T.

The secondary phase MgO, as well as the densification, can have a significant influence on the Jc. To this end hydrogen gas may act as an oxygen getter, and thus the influence of hydrogen containing argon gas on the structural and superconducting properties of MgB2 wires was investigated. Lattice parameters were independent of the concentration level of hydrogen in argon, while the fraction of MgO was sensitive. The MgO particles observed by scanning transmission electron microscopy were 30-50 nm in size, which is considerably larger than the approximate size range of flux pinning centers. The sintering atmosphere conditions are a crucial issue for MgB2 wires.

Finally, the effects of the starting Mg size on the superconducting properties of Mg diffusion processed MgB2 bulks were studied. All samples were fabricated from both Mg powder and Mg lumps by using the diffusion process. From detailed synchrotron X-ray refinement and analysis of Raman spectra, it was found that the lattice disorder was increased for the sample fabricated from Mg powder. This result was further confirmed by the enhancement of the upper critical field, as estimated by direct resistive transition analysis or by critical current analysis using the percolation model. On the other hand, the area fraction from Rowell’s method for the sample fabricated from Mg powder is much larger than that of the Mg lump sample and almost comparable to the value for well-connected MgB2 thin film. The starting Mg size is not only critical for the enhancement of high field properties because it increases the internal strain, but also because it enhances the connectivity.

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