New Type Of Mgb <sub> 2 </ Sub> Series Of Superconductors, Superconductivity And Microstructure Research

In this dissertation, the superconducting properties, structural and microstructural properties were studied in newly discovered superconductor MgB₂ and doped Mg₂ polycrystalline bulk samples. We first investigated the changes of structural and microstructural properties in the undoped Mg₂ superconductor under lower temperature by X-ray diffraction and Raman spectroscopy; Then, the Cr doped, Ag and Al double doped, Zn and Ni double doped Mg₂ polycrystalline samples were, respectively, synthesized and the variations of superconducting properties, structural and microstructural properties after the doping were studied in detail. The dissertation is composed of five chapters which were, respectively, abstracted as following:In Chap. 1, the basic conceptions, discovery and history of development about superconductor were first introduced, respectively. Then, the newly research developments in the newly discovered Mg₂ superconductor were reviewed, which involve the studies about synthesis methods, electronic energy gap, isotope effects, transport properties, element substitution effects, pressure effects and Raman spectrum. Finally, the superconducting mechanism and the problems in basic research about Mg₂ superconductor were summarized on the basis of above review.In Chap. 2, the temperature dependences of the Raman spectrum and lattice parameters of polycrystalline Mg₂ have been investigated by means of Raman spectroscopy and X-ray diffraction. The lattice parameters show an approximate by linear change with the temperature decrease, giving different thermal expansions along the a- and c-axes which is caused by the comparatively weak metal-boron bonding in Mg₂. The grain size of Mg₂ determined by means of X-ray diffraction is around 45nm for both [100] and [001] directions. There is no evidence for any structural transition while the temperature changes from 300K down to 12K. An anomalous Raman band at 603 cm^-1 is observed, which is consistent with the theoretical prediction for the E_2g in-plane boron stretching mode. The Raman...