Structure and properties of MgB2bulks: Ab-initio simulations compared to experiment

Analysis of XRD patterns by Rietveld refinement has been shown that the main phase of superconducting MgB$_{2}$-based bulk materials (with high level of superconducting characteristics) has AlB$_{2}$ type structure and near MgB$_{1.8-1.68}$O$_{0.2-0.32}$ stoichiometry. The materials demonstrated the critical current densities up to 0.9 – 0.4 MA/cm$^{2}$ jc (at 0 - 1 T, 20 K); up to 15 T B$_{c2}$ (at 22.5 K) and B$_{irr}$ (at 18 K). The ab-initio simulation confirmed (1) benefits in binding energy and enthalpy of formation if stoichiometry of the solid solution is near MgB$_{1.75}$O$_{0.25}$; (2) energetic advantage in case if impurity oxygen present only in each second boron plane of MgB2 cell while the first boron plane of the same cell stays pristine and location of substituted oxygen atoms in the nearby positions. Besides, the results of ab-initio modeling allow explanation of the tendency towards segregation of O-impurity in MgB$_{2}$ structure during synthesis or sintering, and formation of Mg-B-O inclusions or nanolayers (with MgO type of structure) which effect pinning. Calculated transition temperatures, T$_{c}$, for MgB$_{1.75}$O$_{0.25}$ occurred to be 23.3 K, while for MgB$_{2}$ it was 21.13 K only. Experimental T$_{c}$ of the bulk materials was 35.7-38.2 K

Formation of magnesium diboride-based materials with high critical currents and mechanical characteristics by high-pressure synthesis

The developed method of high-pressure synthesis (HPS) allows producing nanostructural highly dense material based on MGB2, which possesses the highest superconducting and mechanical characteristics among the known world analogues, in the form of blocks that are suitable for application in SC electromotors and pumps. Additions of Zr can increase critical current density (jc) of synthesized at 2 GPa and 750-800 °C MGB2 in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen forming the ZrH2. The formation of ZrB2 phase at higher synthesis temperatures (about 950 °C) in HPS MGB2 does not result in the jc increase. Some increase in jc of HPS MGB2 at 10 K in the fields higher than 8 T was observed when nano-SiC was added. The additions of Zr, Ta or Ti can prevent the harmful MgH2 impurity phase from appearing and hydrogen from being introduced into the material structure. Besides, the presence of additions in HPS MGB2 promotes the formation of a larger amount of Mg-B (most likely MGB2) inclusions in the Mg-B-O material \u27\u27matrix\u27\u27 that in turn leads to the increase of jc of the material in magnetic fields

Structure and Properties of MgB2: Effect of Ti-O and TiC Additions

IEEE The effects of the additions of powdered titanium carbide (TiC) and polyvalent titanium oxides (Ti-O) to MgB2 (synthesized under high pressure (2 GPa)-high temperature (800 and 1050 & #x00B0;C) conditions) on the critical current density, jc, critical magnetic fields, BC2 and Birr, and the related transformations in the materials structures are reported. The superconducting characteristics are compared with that of MgB2 with titanium (Ti) additions synthesized under the same conditions. The synthesis temperature, the ability of Ti-containing compounds to act as a getter, and the different diffusion rates of Mg and B into grains of these additions influence the distribution of the elements (boron, magnesium and impurity oxygen) in the structures of MgB2-based materials. This in turn affects the formation of pinning centers, the resulting flux pinning and eventually the superconducting properties (jc, BC2, Birr)

High-pressure synthesized nanostructural magnesium diboride-based materials for superconductive electromotors, generators and pumps

Additions of Zr can increase critical current density (jc) of high-pressure synthesized MgB2 (HPS-MgB2) in the same manner as additions of Ta or Ti, i.e. due to the absorption of impurity hydrogen (to form ZrH2). The formation in HPS-MgB2 of ZrB2phase at higher synthesis temperatures (about 950 °C) does not result in the jc increase. Some increase in jc of HPS-MgB2 at 10 K in the fields higher than 8 T was observed when nano-SiC was added. The additions of Zr, Ta or Ti can prevent the harmful MgH2 impurity phase from appearing and may prevent hydrogen from being introduced into the material structure and besides, their presence in HPS-MgB2 promotes the formation of a higher amount of MgB (most likely MgB2) inclusions in the MgBO material matrix that in turn leads to the increase of jc in magnetic fields. The high level of superconductive (SC) and mechanical characteristics attained for HPS-MgB2 and the possibility to manufacture large samples make its application in the superconductive electromotors, generators, pumps, etc., very promising