Structure and superconducting characteristics of magnesium diboride, substitution of boron atoms by oxygen and carbon

An x-ray analysis of MgB2-based materials shows that they contain MgB2 and MgO phases. According to a quantitative Auger analysis (taken after removing the oxidized surface layer by Ar ion etching in the microscope chamber) the MgB2 phase contains some amount of oxygen that approximately corresponds to the composition MgB2.2-1.7O0.4-0.6. Rietveld refinement of the MgB2 phase, based on EDX data with varying B/O content, leads to the composition MgB1.68-1.8O0.2-0.32. Ab-initio modelling of boron substitution by oxygen in MgB2 (ΔH f = -150.6 meV/atom) shows that this is energetically favourable up to the composition MgB1.75O0.25 (ΔH f = -191.4 meV/atom). In contrast to carbon substitution, where very small levels of doping can dramatically affect the superconducting characteristics of the material with concomitant changes in the electron density, oxygen substitution results in very little change in the superconducting properties of MgB2. The formation of vacancies at the Mg site of both MgB2 and substituted MgB1.75O0.25 was modelled as well, but has shown that such processes are energetically disadvantageous (ΔHf of Mg0.875B2 and Mg0.75B1.75O0.25 are equal to -45.5 and -93.5 meV/atom, respectively)

Structure and superconducting characteristics of magnesium diboride, substitution of boron atoms by oxygen and carbon

An x-ray analysis of MgB2-based materials shows that they contain MgB2 and MgO phases. According to a quantitative Auger analysis (taken after removing the oxidized surface layer by Ar ion etching in the microscope chamber) the MgB2 phase contains some amount of oxygen that approximately corresponds to the composition MgB2.2-1.7O0.4-0.6. Rietveld refinement of the MgB2 phase, based on EDX data with varying B/O content, leads to the composition MgB1.68-1.8O0.2-0.32. Ab-initio modelling of boron substitution by oxygen in MgB2 ([Delta]Hf =-150.6 meV/atom) shows that this is energetically favourable up to the composition MgB1.75O0.25 ([Delta]Hf =-191.4 meV/atom). In contrast to carbon substitution, where very small levels of doping can dramatically affect the superconducting characteristics of the material with concomitant changes in the electron density, oxygen substitution results in very little change in the superconducting properties of MgB2. The formation of vacancies at the Mg site of both MgB2 and substituted MgB1.75O0.25 was modelled as well, but has shown that such processes are energetically disadvantageous ([Delta]Hf of Mg0.875B2 and Mg0.75B1.75O0.25 are equal to -45.5 and -93.5 meV/atom, respectively)

Structure and superconducting characteristics of magnesium diboride, substitution of boron atoms by oxygen and carbon

An x-ray analysis of MgB2-based materials shows that they contain MgB2 and MgO phases. According to a quantitative Auger analysis (taken after removing the oxidized surface layer by Ar ion etching in the microscope chamber) the MgB2 phase contains some amount of oxygen that approximately corresponds to the composition MgB2.2-1.7O0.4-0.6. Rietveld refinement of the MgB2 phase, based on EDX data with varying B/O content, leads to the composition MgB1.68-1.8O0.2-0.32. Ab-initio modelling of boron substitution by oxygen in MgB2 ([Delta]Hf =-150.6 meV/atom) shows that this is energetically favourable up to the composition MgB1.75O0.25 ([Delta]Hf =-191.4 meV/atom). In contrast to carbon substitution, where very small levels of doping can dramatically affect the superconducting characteristics of the material with concomitant changes in the electron density, oxygen substitution results in very little change in the superconducting properties of MgB2. The formation of vacancies at the Mg site of both MgB2 and substituted MgB1.75O0.25 was modelled as well, but has shown that such processes are energetically disadvantageous ([Delta]Hf of Mg0.875B2 and Mg0.75B1.75O0.25 are equal to -45.5 and -93.5 meV/atom, respectively)

Higher borides and oxygen-enriched Mg-B-O inclusions as possible pinning centers in nanostructural magnesium diboride and the influence of additives on their formation

The study of high pressure (2 GPa) synthesized MgB2-based materials allows us to conclude that higher borides (with near MgB12 stoichiometry) and oxygen-enriched Mg-B-O inclusions can be pinning centers in nanostructural magnesium diboride matrix (with average grain sizes of 15-37 nm). It has been established that additions of Ti or SiC as well as manufacturing temperature can affect the size, amount and distribution of these inclusions in the material structure and thus, influence critical current density. The superconducting behavior of materials with near MgB12 stoichiometry of matrix is discussed.Comment: 4 pages, 1 figues, presented at VORTEX VI-2009, accepted for Physica

The early Upper Palaeolithic of the Tunka rift valley, Lake Baikal region, Siberia

This paper presents recent results of geological and archaeological research at Late Pleistocene sites in the Tunka rift valley (Lake Baikal region, southern Siberia), including new radiocarbon dating of the Palaeolithic layers at Bol'shoi Zangisan, Slavin Yar and Tuyana. The sites range in age from ?26 to 45 ka 14C BP and represent the earliest evidence of human habitation in the area. Numerous faunal remains have also been identified in the archaeological horizons from which palaeoenvironmental conditions can be reconstructed. These data also provide important new information about the age, context, and development of an early microlithic industry in the Tunka-Pribaikal'e region during the late Karginskii interstadial, attributed to Marine Isotope Stage 3 (MIS3). Although further research is needed to verify the reconstructed site age models, archaeological evidence recovered at Tuyana and Bol'shoi Zangisan represent among the oldest known occurrences of microcore-microblade technology in North Asi

Influence of Oxygen and Boron Distribution on the Superconducting Characteristics of Nanostructural Mg-B-O Ceramics

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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)

Temperature–pressure induced nano-structural inhomogenities for vortex pinning in bulk MgB2 of different connectivity

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