Electronic correlations and crystal structure distortions in BaBiO3

BaBiO3 is a material where formally Bi4+ ions with the half-filled 6s-states form the alternating set of Bi3+ and Bi5+ ions resulting in a charge ordered insulator. The charge ordering is accompanied by the breathing distortion of the BiO6 octahedra (extension and contraction of the Bi-O bond lengths). Standard Density Functional Theory (DFT) calculations fail to obtain the crystal structure instability caused by the pure breathing distortions. Combining effects of the breathing distortions and tilting of the BiO6 octahedra allows DFT to reproduce qualitatively experimentally observed insulator with monoclinic crystal structure but gives strongly underestimate breathing distortion parameter and energy gap values. In the present work we reexamine the BaBiO3 problem within the GGA+U method using a Wannier functions basis set for the Bi 6s-band. Due to high oxidation state of bismuth in this material the Bi 6s-symmetry Wannier function is predominantly extended spatially on surrounding oxygen ions and hence differs strongly from a pure atomic 6s-orbital. That is in sharp contrast to transition metal oxides (with exclusion of high oxidation state compounds) where the major part a of d-band Wannier function is concentrated on metal ion and a pure atomic d-orbital can serve as a good approximation. The GGA+U calculation results agree well with experimental data, in particular with experimental crystal structure parameters and energy gap values. Moreover, the GGA+U method allows one to reproduce the crystal structure instability due to the pure breathing distortions without octahedra tilting

Dynamic model of cold strip rolling

A dynamic model of the cold rolling process at the continuous rolling mill that combines the model of the electromechanical system and the model of the deformation zone has been developed. The paper presents the results of the study of kinematic parameters, force and energy parameters of the deformation zone with regard to the fluctuating nature of the process variables and to the dynamic processes running in the electromechanical system. It is shown that the variations in the stated parameters are of fluctuating unsteady nature, which has an adverse effect on operation of equipment and quality of finished products

Competency model for science and technology (S&T) personnel for the iron and steel industry

The article presents the competency model for S&T personnel for the iron and steel industry. The study performed has made it possible to determine a cluster of competencies required for innovative development of production involving development of energy-efficient technologies and high-performance steels. The results of the performed analysis have shown that for highly qualified professionals it is not sufficient just to have professional knowledge and demonstrate personal efficiency and ability to work in a team. They should also have professional engineering excellence skills, management skills and should be innovation-focused

Transition state method and Wannier functions

We propose a computational scheme for materials where standard Local Density Approximation (LDA) fails to produce a satisfactory description of excitation energies. The method uses Slater's "transition state" approximation and Wannier functions basis set. We define a correction to LDA functional in such a way that its variation produces one-electron energies for Wannier functions equal to the energies obtained in "transition state" constrained LDA calculations. In the result eigenvalues of the proposed functional could be interpreted as excitation energies of the system under consideration. The method was applied to MgO, Si, NiO and BaBiO$_3$ and gave an improved agreement with experimental data of energy gap values comparing with LDA.Comment: 13 pages, 6 figures, 1 tabl

Wannier functions and exchange integrals: The example of LiCu2_{2}O2_{2}

Starting from a single band Hubbard model in the Wannier function basis, we revisit the problem of the ligand contribution to exchange and derive explicit formulae for the exchange integrals in metal oxide compounds in terms of atomic parameters that can be calculated with constrained LDA and LDA+U. The analysis is applied to the investigation of the isotropic exchange interactions of LiCu$_{2}$O$_{2}$, a compound where the Cu-O-Cu angle of the dominant exchange path is close to 90$^{\circ}$. Our results show that the magnetic moments are localized in Wannier orbitals which have strong contribution from oxygen atomic orbitals, leading to exchange integrals that considerably differ from the estimates based on kinetic exchange only. Using LSDA+U approach, we also perform a direct {\it ab-initio} determination of the exchange integrals LiCu$_{2}$O$_{2}$. The results agree well with those obtained from the Wannier function approach, a clear indication that this modelization captures the essential physics of exchange. A comparison with experimental results is also included, with the conclusion that a very precise determination of the Wannier function is crucial to reach quantitative estimates.Comment: 8 pages, 8 figure

Chatter prevention in stands of continuous cold rolling mills

The article states new results of the theoretical and applied research on prevention of chatter in stands of continuous cold rolling mills. It was found that chatter occurs under those process conditions, which involve violation of a condition of strip rolling with tension because of ongoing change of the process state, when the process proceeds now with tension and now with pushing because of variation of strip volumes per second in a negative or positive range of values. It is demonstrated that it is possible to influence effectively the difference of strip volumes per second and prevent chatter by decreasing reduction in a stand subject to chattering and simultaneously increasing reduction in adjacent stands, and also by changing the ratio of rolling speeds in these stands of the mill. This has led to development of process conditions for thin steel strip rolling on the 5-stand mill 1700 of PAO Severstal, enabling rolling speed increase and process stabilization