Magnetic states of iron ions in Bi<Subscript>0.75</Subscript>Si<Subscript>0.25</Subscript>FeO<Subscript>3 − <Emphasis Type="Italic">y</Emphasis></Subscript> perovskites

Hyperfine interactions on ⁵⁷Fe nuclei in cubic perovskite Bi_0.75Sr_0.25FeO_{3 ? y} in the temperature range 87–700 K are studied using Mössbauer spectroscopy. The temperature of the magnetic phase transition (the Neel point T _N ) of bismuth ferrite is T _N = 670(3) K. Below T _N , the experimental spectra demonstrate a partially resolved magnetic hyperfine structure with broadened lines, which is well described by superposition of four sextets. The values of the hyperfine magnetic field B and the isomer shift δ at room temperature initiated that all iron ions are in the trivalent state. Here, three sextets with the equal isomer shifts (δ₁ ≈ δ₂ ≈ δ₃ = 0.38 mm/s correspond to the iron ions in the octahedral oxygen environment; in the fourth sextet, the iron ions are in the square-pyramidal environment (δ₃ = 0.25 mm/s).     

Generation and accumulation of point defects in FCC single crystals upon plastic strain

A model of point defect generation and accumulation in a FCC crystal upon strain with a constant rate at room temperature is suggested in the present work. The model results are in agreement with the available experimental data. The influence of point defects on the formation of fragmented substructures is analyzed. The model is verified using experimental data on the stress‒strain curve, dislocation density, vacancy concentration, and misorientation angle.     

Two-parameter pseudopotential

A new physical model for determining the parameters of the two-parameter pseudopotential based on the Heine-Abarenkov approach is proposed, which in fact made it possible to pass to the one-parameter pseudopotential. The pseudopotential parameters are determined for atoms and ions of the second and third periods and Sr II ion. It is shown that the proposed two-parameter pseudopotential with determined parameters allows calculation of the position of excited one-electron levels of ions with good accuracy.     

Flow of Colloid Liquid in Horizontal Cell under Heating from Sidewall

Based on numerical simulation, the influence of sedimentation on the convective flow of colloid liquids filling a horizontal cell heated from the sidewall is studied. The set of nonlinear equations is solved by the finite-difference method using explicit schemes. Three convective patterns differing in spatial structure and behavior in time are distinguished. The transition between the patterns is accompanied by a jump in the dimensionless heat flow. Bifurcation diagrams of the convection patterns (the dependences of the heat flow intensity on the Rayleigh number) are given. It is shown that the weak flow of a colloidal suspension exists at a low temperature gradient, the intensity of which is several orders of magnitude lower than the intensity of the flow of a homogeneous liquid under the same parameters. The concentration in the flow with a weak intensity is redistributed in such a way that the density gradient becomes almost vertical, and the heat flow across the layer is absent at the same time. The transition from a weak to a strong one-vortex flow filling the entire cell proceeds abruptly. It is found the threshold of the transition from a weak to intense flow depends on the Boltzmann number characterizing the degree of gravitational stratification. One more flow, namely, a three-vortex flow with an intermediate intensity is generated upon a decrease in the Rayleigh number. Stream-function and concentration fields are manifested for all the observed types of flows.     

Stability of a laminar rivulet liquid flow in a cylindrical duct in the approximation of one-dimensional waves

The problem of the stability of a viscous laminar liquid flow with a liquid free surface in an inclined duct is theoretically considered. Since the dependence of the flow rate on the free-surface height is not monotonic (the highest flow rate in a cylindrical duct is observed at H_*=1.7R), primary attention is given to the region H>H_*. It is proved that there is aw region of instability: for an arbitrarity low Reynolds number, there is a free-surface level above which the flow becomes unstable against one-dimensional disturbances. When the height of the liquid layer is close to the vertical dimension of the duct, the one-dimensional disturbances propagate mainly upstream (for moderate Reynolds numbers). Hence it follows that there is not steady regime of liquid flow from a fully filled duct with an open end. Kutateladze Institute of Thermal Physics, Siberian Division, Russian Academy of Sciences, Novosibirsk 630090. Translated from Prikladnaya Mekhanika i Tekhnicheskaya Fizika, Vol. 40, No. 3, pp. 90–96, May–June, 1999.     

Invariant integral: The earliest works and most recent application

The present paper embraces mainly the three-year period of 1966 to 1968 when the invariant integral of fracture mechanics appeared and became popular, and the last two years of 2015 to 2016 when the neoclassic cosmology based on the invariant integral came up. A mention is given to the previous works of Euler, Cauchy, Maxwell, Nother, Gunther and Eshelby who dealt with invariant integrals in mathematics, hydrodynamics, electrodynamics, and the theory of dislocations. A brief review is given of the creation of the invariant integral of fracture mechanics under static and dynamic conditions for a solid continuum including elastic, plastic and viscoelastic materials, as well as of some of its most important applications, ramifications and generalizations for other physical fields. The initial phase of the expansion and revolution of the large-scale universe is studied in the framework of the neoclassic approach, including the Big Bang and the Dark Energy; it is shown that the spheroidal shape of the universe assumed at the Big Bang retains its eccentricity constant in the initial phase. The assumption of a superphoton as a primordial universe was analyzed.     

Determining the Size of Pores in a Partially Transparent Ceramics from Total-Reflection Spectra

A technique is proposed for determining the pore-size distribution based on measuring the dependence of total reflectance in the domain of partial transparency of a material. An assumption about equality of scattering-coefficient spectra determined by solving the inverse radiation transfer problem and by theoretical calculation with the Mie theory is used. The technique is applied to studying a quartz ceramics. The poresize distribution is also determined using mercury and gas porosimetry. All three methods are shown to produce close results for pores with diameters of <180 nm, which occupy ~90% of the void volume. In the domain of pore dimensions of >180 nm, the methods show differences that might be related to both specific procedural features and the structural properties of ceramics. The spectral-scattering method has a number of advantages over traditional porosimetry, and it can be viewed as a routine industrial technique.     

Evolution of Shock Waves in Hot-Pressed Ceramics of Boron Carbide and Silicon Carbide

Technical Physics - In this paper we studied the evolution of shock compression waves in hot-pressed ceramics based on boron carbide and silicon carbide at a maximum compressive stress of 32 and 34...     

Crack growth in metals at elevated temperature

Translated from Zhurnal Prikladnoi Mekhaniki i Tekhnicheskoi Fiziki, No. 5, pp. 144–150, September–October, 1989.