Andrei A. Snarskii

Thermoelectric figure of merit for bulk nanostructured composites with distributed parameters

The effective properties of composites whose structure includes nanocontacts between bulk-phase macrocrystallites are considered. A model for such a nanostructured composite is constructed. Effective values of the thermoelectric power, thermal and electrical conductivities, and thermoelectric figure of merit are calculated in the mean-field approximation.

Limiting values of the quality factor of thermoelectric composites

It is shown that consideration of the effect of thermoelectric phenomena on effective conductivity and thermal conductivity can lead to “rigid” limitations on the effective thermoelectric quality factor. In some cases, a limiting value of the quality factor exists irrespective of how large the quality factor of the phases is.

The effective properties of macroscopically nonuniform ferromagnetic composites: Theory and numerical experiment

Various theoretical models (self-consistent field, local linearization, and percolation theory methods and an analytic solution of the linear problem for an ordered medium) for calculating the magnetostatic properties of two-phase composites containing one ferromagnetic phase were considered. The concentration and field dependences of the effective magnetic permeability were found. A method for determining the coercive force and remanent magnetization as functions of the ferromagnetic phase concentration was suggested. Numerical experiments were performed for composites with a periodic distribution of circular inclusions. The results were compared with the analytically calculated effective magnetic permeability.

ORIGIN OF GIANT THERMOPOWER IN YBA2CU3O7-X FILMS

A simple explanation of the origin of unusually large voltage signals observed on high‐Tc superconducting thin films of composition YBa2Cu3O7−x (with c axis tilted away from the film surface normal) has been given. The phenomenon is suggested to be due to an anisotropy in the Seebeck coefficient. This treatment leads to an expression which captures the essential results of the experiments.

Effect of single-particle magnetostriction on the shear modulus of compliant magnetoactive elastomers

The influence of an external magnetic field on the static shear strain and the effective shear modulus of a magnetoactive elastomer (MAE) is studied theoretically in the framework of a recently introduced approach to the single-particle magnetostriction mechanism [V. M. Kalita et al., Phys. Rev. E 93, 062503 (2016)10.1103/PhysRevE.93.062503]. The planar problem of magnetostriction in an MAE with magnetically soft inclusions in the form of a thin disk (platelet) having the magnetic anisotropy in the plane of this disk is solved analytically. An external magnetic field acts with torques on magnetic filler particles, creates mechanical stresses in the vicinity of inclusions, induces shear strain, and increases the effective shear modulus of these composite materials. It is shown that the largest effect of the magnetic field on the effective shear modulus should be expected in MAEs with soft elastomer matrices, where the shear modulus of the matrix is less than the magnetic anisotropy constant of inclusions. It is derived that the effective shear modulus is nonlinearly dependent on the external magnetic field and approaches the saturation value in magnetic fields exceeding the field of particle anisotropy. It is shown that model calculations of the effective shear modulus correspond to a phenomenological definition of effective elastic moduli and magnetoelastic coupling constants. The obtained theoretical results compare well with known experimental data. Determination of effective elastic coefficients in MAEs and their dependence on magnetic field is discussed. The concentration dependence of the effective shear modulus at higher filler concentrations has been estimated using the method of Padé approximants, which predicts that both the absolute and relative changes of the magnetic-field-dependent effective shear modulus will significantly increase with the growing concentration of filler particles.

Single-particle mechanism of magnetostriction in magnetoactive elastomers.

Magnetoactive elastomers (MAEs) are composite materials comprised of micrometer-sized ferromagnetic particles in a nonmagnetic elastomer matrix. A single-particle mechanism of magnetostriction in MAEs, assuming the rotation of a soft magnetic, mechanically rigid particle with uniaxial magnetic anisotropy in magnetic fields is identified and considered theoretically within the framework of an alternative model. In this mechanism, the total magnetic anisotropy energy of the filling particles in the matrix is the sum over single particles. Matrix displacements in the vicinity of the particle and the resulting direction of the magnetization vector are calculated. The effect of matrix deformation is pronounced well if the magnetic anisotropy coefficient K is much larger than the shear modulus µ of the elastic matrix. The feasibility of the proposed magnetostriction mechanism in soft magnetoactive elastomers and gels is elucidated. The magnetic-field-induced internal stresses in the matrix lead to effects of magnetodeformation and may increase the elastic moduli of these composite materials.

Effective Conductivity of Percolation Media

The analogy between percolation theory and theory of second-order phase transition is introduced. Effective conductivity is considered as the order parameter of phase transitions. Calculations of critical indexes are provided. Different models of percolation media are considered. Hierarchical model of percolation structure is introduced.

INVESTIGATION OF THE DYNAMICS OF BODY OXYGEN STATUS UPON LIMITED LOADS

This paper discusses the dynamics of oxygen partial pressure in intercellular fluid upon a limited load and a subsequent post-ischemic hyperemia in healthy individuals and individuals with vascular pathologies of the lower limbs. Within the framework of the phenomenological approach, which takes into consideration the qualitative peculiarities of the gas exchange processes in the body, a formal theory describing the dynamics of oxygen (O2) concentration dependently on the conditions of blood supply to the tissues has been developed. It was shown that certain parameters, for instance the relaxation time upon the conditions of artificial ischemia and the characteristic time of return of spontaneous blood circulation in the post-ischemic period, depend on the degree of pathology of the vascular system and may be used for assessing the functional status of a biological body.

Temperature-dependent magnetic properties of a magnetoactive elastomer: Immobilization of the soft-magnetic filler

Magnetic properties of a magnetoactive elastomer (MAE) filled with {\mu}m-sized soft-magnetic iron particles have been experimentally studied in the temperature range between 150 K and 310 K. By changing the temperature, the elastic modulus of the elastomer matrix was modified and it was possible to obtain magnetization curves for an invariable arrangement of particles in the sample as well as in the case when the particles were able to change their position within the MAE under the influence of magnetic forces. At low (less than 220 K) temperatures, when the matrix becomes rigid, the magnetization of the MAE does not show a hysteresis behavior and it is characterized by a negative value of the Rayleigh constant. At room temperature, when the polymer matrix is compliant, a magnetic hysteresis exists and exhibits local maxima of the field dependence of the differential magnetic susceptibility. The appearance of these maxima is explained by the elastic resistance of the matrix to the displacement of particles under the action of magnetic forces.

Nonlinear Properties of Composites

Up to now, in the calculation of the effective properties of composites it has been always assumed that local volt-ampere characteristics are linear, i.e., Ohm’s law is valid for each of the phases.