Hopping and optical absorption of electrons in nano-porous crystal 12CaO·7Al2O3

Recently it has been discovered that a nano-porous main group oxide 12CaO·7Al₂O₃ (C12A7) can be converted from a wide-gap insulator to a good transparent conductor. Using ab initio modelling we explain good conductivity of this material by very small barriers for hopping of localised electrons between neighbouring positive cages. We show that optical absorption of C12A7 in infrared region and at energies higher than 2.7 eV is due to inter-cage and intra-cage electron transitions, respectively. The proposed mechanisms can be useful in further search for conducting transparent media.     

On a peculiar feature of longitudinal differential protection of generators reacting to vector values of currents

The possibility of activation delay of longitudinal differential protection of generators reacting to vector values is considered. This work presents calculated data for currents of three-phase short circuits for a turbogenerator operating in combination with a transformer as a function of number of short circuited turns in generator winding.     

Application of a new algorithm using Doppler information to retrieve complex wind fields over the Black Sea from ENVISAT SAR images

Several algorithms have been proposed to retrieve near-surface wind fields from C-band synthetic aperture radar (SAR) images acquired over the ocean. They mainly differ in the way they retrieve the wind direction. Conventionally, the wind direction is taken from atmospheric models or is extracted from the linear features sometimes visible in SAR images. Recently, a new wind retrieval algorithm has been proposed, which also includes the Doppler shift induced by motions of the sea surface. In this article, we apply three wind retrieval algorithms, including the one using Doppler information, to three complex wind events encountered over the Black Sea and compare the SAR-derived wind fields with model wind fields calculated using the high-resolution weather research and forecasting (WRF) model. It is shown that the new algorithm is very efficient in resolving the 180° ambiguity in the wind direction, which is often a problem in the streak-based wind retrieval algorithms. However, the Doppler-based algorithm only yields good results for wind directions that have a significant component in the look direction of the SAR antenna. Furthermore, it is dependent on good separation of the contributions to the Doppler shift induced by surface currents and wind-related effects (wind drift and wind-sea components of the ocean wave spectrum). We conclude that an optimum wind retrieval algorithm should consist of a combination of the algorithms based on linear features and Doppler information.     

Thermoelastic State of a Half Space Containing a Thermally Active Circular Crack Perpendicular to Its Edge

The problem of stationary thermal conductivity and thermoelasticity is solved by the method of boundary integral equations for a semiinfinite body containing a circular crack perpendicular to its edge provided that temperature (or a heat flow is specified) on the crack surfaces. The boundary of the body is unloaded and either is thermally insulated or its temperature is equal to zero. We study the influence of the depth of location of the circular crack on the stress intensity factor for a constant temperature (or heat flow) specified on the crack surfaces. Under thermal loading (unlike the case of constant force loading), the stress intensity factors attain their maximum values on the side of the half space opposite to its boundary. __________ Translated from Fizyko-Khimichna Mekhanika Materialiv, Vol. 41, No. 2, pp. 16–22, March–April, 2005.     

Pressure Effects on Superconducting and Magnetic Transitions in Layered Sodium Cobalt Oxides, NaxCoO2

Magnetoresistance for field close to the parallel to the superconducting planes direction has been studied in organic superconductor κ-(BEDT-TTF)₂Cu[N(CN)₂]Br. The results show a first order phase transition between the superconducting state and the normal state at the upper critical field H_c2. The 1^st order transition is evidenced by the hysteretic behavior in the magnetoresistance as a function of field or angle between the field and the plane. A possible model is discussed.     

Thermoelastic state of a half space containing a thermally active elliptic crack perpendicular to its boundary

By the method of boundary integral equations, we solve the problem of stationary heat conduction and thermoelasticity for a semiinfinite body containing an elliptic crack perpendicular to its boundary in the case where the crack surfaces are kept at a certain temperature. The boundary of the body is unloaded and either thermally insulated or kept at temperature equal to zero. We study the influence of crack depth and its location on the stress intensity factor at a given constant temperature. __________ Translated From Fizyko-Khimichna Mekhanika Materialiv, Vol. 42, No. 2, Pp. 45–52, March–April, 2006.     

Boundary layers in fibrous composite materials

Many studies in the theory of composite materials are based on the homogenization approach, which consists of the substitution of the original heterogeneous medium by a homogeneous one with certain effective properties. Though this procedure works well for the entire composite solid, it cannot be applied in the vicinity of the outer boundary. The transmission of an external load applied at the boundary to the inner domain of the material occurs by a redistribution of stresses between the constitutive components (inclusions and matrix) and involves strong singularities in the local stress field, which may result in microscopic failure of the composite structure. In the present paper, we propose an approximate analytical procedure, allowing determination of the stress?Cstrain field in the vicinity of the outer boundaries of fibre-reinforced composite materials. It is also shown that controlled decrease in bonding between the components leads to a more uniform redistribution of local stresses, which can essentially reduce the risk of failure.