Teoretske granice električne provodljivosti sinterovanih materijala i njihov odnos prema Youngovom modulu

The constraints to the effective electrical conductivity were derived as function of distribution of material electrical conductivity throughout the sample. If the sample consists only of homogeneous isotropic matrix and pores, the constraints to effective electrical conductivity take the form very similar to the constraints derived previously for the effective Young’s modulus of porous materials. The differences occurring in expressions are due to the fact that elastic properties of an isotropic matrix material are characterized by two quantities, e.g. Young’s modulus and Poisson’s ratio, while the conductivity is characterized only by one quantity. But regardless of the small differences, in the both cases the key role in determining the properties is played by the minimum load-bearing cross section of the sample considered.Zapreke učinkovitoj električnoj provodnosti izvedene su kao funkcije distribucije električne provodnosti i poprimaju oblik vrlo sličan onim smetnjama koje su ranije izvedeni za učinkovit Youngov modul poroznog materijala. Razlike koje se pojavljuju u izrazima nastale su zbog činjenice da su svojstva elastičnosti nekog izotropnog matričnog materijala okarakterizirane dvjema količinama, npr. Youngovim modulom i Poissonovim odnosom, dok je provodnost okarakterizirana samo jednom količinom, ali bez obzira na male razlike, u obadva ta slučaja ključnu ulogu u određivanju svojstva igra primjer uzorka pod minimalnim naponom

Effect of local treatments of convection upon the solar p-mode excitation rates

We compute, for several solar models, the rates P at which the solar radial p modes are expected to be excited. The solar models are computed with two different local treatments of convection : the classical mixing-length theory (MLT hereafter) and Canuto, Goldmann and Mazzitelli(1996, CGM hereafter)'s formulation. For one set of solar models (EMLT and ECGM models), the atmosphere is gray and assumes Eddington's approximation. For a second set of models (KMLT and KCGM models), the atmosphere is built using a T(tau) law which has been obtained from a Kurucz's model atmosphere computed with the same local treatment of convection. The mixing-length parameter in the model atmosphere is chosen so as to provide a good agreement between synthetic and observed Balmer line profiles, while the mixing-length parameter in the interior model is calibrated so that the model reproduces the solar radius at solar age. For the MLT treatment, the rates P do depend significantly on the properties of the atmosphere. On the other hand, for the CGM treatment, differences in P between the ECGM and the KCGM models are very small compared to the error bars attached to the seismic measurements. The excitation rates P for modes from the EMLT model are significantly under-estimated compared with the solar seismic constraints. The KMLT model results in intermediate values for P and shows also an important discontinuity in the temperature gradient and the convective velocity. On the other hand, the KCGM model and the ECGM model yield values for P closer to the seismic data than the EMLT and KMLT models. We conclude that the solar p-mode excitation rates provide valuable constraints and according to the present investigation cleary favor the CGM treatment with respect to the MLT.Comment: 4 pages, 3 figures, proceedings of the SOHO14/GONG 2004 workshop "Helio- and Asteroseismology: Towards a Golden Future" from July 12-16 2004 at New Haven CT (USA

White dwarf envelopes: further results of a non-local model of convection

We present results of a fully non-local model of convection for white dwarf envelopes. We show that this model is able to reproduce the results of numerical simulations for convective efficiencies ranging from very inefficient to moderately efficient; this agreement is made more impressive given that no closure parameters have been adjusted in going from the previously reported case of A-stars to the present case of white dwarfs; for comparison, in order to match the peak convective flux found in numerical simulations for both the white dwarf envelopes discussed in this paper and the A-star envelopes discussed in our previous work requires changing the mixing length parameter of commonly used local models by a factor of 4. We also examine in detail the overshooting at the base of the convection zone, both in terms of the convective flux and in terms of the velocity field: we find that the flux overshoots by approximately 1.25 H_P and the velocity by approximately 2.5 H_P. Due to the large amount of overshooting found at the base of the convection zone the new model predicts the mixed region of white dwarf envelopes to contain at least 10 times more mass than local mixing length theory (MLT) models having similar photospheric temperature structures. This result is consistent with the upper limit given by numerical simulations which predict an even larger amount of mass to be mixed by convective overshooting. Finally, we attempt to parametrise some of our results in terms of local MLT-based models, insofar as is possible given the limitations of MLTComment: Accepted for publication in MNRAS; 11 pages, 5 figures, 3 table

Angle dependence of Andreev scattering at semiconductor-superconductor interfaces

We study the angle dependence of the Andreev scattering at a semiconductor-superconductor interface, generalizing the one-dimensional theory of Blonder, Tinkham and Klapwijk. An increase of the momentum parallel to the interface leads to suppression of the probability of Andreev reflection and increase of the probability of normal reflection. We show that in the presence of a Fermi velocity mismatch between the semiconductor and the superconductor the angles of incidence and transmission are related according to the well-known Snell's law in optics. As a consequence there is a critical angle of incidence above which only normal reflection exists. For two and three-dimensional interfaces a lower excess current compared to ballistic transport with perpendicular incidence is found. Thus, the one-dimensional BTK model overestimates the barrier strength for two and three-dimensional interfaces.Comment: 8 pages including 3 figures (revised, 6 references added