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J.E. LUGO+, J.A. DEL RIO*, J. TAGUENA-MARTINEZ* AND J.A. OCHOA-TAPIA**. +Facultad de Ciencias, Universidad Aut6noma del Estado de Morelos, 62210 Cuernavaca, Morelos, Mdxico. *Laboratorio de Energfa Solar, IIM-UNAM A.P. 34, Temixco, 62580 Morelos, Mexico. **Area de Ingenierfa Qulrmica, Universidad Aut6noma Metropolitana, Iztapalapa, A.P. 55534, 04390 M6xico, D.F. M6xico.

ABSTRACT In previous works we have obtained an expression for the effective electrical conductivity of a columnar model simulating porous silicon. We used the averaging volume method that has proven to be successful in treating fluid transport in porous media. With this method vie can calculate the bulk and the surface contribution to an effective transport property. The axial component can be solved analytically, but in the XY plane the calculatio•i can only be performed numerically. However there is a certain approximation called Chang's cell (valid for high porosities) where the transverse component is also analytical. The extension of our original approach to find the axial component of the effective dielectric function is relatively simple, but the transverse component calculation presents interesting features.

INTRODUCTION. Porous silicon (PS) could have relevant applications and presents interesting scientific challenges. An interesting polemic regarding the origin of its luminescence is taking place. On one hand the luminescence could be due to quantum confinement [1] but on the other hand, it could be due to new compounds formed on the surface (in particular siloxenes) [2]. First principle electronic structure calculations of silicon quantum wires indeed confirm that the gap widens and shifts to become direct. However, surface effects cannot l:e ignored, as the luminescence efficiency in PS is closely related the H passivation of the surface and the aging (oxidation) of the samples degrades the luminescence [3]. Most of the work has been directed to study optical properties. Particularly, from the above mentioned first principles calculations in a silicon quantum wire the imaginary dielectric function E2 (w) shows the expected peak in the visible region [41. Theoretically, we have calculated the static and the dynamic effective electrical conductivity for a periodic array of crystalline silicon columns, with a classical approach called volume averaging method [5], [6]. From our static results, we concluded that at low porosities (much lower than the luminescence range) the surface contribution could increase the bulk value for the effective conductivity. Nevertheless, passing the percolation threshold, the conductivity decreases with the porosity to reach zero at 100 %. From our dynamic results we conclded that the surface effects, similarly to the effect of quantum confinement, can shift the response peak to approach the experimental data. Now, we are addressing the calculation of the effective dielectric function using the same average method, which can be extended to study electrodynamic interaction with matter [7]. The advantage of th