COMBUSTION OF POROUS SILICON IN AN OXYGEN ATMOSPHERE WITH A PRESSURE FROM 2 TO 33 BAR

Combustion in an oxygen atmosphere of a layer of porous silicon (pSi), which was formed by electrochemical etching of monocrystalline silicon wafers, was investigated. Layer thickness ranged from 20 to 135 mu m, porosity ranged from 50 to 70%. Four typical pSi combustion modes were determined and classified: a) smoldering, when combustion spreads over the surface of porous layer, mainly along the edges of the sample; b) frontal mode, when combustion propagates along the sample; c) combined mode, when particles of porous layer are ejected into the space above the sample behind the reaction front, forming a reacting aerosol above the surface of the wafers; and d) aerosol mode, in which aerosol combustion area is formed practically at the reaction front of surface combustion. It was established that the minimum ignition pressure decreases significantly with an increase in porous layer thickness, from 17.5 bar at a thickness of 19 mu m to 1.64 bar at a thickness of 61 mu m. Surface combustion velocities of porous silicon layer of 0.15 to 7.6 m/s were determined; it was demonstrated that the combustion velocity increases with increasing thickness of the porous layer. Analysis of integral radiation intensity and the velocity of pressure rise demonstrated a sharp increase in energy release with an increase in the initial oxygen pressure. When the pressure is increased by 5 times (from 5 to 25 bar), the radiation intensity increases by 1400 times, and pressure growth velocity increases by 75 times. Combustion propagation velocity along the sample increases with only an order of magnitude and in a linear manner. The emission spectra were recorded and the temperature in the combustion zone was estimated based on the shift of spectra maximums, and the temperature was measured by the photoemission method.