Mechanical Properties and Drying Methods

The thermal expansion coefficient of bulk silicon is positive at RT (2.6 x 1CT6 K-1), but becomes negative below 120 K. The thermal expansion coefficient of micro PS for heating from 290 to 870 K is found to be negative (-4.3x 10 6 KT1), which can be ascribed to hydrogen desorption and oxidation of the inner surface [Di7]. For meso PS the thermal expansion coefficient was found to increase with porosity in the temperature regime between 90 K and 300 K, from 0.4xl0-6 K 1 to 

6x10 6 K 1 [Fa8]. The latter effect, which has been ascribed to surface stress, also, is found to be reversible, in contrast to measurements at higher temperatures. 

The elastic properties of PS depend on its microstructure and porosity. The Young s modulus for meso PS, as measured by X-ray diffraction (XRD) [Ba8], acoustic wave propagation [Da5], nanoindentation [Bel3] and Brillouin spectroscopy [An2], shows a roughly (1-p)2 dependence. For the same values of porosity (70%), micro PS shows a significantly lower Young s modulus (2.4 GPa) than meso PS (12 GPa). The Poisson ratio for meso PS (0.09 for p=54%) is found to be much smaller than the value for bulk silicon (0.26) [Ba8]. 

The hardness, defined as the resistance to plastic deformation, of microporous silicon decreases with porosity p from the bulk silicon value of about 11.5 GaP to values around 4 GaP for porosities in the order of 75% following a (1-p)2/3 dependence. For porosities above 75% a further decrease in hardness is observed. The hardness of PS formed on highly doped p-type substrates is found to be somewhat less than that observed for low doped substrates, which may be caused by the more columnar structure of meso PS [Du5]. 

The thermal conductivity of bulk silicon (148 W K m ) is dominated by phonons electronic contributions are negligible. Due to restrictions of the mean free path of phonons in the porous network the thermal conductivity of micro PS is reduced by two or three orders of magnitude at RT, compared to the bulk value. Because of the larger dimensions of its network, meso PS shows a thermal conductivity several times larger than that of micro PS, for the same value of porosity. Thermal oxidation at low temperatures (300°C) is found to decrease the thermal conductivity of meso PS by a factor of about 0.5 [Pe9]. In contrast to bulk Si the thermal conductivity of PS is found to decrease with decreasing temperature [Be21, La4, Ge9, Lyl]. 

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