Porous glasses relaxation response

Non-Debye dielectric relaxation in porous systems is another example of the dynamic behavior of complex systems on the mesoscale. The dielectric properties of various complex multiphase systems (borosilicate porous glasses [153-156], sol-gel glasses [157,158], zeolites [159], and porous silicon [160,161]) were studied and analyzed recently in terms of cooperative dynamics. The dielectric response in porous systems will be considered here in detail using two quite different types of materials, namely, porous glasses and porous silicon. 

We measured the temperature-dependance of the spin-lattice relaxation time, for various alumino-silicate aerogels, corresponding porous glasses and crystalline counterparts. The purpose of these experiments is threefold (i) to compare the relaxation response of these very porous amorphous materials to the general one of more classical glasses, (ii) to see whether fractons, whose vibrationnal amplitudes are large, contribute to relaxation mechanisms, (iii) to follow - through variations of the density - the dependance of this dynamical property on the structural parameters, (iv) to test the theoretical predictions about relaxation in disordered systems proposed by R. Orbach and S. Alexander. 

Aveston eta/. [50] suggested that this is not necessarily always the case, and that the overall orientation efficiency would also depend on the response of the matrix to the local flexural stresses. If the matrix is sufficiently weak, it will crumble, and the flexural stresses will be effectively relaxed. They thought this to be the case in the carbon fibre reinforced cement tested in their work. Stucke and M umdar [52] applied this mechanism to account for the embrittlement of glass fibre reinforced cement and suggested that the densification of the ageing matrix around the fibres leads to a build-up of flexural stresses in the fibres in the cracked zone, which in turn results in premature failure. In the younger composite, the matrix interface is more porous and weaker, and crumbles before any significant flexural stress can develop in the fibres. 

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