Acoustic and thermal properties of silica aerogels and xerogels S. Caponi, 1 G. Carini, 2 G. D’Angelo, 2 A. Fontana, 1 O. Pilla, 1 F. Rossi, 1 F. Terki, 3 G. Tripodo, 2 and T. Woignier 3 1 INFM and Dipartimento di Fisica, Università di Trento, 38050 Povo, Trento, Italy 2 Dipartimento di Fisica, Università di Messina, 98166 S. Agata, Messina, Italy 3 Laboratoire des Verres, UMR 5587, Université Montpellier II, F34095, Montpellier Cedex, France (Received 2 April 2004; published 15 December 2004) Comparative measurements of Brillouin light scattering and ultrasounds in a wide class of silica aerogels and xerogels show the existence of distinct mechanisms governing the temperature behaviors of the acoustic attenuation in the different frequency ranges. In the MHz range the attenuation is mainly regulated by dynami- cal mechanisms due (i) to thermally activated local motions of structural defects typical of vitreous silica at low temperatures and (ii) to relaxations of “extrinsic” defects at high temperatures, i.e., the hydroxyl groups covering the inner surface of the pores in aerogels. In the MHz range, instead, the attenuation is dominated by a temperature independent, or “static,” process due to the scattering of phonons by pores. By growing the density of gels up to values close to that of dense vitreous silica, the acoustic attenuation shows a strongly temperature dependent behavior. The sound velocity scales with the density of the system following a power law, which is in good agreement with the predictions of a model describing silica gels in terms of a disordered network of microrods or microplates. The same law also permits to account for the temperature dependence of the sound velocity which reflects very closely the behavior observed in dense vitreous SiO 2 . Finally the analysis of the low temperature specific heat 1.5–20 K reveals that, for densities larger than about 1000 kg m -3 , the vibrational dynamics of these porous systems tends to reproduce the one of vitreous SiO 2 . DOI: 10.1103/PhysRevB.70.214204 PACS number(s): 61.43.Fs, 78.35.+c, 62.80.+f, 65.40.Ba I. INTRODUCTION It is well known that disordered solids exhibit properties in the low frequency vibrational dynamics which are not observed in their crystalline counterparts. 1–3 Among them, the specific heat of amorphous materials is much larger than the Debye value C D , 4 the thermal conductivity shows a pla- teau between 5 and 20 K, 4 and the density of vibrational states, g, exhibits a broad excess band in the plot of g /  2 referred to as Boson peak (BP). 5–7 The nature of these excess modes has been the object of different specula- tions particularly in the case of vitreous silica v-SiO 2 , which is considered the prototype of strong glasses. All these anomalies are usually ascribed to the nanometric length-scale and deal with the not yet completely understood vibrational dynamics of amorphous materials and in particular with the nature and the attenuation mechanisms of vibrational modes. 8–11 The questions at issue are without doubt relevant for the physics of disordered systems and a possible way to have an insight on this problem is the study of porous sys- tems having a solid structure based on a connective back- bone whose size can be reduced to a nanometric level. For this purpose, good candidate materials are silica aerogels and xerogels, highly porous solids whose density can be changed in the wide range from about 100 to 2200 kg/m 3 . We have studied samples with densities in the range between 500 and 2200 kg/m 3 , as a consequence of a controlled sintering pro- cedure leading to modifications of their “texture” and of the network connectivity. 12,13 The sample density was always equal or greater than 500 kg/ m 3 to avoid the fractal phenom- enology. In light silica aerogels having a density lower than 500 kg/m 3 , in fact, the dominant contribution to the vibra- tional dynamics has been attributed to localized modes (frac- tons), supported by a network having a fractal mass distribu- tion over distances smaller than a characteristic length  (the size of the fractal cluster). 14 A well-defined phonon-fracton crossover, whose frequency increases with increasing density (or decreasing characteristic length), is exhibited in the den- sity of vibrational states determined by inelastic neutron and Raman scattering. 15,16 In the frequency region below the phononfracton crossover corresponding to length scales larger than , the aerogels can be considered as a system supporting the propagation of acoustic vibrations. Neverthe- less, also samples with densities higher than 500 kg/ m 3 , show unexpected properties when compared to bulk glasses: (i) the low temperature specific heat is not always an excess respect to the Debye value C D , but can be much lower than C D ; 17,18 (ii) the presence of strong quasielastic scattering in the low frequency region of the Raman and neutron spectra. 19–22 In addition to this, the inelastic characteristics (i.e., the dissipation mechanisms of elastic energy) of fractal and densified silica gels appear to be governed by the relax- ations of locally mobile particles. Acoustic measurements carried out below 100 kHz in fractal silica aerogels in a re- stricted temperature range 23 were unable to establish if the observed relaxational contributions to the sound attenuation arise from the structural (bulk) defects of v-SiO 2 , from sur- face relaxors or eventually from adsorbed molecules within the large surface of inner pores. Experiments of Brillouin spectroscopy performed in fractal and densified gels at room temperature 24,25 revealed the presence of relaxations tenta- tively ascribed to the organic groups covering the surface of the silica backbone. Now, ultrasounds and hypersounds by Brillouin light scattering probe the relaxation processes im- posing their length scales in the experiment. The relevant length scale (i.e., the size of the relaxing molecular groups involved) is clearly of local nature, implying that the probe is PHYSICAL REVIEW B 70, 214204 (2004) 1098-0121/2004/70(21)/214204(8)/$22.50 ©2004 The American Physical Society 214204-1