Giovanni Carini

Effect of cation sizes on tunnelling states, relaxations and anharmonicity of alkali borate glasses

The relaxation losses and the corresponding velocity variations, observed at ultrasonic frequencies in (M2O)0.14(B2O3)0.86 alkali borate glasses (M = Li, K, Cs) between 1.5 and 300 K, have been modelled by an asymmetric double-well potential model having a distribution of both the barrier potential and the asymmetry. It is shown that the relaxation strength C* and the spectral density of asymmetries f0 decreases markedly with decreasing cation size. Below 10 K the sound attenuation is regulated by the phonon-assisted relaxation of tunnelling systems and exhibits a tunnelling strength C, ranging between 10−4 and 10−3. At variance with the behaviour observed for C*, C slightly increases with decreasing cation size and is more than one order of magnitude smaller than C*. It is concluded that, differently from classical relaxing states, tunnelling systems are independent of bond strengths and of structural changes characterizing a glassy network, confirming their inherent universality. Above about 120 K the ultrasonic velocity is mainly regulated by vibrational anharmonicity and shows a nearly linear decrease as the temperature is increased, the slope scaling with the cation size. Taken together, the observations point to the existence of a distinct correlation between anharmonicity and local mobility in the glassy network.

Low temperature specific heats of permanently densified glassy GeO2

Specific heat was measured over the temperature range 0.4–25 K in GeO2 glasses, compacted at pressures of 2 and 6 GPa to achieve more than 20% densification. Above 1 K, specific heat C was reduced by up to a factor of 2.5 with increasing density; the variation being stronger than that observed below 1 K. The broad peak observed above 1 K in a C/T 3 vs. T plot shifted to higher temperatures with increasing density, exhibiting changes which cannot be explained by the modifications of the elastic continuum. In addition, the peak showed no changes in shape, clearly indicating that the overall distribution of the underlying vibrational modes was independent of density. The temperature dependence of C over the explored range was in qualitative agreement with the predictions of the Soft Potential Model.

Low temperature heat capacity of permanently densified SiO2glasses

A study of low temperature specific heat capacity (1–30 K) has been performed on samples of vitreous SiO2, which have been compacted under pressures up to 8 GPa to explore different glassy phases having growing density. Increasing densification by more than 21% leads to a progressive reduction of the specific heat capacity C p and to a shift from 10 K up to about 17 K of the broad hump, the calorimetric Boson peak (BP), observed above 1 K in a C p (T)/T 3 vs. T plot. The revealed changes are not accounted for by the modifications of the elastic continuum, implying a nature of additional vibrations at variance with the extended sound waves. Increasing atomic packing of the glassy network leads to a progressively decreasing excess heat capacity over that of α-quartz, a crystalline polymorph of SiO2. By using the low-frequency Raman intensity measured in these glasses to determine the temperature dependence of the low temperature heat capacity, it has been evaluated the density of low-frequency vibrational states. The observations are compared with some theoretical pictures explaining the nature of the BP, disclosing qualitative agreement with the predictions of the Soft Potential Model and the results of a simulation study concerning the vibrations of jammed particles. This finding leads to evaluate a nanometer length scale which suggests the existence of poorly packed domains formed from several n-membered rings involving SiO4 tetrahedra. These soft regions are believed to be the main source of low-frequency vibrations giving rise to the BP.

Origin of excess low-energy vibrations in densified B2O3glasses

Low-temperature experiments of Raman scattering and heat capacity have been performed in a B2O3 glass, pressure quenched from 1200 °C in order to obtain the density as largest as possible (ρ = 2373 kg/m3). When compared to those of compacted B2O3 glasses having smaller density, the Raman spectrum of this glass exhibits a strong decrease of the intensities of the Boson peak and the band at 808 cm−1, both the features being determined by the decrease of the boroxol ring population. Moreover, the Boson peak exhibits a large shift to 68 cm−1 (from 26 cm−1 observed in normal vitreous B2O3). The high atomic packing of the glassy network also leads to a marked decrease of the excess heat capacity over the Debye T3-behaviour characterizing the crystal. The density g(ν) of low-frequency vibrational states has been assessed by using the low-frequency Raman intensity to determine the temperature dependence of the low-temperature heat capacity. The observations performed over a wide range of glass densities are compared to the predictions of theoretical models and computer simulations explaining the nature of the Boson peak. Consistency with the results of a simulation study concerning the vibrations of jammed particles leads to evaluate a nanometre length scale which suggests the existence of poorly packed domains formed from several connected boroxols. These soft regions are believed to be the main source of low-frequency optic-like vibrations giving rise to the Boson peak.

New insights on the specific heat of glasses

The physical properties of disordered systems are in the focus of a large research effort. In particular, one of the open problems related to glasses is their excess of modes in the THz frequency range over the vibrational contribution predicted by the Debye model. In fact, one could expect the continuum approximation underlying the Debye model to hold in glasses at long wavelengths at least as well as in crystalline solids. Experiments, instead, seem to indicate that the specific heat at low temperature (a few Kelvin) and the density of vibrational states at low frequency (a few terahertz) are very different from the Debye prediction. We present here a detailed analysis of specific heat measurements of vitreous GeO2, a prototype of strong glasses, and of permanently densified vitreous GeO2. Our data give experimental evidence that glasses do not show any excess of vibrational modes when compared to their crystalline counterparts of similar mass density.

Ultrasonic and hypersonic behaviours of borate glasses

Comparative measurements of Brillouin light scattering and ultrasound in (K2O)0.04(B2O3)0.96 and (Ag2O)0.14(B2O3)0.86 borate glasses as a function of temperature between 1.5 and 300 K reveal that distinct mechanisms regulate the temperature behaviours of the acoustic attenuation. In the MHz range the attenuation and the sound velocity are mainly governed by (i) quantum-mechanical tunnelling below 20 K, (ii) thermally activated relaxations between 20 and 200 K and (iii) vibrational inharmonicity at even higher temperatures. In the GHz range and in the temperature interval between 77 and 300 K, additional contributions besides the relaxation process must be taken into consideration to account for the hypersonic attenuation.

Effect of Blending on the Structure of Thermoplastic Interpenetrating Polymer Networks

The infrared absorption spectra of thermoplastic interpenetrating polymer networks based on semicrystalline polyurethane (CPU) and a styrene/acrylic acid block‐copolymer (S‐b‐AA(K+), salt form) have been measured over the spectral range between 4000 and 600 cm−1 in an attempt to study their structure. The analysis of the spectra of the individual polymers and CPU/S‐b‐AA(K+) blends has shown the mutual influence of components on the formation of a network of intra and intermolecular physical bonds between functional groups of the components. In close agreement with these observations, the composition behaviors of experimental results of density, tensile strength, differential scanning calorimetry, and dynamic mechanical spectroscopy reveal deviations from a simple additive law which have been associated to a weak affinity of the individual components. It has been suggested that the existence of specific weak interactions (H‐bonds) between the functional groups of the two components ensure the formation of a mixed microphase. In Commemoration of the Contribution of Professor Valery P. Privalko to Polymer Science.