RICERCA ITALIANA     

Research program

Vibrational dynamics and relaxation in densified glasses and confined disordered systems

University Co-ordinator

Università degli Studi di TRENTO - FISICA - ()

Research Unit Leader

Aldo Fontana

Description

The research unit in Trento will contribute to the project by:

1. Production of the permanently densified samples by the multi-anvil method, and by chemical reaction.

The glasses will be prepared from dehidrated powders of B2O3 and GeO2, having a degree of purity of 99.99%, in order to minimise (a few ppm) the OH content. This precaution is necessary because Boron and Germanium oxides are very hygroscopic and the presence of OH groups in the structure causes important alterations of their physical properties. Each glass will be stabilised by annealing at a temperature about 20-30 K above the calorimetric glass transition Tg, in high-purity nitrogen atmosphere, and subsequently cooled and kept at room temperature, so as to avoid undesired effects (due to different thermal histories) on the studied quantities. A gradual density increase will be achieved by putting the glasses (obtained by melt-quenching) under high pressure in the range 1-10 GPa, by means of a multi anvil apparatus, at temperatures slightly lower than the glass temperature Tg. Chemically densified polymeric glasses will also be produced (see later).

2- Study of the vibrational density of states of permanently densified glasses, by Raman and neutron scattering, and thermal measurements.

The main objective will be a systematic study of the low-energy dynamics in permanently densified glasses, and how it is linked to the elastic constants of the systems [2,3]. In particular, the investigation will be concerned primarily on two classes of glasses: (a) GeO2, B2O3, and sodium silicate glasses; (b) polymeric glasses.
Raman, light and neutron scattering will be used. These techniques cover almost the entire range of energy of interest, as regards both vibrations and relaxations, which may play an active role.

(a) GeO2, B2O3, and sodium silicate glasses. These are prototype strong glasses. Preliminary studies by Raman and neutron scattering on GeO2 [L. Orsingher, PhD Thesis, to be published], evidence an anomalous decrease of the BP intensity with increasing system density. There are controversial interpretations of these results. Therefore, it is necessary to extend the investigation to other systems, by means of different and complementary techniques. In particular, in cooperation with the other research units taking part in the project, we will compare the results obtained by: Raman and neutron scattering (in cooperation with the unit of Perugia), specific heat and ultrasound attenuation (in cooperation with the unit of Messina), structural X ray measurements (in cooperation with the unit of Camerino).

(b) The polymeric system that will be investigates is a mixture of epoxide- diglycidyl ether of bisphenol-A (DGEBA), and aliphatic amine - diethylenetriamine (DETA). During the chemical reaction the polymer, while keeping the same structure, progressively densifies passing from liquid to glassy solid. The reaction time may be very long, up to 10 days, depending on the relative abundance of the components. This will allow us to study the system as a function of its density in a continuous way. Moreover, the system has us the (unique) property of avoiding spurious experimental effects. For example, since the sample remains the same during densification, the experimental parameters are unchanged, so that the various scattering intensities are directly comparable.

Study of the dynamical structure factor.

We will investigate the S(Qw) of densified samples by means of inelastic scattering of X rays (ESRF, Grenoble) and UV at the IUVS line of ELETTRA and by the HIRESUV spectrometer (in collaboration with the unit of L’Aquila), in order to get information on the attenuation mechanisms of acoustic waves, as well as on the correlation between fragility and non-ergodicity factor. The quantities required for these studies are S(Qw) and the fragility. The latter quantity will be evaluated by Differential Scanning Calorimetry at Messina.
In particular, by Brillouin scattering of X rays, of UV and visible light, we will investigate how and to which extent the different attenuation mechanisms contribute, taking into consideration the influence of the molecular network, and/or of the progressive permanent densification of the glass. In fact, contrary to the wealth of data available from ultra-sound experiments, very few experiments have been performed in the 1 GHz-1 THz range [9]. For this study, we will collect Brillouin spectra of ultraviolet synchrotron radiation (at Elettra) in the wavelength range 160-240 nm, and with the HIRESUV apparatus (research unit of L'Aquila) for wavelengths of 244 nm and 532 nm , and X-ray scattering at ESRF.
The following glasses will be studied: GeO2, SiO2, B2O3, BO3, and silicate glasses of the type (1-x)SiO2-xNa2O at different Na2O concentrations. In the latter glasses, Na2O acts as network modifier, and tends to shift the BP to high energy. Preliminary X-ray scattering measurements have demonstrated the feasibility of the experiment

4- Numerical simulation

(a) Simulations will be performed of densified GeO2, adapting and extending to this glass the algorithms previously developed for simple model glasses. In fact, it has been shown [CITARE?] that in order to equilibrate a glass former at temperatures well below the glass transition, the molecular dynamics simulations are not efficient. We will employ a non-local Monte Carlo method, that is capable of yielding equilibrated (or quasi-equilibrated) structures, which in turn yield the vibrational density of states and S(Qw) either by direct diagonalisation of the dynamical matrix or by the method of moments.

(b) We will also perform molecular dynamics simulations on the glass LiO2-2B2O3. In this glass, the crossover between the power laws gamma propto Q^2 and gamma propto Q^4 has been observed at a value Q0 which is one order of magnitude greater than for SiO2 [17] (Q0=1.5 nm^-1 instead of 0.15 nm^-1), and so accessible to simulation. We intend to check whether the experimentally observed crossover is reproduced by simulation and, in case of positive answer, we will try to understand which are the characteristics of the inter-atomic potentials that produce such relevant change.