M. Cutroni

The α-relaxation process in simple glass forming liquid m-toluidine. I. The temperature dependence of the dielectric response

Wide band dielectric spectroscopy has been used to study the bulk dynamics of the simple supercooled liquid m-toluidine. Following the temperature and frequency dependence of the dielectric response we find a stretched relaxation process, the stretching parameter of which is temperature dependent. The observed decoupling between rotational and translational diffusion parameters at low temperatures may be the result of heterogeneous dynamics.

Dynamics of glassy and liquid m-toluidine investigated by high-resolution dielectric spectroscopy

The glass-former m-toluidine displays the characteristic properties of a fragile supercooled liquid, which suggest the existence of a slow secondary relaxation process. In view of the recently realized importance of such a secondary relaxation feature, we have conducted a dielectric search for the secondary process in viscous and glassy m-toluidine. Based on high-resolution experiments on the distilled liquid, a secondary process can be identified which has the properties typical of a Johari-Goldstein beta relaxation. As a result, the previous hypothesis that the methyl group might be responsible for suppressing the secondary dynamics in glassy m-toluidine no longer holds.

Mechanical and dielectric behaviour of some ionic glasses

Abstract Mechanical and electrical relaxation processes are compared on identical superionic glasses in which the glassy matrix AgPO3 was doped with various kinds of salt dopant. It was found that the same values characterize the a.c. conductivity as well as the mechanical spectra. Above T≈10K there are two different activated contributions to the acoustic absorption: one, dominant at high temperatures/low frequencies is characterized by a linear decrease of the activation energy as the salt dopant is increased; the second, dominant at intermediate temperatures/frequencies is characterized by activation energy values one order of magnitude smaller. At the same time the log–log representation of the experimental a.c. conductivity vs. frequency shows a double power law behaviour. Two different relaxing processes can account for the activation energy values and for the double power law behaviour of the a.c. conductivity.

Frequency and temperature dependence of a.c. conductivity of vitreous silver phosphate electrolytes

Abstract The a.c. conductivity behaviour in the 20–300 K temperature range has been investigated for (Ag 2 S) x (AgPO 3 ) 1− x and (Ag 2 SO 4 ) x (AgPO 3 ) 1− x glasses at various salt contents ( x ). The temperature dependence at selected frequencies in the radioand micro-wave region displays several relaxational contributions which are indistinct in the frequency domain. The low temperature experimental data are discussed and a proposed ‘new universality’ has been examined.

Thermodynamic study of alkyl-cyclohexanes in liquid, glassy, and crystalline states

The specific heat of some alkyl-cyclohexanes in their liquid, supercooled liquid, crystalline, and (for the first time) glassy states has been measured by quasiadiabatic calorimetry. Thermodynamic properties as well as the glass forming ability have been studied as a function of systematic changes of the molecular structure. Only one stable crystalline phase is observed experimentally for ethylcyclohexane, propylcyclohexane, and butylcyclohexane. In the case of methylcyclohexane, experimental evidence is provided of a crystal-to-crystal transition at temperatures just below the melting.

Slow dynamics of supercooled m-toluidine investigated by mechanical spectroscopy

Dynamics of supercooled m-toluidine close to the glass transition have been investigated by dynamic shear modulus measurements and stress relaxation experiments. The viscoelastic response of this material follows time-temperature-superposition in the temperature range investigated. Comparison with results at ultrasonic frequencies suggests the existence of a secondary relaxation. A change of the temperature dependent viscosity from a Vogel-Fulcher-Tammann behavior to another regime at low temperatures is also found. Compared to most inorganic glass formers, the viscosity of m-toluidine at the glass transition is approximately two orders of magnitude lower. The shear relaxation times are characterized by the same temperature dependence as the viscosity. They are in reasonable agreement with the results of previous ultrasonic measurements. The conclusions of the present work agree with recent results obtained by high resolution dielectric spectroscopy.

Ionic conduction in silver phosphate glasses doped with silver sulphide

Abstract The dielectric properties of (Ag2S) x ( AgPO3)1‐x and AgPO3 ionic glasses have been investigated at low frequencies (10 Hz‐10 MHz) by complex impedance measurements in the temperature range between 20 and 330 K and at microwave frequencies (8.2–18 GHz) using a waveguide transmission line and also a cavity resonator at 9 GHz from 77 to 300 K. Analysis on the basis of the jump relaxation model has shown that translational and reorientational hopping motions of mobile Ag+ ions are responsible for the observed frequency‐dependent ac conductivity. The temperature dependence of the ac conductivity at fixed frequencies can be fitted using three relaxation processes with characteristic activation energies which differ from each other by an order of magnitude and are closely similar to those obtained from previous ultrasonic attenuation studies on the same glasses. The highest energy (about 0–4 eV) has been associated with the translational hopping motion, responsible for the long‐range ionic diffusion th...

Ion dynamics and devitrification in 0.75AgI–0.25Ag2MoO4 fast ion conducting glass: an XRD, DSC and 109Ag NMR study

Abstract Structural changes and cation dynamics of a fast ion conducting glass 0.75AgI–0.25Ag 2 MoO 4 have been measured by differential scanning calorimetry (DSC), X-ray diffraction (XRD), and 109 Ag solid-state nuclear magnetic resonance (NMR) both in the glassy and supercooled liquid states. In the glassy state, two Ag + populations exist with very different mobilities which are related to different co-ordination spheres. Above the glass transition temperature, the sample undergoes phase separation and gives β-AgI and 2AgI–Ag 2 MoO 4 (2:1) crystals. Cations dynamics decrease and chemical shift anisotropy structures arise. The ionic conduction is based on Ag + sites with mixed co-ordination, and no evidence of AgI clustering is found.

Ionic conduction and dynamical regimes in silver phosphate glasses

Abstract Dielectric measurements on ion conducting silver metaphosphate glasses, pure and doped with silver sulphide or silver sulphate, have been performed in the microwave frequency region up to 60 GHz in a wide temperature range between 77 and 413 K. With the current extension of previous measurements, a frequency range larger than 10 decades has been investigated and several distinct conductivity regimes have been observed. Above the low frequency dc conductivity value, the progressive increase of conductivity with frequency has been found to obey first a power law with exponent p ∼2/3 and later another power law with exponent q ∼4/3, nearly independent of temperature and of composition. In the low temperature region a nearly constant loss contribution to the conductivity has been also detected.

On the features of the dielectric response of supercooled ethylcyclohexane

The main relaxation observed in supercooled ethylcyclohexane in the 50 Hz–20 kHz frequency range using high-resolution dielectric spectroscopy displays an unusual relaxation pattern. The slope of the dielectric loss profile above the peak frequency, progressively tends to zero with increasing temperature. The observed features could be compatible with the existence of different underlying processes. A simple approach based on scaling is exploited to provide relevant information on the properties of the primary and secondary relaxation in ethylcyclohexane. The apparently anomalous behaviour is shown to be due to the superposition of a secondary process to the dielectric α-process and support the thesis that the β-relaxation observed in the glassy phase could have an intramolecular origin.