



Dielectric relaxation at microwave frequencies: analysis of dispersion domains
The study of the dielectric relaxation of polar systems in liquid state leads to the knowledge of mechanisms governing interactions between the different dipolar species present in the structure. From the experimental data deduced at fixed microwave frequencies as a function of temperature, the dispersion spectrum profile at constant temperature can be reconstructed for a wide range of frequencies. Both the static dielectric constant and the high frequency permittivity can be determined, as well as the relaxation times distribution.
The foundation of this frequencytemperature comparative method is exclusively based on the thermal behaviour of the reduced orientation polarization. Besides, the detailed analysis of the dielectric relaxation phenomenon as a function of temperature allows one determination of the activation energy as well as anomalies of the macroscopic relaxation time in the low temperature range, where overcooling effects predominate. This study facilitates analysis of the polarization mechanisms departing from the different relaxation processes present in the dispersion spectrum.
 Dielectric relaxation and calorimetric behaviour: electromagnetic characterisation of phase and state transitions.
This research line is centred in the dielectric characterization of polar systems containing water. It provides information for the structural study of phase and state transitions, taking into account that the response to an electromagnetic signal is different depending on whether it deals with free water or water bound to the crystalline structure.

Particularly, the comparative analysis of dehydration processes, either by means of this method, either by other complementary techniques such as the differential thermal analysis and the thermo gravimetric analysis, allows one to determine any kind of phase change, or even to reveal the existence of metastable states. Besides, the different dielectric behaviour between free and bound water makes possible some industrial applications for different dielectric media presenting moisture.


Comparison of these results with the ones deduced by other experimental methods allows one to get a deeper knowledge of the dielectric structure thanks to the establishment of correlations between the magnitudes present in the dielectric dispersion phenomenon. All of them are necessary for electromagnetic detection of nonelectrical magnitudes such as density, moisture and the ratio of the possible dispersion domains associated with the main constituents of a nonhomogeneous mixture.

 Electromagnetic characterisation of dielectrics in time domain. Dielectric relaxation analysis.
Dielectric characterisation by time domain reflectometry (T.D.R.) makes possible the electromagnetic study of a dielectric response to a transient signal. The technique allows one to obtain the complex dielectric permittivity in a wide frequency range (DC18 GHz) with only one experimental measurement, and to analyse both the relaxation phenomenon and the different dispersion domains.
The experimental procedure is based on the study of the reflection of a step voltage which propagates along a coaxial line partially filled with dielectric. The reflection coefficient G (t) can be determined from the incident and reflected voltage signals, V + (t) and V  (t), in the airdielectric interface. Knowledge of G (t) allows one to obtain the complex dielectric permittivity e * ( w ) and other useful parameters for the characterization of dielectrics, in the frequency domain.

Treatment of the experimental data requires calculation by numerical methods based on a direct deconvolution in time domain, which provide the dielectric spectrum in a frequency range. By means of Montecarlo methods, the high frequency noise present in the measurements can be filtered out.
This kind of experimental measurements allows one to study the temporal response by analyzing the correlation function F (t) , which describes the temporal relaxation of the orientational polarization.Analysis of the dielectric dispersion spectrum constitutes an essential procedure to treat non homogeneous dielectric mixtures with two or more components under the hypothesis of a homogeneous effective dielectric. The study leads to modeling the homogeneous, effective mean. 

The method has been applied to ceramics used in dielectric resonators, as well as systems constituted by a polyester substrate, charged either with barium or calcium tytanates, either with both. This latter case requires a more elaborated treatment based on a mixture law for three components.
The research activity of the group members also includes other lines gathered in the following groups:
Grupo de Fibras y Guías Ópticas Activas 



