M. Maglione

Radio-frequency dielectric measurements at temperatures from 10 to 450 K

A coaxial air line was constructed to connect a radio-frequency impedance analyzer and a temperature-stabilized sample holder. It is suitable for dielectric measurements in the frequency range 1 MHz-1 GHz and at temperatures between 10 and 450 K. The dielectric dispersion of Fe-doped BaTiO_3 and Na-doped KCN is presented. The results demonstrate the capability of this setup when investigating materials with high as well as with low dielectric constants.

High frequency dielectric relaxation in BaTio3 derived materials

Abstract A systematic study of the dielectric relaxation phenomenon has been carried out for ceramics with compositions derived from barium titanate in the frequency and temperature ranges 1 MHz–1 GHz and 250–500 K, respectively. Whatever the composition may be, a minimum of the relaxation frequency and a maximum of the dielectric dispersion appears at each phase transition temperature. The dipole-type relaxation is correlated to coherent displacements of the ions in the octahedron sites ordered either along chains or in 3D-lattice.

Molecular Mobility in Glassy Bread: A Multispectroscopy Approach

ABSTRACT The molecular mobility in low-moisture (<9%, web) white bread was studied as a function of temperature using pulsed-proton nuclear magnetic resonance (NMR), and dielectric and dynamic mechanical spectroscopies. The water was mobile, even in glassy samples. Different processes below glass transition temperature (sub-Tg) were observed, and a relaxation map of the studied system was drawn. These results have been interpreted and extrapolated to suggest that the Tg is not a universal predictive parameter for the physical stability of glassy food.

Dielectric and E.P.R. studies of Mn-doped barium titanate

Manganese introduced in BaTiO3 as a substituant of titanium is able to trap the electrons produced during a sintering under a very low oxygen pressure. However it strongly modifies the vibrational ...

Relaxations Below Glass Transition Temperature in Bread and Its Components

ABSTRACT Dry glassy bread, cooked gluten, and gelatinized wheat starch-sucrose mixtures were prepared. Dielectric properties were studied as a function of temperature at different frequencies ranging from 100 to 1,000 kHz. Both bread and starch samples exhibited a tan δ peak at -53°C at 10 kHz, which is associated with a secondary relaxation characterized by an activation energy of 50 kJ/mol. The gluten sample did not show any relaxation in the temperature range studied. The magnitude of the relaxation peak was sensitive to the sucrose concentration of the starch sample. The higher the sucrose content, the greater the amplitude of the peak. Such an effect is attributed to a greater β-relaxation induced by an increasing number of relaxing units or increasing size of the loosely packed regions of the glass. However, it cannot be concluded whether the relaxation is associated with a group present on both starch and sucrose or that the sucrose has an enhancing effect on starch mobility.

Dielectric relaxation and conductivity in ferroelectric perovskites

Abstract Ferroelectric ABO3 perovskites are usually well known for their high dielectric susceptibility. Under selected impurity substitutions their conductivity may be strongly increased. The interaction between electronic conductivity and lattice polarizability may take place over different length scales. At high temperatures (T > 300 K), this interaction length is macroscopic leading to space charges. At low temperatures (T < 50 K), the polaronic interaction is restricted to a few unit cells. In the special case of SrTiO3, these polaronic excitons are sensitive to the quantum fluctuations which lead to the new concept of quantum polarons.

Reliability of radio frequency dielectric experiments in perovskites

Abstract Several radio frequency experiments with perovskites are reviewed. The devices used by several groups were very different, and the chemical nature and shape of the perovskite samples were intentionally changed. Despite this, sub-gigahertz relaxation is always reported. We describe and overcome all the possible extrinsic contributions to this relaxation, and we conclude that it is related to the perovskite structure.

Dielectric relaxation in crystals and ceramics derived from BaTiO3

Abstract A dielectric relaxation is reported in a crystal of BaTiO3 and in ceramics with composition (Ba1-yCay)TiO3 (0≤y≤0.2) in the frequency and temperature ranges 106 to 109 Hz and 350 to 450 K, respectively. A minimum of the relaxation frequency occurs close to the ferroelectric Curie temperature. The relaxation is of dipolar-type.

Electrostatic model and NMR results for EFG tensors in tetragonal BaTiO3

Abstract We present 47,49Ti and 135,137Ba NMR second-order quadrupolar rotation patterns in a tetragonal single domain crystal of BaTiO3. These data will be analysed in terms of a ionic polarizable point multipole model.

Electrical transport properties of Mn- and Ni-doped barium titanate at high temperature

Abstract The dependence on temperature and oxygen pressure of the electrical transport properties of the ferroelectric compound BaTiO3 doped with manganese or nickel is investigated in the temperature range 800–1100°C with an oxygen partial pressure continuously varying between 0.21 and 10−15 atmosphere. A complex impedance analysis method has been improved for the measurements. A point defect model is proposed to discuss the different behaviour of the two dopants. The calculated conductivity is in very good agreement with the experimental data over the full temperature and oxygen partial pressure ranges. The band gap derived from this work (2.96 eV) is in good agreement with the already reported values. The ionisation energy levels of these two impurities in the band gap have been determined and compared with the vlues deduced from the MS-LSD calculations.