J.V. Zanchetta

Electron Spin Resonance and Dilatometric Studies of Ancient Ceramics Applied to the Determination of Firing Temperature

Electron spin resonance (ESR) and dilatometric studies have been performed on clay fired to various temperatures. ESR spectra are very sensitive to the heat treatment and the variations of spectroscopic parameters are discussed as functions of the firing temperatures. The study shows that these ESR parameters can be used for a quantitative determination of the firing temperature. In the dilatometric analysis, the change of the length as well as the parameters related to the quartz transition, are sensitive to the heat treatment. The most significant parameters are selected for this study. The dilatometric determination of the firing temperature is in good agreement with the ESR results. After a control experiment performed by both methods on a known clay sample, we finally study an archaeological sample as an example. Its firing temperature was determined with satisfying accuracy.

Lineshapes of ESR signals and the nature of paramagnetic species in amorphous molybdenum sulfides

Abstract Amorphous and poorly crystallized molybdenum sulfides were studied by ESR. Qualitative analysis of the spectra suggests the presence of three paramagnetic species. A simulation of the ESR spectra was attempted on this basis. ESR lines were calculated over the stoichiometry range MoS3→ MoS2. A good representation of the main part of the experimental lines was obtained. There is no significant variation of the g values of the various components of the spectra during the transformation from MoS3 to MoS2. The first signal is attributed to sulfur centers. The two others are assigned to metal centers. One is due to Mov sites in a chain-like structure, observed in the amorphous phase of MoS3. The other is due to Mov defects in a layered MoS2 microstructure. The variation of the site concentration was calculated for all compounds, from MoS3 to MoS2.

COOPERATIVE OR NONCOOPERATIVE DIELECTRIC RELAXATION IN IONIC SOLIDS : A DISCRIMINATING EXPERIMENTAL APPROACH

Calculations, confirmed by experimental evidence on a polymer and on two ionically conducting solids, show that the variation of thermally stimulated depolarization currents (TSDC) with the heating rate depends on the polarization decay function that is assumed. This permits discrimination between models usually proposed for describing the non-Debye behavior of low-frequency dielectric relaxation in solids. It is also demonstrated that consideration of the kinetics involved in nonequilibrium TSDC spectroscopy is an essential, and complementary, experimental tool for dielectric measurements carried out under isothermal conditions in the frequency domain in establishing models of relaxation.

Polarization conductivity in dehydrated zeolites NaY and HY

Abstract The ionic conductivity of the dehydrated faujasite zeolites NaY and of its protonated form HY is determined experimentally by low-frequency impedance spectroscopy. Experiments were carried out in the frequency range 10 Hz-10 MHz at temperatures from ambient to 634 K. It is clearly shown that the measured conductivity could be considered as a sum of two terms. The dc term obeys the Arrhenius equation while the polarization term could be expressed as Aω8. The conduction phenomena, including the polarization processes, are ascribed to cation movements localized in supercages. Moreover, it is found that the parent zeolite NaY is more conductive than its protonated form HY.

Etude par resonance paramagnetique electronique de sulfures de molybdene et de tungstene

Abstract Chalcogenides MeS 3− x (Me = Mo, W; 0 ⩽ x ⩽ 1) have been prepared by the thermal decomposition of tetrathiometallates. These compounds are largely amorphous; desulfurization as well as crystalline organization occur when the heat treatment temperature ( TT ) increases. At 1000°C a perfect hexagonal arrangement can be obtained ( x = 1). ESR spectra of amorphous species having x in the range 0–1 have been recorded. A part of the ESR signals is ascribed to sulfur chains whilst the other part is attributed to Me (V) species. The intensity of both series of ESR lines is a decreasing function of TT . The spectra are characteristic of Me (V) in an axial symmetry environment. At TT ≅ 1000°C a narrow ESR signal remains, which is assigned to electron spin centers of impurities ( g -value close to that of the free electron). The results are explained either in terms of continuous structural evolution of the solid during heat treatment or in terms of a biphasic mixture of amorphous MoS 3 and MoS 2 . The variation of the spin concentration as a function of TT is reported and compared with the results previously obtained from a study of the transport properties of these solids. In all cases the values obtained for Me = W are smaller than those observed on Me = Mo.

Mn2+ electron paramagnetic resonance study of a sodium borosilicate glass prepared by the sol-gel method

Abstract Sodium borosilicate glasses were prepared by the sol-gel method and doped with a paramagnetic manganese probe. X-band electron paramagnetic resonance (EPR) experiments, carried out on powdered samples, showed that even for molar ratios higher than 10%, manganese can be inserted in the glass. EPR results obtained on different doped silica glasses showed that the ability of Mn 2+ to be inserted in the glassy network is connected with the presence of alkali species. The bulk location of the probe was confirmed by 29 Si nuclear magnetic resonance investigations. EPR results indicated that manganese is located in distorted sites in an octahedral silica environment.

Dielectric relaxation in ionic solids: experimental evidences

The comparison of different experimental methods of dielectric relaxation measurements applied to diverse ionic solids clearly demonstrates that ac conductivity dispersion observed in these solids is of dielectric nature. Then, by analysing the dielectric signal in terms of distribution of relaxation times, we show that important information on material can be obtained. They are for instance the interaction energy between the ionic charges and their surrounding network or the different types of sites where ions are embedded.

29Si and 129Xe NMR of Mn2+ doped silica xerogels

Abstract Paramagnetic Mn2+ ions were incorporated in silica gel during the preparation of the sol. Doped silica xerogels were obtained by heat treatment at different temperatures and studied by 29Si and 129Xe nuclear magnetic resonance (NMR) experiments. Comparison between 29Si NMR magic angle spinning (MAS) spectra of doped and undoped xerogels showed that there is a significant effect of the paramagnetic probe on Q2, Q3 and Q4 peaks. Reduction of the 29Si-T1 relaxation time in the presence of the Mn2+ probe was observed. 129Xe NMR spectra were recorded on the xerogels at variable Xe pressures. Results were consistent with 29Si NMR data and confirmed the localization of the paramagnetic probe in surface sites as shown by electron paramagnetic resonance (EPR) of Mn2+.

Frequency dependent protonic conduction in N2H5Sn3F7 glass and RbHSeO4 single crystal

Abstract We applied to two different solid compounds, N2H5Sn3F7 glass and RbHSeO4 crystallised, known as protonic conductors, the model of ac conductivity proposed in a previous paper. The fit to experimental data, by the theoretical equations, allows us to obtain characteristics physical values (potential energy and number of protons involved in conductivity) of these solids related to the ac conductivity and to compare them with the previous works about protons in these materials. Some conclusions are encouraging and it would confirm the validity of the model.

Transport properties of tungsten sulfides WS2 + x

Abstract The temperature dependence of DC conductivity of tungsten sulfides WS2 + x (0 Analyses of the results show that the conductivity is correctly explained by a mechanism of electronic hops between localized states. Within such a model the number of sites, independent of measuring temperature, is calculated; the value of the parameter WM, the energy required to move two electrons from infinity to the sites, is also evaluated. A quantitative approach to explain the experimental results to the complex permittivity, is proposed.