S. Villain

Relations between microstructure, electrical percolation and corrosion in metal—insulator composites

Five series of compacted granular metal–polymer composites were prepared in a wide range of metal volume fractions. The metals are Al, Fe, Ni, W and Zn powders. The polymer in powder form is the poly-phenylsulfur [-C6H4S-]n, noted as PPS. Using electrical complex impedance spectroscopy (ECIS) measurements, the a.c. electrical properties of these composites were analyzed as a function of metal volume fraction and of working frequencies. Each material was characterized by scanning electron microscopy to determine the distribution and morphology of the particles. Close to the percolation threshold, abnormal electrical behavior was observed and interpreted using scanning electron microscopy (SEM) analysis. Two types of modeling calculation are proposed to describe the electrical properties.A model inspired by the Effective Medium Approximation (EMA) improves the modeling approach: the mechanically induced modification of grain size and distribution is interpreted in terms of new modeling parameters governing the evolution of the conductance. A description of electrical behavior close to the percolation threshold is proposed using percolation theory. Critical exponents are determined above and below the percolation compositions. Finally, a study of corrosion behavior for Zn based composites is presented and correlated with the initial electrical behavior of these composites.

Structural, vibrational study and UV photoluminescence properties of the system Bi(2−x)Lu(x)WO6 (0.1 ≤ x ≤ 1)

The bismuth lutetium tungstate series Bi(2−x)Lu(x)WO6 with 0.1 ≤ x ≤ 1 were synthesized by solid state reaction of oxide precursors at 1000 °C for 3 h. The as-prepared polycrystalline compounds were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy and photoluminescence (PL) analyses. Biphasic samples were obtained in the composition range of 0.1 ≤ x ≤ 0.3. Solid solutions were obtained in the composition range of 0.4 ≤ x ≤ 1, and their monoclinic crystal structure was refined using the Rietveld method. SEM micrographs showed that solid solutions presented homogeneous morphologies. Attributions of Raman vibrational modes were proposed. A shift in the vibrational wavenumber depending on the lutetium composition was observed. The specific broadening of the spectral bands was interpreted in terms of long range Bi/Lu disorder and local WO6 octahedron distortions in the structure. The PL experiments were performed under UV-laser light irradiation. Each PL band was decomposed into three Gaussian components with energies close to 1.25, 1.80 and 2.1 eV. Their integrated intensities increased with the value of x. The presence of the near infrared band at 1.25 eV is discussed.

Microstructure modifications and modulated piezoelectric responses in PLZT/Al2O3 composites

Abstract Piezoelectric ceramics for acoustic applications have been prepared by mixing the piezoelectric phase Pb1−1.5xLax□x/2(Ti1−yZry)O3 (PLZT) with variable fractions of Al2O3. The samples are in form of pellets and polarized at high temperature. After thermal treatment, X-ray diffraction and scanning electron microscopy have been used to determine the phase and morphological modifications. The morphotropic PLZT initial phase disproportionates into modified PLZT and ZrO2 phases. Using electrical impedance spectroscopy, the resonance frequencies of the composite system have been determined and analyzed. As the Al2O3 volume fractions increase, the resonance frequency and the amplitude of the electrical response both decrease. An interpretation of the role of Al2O3 additions is proposed in terms of phase and microstructure modifications. Using LRC electrical equivalent circuits, the impedance variations close to the resonance frequency are modeled: the increase of the resistance R (electrical losses) and of the capacitance C are respectively correlated to the change in microstructure and in nature of PLZT initial phase.

Degradation mechanism of electrodes subjected to alternating potentials: modeling and protection

Abstract Non-symmetrical degradation of electrodes working in alkaline solutions and submitted to alternative potentials is analyzed from electrical conductance Σ analyses. Degradation rate of pure metal or composite electrodes is measured as a function of the working time t and of the period N of the applied potential. Logarithmic representations of the decrease of Σ ( t , T ) have been validated from elemental modeling. In a first approach, the degradation is describe using the Avrami model. Then, a three-steps semi-empirical model is proposed, that seems better adapted to our problem. The first step corresponds to the incubation of the corrosion, and the two other steps correspond to the growth of corrosion products, first on the surface of the electrode (2D corrosion), then in the bulk (3D corrosion). The lifetimes of the coatings are also connected with the frequency ν . A mechanism of protection is proposed.

New Method for Preparation of Polycrystalline Langasite for Gas Sensors: Structural Studies

Langasite La3Ga5SiO14 (noted LGS) is a piezoelectric material presenting a strong electromechanical deformation at high temperature (up to 1,000–1,200°C). Classical solid state reaction at high temperature (1,450°C) is generally needed to synthesize the LGS phase. In this study, we present a new synthesis way for polycrystalline langasite (LGS), involving a series of specific thermal cycles at moderate temperatures (<1,200°C). The final langasite phase is then characterized by X-ray diffraction, scanning electron microscopy and infrared spectroscopy. FTIR observed vibration modes are commented in relation with literature results.

Characterisation of Thin Films of the Ferroelectric Material SrBi 2 Ta 2 O 9 Obtained by Sol-Gel Methods on Sr 2 RuO 4 (001) Single Crystal Substrate

SrBi 2 Ta 2 O 9 films have been deposited by a spin-coating method on an oxide electrode, Sr 2 RuO 4, chosen to minimise the fatigue phenomenon and for the possibilities of epitaxy between the substrate and the film. X-Ray Diffraction, Scanning Electron Microscopy, Atomic Force Microscopy and Transmission Electron Microscopy, with both medium and high resolution, were used to study the morphology, structure and defects of the obtained films. The films are polycrystalline with grains having an average size of 50 nm. Images and diffraction patterns of SrBi 2 Ta 2 O 9 grains on Sr 2 RuO 4 show chiefly four orientations, namely (110), (100), (001) and (105), which illustrate possibilities of epitaxy between the film and the substrate.