Gilles Taillades

29Si NMR structural studies of ionically conductive silicon chalcogenide glasses and model compounds

Abstract The structure of ionically conductive glasses in the systems (Li2S)x(SiS2)1−x (0.2 ≤ x ≤ 0.6), (Na2S)x(SiS2)1−x (0.1 ≤ x ≤ 0.6), (Ag2S)x(SiS2)1−x (0.5 ≤ x ≤ 0.6) and (Li2S)y(Na2S)1−y)0.5(SiS2)0.5 (0 ≤ y ≤ 1), (Li2Se)x(SiSe2)1−x (0.23 ≤ x ≤ 0.70), and (Na2Se)x(SiSe2)1−x (0.4 ≤ x ≤ 0.6), prepared by twin-roller quenching, is discussed on the basis of solid-state 29Si magic angle spinning (MAS) nuclear magnetic resonance (NMR) results. As in the well known stoichiometrically analogous oxide systems, the 29Si chemical shifts are sensitively affected by differences in the structural environments present. For both the sulfide and selenide systems, the NMR spectra permit easy distinction between corner- and edge-shared silicon tetrahedra. In addition, secondary chemical shift effects are observed, reflecting the number of bridging versus non-bridging chalcogen atoms. The sign and magnitude of these chemical shift trends can be rationalized on the basis of bond ionicities using a semi-empirical theory approach. The main conclusion concerning the structures of these glasses are the following. (1) The introduction of alkali chalcogenides into the network of silicon chalcogenide glasses generates non-bridging sulfur and selenium sites, with preferential destruction of edge-sharing SiX 4 2 tetrahedra. (2) The distribution of the non-bridging selenium sites is closer to random than to ordered. (3) The tendency of forming edge-sharing units decreases in the order S → Se → O and Na → Li → Ag. (4) Mixed LiNa thiosilicate glasses are structurally more closely related to binary lithium thiosilicate glasses than to binary sodium thiosilicate glasses.

The structure of ionically conductive chalcogenide glasses : a combined NMR, XPS and ab initio calculation study

Abstract This paper reports on the structural investigation of lithium and sodium thiosilicate crystals and glasses by means of X-ray photoelectron spectroscopy and ab initio calculation. The results are analysed in conjunction with previously reported 29 Si NMR data. While NMR proved to be an effective tool for the quantitative discrimination of edge- and corner-sharing tetrahedra existing in these materials, X-ray photoelectron spectroscopy (XPS) gives information on the nature of SiS bonds, i.e. bridging and non-bridging bonds. The main result is the noticeable difference existing between the structures of lithium and sodium thiosilicate glasses, which, according to XPS data, is due to different electronic redistributions over the network when one or the other alkali is added, the sodium addition resulting in a change in the electronic distribution over the entire network.

Alkali ion distribution in mixed-alkali chalcogenide glasses: 23Na-{7Li} spin echo double resonance NMR studies of the system [(Na2S)1-y(Li2S)y]0.5(GeS2)0.5

Abstract The relative arrangement of sodium and lithium ions in mixed alkali thiogermanate glasses of composition [(Na2S)1-y(Li2S)y]0.5(GeS2)0.5 (y = 0.20, 0.40, 0.50, 0.60, 0.80) has been studied by 23Na-{7Li} spin echo double resonance (SEDOR) spectroscopy. The results indicate that lithium and sodium cations are intimately mixed and not part of microscopically or macroscopically segregated phases. In a more quantitative analysis, the experimental results are compared with various Li Na cation distribution models. While a complete quantitative description would require independent knowledge of the overall spatial cation distribution in these glasses, the SEDOR data are found to be consistent with a distribution of the alkali ions that is close to homogeneous and not clustered. On the other hand, if clustering did occur, the SEDOR data would give proof of preferred interactions among like cations (‘like cation clustering’). There is no evidence for the cation pairing models previously invoked to describe the structure of mixed alkali glasses.

A 119Sn solid-state nuclear magnetic resonance study of crystalline tin sulphides

Non-spinning and magic angle spinning (MAS) 119Sn nuclear magnetic resonance (NMR) spectra of the binary tin sulphides SnS and SnS2 as well as of a number of stoichiometric compounds from the ternary system Na2S-SnS2 have been recorded. The isotropic chemical shift was found to cover a range of more than 800 ppm and allows us to distinguish between different coordination numbers of Sn(IV) present in tin sulphides. Moreover, the chemical shift anisotropy (c.s.a.) is shown to be a sensitive indicator for deformations in the coordination sphere of the tin atom and can be discussed with respect to the known structures of the compounds.

Preparation of Thin Proton Conducting Membranes by Means of EPD

The present study uses electrophoretic deposition of yttrium doped barium cerate to prepare SOFC half cells on nickel/barium cerate (Ni/BaCe) and nickel / yttria stabilized zirconia (Ni/YSZ) substrates. Therefore we investigated charging and agglomeration behaviour of nanosized barium cerate powder in different solvents by means of a micro electrophoresis experiment and dynamic light scattering. Stable suspensions of barium cerate were prepared and deposition kinetics was examined prior to the preparation of thin membranes on the two cermets. Tests with different solvents and additive combinations showed that methyl-ethyl-keton (MEK) with addition of polyacrylic acid (PAA) is the best combination for the use of the suspension for electrophoretic deposition. Electrophoretic deposition in this case is a simple and low cost method to produce homogenous and dense layers in the scale of microns.

Chemically Stable Electrolytes and Advanced Electrode Architectures for Efficient Proton Ceramic Fuel Cells

Protonic Ceramic Fuel Cells (PCFC) are especially attractive for operation at intermediate temperatures of 600 °C and below. In comparison to their oxygen ion conducting counterparts (SOFC), in PCFCs the water is produced at the cathode preventing fuel dilution. High conversion efficiency may thus be reached [1]. Nevertheless, the current status of PCFC is far beyond expectations. The first reason is the lack of proton conductor materials having requirements for long term operation: high chemical stability and proton conductivity in the order of 10 S cm.