Glass Formation Structure And Electrical Properties Of Glasses In The Ag As Se System

Author : Maxwell Adam Thomas Marple
Publisher :
ISBN 13 : 9780438628403
Total Pages : pages
Book Rating : 4.6/5 (284 download)

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Download or read book The Structural Influence on Fast Ion Conducting Chalcogenide Glasses written by Maxwell Adam Thomas Marple and published by . This book was released on 2018 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Solid state batteries are a safer alternative to the current liquid battery technology, although for efficient performance the solid electrolyte must have high room temperature ionic conductivity. Glassy solid electrolytes are superior alternatives to their crystalline counterparts owing to their lack of grain boundaries that can act as a source of resistance for ionic current and as potential pathways for Li dendrite growth. Chalcogenide glasses are favorable materials for solid electrolytes as they have higher ionic conductivity compared to oxides, which can be ascribed to the greater polarizability of the chalcogens and are therefore, the most likely materials to satisfy the requirements for solid state battery applications. Chalcogenide glasses are an important class of materials that are sulfides, selenides or tellurides of group IV and/or V elements, namely Ge, As, P and Si with minor concentrations of other elements such as Ga, Sb, In. These glasses have found a variety of technological applications in the fields of optoelectronics, remote sensing, memory and energy storage. The unique compositional flexibility of chalcogenide glasses in the form of continuous alloying enables tuning of their optical, electronic, thermo-mechanical and other properties. The structure of these glasses are characterized by their covalently bonded networks that largely obey the 8–N coordination rule, violation of chemical order, and structural peculiarities such as the formation of homopolar bonds, molecular and other low-dimensional structural units. All of these are expected to control a wide range of physical properties relevant to various technological applications hence complete knowledge of the atomic structure of these materials is therefore of key importance in understanding and formulating accurate predictive models. The first chapter of this dissertation details the application of high-resolution two-dimensional nuclear magnetic resonance (2D NMR) and Raman spectroscopy to investigate the structure of Si[subscript x]Se[subscript 1-x] glasses as this glass-forming system forms the basis for the classic glassy Li-ion conducting chalcogenides via modification of the network by incorporation of Li2S. The results indicate that the structure of these glasses consists of a network with nearly perfect short-range chemical order, but with strong intermediate-range clustering. Initial addition of Si to Se results in cross-linking of Se chain segments with nanoclusters of corner- and edge-shared SiSe[subscript 4/2] tetrahedra. These clusters percolate via coalescence near x ≥0.2 to finally form a low-dimensional network with high molar volume, at the stoichiometric composition (x=0.33) that is composed of chains of edge-sharing tetrahedra cross-linked by corner-shared tetrahedra. This structural evolution can explain the compositional variation of the glass transition temperature and the molar volume of these glasses. The structure-property relationship for ionic conductivity in chalcogenide glasses is explored next, in a Ag-ion conducting system that has technological application in conductive bridge random access memory. Novel homogeneous glasses in the ternary system Ag2Se-Ga2Se3-GeSe2 (AGGS) are synthesized and studied using Raman, 77Se, [superscript 71/69]Ga, and 109Ag NMR spectroscopy. The structure of these glasses consists primarily of a network of corner sharing (Ga/Ge)Se[subcript 4/2] tetrahedra with a small fraction of homopolar Se-Se bonds. Compositional modification of the atomic structure follows the charge compensated network model with Ag2Se acting as a network modifier, forming non-bridging Se in glasses with Ag/Ga >1, while Ga2Se3 plays the role of an intermediate glass former. Electrical Impedance Spectroscopy (EIS) reveals the ionic conductivity of the AGGS glasses to be quite high at ambient temperature, reaching up to 10−4 S/cm for glasses with the highest Ag content. Transference number measurements using the electromotive force (EMF) method as well as variable temperature 109Ag NMR line shape studies indicate that the conductivity is predominantly ionic in nature. The high ionic conductivity can be related to a heavily modified structural network that results in a potential energy landscape with many suitable hopping sites for the Ag ions. The structural characteristics and electrical properties of the AGGS glasses are used to guide the development of an analogous Li containing system. Two glass systems are investigated, the first is the stoichiometric Li2S-Ga2Se3-GeSe2 system where the Li2S content is varied to study the influence of Li concentration on ionic conductivity. The structure is characterized using Raman and one– and two– dimensional [superscript 6/7]Li, 77Se, and 71Ga NMR spectroscopy and can be described as a charge-compensated network consisting of corner sharing (Ga/Ge)(Se,S)[subscript 4/2] tetrahedra with Li acting as a network modifier, charge compensating the non-bridging Se and S. These non-bridging units are found to have the greatest influence towards maximizing ionic conductivity as they depolymerize the network and alter the Li-ion dynamics. The dc conductivity data indicate a rapid rise in Li-ion mobility with increasing temperature and, more interestingly, with Li concentration. The compositional variation of E[subscript dc] indicates the formation of a low-energy barrier (~0.35 eV) percolation pathway for Li-ion hopping through the glass network. The highest room temperature ionic conductivity of ~ 10−4 S/cm is found in the glass with the highest Li2S content, suggesting that the concentration of the Li ions, rather than their mobility, is the limiting factor for achieving high ionic conductivity in this system. Besides Li concentration, the nature of the chalcogen atoms in the network is found to have an important influence on Li mobility. This phenomenon is investigated in 40%Li2S-60%Ge(S,Se)2 glasses, as a function of the S/(S+Se) ratio. The network structure of these glasses consists of corner-sharing GeS[subscript x/2]Se[subscript (4-x)/2] tetrahedra, with S and Se being randomly distributed over all bridging and non-bridging environments. While these glasses are found to have comparable ionic conductivity that varies little with composition, the activation energy and the pre-exponential factor display a nonlinear variation with the S/(S+Se) ratio and are shown to be related to the progressive phonon softening, as Se replaces S in the mixed-chalcogen network. When taken together, these results suggest that the phonon softening of the structural network of solid electrolytes, induced via compositional modification, can be used to tune their electrical properties.