Revealing ordering and structural changes at glass transition
- PDF / 3,296,034 Bytes
- 6 Pages / 612 x 792 pts (letter) Page_size
- 99 Downloads / 180 Views
Revealing ordering and structural changes at glass transition Michael I. Ojovan1 Department of Materials, Imperial College London, South Kensington Campus, Exhibition Road, London, SW7 2AZ, United Kingdom ABSTRACT Ordering types in the disordered structure of amorphous materials and structural changes which occur at glass-liquid transition are discussed revealing medium range order and reduction of topological signature of bonding system. INTRODUCTION Glasses are solid amorphous materials. The internal structure of glasses is represented by a topologically disordered 3-dimensional network of interconnected microscopic structural units. Glasses are typically formed on rapid cooling of melts in order to avoid crystallisation processes to occur. It has long been assumed that the glassy state is characteristic of special glass-forming or network materials such as covalent substances which exhibit a high degree of structure organization at atomic length scales. After the discovery of metallic glasses it was realised that practically any molten substance can be transformed to a glass if cooled sufficiently fast [1]. The liquid-glass transition has been considered as a second order phase transition in which a supercooled melt yields, on cooling, a glassy structure and properties similar to those of crystalline materials e.g. of an isotropic solid material [2]. Glasses can be transformed into melts on increase of temperature with glass transition temperature (Tg) to separate the solid-like behaviour of amorphous materials from the liquid-like. Both glasses and liquids are isotropic, e.g. the translation-rotation symmetry of particles is unchanged at the glass-liquid transition. However glasses have elastic properties of isotropic solids which are qualitatively different compared liquids. The difficulty to understand the glassliquid transition is because of almost undetectable changes in the structure of amorphous materials in contrast to qualitative changes in characteristics and extremely large change in the time scale of relaxation processes. We aim to discuss ordering types and structural changes which occur at glass-liquid transition. This will reveal ranges of sizes with medium range order (MRO) and reduction of topological signature of disordered bonding lattices at glass-liquid transition. STRUCTURAL FEATURES OF GLASSES AND MELTS Both glasses and melts possess short-range order (SRO) with a typical radius about several angstroms. SRO structural groups in commercial glasses are usually tetrahedral Si, B, Al, Fe, P surrounded by 4 oxygen atoms (tetrahedral coordination) or B surrounded by 3 oxygen atoms (trigonal coordination). The tetrahedra and trigonal species in glass link to each other via bridging oxygen bonds. The remaining non-bridging oxygen (NBO) atoms effectively carry a 1
E-mail: [email protected]. Current address: Department of Nuclear Energy, International Atomic Energy Agency, A2644 WTS, Vienna International Centre, PO Box 100, Vienna, 1400 Austria. Email [email protected]
negative charge and io
Data Loading...
