Konzeption hochmoduliger Gläser mittels thermodynamischer und strukturchemischer Methoden

Philipps, Katharina; Conradt, Reinhard (Thesis advisor); Roos, Christian Hans-Georg (Thesis advisor); Wondraczek, Lothar (Thesis advisor)

Aachen (2018)
Dissertation / PhD Thesis

Dissertation, RWTH Aachen University, 2018

Abstract

The objective of this thesis, the conception of high modulus glass compositions, requires first of all a fundamental comprehension of the mechanisms responsible for those properties in glassy materials. Within this thesis, elastic properties of crystalline and glassy silicates have been compared and studied on the basis of a to the real volume normalized molar volume. The results show us that the main influencing factors are the short and medium range order; where short range order refers to the nature of the cation coordination polyhedra and medium range order to the nature of linkage among them (e.g. via corners, edges, faces). Long range order, which we do not find in glassy material, is irrelevant. As a measure of these structural influences a normalized molar volume, that is comparable to an ionic packing density, can be used. Elastic moduli are a linear function of this parameter. Different master curves are existing for various glass building systems (e.g. silicates, borates).The elastic moduli of multicomponent glass compositions are functions of the elastic moduli of constitutional compounds in amorphous state. These stoichiometric compounds represent phases in thermochemical equilibrium. The results of this work show that a linear superposition of moduli of these glassy compounds yields the elastic moduli of multicomponent glasses. Densities of these compositions can also be calculated by a linear superposition of molar masses and volumes of the individual phases. These results are verified for different industrial glass compositions, flat glasses and reinforcement fibres, as well as self-molten compositions. A database including elastic moduli and densities of 15 stoichiometric phases in glassy state has been developed. Data were taken from literature and complemented by own measurements on glasses corresponding to one of those phases. Taking these results, a new high modulus glass composition is developed. A density as low as possible and a moderate liquidus temperature could also be achieved. The quaternary composition out of the system CaO - MgO - Al2O3 - SiO2 has a Young’s modulus of 99.9 GPa and a density of 2.64 g/cm3; this corresponds to a ratio Young’s modulus to density of about 37.84·106 m2/s2. The liquidus temperature is 1402 °C. In addition, an established industrial composition is being modified regarding its elastic properties. Only small variations in oxide compositions have a big influence on phase composition and with that an improvement in elastic modulus. An increase of about 6 GPa up to 101 GPa could be measured.