Lecture5 Glass Formation 1 Structure

Information about Lecture5 Glass Formation 1 Structure

Published on January 11, 2008

Author: Penelope

Source: authorstream.com

Content

MSE 423:  MSE 423 Glass Formation 1: Structural Approach Understand and be able to apply the relationships between atomic level structure and ease at which a system will form glass Understand and be able to apply Zacharaisen’s rules for glass formation Understand and be able to apply Sun’s Bond strength criteria Understand and be able to apply Dietzel’s Field Strength model Structural Approach to Glass Formation:  Structural Approach to Glass Formation Structural Approach to Glass Formation:  Structural Approach to Glass Formation Computer Simulation of Glass Structure:  Computer Simulation of Glass Structure Structural Approach to Glass Formation:  Structural Approach to Glass Formation Glass Formation results when Liquids are cooled to below TM (TL) sufficiently fast to avoid crystallization Nucleation of crystalline seeds are avoided Growth of Nuclei into crystallites (crystals) is avoided Liquid is “frustrated” by internal “structure that hinders both events Structural Approach to Glass Formation What internal structures promote glass formation? How can structures be developed that increase viscosity and frustrate crystallization processes? Structural Approach to Glass Formation:  Structural Approach to Glass Formation Using structure to promote glass formation Develop atomic bonding structures in the system that produce large viscosity near the melting point Silicate liquids and glasses Develop large molecular structures that due to their size prevent, frustrate, organization, onto the crystalline lattice Polymeric liquids with large polymer chains Develop complex local and variable structures in the liquid that on cooling have a large number of possible structural motifs to follow and as a result no one structure is favored over another Molten salt liquids with a number of components Zacharaisen’s Rules for Glass Formation:  Zacharaisen’s Rules for Glass Formation Glass formation requires long range continuous bonding in the liquid to: Produce high viscosity 3 - Dimensional bonding Strong individual bond strength “Open” structure that is not efficiently packed Corners of polyhedra are shared to increase “connectivity” Bonds for bridges between corner sharing polyhedra Zacharaisen’s Rules for Glass Formation:  Zacharaisen’s Rules for Glass Formation 1. Oxygen atoms are linked (bonded) to no more than two atoms 2. Oxygen coordination around glass forming cations is small, 3, 4 3. Cation polyhedra share corners and not edges or faces 4. At least three corners are shared Zacharaisen’s Rules for Glass Formation:  Zacharaisen’s Rules for Glass Formation Apply these rules to the strong glass formers: SiO4/2 B2O3 or BO3/2 Apply these rules to the modifiers: BaO Na2O Zacharaisen’s Rules for Glass Formation:  Zacharaisen’s Rules for Glass Formation SiO4/2 Zacharaisen’s Rules for Glass Formation:  Zacharaisen’s Rules for Glass Formation B2O3 or BO3/2 Zacharaisen’s Rules for Modifiers:  Zacharaisen’s Rules for Modifiers Ca1O1 (CaO) Closed-packed cubic Ba occupying all octahedral sites Octahedral sites = Ca = O Zacharaisen’s Rules for Modifiers – M2O:  Zacharaisen’s Rules for Modifiers – M2O Na2O1 (Na2O) Closed-packed cubic Na occupying tetrahedral sites Tetrahedral sites = 2 x O = Na Sun’s Bond Strength Model:  Sun’s Bond Strength Model Glass Formation is brought about by both: Connectivity of Bridge Bonds Strong Bonds between atoms (ions) Sun Classified oxide according to their bond strengths Glass formers form strong bonds to oxygen – rigid network, high viscosity Modifiers from weak bonds to oxygen – Disrupt, modify, network Intermediates form intermediate bonds to oxygen – can’t form glasses on their own, but aid with other oxides to form glasses Sun’s Bond Strength Model:  Sun’s Bond Strength Model Glass formers > 80 Kcal/mole bond strength with oxygen B2O3, SiO2, Geo2, P2O5, Al2O5…. > 70 kcal/mol bond strength with oxygen TiO2, ZnO, PbO…. < 60 kcal/mole bond strength with oxygen Li2O, Na2O, K2O, MgO, CaO…. Glass Formers – form glasses on their own:  Glass Formers – form glasses on their own Intermediate Oxides – assist in glass formation:  Intermediate Oxides – assist in glass formation Modifying oxides – degrade glass formation:  Modifying oxides – degrade glass formation Classifying Oxides:  Classifying Oxides How would each of the following be classified? SiO2, B2O3, P2O5 TiO2, PbO Na2O, CaO, ZnO Dietzel’s Field Strength Criteria:  Dietzel’s Field Strength Criteria Sun classifies Al as a glass former Al2O3 does not form glass at any quenching rate More factors are important than just bond strength Small cations with high charge – glass formers Large cations with small charge – modifiers Medium sized cations with medium charge - intermediates Dietzel’s Field Strength Model:  Dietzel’s Field Strength Model Intermediates – assist in glass formation:  Intermediates – assist in glass formation Glass forming oxides – form glass on their own:  Glass forming oxides – form glass on their own Glassforming compositions:  Glassforming compositions How would you classify the following compositions? Glassforming or not? 0.15Na2O + 0.35Al2O3 + 0.50SiO2 0.35Na2O + 0.15CaO + 0.25Al2O3 + 0.25SiO2

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