21. Introduction to Glasses

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Session Overview

Modules	Amorphous Materials
Concepts	amorphous solids, glass formation, silicate and metallic glass, engineering glass properties: network formers, network modifiers
Keywords	glass, amorphous solid, disordered solid, short-range order, metalloid, atomic mobility, rate of cooling, viscosity, silicate glass, network former, network modifier, solidification, molar volume, supercooled liquid, coefficient of thermal expansion, melting point, glass transition temperature, quenching, excess volume, processing temperature, chain scission, fluidity, bridging oxygen, terminal oxygen, index of refraction, metallic glass, free volume
Chemical Substances	SiO₂/silicates, B₂O₃/borates, GeO₂/germanates, P₂O₅/phosphates, V₂O₅/vanadates, As₂O₅/arsenate, SbO₅/stibnates, polymers, sulfur (S), quartz, Fe₈₀B₂₀, calcium oxide (CaO), lithium oxide (Li₂O), sodium oxide (Na₂O), magnesium oxide (MgO), lanthanum oxide (La₂O₃), yttrium oxide (Y₂O₃), scandium oxide (Sc₂O₃), lead oxide (PbO), tin oxide (SnO), cristobalite, water
Applications	bottles, jars, food and beverage containers, windows, glass cookware, plastic wrap, food canning, optical fibers, computer screens, anti-theft tags, mercury thermometer, lead crystal

Prerequisites

Before starting this session, you should be familiar with:

Shapes of molecules and factors affecting their mobility (Session 8 through Session 12)
Structure, properties, and methods of characterizing crystals (Session 15 through Session 20)

Learning Objectives

After completing this session, you should be able to:

Explain why materials form amorphous rather than crystalline solids.
Compare crystalline and amorphous solids in terms of their composition, molar volume, atomic structure, transition temperature, band gap, and desirable impurities.
Describe the effect of network modifiers at the molecular level.
Design a processing sequence for glass to achieve a given set of physical properties.

Reading

Archived Lecture Notes #7 (PDF)

Book Chapters	Topics
[JS] 4.5, "Noncrystalline Solids – Three-Dimensional Imperfections."	Random network theory of solids; long-, medium-, and short-range order; oxide, semiconductor, and metallic glasses; network formers and modifiers
[JS] 12.2, "Glasses – Noncrystalline Materials."	Network formers, modifiers, and intermediates; commercial silicate glasses; nonsilicate glasses; applications of amorphous solids

Lecture Video

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Resources

Lecture Slides (PDF - 1.0MB)

Lecture Summary

Amorphous solids lack long-range order, but may have small regions of local order surrounded by a non-crystalline network. Commonly called "glass", they may form from inorganic compounds (e.g. SiO₂/silicates, B₂O₃/borates, GeO₂/germanates, P₂O₅/phosphates, V₂O₅/vanadates, As₂O₅/arsenate, SbO₅/stibnates), organic compounds (e.g. polymers), elements (e.g. sulfur), and even metal alloys (e.g. Fe₈₀B₂₀). Glasses form when liquids with low atomic mobility are cooled too quickly to create an ordered crystal. The rate of cooling determines the transition temperature, excess volume, and degree of order present in the resulting glass. The addition of network modifiers (e.g., calcium oxide (CaO), lithium oxide (Li₂O), sodium oxide (Na₂O), magnesium oxide (MgO), lanthanum oxide (La₂O₃), yttrium oxide (Y₂O₃), scandium oxide (Sc₂O₃), lead oxide (PbO), tin oxide (SnO)) increase fluidity in liquid glass via chain scission, facilitating processing at lower temperatures.