L18Glass145s09

L18Glass145s09 - Lecture 18 Noncrystalline Solids...

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Lecture 18, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Reading assignment : C&R §3.21; §4.6, 4.11-4.12; §11.13-11.17; §14.7; OGC p.887 Learning objectives: Understand the differences in structure between crystalline and glassy materials Understand the nature of the glass transition Describe how the structural features of a polymer can affect its glass transition and melting temperatures Lecture 18: Noncrystalline Solids & Semicrystalline Solids 1 Obsidian, a naturally occurring volcanic glass
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Lecture 18, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Covalent crystals Although bonding influences local symmetry … Noncrystalline solids (glasses) have structures that satisfy local bonding requirements without 3-D periodicity Ionic crystals Metallic crystals 2
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Lecture 18, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Crystalline vs. Noncrystalline Structures [C&R §3.21] Crystalline structures Short-range order (SRO) – local bonding Long-range order (LRO) 3-D, periodic arrangement of atoms Non-crystalline structures (glasses) SRO satisfies local bonding No LRO “random network” C&R Figure 3.40 3
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Lecture 18, spring 2009 ENGR 145, Chemistry of Materials Case Western Reserve University Oxides; also some sulfides, selenides, … E.g. SiO 2 Si-O tetrahedra (CN Si = 4) … … sharing corners (CN O = 2) … … as in crystalline forms of SiO 2 ( e.g. quartz) Lack of crystallinity made possible by Rotation about the Si-O-Si bond Some variability in the Si-O-Si bond angle satisfies local bonding requirements without 3-D periodicity Inorganic Glasses Fiber optic wave- guides of silica glass (OGC Figure 21.31) 4
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L18Glass145s09 - Lecture 18 Noncrystalline Solids...

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