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Crystalline Versus Amorphous Solids

Crystalline Versus Amorphous Solids. Liquids, upon cooling, tend to crystallize. This means that atoms weakly bound in the liquid in a random-like manner arrange them- selves into well defined, periodic positions. In order to do so effectively:

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Crystalline Versus Amorphous Solids

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  1. Crystalline Versus Amorphous Solids • Liquids, upon cooling, tend to crystallize. This means that atoms • weakly bound in the liquid in a random-like manner arrange them- • selves into well defined, periodic positions. • In order to do so effectively: • The liquid needs to be sufficiently fluid (low viscosity) to allow • the atoms to rearrange themselves effectively during cooling • through the melting point • 2. The cooling rate needs to be sufficiently slow that the basic atomic units or molecules have sufficient time to re-arrange themselves

  2. Glass Transition Temperature, Tg At Tg,  ~ 104 -106 Ns/m2 Below Tg, atomic rearrange- ments are frozen in. Rigid fluid “Moon rocks” were produced millions of years ago TM-melting point

  3. Crystalline Versus Amorphous Solids Solids with simple structures and non-directional bonds, e.g. metals and alkali halides, have very low viscosity fluids above the melting point and easily crystallize upon cooling. Solids with complex structures and strong, highly directional bonds, e.g. silicates, polymers, have high viscosity fluids and tend to form amorphous or glassy solids

  4. A Crystalline Silicate Si O

  5. Crystalline Versus Amorphous Silicates Silicate melts tend to be highly viscous Variable bond angle & length Ordered SiO4 tetrahedra Disordered SiO4 tetrahedra

  6. Viscosity Measure of resistance to flow: elongation or strain,  = ΔL/L = d/dt Liquid flow requires breaking and reformation of bonds

  7. Viscosity-Temperature Relations

  8. Soda lime glass – strain rate Soda lime glass at 900ºC at its working point: Strain rate d/dt = / = (10-4N/m2)/(10-4Ns/m2) = 1 s-1 Glass rod doubles in length in one second at this small stress

  9. Soda Lime Glass at RT – strain rate (max)= 108N/m2 before breakage; (RT)= 1020Ns/m2 d/dt = 10-12 s-1 wait 1000 yr for 1% strain!

  10. Optical Fiber Puller Viscosity control Pulling rate Key for strength http://www.nasatech.com/Briefs/Dec98/MFS26503.html

  11. Silicon-Oxygen network Bridging oxygens Two dimensional schematic of network of SiO4 tetrahedra. Note: each Si has 4 O neighbors and each O, 2 Si neighbors Common network formers: SiO2, B2O3, P2O5

  12. Glass Modifiers (N2O, K2O, Li2O, CaO, MgO and PbO). Na+ ions Bridging oxygen Non-bridging oxygens SiO2 network Modified with addition of Na20 Disrupt 3 dimensional covalent network reduceTM and Tg

  13. Soda Glass

  14. Viscosity-Temperature-Modifier Relations 1 Pa-s = 106 N-s/m2 Note effect of B2O3 on 

  15. Glass Formation and Fabrication • Three basic steps in the production of glass: • the melting of e.g. quartz sand (minute crystals of silica), • the shaping of the glass while in a viscous state. Sufficient viscosity to enable handling and shaping of article • the controlled cooling of the shaped article thereby allowing the article to form without large residual stresses

  16. Property-Composition Relations

  17. Glasses – High Strength • Glass has “no” crystal structure: • slip cannot take place. • strong bonding between atoms, • very high compressive strength and theoretical tensile strength of about 107 kN/m2 (significantly higher than that of steel). • Cracks or imperfections in glass permit stress concentrations to localize and exceed bond strength between atoms crack propagation. • in actual practice, the strength of glass is, by a factor of 100 to 1000, less than the theoretical strength, and glass is brittle.

  18. Strengthened Glass Glass remains extraordinarily strong in compression but becomes weak in tension. Strengthening: pre-stress glass object by inducing compressive strains in exterior and thereby counteract tensile stresses which develop under tension. • Cool surface of glass preferentially • Ion exchange surface with larger alkali ion such as K. • Coat surfaces to protect against scratches on surface

  19. Rapid Cooling Rates Splat cooling Spin cooling Vapor deposition

  20. Metallic Glasses Reference: Masuhr A, Busch R, Johnson WL. "Rheometry and Crystallization of Bulk Metallic Glass Forming Alloys at High Temperatures." Materials Science Forum. Barcelona, Spain. Switzerland: Trans Tech Publications, 1998: 779-84.

  21. Amorphous SiO2 - MOSFET Metal The Si/SiO2 interface is one of the most important structures technologically Note: Form MOS structure: Metal-Oxide-Semiconductor. Key element of MOSFET http://www.research.ibm.com/amorphous/

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