© H.L. Tuller-2003 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: 1.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

© H.L. Tuller-2003 Glass Transition Temperature, T g At T g,  ~ Ns/m 2 Below T g, atomic rearrange- ments are frozen in. Rigid fluid “Moon rocks” were produced millions of years ago T M -melting point

© H.L. Tuller-2003 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 Crystalline Versus Amorphous Solids

© H.L. Tuller-2003 A Crystalline Silicate Si O

© H.L. Tuller-2003 Ordered SiO 4 tetrahedraDisordered SiO 4 tetrahedra Crystalline Versus Amorphous Silicates Silicate melts tend to be highly viscous Variable bond angle & length

© H.L. Tuller-2003 Viscosity Measure of resistance to flow: Liquid flow requires breaking and reformation of bonds elongation or strain,  = ΔL/L = d  /dt

© H.L. Tuller-2003 Viscosity-Temperature Relations

© H.L. Tuller-2003 Strain rate d  /dt =  /  = (10 -4 N/m 2 )/(10 -4 Ns/m 2 ) = 1 s -1 Glass rod doubles in length in one second at this small stress Soda lime glass at 900ºC at its working point: Soda lime glass – strain rate

© H.L. Tuller-2003  (max)= 10 8 N/m 2 before breakage;  (RT)= Ns/m 2 d  /dt = s -1 wait 1000 yr for 1% strain! Soda Lime Glass at RT – strain rate

© H.L. Tuller-2003 Optical Fiber Puller Pulling rate Viscosity control Key for strength

© H.L. Tuller-2003 Two dimensional schematic of network of SiO 4 tetrahedra. Note: each Si has 4 O neighbors and each O, 2 Si neighbors Silicon-Oxygen network Bridging oxygens Common network formers: SiO 2, B 2 O 3, P 2 O 5

© H.L. Tuller-2003 Glass Modifiers ( N 2 O, K 2 O, Li 2 O, CaO, MgO and PbO ). SiO 2 network Modified with addition of Na 2 0 Bridging oxygen Non-bridging oxygens Na+ ions Disrupt 3 dimensional covalent network reduceT M and T g

© H.L. Tuller-2003 Soda Glass

© H.L. Tuller-2003 Viscosity-Temperature-Modifier Relations 1 Pa-s = 10 6 N-s/m 2 Note effect of B 2 O 3 on 

© H.L. Tuller-2003 Glass Formation and Fabrication Three basic steps in the production of glass: (1) the melting of e.g. quartz sand (minute crystals of silica), (2) the shaping of the glass while in a viscous state. Sufficient viscosity to enable handling and shaping of article (3) the controlled cooling of the shaped article thereby allowing the article to form without large residual stresses

© H.L. Tuller-2003 Property-Composition Relations

© H.L. Tuller-2003 Glass has “no” crystal structure: slip cannot take place. strong bonding between atoms, very high compressive strength and theoretical tensile strength of about 10 7 kN/m 2 (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. Glasses – High Strength

© H.L. Tuller-2003 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. Strengthened Glass Cool surface of glass preferentially Ion exchange surface with larger alkali ion such as K. Coat surfaces to protect against scratches on surface

© H.L. Tuller-2003 Rapid Cooling Rates Splat cooling Spin cooling Vapor deposition

© H.L. Tuller-2003 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: Metallic Glasses

© H.L. Tuller-2003 The Si/SiO 2 interface is one of the most important structures technologically Note: Form MOS structure: Metal-Oxide-Semiconductor. Key element of MOSFET Metal Amorphous SiO 2 - MOSFET