Structure of Amorphous Materials -2

1. Structure of Amorphous Materials -2 Oxide glasses Metallic glasses Amorphous Polymers Silicon

2. Silica - SiO2 Amorphous silica Crystalline SiO2 O Si

3. SiO2 - ideal structure characteristics - continuous random network (CRN) • Basic unit - tetrahedron with Si at the center and O at corners • Each corner is shared by two tetrahedrons • No edges or faces are shared • Two dimensional depiction

4. SiO2 - radial distribution function and cooling rate effects Need to define partial g(r)s - gSiSi(r), gSiO(r), gOO(r) The structure and thus properties depend on the cooling rate

5. Network modifiers • Replacing cations with cations of lower valency (e.g. +3 into +2) introduces breaks in the network. • This lowers the glass transition temperature and modulus and thus allows to process material at lower temperature Most commercially used glasses are with network modifiers

6. Metallic Glasses TEM image of amorphous zirconium alloy • Metallic glasses are made by rapid cooling of a metallic liquid such that there is not enough time for the ordered, crystalline structure to nucleate and grow. In the original metallic glasses the required cooling rate was as much as a million degrees Celsius per second! Recently, alloys have been developed that form glasses around 1-100 degrees per second cooling rates. • Typically the best glass formers are multicomponent materials such as Zr-Ti-Cu-Ni-Al alloy. • Metallic glasses can be quite strong yet highly elastic, and they can also be quite tough. Furthermore above the the glass transition temperature a metallic glass becomes quite soft and flows easily allowing to form complex shapes. Schematic of a two component glass

7. Mechanical Properties of Bulk Metallic Glasses (BMG) • High yield strength, fracture toughness • High elastic strain limit (2%) • Excellent processibility

8. Mechanical deformation of metallic glasses Local plastic deformation and shear band formation Unresolved questions • How does thermo-mechanical history affect the structure of a metallic glass the plastic deformation behavior? • Is there an ideal way to structurally characterize metallic glasses so to get the best structure-property understanding?

9. Polymer chain structure - Gaussian coil Model: N+1 beads (mers) connected by N links (bonds) of length b0 with random orientation - equivalent of a random walk Vector representing nth link End to end distance Since link orientations are random an average over all conformations (denoted by )

10. End to end distance The average end to end distance is zero but the average distance square is not - it measures the size of the polymer coil . The last equality comes for the fact that the average dot product of two randomly oriented vectors is zero Real chains are typically more rigid that a model one

11. End to end distance distribution Probability of having a chain with and to end distance R is a Gaussian distribution Long chains form an entangled network

12. Chain rigidity Rigid chain Flexible chain Rigid chains have larger end-to-end distance for the same contour length, but at large scale they are flexible coils anyway

13. Specific chemical structure, tacticity and ability to crystallize Chains with a regular attachment (isotactic or syndiotactic) of side groups can crystallize Chains with irregular side groups (atactic) can not crystallize Flexible chains are easier to crystallize

14. Semicrystalline polymers A mixture of crystalline regions (lamellae) separated by amorphous regions

15. Amorphous Silicon (aSi) Largely four-fold coordinated network, with some free-fold coordinated atoms (inducing dangling bonds). To eliminate dangling bonds that act as electron traps aSi is hydrogenated. Hydrogen saturates dangling bonds Thin-film amorphous Silicon (a-Si) have good photovoltaic characteristics, are mounted on flexible backings are do not fracture as easily as crystalline Si, which allows them to be formed to fit applications with the bending inherent when used in building materials. Amorphous solar cells do not convert sunlight quite as efficiently as crystalline Si cells, however, they require considerably less energy to produce, and are superior to crystalline cells in terms of the time required to recover the energy cost of manufacture.  Amorphous silicon is gradually degraded by exposure to light. This phenomena is called the Staebler-Wronski Effect (SWE). 

16. Amorphous carbons: property vs. sp2/sp3 content

17. Bonding and mechanical properties of amorphous networks Constrain model - each bond and bond angle represent a constrain in the amorphous network It can be shown that below average coordination, ca, of 2.4 network can be deformed with no energy cost. Based on this modulus is then equal to E=E0{(ca -2.4)/(4-2.4)}1.5 where 4 corresponds to fully coordinated network  higher coordination  larger modulus