The Applications of Nano Materials

1. The Applications of Nano Materials Department of Chemical and Materials Engineering San Jose State University Zhen Guo, Ph. D.

2. How to study Nanomaterials Part I -- Done Basic Materials Science Principles Microstructure Materials Properties Applications Processing Part II – Done Part III – This one

3. The Applications of Nano Materials Electronics Magnetic Device Structure Nano Materials Applications Daily Life consumable Optics Renewable Energy MEMS Bio Device

4. Applications of Nano Materials

5. Session 9 – Nano Structural Materials Briefing of Deformation Mechanism Grain size verse Mechanical Behavior Nano-grained structural Materials Classic Hall-Patch relation New Deformation Mechanism in nano grain

6. Ceramics s F F Metal Polymer L0 L e Mechanical Properties of Materials • Stress and Strain: -- Stress s = F / A -- Strain e = (L-L0)/L0 • Hook’s Law: In Elasticity region: s = E * e • Competition between Strength and Ductility

7. Atomic Model and Bonding • Atomic Level, this is nothing but stretching the bonding • Session III, inter-atomic potential and thus force for bonding: • At small Da: Courtesy from T. H. Courtney: Mechanical Behavior of Materials

8. But Reality said otherwise • Defect, especially dislocation dominated deformation mechanism • Materials yielding – Dislocation movement causing plastic deformation • This lower the strength but improve the ductility

9. Dislocations • Dislocation is one kind of line defect with missing one roll of atoms • It can glide along slip plane under shear stress or climb under stress and temperature • Dislocation movement can cause material yield and plastic deformation Courtesy from T. H. Courtney: Mechanical Behavior of Materials

10. Dislocations and Plastic Deformation • The stress required to mobilize a dislocation is much lower than breaking bonds. • Once dislocation motion started, materials is yielding. • Dislocation will encounter many obstacles that required higher stress for continuous motion, this will lead to hardening. • Once dislocation moved to the boundary, this caused permanent plastic deformations. Courtesy from T. H. Courtney: Mechanical Behavior of Materials

11. Uni-axial Tensile Deformation

12. Competition between Strength and Toughness • Yield Strength decrease with temperature while fracture strength will keep almost same • The room for plastic deformation is smaller and smaller. • When sy > sF, materials will fracture without any plastic deformation => completely brittle mode Ductile Brittle Strength sF sy Temperature

13. Grain Refinement • Yield Strength increase will inevitably lead to a decrease in ductility • Grain refinement is the only way to improve strength and toughness simultaneously. • Ultra-fine grained structural materials is one of the focus area for this century Toughness Grain Size Refinement Strength

14. Forging Rolling ECAP F F How to Refine Grain Size? • Thermomechanical Processing • Severe plastic deformation Recrystrallization • Appropriate temperature

15. Grain Size verse Strength • Grain boundary is a very effective obstacles to block dislocation motions. • Dislocations emitted from neighboring grains is a function of number of piled up dislocations which is proportional to grain size d • Hall-Patch Equation: • d => y Courtesy from T. H. Courtney: Mechanical Behavior of Materials

16. Grain Size verse Toughness • Brittle fracture cleaved along cleavage plane (In most cases close packed one). • Cleavage plane will have to change directions at grain boundary since adjacent grains have different orientations. • Smaller grains offer more zig-zag cleavage surface and thus required larger surface area. Fracture strength is also a function of grain size d => f, surface area 

17. Ultrafine to Nano Grain Materials • Dislocation theory predicts ultra strong but also brittle materials due to lack of mechanisms for hardening and strain relaxation. • In reality of nano crystalline materials, grain boundary mechanism kicked in. - Courtesy from Takaki, et al.

18. What Happened in Nano Scale? • Grain Refinement has moved from micro crystalline to nano crystalline. • Key Questions remained to be answer. • At what length scale, classic Hall-Patch equation is broken down? • What is the controlling deformation and fracture mechanisms for nano grained materials? • What is the role of grain boundary when majority of atoms are belong to grain boundary in nano grained materials?

19. Limits of Grain Refinement • Yield strength at very small grain size deviated from Hall-Patch Equation, Saturated at a steady state value and even decrease following an inverse Hall-Patch Equations. • We also lost ductility at smaller grains - Courtesy from Takaki, et al.

20. Inverse Hall Patch Equations • Inverse Hall-Patch relations showed yield strength or hardness decrease when grain size decreased to over a critical number. • What’s new in nano structural materials • Dominant Deformation Mechanism • Large Grain: Intra grain, dislocation-dislocation • Ultra Fine Grain: Intra grain, dislocation-grain boundary • Nano Grain: Inter grain, grain boundary movement • Possible New Mechanisms for Nano-grained materials: • -- Grain Boundary Sliding • -- Diffusional Creep along Grain Boundary • -- Grain Boundary serve as a dislocation source

21. Dislocation-Dislocation Interaction • In normal grain size structural alloys, there are usually dozens or hundreds dislocations called dislocation forest. • When a dislocation started to move under critical shear stress, it will soon stop in front of a series dislocation • Further movement of dislocation will need larger stress to overcome those dislocations => Hardening Courtesy from T. H. Courtney: Mechanical Behavior of Materials

22. Dislocation-Grain Boundary Interaction • Dislocation density normally ranges between 1010-1014/m2 • For 100nm grain size, dislocations per unit area is hardly 1. • So for submicron grain size, dislocation-dislocation interaction is no longer dominant due to lack of dislocation in smaller grains • Instead, grain boundary becomes the main obstacles for dislocation motions. Courtesy from T. H. Courtney: Mechanical Behavior of Materials

23. Grain Boundary Sliding • Unlike bulk, grain boundary is an incoherent interface between two neighboring grains. • Grain boundary is really a 2-D defect with full of voids, dislocations and other geometric necessary defect to bridge two grains that has completely different crystallographic orientations. • So grain boundary is viscous and easy to get sliding under stress and high temperature.

24. Diffusional Creep along Grain Boundary (Coble Creep) • Since grain boundary is 2-D defect, it is also a fast diffusion path for atoms. Dgb >> Dbulk (106*Dbulk) • It is dominant at low temperature when Dbulk is still small, d/L is 1nm / grain size • But in Nano structural materials, grain size is smaller so Coble creep is more important

25. Grain Boundaries as Source of Dislocations • Grain Boundaries are 2-D defect • It can serve both as source and drain of dislocations. • When grain size is big, the # of dislocation per grains are large, grain boundaries are usually considered as annealing place for dislocations or obstacles for its motions • When grain size is small, not many dislocations inside grains => GB as source of dislocations. • Geometric necessary dislocations are formed due to GB deformation