1. What are dislocations? Recap of 3.14/3.40 Lecture on 9/27/2012 Sangtae Kim Oct/2/2012

2. What are dislocations? Why are they important? How do they move in the crystal? A research topic related to dislocations Contents

3. What are dislocations? 1D Line defects, in which crystal registry is lost A machine to cut bonds on one plane, and then re- stitch them together, one by one – Dislocation Glide Carry local deformation and stress Edge, screw, mixed dislocations

4. Edge dislocation Simple cubic illustration Illustration with bonds Local environment is different only at the core. Localized shear b perpendicular to line direction

5. Edge dislocation Simple cubic illustration Illustration with bonds Local environment is different only at the core. Localized shear b perpendicular to line direction

6. Screw dislocation view from top

7. Screw dislocation view from top b parallel to line direction

8. Screw dislocation

9. Fantastic!

10. In reality Mixed dislocations – b at some angle to the line direction. -- e.g. dislocation loops from the Frank Reed source

12. Why are they important? It controls the yield strength and subsequent plastic deformation at ordinary temperature. Electric defects in semiconductors and optical materials – they are undesirable there. We can understand 2D defects as a set of 1D defects

13. How do they move? Glide - slip plans Climb - 1D defects: vacancy, interstitials In reality, there are bits of both + entanglement

14. Glide Motion

16. Slip

17. Climb Collective vacancy motion/interstitials required. So to harden a material, we want more climb, instead of glide to harden a material.

18. Hardening Mechanism

19. A Research Question LiCoO2: typical battery cathode material Li removal with no structural change Transition Metal Alkali Metal (Na or Li)

20. A Research Question Iso-structural NaCoO2 shows change.

21. O3 – P3 transformation A ---------------- A B ---------------- B C ---------------- B A ---------------- C B ---------------- C C ---------------- A A ---------------- A B ---------------- B O3 LayeredP3 Layered Na TM

22. O3 – P3 transformation A ---------------- A B ---------------- B C ---------------- B A ---------------- C B ---------------- C C ---------------- A A ---------------- A B ---------------- B O3 LayeredP3 Layered Na TM

24. Sangtae Kim, stkim@mit.edu

25. Edge dislocation Simple cubic illustration

26. Edge dislocation Simple cubic illustration