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

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

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

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

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

Screw dislocation view from top

Screw dislocation view from top b parallel to line direction

Screw dislocation

Fantastic!

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

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

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

Glide Motion

Slip

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

Hardening Mechanism

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

A Research Question Iso-structural NaCoO2 shows change.

O3 – P3 transformation A A B B C B A C B C C A A A B B O3 LayeredP3 Layered Na TM

O3 – P3 transformation A A B B C B A C B C C A A A B B O3 LayeredP3 Layered Na TM

Sangtae Kim,

Edge dislocation Simple cubic illustration

Edge dislocation Simple cubic illustration