What are dislocations? Recap of 3.14/3.40 Lecture on 9/27/2012

Sangtae KimOct/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

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

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 ---------------- CC ---------------- A

A ---------------- A

B ---------------- B

O3 Layered P3 Layered

Na

TM

O3 – P3 transformation

A ---------------- A

B ---------------- B

C ---------------- B

A ---------------- C

B ---------------- CC ---------------- A

A ---------------- A

B ---------------- B

O3 Layered P3 Layered

Na

TM

Sangtae Kim, stkim@mit.edu

Edge dislocation

• Simple cubic illustration

Edge dislocation

• Simple cubic illustration