Nucleation, Crystallization, and Growth

Question 1

Nucleation

Answer 1

Initial formation of one phase inside another

Question 2

All nucleation processes have:

Answer 2

Energetic barrier against formation

Question 3

Two types of nucleation

Answer 3

Homogeneous, heterogeneous

Question 4

Homogeneous nucleation

Answer 4

Spontaneous particle formation

Question 5

Heterogeneous nucleation

Answer 5

Formation of particle on a seed of foreign material

Question 6

Why seed important in heterogeneous nucleation?

Answer 6

Seed provides lower barrier to formation at interface

Question 7

Majority of nucleation processes are of what type

Answer 7

Heterogeneous

Question 8

Understanding nucleation rate diagram

Answer 8

Heterogeneous nucleation peaks at a higher temperature so it is easier. Supercooling is required to initiate homogeneous nucleation.

Question 9

Why nucleation limited on diagram?

Answer 9

Lowered concentration of nucleating material after phase change, and also atoms prefer to grow on existing nuclei.

Question 10

Weaker bond energies in a material mean (wrt supercooling)

Answer 10

More supercooling is needed to induce homogeneous nucleation

Question 11

What limits nucleation in slightly undercooled liquids?

Answer 11

Surface tension of small particle radius

Question 12

What produces nucleation in slightly undercooled liquids?

Answer 12

Bulk fluctuations overcoming surface tension barrier to nucleation

Question 13

Undercooled liquid is considered to be (wrt stability)

Answer 13

metastable phase – free energy not minimum

Question 14

The free energy change of nucleation is labeled:

Answer 14

\DeltaG_r

Question 15

The free energy change of phase formation is labeled:

Answer 15

\DeltaG_v

Question 16

Critical radius, how labeled

Answer 16

r*

Question 17

How r* derived

Answer 17

Maximum value of free energy of nucleation

Question 18

What controls surface tension of particles

Answer 18

Fluctuations in bulk or seed

Question 19

\sigma in nucleation equations

Answer 19

Surface tension

Question 20

Nr*/N

Answer 20

Number of particles of critical radius compared to total number of particles

Question 21

Nr*/N implications for density

Answer 21

Higher density results in less nucleation

Question 22

Nr*/N implications for phase transformation energy

Answer 22

Higher phase transformation energy change increases particles of critical radius

Question 23

Nr*/N unincluded effect

Answer 23

More prior particles result in more collisions resulting in more nucleation

Question 24

Nr*/N implication for temperature

Answer 24

Decrease of temperature increase Nr* because of increase in undercooling which increases phase transformation change in free energy

Question 25

Example of liquid mobility effect on undercooling

Answer 25

High atom mobility in molten metal means that undercooled metals nucleate rapidly.

Question 26

Undercooling in glass

Answer 26

Glass is viscous and has low mobility, resulting in fewer clusters forming.

Question 27

Nucleation rate is labeled:

Answer 27

I

Question 28

Rate at which atoms reach the critical nucleus is labeled:

Answer 28

R_(r^*)

Question 29

Crystallization

Answer 29

Phase transition of a disordered fluid to a crystal

Question 30

When crystallization starts

Answer 30

As soon as nucleation seeds form

Question 31

Types of crystallization

Answer 31

Cooling, evaporation, precipitation

Question 32

Cooling crystallization

Answer 32

Result of decrease in temperature dependent solubility of a solute

Question 33

Evaporation crystallization

Answer 33

Result of evaporation of solvent

Question 34

Two types of precipitation

Answer 34

Reactive crystallization, anti-solvent

Question 35

Reactive crystallization

Answer 35

Chemical reacts with solute causing insoluble solid to form

Question 36

Anti-solvent

Answer 36

A solvent which binds better to a solute than the initial, causing the first solvent to crystallize

Question 37

Mechanism by which polymers can locally crystallize

Answer 37

Chain folding

Question 38

What kind of polymer structures crystallize better?

Answer 38

Linear polymer structure

Question 39

Two types of polymer crystallization

Answer 39

Solution, melt

Question 40

Solution crystallization

Answer 40

Caused by chain folding

Question 41

Melt crystallization

Answer 41

Happens across molecules by the lineup of chains

Question 42

Vi

Answer 42

Rate of crystallization for component i

Question 43

Rate of crystallization for component i

Answer 43

Vi

Question 44

Growth in solution vs. growth in melt

Answer 44

Solution growth is slow; melt growth is fast

Question 45

Why solution growth slow

Answer 45

Large density diff. b/t solution and crystal

Question 46

Why melt growth fast

Answer 46

Comparable density b/t solution and crystal

Question 47

What controls growth in solution

Answer 47

Atomic movements from solution to crystal (i.e. diffusion)

Question 48

What controls growth in melt

Answer 48

Heat flow controls growth in melt

Question 49

Two examples of crystal growth

Answer 49

Bridgman growth & Czochralski growth

Question 50

Bridgman growth

Answer 50

Single seed crystal is cooled at one end of a container.

Question 51

Czochralski growth

Answer 51

Liquid is undercooled; crystal is pulled up at rate of growth. Interface between crystal and melt is effectively stationary.

Question 52

Growth rate of CZ process determined by

Answer 52

Undercooling at interface

Question 53

Undercooling at CZ interface controlled by

Answer 53

Heat flow, via conduction heat transfer (crucible) and convection (liquid), heat loss from crystal

Question 54

Changes occurring during CZ process

Answer 54

Crystal surface area expands, so heat loss from crystal changes, and liquid level drops, so coupling between heater and melt changes

Question 55

Coarsening growth

Answer 55

Particles smaller than r* shrink and larger than r* grow

Question 56

What controls surface tension of particles

Answer 56

Fluctuations in bulk or seed