Lecture 5 Flashcards
What do phase diagrams NOT show?
Kinetic information
What do phase diagrams show?
equilibrium states
At what rate does the Arrhenius equation increase as the temperature increases?
Exponentially
How many steps do phase transformations typically take?
more than one step
What is the processes of nucleation and growth?
The nucleation of many small nuclei then the nuclei either grow or shrink
Why does the nucleation need to overcome the nucleation barrier?
The presence of interfacial energy
When and why does crystallization from melt (solidification) happen for solid-liquid phase transformation?
-Temp is less than melting temp
- It has a driving force delta G
Where can nucleation happen for homogenous nucleation?
everywhere (random location), no specific location
Why does the nucleus want to grow in homogenous nucleation?
solid is more stable than liquid at T<Tm
For homogeneous nucleation, what happens at T<Tm?
a solid nucleus with a spherical radius of r nucleates in a liquid
What must the nucleus overcome in homogenous nucleation?
solid-liquid interface energy
What happens at high T for homogenous nucleation?
nucleation barrier is high so crystallization rate is slow
What happens at low T for homogenous nucleation?
atom diffusion is slow (slow kinetics), so crystallization rate is also low
What occurs at optimum T for homogenous nucleation?
maximum nucleation rate
What happens with homogenous nucleation if Ysl is large?
It is difficult
Why does the homogenous nucleation rate increase sharply over a very small undercooling/temperature window?
The (DeltaT)^2 term is inside the exponential term
What kind of clusters (new solid nuclei) grow into stable nuclei?
Ones that reach critical size
