Course D - Microstructure Flashcards
how should you prepare a sample for reflected light microscopy
1) mount, grind, polish the sample to achieve a flat, level surface
2) etch the surface to highlight important features - usually done using chemicals
what does etching do
- etching is treating a sample with a chemical to highlight features, the etching process occurs quicker at grain boundaries and other impurities so they will reflect light differently
what sort of samples are required for transmitted light microscopy
- Transmitted light microscopy relies on detecting the light that has passed through a sample
- very thin samples required
- good for showing grain sizes
what causes contrast in transmitted light microscopy
- differences in absorption
- differences in birefringence
what properties must a material sample have for SEM
- should be prepared through polishing and etching
- cannot be conductive
what is atomic force microscopy (AFM)
- a very sharp tip is mounted on a cantilever and moved across the surface of a material
- this measures the height of the sample
- atomic level resolution can be achieved
what do pressure-temperature phase diagrams show
- for a given composition they show which phase is most THERMODYNAMICALLY stable at some temperature and pressure
why is the most thermodynamically stable phase not necessarily the phase present at a given temp/pres.
- kinetics can prevent a phase from forming
what do temperature-composition phase diagrams show
- they show which phase is most stable at some temperature and composition for a constant, given pressure
what do we mean when we talk about a system
“The subject of a thermodynamic analysis”
what are the 4 types of equilibrium (or not equilibrium) that a phase can be in at any given point
1) stable equilibrium - in a potential energy minimum, the overall minimum for the system
2) unstable equilibrium - at a potential energy maximum, unstable to any perturbations
3) not in equil - not at a max/min, there’s a driving force
4) metastable equil. - stable to small perturbations but not lowest energy state, local potential energy min. but not overall min.
what are the first and second laws of thermodynamics
1st law of thermodynamics: Total energy of universe is conserved
2nd law of thermodynamics: Entropy of the universe cannot decrease
what is the internal energy, U of a system, give the differential form of the equation for U
internal energy of a system, U = potential energy + kinetic energy
dU = δq + δw
q = heat
w = work done
define heat, give a differential equation for it
“The energy that ‘flows’ across a system boundary in response to a temperature gradient”
δq = CdT
C = heat capacity
define work done, give a differential equation for it
“the energy that flows across a system boundary in response to a force moving through a distance”
δw = -p dV
p = pres.
V = vol.
using the differential definitions for heat and work done, redefine the differential equation for internal energy
we know
dU = δq + δw
so
dU = CdT - pdV
give the overall equation for enthalpy, H
H = U +PV
give a differential form of the equation for enthalpy
dH = dU + PdV + VdP
and dU = CdT - pdV
so
dH = CdT + VdP
i.e. enthalpy is the heat transferred at a constant pressure
define entropy, how does it link to the 2nd law of thermodynamics
entropy, S, is a measure of disorder
from 2nd law of TD we get
dS(univ) > 0
what is entropy at equilibrium
dS = δqrev / T
define Gibbs free energy and give both differential and non-differential eq.’s
Gibbs free energy, G, is the energy available to do useful work
- G allows us to find the equil. state of a system, using only properties of the system, not the surroundings
G = H -TS
dG = dH - TdS - SdT
we know dH = δq + VdP
so
dG = δq + VdP - TdS - SdT
at const. temp. and pres.
dG = δq -TdS
what can we say about G at equil for a single phase
dG = 0
Gibbs free energy, G, tends to a minimum at equil.
for spontaneous processes dG<0
for a single phase of constant composition, what can we say about the variation of G with temp
- it will ALWAYS decrease
G = H-TS
its a straight line
y-int = H
grad at any point (draw tangent) = -S
what can we say about which phase is most stable at a temperature for a system of two phases of const. identical comp (e.g. liquid, solid of same phase)
link to G-T graph
- we have two curves on a G-T graph
- whichever is the lowest G phase at any point is the more stable phase
- the point at which the two lines cross is the equil. temp
phase 1: G1 = H1 - TS1
phase 2: G2 = H2 - TS2
difference in G:
ΔG = ΔH - T ΔS
we are interested in the sign of ΔG