diffusion - iowa state universitynano.engineering.iastate.edu/courses/mate271/week6.pdf ·...
TRANSCRIPT
Material Sciences and Engineering, MatE271 1
Material Sciences and Engineering MatE271 1Week6
Diffusion
Atomic Motion in Solids
Material Sciences and Engineering MatE271 Week 6 2
Diffusion in Materials
A. Atoms must be able to move around (diffusion)
Diffusion occurs in solids, liquids and gases:
� Redistribution of non-uniform chemical species (impurity diffusion or interdiffusion)
� Random atomic movement can also occur inchemically uniform materials (self diffusion)
Q. How do changes in microstructure and chemicalcomposition actually occur?
Material Sciences and Engineering, MatE271 2
Material Sciences and Engineering MatE271 Week 6 3
Diffusion is �driven� by Nonuniformity
A B
Distance (x)
Conc
entr
atio
n of
�A�
Time
TemperatureA B
Distance (x)Co
ncen
trat
ion
of �
A�Concentration Profile
Material Sciences and Engineering MatE271 Week 6 4
Interdiffusion forming a solid solution
Initial
Intermediate time
Much longer time
What is this time scale?
Material Sciences and Engineering, MatE271 3
Material Sciences and Engineering MatE271 Week 6 5
Diffusion?
Diffusion is necessary for:� Redistribution of chemical species� Physical changes in microstructure� Densification of powder compacts� Deformation at high temperature (creep)� Formation of solid state reaction products� One kind of conductivity in ceramics (ionic)
Material Sciences and Engineering MatE271 Week 6 6
Atoms would not move around because
there would be no places for them to move to
(all sites would be occupied)-- �locked in place�
Diffusion:
Perfect crystal:
� Point defects must be present in a crystal to permit atomic movement (diffusion)
In a way, atomic diffusion is actually the movement of defects.
Material Sciences and Engineering, MatE271 4
Material Sciences and Engineering MatE271 Week 6 7
Diffusion Mechanisms
I. Vacancy diffusiono Only adjacent atoms can move into a vacancyo Vacancy moves in opposite direction of atomic
motiono Rate depends on concentration of vacancies
Atomic flux
Vacancy flux
Material Sciences and Engineering MatE271 Week 6 8
II. Interstitial Diffusiono Atom can move into any adjacent empty
interstitial position (usually smaller atoms)o Rate depends on concentration of interstitial
atoms� (Usually faster than vacancy diffusion)
Diffusion Mechanisms
Material Sciences and Engineering, MatE271 5
Material Sciences and Engineering MatE271 Week 6 9
o Would you expect vacancy or interstitial diffusion to be faster?
o Why?
Diffusion Mechanisms
Material Sciences and Engineering MatE271 Week 6 10
Net migrationafter n jump
� After many random jumps by an atom, it�s displacment� can be calculated by the theory of �random walks�
Diffusion occurs by random jumps
Material Sciences and Engineering, MatE271 6
Material Sciences and Engineering MatE271 Week 6 11
- The rate of diffusion is characterized by describing atomic
fluxes at particular locations in the material
- Critical quantities
J = atomic flux (atoms/m2-s, kg/ m2-s)
(dc/dx) = concentration gradient
(atoms/m4)
D = diffusion coefficient (m2/s)
Quantitative Description of Diffusion
area
J
c
dc/dx x
Fick�s first law: J = - D (dc/dx)
Material Sciences and Engineering MatE271 Week 6 12
Interrelating the quantities
� Fick�s first law: J = - D (dc/dx)
(negative sign indicates that the direction of diffusion flux
is �down� the concentration gradient from high to low
concentration)
� For steady state diffusion (local flux doesn�t change
with time), Fick�s First Law can be solved directly
Material Sciences and Engineering, MatE271 7
Material Sciences and Engineering MatE271 Week 6 13
Hydrogen (H) gas can be purified by passing atomic hydrogen through a thin sheet of palladium (Pa) at 700oC in a paladium diffusion cell. If the impure hydrogen gas is maintained at 1 atm on one side of a 1 mm thick Pd sheet (A=1 m2), and the pressure on the purified side is maintained at 0.1 atm by pumping, what is the mass of the hydrogen purified in 1 hr? Assume steady state conditions. The concentration of H2 at 1 atm is 9.0x10-3 gm/cm3 and D(H) in Pa is 1.2x10-6 cm2/sec.
Example
Material Sciences and Engineering MatE271 Week 6 14
Non-steady state diffusion
The diffusion flux at a particular point varies with time
� � (There is a net accumulation or depletion of the diffusing species at a given location)
� � i.e., local concentration of diffusing species changes with time as diffusion proceeds
� � This is the most common situation
What is this time scale?
Material Sciences and Engineering, MatE271 8
Material Sciences and Engineering MatE271 Week 6 15
Non-steady state diffusion
� Fick�s Second Law governs
� Many solutions exist for particular geometries (initial and boundary conditions)
� Diffusion from a constant source into an semi-infinite solidBC-1: For t = 0, C = C0 at 0 ≤ x ≤ ∞ BC-3: C = C0 at x = ∞
BC-2: t > 0, C = Cs at x = 0
(Cx - C0) = 1 - erf x(Cs-C0) 2√Dt
∂C = D ∂2C∂t ∂x2 x
C(∞,t)=Co
C(x, t)=Cx ?
C(0,t)=Cs
C(x,0)=Co
Material Sciences and Engineering MatE271 Week 6 16
Non-steady state diffusion
x = 0
Cs
C0
to< t1 < t2 < t3
t1t2
t3
Cx
x
C
C(0,t)=Cs C(∞,t)=Co
C(x,0)=Co
to
C(x, t)=Cx ?
(Cx - C0) = 1 - erf x(Cs-C0) 2√Dt
Material Sciences and Engineering, MatE271 9
Material Sciences and Engineering MatE271 Week 6 17
Example
For some applications (e.g. gears), it is necessary to harden the surface of a steel (Fe-C alloy) above that of its interior. One way of accomplishing this is by increasing the surface concentration of carbon in the steel (as we will see later) using a process termed carburizing. In carburizing the steel piece is exposed, at elevated temperature, to an atmosphere rich in a hydrocarbon gas, such as methane (CH4).
Surface Treatment of Steel:
Material Sciences and Engineering MatE271 Week 6 18
Example: Surface Treatment of Steel:
Consider on such alloy that initially has a uniform carbon concentration of 0.25 wt% and is to be treated at 950° C. If the concentration of carbon at the surface is suddenly brought to and maintained at 1.20 wt%, how long will it take to achieve a carbon content of 0.80 wt% at a position 0.5 mm below the surface? The diffusion coefficient for C in Fe at this temperature is 1.6 x 10-11 m2/sec. Assume piece is semi-infinite.
Material Sciences and Engineering, MatE271 10
Material Sciences and Engineering MatE271 Week 6 19
Factors that Influence Diffusion
I. Diffusing Species� magnitude of diffusion coefficient, D - indicates
the rate at which atoms diffuse� both diffusing species and host material
influence the coefficient� Relative sizes of atoms� �Openess� of lattice� Ionic charges
Material Sciences and Engineering MatE271 Week 6 20
Factors that Influence Diffusion
For example:
� For the host species of iron:- Self diffusion at 500°C (Fe moving in Fe)
D = 1.1 x 10-20 m2/s (vacancy diffusion)
- Carbon interdiffusion at 500°C (C moving in FeD = 2.3 x 10-12 m2/s (interstitial diffusion)
� Atomic Size/Mechanism
This shows the contrast between rates of
vacancy and interstitial diffusion
Material Sciences and Engineering, MatE271 11
Material Sciences and Engineering MatE271 Week 6 21
Factors that Influence Diffusion
II. Temperature� very strong effect on the diffusion coefficient:
� A large activation energy results in a small D
D = Do exp −Q d
RT� �
� �
Do = T independent preexponential Qd = the activation energy for diffusion (J/mol, or eV/atom)
R = the gas constant, 8.31 J/mol - K or 8.662 x 10-5 eV/ atom − KT = absolute temperature, (K)
ln D = ln Do − −Qd
R1T
� �
� �
Plot ln D vs 1/T - get straight line(to measure activation energy and Do)
Material Sciences and Engineering MatE271 Week 6 22
Example:
o Using data from Table 5.2, compute the diffusion coefficient of C in α−Fe and γ−Fe at 900ºC.
Material Sciences and Engineering, MatE271 12
Material Sciences and Engineering MatE271 Week 6 23
Solution:
D = Do exp −Q d
RT� �
� �
Do = T independent preexponential Qd = the activation energy for diffusion (J/mol, or eV/atom)
R = the gas constant, 8.31 J/mol - K or 8.662 x 10-5 eV/ atom − KT = absolute temperature, (K)
o C in α−Fe (BCC) at 900ºC D = 6.2x10-7 m2/sec exp (-0.83 eV/atom / (1173K)(8.62x10-5 eV/atom-K)D = 1.7x10-10 m2/sec
o C in γ−Fe (FCC) at 900ºCD = 2.3x10-5 m2/sec exp (-1.53 eV/atom / (1173K)(8.62x10-5 eV/atom-K)D = 5.9x10-12 m2/sec
Material Sciences and Engineering MatE271 Week 6 24
What does this tell you about interstitial sites in BCC and FCC?�.
� BCC more open than FCC for interstitial diffusion�i.e. it is easier to move from one interstitial site to another in BCC
� But it does not say anything about the sizeor number of interstitial sites in each�.actually, as you will see, FCC has bigger (and more) interstitial sites
Material Sciences and Engineering, MatE271 13
Material Sciences and Engineering MatE271 Week 6 25
(Besides through volume of the crystal)
� Atomic migration often occurs more rapidly along so-called �short circuiting paths�
� Dislocations
� Grain boundaries
� External surfaces
� However, there is usually small total area
for this to occur - so not always important
Other Diffusion Paths
Material Sciences and Engineering MatE271 Week 6 26
Volume, grain boundary and surface diffusion
surface
Grain boundary
volume
Ag in Ag
Material Sciences and Engineering, MatE271 14
Material Sciences and Engineering MatE271 Week 6 27
Diffusion and Materials Processing
o Properties of materials are altered through diffusion
� steelmaking� sintering� semiconductor doping
o �Heat treatment� is used to allow these to occur over a reasonable time frame.
Material Sciences and Engineering MatE271 Week 6 28
Summary� Recognize various imperfections in crystals
� Point imperfections
� Impurities
� Line imperfections (dislocations)
� Bulk imperfections
� Define various diffusion mechanisms
� Identify factors controlling diffusion processes