PPT - Diffusion In Solids | Mass Transfer - Chemical Engineering PDF Download

 Page 1


DIFFUSION IN SOLIDS DIFFUSION IN SOLIDS
q FICK’S LAWS
q KIRKENDALL EFFECT
q ATOMIC MECHANISMS
Page 2


DIFFUSION IN SOLIDS DIFFUSION IN SOLIDS
q FICK’S LAWS
q KIRKENDALL EFFECT
q ATOMIC MECHANISMS
Oxidation
Roles of Diffusion
Creep
Aging Sintering
Doping Carburizing
Metals
Precipitates
Steels
Semiconductors
Many more…
Many mechanisms
Material Joining Diffusion bonding
q Diffusion is relative flow of one material into another
Ø Mass flow process by which species change their position relative to their neighbours.
q Diffusion of a species occurs from a region of high concentration to low concentration
(usually). More accurately, diffusion occurs down the chemical potential (µ) gradient.
q To comprehend many materials related phenomenon (as in the figure below) one must understand
Diffusion.
q The focus of the current chapter is solid state diffusion in crystalline materials.
q In the current context, diffusion should be differentiated with flow (of usually fluids and
sometime solids).
Page 3


DIFFUSION IN SOLIDS DIFFUSION IN SOLIDS
q FICK’S LAWS
q KIRKENDALL EFFECT
q ATOMIC MECHANISMS
Oxidation
Roles of Diffusion
Creep
Aging Sintering
Doping Carburizing
Metals
Precipitates
Steels
Semiconductors
Many more…
Many mechanisms
Material Joining Diffusion bonding
q Diffusion is relative flow of one material into another
Ø Mass flow process by which species change their position relative to their neighbours.
q Diffusion of a species occurs from a region of high concentration to low concentration
(usually). More accurately, diffusion occurs down the chemical potential (µ) gradient.
q To comprehend many materials related phenomenon (as in the figure below) one must understand
Diffusion.
q The focus of the current chapter is solid state diffusion in crystalline materials.
q In the current context, diffusion should be differentiated with flow (of usually fluids and
sometime solids).
Ar H
2
Movable piston
with an orifice
H
2
diffusion direction
Ar diffusion direction
Piston motion
Piston moves in the
direction of the slower
moving species
q When a perfume bottle is opened at one end of a room, its smell reaches the other end via
the diffusion of the molecules of the perfume.
q If we consider an experimental setup as below (with Ar and H
2
on different sides of a
chamber separated by a movable piston), H
2
will diffuse faster towards the left (as
compared to Ar). As obvious, this will lead to the motion of movable piston in the direction
of the slower moving species.
q This experiment can be used to understand the Kirkendall effect.
Page 4


DIFFUSION IN SOLIDS DIFFUSION IN SOLIDS
q FICK’S LAWS
q KIRKENDALL EFFECT
q ATOMIC MECHANISMS
Oxidation
Roles of Diffusion
Creep
Aging Sintering
Doping Carburizing
Metals
Precipitates
Steels
Semiconductors
Many more…
Many mechanisms
Material Joining Diffusion bonding
q Diffusion is relative flow of one material into another
Ø Mass flow process by which species change their position relative to their neighbours.
q Diffusion of a species occurs from a region of high concentration to low concentration
(usually). More accurately, diffusion occurs down the chemical potential (µ) gradient.
q To comprehend many materials related phenomenon (as in the figure below) one must understand
Diffusion.
q The focus of the current chapter is solid state diffusion in crystalline materials.
q In the current context, diffusion should be differentiated with flow (of usually fluids and
sometime solids).
Ar H
2
Movable piston
with an orifice
H
2
diffusion direction
Ar diffusion direction
Piston motion
Piston moves in the
direction of the slower
moving species
q When a perfume bottle is opened at one end of a room, its smell reaches the other end via
the diffusion of the molecules of the perfume.
q If we consider an experimental setup as below (with Ar and H
2
on different sides of a
chamber separated by a movable piston), H
2
will diffuse faster towards the left (as
compared to Ar). As obvious, this will lead to the motion of movable piston in the direction
of the slower moving species.
q This experiment can be used to understand the Kirkendall effect.
A B
Inert Marker is basically a thin rod of a high melting material, which is insoluble in A & B
Kirkendall effect
§ Let us consider two materials A and B welded together with Inert marker and given a
diffusion anneal (i.e. heated for diffusion to take place).
§ Usually the lower melting component diffuses faster (say B). This will lead to the shift in
the marker position to the right.
§ This is called the Kirkendall effect.
Direction of
marker motion
Page 5


DIFFUSION IN SOLIDS DIFFUSION IN SOLIDS
q FICK’S LAWS
q KIRKENDALL EFFECT
q ATOMIC MECHANISMS
Oxidation
Roles of Diffusion
Creep
Aging Sintering
Doping Carburizing
Metals
Precipitates
Steels
Semiconductors
Many more…
Many mechanisms
Material Joining Diffusion bonding
q Diffusion is relative flow of one material into another
Ø Mass flow process by which species change their position relative to their neighbours.
q Diffusion of a species occurs from a region of high concentration to low concentration
(usually). More accurately, diffusion occurs down the chemical potential (µ) gradient.
q To comprehend many materials related phenomenon (as in the figure below) one must understand
Diffusion.
q The focus of the current chapter is solid state diffusion in crystalline materials.
q In the current context, diffusion should be differentiated with flow (of usually fluids and
sometime solids).
Ar H
2
Movable piston
with an orifice
H
2
diffusion direction
Ar diffusion direction
Piston motion
Piston moves in the
direction of the slower
moving species
q When a perfume bottle is opened at one end of a room, its smell reaches the other end via
the diffusion of the molecules of the perfume.
q If we consider an experimental setup as below (with Ar and H
2
on different sides of a
chamber separated by a movable piston), H
2
will diffuse faster towards the left (as
compared to Ar). As obvious, this will lead to the motion of movable piston in the direction
of the slower moving species.
q This experiment can be used to understand the Kirkendall effect.
A B
Inert Marker is basically a thin rod of a high melting material, which is insoluble in A & B
Kirkendall effect
§ Let us consider two materials A and B welded together with Inert marker and given a
diffusion anneal (i.e. heated for diffusion to take place).
§ Usually the lower melting component diffuses faster (say B). This will lead to the shift in
the marker position to the right.
§ This is called the Kirkendall effect.
Direction of
marker motion
q Mass flow process by which species change their position relative to their neighbours.
q Diffusion is driven by thermal energy and a ‘gradient’ (usually in chemical potential).
Gradients in other physical quantities can also lead to diffusion (as in the figure below). In
this chapter we will essentially restrict ourselves to concentration gradients.
q Usually, concentration gradients imply chemical potential gradients; but there are
exceptions to this rule. Hence, sometimes diffusion occurs ‘uphill’ in concentration
gradients, but downhill in chemical potential gradients.
q Thermal energy leads to thermal vibrations of atoms, leading to atomic jumps.
q In the absence of a gradient, atoms will still randomly ‘jump about’, without any net flow of
matter.
Diffusion
Chemical potential
Electric
Gradient
Magnetic
Stress
§ First we will consider a continuum picture of diffusion
and later consider the atomic basis for the same in
crystalline solids. The continuum picture is applicable
to heat transfer (i.e., is closely related to mathematical
equations of heat transfer).