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Sub-topics
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Fuel cells
Casting
Solidification
CONCEPT OF FUEL CELLS
International concerns regarding the emission of
greenhouse gases and the trend toward distributed
power generation are of current interest to the
technical community.
A fuel cell is an electrochemical cell
that produces electricity from a
replenishable fuel tank.
Fuel cells can operate virtually continuously as long as the necessary flows
(reactions) are maintained (they consume reactant from an external source,
which must be replenished).
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FUEL CELL
The electricity is generated
through the reaction, triggered
in the presence of an
electrolyte, between the fuel
(on the anode side) and an
oxidant (on the cathode side).
The reactants flow into the
cell, and the reaction products
flow out of it, while the
electrolyte remains within it.
Many combinations of fuels
and oxidants are possible.
A hydrogen fuel cell uses
hydrogen as its fuel and
oxygen (usually from air) as
its oxidant.
Other fuels include
hydrocarbons and alcohols.
Other oxidants include
chlorine and chlorine dioxide
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MEMBRANE FUEL CELL
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FUEL CELLS DESIGN
Interconnection
Electrolyte
Schematic diagram of
a SOFC bundle
configuration
Air
Electrode
Air
Flow
Fuel Electrode
The electrolyte must conduct
ions, but not electrons,
while the electrodes must conduct the
electrons generated by the electrode
reactions. In addition, the tubes in
structural components must be gastight
and mechanically stable at high
temperatures.
Developing the technology for
producing components that meet
these property requirements requires
processing schemes that produce
specific types of micro- and
macrostructures.
This requires minimizing
thermal expansion differences
among the components, and
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developing gastight seals for the high
temperature use.
FUEL CELLS (CONT)
In addition, the composite components
must be chemically compatible with
each other and with the fuel.
Recent advances in materials selection
and microstructure, combined with
fabrication of electrode-supported
thin-electrolyte planar geometries, has
resulted in tremendous performance
gains.
In addition to oxide ceramics,
silicon-based ceramics such as
SiC, Si3N4, and sialons
along with other borides,
carbides, nitrides, silicides, and
diamond and diamond-like
materials are now common high T
materials of scientific and
technological interest in both bulk
and coating configurations
Current advanced planar SOFCs
have demonstrated ~2 W/cm2 at the cell
level, at 700°C.
These power densities are greater than previous generation cells at 1000°C,
thus, providing the opportunity to utilize less expensive metal interconnects.
However, the use of metal interconnects brings with it new challenges in
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high temperature corrosion prevention.
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HEAT
What is a heat?
Heat is atoms in motion.
In solids, atoms vibrate about their mean
position with a frequency v (about 1013
/second) with an average energy (kinetic +
potential), of RT.
Heat from the sun is the
driving force of life on Earth.
In physics and thermodynamics,
heat is the process of energy transfer from one body or system due
to thermal contact, which in turn is defined as
an energy transfer to a body
in any other way than due to work performed on the body.
Temperature
is used as a measure of the internal energy or enthalpy, that is the
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level of elementary motion giving rise to heat transfer.
CASTING
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CASTING PROCESS
Casting
is a fabrication
process whereby a
totally molten
metal is poured
into a mold cavity
having the desired
shape
Casting techniques are employed when
(1) the finished shape is so large or complicated that any other
method would be impractical,
(2) a particular alloy is so low in ductility that forming by either
hot or cold working would difficulties, and
(3) in comparison to other fabrication processes, casting is the
most economical.
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SAND MOLD CASTING
A two-piece mold is formed by
packing sand around a pattern
that has the shape of the
intended casting
The sand casting process
involves the use of
a furnace, metal, pattern, and
sand mold.
The metal is melted in the
furnace and then ladled and
poured into the cavity of the
sand mold, which is formed
by the pattern. The sand
mold separates along a
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parting line and the solidified
casting can be removed.
INJECTION MOLDING
Injection molding is the most
commonly used manufacturing
process for the fabrication of
plastic parts.
The injection molding process
requires the use of
an injection molding machine,
raw plastic material, and a
mold.
The plastic is melted in the
injection molding machine and
then injected into the mold,
where it cools and solidifies into
the final part.
The common thin-walled products include different types of open
containers, such as buckets. Injection molding is also used to produce
several everyday items such as toothbrushes or small plastic toys.
Many medical devices, including valves and syringes, are manufactured
using injection molding as well.
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DIE CASTING
Die casting is a process that can
produce geometrically complex metal
parts through the use of reusable
molds, called dies.
The die casting process involves the
use of a furnace, metal, die casting
machine, and die.
The metal, typically a non-ferrous
alloy such as aluminum or zinc, is
melted in the furnace and then
injected into the dies. After the
molten metal is injected into the
dies, it rapidly cools and solidifies
into the final part, called the
casting.
Metal housings for a variety of appliances and equipment are often die
cast. Several automobile components are also manufactured using die
casting, including pistons, cylinder heads, and engine blocks.
Other common die cast parts include propellers, gears, bushings, pumps,
and valves.
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CENTRIFUGAL CASTING
Centrifugal casting,
sometimes called
rotocasting, is a metal
casting process that uses
centrifugal force to form
cylindrical parts.
This differs from most
metal casting processes,
which use gravity or
pressure to fill the mold. In
centrifugal casting, a
permanent mold made
from steel, cast iron, or
graphite is typically used.
The casting process is usually performed on a horizontal centrifugal
casting machine (vertical machines are also available).
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WHY IS PROCESS OF SOLIDIFICATION IMPORTANT?
Solidification
is an important industrial
process since most metals are
melted and then cast into a
semi-finished or finished
shape.
http://www.youtube.com/watch?v=kJGlg
SZe4k4&feature=related
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WHY TO STUDY SOLIDIFICATION?
80% of ALL industry involves a casting and
solidification process of materials in various ways
The initial microstructure of the material forms
during solidification process where the melted
alloy becomes a (crystalline) solid
During the last century, by examining metal
alloys with an optical microscope after polishing
and etching the surface, it was discovered that
the microstructures influenced the material's
properties .
Clearly, it is important to understand this
subject
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NUCLEATION AND GROWTH OF GRAINS
When a liquid solidifies, solid first has to appear from
somewhere, after which the interface between solid
and liquid can migrate to enable atoms to switch from
one phase to the other at the boundary –
the two stages are
nucleation and growth
SOLIDIFICATION OF METALS
The steps of solidification:
Thermal gradients define the shape of each grain.
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SOLIDIFICATION: METAL CASTING
In casting, a liquid above its melting point is poured into
a mold where it cools by thermal conduction – it is
relatively cheap and well suited for complex 3-d shapes
New solid forms by
nucleation – new
crystals form in the
melt, on the walls of the
mold, or on foreign
particles
Crystals grow in
opposing directions and
impinge on one another
to form grain boundaries
FUNDAMENTALS
Solidification is a change from liquid to solid state
Recall the atomic arrangements in a liquid and solid
2 step process of NUCLEATION and GROWTH
Solidification - the liquid cools to just below its freezing (or
melting) temperature, because the energy associated with
the crystalline structure of the solid is less than the energy
of the liquid.
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FORMATION OF STABLE NUCLEI
Two main mechanisms:
Homogenous and heterogeneous.
Homogenous Nucleation :
Metal itself will provide atoms to form nuclei.
Metal, when significantly cooled (below freezing T), has
several slow moving atoms which bond each other to form
nuclei.
Cluster of atoms below critical size is called embryo
(continuously being formed and re-dissolved in a molten
metal) .
If clusters of atoms reach critical size, they grow into
crystals. Else get dissolved.
Cluster of atoms that are grater than critical size are called
nucleus.
The critical radius is the minimum size of a crystal that must be formed by
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atoms clustering together in the liquid before the solid particle is stable and
begins to grow.
THE RELATIONSHIP BETWEEN FREE
ENERGY AND TEMPERATURE
At the melting point, both
phases have the same free
energy and can co-exist
Above the melting point,
liquid is in the state of lower
free energy;
If a liquid is cooled beyond its
melting point, its free energy
is greater than that of a solid;
The system can release energy if it
solidifies – this is the driving
force for phase transformation
Energy difference
between the liquid and
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the solid is the driving
force for solidification.
ENERGIES INVOLVED IN HOMOGENOUS
NUCLEATION
Two kinds of energy should be considered
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FREE ENERGY CHANGE
Retarding energy
Driving energy
Energy opposing to
the formation of
embryos, the
energy to form
the surface of
these particles ~
specific surface free
energy
Energy is
released by the
liquid to solid
transformation
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TOTAL FREE ENERGY
Total free energy associated with the formation
of embryo
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CONDITIONS FOR NUCLEATION
Stable cluster
nucleation
solidification
Assume spherical cluster of radius R
Total energy = Volume energy (negative) + Surface energy (positive)
Total energy ET= 4/3πR3 Gv + 4πR2 γ
dET/dR = 0 for energy to be minimum
dET/dR = 4 πR2 Gv + 8πR γ=0
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r* is the critical radius for a stable nucleus
CRITICAL RADIUS AND TEMPERATURE
The greater the degree of undercooling, the
greater the change in volume free energy.
Surface energy does not change much with T.
Cluster stability depends
on energy:
Energy change is positive:
instable cluster
Energy change is negative:
stable cluster
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CRITICAL RADIUS VERSUS UNDERCOOLING
Homogeneous nucleation
occurs when the undercooling becomes large enough
to cause the formation of a stable nucleus.
The latent heat of fusion (entalphy) represents the heat given o¤
during the liquid-to-solid transformation.
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UNDERCOOLING
The undercooling (T) is the difference between the
equilibrium freezing temperature and the actual
temperature of the liquid.
As the extent of undercooling increases,
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the thermodynamic driving force for the formation of a solid phase from the
liquid overtakes the resistance to create a solid-liquid interface.
PROBLEM
Problem: Calculate the critical radius of a homogeneous nucleus
that forms when pure liquid copper solidifies. Assume T of
undercooling = 0.2 Tmelt
Calculate the number of atoms in the critical-sized nucleus.
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HETEROGENEOUS NUCLEATION
Contact angle
between solid
and liquid
The solid
nucleating agent
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must be wetted
by
the liquid metal.
DOES WATER REALLY FREEZE AT 0 C?
This process is dependent on the contact
angle for the nucleating phase and the
surface on which nucleation occurs.
a radius of curvature
greater than the critical radius
is achieved with very little total
surface between the solid
and liquid.
Relatively few atoms must cluster
together to produce a solid particle that
has the required radius of curvature.
The rate of nucleation (the number of
nuclei formed per unit time) is a function of Much less undercooling is required
to achieve the critical size, so
temperature.
Prior to solidification, there is no nucleation. nucleation occurs more readily.
As T drops, the driving force for nucleation
a typical rate of nucleation32
increases; however, as T decreases, atomic
reaches a maximum at some T
diffusion becomes slower, hence slowing the
below the transformation T
nucleation process.
GROWTH OF CRYSTALS AND FORMATION OF
GRAIN STRUCTURE
Nucleus grow into crystals in different orientations.
• Crystal boundaries are formed when crystals
join together at complete solidification.
• Crystals in solidified metals are called grains.
• Grains are separated by grain boundaries.
• More the number of nucleation sites available,
more the number of grains formed.
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When will we obtain fine-grained structures?
SOLIDIFICATION(COOLING) CURVES
Pure metal
Alloy
L
L
Soldification
begins
TL
L
L+S
S
Tm
TS
S
Solidification
complete
Alloys are used in most engineering applications.
• Example:
Cartridge brass is binary alloy of 70% Cu and 30% Zinc.
Iconel is a nickel based superalloy with about 10 elements.
S
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RATE OF
TRANSFORMATION
The rate of nucleation
(the number of nuclei
formed per unit time) is
a function of temperature.
Prior to solidification there is
no nucleation. At T above the freezing point, the rate is zero.
As the temperature drops, the driving force for nucleation
increases.
However, as the temperature becomes lower, atomic
diffusion becomes slower, hence slowing the nucleation
process.
Thus, a typical rate of nucleation reaches a maximum at
some temperature below the transformation temperature
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COARSE-GRAINED OR FINE-GRAINED?
The size of the particles depends on
transformation temperature.
For transformations that occur at T near
to melting point corresponding to low
nucleation and high growth rates, few
nuclei form that grow rapidly.
Thus, the resulting microstructure
will consist of few and relatively large
phase particles (e.g., coarse grains).
For transformations at lower T, nucleation rates are high and growth
rates low, which results in many small particles (e.g., fine grains).
When a material is cooled very rapidly to a relatively low T where the
rate is extremely low, it is possible to produce nonequilibrium phase
structures
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CAN MATERIALS BE STRENGTHENING
DURING SOLIDIFICATION?
Grain structure of Aluminum
cast with and without grain
refiners.
When a metal casting freezes,
impurities in the melt and
walls of the mold in which
solidification occurs serve as
heterogeneous nucleation sites.
To produce cast ingots with fine grain size,
grain refiners are added.
Example: For aluminum alloy, small amount of Titanium, Boron or
Zirconium is added.
The greater grain boundary area provides grain size
strengthening in metallic materials.
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WHAT IS DENDRITE?
If the liquid is undercooled, a protuberance
on the solid-liquid interface can grow
rapidly as a dendrite.
The latent heat of fusion (enthalpy) is
removed by raising the temperature of
the liquid back to the freezing temperature.
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SOLIDIFICATION IN CASTING
Dendritic growth continues until the undercooled
liquid warms to the freezing temperature. Any
remaining liquid then solidifies by planar growth.
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TYPES OF GRAINS
Equiaxed Grains:
M Crystals, smaller in size, grow equally
in all directions.
M Formed at the sites of high
concentration of the nuclie.
Columnar Grains:
M Long thin and coarse.
M Grow predominantly in one direction.
M Formed at the sites of slow cooling
and steep temperature gradient.
M Example: Grains that are away from
the mold wall.
Columnar Grains
Equiaxed Grains
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VIEW OF THE SOLIDIFIED INGOTS
The colomnar grains have
grown perpendicular to the
mold faces since large thermal
gradients are presented in
those directions
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SOLIDIFICATION OF SINGLE CRYSTALS
The most widely used technique for making single-crystal silicon is the
Czochralski process,
in which a seed of single-crystal silicon contacts the top of molten silicon.
As the seed is slowly raised, atoms of the molten silicon solidify in the
pattern of the seed and extend the single-crystal structure.
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