Transcript Water atomization

Powder fabrication
Basic principle: delivering energy to the system to create new surface.
How can the energy be introduced?
1. Mechanical energy (mechanical comminution, milling)
2. Thermal energy delivered for melting, chemical transformations
and evaporation
3. Electrical energy (electrolytic deposition)
Methods of powder fabrication
1. Mechanical (milling, attritioning, mechanical alloying)
2. Electrolytic deposition (electrolysis)
3. Chemical methods (reduction, decomposition, etc.)
4. Atomization methods (gas atomization, water atomization)
5. Vapor phase condensation
Milling
Suitable for brittle materials.
a jar mill
Attritioning, mechanical alloying
Repeated cold welding and fracturing: a way to
microstructural refinement.
A viable method for alloys and composites.
Drawbacks: contamination
Chemical methods
1. Oxide reduction
FeO+H2 = Fe + H2O
2. Carbonyl process
Fe(CO)5 = Fe + 5 CO
3. Precipitation from a gas (involves chemical reactions)
A particle of carbonyl Ni
MoO3 (gas) + 3H2 (gas) = Mo (gas) + 3H2O
CuCl (gas) + ½ H2 = Cu (gas) + HCl (gas)
4. Precipitation from a liquid: spray pyrolysis, hydrothermal
precipitation, sol-gel synthesis.
5. Solid-solid reactive synthesis
Ni + Al = NiAl + Q (heat)
Electrolytic methods: precipitation of a metallic element at
the cathode of an electrolytic cell.
The cycle begins with dissolution of
the anode under an applied voltage.
Transport through the electrolyte is
used to purify the deposit.
Electrolytic powders are of high
purity with a dendritic, sponge-like
morphology.
The most common application is in the production of copper powder.
Pd, Cr, Fe, Zn, Mn, Ag can be produced as well.
Electrolytic methods
A question:
What difficulties might occur in making compound or alloy powder (for
example, brass, an alloy of copper and zinc) by electrolytic technique?
Atomization: disintegration of a melt into droplets that freeze
into particles.
Rayleigh instability: a classic problem in physics.
Explains why and how a falling stream of fluid breaks
up into smaller packets with the same volume but
less surface area.
History: the Plateau-Rayleigh instability is named for Joseph Plateau
and Lord Rayleigh. In 1873, Plateau found experimentally that a
vertically falling stream of water will break up into drops if its length is
greater than about 3.13 to 3.18 times its diameter. Later, Rayleigh
showed theoretically that a vertically falling column of non-viscous
liquid with a circular cross-section should break up into drops if its
length exceeded its circumference, or π D.
The ligament (cylinder) decomposes into spheres during
atomization.
The volume is constant, the surface energy is preserved
or reduced. Can you derive these equations?
D = 3/2 d and N=4L/9d
The final particle size is 1.5 times the original ligament diameter.
Implication: if small particles are to be produced, attention is
required to production of small-diameter ligaments.
Gas atomization
The liquid material is disintegrated by
rapid expansion out of a nozzle.
The powders have spherical shape, good
packing, flow properties and high purity.
Vertical gas atomizer
Water atomization
Water is used to disintegrate the melt stream.
The powders are cooled rapidly, hence their
surface is rough and the shape is irregular.
The atomization liquid has greater efficiency
in transferring velocity into small powders.
Water atomization
A question
Because of rapid cooling in water atomization, the particle
surface is rough and the powder shape is irregular.
What production conditions would favor the formation of a
spherical particle shape using water atomization?
Water and Gas Atomized 17-4 PH Stainless Steel Powders
www.mathson.com/pu_techdoc1.h
Gas atomized Mg
Centrifugal atomization
Centrifugal force is used to break up
the liquid as it is removed from the
periphery of a rotating electrode or
spinning disk/cup.
Offers a clean powder with a spherical
shape and easy flow.
Disadvantages: large particle size, high
cost.
Suitable for high-temperature of
reactive materials when contacts
with a crucible should be avoided.
Drawbacks of atomization methods
1. Powder size limit. When atomization gas or liquid flow rate increases,
the particle size decreases, however, there is a limit (limited impact
velocities). Possible solutions: supersonic rocket nozzles, two-step
atomization.
2. Chemistry control (segregation of additives)
3. Surface oxidation in water atomization.
Vapor phase condensation
A material source is heated and
evaporated.
Low pressure
10% of
atmospheric
pressure,
inert gas (He, Ar)
This method is most suitable to make
ultra pure fine metal powders.
Energy: Joule heat, laser beam,
electron beam.
The metal atoms that boil off from the
source quickly loose their energy.
They form clusters of atoms in the
gas phase by a process called
nucleation. These clusters of atoms
grow by adding atoms from the gas
phase and by coalescence.
The clusters are collected on a cold
finger. A cold finger is simply a
cylindrical device cooled by liquid
nitrogen.
Nanoscale and Submicron metal particles
1. Vapor phase condensation.
2. Solution routes with controlled precipitation (ex. AgNO3 dissolved in
ethylene glycol to produce silver).
3. Decomposition reactions: ex. Co2(CO)8 (see figure below).
Co nanopowder
www.azonano.com
Homework:
Compile a table: material – common production methods for
Fe, W, Ni, Ti, Cu, Ag.