Note: Descriptions are shown in the official language in which they were submitted.
D-24,689
METHOD FOR MAKING ULTRAFINE METAL POWDER
FIELD OF INVENTION
The present invention relates to a process for
making rapidly cooled fine metal powders.
BACKGROUND OF T~IE INVENTION
U.S. patent 3,646,177 to Tnompson discloses a
method for producing powdered metals and alloys that are
Eree from oxidation by a process which involves
atomi~ing molten metal with a fluid j~t to form discrete
particles of the molten metal. The jet is directed into
a reservoir of an inert cryogenic liquid to solidify the
particles and prevent oxidation during cooling.
U.S. patent 4,069,045 to Lundgren describes a
process wherein a jet of molten metal is impinged
against a rotating flat disc. Relatively thin, brittle,
easily shattered, and essentially dentrite free metal
flakes are obtained. These flakes are also described in
.S. patent 4,063,9 42 to Lundgren~
U.S. patent 4,221,587 to Ray relates to a method of
making powder by impinging a jet oE molten alloy at an
acute angle against -the inner surface of a rotating
cylindrical chill body. As set forth in column 5, the
impinging molten metal breaks into a stream of discrete
droplets which bounce off the surface and move in the
direction of the chill surface. Upon impact wi-th the
chill surface, -the droplets are solidified at a rapid
~3~
D-24,689
--2--
rate. As set forth in column 6, "the glassy metal
powder particles ... have relatively sharp notched edges
which enable the particles to interlock during
compaction." As se-t forth in the first example, the
particle size of the powder is such that 90~ oE the
particles have a particle slze range between about 25
and 300 microns. In the second e~ample, the
par-ticle size of the powder ranges between 100 and 1000
microns. Herbert Herman and Hareesh Bha-t, in an article
entitled "Metastable Phases Produced by Plasma Spraying"
appearing in the proceedings of a symposium sponsored by
the T~S-AIME alloy Phases Committee at the Fall meeting
of the Metallurgical Society of AIME, Pittsburgh,
Pennsylvania, October 5-9, 1980 describes -the high
velocity deposition of plasma-melting particles on a
substrate. On page 118, the article indica-tes that good
physical and thermal contact should exist between the
solidifying liquid and substrate. Li~uid spreading
occurs away from the impact point. As illustrated in
the drawings, the particles have a flat surface adjacent
the substrate with a central raised core region and a
circular rim area.
DRAWINGS
Figure 1 is a schematic drawing of a device
including a plasma spray apparatus and drum.
Figure 2 is a schematic drawing of a device
including a plasma spray apparatuC~ substra-te and gas
discharge device.
Figure 3 is a schematic drawing oE plasma spray
apparatus, endless belt, and discharge device for
substrate material.
SUMMARY OF INVENTION
Atomi2ed metal or metal alloy powders in the one to
ten micrometer size range are desirable for many
applications such as electrostatic copying and ~or rapid
D-24,689 -3-
solidification processes. Elowever, particles in this
size range are very difficult to obtain. Standard
atomization techniques, whefe a gas or liquid impinges a
molten metal exiting an orifice, are not eEfective in
producing particles in the above range. Agglomeration
and plasma melting methods are also not effective since
it is necessary to start with particles or agglomerates
of about the same si~e as the ending particles.
Agglomerates oE very small si~e are difficult to form
and uniformity in particle composition is even more
difficult to obtain. In general, very fine powders are
often high in atrnosphere impurities such as oxygen,
nitrogen or impurities Erom the grinding medium used to
obtain the small size.
In accordance with the presen-t invention, there is
provided a fine powder wherein a substantial portion of
the particles have smoothly curvilinear surfaces and an
average particle size less than about ten micrometers.
Also, in accordance with the present inven-tion,
there is provided a process for making very fine metal
powder. A high velocity stream of molten metal droplets
is directed toward a repellent surface. The molten
droplets are impac-ted against the surface to fragment
the droplets and form still molten fragmented portions
which are rapidly cooled to form a fine metal powder.
The resulting powder comprises particles less than about
ten micrometers with curvilinear surface.
DETAILED DESCRIPTION
High velocity streams of molten metal droplets may
be ~ormed by thermal spraying. A wide range of
materials, both organic and inorganic may be thermally
sprayed. Typical organic materials include high melting
polymers such as high temperature aromatic polyester
plastics. One such polymer is sold under the trade name
~3~
D-24,689
--4--
EKO~OL*by the Carborundum Company. Inorganic materials
for thernal spraying include ceramlcs and cermets.
The preferred powders are metals and metal alloys.
Low melting metals or alloys may include zinc, lead,
silver or gold. Higher melting point metals and àlloys
typically contain copper, cobalt, iron and nickel may be
used. The re~ractory metals and alloys which typically
have melting points in excess of 1800 degrees centirade
are of particular interest. The refractory type metals
include molybdenum, niobium, tungsten, tantalum,
chromium alloys and mixtures thereof. The term metals
include elemental metals, alloys, pure or mixed oxides,
borides, carbides and nitrides of metal with or without
additives.
Since the powders of the present invention are
produced by rapid cooling, at least some of the powders
contain particles having amorphous phases or metastable
crystal structures. Metal alloys which are most easily
obtained in the amorphous state by rapid quenching or by
deposition techniques are mix-tures of transition metals.
The cooling rate necessary to achieve the amorphous
state depends on the composition of the alloys.
Generally, there is a small range of compositions
surrounding each of the known compositions where the
amorphous state can be obtained. However, apart rom
quenching the alloys, no practical guideline is kno-~n
for predicting with certainty which of the multitude of
different alloys will yield an amorphous me-tal with
given processing conditions. Examples of arnorphous
alloys formed by rapid quenching are described in U.S.
patent 3,856,513 to Ohen et al, U.S. patents 3,427,154
and 3,981,722, as well as others.
The amorphous and crystalline state are
distinguished most readily by differences in X-ray
diffraction measurement. Diffraction patterns of an
* Trade Mark
~ !; ` . ~
D-24,689
--5--
amorphous substance reveal a broad halo similar to a
liquid. Crystalline materials produce a line or
broadened line diffrac-tion pattern. The amorphous
alloys provided by the present invention appear to be
liquid when studied from x-ray diffraction patternsl but
the alloy is soli~ when studied in terms of hardness anc1
viscosity. An amorphous alloy structure is inherently
metastable, i.e., the sta-te is non-equilibrium. Since
the atoms of the amorphous structure are no-t arranged in
a periodic array, there is at any temperature a tendency
of the amorphous structure to transform toward the
crystalline structure oE the equilibrium state through
diffusion or segregation of components of the alloy.
The rapidly cooled powder particles of the present
invention preferably have a particle size distribution
wherein at least about 80 percent of the particles have
an average particle size less than about 10 microns.
Depending on the composition and exact conditions of
powder formation, even smaller particle size
distributions wherein at least 90 percen-t oE the
particles have an average particle size less -than about
10 microns may be formed. Another particle distribution
includes greater than about 80 percent of the particles
having average particle size grea-ter than about 0.5 and
less than about 8 microns.
The particles of the presen-t inven-tion are
preferably cooled from ultrafine portions of molten
materials to give a charac-teristic curvilinear surface
to the particles. Due to surface tension, airborn
3() molten material tends to contract un-til the smallest
surEace area consistent with its volume is occupied.
Due to the repellent nature of the repellent surface
droplet forma-tion is favored. The -tendency of the
molten material is to form spheres. If the rapidly
cooled part;cles solidify prior to assuming the shape of
D-24,689
a sphere or molten particles collide during cooling, the
molten portions may form elliptically shaped or
elonga-ted particles with rounded ends.
The powders of the present invention differ from
milled or fractured powders which are characterized by
an irregularly shaped outline which may have sharp or
rough edges.
According to -the srunauer, Emmett and Teller tBET)
method and equation for determining the surEace area and
diameter, the particles of the present invention exhibit
BET diameters from about 1/2 micrometers to about 10
micrometers.
A scanning Election Micrograpn (SE~) photo of
molybdenum powder oE the presen-t invention has particles
which have substantially smoothly curvilinear surfaces.
The particles appear as small blobs or globs which are
spheroidally and ovoidally shaped with arcua-te and
curved surfaces. The particles comprise cells of from
about 0.01 to about 0.1 micrometers which are indicative
oE rapid cooling.
In preparing the powders of the present invention,
a high velocity stream of molten metal droplets is
Eormed. Such a stream may be Eormed by any thermal
spraying technique such as electric-arc spraying,
combustion spraying and plasma spraying. Typically, the
velocity oE the molten drople-ts is greater than about
100 meters per second, preferably greater than about 200
meters per second, and more preferably greater than 250
meters per second. Velocities on -the order oE 900
meters per second or grea-ter may be achieved under
certain conditions which Eavor these speeds which may
include spraying in a vacuum.
In the preferred process of the present invention,
a powder is fed through a thermal spray appara-tus. Feed
3'~
D-24,689
--7--
powder is entrained in a carrier gas and then fed
through a high temperature reactor. The temperature in
the reactor is preferably above the melting point of the
highest melting componen-t of -the metal powder and even
more preferably above the vaporization point of the
lowest vaporizing componen-t of the material to enable a
relatively short residence time in the reaction zone.
The stream of dispersed entrained mol-ten me-tal
droplets may be produced by plasma-jet torch or gun
apparatus of conventional nature. Typical plasma jet
apparatus is of the resistance arc or induction typeO
In general, a source of metal powder is connected to a
source of propellant gas. A means is provided to mix
the gas with the powder and propel the gas with
entrained powder through a conduit communicating with a
nozzle passage of the plasma spray apparatus. In the
arc type apparatus, the entrained powder may be fed into
a vortex cham~er which communica-tes with and is coaxial
with the nozzle passage which is bored centrally through
the nozzle. In an arc type plasma apparatus, an
electric arc is maintained between an interior wall of
the nozzle passage and an electrode present in the
passage. The electrode has a diameter smaller than the
nozzle passage with which it is coax.ial to so tha-t the
gas is discharged from the nozzle in the form of a
plasma jet. The current source is normal.ly a DC source
adapted to deliver very large currents at relatively low
vol-tages. By adjusting the magnitude of the arc power
and the rate of gas flow, torch temperatures can range
from 150 degrees centigrade up to about 15,000 degrees
centigrade. The apparatus generally mus-t be adjus-ted in
acco.rdance with the melting point of the powclers being
sprayed and the gas e:mployed. In general, the electrode
may be re-tracted within the nozzle when lower melting
powders are utilized with an inert gas such as nitrogen
~3~
D-24,689
--8--
while the electrode may be more fully extended within
the nozzle when higher melting powders are utilized with
an inert gas such as argon.
In the induction type plasma spray apparatus, me-tal
powder entrained in an inert gas is passed at a high
velocity through a strong magne-tic ~ield so as to cause
a voltage to be generated in the gas. The current
source is adapted to deliver very high currents, on the
order o~ 10,000 amperes, although the voltage may be
relatively low such as 10 volts. Such currents are
required -to generate a very strong direct magnetic field
and create a plasma. Such plasma devices may include
additional means for aiding in the initiation of a
plasma generation, a cooling means for the torch in the
~orm o~ annular chamber around the nozzle.
In the plasma process, a gas which i5 ionized in
the torch regains its heat o~ ionization on exiting the
nozzle to create a highly intense flameO In general,
the ~low of gas through the plasma spray apparatus is
e~ected at speeds a-t least approaching the speed of
sound. The typical torch comprises a conduit means
having a convergent portion which converges in a
downstream direction to a throat. The convergent
portion communicates with an adjacent outlet opening so
that the discharge o~ plasma is ef~ected ollt the outlet
opening.
Other types of torches may be used such as an
oxy-acetylene type having high pressure ~uel gas flowing
through the nozzle. The powder may be introduced into
the gas by an aspirating eEfect. The fuel is iynited at
the nozzle outlet -to provide a high temperature flame.
Pre~erably the powders utilized for -the torch
should be uniform in size, and composi-tion and
relatively free flowing. Flowability is desirable to
aid in the transpor-tation and injection of the powder
into the plasma ~lame. In general, ~ine powders (less
3~
D-24,689
_g _
than 40-micrometers average diameter) do not e~hibit
good ~low characteristics. A narrow size distribution
is disirable because, under set flame conditions, the
largest particles may not melt completely, and the
smallest particles may be heated to the vaporization
point. Incomplete melting is a detriment to -the product
uniformity, whereas vaporization and decomposition
decreases process efficiency. Typically, the size
ranges Eor plasma feed powders are such that 80 percent
of the particles fall within a 30 micrometer diameter
range with the range of substantially all -the particles
within a 60 micrometer ranqe.
U.S. Patent 3,909,241 to Cheney et al describes a
process for preparing smooth, substantially spherical
particles having an apparent density of at least 40
percent of the theoretical density of the material. sy
plasma densifying an agglomerate obtained by spray
drying, me-tals which typically will not alloy in a melt
may be intimately mixed in non-equilibrium phases to
form a uniform powder composition.
The stream of entrained molten metal droplets which
issues from the nozzle tends to expand outwardly so that
the density of the droplets in the stream decreases as
the dis-tance from the nozzle increases. Prior to
impacting the repellent surface, the stream typically
passes through a gaseous atmosphere which tends -to cool
and decrease the velocity of the droplets. As the
atmosphere approaches a vacuum, the cooling and velocity
108s iS diminished. It is desirable that -the nozzle be
positioned sufficiently close to the repellent surface
so that the droplets are in a molten condition during
impact. IE the nozzle is too far away, the droplets may
solidify prior -to impact. IE the nozzle is too close
the droplets may impinge on previously sprayed mol-ten
droplets so as to form a pool of molten ma-terial or
D-24,689 -10-
increase the droplet size. It is generally desriable
that the stream flow in a radial direction toward the
repellent surface if the surEace is curved, and in a
normal direction, if the surface is flat.
The repellent surface is preferably a sur~ace that
is not weted by the molten material so as to increase
the propensity of the material to form droplets on the
sur~ace. The wettability and relative surEace energy of
molten metal and a surface can be determined by
measuring the contac-t angle between the liquid phase of
the molten metal and the surface through the liquid
phase. To Ea~or droplet formation it is preferably to
have con-tact angles greater than about ninety degrees.
Typical surfaces may include ceramics such as alumina,
silicon nitride, quartz; metal surfaces such as
aluminum, copper, and inert solids which may be liquid
or solid at room temperatures such as dry ice (CO2) or
normal ice (H2O). The surfaces are preferably smooth.
Molten droplets which impact the repellent surface
are Eragmented to form molten fragmented portions which
are typically a-t leas-t about one third the volume of
the original droplet. After impact, the molten
fragmented portions solidify to Eorm the powder of the
present invention which has substantially smoothly
curvilinear surfaces. The molten fragmented portions
may be cooled by contact with the repellent surface or
by an atmosphere near the repellent surface. The
cooling medium, either repellent surface or atmosphere
is preferably below the solidification temperature of
the molten materialO When a cooling a-tmosphere is
uti]i~ed, the fragmented particles may solidiEy after
bouncing or rebounding off the surface. When the
repellent surface is -the primary cooling medium, the
major quenching may occur on or closely adjacent -the
surface.
D 24,6~9
It is theorized that the particles tend toward
sphericity due ~.o the fact that molten fragmen-ts on the
surface tend toward sphericity due to the repellent
nature of the surface and rebounding molten fragments
tend toward sphericity due to the tendency to contrac-t
to the smallest sur~ace area consistent with volume. It
is believed that the high velocity tends to promote
fragmentation of the particles. As droplets impact the
surface, the component of velocity in the direction of
:Eliyht is immediately changed -to a velocity componen-t in
a direction which is parallel to or at a slight angle to
the surface. This force tends to promo-te fragmen-tation
of the droplets.
It is preferable -that the rebounding fragmented
molten portions and solidified particles have a
component of velocity in a given direction normal to the
stream direction so as to remove fragmented portions
from the path of oncoming droplets. If the nozzle is
stationary with respect to the repellent surface, this
may be accomplished by passing an inert gas over -the
surface at a velocity sufficient to remove fragmented
portions. The nozzle or the surface may also be moved
relative to each other so as to remove fragmented
portions from the oncoming stream of en-trained
particles. To prevent impingement of droplets on
fragmented portions, it is desirable that the previously
fragmented droplets be passed out o:E the range of the
oncoming droplets.
Figure 1 describes an apparatus for carrying out
the method fo the present inven-tion. There is shown a
plasma gun schematically represented at 15. The gun 15
includes a nozzle radially directed at repellent surface
17 which is in the form of a drum. A source of high
pressure gas 19 communicates with a powder source 21 for
entraining metal powder. The entrained powder is fed to
nozzle 15. A source of D.C. powder 23 is electrically
connected between the nozzle 15 and the elements 23 for
D-24,689 -12-
forming plasma 25. After impacting the surface 17,
fragmented portions are collected in a container 27.
The drum is rotated so as to impart a tangential
component oE velocity -to rebounding particles and remove
the fragmen-ted portions 31 from -the path of -the oncoming
entrained droplets.
Figure 2 illustrates another embodiment of the
presen-t invention where a nozzle 51 directs a plasma
stream 53 against a rota-ting disc repellen-t surEace 55.
Another nozzle 57 is directed at the loca-tion cf impact
so as to direct a stream of inert yas 61 at rebounding
Eragmented portions 65 which are propelled toward
container 59 where collected.
Figure 3 illustrates another embodiment where
plasma 70 from nozzle 71 is directed against a moving
bed of repellent material 75 such as dry ice. The
ma-terial 75 is deposited from hopper 77 at one end of
the moving endless belt 79. The plasma 70 is directed
at the moving bed so as to Eorm fragmented portions 85
which are collected in container 81 at the other end of
the endless belt 79.
In Figure 1 through 3 the velocity of the molten
droplets in the respective plasma streams 25, 53, 70 is
sufficient so that upon impacting respective repellent
25 surfaces 17, 55 and 75 the droplets form fragmented
portions. The surEaces 17, 55 and 75 are sufficiently
repellent so as to favor drople-t Eorma-tion. Droplets of
higher viscosities may require higher velocities for
fraymenting droplets.
It is contemplated that a -turbulent yaseous medium
adjacent repellen-t surEace may aid the solidification o:E
rebounding particles. A turbulent gaseous rnedium or
permitting the rebounding fragmented portions to fall
away from the surface under the influence of yravity may
enhance the solidification oE the fragmented portions
~L~3~7~
D-24,6~9 -13-
awa~ from the surface and thus permit the utilization of
less repellent surfaces. The use of a vacuum and
permit-ting fragmented molten portions to fall back onto
the repellent surface may en'nance the solidification of
the fragmented portions on the surface. In this later
case, a highly repellent sur~ace may be desirable.
EXAMPLE 1
~ _.
A Baystate, PGl20-4* plasma gun is mounted in a
chamber about 4 to about 6 inches from a block of dry
ice. Agglomerated molybdenum powder (99.9 percent
molybdenum) having a size distribution of about 56
percen-t -270 + 325 and about 44 percent -325 mesh is fed
to the gun at the rate of 8.85 pounds per hour entrained
in argon at about lO cubic feet per hour. The argon
plasma gas is fed to the torch at the rate of about 63
cubic feet per hour. The torch power is about 30 volts
at 600 amperes. The chamber has a nitrogen atmosphere.
The powder is sprayed in a normal- direction onto a block
of dry ice as the nozzle is moved back and for-th over
the block. About 85 grams of molybdenum powder is
collected. A Scanning Electron Micrograph indicates
that about 90 percent of the particles appear to be less
than about lO micrometers. At least a portion of the
particles appear to have a cellular structure with the
25 cells being from about 0.01 to 0.1 micrometers in size.
The particles have smooth curvilinear surEaces tending
toward sphericity. The particles which are most
rapidly cooled appear to have amorphous properties.
EXAMPLE 2
In a manner similar to example 1, copper powder
having a starting size oE about 30 to 40 micrometers is
reduced to copper particles having a particle size of
about 1 to about 10 micrometers. The star-ting powder
*Trade Mark
~3~
D-24,639 -14-
has a size distribution of 100 percent less than 270
mesh. The apparatus used is as described in Example 1
except the powder feed rate is 5.7 pounds per hour,
plasma gas feed ra-te is 60 cubic feet per hour, and
about 405 grams oE -the powder is collected. The final
powder exhibits the curvilinear structure similar to the
powder structure as of Example 1.
EX~MPLE 3
In a manner similar to Example 2, a powder
consisting of nickel, chromium, and boron is plasma
sprayed. The resulting powder which tends toward
sphericity has an amorphous metastable structure.
EXAMPLE 4
In a manner similar to Example 1, the dry ice bed
is replaced with a ceramic substrate comprising quartz
which has a high thermal shock resistance. The
substrate surface is smooth and the cooling gas of
nitrogen is directed at the surface in the impact area
in a direction tangential to the plasma stream.
Rebounding fragmented particles which are collected
exhibit the spherical powder shape and have an average
particle size less than about 10 micrometers~
