Note: Descriptions are shown in the official language in which they were submitted.
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PLASMA SPRAYING DEVICE AND METHOD
FIELD OF INVENTION
[0001] The present invention is in the field of plasma spray
technology. In
particular a plasma generating device and a method for spraying flowable
materi-
als are disclosed.
BACKGROUND
[0002] Plasma spraying devices are used for spraying various flowable
mate-
rials, such as powdered materials (or simply powders), in a number of applica-
tions, including, for example, in connection with coating applications. Such
de-
vices typically comprise a cathode, an anode, and a plasma channel extending
between the cathode and through the anode. During operation, a plasma-
generating gas is supplied to the plasma channel. The electrical arc formed be-
tween the cathode and the anode heats the gas flowing through the plasma chan-
nel, forming a plasma flow (sometimes also called a plasma stream or plasma
jet).
The plasma flow exits the device through an outlet in the anode at the end of
the
plasma channel. Several different types of plasma spraying devices are known.
These types may be characterized by the position at which a flowable material
is
introduced (or injected) into the plasma flow. The following discussion
relates to
powder spraying devices. However, a person of skill in the art will appreciate
that
other materials may be used for spraying.
, [0003] In one type of device, the powder is introduced into the plasma
flow at
the anode area. In some devices of this type, the powder is introduced into
the
plasma flow through inlets in the anode, as disclosed in, for example, U.S.
Pat.
Nos. 3, 145,287, 4,256,779, and 4,445,021. In other devices of this type the
pow-
der is introduced into the plasma flow by feeders located outside the plasma-
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generating device, as disclosed, for example, in U.S. Pat. No. 4,696,855. Typi-
cally, the powder is injected substantially perpendicular to the plasma flow.
[0004] One advantage associated with devices of this type is that when
the
powder is injected into the plasma flow, the plasma flow is fully developed
and
has certain known properties, such as temperature, velocity, energy, etc.
These
properties depend on, and can be controlled by, the internal geometry of the
plasma channel, the nature of the gas used to generate the plasma, the
pressure
with which the gas is supplied, the difference in electric potential between
the
cathode and the anode, etc. Another advantage of supplying the powder at the
anode area is that the formation of plasma flow is unaffected by the powder.
[0005] However, introducing the powder at the anode area has
disadvantages.
Typical powders have particles of different sizes. When such powder is
injected
into the plasma flow, heavier particles, which have higher kinetic energy,
reach
the center of the plasma stream faster than lighter particles. Therefore, the
lighter
particles may reach the center of the plasma flow in the relatively cold zones
of
the plasma flow located further away from the anode, or the lighter particles
may
remain on the periphery of the plasma flow never reaching its center. This
creates
two undesired effects. First, there is a low level of homogeneity of the
powder in
the flow because the heavier particles are subjected to a higher temperature
for a
longer period of time compared to the lighter particles. The lighter particles
may
not be sufficiently heated for the coating applications. Second, the
distribution of
the coating is not uniform, and some particles may simply miss the surface to
be
coated, which leads to poor material economy. In other words, the powder-
sprayed coating is produced using only a portion of the supplied powder. This
is
particularly disadvantageous when expensive powders are used. The problem can
be mitigated to some extent by using powders with particles of equal mass. How-
ever, such powders are more expensive to manufacture and using them may not be
a viable alternative for all applications.
[0006] To avoid problems associated with the substantially
perpendicular in-
jection of powder in the anode area of the plasma channel, attempts have been
made to provide a longitudinal powder supply channel. The powder supply chan-
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nel is arranged inside the plasma channel and is surrounded by the plasma flow
during operation of the device. The outlet of the powder supply channel is in
the
anode area of the plasma channel. This interior powder supply channel,
arranged
inside the plasma channel, prevents adequate heating of the plasma flow and,
in
general, has undesirable effects on the plasma flow properties.
100071 A further disadvantage associated with introducing the powder at
the
anode is that a large amount of energy is needed to maintain the high
temperature
and specific power (power per unit of volume) of the plasma flow so as to
obtain a
highly homogeneous coating. It is believed that the cause of this problem is
that
the temperature and velocity distribution of the plasma flow is virtually
parabolic
at the outlet of the plasma channel where the powder is injected. Thus, the
tem-
perature and velocity gradient and the thermal enthalpy of the plasma flow are
inversely proportional to the diameter of the plasma flow. To increase the
homo-
geneity of the sprayed coating, it is therefore necessary to increase the
diameter of
the plasma flow, which in turn requires a lot of energy.
[0008] In a second type of device, the powder is supplied at the inlet
of the
plasma channel, at the cathode. In these devices, the electric arc heats both
the
plasma generating gas and the powder. The cathode area is considered to be a
cold zone, which allows the powder to be introduced in the center of the
plasma
flow. Examples of devices of the second type are disclosed in, for example,
U.S.
Pat. No. 5,225,652, U.S. Pat. No. 5,332,885, and U.S. Pat. No. 5,406,046.
100091 When plasma is generated by supplying a plasma generating gas to
the
plasma channel and heating the gas with an electric arc of a predetermined dis-
charge current, only a small portion of the gas forms the center of the plasma
flow
where the temperature is high. The remaining gas flows closer to the walls of
the
plasma channel, where the temperature is lower, forming the cold layer of the
plasma flow. The cold powder particles interfere with the temperature increase
of
the plasma in the flow, and the powder in the periphery of the flow never
reaches
the desired temperature. Because of this temperature distribution in the
plasma
flow, only a small portion of the powder, supplied at the inlet of the plasma
chan-
nel, flows in the high temperature center of the plasma flow and is
sufficiently
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heated by the electric arc. The remaining powder flows in the cold layer of
the
plasma flow. This causes an uneven heating of the powder, which affects the
quality of the surface coating. Furthermore, there is a risk of the plasma
channel
being clogged by the powder, which has a detrimental effect on the conditions
required for a stable plasma flow.
[0010] Increasing the transfer of mass to the central part of the
channel by
increasing the rate of the gas and powder flows is not a practicable
alternative.
When the flow of the gas and powder increases, while the current remains con-
stant, the diameter of the electric arc decreases, which just aggravates the
problem
of the powder accumulating in the cold layer along the plasma channel walls.
Furthermore, for those particles that end up in the center of the plasma flow,
the
time spent in the plasma flow decreases, because the velocity of those
particles
increases. Therefore, the amount of the powder in the high temperature plasma
flow center cannot be increased if the current remains constant. Increasing
the
operating current, however, causes disadvantages associated with both the
design
and handling of the plasma-spraying devices.
[0011] In devices of a third type, a portion of the plasma channel is
formed by
intermediate electrodes electrically insulated from the anode and the cathode.
The
powder is introduced into the plasma flow in the portion of the plasma channel
formed by the intermediate electrodes, typically between two electrodes. Thus,
the powder is supplied neither at the inlet of the plasma channel nor at the
outlet
of the plasma channel. Examples of devices of the third type are disclosed in,
for
example, U.S. Pub. No. 2006/0091116A1.
[0012] The device disclosed in U.S. Pub. No. 2006/0091116A1 has two
plasma channel sections. The section of the plasma channel located upstream
from the powder feeder is formed by one or more intermediate electrodes and is
used to create optimal conditions in the plasma flow. In particular, during
opera-
tion, the plasma is heated to a temperature sufficient to melt the powder
through-
out the entire cross section of the plasma channel. This eliminates the
problem
associated with powder particles traveling in the cold layer of the flow, and
re-
duces the risk of clogging when particles stick to the walls of the plasma
channel.
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The section located downstream from the powder feeder is also formed by one or
more intermediate electrodes and is used to achieve a high level of
homogeneity
and temperature of the powder particles in the flow thus obviating the
problems
associated with supplying the powders at the anode. By controlling the
properties
of the downstream section, such as its length and the number of intermediate
elec-
trodes forming the section, optimal conditions of the powder are achieved.
These
conditions include velocity and temperature level necessary to obtain the
required
adhesion, structure, and porosity in the sprayed coating for a specific
combination
of the power material and the coating application. However, because the
velocity
to of the plasma flow and the powder particles that it carries is
relatively low, the
powder particles have low kinetic energy when they exit the device.
[0013] To achieve higher velocities of powder particles, some spraying de-
vices use throttling portions. For example so-called cold spray or velocity
spray
devices pressurize a relatively cold gas carrying a powder and then use a
throttling
portion to accelerate the gas carrying the powder to high velocities. Such
devices
use the kinetic energy of the powder particles for coating. Throttling
portions
have been long known in the art. Briefly, they are used to convert pressure of
a
gas flow into velocity. Throttling portions were first used in jet engines,
but now
they are also used in plasma generating devices. A known variation of a
throttling
portion is the supersonic nozzle (also called the de Laval nozzle), which is
capable
of accelerating the plasma flow to supersonic speeds. U.S. Patent No.
8,105,325
discloses the use of the supersonic nozzle in a multi-electrode plasma
generating device used for cutting, evaporating, and coagulating biological
tis-
sues. U.S. Patent No. 8,105,325, however, is not concerned with features of
the throttling portion useful for spraying applications, such as the drop in
the static
pressure of the plasma flow that facilitates the injection of powders and the
ability
to use nanoparticles for spraying.
[00141 Plasma spraying devices that use throttling portions may fall into
any
of the three categories set forth above. However, because of their use of the
throt-
tling portions, they are discussed separately. U.S. Pub. No. 2006/0108332 dis-
closes the use of a throttling portion in a plasma spraying device. In
particular,
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this publication discloses a throttling portion which is located essentially
in the
end of the plasma channel closest to the cathode. During operation of this
device,
after the plasma generating gas is briefly heated by a cathode in the heating
cham-
ber near the cathode, the gas passes through the throttling portion. The
throttling
portion increases the speed of the gas, in some embodiments beyond the speed
of
sound, and decreases the static pressure of the gas. The powder is injected
into
the plasma flow after the plasma passes the throttling portion at a point in
the
plasma channel where the plasma reaches its maximum speed and has minimum
static pressure. However, because the throttling portion is arranged
essentially at
to the cathode end of the plasma channel, the plasma flow is heated by the
electric
arc only while it passes through the throttling portion. Accordingly, the
plasma
reaches the speed of sound while it is essentially cold. Because the speed of
sound is higher at higher temperatures, the absolute speed that the plasma
generat-
ing gas achieves is relatively low. Due to the relatively low speed the plasma
does not achieve a high power density. Furthermore, because the powder is in-
jected in the area of the anode in the device disclosed in U.S. Pub. No.
2006/0108332, the device exhibits limitations generally associated with the de-
vices of the first type discussed above.
10015] U.S. Pub. No. 2006/0037533 discloses the use of a throttling
portion in
a thermal spraying device. The device comprises (1) a heating module used for
heating a flow of gas (or plasma, in some embodiments), (2) a forming module
used to decrease the static pressure and increase the speed of the gas stream;
(3) a
powder feeding module that is used to inject powder into the flow; and (4) a
barrel
module used to carry the powder in the stream, so that the powder achieves
neces-
sary properties. The publication discloses a number of different ways of imple-
menting a heating module. For example, in some embodiments the heating mod-
ule is a combustion type heating module, which heats the gas by combusting
acetylene. After the gas is heated to 3,100 C, it is passed to the forming
module.
After the velocity and pressure of the gas flow are transformed by the forming
module, the powder is injected into the gas flow in the powder feeding module.
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The powder particles, carried by the gas flow, achieve properties required for
a
particular spraying application in the barrel module.
[0016] U.S. Pub. No. 2006/0037533 discloses another embodiment of the
heating portion implemented as a multielectrode plasma torch This plasma torch
has a cathode, an anode, and a plurality of intermediate electrodes. The anode
and
the intermediate electrodes form a plasma channel. The publication further dis-
closes a throttling portion, distinct from the one in the forming module,
located
essentially in the end of the plasma channel closest to the cathode. During
opera-
tion of this heating module, after the plasma generating gas is heated by the
cath-
to ode in a heating chamber near the cathode, the gas passes through the
throttling
portion. The throttling portion accelerates the flow, in some embodiments
beyond
the speed of sound, and decreases the static pressure of the gas.
[0017] Some devices, such as the one disclosed in U.S. Pub. No.
2006/0091116A1 discussed above, provide for injection of different flowable ma-
terials. This feature is desirable for some plasma spraying applications.
[0018] Accordingly, presently there is a need for a plasma spraying
device
that overcomes the limitations of the currently known devices by maximizing
the
energy density of the device while enabling control of both kinetic and
thermal
energy of the plasma flow carrying the powder particles at the outlet of the
device.
In particular, there is a need for a plasma spraying device and method that
gener-
ates a plasma flow having a temperature and speed that enables one or more
flow-
able materials to be injected into the plasma flow by applying a relatively
low
pressure, while also enabling control of the characteristics of the plasma and
the
flowable materials when they exit the plasma channel.
SUMMARY OF THE INVENTION
[0019] The present invention provides a plasma generating device
comprising
an anode, a cathode and a plasma channel, extending longitudinally between the
cathode and anode. The plasma channel has an outlet opening at the anode end
of
the device and a throttling portion. A part of the plasma channel is formed by
two
or more intermediate electrodes electrically insulated from each other and the
an-
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ode. The throttling portion of the device divides the plasma channel into a
high
pressure portion positioned on the side of the throttling portion closest to
the cath-
ode and a low pressure portion positioned on the side of the throttling
portion
closest to the anode. The throat of the throttling portion has a cross
sectional area
transversely to the longitudinal direction of the plasma channel smaller than
both
the minimum cross sectional area of the high pressure portion and the minimum
cross sectional area of the low pressure portion. In the low pressure portion
of the
plasma channel, the device also has one or more flowable material injectors
which
include a flowable material chamber having an inlet and a flowable material
feeder connecting the flowable material chamber to the plasma channel.
Together
the flowable material feeder and the flowable material chamber are referred to
as
the flowable material injector.
[00201 In the plasma spraying device, the high pressure portion of the
plasma
channel is formed by at least one, but preferably two or more, intermediate
elec-
trodes. This enables the plasma flow that reaches the throttling portion to be
suf-
ficiently heated to achieve a high level of homogeneity of a given flowable
mate-
rial, such as powder, injected into the plasma flow. The low pressure portion
of
the plasma channel is formed by at least one, but preferably two or more,
interme-
diate electrodes. This enables sufficient heating of the flowable material for
a
given spraying application after the flowable material is injected into the
plasma
flow.
[0021] During operation, a plasma generating gas is supplied to the
plasma
channel. As the gas flows through the plasma channel, it is heated by an
electric
arc formed between the cathode and the anode. The temperature increase of the
electric arc results in gas ionization and plasma formation. The static
pressure of
the plasma in the high pressure portion of the plasma channel is relatively
high.
As the plasma passes through the throttling portion, its velocity pressure
increases
and the static pressure decreases. The increase in the velocity pressure may
accel-
erate the plasma flow to supersonic speeds. At the end of the throttling
portion
closest to the anode, the static pressure of the plasma is at its minimum. The
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flowable material is injected into the plasma flow in the low pressure
portion,
which, due to the low static pressure of the plasma, requires minimal
pressure.
[0022] The injection of the flowable material in the described device
results in
a high level of homogeneity of the flowable material because the plasma is
suffi-
ciently heated while passing through the high pressure portion. Because the
flow-
able material carrier gas mixes with hot plasma, the temperature of the plasma
drops and is lower than the temperature of the plasma before the flowable
material
is injected. For some spraying applications a high temperature of the flowable
material particles may be required. As the aggregate flow of plasma carrying
the
particles of the flowable material passes through the remaining portion of the
low
pressure portion, the electric arc heats the plasma, which heats the
particles.
[0023] In some embodiments, the device enables injection of two or more
flowable materials. In an exemplary embodiment that enables injection of two
flowable materials, the device comprises a second flowable material injector.
The
second flowable material injector is arranged in a way that enables the
particles of
the injected first flowable material to be sufficiently heated before the
second
flowable material is injected. At the same time, the second flowable material
in-
jector is arranged in such a way that enables particles of both flowable
materials
to be sufficiently heated for a given spraying application before the
aggregate
flow exits the device. In some embodiments, the device may comprise additional
flowable material injectors provided that the following conditions are met:
(1)
plasma and particles of all flowable materials injected upstream from a given
flowable material have to be sufficiently heated before the given flowable
mate-
rial is injected in the flow and (2) particles of all flowable materials
injected up to
a point must be sufficiently heated (i) before exiting the device for a given
spray-
ing application, or (ii) before injecting another flowable material. Note that
it
may not be necessary to heat particles of a flowable material to the
temperature
required when the flow exits the device before injecting another flowable
material
because particles of both flowable materials are heated following injection of
the
second flowable material. Accordingly, the invention also provides a method of
plasma-spraying one or more flowable materials comprising creating a plasma
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flow which is heated to at least 10,000 K and thereafter increasing the
velocity
pressure of the heated plasma in the flow while concurrently decreasing the
static
pressure of the heated plasma in the flow, whereupon one or more flowable Mate-
rials are injected into the flow of plasma. In the method of the invention,
after
each flowable material is injected, the particles of all flowable materials in
the
flow are heated to an appropriate temperature by heating the plasma in the
flow
before being output in the plasma.
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] FIG. 1 illustrates a cross sectional longitudinal view of an
embodiment
of the device of the invention with a single flowable material injector;
[0025] FIG. 2 illustrates a cross sectional longitudinal view,
transversely to
the view illustrated in FIG. 1 of an embodiment of the device of the invention
with a single flowable material injector;
[0026] FIG. 3 illustrates a feeder that is angled in the direction opposite
to the
plasma flow;
[0027] FIG. 4 illustrates a feeder that is angled in the direction of
the plasma
flow;
[0028] FIG. 5 illustrates a cross sectional longitudinal view of an
embodiment
of the device with multiple flowable material injectors;
[0029] FIG. 6 illustrates a water divider of the cooling system used to
cool a
throttling portion; and
[0030] FIG. 7 illustrates a water divider of the cooling system used to
cool the
anode and other intermediate electrodes.
DESCRIPTION OF PREFERRED EMBODIMENTS
[0031] FIGs. 1 and 2 illustrate one embodiment of a plasma-spraying
device
according to the invention. The embodiment depicted in FIGs. 1 and 2 is a pow-
der spraying device with a single flowable material injector. However, it
should
be understood that this is an exemplary embodiment and is not meant to limit
the
scope of the invention to the use of a powder or to the use of a single
flowable
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material or a single injector. For purposes of this disclosure, the expression
"flowable material" is defined as any material that flows in a vessel under
pres-
sure. Flowable materials include, but are not limited to, liquids, gases, or
particles
of solid materials carried by a fluid. The term "powder" in the present
disclosure
should be understood as small particles of a material that can be carried by a
fluid,
such as a gas; for the purposes of this disclosure, a "powder" is a flowable
mate-
rial. Another variation of a flowable material is a solution of powder
particles,
such as nanoparticles, in a liquid precursor used, for example, in a spraying
tech-
nique known as Suspension Precursor Plasma Spray (SPPS). During operation,
such a solution is atomized and injected into the plasma flow as a flowable
mate-
rial.
100321 FIG. 1
shows a longitudinal cross-section of the device. Although the
following discussion refers to the use of a powder, it shall be understood
that any
other type of flowable material may be used. In the embodiment of FIG. 1, a
cas-
ing 2, a flowable material assembly 60, a washer 56, and a casing 48 form the
outside of the device. In this embodiment the device is cylindrical and all
ele-
ments are annular and are arranged coaxially. In other embodiments, however,
the device may not be cylindrical and a different internal or external
geometry
may be used. The device comprises a cathode 4, preferably made of tungsten con-
taining lanthanum, which is arranged in a cathode holder 6, and an anode 8.
Insu-
lator element 10 surrounds a portion of cathode 4 furthest from anode 8 and a
por-
tion of the cathode holder 6. Insulator element 10 provides both thermal and
elec-
trical insulation of cathode 4.
[00331 Annular
intermediate electrodes 12, 14, 16, 18, 20, 22, and 24 and an-
ode 8 form a plasma channel 26. Plasma channel 26 has an inlet 32 at the end
closest to cathode 4 and an outlet (or opening) 34 at the end furthest from
cathode
4. Annular insulators 36, 38, 40, 42, and 44 are located between intermediate
electrodes 12, 14, 16, 18, 20, 22, and 24 and provide electrical insulation
between
adjacent intermediate electrodes. Annular insulator 46 is located between
inter-
mediate electrode 24 and anode 8 and provides electrical insulation between
them.
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[0034] Intermediate electrode 12, which is furthest from anode 8, forms
a
plasma chamber 28 around cathode tip 30. The plasma chamber 28 is connected
to the inlet of the plasma channel 32. Space 61 provides a passage for plasma
generating gas to plasma chamber 28.
[0035] FIG. 2 shows a longitudinal cross section that is transverse to the
lon-
gitudinal cross section shown in FIG. 1. FIG. 2 illustrates portions of the
plasma
channel. Intermediate electrode 18 forms a throttling portion 80. Throttling
por-
tion 80 divides the plasma channel 26 into two portions: a high pressure
portion
82 and a low pressure portion 84. The high pressure portion 82 is formed by
one
or more intermediate electrodes. Preferably, the high pressure portion 82 is
formed by two or more intermediate electrodes. In the embodiment shown in
FIGs. 1 and 2, the high pressure portion 82 of plasma channel 26 is formed by
three intermediate electrodes 12, 14, and 16. The high pressure portion should
have a length sufficient to ensure that when a powder is injected into the
plasma,
the plasma has a temperature sufficient to melt the powder across the entire
cross
section of plasma channel 26.
[0036] The low pressure portion 84 is formed by at least one, but
preferably
two or more, intermediate electrodes. In the embodiment shown in FIGs. 1 and
2,
the low pressure portion 84 is formed by three intermediate electrodes 20, 22,
and
24. The low pressure portion 84 should have a length sufficient to ensure that
particles of the powder carried by the plasma are heated to the temperature re-
quired for a given spraying application.
[0037] The throttling portion 80 has an hourglass shape. The narrowest
part
of the throttling portion 80 is throat 86, which divides the throttling
portion into a
converging portion 88 and a diverging portion 90. In the preferred embodiment
the throttling portion 80 is a supersonic nozzle, also known as a de Laval
nozzle.
(For the purposes of clarity, in this disclosure, unless otherwise specified,
the
phrase "cross sectional area" means "cross sectional area transversely to the
longi-
tudinal direction of the plasma channel 26.") The cross sectional area of
throat 86
is smaller than both (a) the cross sectional area of the high pressure portion
82 and
(b) the cross sectional area of the low pressure portion 84. In the preferred
em-
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bodiment, the cross sectional area of the high pressure portion 82 is smaller
than
or equal to the cross sectional area of the low pressure portion 84. In other
em-
bodiments the cross sectional area of the high pressure portion 82 is greater
than
the cross sectional area of the low pressure portion 84.
100381 During operation, after the initial startup of the device, an
electric arc
between the cathode 4 and the anode 8 is maintained. The plasma generating gas
flows in the plasma channel 26 from the inlet 32 to the outlet 34. The
electric arc
heats the plasma generating gas causing ionization of the plasma generating
gas,
which results in generation of plasma. The direction in which the plasma tray-
erses the plasma channel 26, from the inlet 32 to the outlet 34 is referred to
as the
direction of the plasma flow.
[0039] The plasma generating gas is supplied through the space 61 to
the inlet
32 of plasma channel 26 under pressure. The total pressure of the plasma
consists
of the velocity pressure and the static pressure. In the context of this
description,
the velocity pressure refers to the pressure that pushes the plasma flow along
the
plasma channel and the static pressures refers to the pressure that the plasma
ex-.
erts on the walls of the plasma channel. The velocity pressure of the plasma
is
proportional to the velocity of the plasma flow squared. Conversely, the
velocity
of the plasma flow is proportional to the square root of the velocity pressure
of the
plasma. As the plasma enters the converging portion 88, the velocity pressure
of
the plasma increases because the mass flow rate (mass per time) is constant.
At
the throat 86, where the cross sectional area of the plasma channel is
minimal, the
plasma velocity becomes transonic, Mach 1, a condition called a choked flow.
As
the cross sectional area increases in the diverging portion 90, the plasma
continues
to expand so that the static pressure of the plasma decreases and the velocity
pres-
sure of the plasma increases. In the diverging portion 90, the velocity of the
plasma flow increases to supersonic velocities, Mach > 1Ø At the same time,
in
the diverging portion 90, the static pressure of the plasma decreases. The
total
pressure of the plasma remains substantially constant.
[0040] The velocity pressure of the plasma and the velocity of plasma flow
reach their maximum at the end of the throttling portion 80 closest to anode
8.
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Conversely, the static pressure of the plasma reaches its minimum at the end
of
the throttling portion 80 closest to anode 8. The physical process that the
plasma
undergoes when passing through the throttling portion 80 is isentropic,
meaning
that the entropy of the plasma does not change. Essentially, the throttling
portion
80 increases the velocity pressure of the plasma and decreases the static
pressure
of the plasma relative to the pressures observed in high pressure portion 82.
100411 Accordingly, the high pressure portion 82 is characterized by
(1) high
static pressure of the plasma, which is preferably in the range of 5-100 Bar;
(2)
low velocity pressure of the plasma, and (3) low velocity of the plasma flow.
The
to average temperature of the plasma flow in the high pressure portion is
preferably
10-20 kK. If argon is used as a plasma generating gas, the electric field of
the
plasma is preferably 5-50 V/nun. The power density of the plasma in the high
pressure portion is preferably in the range of 0.5-10 kW/mm3.
100421 When the plasma enters the converging portion 88 of the
throttling
portion 80, its temperature is preferably 10-20 kK. When plasma exits the di-
verging portion 90 of the throttling portion 80, its temperature drops
preferably to
8-13 kK. The velocity of the plasma at the end of the diverging portion 90
clos-
est to the anode 8 is preferably 1-10 km/s, with the Mach number in the range
of
1.2-3. The pressure of the plasma in the diverging portion 90 is preferably in
the
range of 1-5 Bar.
[00431 The low pressure portion 84 is characterized by (1) low static
pressure
of' the plasma, which is preferably close to atmospheric pressure, (2) high
velocity
pressure of the plasma, and (3) high velocity of the plasma flow that results
in a
high average powder velocity that is preferably 400-1,000 m/s. The average tem-
perature of the plasma flow is preferably in the range of 10-15 kK. The
average
powder temperature is the melting temperature of the powder. The electric
field
of the plasma in the low pressure portion 84 is preferably 1-10 V/mm. The
power
density of the plasma in the low pressure portion 84 is preferably in the
range of
0.2-0.8 kW/mM3.
[0044] In the embodiment shown in FIG. 2 the powder enters the device
through two powder inlets 94 and 95. In other embodiments a different number
of
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powder inlets can be used. The powder inlets 94 and 95 are connected to a pow-
der chamber 96. The powder chamber 96 is arranged around the intermediate
electrode 18 and facilitates a uniform distribution of the powder particles
along
the circumference of the device. The powder feeder 98 connects the powder
chamber 96 to the plasma channel 26. In the preferred embodiment the powder
feeder 98 connects to the plasma channel 26 at the end of the low pressure
portion
84 closest to the throttling portion 80. In other embodiments, the powder
feeder
98 can connect the plasma channel 26 to the powder chamber 96 at other points
along the plasma channel 26 in the low pressure portion 84, provided that
powder
particles spend enough time in the plasma channel 26 to achieve required
charac-
teristics, such as temperature, velocity, and homogeneity.
[0045] In the preferred embodiment, feeder 98 is a slit. In other
embodi-
ments, the powder feeder 98 may be implemented as a plurality of channels con-
necting the powder chamber 96 and the plasma channel 26. In yet other embodi-
ments, powder feeder 98 may be any aperture or a plurality of apertures that
pro-
vide communication between the powder chamber 96 and the plasma channel 26.
FIGs. 1 and 2 illustrate the embodiment in which the slit 98 is perpendicular
to the
axis of the device. However, this angle does not produce the best distribution
of
the powder particles in the plasma for all types of powder. As mentioned
above,
for a high quality coating, it is preferable that powder particles be
uniformly dis-
tributed in the plasma flow. Using the embodiment illustrated in FIGs. 1 and 2
to
spray powder having relatively heavy particles may result in the particles
from
different directions colliding in the center of plasma channel 26. Using the
em-
bodiment illustrated in FIGs. 1 and 2 to spray powder having relatively light
parti-
cles may result in the particles being pushed to the walls of the plasma
channel by
the plasma flow before they can even reach the center of the flow. To achieve
a
more uniform distribution of the powder particles, the feeder 98 may be
angled.
FIG. 3 illustrates the embodiment in which the feeder 98 is angled in the
direction
opposite to the plasma flow. This embodiment is desirably used for powders
with
lighter particles. FIG. 4 illustrates the embodiment in which the slit 98 is
angled
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in the direction of the plasma flow. This embodiment is desirably used for pow-
ders with heavier particles.
100461 As mentioned above, the plasma in plasma channel 26 is heated by
the
electric arc established between cathode 4 and anode 8. In the preferred
embodi-
ment, the temperature of the plasma entering the throttling portion 80 is
10,000 K
or above. This temperature increase occurs while the plasma passes through the
high pressure portion 82. The temperature of the plasma entering the
throttling
portion will depend on the characteristics of the high pressure portion 82, in
par-
ticular its length, which depends on the geometry and the number of the
interme-
diate electrodes used to form the high pressure portion 82.
[0047] When the powder is injected into plasma channel 26, it is
delivered by
a cold carrier gas. As the cold carrier gas mixes with the heated plasma, the
tem-
perature of the plasma in the flow significantly drops and becomes lower than
its
temperature before the powder is injected. For some coating applications, the
=
plasma in the flow has to be heated in the low pressure portion 84, after the
pow-
der is injected, so that the powder particles achieve the required temperature
and a
high level of homogeneity when exiting the device at outlet 34. The electric
arc
established between the cathode 4 and the anode 8 heats the plasma that
carries
particles of the injected powder flowing along the low pressure portion 84. In
the
preferred embodiment, the temperature of the plasma exiting the device at the
outlet 34 is 10,000 K or above. The temperature of the powder particles
depends
on the time that they spend in the plasma flow, which is controlled by the
length
of the low pressure portion 84. Some types of powder particles, such as
nanopar-
ticles, may evaporate if exposed to the temperature of the heated plasma and
then
kept at that temperature for a certain period of time. A supersonic speed of
the
plasma in the low pressure portion 84 enables such particles to melt to the
desired
consistency without evaporating. Note however that the particles of powder
travel
with lower speed than the plasma in the flow due to non-ideal transfer of the
ki-
netic energy from the plasma to the powder particles. For a given type of
powder,
the temperature to which its particles are heated in the low pressure portion
84 and
the time it takes for the particles to traverse the low pressure portion 84
may be
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controlled by the geometry and number of the intermediate electrodes that form
the low pressure portion 84.
100481 Coating with nanoparticles presents additional problems. In
particular,
because of the low mass of nanoparticles, they cannot gain enough momentum to
achieve the desired penetration of the plasma flow, even with relatively low
static
pressure of the plasma flow in the low pressure portion 84. For such nanoparti-
cles, this and other embodiments of the device may be used with SPPS. With
SPPS the flowable material that is injected into the plasma flow is an
atomized
solution of nanoparticles with a liquid precursor. When the atomized solution
is
injected into the plasma flow, the precursor quickly evaporates, leaving the
nanoparticles in the plasma flow to be heated and accelerated.
100491 Turning to FIG. 2 again, during operation, all elements, but
especially
the anode 8 and the intermediate electrode 18, which electrode 18 forms the
throt-
tling portion 80, become heated. For the cooling of the intermediate electrode
18,
a coolant, preferably water, is supplied through the inlet 64. The coolant
flows
through a longitudinal coolant channel 65 in the direction of the plasma flow.
The
longitudinal coolant channel 65 connects to the circular coolant channel 66
(shown in FIG. 1) that surrounds the intermediate electrode 18, preferably at
the
cross section of the throat 86. The coolant then flows in the direction
opposite to
the plasma flow through another longitudinal coolant channel 67 connected to
the
circular channel 66. The coolant exits the device through the outlet 68.
Coolant
divider 15, shown separately in FIG. 6, together with other elements, forms
the
coolant channels 65, 66, and 67. The cooling system of the anode 8 is similar.
A
coolant, preferably water, enters the device through the inlet 70. The coolant
then
flows in the direction of the plasma flow through a longitudinal coolant
channel
71. Then, the coolant flows in a circular channel 72 around the anode (shown
in
FIG. 1). After that, the coolant flows in the direction opposite that of the
plasma
flow through another longitudinal channel 73, and then exits the device
through
the outlet 74. Coolant divider 17, shown separately in FIG. 7, together with
other
elements, forms the coolant channels 71, 72, and 73. In some embodiments the
same coolant is used for cooling the anode 8 and the intermediate electrode
18. In
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other embodiments different coolants are used for cooling the anode 8 and the
intermediate electrode 18.
100501 FIG. 5 shows an embodiment of the device with two flowable
material
injectors. In this embodiment, the high pressure portion 82 is formed by the
same
intermediate electrodes 12, 14, 16 and the same insulators 36, 38, and 40 as
in the
first embodiment described above. The low pressure portion 84 is formed by in-
termediate electrodes 20, 22, 24, 140, 142, and 144, the anode 8, and
insulators
42, 44, 46, 152, 154, 156. The portion of the plasma channel 160 between the
feeder 98 and the feeder 128 is formed by at least one, but preferably two or
more
io intermediate electrodes. In the embodiment shown in FIG. 5, the portion
160 is
formed by three intermediate electrodes 20, 22, 24. The portion of the plasma
channel 162 between the feeder 128 and the opening 34 in the anode 8 is also
formed by at least one, but preferably two or more intermediate electrodes. In
the
embodiment shown in FIG. 5, the portion 162 is formed by three intermediate
electrodes 140, 142, 144 and the anode 8. In general, in embodiments that have
two or more flowable material injectors, the adjacent feeders are separated by
at
least one, but preferably two or more intermediate electrodes.
100511 Turning back to FIG. 5, portions 160 and 162 are provided with a
cool-
ing system. As depicted, each portion has its own cooling system. The cooling
system comprising inlet 70, channels 71, 73, and 72 (which is not shown in
FIG.
5), and outlet 74 is used to cool intermediate electrode 146, in accordance
with the
above description. A similar cooling system comprising inlet 130, channels
131,
132, and a channel surrounding the anode (which is not shown in FIG. 5) and
the
outlet 134 is used to cool the anode 8 in accordance with the above
description.
In the embodiments that have two or more flowable material injectors, there
may
be a cooling system for each group of electrodes forming a portion of the
plasma
channel between each adjacent pair of feeders. These cooling systems may use
different cooling agents, and may operate independently from each other.
[0052] In embodiments with multiple flowable material injectors, the
corre-
sponding feeders may or may not be angled in the same manner. For example, in
the embodiment of FIG. 5, both the feeder 98 and the feeder 128 are
perpendicular
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to the direction of the plasma flow. In other embodiments one feeder may be an-
gled in the direction of the plasma flow as shown in FIG. 4, and another
feeder
may be angled in the direction opposite to the plasma flow as shown in FIG. 3.
In
some other embodiments, both feeders may be angled in the same direction but
at
a different angle.
[0053] The operation of the embodiment shown in FIG. 5 is similar to
the
operation of the embodiment shown in FIGs. 1 and 2 described above. In fact,
the
processes that occur upstream from the feeder 128 are substantially the same.
During operation, after a first flowable material has been injected into the
plasma
flow through the feeder 98, the particles of the first flowable material
carried by
the plasma traverse the low pressure portion of the plasma channel. At the
point
where the particles of the first flowable material have been heated to an
appropri-
ate temperature, a second flowable material is injected into the plasma flow
through the feeder 128. The feeder 128 is connected to a second flowable mate-
rial chamber 126. The second flowable material is supplied to the second flow-
able material chamber 126 through inlets 124 and 125. The particles of the two
flowable materials carried by the plasma traverse the remainder of the plasma
channel and exit through the opening 34 of the anode 8.
[00541 The length of portion 160, between the feeders 98 and 128,
depends on
the properties of the first flowable material. It is controlled by the number
and
geometry of the intermediate electrodes used to form portion 160. When the sec-
ond flowable material is injected into the flow, its particles, together with
the par-
ticles of the first flowable material, are heated by the plasma in the flow.
The
length of portion 162, between the feeder 128 and the opening 34 of the anode
8,
depends on the properties of the second flowable material. It is controlled by
the
number and geometry of the intermediate electrodes used to form portion 162.
The length of portion 162 is selected so that particles of the second flowable
mate-
rial (heated together with particles of the first flowable material) achieve
charac-
teristics required by a particular spraying application by the time they reach
the
opening 34 of anode 8. The sum of lengths of portions 160 and 162 is selected
so
that particles of the first flowable material achieve characteristics required
by the
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particular spraying application by the time they reach opening 34 of anode 8.
Note that particles of the first flowable material are heated in portion 160,
then
they are cooled in the area where feeder 128 connects to the plasma channel
26,
and then they are heated again (together with the particles of the second
flowable
material) in portion 162. The length of portion 160 can be determined from the
total sum of lengths of portions 160 and 162 and the length of portion 162. In
embodiments with more than two flowable materials injectors, the lengths of
vari-
ous portions may be determined in the same manner.
10055) The device of the invention may be used for applications other
than
coating by plasma spraying. For example, one possible application is Plasma En-
hanced Chemical Vapor Deposition (PECVD). Briefly, Chemical Vapor Deposi-
tion (CVD) is a method of deposition of thin films, in which particles are
heated
and propelled towards the surface to be coated, and as a result of their high
energy
a chemical reaction occurs on the surface to be coated and a film is formed.
One
application for which CVD is used is deposition of diamond films. To achieve
the
deposition of a diamond film on the surface, the conditions required for the
growth of a diamond have to be created on the surface. Plasma generating de-
vices can be used to create such conditions. Argon plasma, for example,
sustains
the temperature of 10,000 K or above. At this temperature, hydrogen and some
hydrocarbon are decomposed into atomic hydrogen and atomic carbon. The
plasma flow accelerates atomic hydrogen and atomic carbon toward the surface,
on which the atomic carbon (in the presence of certain precursors) begins to
form
the diamond film. Using the embodiment with a single flowable material
injector
separates hydrocarbon and hydrogen from the cathode, thus preventing the cath-
ode erosion, and at the same time ensures that the two gases spend enough time
in
the heated plasma to completely decompose into the elemental particles.
[0056] An important factor in CVD is the thickness of the layer in
which the
diamond grows, called a boundary layer. The thickness of the boundary layer
determines the rate of the diamond film growth, and ideally it should be as
thin as
possible. The thickness of the layer is inversely proportional to the square
root of
the velocity of the plasma flow that is used to deliver the elemental
particles. Ac-
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celerating the plasma with a throttling portion to supersonic speeds,
therefore,
facilitates the formation of a thinner boundary layer.
[0057] Embodiments of the device of the invention may also be used for
de-
struction of hazardous materials or waste in solid, liquid and gaseous forms.
For
example, an embodiment of the plasma generating device may be integrated in a
waste management system or a motor vehicle exhaust system. At high tempera-
tures, the organic materials are pyrolysed. Then, in the cooler sections of
the sys-
tem the elemental particles and ions may recombine prior to rapid alkaline
quench
to form simple molecules. The resulting end products include gases consisting
of
argon, carbon dioxide and water vapor and aqueous solutions of inorganic
sodium
salts.
[0058] It is also possible to combine the function of destruction
hazardous
materials and waste with CVD. For example, when using an embodiment of the
device, the destruction chamber may include a substrate holder where the dia-
monds can be grown from the elemental carbon.
[0059] Embodiments of the device with multiple flowable material
injectors
enable the use of the device in applications for which the embodiments of the
de-
vice with a single flowable material are not suitable.
[0060] For some coating applications, such as for example thermal
barrier
coating (TBC) used to coat turbine parts, it is necessary to have two layers
of
coating: the top coat and the bond coat. The bond layer is necessary due to a
mismatch of thermal expansion coefficients of the coated metal and the top
coat,
which is typically ceramic. The bond coat is applied first to the metallic
surface
to be coated. The bond coat serves as an adhesive layer between the top coat
and
the metallic surface. For better adhesion as well as for producing a thicker
coat-
ing, a method known as Graded Functional Coating is used. With Graded Func-
tional Coating, two powders are injected into the plasma flow. However, the
rela-
tive amounts of the injected powders vary with time. In the beginning of the
coat-
ing process, only the powder used to form the bond coat is injected.
Gradually,
the fraction of the powder used to form the top coat is increased, while the
frac-
tion of the powder used to form the bond coat is decreased. Finally, only the
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powder used to form the top coat is supplied. The powders used for the
formation
of the two coats have very different characteristics, such as particle size,
melting
point, etc. Using the embodiment of the device with a single flowable material
injector would require optimizing the parameters of the device for two
different
powders. Even if satisfactory parameters are found, the performance of such de-
vice would not be optimal.
[0061] Using the device with multiple flowable material injectors
enables
creation of a device that accomplishes optimal conditions for spraying both
pow-
ders. Turning, for example, to the embodiment of the device shown in FIG. 5,
the
powder used to form the top ceramic layer has a higher melting point than the
powder used to form the bond layer. Accordingly, the powder used to form the
top layer is supplied to the upstream flowable material injector through the
inlets
94 and 95. The powder used to form the bond coat is supplied to the downstream
flowable material injector through the inlets 124 and 125. The lengths of the
re-
spective portions 160 and 162 may be configured to ensure that both powders
spend optimal time in the plasma channel 26 before exiting from the outlet 34
of
anode 8 and then exit the device from outlet 34 at optimal temperature.
f00621 The foregoing description of the embodiments of the present
invention
has been presented for purposes of illustration and description. It is not
intended
to be exhaustive nor to limit the invention to the precise form disclosed.
Many
modifications and variations will be apparent to those skilled in the art. The
em-
bodiments were chosen and described in order to best explain the principles of
the
invention and its practical applications, thereby enabling others skilled in
the art
to understand the invention. Various embodiments and modifications that are
suited to a particular use are contemplated. It is intended that the scope of
the in-
vention be defined by the accompanying claims and their equivalents.
