U.S. patent application number 15/335992 was filed with the patent office on 2017-05-04 for apparatus for indirect atmospheric pressure plasma processing.
This patent application is currently assigned to VITO NV. The applicant listed for this patent is VITO NV. Invention is credited to Jan COOLS, Erwin VAN HOOF, Annick VANHULSEL.
Application Number | 20170125221 15/335992 |
Document ID | / |
Family ID | 54365046 |
Filed Date | 2017-05-04 |
United States Patent
Application |
20170125221 |
Kind Code |
A1 |
VANHULSEL; Annick ; et
al. |
May 4, 2017 |
APPARATUS FOR INDIRECT ATMOSPHERIC PRESSURE PLASMA PROCESSING
Abstract
Apparatus for plasma processing of a continuous fiber,
comprising a first and a second plasma torch. Each plasma torch
comprises oppositely arranged electrodes to define a plasma
discharge chamber between the electrodes. The plasma discharge
chamber comprises an inlet and an outlet for passing a plasma
forming gas between the electrodes. The apparatus further comprises
an afterglow chamber in fluid communication with the outlets of the
plasma discharge chambers, which comprises a substrate inlet and a
substrate outlet arranged at opposite sides of the outlets of the
plasma discharge chambers. A transport system is configured to
continuously transport the fiber from the substrate inlet to the
substrate outlet through the afterglow chamber. The substrate inlet
comprises an aperture having a cross-sectional size substantially
smaller than a cross-sectional size of the afterglow chamber. The
outlets of the plasma torches face each other and exhaust plasma
activated species into the afterglow chamber.
Inventors: |
VANHULSEL; Annick; (Mol,
BE) ; VAN HOOF; Erwin; (Mol, BE) ; COOLS;
Jan; (Mol, BE) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
VITO NV |
Mol |
|
BE |
|
|
Assignee: |
VITO NV
Mol
BE
|
Family ID: |
54365046 |
Appl. No.: |
15/335992 |
Filed: |
October 27, 2016 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01J 37/32532 20130101;
H05H 2001/245 20130101; H05H 2240/10 20130101; H01J 37/3244
20130101; H05H 1/2406 20130101; H01J 37/32009 20130101; H05H
2001/2456 20130101; H05H 2245/123 20130101; H05H 2001/2412
20130101; H05H 2001/2431 20130101 |
International
Class: |
H01J 37/32 20060101
H01J037/32 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 28, 2015 |
EP |
15191844.8 |
Claims
1. An apparatus for plasma processing of a continuous fiber,
comprising a first plasma torch and a second plasma torch, wherein
each of the first plasma torch and the second plasma torch
comprises: a first electrode and a second electrode, wherein the
first electrode and the second electrode are spaced apart and are
concentric on an axis, and a plasma discharge chamber between the
respective first and second electrodes, wherein the plasma
discharge chamber comprises an inlet and an outlet for passing a
plasma forming gas between the first and second electrodes, wherein
the apparatus further comprises: an afterglow chamber in fluid
communication with the outlets of the plasma discharge chambers of
the first and second plasma torches, wherein the afterglow chamber
comprises a substrate inlet and a substrate outlet, and a transport
system configured for continuous transport of the continuous fiber
from the substrate inlet to the substrate outlet through the
afterglow chamber and such that the continuous fiber is kept remote
from the plasma discharge chambers while being processed by plasma
activated species flowing from the outlets of the plasma discharge
chambers into the afterglow chamber, wherein the outlets of the
plasma discharge chambers of the first plasma torch and the second
plasma torch are arranged opposite one another, with the afterglow
chamber interposed between the outlets of the plasma discharge
chambers of the first and second plasma torches, wherein the
outlets of the plasma discharge chambers of the first and second
plasma torches are interposed between the substrate inlet and the
substrate outlet, wherein the substrate inlet comprises an inlet
aperture having a cross-sectional size substantially smaller than a
cross-sectional size of the afterglow chamber at the outlet of the
plasma discharge chamber of the first plasma torch.
2. The apparatus of claim 1, wherein the first and second plasma
torches are axially aligned.
3. The apparatus of claim 1, wherein the inlet aperture is aligned
with a delimiting wall of the outlet of each of the plasma
discharge chambers along a transport direction of the continuous
fiber, such that the afterglow chamber is made to extend only at a
downstream side of the outlets of the plasma discharge
chambers.
4. The apparatus of claim 1, wherein a cross-sectional size of the
inlet aperture is equal to or smaller than 50% of a cross-sectional
size of the afterglow chamber.
5. The apparatus of claim 1, wherein the substrate inlet ensures
contactless entrance of the continuous fiber into the afterglow
chamber.
6. The apparatus of claim 1, wherein the inlet aperture has
dimensions to minimise air entrainment by the continuous fiber into
the afterglow chamber.
7. The apparatus of claim 1, wherein the substrate inlet is tubular
comprising a first lumen, the transport system being operable for
transporting the continuous fiber through the first lumen, and
wherein the inlet aperture corresponds to the first lumen.
8. The apparatus of claim 7, wherein the first lumen has a length
in a transport direction of the continuous fiber equal to or larger
than twice a cross sectional size of the first lumen.
9. The apparatus of claim 7, wherein the afterglow chamber is
tubular comprising a second lumen, wherein the second lumen fluidly
communicates with the first lumen, wherein the transport system is
operable for transporting the continuous fiber through the second
lumen, and wherein a cross sectional size of the second lumen is at
least twice a cross-sectional size of the first lumen.
10. The apparatus of claim 9, wherein the second lumen comprises a
longitudinal axis extending between the substrate inlet and the
substrate outlet.
11. The apparatus of claim 9, wherein the afterglow chamber is
cylindrical.
12. The apparatus of claim 11, wherein the first lumen has a
circular cross section having a diameter substantially smaller than
a diameter of the afterglow chamber.
13. The apparatus of claim 1, comprising a device operable to
inject an inert gas on a surface of the continuous fiber at the
substrate inlet.
14. The apparatus of claim 1, wherein each of the plasma discharge
chambers defines an axis of flow of the plasma forming gas, the
axis of flow being perpendicular to a transport direction of the
continuous fiber in the afterglow chamber.
15. The apparatus of claim 1, wherein the outlets of the plasma
discharge chambers of the first and second plasma torches face one
another and exhaust plasma activated species into the afterglow
chamber.
16. The apparatus of claim 1, wherein the first and second
electrodes are cylindrical.
17. The apparatus of claim 1, wherein the afterglow chamber
comprises a transparent wall.
18. The apparatus of claim 1, comprising a control unit coupled to
the first electrode of each of the first plasma torch and the
second plasma torch, wherein the control unit is operable to
sustain an atmospheric pressure plasma discharge in the plasma
discharge chambers of the first and second plasma torches.
19. A method of atmospheric pressure plasma processing of
continuous carbon fibers, comprising transporting the carbon fiber
through the apparatus of claim 1.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application claims priority to European Patent
Application No. 15191844.8, filed Oct. 28, 2015, which is hereby
incorporation by reference.
FIELD OF THE INVENTION
[0002] The present invention is related to apparatuses and methods
for indirect atmospheric pressure plasma processing, in particular
where the substrate to be plasma processed is kept remote from the
plasma discharge zone.
[0003] With indirect or remote plasma treatment of substrates, as
opposed to in-situ plasma treatment, the substrate is not passed
through the plasma discharge zone, in which an atmospheric pressure
plasma is maintained between electrodes and activated species are
formed. Instead, the substrate is positioned at a location remote
from the plasma discharge zone and the plasma-activated species are
transported to the remote location where they are made to react
with the substrate. Remote plasma treatment is often preferred over
in-situ treatment, in particular for cases in which in-situ plasma
treatment would cause charging of the substrate surface and
therefore undesirable interaction with the electric field of the
plasma discharge. This is particularly the case for substrate
materials having at least some degree of electrical
conductivity.
[0004] Background Art An apparatus for indirect or remote
atmospheric pressure plasma processing is known from WO 2009/080662
Feb. 7, 2009. The apparatus comprises a multitude of single
micro-channels in which a plasma is formed and which are
circumferentially arranged around a treatment zone. High gas
velocities up to transonic flow conditions in the discharge zone
are generated while maintaining moderate flow rates. The resulting
superimposition of high drift velocity in the process gas flow and
the inherent diffusion movement results in a prolonged displacement
distance of activated species into the treatment zone. The
treatment zone is cylindrical and wrapped or enveloped by the
plasma micro-channels. A carrier gas with particulate material is
made to flow through the treatment zone. The process gas with
activated species admixes with the carrier gas in the treatment
zone to perform a surface treatment of the particulate material. A
drawback of the above apparatus is that the concentration of plasma
activated species in the treatment zone is not uniform in a radial
direction.
[0005] U.S. 2003/0051993 Mar. 20, 2003 describes an apparatus for
atmospheric plasma processing of a PAN fiber. The PAN fiber is
drawn through a cylindrical hull. A number of plasma discharge
forming capillaries are arranged radially around the cylindrical
hull. A drawback of the above apparatus is that the surface
activation of the PAN fiber is low due to air entrained with the
PAN fiber. For an effective plasma treatment, a long chamber is
required with a large number of plasma capillaries, or the
transport speed of the fiber must be kept low.
[0006] U.S. Pat. No. 8,227,051 Jul. 24, 2012 describes in relation
to FIG. 2B an indirect exposure plasma treatment of a carbon fiber.
The fiber is pulled or placed into the exhaust flow from an
atmospheric plasma device exposing the fiber to contact with the
convected chemical active species generated by the plasma. The
atmospheric pressure plasma device is configured to operate using
background gas preferably comprising air, or any other oxygen
containing gas mixtures including pure oxygen, that promotes the
transport of short-lived reactive oxidative species to the fiber
via a sufficiently high exhaust velocity. The plasma operating
conditions including the size of the plasma volume, the composition
of the processing gas, gas flow rates, and the energizing
conditions of the electrical device generating the plasma, are
adjusted to yield the desired surface modifications within the
required residence time. Deleterious effects on fiber surface
topography are minimized by the indirect exposure process because
the fibers are located away from the bulk of the plasma and do not
undergo direct ion bombardment. In the apparatus as depicted in
FIG. 2B of the above document, an inhomogeneous treatment of the
carbon fiber surface is obtained, since the side of the fiber
facing the plasma discharge apparatus is more exposed to the plasma
activated species than the side opposite the plasma discharge
apparatus. As a result, the residence time of the carbon fiber must
be prolonged, or the fiber must be turned and pulled a second time
through the same apparatus.
SUMMARY OF THE INVENTION
[0007] An objective of aspects of the present invention is to
overcome one or more of the above drawbacks. One objective of
aspects of the invention is to improve uniform and homogeneous
plasma processing of the substrate surface. Another objective of
aspects of the invention is enabling a prolonged and more intimate
contact between the reactive species exhausted from the plasma
discharge and the substrate. Yet another objective is to improve
plasma processing of the substrate surface, in particular for
non-oxidative plasma treatments, i.e. treatments involving a
substantially oxygen-free plasma forming gas.
[0008] According to a first aspect of the invention, there is
therefore provided an apparatus for plasma processing of a
substrate transported continuously through the apparatus, as set
out in the appended claims. Apparatuses according to aspects of the
invention comprise a first plasma torch. The first plasma torch
comprises a first electrode and a second electrode arranged
opposite the first electrode to define a first plasma discharge
chamber between the first and second electrodes. The plasma
discharge chamber comprises an inlet and an outlet for passing a
plasma forming gas between the electrodes. The apparatus further
comprises a control unit coupled to one or both the electrodes and
operable to maintain an atmospheric pressure plasma discharge in
the first plasma discharge chamber. The first plasma torch is
therefore operable to exhaust plasma activated species through the
outlet of the first plasma discharge chamber.
[0009] The apparatus further comprises an afterglow chamber
downstream of the first plasma torch and in fluid communication
with the outlet of the first plasma discharge chamber. A transport
means is provided for continuous transport of the substrate through
the afterglow chamber and such that the substrate is kept remote
from the first plasma discharge chamber while being processed by
plasma activated species exhausted from the outlet of the first
plasma discharge chamber into the afterglow chamber.
[0010] According to a first aspect of the present invention, the
afterglow chamber extends between a substrate inlet and a substrate
outlet arranged at opposite sides of the outlet of the first plasma
discharge chamber. The substrate inlet advantageously comprises an
inlet aperture having a cross-sectional size substantially smaller
than a cross-sectional size of the afterglow chamber. The
cross-sectional size of the afterglow chamber can be assessed in
correspondence of the outlet of the first plasma discharge chamber.
The cross-sectional size can refer to an area, or clearance, such
as a height, or diameter. Advantageously, the cross-sectional size
is defined in a plane perpendicular to a transport direction of the
substrate. Advantageously, the inlet aperture is aligned with a
delimiting wall of the outlet of the first plasma discharge
chamber, such that the afterglow chamber is made to extend at a
downstream side only of the outlet of the first plasma discharge
chamber.
[0011] The reduction in aperture of the afterglow chamber at the
substrate inlet ensures that the afterglow zone is made to
propagate further downstream along a transport direction of the
substrate. An increased plasma treatment efficiency is thereby
obtained.
[0012] By appropriate selection of the aperture size, it becomes
possible to reduce or minimise air entrainment by the substrate in
the afterglow zone.
[0013] According to a second aspect of the present invention, which
can be provided in addition to, or independently of the first
aspect above, a second plasma torch is provided, which can be
identical to the first plasma torch. The second plasma torch is
aligned with and arranged opposite the first plasma torch, such
that the outlets of the plasma discharge chambers of the respective
plasma torches face each other and exhaust plasma activated species
into the afterglow chamber interposed between the first and second
plasma torches. A more intense afterglow stream is thereby
provided, which furthermore allows for uniform treating continuous
fibers along 360.degree. of the circumference.
[0014] According to a third aspect of the invention, there is
provided a method for plasma treatment of continuous fibers, such
as but not limited to carbon fibers and polymeric fibers.
[0015] Methods for indirect or remote atmospheric pressure plasma
treatment of a substrate are described herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Aspects of the invention will now be described in more
detail with reference to the appended drawings, wherein same
reference numerals illustrate same features and wherein:
[0017] FIG. 1 represents a cross section side view of an apparatus
for atmospheric pressure plasma processing of a film substrate
according to aspects of the invention;
[0018] FIG. 2 represents a cross-section side view of another
apparatus for atmospheric pressure plasma processing of a substrate
according to aspects of the invention;
[0019] FIG. 3 represents a cross-section view of yet another
apparatus for atmospheric pressure plasma processing of a fiber
according to aspects of invention, comprising two oppositely
arranged cylindrical plasma torches.
DETAILED DESCRIPTION
[0020] Referring to FIG. 1, an apparatus 10 for plasma processing
of a continuous substrate 16, such as but not limited to films and
foils, comprises a pair of oppositely arranged electrodes 11 and
12. Electrodes 11 and 12 are planar and extend parallel to each
other. They are spaced apart to define a plasma discharge chamber
13 between the electrodes 11, 12. Advantageously, dielectric layers
14 cover one or both electrodes 11, 12 at the side facing the
plasma discharge chamber 13. In such case, the dielectric layers 14
form walls of the chamber 13. Dielectric materials include
borosilicate glass, quartz, and alumina.
[0021] Chamber 13 comprises an inlet 131 through which a plasma
forming gas 133 is made to enter the chamber. The plasma forming
gas is one which is able to create a plasma discharge in chamber 13
under an electric field generated by the electrodes 11, 12. The
plasma forming gas is advantageously a non-oxidizing gas,
advantageously a gas which is substantially oxygen-free.
Non-limiting examples of plasma forming gases are nitrogen
(N.sub.2), argon (Ar), helium (He) and neon (Ne), or combinations
thereof.
[0022] The plasma forming gas is supplied to the chamber 13 at
substantially atmospheric pressure. Suitable pressures may vary
between about 0.5 bar below and about 0.5 bar above atmospheric
pressure. The plasma forming gas may be supplied at ambient
temperature (15.degree. C.-30.degree. C.) to the chamber 13.
Alternatively, it is possible to heat the gas stream 133 to an
elevated temperature prior to supplying it to the chamber 13.
Elevated temperatures possibly range between 30.degree. C. and
400.degree. C., advantageously between 50.degree. C. and
300.degree. C.
[0023] The plasma forming gas enters the chamber at the inlet 131
and is made to flow along an axis 136 of chamber 13 until an outlet
132 arranged downstream of the electrodes 11, 12. The inlet 131 and
the outlet 132 of the plasma discharge chamber 13 are defined by
the extent of the plasma discharges taking place in chamber 13,
i.e. it is assumed in the present description that the plasma
discharge chamber 13 corresponds to and is delimited by the plasma
discharge zone. Generally, the plasma discharge zone will be
maintained in an area delimited by the electrodes 11, 12.
[0024] It will be convenient to note that, since the electrodes are
planar, the chamber 13 (as well as electrodes 11, 12 and dielectric
layers 14) extends in a direction perpendicular to axis 136, i.e.
perpendicular to the plane of FIG. 1. The gap of chamber 13 between
electrodes 11, 12 (between dielectric layers 14) typically is
between 0.5 mm and 5 mm, advantageously 3 mm or less.
[0025] The plasma forming gas stream 133 can be loaded/enriched by
at least one liquid or gaseous monomer added as a precursor to the
plasma forming gas. The precursor can be activated by the plasma
discharge to e.g. form radicals which initiate chemical reactions
with the substrate 16. A stream 134 of precursor can be injected in
the plasma forming gas stream 133 by known methods, such as through
an atomizer 135, e.g. to form an aerosol which is carried with the
plasma forming gas stream into the chamber 13. Non-limiting
examples of precursors are methane (CH.sub.4) and acetylene
(C.sub.2H.sub.2).
[0026] The electrodes 11, 12 are coupled in an electric circuit
including a control unit 15 which is operable to generate an
electric/electromagnetic field between the electrodes 11, 12 that
generates a plasma discharge in the chamber 13. By way of
non-limiting example, one electrode 12 can be connected to electric
ground, whereas the other electrode 11 is supplied with an
Alternating Current (AC) or pulsed Direct Current (DC) high
electric voltage generated in control unit 15. Suitable voltage
differences between the electrodes 11, 12 range between 1 kV and
100 kV. Suitable frequencies (either AC or pulsed DC) range between
1 kHz and 200 kHz, advantageously between 5 kHz and 100 kHz.
[0027] The plasma setup of FIG. 1 is referred to as a parallel
plate dielectric barrier discharge apparatus. The apparatus
operates as a plasma torch which creates plasma activated species
in the plasma discharge chamber. These species are carried by the
plasma forming gas stream to the outlet 132 of chamber 13 where
they are made to react with the substrate 16. It will be convenient
to note that, although dielectric barrier discharge plasma
processing provides advantageous operation, the present invention
is not limited thereto and other kinds of plasma discharge, such as
e.g. glow discharge or corona discharge may be contemplated.
[0028] Generally, the plasma activated species exiting the plasma
discharge chamber retain their reactivity for a short period. A
zone directly downstream of the outlet 132 of the plasma discharge
chamber 13, where electromagnetic fields that sustained the plasma
are absent or insufficient to maintain any plasma discharge, but
where the plasma activated species are still reactive, is referred
to as the afterglow zone. In the afterglow zone, the plasma
activated species exiting the plasma discharge chamber react with
other molecules, such as substrate molecules or recombine with
molecules present in the plasma forming gas or other gas present in
the afterglow zone.
[0029] The plasma treatment apparatus 10 is designed to treat
substrate 16 in the afterglow zone, at a location remote from the
plasma discharge chamber 13. To this end, substrate 16 is
transported in proximity of the outlet 132 of the plasma discharge
chamber 13, but without entering or contacting chamber 13 or the
plasma discharge. Generally, the transport direction of substrate
16 is perpendicular to the axis 136 of flow of the plasma forming
gas in chamber 13. By way of example the substrate 16 may be
unwound from spool 165, guided along guide/tensioning drums 163 and
161 upstream of the plasma torch 10 and further along
guide/tensioning drums 162 and 164 downstream to eventually be
wound on a take-up spool 166.
[0030] According to an aspect of the invention, a chamber 17 is
provided downstream of the plasma discharge chamber 13, which
allows for confining the afterglow. In the example of FIG. 1, the
afterglow chamber 17 is defined/delimited by the substrate 16 on
the one hand (in the assumption that substrate 16 is an impermeable
film), and a confinement wall 174, advantageously made of a
dielectric material, arranged opposite substrate 16 and
advantageously parallel thereto. Substrate 16 is transported at a
side opposite the outlet 132 of plasma discharge chamber 13. Wall
174 extends from outlet 132 along a direction advantageously
parallel to the transport direction of substrate 16. The substrate
16 and wall 174 hence form a channel-shaped chamber 17 which
advantageously guides the afterglow stream along the substrate 16.
Afterglow chamber 17 comprises an inlet 171 for the plasma
activated stream in fluid communication with and which
advantageously corresponds to the outlet 132 of chamber 13 and an
inlet 172 for the substrate 16, separate from inlet 171. Substrate
inlet 172 is advantageously located at an upstream side of outlet
132 opposite wall 174, such that chamber 17 extends at the
downstream side of the outlet 132 only. Both the afterglow stream
and the substrate are transported in an advantageously same
direction until outlet 173 of the chamber 17.
[0031] A shielding member 175 is advantageously provided at the
substrate inlet 172. Shielding member 175 defines a wall 176 which
advantageously extends between the outlet 132 of the plasma
discharge chamber 13 and the substrate inlet 172, and reduces a
clearance G1 between substrate 16 and the shielding member 175 at
the substrate inlet 172 compared to a height G2 of the afterglow
chamber 17. Height G2 can conveniently be assessed at the outlet
132, or further downstream, particularly in cases where the
afterglow chamber would have a constant cross section. Wall 176 is
advantageously aligned with a wall of the outlet 132.
[0032] One advantage of shielding member 175 is to ensure that the
chamber 17 and hence the afterglow zone extends to the downstream
side of the outlet 132 only. This results in a prolonged and more
intimate contact between the reactive species present in the
afterglow zone and the substrate 16. Another advantage of shielding
member 175 is to reduce and/or minimise air entrainment by
substrate 16 into the afterglow chamber 17. Air comprises oxidative
species, such as oxygen, which neutralise the plasma activated
species leading to reduced efficiency of the plasma treatment.
Furthermore, the air entrained by the substrate 16 forms a boundary
layer on the substrate surface hindering contact with the reactive
species present in the afterglow zone.
[0033] As yet a further advantage, shielding member 175 avoids the
necessity that the entire plasma processing zone be put under inert
atmosphere. Therefore, aspects of the present invention allow for
reducing gas consumption and therefore allow more economical plasma
processing.
[0034] Advantageously, the clearance G1 is equal to or smaller than
50% of the clearance G2, advantageously equal to or smaller than
30%, advantageously equal to or smaller than 20%, advantageously
equal to or smaller than 10% of clearance G2. The clearance G1 is
advantageously equal to or smaller than 2.5 mm, advantageously
equal to or smaller than 1 mm, advantageously equal to or smaller
than 0.5 mm, advantageously equal to or smaller than 250 .mu.m. The
clearance G1 can be as small as 10 .mu.m.
[0035] Advantageously, the clearance G2 is equal to or smaller than
10 mm, advantageously equal to or smaller than 7 mm, advantageously
equal to or smaller than 5 mm. G2 is suitably at least 1 mm.
Advantageously, the afterglow chamber 17 extends over a distance L2
between the outlet 132 of the plasma discharge chamber and the
outlet 173. The length L2 of the afterglow chamber is
advantageously at least 100 mm, advantageously at least 200 mm,
advantageously at least 500 mm.
[0036] It will be convenient to note that either one or both the
afterglow chamber 17 and tunnel of the substrate inlet 172 can have
a constant cross-section.
[0037] In an aspect of the present invention, it is advantageous to
have substrate 16 pass through the substrate inlet 172 in a
contactless manner. That is, substrate 16 enters the afterglow
chamber 17 without contacting the shielding member 175 or the
shielding wall 176, such that a clearance G1 is advantageously
always present. In order to further reduce air entrainment,
shielding member 175 advantageously extends a distance L1 upstream
along the transport direction of substrate 16. The clearance G1 may
be maintained along the entire length L1 of shielding member 175.
As a result, the substrate inlet 172 may be shaped as a tunnel with
clearance G1, instead of just being an aperture or diaphragm. The
length L1 of the tunnel is advantageously at least twice the
clearance G1, advantageously at least three times G1,
advantageously at least five times G1. A suitable length L is 10 to
20 times G1.
[0038] In one aspect, the air entrainment by the substrate 16
through the substrate inlet 172 can be substantially completely
suppressed by using a gas knife as shown in FIG. 2. Gas knife 18
injects a stream 181 of an inert or non-oxidising gas, such as
nitrogen gas, at the inlet 172. The stream 181 impinges on the
substrate 16 to remove any entrained air.
[0039] Referring to FIG. 2, in case substrate 16 would be porous, a
channel wall 177 is advantageously arranged opposite wall 174 and
outlet 132 to confine the afterglow chamber 17. The substrate 16 is
transported along the afterglow chamber 17 between walls 174 and
177. It will be convenient to note that in such case the clearances
G1 and G2 are determined as from wall 177 instead of substrate
16.
[0040] FIG. 2 shows an alternative type of plasma torch 20, which
differs from the plasma torch of apparatus 10 in that electrodes 11
are arranged at opposite sides of a central electrode 12. The outer
electrodes 11 are advantageously supplied with high voltage,
whereas the central electrode 12 is connected to ground. The
central electrode 12 can comprise an internal lumen 121
advantageously extending until the outlet 132 of the plasma
discharge chamber. The stream 134 of precursors is supplied through
the internal lumen 121 and injected directly in the afterglow zone
(chamber 17), where the precursors can react with the plasma
activated species exhausted from the plasma discharge chamber. Such
a setup is particularly suited in cases wherein it is not desired
that the precursors be broken down by the plasma discharge.
[0041] The plasma torch 20 can be provided both as a parallel plate
device, with planar electrodes 11 and 12, or as a cylindrical
device, wherein electrodes 11 and 12 are circular and concentric,
extending along axis 136.
[0042] Referring to FIG. 3, for cases in which the substrate 16 is
to be plasma treated at both sides, it is advantageous to provide
two plasma torches 31 and 32 arranged oppositely one another.
Plasma processing apparatus 30 therefore comprises a first plasma
torch 31, similar to anyone of the torches already described above.
Plasma torch 31 shown in FIG. 3 is cylindrical and may have a same
structure as torch 20 shown in FIG. 2. A second plasma torch 32,
advantageously identical in structure as torch 31, is aligned with
torch 31. Torch 32 comprises electrodes 21 and 22 spaced apart to
define a plasma discharge chamber 23. A dielectric layer 24 is
advantageously provided between either one of the electrodes and
the plasma discharge chamber 23 as described. Plasma torches 31 and
32 are aligned on a same axis 136 and such that the respective
outlets 132, 232 of the plasma discharge chambers 13, 23 are facing
each other. The plasma activated species from plasma discharge
chambers 13 and 23 are therefore exhausted towards each other in
the afterglow chamber 17.
[0043] The afterglow chamber 17 is arranged between the outlets 132
and 232, and extends from the outlets downstream along a transport
direction 26 of the substrate 16. The afterglow chamber 17
therefore receives plasma activated species from both plasma
torches 31 and 32 so that a highly concentrated and uniform
afterglow zone in chamber 17 can be obtained. The substrate 16
enters chamber 17 from a substrate inlet 172 having a reduced
clearance as described above.
[0044] The plasma apparatus 30 is particularly suited for plasma
processing of fibers, which require a 360.degree. treatment of the
fiber surface. In such case, torches 31 and 32 can be cylindrical,
with concentric electrodes 11 and 12, and 21 and 22, all aligned on
axis 136. With cylindrical plasma torches, the afterglow chamber 17
can be cylindrical as well, with fiber 16 being transported along
the axis of the cylindrical chamber 17. In such case, wall 174 is
advantageously tubular with circular cross-section.
[0045] A cylindrical afterglow chamber can comprise an upstream end
at the outlets 132 and 232 of the plasma discharge chambers, which
is defined by a shielding member 175 closing chamber 17 except for
a small aperture through it which forms the substrate inlet 172.
Substrate inlet 172 is advantageously aligned with the axis of tube
174. By so doing, the afterglow is conveyed through tube 174 in the
same direction as the substrate 16 to obtain a longer afterglow
zone along the substrate 16 and therefore a longer contact
time.
[0046] It will be convenient to note that the values for the
clearances G1 and G2 as indicated above advantageously apply to the
diameters of the inlet 172 and the tube 174.
[0047] By appropriate selection of dimension of the plasma torches
31 and 32, and the processing parameters such as plasma forming gas
flow, a uniform afterglow zone in chamber 17 can be obtained
allowing for a uniform 360.degree. treatment of the fiber 16.
[0048] Elements of the plasma processing apparatuses described in
relation to FIGS. 1 through 3 can be interchanged. In particular,
two parallel plate plasma torches as in FIGS.
[0049] 1 and 2 can be arranged oppositely as with the plasma
torches 31 and 32 of FIG. 3 to obtain an afterglow channel 17 with
rectangular cross-section and uniform afterglow zone, allowing the
simultaneous treatment of a plurality of fibers.
[0050] Advantageously, the wall 174 and/or 177 of the afterglow
chamber 17 is at least in part made of a transparent material, such
as quartz glass. The transparent wall allows for checking the
colour and/or the length of the afterglow zone, which may be an
indication of the purity of the gases used.
[0051] Apparatuses according to aspects of the invention are
particularly useful for plasma processing of carbon fibers. The
fibers are drawn or pulled through the afterglow chamber and made
to react with reactive species present in the afterglow zone. The
fibers do not enter or come in contact with any of the plasma
discharge zone(s) and do not suffer from charging effects due to
the plasma discharge.
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