Coating processes – Direct application of electrical – magnetic – wave – or... – Plasma
Reexamination Certificate
1999-04-09
2001-10-09
Padgett, Marianne (Department: 1762)
Coating processes
Direct application of electrical, magnetic, wave, or...
Plasma
C427S536000, C427S579000, C427S539000, C216S071000
Reexamination Certificate
active
06299948
ABSTRACT:
BACKGROUND OF THE INVENTION
(i) Field of the Invention
The present invention relates to a method and to a device for creating a uniform discharge at atmospheric pressure (non-filamentary) in a gas, in particular in nitrogen or in a gas mixture based on nitrogen, and to its application in particular to surface treatment using a plasma at atmospheric pressure or alternatively to the breakdown of effluents.
(ii) Description of Related Art
One of the applications relates more particularly to treatments for modifying the surface characteristics of a polymer film with a view, for example, to modifying its wettability or forming chemical bonds capable of improving the adhesion of a future coating.
In methods for surface treatment using a plasma, the surface to be treated is traditionally brought into contact with a plasma created by ionizing a treatment gas so as to create excited chemical species and electrons, which in particular can collide with third species (in particular neutral ones).
These collisions can cause energy to be transferred to the species so as to form new chemically active species, in particular so-called “metastable” species, atomic species, radicals, ions and electrons. Chemically active species with a long lifetime can in turn collide with third species, which can thus create other active species and electrons.
The active species can interact with the surface of an article in contact with the gas, which makes it possible to modify the surface characteristics of this article, on the one hand creating new molecular structures capable of interacting with the molecules of a coating deposited later, and on the other hand modifying the morphology of the material and, in particular, the mobility of chains or chain segments in the case of polymers.
It is known that electric discharges at atmospheric pressure have the advantages, on the one hand, of not requiring the use of bulky and expensive devices for creating a powerful vacuum in the treatment reactor in which the discharge is formed and, on the other hand, of permitting continuous treatments which are compatible with productivity requirements.
The electric discharges employed at atmospheric pressure, such as “corona” discharges, are generally referred to as “filamentary” since they are generated in the form of sorts of current microchannels, typically some hundred microns in diameter, which develop randomly in space and time between the two electrodes.
However, it has been found that such a “filamentary” discharge technique is not efficient enough at the microscopic level because it has a number of drawbacks at the microscopic level. Specifically, the discharge created in this way is microscopically nonuniform since, firstly, between two filaments, the surface has untreated regions and furthermore, even at the individual filaments, the treated surface is liable to be degraded by an excessive number of polymer chain breaks or even local heating.
Attempts have been made to overcome this drawback by subjecting the surface to be treated to a uniform discharge, that is to say a discharge in which the excited chemical species are regularly distributed over the surface to be treated, while of course maintaining atmospheric pressure conditions (specifically, attempts could have been made to make the discharge uniform by reducing the pressure).
It is thus known to produce a uniform discharge at atmospheric pressure in a neutral gas, essentially helium (reference may for example be made to documents U.S. Pat. No. 5,456,972 and EP-A-346 055). However, for treating surfaces with such a plasma created in a gas based on nitrogen, at least one electrode in the form of a grill is still used.
However, work carried out by the Applicant Company has shown that such electrodes do not make it possible for the uniformity of the discharge created to be improved satisfactorily at the microscopic level.
SUMMARY AND OBJECTS OF THE INVENTION
The object of the invention is to overcome this drawback and to provide a method for creating a uniform electric discharge in nitrogen, or a gas containing nitrogen, making it possible in particular to improve the conditions of surface treatment with a plasma at atmospheric pressure.
It therefore relates to a method for creating an electric discharge in an initial gas which is at atmospheric pressure and lies between two exciting electrodes, by applying a supply voltage to the two electrodes, which is characterized in that the supply voltage is an AC voltage whose amplitude and frequency are adapted in order to maintain at least a portion of the components of the gas in the excited state, and/or the presence of electrons, between two successive half-cycles of the supply voltage.
As will be illustrated in more detail further on in the present application, the method according to the invention avoids the conditions of strong field and breakdown voltage typically characteristic of the filamentary mode, by making it possible to create and maintain, between two discharges (i.e. between two half-cycles), sufficient quantities of metastable species and electrons for each discharge to start with a low electric field value (the metastable species relax, for example, on contact with a third species to create an electron and an ion of this third species itself or of the metastable species). The electrons lead to avalanches which produce ions, accelerated toward the cathode, which cause secondary emission of electrons at the cathode, etc.
The method for creating a discharge according to the invention may further- have one or more of the following characteristics, taken individually or in any technically feasible combinations :
the supply voltage lies between about 5 kV and 30 kV, with a more preferential embodiment in the range of from 10 kV to 25 kV, and the frequency of the voltage lies between about 200 Hz and 35 kHz, with a preferential range of less than or equal to 15 kHz;
the initial gas is introduced into the inter-electrode space with a gas velocity of between 0 m/s and 10 m/s, with a preferential range of less than or equal to 5 m/s.
the initial gas includes one or more of the gases in the group formed by nitrogen, silicon precursor gases, oxygen and gases capable of releasing oxygen such as N
2
, CO
2
, NO
2
, H
2
O . . . ;
the thickness of the gas space between the exciting electrodes is between about 0.5 and 5 mm;
at least one of the electrodes is covered with a dielectric or semiconductor material (such as alumina, glass, polymer, etc.), the thickness of which advantageously lies in the range of from a few tens of microns to 1 cm, and preferably in the range of from 500 microns to 2 mm.
The invention also relates to a device for creating an electric discharge in a gas at atmospheric pressure, for implementing a method as defined above, including two exciting electrodes, a source for supplying the inter-electrode space with an initial gas and a voltage supply source which is connected to the exciting electrodes, and which is characterized in that the voltage supply source is capable of delivering an AC voltage whose amplitude and frequency are adapted in order to maintain at least a portion of the components of the gas in the excited state, and/or the presence of electrons, between two successive half-cycles of the voltage.
The invention also relates to a method for surface treatment with a plasma at atmospheric pressure, including the step of introducing a treatment gas into a treatment reactor, in which a surface to be treated is arranged between two exciting electrodes and applying a supply voltage to the two electrodes so as to cause the appearance of an electric discharge in the treatment gas, and which is characterized in that the supply voltage is an AC voltage whose amplitude and frequency are adapted in order to maintain at least a portion of the components of the treatment gas in the excited state, and/or the presence of electrons, between two successive half-cycles of the supply voltage.
The method for surface treatment according to the invention may furthermore have one or more of the follo
Gat Eric
Gherardi Nicolas
Gouda Gamal
Massines Francoise
Villermet Alain
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Padgett Marianne
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