WO2006004399A2

WO2006004399A2 - Method and means for generation of a plasma at atmospheric pressure

Info

Publication number: WO2006004399A2
Application number: PCT/NL2005/000463
Authority: WO
Inventors: Yves Lodewijk Maria Creyghton; Ales Fiala; Timo Huijser; Marius Pieter De Goeje
Original assignee: Nederlandse Organisatie Voor Toegepast-Natuurwetenschappelijk Onderzoek Tno
Priority date: 2004-06-30
Filing date: 2005-06-30
Publication date: 2006-01-12
Also published as: EP1774839A2; WO2006004399A3; NL1026532C2

Abstract

Method and means for the generation of a plasma, for e.g. disinfecting, cleaning or coating an object (11), comprising an initializing step, in which the initial electric discharge (Pi) is realized in an initial gas (Gi), followed by a process step, in which the initial discharge initiates or propagates an electric process discharge (Pp) in a process gas (Gp). The initial electric discharge is realized between a first set of electrodes (1,2) at which an electric initial voltage (Vi) is applied, and the electric process discharge between a second set of electrodes (1, 2, 3, 4) to which an electric process voltage (Vp) is applied. The pressure (pi) of the non-electronegative initial gas is about atmospheric during the initiation step. The initial voltage comprises a voltage pulse, while the pressure of the process gas comprises a pressure pulse.

Description

Title: Method and means for generation of a plasma at atmospheric pressure

BACKGROUND OF THE INVENTION

In a non-thermal plasma free electrons are generated with sufficient energy for ionization, dissociation and excitation effects while the majority of the neutral gas particles keeps a relatively low temperature. In such a plasma, the following physical and chemical effect scan occur: the forming of oxidizing substances with a cleaning effect, UV generation with a disinfecting effect and the dissociation of gases leading to the forming of particles or deposition of coatings. The temperature of electrons, vibrationally excited molecules and the normal kinetic gas temperature, respectively Te, Tv and T, are so different that Te is larger than Tv and Te is much larger than T, through which a high selectivity of chemical reactions can be reached. Non-thermal plasma technics are interesting for industrial use, because synthetic surfaces, textile fibres and also foodstuffs can be treated without the degradation of the surface of these substances which would occur at a high temperature. A spatially equal distribution of the plasma is essential for many applications. In general, spatially homogeneously distributed non¬ thermal plasmas are generated at reduced pressure, viz. with the help of vacuum equipment. For many of the possible industrial uses a batchwise vacuum treatment is too expensive. Therefore, a great need exists for atmospheric, spatially distributed, non- thermal plasmas, often indicated as 'atmospheric glows'. A useful definition of atmospheric glow is: 'An electric discharge at a gas pressure in the order of 1 bar, with a low ionization degree (Ne/N<l/10000), in which the temperature (the average kinetic energy) of the electrons is many times larger than the neutral gas temperature and the spatial structure is not restricted to one or few non-thermal filaments (streamers)'.

Atmospheric glows can be applied for treatment of a surface or a gas. Examples of surface treatment are:

Oxidative cleaning or microbiological inactivation. Possible applications are cleaning/disinfection of a foodstuff or complete cleaning and/or sterilization of a packing material.

- Activation of polymeric surfaces, including fibres, yarns and fabrics of textile products after which this surface is better suitable for coating. - Deposition of thin layers, e.g. the applying of coating at textile fibres for reduction of the number of fibre pores or the applying of gas barrier coatings- at synthetic packing materials.

Examples of gas treatment are:

- Production of small particles for the composition of new materials, such as e.g. aluminium particles for improved properties of pyrogenous substances, magnesium particles for storage of hydrogen and carbon chains for the improvement of the strength properties of synthetic materials. - Gas cleaning through chemical reaction of active plasma components with

NO, NO2, soot particles, aromatic substances, (micro) biological pollutions etc.

Among others, the next methods and means are known for the generation of a plasma (glow):

Capacitive linked plasmas in plan paralleled configuration, i.e. a configuration in which atmospheric plasma glows are generated within electrode configurations with a homogenous geometric electric field and in which the material to be treated covers one of the electrodes. Disadvantages of this method are: o The electrode distance is restricted to ca. 10 mm. The application is restricted to treatment of flat objects or films running over a roll. It is not possible to treat in an effective way articles with a non-flat structure. o The plasma is only homogeneous at a specific gas composition. For an important part (>95%), the gas has to consist of a non electronegative gas such as Helium or Argon. The maximum concentration of pre-cursor S₅ additives used e.g. for forming of particles or layer deposition, is very much restricted because of the occurrence of filamentation in the plasma. o The power density is very much restricted. Above a certain power density thermal filamentary discharges will occur. - Plasmas -generated by microwave radiation. This method is in principle better suitable for a homogenenous excitation of electrons in a gas than capacitive or inductive linked electric fields. However, above a certain electron density the microwave radiation does not longer penetrate to the bulk of the plasma. For an effective treatment of gases and surfaces higher electron densities are necessary.

- Thermal atmospheric plasma jets, by which thermal plasma are generated in a nozzle (efflux opening). For a plasma spray technology apre-cursor material is added to the gas streaming through the nozzle (in the form of gas, powder or liquid droplets). The common atmospheric plasma jets are unsuitable for the forming of (non-thermal) atmospheric glows.

- Non-thermal atmospheric glow jets, developed at the university of Californie. Use is made of a high flow Helium gas to avoid thermal discharges. A cylindrical version of this jet has been described in patent US 5,929,523 and a planar version in US 6,262,532. Disadvantages are: o Because of the continuous high flow speed of inert gas (Helium, Argon) the operational costs are very high when applying this method. o The density of active plasma particles decreases very strongly with the distance to the efflux opening (nozzle) of the jet. o The occurrence of thermal arcs in the electrode configuration is avoided by use of a specific combination of a frequency (typical 13 MHz) and gas flow. The intensity of the treatment can only be varied by adjustments of the geometry: electrode dimensions or the distance from the nozzle to the surface to be treated.

The non-thermal atmospheric plasma jet has been improved according to a design of Matsushita that is described in patent US 6,429,400. In the cylindrical electrode configuration of this jet the outside electrode of the plasma has been separated by a cylindrical tube of isolating material. In this way, a jet has been presented that can be described as 'dielectric barrier discharge jet' (DBD jet). The dielectric barrier avoids thermal disruptive discharge. Compared to a jet without dielectric barrier, the occurrence of an atmospheric glow in this DBD jet is less dependent on the used frequency and gas flow conditions. However, an important problem, the rapid decrease of the concentration of active plasma particles as function of the distance to the nozzle, remains.

Summarizing, the method and means for the generation of atmospheric plasma glows presented up here, have important shortcomings:

- For in situ treatment in a plasma, the volume is restricted by the maximum electrode distance of ca. 10 mm.

- At the treatment in a streaming plasma with jet configurations, the density of active plasma particles decreases very rapidly with the distance to the efflux opening (nozzle) of the jet.

- The known machines are not suitable for the treatment of twofold curved, rough or porous surfaces.

SUMMARY OF THE INVENTION

The aim of the method and means described hereinafter is the generating of an atmospheric glow (non-thermal, spatially well distributed, atmospheric discharge) with a large volume.

The method summarized below for the generation of a plasma comprises an initialization step, in which an initial electric discharge is realized in an initial gas, followed by a process step, in which the initial discharge realizes (initializes and/or propagates) a process discharge in a process gas. Preferably, the initial electric discharge is realized by applying an electric initial voltage between a first set of electrodes, while the electric process discharge is realized by applying an electric process voltage between the second set of electrodes. The pressure of the initial gas during the time of the initiation step is preferably about atmospheric, while the initial gas is preferably a non-electronegative gas such as Nitrogen, Helium or Argon. The initial voltage preferably comprises at least one voltage pulse for the forming of an initial discharge (or initial plasma). The pressure of the process gas to be added after the inflow of the initiation gas is preferably built up very rapidly and preferably comprises at least one pressure pulse, preferably following (the end of) the voltage pulse(s) of the initial charge.

By means of a number of figures the operation of the above summarized method in a number of different devices will be discussed in detail.

FIGURES Figure 1 shows a first exemplary embodiment of a device which is suitable for the realization of the above summarized method.

Figure 2 shows an example of the course of the gas pressure of the initiation gas respectively the process gas and of the electric initiation and process charge.

Figure 3 shows a second exemplary embodiment of a device which is suitable for the realization of the above summarized method.

Figure 4 shows a third exemplary embodiment of a device which is suitable for the realization of the above summarized method.

Figure 5 shows a fourth exemplary embodiment of a device which is suitable for the realization of the above summarized method. Figure 6 shows a fifth exemplary embodiment of a device which is suitable for the realization of the above summarized method.

The figures 1 and 3 to 6 inclusive show an housing 5 in which a plasma can be generated. First, the initial electric discharge is realized in an initial gas Gi. Next, the initial discharge initiates an electric process discharge in a process gas Gp. The process gas Gp is, preceding the initial gas discharge (Pi), present in the housing 5 and/or is supplied at a controlled moment t2 to the gas control unit 6. The initial electric discharge is realized between a set of electrodes, the ignition or initiation electrodes 1 and 2 at which — by a controllable current source 7 — an electric initial voltage (or voltage pulse) Vi is applied. The pressure (pi) of the initial gas - controlled by the gas control unit 6 - is about atmospheric in the time of the initiation step (ca. 1 bar or 1 atm). The initial gas is preferably a non-electronegative gas. Through the initial discharge Pi the electric process discharge PP is then 'lighted' between a second set of electrodes when an electric process voltage Vp is placed between those electrodes by the controllable current source 7, while furthermore the gas pressure increases in the process discharge Pp. As a result of this increase of gas pressure the volume of the process discharge Pp increases rapidly. The pressure increase in the process discharge Pp occurs as a result of thermodynamic expansion and/or the with the help of the unit 6 to the housing supplied process gas Gp.

The control units 6 and 7 are controlled by e.g. a process control unit 9. Initiation and/or process residues can, as much as not used for e.g. disinfecting, cleaning or coating of an object 11, be removed through an exhaust opening 10.

In figure 1 - which will be discussed in detail below - the electrodes 1 and 2 form the ignition/initiation electrode pairs and the electrodes 3 and 4 the process electrode pairs. In the figures 3 to 6 inclusive one of the electrodes always serves as ignition/initiation electrode and then as process electrode. In this way, the first set of electrodes, the ignition/initiation electrodes, are formed in figure 3 by the electrodes 1 and 2, and the second set of electrodes, the process electrodes, by the electrodes 2 and 3 (electrode 2 thus first serves as initiation electrode and during the process step as process electrode). Also in figure 4 electrode 1 serves as initiation electrode (together with electrode 2) and as process electrode (together with electrode 4). In figure 5 the electrodes 1 and 2 form the first set of (initiation) electrodes and 2 and 4 the second set of (process) electrodes, where electrode 2 is common, which is also the case in figure 6.

Figure 2 illustrates the course of the gas pressure of the initiation gas respectively the process gas in the housing 5 of the electric initiation and process charge. As from a moment t0 the initiation gas Gl is entered into the housing 5; the pressure pi of that gas is ca. 1 bar. When the housing 5 between initiation electrodes 1 and 2 has been filled sufficiently, a voltage Vi is placed between the electrodes 1 and 2 - at a moment tl - through which a gas discharge between those electrodes occurs. It should be noted that both at the initiation electrodes and the process electrodes per pair always one electrode is enclosed by a 'dielectric barrier' of e.g. ceramic material, to prevent 'short circuit' between those electrodes with as result a strong conductive and completely ionized arc discharge or spark (an arc discharge of short duration).

If the initial gas discharge Pi is 'running', the current strength Ii will increase - around a moment t2 — and the voltage Vi will decrease (but not in the extreme extent which occurs at the start of an arc discharge), which phenomenon can be used as input for the control unit 9, and which, after the receiving of that input, is able to control the gas control unit 6 and the current source 7 as is illustrated in figure 2. From about the moment t2 or, after some delay (moment t3) the gas pressure p_p in the process discharge Pp is raised in a short time. As a result of that, the initiation gas Gi is driven away and the electric plasma between the initiation electrodes is moved to a plasma between the respective process electrodes, which plasma, after being propagated from the initial plasma, is maintained by the applied voltage Vp and by the process gas Gp between the process electrodes. When the process plasma is not necessary anymore, the electric voltage Vp is taken away at moment t4 and the residue of the process gas can flow away via the exhaust opening (moment t5).

So, first a relatively small quantity of (expensive) inert gas Gi for initiation is supplied. Through the initial gas discharge Pi the real process plasma Pp is -after supply of the process gas Gp- ignited. The plasma power increases at the moment that voltage Vi decreases and the current Ii increases. The process pressure pulse, caused via the unit 6, can be supplied at the moment that the plasma power strongly increases (t2) or short after that (t3). That process pressure pulse is added to the pressure pulse that is built up by the plasma itself as from t2. The initial gas Gi and the process gas Gp are mixed naturally or the initial gas Gi is pressed outside by the process gas Gp and the plasma Pp itself (what is just advantageous if it is about surface treatment) of e.g. the object 11 in figure 1. Besides, the initial pressure pulse can temporary be considerable larger than 1 bar. During the initiation plasma phase the applied electric field is e.g. >2 kV/cm and during the process plasma phase this electric field decreases to ca. 0,5 kV/cm (at an Argon air mixture). The current increases to e.g. ca. 10-100 Amp, dependent on the spatial dimensions of the plasma and of the electro negativity of the gas.// For the continuous treatment of a gas or surface the course of charge, current and gas supply (gas pressure) can have a repeated pulsated character, comprising one or more pulses. A practically possible total electric pulse duration is e.g. 1-10 millisecond and a practically possible pulse repeat frequency 10-100 Hz (period 10-100 millisecond).

Preferably, the period without voltage between the voltage pulses will be longer than the duration of the pulses themselves.

In figure 1 the initial electric discharge is realized between the initiation electrode 1 and the second initiation electrode 2, to which an electric initial voltage (or voltage pulse) Vi is applied. Through the initial discharge Pi the electric process discharge Pp is then 'lighted' between the first process electrode 3 and the second process electrode 4 when an electric process voltage Vp is placed between those process electrodes 3 and 4, while furthermore the process gas below with the help of a pressure pulse is supplied into the housing 5 (see figure 2). The so generated plasma can be used for e.g. the disinfection, cleaning or coating of the object 11, dependent on the composition of the process gas Gp.

Figure 3 shows a second exemplary embodiment of a device which is suitable for the realization of the present method to generate a plasma for e.g. disinfecting, cleaning or coating of an object that is being exposed to the process plasma Pp. The initial electric discharge is realized between the first ignition electrode 1 and the second ignition electrode 2, to which an electric initial voltage Vi is applied. Through the initial discharge Pi the electric process discharge Pp is then 'lighted' (started) between the first ignition electrode 1 and the — in figure 1 so-called - first process electrode 3 when an electric process voltage Vp is placed between those process electrodes 1 and 3, while furthermore the process gas under pressure (pressure pulse) is supplied into the housing 5.

Figure 4 shows a third exemplary embodiment of a device which is suitable for the realization of the above summarized method. The initial electric discharge is realized between the first ignition electrode 1 and the second ignition electrode 2 at which an electric initial voltage (pulse) Vi is put. Through the initial discharge Pi the electric process discharge Pp is then 'lighted' between the first ignition electrode 1 and the —in figure 1 so-called- second process electrode 4 when an electric process voltage Vp is placed between those electrodes 1 and 4, while furthermore the process gas under pressure (pressure pulse) is supplied into the housing 5.

After the forming of particles in the first process discharge Pp, it could be desirable to treat these particles additionally with a plasma in a second, additional process discharge Ppbis. Examples are: a coating of iron particles in the second process step being made in a first process step to keep their unique magnetic properties in a solid substance matrix . A second example is the production of iron particles in a first pulsated process step and use of these particles as catalyst for the growth of carbon nano tubes in the second process step.

The second, additional process discharge Ppbis can be realized at the flow side of electrode 4, thus electrode 4 is getting a double function.

Figure 5 shows a fourth exemplary embodiment of a device which is suitable for the realization of the above summarized method. In this case the object to be treated is a sheet 11 of e.g. synthetics or textile to be conducted through the housing 5. The initiation electrodes 1 and 2 are coaxial in relation to each other, while electrode 1 has been covered with a dielectric barrier. The form of the ring-shaped electrode 2 is such that the radial distance to the central electrode 1 increases gradually with the position along the axial axis. At the third electrode 4, the process electrode that is situated at the flow side of the process housing 5, an electric potential is applied which is equal to that of electrode 2. In this exemplary embodiment the process electrode 4 consists of a grid of tubes being covered by a dielectricum; if required the temperature of those tubes can be controlled by means of a liquid conducted through those tubes.

After the application of voltage Vl between electrodes 1 and 2, ionization starts in this area with maximum field strength. Although the plasma Pi remains non-thermal (the gas temperature is much lower than the temperature of the electrodes), the plasma will expand by the warming up in the direction of electrode 4 and will form there a process plasma Pp. In the initiation phase the narrow part of the electrode configuration 1-2 is first filled with a small amount non-electronegative atmospheric gas Gi, e.g. Nitrogen, Helium or Argon. In these gases a relatively large density of electrodes is reached, by which the expansion of the ionization area is stimulated to a large volume. Besides, Nitrogen has many internal energy positions (vibrational and electronic) that are also advantageous for the maintenance of the plasma in a large volume. After the application of the (R-F) voltage at electrode 1, the generation of the plasma can be observed by means of detection of a sudden increase of the discharge current (from Ii to Ip in figure 2) and/or decrease of the voltage (from Vi to Vp in figure 2). After an adjustable short period (e.g. 0,1-2 ms) a high pressure pulse Pp is generated by means of the gas control unit 6 (a fast gas valve in the gas supply pipe). The pulsated gas is preferably led via the opening between electrodes 1 and 2 in the housing 5. The composition of this gas Gp is application dependent. For cleaning applications an oxygenous gas, e.g. air, can be used. The plasma Pi expands from the 'plasma jet configuration', formed by the coaxial electrodes system 1-2, in the direction of the electrode grid 4. During that plasma expansion the surface of the plasma border layer extends rapidly. The extension of the plasma border layer is stimulated by:

- the presence of an initially high density of electrons and excited molecules in the non-electronegative atmospheric gas Gi;

- application of the high pressure pulse of the process gas Gp;

- the electric field existing between electrode 2 and electrode 4.

For applications such as the forming of particles and deposition a pre-cursor gas can be added or not. Addition of a pre-cursor in a carrier gas (Nitrogen, Helium, Argon) can take place via the gas control unit 6 or e.g. via small openings in the diverging part of electrode 2. The most appropriate moment of addition (in the time scheme of figure 2) has to be determined in a more detailed way by investigation. For possible forming of carbon particles, methane or acetylene can be used, for the forming of aluminium particles A1C13, for the forming of iron particles Ferrocene (C₁₀H₁₀Fe), while for the forming of Si_xOy layers TEOS or HMDSO can be used. Via a crack-shaped opening in the housing 5 e.g. a textile product 11 which has to be treated by the process plasma, can be introduced. Preferably, the product 11 is treated close to the active border layer that is generated in the plasma/gas Pp close to the dielectric covering electrode 4.

Figure 6 shows a fifth exemplary embodiment of a device which is suitable for the realization of the above summarized method. This variant comprises a device for the generation of a non-thermal plasma within e.g. a synthetic packing material 11. The initial plasma Pi is generated between a first ignition electrode (initiation electrode) 1 and the second ignition electrode (initiation electrode) 2 having the form of a coaxial grid here, on which - by a controllable current source 7 - an electric initial voltage pulse Vi is put. The pressure (pi) of the initial, non-electronegative gas — controlled by the gas control unit 6 - is at the time of the initiation step also here ca. 1 Bar. Through the initial discharge Pi the electric process discharge Pp is then 'lighted' between the first ignition electrode 1 and the process electrode 4 -situated here at the outside of the isolated housing 5 — after which an electric process voltage Vp is placed between those electrodes 1 and 4 by the controllable current source 7, while the process gas Gp under pressure is supplied into the housing 5 under control of the unit 6. The initiation and/or process residues can, as far as not used for e.g. the disinfecting, cleaning or coating of the packing material 11, be removed, in this case via the fill opening 10.

Claims

CLAIM S

1. Method for the generation of a plasma, comprising an initializing step, in which an initial electric discharge (Pi) is realized in an initial gas (Gi), followed by a process step, in which the initial discharge realizes an electric process discharge (Pp) in a process gas (Gp).

2. Method according to claim 1, in which the initial electric discharge is realized between a first set of electrodes (1,2) to which an electric initial voltage (Vi) is applied, and the electric process discharge between a second set of electrodes (1,2,

3,

4) to which an electric process voltage (Vp) is applied. 3. Method according to claim 1, where the pressure (pi) of the initial gas during the initiation step is mainly atmospheric.

^■ 4. Method according to claim 1, where the initial gas is a non-electronegative gas.

5. Method according to claim 2, where the initial voltage comprises at least one voltage pulse.

6. Method according to claim 1, where the pressure of the process gas comprises at least one pressure pulse.

7. Method according to claim 5 and 6, where the first voltage pulse of the initial voltage (Vi) precedes the first pressure pulse of the process gas (Gp).

8. Device for the generation of a plasma, comprising means for the generation of an initial electric discharge (Pi) in an initial gas (Gi) and means for initiating and/or propagating an electric process discharge (Pp) in a process gas (Gp) with the help of the initial discharge.

9. Device according to claim 8, comprising means for the generation of the initial electric discharge between a first set of electrodes (1 ,2) by means of the application of an electric initial voltage (Vi), and means for the generation of the electric process discharge between a second set of electrodes (1,2,3,4) by means of the application of an electric process voltage (Vp).

10. Device according to claim 8, comprising means for supplying the initial gas during the initiation step at mainly atmospheric pressure.

11. Device according to claim 9, where the initial voltage comprises at least one voltage pulse.

12. Device according to claim 9, where the pressure of the process gas comprises at least one pressure pulse.

13. Device according to claim 11 and 12, where the first voltage pulse of the initial voltage (Vi) precedes the first pressure pulse of the process gas (Gp).

14. Device according to claim 9, where the initial gas (Gi) and/or the process gas (Gp) is supplied via a space between the first set of electrodes (1,2).