Atmospheric barrier discharge reactor for surface processing.

Nehra, Vijay ; Kumar, Ashok ; Dwivedi, H.K. 等

Introduction

Since the last decade, plasma assisted surface treatment has been playing a vital role in the microelectronics industry [1], material processing [2], textile sector [3-4] and biomedical engineering [5-8]. The remarkable features of cold plasma such as low temperature, presence of energetic & chemically active species and high chemical selectivity, which are not attainable by other competing methods, make cold plasma well-suited in these fields. It provides an environment friendly and economic way to modify material surfaces at microscopic level without resorting to mechanical operation or using wet chemicals.

Low pressure glow discharge plasma [9-13] is sustained between two electrodes extending into a nearly evacuated glass tube. It is produced at reduced pressure and assures the highest possible uniformity of any plasma treatment. It is of great interest in fundamental research such as in the microelectronic industry and material technology but has only limited applicability in the textile sector. Although with glow discharges, it is possible in a well controlled and reproducible way to clean, activate, etch or otherwise modify the surfaces of plastic, metal or ceramic materials to improve their bonding capabilities or to acquire totally new surface properties; but the glow discharge plasmas have never been able to get a foothold in the textile processing sector because of their fundamental incompatibility and several serious drawbacks. These plasmas need to be contained in costly air tight enclosures (massive vacuum reactors) making them highly expensive, time consuming and impractical to treat work piece by batch processing. Moreover, the access for observation or sample treatment is limited and the density of activated particles is also relatively low. Therefore, recently, there has been a growing interest in the generation and sustenance of cold plasmas at atmospheric pressure [14-22], mainly because of non-requirement of a vacuum system, thus leading to high throughput and cost reduction. In an atmospheric plasma reactor, continuous processing at the point of manufacture is possible with a higher activated particle density. In fact, atmospheric plasma processing has got a rebirth in the textile arena, with the earlier unsuccessful approach of using vacuum plasmas replaced by novel atmospheric plasmas reactor. The economic and operational advantages of operating at elevated pressure at or near 1 atm have led to the development of a variety of atmospheric plasma based surface treatment reactors for textile processing applications. Here, efforts have been made to study design consideration of a continuous process DBD reactor suitable for textile treatments. System description and process parameters required for design of DBD setup have been discussed. Moreover, DBD treatment set up has been presented.

This paper is organized as follows: after a brief introduction in Section 1, description of plasma generation systems and process parameters of DBD reactor are presented in Section 2. Further in section 3, DBD treatment setup design is dealt. Section 4 discusses the industrial applications of the DBD reactor.

Description of plasma generation System

The block diagram layout of typical DBD generation system for surface modification is shown in figure 1. It consists of five modules: gas handling system, power supply, impedance matching network, DBD plasma rector and system controller. For a given plasma generation system, the outcome of the process is strongly dependent on its process parameters. The various electrical, kinetic and surface process parameters playing vital role in the design of DBD assisted plasma processing have been elaborated in figure 2. The electrical parameters deal with the selection of suitable DBD reactor geometry, discharge excitation parameters, power and frequency etc. The kinetic parameters regulate the gas handling system and consist of a precursor gas supply of different working media or their combination, depending on the application. This also consists of mass flow controllers, process chamber and pressure handling system. The surface parameters deal with substrate system and include the materials to be processed, such as conducting or non-conducting substrates, films, fabrics, papers, webs and two dimensional sheets etc. It also deals with the process temperature and the position of the substrate.

[FIGURE 1 OMITTED]

[FIGURE 2 OMITTED]

Design of DBD treatment set up

Figure 3 gives a schematic setup of the installation for DBD reactor. This involves following components, mainly designing of gas handling system, selection of suitable DBD plasma reactor, power supply and impedance matching network. Various optional parts may be added in addition to adapt a base system for different applications or substrates.

The gas handling system consists of an assembly of ultra-high purity gas line, which allows controlled entry of gas from high pressure cylinder to the process chamber. The schematic of gas filling system is shown in figure 4. The gas handling system consists of suitable gas cylinders supported by gas plumbing line, valves, mass flow controllers and necessary accessories. Unique reactions may be promoted by the appropriate choice of reactant gases and process parameters. Different gases or a combination of them such as oxidizing gases (O2, air, H2O, N2), reducing gases, noble gases, active gases, fluorinated gases, and polymerizing gases etc may be used in plasma processing techniques, depending upon the desired surface modification. The gas handling system ensures a regular supply of appropriate working media in the reactor.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

Further, the DBD reactor can be realized in different geometrical configurations. In fact, three basic types of DBD arrangements have been distinguished namely, the classical volume, surface, and coplanar discharge. The volume discharge configurations in planar geometry are suitable for continuous processing. The most commonly preferred DBD reactor configurations [23-30] for surface treatment of films and fibers include: the planar parallel-plate DBD and cylindrical annulus reactor. The former is used for fast-moving webs which are not electrostatically charged and do not adhere to the surface of DBD electrodes over which they pass. The latter is used for webs which are electrostatically charged or have high surface energies and adhere to the surface of DBD electrodes. In this configuration, a rotating drum moves the film or web through an annulus of DBD plasma generated between a cylindrical electrode and the drum, while the tension on the film or web is maintained by idle rollers.

However, planar arrangement may contain either a single dielectric layer between the two electrodes or on both the electrodes, or in between two metal electrodes. The plasma is generated between two parallel plane electrodes contained in a safety enclosure. The high and low voltage electrodes, both are covered with a dielectric layer. The height of the plasma discharge gap is adjustable. These reactors are normally enclosed in a light plastic or a metal sheet chamber.

DBDs exhibit two major discharge [31] modes: either filamentary mode or homogenous glow discharge. The filamentary mode is the common form of discharge, comprising of randomly distributed millions of micro-discharges spread over the electrode surface. The homogenous glow discharge mode is also known as atmospheric pressure glow discharge mode. It is suitable for a homogenous treatment of surface or for the deposition of thin films. The filamentary mode is non-uniform and creates uneven treatment such as pitting or pin holes. At high power levels, these pits can burn the surface of the substrate, leaving holes in the work piece. On the contrary, atmospheric pressure glow discharge mode offers a uniform and stable surface treatment. It causes less surface physical damage and is therefore very promising to support material processing.

Applications of DBD reactor in surface treatment

The DBD reactor may be used in several scientific and industrial applications such as: surface treatment of textile substrate and for modifying the surface properties of film, web, fabric, woven and non-woven textile substrates, polymers for a variety of subsequent conversion processes such as printing, bonding etc. It may also be applied to plasma processing of conductive and non conductive films to increase the surface energy, improve the wettability, wickability, printability, bonding of fabrics, cleaning and decontamination of surfaces, adhesion on polymer surfaces and to impart stable change of surface tension of treated materials.

The surfaces to be treated may be moved continuously and exposed to the energetic environment of the active species (atoms, radicals, excited species, and ions) generated in the dielectric barrier discharge, which produce the desired effect on the surfaces. The dynamic and energetic environment of DBD plasma interacts with the surfaces of the substrates to be structured and modifies only the top few nanometer layers. This allows the surface properties of any material to be changed without altering or degrading the bulk properties. The chemical functionality or the morphology of a fiber surface may be altered in order to impart various specific properties.

Further, this reactor may be used in generation of intensive UV and VUV photons using noble gases. These UV photon ay be applied in the field of material processing, in particular, for surface modification, polymer etching, or alternatively for the deposition of thin metallic layers and for the polymerization of lacquers.

Conclusion

The DBD reactor design for surface treatment at atmospheric pressure has been presented. In addition, the major components of this system i.e. gas filling system, power supply; matching network, DBD plasma rector, and controller have been discussed along with possible industrial applications. There is a great potential for DBD reactor in the future for surface modification as the DBDs produce highly nonequilibrium plasma conditions in a controllable way at atmospheric pressure, and at moderate gas temperature. It shows an ease of flexibility with respect to geometrical shapes, operating medium and operating parameters. Further, there is also an easy scaling up of conditions optimized in a small laboratory set up to large industrial installations. DBD reactor at atmospheric pressure has the possibilities of replacing the existing batch process and will thus find diverse applications in various scientific and industrial areas.

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Vijay Nehra (#), Ashok Kumar *, H K Dwivedi ** and Sandeep K. Arya (#)

(#) Department of Electronics and Communication Engineering, Guru Jambheshwar University of Science and Technology, Hisar, India Email: nehra_vijay@yahoo.com

* YMCA Institute of Engineering & Technology, Faridabad, India

** R& D Head (PDP), Samtel Color Limited, Ghaziabad, UP, India