PLASMA PROCESSING UPDATE

A newsletter from the

Facilitation Centre for Industrial Plasma Technologies,

Institute for Plasma Research

Issue 45 January - April 2004

Editor's Note

Barrier Coatings on Food Packaging

Development of Dusty Plasma System for SINP

Exploring Electromagnetic Analysis in ANSYS

Pulsed Thermography and Non Destructive Testing of Coatings

Editor's note

Food researchers face numerous challenges when trying to develop a product that retains its quality during its shelf life. Shelf life is determined not only by a food's chemical nature, but also by the way it has been processed, packaged, distributed and stored. Special grades of polyethylene terephthalate (PET) have been developed for such packaging applications. However, when beverages and food products have to be stored, the properties of PET must be enhanced. This is made achievable by constructing multilayered coatings. Many thin film-coating processes have been adapted to barrier coating applications. Some of these processes are described in this article which, is an excerpt taken from the forthcoming book: Plasma Sciences and the Creation of Wealth by Prof. P. I. John, Published by Tata-McGraw Hill, New Delhi.

Recent interest in plasma technology has focused increasingly on the area of dusty plasmas, indeed this field has been on of the fastest growing areas of physics research in recent years. The study of dusty plasmas has a broad range of applications including interplanetary space dust, comets, planetary rings, dusty surfaces in space, and aerosols in the atmosphere. A laboratory scale dusty plasma system was designed and fabricated by FCIPT and supplied to Saha Institute of Nuclear Physics, Kolkata for research. Mr. Manoj Garg gives a vivid description of the system.

ANSYS is a computer software used across a broad spectrum of industries for various engineering design applications. Its open & flexible simulation solutions provide a common platform for fast, efficient & cost-effective product development, from design concept to final-stage testing & performance validation. This software enables to build computer models or transfer CAD models of structures, products, components, or systems. Also, one can study the physical responses, such as stress levels, temperature distributions, or the impact of electromagnetic fields, etc. It also helps to carry out prototype testing in environments where it otherwise would be undesireable or impossible (for example: biomedical applications). Mr. Ravi Prakash describes the use of ANSYS for electromagnetic analysis. This group undertakes design of various types for engineering applications.

For many years, X-ray or ultrasound were the preferred NDT techniques. Pulsed thermography, however, has recently conquered a remarkable share of NDT market. This completely non-contact NDT technique offers considerable advantages with regard to speed, cost and portability as well as safety. Pulsed thermography uses energy that is generated through a brief light flash and is then converted to heat on the sample surface. This heat is conducted into the sample and the time-dependent change of heat distribution of the surface is detected using a sequence of IR images. Dr.Govindrajan elaborates on the various applications of pulsed thermography and their work at Institute for Plasma Research.

Dr. K. S. Ganesh Prasad

Editorial Assistance: Alphonsa Joseph

Facilitation Centre for Industrial Plasma Technologies

The Institute for Plasma Research (IPR) is exclusively devoted to research in plasma science, technology and applications. India’s first high temperature plasma device `Aditya Tokamak’, built at IPR, produces plasmas at 5 million degrees temperature - comparable to that of the sun. An advanced fusion device with superconducting magnets, capable of steady state operation is under fabrication. IPR is also active on a broad front of fundamental studies. It has engineering groups skilled in technologies of Superconducting Magnetics, Ultra High Vacuum, Pulsed Power, Microwave and RF, Computer-based Control and Data Acquisition, theory and computer simulation complement experimental programmes.

The Facilitation Centre for Industrial Plasma Technologies (FCIPT) links the Institute with the Indian industries and commercially exploits the IPR knowledgebase. FCIPT interacts closely with entrepreneurs through the phases of development, incubation, demonstration and delivery of technologies. Complete package of a broad spectrum of plasma-based industrial technologies and facilitation services is offered.

Some of the recent FCIPT achievements are: plasma nitriding of industrial components to increase wear resistance and hardness, coating of quartz-like films on brassware to inhibit oxidation and tarnishing, ceramic synthesis and processing, plasma ion implantation and ion plating for surface engineering, thermal plasma technologies for smelting of minerals and waste treatment etc.

The Centre has process development laboratories, jobshops and material characterisation facilities. The process development laboratory exploits the areas of expertise in plasma and other allied fields of the institute in developing new plasma based technologies for the industry. The jobshop executes job work for surface and material treatment on an industrial scale to promote the acceptance of plasma based technologies and to generate techno-commercial data relevant to entrepreneurs. The advanced instruments in the material characterisation facility are open to users from industry, research establishments and universities.

This newsletter is designed to help you keep abreast with the developments in the important field of plasma assisted manufacturing and to look for new industrial opportunities. We would be very happy to have you write to us on ways of improving this service or visit us for further discussions.

Please visit our website: http://www.plasmaindia.com

Barrier Coatings on Food Packaging

In the southern Indian state of Kerala, food articles were traditionally packed in banana leaves, made pliant by steaming. Banana leaf can be called an active packaging since it infuses the packed food with its own aroma, and a nice one too. Food packaging has travelled from banana leaves to PET (Polyethylene terephthalate), going full cycle from natural to synthetic organics, driven by consumer preferences and the changing nature of both food and life-styles.

Food packaging has many functions. Food deteriorates by physical, biochemical and microbiological routes independently or in concert [1]. The perishability derives from chemical changes, which include oxidative reactions, lipid oxidation, and enzymatic actions as well as microbiological attack. All of these can degrade food and make it unfit or unsafe for human consumption.

Packaging material must fulfill many specifications. If the physical or chemical deterioration is related to the equilibrium moisture content, the barrier properties of the package relating to water vapour will be of major importance in maintaining or extending shelf life. The oxygen concentration in a permeable package will directly affect the rate of oxidation of oxygen-sensitive nutrients such as vitamins, fatty acids and proteins. The loss of specific aroma or flavour constituents due to permeation can also lead to a reduction of product quality. Microbiological action accelerates in the presence of oxygen content in the package or with the permeation of fresh oxygen into the package.

Polyethylene terephthalate, commonly known as PET made its commercial debut in 1974. It has now become the preferred packaging material and is a ubiquitous part of the 21st century supermarket landscape. PET has replaced glass as the standard packaging for products such as carbonated soft drinks, bottled water and cooking oil. PET has many advantages in terms of strength, clarity, lightness and low cost. However it has a major limitation due to its permeability to oxygen and CO₂, which makes the PET container unsuitable for sensitive products like wine and beer, which demands extended shelf life. The problem is further compounded by the fact that gas permeability is directly proportional to surface area. Hence smaller the container, the greater the rate of gaseous passage and product degradation.

Industry is experimenting with various ideas to enhance the gas barrier properties of PET and other relatively inexpensive polymer packaging materials. Polymer properties can be improved by addition of an inorganic film on their surface. The inorganic film can serve as a gas diffusion barrier, as well as strengthen the polymer. Several types of coatings, which meet the above requirements, have been developed for food packaging. These include 1 - 6 µm thick SiO_x and Al₂O₃ as well as carbon film coatings. The development of the coating process has taken two directions. In one, the traditional thin film web-metalising process has been modified into handling oxide coatings. In the other, the finished products like bottles or containers are coated either inside or outside.

SiO_1.8 is the widely used glass barrier coating because of its unusual glass-forming ability. The film is polymer-like and the lower oxygen stoichiometry improves barrier properties. It has flexible bonds, which imparts high elongation for the coating and resistance to crystallisation during coating. The low refractive index inhibits glare from the surface. The oxygen transmission of coated film is in the range of 0.02-0.06 g/100 square inch/day and the water vapour transmission is 0.05-0.07 g/100 square inch/day. Additives like oxides of magnesium, barium, boron, germanium, zinc and titanium can enhance the barrier and chemical resistance properties.

The barrier coating can be applied externally or internally. The choice is determined by a number of considerations. Different areas of a PET container have different gas permeability, due to variances in the extent of biaxial orientation of molecules within the plastic itself. External coatings are more prone to mechanical damage. However, they give an option of introducing a second process to increase the coating’s resistance to scratching. Internal coating has the added advantage of providing a barrier to migration from the bottle material itself. However, the coating being in contact with the product, their mutual interaction must be considered. Internal coatings on refill bottles are subjected to very aggressive cleaning processes, which may cause the coating to break away.

Many thin film coating processes developed for more demanding applications in optical devices and surface engineering products have been adapted to barrier coating applications. These are primarily reactive sputter deposition or electron beam evaporation of aluminium or SiO₂, Plasma Enhanced Chemical Vapour Deposition (PECVD) of SiO₂ and diamond like carbon.

In the Dual Magnetron Sputtering, two magnetron sources are connected to a bipolar pulse generator so that each magnetron alternatively acts as the cathode and an anode of magnetron discharges [2]. This process significantly reduces arcs prevalent in dielectric coating with the result that reactive sputtering of oxides at high rates becomes possible. The reactive sputtering of oxides lead to the formation of negative ions of oxygen, which, accelerated by the cathode fall, impinge the substrate with high energy. As a result, very dense layers with high hardness are deposited [3].

Evaporative PVD is done in an atmosphere permeated by metal vapours produced by arc evaporation on a metal surface or by irradiating the surface by electrons beams produced by hollow cathode discharge. The very high density that can be produced by electron beam discharges is due to the magnetic trapping of the primary electrons by a longitudinal magnetic field [4]. Low energy electrons produced by the ionisation of neutral atoms and scattering of the energetic electrons along with the ions diffuse across the magnetic field and permeate to the substrate region. Deposition rates of 100 nm/sec are common.

Hollow Cathode Activated Deposition (HAD) process is based on the reactive evaporation of oxide or metal at high rates in combination with a hollow cathode plasma activation. The hollow cathode plasma source generates an arc discharge plasma with very high plasma densities of the order of 10¹²cm^-3. The typical deposition rates are 100 - 150 nm/s for Al₂O₃ and 300 – 600 nm/s for SiO_x. The deposited layers show a dense, amorphous structure. The microhardness is typically 6 GPa for Al₂O₃ layers and 3 GPa for SiO_xrespectively. Both oxides perform well against abrasion.

Plasma polymerisation is the process of building up polymer-like layers of organic or inorganic materials on substrates [5]. This process belongs to the class of plasma enhanced chemical vapour deposition processes (PECVD). In PECVD, the vapours of the desired ingredient are introduced into a plasma where the electrons ionise or fragment the molecules into radicals. These active molecules can undergo chemical reactions on the surface or in the vapour phase and finally deposit as films. The nucleation process depends on the surface morphology and presence of foreign atoms on the surface. Plasma deposited organo-silicon films can be deposited by dissociating silicone resins in a plasma and reacting the silicon atoms with oxygen, nitrogen or their combination to deposit silicon dioxide, silicon nitride or silicon oxynitride films. The precursors for diamond like carbon films are organic gases like acetylene.

Plasma Impulse CVD (PICVD) process is a significant improvement over conventional CVD. The plasma is pulsed by pulsing the power source, typically RF or microwave. This allows for ions to reach lower energies during the coating process. The coating is built up in a series of small steps, which produces an extremely dense and homogeneous coating. The chemical composition of the reaction mixture can be changed between pulses. Consequently, in the course of one process operation, different layers can be combined to produce a made-to-measure multilayer system. PICVD process for SiO₂ and TiO₂ has been applied to a broad range of plastics (e.g. PET, PMMA, PC, COC, PP and HDPE).

Diamond like carbon coating has also been used in barrier films. The coating process consists of the deposition of a very thin, diamond-like layer of carbon on the interior of the PET container. The bottle is first enclosed in a vacuum chamber. Acetylene (C₂H₂) gas is then injected into the bottle. Radio frequency energy is next applied to create a low temperature plasma state. The carbon ions coalesce on the inner surface of the bottle in an amorphous structure. Finally, waste gases are purged with nitrogen before the newly coated bottle emerges from the chamber. The final thickness of the coating is between 0.02 and 0.04 microns.

References

[1] Man CMD, Jones AA (Eds) Shelf Life Evaluation of Food, Blackie Academic & Professional, 1997

[2] Metzner Chr, Scheffel B, Goedicke K, Surf. Coat. Technol. 86/87, 769, 1996

[3] Schiller S, Kirchoff V, Schiller N, Morgner H, Proc. European Research Society Spring Meeting, June 1-4, p 205, 1999

[4] Schiller S, Heising U, Panzer S, Electron beam Technology, Wiley, New York, 1982

[5] Biderman H, Osada Y, Plasma Polymerisation Processes, Elsevier, New York, 1992

Development of Dusty Plasma System for SINP

Dusty plasma is a flourish subject for research field. Dusty plasmas occur frequently in space, and are thought to play an important role in the formation of astronomical entities like stars, planets etc. Most research in space plasmas has focused on Saturn's rings, and in the tails of comets. In these two regions, there is a high density of dust, and dusty plasma interaction may be responsible for much of the structure observed by spacecraft missions. The industrial community has also encouraged the study of dusty plasma. Plasmas are used to produce microchips, thin film coatings and hardened metals. Dusty plasma is three-component plasma, which consists of micron sized charged dust particles, ions, and electrons. Sometimes its called complex plasma. Dust particle can be of different size distribution.

When dust is introduced in the plasma it gets charged due to interaction with background plasma. Because of the high mobility of electrons dust gets negatively charged according to plasma parameters as shown in Figure 1. The levitation of dust particles is basically due to balancing of force due to sheath electric field and gravitational field. Several other forces also affect it as shown in Figure .2.

Fig.1 Fig.2

A typical experimental set-up of dusty plasma system designed, developed at FCIPT was supplied to Saha Institute for Nuclear Physics. The system is shown in Figure 3. The system consists of a S.S. vacuum chamber of 300 mm diameter and 300 mm height. The system is evacuated by rotary pump. The discharge is produced between a cylindrical hollow cathode and axial anode with argon as the plasma forming gas. The desired pressure is attained by suitably feeding Ar gas into the chamber through a gas dosing valve and adjusting the pumping speed of the rotary pump. Laser (He-Ne, l = 543 nm) light with cylinderical lens together with CCD camera forms the diagnostics. The forward scattered light is captured into PC with the help of digital video editor card.

Fig. 3 Dusty Plasma Experimental Set-up Fig. 4. I -V Characteristics of langmuir probe

Plasma parameters viz. electron temperature, ion density are measured in this system with the help of Langmuir probe circutary. Figure 4 shows the I ~ V characteristics of langmuir probe. The electron temperature is measured by calculating the slope of the transition region of the I-V curve. The ion density was calculated by substituting the values of electron temperature and ion saturation current, which was measured by langmuir probe. The measured value of electron temperature in the normal experimental conditions is in range of 2 - 6 eV and ion density of the order 10⁹ per cc.

Exploring Electromagnetic Analysis in ANSYS

The method of finite elements is being used commonly for solving problems in structural, thermal and fluid domains. In general, software packages like ANSYS, ANSOFT, NISA, IDEAS, ProE, etc., are extensively used for the purpose. A detailed study has been explored at IPR on the enhanced feature of solving electromagnetic problems that are available with the multiphysics module of ANSYS software. The electromagnetic domain in ANSYS includes solving of problems in the fields of :

· Electrostatics	· Magnetostatics
· Electromagnetics	· MEMS
· High voltage simulations	· Circuit analysis
· RLC parameter calculation	· HF analysis
· Coupled Field Interactions

With the wide applications on the areas of electromagnetics and their applications in the magnetic fusion research at IPR, it becomes inevitable to use such software packages like ANSYS and ANSOFT in addition to the user made electromagnetic codes that are developed and used at IPR. A number of benchmark problems in all the above fields has been solved using ANSYS and the results are compared with the standard or calculated results. The solutions obtained from ANSYS for problems in RLC parameter calculations, electrostatic and magnetostatic have revealed fairly good agreements with the calculated results.

Electromagnetic Analysis :

A major application of the software is to calculate eddy currents in the surrounding conducting structures of the toroidal plasma in Steadystate Superconducting Tokamak(SST-1). The toroidal plasma may move vertically or radially due to instabilities causing an induction of transient currents in the surrounding structures. This may lead to disruption conditions, that is, the plasma current of about 220 kA will suddenly disrupt to zero within few tenths of milliseconds thus inducing a huge currents onto the structures surrounding the plasma. The output of the induced currents on the surrounding stabilizer structures of plasma is shown in Figure 1 and 2.

Eddy Current Analysis on Plasma Facing Components of SST-1

Fig.2 Inner Passive Stabilizer

Fig.1 Outer Passive Stabilizer

Coupled Field Circuit Simulation:

Typical cases like current distribution phenomenon on super-conducting components like the current leads of SST-1, or the joints of TF coils of SST-1 becomes necessary for the calculation of joint resistance or heat loss or force distribution acting on the joint with the external magnetic field and so on. A coupled physics circuit simulation is performed using ANSYS for solving such critical problems. The ANSYS Circuit capability allows the user to combine both lumped elements where appropriate, with a "distributed" finite element model in regions where characterization requires a full finite element solution.

Micro-electromechanical systems (MEMS):

With the advent of wide applications on micro-electromechanical systems (MEMS) on the optical switches, high frequency RF components, the coupled analysis for such micro-components becomes essential. A typical analysis for a electro-thermal actuator for micro-electromechanical systems (MEMS) is carried out using ANSYS and the results are shown in Figure 3. The electro-thermal actuator of about 200 micron size has deflected to about 11 microns, which is near to the expected value from various experiments conducted elsewhere, when the applied voltage is 5 volts between the arms.

MEMS Analysis

Fig.3 Electro-thermal Actuator

Applications :

Thus, the above illustrated Electromagnetics simulation work on ANSYS brings out the confidence on solving coupled field applications with electromagnetics for various applications apart from structural, thermal and fluid works on ANSYS.

Pulsed Thermography and Non Destructive Testing of Coatings

From time immemorial surfaces have been coated with material different from the substrate, not only for improving aesthetics but also for increasing the resistance for corrosion, rusting, photo degradation, weathering etc. Coating may also be performed to change the absorption and reflection coefficients of surfaces. The coated material may be epoxies, polymers, paints, ceramics, metals etc. The coated substrate may be any material like metals, wood, plastics, paper, ceramics etc.

In many of these coating processes it becomes imperative to control coating properties, like thickness of the coating, coating and adhesion defects, micro structural changes, aggregation of pores as well as metal and oxide inclusions, during manufacturing to deliver the designed functionalities. Hence, it becomes necessary to have some non-destructive evaluation and testing (NDE and NDT) method, not only to validate the design but also during production, preventive maintenance and lifetime assessment stages of these coatings.

Though conventional NDE techniques like radiography, ultrasonic, eddy current measurements can be pressed in to service, they all have their own advantages and disadvantages [1]. For this purpose one looks for a technique which is simple, cost effective, fast and accurate in evaluating the parameters of coating. The newly emerging technique called Pulsed Thermography has all these advantages.

Pulsed Thermography:

Thermography is a technique based on the measurement of surface temperature of objects using infrared cameras. This is a non-invasive, non-destructive and non-contact measurement technique. In Pulse Thermography (PT) the surface is pulse heated by sources, like flash lamps, halogen lamps, air or water jet etc., and the resulting thermal transient at the surface is measured by an infrared camera [2]. The heat flow into the surface is perturbed by the presence of subsurface defects, owing to the difference in the thermal properties of the defects as compared to the bulk. This is detected as the temperature contrast at the surface that is recorded by the camera. A typical set up is shown in Fig. 1.

Fig. 1 Block diagram of experimental setup of Pulsed Thermography.

Contents

Editor's note

Facilitation Centre for Industrial Plasma Technologies

Development of Dusty Plasma System for SINP