Plasma Treatment of Textiles
.
DR.Mohamed A. Ramadan
Associate prof.- Textile Research Division
National Research center
1-History of Plasma
Plasma is by far the most common form of matter. Plasma in the stars and in the tenuous space between them makes up over 99% of the visible universe and perhaps most of that which is not visible (Fig.1).
Fig. 1
On earth we live upon an island of “ordinary” matter. The different states of matter generally found on earth are solid, liquid, and gas. We have learned to work, play, and rest using these familiar states of matter. Sir William Crookes, an English physicist, identified a fourth state of matter, now called plasma, in 1879.The word “PLASMA” was first applied to ionized gas by Dr. Irving Langmuir, an American chemist and physicist, in 1929.While his relating the term to blood plasma has been acknowledged by colleagues who worked with him at the General Electric Research Laboratory, the basis for that connection is unclear. One version of the story has it that the similarity was in carrying particles, while another account speculated that it was in the Greek origin of the term, meaning “to mold,” since the glowing discharge usually molded itself to the shape of its container. In any case, it appears that the first published use of the term was in Langmuir’s “Oscillations in Ionized Gases,” published in 1928 in the Proceedings of the National Academy of Sciences. Thus the term “plasma” was first used to describe partially (if not weakly) ionized gases. The term plasma apparently did not find immediate widespread use in the scientific community. It did eventually catch on, however, but in some cases the term was inappropriately limited to highly ionized gas.
2-What is Plasma?
In physics and chemistry, a plasma is typically an ionized gas. Plasma is considered to be a distinct state of matter, apart from gases, because of its unique properties. “Ionized” refers to presence of one or more free electrons, which are not bound to an atom or molecule. The free electric charges make the plasma electrically conductive so that it responds strongly to electromagnetic fields. Plasma typically takes the form of neutral gas-like clouds (e.g. stars) or charged ion beams, but may also include dust and grains (called dusty plasmas). They are typically formed by heating and ionizing a gas, stripping electrons away from atoms, thereby enabling the positive and negative charges to move more freely. In an ordinary gas each atom contains an equal number of positive and negative charges; the positive charges in the nucleus are surrounded by an equal number of negatively charged electrons, and each atom is electrically “neutral.” A gas becomes a plasma when the addition of heat or other energy causes a significant number of atoms to release some or all of their electrons. The remaining parts of those atoms are left with a positive charge, and the detached negative electrons are free to move about. Those atoms and the resulting electrically charged gas are said to be “ionized.” When enough atoms are ionized to significantly affect the electrical characteristics of the gas, it is a plasma. Plasma consists of a collection of free-moving electrons and ions - atoms that have lost electrons. Energy is needed to strip electrons from atoms to make plasma. The energy can be of various origins: thermal, electrical, or light (ultraviolet light or intense visible light from a laser). With insufficient sustaining power, plasmas recombine into neutral gas. Plasma can be accelerated and steered by electric and ma gnetic fields which allows it to be controlled and applied. Plasma research is yielding a greater understanding of the universe. It also provides many practical uses: new manufacturing techniques, consumer products, and the prospect of abundant energy (fig.2).
Fig. 2
3-Plasma Parameters
3-1-Degree of ionization
For plasma to exist, ionization is necessary. The word “plasma density” by itself usually refers to the electron density, that is, the number of free electrons per unit volume. The degree of ionization of a plasma is the proportion of atoms which have lost (or gained) electrons, and is controlled mostly by the temperature. Even a partially ionized gas in which as little as 1% of the particles are ionized can have the characteristics of a plasma (i.e. respond to magnetic fields and be highly electrically conductive). The degree of ionization, a is defined as a = ni / (ni + na) where ni is the number density of ions and na is the number density of neutral atoms. The electron density is related to this by the average charge state <Z> of the ions through ne=<Z> ni where ne is the number density of electrons (fig.3).
Fig.3
3-2-Temperatures
Plasma temperature is commonly measured in kelvins or electronvolts, and is (roughly speaking) a measure of the thermal kinetic energy per particle. In most cases the electrons are close enough to thermal equilibrium that their temperature is relatively well-defined, even when there is a significant deviation from a Maxwellian energy distribution function, for example due to UV radiation, energetic particles, or strong electric fields. Because of the large difference in mass, the electrons come to thermodynamic equilibrium among themselves much faster than they come into equilibrium with the ions or neutral atoms. For this reason the ion temperature may be very different from (usually lower than) the electron temperature. This is especially common in weakly ionized technological plasmas, where the ions are often near the ambient temperature. Based on the relative temperatures of the electrons, ions and neutrals, plasmas are classified as thermal or non-thermal. Thermal plasmas have electrons and the heavy particles at the same temperature i.e. they are in thermal equilibrium with each other. Non thermal plasmas on the other hand have the ions and neutrals at a much lower temperature (normally room temperature) whereas electrons are much “hotter”.Temperature controls the degree of plasma ionization. In particular, plasma ionization is determined by the electron temperature relative to the ionization energy (and more weakly by the density) in accordance with the Saha equation. A plasma is sometimes referred to as being hot if it is nearly fully ionized, or cold if only a small fraction (for example 1%) of the gas molecules are ionized (but other definitions of the terms hot plasma and cold plasma are common). Even in a “cold” plasma the electron temperature is still typically several thousand degrees Celsius. Plasmas utilized in plasma technology (“technological plasmas”) are usually cold in this sense.
3-3-Electro magnetic Fields
In analysis, plasmas are far harder to model than solids, liquids, and gases because they act in a self-consistent manner. The separation of electrons and ions produce electric fields and the motion of electrons and ions produce both electric and magnetic fields. The electric fields then tend to accelerate plasmas to very high energies while the magnetic fields tend to guide the electrons. Both of these mechanisms,the accelerated (or fast) electrons and the magnetic fields produce what is called sychrotron radiation, so called because it was first discovered in large magnetized containers of electrons beams in laboratories on earth. Because of their self-consistent motions, plasma are rampant with instabilities, chaosity, and nonlinearities. These also produce electric and magnetic fields but also electromagnetic radiation. For example, all beams of electrons produce microwaves.
4-Common Artificial Plasma.
Most artificial plasmas are generated by the application of electric and/or magnetic fields. Plasma generated in laboratory setting and for industrial used can be generally categorized by:
1) The type of power source used to generate the plasma; DC, RF and microwave.
2) The pressure at which they operate; vacuum pressure
(< 10mTorr), moderate pressure ( ~ 1 Torr), and atmospheric pressure (760 Torr).
3) The degree of ionization within the plasma; fully ionized,
partially ionized.
4) The temperature relationships within the plasma; Thermal plasma (Te = Tion = Tgas), Non-Thermal or ‘cold’ plasma (Te >> Tion = Tgas)
5) The electrode configuration used to generate the plasma.
6) The magnetization of the particles within the plasma; Magnetized (both ion and electrons are trapped in larmour orbits by the magnetic field), partially magnetized (The electrons but not the ions are trapped by the magnetic field), non-magnetized (the magnetic field is too weak to trap the particles in orbits but may generate lorentz forces).
5-Examples of Industrial/Commercial Plasma
Low Pressure Discharges:
Glow Discharges: Non-thermal plasmas generated by the application of DC or low frequency RF (<100 kHz) electric field to the gap between two metal electrodes. Probably the most common plasma it is type of plasma generate within fluorescent light tubes.
Capactively Couple Plasmas (CCP): Similar to Glow Discharges but generated with high frequency RF electric fields, typically 13.56 MHz.It differs from Glow discharges in that the sheaths are much less intense. These are widely used in the micro
fabrication and integrated circuit manufacturing
industries for plasma etching and plasma enhanced chemical vapor deposition.
Inductively Coupled Plasmas (ICP): Similar to a CCP and with similar applications but the electrode consists of a coil wrapped around the discharge volume which inductively excites the plasma.
Atmospheric Pressure
Arc: This is a high power thermal discharge of very high temperature ~10000K. It can be generated using various power supplies. It is commonly used in metallurgical processes. For example it is used to melt rocks containing Al2 03 to produce Aluminum.
Corona:This is a non-thermal discharge generated by the application of high voltage to sharp electrode tips. It is commonly used in ozone generator and particle precipitators.
Dielectric Barrier Discharge (DBD):Invented by Siemans this is a non-thermal discharge generated by the application of high voltages across small gaps wherein a non-conducting coating prevents the transition of the plasma discharge into an arc. It is often mislabeled ‘Corona’ discharge in industry and has similar application to Corona Discharges. It is also widely used in the web treatment of fabrics. The application of the discharge to synthetic fabrics and plastics functionalizes the surface and allows for paints, glues and similar materials to adhere.
6-Plasma Chemistry
Most processing plasmas are created in a molecular gas or a gas mixture that contains molecules. There are therefore a very large number of constituent species. The energy required to remove a single electron from a molecule or atom is known as the ionization potential. The energies required to break molecules into various constituent parts are called the dissociation energies. Just as in a neutral gas, the moving charged and neutral particles are colliding or interacting and reacting. The presence of energetic charged particles, particularly the electrons, lead to unique plasma chemistry. There are a huge number of interactions that can take place, particularly in a molecular plasma. Table 1 lists some of the important types of reactions that occur in plasmas where most of the collisions involve only two particles.
