The Gel Point is the point at which an infinite polymer network first appears. If we assume we are able to measure the extent of reaction, p, defined as the fraction of monomers that appear in cross-links, we can determine the gel point.[1] [2] The critical extent of reaction for the gel point to be formed is given by:
• pc=1/(N-1)≈1/N
For example, a polymer with N≈200 is able to reach the gel point with only 0.5% of monomers reacting. This shows the ease at which polymers are able to form infinite networks. The critical extent of reaction for gelation can be determined as a function of the properties of the monomer mixture,r, ρ, and f:
• pc = 1/(r+rp(f-2))0.5
Thursday, July 31, 2008
Tuesday, July 29, 2008
Plasma Heating
In an operating fusion reactor, part of the energy generated will serve to maintain the plasma temperature as fresh deuterium and tritium are introduced. However, in the startup of a reactor, either initially or after a temporary shutdown, the plasma will have to be heated to 100 million degrees Celsius.
In current tokamak (and other) magnetic fusion experiments, insufficient fusion energy is produced to maintain the plasma temperature. Consequently, the devices operate in short pulses and the plasma must be heated afresh in every pulse.
Ohmic Heating:
Since the plasma is an electrical conductor, it is possible to heat the plasma by passing a current through it; in fact, the current that generates the poloidal field also heats the plasma. This is called ohmic (or resistive) heating; it is the same kind of heating that occurs in an electric light bulb or in an electric heater.
The heat generated depends on the resistance of the plasma and the current. But as the temperature of heated plasma rises, the resistance decreases and the ohmic heating becomes less effective. It appears that the maximum plasma temperature attainable by ohmic heating in a tokamak is 20-30 million degrees Celsius. To obtain still higher temperatures, additional heating methods must be used.
Neutral-Beam Injection:
Neutral-beam injection involves the introduction of high-energy (neutral) atoms into the ohmically -- heated, magnetically -- confined plasma. The atoms are immediately ionized and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, thus increasing the plasma temperature.
Radio-frequency Heating:
In radio-frequency heating, high-frequency waves are generated by oscillators outside the torus. If the waves have a particular frequency (or wavelength), their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma
In current tokamak (and other) magnetic fusion experiments, insufficient fusion energy is produced to maintain the plasma temperature. Consequently, the devices operate in short pulses and the plasma must be heated afresh in every pulse.
Ohmic Heating:
Since the plasma is an electrical conductor, it is possible to heat the plasma by passing a current through it; in fact, the current that generates the poloidal field also heats the plasma. This is called ohmic (or resistive) heating; it is the same kind of heating that occurs in an electric light bulb or in an electric heater.
The heat generated depends on the resistance of the plasma and the current. But as the temperature of heated plasma rises, the resistance decreases and the ohmic heating becomes less effective. It appears that the maximum plasma temperature attainable by ohmic heating in a tokamak is 20-30 million degrees Celsius. To obtain still higher temperatures, additional heating methods must be used.
Neutral-Beam Injection:
Neutral-beam injection involves the introduction of high-energy (neutral) atoms into the ohmically -- heated, magnetically -- confined plasma. The atoms are immediately ionized and are trapped by the magnetic field. The high-energy ions then transfer part of their energy to the plasma particles in repeated collisions, thus increasing the plasma temperature.
Radio-frequency Heating:
In radio-frequency heating, high-frequency waves are generated by oscillators outside the torus. If the waves have a particular frequency (or wavelength), their energy can be transferred to the charged particles in the plasma, which in turn collide with other plasma
Tuesday, July 22, 2008
Surface Activation Techniques
Surface activation is a part of surface chemistry. Some time surface chemistry is used to activate the surface of some thing in order to achieve good adhession and bonding property.
To achieve this several techniques are followed. These are:
1. Mechanical Etching: To get a rough surface as wel as higher energy stored at the surface in terms of electrostatic energy.
2. Chemical Etching: Some times suitable chemicals are used to activate and rough the upper surface.
3. Plasma Etching: Plasma etching is a form of plasma processing in which a high-speed stream of plasma is shot (in pulses) at a sample. The atoms of the shot element embed themselves at or just below the surface of the target. The physical properties of the target are modified in the process. Plasma systems ionize a variety of source gases in a vacuum system by using RF excitations. The frequency of operation of the RF power source is frequently of 13.56 MHz, chosen by the Federal Communications Commision (FCC) for industrial and scientific use. Nevertheless, it can be used lower frequencies (kilohertz) or higher (microwave). The mode of operation of the plasma system change if the operating pressure change. Also, it is different for different structures of the reaction chamber. Standard plasma etching work with very high pressures. In the simple case, the electrode structure is symmetrical, and the sample is placed upon the grounded electrode. Free radicals such as fluorine or chlorine are created in the plasma and react at the sample surface.Plasma processing is a plasma-based material processing technology that aims at modifying the chemical and physical properties of a surface.
Plasma processing techniques include:
• Plasma activation
• Plasma modification
• Plasma functionalization
• Plasma polymerization
• Plasma Surface Interactions
To achieve this several techniques are followed. These are:
1. Mechanical Etching: To get a rough surface as wel as higher energy stored at the surface in terms of electrostatic energy.
2. Chemical Etching: Some times suitable chemicals are used to activate and rough the upper surface.
3. Plasma Etching: Plasma etching is a form of plasma processing in which a high-speed stream of plasma is shot (in pulses) at a sample. The atoms of the shot element embed themselves at or just below the surface of the target. The physical properties of the target are modified in the process. Plasma systems ionize a variety of source gases in a vacuum system by using RF excitations. The frequency of operation of the RF power source is frequently of 13.56 MHz, chosen by the Federal Communications Commision (FCC) for industrial and scientific use. Nevertheless, it can be used lower frequencies (kilohertz) or higher (microwave). The mode of operation of the plasma system change if the operating pressure change. Also, it is different for different structures of the reaction chamber. Standard plasma etching work with very high pressures. In the simple case, the electrode structure is symmetrical, and the sample is placed upon the grounded electrode. Free radicals such as fluorine or chlorine are created in the plasma and react at the sample surface.Plasma processing is a plasma-based material processing technology that aims at modifying the chemical and physical properties of a surface.
Plasma processing techniques include:
• Plasma activation
• Plasma modification
• Plasma functionalization
• Plasma polymerization
• Plasma Surface Interactions
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