Microplasma Flashcards
What is plasma?
The fourth state of matter; ionized gas.
Ways of creating plasma?
Gas discharge is the most common way of ionizing gas. There are two mechanisms: Townsend and Streamer mechanism.
Townsend mechanism
Gas ionization process where free electrons are accelerated by an electrical field, collide with gas molecules and consequently free additional electrons.
Steamer mechanism
Streamers are plasma channels composed ofionized air molecules, which repeatedly strike out from the electrode into the air. Creates strictlz non-thermal plasma.
Types of plasma
Thermal, non-thermal
High-low density/temperature/pressure
Thermal plasma
Characterized by thermal equilibrium between electrons, ions and neutrals.
High temperature (10s of thousands of degrees K) and difficult to control.
High cost
Non-thermal pasma
Energy is delivered primarily to the electrons insted of heating up gas, so Te is the highest.
Lower temeprature, sometimes room temperature.
Lower costs.
Dielectric barrier discharge, reactor configurations
Consists of two electrodes stressed by high voltage, a dielectric barried that prevents the formation of gas spark and a gas gap in which the plasma forms.
Reactor configuration differ based on barrier position. Operational parameters: Pressure 0.1-3 bar Gap size: 0.1-5 mm Voltage applied: 1-30 kV Frequency 50-10^6 Hz
Microdischarge filaments
What creates plasma. Generally a cylindrical shape with a radius ~100 μm and duration~ 10–50 ns (completely filamentry discharge).
In packed bed reactors, a combined filamentry-predominant surface discharge is formed around the packings. This creates slightly different plasma, there is also interaction between the plasma and the packings.
Corona discharge, gliding arc, plasma jet, transient spark
Corona - happens at very sharp edges or tip of electrode where electric field is very divergent.
Gliding arc - arc ignition, gas pushes the arc and it glides to the top and is cur off in a certain position; non-thermal plasma, while arc is thermal plasma
Transient spark - non-thermal, but spark is thermal
Microplasma
A special category of plasma confined within sub milimeter length scale in at least one dimension. It’s obtained by increasing surface-to- volume ratio and decreasing electrode spacing.
Characteristics: high-pressure operation reduced voltage non-equilibrium chemistry microscale geometry
Microplasma is generated through gas discharge, a strong electrical field is needed.
Breakdown voltage - voltage necessary to start a discharge orelectric arc, between two electrodes in a gas; depends on gas composition, pressure amd distance between electrodes, is given by Paschen’s curve.
Paschen’s curve
With constant gap length, the voltage necessary decreases as the pressure decreases, and then increases gradually, exceeding its original value.
With a constant pressure, the voltage needed reduces as the gap size reduces but only to a point. As the gap is reduced further, the voltage required rises and again exceeds its original value.
Lowest breakdown voltage at atm pressure is obtained by confining plasma to a very small space.
G-L microfluid plasma reactors
DBD based in-situ generation of plasma - liquid and gas bubbles are in microchannel, an external electric field is applied and plasma is generated inside of the bubbles
Plasma bubble injection - plasma is injected into the channel
Non-thermal plasma chemistry
Initiated by electron impact, the rate of the process depends on the number of electrons with sufficient energy.
Inside of the plasma it isn’t possible to measure electron energy, so we measure EEDF - electron energy distribution function which describes the probability density for an electron to have the needed energy.
Collision process
Electrons acquire energy from an external electric field.
The collision can be:
Elastic - a very small amount of Ek is transferred and electrons are scattered
Inelastic - almost all Ek is transferred and heavy particles are ionized
