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
Primary process
Depends on the cross-section and the electron energy (EEDF, E/N)
Ionization, dissociative ionization, electron attachment, dissociative electron attachment, excitation, dissociation.
EEDF
Electron mean energy is determined by reduced electrical field E/M
Secondary process
Reactive species generated from primary process collide with heavy particles. Collision has low Ea so the reaction rate is larger.
Plasma catalysis
Can have two configurations:
Two-stage - catalyst is in a post plasma zone
Single stage - catalyst is inside the plasma zone
The goal is to find synergy between the plasma and catalyst.
Plasma to catalyst interaction
Hot spot formation Change in morphology and physicochemical characteristics Change in surface process: Reduce coke formation and poisoning etc Reaction path way Lower activation barrier
Catalyst to plasma interaction
Electric field enhancement Formation of microdischarges in pores Change in discharge type Change in plasma density Adsorption of plasma generated species
Why do we perform reactions in plasma?
Mild reaction condition (low temperature, atmospheric pressure
Instant control (on-off, modulation)
Available in small scale, portable, for remote areas
Easy to be coupled with renewable energy
Green environmental profile
Process versatility
Low investment and operation cost
Electrical diagnostics
Methods:
Measurement of electric characters (V,I)
Circuit analysis
Energy calculation
Optical diagnostics
Optical emission spectroscopy (OES)
Optical absorption spectroscopy (OAS)
Laser induced fluorescence (LIF)
Additional methods
Intrusive: Langmuir probe etc,
Non-intrusive: Not interfere with plasma itself, suitable for hush environment (exposure to high voltage, strong electric field, possible high temperature).
DBD reactor
When there is no plasma it can be represented by two capacitors in a series. The green dash line relresents bhrning voltage - voltage at which plasma is generated
Lissajous figure
A-B, D-C - reactor capacitance
B-C, D-a - dielectric layer capacitance (effective capacitance)
Ina apacked bed reactor it is an almond shape, because the effective capacity gradually changes and the slope increases. The packings occupy space, which means less gaps and the breakdown voltage is changed.
Te
Electron temperature (Te) is one of the most informative and basic parameters in
plasma because electrons are not only involved in the excitation, dissociation, and ionization
of atoms and molecules but also govern the chemical reactions inside the plasma. It shows the state of plasma and is a measure of Ek. Rekated to electronic exitation temperature.
Electrostatic Langmuir probe
For measurement of electron temperature. The probe is inserted and generates current , the voltage changes typically linearly and an analysis is done by looking into the 2nd derivative.
The method is limited due to
• the difficulty in analysis in the presence of negative ions,
• In case of fast change of plasma parameters,
• its difficult use for small volume plasmas
Laser induced Rayleigh scattering
For measurement of gas temoerature. A laser source of 532 nm goes through plasma and interacts with species inside.
2 ways of interaction: reflection and transmission
2 signals: background and plasma-on
OES
Optical radiations emitted from the plasma are guided into spectrometric system, followed by detection with opto-electric receivers, which means that the plasma is not perturbed with the measurement.
If the plasma is in thermodynamic equilibrium, the observed line intensities give the number densities of the excited states, which directly shows the temperature as the slope of the Boltzmann plot, because they obey the Boltzmann distribution.
If the radiation emission in the state of non-equilibrium, the excitation kinetics in the plasma must be
understood
Collision Radiative model
An excitation kinetic model that describes the relationship between the excited-state populations and the plasma parameters is described. All the ions in the plasma are assumed to be singly charged ones, and the discharge plasma of a pure monatomic gas species.
Contains the sum of all processes:
excitation and deexcitation by electron collisions
population by radiative decay from the higher level q >p and depopulation to the lower level
ionization loss from the level p bybelectron collisions and its reversal process, three-body recombination
radiative recombination to the level p.
FTIR
Used to monitor the progress of the reaction and identify the various intermediate species that are formed during the course of the reaction.
Advantages:
Majority of molecules in NTP-plasma absorb mid-infrared light, making it a highly useful tool.
Highly sensitive, and could be quantitative.
Relatively inexpensive compared to other methods.
Disadvantages:
Cannot detect atoms or monoatomic ions as single atomic entities contain no chemical bonds.
Water vapour containing mixtures are very difficult to analyse as water is a strong IR absorber
Cannot detect non-polar molecules
TDLAS
TDLAS is focused on a single absorption line in the absorption spectrum of a particular species of interest. The wavelength of a diode laser is tuned over a particular absorption line of interest and the intensity of the transmitted radiation is measured.
The transmitted intensity can be related to the concentration of the species present by the Beer-Lambert law.
One component = one laser
