Microplasma

Question 1

What is plasma?

Answer 1

The fourth state of matter; ionized gas.

Question 2

Ways of creating plasma?

Answer 2

Gas discharge is the most common way of ionizing gas. There are two mechanisms: Townsend and Streamer mechanism.

Question 3

Townsend mechanism

Answer 3

Gas ionization process where free electrons are accelerated by an electrical field, collide with gas molecules and consequently free additional electrons.

Question 4

Steamer mechanism

Answer 4

Streamers are plasma channels composed ofionized air molecules, which repeatedly strike out from the electrode into the air. Creates strictlz non-thermal plasma.

Question 5

Types of plasma

Answer 5

Thermal, non-thermal

High-low density/temperature/pressure

Question 6

Thermal plasma

Answer 6

Characterized by thermal equilibrium between electrons, ions and neutrals.
High temperature (10s of thousands of degrees K) and difficult to control.
High cost

Question 7

Non-thermal pasma

Answer 7

Energy is delivered primarily to the electrons insted of heating up gas, so Te is the highest.
Lower temeprature, sometimes room temperature.
Lower costs.

Question 8

Dielectric barrier discharge, reactor configurations

Answer 8

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

Question 9

Microdischarge filaments

Answer 9

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.

Question 10

Corona discharge, gliding arc, plasma jet, transient spark

Answer 10

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

Question 11

Microplasma

Answer 11

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.

Question 12

Paschen’s curve

Answer 12

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.

Question 13

G-L microfluid plasma reactors

Answer 13

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

Question 14

Non-thermal plasma chemistry

Answer 14

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.

Question 15

Collision process

Answer 15

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

Question 16

Primary process

Answer 16

Depends on the cross-section and the electron energy (EEDF, E/N)
Ionization, dissociative ionization, electron attachment, dissociative electron attachment, excitation, dissociation.

Question 17

EEDF

Answer 17

Electron mean energy is determined by reduced electrical field E/M

Question 18

Secondary process

Answer 18

Reactive species generated from primary process collide with heavy particles. Collision has low Ea so the reaction rate is larger.

Question 19

Plasma catalysis

Answer 19

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.

Question 20

Plasma to catalyst interaction

Answer 20

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

Question 21

Catalyst to plasma interaction

Answer 21

Electric field enhancement
Formation of microdischarges in pores
Change in discharge type
Change in plasma density
Adsorption of plasma generated species

Question 22

Why do we perform reactions in plasma?

Answer 22

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

Question 23

Electrical diagnostics

Answer 23

Methods:
Measurement of electric characters (V,I)
Circuit analysis
Energy calculation

Question 24

Optical diagnostics

Answer 24

Optical emission spectroscopy (OES)
Optical absorption spectroscopy (OAS)
Laser induced fluorescence (LIF)

Question 25

Additional methods

Answer 25

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).

Question 26

DBD reactor

Answer 26

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

Question 27

Lissajous figure

Answer 27

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.

Question 28

Te

Answer 28

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.

Question 29

Electrostatic Langmuir probe

Answer 29

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

Question 30

Laser induced Rayleigh scattering

Answer 30

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

Question 31

OES

Answer 31

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

Question 32

Collision Radiative model

Answer 32

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.

Question 33

FTIR

Answer 33

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

Question 34

TDLAS

Answer 34

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