low temperature plasma chemical processes at atmospheric pr

1
Low Temperature Plasma Chemical Processes at
Atmospheric Pressure
Professor Sergey E. Alexandrov
2
Plasma can be defined as a quasi-neutral gas of
charged and neutral particles characterized by a
collective behavior
State of matter versus temperature
3
PLASMA TYPES
Plasma types by electron density and temperature
The plasma state exists in natural form in the
cosmos or is created under unique conditions for
specific purposes. The plasmas found in nature
cover a very large range of electron densities
and temperatures.
4
PLASMA TYPES
Taking into account the wide ranges of
parameters, the plasmas are classified into
several categories Plasmas in complete
thermodynamic equilibrium - CTE plasmas. In a CTE
plasma all temperatures discussed previously are
equal. Tg Tex Tion Td Tr Te Plasma in
local thermodynamic equilibrium - LTE plasmas
These are plasmas in which all temperatures,
except the radiation temperature, Tr, are equal
in each small volume of the plasma. Plasmas
that are not in any local thermodynamic
equilibrium - non-LTE plasmas. These plasmas, are
also named cold plasmas.
5
Thermal Plasmas
Cold Plasmas
Exist under two circumstances When the heavy
particles are very energetic, at temperatures of
the order of 106-108 K (102-104 eV) When the
pressure is atmospheric, even at temperatures as
low as 6000 K. For example, in electric arcs, or
in plasma jets operating at pressures of about I
atm, the temperature of the electrons is
approximately equal to that of the gas, Te Tg.
The temperature of the gas in the center of these
plasmas can reach values of 20,000 to 30,000 K.
Such LTE-plasmas at atmospheric pressure are
called thermal plasmas.
In low-pressure discharges, thermodynamic
equilibrium is not reached, even at a local
scale, between the electrons and the heavy
particles. In the non-LTE plasmas the temperature
of the electrons is much higher than that of the
heavy particles and Te gtgt Ti gt Tg gt Tex. The
electrons can reach temperatures of 104-105 K
(1-10 eV), while the temperature of the gas, Tg,
can be as low as room temperature. Therefore,
such plasmas are called cold plasmas. The cold
plasmas have been developed specifically and
purposefully based on their non-equilibrium
properties and their capability to cause physical
and chemical reactions with the gas at relatively
low temperatures.
6
Electron and ion temperatures as a function of
pressure
The temperatures tend to equilibrate as the
interaction between the two systems, that is,
electrons and heavy particles, increases. This
happens if either the pressure or the density of
the electrons in the plasma increase
7
Objectives
Non-equilibrium nonthermal plasma is generated in
the most common approach due to electrical
discharges in the gas phase at low pressure. In
this case effective electron temperature is much
higher than the gas temperature thus electrons
due to inelastic collisions with precursor
molecules provide formation of chemically active
species participating in the chemical reactions
under low temperature conditions. However the
use of low pressure nonthermal plasma is limited
because of high cost of vacuum equipment required
for its realization. In this connection the
development of atmospheric pressure non-thermal
plasma sources and industrial technologies based
on their use is of great interest.
8

• OVERVIEW OF PARAMETERS OF VARIOUS ATMOSPHERIC
  PRESSURE DISCHARGES

9
Development of various technologies for surface
treatment based on the use of low temperature
plasma at atmospheric pressure

• 
  Applications
• Cleaning of metal surfaces before painting (oil
  removing)
• Modification of wood surface
• Treatment of the surfaces of polymeric materials
  (improving
• adhesion of further deposited metal films,
  variation
• of hydrophilic or hydrophobic properties,
  cleaning of the surface)
• Low temperature deposition of thin layers of
  different materials
• (on glass, polymers, metals)

10
Dielectric Barrier Discharge
Common dielectric-barrier discharge
configurations
11
Atmospheric pressure PECVD of silicon dioxide
films using DBD discharges
A design of the precursor distributer for remote
atmospheric pressure PECVD of silicon dioxide
films
12
APECVD of silicon dioxide films from HMDSO and
HMDS
Dependence of growth rate on deposition
temperature. 1 HMDS, 2- HMDSO
Dependence of breakdown strength and refractive
index on electrical power.
13
APECVD of diamond-like layers
Schematic diagram of the reactor used for APECVD
of diamond films 1 HV electrode 2 ground
electrode 3 dielectric 4 reaction
14
Plasma enhanced CVD on polymers
Scanning electron microscopic image of polymer
surface (polyimide) after plasma deposition of
silicon oxide (the thickness of dendrite
branches is less than 50 nm)
15
Schematic view of model reactor for surface
modification
16
RF Discharges
RF needle plasma source
RF planar plasma source
17
Plasma Induced Surface Modification of Wood
Plasma modified samples exhibited very high water
contact angle values (contact angle 130
degrees) in comparison to the unmodified samples
(contact angle 15 degrees), indicating the
presence of a hydrophobic surface.
Schematic of capacitively-coupled cold plasma
reactor (A.R. Denes et al, Holzforschung, 53
(1999)318-326)
18
Plasma Induced Surface Modification of Wood
AFM images of unmodified wood surface (a), 5
minute plasma modified wood surface (b), 10
minute plasma modified wood surface (c).
19
PLASMA-MODIFIED WOOD FIBERS AS FILLERS IN
POLYMERIC MATERIALS
Strength vs. proportion of filler in composites
based on polyethylene for untreated (1) and
plasma-treated (2) wood fibers.
Schematic diagram of a reactor with vibrated bowl
20
Plasma Induced Surface Modification of Wood
1. Possibility to change sufficiently hydrophilic
and hydrophobic properties of wood surface (wood
protection, wood painting, more than 40 increase
in strength of glued laminated beam and
glu-lam) 2. Possibility to produce strong
composite materials with wood fillers 3.
Bacterial decontamination of wood surface
21
Air cleaning from the hazardous air pollutants
(HAPs), especially from volatile organic
compounds (VOCs), in a Non-Thermal Plasma at
atmospheric pressure
22
Conventional Pollution Removal Technologies

• process is selective to removal compound
• additional components, which need periodic
  replacement or regeneration
• process is discontinuous
• low efficiency of process

23
DBD reactor was used to destroy

• formic acid
• acetone
• toluene
• white spirit
• organosilicon compound hexamethyldisiloxane,
  Decamethylcyclopentasiloxane (D5)
• Organo-chloride perchloroethylene

Decomposition efficiency of VOCs 100
24
Gas phase analysis by IR Fourier (-transform)
spectrometer
IR Fourier(-transform) spectrometer 1201?
Multipass absorption cell 4.8 m
25
The main products of plasma decomposition of VOC
IR-spectrum of the products of D5 decomposition
in DBD plasma process (various applied voltage)
26

• Others applications of non-thermal plasmas
  sustained
• at atmospheric pressure
• Development of devices for removing of volatile
  organic
• compounds from air ( toxic VOCs, causing smell
  and etc.)
• - Bacterial decontamination of air
• Reduction of toxic gases in car exhaust line
  (possible)
• - Water purification from organic impurities.

27
Reduction of odors in air
Variation of FTIR spectra of the gas phase on
applied voltage.
FTIR spectra of air saturated with VOCs before
and after plasma treatment.
28
Reduction of odors in air
View on the model plasma reactor through
ventilation window of some device when plasma
switch and switch on (blue emission).
Dependence of degradation rate of VOCs during
plasma treatment on applied voltage
29
Water purification from organic impurities and
bacterial decontamination of water
Experimental set up for plasma treatment of fog.
HV discharge in water.
Up to 100 of phenol is destroyed
30
Atmospheric pressure PECVD reactor for synthesis
of nanoparticles
One of the promising directions is development
of atmospheric pressure PECVD technologies. Such
processes do not require expensive vacuum
equipment and can deposit quality oxide and
nitride films with high growth rates. Also
because of higher partial pressures of the
precursors one can expect to apply successfully
such processes for homogeneous formation of
nanoparticles. In figure one can see schematic
diagram of an AP PECVD reactor for synthesis of
nanoparticles of various materials developed.
31
Typical structure of SiO2 nano-powder obtained by
RPACVD of D5 at atmospheric pressure
SEM photography of SiO2 nanopowder formed.
32
YOU ARE WELCOME TO CO-OPERATION
THANKS FOR YOUR ATTENTION !