Review
Atmospheric pressure plasmas:A review
Claire Tendero a,*,Christelle Tixier a ,Pascal Tristant a ,Jean Desmaison a ,Philippe Leprince b
a
Laboratoire Sciences des Proce ´de ´s Ce ´ramiques et Traitements de Surfaces,UMR CNRS 6638,Universite ´de Limoges,France
1
b
翻译英文名Laboratoire de Physique des Gaz et des Plasmas,UMR CNRS 8578,Universite ´Paris XI,Orsay,France 2
Received 17July 2005;accepted 5October 2005
Available online 18November 2005
Abstract
This article attempts to give an overview of atmospheric plasma sources and their applications.The aim is to introduce,in a first part,the main scientific background concerning plasmas as well as the different atmospheric plasma sources (description,working principle).The cond part focus on the various applications of the atmospheric plasma technologies,mainly in the field of surface treatments.
Thus this paper is meant for a broad audience:non-plasma-specialized readers will find basic information for an introduction to plasmas whereas plasma spectroscopists who are familiar with analytical plasmas may be interested in the synthesis of the different applications of the atmospheric pressure plasma sources.D 2005Elvier B.V .All rights rerved.
Keywords:Plasma;Atmospheric;Review;Surface treatment;DBD;Corona;Torch
Contents 1.Introduction ..............................................................32.
Part A.Basic 32.1.Definitions ........................
...................................32.1.1.Plasma generation ..................................................32.1.2.Plasmas clas
<32.2.Atmospheric pressure plasmas:LTE or non-LTE?....
...................................42.2.1.LTE plasmas .....................................................42.2.2..42.2.3.Atmospheric pressure plasmas ............................................52.3.Overview of various atmospheric plasma sources ....
...................................52.3.1.DC and low frequency discharges ..........................................52.3.2.82.3.3.Microwave induced plasmas (MIPs).........................................112.3.4.Summary .......................................................163.
Part B.Applications of the various atmospheric plasma sources ...................................173.1.Spectroscopic analysis .....................................................183.2.Gas treatments ......................
...................................183.2.1.Gas cleaning .....................................................183.2.2.
...................................
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0584-8547/$-e front matter D 2005Elvier B.V .All rights rerved.doi:10.1016/j.sab.2005.10.003
*Corresponding author.Tel.:+33616777347;fax:+33555423680.E-mail address:tendero _claire@yahoo.fr (C.Tendero).1
ENSIL,16rue d’Atlantis,Parc d’ESTER Technopole,BP 6804,87068Limoges,France.2
LPGP,Universite ´Paris Sud,Bat 210,ave G.Clemenceau,91405Orsay Cedex,France.Spectrochimica Acta Part B 61(2006)2–
30
3.3.Material processing (20)
3.3.1.Surfaces treatments (20)
3.3.2.Surface coating (21)
3.3.3.Bulk material treatments (24)
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3.4.Lamps (25)
4.Conclusion (25)
gentle什么意思Acknowledgments (27)
References (27)
1.Introduction
Plasmas are chemically active media.Depending on the way they are activated and their working power,they can generate low or very high‘‘temperatures’’and are referred correspond-ingly as cold or thermal plasmas.This wide temperature range enables various applications for plasma technologies:
surface coatings,waste destruction,gas treatments,chemical synthesis, Nevertheless,many of the techniques have not been industrialized,albeit environmental norms are strictly followed.
Thermal plasmas(especially arc plasma)were extensively industrialized,principally by aeronautic ctor.Cold plasma technologies have been developed in the microelectronics but their vacuum equipment limits their implantation.
To avoid drawback associated with vacuum,veral labora-tories have tried to transpo to atmospheric pressure process that work under vacuum for the moment.Their rearches have led to various original sources that are described here.
This paper is a review about atmospheric plasmas.It is mainly about the technologies that are still under development in laboratories whereas arc and inductive plasmas are briefly discusd.
After a summary about the different kinds of plasmas,the various sources are described in term of design,working conditions and plasma properties.Then the study focus on their applications(spectroscopic analysis,gas treatments and materials proceedings)and concludes with a comparative synthesis of each system(applications,advantages,limits).
2.Part A.Basic and fundamentals
2.1.Definitions
Plasma is a more or less ionized gas.It is the fourth state of matter and constitutes more than99%of the univer.It consists of electrons,ions and neutrals which are in funda-mental and excited states.From a macroscopic point of view, plasma is electrically neutral.However,it contains free charge carriers and is electrically conductive.
2.1.1.Plasma generation
A plasma is created by applying energy to a gas[1]in order to reorganize the electronic structure of the species(atoms, molecules)and to produce excited species and ions.This energy can be thermal,or carried by either an electric current or electromagnetic radiations.
The atmospheric plasmas described in this paper are generated from electrical energy.The electric field transmits energy to the gas electrons(which are the most mobile charged species).This electronic energy is then transmitted to the neutral species by collisions.The collisions[2]follow probabilistic laws and can be divided in:
英语拼写规则&Elastic collisions:they do not change the internal energy of the neutral species but slightly ri their kinetic energy
&Inelastic collisions:when electronic energy is high enough, the collisions modify the electronic structure of the neutral species.It results in the creation of excited species or ions if the collisions are energetic enough.
Most of the excited species have a very short lifetime and they get to ground state by emitting a photon.The‘‘metastable species’’are also excited states but with a long lifetime becau their decay by emission of radiation is hampered as there are no allowed transitions departing from the respective state:decay can only take place by energy transfers through collisions.
2.1.2.Plasmas classification
Depending on the type of energy supply and the amounts of energy transferred to the plasma,the properties of the plasma change,in terms of electronic density or temperature.The two parameters distinguish plasmas into different categories, prented in e Fig.1.The atmospheric plasma sources described in this paper are suppod to be positioned near the glow discharges and the
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arcs.
Fig.1.2D classification of plasmas(electrons temperature versus electrons density)[5].
C.Tendero et al./Spectrochimica Acta Part B61(2006)2–303
In this classification,a distinction can be made between: &Local thermodynamic(or thermal)equilibrium plasmas (LTE)
&Non-local thermodynamic equilibrium plasmas(non-LTE).
2.2.Atmospheric pressure plasmas:LTE or non-LTE?
The Local Thermodynamic Equilibrium notion[3]is really important,especially for a spectroscopic study of the plasma, since the determination of the plasma parameters(particles distribution functions;electron,excitation,)is bad on relationships which differ for plasmas in LTE or not.
2.2.1.LTE plasmas
LTE plasma requires that transitions and chemical reactions are governed by collisions and not by radiative process. Moreover,collision phenomena have to be micro-reversible.It means that each k
ind of collision must be balanced by its inver(excitation/deexcitation;ionization/recombination;ki-netic balance)[4].
Moreover LTE requires that local gradients of plasma properties(temperature,density,thermal conductivity)are low enough to let a particle in the plasma reach the equilibrium: diffusion time must be similar or higher than the time the particle need to reach the equilibrium[5].For LTE plasma,the heavy particles temperature is clod to the electrons temper-ature(ex:fusion plasmas).
According to the Griem criterion[6],an optically thin homogeneous plasma is LTE if the electron density fulfills:
n e¼9:1023
E21
E Hþ
3kT
E Hþ
mÀ3
ÀÁ
where
alessi˝E21reprents the energy gap between the ground state and the first excited level,
˝E H+=13.58eV is the ionization energy of the hydrogen atom
˝T is the plasma temperature.
This criterion shows the strong link that exists between the required electron density for LTE and the energy of the first excited state.
Tho rules for LTE are very strict.Thus most of the plasmas deviate from LTE,especially all types of low density plasma in laboratories.
2.2.2.Non-LTE plasmas
Departure from Boltzmann distribution for the density of excited atoms can explain the deviation from
LTE.Indeed,for low-lying levels,the electron-induced deexcitation rate of the atom is generally lower than the corresponding electron-induced excitation rate becau of a significant radiative deexcitation rate[4].
Another deviation from LTE is induced by the mass difference between electrons and heavy particles.Electrons move very fast whereas heavy particles can be considered static: electrons are thus likely to govern collisions and transitions phenomena.Deviations from LTE are also due to strong gradients in the plasma and the associated diffusion effects.
It has been shown that the LTE distribution can be partial. For example,LTE can be verified for the levels clo to ionization threshold[7](e.g.,5p levels and higher,in argon plasma):such plasmas are pLTE(partial LTE).
The non-LTE plasmas can be described by a two-temperature model:an electron temperature(T e)and a heavy particle temperature(T h).Regarding the huge mass differ-ence between electrons and heavy particles,the plasma temperature(or gas temperature)is fixed by T h.The higher the departure from LTE,the higher the difference between T e and T h is.
Table1sums up the main characteristics of LTE and non-LTE plasmas.More details on LTE and devia
tions from LTE are developed in the books by Huddlestone and Leonard[8], Griem[9],Lochte-Holtgreven[10]and Mitchner and Kruger [11]
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Fig.2.Evolution of the plasma temperature(electrons and heavy particles)with the pressure in a mercury plasma arc[5].
Table1
Main characteristics of LTE and non-LTE plasma
LTE plasmas Non-LTE plasmas Current
name
Thermal plasmas Cold plasmas Properties T e=T h T e H T h
High electron density: 1021–1026mÀ3Lower electron density: <1019mÀ3
Inelastic collisions between electrons and heavy particles create the plasma reactive species whereas elastic collisions heat the heavy particles(the electrons energy is thus consumed)Inelastic collisions between electrons and heavy particles induce the plasma chemistry. Heavy particles are slightly heated by a few elastic collisions(that is why the electrons energy remains very high)
Examples [117]Arc plasma(core)Glow discharges
T e=T hå10,000K T eå10,000–100,000K
T hå300–1000K
C.Tendero et al./Spectrochimica Acta Part B61(2006)2–30
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2.2.
3.Atmospheric pressure plasmas
Fig.2shows the influence of the pressure on the transition from a glow discharge(T e>T h)to an arc discharge.
Low pressure plasmas(10À4to10À2kPa)are non-LTE. Heavy particles temperature is lower than the electronic one. The inelastic collisions between electrons and heavy particles are excitative or ionizing.The collisions do not ri the heavy particles temperature.
When the pressure becomes higher,collisions intensify. They induce both plasma chemistry(by inelas
tic collisions) and heavy particles heating(by elastic collisions).The difference between T e and T h is reduced:plasma state becomes clor to LTE but does not reach it.The significant gradient of properties in plasma restricts a particle,moving in the discharge,achieving equilibrium.
The density of the feeding power influences a lot the plasma state(LTE or not).On the whole,a high power density induces LTE arc plasmas)whereas non-LTE plasmas are favored by either a low density of feeding power or a puld power supply.In this latter ca, the short pul duration prevents the equilibrium state from establishing.
Finally,it is important to note that an atmospheric plasma jet can be divided in two zones:
&a central zone or plasma core which is LTE
&a peripheral zone which is non-LTE.In this plume,heavy particles temperature is much lower than electrons one.
Indeed,for a free-burning argon arc[6],operating conditions(a pressure of300kPa,currents of300to400 A)are necessary to reach a LTE state in the central portion. The conditions lead to an electron density of1024mÀ3in the center.Departures from LTE occur in the outer regions of such arcs where the electron density decreas below 1024mÀ3.
Thus,the local thermodynamic equilibrium is a primor-dial notion since it induces the temperature of the plasma. It strongly depends on the kind of plasma source and is determining for its applications.In the next part,the various atmospheric plasma sources are described in terms of design,operating conditions(power supply,).
2.3.Overview of various atmospheric plasma sources
The excitation frequency is important since it influences the behavior of the electrons and the ions.Fig.3shows an example of the variation range for f pe(frequency of the electrons in the plasma)and f pi(ions frequency)in cold low discharges).
The atmospheric plasma sources can be classified regarding their excitation mode.Three groups are then highlighted:
&the DC(direct current)and low frequency discharges;
&the plasmas which are ignited by radio frequency waves;
and
&the microwave discharges.
Table4reports the main characteristics of the various plasma sources.
Among them,the emerging of original microplasmas[12] is quite interesting.The trend of miniaturization of plasma systems is important in order to create low-powered directive portable systems and to reduce instrument and operation costs.
2.3.1.DC and low frequency discharges
Depending on their design,the DC and low frequency discharges can work either with a continuous or a puld mode.
A puld working mode enables the injection of large energy amounts in the discharge while the system warming up is limited.On the other hand,a puld power supply is technically more complex than a DC source and compromis the reproducibility of the process.
2.3.1.1.Continuous working mode:the arc plasma torches.The arc plasma torches[13]are fed by a DC power supply.They can
be Fig.3.Electrons and ions frequencies in cold plasmas[99].
C.Tendero et al./Spectrochimica Acta Part B61(2006)2–305
divided into two categories:current-carrying arc and transferred arc (e Fig.4).They both consist of:&a cathode where electrons are emitted;&a plasma gas injection system;and &a nozzle which confines the plasma.
In a current-carrying arc torch,the nozzle which is positively polarized is the anode.In the ca of a transferred arc torch,the treated material is the anode whereas the nozzle is at a floating potential.
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The arc is ignited between the cathode and the anode and ionizes the plasma gas.The plasma temperature varies from 8000K (plasma envelop)to 15000K (plasma core)which enables high temperature applications (u of the thermal effect of the plasma).An arc plasma is a very conductive media (I =50–600A).The gas is highly ionized and the electronic density is about 3.1023m À3.
Through the years,the arc plasma torches have been improved and are strongly implanted in industries:
&a high-energy,high velocity torch:Plazjet [14](e Fig.5);&a structure with three cathodes and a gmented anode:TRIPLEX (Fig.6);
&a torch working with a long plasma column stabilized by vortices [15];and
&A miniature al-less plasma torch designed by CEA [15](licend by Europlasma).
2.3.1.1.1.Pencil-like torches.Tho last 5years,low-powered flexible and innovative arc plasma torches have been commercialized:
&Plasmapen and Plasmapen Xtension to increa the size of the treated surface (PVA-TEPLA [16]):
e Fig.7;&Plasma-Jet R from Corotec Corporation [17];
&Openair technology (patterned by Plasmatreat R [18])is well implanted in production lines (automobile,textile,packaging ...).
They all u a homogeneous,low-powered,current-carrying arc plasma jet to prepare a surface prior to joining it with adhesives,coating it,or printing upon it.
On the opposite of the classical arc plasma torches,the discharge generates very little heat,which allows surface treatment of various materials including low temperature degradable materials (polymers).
The classical arc torches can be classified as LTE discharges.They are characterized by rather high temperatures and are
ud
Fig.5.High velocity Plazjet [100](Tafa,
Praxair).Fig.6.Sultzer Metco Triplex II Plasma Spray Gun [101]
.
Fig.4.Principle of arc plasma torches (left:current-carrying arc,right:transferred arc).
C.Tendero et al./Spectrochimica Acta Part B 61(2006)2–30
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