Dust formation : speculated mechanism

iiiii TWGR

dt

dN ~~~~

Ni = density of particles with a size iR = nucleation rate (estimated from the chemical kinetics model)G = coagulation/agglomeration rate (two particles larger particles)W = growth rate (surface growth - heterogeneous chemistry)T = particle losses due to transport : diffusion, thermophoresis, drag, ...

C, C2, C3

Ar+/H3+ sputtering/chemical sputtering/erosion

Gas phaseChemistry nucleatio

n

Surface growth

CoagulationAgglomeratio

n

Estimation of discharge main characteristics: flux and ion energy distribution or ion average energy on the cathode

Extraction of C1, C2 et C3 from the substrate surface

Chemistry and molecular growth Formation of Cn=1,nl clusters, where nl is arbitrary chosen (nl=30 or 60)

Nucleation of carbon dusts from clusters: Assumption of ‘Largest Molecular Edifice’

Growth, charging, transport and wall losses of dusts

Feed back on the gas phase chemistry heterogeneous process

Size distribution of dusts

Model of nucleation, growth and transportof dust in DC discharges ignited in Ar/H2 (2)

Molecular growth modelling of carbon clusters and dusts

ni,z = density of the cluster Ci of charge zNnWt

nln

zn

l

l

)(,

Nucleation

CNEznnDt

n

....

ANMEznnDt

....

Dust Transport

N = nucleationC = coagulationA = condensation

iziiizi WEnznDt

n

,,

Molecular growth

MobilityDiffusion Gas phase chemistry and molecular growthProduction rate of the Ci cluster

Determination of the average diamater dp

clusters

• Molecular growth of clusters

– Rates computed according to formation enthalpies

– Clusters have configurational isomers (chains, rings, multi-cycles) distinguished by cyclization entropy (20 kcal/mol/cycle)

– Extrapolation for unknown values according to cluster periodicities

Bernholc & Schweigert models (classical models) (**):

Carbon cluster growth reactions**

• Growth = one single process (Cn + Cx Cn+x), but take into account the stability of the Cn clusters

• First version of the model took into account neutral clusters

Low pressure discharge : p=1-10 Pa

Diffusion characteristic time =1-10 ms very short as compared to the growth chemistry no possibility for growth of neutral

Need for species with higher residence time :

Negative clusters

And

Trapping electric field configuration

Back to some basic discharge physics

Molecular growth modelling of neutral carbon clusters and dusts

Electric field reversal and molecular growth of negative

clusters• Charging of dust particles only effective if electric field is

confining !• Where is the confining electric field ? Kolobov & Tsendin, Phys.

Rev. A 46 7837, Boeuf &PitchFord, J. Phys. D, (1994)– Self-consistent electric field reversal: confinement– Three electron populations: energetic, passing, trapped

NG: Negative glow / FDS: Faraday Dark Space / PC: Positive Column

and negative ions

Ee

xsheath dc

E0

x0 x1

~ < 1 V

NG FDS PC

Trapped electrons (ne)

R

Energetic electrons ()

Passing electrons (j)

2 V0/dc

Negative carbon cluster growth reactions

• Attachment Cn + e- Cn-

– Rates computed according to electronic affinities

• Charge exchange Cn- + Cx Cn + Cx

-

– Electronic affinities

From Y. Achiba et al., J. Elect. Spect. Related Phen. 142, 231 (2005)

• Dust agglomeration (sticking)

• Detachment Cn- + e- Cn + 2e-

kT

HA

ijji

ji

eRT

3

carbon particles aerosol dynamic in a DC dicharge

Particle charging is a key point :==> Enhanced particle charging insures a significant trapping and long residence time

==> Enhanced particle charging prevents coagulation and growth

ZU=zV

Z Z' kcoag

====> Z+Z'

)',(

)0,0()',(

zzw

kzzk coag

coag

th

el

th

el

UU

UU

zzw1exp

)',(

th

elU

U Kcoag(z,z’)

The only way to have growth ==> charge fluctuation and electron depletion

Possible because particle charg ing is a discrete process Dynamic fluctuation of small particles between positively and negatively charged states

Coagulation takes place between two particles that has opposite instantanous charges or no charge involve small particles.

coag<<fluctuation<<trans

Transport feels the average charge

Coagulation feels the fluctuations

ii

growthigrowth

i

coagicoag

i

iiii

n

II

n

wqwq

n

wqwq

n

FdivqJdiv

dt

qd

)((

2

2

2

)(exp

2

1),(

qq

qq

th

ele

U

U

T

Tf ,Fluctuation

Molecular growth of negative clusters

Negative clusters have significant densities

Growth rate is a function of the electric field profile in the discharge

An accurate knowledge of the field profile is required

Dust density

Electric field reversal <=> electron average energy in the NG E <e>

CathodeAnode

E np

np|max=1013 cm-3

np|max=5x1011 cm-3

<e>=0.1 eV <e>=1 eVField reversal

0 6 120

5

10

15

20

25

char

ge

position (cm)

0.03 eV 0.1eV 1eV

0 6 12

1.0x10-7

2.0x10-7

3.0x10-7

4.0x10-7

5.0x10-7

6.0x10-7

7.0x10-7

dia

mè

tre

(cm

)position (cm)

0.03 eV 0.1 eV 1 eV

CathodeAnode

<e>

Dust average charge and diameter

Cathode

Anode

It is indeed possible to explain particle formation through negative ion driven molecular growth Discharge dynamic (field reversal) and sputtering kinetics are key-points

Pbs : we need better description of the growth kinetics : Model 1 hour for dust formation (instead of few minutes)Take into account the size and charge distributions

CASIMIR Device (Chemical Ablation, Sputtering, Ionization, Multi-wall Interaction, and Redeposition)

3rd module : Redeposition chamber

- Collection of the deposit : filter and substrate)

2nd module :Microwave plasma source

"surfaguide"Decoupling gas phase and surface process

1st module :Sputering/erosion of carbon susbtarte

(H2/Ar plasmas)- Multipolar microwave discharge- Gaz = H2/Ar, Pressure 10-2 mbar- carbon Substrate(Controled temperature and voltage)

Measurement techniques Mass spectrometer / ion energy analyzer

- Detection of neutral and radivcalar species in the plasma (m/z 1-500 uma)- Detection of positive et négative ions- Measurement of IEDF (+/- 1000 eV)

Optical Emission Spectroscopy (H/D et carbonated species) (temperature and density measurements and characterization of plasma species in CASIMIR)

Analysis of the deposit microstructure by SEM and Raman

Results

I. Mass spectrometry:

Polarisation Sheath

graphite disc substrate

Photography of the negatively polarized disc substrate in Ar/H2

PolarisationSheath

Plane Substrat

Photography of the plane polarized substrate in Ar/H2 plasma

Resultts

b) Mass spectrometry and IEDF measurements : Ions in the discharge

0 10 20 30 40 50

0,0

5,0x105

1,0x106

1,5x106

2,0x106

2,5x106

3,0x106

H3

+

H2

+

H+

Inte

nsité

[c/s

]

m/z [u.m.a]

0 10 20 30 40 50

0

1x106

2x106

3x106

4x106 D3

+

D2

+D+

Inte

nsi

té [c

/s]

m/z [u.m.a]

0 10 20 30 40 50

104

105

106

H2O

N2

Ar

Ar (Ar2+)

Inte

nsi

té [c

/s]

m/z [u.m.a]

H+, H2+, H3

+ mass spectra (0,60 kW, 100 sccm)

D+, D2+, D3

+ mass spectra (0,60 kW, 100 sccm)

Ar2+, Ar+ mass spectra (0,60 kW, 10 sccm)

0 2 4 6 8 10

0,0

5,0x102

1,0x103

1,5x103

2,0x103

2,5x103

3,0x103

Inte

nsité

[c/

s]

m/z [u.m.a]

D- mass spectrum (0,60 kW, 100 sccm)

I. Spectromètre de masse / analyseur d’énergie :

Results

c) IEDF

D+ and Ar+ IEDF’s

D+ Ar+

Results

I.2) deuxième études : sur la tête 1-500 uma

0 20 40 60 80 100102

103

104

105

106

107

Inte

nsi

té [c

/s]

m/z [u.m.a]

Ar H

2

Ar/H2

a) Hydrocarbon production through erosion/sputtering in CASIMIR

Mass spectra in H2, Ar, et Ar/H2

plasma

I. Carbon detection :

(1) : E between 9,8 and 14,25 eVCH3 + e- => CH3

+ + 2 e- (in the plasma)(2) : E > 14,25 eVCH4 + e- => CH3

+ + H + 2 e- (in the analyzer)

5 10 15 20 25 30 35 40100

101

102

103

104

Inte

nsité

[c/s

]

Energie électronique [eV]

seuil_plasma_Ar/H2_on_pol_115mA_960V seuil_plasma_Ar/H2_off_pol

Threshold mode detection of CH3 radical CH3

Detection of C, CH, CH3,CH4 et C2

ResultsI. Mass spectrometry: b) Effetc of the polarisation on the erosion yield

Voltage contrôle microarcs

600 V – 2 A

Mass spectrum in H2 plasma With and without polarisation (Alim1)

10 11 12 13 14 15 16 17 18 19 20101

102

103

104

105

106

Inte

nsité

[c/s

]

m/z [u.m.a]

plasma Ar/H2 sans polarisation (Alim1)

Plasma A/H2 avec pol U=240V

10 11 12 13 14 15 16 17 18 19 20101

102

103

104

105

106

Inte

nsité

[c/s

]

m/z [u.m.a]

plasma_Ar/H2_240V_Alim1

plasma_Ar/H2_35mA_600V_Alim2

Comparaison of masse spectra obtained with the two contrôle modes in Ar/H2 plasma

Courant contrôle 300 mA – 1000 V