L.B. Begrambekov

Plasma Physics Department,Moscow Engineering and Physics Institute, 115409 Moscow, Russia

Peculiarities, Sources and Driving Forces of Hydrogen Trapping in

Pyrolytic Graphite, CFC and Thin Films under Low-Energy Irradiation

OutlineOutline

1. Experimental devices and methods

2. Hydrogen trapping in PG and CFC under irradiation by D2-plasma ions and electrons

3. Hydrogen trapping in PG and CFC under irradiation in D2-plasma with oxygen impurities

4. Hydrogen trapping in the deposited carbon films

5. Conclusion. Sources, driving forces and mechanisms of hydrogen trapping in carbon materials

1 - heated cathode,2 - sample heater,3 - sample,4 - anode,5 - mass-spectrometer,6 - plasma chamber,7 - plasma,8 - vacuum vessel,9 - to the vacuum pumping system.

The scheme of thermal desorptional standThe scheme of thermal desorptional stand

Ion source

Н2+ ion flux

Substrate

Carbonflux

Carbon

evaporator Neutral particle flux from the gas phase

Plasma

Scheme of carbon film deposition systemScheme of carbon film deposition system

Hydrogen trapping in PG and CFCHydrogen trapping in PG and CFC

Hydrogen trapping in PG and CFC under Hydrogen trapping in PG and CFC under irradiation by ions and electrons of irradiation by ions and electrons of D2-plasma

1.  Hydrogen trapping takes place when energy of impinging ions approaches zero;2.  Hydrogen is trapped under irradiation by plasma electrons;3.  Trapping of deuterium originated from the layer of surface sorption constitutes presumable part of entire deuterium trapping under low energy irradiation.  But the amount of deuterium additionally trapped in longer experiments is practically the same for ions with different energies

0 200 400 600 800 10000,0

0,5

1,0

1,5

Des

orpt

ion,

x10

21 a

t m

-2

CFC PG

Ion energy, eV

10

Electron energy, eV

7,6x1018 at m-2

TDS spectra of deuterium from CFCTDS spectra of deuterium from CFC

• TDS spectra of deuterium as D2 from the samples irradiated by electrons and by low energy ions are similar

• One can conclude that the traps of the same type are formed in both cases

400 600 800 1000 1200 1400 16000,0

0,5

1,0

1,5

2,0

Des

orpt

ion

, x1

019 a

t m

-2 s

-1

Temperature, K

el. 10 eV fl.pt. 12 eV / at 50 eV / at 100 eV / at 200 eV / at 500 eV / at 800 eV / at 1000 eV / at

Spectra of thermal desorption of deuterium as D2 from CFC irradiated

by D2 plasma with different energies (j=1020 at/m2s).

Hydrogen trapping in PG and CFC under Hydrogen trapping in PG and CFC under irradiation by ions and electrons of irradiation by ions and electrons of D2-plasma

1.  Electrons and low energy ions cannot creat traps through knock out collisions with carbon atoms.2. Deuterium sorbed on the surface act as the source for trapping in both cases.3. Energy of inelastic interactions of electrons and low energy ions with the surface act as a driving force of deuterium trapping. 4. It provides creation of active centers which initiate dissociation of sorbed deuterium, penetration of deuterium ions into graphite and their trapping in specific low energy traps.5. The term “potential trapping “ is proposed for this type of trapping.6. Contrary, the term ”kinetic trapping” could be used for trapping of fast ions

0 200 400 600 800 10000,0

0,5

1,0

1,5

Des

orpt

ion,

x10

21 a

t m

-2

CFC PG

Ion energy, eV

10

Electron energy, eV

7,6x1018 at m-2

Hydrogen trapping in dependence on Hydrogen trapping in dependence on irradiation ion fluxirradiation ion flux

Thermal desorption of deuterium as D2 from CFC irradiated with different ion flux density 2×1019at/m2s and 1×1020at/m2s and difference between them.

1. Under irradiation at equal fluences deuterium trapping is higher, when ion flux density is smaller (Irradiation time is longer).

2. Kinetic trapping does not depend on irradiation time.

3. Potential trapping is time dependent process.

4. Potential trapping constitutes presumable part of entire deuterium trapping under low energy irradiation.

5. Deuterium atoms penetrating surface by potential mechanism can fill kinetic traps 

0 200 400 600 800 10000,0

0,5

1,0

1,5

2,0

Des

orpt

ion,

10

21 a

t m

-2

Ion energy, eV/at

j=2x1019 at m-2 s-1

j=1x1020 at m-2 s-1

Difference = -

Electron energy, eV

10

(2,2 ± 0,6) x 1020

0 1 2 3 4 5 6 70,0

0,5

1,0

1,5

D2

CO

De

sorp

tion

, x

10

21 a

t m

- 2

O2 concentration, %

Deuterium and CO retention in dependence on Deuterium and CO retention in dependence on oxygen concentration in oxygen concentration in D2-plasma

Ion energy is 50 eV/at. Flux is 11020 m-2s-1. Fluence is 51023 m-2.

1.Deuterium trapping is influenced by oxygen impurities in D2-plasma.

2.Deuterium trapping increases oxygen trapping.

3.Oxygen activates potential mechanism of trapping and thus enhances deuterium trapping in CFC.

4.At the same time, presence of oxygen decreases concentration of deuterium in the sorbed layer on the surface leading to decrease of its trapping.

5.This controversial influence explains appearance of maximum at dependency of deuterium trapping on oxygen concentration.

400 600 800 1000 1200 1400 16000,0

0,5

1,0

1,5

2,0

2,5

Des

orpt

ion

, x10

19 a

t m

-2 s

-1

Temperature, K

el. 10 eV fl.pot. 12 eV / at 50 eV / at 100 eV / at 200 eV / at 400 eV / at 600 eV / at 800 eV / at

400 600 800 1000 1200 1400 16000,0

0,5

1,0

1,5

2,0

Deso

rptio

n , x

1019

at

m-2

s-1

Temperature, K

el. 10 eV fl.pt. 12 eV / at 50 eV / at 100 eV / at 200 eV / at 500 eV / at 800 eV / at 1000 eV / at

Spectra of thermal desorption of deuterium as D2 from CFC irradiated

by D2 plasma with different energies (j=1020 at/m2s).

Spectra of thermal desorption of deuterium as D2 from CFC irradiated by D2+4.3%O2 plasma with different energies (j=1020 at/m2s).

TDS spectra of deuterium from CFCTDS spectra of deuterium from CFC

TDS spectra of COTDS spectra of CO

400 600 800 1000 1200 1400 16000

1

2

3

4

5

De

sorp

tion

, x1

01

9 a

t m

-2s-1

Temperature, K

electr. 50 eV 100 eV 200 eV 400 eV 600 eV 800 eV

Plasma concentration is D2+4.3%O2, Flux is 11020 m-2s-1, Fluence is 51023 m-2

•TDS spectra of CO from the samples irradiated by electrons and by ions are rather similar

•Potential trapping is the main mechanism of oxygen trapping under electron irradiation as well as under ion irradiation in entire investigated diapason of ion energies.

Hydrogen trapping in deposited carbon films

Carbon film deposition in resudual gasCarbon film deposition in resudual gas

400 600 800 1000 12000

1

2

3

H d

esor

ptio

n ra

te, 1

016 H

*cm

-2*m

cm-1*s

-1

Temperature, K

H/C = 0.1 – 0.12 is constant O/C = 0.03 – 0.04 is constant Shapes of thermodesorption

spectra are similar for all deposition conditions, and have one main maximum at 1050 K.

Hydrogen trapping mechanism is the same for all films.

Sorbed layer of water molecules is the source for hydrogen trapping.

Energy of inelastic collisions of water molecules with the surface act as a driving force of deuterium trapping.

10 1000,0

0,1

0,2

0,3

0,4

Ratio of deposition fluxes Hf/C

f

H/C

rat

io i

n fi

lms

Carbon film deposition in hydrogen atmosphereCarbon film deposition in hydrogen atmosphere

0 2 4 6 80,0

0,1

0,2

0,3

0,4

0,5

Hyd

roge

n co

ncen

trat

ion,

H/C

Time of single layer deposition, s

104 105 1060,0

0,1

0,2

0,3

0,4

0,50.04 nm/s2.4 Pa120 min

0.06 nm/s1.2 Pa60 min

0.07 nm/s0.15 Pa60 min

Hyd

roge

n co

ncen

trat

ion,

H/C

Deposition fluxes ratio Hf/C

f

0.05 nm/s0.15 Pa120 min

H/C ratio does not depend on hydrogen pressure

H/C ratio of the films increases with decrease of deposition rate and reaches 0.4 at the deposition rate of 0.07 nm/s

Hydrogen is trapped into the film from the constant concentration layer sorbed on the surface.

Dependence of H/C on the time of single layer deposition (t): H/C=0.4(1-exp(-Awt)), where A is hydrogen concentration in the sorbed layer on the surface, w is the probability of an atom being trapped. 0.4 is maximum hydrogen concentration in the films

Carbon film deposition in hydrogen atmosphere.Carbon film deposition in hydrogen atmosphere. TDS spectraTDS spectra

Shape of TDS spectra does not depend on deposition rate

Narrow peak at 1400 – 1500 K region appears in the TDS spectra of the films deposited with lowest deposition rates.

300 600 900 1200 15000

1

2

3

4

0.56 nm/s 0.4 nm/s 0.07 nm/s 0.04 nm/s

Des

orpt

ion

rate

, 1016 c

m-2*m

cm-1*s

-1

Temperature, K

Carbon film deposition under assisting plasma Carbon film deposition under assisting plasma irradiationirradiation

0,0 0,5 1,0 1,50,0

0,1

0,2

0,3

0,4

0,50.07 nm/s200 eV/H

2,1x1019 H*cm-2*s-1

Hyd

roge

n co

ncen

trat

ion,

H/C

Deposition fluxes ratio 105 Hf/C

f

0.14 nm/s50 eV/H

3x1018 H*cm-2*s-10.07 nm/s100 eV/H

2,1x1019 H*cm-2*s-1

0.06 nm/s0.15 Pawithout plasma

0.14 nm/s200 eV/H

7.5x1018 H*cm-2*s-1

Low energy ions (50, 100 eV/H) do not make sufficient contribution in hydrogen trapping.

200 eV/H ion irradiation leads to an increase of the H/C ratio from 0.2 to 0.4. They penetrate into the films due to their kinetic energy.

TDS spectra of hydrogen from the films, TDS spectra of hydrogen from the films, deposited with accompanying plasma irradiationdeposited with accompanying plasma irradiation

300 600 900 1200 15000

1

2

3

Des

orpt

ion

rate

, 1016

cm

-2*m

cm-1*s

-1

Temperature, K

50 eV/H 200 eV/H without plasma

New high temperatures peaks appear due to accompanying ion irradiation. It shows that graphitization of the growing carbon layers occurs.

Consequent carbon layer irradiation in Consequent carbon layer irradiation in deuterium plasmadeuterium plasma

Deuterium part in total H+D concentration is small.

Hydrogen concentration in the remaining part of the film increases under high energy irradiation.

High energy ion bombardment increases equilibrium trap concentration in entire film.

Some fraction of hydrogen released from sputtered layers fills the new formed traps in the remaining part of the film

without irradiation 100 eV/D 400 eV/D0,10

0,15

0,20

0,25

0,30

H/C (H+D)/C

H/C

ra

tio in

th

e f

ilms

ConclusionsConclusions

1. Deuterium sorbed on the surface act as the main source for trapping in both in deposited carbon films and in graphites (CFC) irradiated by electrons, low energy deuterium ions and oxygen ions (atoms).

2. Energy of inelastic interactions of these particles with the surface act as a driving force of trapping of deuterium originated from sorbed surface layer.

3. Oxygen sorbed on the surface act as the source for oxygen trapping. 4. Energy of inelastic interactions of oxygen ions and atoms with the

surface act as a driving force of trapping of oxygen originated from sorbed surface layer.

5. The term “potential trapping “ is proposed for this type of trapping.6. The term ”kinetic trapping” could be used for trapping in the traps

created at the expense of kinetic energy of fast ions. 7. Deuterium atoms penetrating surface by potential mechanism can fill

kinetic traps