L'B' Begrambekov

1
Peculiarities, Sources and Driving Forces of
Hydrogen Trapping in Pyrolytic Graphite, CFC and
Thin Films under Low-Energy Irradiation

• L.B. Begrambekov

Plasma Physics Department, Moscow Engineering and
Physics Institute, 115409 Moscow, Russia
2
Outline

• Experimental devices and methods
• Hydrogen trapping in PG and CFC under irradiation
  by D2-plasma ions and electrons
• Hydrogen trapping in PG and CFC under irradiation
  in D2-plasma with oxygen impurities
• Hydrogen trapping in the deposited carbon films
• Conclusion. Sources, driving forces and
  mechanisms of hydrogen trapping in carbon
  materials

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The scheme of thermal desorptional stand
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.
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Scheme of carbon film deposition system
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Hydrogen trapping in PG and CFC
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Hydrogen trapping in PG and CFC under 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
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TDS 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

Spectra of thermal desorption of deuterium as D2
from CFC irradiated by D2 plasma with different
energies (j1020 at/m2s).
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Hydrogen trapping in PG and CFC under 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
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Hydrogen trapping in dependence on irradiation
ion flux

• Under irradiation at equal fluences deuterium
  trapping is higher, when ion flux density is
  smaller (Irradiation time is longer).
• Kinetic trapping does not depend on irradiation
  time.
• Potential trapping is time dependent process.
• Potential trapping constitutes presumable part of
  entire deuterium trapping under low energy
  irradiation.
• Deuterium atoms penetrating surface by potential
  mechanism can fill kinetic traps 

• Thermal desorption of deuterium as D2 from
  CFC irradiated with different ion flux density
  21019at/m2s and 11020at/m2s and difference
  between them.

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Deuterium and CO retention in dependence on
oxygen concentration in D2-plasma
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.
Ion energy is 50 eV/at. Flux is 1?1020 m-2s-1.
Fluence is 5?1023 m-2.
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TDS spectra of deuterium from CFC
Spectra of thermal desorption of deuterium as D2
from CFC irradiated by D2 plasma with different
energies (j1020 at/m2s).
Spectra of thermal desorption of deuterium as D2
from CFC irradiated by D24.3O2 plasma with
different energies (j1020 at/m2s).
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TDS spectra of CO

• 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.

Plasma concentration is D24.3O2, Flux is
1?1020 m-2s-1, Fluence is 5?1023 m-2
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Hydrogen trapping in deposited carbon films
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Carbon film deposition in resudual gas

• 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.

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Carbon film deposition in hydrogen atmosphere

• 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/C0.4?(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

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Carbon film deposition in hydrogen atmosphere.
TDS 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.

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Carbon film deposition under assisting plasma
irradiation

• 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.

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TDS spectra of hydrogen from the films, deposited
with accompanying plasma irradiation
New high temperatures peaks appear due to
accompanying ion irradiation. It shows that
graphitization of the growing carbon layers
occurs.
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Consequent carbon layer irradiation in deuterium
plasma

• Deuterium part in total HD 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

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Conclusions

• 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).
• Energy of inelastic interactions of these
  particles with the surface act as a driving force
  of trapping of deuterium originated from sorbed
  surface layer.
• Oxygen sorbed on the surface act as the source
  for oxygen trapping.
• 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.
• The term potential trapping is proposed for
  this type of trapping.
• The term kinetic trapping could be used for
  trapping in the traps created at the expense of
  kinetic energy of fast ions.
• Deuterium atoms penetrating surface by potential
  mechanism can fill kinetic traps