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L.B. Begrambekov

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. Outline. Experimental devices and methods

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L.B. Begrambekov

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  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

  3. 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.

  4. Scheme of carbon film deposition system Carbon evaporator Neutral particle flux from the gas phase Ion source Plasma Carbon flux Substrate Н2+ ion flux

  5. Hydrogen trapping in PG and CFC

  6. 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

  7. 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 (j=1020 at/m2s).

  8. 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

  9. 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 2×1019at/m2s and 1×1020at/m2s and difference between them.

  10. 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.

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

  12. 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 D2+4.3%O2, Flux is 11020 m-2s-1, Fluence is 51023 m-2

  13. Hydrogen trapping in deposited carbon films

  14. 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.

  15. 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/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

  16. 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.

  17. 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.

  18. 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.

  19. Consequent carbon layer irradiation in deuterium 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

  20. 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

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