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1 PIII for Hydrogen Storage Child's Plasma Sheath Model and Theoretical Considerations Emmanuel Wirth, prof. L. Pranevičius, Project: “ORGANIZATION OF HYDROGEN ENERGY TECHNOLOGIES TRAINING” Project code: BPD2004-ESF /0045

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2 Outline of the presentation Current plasma hydriding /PIII Simple Model of the sheath : Child law Calculations Conclusions

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3 DC/AC Plasma Treatment/ PIII Near anode there is a negative space charge Near cathode, the cathode sheath is a zone of intensive ionisation Not suitable for good extraction of H Substrate at the Anode Substrate at the Cathode If AC A and C are equivalent

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4 Plasma Immersion Ion Implantation System Negative bias of the substrate electrons near the substrate are rejected positive charge space (sheath) near the substrate Plasma created by an auxiliary device Plasma Surface is the Source of ions Plasma CathodeAnode Substrate U < 0

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5 Child Langmuir law C.D.Child, Phys. Rev. 32 (1911) 492. and I.Langmuir, Phys. Rev. (Ser.II) 2 (1913) 450. Maximum ion current Solution of the the Poisson's Equation ε 0 = F.m -1 m i = kg (for H) e = C Only when space charge ≠ 0 V 0 = Absolute value of sheath potential drop s = length of the sheath

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6 Child law sheath After a transition time the sheath become a Child law sheath: Debye length: Screening distance over which external electrical field are excluded in the plasma

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7 Values of ionic current densities J i strongly depends on electronic density (= ion density in plasma) If P , J i ( constant ionization ratio) This is the very maximum that you can reach!

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8 Values of the sheath length Sheath Size depend on n e The device should be bigger than the sheath assumption: kTe ≈ 2 eV

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9 Effect of the electronic density on the sheath length Many parameters influences the e- density (P, type of gas, geometry of chamber,..) but if P , n e , s Sheath can be in order of meters The less the e- density is the bigger the sheath is

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10 2 cases for the sheath Collisionless regime If s < λ, ions pass the sheath like in vacuum (no energy loss) Collisional regime If s > λ, ions perform collisions Some energy is lost You cannot reach the maximum voltage You do not obtain the theoretical maximum ions flux U < s

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11 Comparison Sheath Length/Mean Free Path (1) Mean free path for hydrogen plasma depending on the pressure S= cross section N= number of particles per unit volume Distance between 2 collisions Calculation for Hydrogen gas

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12 Comparison Sheath Length/Mean Free Path (2) In some case you may have s > λ If P mean free path you have collision in sheath If P you avoid collisions but ion flux

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13 Possible PIII systems + : Easy to use - : Pressure must be higher (λ), collisions problems PIII + : Higher plasma density, higher ion flux - : Contamination of the magnetron, λ The exact ion energy, flux cannot be known

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14 Conclusion (1) PIII is better than DC/AC Plasma Treatment for Hydriding Independent control of parameters But The right choice of P and V must be done If P too high Collisions:Loss of E (s/ If P too low low ion flux If V too high sheath > chamber If V too low low ion energy

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15 Conclusion (2) Main Parameter in PIII: Gas Pressure Voltage Pulse width (in case of pulsed plasma) Furthers informations? A.Anders, Surf. Coat. Technol.183 (2004)

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