PIII for Hydrogen Storage

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PIII for Hydrogen Storage

• Child's Plasma Sheath Model and Theoretical
  Considerations

Emmanuel Wirth, prof. L. Pranevicius ,
2006-01-27 Project ORGANIZATION OF HYDROGEN
ENERGY TECHNOLOGIES TRAINING Project code
BPD2004-ESF-2.5.0-01-04/0045
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Outline of the presentation

• Current plasma hydriding /PIII
• Simple Model of the sheath Child law
• Calculations
• Conclusions

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DC/AC Plasma Treatment/ PIII
If AC A and C are equivalent

• 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

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Plasma Immersion Ion Implantation System

• Plasma created by an auxiliary device
• Plasma Surface is the Source of ions

• Negative bias of the substrate ? electrons near
  the substrate are rejected ? positive charge
  space (sheath) near the substrate

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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

e0 8.854 . 10-12 F.m-1 mi 1.67. 10-27 kg
(for H) e 1.602. 10-19 C
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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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Values of ionic current densities

• Ji strongly depends on electronic density ( ion
  density in plasma)
• If P ?, Ji ? ( constant ionization ratio)

This is the very maximum that you can reach!
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Values of the sheath length

• Sheath Size depend on ne
• The device should be bigger than the sheath
• assumption kTe 2 eV

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Effect of the electronic density on the sheath
length
The less the e- density is the bigger the sheath
is

• Sheath can be in order of meters

• Many parameters influences the e- density (P,
  type of gas, geometry of chamber,..) but if P ?,
  ne ?, s ?

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2 cases for the sheath

• Collisionless regime
• If s lt ?, ions pass the sheath like in vacuum (no
  energy loss)

• Collisional regime
• If s gt ?, ions perform collisions
• Some energy is lost
• You cannot reach the maximum voltage
• You do not obtain the theoretical maximum ions
  flux

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U lt 0
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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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Comparison Sheath Length/Mean Free Path (2)

• In some case you may have s gt ?
• If P ? mean free path ? you have collision in
  sheath
• If P ? you avoid collisions but ion flux ?

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Possible PIII systems
PIII
Higher plasma density, higher ion flux -
Contamination of the magnetron, ?

• Easy to use
• - Pressure must be higher (?), collisions
  problems

The exact ion energy, flux cannot be known
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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 ? CollisionsLoss of E (s/?)If P
  too low ? low ion flux
• If V too high ? sheath gt chamberIf V too low ?
  low ion energy

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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) 301-311