PIII for Hydrogen Storage

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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

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

DC/AC Plasma Treatment/ PIII
Substrate at the Cathode Substrate at the Anode 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

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

Child Langmuir law C. D. Child, Phys. Rev. 32 (1911) 492. and I
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 Only when space charge ≠ 0 V0 = Absolute value of sheath potential drop s = length of the sheath ε0= F.m-1 mi = kg (for H) e = C

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

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!

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

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 

2 cases for the sheath Collisionless regime Collisional 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 s + + + + + - - - - - + + + + + + - - - - + + + + + + U < 0

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

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 

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

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

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