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

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

3. 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

4. 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

5. 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

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

7. 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!

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

9. 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 

10. 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

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

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 

13. 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

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

15. 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)