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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

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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**

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

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**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

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**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

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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 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

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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 > λ If P mean free path you have collision in sheath If P you avoid collisions but ion flux

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**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

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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 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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**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)

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