M. Rutigliano , D. Santoro , and M. Balat-PichelinM. Rutigliano a, D. Santoro , and M. Balat-Pichelin b a CNR–IMIP, Istituto di Metodologie Inorganiche e dei Plasmi, Via G. Amendola

M. Rutigliano a, D. Santoro a , and M. Balat-Pichelin b

a CNR–IMIP, Istituto di Metodologie Inorganiche e dei Plasmi, Via G. Amendola 122/D, 70126 Bari, Italy

b Laboratoire Procédés, Matériaux et Energie Solaire, PROMES-CNRS, 7 rue du four solaire, 66120 Font-Romeu Odeillo, France

[email protected]

ICTP-IAEA Conference on Models and Data for Plasma-Material Interaction in Fusion Devices 07 November 2014

Outline


Introduction

Experiment Setup Evaluation method for recombination coefficient

Results

Simulations

Molecular Dynamics (MD)calculations

Results Comparison experimental vs theoretical for recombination coefficient

Summary

Introduction


The realization of nuclear fusion reactors like ITER has revealed the need of new data on hydrogen recombination on several kinds of materials at high temperature level.

In the literature, one can find only data at low temperature (around 300°K) for silica, stainless steel, several pure metals and carbon. For a same material, data are widely scattered depending on the gas temperature and gas pressure and on the surface temperature of the material.

According to the literature review, it seems necessary to perform new measurement of the recombination coefficient of atomic hydrogen at higher temperature levels.

Experimental Setup


1 - concentrator 2 - shutter 3 - mass flowmeter 4, 8 and 9- mirrors 5 - quartz reactor 6 - waveguide 7 - optical pyrometer 10 - magnetron 11 - support bracket 12 - monochromator 13 - CCD 14 - computer 15 - vacuum pump 16 - control valve 17 - pressure gauge

MESOX set-up (Moyen d’Essai Solaire d’OXydation) at the focus of the 6 kW Odeillo solar furnace

[email protected]

Experimental evaluation of recombination coefficient γH


Actinometry, Optical Emission Spectroscopy :

• the intensity ratio profile IH/IH2 of the two lines, Hβ at 486.1 nm and H2 at 492.7 nm was used for the determination of γH as these two lines can be acquired simultaneously.

Resolution of the diffusion equation at steady state : • mean free path of H atoms <<< reactor radius

• some hypotheses: convective transfer negligible, radial gradient negligible, the stability of the ratio IH/IH2 in the reactor (without sample) allows neglecting the recombination in volume and on the reactor wall

Finally :

γH =4.DH,H2

V.L

IH

IH2 x= L

IH

IH2 x=0

Ts

TL−1

γH=Hrec/Htot

Tungsten sample


AFTER

BEFORE

Heat treatment in H2 plasma at 2143°K

XRD pattern

The chemical composition of the W sample is: W 99.98%, Mo 0.004%, Fe 0.002%, P 0.002% and other components (0.012%)

Experimental Results for the recombination of hydrogen atoms


Process activation energy 14 kJ/mol

Recombination of atoms on surfaces


H H

H2

H

H

H

H

H2

H2

LANGMUIR-HINSHELWOOD

ELEY-RIDEAL

HOT ATOM

Semiclassical Molecular Dynamics calculations


The dynamical simulation of surface processes is worked out through three main steps:

building up of a 3D lattice crystal and phonon dynamics determination

determination via DFT calculations of Potential Energy Surface(PES)where the reaction takes place

Trajectory Propagation

G. D. Billing 2000 Dynamics of Molecule Surface Interactions John Wiley & Sons, New-York

Semiclassical collisional method

Recombination Probability: site and surface temperature effect


10-1.0 100.06 7 2 3 4 5 6 7 2 3 4 5 6 7

Kinetic Energy (eV)

0.0e0

2.0e-3

4.0e-3

6.0e-3

Rec

ombi

natio

n Pr

obab

ility

10-1.0 100.06 7 2 3 4 5 6 7 2 3 4 5 6 7

Kinetic Energy (eV)

0.00

0.01

0.02

0.04

Rec

ombi

natio

n Pr

obab

ility

3F SITE T SITE

TS=1000K TS=700K

3F T

Factors Influencing the surface temperature effect

The effect of surface temperature on the reaction dynamics is not wholly predictable due to the influence of different chemico-physical factors at both microscopic and macroscopic levels. The main factors include:

energy exchange mechanism between the chemisorbed species and the substrate

mobility of adsorbed species

surface coverage

surface structural modifications

reaction mechanism

Probability for other surface processes


TS=1000K

3F SITE T SITE

Had*W(1110) + Hgas ”products”

10-1.0 100.02 3 4 5 6 7 2 3 4 5 6 78

Kinetic Energy (eV)

0.00

0.20

0.40

0.60

0.80

1.00

Prob

abili

ty

Had + Hgas

[H2]ad

Had + Had

10-1.0 100.06 7 2 3 4 5 6 7 2 3 4 5 6 7

Kinetic Energy (eV)

0.00

0.20

0.40

0.60

0.80

1.00

Prob

abili

ty

Had + Hgas

[H2]ad

Had + Had

Recombination coefficient: theoretical versus experimental values


(γH)th

3F T

TS=Tgas=1000K 0.19 0.35

TS=Tgas=700K 0.17 0.32

TS=1000K Tgas= 800K 0.15 0.30

TS= 700K Tgas=800K 0.18 0.34

TS (K)

(γH)exp

700

0.169

0.170

0.181

0.185

1045 0.396

1075 0.416

1325 0.507

1350

0.581

0.588

In the experiment the gas temperature was assumed to be around 700 K for similar surface temperature and 800 K for surface temperatures up to 1350 K.

M. R., D. Santoro, M. Balat-Pichelin, Surface Science 628 (2014) 66

Comments: theoretical versus experimental

❶ In MD simulations, we considered a perfect (110) crystal while in the sample used in the experiment, there are other crystalline planes, the considered surface plane can have some defects and the sample contains, some impurities.

❷ The adsorption density considered in the calculations is different from that of the real surfaces in which different sites can be occupied by hydrogen atoms at the same time altering the reaction dynamics. This latter circumstance could lead the activation of Langmuir–Hinshelwood mechanism. This issue requires further investigation.

❸ In the experiment it was observed that recombination acts for approximately 3 mm above the surface. This distance is too large to ascribe the decrease in the flux intensities just to the surface processes. In fact, surface processes act for distances of few Å from the surface, while for larger distances gas-phase atomic recombination can be effective.

Recombination Reaction Energetics


Ekin(eV) ∆Eph Evib(%) Erot(%) Etr(%)

4.0 <10-3 38 37 24

5.0 <10-3 42 44 13

6.0 <10-3 40 53 6

3F SITE

0.1 10.00

0.10

0.20

0.30

0.40

0.50

0.60

∆Eph

Etr

Evib

Tota

l Ene

rgy

Frac

tion

Kinetic Energy (eV)

ErotT SITE

Vibrational Distribution of H2 formed molecules


T SITE

100.06 7 8 2 3 4 5 6 7 8Kinetic Energy (eV)

0.0

0.2

0.4

0.6

P(v)

v=0v=1v=2v=3v=4v=5v=6

100.06 7 8 2 3 4 5 6

Kinetic Energy(eV)

0.0

0.1

0.2

0.3

P(v)

v=7v=8v=9v=10v=11v=12

3F SITE

0 1 2 3 4 5 6 7 8 9 10 11 12Vibrational Number

0.0

0.1

0.2

0.3

P(v)

Ekin=5 eV

0 1 2 3 4 5 6 7 8 9 10 11 12Vibrational Number

0.00

0.05

0.10

0.15

P(v)

Ekin=6 eV

Summary


The recombination coefficient of hydrogen atoms on W(110) has been evaluated by using a method based on the actinometry technique.

The experimental results obtained for the recombination coefficient of H atoms on W were presented in the high temperature range and have shown that this material is very catalytic with values going from 0.17–0.18 at 700 K up to 0.59 at 1350 K.

MD simulations on the two most active sites on W(110) surface for the surface temperatures of 700 and 1000 K and a gas temperature of 700, 800 and 1000 K have been presented.

A quite good agreement has been found between the experimental and the theoretical γH.

The energy transferred to the surface during recombination reaction is negligible.

H2 molecule are formed in excited vibrational levels.

Acknoldegments


This research was supported by :

CNRS, France (ref. EDC 25071) and

CNR, Italy (Bilateral Project n. 9 2012-2013)

through the funding project: “HYDROGEN ATOMS RECOMBINATION ON TUNGSTEN FOR ITER - EXPERIMENTS AND MOLECULAR DYNAMICS SIMULATION”

Documents

M. Rutigliano , D. Santoro , and M. Balat-PichelinM. Rutigliano a, D. Santoro , and M. Balat-Pichelin b a CNR–IMIP, Istituto di Metodologie Inorganiche e dei Plasmi, Via G. Amendola