ADSORPTION STATESOF PROTIUM AND DEUTERIUM

IN POLYMER HYDROCARBON FILMSFROM T-10 TOKAMAK

V.G. Stankevich1, N.Yu. Svechnikov1, L.P.Sukhanov1, K.A .Menshikov1, A.M. Lebedev1, B.N. Kolbasov1,

Y.V. Zubavichus1, D. Rajarathnam2

1 Russian Research Center Kurchatov Institute, Moscow 123182, Russia

2 National University of Singapore, Singapore 117576

ASEVA WORKSHOP 2008WS-23 9th International Workshop on Hydrogen Isotopes in Fusion Reactor MaterialsSalamanca, Spain, June 2-3, 2008

Our goals

Search for ways to decrease tritium accumulation rate inside the vacuum vessel

Investigation of the electronic states of Tokamak erosion products

Determination of the hydrogen-carbon bonding states for hydrogen isotopes, and their thermal stability

Flakes’ formation conditionsTokamak T-10 (RRC Kurchatov Institute)

The total duration of VV conditioning modes and plasma discharges in 2002:

♦ heating up to 200С – 897 hours; ♦ inductive discharges – 35 hours H2 plus 270

hours (99% D2 + 1% H2); ♦ Не glow discharges – 86 hours; ♦ D- plasma discharges – 1620 s.

minor radius 0.39 m

major radius 1.5 m

minor radius of plasma 0.35 m

toroidal field 2.8 T

plasma current 200 - 400 kA

discharge time 1 s

electron temperature of core plasma 1 keV

ion temperature 450 - 700 eV

Movable limiter and stationary annular diaphragm, made of a fine grain graphite MPG-8.

Samples

H/C = 0.1 - 0.2Thickness 20–30 µm, size S ≈0.5 cm2

Flakes were collected in the shadowed areas, between two sidewalls forming the first wall where temperature was close to room temperature.

The color of flakes strongly varies with D/C ratio:

dark-brown D/C = 0.2 - 0.4reddish-gold D/C = 0.5 - 0.8

Plasma facingside of flakes

Structure of soft a-C:H films is rather close to that of flakes

Experimental methods

• Thermal desorption spectroscopy (TDS) of H2, D2, HD gases

• X-ray Diffraction

Thermodesorption spectra

Heating rate dT/dt = 10 K/min

500 600 700 800 900 1000 1100

0

20

40

60

80

100

Inte

ns

ity,

n

A

T, K500 600 700 800 900 1000 1100

0

20

40

60

80

100

Inte

ns

ity

, n

A

T, K500 600 700 800 900 1000 1100

0

20

40

60

80

100

Inte

ns

ity,

n

A

T, K

H2

HD

D2

TDS curves for D2(H2) consist of 2 groups of peaks: at 450–800 K and 900–1000 K

Comparison of gold and dark flakes

The whole TDS structure of gold and dark flakes cannot be regarded as totally different, i.e. their adsorption sites have similar features

for H2 isotope for D2 isotope

Deuteriumthermodesorption spectra [1]:

а) gas charged graphite

[1] H. Atsumi et al., J. Alloys Comp. 356 (2003) 705

b) ion implanted graphite

с) nano-structured graphite milled in hydrogen atmosphere

Graphite milled in D2-atmosphere [2] Raman spectra

crystallites less than 40 Å

TDS spectra

As a result, two main adsorption states of hydrogen isotopes were revealed from TDS.

Similarity of spectral features allow to use data already reported on activation energies for

interpretation of the present TD spectra.

[2] Orimo et al., J.Appl.Phys. 90 (2001) 1545.

X-ray diffraction

● The sample is essentially non-crystalline.

The XRD profile can be deconvolved into 2 Gaussians which correspond to the interplanar distances of 0.77 nm (large peak) and 0.28 nm (small peak).

● The carbon flakes differ substantially from graphite (graphene layers are observed at d (002) = 0.335–0.345 nm; in-plane hexagonal structure at d(100) = 0.214 nm) and are amorphous.

The dominant diffraction component at d=0.77 nm corresponds to a certain

characteristic dimension of poorly ordered structure of flakes.

Two mechanismsof thermal desorption

of H2, D2, HDin flakes

500 600 700 800 900 1000 1100

0

20

40

60

80

100

D2

D2

HD

HD

H2

Inte

nsit

y,

nA

T, K

H2

Hopping diffusionbetween weakly bonded states on structural elements (nanopores), followed by a fast pair

recombination(2-nd order reaction)

Resonance mechanismfor strongly bonded states

(1-st order reaction)

CONCLUSIONS

1. Two main adsorption states of hydrogen isotopes were revealed from TDS :

Temperature 450 – 600 К 900 – 1000 К

Adsorption state Weak (“physisorption”) Strong (chemisorption)

Desorption Mechanism Hopping diffusion Resonance type mechanism

Activation energy ≈ 0.65 eV/H ≈ 1.25 eV/H

Genesis of adsorption state

H2 cleaning discharge

&

storage at atmosphere ?

D- plasma discharge

&

(99% D2 + 1% H2) cleaning

2. Carbon flakes differ substantially from graphite and are amorphous. The dominant diffraction component at d=0.77 nm corresponds to a certain poorly ordered structure of flakes.

TEAM

Gracias por su atención

Infrared reflectance spectra of golden and dark flakes

●Spectral differences between golden and dark flakes are correlated with concentration differences of carbon deposits and to the degree of C–H hybridization. The dark flakes have less hydrogen adsorption which could be lead to much carbon – carbon network. Dark films

have a more fragile and weak C–H, C–C, O–H, C=O interconnected adsorbates, i.e. more

short carbon network structures composed mainly of C–H aromatic modes at 700–900 cm-1.

● The CD2,3 modes around 2200 cm-1 (main D-tracing modes) are weaker for dark flakes, but their shape is similar to that of golden flakes, therefore these modes are not introduced into the carbon net, but form the CD2, CD3 end-groups connected to the disordered carbon

network.

Low energy part of spectra

4000 3500 3000 2500 2000 1500 1000 500-0,2

-0,1

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,74000 3500 3000 2500 2000 1500 1000 500

0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

C=C CHCOOH

CD2

C=OOH CH1,2,3

CD2,3

0.5

0.4

0.3

0.2

0.1

0

IR

re

flec

tan

ce DARK

GOLD

Wavenumbers, cm -1

0.6

1800 1600 1400 1200 1000 800 600

1595

1570

1355

894

827

753

661

17111722

14001252

985

1087

1058

1053

dark, Rx10

golden

0.25

0.20

0.15

0.10

0.05

0.00

Ref

lect

ance

Wavenumber, cm-1