EU-PWI TF meeting, Ljubljana 13-15 November 2006 Studies of Plasma-Wall Interactions Association IPPLM / EURATOM, Poland Presented by Elzbieta Fortuna

EU-PWI TF meeting, Ljubljana 13-15 November 2006

Studies of Plasma-Wall Interactions

Association IPPLM / EURATOM, Poland

Presented by Elzbieta Fortuna

Association

EURATOM-IPPLMWarsaw University of Technology


Selected Research Areas

• Studies of material erosion and re-deposition on plasma-facing components from the TEXTOR tokamak

• Novel W-Cu composites

• Nanomaterials

Outline


Studies of material erosion and re-deposition on plasma-facing components from the TEXTOR tokamak

Work programme:

a) Characterization of erosion and re-deposition zones of various graphite PFCs

b) Characterization of co-deposits, flakes and dust

c) Characterization of plasma-induced damage in high Z metals used as limiters

d) Searching for evidence of brittle destruction occurring on PFCs

In cooperation with :

Alfvén Laboratory, Royal Institute of Technology (KTH), (VR), Sweden

Institut für Plasmaphysik, Forschungszentrum Jülich GmbH (FZJ), Germany


Characterization of plasma-induced damage in tungsten coatings of graphite limiters

Experimental

VPS tungsten coatings with a PVD Re/W sandwich interlayer on graphite limiter blocks (EK 98) were studied after the experimental campaign in the TEXTOR tokamak).

The limiters were exposed to a variety of plasma operation scenarios in the presence of either boronised or siliconised walls. During some discharges the heat loads to the limiter exceeded 20 MW/m2 causing the surface temperature rise to above 3600 K (leading to overheating and damage of several limiter blocks).

The examination was focused on:

• micro-structure of the interface layer

• chemical and phase composition of co-deposited films


Structure of VPS-W coating and Re-W interface to carbonInitial structure

SEM image of the initial structure of PVD interlayer and distribution of rhenium, tungsten and carbon

TEM images of the Re-W layers interface


Structure of VPS-W coating and Re-W interface to carbonOverheated part of a coating

(σ)

SEM image of the degenerated structure of PVD interlayer together with Re and C line scan

25 m Tile side

Nano-hardness measurements results

VPS tungsten H = 9 GPa

W-Re zone H = 24 GPa

(under maximum load 40 mN )

Re+W

VPS tungsten


Material mixing on plasma-facing surfaces

XRD pattern showing the phase composition of the plasma-facing surface


Conclusions

1. XRD study has proven the co-existence of several major phases in co-deposited films: metallic W, carbides (WC and W2C) and boride (W2B).Carbon, boron and silicon are the major plasma impurities in co-deposited films on top of the limiter. The content of deuterium ranges from 5x1014 cm-2 in the overheated area to 1x1018 cm-2 in carbon-rich films in the deposition zone.

2. In the overheated region the W-Re sandwich interlayer has been transformed into a brittle inter-metallic phase containing 35-56 wt.% of W. Transverse cracks in the phase layer usually end at the W – Re/W interface. In some cases the cracks propagate along this interface causing delamination. Beneath W-Re layer several micron thick zone with increased carbon concentration were detected.


Novel W-Cu composites

The main goal of this project is to develop new, modern functionally graded W-Cu composites for application in fusion reactors (divertor). Such composites could be also of interest for electrical contacts, spark erosion electrodes and heat sink materials.

Relevance of the project

• The plasma facing materials (PFM) foreseen for ITER are Be, CFC and tungsten. As a heat sink material the Cu alloy is proposed.

• The main problem in the development of W/Cu joints is the large difference in the coefficients of thermal expansion (CTE) (Cu 4W) and elastic modulus (ECu 0.2EW).

• This difference generates large stresses at the W/Cu interfaces during manufacturing and/or during operation of PFCs.

• These stresses may lead to cracking, delamination and a reduced lifetime of the components.

• FGMs can reduce the thermal stresses and increase lifetime.


The arc discharges that take place between the powder particles accelerate mass transport and enhance the process of activation of the powder surface by removing oxides and contaminants. Thanks to the use of capacitors as the source of electric energy, periodically repeated current pulses of a duration of several hundred microseconds and an amplitude of tens kiloamperes are used. The total sintering time is about ten minutes.

This method utilizes a pulsed high electric current discharge to heat the powder subjected to pressing. The powder mixture is loaded into a graphite die, between two graphite punches. The powder is heated by the pulse high-current electric discharges generated by discharging of a 300 μF capacitor, charged to a maximum voltage of 10kV.

Fabrication technique - Pulse Plasma Sintering


Apparatus for pulse plasma sintering

Parameters

Impulse duration (s) 600

Maximum pulsed current (kA) 60

Maximum discharge energy (kJ) 15

Maximum impulse frequency (Hz) 2

Maximum discharge voltage (kV) 10

Maximum sample diameter [mm] 50

This technique can be used for sintering of a wide range of materials e. g. metals (tungsten, titanium, iron and its alloys), ceramics (Al2O3, TiN, TiB2) metal-ceramics composites (WC-Co, NiAl-TiC, NiAl-Al2O3) as well as for SHS (self propagating synthesis). It enables to receive sinters with density approaching 99% of theoretical in several minutes time

Parameters of the pulse plasma sintering apparatus


50 m 50 m

100 m 5 m

W-Cu composite/tungsten plate joint

Composites microstructure

W‑75vol.%Cu W‑50vol.%Cu


Properties W-50vol.%Cu W-75vol. %Cu

Density [g/cm3] 13.9 [98.6%TD] 11.3 [98.5% TD]

Hardness HV1 230 150

UTS [MPa] 500 385

CTE [1/K]at 500oC

7x10-6 15x10-6

Composites properties


Summary

The Pulse Plasma Sintering might be a perspective technology for W-Cu composites manufacturing. The W-50vol.%Cu and W-75vol.%Cu composites have been successfully fabricated by this method by sintering at 900oC for 300s. The results obtained in this study pave the way for sintering gradient composite materials.

The first experiments, using the PPS devise to join the composite material to tungsten plate, show that this method could also be used for joining W-Cu and W elements.


Application of hydrostatic extrusion for particle and grain size refinement in materials relevant to the fusion technologies

The aim of the present project is to describe the thermal stability of fusion relevant materials such as EUROFER 97 processed by hydrostatic extrusion and to determine the factors responsible for thermal stability improvement. The results of the investigations will allow to optimize the material microstructure and the processing route.

Eurofer 97


Effect of hydrostatic extrusion on mechanical properties and thermal stability of Eurofer 97

HV 0,2 = f(e)

200

220

240

260

280

300

320

340

360

380

0 0,5 1 1,5 2 2,5 3 3,5 4

True strain

Mic

roh

ard

nes

s H

V 0

,2

HV 0,2 = f(e)

200

250

300

350

400

450

0 100 200 300 400 500 600 700

Annealing temperature [oC]

Mic

roh

ard

nes

s H

V 0

,2

Thermal stability of HE processed Eurofer 97

Microhardness vs. true strain


Thank you for your kind attention

Documents

EU-PWI TF meeting, Ljubljana 13-15 November 2006 Studies of Plasma-Wall Interactions Association IPPLM / EURATOM, Poland Presented by Elzbieta Fortuna