Ceramic Routes for Fabrication of SiCfSiC Composites

1
Joef Stefan Institute
Ceramic Routes for Fabrication of SiCf/SiC
Composites
S. Novak1, G. Draic1. K. Mejak1, T. Topliek1,
T. agar2, M. Ravnik2 1 Department for
Nanostructured Materials, Joef Stefan Institute,
Slovenia 2 Department for Reactor Physics, Joef
Stefan Institute, Slovenia

• Densification of the SiC-based matrix
• LIMITATIONS
• lt 1500C
• Use of nano-powder
• Low-activation additives
• Limited number of
• elements allowed

INTRODUCTION
Due to the extremely low neutron-activation of
SiC, SiC-based composites are considered as an
attractive alternative to steel based materials
(e.g. Eurofer) for the first-wall blanket of the
future fusion reactors. However, the current
SiCf/SiC composites, produced by the CVI process
does not meet all the requirements (certain
amount of internal porosity and high processing
cost). Consequently, alternative techniques for
the material production are under development.
AIM
The main problem that needs to be solved in the
case of the ceramic routes is high sintering
temperature of SiC and high neutron activation of
commonly used sintering additives. The main
stress in the present investigation was therefore
to find a sintering additive from a limited
number of low-activation elements, that would
enable sintering of SiC to high density at
temperature below 1500C. Candidate sintering
aids, alternative to conventionally used, was
selected on the base of activation cross-section
data and on the survey of relevant phase
diagrams.
CERAMIC ROUTE TO CMC

- Wetting - Coating
Low-activation additives
Fracture surface of the sample with 0.5 mm SiC
(SEM)
Polished sample with 0.5 mm SiC (SEM)
Continuous SiC fibres / woven
SiC powder
Al-phosphateMg-phosphate
- 0.5 mm- 50 nm
Liquid SAA
SiC fibre
SiC-based matrix
Infiltration
Densification
Suspension
50 nm
15 mm
(!!) lt 1500C
Vacuum Slip Infiltration (VSI)
Polished cross-section of the sample with 50 nm
SiC (SEM)
Interface between SiC-fibre and matrix with 50 nm
SiC (TEM)
Electrophoretic Infiltration (EPI)
SiC-fibres
SiC-fibre
SiC deposit
Counterelectrode
SiCf/SiC
Good adhesion of the matrix material to the
fibers (SEM)
Crack deflection at the fibre (SEM)
SiC bundle with SiC deposit
Deposition cell
SiC particles at SiC fiber
Massive SiC deposit on SiCf/SiC after 10 min of
deposition at 60 V
Saturated gamma activities of impurities found in
used SiC powders irradiated in the central
channel of TRIGA Mark II reactoras a function of
cooling time.
SiC fibres are conductive and hence applicable as
an electrode in electrophoretic deposition or
infiltration. SiC particles and Al-phosphate are
deposited from ethanol suspension and after
firing produce a dense SiC-ceramics.
CONCLUSIONS

• Using nano-sized SiC powder and Al-phosphate as
  transient sintering additive (TSA), SiC-based
  ceramics can be densified at 1450C.
• Electrophoretic deposition was confirmed to be a
  promising technique for fast infiltration of
  ceramic suspension into SiC-fibre substrate as
  well as to produce gas-impermeable coating at the
  SiCf/SiC composite.
• Effective filling the SiC-fibre woven can be
  achieved by modification of the fibres surface
  using appropriate surface active agents. The
  critical parameters are particles surface charge
  and conductivity of the SiC-ethanol suspension.
• Oxide layer at the SiC particles hinders the
  deposition and have also a detrimental effect on
  densification and properties of the sintered
  material.
• Impurities in the raw materials used in this
  phase of the investigation, cause high
  activation.

• Further work
• - Replacment of Al with Mg
• Use of high-purity raw materials
• ...

Acknowledgement The work was performed in the
frame of the project Novel processing of SiC/SiC
by slip infiltration of SIC-preforms with SiC,
Euratom-FU06-CT-2003-000323 (2004) and Fusion
association contract MHST FU06-CT-2004-00083-UT2
(2005)