B. Riccardi

Joining of SiC/SiCf ceramic matrix composites for fusion reactor blanket applications

Abstract Using a preceramic polymer, joints between SiC/SiCf ceramic matrix composites were obtained. The polymer, upon pyrolysis at high temperature, transforms into a ceramic material and develops an adhesive bonding with the composite. The surface morphology of 2D and 3D SiC/SiCf composites did not allow satisfactory results to be obtained by a simple application of the method initially developed for monolithic SiC bodies, which employed the use of a pure silicone resin. Thus, active or inert fillers were mixed with the preceramic polymer, in order to reduce its volumetric shrinkage which occurs during pyrolysis. In particular, the joints realized using the silicone resin with Al–Si powder as reactive additive displayed remarkable shear strength (31.6 MPa maximum). Large standard deviation for the shear strength has nevertheless been measured. The proposed joining method is promising for the realization of fusion reactor blanket structures, even if presently the measured strength values are not fully satisfactory.

Status of the European R&D activities on SiCf /SiC composites for fusion reactors

Abstract Silicon carbide composites are a candidate for fusion reactors structural material because of their low activation and after heat properties and good mechanical properties at elevated temperatures. These materials, to be more suitable with their use for fusion energy production, need a strong R&D effort in order to solve some critical issues such as thermal conductivity and radiation stability, hermeticity, chemical compatibility with the fusion environment, the capability to be formed in complex geometries, the joining process and long production time. Constant progress in the fibre quality and matrix–fibre interfaces contribute to support the use of SiCf/SiC composites as structural material for fusion application. This paper presents an overview of the current status of the European R&D activities on SiCf/SiC composites focussing on reactor design studies, composites manufacturing, material characterisation in particular after irradiation, chemical compatibility with different blanket environments and development of joining techniques.

SiC–SiCf CMC manufacturing by hybrid CVI–PIP techniques: process optimisation

SiC–SiCf ceramic matrix composites (CMC) are candidate structural material for fusion power reactor applications because of their favourable thermo-mechanical and low-activation properties. Among their different manufacturing techniques, present, the most employed ones are chemical vapour infiltration (CVI) and polymer infiltration and pyrolysis (PIP). These two techniques are based on the common principle of filling the porosity among the fibres with SiC resulting from precursor decomposition. CVI process deposits high purity crystalline SiC with good properties onto fibres whereas PIP leaves lower characteristic amorphous SiC with traces of oxygen between fibres. PIP, on the other hand, seems to be much more industrially effective than CVI. In the attempt to maximise the properties and reduce costs, some work has been done on the so called ‘hybrid techniques’ in which CVI and PIP are both employed. The work performed by ENEA and FN S.p.A. consists of a series of combined CVI–PIP process cycles and the subsequent product characterisation.

Manufacturing and testing of monoblock tungsten small-scale mock-ups

In the frame of the European Technology R&D program for ITER, and in the area of High Heat Flux plasma facing Components (HHFC), representative small-scale mock-ups were manufactured and tested to compare different concepts and joining technologies (i.e. active brazing, Hot Isostatic Pressing (HIPping), diffusion bonding, etc.). On the basis of the results obtained by thermal fatigue tests, the monoblock concept resulted to be the most promising one, particularly when the HIPping manufacturing technology is used. Within this program, ENEA manufactured tungsten monoblock mock-ups by using the HIPping technology. The mock-ups were tested with respect to thermal fatigue and, upon screening tests, the best performance obtained was 15 MW/m2 for 200 cycles. One of these mock-ups was enclosed in the ‘Paride’ neutron irradiation campaign. This campaign has the scope of enlarging the available database on the performance degradation of prototypical basic solutions of the ITER plasma facing components due to neutron irradiation. The manufacturing procedure and the results of the screening and fatigue thermal tests performed on the ENEA mock-ups are reported in this paper.

Joining of SiC based ceramics and composites with Si-16Ti and Si-18Cr eutectic alloys

Silicon carbide and SiCf/SiC ceramic matrix composites are attractive materials for energy application because of their chemical stability and mechanical properties at high temperature. For these materials advanced joining techniques are under development and, among them, brazing is one of the most promising. In this paper a brazing technique based on the use of the Si-16Ti (at.%) and the Si-18Cr (at.%) eutectic alloys (with 1330°C and 1305°C melting points, respectively) is presented and discussed. The eutectic alloys were prepared by several melting operations, reduced in powders and finally used for the joining experiments. The brazing trials led to joints without discontinuities and defects at the interface and with fine eutectic structures exhibiting a morphology comparable with that of the starting alloys. Microstructure and nanochemistry investigations showed neither interdiffusion nor phase formation at the interface leading to the conclusion that direct chemical bonds are responsible for the adhesion. Joint specimens showed high shear strength with failure occurring mainly in the base material.

Low activation brazing materials and techniques for SiCf/SiC composites

Abstract A low activation brazing technique for silicon carbide fiber reinforced silicon carbide matrix composites (SiCf/SiC) is presented; this technique is based on the use of the 78Si–22Ti (wt%) eutectic alloy. The joints obtained take advantage of a melting point able to avoid composite fibre-interface degradation. All the joints showed absence of discontinuities and defects at the interface and a fine eutectic structure. Moreover, the joint layer appeared well adherent both to the matrix and the fibre interphase and the brazing alloy infiltration looked sufficiently controlled. The joints of SiCf/SiC composites showed 71±10 MPa almost pure shear strength at RT and up to 70 MPa at 600 °C.

Mechanical characterisation of fusion materials by indentation test

FIMEC is an indentation test, which permits the evaluation of yield and ultimate tensile stress and to draw indication about the ductile to brittle transition temperature. The apparatus has been recently implemented with a feedback system to maintain strictly constant the penetration speed of cylindrical punch during the test. So, experiments performed on several materials, as those for first wall and blanket, show that the scattering between data from FIMEC and from standard tensile test is reduced within the range ±0.06, i.e. it is comparable with the scattering obtained in different tensile tests on the same material. A numerical simulation has been carried out to understand the basic mechanism of the process.

Latest liquid lithium target design during the key element technology phase in the international fusion materials irradiation facility (IFMIF)

Abstract International Fusion Materials Irradiation Facility (IFMIF), being jointly developed by EU, JA, RF and US, is a deuteron–lithium (Li) stripping reaction neutron source for fusion materials testing. In 2002, a 3 year Key Element technology Phase (KEP) to reduce the key technology risk factors was completed. A liquid Li target has been designed to produce intense high energy neutrons (2 MW/m 2 ) up to 50 dpa/year by 10 MW of deuterium beam deposition which corresponds to an ultra high heat load of 1 GW/m 2 . This paper describes the latest design of the liquid Li target system reflecting the KEP results and future prospects.

Activities on IFMIF lithium target at ENEA

Abstract The status of R&D activity ongoing at ENEA on the lithium target system of the international fusion materials irradiation facility (IFMIF) is reported. The activity has been launched in year 2000 in the frame of IFMIF key element technology phase in order to reduce the key technology risk factors and to guarantee the required availability and reliability of the IFMIF liquid Li target system. The items discussed in the paper are related to the Li jet flow stability numerical analysis, water flow simulation experiment, backplate remote handling simulation and Li target safety analysis.

Characterisation of deuterium transport in the fibres and matrix of a 3D-SiCf/SiC composite

A non-stationary heterogeneous transport model has been used to simulate a series of deuterium absorption–desorption tests carried out with two sets of 3D-SiCf/SiC composite specimens over a temperature range of 675–1029 K and driving pressures ranging from 13 to 101 kPa. The deuterium transport parameters corresponding to each phase of the composite have been derived showing the following Arrhenius tendencies for the diffusivity and solubility in the matrix: Dm (m2 s−1)=9.0×10−8 exp(−39.0 (kJ mol−1)/RT), Sm (mol m−3)=1.9×105 exp(−70.7 (kJ mol−1)/RT), and the fibre: Df (m2 s−1)=1.6×10−11 exp(−91.0 (kJ mol−1)/RT), Sf (mol m−3)=2.1×101 exp(−28.5 (kJ mol−1)/RT), the solubilities being referred to 105 Pa. The different manufacturing procedures followed in the preparation of the specimens have not provoked a noticeable variation of the deuterium transport characteristics of the matrix or the fibres. The results are compared with the parameters previously obtained for different types of silicon carbide and the same material with a homogeneous transport model. The differences found are explained on the basis of the physico–chemical phenomena foreseen within its particular structure.