26aD59P
SiC材料の電気特性に及ぼすマクロ・ミクロ損傷の影響
Effects of macro/micro damages on electrical properties of SiC materials
野澤貴史、大久保成彰、小沢和巳、谷川博康
Takashi NOZAWA, Nariaki OKUBO, Kazumi OZAWA, Hiroyasu TANIGAWA
原子力機構
JAEA
Introduction
A silicon carbide fiber reinforced siliconcarbide matrix (SiC/SiC) composite is a promising
candidate of the functional structure of the liquid
breeding DEMO blanket. Of many functions torealize this concept, electrical insulation is the mostessential and it is recognized that damage toleranceof the electrical properties needs to be understoodsince overall function of the component can beensured by structural stability. This study therefore
aims to identify the effects of macro/micro damages
on the electrical properties of various SiC materials.
Effect of macro damage on SiC/SiC composites
SiC/SiC composites inherently have muchinternal flaws, which are progressively propagatedby mechanical loading. The electrical properties aretherefore subject to the evolution of these macro
cracks. The material tested was highly-crystalline
Tyranno-SA3 SiC fiber reinforced chemically vapor
infiltrated (CVI) SiC matrix composite withmulti-layered SiC/pyrolytic carbon (PyC) interface.
The macro damage was induced by tensile loading
at different stress levels. The electrical properties
were then evaluated by the frequency sweep AC
impedance method (DC level: 0 V, amplitude: 100
mV, start and end frequencies: 1 MHz and 1 Hz).
First, we identified that the SiC/SiC composite
tested was electrically much conductive (~5 Ohm).
This is because PyC as the fiber/matrix (F/M)
interface is more conductive than SiC monolith.
From this aspect, it is believed that micro-cracking
caused electrical disconnection at the F/M interface.
However, at least below proportional limit stress(PLS), which is considered as the first fiber sliding
stress for the latest composites with high interfacial
friction, there was no marked change in theelectrical conductivity. Even if the matrix cracking
occurred, the conductive PyC interfacial materials
still contact to the intact SiC fibers until sliding.
In contrast, above the PLS, the electricaldegradation was detected. In this case, fiber sliding
and subsequent fiber break progressively occurreduntil the fracture strength. Physical discontinuity of
the fibers finally caused electrical degradation.
Effect of micro damage on SiC ceramics
Due to less resistivity of the composite, it isconsidered that the high-resistivity SiC over-coatingmust be effective for the insulator application.
Understanding fundamental electrical behavior of
SiC itself is therefore more important. In addition,irradiation-induced damage needs to be considered.
Of particular emphasis is that identifying the effectof displacement damage as well as the effect ofionization is essential toward the DEMO design.
160 MeV Xe14+ ions were irradiated at room
temperature with chemical vapor deposited (CVD)
SiC by the Tandem accelerator at Tokai, JAEA.
The AC impedance was measured by the guard ring
method after each irradiation test. Pt electrodes
were deposited on both surfaces of the 3 mm-dia.
50 µm-thick small disk specimen.
Some degradation of electrical conductivity of
SiC were identified after irradiation. Fig. 1 shows
indicative radiation induced electrical degradation(RIED) of CVD SiC. With increasing irradiation
fluence, RIED monotonically increased. This trend
is qualitatively quite consistent with the previous
results by gamma-ray and 14 MeV neutron
irradiation experiments.
Au wire Irradiation region

200nmt-Pt
electrodes

3mmφ CVD-SiC disk 50µmt

Fig. 1 Effect of Xe14+ on electrical conductivity