J. Plasma Fusion Res. SERIES, Vol. 9 (2010)

Electrical insulating performances of SiC materials
under gamma-ray irradiation
Teruya TANAKA1, Wataru KADA2, Tatsuya HINOKI3, Toshiji IKEDA4, Kazuya SHIMODA3,
Min-Soo SUH 3, Fuminobu SATO2, Toshiyuki IIDA2, Akira KOHYAMA5, Takeo MUROGA1
1

Fusion Engineering Research Center, National Institute for Fusion Science, Toki, Gifu 509-5292, Japan
2

Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871, Japan
3

Institute of Advanced Energy, Kyoto University, Uji, Kyoto, 611-0011, Japan

4

Institute of Scientific and Industrial Research, Osaka University, Ibaragi, Osaka 567-0047, Japan5

Muroran Institute of Technology, 27-1 Muroran, Hokkaido 050-8585, Japan567-0047, Japan(Received: 30 October 2009 / Accepted: 26 February 2010)

For evaluation of electrical insulating performances of SiC materials in radiation and high temperatureenvironments, electrical conductivities of single crystal and sintered SiC materials were measured undergamma-ray irradiations at temperatures of up to ~450 oC. The radiation induced conductivities (RICs) evaluated forsingle crystal SiC plates were 2.8 x 10-8 – 1.3 x 10-7 S/m under the irradiations of 2.3 Gy/s at room temperature. The
RICs in the sintered SiC plates were almost below measurable level due to their high inherent conductivities. The
magnitudes of the RICs did not increase significantly with temperature. Extrapolation from the present data
indicates that the maximum magnitude of RIC in SiC materials would be the order of 10-4 S/m for a dose rate ofseveral kGy/s at a first wall of a fusion reactor. The magnitude is adequately lower than the allowable electricalconductivities in the SiC flow channel inserts (FCIs) for the dual-cooled lithium lead (DCLL) blanket design.
Keywords: Silicon carbide, Electrical conductivity, Radiation induced conductivity, Gamma-ray
irradiation

performances will be degraded by radiation andtemperature effects under a fusion reactor environment.
Radiation effects on the electrical performances have beenstudied profoundly for ceramic materials of Al2O3, MgO
etc. used in diagnostics systems [4-7]. Although a few data
for sintered SiC materials have been reported [8], similar
database is required to be constructed also for SiC
materials.
In the present study, degradation of the electricalinsulating performances of SiC materials due to radiationinduced conductivities (RICs) and the temperaturedependences were examined by gamma-ray irradiations attemperatures of up to ~450 oC. The irradiations were
performed on monolithic sintered plates studied for the
FCIs and especially on high-purity single crystals tounderstanding the fundamental electrical properties of SiC
under a high temperature and radiation environment.

1. Introduction

Silicon carbide (SiC) materials have been studied in
the fusion reactor development especially as a blanket
material for the superior properties of high-temperaturestrength, irradiation resistance, chemical stability, lowactivation etc [1, 2]. In the conceptual design of the Dual
Cooled Li-Pd (DCLL) blanket system, install of SiC flow
channel inserts (FCIs) has been proposed for highefficiency operation at higher temperature region. The
design indicates that the maximum coolant temperature of~800 oC could be achieved by attenuating the temperatureof ferritic steel blanket walls with the SiC FCIs. In thisconcept, electrical insulation is also a required function of
the SiC FCIs for suppression of MHD pressure drop, sinceelectrically conductive liquid Li-Pb coolant flows throughthe FCIs across strong magnet fields. The maximum
allowable electrical conductivity for the FCIs has been
limited to 100 S/m [3].
Pure ceramic materials generally indicate adequate
electrical insulating performances. However, the SiC
materials studied for the FCIs contain sintering additives in
the fabrication process. While the additives improve themechanical properties, fabrication process etc. [9], theymight affect the electrical insulating performance
significantly. In addition, the electrical insulating

2. Experiment

Measurements of electrical conductivities were
performed for two types of single crystal SiC plates andthree types of sintered SiC plates. The single crystal SiC
plates of 10 mm x 10 mm were prepared by cutting a 50
mmI high-purity semi-insulating wafer and a 75
mmIn-type semiconductor wafer supplied from Cree, Inc.

author’s e-mail:teru@nifs.ac.jp282©2010 by The Japan Society of Plasma
Science and Nuclear Fusion Research

T. Tanaka et al., Electrical Insulating Performances of SiC Materials under Gamma-Ray Irradiation

Fig.1

plates. Electrical conductivities of the SiC plates were
evaluated from relations between bias voltages andcurrents flowing through the plates.
Examination of electrical insulating properties undergamma-ray irradiations were performed at the 60Co
gamma-ray irradiation facility of the Institute of Scientificand Industrial Research, Osaka University. The SiC plate
was set in a vacuum chamber evacuated to < 10-3 Pa. The
averaged dose rate estimated with the MCNP Monte Carlocode [10] was 2.3 Gy/s in a SiC plate at the specimen
position. Magnitudes of radiation induced conductivities(RICs) of all single crystal and sintered SiC plates were
evaluated from differences between conductivities
measured with and without gamma-ray irradiations. Bychanging the position of the gamma-ray source, the dose
rate dependence of the RIC was examined for asemi-insulating single crystal SiC sample in the region of
0.01–2.3 Gy/s at room temperature. To examine thetemperature effects on RICs, the semi-insulating single
crystal and Hexoloy/NITE/Hexoloy sandwiched SiC plates
were heated up to ~450 oC and the conductivities were
measured under the gamma-ray irradiations.
Table 1 lists the specimen names and examinations
performed in the present study.

Schematic drawing of electrical conductivity
measurement of SiC materials undergamma-ray irradiation.

The thickness of the plates was 0.39 mm and the crystal
structure was 4H-SiC. Sintered SiC plates were monolithic
Hexoloy-SA® commercially obtained from Hitachi
chemical Co., Ltd. and monolithic NITE-SiC (NITE;
Nano-Infiltration Transient-Eutectic Phase) fabricated in
Kyoto University [9]. The dimensions of the sintered plates
were 10 mm x 10 mm x 1 mm. In addition to the above
monolithic sintered plates, sandwiched plates consisting ofHexoloy-SA (0.45 mm in thickness) / NITE-SiC (0.1 mm)/ Hexoloy-SA (0.45 mm) layers were also prepared withthe same dimensions.
Schematic drawing of the conductivity measurements
is shown in Fig. 1. On the single crystal SiC plates, Al or Pt
electrodes were deposited by resistance heating or DC
sputtering. The diameters were 5 mm for the current
measurement side and 8 mm for the bias voltage side. On
the sintered monolithic and sandwiched SiC plates, Pt
electrodes were deposited by DC sputtering. The sizes
were 5 x 5 mm2 for the current measurement side and 9 x 9mm2 for the bias voltage side. Guard electrodes were also
deposited around the current measurement electrodes toavoid surface currents flowing across the edges of theHigh-purity
singlecrystalSintered

3. Results

Fig. 2 shows the current-voltage (I-V) curves of thesingle crystal SiC plates measured without and under the
gamma-ray irradiations at room temperature. In thesemi-insulating #1 and #2 plates, both the currents without
and under irradiations showed the ohmic characteristics(Fig. 2 (a)). The inherent electrical conductivities of thesemi-insulating plates without gamma-ray irradiations
were 4.0 x 10-14 and 1.0 x 10-13 S/m, respectively. The
orders of the inherent conductivities were almost similar to
those of oxide ceramic materials such as Er2O3 and Y2O3
examined in our previous studies [11]. Under the
gamma-ray irradiations of 2.3 Gy/s, the electrical

Table 1 SiC specimens and examinations performed in present study.
Without
Under gamma-ray irradiation
irradiation
Specimen
RIC
Dose rate
Temperature
Inherent
evaluation for
dependence
Dependence
conductivity
2.3 Gy/s
SemiOOO
RT - 450 oC
insulating #1
Semiinsulating #2
n-type

O

O

-

-

O

O

-

-

Hexoloy-SA

O

-

-

-

NITE-SiC
Hexoloy/NITE/Hexoloy

O

O

-

-

O

O

-

RT - 450 oC

283

T. Tanaka et al., Electrical Insulating Performances of SiC Materials under Gamma-Ray Irradiation

similar to that obtained without irradiation. The
conductivities for the bias voltage of -3 V were 1.1 x 10-8
S/m without irradiations and 3.9 x 10-8 S/m under theirradiation, respectively. From the difference, themagnitude of the RIC in the n-type plate was evaluated tobe 2.8 x 10-8 S/m for the dose rate of 2.3 Gy/s.
The inherent electrical conductivities of the sinteredmonolithic SiC plates without irradiations were 1.1 x 10-6
S/m in the Hexoloy-SA plate and 3.2 x 10-2 S/m in the
NITE-SiC plate at room temperature. In the
Hexoloy/NITE/Hexoloy sandwiched plate, the inherent
conductivity was dominated by the Hexoloy-SA layerswith a higher insulating performance and the value was 1.3
x 10-6 S/m at room temperature. Magnitudes of RICs were
examined for the NITE-SiC and Hexoloy/NITE/Hexoloy
sandwiched plates at room temperature. In the case of the
NITE-SiC plate, an increase in the conductivity under the
gamma-ray irradiation could not observed since theinherent conductivity was relatively high and the
conductivity increased easily by a thermal conduction fromthe radiation source. In the measurement for the
sandwiched plate, the RIC was evaluated to be 1.6 x 10-8
S/m under the irradiation of 2.3 Gy/s. However, since themagnitude of the RIC was ~1/100 of the inherent
conductivity, the measurements with more accurate
temperature control might be required for evaluation of thefactor of the value. Table 2 summarizes the abovemeasurement results at room temperature.
The dose rate dependence of the RIC in thesemi-insulating #1 plate is plotted in the Fig. 3 with theresults described in the above paragraphs. While the samesemi-insulating sample (semi-insulating #1) was examined,
the magnitude of RIC in the dose rate dependence
measurement was 1.3 x 10-7 S/m for the dose rate of 2.3
Gy/s, which was three times larger than the result of theI-V curve measurement described in the above paragraph.
The magnitude of the RICs might be affected by acondition and behavior of electron trapping levels in the
crystal. In Fig. 3, the RICs of the SiC plates are comparedwith those of several oxide ceramic materials obtained in
our previous experiments [11]. Magnitudes of RICs of
Al2O3 materials, which have been studied most profoundly,
were almost the same order as those of the Y2O3 material
plotted in Fig. 3 [12]. In the present evaluation, the RICs of
the SiC plates were 2-3 orders higher than those of theoxide ceramic materials. It is considered that the drift
distance of radiation excited charge carriers in the SiC
materials is 2-3 orders larger compared with that in theoxide ceramic materials.
The temperature dependences of the electricalconductivities of the SiC plates are shown in Fig. 4. The
applied bias voltage was 250 V or -250 V for thesemi-insulating #1 plate and +1 V for the
Hexoloy/NITE/Hexoloy plate. The conductivity of thesemi-insulating #1 plate without a gamma-ray irradiation(a)Electrical current (nA)

800
600
400
200

㼁㼚㼐㼑㼞㻌㼕㼞㼞㼍㼐㼕㼍㼠㼕㼛㼚㻌㻿㼑㼙㼕㻙㼕㼚㼟㼡㼘㼍㼠㼕㼚㼓㻌㻏㻝㻌㻿㼑㼙㼕㻙㼕㼚㼟㼡㼘㼍㼠㼕㼚㼓㻌㻏㻞㼃㼕㼠㼔㼛㼡㼠㻌㼕㼞㼞㼍㼐㼕㼍㼠㼕㼛㼚㻌㻿㼑㼙㼕㻙㼕㼚㼟㼡㼘㼍㼠㼕㼚㼓㻌㻏㻝㻌㻿㼑㼙㼕㻙㼕㼚㼟㼡㼘㼍㼠㼕㼚㼓㻌㻏㻞60

Co gamma-ray
2.3 Gy/s

0

-200
-400
-600
-800
-400

-200

(b)

Electrical current (nA)

400

Under irradiation
Without irradiation

15
5

200

㻌

20
10

0

Bias voltage (V)60

Co gamma-ray
2.3 Gy/s

0
-5
-10
-15
-20
-6

-4

-2

0

2

4

6

Bias voltage (V)Fig.2

Current-voltage (I-V) curves of (a)semi-insulating and (b) n-type single crystal
SiC plates measured under gamma-ray
irradiation and without irradiation at room
temperature.

conductivities of the semi-insulating plates increased to 4.3
x 10-8 S/m and 3.7 x 10-8 S/m. From the difference in
conductivities measured without and under the irradiations,the magnitudes of the radiation induced conductivities(RICs) in the semi-insulating plates were evaluated to be
3.7 – 4.3 x 10-8 S/m for the dose rate of 2.3 Gy/s. In themeasurement of the I-V curve of the n-type single crystal
SiC plate, the current increased rapidly with the bias
voltages of higher than +2 V and lower than -3 V (Fig. 2(b)). It is considered that a rectification behavior of then-type semiconductor did not appear for the low bias
voltages due to the electrode material selected in thepresent evaluation. Increase of the electrical current in the
n-type plate was observed under the gamma-ray irradiationand the shape of the I-V curve under the irradiation was284

T. Tanaka et al., Electrical Insulating Performances of SiC Materials under Gamma-Ray Irradiation

Table 2 Electrical conductivities of SiC specimens before gamma-ray irradiation and magnitudes of radiation induced
conductivities (RICs) for dose rate of 2.3 Gy at room temperature.
Inherent
RIC for 2.3 Gy/sSpecimenconductivity
at room temperature
(S/m)
(S/m)
Semi-14
4.3 x 10-8
4.0 x 10
insulating #1
High-purity
Semi3.7 x 10-8
1.0 x 10-13
single crystal
insulating #2
1.1 x 10-8
2.8 x 10-8
n-typeHexoloy-SA
1.1 x 10-6
NITE-SiC

3.2 x 10-2

Under measurable
level

Hexoloy/NITE/Hexoloy

1.3 x 10-6

1.6 x 10-8Sinteredincreased drastically with temperature. In contrast,
significant increase in the conductivity was not observed
up to ~300 oC under the irradiation. The conductivity under
the irradiation was 1.3 x 10-7 S/m at room temperature anddecreased gradually to 2.6 x 10-8 S/m at ~200 oC. It could
be considered that shallow trapping levels became vacantby thermal excitation and the drift distance of radiation
excited charge carriers decreased due to trapping to thevacant levels. At temperatures of >300 oC, theconductivities obtained without and under the irradiation
were almost same magnitudes and increased withtemperature. The results indicate that the magnitude of RICof the high-purity single crystal SiC material does notincrease significantly with temperature. This temperaturedependence of the RIC is similar to that of other oxide
ceramic materials [11]. From exponential curve fitting forthe temperature dependences, the electrical conductivity inthe semi-insulating #1 plate was considered to be
dominated by a shallow energy level under 200 oC. The

estimated donor (or acceptor) energy level using theequation of V V0 exp (-Ed/kT) was 0.2-0.4 eV due toscattering in the plots. In this equation, Vis conductivity
and Ed is the dominant energy level in the material. For theregion higher than 200 oC, the dominant energy level was
calculated by using the equation for an intrinsic
semiconductor, i.e. V V0 exp (-Eg/2kT), and estimated tobe 3.4 eV. Eg is the band gap energy of the material. The
value was close to 3.3 eV in literature [13].
The temperature dependence of the conductivity was
examined also for the sandwiched plate of
Hexoloy/NITE/Hexoloy, whose insulating property isdominated by the Hexoloy-SA layers. Since the inherent
conductivity without the irradiation was relatively high,significant increase due to the gamma-ray irradiation was
not observed up to ~450 oC except for 1.6 x 10-8 S/mmeasured at room temperature described above. The
temperature dependence of the conductivity of the
NITE-SiC plates were also plotted in the Fig. 3 forFirst wall

Fig.3

-7

10

-8

10

㻌

Fission reactor(JMTR)-9

10

SiC(Semi-insulating)
SiC (n-type)
SiC (Hexoloy/
NITE/Hexoloy)
Y2O3
㻌Radiation induced conductivity (S/m)

-6

10

10

-10

10

-11

10

-12

10

-13

10

-14

60

Co
gamma-ray(Osaka Univ.)

DT neutron
(FNS)

-4

10

-3

10

-2

10

-1

0

CaZrO3Er2O3

1

2

10 10 10 10Dose rate (Gy/s)

10

3

10

4

Magnitudes of radiation induced conductivities (RICs) in the SiC materials obtained at room temperature andcomparison with those of oxide ceramic materials [11]. All data for the oxide ceramic materials have beenobtained at low temperatures from room temperature to 50 oC.

285

T. Tanaka et al., Electrical Insulating Performances of SiC Materials under Gamma-Ray Irradiation

Electrical conductivity (S/m)

1

Fig.4

10
0
10
-1
10
-2
10
-3
10
-4
10-5
10
-6
10-7
10
-8
10
-9
10
-10
10
-11
10
-12
10
-13
10
-14
10




Without irradiation
Semi-insulating #1
Hexoloy/NITE/Hexoloy
NITE-SiC(G H9(G H9

Under Irradiation
Semi-insulating #1(Bias: +250 V)(Bias: -250 V)
Hexoloy/NITE/Hexoloy
NITE-SiC(J H9(G H9

0

100

200

300

o

400

Temperature ( C)500

Temperature dependence of electrical conductivities of SiC materials under the gamma-ray irradiations andwithout irradiation. Plots of the Hexoloy/NITE/Hexoloy specimen at 400 and 450 oC might be several tens
percent smaller due to the electrical resistance of the electrode. Ed: Estimated donor (or acceptor) level
dominating conductivity. Eg: Estimated band gap energy.

+250 V. The drift distance is considered to be longest in thehigh-purity single crystal due to the minimum trapping rateby impurities, defects and grain boundaries, and themagnitude of RIC would be largest.
Although the maximum temperature was ~450 oC in
the present experiments due to the heater performance andlimitation of irradiation time, it is considered possible toextrapolate the present results to a fusion blanket condition
with a higher dose rate and temperature. In a fusion
blanket, the highest dose rate is estimated to be several
kGy/s at the fist wall [14]. Assuming that the RICs arealmost proportional to a dose rate and weakly depend onradiation species, the magnitude at the first wall is
extrapolated to be order of 10-4 S/m from the present
evaluation for the high-purity single crystal plates. Thismagnitude is adequately small compared with therequirement on the SiC FCIs for the DCLL blanket, i.e.
100 S/m. Under the radiation environment of the blanket,
the RIC would dominate the electrical insulating
performance of the semi-insulating single crystal plates
up to ~500 oC. In the case of the Hexoloy/NITE/Hexoloy
plate, the RIC would be dominant up to ~100 oC. In ahigher temperature region, the insulating performances
would be dominated by the inherent electricalconductivities, i.e. thermal excitation of electrons.

comparison. For the NITE-SiC plate, the conductivity
measurement under the irradiation was performed only atroom temperature and significant increase could not be
observed. In the measurement circuit, a maximum
electrical resistance of the thin Pt electrode was ~25:Therefore, it is considered that the plots of the
Hexoloy/NITE/Hexoloy sample at 400 and 450 oC in Fig.
3 were several tens percent lower than actual magnitudes,
while they indicate that the RIC does not increasesignificantly at the high temperatures. The energy levels
dominatingtheconductivities
ofthe
Hexoloy/NITE/Hexoloy and NITE-SiC plates were
estimated to be ~0.73 eV and ~0.09 eV by using theequation of V V0 exp (-Ed/kT).

4. Discussions

The magnitude of radiation induced conductivity(RIC) is considered to be almost proportional to the drift
distance of radiation excited charge carriers of electrons
and holes. Assuming the energy for an electron-hole pair
production is three times larger than the band gap energy,
the density of the pairs are calculated to be ~5 x 1021
pairs/m3/s under the gamma-ray irradiation of 2.3 Gy/s. If
all the electrons and holes are collected by electrodes in thesemi-insulating #1 plate with the thickness of 0.39 mm, theconductivity will be 4.9 x 10-7 S/m for the bias voltage of+250 V. However, the maximum RIC was 1.3 x 10-7 S/m inthe present measurement. Also assuming that a drift
distance of electrons is significantly longer than that ofholes and dominates the magnitude of RIC, this indicatesthat the drift distance of electrons is almost 0.39 x (1.3 x
10-7 / 4.9 x 10-7) mm, i.e. ~100 nm for the bias voltage of

5. Conclusion

Electrical insulating performances of single crystal
and sintered SiC materials were examined by conductivity
measurements under gamma-ray irradiations attemperatures up to ~450 oC. The radiation induced
conductivities (RICs) evaluated for the semi-insulating

286

T. Tanaka et al., Electrical Insulating Performances of SiC Materials under Gamma-Ray Irradiation

single crystal plates, in which the magnitudes of RICs are
considered to be largest, were 2.8 x 10-8-1.3 x 10-7 S/munder the irradiations of 2.3 Gy/s at room temperature. For
the sandwiched plate of sintered SiC layers (Hexoloy-SA/
NITE-SiC/ Hexoloy-SA), the RIC was 1.6 x 10-8 S/m. In
the NITE-SiC plate, the RIC was under measurable level
due to the high inherent conductivity. The maximum
magnitude of RIC extrapolated from the present results
would be the order of 10-4 S/m for the dose rate of several
kGy/s at the first wall of a fusion blanket. The value isadequately lower than the allowable electricalconductivities in the SiC FCIs for the DCLL blanket design,i.e. 100 S/m. Significant increases in the magnitudes of the
RICs were not observed at temperatures of up to 450 oC. At
a high temperature region in a fusion blanket, thermal
excitation of electrons would dominate the insulating
performances of SiC materials. Influences of radiation
damages on electrical properties will be issues in
irradiation effects to be studied for the electricalapplications of SiC materials in a fusion blanket.

[14]

Acknowledgement

The authors would like to express their thanks to Mr.
M. Taguchi of Mitsubishi Electric Corporation (formallystudied at Osaka University) for preparation and dataacquisition in the gamma-ray irradiation experiments. This
work has been performed under the research programs of
National Institute for Fusion Science, NIFS08UCFF004
and NIFS07KFRF035, and the collaborative research
program of Institute of Advanced Energy, Kyoto
University.

[1]

P. Norajitra, L. Buhler, U. Fischer, S. Gordeev, S. Malang
and G. Reimann, Fusion Engineering and Design, 69 (2003)
669-673.
[2] M. Abdou, D. Sze, C. Wong, M. Sawan, A. Ying, N.B.
Morley and S. Malang, Fusion Sci. Tech., 47(2005)
475-487.
[3] S. Smolentsev, N.B. Morley and M. Abdou, Fusion Sci.
Tech. 50 (2006) 107-119.
[4] R.W. Klaffky, B.H. Rose, A.N. Goland and G.J. Dienes,
Physical Review B 21 (1980) 3610-3634.
[5] G.P. Pells,, J. Nucl. Mater. 184 (1991) 177-182.
[6] E.R. Hodgson, J. Nucl. Mater. 212-215 (1994) 1123-1127.
[7] D.P. White, L.L. Snead, S.J. Zinkle and W.S. Eatherly, J.
Appl. Phys. 83 (1998) 1924-1930.
[8] L.L. Snead, Journal of Nuclear Materials 329-333 (2004)
524-529.
[9] A. Kohyama, Key Engineering Mater., 287 (2005) 16-21.
[10] J.F. Briesmeister, MCNP-A general Monte Carlo n-particle
transport code, LA-12625-M, 2000.
[11] T. Tanaka et al., Journal of Nuclear Materials 367-370(2007)1155-1159.
[12] T. Shikama, JAERI-Research 98-053 (1998) 11-18.
[13] M. Rogalla. K. Runge, A. Soldner-Rembold, Nuclear
Physics B 78 (1999) 516-520.

287

L.A. El-Guebaly, The ARIES Team, Fus. Eng. Des. 38(1997) 139-158.