24P051-P
Electrical insulating performance of Er2O3 coatings by MOD method
ＭＯＤ法による酸化エルビウム被覆の電気絶縁特性
Teruya Tanaka1, Fuminobu Sato2, Toshiyuki Iida2, Takeo Muroga1
田中照也1，佐藤文信2，飯田敏行2，室賀健夫11

National Institute for Fusion Science322-6, Oroshi-cho, Toki, Gifu 509-5292, Japan
2
Graduate School of Engineering, Osaka University
2-1, Yamada-Oka, Suita, Osaka 565-0871, Japan1
核融合科学研究所 〒509-5292 岐阜県土岐市下石町322-62
大阪大学大学院工学研究科 〒565-0871 大阪府吹田市山田丘2-1
Electrical insulating performances of erbium oxide (Er2O3) ceramic coatings fabricated by the metalorganic decomposition (MOD) method with the baking temperature of 600 oC have been measured to
examine the applicability to suppression of an MHD pressure drop in a liquid Li cooled blanket system.
The electrical conductivity of a 0.5 μm thick coating sample was 5.6 x 10-12 S/m at room temperature andthe value was almost one order higher than that of sintered Er2O3 disc sample. Increase of the conductivity
under a gamma-ray irradiation was almost same level as that of the sintered disc sample. The electricalbreakdown in the 1 μm thick coating sample was observed for applied voltages of 29 - 40 V. The obtained
results indicate that the MOD Er2O3 layer baked at ~600 oC could have superior electrical insulating
performances.

1. Introduction
Fabrications of large area electrical insulating
ceramic coatings by the metal organic
decomposition (MOD) method have been studied
for suppression of an MHD pressure drop in aliquid Li cooled blanket system. Erbium oxide(Er2O3) has been selected as one of candidates forthe insulating coating in a liquid lithium coolant
from the view point of the chemical stability. Inthe MOD process, ceramic coatings are obtainedby immersion of a substrate plate into an organic
liquid containing Er and baking the substrate
plate in an oxidation atmosphere at >~500 oC.
While it has been confirmed that the superior
crystalinity could be obtained by baking at ahigher temperature in our previous studies [1], it
would be preferable to keep the baking
temperature similar to a blanket operation
temperature to avoid significant mechanicalproperty changes in structural materials of ablanket. Electrical insulating performances of theMOD Er2O3 coatings fabricated with therelatively low baking temperature of 600 oC havebeen examined in the present study.
2. Experiments
Coatingsamplesforconductivity
measurements were fabricated on substrates of
SiO2 plates. An electrode layer was made on thesubstrate surface by DC sputtering of Pt. Er2O3
coating layers were fabricated on the Pt coatedsubstrates by immersing into an MOD liquid andbaked at 600 oC in air. The thickness of thecoating layer was 0.5 μm. On the coating layer, 2mmφ electrodes were made by DC sputtering ofPt. The electrical conductivity was measured byapplying a bias voltage to the Pt layer on the SiO2
substrate and measuring an electrical current at
the 2 mmφ electrode on the Er2O3 coating layer.
The measurement was performed at room
temperature in a vacuum of ~2.5 x 10-2 Pa.
Increase of the conductivity in a radiation
environment, i.e. radiation induced conductivity(RIC), has also been measured under a 60Co
gamma-ray irradiation with a dose rate of ~2Gy/s.
In addition to the electrical conductivities, abreakdown voltage of a coating layer is also an
important property for the suppression of an
MHD pressure drop in a blanket system. Acoating sample for the examination of abreakdown voltage was fabricated on a polished
SUS430 substrate with the thickness of 1.0 μm. A
bias voltage was applied to the SUS430 substrate
and a current flowing through the tungsten needle
probe touching the coating surface was measured
at room temperature in air.
3. Results and discussion
The electrical conductivity of the Er2O3 coating
sample at room temperature was measured for abias voltage of <0.5 V, i.e. electric field of <14. Conclusion
Electrical insulating performances of Er2O3coatings fabricated by the MOD method have been
examined at room temperature. The coating
samples in the present study have been baked at 600oC which could avoid significant mechanicalproperty changes of structural materials of a blanketsystem. While the electrical conductivity of 0.5 μm
thick MOD Er2O3 coating was almost one orderhigher than that of sintered Er2O3 disc sample, it
still showed a superior insulating performance. The
increase of the electrical conductivity of the MOD

coating under a gamma-ray irradiation of ~2 Gy/s,i.e. radiation induced conductivity, was almostsame as that of sintered disc sample. The electricalbreakdown voltage examined for the 1.0 μm thickMOD coating was significantly higher than thepotential difference expected in the Li cooled
blanket system. The examination of the insulating
performances and their stability at the blanket
temperature of ~650 oC is being performed atpresent.
15
Without irradiation
Under irradiation

Current (pA)

10Thickness: 0.5 μm

5 Electrode: 2mmφ0-5-10-15-0.6 -0.4 -0.2 0.0 0.2
Bias voltage (V)

0.4

0.6

Fig.1. Voltage-current curves obtained in conductivity
measurements without irradiation and under agamma-ray irradiation with a dose rate of ~2 Gy/s.

Current (A)kV/mm. The current flowing through the coatinglayer was almost proportional to the bias voltage
and the evaluated conductivity was 5.6 x 10-12 S/m(Fig.1). The conductivity measured for a sintered
Er2O3 disc sample was in the order of 10-13 S/maround room temperature [2]. The conductivity of
the MOD coating was almost one order higher thanthat of the sintered disc.
The conductivity of the MOD coating increased
to 1.05 x 10-11 S/m under the gamma-ray irradiationof ~2 Gy/s and the magnitude of the radiationinduced conductivity (RIC) was evaluated to be 4.9
x 10-12 S/m. The magnitude of the RIC in the MOD
coating is almost same level compared with that in
the sintered Er2O3 disc sample. From the previous
measurement of RICs at high temperatures, it isconsidered that magnitudes of RICs reflect drift
behaviors of charge carriers inside grains. The
present result of the RIC measurement indicatesthat the electrical insulating property of the Er2O3grains grown by the MOD method at 600 oC issimilar to that of Er2O3 powders which are used infabrication of the sintered disc sample and show
higher crytalinities. The one order higher
conductivity of the MOD coating layer measured
before the gamma-ray irradiation might be due toelectrical currents through grain boundaries.
The breakdown voltage of a 1 μm thick Er2O3
coating on a SUS430 substrate was examined byincreasing the magnitude of the DC bias voltage bysteps of 1 V. Responses obtained at fourmeasurement points on the coating are shown in Fig.
2. The breakdown voltages that the coatingcompletely lost the insulation were 29 – 40 V.
These applied voltages correspond to the electricalfield strengths of 29 – 40 kV/mm in the coating
layer. For the condition of a magnetic field of 10 T,
coolant flow velocity of 1 m/s and duct width of 0.1m, the electrical potential different in the blanket
duct will be ~1 V [3, 4]. The measured breakdown
voltage is significantly higher than the expected
potential difference.

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

Measurement
position
#1
#2
#3#40

5 10 15 20 25 30 35 40 45
Bias voltage (V)Fig.2. Responses in a breakdown voltage
measurement.

Acknowledgment
The authors would like to express their gratitude
to Dr. D. Zhang for fabrication and property
improvement of the coating samples. This study has
been conducted under the NIFS research programof UFFF021.
References[1] D. Zhang et al.: Journal of Nuclear Materials 417(2011) 1249.
[2] T. Tanaka et al.: Journal of Nuclear Materials
367–370 (2007) 1155.
[3] H. Hashizume: Fusion Engineering and Design 81(2006) 1431.
[4] S. Malang et al.: Fusion Engineering and Design 27(1995) 570.