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Journal of Nuclear
Materials 217 (1994) 32-47
Review article
Electrical breakdown of insulating ceramics
in a high-radiation field
Y. Chen a*b,F.W. Clinard ‘, B.D. Evans d, E.H. Farnum e, R.H. French ‘,
R. Gonzalez g, J.J. O’Dwyer h, F.W. Wiffen i, X.F. Zong j
a USDepartment of Energy, ER 131, Office of Basic Energy Sciences, Washington DC 20585, USA
b Oak Ridge National Laboratory, Solid State Division, P.O. Box 2008, Oak Ridge, TN 378316031, USA
’ Los Alamos National Laboratory, P.O. Box 1663, MS K-762, Los Alamos, NM 87545, USA
d Boeing Defense & Space Group, P.O. Box 3999, MS 9E-XWj Seattle, WA 98124-2499, USA
e Los Alamos National Laboratory, P.O. Box 1663, MS K-762, Los Alamos, NM 87545, USA
’ E.I. DuPont de Nemours & Co., Central Research & Development, Experimental Station Bldg. E356 /323, Wilmington,
DE 19880-0356, USA
8 Universidad Carlos III, Departamento de Ingenieria, Escuela Politecnica Superior, Avda de1 Mediterraneo, 20, 28913 Leganes,
Madrid, Spain
h State University of New York, Department of Physics, Oswego, NY 13126, USA
i US Department of Energy, ER-543, Office of Fusion Energy, Washington DC 20585, USA
’ Fudan University, Department of Materials Science & Engineering, Shanghai 200433, China
Received 21 January 1994; accepted 21 June 1994
Abstract
This report addresses a recently reported phenomenon - that a simultaneous application of energetic particle
radiation, electric field, and elevated temperature for an extended period of time has a permanent adverse effect on
insulating ceramics, including electrical breakdown. This behavior poses a serious challenge for fusion devices, which
require electrical insulators in several key components. The summary and recommendations
developed here are
based largely on the proceedings of a research assistance task force meeting entitled “Electrical Breakdown of
Ceramics in a High-Radiation
Field”. Since this is a rapidly expanding field, this report attempts to include
highlights of pertinent studies reported at recent international meetings. In one recent meeting the fusion materials
community recommended that this effect be referred to as radiation-induced
electrical degradation CRIED).
1. Introduction
This report is based primarily on the proceedings
and recommendations
of a 1991 Research Assistance
Task Force meeting on “Electrical Breakdown of Ceramics in a High Radiation Field” [l], sponsored by the
US Department of Energy. It includes recent studies
pertaining to this topic. This meeting was held in
response to recent reports that a simultaneous application of energetic particle Radiation, Electric field, and
elevated Temperature (to be referred to as Rad-E-T)
for an extended period of time has an adverse effect on
insulating ceramics and can lead to electrical break-
down [2-41. These reports characterize
the phenomenon as an unexpected increase in DC electrical
conductivity, over and above that previously known.
Above a critical dose the conductivity increases with
irradiation.
The previously known conductivity increase, Au, known as radiation-induced
conductivity
(RIG), was described by the phenomenological expression Au = kRs where R is the ionizing radiation dose
rate [S-7]. During the meeting, it was recommended
that this new effect be generalized and termed radiation-enhanced electrical degradation, in order to distinguish it from RIC. It has since been accepted that
the phenomenon be referred to as radiation-induced
0022-3115/94/$07.00 0 1994 Elsevier Science B.V. All rights resewed
SSDI 0022-3115(94)00338-O
33
Y. Chen et al. /Journal of Nuclear Materials217 (1994) 32-47
electrical degradation (RIED). The RIED acronym is
used in this paper. It was reported at the meeting that
RIED takes place after an incubation period with
application of Rad-E-T. In contrast to RIC, removal of
radiation does not return the conductivity to its preirradiation level. Furthermore, it was reported that the
degraded ceramic insulator cannot be readily annealed
or otherwise returned to its original low conductivity.
This behavior poses a serious challenge for fusion
devices, which require electrical insulators for several
applications
including diagnostic systems, radiofrequency and neutral beam heating systems, magnetic
coil insulators, and toroidal current breaks.
The problem of selecting suitable electrical insulating materials for fusion applications may be far more
complex than previously anticipated [g-lo]. Selection
criteria, in addition to resistance to radiation-induced
swelling, changes in strength, resistance to thermal
shock, thermal conductivity, electrical conductivity and
dielectric loss tangent. In addition, chemi-physical
near-surface alterations may have to be expanded to
include more subtle synergistic effects due to combinations of environmental parameters such as electric field
strength, temperature, and accumulated irradiation fluence measured as ionizing dose and displacements per
atom (dpa). The observations reported at the meeting
showed a large diversity, with conductivities for crystalline sapphire and polycrystalline alumina ((Y-AlzOs)
samples having increased by several orders of magnitude from the reported pre-irradiation
levels ranging
from 10W14-10-10 (a m)-’ to 10-3-10-5
(0 ml-’
under prolonged energetic electron irradiation near
750 K as the accumulated irradiation level approached
lop3 dpa [l-4,8]. Other results indicate conductivity
reaching 1O-5 (Cl m)-’ as fission-spectrum neutron
dpa approached unity, and total background gamma
approached 2.4 x 10 lo Gy at temperatures
between
600 and 1000 K [l]. These conductivity values pose a
challenge to reactor design and materials efforts. The
meeting served two main purposes: (1) discussion of
past and current results related to the extent of understanding of RIED; and (2) identification of opportunities and methodologies to serve as guidelines for research toward the understanding and solutions to this
problem.
This report emphasizes the importance of in-situ
studies. Traditionally, radiation damage to a material
has been studied as a post-mortem event: the material
is first subjected to irradiation and the resultant effect
of the damage is later studied by experimental measurements and analyses, often in response to an applied external perturbation. The perturbation can be a
beam of light, an electric field, or perhaps even the
same type of beam that caused the damage in the first
place, such as electrons, or neutrons. However, in
certain situations the net effects of the perturbation
applied during irradiation can be drastically different
from those measured after irradiation. Specifically, the
effects of Rad-E-T are not the same as those observed
if the sample is first irradiated at an elevated temperature and then subjected to an electric field. One of the
conclusions of the meeting is that there is a dearth of
in-situ irradiation effects data base on dielectric materials, especially as it pertains to the use of these materials for fusion.
2. Characteristics of the fusion environment
The expected applications and operating conditions
for ceramic insulators in fusion power systems [11,12]
are as follows:
Insulators for lightly-shielded magnetic coils. Coils
such as those used in divertors and for plasma stabilization may be located near the first wall. Insulators
may be in either solid or powder form, depending on
magnet design. Operating temperatures, will vary from
slightly above to well above room temperature depending on extent of cooling supplied. First-wall-like radiation conditions (about 1019 n m-’ s-l and lo4 Gy/s
gamma) will be imposed on these insulators. The likely
mode of failure is RIC, perhaps accompanied by RIED.
Windows for electron cyclotron resonant heating
(ECZW) systems. ECRH windows must retain their
transparency to electromagnetic beams of frequency
about 100 GHz. Retention of high thermal conductivity
is necessary to avoid thermal stress-induced mechanical failure resulting from absorption of energy from the
beam. Attempts will be made to locate the windows
well away from the first wall, but too remote a location
will compromise operation of the ECRH system. The
present expected dose rates for the ECRH windows
are several orders of magnitude lower than that at the
first wall (i.e. < 1014 n mP2 s-l).
Standoffs for ion cyclotron resonant heating antennas. These antennas will operate at about 100 MHz,
and must be mounted inside the first wall. There will
also be windows further along the coax lines behind
the first wall. Thus radiation fields will be intense and
operating temperatures will be high. In addition to
radiation damage, the standoffs may be damaged by
sputtering and/or by deposition of impurities.
Insulators for
neutral beam
injectors (NBZJ. Al-
though remote from the first wall, the massive insulators for the neutral beam injector sources will be
exposed to low-level radiation resulting from streaming
down the injector ducts. The insulators themselves may
be made of lower grade ceramics in order to enable
fabrication of the very large insulator/electrode
stacks;
if it is necessary to use radiation-sensitive silicate-based
materials, then significant damage may occur.
34
Y Chen et al. /Journal of Nuclear Materials217 (1994) 32-47
Insulating coating for suppression of magnetohydrodynamic (MHD) forces. Liquid metal coolant pumped
through the blanket will be impeded by MHD forces
unless the metal pipes are lined with an insulating
coating. Electrical requirements are modest, but cracking and spalling of the coating may occur if its bond is
weakened by radiation. Differential swelling is a particular problem for this application. Radiation fields will
be about an order of magnitude lower than those at
the first wall, but will still be high enough to damage
both metals and ceramics.
Toroidal current breaks. It may be necessary to divide the metal structure of the torus into electricallyisolated sections (in some designs) to minimize damage
from disruptions. Insulating current breaks may take
the form of thin ceramic layers or insulated bolts.
Radiation fields will be as high as 10% of first wall
level. The most serious problem is mechanical failure
(flaking or fracture), which could lead to loss of electrical insulation.
Insulators and optical components for diagnostic systems. A number of dielectric materials are specified for
use in diagnostic systems. Included are: probes to measure magnetic field intensity at the first wall, where
temperatures may exceed 1000 K, windows and reflectors, likely located near the first wall; and optical
fibers, used widely throughout the reactor. Of these,
the most radiation-sensitive
components are windows
and fibers, where SiO,-based materials are specified,
and severe damage can be expected [13,14]. Remoteness from the first wall, frequent changeout, and periodic thermal annealing will be employed where possible to achieve viable performance. While more resistant to the effects of irradiation, ceramic insulators for
other diagnostic systems will be less remote, and their
performance is also a concern.
Insulators for superconducting toroidal field (TF)
coils. Designs of TF coils usually specify polymer insu-
lators. However, the known sensitivity of organic insulators to radiation damage, coupled with concern about
generation of radiolytic gases, is increasingly directing
attention to ceramic insulators. Radiation fields in
these shielded magnets are low (about 1015 n me2 s-l
and 1 Gy/s), but their effect on conventional ceramics
will be magnified by the greatly increased retention of
displacement damage at 4 K. If construction requirements dictate that the insulator be injected into the TF
coil structure, then inorganic cements can be considered; however, the relatively high radiation sensitivity
of traditional water-bearing silicate-based cements must
be taken into account.
3. Background studies and recent work
All solids conduct electricity to a greater or lesser
extent, and all will suffer some form of electrical (or
dielectric) breakdown in a sufficiently strong electric
field [15]. In the case of insulators, such electrical
breakdown is characterized by a rapid increase in the
electrical conductivity [15-181. In this section, we shall
provide (a) the background information regarding electrical breakdown of oxides at high temperatures in the
absence of irradiation; (b) a brief background of materials requirements for fusion devices; (c) electrical conductivity at high temperatures in the absence of radiation; and (d) a brief description of RIC.
3.1. Breakdown at high temperatures without irradiation
In the last two decades, the need for ceramics as
insulators at high temperatures in a number of advanced energy systems has become very critical. Investigations of the electrical properties of several oxides
[19-271, including MgO, Al,O,, MgAl,O,, Y,O,,TiO,,
CeO,, MgSiO, and SiO,, at high temperatures (13001500 K) have been reported. The results led to a new
level of understanding of the fundamental mechanisms
and phenomena that cause the breakdown of the insulating properties of oxide ceramics. In the breakdown
process, the electrical current increases until the material evaporates along the path of least electrical resistance and leaves behind a large channeled gap. During
the current increase the temperature of the sample was
determined to rise above the ambient temperature.
The observation and new understanding of the breakdown phenomena can be summarized as follows.
(1) Formation of impurity precipitates and dislocations. The breakdown process frequently is preceded by
the formation of a dark coloration in the crystal [19].
Fig. 1 shows a cross-sectional view of a MgO: Mn
sample heat-treated at 1050°C for 2 h with a 1100 V
potential applied between a point contact and a planar
platinum electrode [21]. This figure illustrates the electric-field flux lines and the electrolytic-coloration
effects observed in the crystal. The similarity between
the shape of the altered region and the E-field distribution is noted. The narrow top portion of the
‘volcano-shaped’ dark region is located at the position
where the cathode point contact was made. Near the
broad bottom portion of the cone, where the coloration
has not proceeded to the point of making the entire
area completely opaque, the altered portion of the
crystal was a dark brownish, amber color. Coloration
generated in such manner has been shown to arise
from Mie scattering from metallic precipitates that are
produced by electrolytic reduction of impurities in the
crystals.
In Fe-doped MgO ( N 1700 ppm) TEM micrographs
revealed a variety of precipitates and dislocation tangles [19]. Analytical transmission electron microscopy
(TEM) has provided an identification of the chemical
composition and structure of the precipitates and has
‘ANODE
MgU: Mn
Fig. 1, A sketch of the sample and electrode ~n~~rat~on
showing a calculated representation of the electric-field flux
lines present in the sample (left); photograph of an MgO: Mn
crystals following a heat treatment at 1323 K and an applied
voltage at 1100 V for 2 h (right). (See Ref. 1211.)
demonstrated that they are formed coherently in the
crystal lattice. Microdiffraction and X-ray fluorescence
experiments indicated that the precipitates are bodycentered iron primarily, and in some cases, Fe0 and
MgFe,O,, all of which are coherent within the lattice.
In undoped ctystals, no Mie scattering was observed
and the primary defects formed were dislocations.
There were discussions at the meeting as to whether
the formation of metallic colloids is a necessary condition for breakdown. There was no uniform agreement.
Most of the participants believed that it was not. It was
then pointed out that there were counter-examples: in
undoped high-purity MgO crystals, electrical breakdown did occur without the fo~ation
of metallic colloids 1221.
(2) Ph~sicffl ~ru~eters a~ec~n~ elective breakbowm Several parameters were found to affect the
efectrical characteristics and influence the electrical
breakdown process of MgO and other oxide crystals
[24]. They are temperature, surface preparation, etectric field, field reversal, ac and dc, ambient atmosphere, electrode material, and impurity content in the
sample. Some parameters are more critical than others. Parameters such as surface preparation are obvious villains, It has been shown that chemically etched
surfaces are much more resistant to electrical breakdown than mechanically polished surfaces [22-261. At
elevated temperatures dislocations generated by mechanical polishing in the surface region of MgO crystals propagate into the bulk material [28,29] and can
therefore provide the ~nd~t~ons conducive to electrical
breakdown.
These studies on electrical breakdown were primarily confined to dc conditions. Application of ac did not
produce breakdown. This obsetvation was not surprising, in view of the model that was deduced. The
breakdown was clearly demonstrated to be a space
charge problem, as shown in the following segment.
(3) A space charge problem. In view of the production of precipitates, it was tempting to interpret the
increase in conductivity as due to the inter-twinning of
precipitates. It is realized that the population of the
precipitates, if any, is low - typically no more than lOi
cm-‘. Furthermore, a simple experiment appears to
discard this likelihood. When the conductivity begins to
increase rapidly, the polarity of the electric field is
reversed [221. The result is that the eiectrical conductivity decreases instead of continuing to increase, as
would be expected if the current increase is due to the
creation of a conducting path, due to metallic precipitates. Reversal can be administered as many times as
desired with the same general result. The results are
shown in Figs. 2 and 3.
When a moderate dc electric field was applied to a
nickel-doped MgO crystal at 1473 K, a slight decrease
in current was observed martially. Then the current
increased slowly at first and subsequently exponentially
until the sample experienced electrical breakdown and
could no longer be used as an electrical insulator.
However, in the experiment shown in Fig, 2 the sample
-20
I
0
I
I
io
M
TIME lhl
I
I
I
XJ
Fig. 2. Current vs. time at 1473 K with E = 1SOkV/m applied
to MgO: Ni for several polarity reversals. (After Ref. (251.)
Y. Chen et al. /Journal
36
0
ofNuclear
‘
I
I
I
Materials 217 (1994) 32-47
source of the material, the crystai-growth conditions,
and the experimental conditions were identical. The
results are shown in Fig. 4. Crystals doped with V, Cr.
Fe, or Ni were more susceptible to breakdown. Doping
with Co, H, or Cu suppressed breakdown. Doping with
Cu provided the most dramatic effect; Cu suppressed
the breakdown characteristic time by a factor of about
20.
3.2. Insulating materials for fusion power systems
cl*
0
40
9
nrh FIELD
REVERSAL
Fig. 3. Characteristic time for breakdown versus the nth field
reversals. (After Ref. L251.1
was not allowed to proceed to breakdown since this
would have destroyed it. When the current reached a
certain level, in this case 18 mA, the appIied field was
reversed, because at this current the breakdown was
imminent and the sample temperature began to significantly exceed the ambient temperature. The time required for the current to reach 18 mA was considered
to be the characteristic time for breakdown. Fig. 2
illustrates the current behavior of several field reversals. Fig. 3 is a plot of the characteristic time for
breakdown as a function of chronological field reversals. The initial decreasing time trend is evident. After
the fifth reversal, the characteristic time remains constant. These results suggest that the breakdown is not
due to creation of conducting paths via precipitates,
but is a manifestation of creation of space charge.
Other features of conductivity increase at high temperatures are consistent with a double-injection model
[25] in which hole injection at the anode is augmented
by the accumulation of negative charge through some
form of ion or vacancy migration. The resultant increase in hole concentration
demands an equal increase in electron concentration
from the cathode.
Hence, there is creation of an exponentially increasing
current which, unchecked, will lead to electrical breakdown.
Suppression and enhancement of breakdown by impurity doping. A study on the effect of impurities on the
characteristic time for electrical breakdown was performed in MgO crystals 1231. This host was chosen
because of its high melting temperature, simple crystalline structure, high purity, and availabili~ as a highquality refractory crystal. Crystals doped indi~dually
with H, Cu, Co, Fe, Ni, Cr, and V were used. The
The dielectric materials requirements for fusion systems have been summarized in several publications in
which both the general and more specific aspects were
treated [30-331. These devices require electrical insulators in diagnostic systems, radiofrequency and neutral
beam systems, and in magnetic assemblies. During the
operation of such devices, the insulating systems are
bombarded by neutrons and gamma rays (which result
in Compton electrons) resulting directly or indirectly
from the D-T fusion reaction. Alpha particles from
the D-T reaction are stopped in the plasma or at the
first wall of the reactor and will not generally bombard
the ceramic insulators.
One of the important findings in the study of the
high-temperature electrical breakdown without irradiation is that of ail the oxides studied, sapphire {crystalline alurni~~ is the only material immune to electrical
brea~o~n,
within the experimental time scale [261.
Therefore, alumina is a leading material candidate for
sub-systems in fusion devices.
3.3. Conductivi~ at high temperatures ~without irradiation)
Experimental studies of electrical conduction in insulators at elevated temperatures is important for many
20
0
0
too
200
300
400
TIME b,
Fig. 4. Current versus time at T = 1473 K with E = 150 kV/m
applied to the doped and undoped MgO samples. Crystals
doped with V, Fe, or Ni have lower characteristic time for
breakdown than the undoped ones. The crystals ~nta~ning
Co, H, or Cu have higher characteristic time for breakdown.
(After Ref. [23].)
37
modem energy and electronic systems, and has received much attention for many years. Theoretical descriptions often deviate significantly from experimental
measurements, especially in the wider band gap materials, because of the presence of atomic and structural
defects which are ill-characterized and can give rise to
donors, acceptors, and charge trapping levels within
the band gap. Fig. 5 shows the spread of conductivity
versus temperature curves reported for sapphire [34],
compared to results obtained by Will-deforenziJanora [35]. For example, at 1000 K the conductivity
ranges over six orders of magnitudes, varying from
lo-l3 to 10F6 (fi m)-‘. A similarly wide range of
conductivities has been reported for other insulators,
such as MgO, BeO, Y,O,, AlN, BN, and Si3N, [36].
This range of experimental variations appears to be
due to different material purities, preparation procedures, material forms (single crystal, polycrystalline,
sintered, fused, etc.), thermochemical history, and experimental techniques. The latter may depend on the
number of contacts, with or without guard ring, AC or
DC, atmosphere, or vacuum. Therefore one of the key
Tem~rature (‘C)
2000
I
45
I
1000
200
MO
I
t
15
10
iovr
20
I
25
(K-1)
Fig. 5. ~onduct~~~ versus temperature for past measurements of ru-AlzO, (Ref. [36]) compared to the recent measurements of Will, deLorenzi and Janora (Ref. f3511, performed on a single crystal at 10m7 Ton vacuum ( - 40 ppm
impurities); and Pells on Vitox alumina (99.9% pure polycrystailine, less than 2 pm grain size (major impurity 500 wppm
MgO) under unspecified vacuum (Ref. [941X
TEMPERATUREPC)
2000
i 000600400
200
10‘2
100
50
2(
-0- Will et al
--CL- fiod@m
-A- Pelts et al
-+
Klaffkyet al
IO-‘6
0.5
1 .o
1.5
2.0
2.5
3.0
3 5
T“ (K’x103)
Fig. 6. Conductivity versus temperature for alumina, after Will
et al. (Ref. [35]), Pelts (Ref. [32]), Hodgson (Ref. [ZD, and
Klaffky et al. (Ref. [71X
characteristic of electrical measurements has been the
large variability.
There were several reports at the meeting on conductivity versus temperature measurements, shown in
Fig. 6. Also shown is the recent curve by Will et al.
made on high-purity sapphire crystal, employing experimental techniques along the lines suggested by Frederikse and Hosler [37] to eliminate surface and gas
leakage conduction. These crystals were taken from
boules grown by the Schrnid-Viechnicki technique and
obtained from Crystal Systems, Newton, MA. Using
high-purity samples and paying careful attention to
parasitic leakage paths, they obtained conductivity values lower than those previously reported; above 1000 K
the electronic conduction mechanisms dominate over
ionic conduction and the activation energy approaches
E,/2, as suggested by theory. This value is higher than
previously reported values. Other studies report higher
conductivities for both high-purity single crystals as
well as commercial polycrystalline alumina. Therefore
it may not be surprising that variability in conductivities can prevail when the materials are placed in a high
radiation field, even apart from the RiC dose-rate
dependent conductivity. This, in fact, was a major
concern at the meeting.
Fig. 6 also shows the data from Klaf&y et al. on the
temperature-dependent
conductivi~
of undoped
~z~hralski-grown
AlzO, single crystals measured in
the presence of irradiation with 1.5 MeV electrons,
albeit restricting the accumulated electron fluence to
low values, i.e. < 1 X 1Or6 e/0$.
The salient feature
is that in the presence of ionizing radiation at a dose
38
Y Chen et al. /Journal of Nuclear Materials217 (1994) 32-47
rate of 6.6 X 10’ Gy/s the conductivity
about eight orders of magnitude.
is increased
3.4. Radiation-induced conductivity (RIG)
In earlier measurements, insulators were subjected
to high-temperature and high-radiation exposure, to be
followed by subsequent measurements of the electrical
conductivity. This procedure, known as post-irradiation
conductivity measurement, was recognized at an early
stage in the fusion program to lead to erroneous conclusions. It was also recognized that there would be
substantial changes in the insulating properties of dielectrics during operation in the elevated-temperature,
high-radiation environment of a fusion reactor. Experiments using electron accelerators were performed on
sapphire to study RIC effects [7]. The enhancement of
the dc electrical conductivity in insulators during irradiation was recognized as a potential problem fairly
early in the design of fusion systems. RIC is a direct
result of the creation of electron-hole pairs by ionizing
radiation. The total electron dose for these experiments was relatively low, however, and the experiments
were confined primarily to studies of the RIC dependence on the temperature and beam intensity [7]. From
these studies, values of the constants K and 6 near 750
K for undoped crystalline sapphire were determined;
K = 8 x lo-l3 and 6 = 0.85, when the dose rate, R, is
expressed in units of rad/s and conductivity in (fl
cm)-‘. Pells [l] has reported that 6 for Al,O, can vary
between 0.5 and 1.3. The RIC effect reaches equilibrium rapidly, either increasing or decreasing and does
not seem to present great challenge to reactor designers.
3.5. Radiation-induced
tions
conductivity by proton irradia-
Farnum et al. [38] measured RIC in sapphire from
100 to 10 MHz during irradiation with 3 MeV protons.
A thin sample was employed, so that the protons
passed through the material. In one series of experiments the beam was repeatedly cycled on and off while
the conductivity was monitored, with the result shown
in Fig. 7. Conductivity increased sharply at the beginning of the irradiation, but then decayed exponentially
with time. When the beam was turned off, conductivity
returned to its near-zero starting value. As the experiment progressed, RIC dropped well below the initial
value, but remained above the beam-off reading. These
results were interpreted in terms of a balance between
the generation of electrons and holes, and their trapping and recombination at displacement-type
defects,
which built up as the radiation dose increased.
Proton irradiation caused an immediate increase of
the loss tangent from about low4 to more than 1.0. As
Time (ks)
Fig. 7. Ohmic electrical conductivity in sapphire at 100 kHz
versus chronographic time, during irradiation with 3 MeV
protons of beam current 0.6 PA/cm’ at 300 K.
was the case for the resistivity component of loss, the
increase in loss tangent lessened at longer times. Such
a large increase in loss tangent could cause unacceptable degradation in performance of insulators used for
high-frequency applications.
3.6. Radiation-induced conductivity in a reactor.
Insulating windows to a fusion reactor chamber
must transmit radio frequency energy at a loss that
does not become excessive. Stoller et al. [39] measured
the change in dielectric loss tangent of alumina at 100
MHz during irradiation in the gamma irradiation facility at the Oak Ridge National Laboratory and in the
mixed gamma and neutron environment of a TRIGA
fission reactor. Under gamma irradiation alone, there
was no significant increase in the loss tangent; however, the mixed gamma-neutron
irradiation field produced an increase in the loss tangent by a factor of
about 15 to a final value of about 10m4. This loss-tangent degradation is high enough to cause concern for
the design of radio frequency windows.
4. Effects due to simultaneous
application
of Rad-E-T
In this section we shall provide a brief overview of
recent studies on the effects of combined Rad-E-T in
ceramics, with emphasis on electron and neutron irradiations. For a full account of this work, the reader
should consult the original papers where details are
available. Nevertheless, even the abbreviated accounts
given here illustrate that the knowledge of Rad-E-T
effects on the insulating property of dielectrics is scant.
40
1
10
t
cl
10
20
30
/ *
40
50
60
t(h)
Fig. 8. Current versus time for a single-crystal alumina sample
obtained from R~iti-Union Carbide uv grade during electron
irradiation at 773 K with 1.8 MeV electrons with an applied
field of 130 kV/m (Ref. [21).
4.1. RIED by electron irradiation
Hodgson first extended the earlier studies to higher
doses and demonstrated that the problem of electrical
insulation for fusion power systems extends beyond the
RIC effects [2-41. Sapphire crystals were subjected to
simultaneous application of Rad-E-T, in order to simulate fusion reactor conditions. It was something of a
surprise to the ceramics and fusion community when
the failure of sapphire insulation under bombardment
by high-energy electrons was announced 12,331. The
sample was held at 773 K at a field strength of 130
kV/m and subjected to irradiation with 1.8 MeV electrons from a HVEC Van de Graaff accelerator. Initially, the simultaneous application of Rad-E-T generates a RIC which remains constant for a long period of
time. Based upon previous e~erimentally
determined
f7] values of I( and S, the value of RIC at the dose rate
employed (R = 1 x lo6 Gy/h) was expected to be - 1
x 10e6 (ti m)-‘. This value was appro~mately
the
RIC level observed. At some critical dose corresponding to about 30 h of irradiation, shown in Fig. 8, the
electrical conductivity began to increase, signaling the
onset of what was reported to be an electrical breakdown process. At about 70 h the conductivity had
increased to 8 x lo-’ (fi m)-‘. The increase appeared
to be in the base conductivity and not an increase in
RIC, since post-irradiation measurements give a conductivity that was reduced only by the RIC. It was
reported that the increase in conductivity was attended
by a dark coloration, which Hodgson associated with
the presence of colloids [40]. He suggested that the
colloid production plays an important role in the RIED.
The RIED was found to be a strong function of the
applied electric field. Under identical conditions, but
without the electron irradiation, there would be no
increase in the electrical conductivity for the duration
of the experiment [26]; therefore, no degradation would
occur. Thus the RIED is linked to bombardment with
energetic electrons.
Hodgson [2-41 also reported the same effect in
single-crystal MgO and in single-crystal and polycrystalline spinel. In addition, similar results were obtained
for conductivities measured by ac voltages up to 126
MHz. However, RIED was not reported during irradiations with 0.3 MeV electrons, which is below the
reported A&O, displacement damage threshold for
energetic electrons, 0.40 It 0.04 MeV [41,423. Therefore, on the basis of this energy dependence, I-Iodgson
concluded that both elastic collision and ionization
events are necessary for electrical breakdown.
Recently Zong et al. [43] subjected single crystals of
sapphire to electron Rad-E-T under experimental conditions similar to Hodgson’s investigations [2-41, but
not identical. The samples were irradiated in air, instead of vacuum. The energy of the electrons was 1.8
MeV. The dose rate and the fluence were higher than
those of the previous study, and only the center portion
of the sample disk was irradiated. Fig. 9 illustrates a
semi-log plot of the dc conductivity of a sapphire
crystal at various stages of electron irradiation. The c
Cdurins had)
Electron Dose (e7m2)
(X@)
Fig. 9. Semilog plot of etectrical conductivity versus electron
(1.8 MeV) dose for an A1,Os crystal, obtained from the
Shanghai Institute of Optics and Fine Mechanics (SIOFM)
during irradiation (top curve), and after irradiation followed
by stabilization of the current (bottom curve). (See Ref. 1431.)
Y:Chm etal,
/Journal of Nuclear Materials 217 (1994) 32-47
40
axis of the crystal was perpendicular
to the sample
surface, and parallel to the applied electric field and
the incoming electron beam. During irradiation the
temperature of the sample was heid at 773 K and an
electric field of 2120 V/cm was maintained. The beam
current was about 4 PA/cm’, or 2.5 x 1013 e cm-’
S -I.
The
top curve illustrates the ~ndu~tivi~
during
irradiation, represented by o. Obviously these values,
which include RIG, depend on the intensity of the
beam current, which was maintained at a constant
Ievel. No data were taken initially during irradiation,
but the conductivity was projected by the dotted line.
The beam current was intense, reflected by the nearly
flat curve throughout; clearly the conductivity was
dominated by RIC. These results confirmed that at
some critical electron fluence during Rad-E-T, the
post-irradiation
electrical
conductivity
increased
rapidly, albeit the critical dose was a factor of 10
higher than that observed by Hodgson. The conductivity did not increase indefinitely but saturated at - 2 X
lo-’ (fI ml-‘. Two main conclusions emerged from
this study: First, the mechanism for RIED was attributed to the charge of electrons and holes created
during irradiation, rather than due to displacements of
indigenous ions by elastic collisions with the energetic
bombarding
electrons. Second, the primary defect
which leads to RIED was deduced to be dislocations
produced during Rad-E-T. TEM revealed a large average dislocation density, non-unifo~ity
distributed
throughout the degraded area, with an overall density
of - lo9 cmm2, as opposed to - IO4 cm-* at the
periphery of the sample which was not subjected to
electron bombardment or to an electric field. No second phase was observed. The concentration of point
defects, as characterized by optical absorption and
electron paramagnetic resonance, was not detectable
in this crystal.
2.5~~05
.?E-05
I!!
e
‘7
3,@
s
3
1.5E-05
s
Fig. IO. Electrical conductivity of an alumina crystal as a
function of days with the reactor running at full power. An
ebctric field of 0.5 kV/m was applied to the sample during
irradiation and the temperature was held at 620 K (after Ref.
1481).
Furthermore they pointed out two serious flaws in
Hodgson’s experiments. First, no meaningful conclusions can be obtained from his energy dependence
experiment, because the sample he used for the 0.3
MeV electrons was too thick - much thicker than the
range of the electrons. Second, the spectrum he attributed to the optical absorption of the F’ center
(anion vacancy with one-electron)
was erroneously
identified. The fulI-width-at-half-maximum
of the absorption band was far too large. In addition, no anion
vacancies were observed after RIED in Zong’s samples, nor in the work in Refs. [56,57]. Zong et al. [43]
pointed out that there should be negligible anionvacancy concentration after RIED at 773 K, because
oxygen interstitials at that temperature are highly mobile. In brief, the production rate of anion vacancies
was much smaller than the thermal annihilation rate.
4.2. RIED by neutron irradiation
In-situ electrical conductivity measurements were
also performed in fission reactors and in a spallation
neutron source. Shikama et al. performed Rad-E-T
experiments on alumina crystal, Kyocera SA 100 (99.9%
pure), in the JMTR water-cooled fission reactor at the
Oarai Establishment
of Japan Atomic Energy Research Institute [44-491. The neutron flux at the specimens with JMTR operating at full power (50 MW) was
estimated to be 3.4 X 1017 and 1.8 X lOi n me2 s-i
for the fast (E > 1.0
MeV) and thermal (E < 0.63 eV)
respectively. The flux for E > 0.1 MeV was about a
factor of 10 higher. The gamma dose rates at the
position used in this facility were estimated to be about
2.8 x lo3 Gyfs at reactor full power. The sample temperature was 600-630 K and the applied electric field
was 500 kV/m. The results are shown in Fig. 10.
Initially the electrical conductivity was due primarily to
RIC. The conductivity was essentially constant at about
6 X 10v6 (fI ml-” up to about 5 days at full reactor
power. This RIC value agrees with the phenomenlogical I@ expression with known values of K and 6 [7].
During this period the specimen had accumulated 2 x
1O23 n/m2 fast neutrons (E: > 1.0 MeV) and (2-6) x
10’ Gy of gamma rays. However, the conductivity
began to increase after 10 reactor full-power days, and
continues to increase even after 44 days at full power.
At a neutron dose of 2 x 1O24 n/m2 and a gamma
dose of (2-6) x 10” Gy, the conductivity had increased
to 2 X 1O-5 (fl m)-‘.
In their second experiment the temperature and the
electric field more closely approximated those of the
eiectron irradiations. The fluxes were 3.4 X 10” and
1.8 X 10” n m-* s-l for fast (E > 1.0 MeV) and
thermal (E < 0.6286 eV) neutrons respectively. The
ionizing dose rate (gamma rays) was estimated to be
5.3 x lo3 Gyfs. The specimen was, however, polycrys-
X Chen et al. /Journal
ofNuclear Materials 217 (1994) 32-47
I
l
**
1
I
I
I
10
30
1
3
0.3
REACTORFULL POWER DAYS (RFPD)
I
/
loo
Fig. 11. Electrical conductivity of two polycrystalline alumina
cxystals as a function of days with the reactor running at full
power. An electric field of 500 W/m was applied to one
sample, and na field was applied to the other, during irradiation and the temperature was held at 770 K (after Ref. [48]).
talline Kyocera A 479s~ alumina with 99.5% purity.
The results are shown in Fig. 11. The conductivity
began to increase at about 3 days, and continued to
increase throughout the experiment, which lasted at
least 40 days. This second experiment confirmed the
conductivity increase observed in the first experiment.
On the other hand, in a much earlier study, Ranken
and Veca [SO] found no conductivity increase when
they irradiated plasma arc sprayed ceramic test specimens of alumina and yttria with a fast neutron dose at
6 x 1O24 n m-‘” at irradiation temperatures of about
1000 K and with an applied field of 40 kV/m. Rather,
they observed a general decrease in conductive
as a
function of irradiation time, or dose.
Recently Farnum et al. [Sl] measured electrical
~ondu~tivi~ of afmina
and sapphire under applied
fields of up to 185 kV/m during elevated-temperature
irradiation with spallation neutrons and accompanying
ionizing radiation. Up to a level of 2 X 10z3 n/m2 no
RIED was observed in alumina irradiated in argon at
668, 888, and 928 K with an applied DC voltage of 50
and 150 kV/m; in fact, ~ondu~tivi~ decreased with
time in a manner reminiscent of that observed in
sapphire during irradiation with 3 MeV protons [381.
However, in sapphire irradiated at the same temperatures with an applied AC voltage of 185 kV/m at 20
kHz, a RIED-like increase in conductivity was observed. The sapphire was irradiated in an evacuated
capsule. Farnum et al. pointed out that this effect
could have been caused by degradation of the vacuum
in the capsule and that post-irradiation
conductivity
measurements would be necessary to verify these results.
Measurements by Shikama and Pells [52-54) reported the fo~ation of aluminum colloids in at-alumina
by 1.0 MeV electron irradiation in a hip-voItage elec-
41
tron microscope. Pelis [55] used 18 MeV protons to
irradiate alumina and spinel. For irradiation temperatures above 673 K and proton doses up to 7.5 x 10”
m -‘, there were large increases in the intrinsic conductivi~ that were not reduced by annealing at 923 K.
More recently Kesternich et al. 1561 have proposed
an alternative inte~retation
for the previously reported anomalous conductivity increase. in their experiment, pulsed 28 MeV alpha particles were used to
irradiate _ 150 Frn thick poiycrystalline alumina and
silicon nitride in vacuum at 773 K with an electric field
of 350 kV/m. An increase of lo4 in conductivity was
observed in the alumina sample and 10J in the silicon
nitride specimen under certain experimental conditions. However, the ~ondu~ivi~ of the alumina saturated thereafter. The authors concluded that the increase in conductivity was not due to degradation of
the bulk material, but was a result of radiation-induced
surface contamination. In a similar study by Jung et al.
1571,
a polycrystalline alumina with a thickness of 250
pm was irradiated with 10.7 MeV protons at 800 K
with an electric field of 320 kV/m. Again an increase
in conductivity was reported. They attributed the increased conductivity to radiation-induced
surface contamination consisting of carbon. To demonstrate this
point, they (1) deposited carbon films on their samples
with and without electric field and (2) performed RIED
experiments at 523 and 723 K. The results were optical
absorption spectra similar to that obtained by Hodgson
1401;in fact, the RIED performed at 523 K was practically identical. Therefore, Jung et al. [57j strongly suggested that the RIED observed by Hodgson was due to
surface ~n~aminations
during irradiation. Regardless
whether the increased electrical condu~iti~~ is due to
RIED or surface ~ntamination,
the implication of
their conclusions in these studies is likely to add another dimension to ceramic degradation under irradiations: surface conductivity resulting from gas-environment impurity contamination.
At the sixth Inte~at~onal
Conference on Fusion
Reactor Materials (ICFRM-6) Hodgson proposed an
explanation as to why in certain cases RIED was not
observed. He proposed that dose rates and irradiation
temperature play an important role [%I. He reported
that for higher dose rates, as typically encountered in
pulsed accelerators, equivalent damage requires higher
dose, thereby accounting for the report that RIED was
not observed in some accelerator experiments. In addition, Zinkle and Hodgson [591 reported that for irradiation-temperature
dependence a high temperature cutoff at _ 823 K was observed. The updated curve is
shown in Fig. 12.
Recently a joint USDOE-Japan
Monbusho meeting
entitled “Dynamic Effects of I~adiation in Ceramics”
Y. Chen et al. /Journal
of Nuclear Materials 217 (1994) 32-47
-I
or proton fluence into neutron fluencc
requires assumptions about damage equivalency which
may not be valid. Furthermore it was recognized that
the experimental conditions were quite dissimilar. For
example, the temperatures
and the electric field
strengths were markedly different. As seen in Fig. 13,
there is nothing that would suggest a trend, when the
data are plotted as a function of damage level. It is
clear that further understanding
is needed before
RIED effects can be explained. Attempts to correlate
RIED data were further explored at the IEA workshop
described in the next section.
An International
Energy Agency (IEA) workshop
was held in conjunction with the ICFRM-6 meeting.
This workshop, organized by S.J. Zinkle, E.H. Farnum,
and F. Clinard, Jr., was held during September 27-29,
1993 to review current work in insulating ceramics for
fusion applications. The workshop mainly focused on
the new and apparently conflicting data on RIED
under particle irradiation. As a result of the discussions, an international
round robin experiment was
proposed to address the question of atmospheric and
surface contamination effects on the RIED measurements. The following environmental
conditions were
specified: irradiation temperature, 450°C; electric field
> 200 kV/m; damage rate between 10e9 and lop8
dpa/s; material Wesgo AL995 from a batch obtained
by R. Stoller at ORNL. In addition, experimental
guidelines were developed, that included measurement
of the sample surface temperature, utilization of a
guard-ring geometry, measurement of inter-electrode
resistance during the experiment, verification of ohmic
behavior, full documentation and reporting of sample
history and experimental
conditions, and detailed
post-irradiation
examination (including TEM). The
lating electron
-1
I-
I
200
I
300
I
I
LOO
I
I
-J
500
T f"C1
Fig. 12. The total dose required to degrade samples of single
(SC) and polycrystalline (PC) alumina to 10T6 (R m)-’ as a
function of irradiation temperature at lo-” dpa s-l.
was held to address in-situ studies of physical properties of ceramics during irradiation [60]. Most of the
data presented at the meeting were results from ongoing measurements. It is clearly too early to present
concrete interpretations. Fig. 13, assembled by Famum
[60], illustrates an attempt to compare all RIED experiments under one common denominator, ‘equivalent
neutron fluence’. It is clear from this figure that trans-
Displacement Damage
1oJo’s
104
IOJ
(dpa)
1o’2
IO-’
IO0
-
r
t
Fmum.
-
Y
3
IO"
1oz2
10'3
Equivalent Neutron Fluence
102'
1
Neutron,
330 K. 000 V/cm
,Y.“oY.
Nwbon.
720 K. 16OVlc.m
Neuaon.
I20
-
IVIDOV,
-
StIIkuna.
Nwaon.
-
Shfkmn..
N.“trcm,
-
Ranken. Neutron.
1070
K. 100 V/cm
--+-
R.nk.n.
,070
K. 400 Vkm
-
Linkle.
-
Hd‘,ron.
-c
Pa*.
,,.“a~“.
22 M.V
H..
El.ctron.
Proton.
K. 110 “/cm
(I20 K. 5 Vlcm
,,I
no
K. 6000
SM) K. ,000
720 K. ,200
K. 6000
v,cm
Vlcm
“lcm
wcm
5
n/m*
Fig. 13. Electrical conductivity versus equivalent neutron fluence for several investigations. (After Ref. 1601.)
Y. Chen et al. /Joumal
of Nuclear Materials217 (1994) 32-47
participants at the workshop announced plans to conduct such a round robin experiment during the coming
year.
5. Scientific issues and opportunities
The fusion power plant environment commands one
of the most demanding sets of materials requirements
of any advanced technology involving ceramics. In support of various critical components in the reactor, the
ceramics must overcome a combination of hostile environmental conditions. Their successful application requires materials integrity to extend over the life of the
reactor. Therefore an understanding
of the critical
scientific issues in conjunction with the fusion operating conditions is essential in order to advance the
materials technology to meet these demands. We shall
address several of these key issues below:
43
tal conditions, such as the temperature and dose rates,
are unchanged. However, as pointed out in an earlier
section, when the electron irradiation extends to a
(critical) dose substantially beyond that used by Klaffky
et al. [7], the electrical conductivity begins to increase.
This sequence indicates that there exists an incubation
period during which the electrical conductivity remains
constant, or may even decrease. It is therefore essential to know: What defects (point and/or extended), if
any, are formed during the incubation period? Hodgson
suggested that aluminum colloids play an important
role in the RIED. Is the formation of metallic precipitates during Rad-E-T a necessary condition for electrical
breakdown? What are the dynamical processes involving
these defects that trigger the increase in the e~ect~cal
conductiui~? Can doping with impu~ties lead to suppression of the RIED mechanism? These are fundamental
issues which will provide a basis for an understanding
of the RIC-RIED mechanisms.
5. Z. Rad~t~on damage
5.2. Tem~rature
During the operation of the fusion reactor, the
insulating ceramics are bombarded by neutrons and
gamma rays (which result in Compton electrons) resulting directly or indirectly from the D-T fusion reaction.
Alpha particles from the D-T reaction are stopped in
the plasma or at the system first wall and will not make
contact with the ceramic insulators. At this time, it
appears likely that in a fusion chamber RIED would be
induced by either the neutrons, or the gamma rays, or
the synergism with both radiation fields. Is the RIED
due to a radiation damage mechanism? There are other
conductivi~ processes operating in insulators such as
space charge limited conductivity,
Frenckel-Poole
emission, tunnel or field emission [61]. If it is a result
of the radiation, then one may focus on the mechanisms and the corresponding cross sections in producing defects in insulating crystals by neutrons and gamma
rays. Therefore one of the important issues is: Zs RL?CD
The temperature
of application for the ceramic
materials in fusion devices is expected to be between
room temperature and 1100 K, although in some situations, cryogenic temperatures will be used. This moderate temperature range may actualIy be an impediment
in several ways. First, these temperatures may be sufficiently high to generate thermally activated defects at a
rate which is more prominent than at lower temperatures. Secondly, many thermally activated processes,
such as atomic diffusion, which can lead to dynamic
repair of damage at the atomic level, may not be
suf~ciently prominent at these temperatures. In fact, it
appears that the defect production in the 800 K regime
overshadows the annealing mechanism. Thirdly, the
strength of alumina, for example, may actually undergo
a minimum at u 800 IS because it occurs before the
onset of appreciable radiation heat transfer which
serves to diminish thermal gradients and therefore
stresses in the materials. Indeed there are indications
that degradation of physical properties in insulating
ceramics may be avoided by the judicious choice of the
ambient temperature. Operating at lower or higher
temperatures
may provide better performance with
respect to RIED 1591.
a result of an elastic collision between an incoming
energetic particle and a lattice ion, or caused by an
ionization process? In the Hodgson experiment,
1.8
MeV electrons were used. These electrons have enough
energy to displace lattice ions by elastic collisions
162,631. On the other hand, the ionization-loss mechanism of charged particles may also cause displacements
of indigenous ions, as is well-known in the alkali halides
and many silica-based compounds 164-671. Indeed,
electron-hole
pairs are created in abundance. Determination of the triggering mechanism would shed light
on whether the neutrons, gamma rays, or both, are
detrimental to RIED in ceramics.
In Rad-E-T experiments using electrons, such as
performed by Klaffky et al. [7] and by Hodgson 12-41,
the RIC remains essentially constant when experimen-
de~~ence
of ~~-~~D
5.3. Mechanical stresses
The ability of a ceramic to maintain structural integrity under the swelling and applied loads is very
important to the application 1681.As noted in Section
3.2, a-alumina is the only ceramic investigated which
does not suffer electical breakdown at high temperatures in the absence of radiation. It is likely that
c&l,O,
is also more resistant to electrical breakdown
44
1’. Cl~enet (11./fournal
of Nuclear Materials 217 11994) 32-47
in a radiation field than other oxides. However, the
shortcoming of this material is that it swells under
irradiation. Conversely, MgAl,O, spine1 is resistant to
radiation-induced
swelling, but the characteristic time
for elevated temperature electrical breakdown, even in
a radiation-free environment, is very short [%I. It is
encouraging that certain additives in a-alumina can
serve to suppress swelling [SO].
5.4. Impurities and transmutation effects
Since the largest body of knowledge exists for the
traditional
high-temperature
ceramic materials, cyAI,O,, MgAI,O,, and MgO, these materials warrant
new research on their processing to improve their
RIED performance. There have been suggestions that
the onset of colloid formation is associated with certain
trace element impurities. There has been previous work
on ultra-high purity synthesis and processing of CLalumina in attempts to define the current limits on the
extrinsic and intrinsic nature of its properties 169,701.
Currently all nominally pure cu-A120, has approximately 100 ppm total impurity content [71], while from
estimates of defect formation energies, the onset of
intrinsic behavior should occur at trace impurity contents below 1 ppm. With the routine achievement of
ppb levels of contamination
in silicon, an intrinsic
a-alumina is not unobtainable. This material may show
a greater stability and delayed onset of the colloidal
precipitation process. In addition to a better understanding of the role of trace impurities in the RIED
process, the possibility exists that intentionally doping
a-alumina with impurities may achieve a fully compensated material which may neutralize the onset of RIED.
The role of impurities in RIED has yet to be
investigated. Traditionally impurities (or dopants) have
always played a major role not only in the radiation
damage of solids, but also in the electrical characteristics of insulators [23,72-831. There is no reason to
believe that in RIC-RIED, the situation would be any
different. Indeed, in the case of electrical breakdown
of ceramics, it has been shown that the positive effect
of impurities in MgO crystals is dramatic (Fig. 4).
Doping the crystals with copper impurities improved
the breakdown longevity by a factor of 20. In a real
situation, this magnitude of improvement can often
translate from a failing to a passing grade. The understanding of the mechanism of any enhancement (or
suppression) of RIED could be elusive, but nevertheless essential. From a technological point of view, an
understanding of the mechanisms of RIED will serve
to guide the usage (or non-usage) of impurities pertaining to possible suppression of RIED.
The transmutation of host nuclei (or even impurities) under particle bombardment may produce daughters, which are highly radioactive (tong decay lifetimes)
or serve to enhance RIED. These conditions would
tend to rule out certain types of ceramic hosts, or
impurities.
5.5. Atomic and electronic st$~ct~re~~
Extensive work is needed to determine the atomic
structure
of the materials at all stages in the
RIC/RIED
process, covering the role of extrinsic impurities and intrinsic atomic defects, to the nucleation
and growth processes of RIC/RIED.
This will require
high sensitivity experimental techniques that may include magnetic resonance, electron microscopy, EXAFS, X-ray diffraction, optical spectroscopy, and
positron annihilation spectroscopy, to study the atomic
structure of materials. In addition, modeling including
atomistic simulations of defect interactions, and quantum-based calculations of defect formation and radiation damage in a highly ionizing and displacive radiation field, is desirable. This work should apply to both
in-situ and ex-situ studies and encompass pulsed techniques to understand the myriad of atomic structural
phenomena which are occurring in the fusion reactor
environment. An understanding
of this sort will also
illuminate the possible recovery processes which are
competing to reduce the RIED sensitivity of ceramics
and which can be advantageously tailored in the final
materials.
The fusion environment involves moderately high
temperatures and high levels of particle and ionizing
radiation. The conductivity behavior of ceramics is
impuri~/defect
dependent, and temperature dependent. In addition to the normal thermal carrier excitation across a band gap, increase in temperature decreases the band gap in ceramics. This temperature-dependent change in the electronic structure, arising
from thermal lattice expansion and electron-phonon
interaction, plays an important role in the intrinsic
high-temperature
electronic conductivity of ceramics
[84,85]. This is an example where more understanding
of the electronic structure is needed. The first-order
effect, more thermal creation of carriers, supplies an
incomplete picture. Besides the temperature dependence of the intrinsic and extrinsic electronic structure
of materials, the high level of ionizing radiation leads
to high levels of radiation-induced
electron-hole pairs
in the electronic structure of the materials.
The electronic structure is closely coupled to the
atomic structure. As atomic defects accrue, their bonding can be substantially different from the equilibrium
room-temperature
material, opening up more avenues
for new electronic processes to occur. Expanding our
understanding of the electronic properties will invoive
both theoretical calculations of the temperature-dependent equilibrium and non-equilibrium
structure of
ceramics [86], and experimental determinations of the
45
Y. Chen et al. /Journal of Nuclear Materials217 (1994) 32-47
electronic structure directly. This knowledge can also
be coupled to understanding
the effects of other
stresses on the material, such as radiation-induced
swelling. Important issues will be: What are the electronic carriers? Are they polaronic, large or small? What
is the role of the lattice stabilization of the electronic
(polaronic) carriers in initiating other processes which
distort the atomic structure? Insight into the degrada-
tion of the insulator to a metallic state, in terms of
changes in the atomic bonding and electronic structure
will be the context for understanding
the myriad of
competing processes in fusion reactor ceramics.
5.6. Defect chemistry
Defect chemistry of ceramics corresponds to the
intermediate stage at which thermally created (or radiation-induced)
defects and defect clusters react en
route to either annihilation or agglomeration to form
precursors or nuclei for precipitates. Defect chemistry
studies supply information on the behavior and of
defect processes in ceramics under a large variety of
conditions, for example, temperature, oxidizing or reducing atmosphere, and irradiation. Therefore these
studies can play a very strong role in determining the
nature of the processes occurring in ceramics during
RIC/RIED.
The methods of defect chemistry involve
in-situ conductivity measurements or variants thereof.
Defect chemistry supplies information on: (1) defect
dependence on temperature and oxygen partial pressure, (2) interaction of defects and impurities, and (3)
dopant effects. Furthermore, defect chemistry can help
to understand the precursors to colloid formation, or
defect annihilation and colloid disappearance. Asides
from the measurement
of electrical conductivity at
various temperatures, defect chemistry has not been
applied to fusion system materials. Therefore this area
provides many fruitful opportunities.
6. Recommendations
6.1. Standardization
A wide range of conductivity values were obtained
under very diverse experimental conditions (see Fig.
131. In addition, a review of the literature has revealed
how difficult these measurements in ceramics can be,
even without an impressed radiation field. For these
reasons it is recommended that the RIC/RIED
observations in insulating materials be performed and repeated, focusing on three elements: (1) standardization
of measurement technique, (2) standardization and reproducibility of samples, and (3) parallel multiple laboratory/investigator
participation, with at least one set
of measurements, for interlaboratory
der similar parameters.
comparison,
un-
6.2. New materials with minimal ?UC-RIED
Previous studies have shown that oxide ceramics
subjected to moderate electric fields at high temperatures, but in the absence of radiation, are susceptible
to electrical breakdown [19-271. It is believed that the
addition of radiation would serve to accelerate the
breakdown process. Therefore it is essential to select
for fusion candidates which are least susceptible to
electrical breakdown in the absence of radiation. CXAl,O, (alumina) is such a material. Even though the
conductivity of alumina was found to degrade under
extended electron irradiation, it is still likely to be
more resistant to RIED than other oxides which readily breakdown without radiation, such as MgO (magnesia) and MgAl,O, (spinel). Therefore alumina is still
a candidate for fusion applications, notwithstanding its
undesirable characteristic of anisotropic swelling and
strength loss under prolonged neutron irradiation.
In order to pursue materials which are least susceptible to RIED, it is necessary to understand the cause
of the phenomenon. Therefore it is recommended that
different materials be studied, in order to establish a
data base for categorizing and understanding
those
materials which are resilient to RIED and those which
are not. Various materials have been suggested, Si,N,,
Sic, AlN, ScO,, Ta,O,, and La,O, [87,88]. The area
of new materials is a fertile one from a solid-state
chemistry approach, to establish a data base for establishing structural families which are susceptible to
RIED, and to establish structure-property
relationships in order to target certain structural families for
Rad-E-T for application.
The second area of new materials research involves
developments in materials processing. For example new
forms of materials preparation such as spray deposition
[89], the formation of gradient materials or the development of thin film forms of materials which have
novel and non-equilibrium structures may hold promise.
Some of these microstructurally distinct materials may
show enhanced resistance to RIED. These materials
might be re-engineered for their radiolytic properties.
Acknowledgement
The task force meeting on “Electrical Breakdown
of Ceramics in a High Radiation Field” was sponsored
jointly by the Office of Basic Energy Sciences (OBES)
and the Office of Fusion Energy (OFE) of the US
Department of Energy. It was held May 28-31, 1991 in
Vail, Colorado and co-chaired by Y. Chen of OBES
and F.W. Wiffen of OFE. The US Department of
Y. Chen et al. /Journal
36
Energy is grateful for the valuable contributions
participants at the meeting. They were:
of Nuclear Materids 217 (1994) 32--i/7
of the
M.M. Abraham, Oak Ridge National Laboratory
L.A. Boatner, Oak Ridge National Laboratory
Y. Chen, US. Department
of Energy and Oak
Ridge National Laboratory
F.W. Chnard, Los Alamos National Laboratory
R. Cooper, University of ~el~u~c~
Australia
A. Dragoo, U.S. Department of Energy
B.D. Evans, Boeing Aerospace and Electronics
E. Farnum, Los Alamos National Laboratory
R.H. French, E.I. DuPont de Nemours & Co.
R. Gonzalez, Universidad Cados III, Madrid, Spain
E. Hodgson, CIEMAT, Madrid, Spain
N. Itoh, Nagoya University, Japan
J.J. O’Dwyer, State University of New York-Oswego
H. Ohno, Japan Atomic Energy Research Institute,
Japan
G.P. Fells, Harwelt Laboratory, United Kingdom
C.J. Pogatshnik, Southern Bhnois University
W. Ranken, Los Alamos National Laboratory
T. Shikama, Tohuku University, Japan
R. Staller, Oak Ridge National Laboratory
K.L. Tsang, Synchrotron Radiation Research Laboratory, Taiwan
W.P. Unruh, Los Alamos National Laboratory
W.J. Welter, Pacific Northwest Laboratory
F.W. Wiffcn, U.S. Department of Energy
M. Zahn, Massachusetts Institute of Technology
H.R. Zeller, Asea Brown Boveri, Switzerland
X.Z. Zong, Fudan University, China
Research at the Oak Ridge Nationai Laboratory
was sponsored by the Division of Materials Sciences, US Department of Energy under contract
No. DE-AC05840R21400
with Martin Marietta
Energy Systems, Inc. Research at the Universidad
Carlos III was supported by the Comision Interministerial de Ciencia y Tecnologia (CICYT) of Spain
and by the Comunidad
Autonoma
de Madrid
(CAMI*
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