Chiken Kinoshita

Defect production in ceramics

A review is given of several important defect production and accumulation parameters for irradiated ceramics. Materials covered in this review include alumina, magnesia, spinel, silicon carbide, silicon nitride, aluminum nitride and diamond. Whereas threshold displacement energies for many ceramics are known within a reasonable level of uncertainty (with notable exceptions being AIN and Si3N4), relatively little information exists on the equally important parameters of surviving defect fraction (defect production efficiency) and point defect migration energies for most ceramics. Very little fundamental displacement damage information is available for nitride ceramics. The role of subthreshold irradiation on defect migration and microstructural evolution is also briefly discussed.

Why is magnesia spinel a radiation-resistant material?

Side-by-side irradiation of stoichiometric MgAl 2 O 4 and α-Al 2 O 3 in JOYO shows that the radiation-induced microstructural evolution to exposure of ≤6 dpa proceeds by very different paths in these two materials. The large difference in dislocation loop evolution appears to account for the ease of void swelling in α-Al 2 O 3 and the strong resistance to void formation in MgAl 2 O 4 . Irradiation of MgAl 2 O 4 to much higher exposure (22-230 dpa) in FFTF confirms the details of the dislocation evolution, which involves a progressive change in Burgers vector and habit plane as interstitial loops increase in size. Constraints unique to the MgAl 2 O 4 crystal structure do not allow the formation of dislocation network structures that favor swelling

Potential and limitations of ceramics in terms of structural and electrical integrity in fusion environments

Copyright (c) 1996 Elsevier Science B.V. All rights reserved. The characteristic behavior of the nucleation and growth process of defect clusters in various ceramic materials irradiated with electrons, ions and neutrons is a good measure for evaluating their structural integrity in fusion energy devices. In order to provide insight on the origin of the structural integrity, this review treats the characteristic difference in defect cluster evolution in MgAl 2 O 4 , a-Al 2 O 3 and MgO. This review also includes experimental results which show the importance of irradiation with concurrent transmutation-produced gases and of irradiation spectrum. Further attention is focused on the dynamic effects of irradiation, such as radiation induced conductivity and radiation induced electrical degradation (RIEDr during irradiation with electrons, ions, fission neutrons and fusion neutrons. Since the phenomenon of RIED evokes serious technological problems for applications of ceramics in irradiation environments, this paper heavily emphasizes on the nature and possible mechanisms of RIED. Data obtained from recent experiments that used standardized procedures for in-situ measurements of electrical conductivity are discussed.

Radiation-induced defect clusters in fully stabilized zirconia irradiated with ions and/or electrons

Abstract Microstructure evolution of yttria-stabilized cubic zirconia (YSZ), ZrO 2 –13 mol% Y 2 O 3 , was investigated through transmission electron microscopy under irradiation with electrons and/or ions. Anomalous formation of large defect clusters was found under electron irradiation subsequent to ion irradiation, such as 300 keV O + , 100 keV He + and 4 keV Ar + ions. Such defect clusters were not formed solely with ion irradiation. The extended defect clusters possess strong black/black lobes contrast, and are observed preferentially around a focused electron beam at or near dislocations at temperature less than 520 K. The defect clusters were transformed into dislocation network when they reached a critical diameter of about 1.0–1.5 μm, and processes of nucleation, growth and transformation were repeated under electron irradiation. The defect clusters are assumed to be oxygen platelets induced through selective displacements of oxygen ions in YSZ with electron irradiation. An important role of the accumulation of electric charges due to the selective displacements in YSZ is also discussed.

Radiation-induced amorphization and swelling in ceramics☆

Irradiation-induced amorphization and swelling of ceramics including graphite have been studied through in-situ observation of electron microscopy, electron energy loss spectroscopy and convergent beam electron diffraction. Amorphization occurs below 500 K for highly-oriented pyrolytic graphite (HOPG) and below 300 K for SiC under electron irradiation. The amorphization fluences of HOPG and SiC required for full amorphization show no flux dependence. The displacement threshold energy is determined to be 12 eV for HOPG from the electron energy dependence of the amorphization fluence which is equivalent to 1 dpa. An atomistic model for describing the irradiation-induced amorphization and swelling of graphite is proposed. A high concentration of C2 molecules and their clusters between basal planes induce abrupt swelling along the c-axis within 4 × 10−3 dpa and fade the periodical structural image at 4 × 10−2 dpa. The concentration of vacancies increases gradually up to 1 dpa at which amorphization is completed.

Role of irradiation spectrum in the microstructural evolution of magnesium aluminate spinel

Abstract In-situ observations of the nucleation and growth of defect clusters in MgAl2O4 were made in transmission electron microscope-accelerator facilities using concurrent irradiation with ions and electrons to obtain insight into the role of the irradiation spectrum. Various kinds of 30 or 300 keV ions (He+, O+, Mg+, Ar+ and Xe+) and 200 keV or 1 MeV electrons were used to provide a wide range of nuclear and ionizing stopping powers as well as a various effects of displacement cascades. Dislocation loops were formed both inside and outside the electron beam at 870 K under concurrent irradiation with 30keV ions and 1 MeV electrons. We observed a variety of microstructures that formed during concurrent irradiation with 300 keV ions and 200 keV electrons. These include preferential formation of cavities, suppression of dislocation loop formation and preferential formation of loops at the periphery of the focused electron beam. We discuss the irradiation spectrum effects in terms of the ionizing, displaci...

A study of amorphization and microstructural evolution of graphite under electron or ion irradiation

In order to clarify the mechanism of irradiation-induced phenomena, such as amorphization,swelling and microstructural evolution in highly oriented pyrolytic graphite, in situ observation through high-voltage and high-resolution electron microscopy, electron energy loss spectroscopy and convergent-beam electron diffraction has been performed under irradiation with 16–160 fJ electrons or 4.8 fJ He+, Ar+ or Xe+ ions in the temperature range from 110 to 470 K. The induced amorphization with electron irradiation accompanies swelling along the c-axis due to the accumulation of di-carbon molecules and their clusters between the basal planes, and is fully completed with excess vacancies introduced by atom displacements of around 1 dpa. Heavy-ion irradiations were also performed to understand the effects of the impact and/or the deposited energy density in cascades and the projected range of ions on the irradiation-induced phenomena. Twinning, amorphization and microcrystal evolution in the amorphized region emerge, depending on the heterogeneous damage rate with respect to the depth from the incident surface of ions. The microstructural evolution under ion irradiation is successfully analyzed in terms of the damage profile and the deposited energy density, which are directly related to the mass and the energy of projectiles.

Kinetics of point defects in electron irradiated MgO

Abstract The kinetic behavior of point defects in MgO has been studied through the observation of the nucleation and growth processes of dislocation loops during electron irradiation in a high voltage electron microscope. Results have been obtained on dislocation loop formation and growth in MgO single crystals obtained from three sources during irradiation at temperatures from 25 to 1000°C. Circular and elongated dislocation loops with 1 2 〈110〉 Burgers vectors lying on {110} planes are formed in Norton MgO and ORNL and Tateho MgO, respectively. The loop nucleation stage is complete early in the irradiation and the density of loops reaches a saturation value CL. The diameter of loops D increases with irradiation time t. The dependence of CL and D on t and electron flux φ at various temperatures T is expressed as follows: C L ∝ t 0 φ −1 2 and D ∞ t 1 2 − 2 3 at T ≲ 600° C and D tφ −1 2 at T ≳ 600° C . Experimental results below ≈ 600°C are successfully analyzed using a theory based on a nucleation mechanism controlled by interstitial motion in which one or two pairs of Mg- and O-interstitials serve as stable nuclei for interstitial loops. The growth kinetics of loops above ~ 600°C are understood in terms of the steady state behavior of interstitials and vacancies with high mobilities, and are controlled by the vacancy species (Mg or O), the product of whose mobility and displacement rate has the lower value.

VACANCY CONCENTRATION AND ARRANGEMENT OF ATOMS AND VACANCIES IN METALS AND ALLOYS

Abstract With a method of statistical thermodynamics fundamental equations are derived which describe the arrangement of atoms and vacancies in the alloys in the state of thermal equilibrium, where an ordering or a clustering can take place. The solutions of these equations give, among other things, a more precise description on the concentration of vacancies than the classical ones. The vacancy concentration in pure metals may almost precisely be expressed by the usual approximate expression, but in dilute alloys it is not always expressed by Lomers expression. The fraction of vacant sites in the alloys, furthermore, is very unlikely to be expressed by an Arrhenius type of equation, but it decreases or increases, according as a result of ordering or clustering. In binary alloys with short range orders or clusters the probability that one of the nearest neighbor sites of a vacancy is occupied by an atom of any particular kind does not always vary monotonically with temperature but in some alloys it increases after decreasing or decreases after increasing. A possibility for an interpretation of the anomalous behaviors of αCu-Al alloys is pointed out.

Effect of damage cascades on the irradiation-induced amorphization in graphite

Abstract In-situ observation of the irradiation-induced amorphization and measurements of the critical amorphization dose in graphite under irradiation with ions and electrons have been performed to reveal the effects of damage cascades. The critical amorphization dose ( D c ) increases exponentially with temperature and shows an apparent critical amorphization temperature ( T c ). T c increases with increase in mass of projectiles and with decrease in energy of ions. Dose rate dependence of D c and T c was also observed. No amorphization was detected above 860 K. Accumulation of vacancies and/or interlayer carbon molecules is attributable to the amorphization, while annealing of amorphous regions is dominated by kinetics of lattice defects. Defect annealing is associated with recombination of interlayer carbon molecules with vacancies. Damage cascades leave high density of interlayer molecular clusters ((C 2 ) n , n = 1, 2, 3…) and stable di-vacancies, resulting in stable amorphous regions at high temperature.