Radiation damage mechanisms in oxide ceramics are briefly discussed for irradiation by electrons, ions and neutrons. The effects of electron, ion and neutron irradiation on the evolution of the damage microstructure of MgO, A12O3 and MgAl2O4 are followed in detail. Particular attention is paid to differences in the microstructure produced by ions of varying mass and energy. The effects of damage microstructure on other properties are indicated.

Polycrystalline samples of alpha-alumina have been irradiated with various ions ranging from 3.6 MeV Fe+ to 1 MeV H+ ions at 650°C. Cross-section transmission electron microscopy was used to investigate the depth-dependent microstructure of the irradiated specimens. The microstructure following irradiation was observed to be dependent on the irradiation spectrum. In particular, defect cluster nucleation was effectively suppressed in specimens irradiated with light ions such as I MeV H+ ions. On the other hand, light ion irradiation tended to accelerate the growth rate of dislocation loops. The microstructural observations are discussed in terms of ionization enhanced diffusion processes.

Measurements of marker spreading following 2.0 MeV Kr+ irradiation at 25 to 1300°C have been performed on MgO samples containing O18, Ca and Zn buried tracer atoms. Ion beam mixing at room temperature on both sublattices was approximately 2.0 to 3.0 A5/eV. From 600 to 1000°C, the apparent activation enthalpy for diffusion on the anion sublattice (O18) was 0.35 eV and the diffusion coefficient was linear in the irradiation flux. From 1150 to 1300°C the measured activation enthalpy was 4.1 eV and the diffusion coefficient was proportional to the square root of flux. The measured activation enthalpy on the cation sublattice was roughly 0.30 eV up to 700°C for both Ca or Zn doped samples. Measurable extrinsic thermal diffusion from vacancies present for trivalent impurity charge compensation occurred above this temperature, complicating the analysis at higher temperatures. The observed kinetics in the lower temperature range are most likely controlled by interstitial loop formation. In the higher temperature range, vacancy traps with a binding energy of approximately 2 eV could account for the high activation enthalpy.

A thin amorphous film of alumina was irradiated with 2-MeV He+ ions at ~400°C up to a damage level of about 0.01 displacements per atom (dpa). The alumina films were sufficiently thin (~1.8 μm) to allow the ion beam to be completely transmitted through the specimen. An electric field of μ280 V/;mm (dc) was applied continuously during the irradiation. Radiation induced electrical degradation (RIED), i.e. a permanent increase in the conductance of the film, was observed in specimens irradiated at temperatures near 400 to 450°C but did not occur in a specimen irradiated above 500°C. An investigation by transmission electron microscopy found no evidence for colloid formation. The observed increase in the conductance of the alumina film may be due to radiation-induced microcracking.

Nanocomposites of iron in sapphire (α-A12 O3) prepared by ion implantation have been studied as a model to investigate the potential of such materials for applications in high technology areas. The implantation was performed with 160 keV ions at several doses; the nanocomposites were then annealed at selected temperatures between 700 and 1400°C in an Ar-4&H2 atmosphere for 1 hour. Rutherford backscattering spectroscopy and high resolution transmission electron microscopy (TEM) were used to characterize the structure of these nanocomposites. Measurements showed that damage depth extended to about 300 nm and the embedded iron extended to about 200 nm. This region became amorphous when the fluence reaches 2x1017 Fe/cm2 at this energy. At this dose, oriented precipitates with diameters of 2 to 3 nm were identified by TEM techniques as α-Fe which had the following orientation relationship with the sapphire matrix: <111>Fe||<310>Sapphire and (011)Fe||{006}sapphire. Thermal annealing could be used to restore the crystallinity to the damaged near-surface region, to form the metallic colloids, and also to coarsen the precipitates. The optical density and luminescence spectra were also measured. The predominant defects were oxygen vacancies with two electrons (F center) at the known absorption peak of 200 nm.

Single crystals of CaF2 were irradiated with 5.8 MeV alpha particles from a 244Cm source, and the absorbance spectra were recorded as a function of dose and temperature. Absorbance from the metal colloid band increases with dose at all temperatures. The colloid band exhibits a sharp increase in rate of growth over a narrow temperature range with a peak at 150°C. After a dose of 1.4 x 1016 α/cm2 at 150°C, the intensity of the colloid band is a factor of 7 to 8 higher than the intensity produced at 100 and 200°C. The optimum temperature (150°C) for colloid formation in CaF2 under the alpha-irradiation conditions (σ5 x 108 rad/h) of this study is significantly higher than the peak temperature (60°C) reported for colloid production in electron irradiated CaF2 at slightly higher dose rates (σ109 rad/h). Cryogenic transmission electron microscopy of the irradiated crystals reveals dislocation loops and small dark clusters as the dominant microstructural features.

The study of the photoplastic effect (PPE) on ice is essential for both fundamental and applied reasons. It is important for an understanding of dislocation motion as well as the flow of glaciers in cold regions that occurs under intensive solar radiation. It was found that the illumination of ice with UV light (λ < 300 nm) leads to its irreversible hardening1. A prolonged irradiation with a total light exposure of about 8.10−5 J/cm2 at λ= 260 nm can change the creep rate up to 60 percent. Even more pronounced PPE was found in HCI-doped ice with the concentration, n= 1018 cm−3. It is suggested that PPE is caused by the excitation of “autoionization” reaction which was found responsible for the photoconductivity of ice.

This paper deals with the study of the tunneling states (TS) in neutron-irradiated quartz. Ultrasonic attenuation measurements are carried out for a dose of 1.2 x 1019 n/cm2, before and after heat treatment at 700°C and 840°C. The data give evidence for significant changes in the distributions of the tunneling parameters upon heat treatment. These modifications provide interesting information about the nature of the tunneling states and will be discussed in view of the possible relation between the TS and the twin domains: the influence of heat treatment on the distribution of the tunneling states can be related to changes in the twin formation upon annealing.

The progress of heterogeneous amorphization of electron-irradiated quartz was studied in situ by high-resolution electron microscopy (HREM). The transformation front appears surprisingly sharp when viewed close to edge-on, but the apparent morphological evolution when aperiodic and periodic material overlap in projection is one in which gradual changes in image regularity are observed. Electron-microscopical contrast obtained in HREM irradiations was interpreted quantitatively, through the use of image contrast simulations. When the amorphized portion occupies less than about two-thirds of the total thickness, the HREM images continue to appear crystalline. Only when the amorphized thickness fraction exceeds about two-thirds does the resultant image contrast show mainly non-crystalline features.

Electron beam irradiation effects in SiO2 have been studied by STEM (Scanning Transmission Electron Microscopy). Oxygen loss and the corresponding transformation from SiO2 to Si in SiO2 are confirmed and consistent with previous reports 1,2. A “flower-like” Si rich area, which might not be observed in STEM BF (Bright Field) or ADF (Annular Dark Field) images, was found in Si plasmon energy filtered images. Quantification of the observations and the formation mechanisms leading to the Si-rich area are discussed.

The first sharp diffraction peak from various forms of aperiodic silica was studied. Materials examined include vitreous silica, electron-metamict quartz, neutron-metamict quartz, and Si+ ion-implanted quartz. The peak is attributed to the dominant structural rings existing in the materials, or modification thereof. The anomalous behavior of this peak is also discussed.

“Complex” ceramics, for the purpose of this paper, include materials that are generally strongly bonded (mixed ionic and covalent), refractory and frequently good insulators. They are distinguished from simple, compact ceramics (e.g., MgO and UO2) by structural features which include: 1.) open network structures, best characterized by a consideration of the shape, size and connectivity of coordination polyhedra; 2.) generally, complex compositions which characteristically lead to multiple cation sites and lower symmetry; 3.) directional bonding; 4.) bond-type variations, from bond-to-bond, within the structure. The heavy particle irradiations not only include ion-beam irradiations, but also recoil-nucleus damage resulting from a-decay events from constituent actinides. The latter effects are responsible for the radiation-induced transformation to the metamict state in minerals. The responses of these materials to irradiation are complex, as energy may be dissipated ballistically by transfer of kinetic energy from an incident projectile or radiolytically by conversion of radiation-induced electronic excitations into atomic motion. This results in isolated Frenkel defect pairs, defect aggregates, isolated collision cascades or bulk amorphization; all may occur concurrently. Thus, the amorphization process is heterogeneous. Only recently have there been systematic studies of heavy particle irradiations of “complex” ceramics on a wide variety of structure-types and compositions as a function of dose and temperature. In this paper, we review the conditions for amorphization for the tetragonal orthosilicate, zircon [ZrSiO4]; the hexagonal orthosilicate/phosphate apatite structure-type [X10(ZO4)6(F,Cl,O)2]; the isometric pyrochlores [A1-2B2O6(O,OH,F)o-1pH2O] and its monoclinic derivative zirconolite [CaZrTi2O7]; the olivine (derivative - hcp) structure types, α-VIA2IVBO4, and spinet (ccp,) γ-VIA2IVBO4.

Amorphization, like glass-formation, represents fundamentally a failure to crystallize. The problem is to understand how atoms can rearrange themselves, perhaps within the confines of unaffected surrounding crystal, after a local disordering event. Some ceramics, like alkali halides and oxides with the rocksalt structure, appear almost impossible to amorphize, forming localized aggregate defects (voids, dislocation loops, colloids) and even decomposing (as a response to radiation disorder), rather than structurally reordering (or disordering). Other network solids such as silicas and silicates readily amorphize. In this study, we attempt to establish topology and structural freedom as criteria for amorphization of network solids.

A model based on the assumption of a Gaussian temperature distribution in the thermal spike well accounts for the basic features of latent track formation. In good agreement with the observations it predicts a logarithmic variation of the track size for 2.7≥Se/Set≥1 and a linear variation for Se/Set≥2.7, where Se is the electronic stopping power and Set is a threshold value. The model also provides a quantitative relation between Set and the thermal properties of the target. A weak temperature dependence of track formation is predicted in agreement with the experiments.

Zircon (ZrSiO4) samples were irradiated at 100K with 400 keV Ar+ and Xe+ ion beams to fluences ranging from 5x1013 to 5x1015 ions/cm2. Rutherford backscattering spectroscopy (RBS) experiments were completed to study the dechanneling of He ions in the irradiated zircons. Cross-sections of some irradiated samples were prepared, and the zircon microstructure was examined by highresolution transmission electron microscopy (HRTEM). At doses greater than 8x1014 ions/cm2 or 0.8 dpa (displacements per atom), RBS channeling experiments showed the presence of a disordered or amorphous layer. The electron microscopy confirmed the presence of an amorphous layer extending over a depth of 300 to 350 nm (Ar+ irradiation) and 200 nm (Xe+ irradiation) in agreement with the damage layer depth calculated by TRIM. At depths extending beyond the damage peak, TEM reveals an amorphous layer with “islands” of crystalline material of σ 30 nm in size. These experiments show that RBS and TEM are complementary techniques in investigating radiation effects in irradiations of bulk ceramic.

The effects of implanted ion chemistry and displacement damage on the amorphization threshold dose of SiC were studied using cross-section transmission electron microscopy. Room temperature as well as 200 and 400°C irradiations were carried out with 3.6 MeV Fe, 1.8 MeV Cl, 1 MeV He or 0.56 MeV Si ions. The room temperature amorphization threshold dose in irradiated regions well separated from the implanted ions was found to range from 0.3 to 0.5 dpa for the four different ion species. The threshold dose for amorphization in the He, Si and Fe ion-implanted regions was also σ0.3 to 0.5 dpa. On the other hand, the amorphization threshold in the Climplanted region was only about 0.1 dpa. The volume change associated with amorphization was σ17&. No evidence for amorphization was obtained in specimens irradiated at 200 or 400°C.

A transmission electron microscope interfaced with ion accelerators (TEM-Accelerator Facility) in JAERI-Takasaki has been used to get insights into the irradiation-induced amorphization of highlyoriented pyrolytic graphite. Amorphization is induced through lattice disorder under irradiation with electrons or ions at room temperature. The displacement threshold energy is determined as 27 eV and 28 eV from the electron-energy dependence of the amorphization fluence at room temperature and 373 K, respectively. The amorphization fluences in dpa are almost identical for irradiation with various kinds of ions within the experimental error. It indicates the thermal spikes do not have an important role for the amorphization over the energy density between 10-4 to 10-2 eV/atom.

Ca2La8(SiO4)6O2 thin crystals become amorphous under ion beam irradiation. The ion dose required for complete amorphization of the thin crystal (critical amorphization dose, Dc) increased with the increasing irradiation temperature and decreased with ion mass at elevated temperatures. Samples irradiated with 1-1.5 MeV Ar+, Kr+ and Xe+ ions to doses much lower than Dc, in the temperature range from 20 to 498 K were used for a detailed HRTEM investigation to study the amorphization process. The residual collision cascade damage after irradiation appeared as nanometer scale amorphous domains. The images of these domains are extremely sensitive to the sample thickness. Small domains of cascade size were found only at the very thin edge of the sample. In thicker regions, amorphous domains appear after higher doses as the result of cascade overlap in projection. At higher temperatures, the observed amorphous domains are smaller indicating thermal recovery at the amorphous/crystalline interface. The amorphous domains are also larger in size after irradiation with ions of higher mass at a fixed ion dose. These results are consistent with the Dc-temperature curves determined by in situ TEM with the HVEM-Tandem Facility at Argonne National Laboratory. The width of the amorphous rim along the edge of the specimen grew with increasing ion dose suggesting that amorphization also proceeds from the sample surface. Images of the collision cascade damage were compared to the cascade sizes calculated with the TRIM code. Some digitally acquired HRTEM images of the cascade damage were processed to reveal more detailed information.

Berlinite (AIPO4) is isostructural with α-quartz. Like α-quartz, berlinite undergoes a pressure-induced amorphization at 15 ±3 GPa; however, upon release of the pressure, unlike α-quartz which remains amorphous, berlinite returns to the original crystalline structure of the single crystal. Berlinite was irradiated with 1.5 MeV Kr+ at temperatures ranging from 20 to 600K. The onset of amorphization was examined by monitoring the electron diffraction pattern by in situ transmission electron microscopy (TEM) at the HVEM-Tandem Facility at Argonne National Laboratory. The berlinite was easily amorphized at 20K at a relatively low dose of 4x1013 ions/cm2 or 0.05 dpa (displacements per atom). The critical amorphization dose increases with the sample temperature. These experiments also showed that the focused electron beam can locally amorphize the berlinite. After these irradiations, berlinite remained amorphous. At 500 °C, berlinite began to recrystallize: small areas of crystalline material appear in the aperiodic matrix. These results suggest that pressure-induced amorphization and ion-beam induced amorphization, in the case of berlinite, are different processes that result in two different aperiodic structural states.

Single crystalline samples of magnesium aluminate spinel (MgAl2O4), <100> oriented, were irradiated at 100 K and 670 K with 370-400 keV Xe ions to doses of (1-2)x1016 Xe/cm2. The microstructures of irradiated samples were subsequently examined by cross-sectional transmission electron microscope. A uniform layer of amorphous phase was observed on the surface of spinel irradiated at 100 K. At the end of the damage range underlying the amorphous layer, a disordered transition layer resided on the undamaged substrate. Both high resolution electron microscopy and microdiffraction revealed that the transition layer retained single crystallinity with epitaxial relationship to the underlying substrate. However, the intensity of <220> reflections in the transition layer was significantly weaker than that of the undamaged spinel. No evidence of amorphization was found in the spinel sample irradiated at 670 K to a dose of 2x1016 Xe/cm2. The <220> reflections exhibit only limited diminution in the heavily damaged region. The observation of reduced intensity of <220> reflections or absent reflections suggests that spinel experiences a structural transition from its original cubic phase (a=0.808 nm) to a new cubic phase (a=0.404 nm). A transition sequence from the original phase to a metastable phase and then to an amorphous phase has been observed. The temperature dependence of metastable and amorphous phase formation has revealed that the accumulation efficiency of cation disorder decreases with increasing irradiation temperature due to the enhancement of interstitial-vacancy recombination.