Linn W. Hobbs

RADIATION EFFECTS IN CRYSTALLINE CERAMICS FOR THE IMMOBILIZATION OF HIGH-LEVEL NUCLEAR WASTE AND PLUTONIUM

This review provides a comprehensive evaluation of the state-of-knowledge of radiation effects in crystalline ceramics that may be used for the immobilization of high-level nuclear waste and plutonium. The current understanding of radiation damage processes, defect generation, microstructure development, theoretical methods, and experimental methods are reviewed. Fundamental scientific and technological issues that offer opportunities for research are identified. The most important issue is the need for an understanding of the radiation-induced structural changes at the atomic, microscopic, and macroscopic levels, and the effect of these changes on the release rates of radionuclides during corrosion. {copyright} {ital 1998 Materials Research Society.}

Radiation effects in glasses used for immobilization of high-level waste and plutonium disposition

This paper is a comprehensive review of the state-of-knowledge in the field of radiation effects in glasses that are to be used for the immobilization of high-level nuclear waste and plutonium disposition. The current status and issues in the area of radiation damage processes, defect generation, microstructure development, theoretical methods and experimental methods are reviewed. Questions of fundamental and technological interest that offer opportunities for research are identified.

Radiation effects in ceramics

Abstract Ceramics represent a large class of solids with a wide spectrum of applicability, whose structures range from simple to complex, whose bonding runs from highly ionic to almost entirely covalent and, in some cases, partially metallic, and whose band structures yield wide-gap insulators, narrow-gap semiconductors or even superconductors. These solids exhibit responses to irradiation which are more complex than those for metals. In ceramic materials, atomic displacements can be produced by direct momentum transfer to often more than one distinguishable sublattice, and in some cases radiolytically by electronic excitations, and result in point defects which are in general not simple. Radiation-induced defect interaction, accumulation and aggregation modes differ significantly from those found in metals. Amorphization is a frequent option in response to high-density defect perturbation and is strongly related to structural topology. These fundamental responses to irradiation result in significant changes to important applicable properties, such as strength, toughness, electrical and thermal conductivities, dielectric response and optical behavior. The understanding of such phenomena is less well-understood than the simple responses of metals but is being increasingly driven by critical applications in fusion energy production, nuclear waste disposal and optical communications.

Neutron irradiation damage in MgO, Al2O3 and MgAl2O4 ceramics☆

The consequences of point defect aggregation in single-crystal and polycrystalline Al2O3 and MgAl2O4 and polycrystalline MgO, irradiated to fast-neutron (>0.1 MeV) doses ⩽2–3 × 1026nm−2, have now been investigated over a range of temperatures using macroscopic density measurements of swelling and transmission electron microscopy of defect aggregates. Al2O3 irradiated between 925 and 1100 K exhibits 2–4 vol% swelling, dense dislocation networks and aligned voids, while single-crystal MgAl2O4 spinel irradiated under similar conditions, does not swell and forms only isolated interstitial dislocation loops which do not unfault. Planar arrays of voids form either side of grain boundaries in polycrystalline MgAl2O4 spinel, however. MgO irradiated at 430 K swells 2.8% and contains a high density of small elongated loops and exhibits a tendency towards dislocation network formation despite the low irradiation temperature. Al2O3 and MgAl2O4 irradiated at 430 K exhibits dense arrays of small loops. The nature and origin of the observed aggregate defect structures are described and correlated with swelling behaviour and with previous studies which are reviewed.

Asymmetries in dislocation densities, surface morphology, and strain of GaInAs/GaAs single heterolayers

The dislocation densities, surface morphology, and strain of Ga1−xInxAs/GaAs epitaxial interfaces as a function of indium composition and layer thickness have been investigated by transmission electron microscopy, medium energy ion blocking, and double‐crystal x‐ray diffractometry. The electron microscopy shows that in the thinnest dislocated films (90 and 160 nm, x=0.07) 60° α dislocations form first in one 〈110〉 direction at the interface. Surprisingly, however, an asymmetry in residual layer strain is not detected in these samples, suggesting that the dislocations have the same Burgers vector or are evenly distributed between two Burgers vectors. Orthogonal arrays of dislocations are observed in films thicker than 300 nm (60° and edge‐type, x=0.07). In this case, dislocation densities in each 〈110〉 direction are equal to within experimental error while an asymmetry in in‐plane strain is measured (18% and 30% for x=0.07, 300, and 580 nm thick, respectively). An unequal distribution of Burgers vectors of...

18O/SIMS characterization of the growth mechanism of doped and undoped α-Al2O3

Sequential oxidation experiments at 1200°C and 1500°C using16O and >95%18O-enriched environments were conducted on undoped and Y- and Zr-doped β-NiAl and FeCrAl alloys. After oxidation, samples were analyzed by SIMS sputter depth profiling. At 1200°C, a clear pattern was established where the undoped α-Al2O3 was found to grow by the simultaneous transport of both Al and O. Zr-doped α-Al2O3 was found to grow mainly by the inward transport of oxygen. The profiles in all cases indicate O diffusion primarily by shortcircuit pathways. Results at 1500°C (only on β-NiAl) indicated a similar behavior but were less conclusive. Y and Zr were found to segregate to the oxide grain boundaries at 1200°C and 1500°C. The segregation of these dopants is believed to impede the cation transport in the α-Al2O3 scale and thereby change the oxidation mechanism.

The oxidation mechanism of θ-Al2O3 scales

Abstract A nearly single-phase θ-Al2O3 scale has been grown on two FeCrAl alloys, according to a time-temperature-transformation relationship for the formation of external alumina scales. SIMS sputter depth profiles of the 18 O 16 O distribution consistently indicate that θ-Al2O3 scales grow primarily by the outward transport of Al. This mechanism is consistent with the observed scale microstructure and is significantly different from tracer profiles for α-Al2O3 scales.

Structural freedom, topological disorder, and the irradiation-induced amorphization of ceramic structures

Abstract The susceptibility to irradiation-induced loss of long-range translational and orientational order during amorphization of crystalline solids, a process which usually amounts to topological disordering, can be understood on the basis of available structural freedom which depends on the redundancies present in structural connectivity. A surprisingly good correlation has been established between the critical accumulated energy deposited per atom — or displacements per atom (dpa) — required for amorphization during ion irradiation and the calculated structural freedom for the structure type irradiated over a wide range of non-metal structural types. These include AO, AO2, A2O3, ABO3, ABO4, A2B2O7 and A2BO4 compact structure types, in which weakest links can often be identified, as well as less-compact network structures. The present contribution specifically compares the order of susceptibility to amorphization — in these more complex oxide structures and in the simpler network structures Si3N4, SiC, Be2SiO4, AlPO4, Si, SiO2, P2O5 and graphite — on the basis of structural connectivity and discusses the anomalous susceptibility of SiC.

The reactive element effect in commercial ODS FeCrAI alloys

Two commercial oxide dispersion strengthened alumina-forming FeCrAl alloys, Inco alloy MA956 and Kanthal alloy APM, were studied in order to look at the effect of reactive elements on their oxidation behaviour. MA956 has a distribution of Y2O3−Al2O3 particles, while APM has a ZrO2—AI2O3 distribution. Isothermal oxidation at 1000°C and 1200°C showed a reduced oxidation rate for both alloys compared to that of an undoped FeCrAl alloy. In short-term cyclic tests at 1200°C, both alloys exhibited excellent scale adhesion. Using scanning transmission electron microscopy with X-ray energy dispersive spectroscopy, both Y and Zr, respectively, were found to segregate to the oxide grain boundaries and the metal-scale interface after oxidation at 1000°C and 1200°C. These experimental observations are discussed with regard to a new theory to explain the reactive element effect.

The effect of various oxide dispersions on the phase composition and morphology of Al2O3 scales grown on β-NiAl

A series of oxide-dispersedβ-NiAl alloys were oxidized in order to explore the effect of various cation dopants on the ϕ-α phase transformation in the Al2O3 scale and the effect of phase composition on the scale microstructure. Larger ions such as Y, Zr, La, and Hf appeared to slow theϕ- toα-Al2O3 phase transformation, while a smaller ion, Ti, appeared to accelerate the transformation.