L.M. Wang

Radiation stability of gadolinium zirconate: A waste form for plutonium disposition

Zirconate and titanate pyrochlores were subjected to 1 MeV of Kr{sup +} irradiation. Pyrochlores in the Gd{sub 2}(Zr{sub x}Ti{sub 1-x}){sub 2}O{sub 7} system (x=0,0.25,0.5,0.75,1) showed a systematic change in the susceptibility to radiation-induced amorphization with increasing Zr content. Gd{sub 2}Ti{sub 2}O{sub 7} amorphized at relatively low dose (0.2 displacement per atom at room temperature), and the critical temperature for amorphization was 1100 K. With increasing zirconium content, the pyrochlores became increasingly radiation resistant, as demonstrated by the increasing dose and decreasing critical temperature for amorphization. Pyrochlores highly-enriched in Zr (Gd{sub 2}Zr{sub 2}O{sub 7}, Gd{sub 2}Zr{sub 1.8}Mg{sub 0.2}O{sub 6.8}, Gd{sub 1.9}Sr{sub 0.1}Zr{sub 1.9}Mg{sub 0.1}O{sub 6.85}, and Gd{sub 1.9}Sr{sub 0.1}Zr{sub 1.8}Mg{sub 0.2}O{sub 6.75}) could not be amorphized, even at temperature as low as 25 K. (c) 1999 Materials Research Society.

Ion irradiation-induced phase transformation of pyrochlore and zirconolite

Abstract Pyrochlore (Gd2Ti2O7) and zirconolite (CaZrTi2O7) were irradiated with 1.5 MeV Xe+, 1.0 MeV Kr+, and 0.6 MeV Ar+ at temperatures from 20 to 1073 K. Critical amorphization dose increased with increasing temperature. Heavier ion irradiation increased the critical temperature. The extrapolated critical temperatures for amorphization were calculated as: 1300, 1100 and 950 K for pyrochlore irradiated with 1.5 MeV Xe+, 1.0 MeV Kr+ and 0.6 MeV Ar+, respectively; 710 and 654 K for zirconolite irradiated with 1.5 MeV Xe+ and 1.0 MeV Kr+, respectively. At the early stage of irradiation, pyrochlore transformed to a disordered fluorite structure; monoclinic zirconolite first transformed to partially disordered cubic pyrochlore structure followed by a disordered fluorite structure.

Structure and properties of ion-beam-modified (6H) silicon carbide

Abstract The ion-beam-induced crystalline-to-amorphous phase transition in single crystal ( 6 H) α -SiC has been studied as a function of irradiation temperature. The evolution of the amorphous state has been followed in situ by transmission electron microscopy in specimens irradiated with 0.8 MeV Ne + , 1.0 MeV Ar + , and 1.5 MeV Xe + ions over the temperature range from 20 to 475 K. The threshold displacement dose for complete amorphization in α -SiC at 20 K is 0.30 dpa (damage energy=15 eV atom −1 ). The dose for complete amorphization increases with temperature due to simultaneous recovery processes that can be adequately modeled in terms of a single-activated process. The critical temperature, above which amorphization does not occur, increases with particle mass and saturates at about 500 K. Single crystals of α -SiC with [0001] orientation have also been irradiated at 300 K with 360 keV Ar 2+ ions at an incident angle of 25° over fluences ranging from 1 to 8 Ar 2+ ions nm −2 . The damage accumulation in these samples has been characterized ex situ by Rutherford backscattering spectrometry–channeling (RBS/C) along the [0001] direction, Raman spectroscopy, cross-sectional transmission electron microscopy (XTEM), and mechanical microprobe measurements.

Ion irradiation of rare-earth- and yttrium-titanate-pyrochlores

Abstract Pyrochlore, A1–2B2O6(O,OH,F)0–1, is an actinide-bearing phase in Synroc, a polyphase ceramic proposed for the immobilization of high level nuclear waste. Structural damage due to alpha-decay events can significantly affect the chemical and physical stability of the nuclear waste form. Pyrochlore can effectively incorporate a variety of actinides into its structure. Four titanate pyrochlores were synthesized with compositions of Gd2Ti2O7, Sm2Ti2O7, Eu2Ti2O7 and Y2Ti2O2. These samples were irradiated with 1 MeV Kr+ in order to simulate alpha-decay damage and were observed by in situ electron microscopy. Irradiations were conducted from 25 K to 1023 K. At room temperature, Gd-, Sm- and Eu-pyrochlores amorphized at a dose of ∼2×1014 ions/cm2 (∼0.5 dpa) and Y-pyrochlore amorphized at 4×1014 ions/cm2 (∼0.8 dpa). The amorphization dose became higher at elevated temperatures with different rates of increase for each composition. The critical temperatures for amorphization are ∼1100 K for Gd-, Sm-, Eu-pyrochlore and ∼780 K for Y-pyrochlore. The rare-earth-pyrochlores are more susceptible to amorphization and have higher critical temperatures than Y-pyrochlore. The difference in amorphization dose and critical temperature is attributed to the different cascade sizes caused by the different cation masses of the target. Based on a model of cascade quenching, the larger cascade is related to a lower amorphization dose and higher critical temperature. The irradiated materials were studied by electron diffraction and high-resolution electron microscopy. All the pyrochlores transformed to a fluorite substructure prior to the completion of amorphization of the observed regions. This transformation was caused by the disordering between cations and between oxygen and oxygen vacancies. The concurrence of cation disordering with amorphization suggests the partial recrystallization of the displacement cascades. Isolated cascade damage regions were observed by high-resolution electron microscopy, and the cation disordering was associated with the damaged regions.

Ion beam-induced amorphization in MgO–Al2O3–SiO2. I. Experimental and theoretical basis

Abstract Ion beam-induced, crystalline-to-amorphous transition was studied for crystalline MgO (periclase), α-Al 2 O 3 (corundum), SiO 2 (quartz), MgSiO 3 (enstatite), Al 2 SiO 5 (sillimanite, andalusite, kyanite), 3Al 2 O 3 · 2SiO 2 (mullite), Mg 3 Al 2 Si 3 O 12 (pyrope), and Mg 2 Al 4 Si 5 O 18 (cordierite). Samples were irradiated with 1.5 MeV Xe + at temperatures from 15 to 1023 K, and the dose required for amorphization was determined by in situ transmission electron microscopy. Results suggest a parallel between the susceptibility to ion beam irradiation-induced amorphization and the ease of glass formation. The critical amorphization doses of ion irradiation for the crystalline phases are related to the viscosities of melts of equivalent compositions through the activation energies of both processes. Doses required for amorphization have a negative correlation with viscosities at melting temperatures.

Electron and ion irradiation of zeolites

Abstract Three zeolites (analcime, natrolite, and zeolite-Y) were irradiated with 200 and 400 keV electrons. All zeolites amorphized at a relatively low electron fluence. The electron fluences for amorphization by the 200 keV electron irradiation at room temperature were: 7.0×10 19 e − / cm 2 (analcime), 1.8×10 20 e − / cm 2 (natrolite), and 3.4×10 20 e − / cm 2 (zeolite-Y). These doses are equivalent to an energy deposition between 2.6×1010 and 1.27×1011 Gy. An inverse temperature dependence of amorphization dose was observed for all three zeolites, i.e., amorphization dose decreased with increasing temperature. Analcime was also irradiated with 1.5 MeV Kr+ from 300 to 973 K. The amorphization dose by the ion irradiation was constant, ∼1×10 14 ions / cm 2 , which is ∼0.1 dpa of collisional damage and ∼6.25xa0×xa0108 Gy of ionizing energy deposition.

The effect of amorphization on the Cs ion exchange and retention capacity of zeolite-NaY

Abstract Zeolite-NaY is susceptible to both irradiation- and thermally-induced amorphization. Amorphized zeolite-NaY loses approximately 95% of its ion exchange capacity for cesium due to the loss of exchangeable cation sites and /or the blockage of access to exchangeable cation sites. A secondary phase was formed during the ion exchange reaction with cesium. The Cs-exchanged zeolite-NaY phase has a slightly higher thermal stability than the unexchanged zeolite-NaY. A desorption study indicated that the amorphization of cesium-loaded zeolite-NaY enhances the retention capacity of exchangeable Cs ions due to the closure of structural channels.

Irradiation-Induced Nanostructures

This paper summarizes the results of the studies of the irradiation-induced formation of nanostructures, where the injected interstitials from the source of irradiation are not major components of the nanophase. This phenomena has been observed by in situ transmission electron microscopy (TEM) in a number of intermetallic compounds and ceramics during high-energy electron or ion irradiations when the ions completely penetrate through the specimen. Beginning with single crystals, electron or ion irradiation in a certain temperature range may result in nanostructures composed of amorphous domains and nanocrystals with either the original composition and crystal structure or new nanophases formed by decomposition of the target material. The phenomenon has also been observed in natural materials which have suffered irradiation from the decay of constituent radioactive elements and in nuclear reactor fuels which have been irradiated by fission neutrons and other fission products. The mechanisms involved in the process of this nanophase formation are discussed in terms of the evolution of displacement cascades, radiation-induced defect accumulation, radiation-induced segregation and phase decomposition, as well as the competition between irradiation-induced amorphization and recrystallization.

Evolution of hydrogen and helium co-implanted single-crystal silicon during annealing

H+ was implanted into single-crystal silicon with a dose of 1×1016/cm2 and an energy of 30 KeV, and then He+ was implanted into the same sample with the same dose and an energy of 33 KeV. Both of the implantations were performed at room temperature. Subsequently, the samples were annealed in a temperature range from 200 to 450u200a°C for 1 h. Cross-sectional transmission electron microscopy, Rutherford backscattering spectrometry/channeling, elastic recoil detection, and high resolution x-ray diffraction were employed to characterize the strain, defects, and the distribution of H and He in the samples. The results showed that co-implantation of H and He decreases the total implantation dose, with which the surface could exfoliate during annealing. During annealing, the distribution of hydrogen did not change, but helium moved deeper and its distribution became sharper. At the same time, the maximum of the strain in the samples decreased a lot and also moved deeper. Furthermore, the defects introduced by ion i...

First-principles study of electronic properties of La2Hf2O7 and Gd2Hf2O7

The structural and electronic properties of A2Hf2O7 (A=La and Gd) pyrochlore compounds are investigated by means of first-principles total energy calculations. Also, the formation energies of defects are calculated, and the results can be used to explain the stability of pyrochlores. Hybridizations between A 5p and O 2s and between A 5d and O 2p states are observed, but the interaction between A 5p and O 2s orbitals is much stronger in Gd2Hf2O7 than that in La2Hf2O7. Gd2Hf2O7 shows a density of state distribution much different from that of La2Hf2O7. Mulliken overlap population analysis shows that the A–O48f and A–O8b bonds in Gd2Hf2O7 are more ionic than the corresponding bonds in La2Hf2O7, while the Hf–O48f bond in Gd2Hf2O7 is more covalent. These calculations suggest that A–O48f and A–O8b bonds may play important roles in their response to irradiation-induced amorphization observed experimentally.