S.X. 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.

Uniform deposition of ultrathin polymer films on the surfaces of Al2O3 nanoparticles by a plasma treatment

Surface modification of nanoparticles will present great challenges due to their extremely small dimensions, high surface areas, and high surface energies. In this research, we demonstrate the uniform deposition of ultrathin polymer films of 2 nm on the surfaces of alumina nanoparticles. The deposited film can also be tailored to multilayers. Time-of-flight secondary ion mass spectroscopy was used to confirm the pyrrole thin film on the nanoparticle surfaces. Using such a nanocoating, it is possible to alter the intrinsic properties of materials that cannot be achieved by conventional methods and materials.

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.

Localized epitaxial growth of α-Al2O3 thin films on Cr2O3 template by sputter deposition at low substrate temperature

Low-temperature growth of α-Al2O3 films by sputtering was studied with x-ray diffraction and high-resolution transmission electron microscopy (HRTEM). Pure α-Al2O3 film was formed at 400 °C using Cr2O3 as template, whereas amorphous or θ-Al2O3 was formed without Cr2O3. HRTEM revealed localized epitaxial growth of α-Al2O3 on Cr2O3 with the relationship [011]Al2O3/[011]Cr2O3, suggesting the importance of Cr2O3 as a structural template for the growth of α-Al2O3, in addition to other contributions such as good stoichiometry, low sputter pressure, and low deposition rate under optimized deposition conditions. Successful growth of α-Al2O3 by sputtering at 400 °C or below makes the film widely applicable to even glass substrates.

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.

Ion irradiation-induced amorphization of six zirconolite compositions

Abstract Zirconolite is an important phase in Synroc, a polyphase titanate ceramic, designed for the immobilization of high level waste from nuclear fuel reprocessing. Zirconolite is a principal host phase for actinides in Synroc. We have studied radiation effects of six zirconolite compositions: CaZrTi2O7, Ca0.8Ce0.2ZrTi1.8Al0.2O7, Ca0.85Ce0.5Zr0.65Ti2O7, Ca0.5Nd0.5ZrTi1.5Al0.5O7, CaZrNb0.85Fe0.85Ti0.3O7 and CaZrNbFeO7. The samples have been irradiated by 1.0 MeV Kr+in a temperature range from 25 to 973 K and observed by in situ transmission electron microscopy. The radiation-induced crystalline-to-amorphous transformation was studied by electron diffraction and high-resolution electron microscopy. Concurrent with amorphization, monoclinic zirconolite has been found to transform to a fluorite sublattice through cation disordering during irradiation. All the zirconolites amorphized after a similar dose (∼2×10 15 −3.9×10 15 ions / cm 2 ) at room temperature. The amorphization dose increased at elevated temperatures with varying rates of increase for each phase. The critical temperatures for amorphization were 550 K for CaZrNbFeO7, 590 K for CaZrNb0.85Fe0.85Ti0.3O7, 640 K for CaZrTi2O7, 900 K for Ca0.8Ce0.2ZrTi1.8Al0.2O7, 1000 K for Ca0.85Ce0.5Zr0.65Ti2O7, and 1020 K for and Ca0.5Nd0.5ZrTi1.5Al0.5O7. The trend of the critical temperatures indicates that decreasing calcium content increases susceptibility to amorphization. The role of calcium in the radiation-induced amorphous structure is considered to be that of a network-modifier of the aperiodic structure formed by the polyhedra of high-valence cations.

Effects of fission product incorporation on the microstructure of cubic zirconia

Abstract Cesium, iodine and strontium ions have been implanted into yttria-stabilized cubic zirconia (YSZ) to determine the effects of fission product incorporation in YSZ that is considered as an inert nuclear fuel matrix. The ion implantation was conducted at room temperature to 1×1021 ions/m2 for each ion with ion energies ranging from 70 to 400 keV. The peak displacement damage level and the peak ion concentration in YSZ reached 205–330 displacement per atoms (dpa) and 11–26 at.%, respectively. The microstructure of the implanted YSZ was studied by in situ and cross-sectional transmission electron microscopy (TEM). In the iodine and strontium implanted samples, a damaged layer with a high density of defect clusters was observed, while in the cesium implanted specimen, most of the damaged layer is amorphous.Nanocrystalline precipitates were observed in the strontium implanted specimen after annealing at 1273 K. The results are discussed in terms of the ionic size, mobility and the solubility of the implanted species in YSZ.

Amorphization of ceramic materials by ion beam irradiation

Abstract Ion-beam-induced amorphization of a wide variety of ceramic materials has been investigated using in situ TEM with 1.5 MeV Kr+ or Xe+ ions at temperatures between 20 and 1000 K. Except for a few ‘amorphization resistant’ materials which usually have simple crystal structures, most ceramic materials under study amorphized after a fraction of a dpa (displacement per atom) at cryogenic temperatures. In general, critical amorphization dose increases with the irradiation temperature at a rate determined by the kinetics of the amorphization and crystallization processes. Based on a cascade quenching model and an analysis on the structural resistance to recrystallization, a semiempirical parameter which can easily be calculated from both structural and chemical parameters of a material, has been developed to predict the susceptibility of ceramics to amorphization. The calculated results for over ten phases in the Al2O3–MgO–SiO2 system agree quite well with the experimental data. The results for phases in the Al2O3–MgO–SiO2 system have also suggested a parallel in the kinetics between ion-beam-induced amorphization and glass formation. The critical amorphization temperature, above which irradiation-induced amorphization cannot be completed, is found to be closely related to the glass transition temperature. The ratio between glass transition and melting temperatures can also be used to predict the susceptibility of a ceramic material to amorphization, equivalent to the Debye temperature criterion.