Ion Irradiation-induced Amorphization Research Articles

Amorphization resistance of nanocrystalline 3C-SiC implanted with H2+ ions

Compared to single-crystal SiC, nanocrystalline SiC with high densities of stacking faults has been reported to be much more resistant to amorphization under self-ion and electron irradiations. This study examines H2+ ion irradiation-induced amorphization in nanocrystalline 3C-SiC with dense stacking faults using transmission electron microscopy. The results show that full amorphization at room temperature occurs at a comparable dose to that for its single-crystal SiC counterpart under the identical irradiation conditions. Both materials are amorphized as a result of local damage accumulation. The formation of the nucleation sites for amorphization is not appreciably affected by the presence of stacking faults and grain boundaries. The behavior may be attributed to the significant chemical effects of the implanted H atoms that may completely immobilize the point defects in SiC at room temperature. The results suggest cautions be excised to use nanocrystalline SiC materials in high H irradiation environment at room temperature. Further studies of the H behavior at elevated temperatures are warranted.

Heavy Kr ion irradiation-induced amorphization on fluorapatite Ca10-2xLaxNax(PO4)6F2 (x=0.2 and 2)waste form with varying La loading

Polycrystal fluorapatite Ca10-2xLaxNax(PO4)6F2 (x = 0.2 and 2) were successfully synthesized by solid state reaction. The both samples were irradiated with 800 keV heavy Kr2+ ions at room temperature with fluences ranging from 5.0×1013 Kr2+/cm2 to 7.0×1014 Kr2+/cm2, corresponding to peak doses of 0.06–0.84 dpa. the irradiated samples were characterized by grazing incidence X-ray diffraction (GIXRD) and cross-sectional transmission electron microscopy (XTEM). A crystalline to amorphization phase transformation was observed as the irradiation fluence up to 1.0×1014 Kr2+/cm2, and the amorphization fraction became larger with the irradiation fluence increasing. However, the amorphization fraction of both samples are different at the same irradiation fluence, where the fluorapatite Ca10-2xLaxNax(PO4)6F2 with more La doping exhibited less amorphous fraction based on GIXRD measurements. Meanwhile, the nanocrystals existed in the irradiation damaged layer of Ca6La2Na2(PO4)6F2 with irradiation fluence of 7.0×1014 Kr2+/cm2, while a fully amorphous layer was observed in Ca9.6La0.2Na0.2(PO4)6F2 at the same fluence. The results suggest that more fraction of La3+ at AI site of apatite may enhance the irradiation tolerance, especially the amorphization resistance.

In-situ TEM study of the ion irradiation behavior of U3Si2 and U3Si5

U3Si2 and U3Si5 are two important uranium silicide phases currently under extensive investigation as potential fuel forms or components for light water reactors (LWRs) to enhance accident tolerance. In this paper, their irradiation behaviors are studied by ion beam irradiations with various ion mass and energies, and their microstructure evolution is investigated by in-situ transmission electron microscopy (TEM). U3Si2 can easily be amorphized by ion beam irradiations (by 1 MeV Ar2+ or Kr2+) at room temperature with the critical amorphization dose less than 1 dpa. The critical amorphization temperatures of U3Si2 irradiated by 1 MeV Kr2+ and 1 MeV Ar2+ ion are determined as 580 ± 10 K and 540 ± 5 K, respectively. In contrast, U3Si5 remains crystalline up to 8 dpa at room temperature and is stable against ion irradiation-induced amorphization up to ∼50 dpa by either 1 MeV Kr2+ or 150 KeV Kr+ at 623 K. These results provide valuable experimental data to guide future irradiation experiments, support the relevant post irradiation examination, and serve as the experimental basis for the validation of advanced fuel performance models.

Krypton ion irradiation-induced amorphization and nano-crystal formation in pyrochlore Lu2Ti2O7 at room temperature**Project sponsored by the National Natural Science Foundation of China (Grant No. 11205128) and the Fundamental Research Funds for the Central Universities, China (Grant No. 2012121034).

Polycrystalline pyrochlore Lu2Ti2O7 pellets are irradiated with 600-keV Kr3+ ions up to a fluence of 1.45 × 1016 Kr3+/cm2. Irradiation induced structural modifications are examined by using grazing incidence x-ray diffraction (GIXRD) and cross-sectional transmission electron microscopy (TEM). The GIXRD reveals that amorphous fraction increases with the increase of fluences up to 2 × 1015 Kr3+/cm2, and the results are explained with a direct-impact model. However, when the irradiation fluence is higher than 2 × 1015 Kr3+/cm2, the amorphous fraction reaches a saturation of ∼80%. Further TEM observations imply that nano-crystal is formed with a diameter of ∼10 nm within the irradiation layer at a fluence of 4 × 1015 Kr3+/cm2. No full amorphization is achieved even at the highest fluence of 1.45 × 1016 Kr3+/cm2 (∼36 displacement per atom). The high irradiation resistance of pyrochlore Lu2Ti2O7 at higher fluence is explained on the basis of enhanced radiation tolerance of nano-crystal structure.

Swift heavy ion irradiation-induced amorphization of La2Ti2O7

Polycrystalline La2Ti2O7 powders have been irradiated with 2.0GeV 181Ta ions up to a fluence of 1×1013ions/cm2. Radiation-induced structural modifications were analyzed using synchrotron-based X-ray diffraction (XRD), small angle X-ray scattering (SAXS), Raman spectroscopy and transmission electron microscopy (TEM). An increase in the amorphous fraction as a function of fluence was revealed by XRD and Raman analyses and is evidenced by the reduction in intensity of the sharp Bragg maxima from the crystalline regions. Concurrently, diffraction maxima and vibrational absorption bands broaden with the increasing amorphous fraction. The cross-section for the crystalline-to-amorphous transformation (ion tracks) was determined by quantitative analysis of XRD patterns yielding a track diameter of d=7.2±0.9nm. Slightly larger track diameters were obtained directly from TEM images (d=10.6±0.8nm) and SAXS analysis (d=10.6±0.3nm). High-resolution TEM images revealed that single tracks are entirely amorphous without any outer crystalline, disordered shell as found in pyrochlore oxides of the same stoichiometry. The large ratio of ionic radii of the A- and B-site cations (rA/rB=1.94) means that disordering over the A- and B-sites is energetically unfavorable.

First-principles study of disordering tendencies in Gd2B2O7 (B = Ti, Sn, Zr) compounds

This paper performs the density functional theory calculations to obtain some factors influencing the response of pyrochlores Gd2B2O7 (B = Ti, Sn, Zr) to ion irradiation-induced amorphization. The 48f oxygen position parameter x, cohesive energy, bond type and defect-formation energy are discussed. The results show that parameter x can be used to indicate the disordering tendencies within a given pyrochlore family. Bond type, cohesive energy and defect-formation energies can be used to explain some experimental observations, but they are not determined exclusively by radiation “resistance" for a different pyrochlore family.

Ion Irradiation-induced Amorphization in Vanadate-Phosphate Apatites

Extended abstract of a paper presented at Microscopy and Microanalysis 2010 in Portland, Oregon, USA, August 1 – August 5, 2010.

Theoretical investigation of structural, energetic and electronic properties of titanatepyrochlores

Ab initio total energy calculations using the plane-wave pseudopotential method based on densityfunctional theory were carried out to investigate the structural, energetic and electronic propertiesof A2Ti2O7 (A = La, Gd and Yb) pyrochlores. It turned out that the formation energies ofantisite defects are not linearly dependent on the ratio of the cation radii, and,for the three compositions, the cation antisite formation energy is largest forGd2Ti2O7 pyrochlore. It wasindicated that Gd2Ti2O7 compound is the least likely to form defect fluorite structure, which gives rise to the least resistanceto radiation-induced amorphization. DOS analysis showed that stronger interaction exists in theGd2Ti2O7 compound, and its electronic structure is very different from that ofLa2Ti2O7 and Yb2Ti2O7. Our calculations suggested that the electronic structure of the A cation andbond type should be taken into account when explaining the response behavior ofA2Ti2O7 (A = La, Gd, Yb) pyrochlores to ion irradiation-induced amorphization.

Ion beam irradiation in La 2Zr 2O 7–Ce 2Zr 2O 7 pyrochlore

Generally, zirconate pyrochlores do not experience a radiation-induced transformation from the crystalline-to-amorphous state, but rather disorder to a defect fluorite structure-type. Thus Gd 2Zr 2O 7 has been proposed as a nuclear waste form for the immobilization of plutonium because of its radiation “stability”. In contrast, La 2Zr 2O 7 can be amorphized by a 1.5 MeV Xe + ion irradiation (∼5.5 dpa at room temperature), and the critical amorphization temperature is low (∼310 K). In this study we present data on ion beam irradiations of compositions in the solid solution: (La 1− x Ce x ) 2Zr 2O 7 ( x=0, 0.1, 0.2, 1). Ce is used as an analogue element for Pu because of similarities in charge and size. La 2Zr 2O 7 can be amorphized by a 1.0 MeV Kr + ion irradiation at 25 and 293 K at doses of ∼1.19 and ∼3.42 dpa, respectively, confirming that La 2Zr 2O 7 is susceptible to ion irradiation-induced amorphization. With the addition of 10 mol% Ce in lanthanum–zirconate pyrochlore structure, no ion irradiation-induced amorphization has been observed at room temperature. The critical amorphization dose for (La 0.9Ce 0.1) 2Zr 2O 7 at 25 K is ∼3.55 dpa, and with increasing Ce-content, a higher dose (∼5.20 dpa) is required to fully amorphize (La 0.8Ce 0.2) 2Zr 2O 7 at 25 K. No amorphization occurred for Ce 2Zr 2O 7 at 25 K at a dose of ∼7 dpa. These results suggest that the addition of Ce into the La 2Zr 2O 7 structure increases the stability of the La 2Zr 2O 7 waste form in a radiation environment, which may be attributed to the decreasing average radius of cations in the A-site, resulting from the smaller ionic radii of Ce 3+ (0.114 nm) and Ce 4+ (0.097 nm) as compared to La 3+ (0.116 nm). An ion beam-induced anion-disordered pyrochlore was observed prior to the final transformation to a disordered fluorite structure. The local structural evolution upon ion irradiation was also investigated for Ce-doped La 2Zr 2O 7 pyrochlores with different average A-site cation sizes and valence states using electron energy-loss spectrometry (EELS).

Radiation-induced amorphization of rare-earth titanate pyrochlores

Single crystals of the entire series of ${A}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ $(A=\mathrm{Sm}$ to Lu, and Y) pyrochlore compounds were irradiated by 1-MeV ${\mathrm{Kr}}^{+}$ ions at temperatures from 293 to 1073 K, and the microstructure evolution, as a function of increasing radiation fluence, was characterized using in situ transmission electron microscopy (TEM). The critical amorphization temperature, ${T}_{c},$ generally increases from \ensuremath{\sim}480 to \ensuremath{\sim}1120 K with increasing A-site cation size (e.g., 0.977 \AA{} for ${\mathrm{Lu}}^{3+}$ to 1.079 \AA{} for ${\mathrm{Sm}}^{3+}).$ An abnormally high susceptibility to ion beam damage was found for ${\mathrm{Gd}}_{2}{\mathrm{Ti}}_{2}{\mathrm{O}}_{7}$ (with the highest ${T}_{c}$ of \ensuremath{\sim}1120 K). Factors influencing the response of titanate pyrochlores to ion irradiation-induced amorphization are discussed in terms of cation radius ratio, defect formation, and the tendency to undergo an order-disorder transition to the defect-fluorite structure. The resistance of the pyrochlore structure to ion beam-induced amorphization is not only affected by the relative sizes of the A- and B-site cations, but also the cation electronic configuration and the structural disorder. Pyrochlore compositions that have larger structural deviations from the ideal fluorite structure, as evidenced by the smaller $48f$ oxygen positional parameter, x, are more sensitive to ion beam-induced amorphization.

Ion-beam and electron-beam irradiation of synthetic britholite

Britholite, ideally Ca 4− x REE 6+ x (SiO 4) 6O 2 (REE=rare earth elements), has the hexagonal structure of apatite, a candidate waste form for actinides. Two synthetic britholites: Ca 3.05Ce 2.38Fe 0.25Gd 5.37Si 4.88O 26 (N56) and Ca 3.78La 0.95Ce 1.45Zr 0.78Fe 0.14Nd 2.15Eu 0.50Si 6.02O 26 (N88) ( P63; Z=1) were irradiated with 1.0 MeV Kr 2+ and 1.5 MeV Xe + over the temperature range of 50–973 K. The process of ion irradiation-induced amorphization, including the effects of the target mass and the ion mass, and the recrystallization of amorphous domains due to ionizing irradiation were investigated. The critical amorphization temperature, T c was determined to be 910 K for N56 (1.0 MeV Kr 2+), 880 K for N88 (1.0 MeV Kr 2+) and 1010 K for N88 (1.5 MeV Xe +). The sequence of increasing T c correlates with the mass of the incident ion; whereas, the ratio of electronic to nuclear stopping power (ENSP) is inversely correlated with T c. Electron irradiations were conducted on previously amorphized britholite (N56) with an electron flux of 1.07 × 10 25 e −/m 2/s. The ionizing radiation resulted in recrystallization at the absorbed dose of 6.2 × 10 13 Gy. This result suggests that the ionizing radiation can induce recrystallization in silicate apatites, similar to that observed for phosphate apatite.

Low‐energy helium ion irradiation‐induced amorphization and chemical changes in olivine: Insights for silicate dust evolution in the interstellar medium

Abstract— We present the results of irradiation experiments aimed at understanding the structural and chemical evolution of silicate grains in the interstellar medium. A series of He+ irradiation experiments have been performed on ultra‐thin olivine, (Mg,Fe)2SiO4, samples having a high surface/volume (S/V) ratio, comparable to the expected S/V ratio of interstellar dust. The energies and fluences of the helium ions used in this study have been chosen to simulate the irradiation of interstellar dust grains in supernovae shock waves. The samples were mainly studied using analytical transmission electron microscopy. Our results show that olivine is amorphized by low‐energy ion irradiation. Changes in composition are also observed. In particular, irradiation leads to a decrease of the atomic ratios O/Si and Mg/Si as determined by x‐ray photoelectron spectroscopy and by x‐ray energy dispersive spectroscopy. This chemical evolution is due to the differential sputtering of atoms near the surfaces. We also observe a reduction process resulting in the formation of metallic iron. The use of very thin samples emphasizes the role of surface/volume ratio and thus the importance of the particle size in the irradiation‐induced effects. These results allow us to account qualitatively for the observed properties of interstellar grains in different environments, that is, at different stages of their evolution: chemical and structural evolution in the interstellar medium, from olivine to pyroxene‐type and from crystalline to amorphous silicates, porosity of cometary grains as well as the formation of metallic inclusions in silicates.

Ion irradiation-induced amorphization and nano-crystal formation in garnets

The radiation susceptibility of garnet structure types (A 3B 2(XO 4) 3, I a3d, Z=8) has been examined by 1.0 MeV Kr 2+ irradiation with in situ transmission electron microscopy over the temperature range of 50–1070 K. The target garnets included five natural garnets: almandine, andradite, pyrope, grossular and spessartine, and five synthetic garnets incorporating various contents of actinides. The synthetic garnets were silicates (N-series) and ferrate–aluminates (G-series). The critical amorphization temperature ( T c), above which amorphization does not occur, were determined to be 1050 K for N77, 1130 K for N56, 1100 K for G3, 890 K for G4 and 1030 K for andradite. T c of the synthetic garnets increased with increasing mass density of the target and as the electronic-to-nuclear stopping power ratio decreased. During ion irradiation of the G3 garnet at a temperature of 1023 K near the T c, nano-crystals were produced in which the unit-cell parameter of the original garnet was halved (∼0.64 nm) as a result of a radiation-induced recrystallization process, without evidence for ordering of the cations.

Heavy ion irradiation effects of brannerite-type ceramics

Brannerite, UTi 2O 6, occurs in polyphase Ti-based, crystalline ceramics that are under development for plutonium immobilization. In order to investigate radiation effects caused by α-decay events of Pu, a 1 MeV Kr + irradiation on UTi 2O 6, ThTi 2O 6, CeTi 2O 6 and a more complex material, composed of Ca-containing brannerite and pyrochlore, was performed over a temperature range of 25–1020 K. The ion irradiation-induced crystalline-to-amorphous transformation was observed in all brannerite samples. The critical amorphization temperatures of the different brannerite compositions are: 970 K, UTi 2O 6; 990 K, ThTi 2O 6; 1020 K, CeTi 2O 6. The systematic increase in radiation resistance from Ce-, Th- to U-brannerite is related to the difference of mean atomic mass of A-site cation in the structure. As compared with the pyrochlore structure-type, brannerite phases are more susceptible to ion irradiation-induced amorphization. The effects of structure and chemical compositions on radiation resistance of brannerite-type and pyrochlore-type ceramics are discussed.

Ion-Induced Amorphization of Murataite

ABSTRACTMurataite A4B2C7O22-x, where A = Na+, Ca2+, REE3+, An3+/4+; B = Mn2+/3+, Zn2+; C = Ti4+, Fe3+, Al3+; 0≤x≤1, is an isometric, derivative of the fluorite-structure. Murataite is potentially suitable as a phase for the immobilization of rare earth (REE) and actinide elements (An). Murataite structures with three-(3C), five-(5C), and eight-fold (8C) multiples of the fluorite unit cell parameters have been identified. Radiation-induced amorphization of murataite has been investigated by 1 MeV Kr+ ion irradiation of three ceramic samples produced by melting in a resistive furnace and a cold crucible at 1400-1600 °C. The 1 MeV Kr+ ion irradiations were performed at room temperature using IVEM-Tandem Facility at Argonne National Laboratory. Radiation damage was observed by in-situ TEM. Initially, the irradiation caused disordering of the murataite structure. Murataite was rendered fully amorphous at a dose of (1.7∼1.9)Á1018 ion/m2. The pyrochlore structure phase (2C) is more radiation resistant to ion irradiation-induced amorphization than the murataite structure. Combining results on murataite with those pyrochlore and fluorite, a generally increasing trend in the susceptibility to ion beam damage is found in the fluorite-related structures as a function of the increasing multiples of the fluorite unit cells.

Ion irradiation-induced amorphization of two GeO 2 polymorphs

The amorphization of two GeO 2 polymorphs, the hexagonal ( P3 2 21) and tetragonal forms ( P4 2 /mnm), were studied using 1.5 MeV Xe +. The polymorphs showed significant differences in their susceptibility to amorphization by irradiation. Hexagonal GeO 2 had a lower amorphization dose (1.7×10 13 ions/cm 2 at room temperature), higher critical temperature (876 K) and higher activation energy for dynamic annealing (0.2 eV) than tetragonal GeO 2 which had a higher amorphization dose (1×10 15 ions/cm 2 at room temperature), lower critical temperature (820 K) and lower activation energy (0.02 eV). Because of their identical composition, the difference in susceptibility to amorphization between the two GeO 2 polymorphs is attributed to the connectivity of polyhedra. The glass-forming ability of hexagonal GeO 2 is much greater than tetragonal GeO 2 because of the three dimensional network formed by corner sharing (GeO 4) tetrahedra. The radiation effects of two GeO 2 polymorphs are also compared to the results of irradiation study of two related materials, rutile (TiO 2) and quartz (SiO 2). Both chemical composition and structure contribute significantly to the susceptibility to irradiation-induced amorphization in these phases.

Ion irradiation-induced amorphization of six zirconolite compositions

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 (∼2×1015−3.9×1015ions/cm2) 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.

Ion irradiation-induced amorphization of CaAl 2O 4

The relationship between irradiation-induced amorphization and glass formation was investigated with CaAl 2O 4, a highly ionic (ionicity=0.68) glass former (Al 2O 3·CaO). Samples were irradiated with 1.5 MeV Xe + from 100 to 673 K. In situ TEM was used to characterize the effects of ion beam damage and the critical temperature for amorphization, T c. The sample became amorphous at 1.2 × 10 14 Xe +/cm 2 at room temperature. T c is 733 K. Two other phases, Al 2O 3·SiO 2 (andalusite) and Al 2O 3·MgO (spinel), are compared with Al 2O 3·CaO. Based on a cascade-quenching model, the susceptibility to radiation-induced amorphization is proposed to be related to the tendency toward glass formation.

Ion irradiation-induced amorphization in the Al2O3–SiO2 system: A comparison with glass formation

The ion beam-induced crystalline-to-amorphous transition has been investigated for crystalline phases in the Al2O3–SiO2 system: Al2O3, SiO2 (quartz), Al2SiO5 (kyanite, andalusite, sillimanite), and 3Al2O3⋅2SiO2 (mullite). Xe+ 1.5 MeV was used to irradiate samples at temperatures from 15 to 1023 K in situ in a transmission electron microscope to determine the critical amorphization doses. The susceptibility to amorphization is (highest to lowest): quartz, sillimanite, kyanite, andalusite, mullite, and alumina. These data are compared to viscosities and activation energies for viscous flow of melts in this system. The doses required for amorphization by ion irradiation are related to the viscosities of the melts. The activation energies for irradiation-enhanced annealing are qualitatively correlated with the activation energies of viscous flow. These results suggest a parallel between ion beam irradiation-induced amorphization and glass formation. Glass-forming “ability’’ correlates with susceptibility to radiation-induced amorphization.

A Model for Irradiation-Induced Amorphization

ABSTRACTA model based on cascade melting and recrystallization is derived to describe ion irradiation-induced amorphization. The accumulation of amorphous volume fraction during irradiation is represented in a single equation. Depending on the extent of recrystallization of a subcascade, the amorphous volume accumulation can be described by a set of curves that change from exponential to sigmoidal functions. The parameters (including temperature, cascade size, crystallization rate, glass transition temperature, dose rate) that affect the extent of recrystallization are included in the model. The model also describes the temperature dependence of critical dose for amorphization.