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During the past half-century, the nuclear fuel cycle has generated approximately 1400 metric tons of plutonium and substantial quantities of the “minor” actinides, such as Np, Am, and Cm. The successful disposition of these actinides has an important impact on the strategy for developing advanced nuclear fuel cycles, weapons proliferation, and the geologic disposal of high-level radioactive waste. During the last decade, there has been substantial interest in the use of the isometric pyrochlore structure-type, A2B2O7, for the immobilization of actinides. Most of the interest has focused on titanate-pyrochlore because of its chemical durability; however, these compositions experience a radiation-induced transition from the crystalline-to-aperiodic state due to radiation damage from the alpha-decay of actinides. Depending on the actinide concentration, the titanate pyrochlore will become amorphous in less than 1000 years of storage. Recently, systematic ion beam irradiations of a variety of pyrochlore compo...

Oriented CoxNi1−x nanoparticles were synthesized by sequential implantation of 90 keV Co and Ni ions at room temperature in the near surface of an yttria-stabilized zirconia single crystal. Transmission electron microscopy and a superconducting quantum interference device magnetometer as well as a vibrating sample magnetometer were utilized to characterize the structural and magnetic properties of the implanted layer. Typical nanoparticles range from 3 to 10 nm in diameter, and some elongated precipitates as long as 25 nm were formed by joining the nanoparticles with nanotwins. Hysteresis loops were measured at both 300 and 10 K, and showed coercivity of 100 or 250 Oe, respectively. Magnetization versus temperature in zero-field-cooled and field-cooled fields was measured. No obvious superparamagnetism was detected below 300 K.

Single crystals of ZrSiO4 (zircon) with a (110) orientation were implanted with 300 keV Pb+ at room temperature to fluences ranging from 1014 to 1017 ions/cm2. The damage accumulation and microstructural evolution were analyzed by cross-sectional transmission electron microscopy (TEM) and glancing-angle x-ray diffraction (XRD). The experimental damage profiles as observed by TEM and XRD methods were compared to Monte Carlo simulations using the SRIM-2000 code. At the lowest ion fluence (1014 ions/cm2), a buried amorphous layer formed in the zircon matrix. The surface layer is highly damaged and consists of zircon nanocrystals. The critical amorphization dose for zircon implanted with 300 keV Pb+ was in the range of 0.25–0.43 displacements per atom. With increasing ion fluence, the thickness of the amorphous layer increased. When the Pb concentration in the substrate exceeded ∼3.5 at. % (i.e., at 1017 ions/cm2), Pb nanoparticles precipitated at room temperature and formed a layer ∼90 nm thick embedded within the amorphous zircon matrix. Effects of the displacement energies employed in the SRIM-2000 simulation on the damage profiles and the critical amorphization dose were also analyzed.

In situ high-pressure X-ray diffraction and Raman spectroscopy were used to determine the influence of composition on the high-pressure behavior of A2B2O7 pyrochlore (A = Eu, Dy; B = Ti, Zr) up to ∼50 GPa. Based on X-ray diffraction results, all compositions transformed to the high-pressure cotunnite structure. The B-site cation species had a larger effect on the transition pressure than the A-site cation species, with the onset of the phase transformation occurring at ∼41 GPa for B = Ti and ∼16 GPa B = Zr. However, the A-site cation affected the kinetics of the phase transformation, with the transformation for compositions with the smaller ionic radii, i.e., A = Dy, proceeding faster than those with a larger ionic radii, i.e., A = Eu. These results were consistent with previous work in which the radius-ratio of the A- and B-site cations determined the energetics of disordering, and compositions with more similarly sized A- and B-site cations had a lower defect formation energy. Raman spectra revealed dif...

Cross-sectional and high-resolution transmission electron microscopy (XTEM and HRTEM, respectively) have been used to characterize ion-beam-induced amorphization of α-quartz irradiated with 1.5 MeV Kr+ ions at room temperature. The accumulation of damage and the growth of the amorphous layers in quartz were studied as a function of ion fluence up to 1.7×1014 ions/cm2. An amorphous band was first observed at the peak displacement damage range, and this region increased in width with increasing ion fluence. These results demonstrate that direct displacement damage by nuclear collision is more efficient than ionization processes in inducing amorphization in quartz. At lower fluences (⩽1.7×1013 ions/cm2), the damage profile observed by XTEM is in excellent agreement with the distribution of displacement damage predicted by TRIM calculations. At higher fluences, the range of the amorphous band measured by XTEM exceeds the depth predicted by TRIM.

Electron-induced amorphization of coesite was studied as a function of irradiation temperature by in situ transmission electron microscopy at an incident energy of 200 keV. Electron-induced amorphization of coesite is induced by an ionization mechanism and is mainly dominated by an interface-mediated, heterogeneous nucleation-and-growth controlled process. Amorphous domains nucleate at surfaces, crystalline-amorphous (c-a) interfaces, and grain boundaries. This is the same process as the interface-mediated vitrification of coesite by isothermal annealing above the thermodynamic melting temperature (875 K), but below the glass transition temperature (1480 K). The interface-mediated amorphization of coesite by electron irradiation is morphologically similar to interface-mediated thermodynamic melting.

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 r...

A plasma polymerization method was employed to deposit an ultrathin pyrrole film of 3 nm onto the surfaces of single wall carbon nanotubes (SWCNTs) and Al2O3 nanoparticles for developing high-strength nanocomposites. The surfaces of plasma coated SWCNTs and Al2O3 nanoparticles were studied by high resolution transmission electron microscopy (TEM) and time-of-flight secondary ion mass spectroscopy. After sintering the SWCNTs-Al2O3 composites at different temperatures (maximum of 1200 °C), the thermal stability of plasma-coated SWCNTs was significantly increased, compared to their uncoated counterparts. After hot-press sintering, the SWCNTs without plasma coating were essentially decomposed into amorphous clusters in the composites, leading to degraded mechanical properties. However, under the same sintering conditions, the plasma surface modified SWCNTs were well preserved and distributed in the composite matrices. The effects of plasma surface coating on the thermal stability of SWCNTs and mechanical beha...

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.

Single crystal 6H‐SiC wafers were fully amorphized at room temperature or 200K using 1.0 or 2.0MeV Au+ ion irradiation. The thickness of the amorphized layers has been determined using Rutherford backscattering spectrometry under ion channeling conditions. Microstructures of the irradiated SiC have been examined using cross-sectional transmission electron microscopy. The depth profiles of both the Si and C atoms have been studied using both x-ray photoelectron spectroscopy (XPS) and time-of-flight energy elastic recoil detection analysis. Neither Si nor C in the amorphized SiC exhibits a significant mass transport by diffusion during the irradiation and subsequent storage at room temperature. There is no observable phase segregation of either Si or C in the amorphized SiC. Ar+ ion sputtering leads to modifications of the composition, structure, and chemical bonding at the 6H‐SiC surface. The Si–Si bonds at the sputtered surface (amorphized) do not appear, as suggested by the XPS; however, Raman backscatte...

Murataite (A3B6C2O22−x∕2,F4¯3m), a derivative of an anion-deficient fluorite structure, has been synthesized as different polytypes as a result of cation ordering. Ion-beam-induced amorphization has been investigated by 1-MeV Kr2+ ion irradiation with in situ transmission electron microscopy. The critical amorphization dose was determined as a function of temperature and the degree of structural disordering. A lower critical amorphization temperature (∼860K) was obtained for the disordered murataite as compared with that of the murataite superstructure (930to1060K). An ion-beam-induced ordered murataite to a disordered fluorite transition occurred in the murataite superstructure, similar to that observed in the closely related pyrochlore structure-type, A2B2O7. The ion-beam-induced defect fluorite structure is more energetically stable in the murataite structure with a higher degree of structural disordering, as compared with the murataite superstructure. This suggests that the degree of intrinsic structu...

Engineering applications of carbon nanofibers and nanotubes require their alignment in specific directions. Single-walled carbon nanotubes can be aligned in a magnetic field due to the presence of small amounts of catalyst elements, such as Ni and Co. However, for carbon nanofibers, their extremely low magnetic susceptibility is not sufficient for magnetically induced alignment. We present a method of solution-coating of NiO and CoO onto the surface of the carbon nanofibers. Due to the NiO- and CoO-coating, these nanofibers can be well aligned in the polymer composites under moderate magnetic field (3 T). Both transmission electron microscopy and scanning electron microscopy results show the well-aligned nanofibers in a polymer matrix. Mechanical testing shows a pronounced anisotropy in tensile strength in directions normal (12.1 MPa) and parallel (22 MPa) to the applied field, resulting from the well-aligned nanofibers in the polymer matrix. The mechanism of magnetic alignment due to coating of NiO and CoO on the nanofiber surface is discussed.