Annealing of isolated amorphous zones in silicon

In situ transmission electron microscopy has been used to observe the production and annealing of individual amorphous zones in silicon resulting from impacts of 200-keV Xe ions at room temperature. As has been observed previously, the total amorphous volume fraction decreases over a temperature range from room temperature to approximately 500 °C. When individual amorphous zones were monitored, however, there appeared to be no correlation of the annealing temperature with initial size: zones with similar starting sizes disappeared (crystallized) at temperatures anywhere from 70 °C to more than 400 °C. Frame-by-frame analysis of video recordings revealed that the recovery of individual zones is a two-step process that occurred in a stepwise manner with changes taking place over seconds, separated by longer periods of stability

Energy dependence of Ge amorphization by Ne, Ar or Kr ion irradiation

Ge has been irradiated at RT by Ne, Ar, or Kr ions of different energies, and the doses required for complete amorphization determined by in situ TEM and electron diffraction. Onset of amorphization was detected after the lowest ion doses reflecting amorphization by individual ions. The ion dose required for complete amorphization increased nearly linearly with ion energy over the range 0.5 to 3.5 MeV for all ions. Amorphization cross sections have been determined for all ions and energies used. The displacements per atom required for complete amorphization decreased with ion energy or mass, owing to decrease in radiation annealing of amorphous volumes as a result of a decrease in fraction of low energy transfers to Ge atoms. Increasing the relative fraction low energy transfers to Ge atoms by simultaneous 1 MeV electron irradiation increased the Kr ion dose required for complete amorphization by as much as a factor of 2.5. The effect is believed to be due to an increase in the fraction of freely migrating defects produced by low energy transfers to Ge atoms

Misty Birtcher, Soprano and Joseph Whitenton, Tenor

Thou shalt break them, from Messiah; Care Selve, from Atalanta / G.F. Händel; Nacht und Träume; Du bist die Ruh; Der Schiffer / Franz Schubert; Intermezzo / Robert Schumann; Aus liebe, from Matthäus-Passion / J.S. Bach; Mandoline / Claude Debussy; Ouvre ton Coeur / Georges Bizet; Musique / Claude Debussy; Pastorale / Georges Bizet; From A Young Man\u27s Exhortation / Gerald Finzi; I Will Not Go, from Troilus and Cressida / Vince Gover; Dammit, Janet!, from The Rocky Horror Picture Show / Richard O\u27Brie

Origin of atomic clusters during ion sputtering

Previous studies have shown that the size distributions of small clusters ( n<=40 n = number of atoms/cluster) generated by sputtering obey an inverse power law with an exponent between -8 and -4. Here we report electron microscopy studies of the size distributions of larger clusters ( n>=500) sputtered by high-energy ion impacts. These new measurements also yield an inverse power law, but one with an exponent of -2 and one independent of sputtering yield, indicating that the large clusters are produced when shock waves, generated by subsurface displacement cascades, ablate the surface

Misty Danielle Birtcher, Soprano

Quanto dolce e quell\u27ardore / Francesco Mancini; Poema en forma de canciones / Joaquín Turina Pérez; Airs chantés / Francis Poulenc; I Hate Music!: A Cycle of Five Kid Songs / Leonard Bernstein; Christmas Lullaby, from Songs for a New World / Jason Robert Brow

Radiation Damage From Single Heavy Ion Impacts on Metal Surfaces

The effects of single ion impacts on the surfaces of films of Au, Ag, In and Pb have been studied using in-situ transmission electron microscopy. On all of these materials, individual ion impacts produce surface craters, in some cases, with associated expelled material. The cratering efficiency scales with the density of the irradiated metal. For very thin Au foils ({approx} 20--50 nm), in some cases individual ions are seen to punch small holes completely through the foil. Continued irradiation results in a thickening of the foil. The process giving rise to crater and hole formation and other changes observed in the thin foils has been found to be due to pulsed localized flow--i.e. melting and flow due to the thermal spikes arising from individual ion impacts. Experiments carried out on thin films of silver sandwiched between SiO{sub 2} layers have indicated that pulsed localized flow also occurs in this system and contributes to the formation of Ag nanoclusters in SiO{sub 2}--a system of interest for its non-linear optical properties. Calculation indicates that, when ion-induced, collision cascades occur near surfaces (within {approx} 5 nm) with energy densities sufficient to cause melting, craters are formed. Crater formation occurs as a result of the explosive outflow of material from the hot molten core of the cascade. Processes occurring in the sandwiched layer are less well understood

Stability of uranium silicides during high energy ion irradiation

Changes induced by 1.5 MeV Kr ion irradiation of both U{sub 3}Si and U{sub 3}Si{sub 2} have been followed by in situ transmission electron microscopy. When irradiated at sufficiently low temperatures, both alloys transform from the crystalline to the amorphous state. When irradiated at temperatures above the temperature limit for ion beam amorphization, both compounds disorder with the Martensite twin structure in U{sub 3}Si disappearing from view in TEM. Prolonged irradiation of the disordered crystalline phases results in nucleation of small crystallites within the initially large crystal grains. The new crystallites increase in number during continued irradiation until a fine grain structure is formed. Electron diffraction yields a powder-like diffraction pattern that indicates a random alignment of the small crystallites. During a second irradiation at lower temperatures, the small crystallizes retard amorphization. After 2 dpa at high temperatures, the amorphization dose is increased by over twenty times compared to that of initially unirradiated material

Structural and elastic properties of Ge after Kr-ion irradiation at room temperature

Changes in the elastic properties of Ge induced by room-temperature irradiation with 3.5-MeV Kr ions have been determined and correlated with changes in the microstructure determined by transmission electron microscopy. Elastic-shear-moduli changes were measured by Brillouin scattering, and changes in local atomic arrangement were determined by Raman scattering. Amorphization decreased the elastic shear modulus of Ge by 17%. The fractional decrease was correlated with the amorphous volume fraction with a cross section of 4.5±0.5 nm2/ion. No change was observed in the shear modulus during void formation and growth. The elastic properties of the voided material are described by the Voigt averaging. However, as the voids evolved into a fibrous spongelike microstructure, a second dramatic elastic softening occurs which we attribute to the inability of the fibrous structure to support shear stresses. Raman scattering showed that, once formed, there was no change in the structure of the amorphous material at the atomic scale during void formation and subsequent void coalescence

Phase Stability under Irradiation of Precipitates and Solid Solutions in Model ALloys and in ODS Alloys Relevant for Gen IV

The overall objective of this program is to investigate the irradiation-altered phase stability of oxide precipitates in ODS steels and of model alloy solid solutions of associated systems. This information can be used to determine whether the favorable mechanical propertiies of these steels are maintained under irradiation, thus addressing one of the main materials research issues for this class of steels as identified by the GenIV working groups. The research program will also create fundamental understanding of the irradiation precipitation/dissolution problem by studying a "model" system in which the variables can be controlled and their effects understood individually