Proximity effect thermometer for local temperature measurements on mesoscopic samples

Using the strong temperature dependent resistance of a normal metal wire in proximity to a superconductor, we have been able to measure the local temperature of electrons heated by flowing a dc current in a metallic wire to within a few tens of millikelvin at low temperatures. By placing two such thermometers at different parts of a sample, we have been able to measure the temperature difference induced by a dc current flowing in the sample. This technique may provide a flexible means of making quantitative thermal and thermoelectric measurements on mesoscopic metallic samples

Phase formation in ion‐irradiated and annealed Ni‐rich Ni‐Al thin films

Phase formation was studied in ion‐irradiated multilayer and coevaporated Ni‐20 at. % Al films supported by Cu, Mo, and Ni transmission electron microscopy (TEM) grids. Irradiation with either 700‐keV Xe or 1.7‐MeV Xe, to doses sufficient to homogenize the multilayers (≥7.5×1015 cm−2), resulted in the formation of metastable supersaturated γ and HCP phases in both film types. Post‐irradiation annealing of multilayers at 450 °C for 1 h transformed the metastable phases to a two‐phase γ+γ′ microstructure. In the absence of Cu, the formation of γ′ appeared to proceed by a traditional diffusional growth mechanism, resulting in small (<50 Å) γ′ precipitates in γ matrix grains. The presence of Cu caused the formation of a dual‐phase γ+γ′ structure (i.e., distinct, equal‐sized grains of γ and γ′) during post‐irradiation annealing. It is suggested that copper affected the nucleation of γ′ precipitates and increased the kinetics of growth resulting in the dual‐phase morphology. Strong irradiation‐induced textures were observed in the multilayers that were less pronounced in the coevaporated films. The texture in the multilayers was attributed to the presence of a slight as‐evaporated texture combined with the enhanced atomic mobility due to the heat‐of‐mixing released during irradiation. The irradiation‐induced texture appeared to be necessary for the formation of the dual‐phase structure since it likely provided high‐diffusivity paths for Cu to diffuse into the film from the TEM grid.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70874/2/JAPIAU-69-4-2021-1.pd

The heat‐of‐mixing effect on ion‐induced grain growth

Irradiation experiments were conducted on multilayer (ML) and coevaporated (CO) thin films in order to examine the role that the heat‐of‐mixing (ΔHmix) has in ion‐induced grain growth. Room‐temperature irradiations using 1.7‐MeV Xe ions were performed in the High Voltage Electron Microscope at Argonne National Laboratory. The ML films (Pt‐Ti, Pt‐V, Pt‐Ni, Au‐Co, and Ni‐Al) spanned a large range of calculated ΔHmix values. Comparison of grain growth rates between ML and CO films of a given alloy confirmed a heat‐of‐mixing effect. With the exception of the Pt‐V system, differences in grain growth rates between ML and CO films varied according to the sign of the calculated ΔHmix of the system. Substantial variations in growth rates among CO alloy films experiencing similar displacement damage demonstrated that a purely collisional approach is inadequate for describing ion‐induced grain growth. Therefore consideration must also be given to material‐specific properties, such as cohesive energy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/70305/2/JAPIAU-70-3-1252-1.pd

The role of gamma rays and freely-migrating defects in reactor pressure vessel embrittlement

Gamma ray effects are often neglected when evaluating reactor pressure vessel (RPV) embrittlement. However, recent analyses indicate that in newer style light water reactors, gamma damage can be a substantial fraction of the total displacement damage experienced by the (RPV); ignoring this damage will lead to errors in embrittlement predictions. Furthermore, gamma rays may be more efficient than fast neutrons at producing freely-migrating defects and as such can impact certain embrittlement mechanisms more effectively than fast neutrons. Consideration of these gamma effects are therefore essential for a more complete understanding of radiation embrittlement

Ion-induced grain growth in multilayer and coevaporated metal alloy thin films

Irradiation experiments were conducted on multilayer (ML) and coevaporated (CO) thin films in order to examine the role that the heat-of-mixing ([Delta]Hmix) has in ion-induced grain growth. Room-temperature irradiations using 1.7 MeV Xe were performed in the High Voltage Electron Microscope at Argonne National Laboratory. The alloys studied (Pt-Ti, Pt-V, Pt-Ni, Au-Co and Ni-Al) spanned a large range of [Delta]Hmix values. Comparison of grain growth rates between ML and CO films of a given alloy confirmed a heat of mixing effect. Differences in grain growth rates between ML and CO films scaled according to the sign and magnitude of [Delta]Hmix of the system (with the exception of the Pt-V system). Substantial variations in growth rates among CO alloy films experiencing similar irradiation damage demonstrated that a purely collisional approach is inadequate for describing ion-induced grain growth and consideration must also be given to material-specific properties. Results from CO alloy films were consistent with a thermal spike model of ion-induced grain growth. The grain boundary mobility was observed to be proportional to the thermal spike-related parameter, F2D/[Delta]H3coh, where FD is the energy deposited in nuclear interactions and [Delta]Hcoh is the cohesive energy.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/29259/1/0000316.pd

Transmission electron microscopy study in-situ of radiation-induced defects in copper at elevated temperatures

Neutrons and high-energy ions incident upon a solid can initiate a displacement collision cascade of lattice atoms resulting in localized regions within the solid containing a high concentration of interstitial and vacancy point defects. These point defects can collapse into various types of dislocation loops and stacking fault tetrahedra (SFT) large enough that their lattice strain fields are visible under diffraction-contrast imaging using a Transmission Electron Microscope (TEM). The basic mechanisms driving the collapse of point defects produced in collision cascades is investigated in situ with TEM for fcc-Cu irradiated with heavy (100 keV Kr) ions at elevated temperature. The isothermal stability of these clusters is also examined in situ. Areal defect yields were observed to decrease abruptly for temperatures greater than 300 C. This decrease in defect yield is attributed to a proportional decrease in the probability of collapse of point defects into clusters. The evolution of the defect density under isothermal conditions appears to be influenced by three different rate processes active in the decline of the total defect density. These rate constants can be attributed to differences in the stability of various types of defect clusters and to different loss mechanisms. Based upon observed stabilities, estimations for the average binding enthalpies of vacancies to SFT are calculated for copper

Origin of atomic clusters during ion sputtering

Previous studies have shown that the size distributions of small clusters ( n&#60;=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&#62;=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

Ion-channeling study of anomalous atomic displacements at the superconducting transition in high-T sub c materials

Ion channeling along the (001) direction in high-quality single crystals of (Y/Er)Ba{sub 2}Cu{sub 3}O{sub 7-x} revealed an abrupt change in displacements in the a-b plane of the Cu and O atoms at the superconducting transition, {Tc}; normal Debye-like'' vibrations were found for the Y/Er and Ba atoms. The anomalous change in Cu-O displacements was found to shift directly with stoichiometry-induced changes in {Tc}, implying a direct link between the observed phonon anomaly and the superconducting transition. Recent measurements of ion-channeling along the (001) axis in (Bi{sub 1.7}Pb{sub 0.3})Sr{sub 2}Ca{sub 1}Cu{sub 2}O{sub x} single-crystals revealed a similar change at {Tc}, suggesting that this phonon anomaly is a general feature of high-{Tc} superconductivity. In order to identify more specifically the crystallographic directions and displacement amplitudes associated with the anomalous phonon behavior, axial channeling scans using RBS, as well as characteristic x-ray production, were taken at several temperatures between 30 and 300K along the (301) and (331) directions of YBa{sub 2}Cu{sub 3}O{sub 7-x} single crystals. Twins present in the specimens, and the existing static atomic displacements present along these directions, caused the channeling to be poorer along these axes compared to the (001) direction. Also, a much stronger dependence of the minimum yield on depth was observed. However, since only one twin variant generally dominated over sufficiently wide areas of the specimens, reasonably good ({approximately}10%) minimum yields could be obtained along the appropriate (331) axis, and detwinned crystals produced good results along (301). 22 refs., 5 figs

The effect of DIGM and irradiation-induced grain growth on interdiffusion in bilayer ion-beam mixing experiments

Experiments were performed demonstrating that ion irradiation enhances diffusion-induced grain boundary migration (DIGM) in polycrystalline Au/Cu bilayers. Here, a model is presented relating film-averaged Cu composition in Au with treatment time, grain size and film thickness. Application of this model to the experimental results indicates that irradiation enhances DIGM by increasing the grain boundary velocity. Effects of DIGM and irradiation-induced grain growth on the temperature dependence of ion mixing in bilayers are discussed