M. A. Kirk

Microstructure of ultrananocrystalline diamond films grown by microwave Ar–CH4 plasma chemical vapor deposition with or without added H2

Ultrananocrystalline diamond (UNCD) films, grown using microwave plasma-enhanced chemical vapor deposition with gas mixtures of Ar–1%CH4 or Ar–1%CH4–5%H2, have been examined with transmission electron microscopy (TEM). The films consist of equiaxed nanograins (2–10 nm in diameter) and elongated twinned dendritic grains. The area occupied by dendritic grains increases with the addition of H2. High resolution electron microscopy shows no evidence of an amorphous phase at grain boundaries, which are typically one or two atomic layer thick (0.2–0.4 nm). Cross-section TEM reveals a noncolumnar structure of the films. The initial nucleation of diamond occurs directly on the Si substrate when H2 is present in the plasma. For the case of UNCD growth from a plasma without addition of H2, the initial nucleation occurs on an amorphous carbon layer about 10–15 nm thick directly grown on the Si substrate. This result indicates that hydrogen plays a critical role in determining the nucleation interface between the UNCD...

Electron‐beam‐induced crystallization of isolated amorphous regions in Si, Ge, GaP, and GaAs

An energetic electron beam has been used to stimulate crystallization of spatially isolated amorphous regions in Si, Ge, GaP, and GaAs at 30 and 300 K. In the four materials it was found that crystallization was induced even when the energy of the electron beam was less than that required to create point defects in the crystalline structure. The rate of crystallization depended on the material and on the electron energy. In all materials, the rate decreases as the electron energy increases from 50 keV (the lowest electron energy used), reaching a minimum value at an electron energy slightly below the displacement threshold voltage. Above the displacement threshold, the regrowth rate again increases with increasing electron energy. The possible role of electron‐beam heating was studied both theoretically and experimentally. Calculations suggested heating effects were negligible and this was confirmed by in situ ion implantations and electron irradiations performed at 30 K, where subthreshold electrons stimulated crystallization. The subthreshold and low‐temperature results are consistent with the model that the crystallization process is dependent on the creation of defects (dangling bonds and kinks) at the crystalline‐amorphous (c‐a) interface. The crystallization stimulated by the subthreshold electron beams suggests that electronic excitation of the bonds along the c‐a interface can induce the amorphous to crystalline transition.

Heavy-ion irradiations of Fe and Fe–Cr model alloys Part 2: Damage evolution in thin-foils at higher doses

A study of heavy-ion damage in Fe and Fe–Cr alloys started in Part 1 1 was continued with an investigation of damage development in UHP Fe and Fe–8%Cr at higher doses up to 2 × 1019 ions m−2 (∼13 dpa). In thin-foil irradiations with 150 keV Fe+ ions at 300°C and room temperature (RT), more complex microstructures started to develop in thicker regions of the foils at doses greater than about 2 × 1018 ions m−2, apparently involving cooperative interaction, alignment and coalescence of smaller loops. First strings of loops all with the same ½⟨111⟩ Burgers vectors formed. In UHP Fe irradiated at 300°C the damage then developed into colonies of resolvable interstitial loops with ½⟨111⟩ Burgers vectors. By a dose of 2 × 1019 ions m−2, large (several hundred nanometre) finger-shaped loops with large shear components had developed by the growth and subsequent coalescence of smaller loops. Similar but finer-scale damage structures developed in UHP Fe irradiated at RT and in Fe–8%Cr irradiated at both RT and 300°C.

The collapse of defect cascades to dislocation loops

We describe a number of experiments that we have recently performed to investigate the collapse of defect cascades to dislocation loops. This important ion and neutron irradiation phenomenon has been studied with in situ ion bombardment using the High Voltage Electron Microscope-Ion Accelerator Facility at Argonne National Laboratory in Cu3Au, Cu, and Fe at temperatures of 30 and 300 K and in Ni at 30, 300 and 600 K. These experiments have demonstrated that individual defect cascades collapse to dislocation loops athermally at 30 K in some materials (Ni, Cu and Cu3Au), while in Fe overlapping of cascades is necessary to produce dislocation loops. A slight sensitivity to the irradiation temperature is demonstrated in Cu3Au and Fe, and a strong dependence on the irradiation temperature is seen in Ni. This phenomenon of cascade collapse to dislocation loops in metals at 30 K provides an understanding for previous neutron irradiation data. The more detailed dependencies of the collapse probability on material, temperature, bombarding ion dose, ion energy and ion mass contribute much information to a thermal spike model of the collision cascade which we will describe.

Removal of stacking-fault tetrahedra by twin boundaries in nanotwinned metals

Stacking-fault tetrahedra are detrimental defects in neutron- or proton-irradiated structural metals with face-centered cubic structures. Their removal is very challenging and typically requires annealing at very high temperatures, incorporation of interstitials or interaction with mobile dislocations. Here we present an alternative solution to remove stacking-fault tetrahedra discovered during room temperature, in situ Kr ion irradiation of epitaxial nanotwinned Ag with an average twin spacing of ~8 nm. A large number of stacking-fault tetrahedra were removed during their interactions with abundant coherent twin boundaries. Consequently the density of stacking-fault tetrahedra in irradiated nanotwinned Ag was much lower than that in its bulk counterpart. Two fundamental interaction mechanisms were identified, and compared with predictions by molecular dynamics simulations. In situ studies also revealed a new phenomenon: radiation-induced frequent migration of coherent and incoherent twin boundaries. Potential migration mechanisms are discussed.

Determination of the neutron flux and energy spectrum in the low-temperature fast-neutron facility in CP-5, calculations of primary-recoil and damage-energy distributions, and comparisons with experiment☆

Abstract We have determined the absolute differential neutron-energy spectrum for the low-temperature fast-neutron irradiation facility in the CP-5 reactor by means of a 20-foil activation technique. This technique employs the most recent version of the SAND-II computer code, which iteratively unfolds the neutron spectrum by fitting the foil activities. A Monte Carlo routine was also employed to calculate standard-deviation errors in each neutron-energy group. Using this differential neutron spectrum we have calculated, for numerous elements, total recoil cross sections, detailed primary-recoil group distributions, total damage-energy cross sections, damage-energy distributions, and an error analysis based on the uncertainties in the neutron spectrum. The significance of this information with respect to the interpretation of various neutron radiation-damage experiments, including sputtering, disordering of ordered alloys, and changes in critical current of A-15 compound superconductors is discussed. A detailed comparison is made among initial resistivity-damage rates for five widely different (well characterized) neutron sources, fission fragments, and heavy ions.

Study of defect evolution by TEM within situion irradiation and coordinated modeling

The paper describes a novel transmission electron microscopy (TEM) experiment with in situ ion irradiation designed to improve and validate a computer model. TEM thin foils of molybdenum were irradiated in situ by 1 MeV Kr ions up to ∼0.045 displacements per atom (dpa) at 80°C at three dose rates −5 × 10−6, 5 × 10−5, and 5 × 10−4 dpa/s – at the Argonne IVEM-Tandem Facility. The low-dose experiments produced visible defect structure in dislocation loops, allowing accurate, quantitative measurements of defect number density and size distribution. Weak beam dark-field plane-view images were used to obtain defect density and size distribution as functions of foil thickness, dose, and dose rate. Diffraction contrast electron tomography was performed to image defect clusters through the foil thickness and measure their depth distribution. A spatially dependent cluster dynamic model was developed explicitly to model the damage by 1 MeV Kr ion irradiation in an Mo thin foil with temporal and spatial dependence of defect distribution. The set of quantitative data of visible defects was used to improve and validate the computer model. It was shown that the thin foil thickness is an important variable in determining the defect distribution. This additional spatial dimension allowed direct comparison between the model and experiments of defect structures. The defect loss to the surfaces in an irradiated thin foil was modeled successfully. TEM with in situ ion irradiation of Mo thin foils was also explicitly designed to compare with neutron irradiation data of the identical material that will be used to validate the model developed for thin foils.

The temperature dependence of heavy-ion damage in iron: A microstructural transition at elevated temperatures

A transition is reported in the dislocation microstructure of pure Fe produced by heavy-ion irradiation of thin foils, which took place between irradiation temperatures (T irr) of 300°C and 500°C. At T irr ≤ 400°C, the microstructure was dominated by round or irregular non-edge dislocation loops of interstitial nature and with Burgers vectors b = ½ ⟨111⟩, although interstitial ⟨100⟩ loops were also present; at 500°C only rectilinear pure-edge ⟨100⟩ loops occurred. At intermediate temperatures there was a gradual transition between the two types of microstructure. At temperatures just below 500°C, mobile ½⟨111⟩ loops were seen to be subsumed by sessile ⟨100⟩ loops. A possible explanation of these observations is given.

In situstudy of self-ion irradiation damage in W and W–5Re at 500 °C

In situ self-ion irradiations (150 keV W+) have been carried out on W and W–5Re at 500 °C, with doses ranging from 1016 to 1018 W+m−2 (∼1.0 dpa). Early damage formation (1016W+m−2) was observed in both materials. Black–white contrast experiments and image simulations using the TEMACI software suggested that vacancy loops were formed within individual cascades, and thus, the loop nucleation mechanism is likely to be ‘cascade collapse’. Dynamic observations showed the nucleation and growth of interstitial loops at higher doses, and that elastic loop interactions may involve changes in loop Burgers vector. Elastic interactions may also promote loop reactions such as absorption or coalescence or loop string formation. Loops in both W and W–5Re remained stable after annealing at 500 °C. One-dimensional hopping of loops (b = 1/2 ⟨111>) was only seen in W. At the final dose (1018W+m−2), a slightly denser damage microstructure was seen in W–5Re. Both materials had about 3–4 × 1015 loops m−2. Detailed post-irradiation analyses were carried out for loops of size ⩾ 4 nm. Both b = 1/2 ⟨111⟩ (∼75%) and b = ⟨100> (∼25%) loops were present. Inside–outside contrast experiments were performed under safe orientations to determine the nature of loops. The interstitial-to-vacancy loop ratio turned out close to unity for 1/2 ⟨111⟩ loops in W, and for both 1/2 ⟨111⟩ and ⟨100⟩ loops in W–5Re. However, interstitial loops were dominant for ⟨100⟩ loops in W. Re seemed to restrict loop mobility, leading to a smaller average loop size and a higher number density in the W-Re alloy.

Defect cascades produced by neutron irradiation in YBa2Cu3O7−δ

Abstract Defect cascades produced by fast neutron irradiation of YBa 2 Cu 3 O 7−δ single crystals were studied by transmission electron microscopy. The visible defects were found to have sizes between 1 and 5 nm. Defect densities were obtained as a function of neutron fluence between 2x10 21 and 8x10 21 m −2 ( E >0.1 MeV) and compared to damage calculations. The measured defect density scales linearly with fluence and amounts to 1x10 22 m −3 at a neutron fluence of 2x10 21 m −2 . The defect stability was studied at room temperature and through annealing to 400°C.