Charles W. Allen

In situ ion irradiation /implantation studies in the HVEM-tandem facility at argonne national laboratory

Abstract The HVEM-Tandem User Facility at Argonne National Laboratory interfaces two ion accelerators, a 2 MV tandem accelerator and a 650 kV ion implanter, to a 1.2 MV high-voltage electron microscope. This combination allows experiments involving simultaneous ion irradiation/ion implantation, electron irradiation and electron microscopy/electron diffraction to be performed. In addition the availability of a variety of microscope sample holders permits these as well as other types of in situ experiments to be performed at temperatures ranging from 10 to 1300 K, with the sample in a stressed state or with simultaneous determination of electrical resistivity of the specimen. This article summarizes the details of the Facility which are relevant to simultaneous ion beam material modification and electron microscopy, presents several current applications and briefly describes the straightforward mechanism for potential users to access this US Department of Energy-supported facility.

The HVEM-Tandem Accelerator Facility at Argonne National Laboratory

Abstract The HVEM-Tandem National User Facility consists of a modified Kratos/AE1 EM7 HVEM with a maximum accelerating voltage of 1.2 MeV, interfaced to both a 2MV National Electrostatics tandem ion accelerator and a 300 kV Texas Nuclear ion accelerator. The latter is being replaced with a 650 kV National Electrostatics accelerator which should be fully operational in FY 1987. These accelerators provide a wide range of ion species with energies from 25 keV to 8 MeV. The combination of HVEM and ion accelerators provides a truly unique capability for ion irradiation/implantation experimentation along with simultaneous microscopy. The HVEM-Tandem Facility currently is employed for a wide range of materials research, including basic in situ studies of mechanical properties, oxidation and hydrogen effects in metals, radiation effects including ion and electron irradiation-induced phase changes and general defect analysis. More than half of these studies are conducted by non-ANL scientists from universities and other national laboratories. Access to the National User Facility is by means of research proposals which are reviewed by a Steering Committee composed of both Argonne and non-Argonne scientists representing the user community.

New instrumentation in Argonne`s HVEM-Tamdem Facility: Expanded capability for in situ ion beam studies

During 1995, a state-of-the-art intermediate voltage electron microscope (IVEM) has been installed in the HVEM-Tandem Facility with in situ ion irradiation capabilities similar to those of the HVEM. A 300 kV Hitachi H-9000NAR has been interfaced to the two ion accelerators of the Facility, with a spatial resolution for imaging which is nearly an order of magnitude better than that for the 1.2 MV HVEM which dates from the early 1970s. The HVEM remains heavily utilized for electron- and ion irradiation-related materials studies, nevertheless, especially those for which less demanding microscopy is adequate. The capabilities and limitations of this IVEM and HVEM are compared. Both the HVEM and IVEM are part of the DOE funded User Facility and therefore are available to the scientific community for materials studies, free of charge for non-proprietary research.

Migration and coalescence of Xe nanoprecipitates in Al induced by electron irradiation at 300 K

Effects of 1 MeV electron irradiation on Xe precipitates in Al, formed by ion implantation, have been observed in situ by high-voltage transmission electron microscopy. Individual Xe precipitates undergo melting and recrystallization, migration which leads to coalescence, and shape changes. These processes are driven by the production of defects without either cascade defect production or the introduction of additional Xe atoms. Precipitate migration is due to an irradiation-induced surface diffusion process on the Xe/Al interfaces. Coalescence of close precipitates is enhanced by directed motion as a result of the net displacement of Al atoms out of the volume between them.

HRTEM analysis of solid precipitates in Xe-implanted aluminum

High-resolution TEM was carried out to determine shape and atomic arrangement of solid Xe precipitates in Al. Polycrystalline Al TEM specimens were implanted with 30 keV Xe{sup +} at RT to a dose of 3x10{sup 20} ions/m{sup 2} and then annealed at 523 K. Below a size 4 nm dia, the Xe precipitates are solid with an fcc crystal structure mesotacticly aligned with the Al lattice. In HRTEM along [011] projection, the difference in the lattice parameters of solid Xe and Al produces a precipitate image dominated by a 2-D Moire pattern that repeats in both the and directions every 3 Al (or 2 Xe) lattice spacings. Multi-slice image simulations, using a 3-D atomic model, demonstrates that the precipitates are tetradecahedra with faces parallel to the dense {l_brace}111{r_brace} planes and the {l_brace}100{r_brace} planes. Off-Bragg illumination of the precipitates minimizes Al lattice fringes and generates precipitate images which are in good agreement with the model.

In situ ion- and electron-irradiation effects studies in transmission electron microscopes

Abstract Interfacing an ion accelerator to a transmission electron microscope (TEM) allows the analytical functions of TEM imaging and diffraction to be employed during ion-irradiation effects studies. The techniques and special procedures for performing quantitative TEM studies employing in situ ion and electron irradiation are summarized in the context of several irradiation-induced amorphization and irradiation-assisted crystallization studies, which illustrate the dynamics of this approach in the materials science of irradiation effects.

IN SITU TRANSMISSION ELECTRON MICROSCOPY EMPLOYED FOR STUDIES OF EFFECTS OF ION AND ELECTRON IRRADIATION ON MATERIALS

This paper summarizes the essential features of the various facilities dedicated to in situ irradiation effects research around the world at the present time, mentions some essential techniques that are involved in this type of research, and describes very briefly some examples of studies employing these facilities and techniques. Microsc. Res. Tech. 42:255–259, 1998.

In situ ion beam research in Argonne`s intermediate voltage electron microscope

Since Fall 1995, a state-of-the-art intermediate voltage electron microscope (IVEM) has been operational in the HVEM-Tandem Facility with in situ ion irradiation capabilities similar to those of the HVEM of the Facility. A 300 kV Hitachi H-9000NAR is interfaced to the two ion accelerators of the Facility, with a demonstrated point-to-point spatial resolution for imaging of 0.25 nm with the ion beamline attached to the microscope. The IVEM incorporates a Faraday cup system for ion dosimetry with measurement aperture 6.5 cm from the TEM specimen, which was described in Symposium A of the 1995 MRS Fall Meeting. The IVEM is now employed for a variety of in situ ion beam studies ranging from low dose ion damage experiments with GaAs, in which damage zones individual displacement cascades are observed, to implantation studies in metals, in which irradiation-induced noble gas precipitate mobility is studied in real time. In this presentation, the new instrumentation and its specifications will be described briefly, several basic concepts relating to in situ experiments in transmission electron microscopes will be summarized and examples of in situ experiments will be presented which exploit the experimental capabilities of this new user facility instrumentation.

Observation of atomic processes in Xe nanocrystals embedded in Al under 1 MeV electron irradiation

Defect introduction processes in two differently sized Xe nanocrystals embedded in Al were observed with in situ high-resolution electron microscopy. The imaging conditions made it possible to follow the apparent movement of Xe atom columns within a nanocrystal. The positions of individual atom columns were measured on captured video images, before and after defect introduction, from intensity traces along the rows of atom columns. For each of these precipitates, a planar defect appears on {1 1 1}. The observed displacement of Xe atom columns approximately corresponds to the projected displacement produced by a Shockley partial dislocation, bounding an intrinsic stacking fault. Comparison of the displacements of atoms in the two nanocrystals shows that the displacements are reduced at the precipitate/matrix interfaces. This implies that there will be a displacement at interfaces that represents some compromise between the strain field of the partial dislocation and the interface compatibility requirement.

In-situ observation of shape and atomic structure of Xe nanocrystals embedded in aluminium

An imaging technique to determine in situ the shape and atomic structure of nanosized Xe crystals embedded in Al is described using high‐resolution transmission electron microscopy (HRTEM). The Xe nanocrystals, with sizes less than 5 nm were prepared by the implantation of 30 keV Xe+ into Al at room temperature. The fcc Xe nanocrystals are mesotactic with the Al lattice and have a lattice parameter ≈ 50% larger than that of Al. HRTEM images of the Xe were not clear in [110] zone axis illumination because of the small number of Xe atoms relative to Al atoms in any atom column. An off‐axial imaging technique that consists of tilting the specimen several degrees from a zone axis and defocusing to suppress the Al lattice fringes is employed for the 110 projection of the Xe/Al system and the structure of the Xe nanocrystals is successfully imaged. The Xe images clearly represent projections of cuboctahedra with faces parallel to eight Al {111} planes truncated by six {100} planes. The results of multislice image simulations using a three‐dimensional atomic model agreed well with the results obtained by the off‐axial imaging technique. The usefulness of the technique is demonstrated with observations of crystal defects introduced into the Xe under intense 1000 keV electron irradiation.