Research

• The IVEM-Tandem consists of a Hitachi H-9000NAR intermediate voltage microscope (IVEM), which is interfaced to two positive ion accelerators: an NEC 2 MV tandem Pellatron and an NEC ion implanter, recently upgraded to 600kV operation. In the past year the Facility serviced 24 research proposals involving 22 faculty members, 9 graduate students, and 5 postdoctoral appointees from 13 universities along with 9 ANL scientists and 14 scientists from other national and industrial laboratories. IVEM, highly stable double tilt heating and cooling holders (300<T<1200 K and 15<T<300 K, respectively) were obtained. A single tilt heating-straining holder was also placed in service. To increase the energy range of positive ions available to users of the IVEM, the NEC implanter was upgraded to 600 kV operation (previously 400kV).
• High resolution and analytical microscopes have been operated successfully serving users from the Materials Science, Energy Technology, Chemistry, Chemical Technology and Biology Divisions at Argonne East, and users from Argonne West. Significant use has also come from visitors, postdocs and graduate students, mostly from area universities, and all in collaboration with Argonne scientists. Recent efforts have been made to reduce acoustic noise levels and improve air flow patterns in laboratory rooms housing the high resolution JEOL 4000EXII and intermediate voltage analytical CM30 microscopes. Measurable improvements in resolution have resulted.
• The sample preparation laboratory was improved by the addition of a plasma etcher to clean TEM samples and holders. Samples now show no sign of contamination for up to eight hours in most of the transmission electron microscopes (TEM). This has significantly increased the ability of researchers to do microanalysis with focused electron beams.
• The image analysis laboratory has acquired and implemented two computer programs that increase our image simulation and analysis capabilities. Simulated images of model defects can be used to identify defect structures in experimental images, and 3D viewing and stereo analysis of experimental image data has been added to identify defect morphology and distribution in three dimensions.

Research

• Structures of grain boundaries in the high temperature superconductor BSCCO. Y. Yan, J. Evetts (University of Cambridge, England), and M. Kirk. A study has been made of grain boundary structure in melt textured BiSrCaCuO (BSCCO). Our TEM data show that the form of the low-tilt angle grain boundaries in the BSCCO system are unusual in exhibiting changes in the local composition as well as in the structure. Although we see local low-T_c impurity 2201 phase at such boundaries, there are also some high-T_c "channels" crossing the boundary plane made up of the 2212 and 2223 phases. The structural models developed by us in the BiSrCaCuO explain why this particular system exhibits good J_c values.
• Columnar Defect structures in high temperature superconductors. Y. Yan and M. Kirk. Irradiation by GeV heavy ions not only leads to the formation of amorphous columnar defects, but also a local oxygen rearrangement into adjacent columns of twinned material. Furthermore TEM in cross-section reveals that the diameters of amorphous columns are variable along the ion path: they can be uniform, modulated, or discontinuous depending on the ion and irradiation energy. These studies provide important information for understanding the detailed mechanisms of magnetic vortex pinning. Portions of these investigations are carried out in collaboration with L. Paulius and W. Kwok (Superconductivity and Magnetism).
• Irradiation cascade defects at elevated temperatures. T. Daulton (Defect and Disordered Materials) and M. Kirk. The formation and annealing of cascade defects in copper at elevated temperatures has been investigated. Detailed in situ video TEM experiments demonstrate that cascade defects have a decreasing probability to form stable defect clusters (dislocation loops and stacking fault tetrahedra) with increasing irradiation temperature, a result with important implications to neutron irradiations of metals at fission and fusion reactor temperatures.
• Search for interstitial cluster defects produced by irradiation cascades at low temperatures. M. Jenkins (University of Oxford, England), H. Fukushima (University of Hiroshima, Japan), and M. Kirk. TEM experiments to search for interstitial loops formed in situ by ion irradiation at low temperature (20K) are in progress. The results will have a strong impact on current theory of damage production under neutron and ion irradiation of metals.
• Effects of electron irradiation on implanted noble gas precipitates in Al. C. Allen and R.l Birtcher (Defects and Disordered Materials), K. Furuya (NRIM, Tsukuba, Japan) and S. Donnelly (U. of Salford, England). High resolution TEM of Xe precipitates in Al has revealed that atomic displacements can profoundly alter the precipitate morphology and crystalline structure, resulting in melting or crystallization, precipitate motion within the Al lattice and coalescence. For example, the kinetics of coalescences of two liquid precipitates and of two solid precipitates have been recorded with atomic resolution.
• Hg implantation in an austenitic stainless steel. C. Allen, A. McCormick and L. Rehn (Defects and Disordered Materials). A study is underway of the behavior of Hg implanted into an austenitic stainless steel in order to determine the conditions under which Hg bubbles and possible intermetallics of Ni-Hg form. This work is of importance to possible problems associated with the target of the next Spallation Neutron Source to be built at ORNL.
• In situ study of dislocation activity in Al/silica heterostructures. C. Allen and H. Schroeder (KFA, JŸlich, Germany). An investigation of dislocation generation and glide associated with differential contraction of Al/silica heterostructures on cooling from elevated temperatures, a problem in the production of device interconnects, is being made.
• In situ study of implantation of Ge in silica. C. Allen; and A. MŸcklich, H. Tyroff, W. Skorupa and B. Schmidt (FZ Rossendorf-Dresden, Germany). Nanocluster formation of Ge was studied in situ at temperatures above and below the melting point of Ge. The kinetics of precipitation were followed.
• Effect of three-fold astigmatism on measurements of rigid shifts at solid interfaces. K. Merkle (Interfacial Materials) and R. Csencsits. Quantification of interfacial free volume by HREM analysis is an important goal of interface science. For example, the grain boundary volume expansion has been directly related to grain boundary energy. Measurements of the lattice fringe shifts across an interface can give a highly precise value of the volume expansion, provided the objective lens aberrations do not give rise to extraneous fringe shifts. Based on atomistic simulations of grain boundaries and multislice image simulations we have quantified the effect of three-fold astigmatism and found that the effect is very significant when small fringe spacings near 0.2 nm and below are considered. It was found that correction of the three-fold astigmatism to a value of 100 nm should be sufficient for most applications. However, at present most high resolution electron microscopes are not equipped with a stigmator for three-fold astigmatism. Fortunately, the value measured for our JEOL 4000EX is relatively low (560 nm), whereas values up to 1400 nm have been reported for other instruments of the same type.
• Structure and composition of very low-friction diamond-like carbon films. R. Csencsits and A. Erdemir (Energy Technology Division). A study is underway of the microstructure of very low friction diamond-like carbon (DLC) films. TEM on these low friction films have found them to be amorphous and to contain 9-10% oxygen, which is unusual for more common DLC films.
• Microstructure of thin films grown by microwave plasma-enhanced chemical vapor deposition (MPCVD) from fullerene C₆₀ purcursors. L-C. Qin; and D. Gruen and A. Krauss (Molecular Control of Materials Synthesis). The microstructure was characerized by scanning electron microscopy (SEM), high-resolution electron microscopy (HREM), and parallel electron energy-loss spectroscopy (PEELS). The films were composed of nanosize crystallites of diamond, and no graphitic phase was observed. The diamond crystallite size measured from lattice images showed that most grains ranged between 3-5 nm, reflecting a t distribution. Electron diffraction gave no evidence of either sp²-bonded glassy carbon or sp³-bonded diamond-like amorphous carbon. The sp² bonding configuration found in PEELS was attributed to grain boundary carbon atoms, which constituted 5-10% of the total. Larger diamond grains tended to be highly faulted.
• Structure-property correlations in ferroelectric thin films. I-F. Tsu; and G-R. Bai, C. M. Foster, and K. L. Merkle (Interfacial Materials). The properties of ferroelectric thin films synthesized by metallic-organic chemical vapor deposition (MOCVD) strongly depend on the nature and crystallinity of the bottom electrode on which they are deposited. Microstructural and analytical TEM techniques were applied to characterize Pb(Zr_xTi_1-x)O₃ (PZT) thin films grown by MOCVD at x near 0.5 on RuO₂ electrodes with (110) and (101) texture. It was demonstrated that the grain morphology, interfacial structure, surface roughness and pyrochlore surface phase formation are strongly dependent on the microstructure of the underlying RuO₂ electrodes. Moreover, energy dispersive x-ray spectroscopy showed cation heterogeneities which implies that there are local variations in ferroelectric properties.
• High temperature mechanical properties of LaSrMnO ceramic. R. Cook; and J. Routbort and K. Goretta (Energy Technology Division), are investigating the defect chemistry of La_1-xSr_xMnO₃ (0.1 < x < 0.3) by examining the PO₂-dependence of its high-temperature creep properties. Mechanical testing has been completed and TEM begun. It has been found that two rate-controlling defects operate in this material.