Competitive segregation of gallium and indium at heterophase Cu–MnO interfaces studied with transmission electron microscopy

This paper concentrates on the possible segregation of indium and gallium and competitive segregation of gallium and indium at atomically flat parallel {111}-oriented Cu–MnO interfaces. The segregation of gallium at Cu–MnO interfaces after introduction of gallium in the copper matrix of internally oxidized Cu–1 at.%Mn could be hardly detected with energy-dispersive spectrometry in a field emission gun transmission electron microscope. After a heat treatment to dissolve indium in the copper matrix, gallium has a weak tendency to segregate, that is 2.5 at.% Ga per monolayer at the interface compared with 2 at.% in the copper matrix. The striking result is that this gallium segregation is observable because it does not occur at the metal side of the interface but in the first two monolayers at the oxide side. Using the same heat treatment as for introducing indium in the sample, but without indium present, gallium segregates strongly at the oxide side of the Cu–MnO interface with a concentration of about 14.3 at.% in each monolayer of the two. In contrast, the presence of gallium has no influence on the segregation of indium towards Cu–MnO interfaces, because the outermost monolayer at the metal side of the interface contains 17.6 at.% In, that is similar to previously found results. This leads to the intriguing conclusions, firstly, that, in contrast with antimony and indium, gallium segregates at the oxide side of the interface and, secondly, that the presence of indium strongly hampers gallium segregation. The results from analytical transmission electron microscopy on gallium segregation are supported by high-resolution transmission electron microscopy observations.

Some aspects of nanocrystalline nickel and zinc ferrites processed using microemulsion technique

Nanocrystalline nickel and zinc ferrites synthesised using a microemulsion technique were characterised by high resolution transmission electron microscopy and vibrating sample magnetometry. A narrow and uniform distribution of crystals of size range 5 – 8 nm, distinguished by a clear lack of saturation magnetisation at 9 kOe, were obtained. Also, no coercivity or remanence was observed.

In-situ TEM analysis of the reduction of nanometre-sized Mn3O4 precipitates in a metal matrix

The objective of the present work is the in-situ study of the transformation of small oxide precipitates in a metal matrix by conventional and high-resolution transmission electron microscopy (HRTEM). As an example the reduction of Mn3O4 into MnO for nano-sized oxide precipitates in a silver matrix was studied in detail. A convenient method for monitoring the reduction process is shown for a large number of precipitates simultaneously. It is based on two-beam dark-field images showing distinct Moiré patterns for the MnO and the various types of Mn3O4 precipitates embedded within an Ag matrix. A controlling factor of the transformation kinetics appeared to be the rate in which the system can relax the strains due to the accompanying volume reduction of the precipitates. Other interesting aspects of the Mn3O4 to MnO transformation scrutinized and explained were the shape change of the precipitates upon reduction and the fact that mixed Mn3O4/MnO precipitates were only detected within a small temperature/time interval. Ostwald ripening of the MnO precipitates was observed as well.

Influence of Interfacial Binding Energy and Misfit on the Shape of the Oxide Precipitates in Metals

Transmission electron microscopy revealed Mn3O4 precipitates with two types of dominant shapes in Pd-3at.%Mn that was internally oxidized in air at 1000°C. One type is octahedrally shaped and bounded by {111} planes of the Mn3O4. These observations were compared with earlier observations in the Ag/Mn3O4 system. The octahedrons show a relatively larger (002) truncation in Pd than in Ag. Further, the second type of precipitate shape, comprising about ⅓ of all the precipitates in Pd, was not observed in Ag. It corresponds to a plate-like structure. HRTEM observations revealed the presence of a square misfit dislocation network with line direction <110> and Burgers vector ½<110> at these interfaces with (002)Mn3O4//{200}Pd. The general conclusion of the present analysis is (i) anisotropy in the interface energy for oxide precipitates in a metal matrix is substantial due to the ionic nature of the oxide, giving well defined shapes associated with the Wulff construction, (ii) the influence of misfit energy on the precipitate shape as bounded by semi-coherent interfaces is important only if sufficient anisotropy in the mismatch is present and if the matrix is sufficiently stiff, and (iii) the stronger coupling strength due to electronic binding effects across the interface in Pd compared to Ag is responsible for the formation of the dislocation network structures at larger misfit.

Nanocavity formation processes in MgO(100) by light ion (D, He, Li) and heavy ion (Kr, Cu, Au) implantation

In studies on the controlled growth of metallic precipitates in MgO it is attempted to use nanometer size cavities as precursors for formation of metallic precipitates. In MgO nanocavities can easily be generated by light gas ion bombardment at room temperature with typically 30 keV ion energy to a dose of 10^16 cm–2, followed by annealing to 1300 K. It has been shown earlier by transmission electron microscopy (TEM) that the cavities (thickness 2–3 nm and length/width 5–10 nm) have a perfectly rectangular shape bounded by {100} faces. The majority of the gas has been released at this temperature and the cavities are stable until annealing at 1500 K. The depth location of the cavities and the implanted ions is monitored by positron beam analysis, neutron depth profiling, RBS/channeling and energy dispersive spectroscopy. The presence of metallic nanoprecipitates is detected by optical absorption measurements and by high-resolution XTEM. Surprisingly, all the metallic implants induce, in addition to metallic precipitates in a band at the mean ion range, small rectangular and cubic nanocavities. These are most clearly observed at a depth shallower than the precipitate band. In the case of gold the cavities are produced in close proximity to the crystal surface. The results indicate that in MgO vacancy clustering dominates over Frenkel-pair recombination. Results of molecular dynamics calculations will be used to discuss the observed defect recovery and clustering processes in MgO