R. Gronsky

Synthesis, characterization, and properties of nanophase TiO 2

Ultrafine-grained, nanophase samples of TiO/sub 2/ (rutile) were synthesized by the gas-condensation method and subsequent in situ compaction. The samples were studied by a number of techniques, including transmission electron microscopy, Vickers microharness measurements, and positron annihilation spectroscopy, as a function of sintering temperature. The nanophase compacts with average initial grain sizes of 12 nm were found to densify rapidly above 500 /sup 0/C, with only a small increase in grain size. The hardness values obtained by this method are comparable to or greater than those for coarser-grained compacts, but are achieved at temperatures 400 to 600 /sup 0/C lower than conventional sintering temperatures and without the need for sintering aids.

Synthesis of B{sub {ital x}}C{sub {ital y}}N{sub {ital z}} nanotubules

We report the successful synthesis of B{sub {ital x}}C{sub {ital y}}N{sub {ital z}} nanotubes. Arc-discharge methods were used to produce stable nanotubule structures identified by high-resolution transmission-electron microscopy. Local electron-energy-loss spectroscopy of {ital K}-edge absorptions for B, C, and N atoms was used to determine the atomic compositions of individual tubules. Tubes of stoichiometry BC{sub 2}N and BC{sub 3} have been observed, in agreement with theoretical predictions.

Insights into the Electrodeposition of Bi2Te3

In this paper, the processes associated with the electrodeposition of bismuth telluride (Bi 2 Te 3 ), a thermoelectric material, are reported along with an analysis of the composition and crystallinity of the resulting films. The electrodeposition can be described by the general reaction 3HTeO 2 + + 2Bi 3+ + 18e - + 9H + → Bi 2 Te 3 + 6H 2 O. Cyclic voltammetry studies of Bi, Te, and Bi/Te dissolved in I M HNO 3 reveal two different underlying processes depending on the deposition potential. One process involves the reduction of HTeO + 2 to Te 0 and a subsequent interaction between reduced Te 0 and Bi 3+ to form Bi 2 Te 3 . A second process at more negative reduction potentials involves reduction of HTeO + 2 to H 2 Te followed by the chemical interaction with Bi 3+ . Both processes result in the production of crystalline Bi 2 Te 3 films in the potential range -0.1 < E < -0.52 V vs. Ag/AgCl (3 M NaCI) on Pt substrates as determined by powder X-ray diffraction (XRD). Electron probe microanalyses and XRD reveal that the films are bismuth-rich and less oriented for more negative deposition potentials.

Ni, Pd, and Pt on GaAs: A comparative study of interfacial structures, compositions, and reacted film morphologies

The reactions between (100) GaAs and the near-noble metals Ni, Pd, and Pt have been investigated by application of high-resolution transmission electron microscopy (TEM), energy-dispersive analysis of x rays in the scanning TEM and Rutherford backscattering spectrometry. Emphasis is placed on the evolution of the phase distributions, film compositions, and interface morphologies during annealing at temperatures up to 480 /sup 0/C. The first phase in the Ni/GaAs reaction is shown to have the nominal composition Ni/sub 3/GaAs. Ternary phases of the type Pd/sub x/GaAs are also found to be the dominant products of the Pd/GaAs reaction. Conversely, only binary phases result from the Pt/GaAs reaction. These observations are used to construct isothermal sections of the M--Ga--As thin-film phase diagrams. The behavior of a thin (1--2 nm) native oxide--hydrocarbon layer during the Ni/GaAs, Pd/GaAs, and Pt/GaAs reactions is also investigated. Only the Ni/GaAs reaction is noticeably impeded in some regions by this intervening layer. In contrast, the Pd/GaAs and Pt/GaAs reactions tend to mechanically disperse the native oxide layers.

The effects of gamma radiation sterilization and ageing on the structure and morphology of medical grade ultra high molecular weight polyethylene

Ultra high molecular weight polyethylene (UHMWPE) that had been sterilized with 25 kGy of gamma radiation and non-sterile material processed from the same batch were studied following five years of ageing in air. Differential scanning calorimetry, small angle X-ray scattering, transmission electron microscopy, Fourier transform infra-red spectroscopy and density gradient column measurements were used to characterize the changes in structure and morphology of the UHMWPE as a result of sterilization and shelf ageing. Significant changes were observed. The UHMWPE oxidized and increased in crystallinity as well as density, with chain scission the dominant response to irradiation. The applicability of the results of this study to the evolution of mechanical properties and structural integrity of medical grade UHMWPE for total joint replacement is addressed.

Structure and composition of NixGaAs

Advanced compound semiconductor devices require increasingly stable, shallow, and uniform metallized layers for ohmic and Schottky contacts. However, the design of new multielemental contact metallization systems is limited by the paucity of information regarding the structure, composition, and stability of phases resulting from the interaction of single metal layers with compound semiconductors. In this letter, the results of a transmission electron microscopy investigation of the Ni/GaAs reaction are presented. The first reaction product is shown to have the composition Ni3GaAs. Based on this composition and lattice parameter measurements, it is proposed that the structure of Ni3GaAs is closely related to that of γ’ Ni3Ga2, a derivative of the hexagonal B8 structure type.

Atomic mechanisms of precipitate plate growth in the AlAg system—I. Conventional transmission electron microscopy

Abstract The detailed interfacial structure of γ′ precipitates in an Al-15 wt%Ag alloy has been studied by conventional transmission electron microscopy. Contrast analyses indicate that there is a strong tendency for single 1 6 〈112〉 Shockley partial dislocations on the precipitate faces to interact, forming multiple-unit ledges which display the contrast behavior of 1 2 〈110〉 dislocations. In addition, both the edges of these precipitates and ledges on the edges are comprised of the same partial dislocations stacked vertically or slightly behind one another. All three variants of Shockley partials have been observed on the same {111} faces of precipitates, and all interfaces of the precipitates display a strong preference for 〈110〉 configurations. The similarities between growth of ledges on the broad faces and the edges of precipitates by a kink mechanism are described and explained.

Atomic mechanisms of precipitate plate growth in the AlAg system—II. High-resolution transmission electron microscopy

Abstract High-resolution electron microscopy was used to study the interfacial structure of γ′ precipitates in an Al-15 wt% Ag alloy aged at 350°C. The results of these studies show that: 1. (1) all ledges are multiples of two {111} planes high, supporting the theory and conventional transmission electron microscopy observations that plate thickening occurs by passage of Shockley partial dislocations on alternate {111} planes 2. (2) most ledges are more than just two planes high, indicating a strong tendency toward diffusional and/or elastic interactions 3. (3) the terraces between ledges are atomically flat and ledges are uniformly stepped-down from the centers to the edges of isolated precipitates as predicted by the general theory of precipitate morphology 4. (4) the {111} planes are continuous across the edges of ledges, indicating that they are largely coherent and not disordered as treated in most kinetic analyses, and 5. (5) the edges of precipitate plates appear to be composed of similar two-plane ledges arranged vertically above one another and hence, may grow by the same mechanism of atomic attachment as ledges on the broad faces. Examination of γ′ plates during early stages of growth indicates that their aspect ratio may deviate from the equilibrium value almost immediately, probably due to the ledge mechanism of growth. Lastly, an atomic model of a γ′ precipitate was used to test the high-resolution images obtained, and illustrate possible atomic growth mechanisms of the ledges.

Initial stages of the Pd-GaAs reaction: Formation and decomposition of ternary phases

Abstract Investigations of the metallurgical reactions occuring at metal-compound semiconductor interfaces are essential for the understanding of ohmic contact and Schottky barrier formation mechanisms. The resulting film and interface morphologies are also important for the performance of discrete devices and integrated circuits. In this study the reactions occurring during thermal annealing of the Pd-GaAs system are investigated. Reported here are the initial stages of the Pd-GaAs reactions as studied by high resolution transmission electron microscopy, electron diffraction and energy-dispersive analysis of X-rays. The first Pd-GaAs reaction product, observed in the low temperature range (less than about 315°C), is the hexagonal ternary phase Pd 5 (GaAs) 2 (phase I with a 0 = 0.673 nm and c 0 = 0.338 nm ). The second stage of the reaction leads to the formation of a second hexagonal ternary phase, Pd 4 GaAs (phase II with a 0 = 0.92 nm and c 0 = 0.370 nm ), and the binary phases Pd 2 Ga and Pd 2 As. The concurrent application of energy-dispersive X-ray analysis and high resolution cross-sectional transmission electron microscopy leads to the unambiguous identification of the ternary Pd 5 (GaAs) 2 and Pd 4 GaAs phases. X-ray diffraction data alone could erroneously lead to the identification of Pd 5 (GaAs) 2 as PdGa and of Pd 4 GaAs as PdAs 2 . In view of this source of ambiguity in previous work, the sequence of reactions in the Pd-GaAs system is re-evaluated and the evolution of the film and interface morphologies is discussed.

Optical diffraction from lattice images of alloys

Abstract Optical diffraction patterns have been obtained from optimum lattice images of partially ordered Cu 3 Au and spinodally decomposed AuNi alloys. These diffraction results show excellent agreement with the corresponding portions of electron diffraction patterns, indicating that a direct correlation exists between the lattice images and the atomic lattices of the alloys studied. It must be borne in mind that in general the optical diffractogram and the electron diffraction pattern may contain different information because of the contrast transfer in the electron microscope, and this will particularly affect reflections with larger scattering angles usually outside the objective aperture. Compared to conventional 100 kV selected area electron diffraction, the optical diffraction technique makes possible an outstanding improvement in field limitation, where the area contributing diffraction information may be reduced by over three orders of magnitude to 10A dia. A quantitative analysis of the spinodal and ordering transformations by combined lattice imaging and optical diffraction provides experimental detail on an extremely localized scale. The results are in agreement with conventional experimental results and modern theories of phase transformations and illustrate an exciting new approach to gaining information about localized atomic mechanisms of phase transformations in alloys.