High-resolution Transmission Electron Microscopy Examination Research Articles

Synthesis and Microstructural Characterization of Unsupported Nanocrystalline Zirconia Thin Films

A polymeric precursor spin‐coating technique is illustrated in which yttrium‐stabilized zirconia (YSZ) thin films are produced on Si, Al2O3, and NaCl at temperatures less than 350°C. High‐resolution transmission electron microscopy (HRTEM) examinations show that the YSZ films are nanocrystalline (grain size of less than 5 nm), fully dense, and have a stabilized cubic fluorite structure. Using the polymeric precursor spin coating method, unsupported nanocrystalline thin films of YSZ with thicknesses ranging from 30 to 1000 nm are prepared by transferring the films from a host substrate to metallic TEM grids with unsupported areas exceeding 1 mm2.

Mechanical properties and microstructure of TiC/amorphous hydrocarbon nanocomposite coatings

Using the techniques of reactive magnetron sputter deposition and inductively coupled plasma (ICP) assisted hybrid physical vapor deposition (PVD)/chemical vapor deposition (CVD), we have synthesized a wide variety of metal-free amorphous hydrocarbon (a-C:H) and Ti-containing hydrocarbon (Ti-C:H) coatings. Coating elastic modulus and hardness have been measured by the technique of instrumented nanoindentation and related to Ti and hydrogen compositions. We show that both metal and hydrogen compositions significantly influence the mechanical properties of Ti-C:H coatings. The microstructure of Ti-C:H coatings is further characterized by transmission electron microscopy (TEM), X-ray absorption near edge structure (XANES) spectroscopy, and extended X-ray absorption fine structure (EXAFS) spectroscopy. XANES spectroscopy and high-resolution TEM examination of Ti-C:H specimens shows that the dissolution limit of Ti atoms in an a-C:H matrix is between 0.9 and 2.5 at.%. Beyond the Ti dissolution limit, precipitation of nanocrystalline B1-TiC cluster occurs and Ti-C:H coatings are in fact TiC/a-C:H thin film nanocomposites. Measurements of the average Ti bonding environment in TiC/a-C:H nanocomposites by EXAFS spectroscopy are consistent with a microstructure in which bulk-like B1-TiC clusters are embedded in an a-C:H matrix.

Aligned silicon carbide nanocrystals at the SiO2/Si interface by C implantation into SiO2 matrices

In this article, we report a specific feature for the distribution of silicon carbide nanocrystals formed by C implantation into SiO2 followed by thermal annealing. Cross-sectional transmission electron microscopy shows that silicon carbide nanocrystals (islands) are buried in the Si wafer at the SiO2/Si interface in a rectangular array (∼40×10 nm in dimension) and with a spacing of ∼20 nm. High-resolution transmission electron microscopy examination shows that silicon carbide nanocrystals are epitaxial on the Si wafer and are absent in the SiO2 matrix. Photoluminescence peaked at 580 nm is observed for samples annealed at 1100 °C under 514 nm excitation, which is thought to arise from the embedded silicon carbide nanocrystals.

Effects of temperature oscillations on the growth of carbon nanotubes by chemical vapor deposition

We have grown carbon nanotubes at oscillatory growth temperature by chemical vapor deposition. The high-resolution transmission electron microscopy (HRTEM) images show that they contain a number of incomplete carbon layers both on the outer and inner periphery and dislocations in the middle part of the carbon nanotubes walls. The defect structure of these nanotubes depends on the period of the T oscillation. In addition, HRTEM examinations of these carbon nanotubes from end-on reveal that some have polyhedral sections and contain dislocations. These features make them more active chemically and might have significant influence on the properties and applications of the carbon nanotubes.

Complex Layer Stacking In Muscovite: A Hrtem Study

Abstract One-, two- and three-layer polytypes are common in muscovite (sheet silicate). Based on high-resolution transmission electron microscopy (HRTEM) and analytical electron microscopy (AEM) examinations, we report in the discovery of complex 5-layer and 7- layer polytypes in natural muscovite for the first time (Fig. 1) and the implications of polytype transitions. Three main structures are found in muscovite crystals in a sample from Oreana, Nevada, which are 1M, 2M1, and a disordered structure often containing short-range ordered stacking sequences. On the scale of a few micrometers, muscovite occurs as ordered crystals, disordered crystals, and crystals with regions of ordered and disordered layer stacking. Short-range ordered 5- or 7-layer repeats are found in some disordered structures where single 4-, 6-, 9- or 10-layer intergrowths occur occasionally with 2- and 3-layer repeats. Such complex polytypes have not been observed before in either natural or synthetic muscovite, although 5- and 7-layer structures are commonly present in trioctahedral micas (i.e., biotite) (Baronnet, 1992).

Properties and Microstructure of Molybdenum Disilicide–β′;‐SiAlON Particulate Ceramic Composites

Particulate ceramic composites that were composed of a combustion‐synthesized β′;‐SiAlON matrix and dispersed MoSi2 particles were hot pressed at 1600°C in a nitrogen atmosphere. The physical and mechanical properties of the composites that contained 15, 30, and 45 vol% MoSi2 were evaluated. The average four‐point bend strength, fracture toughness, and Vickers hardness of the composites were in the ranges of 500‐600 MPa, 3‐4 MP·am1/2, and 11‐13 GPa, respectively. The measured mechanical strength and hardness were very similar to the values that were predicted from the rule of mixtures. The fracture toughness of the combustion‐synthesized β′;‐SiAlON (2.5 MPa·m1/2) was apparently enhanced by the MoSi2 particles that were added. The increase in the fracture toughness was predominately attributed to the residual thermal stress that was induced by the thermal expansion mismatch between the MoSi2 particles and the β′;‐SiAlON matrix. The composites showed improved electrical conductivity and oxidation resistance over monolithic β′;‐SiAlON. High‐resolution transmission electron microscopy examination of the composites indicated that the MoSi2 was chemically well compatible with the β′;‐SiAlON.

96/05301 Shock tube and modeling study of ketene oxidation

In this work, the pulverized coal combustion of Zhundong lignite was conducted in an optically-accessible, downward Hencken flat-flame burner to investigate the incipient formation of ultrafine particulate matter (PM). The ultrafine PM were collected in-flame from distinct positions under 1800 K, 1500 K and 1200 K using a self-designed thermophoretic sampler, together with extensive conventional transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and energy-dispersive X-ray spectroscopy (EDS) examinations. A novel in situ low-intensity phase-selective laser-induced breakdown spectroscopy (PS-LIBS) was further introduced to diagnose the dynamic behavior of particle-phase sodium during the pulverized coal combustion. The primary particles in the collected ultrafine PM have mean diameter of 8–15 nm, with Si and Na as main mineral components based on TEM/HRTEM–EDS results. The formation of ultrafine PM is regarded as a process of multi-component flame syntheses of mineral precursors, in which the competition between the devolatilization and the coalescence of particles are of most significance. Finally, based on an approach of time scale analysis, three characteristic times of minimum mean diameter of primary particles, the initial increment point of particle-phase sodium and the maximum devolatilization are found to be highly correlated.