Neal D. Evans

Mechanical and morphological variation of the human lumbar vertebral cortical and trabecular bone

The nanoindentation technique was used to characterize the variation in the elastic modulus and hardness of human lumbar vertebral cortical and trabecular bone. The elastic modulus (and in most cases, the hardness as well) of axially aligned trabeculae cut in the transverse direction was significantly greater than in other orientations of vertebral cortical and trabecular bone. In all cases, the elastic modulus and hardness of bone in the load-bearing direction was greater than in corresponding bone types cut in the other directions. Scanning electron micrographs of cortical shell revealed the Haversian-like canal systems expected in secondary cortical bone, but it was difficult to differentiate by morphology cortical from trabecular bone in the human lumbar vertebrae.

Valence state mapping of cobalt and manganese using near-edge fine structures

The properties of transition metal oxides are related to the presence of elements with mixed valences. The spectroscopy analysis of the valence states is feasible experimentally, but a spatial mapping of valence states of transition metal elements is a challenge to existing microscopy techniques. In this paper, with the use of valence state information provided by the white lines and near-edge fine structures observed using the electron energy-loss spectroscopy (EELS) in a transmission electron microscope (TEM), a novel experimental approach is demonstrated to map the valence state distributions of Mn and Co using the ratio of white lines in the energy-filtered TEM. The valence state map is almost independent of specimen thickness in the thickness range adequate for quantitative EELS microanalysis. An optimum spatial resolution of approximately 2 nm has been achieved for a two-phase Co oxides.

Adhesion study of polyimide to single-wall carbon nanotube bundles by energy-filtered transmission electron microscopy

High-resolution electron microscopy and energy-filtered imaging methods were used to examine single-wall carbon nanotubes and nanotube reinforced polyimide composites. The nanotubes were studied alone, in a polyimide matrix composite, and after deposition of composite material previously dissolved in a solvent. Energy-filtered images based on nitrogen core loss excitations were used to discern the presence of polyimide. Elemental maps of nanotubes extracted from the composite revealed good wetting of the nanotube surfaces by the polyimide.

Gallium-mediated growth of multiwall carbon nanotubes

Liquid gallium was used as a viable and effective solvent and template for high-yield growth of multiwall carbon nanotubes. The gallium-mediated nanotubes thus obtained differ morphologically from nanotubes obtained by using transition metals as catalysts. The nanotubes have a pin-like morphology, generally composed of an oval-shaped tip filled with liquid gallium and a tapered hollow body. The inner diameter of the tube is so large that the inner/outer diameter ratio is usually larger than 0.9. The tubes are naturally opened at both ends. These gallium-filled nanotubes may be used as a nanothermometer in the temperature range of 30 to 550 °C. This study opens an interesting route for carbon nanotube synthesis.

Exchange coupling in FeTaN/IrMn/FeTaN and NiFe/IrMn/NiFe trilayer films

The dependence of the exchange bias field Heb in FeTaN/IrMn and NiFe/IrMn systems on the microstructure has been investigated. Bilayer and trilayer films of 50 nm thick FeTaN and NiFe and 10 nm thick IrMn were prepared by dc magnetron sputtering. The glass/FeTaN/IrMn/FeTaN trilayer showed that the top FeTaN did not influence Heb in the bottom FeTaN, closest to the substrate, during deposition and annealing. High-resolution transmission electron microscopy results showed limited evidence of epitaxial growth with both columnar single IrMn grains and multiple IrMn grains. In contrast, in the glass/NiFe/IrMn/NiFe trilayer, the top NiFe significantly influenced Heb in the bottom NiFe during deposition and annealing. X-ray diffraction data for both systems showed no detectable changes in either the crystallinity or (111) texture of the IrMn layer during annealing. In the NiFe system, the trend in Heb in the as-deposited and annealed states may be explained by assuming single columnar grains in the IrMn which co...

CF8C plus: a new high temperature austenitic casting alloy for advanced power systems

Abstract A new cast austenitic stainless steel, CF8C plus, has been developed by Oak Ridge National Laboratory and Caterpillar for a wide range of transportation and energy applications. CF8C plus steel has improved high temperature tensile, creep, fatigue, and creep–fatigue properties compared with standard CF8C steel. Changes to the CF8C steel composition, including additions of Mn and N, result in changes to the solidification behaviour and final microstructure of the alloy, which directly relate to the improved mechanical properties. Additionally, CF8C plus is a relatively inexpensive steel which exhibits good castability. The mechanical properties of the alloy have generated significant interest for the production/design of cast components for diesel engine turbochargers and other exhaust components, natural gas reciprocating engines for distributed power, and turbine end covers and casings for land based turbines. In the present paper, the microstructural evolution of CF8C and CF8C plus are presented in more detail, and the mechanical properties of the alloys are compared with each other and other engineering alloys.

Overview of Creep Strength and Oxidation of Heat-Resistant Alloy Sheets and Foils for Compact Heat Exchangers

The Oak Ridge National Laboratory (ORNL) has been involved in research and development related to improved performance of recuperators for industrial gas turbines since about 1996, and in improving recuperators for advanced microturbines since 2000. Recuperators are compact, high efficiency heat-exchangers that improve the efficiency of smaller gas turbines and microturbines. Recuperators were traditionally made from 347 stainless steel and operated below or close to 650 C, but today are being designed for reliable operation above 700 C. The Department of Energy (DOE) sponsored programs at ORNL have helped defined the failure mechanisms in stainless steel foils, including creep due to fine grain size, accelerated oxidation due to moisture in the hot exhaust gas, and loss of ductility due to aging. ORNL has also been involved in selecting and characterizing commercial heatresistant stainless alloys, like HR120 or the new AL20-25+Nb, that should offer dramatically improved recuperator capability and performance at a reasonable cost. This paper summarizes research on sheets and foils of such alloys over the last few years, and suggests the next likely stages for manufacturing recuperators with upgraded performance for the next generation of larger 200-250 kW advanced microturbines.

Amorphization of sapphire during ion beam mixing

Earlier studies indicated that implantation of zirconium into sapphire at room temperature produced an amorphous layer at a critical composition of approximately 6.5% (cation). Further insight into the amorphization of sapphire has been provided by ion beam mixing studies. Bi-layer couples of {approx}80 nm thick polycrystalline ZrO{sub 2} films deposited on the (0001) face of {alpha}-Al{sub 2}O{sub 3} single crystals were irradiated to 4 {times} 10{sup 16} ions/cm{sup 2} with Kr (475 keV, 20{degree}C), or Cr (340 keV, 20{degree}C or {approx}900{degree}C). Transmission electron microscopy showed the unirradiated couples to have sharp, planar interfaces between the films and substrates. Recoil mixing by both ion species gave Zr concentrations greater than 10% (cation) to depths of 10-20 nm. An amorphous layer containing Zr was present at the interface for samples irradiated at room temperature. The sample mixed at the elevated temperature contained a sharp interface similar to the as-deposited sample. The present results suggest that both irradiation-produced damage (defects) and certain chemical species are required to amorphize sapphire.

Austenitic Stainless Steels and Alloys With Improved High-Temperature Performance for Advanced Microturbine Recuperators

Compact recuperators/heat-exchangers increase the efficiency of both microturbines and smaller industrial gas turbines. Most recuperators today are made from 347 stainless steel and operate well below 700°C. Larger engine sizes, higher exhaust temperatures and alternate fuels all demand recuperator materials with greater performance (creep strength, corrosion resistance) and reliability than 347 steel, especially for temperatures of 700–750°C. The Department of Energy (DOE) sponsors programs at the Oak Ridge National Laboratory (ORNL) to produce and evaluate cost-effective high-temperature recuperator alloys. This paper summarizes the latest high-temperature creep and corrosion data for a commercial 347 steel with modified processing for better creep resistanc, and for advanced commercial alloys with significantly better creep and corrosion resistance, including alloys NF709, HR120. Similar data are also provided on small lab heats of several new ORNL modified stainless steels.Copyright

Experimental estimation of the magnetic energy distribution of CoCrTa from EFTEM data

Energy-filtered transmission electron microscopy (EFTEM) was used to estimate the magnetic energy distribution of magnetic CoCrTa thin films. The films were obtained by sputtering onto identical substrates with a CrMo seed-layer. The results show that the coercivity was increased with increasing substrate temperature.