Powder effects in SiC matrix layered structures fabricated using selective area laser deposition vapor infiltration (SALDVI)

Silicon carbide has been deposited by laser-induced chemical vapor infiltration from the gas precursor tetramethylsilane, Si(CH₃)₄, into loosely packed powder layers of SiC, ZrO₂-Y₂O₃, or Mo. The goal is to produce dense layered structures of arbitrary shape by computer controlled laser scanning where the pore spaces between the powder particles are filled with solid material deposited from the gas phase using the selective area laser deposition vapor infiltration (SALDVI) process. Layered samples were fabricated for each powder material using both single line (bar) and multiple line (slab) laser scan patterns and 10 Torr Si(CH₃)₄, 2.5 m/s scan speed, 1000°C target temperature, and 120 m layer thickness. Samples of SiC and ZrO₂-Y₂O₃ are prone to surface cracking in the bar geometry, and cracking and delamination of layers in the slab geometry. Samples fabricated with Mo powder have no cracks or delamination defects in either bar or slab geometry as well as a better surface appearance.     

Conversion of Triglycerides to Hydrocarbons Over Supported Metal Catalysts

The deoxygenation of triglycerides (tristearin, triolein and soybean oil) under nitrogen atmosphere was investigated over 20 wt% Ni/C, 5 wt% Pd/C and 1 wt% Pt/C catalysts. Use of the Ni catalyst resulted in near quantitative conversion of the triglyceride in each case, high yields of linear C5 to C17 alkanes and alkenes being obtained. Oxygen was rejected as CO and CO₂, while small amounts of light alkanes (C1–C4) and H₂ were also formed. ¹³C NMR spectroscopic analysis of the liquid product from soybean oil deoxygenation at intermediate reaction times suggested that one pathway for triglyceride deoxygenation involves liberation of fatty acids via C–O bond scission and concomitant H transfer, followed by elimination of CO₂ from the acids in a later step. Compared to Ni, catalysts containing Pd or Pt supported on activated carbon showed lower activity for both triglyceride deoxygenation and for cracking of the fatty acid chains.     

Novel images of the evolution of stress corrosion cracks from corrosion pits

SCC in turbine disc steels exposed to simulated steam-condensate tends to nucleate preferentially from corrosion pit precursors. The evolution of these cracks is not straightforward and not well understood. In this work, unique three-dimensional X-ray microtomographic images have confirmed that cracks develop predominantly at the shoulder of the pit, near the pit/surface interface, for specimens stressed to 50–90% σ_0.2. In support of this observation, FEA of model pits indicate that strain is a maximum on the pit wall just below the pit mouth. Implications of these observations for the pit-to-crack transition and predictive-modelling of crack nucleation and growth are discussed.     

In Situ Thermocouples in Macro-Components Fabricated Using SALD and SALDVI Techniques. III. Fabrication and Properties of the SiC/C Thermocouple Device

Fabrication of a SiC/C thermocouple embedded in an arbitrarily-shaped SiC macro-component has been demonstrated using an integrated Selective Area Laser Deposition (SALD) with the closely related Selective Area Laser Deposition Vapor Infiltration (SALDVI) process. SALD is used to make the embedded thermocouple devices in situ and SALDVI to fabricate the macro-components. The thermocouple elements, silicon carbide and carbon lines, and the electrical insulation layer, Si₃N₄, have been deposited from tetramethylsilane (TMS), acetylene, and a gas mixture of TMS and ammonia, respectively. It is found that the fabricated in situ thermocouples respond sensitively to temperature variation. Furthermore, the electric signal of the embedded thermocouple is very stable and reproducible in response to thermal cycling. This is not the case when the thermocouple is not embedded in the SiC matrix because of the oxidation of the thermocouple elements.     

Phylogenetic characterization of bacteria in the subsurface microbial culture collection

The Subsurface Microbial Culture Collection (SMCC) was established by the U.S. Dept. of Energy (DOE) and contains nearly 10,000 strains of microorganisms (mostly bacteria) isolated from terrestrial subsurface environments. Selected groups of bacterial isolates from three sample sites situated above geochemically and hydrologically different subsurface environments have been characterized by phylogenetic analysis of 16S ribosomal RNA (rRNA) gene nucleotide sequences. Among these isolates were members of six major phylogenetic groups of bacteria: the high-G+C and low-G+C Gram-positive bacteria; the alpha-, beta-, and gamma-subdivisions of the Proteobacteria; and the Flexibacter/Cytophaga/Bacteroides group. A small number of the SMCC strains may be members of new bacterial genera, but most of them could be placed with reasonable confidence into more than 35 previously described genera. The majority of the Gram-positive isolates were species of Arthrobacter, Bacillus, or Streptococcus, whereas Acinetobacter, Comamonas, Pseudomonas, Sphingomonas, and Variovorax were among the most frequently encountered Gram-negative genera. A high proportion of the strains were placed in fewer than 10 genera, implying that there is substantial duplication within the SMCC at the genus level. When groups of isolates assigned to Acinetobacter, Arthrobacter, or Sphingomonas were analyzed in more detail, however, it was found that each group consisted of subgroups of strains that probably differed at the species level. Restriction endonuclease analysis (applied to the strains from one sample site) indicated that additional diversity was present at the strain level. Most of the SMCC isolates assigned to some genera (e.g., Acinetobacter) were very closely related to previously described species in those genera, but most of the isolates assigned to other genera (e.g., Arthrobacter and Sphingomonas) appeared (or were shown) to be new species, thereby indicating that a reasonable amount of novelty is present within the SMCC at the species level.     

Preparation and characterization of cerium oxide templated from activated carbon

The application of carbon templating to the preparation of cerium oxide and cerium–zirconium mixed oxide was investigated. Impregnation of a highly mesoporous activated carbon (Darco KB-B) with aqueous cerium nitrate, followed by calcination to remove the template, afforded nanocrystalline ceria with a surface area of up to 148 m²/g. Ceria-zirconia with surface area of 148 m²/g was similarly prepared. TEM studies on the ceria product revealed the presence of a polycrystalline phase, comprised of irregular aggregates of ceria crystallites of ca. 6.5 nm diameter. Use of activated carbon fibers (ACFs) as templates afforded ceria with a gross morphology resembling that of the fiber template. The lower surface areas (3–59 m²/g) of the resulting ceria reflect the mainly microporous nature of the ACFs; evidently the Ce nitrate solution is largely unable to penetrate the micropores. TEM showed the ceria fibers to be comprised of crystallites possessing a morphology similar to the Darco KB-B templated ceria.