Improved consolidation of silicon carbide

Alpha silicon carbide powder was consolidated by both dry and wet methods. Dry pressing in a double acting steel die yielded sintered test bars with an average flexural strength of 235.6 MPa with a critical flaw size of approximately 100 micro m. An aqueous slurry pressing technique produced sintered test bars with an average flexural strength of 440.8 MPa with a critical flaw size of approximately 25 micro m. Image analysis revealed a reduction in both pore area and pore size distribution in the slurry pressed sintered test bars. The improvements in the slurry pressed material properties are discussed in terms of reduced agglomeration and improved particle packing during consolidation

Parametric evaluation of ball milling of SiC in water

A statistically designed experiment was conducted to determine optimum conditions for ball milling alpha-SiC in water. The influence of pH adjustment, volume percent solids loading, and mill rotational speed on grinding effectiveness was examined. An equation defining the effect of those milling variables on specific surface area was obtained. The volume percent solids loading of the slurry had the greatest influence on the grinding effectiveness in terms of increase in specific surface area. As grinding effectiveness improved, mill and media wear also increased. Contamination was minimized by use of sintered alpha-SiC milling hardware

Particle size reduction of Si3N4 with Si3N4 milling hardware

The grinding of Si3N4 powder using reaction bonded Si3N4 attrition, vibratory, and ball mills with Si3N4 media was examined. The rate of particle size reduction and the change in the chemical composition of the powder were determined in order to compare the grinding efficiency and the increase in impurity content resulting from mill and media wear for each technique. Attrition and vibratory milling exhibited rates of specific surface area increase that were approximately eight times that observed in ball milling. Vibratory milling introduced the greatest impurity pickup

Quantum Field Theory and Differential Geometry

We introduce the historical development and physical idea behind topological Yang-Mills theory and explain how a physical framework describing subatomic physics can be used as a tool to study differential geometry. Further, we emphasize that this phenomenon demonstrates that the interrelation between physics and mathematics have come into a new stage.Comment: 29 pages, enlarged version, some typewritten mistakes have been corrected, the geometric descrition to BRST symmetry, the chain of descent equations and its application in TYM as well as an introduction to R-symmetry have been added, as required by mathematicia

Atmospheric Sulfur Photochemistry on Hot Jupiters

We develop a new 1D photochemical kinetics code to address stratospheric chemistry and stratospheric heating in hot Jupiters. Here we address optically active S-containing species and CO2 at 1200 < T < 2000 K. HS (mercapto) and S2 are highly reactive species that are generated photochemically and thermochemically from H2S with peak abundances between 1-10 mbar. S2 absorbs UV between 240 and 340 nm and is optically thick for metallicities [SH] > 0 at T > 1200 K. HS is probably more important than S2, as it is generally more abundant than S2 under hot Jupiter conditions and it absorbs at somewhat redder wavelengths. We use molecular theory to compute an HS absorption spectrum from sparse available data and find that HS should absorb strongly between 300 and 460 nm, with absorption at the longer wavelengths being temperature sensitive. When the two absorbers are combined, radiative heating (per kg of gas) peaks at 100 microbars, with a total stratospheric heating of about 8 x 10^4 W/m^2 for a jovian planet orbiting a solar-twin at 0.032 AU. Total heating is insensitive to metallicity. The CO2 mixing ratio is a well-behaved quadratic function of metallicity, ranging from 1.6 x 10^-8 to 1.6 x 10^-4 for -0.3 < [M/H] < 1.7. CO2 is insensitive to insolation, vertical mixing, temperature (1200 < T <2000 K), and gravity. The photochemical calculations confirm that CO2 should prove a useful probe of planetary metallicity.Comment: Astrophysical Journal Lett. in press; important revision includes effect of updated thermodynamic data and a new opacity sourc

Reliability analysis of a structural ceramic combustion chamber

The Weibull modulus, fracture toughness and thermal properties of a silicon nitride material used to make a gas turbine combustor were experimentally measured. The location and nature of failure origins resulting from bend tests were determined with fractographic analysis. The measured Weibull parameters were used along with thermal and stress analysis to determine failure probabilities of the combustor with the CARES design code. The effect of data censoring, FEM mesh refinement, and fracture criterion were considered in the analysis

The Three Loop Equation of State of QED at High Temperature

We present the three loop contribution (order $e^4$) to the pressure of massless quantum electrodynamics at nonzero temperature. The calculation is performed within the imaginary time formalism. Dimensional regularization is used to handle the usual, intermediate stage, ultraviolet and infrared singularities, and also to prevent overcounting of diagrams during resummation.Comment: ANL-HEP-PR-94-02, SPhT/94-054 (revised final version

Cosmicflows-2: SNIa Calibration and H0

The construction of the Cosmicflows-2 compendium of distances involves the merging of distance measures contributed by the following methods: (Cepheid) Period-Luminosity, Tip of the Red Giant Branch (TRGB), Surface Brightness Fluctuation (SBF), Luminosity-Linewidth (TF), Fundamental Plane (FP), and Type Ia supernova (SNIa). The method involving SNIa is at the top of an interconnected ladder, providing accurate distances to well beyond the expected range of distortions to Hubble flow from peculiar motions. In this paper, the SNIa scale is anchored by 36 TF spirals with Cepheid or TRGB distances, 56 SNIa hosts with TF distances, and 61 groups or clusters hosting SNIa with Cepheid, SBF, TF, or FP distances. With the SNIa scale zero point set, a value of the Hubble Constant is evaluated over a range of redshifts 0.03 < z < 0.5, assuming a cosmological model with Omega_m = 0.27 and Omega_Lambda = 0.73. The value determined for the Hubble Constant is H0 = 75.9 \pm 3.8 km s-1 Mpc-1.Comment: Accepted for publication in The Astrophysical Journal. 11 pages, 8Figures, 5 Table