Effects of varying oxygen partial pressure on molten silicon-ceramic substrate interactions

The silicon sessile drop contact angle was measured on hot pressed silicon nitride, silicon nitride coated on hot pressed silicon nitride, silicon carbon coated on graphite, and on Sialon to determine the degree to which silicon wets these substances. The post-sessile drop experiment samples were sectioned and photomicrographs were taken of the silicon-substrate interface to observe the degree of surface dissolution and degradation. Of these materials, silicon did not form a true sessile drop on the SiC on graphite due to infiltration of the silicon through the SiC coating, nor on the Sialon due to the formation of a more-or-less rigid coating on the liquid silicon. The most wetting was obtained on the coated Si3N4 with a value of 42 deg. The oxygen concentrations in a silicon ribbon furnace and in a sessile drop furnace were measured using the protable thoria-yttria solid solution electrolyte oxygen sensor. Oxygen partial pressures of 10 to the minus 7 power atm and 10 to the minus 8 power atm were obtained at the two facilities. These measurements are believed to represent nonequilibrium conditions

Electronic structure and magnetism in two-dimensional hexagonal 5d transition metal carbides, Tan+1Cn (n=1,2,3)

Density functional calculations are used to investigate the electronic structure of two-dimensional 5d tantalum carbides with honeycomb-like lattice structures. We focus on changes in the low-energy bands near the Fermi level with dimensionality. We find that the Ta 5d states dominate, but the extended nature of the wavefunctions makes them weakly correlated. The carbide sheets are prone to long range magnetic order. We evaluate the stability of these states to enhanced electron--electron interactions through a Hubbard U correction. Lastly, we find spin orbit interactions strongly renormalize the band structure for n=2, but play a minor role in n=1 and 3.Comment: 4 pages, 4 figure

Metal Chelates of Diacetylacetanilide

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312 MAX Phases: Elastic Properties and Lithiation

Interest in the Mn+1AXn phases (M = early transition metal; A = group 13–16 elements, and X = C or N) is driven by their ceramic and metallic properties, which make them attractive candidates for numerous applications. In the present study, we use the density functional theory to calculate the elastic properties and the incorporation of lithium atoms in the 312 MAX phases. It is shown that the energy to incorporate one Li atom in Mo3SiC2, Hf3AlC2, Zr3AlC2, and Zr3SiC2 is particularly low, and thus, theoretically, these materials should be considered for battery applications

Study program to develop and evaluate die and container materials for the growth of silicon ribbons

The development and evaluation of proprietary coatings of pure silicon carbide, silicon nitride, and aluminum nitride on less pure hot pressed substrates of the respective ceramic materials, is described. Silicon sessile drop experiments were performed on coated test specimens under controlled oxygen partial pressure. Prior to testing, X-ray diffraction and SEM characterization was performed. The reaction interfaces were characterized after testing with optical and scanning electron microscopy and Auger electron spectroscopy. Increasing the oxygen partial pressure was found to increase the molten silicon contact angle, apparently because adsorbed oxygen lowers the solid-vapor interfacial free energy. It was also found that adsorbed oxygen increased the degree of attack of molten silicon upon the chemical vapor deposited coatings. Cost projections show that reasonably priced, coated, molten silicon resistant refractory material shapes are obtainable

Evaluation of HFMI as a Life Extension Technique for Welded Bridge Details

Published by Elsevier B.V. In this current study, HFMI technique is used to study the possibility to extend the fatigue life of pre-fatigued flange gusset welds typically found in girder bridges. The results from the study are also compared with results found in the literature for other more conventional techniques for retrofitting, e.g. cut-outs. The study also aims to investigate if the IIW HFMI recommendations could be applied for existing steel structures and that equal fatigue strength improvement could be claimed for prefatigued structures. Furthermore, new recommendations for structural hot spot stress type B are suggested for HFMI treated welds, applicable to flange guest welds. The results indicate that the HFMI could be used for welded bridge details rehabilitation as a competing technology with conventional cut-out. Furthermore, the results indicate that the IIW recommendations for HFMI fatigue strength improvement could also be applied for pre-fatigued welded details. \ua9 2019 The Authors

Comparison of thermal stability in MAX211 and 312 phases

The susceptibility of four MAX phases (Ti2AlC, Cr2AlC, Ti3AlC2, and Ti3SiC2) to high-temperature thermal dissociation in vacuum has been investigated using in-situ neutron diffraction. In high vacuum, these phases decomposed above 1400°C through the sublimation of M and A elements, forming a surface coating of MC. The apparent activation energies for the decomposition of sintered Ti3SiC2, Ti3AlC2, and Ti2AlC were determined to be 179.3, -71.9, and 85.7 kJ mol−1, respectively. The spontaneous release of Ti2AlC and TiC from de-intercalation during decomposition of Ti3AlC2 resulted in a negative activation energy

Synthesis and DFT investigation of new bismuth-containing MAX phases

The M(n + 1)AX(n) phases (M = early transition metal; A = group A element and X = C and N) are materials exhibiting many important metallic and ceramic properties. In the present study powder processing experiments and density functional theory calculations are employed in parallel to examine formation of Zr(2)(Al(1−x)Bi(x))C (0 ≤ x ≤ 1). Here we show that Zr(2)(Al(1−x)Bi(x))C, and particularly with x ≈ 0.58, can be formed from powders even though the end members Zr(2)BiC and Zr(2)AlC seemingly cannot. This represents a significant extension of the MAX phase family, as this is the first report of a bismuth-based MAX phase

Optical properties of Ti3SiC2 and Ti4AlN3

The dielectric functions of the MAX phases, Ti3SiC2 and Ti4AlN3, have been determined from first principles calculations. We compared the dielectric functions and the reflectivityspectra of Ti3SiC2 and Ti4AlN3 with those of TiC and TiN. The optical spectra were analyzed by means of the electronic structure, which provides theoretical understanding of the conduction mechanism of these two phases. We found that Ti4AlN3 can be used to avoid solar heating and also increase the radiative cooling due to the increased thermal emittance as compared to TiN. Ti4AlN3 can therefore be a candidate coating material for temperature control of space vehicles