The Measurement of the Thermal Properties and Absorptances of Powders Near their Melting Temperatures

A new technique, using a laser as the heating source, has been adopted to measure the heat capacities, thermal diffusivities, thermal conductivities, and absorptances of powders (especially polymer powders) near their melting temperatures. This makes use of an unsteady state process. The data of the thermal conductivities obtained through this technique below 100°C are in concord with the values obtained through the other techniques, which predicts well for the use of this technique for still higher temperatures, up to the melting temperatures of the powders to be investigated.Mechanical Engineerin

Measurement of the Thermal Conductivity of Powders by Two Different Methods

The thermal diffusivities and thermal conductivities of powders, especially PMMA-coated silicon carbide, at various temperatures, have been tested by two different dynamic methods, the water-bath method and the laser-heated method. The thermal conductivity data found by these two techniques are found to be consistent with each other.Mechanical Engineerin

Models for the Prediction of the Thermal

Five models and eq\1ationsJorthe.predictic;>nof the tbertnal conduc~"ities of powders in the literature are compared with the data obtainedill the experiments of the authors. Anew modified ntodel for the. correlation of the experimental data is presented. Key words: differential scanning calorimetry, porosity, solid content, specific heat, thermal conductivity.Mechanical Engineerin

Parametric Analysis of the Selective Laser Sintering Process

Qualitative and quantitative analyses are required to develop Selective Laser Sintering into a viable Manufacturing process. A simplified mathematical model for sintering incorporating the heat tJ;ansfer equation. and the sintering rate equation, but using temperature independent thermal properties, is presented in this paper. A practical result is the calculation of sintering depthdeftned as the depth of powder where the void fraction is less than 0.1 as a function of control parameters, such as the laser power intensity, the laser scanning velocity, and the initial bedtemperature. We derive the general behavior of laser sintering. A comparison of model predictions with laser sinterlng tests is provided.Mechanical Engineerin

Method of producing high-temperature parts by way of low-temperature sintering

A method of producing high temperature parts by way of relatively low temperature sintering is disclosed. A powder is produced, for example by spray drying a slurry, in which particles of a high temperature material such as a ceramic are partially coated with a low temperature material such as a polymer. The powder is subjected to selective laser sintering, or another heat-based layerwise additive process, to form a green part where the ceramic particles are bound by the polymer. The green part is permeated with a cementing agent having high temperature properties when cured; the cementing agent binds the exposed ceramic surfaces in the green part. The polymer is then removed, leaving the high temperature part. Additional cementing agent, or another high temperature material, may then be added to the part after removal of the polymer.Board of Regents, University of Texas Syste

Binder compositions for laser sintering processes

The development of polymer binder compositions that provide novel binders for high temperature inorganic particulates, especially metal and ceramic particulates is described. These materials are especially useful in a laser beam sintering process known as SLS.TM. that forms accurately shaped high strength green objects. The new binders may be thermally removed in post-shaping operations without significant residual ash. The resulting structures contain only metal, ceramic, or metal ceramic materials. The inventive methods developed for producing the new polymeric binder materials are based in part on using emulsion forms of selected polymers at controlled low molecular weights and high melt flow. An important aspect of the process is the semi-batch addition of selected chain transfer agents to the reacting emulsion. This optimizes coating characteristics of the polymer binder and results in high part strengths of the prototype parts produced in the SLS.TM. process.Board of Regents, University of Texas Syste

Multiple material systems for selective beam sintering

A method and apparatus for selectively sintering a layer of powder to produce a part comprising a plurality of sintered layers. The apparatus includes a computer controlling a laser to direct the laser energy onto the powder to produce a sintered mass. The computer either determines or is programmed with the boundaries of the desired cross-sectional regions of the part. For each cross-section, the aim of the laser beam is scanned over a layer of powder and the beam is switched on to sinter only the powder within the boundaries of the cross-section. Powder is applied and successive layers sintered until a completed part is formed. Preferably, the powder comprises a plurality of materials having different dissociation or bonding temperatures. The powder preferably comprises blended or coated materials.Board of Regents, University of Texas Syste

A Spitzer IRS Spectral Atlas of Luminous 8 micron Sources in the Large Magellanic Cloud

We present an atlas of Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of highly luminous, compact mid-infrared sources in the Large Magellanic Cloud. Sources were selected on the basis of infrared colors and 8 micron (MSX) fluxes indicative of highly evolved, intermediate- to high-mass stars with current or recent mass loss at large rates. We determine the chemistry of the circumstellar envelope from the mid-IR continuum and spectral features and classify the spectral types of the stars. In the sample of 60 sources, we find 21 Red Supergiants (RSGs), 16 C-rich Asymptotic Giant Branch (AGB) stars, 11 HII regions, 4 likely O-rich AGB stars, 4 Galactic O-rich AGB stars, 2 OH/IR stars, and 2 B[e] supergiants with peculiar IR spectra. We find that the overwhelming majority of the sample AGB stars (with typical IR luminosities ~1.0E4 L_sun) have C-rich envelopes, while the O-rich objects are predominantly luminous RSGs with L_IR ~ 1.0E5 L_sun. We determine mean bolometric corrections to the stellar K-band flux densities and find that for carbon stars, the bolometric corrections depend on the infrared color, whereas for RSGs, the bolometric correction is independent of IR color. Our results reveal that objects previously classified as PNe on the basis of IR colors are in fact compact HII regions with very red IRS spectra that include strong atomic recombination lines and PAH emission features. We demonstrate that the IRS spectral classes in our sample separate clearly in infrared color-color diagrams that use combinations of 2MASS data and synthetic IRAC/MIPS fluxes derived from the IRS spectra. On this basis, we suggest diagnostics to identify and classify, with high confidence levels, IR-luminous evolved stars and HII regions in nearby galaxies using Spitzer and near-infrared photometry.Comment: 46 pages, 9 figures; accepted for publication in AJ; abstract abridge

Probing the Type I Seesaw Mechanism with Displaced Vertices at the LHC

The observation of Higgs decays into heavy neutrinos would be strong evidence for new physics associated to neutrino masses. In this work we propose a search for such decays within the Type I seesaw model in the few-GeV mass range via displaced vertices. Using 300 fb−1 of integrated luminosity, at 13 TeV, we explore the region of parameter space where such decays are measurable. We show that, after imposing pseudorapidity cuts, there still exists a region where the number of events is larger than O(10). We also find that conventional triggers can greatly limit the sensitivity of our signal, so we display several relevant kinematical distributions which might aid in the optimization of a dedicated trigger selection

Characterisation of a dipolar chromophore with third-harmonic generation applications in the near-IR

E-2-Tricyanovinyl-3-n-hexyl-5-[4-{bis(4-n-butylphenyl)amino}-2-methoxystyryl]-thiophene, 1, has previously been used to demonstrate applications relying on frequency tripling of 1.55 μm light. Here we report the synthesis and chemical characterisation of 1, along with quantum-chemical calculations and additional experimental investigations of its third-order nonlinear properties that give more insight into its frequency tripling properties. Although 1 can be processed into amorphous films, crystals can also be grown by slow evaporation of solutions; the crystal structure determined by X-ray diffraction shows evidence of significant contributions from zwitterionic resonance forms to the ground-state structure, and reveals centrosymmetric packing exhibiting π–π and C–H···N≡C interactions. Both solutions and films of 1 exhibit near-infrared two-photon absorption into the low-lying one-photon-allowed state with a peak two-photon cross-section of cɑ. 290 GM (measured using the white-light continuum method with a pump wavelength of 1800 nm) at a transition energy equivalent to degenerate two-photon absorption at cɑ. 1360 nm; two related chromophores are also found to show comparable near-IR two-photon cross-sections. Closed-aperture Z-scan measurements and quantum-chemical calculations indicate that the nonlinear refractive index and third-harmonic generation properties of 1 are strongly dependent on frequency in the telecommunications range, due the aforementioned two-photon resonance