Thermal Analysis and Modeling of Steady-State Rod Growth During Gas-Phase Solid Freeform Fabrication

An analysis ofthe steady-state growth ofrods during gas-phase solidfreeform fabrication is presented. It is demonstrated that heat transfer controls the evolution ofshape during laser-induced pyrolysis of slender 3-D structures. Insulating and conductive deposit materials were studied, using both simple analytic and numerical simulations to demonstrate how steady-state rod growth is achieved.Mechanical Engineerin

Cost-performance analysis of silicon carbide fibers

Side-by-side studies of tensile strength and high-temperature exposures were conducted for High-Nicalon Type S, Sylramic, and Laser-Printed Silicon Carbide fibers. This battery of tests supports the well-known superior properties of CVD-derived Silicon Carbide, such as Specialty Materials’ SCS Ultra. SCS Ultra, however, is derived from CVD onto a continuous core filament substrate, hence it can only be produced in large diameter continuous filaments (90-140 µm). In contrast, laser-printing of SiC fibers relies on self-seeded Laser-Induced Chemical Vapor Deposition, which does away with the core filament substrate requirement, achieving much smaller diameters (adjustable between 14 and 50 µm) and growth rates, while replicating SCS Ultra’s superior properties. This paper also discusses economic projections for laser-printed SiC fiber production. Each laser beam produces one continuous filament. Lasers are currently at $100/beam with the cost expected to continue to fall to$50/beam in the next 5 years. Current fiber laser printers are at 384 beams per unit. This means 120 laser printers are needed per metric ton of 25 µm SiC fiber per year. This places capital equipment costs at a fraction of current polymer-derived SiC fibers production, with specific energetic requirements (kW-h/Ton) and raw material exploitation (kg out / kg in) improvements by one order of magnitude or greater. Long-term projections for mass-produced laser-printed fibers present a realistic pathway to $1000/lb CVD-quality SiC fibers

How not to measure the tensile strength of high-modulus fibers

Monofilament tensile strength and Young\u27s Modulus measurements are standardized in ASTM C1557 - 14. The Standard prescribes the use of mounting tabs that are “appropriately designed to be self-aligning if possible, and as thin as practicable to minimize fiber misalignment.” We have now shown through analysis, and verified through experiments, that this method can be expected to have an increasingly negative impact on strength measurement as the Young’s modulus and/or fiber diameter increases. We show that translational and angular misalignments are in fact neither measurable nor controllable in the Standard, and that even half the suggested misalignment tolerance can have an order of magnitude deleterious impact on the measured strength. The Standard notes that there is no standard gage length but that current practice is to use 1 . We have found that, for the test procedure advocated by the Standard, the gage length cannot be considered independently of the fiber diameter, and that there is an optimal ratio that will minimize the influence of perturbations induced by the test apparatus. The standard recommends recovering the fracture surfaces to measure the fiber diameter. However, it acknowledges that stiff fibers tend to shatter upon failure. This is in agreement with our experience with high-modulus Silicon Carbide ceramic fibers tensile tests, which shatter into small fragments, sometimes over the entire gage length. In an attempt to alleviate fiber fragmentation, the standard suggests vacuum grease to dampen fiber fracture. Experiments with fibers 15-50 µm in diameter and a Young’s modulus over 350 GPa resulted in complete fragmentation even when covered with vacuum grease. We found that using wax instead offered a better preservation of the fracture surface allowing for areal measurements. The Standard considers a tensile test to be valid when fracture occurs within the gage length of the test fiber. Our experiments rigorously following the Standard with high modulus (\u3e300 GPa) fibers (diameter 15-50 µm) covered with vacuum grease always resulted in a fractures occurring at the grip over 90% of the time. To prepare samples for the tensile strength measurement procedure, the Standard specifies: “Randomly choose, and carefully separate, a suitable single-fiber from the bundle or fiber spool.” We observed that the process of teasing a monofilament out of a tow is not random. Indeed, the separation of a monofilament from a bundle is a self-selecting process that is biased towards minimally flawed single-fibers. The Standard assumes a Weibull distribution of tensile strengths. This assumption is legitimate and in agreement with statistical failure analysis. A legitimate question is whether eliminating a source of perturbation has an effect on the Weibull statistics. Indeed the case can be made that the current standard, by restricting the measurable tensile strength would have the effect of artificially increasing the Weibull modulus, making the fiber quality appear to be more consistent that it actually is. This article introduces a new testing procedure, which circumvents the limitations imposed by the Standard. Experiments consistently show that the Standard tensile strength measurement procedure systematically underreports, sometimes severely, the strength of high-modulus filaments and overreports the Weibull modulus

Layered Micro-Wall Structures from the Gas Phase

The use of3-D LCVD with volumetric rate feedback was investigated in the fabrication ofmicromechanical wall structures. These were constructed by recursive laser scanning and resulted in layered wall composed ofrecursive line deposition. Experiments were designed to uncover the relationship between scan rate, volumetric deposition rate, pressure and laser powerfor pyrolytic graphite from an ethylene precursor. Results point to a conduction dominated heat transfer which greatly limits the volumetric deposition rate at the wall. This also results in a highly unstable deposition process, since volumetric deposition increases by orders ofmagnitude as soon as rod growth is initiated. An unexpected results ofthis work is the ability to grow rods at an angle to the laser axis, with good control ofthe linear growth rate. This is achieved by adaptive laser scanning during rod growth.Mechanical Engineerin

Laser-CVD silicon carbide fibers as non woven preforms in fiber-reinforced SiC-SiC composites

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High Strength Ceramic Fibers and Methods of Fabrication

A method and apparatus for forming a plurality of fibers from (e.g., CVD) precursors, including a reactor adapted to grow a plurality of individual fibers; and a plurality of independently controllable lasers, each laser of the plurality of lasers growing a respective fiber. A high performance fiber (HPF) structure, including a plurality of fibers arranged in the structure; a matrix disposed between the fibers; wherein a multilayer coating is provided along the surfaces of at least some of the fibers with an inner layer region having a sheet-like strength; and an outer layer region, having a particle-like strength, such that any cracks propagating toward the outer layer from the matrix propagate along the outer layer and back into the matrix, thereby preventing the cracks from approaching the fibers. A method of forming an interphase in a ceramic matrix composite material having a plurality of SiC fibers, which maximizes toughness by minimizing fiber to fiber bridging, including arranging a plurality of SiC fibers into a preform; selectively removing (e.g., etching) silicon out of the surface of the fibers resulting in a porous carbon layer on the fibers; and replacing the porous carbon layer with an interphase layer (e.g., Boron Nitride), which coats the fibers to thereby minimize fiber to fiber bridging in the preform

Characterization of ultra-high temperature materials produced by rapid- laser chemical vapor deposition (R-LCVD)

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Influence of solution parameters for the fast growth of ZnO nanostructures by laser-induced chemical liquid deposition

ZnO nanorods, nanoneedles, nanoparticles and nanoballs were synthesized on fused quartz substrates upon irradiation of a droplet of methanolic zinc acetate dihydrate solution by an infrared continuous wave CO₂ laser for a few seconds. The addition of monoethanolamine and water to the solution improved the alignment of the nanorods and had a significant effect on the volume and morphology of the deposits. An increase of the zinc acetate concentration was found to lead to an increase of the thickness and area covered by the initial ZnO seed layer on which the nanostructures grew. By investigating the crystal structure of the deposits using x-ray and electron diffraction, we were able to show that the nanorods grow along the c axis with a high crystalline quality. Raman and photoluminescence spectroscopy confirmed the high-quality of the grown ZnO nanostructures. As a matter of fact, their photoluminescence spectra are dominated by an intense UV emission around 390 nm

Differential cross section measurements for the production of a W boson in association with jets in proton–proton collisions at √s = 7 TeV

Measurements are reported of differential cross sections for the production of a W boson, which decays into a muon and a neutrino, in association with jets, as a function of several variables, including the transverse momenta (pT) and pseudorapidities of the four leading jets, the scalar sum of jet transverse momenta (HT), and the difference in azimuthal angle between the directions of each jet and the muon. The data sample of pp collisions at a centre-of-mass energy of 7 TeV was collected with the CMS detector at the LHC and corresponds to an integrated luminosity of 5.0 fb[superscript −1]. The measured cross sections are compared to predictions from Monte Carlo generators, MadGraph + pythia and sherpa, and to next-to-leading-order calculations from BlackHat + sherpa. The differential cross sections are found to be in agreement with the predictions, apart from the pT distributions of the leading jets at high pT values, the distributions of the HT at high-HT and low jet multiplicity, and the distribution of the difference in azimuthal angle between the leading jet and the muon at low values.United States. Dept. of EnergyNational Science Foundation (U.S.)Alfred P. Sloan Foundatio