Search

Tantalum carbide ceramics with high volume fractions of the ζ-Ta4C3 phase have been shown to exhibit high fracture strength and toughness as compared to those in absence of this phase. In this work, we investigated how microcracks propagated in this these high toughness ceramics using Knoop and Vickers microindentation. The Knoop indentations demonstrated that cracking preferentially occurred parallel to the lath structure in ζ-Ta4C3; however shorter cracks did form between the laths when a sufficient driving force was present. The resulting crack path was tortuous providing direct evidence for toughening through crack deflection; however, the microscale nature of the work cannot rule out crack bridging as a toughening mechanism as well. Plasticity is also observed under the indents, but is likely a result of the high confining pressures that occurred during indentation allowing for plastic flow.

Coatings with the 0.7BaTiO3–0.3BiScO3 solid-solution composition were formed on palladium and single-crystal (001) SrTiO3 substrates using a polymeric metal citrate precursor. Solutions of TiOCl2, Ba(NO3)2, Sc(NO3)3, and Bi(NO3)3 were mixed with citric acid and polymerized with ethylene glycol. Stable mixed-metal citrate solutions were formed at pH > 9 and used for coatings. The phase and composition of powders and coatings were characterized using DTA, TGA, SEM, TEM, and X-ray diffraction. Single-phase cubic 0.7BaTiO3–0.3BiScO3 solid solutions formed at 600°C. Coatings on Pd using precursors doped with 5 wt% lithium nitrate were dense after sintering at 950°C/1 h. Coatings without lithium nitrate required 1050°C/50 h to densify. Coatings on SrTiO3 heat-treated at 1150°C were dense but formed a (Sc,Ti)-rich second phase.

The superior thermal and optical properties of transparent polycrystalline ceramics make them attractive alternatives to glass-based materials for laser gain media. Fibers have other advantages of compactness, vibration-resistance, and reduced cooling requirements. Recently it was found that surface roughness caused by grain boundary grooving dominated optical scattering even though there were other scattering sources in the fiber. Therefore, a lot of effort went to fabrication of fibers with smooth surfaces. A mechanical polishing method for polycrystalline YAG fibers was developed. The fiber surface roughness was reduced, while maintaining a circular cross-section. Surface-polished 1.5% Ho-doped polycrystalline YAG fiber, 62 mm long with 31 μm diameter, was fabricated, and lasing was demonstrated from this fiber. Effects of surface-polishing on the surface roughness and scattering coefficient are presented, and lasing characteristics are discussed.

A polycrystalline 1.5% Ho: YAG fiber with a diameter of 31 µm was prepared. Surface roughness from grain boundary grooving was reduced by polishing, which decreased the fiber scattering coefficient from 76 msup-1/supto 35 msup-1/sup. Lasing tests were done on this fiber with a SF57 Schott glass cladding. Lasing was confirmed by spectrum narrowing with threshold pump power lower than 500 mW and a slope efficiency of 7%. To our knowledge, this is the first lasing demonstration from a small diameter polycrystalline ceramic fiber.

Spark Plasma Sintering (SPS) consolidated ZrB2+SiC composites using nano-powders (around 40 nm) showed smaller grains compared to those using micron size powders and segregation of SiC into islands is minimal but with higher oxidation of ZrB2 to form ZrO2 in nano-composites. Argon-gas purging prior to SPS consolidation at around 2000 °C and 40 MPa of ZrB2+20vol.%SiC nano-powders was used to minimize the oxidation and obtain fine granules with high densification. The densification of the Argon-gas purged nano-composites is higher compared to those consolidated without Argon gas purging. The EDX analysis showed a strong reduction in the oxygen peak for the Argon gas purged composites. The XRD spectra also support this observation with less ZrO2 phase composition in Argon gas purged composites. The Vickers micro-hardness showed slightly lower values for Argon gas purged composites though they have higher densification.

In multi-dimensional two-phase flows, a local pressure is very important because it directly influences the phase change and it may lead to a great change in the flow field. This in turn puts emphasis on the accurate evaluation of local pressure gradient. This paper presents a new numerical scheme to evaluate the pressure gradient at cell centers on unstructured meshes for a three-dimensional thermal-hydraulic code, named CUPID. The results of the new scheme for a simple test function, a gravity-driven cavity, and a wall boiling two-phase flow are compared with those of the previous schemes in the CUPID code.

For the analysis of transient two-phase flows in nuclear reactor components, a three-dimensional thermal hydraulics code, named CUPID, has been developed. The CUPID code adopts a two-fluid, three-field model for two-phase flows, and the governing equations were solved over unstructured grids, which are very useful for the analysis of flows in complicated geometries. To obtain numerical solutions, the semi-implicit numerical method for the REALP5 code was modified for an application to unstructured grids, and it has been further improved for enhanced accuracy and fast running. For the verification of the CUPID code, a set of conceptual problems and experiments were simulated. This paper presents the flow model, the numerical solution method, and the results of the preliminary assessment.

A two-phase flow analysis code, CUPID, has been developed for a realistic simulation of thermal–hydraulic phenomena in nuclear reactor components. In the CUPID code, a two-fluid three-field model is adopted and the governing equations are solved on unstructured meshes. To obtain the numerical solution, the semi-implicit method of the RELAP5 code was used with some modifications for a cell-centered finite volume method. In this work, a second-order upwind method was implemented for the convective terms of the CUPID code. To get the slopes of the convective quantities, we adopted the Frink’s reconstruction method (1994, AIAA Paper 94-0061) and modified it for an application to arbitrary polyhedral cells. To stabilize the numerical solutions, the Barth and Jesperson’s slope limiter was used. In order to evaluate the enhanced accuracy and ensure the robustness of the implemented scheme, numerical tests were performed using conceptual single- and two-phase flow problems, which include a strong phase change and very heterogeneous phase distributions.

Densification of porous alumina by CrO3 infiltration was investigated on model composites containing a sapphire interface. Gas–solid deposition of Cr2O3 from CrO3 source on sapphire and glass at 600°C was also investigated. The deposited Cr2O3 layers were characterized using X-ray diffraction and transmission electron microscopy. It was found that similar to gas-phase deposition on sapphire, in CrO3-infiltrated porous alumina Cr2O3 grows epitaxially on adjacent alumina surfaces, suggesting a gas–solid condensation mechanism. The deposition rate and morphology of the Cr2O3 film deposited on alumina depend strongly on the crystallographic orientation.

Ultra-High-Temperature Ceramics (UHTCs) such as ZrB2 and HfB2 with incorporation of SiC nanofiller are useful as structural materials for applications in propulsion and thermal protection systems such as turbine-engine hot section components, leading edge of hypersonic vehicles, where extremely high heat fluxes generate very high temperatures and steep temperature gradients [1]. Spark plasma sintering (SPS) technique is used for densifying the UHTCs under the influence of uniaxial pressure and pulsed direct current [2]. Here, we study the densification, grain growth, and microhardness of ZrB2 nanocomposites with 15% and 20% SiC consolidated using SPS.

The advent of high energy, high power fiber lasers hinges on the availability of new fibers and suitable cladding materials. New fiber materials with the needed thermal and optical properties are urgently needed for high powered fiber lasers; a viable cladding process using suitable cladding materials is equally critical. These two fundamental technologies are being developed. Most recently, the technical feasibility of applying optically transparent undoped YAG as the fiber cladding was explored via a single source electron beam deposition process. The technical feasibility of depositing such fiber claddings was successfully demonstrated on various model fibers. Amorphous Y x Al 1-x O 3 coatings with various chemistries and high optical transmittances were deposited and shown to be stable even after high temperature annealing, while retaining the optical quality.

There is a critical demand for high quality, transparent ceramic YAG fibers for high powered fiber lasers. The production of laser quality ceramic fibers hinges on advanced ceramic processing technology, along with the availability of highly sinterable powder with high phase and chemical purity. These two fundamental technologies have been successfully developed at UES. Nd (1.1 a/o) and Yb (1.0 a/o)-doped yttrium aluminum garnet (YAG) fibers with high optical quality were produced by combining UES's tailored powders with advanced consolidation processes including fiber extrusion and vacuum sintering. The as-sintered and as-annealed fibers, approximately 30 microns in diameter, appeared transparent and successfully transmitted laser beams; further development will allow for the production of doped ceramic YAG fiber lasers for advanced high power and high energy fiber laser systems.

Laser quality, polycrystalline oxide fibers offer significant advantages over state-of-the-art silica fiber for high energy lasers. Advanced ceramic processing technology, along with a novel powder production process, has potential to produce oxide fibers with an outstanding optical quality for use in the fiber laser applications. The production of contaminant-free green fibers with a high packing density, as well as uniform packing distribution, is a key factor in obtaining laserquality fibers. High quality green fibers are dependent on the powder quality combined with the appropriate slurry formulation. These two fundamental technologies were successfully developed at UES, and used to produce Yb-doped yttrium aluminum garnet (YAG) fibers with high optical quality, high chemical purity, and suitable core diameters down to 20-30 microns.

Polycrystalline ceramics offer a number of advantages relative to single crystal materials such as lower processing temperatures, improved mechanical properties, and higher doping levels with more uniform distribution of dopants for improved laser performance. Ceramic YAG (Y3Al5O12) and rare earth sesquioxide (RE2O3) fibers promise to enable a number of high power laser devices via high thermal conductivity and higher allowable dopant concentration; however, these materials are not currently available as fine diameter optical-quality fibers. Powder processing approaches for laser quality polycrystalline ceramic fibers are in development at AFRL. Current processing techniques will be reviewed. The effects of a number of processing variables on the resulting fibers as well as preliminary optical characterization will also be presented.

Polycrystalline YAG fibers are of interest for both optical and structural applications. Various processing routes of YAG fibers for structural applications have been explored; however, processing routes for optical quality polycrystalline YAG fiber have not been investigated intensively despite the potential of the material to enable high power lasers. Recent results in the processing of YAG fiber for laser applications are presented and detailed relationship between processes, microstructures, and optical properties of YAG fibers are discussed. Specifically, details of the processes for green fiber preparation, sintering methods, and transparencies depending on the process variables are shown. Our recent advancement in fiber processing prior to sintering has improved the transparency of YAG fiber significantly. Vacuum or air sintering followed by Hot Isostatic Press (HIP) produced fibers with transparency comparable to that of single crystal YAG fiber.

A small section of extruded green fiber of ceramic YAG has been densified by means of laser beam sintering with a 1.6W beam from a CO2 laser, sintered for approximately 1 minute.

Traditional silica fibers currently are unlikely to be able to sustain the powers needed for future Air Force applications. The low thermal conductivity of silica makes it difficult to control thermal gradients within the fibers resulting in failure or degradation in beam quality. While some of these problems can be ameliorated by using longer fibers, this results in problems with nonlinear effects such as stimulated Raman and Brillouin scattering (SRS and SBS). Yttrium aluminum garnet (Y3Al5O12, YAG) has the potential for overcoming these problems due to 1) higher thermal conductivity, 2) reduced thermal lensing, and 3) higher SBS threshold. Polycrystalline YAG has been demonstrated to be a highly efficient and economical laser host material in slab form. Polycrystalline YAG can be doped more uniformly and at higher levels than single-crystals with no dopant loss by zone refinement, has higher fracture toughness than single-crystals, and supports higher power densities. Despite the anticipated advantages, polycrystalline YAG has never been demonstrated in high-power fiber lasers. The development and characterization of YAG fibers for high energy laser applications is the primary goal of our research. Recent results in the processing of optical quality polycrystalline YAG fibers will be presented and discussed.

The fabrication of lightweight mirror assemblages via a replication technique offers great potential for eliminating the high cost and schedule associated with the grinding and polishing steps needed for conventional glass or SiC mirrors. A replication mandrel is polished to an inverse figure shape and to the desired finish quality. It is then, coated with a release layer, the appropriate reflective layer, and followed by a laminate for coefficient of thermal expansion (CTE) tailorability and strength. This optical membrane is adhered to a mirror structural substrate with a low shrinkage, CTE tailored adhesive. Afterwards, the whole assembly is separated from the mandrel. The mandrel is then cleaned and reused for the next replication run. The ultimate goal of replication is to preserve the surface finish and figure of the optical membrane upon its release from the mandrel. Successful replication requires a minimization of the residual stresses within the optical coating stack, the curing stresses from the adhesive and the thermal stress resulting from CTE mismatch between the structural substrate, the adhesive, and the optical membrane. In this paper, the results on replicated trials using both metal/metal and ceramic/ceramic laminates adhered to light weighted structural substrates made from syntactic foams (both inorganic and organic) will be discussed.

Transparent polycrystalline yttrium aluminum garnet (YAG) with a fine grain size (1∼2 μm) was produced via a sinter-HIP process using highly reactive nano-sized powders that were synthesized by partial and fall combustionprocesses at 200° - 240°C. The primary particle size was estimated to be between 30 - 50 nm, and the powders appeared to be weakly agglomerated. The densities after sintering at 1550° - 1650°C were 95.0 - 99.0 % of the theoretical density and the samples appeared translucent. Further HIPing at temperatures between 1500° and 1550°C resulted in completely dense polycrystalline YAG that showed a visibly higher transparency that was comparable to a single crystal YAG. The details of microstructural characterization (SEM), phase identification (XRD), and visible and IR transmittance (Spectrophotometer) are presented.

The present paper reports an on-line recognition method of cursive Korean characters. In the present method, we treat a Korean character pattern as a finite sequence of basic character patterns. After extracting candidate basic character patterns from an input character pattern, we determine basic character patterns making a Korean character from the candidates by connecting processing. We described basic character patterns and their connected patterns used in the present method according to their features. By extracting and connecting basic character patterns based on the descriptions, we improve description ability of patterns and processing speed. Precise description ability of patterns and extracting candidates can remove unstable writing movements and can separate strokes stably.