AlMgB14 based composites have many potential applications in industry, but better understanding of the mechanisms responsible for high hardness and wear resistance is required for the implementation of this technology...
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ISBN:
(纸本)0976205726
AlMgB14 based composites have many potential applications in industry, but better understanding of the mechanisms responsible for high hardness and wear resistance is required for the implementation of this technology. This study was performed to determine if the results of electron probe microanalysis (EPMA) could be correlated with the observed variations in local hardness and toughness. The relationship between microstructure and hardness in AlMgB14 was investigated to improve the processing of this ultra-hard ceramic and its related composites. High-energy milling and subsequent hot-pressing of the powders produced dense composites with high hardness, but inconsistencies in powder processing and microstructure have led to variations in hardness. The morphology of the microstructure and distribution of O and Fe containing impurities were compared to the local hardness.
Directional solidification studies have been conducted using Pb-Bi peritectic alloys over a wide range of compositions, temperature gradients, and growth velocities to characterize the primary α- to primary β-phase ...
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Directional solidification studies have been conducted using Pb-Bi peritectic alloys over a wide range of compositions, temperature gradients, and growth velocities to characterize the primary α- to primary β-phase transitions, which have been observed at both very low and very high velocities. The critical conditions for these transitions correspond to the simultaneous growth of the α and β phases at or close to a single isotherm. The low velocity transition occurs under very specific conditions of composition, temperature gradient, and growth velocity. Since the transition conditions are composition dependent, they change continuously under terrestrial conditions where rejected solute is convectively mixed into the liquid. Detailed experimental studies have been carried out to examine the phase selection in the immediate vicinity of the critical velocity for the α to β transition, and the effect of convection on this transition is examined experimentally in the Pb-Bi system. The dynamic condition, at which both phases are present at the same isotherm, was shown to depend not only on velocities, temperature gradients, and bulk (nominal) alloy compositions, but also on the volume fractions of solid. A quantitative expression for the α- to β-phase transition condition was obtained by using the boundary layer model of fluid flow, which showed good agreement with the experimental results. It is shown that the transition occurs at the volume fraction where the bulk composition reaches the critical composition value predicted by the diffusive model. The modification in the microstructure map for the trailing planar or nonplanar β phase is discussed.
Existing anatomical models are mostly representative of three-dimensional (3D) standing individuals, based on either stylized surface equations (the so-called MIRD models) or tomographic images (the so-called voxel mo...
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Existing anatomical models are mostly representative of three-dimensional (3D) standing individuals, based on either stylized surface equations (the so-called MIRD models) or tomographic images (the so-called voxel models). This paper presents a method to develop a 4D anatomical model using the existing 3D VIP-Man model, the Non-Uniform Rational B-Splines (NURBS) method and clinically obtained respiratory motion data. A lesion in the left lung is simulated to study how the organ motions may affect dose to the lesion. The procedure to simulate the 4D VIP-Man models for Monte Carlo simulations using EGS code is described. This preliminary study demonstrates that the motion of the organs can be modeled in the Monte Carlo code for more accurate dose calculations. Although the study was based on the VIP-Man images, the method can be extended to patient specific models constructed, for example, from multi-slice CT images.
The optical absorption spectra of intersubband transitions in In 0.3Ga0.7As/GaAs multiple quantum dots (MQDs) grown by molecular beam epitaxy were investigated. By varying the number of In 0.3Ga0.7As monolayers deposi...
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Toughening of ultra-hard AlMgB14 was explored through the addition of second phase reinforcements. The high hardness of AlMgB14 combined with its low chemical reactivity with titanium and other metallic alloys makes t...
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ISBN:
(纸本)0976205726
Toughening of ultra-hard AlMgB14 was explored through the addition of second phase reinforcements. The high hardness of AlMgB14 combined with its low chemical reactivity with titanium and other metallic alloys makes these materials promising for high-speed machining, coatings, and other wear-resistant applications. However, actual performance to date has suffered as a result of low toughness. Initial machining and wear tests have had encouraging results and optimization of composition and microstructure of these composites is expected to result in additional performance benefits. High-energy milling was used to produce sub-micron powders that were consolidated into composites, some of which exhibited a maximum hardness of 46 GPa. Additions of TiB2 have increased hardness, toughness, and erosion resistance. Improvements in fracture toughness will expand the array of possible applications for this material.
Analysis of cell regulation requires methods for perturbing molecular processes within living cells with spatial discrimination on the nanometer-scale. We present a technique for ablating molecular structures in livin...
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Novel ZnO nanostructures such as hollow nanospheres, nano-cuges, nanoneedles, tetrupods, nanowires, aligned nanorods and nanotubes are synthesized via thermal evaporation of ZnO and graphite powder mixtures in reduced...
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With the rapid development of high-power white light-emitting diodes (LEDs), advances in packaging are required to further improve the device performance. In this work, an optimized packaging configuration for high po...
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ISBN:
(纸本)1424400848
With the rapid development of high-power white light-emitting diodes (LEDs), advances in packaging are required to further improve the device performance. In this work, an optimized packaging configuration for high power LED lamps with enhanced phosphorescence efficiency is presented based on ray-tracing simulations and experimental results. Figure 1 shows two phosphor arrangements, namely a "proximate phosphor" and a "remote phosphor" arrangement. The surface of the reflector cup is assumed to be either specular or diffuse. Three different encapsulant geometries, i.e., a "flat" (no cap), "convex" (spherical cap with height h = r/2), and "hemispherical" (h = r) top surface, are used in the simulations. Figure 2 shows phosphorescence efficiency with different packaging configurations obtained by ray-tracing simulation. The packaging configuration with remote phosphor, diffuse reflector cup and hemispherical encapsulation shows a 50% enhancement of phosphorescence efficiency compared to conventional packaging configuration. This improvement is attributed to reduced absorption of the phosphorescence by the LED chip and the reduction of deterministic optical modes trapped inside the encapsulant of remote-phosphor and diffuse reflector cup configuration. The angular dependence of the reflectivity for a diffuse Ag reflector and a specular Ag reflector is measured and shown in Figure 3. The roughened Ag reflector has a more than two orders of magnitude higher diffusely reflected power compared with the specular Ag reflector, which leads to the extraction of the trapped optical modes occurring in packaging structures with specular reflector cups. Dichromatic LED lamps comprising an ultraviolet GaN LED (λ = 400 nm) with a blue phosphor and a blue GaInN LED (λ = 470 nm) with a yellow phosphor are fabricated with different phosphor arrangements and reflector cups. The emission spectra of the reference primary LED emitters and of the dichromatic lamps operating at 20 mA are mea
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