Modeling of the Metal Powder Flow with Carrier Gas in Coaxial Nozzle for Direct Laser Deposition Process

In direct laser deposition process, metal powder is directly fed with carrier gas through the coaxial nozzle into the melt pool created by the laser to form the solid parts. Many operational parameters of the process depend on the characteristic of the powder stream structure below the exit of the coaxial nozzle. In this paper, a computational approach is developed for the simulation of the gas-particle flow in the coaxial nozzle. By taking into account the nozzle geometry and operating parameters, such as width and inclination angle of powder passage and carrier gas velocity, the developed computational code allows the simulation, optimization and control of the delivery of the metal powders.Mechanical Engineerin

Numerical and Experimental Analysis of the Powder Flow Streams in the Laser Aided Material Deposition Process

Axial powder stream concentration between the nozzle end and the deposition point is an important process parameter in the laser aided material deposition process. The powder concentration is greatly influenced by the nozzle geometry in use. This paper describes the numerical and experimental analysis of this important parameter in relation to the coaxial nozzle. The experiments are performed with the different nozzle geometries to generate various flow patterns of the gravity fed powder in a cold stream. The results of the experimental analysis are compared with the numerical simulation and found justified. These results are used in concluding the significance of important nozzle parameters for various powder concentration modes.Mechanical Engineerin

Patent Protection, Technological Change and Wage Inequality

We develop a directed-technological-change model to address the issue of the optimal patent system and investigate how the optimal patent system influences the direction of technological change and the inequality of wage, where patents are categorized as skill- and labor-complementary. The major results are: (i) Finite patent breadth maximizes the social welfare level; (ii) Optimal patent breadth increases with the amount of skilled (unskilled) workers; (iii) Optimal patent protection is skill-biased, because an increase in the amount of skilled workers increases the dynamic benefits of the protection for skill-complementary patents via the economy of scale of skill-complementary technology; (iv) Skill-biased patent protection skews inventions towards skills, thus increasing wage inequality; And, (v) international trade leads to strong protection for skill-complementary patents, hence increasing skill premia.Patent Breadth, Skill-Biased Patent Protection, Skill-Biased Technological Change, Wage Inequality, Growth

Directed Technological Change: A Knowledge-Based Model

We develop a knowledge-based growth model to address the issues of directed technological change, wage inequality and economic growth, in which skilled workers are used both in innovation and production. Since skill-biased technological change may lead to a decrease in the average productivity in R&D sectors, scale effect is removed. Free trade between developed countries increases the demand for skilled workers employed in the production of the skill-intensive good, thus promoting skill-biased technological change through the market size e¡èect and an increase in skill premia. In contrast, free trade between developed and developing countries reduces the profits of skill-complementary innovation, since its market is relatively small in the developing country. Thus, international trade may lead to skill-replacing technological change and decrease wage inequality in the developed country. Wage inequality, however, increases in the developing countries since the degree of skill bias of technology in the developing country in the open economy is greater than the one in autarky. Skill-biased technological change has opposite e¡èects on economic growth, therefore trade stimulates economic growth in some circumstances, and hurts it in other circumstances.Directed Technological Change, Wage Inequality, Scale Effect, Trade, Growth

Silicon-Wall Interfacial Free Energy via Thermodynamics Integration

We compute the interfacial free energy of a silicon system in contact with flat and structured walls by molecular dynamics simulation. The thermodynamics integration method, previously applied to Lennard-Jones potentials [R. Benjamin and J. Horbach, J. Chem. Phys. 137, 044707 (2012)], has been extended and implemented in Tersoff potentials with two-body and three-body interactions taken into consideration. The thermodynamic integration scheme includes two steps. In the first step, the bulk Tersoff system is reversibly transformed to a state where it interacts with a structureless flat wall, and in a second step, the flat structureless wall is reversibly transformed into an atomistic SiO2 wall. Interfacial energies for liquid silicon-wall interfaces and crystal silicon-wall interfaces have been calculated. The calculated interfacial energies have been employed to predict the nucleation mechanisms in a slab of liquid silicon confined by two walls and compared with MD simulation results

Direct Printing of Microstructures by Femtosecond Laser Excitation of Nanocrystals in Solution

Direct writing using single or multiple energized beams (e.g. laser, ion or electron beams) provides high feature resolution ( \u3c 1µm) compared with other solution-based printing methods (e.g. inkjet printing). There have been extensive researches on micro/nano additive manufacturing methods employing laser (or optical) and ion/electron beams. Many of these processes utilize specially designed photosensitive materials consisting of additives and effective components. Due to the presence of additive (such as polymer and binders), the effective components are relatively low resulting in high threshold for device operation. In order to direct print functional devices at low cost, there has been extensive research on laser processing of pre-synthesized nanomaterials for non-polymer functional device manufacturing. Pre-synthesized nanocrystals can have better control in the stoichiometry and crystallinity. In addition, pre-synthesis process enjoys the flexibility in material choice since a variety of materials can be synthesized. Femtosecond laser assembly and deposition of nanomaterials can be a feasible 3D micro/nano additive manufacturing approach, although mechanisms leading to assembly and deposition have not been fully understood. In this paper, we propose a mechanism for 2D and 3D deposition of nanocrystals by laser excitation with moderate peak intensities(1011-1012 W/cm2). It is postulated that laser induced charging is responsible for the deposition. The scheme paves the way for laser selective electrophoretic deposition as a micro/nanoscale additive manufacturing approach

Crystallization in Nano-Confinement Seeded by a Nanocrystal -- A Molecular Dynamics Study

Seeded crystallization and solidification in nanoscale confinement volumes have become an important and complex topic. Due to the complexity and limitations in observing nanoscale crystallization, computer simulation can provide valuable details for supporting and interpreting experimental observations. In this article, seeded crystallization from nano-confined liquid, as represented by the crystallization of a suspended gold nano-droplet seeded by a pre-existing gold nanocrystal seed, was investigated using molecular dynamics simulations in canonical (NVT) ensemble. We found that the crystallization temperature depends on nano-confinement volume, crystal orientation, and seed size as explained by classical two-sphere model and Gibbs-Thomson effect