PVC异型材原料粒径分布测试方法的评价

本文介绍了三种PVC异型材原料粒径分布的测试方法:沉降法(检测时间长,重现性差)、筛分法(操作复杂)、激光法(准确方便)。     

High power pulsed lasers

Pulsed high power lasers can deliver sufficient energy on inertial fusion time scales (0.1–10 ns) to heat and compress DT fuel to fusion reaction conditions. Several laser systems have been examined for application to the fusion problem. Examples are Ndglass, CO₂, KrF, and I₂, etc. A great deal of developmental effort has been applied to the Ndglass laser and the CO₂ gas laser systems. These systems now deliver >10⁴ kJ and >20×10¹² W to inertial fusion targets. The Nova Ndglass laser is being constructed to provide >200 kJ and >200×10¹² W of 1 m radiation for fusion experimentation in the mid-1980s. For inertial fusion target gain, >100 times the laser input, it is expected that the laser must deliver 3–5 MJ of energy on the 10–20 ns time scale. This paper reviews the developments in laser technology and outlines approaches to construction of a 3–5 MJ driver.     

Reexamination of slow relaxation of polymer chains in sol-gel transition

Using [2+2] photocycloaddition of coumarin groups attached to the PMMA chain backbone, we can gradually transfer a semidilute copolymer solution to a uniform speckle-free gel. We reexamined the effects of concentration, crosslinking density, solvent quality on the controversial scaling exponent (α_s) between the linewidth (Γ_s) of the slow mode and the scattering vector (q) during the sol-gel transition. Our results suggest that the slow mode is attributed to large thermally agitated density fluctuation and revealed that the variation of α_s is due to the relative length of 1/q to the correlation length (ξ) of the density fluctuation; namely, if ξ>1/q, α_s→3, while when ξ<1/q, α_s→2.     

Percussion Laser-Drilled Holes: Characteristics and Characterization Procedure

Percussion laser drilling, being a thermal process, produces holes having widely differing characteristics than that of the mechanically drilled holes. In the present study, on the percussion laser drilling of through holes in a nickel-based superalloy (SUPERNI 263A), 21 characteristics were identified, and the methods of their determination were proposed. The effect of peak power of laser pulses on the identified hole characteristics were studied.     

Development of theoretical process maps to study the role of powder preheating in laser cladding

Cladding is the process of depositing a superior built-up layer by fusion on a substrate. In blown powder laser cladding process, the powder travels across the laser path, gets heated up by absorbing laser energy, and finally melts on the substrate under the intense laser beam; as the substrate moves away this melt pool solidifies to form a continuous built-up layer. The laser energy is partly absorbed by the solid powder during its flight path (termed preheating) and partly by the top surface of the melt pool. In the present study a two-dimensional conduction heat transfer equation has been solved using finite-volume method to model the cladding process. It is observed that preheating allows higher scanning speed resulting in thin clad layer with low dilution. Preheating also permits high powder feed rate resulting in thick cladding with low dilution.     

Parametric studies of multi-material laser densification

Fabrication of multi-material components via a laser-assisted layer-by-layer fabrication process has been numerically simulated and analyzed using a three-dimensional thermo-mechanical model. Effects of the chamber preheating temperature, laser scanning rate, initial porosity and thickness of each powder layer on the out-of-plane warping and residual thermal stresses of a nickel/porcelain workpiece have been investigated. It is found that warping and residual thermal stresses of the laser-densified multi-material workpiece are more sensitive to the chamber preheating temperature and the thickness of each powder layer than to the laser scanning rate and the initial porosity of the powder layer. The major mechanism responsible for these phenomena is identified to be related to the change of the temperature gradient induced by these laser processing parameters.     

镍基熔覆涂层受冲击载荷作用下的性能研究

45钢及1Cr18Ni9Ti表面激光熔覆WFCL-11涂层，研究了激光熔覆涂层与基体界面及表面显微组织和硬度特点，分析在冲击载荷作用下，显微组织、结合性能、硬度变化等特点。发现了不同基材与熔覆材料对结合面性能的影响。     

Benefits of carbon addition on the hydrogen absorption properties of Mg-based thin films grown by Pulsed Laser Deposition

Mg-Ni thin films were grown using Pulsed Laser Deposition. In situ optical changes from shiny metallic to transparent states were observed for films deposited in vacuum and under an Ar/H₂ gas mixture (93/7%), respectively. Optical changes were also achieved by ex situ hydrogenation under hydrogen gas pressure of 15 bars at 200 °C. However, after ex situ hydrogenation, the optical transmittance of the Mg-based hydrogenated thin films did not exceed 25%. Such limitation was attributed to oxygen contamination, as deduced by High Resolution Transmission Electron Microscopy observations, showing the co-existence of both Mg-based and MgO phases for as-deposited films. A significant decrease in oxygen contamination was successfully achieved with the addition of carbon, leading to the preparation of (Mg-based)-C_x (x < 20%) thin films showing a faster and easier hydrogenation.     

散射法表面粗糙度测量的数学模型分析

对材料的表面粗糙度进行测量时,往往要求采用在线非接触方式。散射法能较好地满足该要求。本文以beckm ann的光散射模型为理论基础,分析建立了散射法表面粗糙度测量的数学模型,并进一步简化了该模型,为应用散射法测量提供了数学依据。