Contact pressure of porous Al2O3 probed by nanoindentations

Contact pressure of porous Al2O3 probed by nanoindentation was investigated by dimensional analysis with special attention paid to scaling effects in the mechanical behavior. It was found that, for sample containing small grains and interconnected pores, the contact pressure is manifest dominated by bonding strength of the porous alumina. Whereas the samples with coarse grain and various porous structures exhibit higher contact pressures and smaller residual deformations, which can be attributed to the mechanical response of the solid-phase under current limited peak loads.

The response of porous Al2O3 probed to nanoindentation

The response of porous Al2O3 to nanoindentation was investigated at microscopic scales (nm-mu m) and under ultra-low loads from 5 to 90 mN with special attention paid to the dependence of the load-depth behaviour to sample porosity. It was found that the load-depth curves manifest local responses typical of the various porous structures investigated. This is particularly clear for the residual deformation after load removal. Similarly, the limited mean pressure of the sample containing small grains and interconnected pores is consistent with its porous structure. By comparison, the samples with larger grain size and various porous structures exhibit higher pressures and smaller residual deformations that can be attributed to the mechanical response of the solid phase. (C) 2007 Elsevier B.V. All rights reserved.

Comparisons of static, quasi-static and dynamic 3D porous media scale network models for two-phase immiscible flow in porous media

A dynamic 3D pore-scale network model is formulated for investigating the effect of interfacial tension and oil-water viscosity during chemical flooding. The model takes into account both viscous and capillary forces in analyzing the impact of chemical properties on flow behavior or displacement configuration, while the static model with conventional invasion percolation algorithm incorporates the capillary pressure only. From comparisons of simulation results from these models. it indicates that the static pore scale network model can be used successfully when the capillary number is low. With the capillary increases due to the enhancement of water viscosity or decrease of interfacial tension, only the quasi-static and dynamic model can give insight into the displacement mechanisms.

A CE/SE Scheme for Flows in Porous Media and Its Applications

In this paper, a new computational scheme for solving flows in porous media was proposed. The scheme was based on an improved CE/SE method (the space-time Conservation Element and Solution Element method). We described porous flows by adopting DFB (Brinkman-Forchheimer extended Darcy) equation. The comparison between our computational results and Ghia's confirmed the high accuracy, resolution, and efficiency of our CE/SE scheme. The proposed first-order CE/SE scheme is a new reliable way for numerical simulations of flows in porous media. After investigation of effects of Darcy number on porous flow, it shows that Darcy number has dominant influence on porous flow for the Reynolds number and porosity considered.

Instabilities of a liquid film flowing down an inclined porous plane

The problem of a film flowing down an inclined porous layer is considered. The fully developed basic flow is driven by gravitation. A careful linear instability analysis is carried out. We use Darcy's law to describe the porous layer and solve the coupling equations of the fluid and the porous medium rather than the decoupled equations of the one-sided model used in previous works. The eigenvalue problem is solved by means of a Chebyshev collocation method. We compare the instability of the two-sided model with the results of the one-sided model. The result reveals a porous mode instability which is completely neglected in previous works. For a falling film on an inclined porous plane there are three instability modes, i.e., the surface mode, the shear mode, and the porous mode. We also study the influences of the depth ratio d, the Darcy number delta, and the Beavers-Joseph coefficient alpha(BJ) on the instability of the system.

Influences Of Precursor Constitution And Processing Speed On Microstructure And Wear Behavior During Laser Clad Composite Coatings On Gamma-Tial Intermetallic Alloy

The effects of constitution of precursor mixed powders and scan speed on microstructure and wear properties were designed and investigated during laser clad gamma/Cr7C3/TiC composite coatings on gamma-TiAl intermetallic alloy substrates with NiCr-Cr3C2 precursor mixed powders. The results indicate that both the constitution of the precursor mixed powders and the beam scan rate have remarkable influence on microstructure and attendant hardness as well as wear resistance of the formed composite coatings. The wear mechanisms of the original TiAl alloy and laser clad composite coatings were investigated. The composite coating with an optimum compromise between constitution of NiCr-Cr3C2 precursor mixed powders as well as being processed under moderate scan speed exhibits the best wear resistance under dry sliding wear test conditions. (C) 2008 Elsevier Ltd. All rights reserved.

溶胶-凝胶多孔二氧化硅减反膜稳定性研究

多孔SiO2膜层经热处理后，具有很高的激光破坏阈值，但是结构中有许多Si-OH亲水基团，导致光学透过率受环境相对湿度的影响很大。实验目的是改善膜层内部结构，使膜层结构中的亲水基团转变为疏水基团。提高膜层的疏水性，增强膜层的透过率稳定性。系统地研究了膜层透过率随时间变化的规律，在氨气和六甲基二硅氮烷(HMDS)混合气氛下热处理膜层，处理后生成Si-O-Si(CH2)3非极性疏水基团，使膜层的疏水性大大提高，因而膜层的透过率稳定性有大幅度提高。稳定性的提高延长了膜层的寿命。处理后膜层的表面粗糙度良好，均方根表

掺入有机硅提高溶胶-凝胶二氧化硅减反膜的稳定性研究

膜层稳定性对于激光器能否长期稳定使用极为重要。多孔SiO。减反膜经热处理后，结构中还存在许多Si—OH亲水基团，透过率稳定性受环境相对湿度的影响较大。向膜层中掺入有机硅，添加疏水基团，提高了膜层的疏水性，增强了膜层的透过率稳定性。膜层中加了Si-CH3疏水基团，膜层的疏水性大大提高；当Si-CH3与二氧化硅悬胶体中的Si的摩尔比为1／5．7时，即Si—CH3质量分数为0．35％时的二氧化硅膜层，其减反效果好，疏水性也高，从而大幅度提高了膜层的透过率稳定性，延长了膜层的寿命，对二氧化硅膜层具有高激光损伤阈值

甲基化对多孔SiO_2增透薄膜光学稳定性能的影响

分别通过引入甲基三乙氧基硅烷(MTES)、二甲基二乙氧基硅烷(DMDES)以及六甲基二硅氮烷(HMDS)组分对SiO2悬胶体进行改性,得到不同甲基化的多孔SiO2改性薄膜。研究了改性薄膜的光学稳定性,抗激光破坏性能以及机械抗擦性能。结果表明,HMDS改性薄膜的光学稳定性最好而机械抗擦性较弱,MTES与DMDES改性薄膜的光学稳定性较低而机械抗擦性良好且均与改性组分的含量有关,甲基化改性薄膜的抗激光破坏性能比未改性薄膜的有所降低。

Blue green emission from a Cu+-Doped transparent material prepared by sintering porous glass

A colorless transparent, blue green emission material was fabricated by sintering porous glass impregnated with copper ions. The emission spectral profile obtained from Cu+ -doped high silica glass (HSG) by 267-mn monochromatic light excitation matches that obtained by pumping with an 800-nm femtosecond laser, indicating that the emissions in both cases come from an identical origin. The upconversion emission excited by 800-nm femtosecond laser is considered to be a three-photon excitation process. A tentative scheme of upconverted emission from Cu+ -doped HSG was also proposed. The glass materials presented herein are expected to find application in lamps, high density optical storage, and three-dimensional color displays.

Spectroscopic properties of Nd3+-doped high silica glass prepared by sintering porous glass

A new kind of Nd3+, -doped high silica glass (SiO2 > 96% (mass fraction)) was obtained by sintering porous glass impregnated with Nd3+, ions. The absorption and luminescence properties of high silica glass doped with different Nd3+, concentrations were studied. The intensity parameters Omega(t) (t = 2, 4, 6), spontaneous emission probability, fluorescence lifetime, radiative quantum efficiency, fluorescence branching ratio, and stimulated emission cross section were calculated using the Judd-Ofelt theory. The optimal Nd3+ concentration in high silica glass was 0.27% (mole fraction) because of its high quantum efficiency and emission intensity. By comparing the spectroscopic parameters with other Nd3+ doped oxide glasses and commercial silicate glasses, the Nd3+-doped high silica glasses are likely to be a promising material used for high power and high repetition rate lasers.

Role of interface absorption in laser-induced local heating of optical coatings

Considering the interface absorption in optical coatings, we propose a model to simulate interface absorption. Calculations are made and the temperature field of several kinds of thin film multilayers, including those of partial reflectivity, high-reflectivity, and antireflectivity coatings are analyzed. The interface absorption is found to greatly influence the temperature distribution within multilayer coatings and to weaken the laser damage resistance of the samples. The real-time results of the photothermal deflection technique for laser induced damage to samples supports the model. (C) 1997 Society of Photo-Optical Instrumentation Engineers.

Laser-induced damage threshold of the antireflection coatings on quartz and sapphire under different laser modes

The laser-induced damage threshold (LIDT) and damage morphology of antireflection (AR) coatings on quartz and sapphire are investigated. A very interesting phenomena is found in the measurement. In the case of a single pulse laser, the LIDT of the AIR coatings on quartz is higher than that of sapphire. On the contrary, for a free-pulse laser, the LIDT of AIR coatings on sapphire is higher than that of quartz. (C) 2004 Society of Photo-Optical Instrumentation Engineers.