Browsing by Subject "Two-stage process"

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Open Access
Post-Treatment od Silicon Nanocrystals Produced by Ultra-Short Pulsed Laser Ablation in Liquid: Toward Blue Luminescent Nanocrystal Generation
(American Chemical Society, 2012-01-11) Alkis, S.; Okyay, Ali Kemal; Ortac, B.
Blue luminescent colloidal silicon nanocrystals (Si-NCs) were produced in a two-stage process. In the first step, synthesis of Si-NCs was achieved by femtosecond pulsed laser ablation of a silicon wafer, which was immersed in deionized water. The size and the structural and the chemical characteristics of colloidal Si-NCs were investigated by TEM and EDAX analyses, and it is found out that the Si-NCs are in spherical shape and the particle diameters are in the range of 5-100 nm. In the second step, ultrasonic waves and filtering chemical-free post-treatment of colloidal Si-NCs solution was performed to reduce the particle size. High-resolution TEM (HRTEM) studies on post-treated colloidal solution clearly show that small (1-5.5 nm in diameter) Si-NCs were successfully produced. Raman spectroscopy results clearly confirms the generation of Si nanoparticles in the crystalline nature, and the Raman scattering study of post-treated Si-NCs confirms the reduction of the particle size. The UV-vis absorption and photoluminescence (PL) spectroscopy studies elucidate the quantum confinement effect of Si-NCs on the optical properties. The colloidal Si-NCs and post-treated Si-NCs solutions present strong absorption edge shifts toward UV region. Broadband PL emission behavior is observed for the initial colloidal Si-NCs, and the PL spectrum of post-treated Si-NCs presents a blue-shifted broadband PL emission behavior due to the particle size reduction effect.
Open Access
Scalable image quality assessment with 2D mel-cepstrum and machine learning approach
(Elsevier, 2011-07-19) Narwaria, M.; Lin, W.; Çetin, A. Enis
Measurement of image quality is of fundamental importance to numerous image and video processing applications. Objective image quality assessment (IQA) is a two-stage process comprising of the following: (a) extraction of important information and discarding the redundant one, (b) pooling the detected features using appropriate weights. These two stages are not easy to tackle due to the complex nature of the human visual system (HVS). In this paper, we first investigate image features based on two-dimensional (2D) mel-cepstrum for the purpose of IQA. It is shown that these features are effective since they can represent the structural information, which is crucial for IQA. Moreover, they are also beneficial in a reduced-reference scenario where only partial reference image information is used for quality assessment. We address the second issue by exploiting machine learning. In our opinion, the well established methodology of machine learning/pattern recognition has not been adequately used for IQA so far; we believe that it will be an effective tool for feature pooling since the required weights/parameters can be determined in a more convincing way via training with the ground truth obtained according to subjective scores. This helps to overcome the limitations of the existing pooling methods, which tend to be over simplistic and lack theoretical justification. Therefore, we propose a new metric by formulating IQA as a pattern recognition problem. Extensive experiments conducted using six publicly available image databases (totally 3211 images with diverse distortions) and one video database (with 78 video sequences) demonstrate the effectiveness and efficiency of the proposed metric, in comparison with seven relevant existing metrics.
Open Access
Synthesis, characterization, and wear and friction properties of variably structured SiC/Si elements made from wood by molten Si impregnation
(2012) Dhiman, R.; Rana, K.; Bengu, E.; Morgen P.
We have synthesized pre-shaped SiC/Si ceramic material elements from charcoal (obtained from wood) by impregnation with molten silicon, which takes place in a two-stage process. In the first process, a porous structure of connected micro-crystals of β-SiC is formed, while, in the second process, molten Si totally or partly infiltrates the remaining open regions. This process forms a dense material with cubic (β-)SiC crystallites, of which the majority is imbedded in amorphous Si. The synthesis of preshaped "sprocket" elements demonstrates that desired shapes of such a dense SiC/Si composite ceramic material can be achieved, thus suggesting new industrial applications. The structure and composition of numerous as-synthesized samples were characterized in detail by using a wide range of techniques. Wear and friction properties were also investigated, with polished samples. The properties found for the present samples are very promising for abrasive applications and for new generation brake systems. © 2011 Elsevier Ltd.