The Role of Silion Oxide Layers in Luminescence of Ensembles of Silicon Quantum Dots

Based on the quantum confinement-luminescence center model, to ensembles of spherical silicon nanocrystals (nc-Si) containing two kinds of luminescence centers (LCs) in the layers surrounding the nc-Si, the relationship between the photoluminescence (PL) and the thickness of the layer is studied with the excitation energy flux density as a parameter. When there is no layer surrounding the nc-Si, the electron-heavy hole pair can only recombine inside the nc-Si, then the PL blueshift with reducing particle sizes roughly accords with the rule predicted by the quantum confinement model of Canham. When there presences a layer, some of the carriers may tunnel into it and recombine outside the nc-Si at the LCs to emit visible light. The thicker the layer is, the higher the radiative recombination rate occurred outside the nc-Si will be. When the central scale of the nc-Si is much smaller than the critical scale, the radiative recombination rate outside the nc-Si dominates, and visible PL will be possible for some nc-Si samples with big average radius, greater than 4 nm, for example. When there is only one kind of LC in the layer, the PL peak position does not shift with reducing particle sizes. All these conclusions are in accord with the experimental results. When there are two or more kinds of LCs in the layer, the PL peak position energy and intensity swing with reducing particle sizes.

激光选区熔化成形316L循环使用粉末特性演变机理研究

在激光选区熔化技术中,激光-粉末的相互作用会对粉末特性产生重大影响,而在粉末循环使用过程中粉末特性的变化规律和演变机理尚不明确。本文利用激光粒度仪、扫描电子显微镜、能量色散型光谱仪研究了激光选区熔化粉末的粒径、物理特性、表面形貌、元素含量、微观组织在循环使用过程中的变化规律。研究结果表明:随着316L不锈钢粉末循环使用次数的增加,粉末的粒径分布、形貌、表面成分、表面微观组织和氧化程度都发生了较大变化;粉末的堆积特性如松装密度、振实密度、流动性也发生了不同程度的变化;另外,循环使用的粉末颗粒表面生成了富含硅、锰元素的圆形氧化斑点。本文将循环粉末中的异形颗粒分为两类——激光诱导熔池溅射颗粒和气体夹带诱导异形颗粒,并详细讨论了两类异形颗粒的形成机理。本文研究结果表明316L奥氏体不锈钢在循环使用过程中会产生弱磁性粉末颗粒。