Large-angle channeling radiation from relativistic electrons in optically transparent crystals

In the work within the frame of quantum electrodynamics are obtained new formulae describing the large-angle photon emission from channeled electrons with taking into account of the dispersion of refractive index. Calculations based on these formulae show that the spectral and angular distributions of large-angle optical and ultraviolet radiation from planar channeled sub-GeV electrons in optically transparent crystal reflect the band structure of transverse energy levels of channeled electrons. Comparison with ordinary Cherenkov radiation spectrum reveals that channeling (depending on the beam energy) leads to sufficient change of the large-angle emission spectrum.     

Thermal radiation effects on Williamson fluid flow due to an expanding/contracting cylinder with nanomaterials: Dual solutions

The phenomena of heat and mass transfer during the flow of non-Newtonian transfer are amongst the core subjects in mechanical sciences. Recently, the nanomaterials are among the eminent tools for improving the low thermal conductivity of working fluids. Therefore, in view of the existing contributions, this article presents a two-dimensional numerical simulation for the transient flow of a non-Newtonian nanofluid generated by an expanding/contracting circular cylinder. This critical review further explores the impacts of variable magnetic field, thermal radiation, velocity slip and convective boundary conditions. The basic governing equations for Williamson fluid flow are formulated with the assistance of boundary layer approximations. The non-dimensional form of partially coupled ordinary differential equations has been tackled numerically by utilizing versatile Runge–Kutta integration scheme. The momentum, thermal and concentration characteristics are investigated with respect to several critical parameters, like, Weissenberg number, unsteadiness parameter, viscosity ratio parameter, slip parameter, suction parameter, magnetic parameter, thermophoresis parameter, Brownian motion parameter, Prandtl number, Lewis number and Biot number. The outcomes of the systematic reviews of these parameters and forest plots are illustrated. The study reveals that multiple solutions for the considered problem occurs for diverse values of involved physical parameters. The computed results indicate that the friction and heat transfer coefficients are significantly raised by the magnetic parameter for upper branch solutions.     

求解水中加肋板声辐射特性的虚拟声源法*

发展一种利用虚拟声源离散声场的方法求解加肋板在水中的声振耦合问题。由波叠加原理和单元体积速度匹配的原则,根据离散的结构单元满足的动力方程和结构与介质的交界相容性条件,确定虚拟声源强度,计算结构的声辐射功率。本文以简支矩形加肋板为例,在不获得结构表面振速和声压的情况下,计算了结构在水中的声辐射功率,并与解析方法计算的结果进行了比较,表明了该方法具有较好的计算精度。     

用于开口声辐射控制的虚拟声屏障实现 方法及机理研究*

建筑物通常留有开口以便人员物料的进出及室内的自然通风采光,但这些开口也是噪声传播的途径。传统被动噪声控制方法需要将开口封闭,且对低频噪声的控制效果不好,故引入有源噪声控制技术降低室内声源通过开口的声辐射。基于惠更斯原理,均布开口的次级源和误差传声器构成的平面型虚拟声屏障可以实现对开口声辐射的有效控制,数值仿真和实验已证明其有效性。将次级源安装在开口边界更有利于保留开口的功能且方便实际安装,但这样的单层边界虚拟声屏障降噪效果存在上限,仅能在低频段实现全局控制。和单层边界次级源相比,双层边界次级源可显著提高降噪量和有效降噪频率上限。该文回顾了开口声辐射有源控制的相关工作,并讨论了未来可能的研究方向。     

Structural changes in Poly(lactic acid)–zeolite nanocomposites exposed to 60Co gamma rays

Poly(lactic acid) (PLA) nanocomposites containing 3%, 5% and 7% zeolite used in the present research study were prepared by solution casting. The PLA nanocomposites were structurally characterized by Scanning Electron Microscopy and Fourier-transform infrared spectroscopy. The effects of the gamma radiation at the absorbed doses of 10, 15 and 20?kGy on the structures and thermal properties of the nanocomposites were investigated in a tetrahydrofuran solvent. Thermodecomposition tests and analyses were carried within 30–500°C range at the rate of 10°C?min^?1 by thermogravimetric analysis (TG). The activation energies for thermal degradation of the nanocomposites were calculated from their TG data by Flynn–Wall–Ozawa method. The G values of the nanocomposites were calculated based on molecular weights which were measured by means of gel permeation chromatography before and after the gamma irradiation. The E_a and molecular weight results showed that addition of zeolite to the PLA affected the radiation resistance of the polymer.     

Bifurcation and extinction limit of stretched premixed flames with chain-branching intermediate kinetics and radiative loss

Premixed counterflow flames with thermally sensitive intermediate kinetics and radiation heat loss are analysed within the framework of large activation energy. Unlike previous studies considering one-step global reaction, two-step chemistry consisting of a chain branching reaction and a recombination reaction is considered here. The correlation between the flame front location and stretch rate is derived. Based on this correlation, the extinction limit and bifurcation characteristics of the strained premixed flame are studied, and the effects of fuel and radical Lewis numbers as well as radiation heat loss are examined. Different flame regimes and their extinction characteristics can be predicted by the present theory. It is found that fuel Lewis number affects the flame bifurcation qualitatively and quantitatively, whereas radical Lewis number only has a quantitative influence. Stretch rates at the stretch and radiation extinction limits respectively decrease and increase with fuel Lewis number before the flammability limit is reached, while the radical Lewis number shows the opposite tendency. In addition, the relation between the standard flammability limit and the limit derived from the strained near stagnation flame is affected by the fuel Lewis number, but not by the radical Lewis number. Meanwhile, the flammability limit increases with decreased fuel Lewis number, but with increased radical Lewis number. Radical behaviours at flame front corresponding to flame bifurcation and extinction are also analysed in this work. It is shown that radical concentration at the flame front, under extinction stretch rate condition, increases with radical Lewis number but decreases with fuel Lewis number. It decreases with increased radiation loss.     

A black hole with quantum core

A model black hole, holding a ‘quantum core’ characterized by the Planck order matter density, is revisited here. Based on the quantum improved Newton’s potential drawn out of the loop quantum cosmology we propose a Schwarzschild class, quantum improved black hole line-element that upholds the existence of Planck-dense quantum matter core. Causality is kept preserved in this proposal. Quite in a natural way the quantum core emerges closely homogeneous in its interior matter distribution. The radius of the quantum core turns out to be necessarily proportional to one-third power of the black hole mass. Hawking process of black hole evaporation leads to a shrinking quantum core, and as the mass of black hole approaches near about the Planck mass, the rate of evaporation diminishes rapidly and eventually leaves a cold remnant having a Planck order mass. Proposed model supports the standard quantum geometrical logarithmic correction to black hole entropy-area law.     

锂离子对稀土掺杂氧化物的红外激光诱导热辐射的影响

由于具有较高的可见波段荧光效率,基于稀土掺杂氧化物的红外激光诱导热辐射具有重大的应用潜力。进一步提高红外诱导热辐射效率具有重大的实用价值。设计了一种改善稀土掺杂氧化物材料中红外诱导热辐射效率的方案,即通过掺杂改性杂质,既可以改变稀土离子周围局域晶场的对称性;同时又引入了晶格缺陷。对应的效果包括：一方面,可以通过增强稀土离子周围的晶场强度来提高稀土元素对于入射光子的吸收能力;另一方面,可以利用晶格缺陷作为猝灭中心来增加材料的热转化能力,最终将显著提高杂质改性材料的光热转化效率,即获得更加高效的红外诱导热辐射材料。为了验证设计方案的可行性,利用溶胶-凝胶方法合成了不同浓度的镱离子和锂离子的共掺杂样品,通过XRD及TEM测试分析了杂质对样品结构的影响,并且基于荧光发射光谱具体研究了杂质掺杂浓度对热辐射效率的影响。该工作为高效稀土掺杂热辐射材料的制备提供一定的参考。     

基于分层远程荧光膜白光LED的制备及光谱性能研究

基于荧光粉分层和远程荧光封装技术,采用热压法制备出双层远程荧光膜,并封装出白光LED。通过荧光分光光度计和可见光光谱分析系统研究了绿色和红色远程荧光膜不同分层顺序及不同发射波长对于白光LED光谱性能的影响。研究发现：蓝-绿-红(B-G-R)膜层封装形式相较于蓝-红-绿(B-R-G)辐射发光效率提高了31.69%,色保真度和色域指数均随着红色远程荧光膜波长的增加而升高,发射波长为660 nm时制备的白光LED色保真度最高值达到91,色域指数最高值达到104,辐射发光效率值则与波长成反比关系;色保真度随着绿色远程荧光膜波长的增加逐渐降低,色域指数则先降低后升高,发射波长为530 nm制备的白光LED具有最高的辐射发光效率,达到300.7 lm·W-1。研究所得出的相关结论对于实际的应用具有一定的参考意义。     

CCD与CMOS像素传感器γ射线电离辐射响应特性对比研究

为了研究CCD与CMOS像素传感器对γ射线电离辐射的响应差异,对比研究了4类像素传感器的结构特点和辐射响应特性。通过辐射实验,对各传感器在不同辐射水平条件下的光子响应事件分布、平均灰度值以及典型光子响应事件进行研究。研究结果表明：光子响应程度均与辐射剂量率相关;CCD像素传感器的沟道传输方式使每列像元间的辐射响应更容易相互干扰;平均灰度值随剂量率的增大存在明显的梯度,各积分周期内输出信号灰度值围绕均值上下波动,CCD像素传感器输出信号灰度浮动范围较小;CMOS像素传感器各像元对光子的响应更加明显,CCD像素传感器各像元的响应信号易与相邻像元发生串扰;各类像素传感器典型辐射响应事件区域中发生光子响应的像元数量随剂量率的增大而增多,响应事件并非单个光子的行为,而是反映了多个光子在区域内同时沉积能量的过程。本研究为开发像素传感器的γ射线辐射探测技术和应用提供了重要的理论分析和实验数据支撑。