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251.
Dynamically tunable terahertz passband filter based on metamaterials integrated with a graphene middle layer
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The dynamic tunability of a terahertz(THz) passband filter was realized by changing the Fermi energy(E_F) of graphene based on the sandwiched structure of metal-graphene-metal metamaterials(MGMs). By using plane wave simulation, we demonstrated that the central frequency( f_0) of the proposed filter can shift from 5.04 THz to 5.71 THz; this shift is accompanied by a 3 dB bandwidth(? f) decrease from 1.82 THz to 0.01 THz as the EFincreases from 0 to 0.75 eV.Additionally, in order to select a suitable control equation for the proposed filter, the curves of ? f and f_0 under different graphene EFwere fitted using five different mathematical models. The fitting results demonstrate that the Dose Resp model offers accurate predictions of the change in the 3 dB bandwidth, and the Quartic model can successfully describe the variation in the center frequency of the proposed filter. Moreover, the electric field and current density analyses show that the dynamic tuning property of the proposed filter is mainly caused by the competition of two coupling effects at different graphene EF, i.e., graphene-polyimide coupling and graphene-metal coupling. This study shows that the proposed structures are promising for realizing dynamically tunable filters in innovative THz communication systems. 相似文献
252.
针对轮式机器人轨迹跟踪控制系统误差收敛速率低、精度和实时性差的问题,采用反演控制算法并结合李雅普诺夫稳定性分析方法对轮式机器人的轨迹跟踪系统进行了优化设计。建立了轮式机器人轨迹跟踪控制系统的运动学模型,并对该模型进行位置偏差分析;在反演控制算法中引入了分部虚拟控制量,并分析和设计了其他间接受控量,提高了算法运行的效率;采用李雅普诺夫收敛定理对系统的收敛性进行分析,根据分析的结果提出了算法更加简单的控制律。利用Matlab软件的Simulink库对设计的轨迹跟踪控制系统试验研究。结果表明,与基于李雅普诺夫直接法或者迭代学习算法设计的轮式机器人轨迹跟踪控制系统相比较,设计的控制系统具有跟踪精度高、收敛速度快、实时性好的优点。 相似文献
253.
The settling velocity of equiaxed dendrites can cause macrosegregation and influence the structure of the equiaxed zone during the casting solidification process. So an understanding of the settling characteristics is needed to predict the structure and segregation in castings. The settling velocity of NH4Cl equiaxed dendrites of non-spherical geometry was studied experimentally in an NH4Cl-70wt.%H2O solution. A calculation formula was proposed to calculate the settling velocity of sediment equiaxed dendrites in a tube filled with saturated solution at a moderate Reynolds number region. The retardation effects of the wall and morphology of the equiaxed dendrite on the settling velocity were taken into account in the development of the calculation formula, and the correction function B of the drag coefficient with consideration of the retardation effects of the wall and morphology of the equiaxed dendrite on the settling velocity of the equiaxed dendrite was calibrated according to the experimental results. A comparison showed that the formula has a good accordance with the experimental results. 相似文献
254.
The effect of two-dimensional shear flow on the stability of a crystal interface in the supercooled melt
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A model is developed based on the time-related thermal diffusion equations to investigate the effects of two-dimensional shear flow on the stability of a crystal interface in the supercooled melt of pure substance. Similar to the three-dimensional shear flow as described in our previous paper, the two-dimensional shear flow can also be found to reduce the growth rate of perturbation amplitude. However, compared with the case of Laplace equation for steady state thermal diffusion field, due to the existence of time partial derivatives of the temperature fields in diffusion equation the absolute value of the gradients of the temperature fields increases, therefore destabilizing the interface. The circular interface is more unstable than in the case of Laplace equation without time partial derivatives. The critical stability radius of the crystal interface increases with shearing rate increasing. The stability effect of shear flow decreases remarkably with the increase of melt undercooling. 相似文献
255.
分别以邻、间、对甲基苯甲酸(o-,m-,p-MA)为第一配体、邻菲罗啉(o-Phen)为第二配体,利用沉淀法合成了一系列稀土铽的三元荧光配合物。对该系列稀土铽荧光配合物的合成条件(配体种类、配体用量、体系pH、反应温度、反应时间)进行了优化。将最优条件下合成的[Tb(p-MA)_3(o-Phen)]荧光配合物粉末应用到常见光滑客体表面潜在手印的显现技术中。从显现的对比度、灵敏度、选择性等方面对手印显现效果进行了详细考察。结果表明,显现后的手印在254nm紫外光的激发下,手印与客体之间的对比反差强烈,乳突纹线连贯清晰、细节特征反映明显,粉末只与乳突纹线部位吸附、基本不与小犁沟部位吸附。因此,本研究中基于铽荧光配合物的手印显现具有较强的对比度、灵敏度、选择性。 相似文献
256.
Absolute stability of the solidification interface in a laser resolidified Zn--2wt.%Cu hypoperitectic alloy
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This paper reports on laser surface remelting experiments performed on a
Zn--2wt.%}Cu hypoperitectic alloy by employing a 5kW CW CO2 laser at
scanning velocities between 6 and 1207mm/s. The growth velocities of the
microstructures in the laser molten pool were accurately measured. The planar
interface structure caused by the high velocity absolute stability
was achieved at a growth velocity of 210~mm/s. An implicit expression
of the critical solidification velocity for the cellular--planar transition was
carried out by nonlinear stability analyses of the planar interface. The results
showed a better agreement with the measured critical velocity than that predicted by
M--S theory. Cell-free structures were observed throughout the whole molten pool at
a scanning velocity of 652~mm/s and the calculated minimum temperature gradient in
this molten pool was very close to the critical temperature gradient for high
gradient absolute stability (HGAS) of the \eta phase. This indicates that HGAS
was successfully achieved in the present experiments. 相似文献
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将两种向列相液晶,4-氰基-4'-庚基联苯和4-氰基-4'-戊基联苯,引入到染料敏化太阳电池(DSC)用偏氟乙烯-六氟乙烯共聚物基准固态电解质中,以期提高DSC的光电转化效率.研究了液晶的引入对电解质中I-/I3-的氧化还原行为、DSC中TiO2/电解质界面的暗反应以及DSC光伏性能的影响.结果表明,尽管液晶的引入会降低电解质中I-/I3-的传输并且使得DSC中暗反应加快,但是DSC的短路电流密度却显著地提高,使DSC的光电转化效率增加.这可能是由于液晶的引入改善了DSC中的界面接触以及增加染料的光吸收引起的. 相似文献