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Observation of Atomic Dynamic Behaviors in the Evaporative Cooling by In-Situ Imaging the Plugged Hole of Ultracold Atoms
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We experimentally observe the dynamic evolution of atoms in the evaporative cooling, by in-situ imaging the plugged hole of ultracold atoms. Ultracold rubidium atoms confined in a magnetic trap are plugged using a blue-detuned laser beam with a waist of 20 m at a wavelength of 767 nm. We probe the variation of the atomic temperature and width versus the radio frequency in the evaporative cooling. Both the behaviors are in good agreement with the calculation of the trapping potential dressed by the rf signal above the threshold temperature,while deviating from the calculation near the phase transition. To accurately obtain the atomic width, we use the plugged hole as the reference to optimize the optical imaging system by precisely minimizing the artificial structures due to the defocus effect. 相似文献
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采用化学还原的方法成功地合成了Nafion聚离子修饰的纳米Pt颗粒,平均粒径为~4nm;由于表面缺电子的特性,纳米Pt粒子与Nafion高分子长链上的-SO3^-基团有较强的结合,使粒子随-SO3^的存在状态而分散,在水溶液中呈现与离子团簇相似的分散状态。FTIR显示Pt颗粒存在表面原子暴露在修饰离子之外,因此,这种粒子在具有功能结构的催化领域有着很好的应用前景;合成过程中反应速率随Nafion含量的降低、pH的增加而增大。 相似文献
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开发高效非贵金属氧还原反应(ORR)催化剂是降低质子交换膜燃料电池(PEMFC)成本,实现其大规模商业化应用的关键.目前,过渡金属-氮-碳(TM-N-C)被认为是最有希望替代Pt的非贵金属催化剂.然而,尽管在半池测试中展现出较好的ORR性能,但当将其组装到PEMFC核心部件膜电极(MEA)中时,其单池性能远低于Pt/C催化剂.因此,未来需要进一步优化TM-N-C在全电池环境中的性能.本文提出了一种原位策略,成功构筑了TM-N-C三维有序一体化ORR电极.通过结合碳纸亲水处理和化学气相沉积技术,在含氧官能团修饰的碳纸(OCP)上原位构建了Co,N共掺杂碳纳米管(N-CNTs@Co)自支撑三维有序一体化电极.该独特的三维有序网络结构不仅使反应物(H2,O2和H2O)和质子(H+和e-)的传输通道处于有序状态,降低了实际工况条件下的浓差极化,还避免了催化层传统制备过程(如涂覆、喷涂和流延法)引起的催化剂活性位点团聚或包埋,从而提高了催化剂的利用率.X射线光电子能谱分析表明,优化后的一体化电极试样(N-CNTs-20@Co/OCP,其中20代表CNT生长时间为20 min)具有最高的吡啶N和石墨化N含量,而吡啶N和石墨化N被普遍认为是N掺杂碳材料在电催化ORR中的活性位点.因此,该N-CNTs-20@Co/OCP一体化电极在酸性(0.1 mol L-1 HC1O4)和碱性(0.1 mol L-1 KOH)介质中均展现出与商用Pt/C(20 wt%)喷涂在CP(0.2 mgPt cm-2)上制备的传统电极相当的ORR性能.密度泛函理论计算进一步揭示了其性能提升的机制:Co纳米颗粒被封装在碳纳米管内部并作为电子供体,通过电子隧穿效应,强化了 N-CNTs@Co/OCP催化剂表面对氧的吸附,进而提高了电催化ORR性能.此外,封装在碳纳米管内部的Co纳米颗粒避免了与电解液的直接接触,从而显著提高了催化剂的稳定性.综上,本文不仅为基于非贵金属ORR催化剂三维有序一体化电极的构筑提供了有益的理论储备和实验技术积累,而且对于降低燃料电池成本以及推动燃料电池产业化进程提供了 一定的参考. 相似文献
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设$mathcal{A}$是一个包含非平凡投影的单位素*-代数.本文证明了一个映射$Phi:mathcal{A}rightarrowmathcal{A}$满足对任意$A,B,Cinmathcal{A}$有$Phi([A,B]_{diamond}circC)=[Phi(A),B]_{diamond}circC+[A,Phi(B)]_{diamond}circC+[A,B]_{diamond}circPhi(C)$当且仅当$Phi$是一个可加的*-导子, 其中$Acirc B=A^{*}B+B^{*}A$和$[A,B]_{diamond}=A^{*}B-B^{*}A$. 相似文献
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Cascaded nonlinear phase shifts may be imposed on the interacting waves during second-harmonic generation (SHG) in a quasi-Phase-matched (QPM) structure, whieh are severe in high-intensity regime and may result in lower conversion efficiency. We propose a configuration of QPM structure with reduced domain-length, which may depress the nonlinear phase shifts to some extent and lead to an improvement on the conversion efficiency. The numerical analyses on the conversion efficiency as well as the relative phase angle are discussed in detail for better understanding of the SHG process. 相似文献
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