999 resultados para Li, Jiang


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An analytic closed form for the second- order or fourth- order Markovian stochastic correlation of attosecond sum- frequency polarization beat ( ASPB) can be obtained in the extremely Doppler- broadened limit. The homodyne detected ASPB signal is shown to be particularly sensitive to the statistical properties of the Markovian stochastic light. fields with arbitrary bandwidth. The physical explanation for this is that the Gaussian- amplitude. field undergoes stronger intensity. fluctuations than a chaotic. field. On the other hand, the intensity ( amplitude). fluctuations of the Gaussian- amplitude. field or the chaotic. field are always much larger than the pure phase. fluctuations of the phase-diffusion field. The field correlation has weakly influence on the ASPB signal when the laser has narrow bandwidth. In contrast, when the laser has broadband linewidth, the ASPB signal shows resonant- nonresonant cross correlation, and the sensitivities of ASPB signal to three Markovian stochastic models increase as time delay is increased. A Doppler- free precision in the measurement of the energy- level sum can be achieved with an arbitrary bandwidth. The advantage of ASPB is that the ultrafast modulation period 900as can still be improved, because the energy- level interval between ground state and excited state can be widely separated.

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Based on the phase-conjugation polarization interference between two two-photon processes, we theoretically investigated the attosecond scale asymmetry sum-frequency polarization beat in four-level system (FASPB). The field correlation has weak influence on the FASPB signal when the laser has narrow bandwidth. Conversely, when the laser has broadband linewidth, the FASPB signal shows resonance-nonresonance cross correlation. The two-photon signal exhibits hybrid radiation-matter detuning terahertz; damping oscillation, i.e., when the laser frequency is off resonance from the two-photon transition, the signal exhibits damping oscillation and the profile of the two-photon self-correlation signal also exhibits zero time-delay asymmetry of the maxima. We have also investigated the asymmetry of attosecond polarization beat caused by the shift of the two-photon self-correlation zero time-delay phenomenon, in which the maxima of the two two-photon signals are shifted from zero time-delay point to opposite directions. As an attosecond ultrafast modulation process, FASPB can be intrinsically extended to any level-summation systems of two dipolar forbidden excited states.

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The dressed four- and six-wave mixings in a V-type four-level system are considered. Under two different dressed conditions, two- and three-photon resonant Autler-Townes splittings, accompanied by enhancement and suppression of wave mixing signal, are obtained analytically. Meanwhile, an electromagnetic induced transparency of multi-wave mixing is presented, which shows multiple peaks and asymmetric effects caused by one-photon, two-photon and three-photon resonances, separately. The slow light propagation multiple region of multi-wave mixing signal is also obtained.

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The near-threshold highly bound states of all three stable isotopic variants of molecular hydrogen have been studied. Numerous perturbations and unexpected transitions are observed as far as 1cm(-1) just below the second dissociation threshold. This complex structure may arise from a combination of nonadiabatic coupling between B, B', C electronic states, perturbations due to. ne and hyperfine interactions, and strong shape resonances. The perturbed near-threshold states and vibrational continuum exhibit finegrained structure, differing greatly between isotopes because of varying nonadiabatic coupling.

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Al-Li合金是近年发展起来的新型航空材料,具有低密度、高强度的特点。目前世界上正就如何进一步提高其断裂韧性,使Al-Li合金尽早走出实验室,获得实际用进行广泛,深入的研究。熔盐电解法就是在这种情况下发展起来的。虽然此法现在尚处于研究的初级阶段,但已以其能够在较简单的设备上制备低Na高纯Al-Li合金的特点受到广泛的重视,是一种很有前途的发展方向。针对我国的技术、设备现状,在参考国外研究结果的基础上,采用熔盐电解法制备Al-Li母合金 → 应用合金的制备方法是可以尽快赶上世界发步伐的有效途径。因此,本文作为整个熔盐电解制备Al-Li合金系统研究的一部分,针对目前在此领域中很多应用基础问题,诸如:作为新的电解体系正在探索中的LiCl-KCl-LiF三元相图,Li在液体Al阴极中的扩散系数,利用熔盐电解法制备低Na高纯Al-Li合金的热力学基础及如何克服LiCl强烈的吸水性给电解工艺带来的种种不便等均未得到系统研究的现状,设计完成了一系列有关熔盐电化学和热力学实验,填补了本领域的一些研究空白,并为进一步系统研究工艺条件提供了重要的参考数据。1.用NH_4Cl氯化Li_2CO_3的研究 根据热力学从理论上论证了在200 ℃左右下述氯化反应:Li_2CO_3 + 2NN_4Cl = 2LiCl + 2NH_3~↑ + H_2O~↑ + CO_2~↑可以进行完全。利用DSC方法测定反应产物LiCl的纯度,并用离子色谱法分析反应产物中CO_3~=的含量,结果均证明:在Li_2CO_3:NH_4Cl = 1:4 (mol)时,Li_2CO_3可以定量转化为LiCl,剩余的NH_4Cl完全分解。根据热重分析结果推测NH_4Cl氯化Li_2CO_3的反应历程为:Li_2CO_3 + 2NH_4Cl = 2LiCl + 2NH_3~↑ + H_2O~↑ + CO_2~↑ NH_4Cl = HCl~↑ + NH_3~↑ 而并非是想像中的:NH_4Cl = HCl~↑ + NH_3~↑ 2HCl + Li_2CO_3 = 2LiCl + H2O~↑ + CO_2~↑利用X-射线衍射方法分析产物,结果亦说明氯化可以成功。将1:4(mol)= Li_2CO_3:NH_4Cl混合样品在差热分析反应炉中直接加热测定反应产物LiCl的溶点,并与纯LiCl样品熔点的测定结果相比较,二者完全一致。以上结果说明:不仅可用此氯化反应产物代替LiCl应用于熔盐电解制备Al-Li合金中,而且还可将其应用于LiCl体系的相图测定中。因Li_2CO_3,NH_4Cl均不吸水,极易处理,因此以上研究结果无论对于Al-Li合金的工艺研究还是其他有关LiCl体系的基础研究都是很有价值的。2.直接氯化法制备LiCl-KCl-LiF三元相图的研究。LiCl-KCl-LiF三元相图是研究此体系电解机制的重要基础。为将以上直接氯化法应用于差热分析中制作此三元体系相图,首先用直接氯化法测定了三个已知二元LiCl体系相图:LiCl-KCl;LiCl-LiF;LiCl-NaCl。与文献结果吻合很好。说明将此法应用于差热分析中制作LiCl体系相图结果是可靠的。在LiCl-KCl-LiF三元相图的测定中共做出七个垂直截面,在各截面上读出等温条件下的相界点投影到浓度三角形中,得到等温投影图。结果说明LiCl-KCl-LiF是固态完全不互溶的三元共晶体系,共有三个液-固两相区;三个液-固-固三相区;一个液-固-固-固四相区,(三元共晶平面)和一个固-固-固三相区。四相点温度为348 ℃,其组成在三相平衡线的交点处,在实验上测出近似等于:41.4KCl + 57.3LiCl + 1.3LiF (mol)。3.氯化物体系中Li~+, Na~+析出电位的比较及其去极化作用的研究。在正常电化序中,Li~+应先于Na~+析出。但在以Al作阴极电解LiCl体系时,则由于Li~+在Al上有较强的去极化作用而提前析出。这是熔盐电解法可以制备低Na,高纯Al-Li合金的基础。本文在理论上对此问题进行了较深入的研究。具体内容包括(1)。测定Li~+, Na~+在二元氯化物体系中的析出电位。通过在Al阴极,Mo阴极上二离子析出电位的比较,确认了Li~+在Al阴极上产生很强的去极化作用是能利用电解法制备低Na高纯Al-Li合金的根本原因。并为在工艺研究中选择合适的电流密度提供了参考依据。(2).根据热力学理论推导出合金化反应产生的自由焓变化与去极化作用的关系:ΔG_x + ΔG_m = -nFΔE。揭示了产生去极化作用的原因。并根据ΔG_x(偏摩尔过剩自由焓)与合金结构的关系提出可以利用二元合金相图推测极化类型及极化大小。并根据动力学原理对温度对极化的影响提出了自己的看法。(3).求出合金化反应的热效应,认为在一定条件下亦可利用此值作为判断去极化作用大小的标准。(4).测定Li~+, Na~+在Al-Cu, -Al-RE合金上的析出电位。结果表明Cu,RE的存在均可加强Li~+在阴极上的去极化作用,进一步加大了Li~+, Na~+析出电位之间的差别,有利于制备更纯的Al-Li 使金。为直接生产Al-Cu-Li, Al-RE-Li三元母合金奠定了基础。(5)测定Li~+在不同组成配比的LiCl-KCl熔体中,在Al阴极上的析出电位,并求出LiCl的离子平均活度系数γ=0.71 (T = 740 ℃), 熔体对理想状态产生负偏离。4.利用阳极计时电位法测定T=720 ℃时Li在液体Al中的扩散系数D_(Li/Al) = 4.94 * 10~(-5)cm~2·s~(-1),与利用Stocks-Einstan公式计算出的理论值D_(Li/Al) = 4.85 * 10~(-5)cm~2·s~(-1)吻合较好。5.在上述理论研究的基础之上进行了工艺初探,所得初步结论有:(1).加入LiF可以提高电效。(2).采用电流密度为1 A/cm~2时,不加搅拌亦可制备出成份均匀,含Li量为10%(w.f)的Al-Li合金。(3).根据实验结果提出 Li在熔体中的熔解可能是影响电流效果的主要原因。

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本论文借助光学显微镜、电镜、X-射线、差热等金相分析技术研究了稀土(La、Ce、Pr、Nd、MM、Ymm)在单向、自由凝固条件下对Al-Li合金凝固组织的影响,进而分析了稀土的作用机理。实验结果表明,稀土对自由凝固的晶粒及二次枝晶、单向凝固的一次枝晶均有细化作用,这种作用同稀土元素、稀土含量及凝固条件有关。分析认为,在合金凝固过程中由于稀土的富集产生“成分过冷”等作用是影响凝固组织的因素。

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本论文较详细地总结了目前文献中有关甲烷氧化偶联催化剂、活性中心及活性氧种、反应机理及动力学和甲烷氧偶联反应(OCM)的影响因素等方面的重要结果;叙述了催化剂的制备和表征方法;系统地研究了Ti-La-Li系多元氧化物催化剂关于OCM反应有关主要活性相及作用机制、催化剂的表面碱性和活性氧种的作用,探讨了Li的含稀土、过渡金属多元氧化物催化剂中的作用,考察了制备方法、焙烧温度和反应条件对OCM反应的影响,同时对催化剂的高温失活机理进行了探讨。