高分散还原态Pt基催化剂的制备及其NO氧化的催化性能

Pt⁰被认为是NO氧化的活性物种,而催化剂的制备方法对活性物种的含量起着决定性作用。本文采用非惰性气氛保护的改性醇还原-浸渍法(MARI)合成了高分散高Pt⁰含量的1% (w, 质量分数) Pt/SiO₂-Al₂O₃催化剂(MA-Pt/SA)。X射线粉末衍射(XRD)、CO-漫反射傅里叶变换红外吸收光谱(CO-DRIFTS)和透射电镜(TEM)表征证实在550 ℃焙烧3 h后催化剂的Pt颗粒仅有3.8 nm。同时,X射线光电子能谱(XPS)和H₂-程序升温还原(H₂-TPR)结果表明催化剂具有高Pt⁰含量(60.3%)。模拟柴油车尾气气氛进行活性测试,并与传统浸渍法制备的1% (w) Pt/SiO₂-Al₂O₃催化剂(C-Pt/SA)对比,结果显示MA-Pt/SA具有优异的催化氧化性能,其NO最大转化率高达74%,比C-Pt/SA的NO转化率高了23%。经670 ℃高温老化15 h后,老化的MA-Pt/SA的NO转化率仍然高达69%。此外NO + O₂共吸附原位漫反射傅里叶变换红外吸收光谱(in situ DRIFTS of NO + O₂ co-adsorption)表明高的Pt分散度和高Pt⁰含量能够促进中间物种桥式硝酸盐的生成及分解,进而导致了优异的NO氧化活性。最后,利用同样方法将Pt的负载量降低至0.5% (w)制备催化剂,NO转化率仍达64%。这种制备方法能够获得低贵金属高性能的Pt基催化剂。     

Au/SiO2纳米多层薄膜的制备及其性质表征

利用多靶磁控溅射技术制备了Au/SiO2纳米颗粒分散氧化物多层复合薄膜.研究了在保持Au单层颗粒膜沉积时间一定时薄膜厚度一定、变化SiO2的沉积时间及SiO2的沉积时间一定而改变薄膜厚度时,多层薄膜在薄膜厚度方向的微观结构对吸收光谱的影响.研究结果表明:具有纳米层状结构的Au/SiO2多层薄膜在560nm波长附近有明显的表面等离子共振吸收峰,吸收峰的强度随Au颗粒的浓度增加而增强,在Au颗粒浓度相同的情况下,复合薄膜光学吸收强度随薄膜厚度的增加而增强.但当金属颗粒的浓度增加到一定程度时,金属颗粒相互接触,没有观察到纳米层状结构,薄膜不显示共振吸收峰特征.用修正后的M-G(Maxwell-Garnett)理论对吸收光谱进行了模拟,得到了与实验一致的结果.     

Au/SiO2纳米多层薄膜的制备及其性质表征

利用多靶磁控溅射技术制备了Au/SiO₂纳米颗粒分散氧化物多层复合薄膜.研究了在保持Au单层颗粒膜沉积时间一定时薄膜厚度一定、变化SiO₂的沉积时间及SiO₂的沉积时间一定而改变薄膜厚度时，多层薄膜在薄膜厚度方向的微观结构对吸收光谱的影响.研究结果表明：具有纳米层状结构的Au/SiO₂多层薄膜在560 nm波长附近有明显的表面等离子共振吸收峰，吸收峰的强度随Au颗粒的浓度增加而增强，在Au颗粒浓度相同的情况下，复合薄膜 关键词： 2纳米复合薄膜')" href="#">Au/SiO₂纳米复合薄膜 多靶磁控溅射 吸收光谱 有效介质理论     

几种电梯运行模式的比较及应用

通过对上班高峰时段的电梯运行情况进行分析,以"最后被运送的乘客的等待时间最短"的"最大最小"原则作为其评价指标,以"电梯运行周期与运行总时间之比等于电梯在一个周期内运送的乘客数与乘客总数之比"的"比例"原则为依据,对高层楼宇中人员流动高峰时段的几种电梯运行方案建立了数学模型进行描述与比较,找到了电梯停靠楼层的最佳方案,并对北京大学第三医院外科楼的运行方案做出定量的数学证明.     

超滤-吸附法制备高纯度L-辛弗林的优化工艺

以枳实提物液为原料,探讨了膜过滤和吸附树脂分离杂质,制备纯化L-辛弗林的新方法并获得纯度高达90%的冻干粉。先用不同截留分子量的超滤膜对枳实提取液进行超滤分级分离,脱除分子量差异大的杂质,再用筛选出的大孔吸附树脂吸附分离超滤透过液中的色素,确定其优化工艺条件,然后用反渗透浓缩及冷冻干燥,获得高纯度L-辛弗林冻干粉。超滤分级富集枳实粗提液中的L-辛弗林的优化条件为:以截留分子量为30kDa和5kDa的超滤膜顺序进行分级分离L-辛弗林浓度为4.5mg/mL的枳实提取液,压力为0.20MPa,获得的枳实超滤液中L-辛弗林透过率达89.7%,多糖、蛋白质和色素截留率分别为95.2%、93.8%和75.7%;D3520大孔吸附树脂脱除超滤透过液中色素的优化工艺条件为:上柱液L-辛弗林浓度为3mg/mL,上柱流速为1BV/h,上柱体积为3BV,获得的脱色液中色素仅残留3.82%,L-辛弗林损失率仅为6.54%;该脱色液经反渗透浓缩及冷冻干燥,制备出L-辛弗林冻干粉,其纯度为89.61%,总收率为77.34%。通过系列超滤膜顺序分级分离枳实提取液中分子量差异大的杂质及采用D3520大孔吸附树脂脱除色素的集成创新方法,制备出高纯度的L-辛弗林,丰富了天然产物分离纯化领域的应用理论和实证,并对分离枳实中L-辛弗林提供指导和参考。     

Co1-xNixSb3-ySey热电输运中晶界和点缺陷的耦合散射效应

结合机械合金化与放电等离子烧结工艺制备了Ni和Se共掺的细晶方钴矿化合物Co_1-xNi_xSb_3-ySe_y,研究了晶界和点缺陷的耦合散射效应对CoSb₃热电输运特性的影响.通过Ni掺杂优化载流子浓度提高功率因子.在x=0.1时,功率因子达到最大值1750μWm^-1K^-2(450℃),是没有掺Ni试样的两倍.晶界和点缺陷的耦合散射机理使晶格热导率急剧下降,其中Co_0.9Ni_0.1Sb_2.85Se_0.15的室温晶格热导率降低至1.67Wm^-1K^-1,接近目前单填充效应所能达到的最低值1.6Wm^-1K^-1,其热电优值ZT在450℃时达到最大值0.53.将Callaway-Von Baeyer点缺陷散射模型嵌入到Nan-Birringer有效介质理论模型,对晶界散射和点缺陷散射的耦合效应对热导率的影响进行了定量分析,模型计算与实验结果符合.理论模型计算表明,当晶粒尺寸下降到50nm同时掺杂引入点缺陷散射后,Co_0.9Ni_0.1Sb_2.85Se_0.15的晶格热导率下降到0.8Wm^-1K^-1. 关键词： 3')" href="#">CoSb₃ Ni和Se掺杂 热电性能 耦合散射效应     

Hysteresis-induced bifurcation and chaos in a magneto-rheological suspension system under external excitation

The magneto-rheological damper(MRD) is a promising device used in vehicle semi-active suspension systems, for its continuous adjustable damping output. However, the innate nonlinear hysteresis characteristic of MRD may cause the nonlinear behaviors. In this work, a two-degree-of-freedom(2-DOF) MR suspension system was established first, by employing the modified Bouc–Wen force–velocity(F –v) hysteretic model. The nonlinear dynamic response of the system was investigated under the external excitation of single-frequency harmonic and bandwidth-limited stochastic road surface.The largest Lyapunov exponent(LLE) was used to detect the chaotic area of the frequency and amplitude of harmonic excitation, and the bifurcation diagrams, time histories, phase portraits, and power spectrum density(PSD) diagrams were used to reveal the dynamic evolution process in detail. Moreover, the LLE and Kolmogorov entropy(K entropy) were used to identify whether the system response was random or chaotic under stochastic road surface. The results demonstrated that the complex dynamical behaviors occur under different external excitation conditions. The oscillating mechanism of alternating periodic oscillations, quasi-periodic oscillations, and chaotic oscillations was observed in detail. The chaotic regions revealed that chaotic motions may appear in conditions of mid-low frequency and large amplitude, as well as small amplitude and all frequency. The obtained parameter regions where the chaotic motions may appear are useful for design of structural parameters of the vibration isolation, and the optimization of control strategy for MR suspension system.     

Influence of ignition condition on the growth of silicon thin films using plasma enhanced chemical vapour deposition

The influences of the plasma ignition condition in plasma enhanced chemical vapour deposition (PECVD) on the interfaces and the microstructures of hydrogenated microcrystalline Si (μc-Si:H) thin films are investigated. The plasma ignition condition is modified by varying the ratio of SiH₄ to H₂ (R_H). For plasma ignited with a constant gas ratio, the time-resolved optical emission spectroscopy presents a low value of the emission intensity ratio of Hα to SiH^* (I_Hα/I_SiH^*) at the initial stage, which leads to a thick amorphous incubation layer. For the ignition condition with a profiling R_H, the higher I_Hα/I_SiH^* values are realized. By optimizing the R_H modulation, a uniform crystallinity along the growth direction and a denser μ c-Si:H film can be obtained. However, an excessively high I_Hα/I_SiH^* may damage the interface properties, which is indicated by capacitance-voltage (C-V) measurements. Well controlling the ignition condition is critically important for the applications of Si thin films.     

结构特征值的实用验证法

本文从矩阵理论出发,介绍了对自振频率进行验证的实用方法。文后,给 出了我国最大跨径刚构桥的有关计算过程。