The bifurcation threshold value of the chaos detection system for a weak signal＊

Recently, it has become an important problem to confirm the bifurcation threshold value of a chaos detectionsystem for a weak signal in the fields of chaos detection. It is directly related to whether the results of chaos detectionare correct or not. In this paper, the discrimination system for the dynamic behaviour of a chaos detection system fora weak signal is established by using the theory of linear differential equation with periodic coefficients and computingthe Lyapunov exponents of the chaos detection system; and then, the movement state of the chaos detection system isdefined. The simulation experiments show that this method can exactly confirm the bifurcation threshold value of thechaos detection svstem.     

Physical mechanism of the chaotic detection of the unknown frequency of weak harmonic signal and effects of damping ratio on the detection results

In the zero-order approximation, we use the perturbation method of parameter with small magnitude to prove that the harmonic frequency in the solution of the equation is close to that of the driving force when the chaotic system from Duffing－Holmes equation stays in the stable periodic state, which is the physical mechanism of the detection of the unknown frequency of weak harmonic signal using the chaotic theory. The result of the simulation experiment shows that the method proposed in this paper, by which one can determine the frequency of the stable system from the number of circulation change of the phase state directionally across a fixed phase state point (x,\dot{x}) in fixed simulation time period, is successful. Analyzing the effects of the damping ratio on the chaotic detection result, one can see that for different frequency ranges it is necessary to carefully choose corresponding damping ratio α.     

岩石孔隙中流体的核磁共振谱研究

岩石孔隙中流体的核磁共振(NMR)谱线表现出严重的增宽效应.固液界面性质是原因之一,当孔隙表面含有磁性杂质时,线宽增加会更加明显.模型分析表明,孔隙形状及其排列方式是另一重要因素.两种分子筛、硅胶以及天然砂岩孔隙中吸附或饱和甲苯与戊醇的¹H及¹³C NMR谱提供了依据.由此推断,岩石孔隙中流体的NMR谱包含丰富的孔隙结构信息.低速魔角旋转(MAS)即能够使岩石中饱和流体的NMR谱线有效窄化.     

新型锂离子电池CaSnO3负极材料的湿化学制备与电化学性能

采用湿化学方法合成了具有钙钛矿结构的CaSnO₃，将其作为锂离子电池的负极活性物质，研究了其电化学性能。结果表明，湿化学方法制备的锡酸钙，粒度分布集中、平均粒径在500 nm左右，在0～1.0 V之间以0.1 C倍率充放电时，其可逆容量达到469 mAh·g^-1，而且循环性能良好。经80次循环后的容量衰减率只有0.57%。从首次放电容量和可逆容量来看，锡酸钙的储锂机制与锡基氧化物材料相似，即:首先是结构的还原并形成金属锡；然后金属锡与锂发生可逆的合金化与去合金化过程。锡酸钙的可逆容量、循环性能都比文献报道的块状锡氧化物或者是无定型锡基复合氧化物好，这说明钙钛矿结构和钙离子的存在可能对改善锡基负极材料的性能是有益的。     

纳米SnO的溶胶-凝胶法制备与电化学性能

以氯化亚锡、草酸和无水乙醇为原料，采用溶胶-凝胶法制备了纳米SnO粉末，并用热重-差热分析、X-射线衍射分析、透射电镜和扫描电镜等多种电化学方法对其进行了表征。结果表明，采用溶胶-凝胶法可以制备出粒度分布较集中，平均粒径在20 nm左右的纳米SnO粉末。该纳米SnO的可逆容量达到607 mAh·g^-1，经80次循环后平均每次循环的容量损失只有0.056%，说明纳米SnO是一种优秀的高容量锂离子电池负极材料。     

锂离子电池用SnO2-聚苯胺复合负极材料的制备与表征

以苯胺、过硫酸铵和SnO₂为原料通过微乳液聚合法合成了SnO₂-聚苯胺的复合材料，并通过X-射线衍射、红外吸收光谱、扫描电镜和电化学测试等手段对所得复合材料进行了表征与分析。结果表明，复合材料中的聚苯胺是无定形的，聚苯胺在反应过程中沉积在SnO₂颗粒上形成SnO₂被聚苯胺包裹的复合材料。电化学测试说明，该复合材料的首次容量达到657.6 mAh·g^-1，经过80次循环后每次循环的容量衰减率仅为0.092%。     

新型锂离子电池CaSnO_3负极材料的湿化学制备与电化学性能

采用湿化学方法合成了具有钙钛矿结构的CaSnO3,将其作为锂离子电池的负极活性物质,研究了其电化学性能。结果表明,湿化学方法制备的锡酸钙,粒度分布集中、平均粒径在500nm左右,在0￣1.0V之间以0.1C倍率充放电时,其可逆容量达到469mAh·g-1,而且循环性能良好。经80次循环后的容量衰减率只有0.57%。从首次放电容量和可逆容量来看,锡酸钙的储锂机制与锡基氧化物材料相似,即:首先是结构的还原并形成金属锡;然后金属锡与锂发生可逆的合金化与去合金化过程。锡酸钙的可逆容量、循环性能都比文献报道的块状锡氧化物或者是无定型锡基复合氧化物好,这说明钙钛矿结构和钙离子的存在可能对改善锡基负极材料的性能是有益的。     

卟啉基超交联聚合物的合成及其对二氧化碳和汞(Ⅱ)离子的吸附北大核心CSCD

超交联聚合物(HCPs)具有较高的比表面积和独特的孔结构,且制备方法简单、合成原料广泛,是多孔有机聚合物的研究热点之一。卟啉是一类由4个吡咯类亚基的α-碳原子通过次甲基桥互联形成的大分子杂环化合物,可与多种金属形成稳定的配合物,在重金属的吸附与检测、气体的吸附与分离等方面具有很好的应用。卟啉结构的HCPs也有了较多的报道,然而当前报道的卟啉结构HCPs几乎均是通过Friedel-Crafts烷基化反应构筑的。Friedel-Crafts酰基化反应在卟啉结构HCPs上报道较少。而通过Friedel-Crafts酰基化反应制备的聚合物表面含大量羰基,为其进一步功能化提供了可能。但是,这类聚合物的比表面积低、孔结构差,而其优良孔结构的构建是亟待解决的问题。为此,本文在四苯基卟啉与1,2,4,5-苯四甲酸酐通过Friedel-Crafts酰基化反应制备的聚合物的基础上,进一步通过席夫碱反应,用含刚性三嗪环的三聚氰胺将其进一步交联,合成了三聚氰胺修饰的卟啉基超交联聚合物。结果发现,三聚氰胺的引入不仅实现了超交联聚合物孔结构的支撑,使得聚合物比表面积极大增长(从2 m^(2)/g增加到118 m^(2)/g),且具有优良的多级孔结构。三聚氰胺修饰卟啉基超交联聚合物的高氮质量分数(46.28%)和多级孔结构使其对CO_(2)(93 mg/g,273 K,101 kPa)和Hg^(2+)(494.1 mg/g,298 K)具有较好的吸附。     

Cross-sample entropy of foreign exchange time series

The correlation of foreign exchange rates in currency markets is investigated based on the empirical data of DKK/USD, NOK/USD, CAD/USD, JPY/USD, KRW/USD, SGD/USD, THB/USD and TWD/USD for a period from 1995 to 2002. Cross-SampEn (cross-sample entropy) method is used to compare the returns of every two exchange rate time series to assess their degree of asynchrony. The calculation method of confidence interval of SampEn is extended and applied to cross-SampEn. The cross-SampEn and its confidence interval for every two of the exchange rate time series in periods 1995-1998 (before the Asian currency crisis) and 1999-2002 (after the Asian currency crisis) are calculated. The results show that the cross-SampEn of every two of these exchange rates becomes higher after the Asian currency crisis, indicating a higher asynchrony between the exchange rates. Especially for Singapore, Thailand and Taiwan, the cross-SampEn values after the Asian currency crisis are significantly higher than those before the Asian currency crisis. Comparison with the correlation coefficient shows that cross-SampEn is superior to describe the correlation between time series.     

含嵌段共聚物“稀固体溶液”中共聚物胶束核的结晶行为

报道了含嵌段共聚物的可结晶型稀固体溶液中，不同共聚物胶束的结晶行为不同；结晶段形成胶束壳和形成胶束核，其结晶行为也相差很大。