CPT原子频标实验研究

介绍了相干布居囚禁（CPT）原子频标的小型、低功耗物理系统,并应用它开展实现了CPT原子频标的激光锁定、微波锁定方案的实验室研究.通过采用电流负反馈将激光频率锁定在原子对激光的吸收峰上,电压负反馈将微波频率锁定在电磁感应透明共振峰上,用该物理系统实现了闭环锁定的实验室桌面CPT频标实验系统.对该实验系统的频率稳定度测量获得200 s内优于5×10^-11τ^-1/2的结果. 关键词： CPT原子频标 频率稳定度 激光稳频     

Effect of Initiator on the Over-Voltage Positive Temperature Coefficient of Linear Low Density Polyethylene/Carbon Black Nano Composites

The composite of linear low density polyethylene (LLDPE) with carbon black (CB) and inorganic flame retardant (aluminum hydroxide, Al(OH)₃) was prepared by melt-blending method. The effect of cross-linking on the stability of positive temperature coefficient (PTC) of composite and the elimination of negative temperature coefficient (NTC) of composite were investigated. LLDPE was chemically cross-linking with different contents of initiator (dicumyl peroxide, DCP). The cross-linking effects of composite were analyzed by gel content analysis, differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). By the effect of DCP, not only the composite appeared a high PTC intensity, but also the NTC effect of composite was eliminated. In this investigation, the optimum PTC intensity of composite reached 5.87 orders of magnitude for the composition of LLD0.l0C33.7A28, and the PTC transitional temperature of composite decreased with increasing of DCP content. In addition, the good reproducibility of composites was proved by thermal cycling, and successfully passed the test of over-voltage resistance.     

ICP-AES测定高纯碲中11种杂质元素

电感耦合等离子体原子发射光谱法 (ICP- AES)同时测定高纯碲中 11种杂质元素。用离峰扣背景法消除背景干扰 ,检出限能满足高纯碲中杂质元素的测定要求 ,加标回收率在 94 .0 %— 10 8%之间。     

Conformational entropy of a pseudoknot polymer

The thermodynamics and kinetics of ABAB pseudoknot formation owing to reversible intrachain reactions are investigated for a flexible polymer based on the off-lattice Monte Carlo simulations. The polymer is made of N hard spheres tethered by inextensible bonds and consists of two reactive pairs AA and BB with binding energies -epsilon1 and -epsilon2, respectively, and three loop lengths (l1, l2, and l3). Although two intermediate states, loops A and B, may be formed, the folding path goes mainly through the intermediate loop whose free energy reduction associated with coil-to-loop crossover is greater. The conformational entropy loss is found to follow DeltaS=alpha ln N+G, where alpha approximately 2.48 for coil-loop crossover and alpha approximately 2.43 for loop-pseudoknot crossover. The constant G depends on the three loop lengths and the two end-to-reactive site lengths (L1 and L2). For a given total loop length, G is maximum when the three loop lengths are equal (l1=l2=l3). When l1=l3, the entropy loss is minimum if l2=0. However, the condition l1 not equal l3 makes G even smaller. This consequence indicates that asymmetry in loop lengths is thermodynamically favorable and this fact is consistent with observations of pseudoknotted RNA structures.     

Fractal aggregates of conjugated polymer in solution state

Semirigid conjugated polymers have received much scientific and technological interest due to their unique electrical and photonic semiconducting properties. Spectroscopic studies have indicated that these polymers underwent interchain aggregation in the solution state even at large dilution; however, the origin of this event and the structure of the resultant aggregates remained the crucial issues to be resolved. In the present study, we revealed that the interchain aggregation of a conjugated polymer, poly(2,3-diphenyl-5-hexyl-1,4-phenylenevinylene) (DP6-PPV), in solutions with chloroform and toluene generated network aggregates with the hydrodynamic radii of several micrometers. Small angle neutron scattering (SANS) demonstrated that the internal structure of these aggregates could be characterized by the mass fractal dimensions of 2.2-2.7. The networks were looser in chloroform but became highly compact in the poorer toluene solvent due to severe segmental association. Increasing the temperature alleviated the segmental association in toluene while largely retaining the mass fractal dimension of the aggregates. However, the interchain aggregation was never completely dissipated by the heating, suggesting the existence of two types of segmental association with distinct stability. The highly stable segmental association that could neither be solvated by chloroform nor be disrupted thermally in toluene was attributed to the pi-pi complex already present in the DP6-PPV powder used for the solution preparation. The chains tied firmly by this complex formed network aggregates in the solution and hence reduced the entropy of mixing of the polymer. In the poorer toluene solvent, further segmental association took place within the preexisting aggregates, making the networks more compact. This type of segmental association could be disrupted by moderate heating, and its occurrence was ascribed to the poor affinity of the aliphatic side chains of DP6-PPV for toluene.     

Graphene films grown on sapphire substrates via solid source molecular beam epitaxy

A method for growing graphene on a sapphire substrate by depositing an SiC buffer layer and then annealing at high temperature in solid source molecular beam epitaxy (SSMBE) equipment was presented. The structural and electronic properties of the samples were characterized by reflection high energy diffraction (RHEED), X-ray diffraction Φ scans, Raman spectroscopy, and near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The results of the RHEED and Φ scan, as well as the Raman spectra, showed that an epitaxial hexagonal α-SiC layer was grown on the sapphire substrate. The results of the Raman and NEXAFS spectra revealed that the graphene films with the AB Bernal stacking structure were formed on the sapphire substrate after annealing. The layer number of the graphene was between four and five, and the thickness of the unreacted SiC layer was about 1--1.5 nm.     

含三唑结构的新型稠合三环2(5H)-呋喃酮衍生物的合成

利用系列含烯二炔结构的2(5H)-呋喃酮衍生物与叠氮化钠发生串联的成环反应, 在优化的反应条件下, 即反应溶剂为DMF、反应时间48 h、反应温度30 ℃时、NaN₃为1.5 equiv., 以中等产率(42%～62%)合成了系列新型的稠合三环2(5H)-呋喃酮衍生物, 其可以进一步高产率地(94%～96%)衍生为吡啶稠合的2(5H)-呋喃酮化合物. 所有新化合物的结构用IR, ¹H NMR, ¹³C NMR, MS, 元素分析等方法进行了表征. 该串联反应合成途径简捷、反应条件温和, 无需添加催化剂, 可为具有含三唑结构的稠杂环化合物合成提供简便的途径.     

过充电保护添加剂1,4-二甲氧基苯的反应机理

采用密度泛函理论(B3LYP/6-311+G(d,p))和MP2/6-311+G(d,p)方法,研究锂离子电池过充电保护添加剂1,4-二甲氧基苯(p-DMOB)的作用机理.计算结果表明,在过充时,p-DMOB优先于溶剂分子(乙基甲基碳酸酯、二甲基碳酸酯、碳酸乙酯)发生氧化反应.用B3LYP和MP2计算所得的p-DMOB理论氧化电位接近,分别为4.12和4.05V(vsLi/Li+).p-DMOB氧化时首先失去一个电子,生成p-DMOB+·正离子自由基,用B3LYP和MP2方法计算所得的相应能量变化分别为701.24和728.27kJ·mol-1.失去电子后苯环的共轭性受到破坏,随后p-DMOB+·苯环上的C―H键发生断裂,失去H+并形成p-DMOB·自由基.用B3LYP和MP2方法计算所得的相应能量变化分别为1349.78和1810.99kJ·mol-1.p-DMOB·自由基很不稳定,会在电极表面发生聚合反应形成聚合物膜,用B3LYP和MP2方法计算所得的相应能量变化分别为-553.37和-1331.20kJ·mol-1.     

单壁碳纳米管储氢的第一原理分子动力学模拟

本文用第一性原理平面波赝势方法模拟研究了手性单壁碳纳米管与氢分子的相互作用,考察了碳纳米管直径对储氢性能的影响.对单壁碳纳米管储氢的模拟结果表明:(1)物理吸附时,H2可以吸附在空腔内,也可以吸附在管与管之间的空隙中,纳米管内部的氢吸附力均高于管外,而“完好无损”的H2分子不能够穿过管壁而进入管内.(2)化学吸附时,碳纳米管对氢的吸附首先出现在管的边缘附近,碳纳米管局部会发生形变,SWCNTs的张力会随C-H键的增加而增大,系统不稳定.(3)随着直径的增加,纳米管内、外的氢吸附力差异减小.     

A novel 1 × 2 optical power splitter with PBG structures on SOI substrate

A novel 1 × 2 optical power splitter in size of 8.0 μm × 4.2 μm is presented in this paper, by using photonic bandgap (PBG) structures on silicon-on-insulator (SOI) substrate. The splitting ratio can be adjusted by changing the air hole position to get wide tuning range. The design is examined by the commercial finite-difference-time-domain (FDTD) software for various splitting ratios. Some approximated formulas are obtained through curve-fitting to facilitate design process.