纳米银/聚乙烯复合体的太赫兹光谱学性质

测定了纳米银/聚乙烯(nano-Ag/PE)复合体的电导率和太赫兹时域光谱(THz-TDS),研究了其导电性质和在远红外区的反常吸收特性;结合有效介质理论和Drude理论,利用简化的公式对太赫兹时域光谱测量结果进行了分析,并通过对电导率测量结果进行拟合分析得到了纳米Ag及其表层氧化物的电导率信息.结果表明,nano-Ag/PE复合体的逾渗阈值在0.11附近(体积分数),n值接近4.5,偏离"普适性行为".这是由于颗粒表面的氧化层及颗粒隧穿导电效应所致.与此同时,nano-Ag/PE复合体在THz波段的反常吸收性质与表面氧化层有关.     

多壁碳纳米管/高密度聚乙烯复合体系在太赫兹波段的光学性质研究

利用太赫兹时域光谱研究了多壁碳纳米管/高密度聚乙烯(MWNTs/HDPE)复合体系的光学性质.第一次使用MG模型提取了不同浓度下MWNTs的光学常数,并利用DL模型对结果进行了解释.     

聚乙烯—碳黑复合材料的太赫兹时域光谱测量研究

利用太赫兹时域光谱(THz-TDS)技术在0.3-2.0 THz频率区间测量了不同碳黑含量(?渍)的聚乙烯-碳黑复合材料光电性质随频率的变化及其与碳黑含量之间的关系. 测量发现, 随碳黑含量的增加, 复合体系的吸收系数逐渐增大, 并伴随折射率的相应增加. 在碳黑含量确定的情况下, 材料的吸收系数随频率的增加而增大, 但折射率随频率的增加而略有减小. 在假定复合体系中孤立碳黑颗粒在外电场作用下的极化过程是引起材料太赫兹频区介电损耗主要因素的情况下, 利用德拜偶极子弛豫理论对聚乙烯-碳黑复合材料的介电行为进行了解释.     

超细颗粒填充高密度聚乙烯复合体的导电和流变学性质(英文)

制备了多壁碳纳米管、石墨和碳黑填充高密度聚乙烯(HDPE)复合体,研究了复合体的导电和流变学性质.利用隧道逾渗模型对关键指数分别为4.4、6.4和2.9的三种复合体的"非普适性"导电行为进行了解释.与此同时,考察了颗粒类型和含量,以及剪切速率对复合体流变学性质的影响.结果表明,复合体系的储能模量在低频区出现"第二平台",而复合黏度则表现出强烈的剪切变稀行为,标志着颗粒在聚合物内部发生聚集形成了网络结构.与石墨和碳黑填充复合体相比,具有更高纵横比的多壁碳纳米管填充复合体具有更高的储能模量和复合黏度.基于Guth-Smallwood理论结合有效介近似的G′r分析结果表明,填充HDPE复合体系的流变学逾渗阈值和导电逾渗阈值吻合良好.     

碳纳米管/高分子复合导电材料的研究进展

从碳纳米管的电性质出发,对以它为填料的导电性复合材料的制备方法和研究进展进行了综述,同时简单介绍了导电性复合材料电性质转变现象的理论分析模型,最后对碳纳米管/高分子复合导电材料的研究前景作了一定探讨。     

聚吡咯微粒掺入的聚苯乙烯膜在电解质溶液中的介电弛豫谱及其解析

在40 Hz～11 MHz频率范围测量了聚苯乙烯膜以及混入聚吡咯粒子的聚苯乙烯膜和电解质溶液构成的体系的介电谱, 发现了特异的弛豫现象: 纯的和掺入导电性聚吡咯后的聚苯乙烯膜分别显示出单一弛豫和双弛豫的不同模式的介电谱. 在Maxwell-Wagner界面极化概念基础上解释了该弛豫机制: 高、低频弛豫分别由膜-液界面极化和膜相本身的不均一性引起的. 将体系进行了模型化, 并利用Hanai理论方法对谱进行了解析, 获得了内部电性质的诸多参数. 对不同聚吡咯掺入量的膜/溶液体系的介电测量和解析结果表明, 电解质溶液的种类、浓度以及膜中混入聚吡咯的量都影响着膜相的介电响应. 这些结论为利用加入导电粒子改善绝缘高分子聚合物的电性质的研究以及制备既具有导电功能又使基体的力学性能得到提高的高分子复合物提供了重要的线索.     

一种提高有机硅导电橡胶复合材料导电和电磁屏蔽性能的方法

描述了一种利用多元复合导电网络提高有机硅橡胶复合材料导电和电磁屏蔽性能的思想。这种含有多元复合导电网络的复合材料是通过将镀银泡沫、导电填料和有机硅橡胶复合在一起而制备的。利用扫描电子显微镜观察了复合材料的微观结构,采用电阻测试仪、矢量网络分析仪、热失重分析仪分别研究了复合材料的导电性、电磁屏蔽性能及热稳定性,并探讨了补强剂SiO2对这些性能的影响。结果表明,2%碳纳米管/导电泡沫/硅橡胶多元复合体系相比于2%碳纳米管/硅橡胶单一复合体系电阻率降低了6个数量级,平均电磁屏蔽效能由12dB提高到52dB,增大了4倍。当导电填料换为炭黑后,2%炭黑/导电泡沫/硅橡胶多元复合体系相比于2%炭黑/硅橡胶单一复合体系电阻率降低了7个数量级,平均电磁屏蔽效能由10dB提高到50dB,增大了5倍。由于多元复合导电网络,材料的导电性能和电磁屏蔽性能大幅度提升。此外,补强剂的加入增大了复合材料的热稳定性,但降低了其导电和电磁屏蔽性能。     

纤维状填料/HDPE复合体系的导电渗流与流变渗流行为的关系

研究了纤维状导电材料不锈钢纤维(SSF)填充高密度聚乙烯(HDPE)导电复合体系的导电渗流与流变渗流行为之间的关系,并与颗粒状导电颗粒炭黑(CB)/HDPE导电复合体系进行了比较.发现当SSF含量极低(0.3vol%)时,SSF/HDPE体系即发生导电渗流现象,且导电渗流转变区域极窄;而仅当SSF含量达到4.8vol%时,该复合体系才表现出流变渗流现象,这一结果与CB/HDPE体系及纳米级导电纤维填充体系截然不同.此外,通过正温度系数效应的研究发现SSF形成的导电通路稳定性高于CB/HDPE体系.我们认为,SSF/HDPE体系呈现的这些特点均与SSF较大的直径及长径比且其导电通路及流变渗流网络的形成机理不同有关.     

非导电球型粒子悬浮液宽频介电谱——薄双电层理论的计算机模拟

以Shilov等提出的带有紧密层表面电导率的非导电球型粒子悬浮液宽频介电弛豫的薄双电层理论为基础, 从电动力学角度解释了粒子分散系两种典型介电弛豫(高频和低频弛豫)的机制. 在此基础上, 利用Mathcad程序将该理论定量程序化并建立了粒子/水相分散系介电谱参数与体系内部相参数的关系. 进而利用该程序模拟了溶液浓度、Zeta电位以及分散粒子半径等内相参数对两种弛豫的影响, 结合该理论阐述了不同环境下这两种弛豫的变化规律, 从而为今后更好地利用这两种弛豫表征纳米至毫米级球形粒子分散系的各相电及界面性质提供了有价值的参考.     

水溶液中碳纳米管的表面增强拉曼光谱研究

合成了碳纳米管和金纳米颗粒的复合物, 测量了水溶液相中复合物的表面增强拉曼光谱, 结果表明, 碳纳米管的巯基化修饰可以提高碳纳米管与金纳米颗粒复合的效率, 随着金纳米颗粒负载量的增加, 碳纳米管的拉曼信号逐渐增强. 加入己二胺分子可以减小金纳米颗粒之间的距离使表面增强效应更显著, 碳纳米管的拉曼光谱得到进一步的增强. 还可进一步在复合体系中加入对巯基苯胺和罗丹明B等小分子拉曼探针, 利用金纳米颗粒的表面增强效应, 这种多元复合体系有望作为多通道拉曼成像探针材料.     

Ground-state spectral features of molecular clusters RDX excited at THz frequencies

Calculations are presented of ground state resonance structure at THz frequencies for molecular clusters of the high explosive RDX using density functional theory (DFT). The spectral features of this resonance structure are due to coupling of resonance modes for ground state excitation. In particular, the coupling among ground state resonance modes provides a reasonable molecular level interpretation of spectral features associated with THz excitation of molecular clusters. THz excitation is associated with frequencies that are characteristically perturbative to molecular electronic states, in contrast to frequencies that can induce appreciable electronic state transition. Owing to this characteristic of THz excitation, one is able to make a direct association between local oscillations about ground-state minima of molecules, either isolated or comprising a cluster, and THz excitation spectra. The DFT software GAUSSIAN was used for the calculations of ground state resonance structure presented here.     

Hydrophobized oxygen cathode of a fuel cell with a liquid electrolyte: Calculating overall currents and thicknesses

The mechanism governing operation of hydrophobized cathodes is discussed. A model is proposed for the active-layer structure. The model consists of equidimensional hydrophobic (agglomerates of polytetrafluoroethylene particles) and hydrophilic (agglomerates of carbon black particles with the catalyst on them) grains. The percolation characteristics of the model are calculated: the presence of a gas cluster and an ionic cluster is established, the specific area of contact between these clusters is determined, the magnitude of ionic conductivity is assayed, and so forth. The “model of cylindrical gas pores” is selected for calculating the overall current. Formulas for the bulk current density are determined. The overall characteristics of a cathode with a platinum catalyst on a carbonaceous carrier (on carbon black) in 7 M KOH at a temperature of 60°C are calculated with allowance made for the fact that the Tafel plots for the process of reduction of oxygen on platinum have two segments with different slopes.     

Frequency and field dependent conductivity in Pd-based metal cluster compounds

Ligand stabilized metal cluster compounds and colloids consist of nanometer-size metal cores surrounded by ligand shells. We study the dielectric properties of randomly packed aggregates of these compounds as a function of temperature T, electric field E and frequency ω. Cores were formed by Pd-atoms and stabilized by a selection of ligands. The core size varied between 2.4 and 3.6 nm (monodisperse) and 18 nm (5–10% dispersion). The dielectric properties of all compounds show scaling: the same functional dependence of the normalized conductivity on normalized frequency irrespective of temperature. The results imply that at room temperature at all frequencies below 1 THz exclusively hopping between the particles is seen. More details of the hopping processes are obtained by varying E. The observed dependences are consistent with stochastic models that use random packing of clusters inside the sample.     

Conductivity of solvated electrons in hexane investigated with terahertz time-domain spectroscopy

We present investigations of the transient photoconductivity and recombination dynamics of quasifree electrons in liquid n-hexane and cyclohexane performed using terahertz time-domain spectroscopy (THz-TDS). Quasifree electrons are generated by two-photon photoionization of the liquid using a femtosecond ultraviolet pulse, and the resulting changes in the complex conductivity are probed by a THz electromagnetic pulse at a variable delay. The detection of time-domain wave forms of the THz electric field permits the direct determination of both the real and the imaginary part of the conductivity of the electrons over a wide frequency range. The change in conductivity can be described by the Drude model, thus yielding the quasifree electron density and scattering time. The electron density is found to decay on a time scale of a few hundred picoseconds, which becomes shorter with increasing excitation density. The dynamics can be described by a model that assumes nongeminate recombination between electrons and positive ions. In addition, a strong dependence of the quasifree electron density on temperature is observed, in agreement with a two-state model in which the electron may exist in either a quasifree or a bound state.     

Pulsed sonoelectrochemical synthesis of size-controlled copper nanoparticles stabilized by poly(N-vinylpyrrolidone)

A novel sonoelectrochemical method for the size-controlled synthesis of spherical copper nanoparticles in an aqueous phase was developed. In this study, poly(N-vinylpyrrolidone) (PVP) was used as the stabilizer for the copper clusters. The copper nanoparticles were characterized by XRD, UV-vis, IR, DLS, TEM, and HRTEM. The PVP was found to greatly promote the formation rate of copper particles and to significantly reduce the copper deposition rate, thereby making monodispersed copper nanoparticles. We could control the particle size by adjusting various parameters such as current density, deposition, temperature, and sonic power, and improve the homogeneity of the copper particles. The results also showed that the transfer rate of PVP-stabilized copper clusters from the cathodic vicinity to the bulk solution played an important role in the preparation of the monodispersed nanoparticles.     

Transport properties of highly ordered heterogeneous ion-exchange membranes

Model "ordered" heterogeneous ion exchange membranes are made with ion exchange particles heaving ion exchange capacity in the range 3 to 2.5 meq/gr (dry basis) and diameters ranging from 37 to 7 microm and 2 component room-temperature vulcanizing silicon rubber as a polymeric matrix, by applying an electric field normal to the membrane surface during preparation. These membranes were shown to have an improved ionic conductivity compared with "nonordered" membranes based on the same ion exchange content (for instance, at 10% resin content "nonordered" membranes show <10(-5) mS/cm while "ordered" membranes have conductivity of 1 mS/cm). The transport properties of ordered membranes were compared with those of nonordered membranes, through the current-voltage characteristics. Limiting currents measured for the ordered membranes were significantly higher than those of the nonordered membranes with the same resin concentration. In addition, higher limiting currents were observed in ordered membranes as the resin particles became smaller. Energy dispersion spectrometry analyses revealed that the concentration of cation exchange groups on the membrane surface was higher for ordered membrane as compared to that of nonordered membranes. This implies that the local current density for the conducting domains at the surface of the nonordered membranes is higher, leading to higher concentration polarization and, eventually, to lower average limiting current densities. The effect of ordering the particles on the membrane conductivity and transport properties was studied, and the advantages of the ordered membranes are discussed.     

Effect of Particle Conductivity on Fe-Si Composite Electrodeposition

Coatings containing Fe-Si or Si particles were electrodeposited on 3.0%（mass fraction） Si steel sheets. The surface morphology, the cross-section and the silicon content of coating have been investigated, respectively. It was found that the number of particles on the coating surface and cross-section significantly decreased with increasing silicon content in the applied particles, leading to a decrease of the silicon content of coatings. About 10.2% silicon content of coatings deposited with Fe-30%Si particles can be obtained, whereas that for Si particles was only 2.9% at a particle concentration of 100 g/L and current density of 2 A/dm2. This is mainly attributed to the conductivity of applied particles. High conductivity can promote the co-deposition of the particles. With increasing silicon content in the particles, their conductivity decreased sharply, resulting in the decrease of silicon content of coatings. Present work may initiate a new method to modify the particle content of the composite coatings via changing the conductivity of the particles during the composite electrodeposition. In this paper, a possible mechanism was proposed to explain the phenomena.     

乐果分子的太赫兹时域光谱研究

研究了有机磷农药乐果在0.2~2.5 THz波段的光谱特性。应用密度泛函理论的Becke-3-Lee-Yang-Parr(B3LYP)方法计算了乐果分子在THz波段的振动吸收谱,同时利用THz时域光谱系统(THz-TDS)测得了乐果在此波段的吸收谱和折射率谱。根据理论计算结果,借助于Gaussian View软件对乐果的THz吸收谱进行了指认,并给出了与光谱特征吸收对应的分子振动构象。研究表明:乐果分子在THz波段存在吸收峰,理论计算与实验结果符合较好,且乐果分子在THz波段的吸收是由分子内和分子间振动共同引起。本研究证明了将THz-TDS技术用于乐果分子探测和识别的可行性,为THz时域光谱技术在其它农药分子识别和残留检测中的应用提供了有益的借鉴。     

In situ fabrication and graphitization of amorphous carbon nanowires and their electrical properties

Individual amorphous carbon nanowires (a-CNWs) were fabricated inside a transmission electron microscope (TEM) by the electron beam induced deposition (EBID) method, and the a-CNWs were graphitized in situ by introducing Fe particles into these a-CNWs and controlled movement of the Fe particles in these CNWs. Detailed structural characterizations and electrical measurements were carried out, and it was found that the current-induced movement of Fe particles has significant effects in purifying the as-fabricated a-CNW, transforming the a-CNW into a graphitized-CNW (g-CNW). Two-terminal current voltage characteristics measurements showed that the g-CNW has a very good electrical conductivity with a resistivity of about 5.3 x 10(-4) Omega cm, a current carrying capacity of at least 4.35 mA, and a current density of 4.6 x 10(8) A/cm(2), and these values are comparable to those of multiwalled carbon nanotubes. Field emission characteristics of both a-CNWs and g-CNWs were also measured, and their respective Fowler-Nordheim plots were found to have basically a linear form.     

Study of the Electrical Conductivity Characteristics of Micro and Nano-ZnO/LDPE Composites

Polyethylene, a thermoplastic resin made by ethylene polymerization, is widely used in electrical insulation. In this study, low-density polyethylene (LDPE) is used as a matrix with micro- and nano-ZnO particles as a filler to produce different proportions of micro- and nano-ZnO composites by melt blending. These samples are characterized by Polarized Light Microscopy (PLM) and FTIR tests, with their conductance measured under different field strengths. The current density vs. electric field strength (J-E) curve of micro- and nano-ZnO composites under different field strengths are measured and analyzed. The J-E curves of different composites at different temperatures are measured to explore conductance with temperature. The results of these tests showed that nano-ZnO composites successfully suppressed conductivity at elevated temperatures and electric field strengths, while micro-ZnO composites increased the conductivity relative to pure LDPE.