Investigation of pseudocapacitance effect and frequency dependence of ac impedance in polyaniline–polyoxometalate hybrids

In polyaniline (PAni) prepared by interfacial polymerization, the conductivity increases with temperature due to the heating induced molecular arrangement which is favorable for charge delocalisation. The hydroxyl groups present in the polyoxometalates-doped PAni hybrids play a vital role in the conductivity mechanism of these materials. The appearance of pseudocapacitance loop at low frequency region is due to the temperature-assisted formation of oxonium ions and the protonation of polymer by this at the electrode–electrolyte interface and it is in agreement with the TGA studies. From the frequency dependant conductivity measurements at high temperatures, it is found that, PAni obeys the correlated barrier hopping model whereas its hybrids follow overlapping large polaron tunneling model.     

Al2O3掺杂对YSZ固体电解质烧结及电性能的影响

研究了用常规共沉淀法掺杂Al2O3对YSZ固体电解质的烧结及电性能的影响．结果表明：适量的Al2O3能提高YSZ材料的烧结性能，促使其致密化，但过量的Al2O3对材料的致密化不利；同时，材料的晶界电导随Al2O3含量的增大表现出先增大后减小的变化趋势，这与Al2O3对YSZ晶界两方面的不同影响有关，Al2O3偏析于晶界一方面能清除晶界上对氧离子电导不利的SiO2，但另一方面也会降低晶界空间电荷层中的自由氧离子空穴的浓度．     

相对湿度对泡沫玻璃导热系数的影响

采用瞬态平面热源法在不同的环境相对湿度下对3对泡沫玻璃的导热系数进行了测量。实验结果表明,在相对湿度较低的情况下,试样的导热系数受湿度影响较小,几乎不随环境湿度变化;在相对湿度较高的情况下,试样的导热系数随湿度的增大而增大,与低湿度时相比导热系数最多增大了近1.5%;此外,实验结果还表明泡沫玻璃的导热系数存在不均匀性,不同区域的导热系数值存在较大差别,最大可相差8.8%。     

Cu-Cr-Zr合金时效后显微硬度和导电率的研究

研究了Cu 0 .3Cr 0 .15Zr 0 .0 5Mg合金时效温度、时效时间和时效前变形量对导电率和显微硬度的影响。结果表明 ,合金 92 0℃固溶后在 4 5 0℃时效可以获得较高的硬度 ,在 6 0 0℃时效可以获得较高的导电率 ;时效前冷变形可以加速第二相的析出 ,大幅度提高合金的导电率 ,合金固溶并 6 0 %形变后在 5 0 0℃时效 0 .5h导电率可达 72 .0 2 %IACS ,而固溶后直接时效仅为 5 1.79%IACS。     

Determination of the thermal conductivity of xenon-helium mixtures at high temperatures by the shock-tube method

The thermal conductivity of gases at high temperatures has been measured by the shock-tube method, which is uniquely suited to measure thermal conductivities of gases at high temperatures above 2000 K. A consistent set of thermal-conductivity data over a wide range of temperatures has been obtained from optimum combinations of shock-tube experiments at high temperatures, previously published data at lower temperatures, and a theoretical correlation of the temperature dependence. In the present study, the thermal conductivity of xenon-helium mixtures has been determined at compositions of 10 and 30 mol% xenon over the temperature range from 300 to 4800 K. Even though there is a large difference between the thermal conductivity of pure xenon and that of helium, it is interesting that the dependences of the thermal conductivity of the mixture on temperature and composition are linear. The experimental results are in good agreement with the predicted values based on the corresponding-states principle and the mixing rule. From these experimental results, interpolating the corresponding-states correlation data, we represent the equation of xenon-helium gas mixtures for thermal conductivity in terms of temperature and composition.     

低温聚苯胺/碳纳米管复合材料的制备

采用原位聚合法合成出了具有较高导电性的聚苯胺及聚苯胺 /碳纳米管复合材料 ,考察了不同碳纳米管添加量对聚苯胺 /碳纳米管复合材料表面形态、材料结构及导电性的影响并进行了表征。结果证实 ,制备出的复合聚苯胺的电导率比所见报道值提高了 1～ 2个数量级 ,为高电导率聚苯胺的合成开辟了更广阔的前景。     

Electrical conductivity based characterization of plain and coarse glass powder modified cement pastes

This paper describes the use of electrical conductivity to characterize plain and coarse glass powder modified cement pastes. It is observed that the glass powder addition facilitates improved hydration of the cement grains. For the proportions investigated in this study, and the particle size of glass powder, this advantage is negated by the reduced amount of hydration products, i.e., the dilution effect. The variation of electrical conductivity and its derivative with time can be related to the various phases in the microstructural development of the paste. It is observed from the time derivative of conductivity plots that the addition of glass powder results only in minor changes in the setting time of the pastes. Higher the glass powder content, higher the normalized conductivity (ratio of conductivity at a certain glass powder content to that of plain paste) at very early times, and then it falls to a value closer to or less than 1.0 at later times. A parallel model is used to represent effective conductivity as a function of the pore solution conductivity, porosity, and pore connectivity factor. The pore solution conductivity increases with increase in glass powder content. The porosity of the pastes reduces with increase in glass powder content at early ages and increases at later ages. A reduced pore connectivity factor is observed for pastes with higher glass powder content at later times. However, this does not imply increased volume of hydration products as is commonly interpreted for normal pastes, but the electrical conduction pathways are made more tortuous by the relatively large volume of un-reacted filler material in the pore structure.     

Reducing thermal conductivity of crystalline solids at high temperature using embedded nanostructures

Thermal conductivity of a crystalline solid at high temperature is dominated by the Umklapp process because the number of high frequency phonons increases with temperature. It is challenging to reduce the thermal conductivity of crystalline solids at high temperature although it is widely known that, by increasing the atomic defect concentration, thermal conductivity of crystalline solids can be reduced at low temperature. By increasing the concentration of ErAs nanoparticles in In 0.53Ga 0.47As up to 6 atom %, we demonstrate a thermal conductivity reduction by almost a factor of 3 below that of In 0.53Ga 0.47As at high temperature. A theoretical model suggests that the mean free path of the low frequency phonons is suppressed by increasing the ErAs nanoparticle concentration.     

几种纳米管导热性的分子动力学模拟

采用平衡态分子动力学方法,计算了不同直径的单壁碳、硅、锗、碳化硅、氮化硼纳米管的导热系数,得到了导热系数随温度、直径变化的曲线;进而根据这些曲线,讨论了直径、温度等因素对几种纳米管导热性的影响,以及不同纳米管之间导热系数的差异。研究结果表明,各单壁纳米管的导热系数均随温度的升高以及直径的增大而降低;在模拟的温度范围(<2200K)内,温度相同时,几种纳米管的导热系数由大到小有如下的排序:碳、氮化硼、硅、锗和碳化硅,其中,锗和碳化硅纳米管的导热性相当。     

纳米流体导热系数实验研究进展

导热系数是反映介质换热能力的主要参数,具有重要的理论和应用意义。详细介绍国内外纳米流体制备及其导热系数的研究进展情况,对比和总结国内外研究者的研究结果。表明:纳米流体能显著提高基液的导热系数,但不同的研究者对于纳米流体导热系数的研究得出的结果存在一定差异,有待于进一步的深入研究。     

Electrical Conductivity of Tungsten in a Continuous Transition from Condensed to Gaseous State

A pulse heating technique is developed that makes it possible to investigate the transition of a metal from a condensed to a gaseous state while maintaining almost uniform temperature and pressure distributions in a sample. By means of the technique, the electrical conductivity of tungsten was measured in a process during which a pressure in the range of 30–100 kbar was applied to the sample and its density decreased from the standard solid density to a density 15–20 times less. Since the pressures are substantially higher than the critical pressure, the transition from a condensed to a gaseous state was continuous. Earlier results have shown that along isobars in the range of 30–60 kbar the density dependence of the electrical conductivity changes radically at a certain density value (at which it has a pronounced knee). At the knee, the density is approximately 10 times less than the standard solid density, and the internal energy is about two times the sublimation energy. The dependence of the electrical conductivity near the knee becomes smoother as the pressure increases. In this paper new results on the conductivity of tungsten at the pressures up to 100 kbar are presented. It is shown that the knee becomes remarkably flatter and smoother than the corresponding low pressure dependence. Nevertheless, the main features of the electrical conductivity dependence observed at low pressures persist at the maximum applied pressure.     

Cu-3.0Ni-0.75Si合金时效析出动力学分析

为研究Cu-3.0Ni-0.75Si合金时效过程中沉淀相的析出与长大规律,及其对合金硬度的影响,采用涡流电导仪和布氏硬度计分别测量合金的电导率和硬度,根据导电率与新相析出量之间的关系分析合金的时效析出动力学过程.结果表明,在350℃下时效,合金硬度随时效时间的延长,先升高后趋于平缓;在450℃、550℃下时效,合金硬度随时效时间的增加快速上升,到达峰值后缓慢下降;时效温度越高,合金硬度峰值越低,但硬度达到峰值所需的时间越短.温度一定,随时效时间的增加,合金电导率在时效初期快速升高,至峰值后趋于平缓.根据Cu-3.0Ni-0.75Si合金在450℃时效过程中电导率的变化,通过Avrami方程推导出相应的相变动力学方程及电导率方程分别为f=1-exp(-0.052 2t0.717 61)和σ=15.2+16.3[1-exp(-0.052 2t0.717 61)],采用相关系数检验法及F检验法对电导率方程的可信性进行检验,结果说明时效析出动力学方程和电导率方程具有一定的可靠性.对比由电导率经验方程得出的电导率理论值与测量得出的实验值,该理论值与实验值有良好的吻合度.     

Thermally activated conduction mechanisms in Silicon Nitride MIS structures

This publication reports on thermally activated currents in n-silicon/Si₃N₄/Al structures. The samples prepared were examined by means of I-V, C-V, ac conductivity and thermally stimulated depolarisation current (TSDC) measurements. The results indicate that DC conductivity is controlled by charge diffusion. Both conductivity and TSDC measurements indicate that mobile carriers which are trapped at the Si/nitride interface for low temperatures are de-trapped when temperature approaches room temperature resulting in a fast increase of DC conductivity. Additionally it is found that at high temperatures the ac conductivity follows a power law with respect to frequency where the exponent is close to two. Implications of this type of variation are also discussed. The results were used in order to explain the contribution of defects to the conduction mechanisms and the device behaviour.     

Photoconductivity of ZnTe thin films at elevated temperatures

Photoconductivity of thermally evaporated ZnTe thin films was studied at different elevated temperatures. A gap type cell configuration with Al electrodes on glass substrates was used. The conductivity was found to obey two distinct conduction mechanisms within the region of applied fields. At low fields the photoconduction is ohmic and at high fields it is of Poole-Frenkel type. With increase of ambient temperatures, the Poole-Frenkel conductivity regions were found to extend to lower fields. The temperature dependence of dark conductivity also was found to be of similar nature. The paper was presented at the 6th Asian Thermophysical Properties Conference (6th ATPC), held at Gauhati University, during 8–11 October 2001.     

Electrical conductivity,crystallinity and glass transition in vulcanised rubber

Temperature variation of electrical conductivity has been measured for rubber vulcanised with different concentrations of sulphur. Change of amorphous character with sulphur concentration has also been measured from the amorphous halo of the Debye-Scherrer x-ray photographs of the samples. The conductivity of vulcanised rubber in glassy state shows a slow and gradual increase; but during glass transition it increases suddenly to a high value which again exhibits gradual increase in high elastic state. X-ray studies show that with increase of sulphur, the amorphous region decreases due to formation of cross-links by the chain units. The decrease of conductivity with increase of sulphur is then due to decrease of amorphous region. The fast decrease of conductivity with sulphur at low concentration in the high elastic state indicates high rate of formation of cross-links at such concentration.     

Pore conductivity control at the hundred-nanometer scale: an experimental and theoretical study

We report on the observation of an unexpected mechanism that controls conductivity at the 100-nm scale on track-etched polycarbonate membranes. Transport measurements of positively charged methyl viologen performed by absorption spectroscopy under various pH conditions demonstrate that for 100-nm-diameter pores at pH 2 conductivity is blocked, while at pH 5 the ions move through the membrane according to diffusion laws. An oppositely charged molecular ion, naphthalene disulfonate, in the same membrane, shows the opposite trend: diffusion of the negative ion at pH 2 and very low conductivity at pH 5. The influence of parameters such as ionic strength and membrane surface coating are also investigated. A theoretical study of the system shows that at the 100-nm scale the magnitude of the electric field in the vicinity of the pores is too small to account for the experimental observations; rather, it is the surface trapping of the mobile ion (Cl- or Na+) that gives rise to the observed control of the conductivity. This surprising effect has potential applications for high-throughput separation of large molecules and bio-organisms.     

Evaluation of thermal conductivity from temperature profiles

A new dynamic technique for the measurement of thermal conductivity is being developed at IMGC. The experiment consists in bringing the specimen to high temperatures with a current pulse and in measuring the temperature profiles during the free cooling period. Different techniques can be used to extract the information on thermal conductivity from the profiles. The numerical computation of thermal conductivity from the experimental temperature profiles in absolute space is possible, but it is difficult and cumbersome because one must know and take into the account the exact position of the infinitesimal elements of the specimen in different profiles. Computations in tube-space (a fictitious space where no thermal expansion occurs) are simpler and lead to less complex numerical computations. Complementary techniques to evaluate thermal conductivity as a function of temperature or at constant temperature are presented with a discussion of advantages and disadvantages of each method. Computer simulations have tested the precision of the complex software. Numerically generated temperature profiles from known thermophysical properties have been obtained and thermal conductivity has been recomputed from the profiles. The relative difference using different computational approaches and different fitting functions is always less than 0.1%.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Hole transport due to shallow acceptors along boron doped SiGe quantum wells

Lateral conductivity and magnetotransport measurements were performed with SiGe single quantum well (QW) structures doped with boron in the QW. The conductivity at low temperatures (T) is shown to be due to hopping over B centers while at higher T, it is due to two-stage excitation: thermal activation of holes from the ground to strain-split B states are followed by hole tunneling into the valence band. The tunneling is due to a potential drop across the QW which is due to hole capture at surface states of the Si cap layer making the surface charged. The external potential applied across the QW essentially changes the lateral conductivity as well as the activation energy. The calculations of band profile, free carrier concentration in the QW and acceptor population, as well as an effect on the transverse electric field were carried out taking into account the charging of surface states.     

The research of manufacture of flexible conductive tracks at room temperature

Nano silver is widely used in conductive lines for its advantages, nano size, low melt temperature and excellent conductivity. In general, nano silver of less than 50 nm need be sintered at 200 centigrade in order to acquire conductivity, but it limits the choice of the substrates, only high temperature resistant substrate can be used, for example, PI film and glass. In order to acquire flexible tracks with excellent conductivity at room temperature on paper and PET, nano silver emulsion of 2 percent silver content was prepared through liquid phase chemical reduction, and then the concentration of silver was improved to 17 percent by centrifuging at high speed. And the emulsion was adjusted to meet the demand of inkjet printing, such as surface tension, and viscosity, pH value, then was jetted onto photo paper and PET film. High conductivity was acquired after treated by 830 nm infrared laser, and the sheet resistance is less than 2 omega/(see symbol). After aging test, the conductivity was stable. The method can be used in solar cells, flexible LED, intelligent packaging and can replace the traditional screen printing method at low cost.     

Solid State Synthesis and Thermoelectric Properties of Mg-Si-Ge System

1.IntroductionMuch attention has been paid to thermoelectric mate-rials for manufacturing thermoelectric energy conversiondevices by utilizing temperature change from waste heatand geothermal sources[1~5].The performance of ther-moelectric materials is usually expressed by the figure ofmerit z,z=α2σ/k,whereαis the Seebeck coefficient,σis the electrical conductivity and k is the thermal con-ductivity.The higher the z,the higher the effectivenessof a material for thermoelectric applications[…