Polymere Nanoverbundwerkstoffe: Chancen,Risiken und Potenzial zur Verbesserung der mechanischen und physikalischen Eigenschaften

Polymer Nanocomposites: chances, risks and potential to improve the mechanical and physical properties The development of nano‐particle reinforced polymer composites is presently seen as one of the most promising approaches of materials for future engineering applications. The unique properties of at least some types of the nano‐particles (e.g., Carbon Nanotubes or Carbon Black) and the possibility of combining them with conventional materials and reinforcements (e.g., carbon‐, glass‐ or aramid‐fibres), has led to an intense research in the field of nanocomposites. Especially Carbon Nanotubes have shown a high potential for an improvement of the properties of polymers. Besides an increase in the electrical conductivity even at an extremely low nanotube content the improvement of the mechanical properties is of special interest. The exceptionally high aspect ratio in combination with a low density and a high strength and stiffness make the carbon nanotubes a most interesting candidate for a reinforcement of polymeric materials. The electrical, mechanical and thermal properties of Carbon Nanotubes open up new perspectives also for their use as multifunctional materials, e.g. conductive polymers with improved mechanical performance. The problem, however, is to transfer the interesting potential regarding the mechanical, thermal and electrical properties to the polymer. Two main issues have to be addressed for a significant improvement of the properties of polymers by adding Carbon Nanotubes: the interfacial bonding and, especially also, a proper dispersion of the individual Carbon Nanotubes in the polymeric matrix.     

Thermal conductivity and electrical resistivity of the T′ phase and the T phase in 2-1-4 oxide superconductors

The temperature dependences of the thermal conductivity,(T), and the electrical resistivity,(T), of sintered samples of Nd_1.85Ce_0.15CuO_4–y (NCCO) and Pr_1.85Ce_0.15CuO_4–y (PCCO) with the T phase, and La_1.85Sr_0.15CuO_4–y (LSCO) and La_1.935Ca_0.065CuO_4–y (LCCO) with the T phase, have been measured. The superconducting transition temperatureT _con , of NCCO, PCCO, LSCO, and LCCO were 26.4, 23.0, 37.1, and 8.8 K, respectively. The enhancement of(T) below justT _con such as seen in 1-2-3 oxide superconductors was not clearly observed.(T) for the samples of NCCO and PCCO showed a pronounced maximum such as that of sapphire crystal around 30 and 40 K, respectively. For these samples of NCCO and PCCO, it has been shown that the analysis based on the various phonon scattering mechanisms is in good agreement with our experiment.     

Thermal Conductivity of Sm3S4 System with Mixed Valence Sm Ions

The thermal conductivity () and electrical resistivity () of mixed-valence compound Sm₃S₄ have been measured in the temperature range 5 to 300 K. The present results and those presented previously [1] for the thermal conductivity between 80 to 850 K are interpreted in terms of the temperature-dependent fluctuating valence of Sm ions. Sm₃S₄ crystallizes in the cubic Th₃P₄ structure, and the cations with different valences occupy equivalent lattice sites. Divalent and trivalent Sm ions are randomly distributed in the ratio of 1:2 over all possible crystallographic cation positions (Sm²⁺ ₂Sm³⁺ ₂S^2– ₄). The behavior of the Sm₃S₄ lattice thermal conductivity _ph is extraordinary since valences of Sm ions are fluctuating (Sm³⁺Sm²⁺) with a temperature dependent frequency. In the interval 20 to 50 K (low hopping frequencies), _ph of Sm₃S₄ varies as _ph T ^–1 (it is similar to materials with static distribution of cations with different valences): at 95 to 300 K (average hopping frequencies 10⁷ to 10¹¹ Hz), _ph changes as _ph T ^–0.3 (it is similar to materials with defects). Defects in Sm₃S₄ appear because of local strains in the lattice by the electrons hopping from Sm²⁺ ions (with big ionic radii) to Sm³⁺ ions (with small ionic radii) and back (Sm²⁺Sm³⁺), at T>300 K (high hopping frequencies), _ph becomes similar to materials with homogenous mixed valence states [1].     

Thermophysical Properties of W–Re Alloys Above the Melting Region

In earlier experiments we have studied pure elements with a fast pulse heating technique to obtain thermophysical properties of the liquid state. We report here results for thermophysical properties such as specific heat and dependences among enthalpy, electrical resistivity, and temperature, for four W–Re alloys (3.95, 21.03, 23.84, and 30.82 at % of Re) in a wide temperature range covering solid and liquid states. Thermal conductivity is calculated using the Wiedemann–Franz law for the liquid alloy, as.well as data for thermal diffusivity for the beginning of the liquid phase. Additionally, data for the entire temperature range studied have been analyzed in comparison with those of the constituent elements, tungsten and rhenium, since both metals have been studied previously with the same experimental technique. Such information is of interest in the field of metallurgy since W–Re alloys of low Re content in the region of mutual component solubility in the solid state are widely used as thermocouple materials for the purposes of high-temperature thermometry.     

Specific Heat,Electrical Resistivity,and Linear Thermal Expansion of the Magnesium Alloy AE42 Measured by Subsecond Pulse Heating

Magnesium alloy AE42 (Mg with 4% Al and 2% rare earths) is used for the production of high-temperature creep-resistant castings. Its thermophysical properties are used as input parameters for the numerical simulation of the casting process by finite methods. Measurements of specific heat capacity, electrical resistivity, and linear thermal expansion of the magnesium alloy AE42 in the temperature range from 550 to 840 K by a transient technique are presented and discussed. Tubular specimens were Joule-heated from room temperature up to melting within 500 ms by a large current pulse. The current and the voltage drop along a defined portion of the specimen were measured by a fast precision data acquisition system. Temperature measurements were made with a high-speed broad-band infrared pyrometer. Thermal expansion was measured by a polarized-beam Michelson-type interferometer. Paper presented at the Seventh International Workshop on Subsecond Thermophysics, October 6–8, 2004, Orléans France.     

Thermal and electrical conductivities of an improved 9 Cr-1 Mo steel from 360 to 1000 K

The electrical and thermal conductivities and Seebeck coefficients of three 9 Cr-1 Mo samples were measured over the temperature range 360–1000 K. All of the samples were in the normalized and tempered condition and two of the samples were from different heats of a new, modified alloy. The thermal conductivity of the third sample, which was from a commercial heat, was found to agree well with the ASME code values for this steel. The two heats of modified 9 Cr-1 Mo were found to have significantly higher thermal conductivities and this difference appears to be due to the lower Si content of the modified alloy. The results were compared with the predictions of standard transport theory and data on bcc Fe. These comparisons show that phonon energy transport is important and quite dependent upon the Si content.     

纳米银镓合金/聚甲基丙烯酸甲酯复合粒子的结构表征

在不加任何引发剂和金属还原剂的条件下, 超声辐射双原位引发乳液聚合制备纳米银镓合金/聚甲基丙烯酸甲酯(Ag-Ga/PMMA)复合粒子。利用HREM、 EDS、 XRD等对复合粒子进行了分析, 表明所制得的乳胶粒子具有典型的核壳结构, 粒径为80~200nm, 分布均匀, 单分散性好, 基本没有出现团聚。乳胶粒子中成壳部分的聚合物产生了一层层有序组装的现象。XRD证明, 有纳米合金Ag_0.72Ga_0.28存在; 此外还出现了2个新的衍射峰, 推断可能是新的银镓合金物质。      

The thermal conductivity of AISI 304L stainless steel

A compilation and critical analysis of the thermal conductivity () of AISI 304 stainless steel (SS) between 100 and 1707 K has been given in the literature. The author represented his recommended values of by an inflection in the A versus temperature relationship between 300 and 500 K. Because a physical mechanism had not been identified that would produce such a temperature dependence in of 304 SS, interest was generated in the possible existence of an as yet undiscovered phenomenon that might cause such an inflection. Consequently, experimental verification of the inflection was sought. The present paper presents recent measurements of , the electrical resistivity, and the absolute Seebeck coefficient of 304L SS from 300 to 1000 K and of the thermal diffusivity () from 297 to 423 K. The values computed from the a measurements were within ± 1.6% of the directly measured An inflection was not observed in the temperature dependence of between 300 and 500 K. After careful evaluation and because a physical mechanism still has not been identified which would produce such an inflection, the authors conclude that the inflection in the vs T relationship reported in the literature was caused by the data analysis technique.     

石墨化温度及掺硅组分对再结晶石墨传导性能及微观结构的影响

用煅烧石油焦作填料 ,煤沥青作粘结剂 ,分别以硅粉、碳化硅和二氧化硅 3种含硅组分作添加剂 ,采用热压工艺制备了再结晶石墨。考察了石墨化温度以及单组元掺硅组分对再结晶石墨的热导率、电阻率和抗折强度的影响及其微观结构的变化。实验结果表明 ,对于掺硅组分相同的再结晶石墨 ,材料的导电、导热性能随着石墨化温度的升高而增强 ,但其力学性能却随之降低。当掺硅粉的热压再结晶石墨再经 2 80 0℃石墨化处理后 ,材料RG Si 48沿石墨层方向的常温热导率可达 3 3 2W /(m·K) ,电阻率为 4.94μΩ·m。对于相同工艺及硅含量的不同掺硅组分再结晶石墨 ,以掺入硅粉对材料综合性能最理想 ,而掺入二氧化硅对材料的综合性能较差。XRD分析表明 ,不论掺硅组分是硅粉、碳化硅还是二氧化硅 ,硅组分最终在再结晶石墨内均以α SiC形式存在 ,甚至在石墨化温度高达 2 80 0℃时 ,再结晶石墨内仍有α SiC存在。对其微晶参数进一步分析表明 ,随着石墨化温度的升高 ,掺杂硅组分的催化作用进一步加强 ,再结晶石墨的微晶尺寸La 迅速增大 ,石墨微晶的晶面层间距d0 0 2 也迅速降低。材料RG Si 48的微晶尺寸La 以及晶面层间距d0 0 2 分别为2 5 7nm和 0 .3 3 5 77nm。     

Measurement of the Electrical Conductivity of Metals in the Vicinity of the Critical Point

Measurement of the electrical conductivity of metal plasmas at near-solid densities is made possible by rapid vaporization of metal wires in water. The water acts as a tamper, slowing the expansion of the plasma and ensuring a well-defined cylindrical boundary. Measurement of the time-varying resistance is straightforward, and the conductivity and density are easily deduced after the column diameter is measured photographically. Temperature may be deduced from the measured energy input with help of an equation of state provided by the LANL SESAME tables. Measurements have been made on copper, aluminum, and tungsten plasmas. The electrical conductivity is almost independent of temperature at the highest densities. Conductivity falls steeply with falling density and reaches a minimum at a few percent of solid density, rising with further reduction in density. Near the minimum, the temperature dependence of the conductivity becomes appreciable.     

空间太阳近紫外辐照对ACR-1白漆表面导电性能的研究

通过试验研究了近紫外辐照对ACR-1白漆电学性能的影响,并用X射线光电子能谱仪和四极质谱仪对ACR-1白漆的组分和状态等进行了分析,进而对其电学性能变化的机理进行了分析。研究发现：ACR-1白漆的表面电阻率随着近紫外辐照度的增加而呈指数规律减小,并在大气环境中存在“回复”效应;氧化锌的分解、吸附氧的减少和氧空位的增多是近紫外辐照后ACR-1白漆表面电阻率减小的主要原因。     

Thermophysical Properties of Solid and Liquid 90Ti–6Al–4V in the Temperature Range from 1400 to 2300 K Measured by Millisecond and Microsecond Pulse-Heating Techniques

The heat capacity and electrical resistivity of 90Ti–6Al–4V were measured in the temperature range from 1400 to 2300 K by two pulse-heating systems, operating in the millisecond and microsecond time regimes. The millisecond-resolution technique is based on resistive self-heating of a tube-shaped specimen from room temperature to melting in less than 500 ms. In this technique, the current through the specimen, the voltage drop along a defined portion of the specimen, and the temperature of the specimen are measured every 0.5 ms. The microsecond-resolution technique is based on the same principle as the millisecond-resolution technique except for using a rod-shaped specimen, a faster heating rate (by a factor of 10,000), and faster data recording (every 0.5 s). Due to the rapid heating with the microsecond system, the specimen keeps its shape even in the liquid phase while measurements are made up to approximately 300 K above the melting temperature. A comparison between the results obtained from the two systems with very different heating rates shows significant differences in phase transition and melting behavior. The very high heating rate of the microsecond system shifts the solid–solid phase transition from the (+) to the phase to a higher temperature, and changes the behavior of melting from melting over a temperature range to melting at a constant temperature like an eutectic alloy or a pure metal.