短碳纤维填充HDPE复合体系的渗流网络与压阻行为

研究了短碳纤维(Shortcarbonfiber,SCF)填充高密度聚乙烯(HDPE)导电复合体系的渗流(Percola-tion)与压阻行为(Piezoresistivebehavior,PRB),发现SCF经物理接触而形成的导电网络是复合材料导电的根源.体系的压阻行为呈现浓度依赖性.受压时SCF间隙的减小与渗流网络的局部破坏-重建过程随填料浓度、载荷大小和力学循环次数的变化而变化,导致PRB表现为电阻负压力系数(NPC)、电阻正压力系数(PPC)或两者兼有的现象.讨论了体系PRB的稳定性,发现由于HDPE基体的塑性永久形变,电阻-时间基线随着压缩循环的进行而发生漂移,多次循环可有效提高体系的压阻稳定性.     

HDPE/CB高分子导电复合体系的单轴形变与压阻行为

根据基体形变及其与渗流网络结构之间的关系,研究了高密度聚乙烯(HDPE)/炭黑(CB)复合材料压阻行为的发生机制及其影响稳定性的力学因素.结果表明,单轴压阻行为的产生源于材料受外力变形而导致的渗流网络微结构变化,且这种变化强烈依赖于填料含量.当填料含量较低时(渗流阈值附近),体系电阻率随压力升高而表现为电阻正压力系数行为;当填料含量较高时,体系电阻率随压力升高显现电阻负压力系数效应.完全卸载的零压力下,电阻基线随压缩循环随循环次数增大而发生漂移,这种漂移与轴向残余压缩应变有关,可以通过增加循环次数来加以稳定.交联可以减小残余压缩应变,并抑制高填充复合体系电阻基线的漂移.     

高中生对化学核心概念掌握情况的测查分析——以“电解质、离子反应、化学键”概念群为例

电解质、离子反应、化学键组成一个核心概念群,是学生掌握相关知识的重难点。对北京市普通高中370名高二学生的测查显示:(1)概念掌握的程度为:化学键<离子反应<电解质。(2)“电解质的概念”与“导电原理”“离子反应概念”“离子共存”的掌握程度存在显著相关;电离与离子反应概念的掌握程度显著相关;“导电原理”与“离子反应概念”“离子共存”“离子键与共价键综合辨析”的掌握程度显著相关。表明电解质的概念、导电原理的理解与掌握是概念体系中的重要纽带。(3)优秀、中等、学困3类学生在解决容易和较难2类概念的相关问题上差异最大,而中等难度上的差异相对较小。基于此,提出了相应的核心概念教学对策。     

聚甲基乙烯基硅氧烷/炭黑复合体系的电阻弛豫行为

考察了聚甲基乙烯基硅氧烷/炭黑(CB)复合交联体系在恒定压应力作用下以及完全卸载后的导电行为,发现电阻在恒定压应力作用下及完全卸载后均随时间非线性下降,呈现典型的电阻弛豫行为.根据电阻弛豫时间以及最大电阻弛豫幅度,分析了导电网络结构变化与电阻弛豫之间的关联,讨论了炭黑含量对电阻弛豫的影响.结果表明,在恒定压应力2~4MPa下,电阻弛豫具有两个与CB含量无关的弛豫时间,对应不同尺寸导电网络的结构变化.完全卸载后,基体形变回复造成导电网络发生结构弛豫,弛豫时间随CB含量增大而延长.     

纳米“鸡蛋盒”的简易新制法

利用氧化铝模板,在聚苯乙烯玻璃化转变温度附近,采用气压方法抽取聚苯乙烯纳米膜,得到类似“鸡蛋盒”的纳米结构。 用原子力显微镜观察了聚苯乙烯纳米膜的表面形貌。 在最佳气体压力20 kPa下,可得到9个/μm2纳米“鸡蛋盒”,其深度约为15 nm,随压力增大或减小,单位面积上纳米“鸡蛋盒”的个数均降低。     

磺化聚苯乙炔/多壁碳纳米管复合材料导电特性和机理的研究

将磺化聚苯乙炔(SPPA)与多壁碳纳米管(MWCNTs)超声共混制备得到SPPA/MWCNTs复合材料. 用X光电子能谱仪、固体紫外-可见分光光度计、X射线衍射仪、四探针、场发射扫描电镜等对复合材料导电特性及机理进行研究. 结果表明: SPPA/MWCNTs 复合材料中SPPA与MWCNTs发生电荷转移而被掺杂, 并且由于SPPA与MWCNTs间的电荷转移, 彼此间存在一定的相互作用力; 复合材料电阻呈负温度系数效应; SPPA/MWCNTs复合材料电导率发生两次突跃. 可能的导电机理为, 复合材料中SPPA不仅被MWCNTs物理填充, 同时还被MWCNTs掺杂, 复合材料中存在两种导电通路, 一是SPPA与MWCNTs的碳原子发生电荷转移而被掺杂, 彼此之间存在一定的相互作用力, 导致SPPA包裹MWCNTs形成独立导体单元, 这种独立单元相互接触形成导电通路; 二是MWCNTs彼此之间相互接触形成导电通路, 并建立了该导电机理的理论模型.     

聚氯乙烯/炭黑导电复合体系的压阻行为

研究了聚氯乙烯/炭黑(PVC/CB)导电复合体系在单轴压力作用下的压阻行为,发现CB含量对电阻-机械响应具有显著的影响.当CB含量低于渗流阈值时,PVC/CB复合材料呈现PPC效应;而高于渗流阈值时,呈现NPC效应.在渗流阈值附近,单轴压缩可诱导NPC效应的出现,或者抑制PPC效应.     

磺化聚苯乙炔/多壁碳纳米管复合材料导电机理研究

将磺化聚苯乙炔(SPPA)与多壁碳纳米管(MWNT)超声共混制备得到SPPA/MWNT复合材料. 用四探针电阻率测试、场发射扫描电镜(FESEM)、XPS、UV-Vis、XRD等方法对复合材料导电机理进行研究. 结果表明, SPPA/MWNT的电导率发生两次突跃；掺杂剂MWNT具有低的临界阈值; 临界阈值附近, 复合材料中MWNT具有不连续分布的现象及复合材料电阻呈负温度系数(NTC)效应; SPPA/MWNT复合材料中MWNT的碳原子对SPPA 进行掺杂. 推测复合材料的导电机理为, 共轭聚合物SPPA不仅被导电粒子MWNT物理填充, 同时还被MWNT的碳原子掺杂, 使复合材料中存在两种导电通路而导电, 一是因被掺杂而成为高电导率主体的SPPA相互接触形成的导电通路, 二是MWNT相互接触形成的导电通路.     

从过量体积推算液相活度系数与汽液平衡数据

本文运用统计热力学导出了一个直接由过量体积数据推算液相活度系数的新模型--EVLAC模型,本模型主要从“双液胞腔”与“局部组成”等概念出发,通过配分函数导出y-x表达式和V^E-x表达式,仅需要一个温度下的V^E-x数据即可推算出不同温度下盼恒温y-x,数据和不同压力下的恒压y-x数据,在不需要增加多元可调参数的情况下,可推广应用至多元体系。     

复合材料柔软性对倒金字塔微结构阵列传感器性能的影响

采用模压成型方法制备了2种柔软性不同的热塑性聚氨酯/短切碳纤维/碳纳米管(TPU/SCF-CNT)复合材料复制物, 其表面上具有倒金字塔微结构阵列, 内部有SCF与CNT共同构成的导电通路. 将复合材料复制物和相应的复合材料平整片封装成柔性传感器. 结果表明, 压力作用下传感器内复制物和平整片之间的接触电阻因倒金字塔底棱的形变而显著降低. 对使用柔软性较高的复合材料封装的传感器, 虽然其相对迟滞稍大, 但压力作用下倒金字塔底棱形变量较大, 且复制物和平整片内导电通路增加量较大, 因此其在0~2.5 kPa的线性区内具有较高的灵敏度(0.32 kPa^?1). 制备的2种传感器均具有快速响应特性, 且能在500 s(约1580次)的循环压缩/释放测试(峰值压力约3 kPa)中保持较稳定的电阻响应. 研究表明, 利用模压成型的表面倒金字塔结构复合材料复制物封装成的柔性压力传感器具有良好的传感性能.     

The piezoresistive behaviors of polyethylene/foliated graphite nanocomposites

A conductive nanocomposite with an ordered conductive network and low-percolation threshold were prepared by adding foliated graphite (FG) nanosheets to high-density polyethylene (HDPE). The piezoresistive behavior of the nanocomposites was investigated under different pressure ranges. There existed a critical pressure below which composite resistance decrease with the increase of pressure and above which resistance increased sharply with increasing of pressure. The critical pressure is a function of the concentration of FG and the sensitivity of the change in resistivity against the applied pressure strongly depended on the FG content. The critical pressures of the nanocomposites with FG concentration at 6 vol.%, 11 vol.%, 12 vol.%, are about 7 MPa, 10 MPa, and 12 MPa, respectively. After repeated pressure treatment, there was a gradual decrease in the change of conducting structure although a permanent damage occurred due to the irreversible deformation of polymer matrix. As a result, the piezoresistive behavior of HDPE/FG nanocomposites tended to be constant under cyclic compression. The behavior was accounted for by an extension of tunneling conduction theory to which provides a good approximation to the piezoresistive effect.     

Effects of ultra-high pressures (>3.6 GPa) on the electrical resistance of polyaniline by in situ FT-IR studies

Electrical resistance measurements and FT-IR spectroscopy of polyaniline were studied in situ under ultra-high pressures generated by a diamond anvil cell (DAC). The electrical resistance of polyaniline decreased as the pressure increased, exhibiting polaron conductor characteristics. Minimum electrical resistance was observed at 3.6 GPa, about three orders of magnitude smaller than that at 1.0 GPa. Changes in electrical resistance were reversible when the pressure was below 3.6 GPa. In situ FT-IR results showed that irreversible chemical changes of the quinoid units in polyaniline molecular chains took place when the pressure exceeded 3.6 GPa, giving rise to a huge increase in electrical resistance.     

Structural Properties of Nitromethane Molecular Crystal under High Pressure： An ab initio Investigation

The pressure-dependent structural properties under hydrostatic pressure up to 120 GPa and the decomposition under uniaxial compression along the b lattice vector up to 40 GPa of nitromethane molecular crystal using ab initio method are presented. The internal molecular bond lengths and bond angles were calculated for different pressures. All bond lengths decrease as the pressures are increased under hydrostatic compression. The obvious rotation of methyl group is 33.89° under hydrostatic pressure at 120 GPa. In addition, we observe the change of C-H bonds, which have been stretched under uniaxial compression along b lattice vector in the range of 0-40 GPa of nitromethane.     

Current-voltage characteristics of thin poly(biphenyl-4-ylphthalide) films

The paper considers the features of the charge transport near to the threshold of the transition of polymer films to the high conductive state, induced by a small uniaxial pressure. The problem has not been solved so far, how the energy structure of a wide-band-gap organic dielectric varies near this threshold. The current-voltage characteristics of poly(biphenyl-4-ylphthalide) films at different uniaxial pressures were measured and analyzed. The interpretation of the obtained results is carried out within the framework of the space charge limited conduction model. The estimation of the injection model of transport parameters such as the charge carrier mobility and concentration, trapping state concentration and others are carried out. The analysis of the obtained results allows to make the following preliminary conclusion. Pressure increase promotes formation of a narrow trap band near the quasi-Fermi level resulting from the increase in the injection. This can give rise to a sharp magnification of the charge carrier mobility and even transition to the metallic state.     

Changes in electrical resistance of carbon‐black‐filled silicone rubber composite during compression

We studied the changes in the electrical resistance of carbon black filled silicone rubber composite, which is the sensitive element of the flexible force sensor, as a function of time during compression. The experimental results show that there is a sudden increase of the electrical resistance along with the sudden increase of the stress immediately after the compression. When the sample strain is kept constant, the electrical resistance and the stress both decay with time. The data of the stress relaxation and the resistance relaxation both can be fitted by the linear combination of two exponential functions. Based on the shell structure theory, the experimental phenomena are explained from the view that the uniaxial pressure induces the changes in the effective conductive paths. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2700–2706, 2007     

Electrically enhanced dewatering (EED) of particulate suspensions

Electrically enhanced dewatering has been characterised from first principles using model kaolinite suspensions in both the coagulated and dispersed state and sludge from a potable water treatment plant. The dewatering properties, namely the compressive yield stress or extent of dewatering (quantified as the applied pressure at an equilibrium solids concentration) and the hindered settling function or the resistance of fluid flow (quantified as the rate of fluid expression at an equilibrium solids concentration), have been measured as a function of electric field strength. For both the dispersed and coagulated kaolinite suspensions, the rate of dewatering was found to improve at all applied pressures and with increasing applied electrical field strength, up to 1250 V m⁻¹. Improvements in the extent of dewatering were also observed but only for the coagulated suspension. The greatest improvements in dewatering were observed at the lowest applied pressures. Improvement in suspension compressibility is only predicted for the specific case where the feed to the process is both coagulated and at a neutral or low pH. For the potable water sludge, improvements in the rate of dewatering were observed at all pressures above 10 kPa. At lower pressures, the low particle surface charge caused a slow onset of electro-osmotic effects. In general, the results indicate that the application of an electric field in situ during dewatering primarily helps to increase the rate of dewatering and the benefit of this methodology is predicted to be for suspensions that exhibit low permeability at low solids concentrations in applications where the pressure is low or the process is gravity driven.     

Experimental contribution to the understanding of transport through polydimethylsiloxanenanofiltration membranes: Influence of swelling,compaction and solvent on permeation properties

This study aims to better understand the permeation properties of polydimethylsiloxane (PDMS) membranes. The compressibility and nanofiltration fluxes were measured for swollen PDMS films using several solvents at applied pressures ranging from 5 to 50 bar. The degree of swelling varied according to the solvent and the pressure applied. To show the correlation between the behaviour of the swollen PDMS under pressure and its permeation performance, the thickness reduction of the membrane was mimicked using uniaxial compression tests. The evolution of the nanofiltration flux as a function of the transmembrane pressure proved to be non-linear. Linearization was achieved by taking into account both the swelling and the thickness reduction previously measured, confirming that these phenomena may have occurred during the nanofiltration experiments. Moreover, the solvents' viscosity and affinity for the polymer were confirmed to have a great influence on their ability to permeate the membrane. Finally, employing the most commonly used models, a study of transport through the membrane led to the conclusion that the experimental results were in agreement with the hydraulic theory of transport.     

Optimization of performance of monolithic capillary column in gas chromatographic separations

Dependence of monolithic column efficiency on column pressure was analyzed using modified Van Deemter relationship with incorporated inlet and outlet column pressures as independent variables. It was demonstrated that the highest column efficiency is observed at high pressures. Inlet and outlet pressure increase has to be controlled in such a way that the relative pressure approaches 1 and the pressure drop across the column is close to zero. Experimental results obtained for open and monolithic capillary columns confirm up to 50% higher column efficiency as compared to column efficiency under standard conditions found using conventional Van Deemter plot. Pressure increase also results in a decrease in the optimal carrier gas velocity and corresponding increase in the analysis time. This drawback can be compensated via an increase in the column temperature.     

Evaporation rates and vapor pressures of the even-numbered C8-C18 monocarboxylic acids

Temperature-dependent vapor pressures of the even-numbered alkanoic monoacids from C8-C18 were measured using temperature-programmed desorption (TPD). In TPD, the evaporation rates from the samples are directly measured and the vapor pressures are subsequently determined from the Hertz-Knudsen equation. Our measurements indicate that the vapor pressures of the solid even-numbered alkanoic acids decrease monotonically with increasing carbon number by more than 6 orders of magnitude going from C8 to C18. The enthalpies of sublimation increase monotonically with carbon number, from approximately 110 to 205 kJ/mol. The liquid-phase vapor pressure was measured for oleic acid, a C18 alkenoic acid. Comparison to the estimated liquid-phase vapor pressure for the corresponding C18 alkanoic acid indicates that the liquid-phase vapor pressures of these two compounds are identical. Our measured solid-phase vapor pressures for the C14 and larger alkanoic acids are lower than in previous studies. We attribute these differences to the influence of residual solvent molecules on the previous measurements, which cause the measured vapor pressures to be too large.