高密度聚乙烯/石墨半导体复合物的压阻特性

研究了聚乙烯/石墨半导体复合物(HDPE/GP) 在轴向压力下的压阻特性。结果表明, 这种复合物的导电性有较显著的压力依赖性。在低压力范围内电阻随压力增加而降低, 在较高压力下则随压力增大而升高, 呈现出所谓的“电阻负压力系数(N PCR ) ”和“正压力系数(PPCR ) ”效应。电阻的压力依赖性, 以及在恒压力作用下表现出来的“电阻蠕变”行为, 被认为与导电粒子网络在应力作用下的破坏与重组有关。      

碳纤维增强水泥基复合材料的压阻效应

试验研究了短切碳纤维增强水泥基复合材料(CFRC)的压阻效应, 获得了正、负两种压阻效应相互转换的全过程。从隧道效应和孔隙的连通性角度对该现象的产生机理进行了探讨。结果表明, 在连续烘干和单向循环加载条件下, CFRC的压阻效应会随含水量变化而发生改变。多数情况下, CFRC的体积电阻率随压应变单调减少, 压阻效应为正。含水量越少, 正压阻效应越明显。当含水量减少到约3.19%~4.04%的范围时, CFRC的体积电阻率随压应变单调增加, 压阻效应为负。与正压阻效应相比, 负压阻效应表现更强。CFRC的正、负压阻效应及其相互转换是隧道效应和孔隙连通性两方面相互影响的必然结果。      

The Colossal Piezoresistive Effect in Nickel Nanostrand Polymer Composites and a Quantum Tunneling Model

A novel nickel nanostrand-silicone composite material at an optimized 15 vol% filler concentration demonstrates a dramatic piezoresistive effect with a negative gauge factor (ratio of percent change in resistivity to strain). The composite volume resistivity decreases in excess of three orders of magnitude at a 60% strain. The piezoresistivity does decrease slightly as a function of cycles, but not significantly as a function of time. The material's resistivity is also temperature dependent, once again with a negative dependence.
The evidence indicates that nickel strands are physically separated by matrix material even at high volume fractions, and points to a charge transport mechanism that causes a large change in conductivity for a small relative change in the distance between filler particles. Combined with the temperature dependence data, this suggests that conduction in this composite material may be dominated by quantum tunneling effects. Based upon a statistical model of junction character distribution, a quantum tunneling percolation model is applied that accurately reflects the mechanical and thermal trends.     

碳纤维增强塑料(CFRP)力阻效应的研究评述

碳纤维增强塑料(CFRP)作为一种先进结构材料,其力阻效应的发现引起了广泛关注。基于现有研究成果,分别概述了连续碳纤维增强塑料和短切碳纤维增强塑料的力阻行为,从微观力阻现象和电场传输物理模型两方面分析了力阻效应产生的机理,并对内嵌式CFRP力阻传感器和CFRP智能表层进行了介绍。在此基础上,总结了制约CFRP在传感领域应用的因素并指出了未来研究方向。     

Negative piezoresistivity in continuous carbon fiber epoxy-matrix composite

The negative and positive piezoresistivity in continuous carbon fiber epoxy-matrix composite has been clarified. The negative piezoresistivity associated with the increase of the through-thickness resistivity upon longitudinal tension and decrease in the through-thickness resistivity upon longitudinal compression is practically attractive for strain sensing and is attributed to the decrease in the degree of contact between fibers of adjacent laminae upon longitudinal tension. This effect is stronger, more reversible and less prone to causing minor damage for the tension case than the compression case. The positive piezoresistivity associated with the longitudinal resistivity increasing upon longitudinal tension is negligibly weak, if any, independent of the number of laminae. The previously reported negative piezoresistivity associated with the longitudinal resistivity decreasing upon longitudinal tension does not occur for a commercially manufactured composite in which the fibers are well aligned.     

Uniaxial pressure dependence of the Debye temperature in metals

The electrical resistance and its temperature coefficient have been measured for a platinum foil as a function of uniaxial pressure over the pressure range 0 to 60 MPa. The measurements were performed at room temperature using the transient hot-strip method. The data are analyzed using the electrical resistivity formula within the Block-Grüneisen approximation. The pressure dependence of the Debye temperature was directly obtained from an expansion of this formula and using the basic definition of the temperature resistivity coefficient. The reliability of the experimental data was then verified using the basic definition of Grüneisen constant. Within the investigated pressure range, the analysis supports the interpretation that the change in resistance of platinum under pressure is mainly due to the change in the amplitude of the atomic vibrations that are directly related to the change in Debye temperature. The pressure dependence of resistance and the Debye temperature of the platinum were reasonably good in spite of the approximations involved.     

Theory of piezoresistivity for strain sensing in carbon fiber reinforced cement under flexure

A theory is provided for piezoresistivity in carbon fiber reinforced cement (with and without embedded steel reinforcing bars) under flexure (three-point bending). The phenomenon, which involves the reversible increase of the tension surface electrical resistance and the reversible decrease of the compression surface electrical resistance upon flexure, allows strain sensing. The theory is based on the concept that the piezoresistivity is due to the slight pull-out of crack-bridging fibers during crack opening and the consequent increase in the contact electrical resistivity of the fiber-matrix interface. This work is an extension of prior theory, which concerns the effect of uniaxial loading on the volume resistance. The extension requires modeling the surface resistance and its change under flexure. The theoretical results on the piezoresistivity, both with and without rebar, are in good agreement with prior experimental results. Differences between theoretical and experimental results are probably due to minor damage and rebar debonding during flexure.     

粒径对Ni/硅橡胶复合材料压敏性及介电性质的影响

采用不同粒径的Ni粉与硅橡胶(110型)按质量比2.4∶1.0制成Ni/硅橡胶复合材料, 分别测量了其压敏导电性及介电性质, 并结合扫描电镜照片对其微观导电机制进行了分析。结果表明随着填料Ni粉粒径的减小, Ni/硅橡胶复合材料的直流电阻率对外加压强更加敏感: 在低压强下, 粒径为74、48和18 μm的样品的电阻率随压强的变化率分别为1.73×104、2.59×104和3.71×10 4 Ω·m/kPa。样品直流电阻率陡降的区域随粒径的减小向压强较小的方向移动, 显示出复合材料的渗流阈值与填充粒子的粒径有关: 粒径越小, 渗流阈值也越小。Ni/硅橡胶复合材料的交流电导率、介电常数、介电损耗均随填料Ni粉粒径的减小而变大: Ni粉粒径为18 μm的Ni/硅橡胶复合材料的电导率约为10-2 S·m-1, 比74 μm粒径样品的电导率(约10-7 S·m-1)提高了5个数量级; 对应的介电常数由约102提高到约103。改变填料Ni粉粒径可以有效地调节复合材料的弹性和压敏、电输运特性。      

乙炔炭黑水泥基复合材料的压敏性

为探讨乙炔炭黑水泥基复合材料的压敏性 , 验证了利用基于埋入式环状电极的四电极法测试复合材料电阻的可行性; 研究了一次加载至破坏时乙炔炭黑水泥基复合材料的电阻率变化规律 , 并通过在弹性阶段的加载考察了乙炔炭黑水泥基复合材料压敏性的重现性 ; 分析了测试电流和试件偏压对压敏性的影响。研究结果表明 ,基于埋入式环状电极的四电极法测试压敏水泥基材料的电阻误差小于 10 % , 用于复合材料电阻的测试是可行的。掺量 15 vol %的乙炔炭黑水泥基复合材料一次加载至破坏 , 电阻率变化率最大可达 55 % , 弹性阶段电阻率变化率可达 35 %以上 , 且其压敏性基本不受测试电流和试件偏压的影响。乙炔炭黑是制备具有高感知灵敏度的压敏水泥基材料的一种有效功能组分。     

小应力下炭纤维增强水泥的压敏性和温敏性

研究了炭纤维增强水泥的导电性，测定了它在小应力范围内恒定载荷和循环载荷下电阻率的变化。结果表明，试块的电阻率与压应力有关，反应了试块内部结构及其变形规律，还研究了温度和电阻率压敏性的关系。     

碳纤维增强混凝土的拉敏特性及梁构件的机敏监测

碳纤维具有良好的导电性,电阻率变化率与所处应力场具有稳定的对应关系,在混凝土结构中构造一定厚度的碳纤维增强混凝土机敏层,并通过实时监测电阻率变化率,可对结构的实时荷载和变形程度进行预报.本工作研究了碳纤维增强混凝土本征拉敏特性规律,并将其应用于钢筋混凝土梁构件中,实现了对构件实时荷载和变形的在线监测.     

Study of the volume fraction,temperature, and pressure dependence of the resistivity in a ceramic-polymer composite using a general effective media theory equation

A quantitative general effective media (GEM) equation is used to describe quantitatively the resistivity of an Fe₃O₄-epoxy composite system over a large range of volume fractions in terms of the resistivities of each component and two percolation morphology parameters. One parameter is the critical (percolation) volume fraction, _c, and the other is an exponent,t. Preliminary models, also based on the GEM equation, are used to describe the positive temperature coefficient of resistivity (PTC) and the piezoresistivity (uniaxial pressure) of the composite when the composition is near the percolation threshold.     

Relation between repeated uniaxial compressive pressure and electrical resistance of carbon nanotube filled silicone rubber composite

Carbon nanotube filled polymer composite can be used as sensitive material of flexible pressure sensor. By using solution mixing method, carbon nanotubes are dispersed into silicone rubber matrix to fabricate the composite. The piezoresistivities of the composite with different carbon nanotube concentrations under repeated compressions are researched quantitatively. The monotonicity of the piezoresistivity is dependent on the content of carbon nanotube and the range of the applied pressure. The reproducibility error of the piezoresistivity decreases with the increase of the compression cycles. The experimental data of the piezoresistivity are fitted by the linear combination of two exponential functions. The piezoresistive mechanism is studied qualitatively by analyzing the changes in the carbon nanotube network.     

碳纤维/水泥基复合材料微观结构及机敏特性

以普通硅酸盐水泥为基体材料, 以碳纤维为功能组分, 采用压力成型法制备了碳纤维增强水泥基复合材料(CFRC) 。用SEM 和孔结构分析仪对复合材料的微观结构进行了分析, 同时研究了其机敏特性。结果表明:较大成型压力制备的复合材料孔隙率明显低于较小成型压力制备的复合材料。不同成型压力制备的复合材料电阻率均随温度升高而呈先增大后减小的趋势。较小成型压力制备的CFRC , 其临界温度为75～100 ℃; 较大成型压力制备的CFRC , 其临界温度为100～120 ℃。循环载荷下, 碳纤维水泥基复合材料电阻的相对变化与载荷之间呈现明显的一一对应关系, 较大成型压力制备的CFRC 在每个循环过程中电阻相对变化的幅度明显大于较小成型压力制备的CFRC , 更适合应用于结构的实时、动态的健康监测和损伤评估。      

Multifunctional thin film sensors based on amorphous diamond-like carbon for use in tribological applications

Amorphous diamond-like carbon films are well known for their excellent tribological properties. In this paper, we will discuss the sensoric properties of multifunctional a-C:H films with respect to force/load/pressure and temperature measurement. It turned out that nanostructured amorphous carbon films showed an impressive piezoresistive effect.Unlike well known strain gages and piezoresistive sensors, which detect a deformation of the base substrate, this novel sensor can be used in a complete stiff arrangement without any elastic joint. For film preparation, rf-plasma CVD processes were used. Besides gas pressure, a variation of the substrate power and bias potential was performed. It turned out that the tribological parameters varied slightly (hardness: 20-30 GPa, friction coefficient: 0.10 to 0.15), whereas the electrical parameters showed remarkable differences with respect to the electrical resistivity and piezoresistivity.This paper will also present different applications of amorphous carbon thin film sensors.     

Ag diffusion in ZnS thin films prepared by spray pyrolysis

ZnS thin films were deposited by spray pyrolysis method on glass substrates. Diffusion of Ag in ZnS thin films was performed in the temperature range 80-400 °C under a nitrogen atmosphere. The diffusion of Ag is determined with XRF, and the obtained concentration profile allows to calculate the diffusion coefficient. The temperature dependence of Ag diffusion coefficient is determined by the equation D = 8 × 10^− 9 exp(− 0.10 eV / kT). It was found that the as-grown undoped high resistive n-type ZnS thin films were converted to the p-type upon Ag doping with a slight increase in resistivity only by rapid thermal annealing at 400 °C in N₂ atmosphere. In addition, the band gap of the p-type film was decreased as compared with the undoped sample annealed under the same conditions. The results were attributed to the migration of Ag atoms in polycrystalline ZnS films by means of both along intergrain surfaces and intragrain accompanied by interaction with native point defect.     

短切炭纤维的CVI处理及其在CFRC中的分散性

采用CVI法对短炭纤维进行表面处理, 借助超声波对其进行预分散, 用新型分散剂羟乙基纤维素(HEC)和超细粉硅灰对其进行分散, 并研究了其在水泥基体中的分散性; 在SEM电镜下观察了短炭纤维增强水泥基复合材料(CFRC)的断口形貌, 用炭纤维质量变动系数定量评价了短炭纤维在CFRC中的分散性。结果表明, 采用CVI预处理和超声波预分散, 在分散剂HEC和硅灰不同掺量下, 炭纤维的分散性均得到显著改善。炭纤维的分散性随HEC掺量的增加而提高, 当HEC掺量为水泥质量的0.6%、 硅灰掺量为水泥质量的10%时, 两种分散剂的协同作用使炭纤维质量变动系数最小, 此时炭纤维在水泥基体中的分散性最理想。      

Structural,Electrical and Magnetic Behaviour of FeTe0.5Se0.5 Superconductor

We report the synthesis as well as structural and physical properties of the bulk polycrystalline FeTe and FeTe_0.5Se_0.5 compounds. These samples are synthesised by the solid state-reaction method via vacuum encapsulation. Both studied compounds are crystallized in a tetragonal phase with space group P4/nmm. The parent FeTe compound shows an anomaly in resistivity measurement at around 78 K, which is due to the structural change along with a magnetic phase transition. The superconductivity in the FeTe_0.5Se_0.5 sample at 13 K is confirmed by the resistivity measurements. DC magnetisation along with an isothermal (M–H) loop shows that FeTe_0.5Se_0.5 possesses bulk superconductivity. The upper critical field is estimated through resistivity ρ (T,H) measurements using Gingzburg–Landau (GL) theory and is above 50 T with 50 % resistivity drop criterion. The origin of the resistive transition broadening under magnetic field is investigated by thermally activated flux flow. The magnetic field dependence of the activation energy of the flux motion is discussed.     

Thermophysical properties of solid and liquid beryllium

A submillisecond resistive heating technique under high pressure (0.12 GPa) has been used to measure selected thermophysical properties of both solid and liquid beryllium. Data have been obtained between room temperature and 2900 K. Results on enthalpy, volume expansion, electrical resistivity, and sound velocity measurements are presented.Paper presented at the Third Workshop on Subsecond Thermophysics, September 17–18, 1992, Graz, Austria.     

Superconductivity and structural instability in EuMo6S8 at high pressures (0–70 kbar)

We report the detailed temperature dependence of the electrical resistivity of EuMo₆S₈ as a function of pressure up to 70 kbar under quasihydrostatic pressure conditions. The effect of an external magnetic field on the electrical resistivity has been studied at selected temperatures and pressures. Our results, which are qualitatively different from those reported under hydrostatic pressure conditions, can be explained on the basis of a proposed coexistence of the untransformed rhombohedral phase and the transformed triclinic phase of EuMo₆S₈.