纳米碳包镍/环氧树脂复合材料的NTC效应及其调节

采用磁场辅助固化工艺,制备了纳米碳包镍/环氧树脂复合材料,用SEM观察了复合材料的微观组织,测量了复合材料的直流电学特性,考察了磁场强度、纳米粒子含量、温度对电性能的影响。在无磁场情况下固化时,纳米粒子在环氧树脂中均匀分布;而施加磁场后,纳米粒子在基体中以纤维状线性排列;施加的磁场越大,纤维组织发育得越粗壮,获得的复合材料的电阻率越小。复合材料的I-U曲线呈现非线性特征,表明电导机制依赖于导电粒子之间的隧道传递,电阻率随温度上升而下降,显示奇怪的负温度系数（NTC）效应,且可由填料含量、磁场强度调节。为解释NTC起源,提出一个修正的量子隧道模型。根据该模型,复合材料的NTC效应归因于占统治地位的电子热活化隧道传递;另一方面,环氧树脂的低膨胀率促成了这一效果。      

导电炭黑填充硅橡胶的电阻温度系数

以炭黑(CB3100)为导电相,硅橡胶为基质制备导电复合材料。研究导电橡胶中炭黑质量分数对电阻温度系数的影响,并用填料对电阻温度系数的影响。以隧道效应理论为基础,给出了导电炭黑填充橡胶的电阻温度系数计算模型,结合实验得到温度对导电炭黑/硅橡胶电阻温度系数的影响主要体现在对其电阻率的影响;基体的体积热膨胀提高复合材料的电阻率,提高了正电阻温度系数;炭黑粒子间的隧道效应降低复合材料的电阻率,增强了负电阻温度系数;在炭黑/硅橡胶中加入少量碳纳米管,利用碳纳米管和炭黑的协同补强效应,使复合材料的导电性和稳定性提高。     

Epoxy Resin/Nano Ni@C Composites Exhibiting NTC Effect with Tunable Resistivity

Epoxy resin/Ni@C nanoparticle composites with aligned microstructure were prepared by using a procedure of magnetic field assisted curing. The results show that the resistivity of composites exhibits negative temperature coefficient (NTC) effect above room temperature, and can be adjusted by varying the content filler and the magnitude of magnetic field applied. Hill's quantum tunneling model was modified to understand the electrical conduction mechanism in the composites. It shows that the NTC effect ascribes to the dominant thermal activated tunneling transport of electron across adjacent nanoparticles, as well as the low thermal expansivity of epoxy resin matrix.     

Smart Behavior of Carbon Fiber Reinforced Cement-based Composite

The electrical characteristics of cement-based material can be remarkably improved by the addition of short carbon fibers.Carbon fiber reinforced cement composite (CFRC) is an intrinsically smart material that can sense not only the stress and strain, but also the temperature. In this paper, variations of electrical resistivity with external applied load, and relation of thermoelectric force and temperature were investigated. Test results indicated that the electrical signal is related to the increase in the material volume resistivity during crack generation or propagation and the decrease in the resistivity during crack closure. Moreover, it was found that the fiber addition increased the linearity and reversibility of the Seebeck effect in the cement-based materials. The change of electrical characteristics reflects large amount of information of inner damage and temperature differential of composite, which can be used for stress-strain or thermal self-monitoring by embedding it in the concrete structures.     

Effect of extensional strain on the resistivity of electrically conductive nitrile-rubber composites filled with carbon filler

The resistivity and mechanical properties of nitrile-rubber based conductive composites filled with short carbon fibres (SCFs) and mixed filler system (SCF + carbon black) are studied as functions of the extensional strain and the strain rate. It has been observed that both strain and strain rate have a strong influence on the resistivity of the composites. The sensitivity of the change in resistivity against the strain and strain rate depends on the concentration as well as the type of conductive filler. SCFs impart higher conductivity to the composite than a blend of SCFs and carbon black at the same level of loading parts per hundred of rubber (p.h.r.). Composites filled with a mixed filler system show high mechanical properties in contrast to those of SCF-filled composites. The change in resistivity with the degree of strain is less pronounced in mixed-filler-filled composites than in only carbon-fibre-filled composites. The mechanical properties of the composites are dependent on the polymer-filler interaction whereas change in resistivity is dependent on the transient arrangement of the conducting components in the polymer matrix. A good correlation exists between mechanical and electrical response to the strain sensitivity.     

多壁碳纳米管增强炭黑/聚丙烯导电复合材料导电行为

为了充分利用不同导电粒子的导电作用,在炭黑(CB)/聚丙烯(PP)导电复合体系中引入了多壁碳纳米管(CNTs)。研究发现：引入的CNTs分散在CB粒子间起到“桥梁”作用,使体系的导电性能得到明显改善,并且CB∶CNTs为19∶1时其协同导电效果最好,该复合体系出现逾渗现象,对应的导电填料体积分数明显降低。在导电填料总体积分数为4.76%时,少量CNTs的引入就可使复合体系的体积电阻率从109Ω·cm下降到105Ω·cm;同时少量的CNTs能明显抑制炭黑/聚丙烯导电复合材料的正温度效应(PTC),使PTC强度从6.10降低到1.48,PTC转变峰温度从166℃升高到174℃。少量的 CNTs可以使PP的结晶温度提高12℃,对PP结晶的成核作用比CB更加明显。复合体系力学性能随导电填料体积分数增加而明显降低,但因为体积电阻率一定时CB-CNTs/PP体系所需导电填料体积分数较CB/PP体系明显降低,因此少量CNTs的引入能够使复合体系的力学性能得到更大程度的保持。     

Pitch-matrix composites for electrical,electromagnetic and strain-sensing applications

Pitch-matrix composites for deicing, electromagnetic shielding and strain sensing have been developed by using carbon fiber (discontinuous) and carbon black as electrically conductive fillers. A composite with carbon fiber (5 vol%) as the sole filler is effective for strain sensing, which functions by the electrical resistivity increasing reversibly with tensile strain. A composite with carbon fiber (3.4 vol%) and carbon black (1.5 vol%) is less effective for strain sensing and is lower in tensile strength, modulus and ductility, but it is lower in the electrical resistivity. A composite with carbon black (7 vol%) as the sole filler is very high in resistivity, but exhibits high storage modulus. Either carbon fiber or carbon black as filler increases the storage modulus, decreases the resistivity, renders the ability to provide EMI shielding and increases the softening temperature.     

基于炭黑填充导电橡胶的压力/温度传感器非线性特性

通过研究电阻率和电阻几何系数对"负电阻-压力系数"(NPCR)和"正电阻-温度系数"(PTCR)的影响,分析了炭黑填充导电橡胶(导电炭黑/橡胶)的压力/温度传感器的非线性特性。结果表明: 导电炭黑/橡胶的NPCR和PTCR效应产生非线性的主要原因为电阻率的非线性变化; 当炭黑体积分数接近渗流体积分数时,其电阻率对体积的变化敏感程度高,此时,导电炭黑/橡胶的NPCR和PTCR效应的非线性特性较强; 由于导电炭黑/橡胶的体压缩系数大于其热膨胀系数,且导电炭黑/橡胶在压力场和温度场下的形变过程不同,导电炭黑/橡胶NPCR效应的非线性强于PTCR效应的非线性。     

Micromechanical deformations in particulate filled thermoplastics: volume strain measurements

Volume strain measurements were carried out on PP composites containing different CaCO₃ fillers. During deformation, a volume increase was detected which could be divided into two linear sections as a function of elongation. Comparison of data with existing theories has shown that in the first part, mostly elastic deformation takes place and the slope can be related to the Poisson's ratio of the composite. Scanning electron microscopy revealed that in the second stage, the dominating micromechanical deformation process is debonding. Void formation is initiated at a certain stress which approximately corresponds to the yield stress of the composites, but data in the literature and model calculations indicate that separation of the matrix/filler interface may start at lower stresses. Initiation stress depends on the particle size of the filler and on interfacial interactions. The rate of volume increase has non-linear dependence on the volume fraction of the filler. Volume strain measurements reflect micromechanical deformations well, but further study is needed to explain contradictions between experimental results and theoretical predictions.     

Tensile properties and fracture behaviour of polypropylene-nickel-coated carbon-fibre composite

Polypropylene-based composites reinforced with nickel-coated carbon fibres have been prepared and the effect of filler content on tensile properties and fracture behaviour at different temperatures and strain rates was investigated. The elastic modulus of such composites is enhanced by two orders of magnitude while the tensile strength and strain-to-break are lowered. The fracture toughness parameters,G _c andK _c, are also enhanced with filler content. The yield stress of this composite showed strain rate and temperature dependence. Activation energy and volume of a single rate-activated yielding process, at relatively high strain rates, were determined. The variations of the measured physical quantities are discussed in terms of the observed composite morphology.On sabbatical leave from the Physics Department, University of Jordan, Amman, Jordan.     

Mechanism of radiation-induced degradation in mechanical properties of polymer matrix composites

Four kinds of polymer matrix composites (filler, E-glass or carbon fibre cloth; matrix, epoxy or polyimide resin) and pure epoxy and polyimide resins were irradiated with ⁶⁰Co -rays or 2 MeV electrons at room temperature. Mechanical tests were then carried out at 77 K and at room temperature. Following irradiation, the Young's (tensile) modulus of these composites and pure resins remains practically unchanged even at 170 MGy for both test temperatures. The ultimate strength, however, decreases appreciably with increasing dose. The dose dependence of the composite strength depends not only on the combination of fibre and matrix in the composite but also on the test temperature. A relationship is found between the composite ultimate strain and the matrix ultimate strain, thus indicating that the dose dependence of the composite strength is virtually determined by a change in the matrix ultimate strain due to irradiation. Based on this finding, we propose a mechanism of radiation-induced degradation of a polymer matrix composite in order to explain the dose dependence of the composite strength measured at 77 K and at room temperature.     

Control of the temperature coefficient of the DC resistivity in polymer-based composites

In this study, the roles of polymer matrices and filler additives in controlling the positive temperature coefficient (PTC)/negative temperature coefficient (NTC) behavior of DC resistivity at high temperature for semicrystalline ethylene vinyl acetate copolymer, amorphous acrylonitrile butadiene copolymer, and their blend composites filled with different carbon fillers like Conductex carbon black, Printex carbon black, and short carbon fiber have been investigated. It is seen that the PTC/NTC behavior of resistivity depends on the characteristics of both polymer matrices and filler additives. The anomaly in the results are due to polymer crystallinity, shape and size of fillers, and their thermal expansion coefficient, that play major role in controlling the PTC/NTC of resistivity at high temperature for the composites. Finally, reproducibility of composite resistivity has been evaluated with their some proposed practical applications. These composites can be used as both PTC and NTC thermistors.     

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。      

Effects of Al2O3 Particulates on the Thickness of Reaction Layer of Al2O3 Joints Brazed with Al2O3-Particulate-Contained Composite Filler Materials

In order to understand the rate-controlling process for the interracial layer growth of brazing joints brazed with active composite filler materials, the thickness of brazing joints brazed with conventional active filler metal and active composite filler materials with different volume fraction of AI203 particulate was studied. The experimental results indicate although there are Al2O3 particulates added into active filler metals, the time dependence of interracial layer growth is t^2 as described by Fickian law for the joints brazed with conventional active filler metal. It also shows that the key factor affecting the interracial layer growth is the volume fraction of alumina in the composite filler material compared with the titanium weight fraction in the filler material.     

Low temperature electrical conductivity of low-density polyethylene/carbon black composites

The study deals with the electrical characteristics of carbon black/low-density polyethylene (CB/LDPE) composites of various CB filler concentrations (10, 15, and 20 wt.%). The DC electrical conductivity was studied as a function of filler concentration in low temperature range 25–285 K. It was found that the composites exhibit negative temperature coefficient of resistivity (TCR) at low temperatures and high enhancement in the electrical conductivity with both temperature and carbon black concentration. The observed increase of conductivity with the filler concentration was interpreted through the percolation theory. The dependence of the electrical conductivity of the given composites on temperature (25–285 K) was analyzed in terms of a formula in consistence with Mott hopping mechanism.Visiting scientist from the Jordan University of Science and Technology, Irbid-Jordan.     

Carbon based conductive photoresist

A conductive photoresist for photolithographic application was studied here. The negative near-UV sensitive epoxy-based photoresist was used as a polymer matrix and conductive carbon black was used as functional filler. DC electrical resistivity of composite as a function of filler concentration has a well-known S-shape. After UV-exposure the resistivity of the composite decreases for almost five orders of magnitude, mostly at percolation threshold (approx. 0.6 vol.%). This effect can be attributed to the fully cross-linked polymer structure formed during UV-exposure of the composite. The resistivity of prepared samples also depend on the state of dispersion of the functional filler obtained using different dispersing additives. Composites with better dispersed particles have lower resistivities. This effect remained below one order of magnitude and decreased after UV-exposure. The composites with carbon black concentration of up to 1.1 vol.% are suitable for spin-coating and photolithography.     

Fabrication process and electrical behavior of novel pressure-sensitive composites

A novel route was developed to fabricate a new pressure-sensitive composite by dispersing homogeneously conductive carbon particles in an insulating silicone rubber matrix. The composites showed a gradual change in electrical resistivity with applied pressure within percolation threshold region at a constant temperature. This type of gradual fall of resistivity with applied pressure is very important to fabricate pressure sensors. Various amounts of carbon particles were dispersed in a rubber matrix to understand the effect of volume fraction of conductive filler with applying external pressure on resistivity. A quantitative general effective media (GEM) theory was used to understand the resistivity of carbon–rubber composites system over a large range of volume fraction of carbon with applied pressure. The use of two different sizes of silicon rubber particles showed a significant effect in gradual fall of resistivity with applied pressure in the narrow range of percolation threshold. However, a large variation in resistivity from 1st measuring to 10th measuring was observed. A significant improvement in successive measuring of resistivity variation from 1st measuring to 10th measuring was observed when composites were fabricated in hexane solvent media. Finally, nano-sized Al₂O₃ was dispersed to control the resistivity variation upon successive measurement and to improve the mechanical properties of the composites. The material was suggested to use as unique materials as pressure sensors in practical applications mainly for robots.     

超导YBa_2Cu_3O_(7+δ)/硅橡胶复合材料的压敏与介电特性

采用YBa_2Cu_3O_(+δ)(简称YBCO)多晶陶瓷超导粉末与硅橡胶(110型)按不同质量比进行配料, 经过特殊的制备工艺, 合成不同含量的超导YBCO/硅橡胶高分子复合材料, 分别测量样品的压敏效应和介电特性. 结果表明, 在不同应力作用下, 样品电阻值的变化范围在1～4个数量级. 样品电阻值随测量温度的降低(300~50 K)呈下降趋势, 测量温度降到90 K时, 样品电阻值发生突变, 但在90~50 K没有观察到超导零电阻现象. 室温下, 样品的介电常数随频率的增加(1 kHz~5 MHz)而减小, 介电损耗随测量频率的增大先增大后减小. 随着YBCO含量的增加, 形成的超电容网络微观结构也就越多, 样品的电阻逐渐减小电流加大, 导致超电容中电解质的极化强度有所增加, 两者共同作用的结果导致样品的介电常数、介电损耗均随着YBCO含量的增加而增大.     

The relationship between quantum transport and microstructure evolution in carbon-sheathed Pt granular metal nanowires

The carbon-sheathed Pt nanowires fabricated by focused ion beam induced deposition were Pt grains in a Ga-doped carbon matrix. The dimensions of the Pt quantum dots and the intergrain coupling strength were modified through annealing the nanowires, and therefore our investigation demonstrates a tunable 'quantum metal' system by experiment via adjusting both the effects of electron-electron (e-e) interaction and the quantum interference. At low temperatures, the as-deposited samples display a [Formula: see text] temperature dependence of resistivity, while the 500?°C annealed samples exhibit a ln(T) temperature dependence of conductivity. For?the 900?°C annealed samples, the conductivity was enhanced by one order of magnitude, accompanied by a metallic temperature dependence of resistivity and a negative magnetoresistance. The interesting transitions from e-e interactions to local voltage fluctuations, from electron localization to delocalization, from tunneling transport between isolated Pt grains to diffusive transport in continuously conductive metal, and from weak antilocalization with spin-orbital scattering to weak localization, are discussed with reference to the evolution of the sample microstructures. The results and discussions may be valuable for understanding how the microstructures influence the manner of electron transport through controlling the intergrain coupling strength, intragrain confinements, and the degree of disorder.