Thermally conducting aluminum nitride polymer-matrix composites

Thermally conducting, but electrically insulating, polymer-matrix composites that exhibit low values of the dielectric constant and the coefficient of thermal expansion (CTE) are needed for electronic packaging. For developing such composites, this work used aluminum nitride whiskers (and/or particles) and/or silicon carbide whiskers as fillers(s) and polyvinylidene fluoride (PVDF) or epoxy as matrix. The highest thermal conductivity of 11.5 W/(m K) was attained by using PVDF, AlN whiskers and AlN particles (7 μm), such that the total filler volume fraction was 60% and the AlN whisker–particle ratio was 1:25.7. When AlN particles were used as the sole filler, the thermal conductivity was highest for the largest AlN particle size (115 μm), but the porosity increased with increasing AlN particle size. The thermal conductivity of AlN particle epoxy-matrix composite was increased by up to 97% by silane surface treatment of the particles prior to composite fabrication. The increase in thermal conductivity is due to decrease in the filler–matrix thermal contact resistance through the improvement of the interface between matrix and particles. At 60 vol.% silane-treated AlN particles only, the thermal conductivity of epoxy-matrix composite reached 11.0 W/(m K). The dielectric constant was quite high (up to 10 at 2 MHz) for the PVDF composites. The change of the filler from AlN to SiC greatly increased the dielectric constant. Combined use of whiskers and particles in an appropriate ratio gave composites with higher thermal conductivity and low CTE than the use of whiskers alone or particles alone. However, AlN addition caused the tensile strength, modulus and ductility to decrease from the values of the neat polymer, and caused degradation after water immersion.     

Electrothermal stress in conducting particulate composites

Electrothermal–mechanical interaction plays an important role in controlling the performance of electromechanical structures and field-assisted processes. The understanding of electrothermal–mechanical behavior of a material requires the analyses of Joule heating and thermomechanical deformation. In this study, we analyze the current-induced thermal stress in a conducting composite consisting of conducting spherical inclusions at dilute concentration. Assuming that there is no interaction among conducting inclusions, we obtain closed-form solutions of local temperature and thermal stress. The thermal stress created by Joule heating is proportional to the square of electric current density (electric field intensity) and the von-Mises stress reaches the maximum value at the interface between the spherical inclusion and the matrix. Large electric current will likely cause local delamination along the interface.     

聚合物基导电复合材料的电阻机械效应(RME)

综述了聚合物基导电复合材料的电阻机械效应(resistancce-mechanical effect,RME),列举了砌RME的各种表现及其可能的内在机制,介绍了改善RME循环稳定性的方法。RME取决于导电填料和基体性质两方面因素,包括填料种类和形态分布、填料体积分数、基体粘弹性、基体与填料的相五作用。其次,应力或应变大小、加栽方式和加栽速率、加工方法以及环境温度和湿度等也是影响RME的重要因素。对于纤维填充复合体系,影响因素还包括纤维取向与长径比、外力方向等。     

Directed assembly of conducting polymers on sub-micron templates by electrical fields

Patterning of conducting polymer into sub-micron patterns over large areas at high rate and low cost is significant for commercial manufacturing of novel devices. Electrophoretic and dielectrophoretic assembly provide an easily scaled approach with high fabrication rates. In this work, electrophoretic and dielectrophoretic assembly were used to assemble polyaniline (PANi) into multiscale sub-micron size patterns in less than 1 min. The process was controlled by assembly time, amplitude, and frequency of the electric field. Dielectrophoretic assembly is preferable for manufacturing as it reduces damage to the templates used to control the assembly. Using this method, sub-micron patterns with dimensions of the assembled PANi down to 100 nm were fabricated over large areas in short times. The assembled PANi was further transferred to other flexible polymer substrates by a thermoforming process, providing a fast, easily controlled and promising approach for fabrication of nanoscale devices.     

Electromagnetic shielding behaviour of conducting polyaniline composites

The paper reports the designing and development of conducting polyaniline composites which show a shielding effectiveness of 4–58 dB against electromagnetic interference at 101 GHz range, depending upon the loading of the conducting polyaniline in polystyrene and polymethylmethaacrylate matrix. The composites can be used for the dissipation of static charge. A static decay time of the order of 0.11–0.02 s for the conducting polyaniline composites is observed when the charge is reduced from 5000 to 500 V.     

Thermal behaviour of high performance conducting composites

Abstracts are not published in this journal This revised version was published online in November 2006 with corrections to the Cover Date.     

Charge dynamics in conducting polyaniline-metal oxalate composites

Polyaniline (Pani) and its metal oxalate composites (∼10 wt.%) of trivalent metal ions of Cr, Fe, Mn, Co and Al were synthesized by chemical oxidative polymerization technique with potassium perdisulphate oxidant in aqueous sulphuric acid medium. These materials were characterized by UV-VIS and EPR spectral techniques. Their d.c. electrical conductivities at room temperature and also as a function of temperature (307-453 K) were measured by four- probe technique. Presence of radical cation/polaron transition was indicated by UV- VIS absorption peak and EPR signals. Further, a close correlation existed between the conductivities and EPR parameters such as line width and peak ratio, which demonstrated that both mobile and fixed spins are involved in these composites. The dependence of conductivity on temperature, when analysed graphically by VRH, GB and TC mechanisms, pointed out that VRH is the predominant charge transport mechanism in these materials.     

Formation of conducting polymer nanostructures with the help of surfactant crystallite templates

An extremely simple approach is described here to synthesize bulk quantities of conducting polymer microspirals assembled from nanofibers by in situ chemical oxidative polymerization in the presence of a conventional surfactant. It is worth noting that the surfactant used in our approach is in crystalline state, which is quite different from micellar state in emulsional polymerization reported previously. The growth mechanism of the conducting polymer is proposed.     

Formation of conducting polymer nanostructures with the help of surfactant crystallite templates

An extremely simple approach is described here to synthesize bulk quantities of conducting polymer microspirals assembled from nanofibers by in situ chemical oxidative polymerization in the presence of a conventional surfactant. It is worth noting that the surfactant used in our approach is in crystalline state, which is quite different from micellar state in emulsional polymerization reported previously. The growth mecha-nism of the conducting polymer is proposed.     

Microengineered silicon double-ended tuning fork resonators

Micromachined silicon sensors employing resonant sensing offer many potential performance benefits over alternative piezoresistive and capacitive techniques. Resonant devices are, however inherently more complicated to design and fabricate. This article provides a brief overview of the principles of resonant sensing and describes a design study involving double-ended tuning-fork (DETF) resonant structures that overcome many practical difficulties due to their dynamically balanced design and mode of operation. The optimum mode is a lateral vibration in the plane of the wafer and this paper also reports a simple method for exciting and detecting lateral vibrations without compromising the degree of dynamic balance of the structure. Test devices have been fabricated in single-crystal silicon     

Electrically Driven Microengineered Bioinspired Soft Robots

To create life‐like movements, living muscle actuator technologies have borrowed inspiration from biomimetic concepts in developing bioinspired robots. Here, the development of a bioinspired soft robotics system, with integrated self‐actuating cardiac muscles on a hierarchically structured scaffold with flexible gold microelectrodes is reported. Inspired by the movement of living organisms, a batoid‐fish‐shaped substrate is designed and reported, which is composed of two micropatterned hydrogel layers. The first layer is a poly(ethylene glycol) hydrogel substrate, which provides a mechanically stable structure for the robot, followed by a layer of gelatin methacryloyl embedded with carbon nanotubes, which serves as a cell culture substrate, to create the actuation component for the soft body robot. In addition, flexible Au microelectrodes are embedded into the biomimetic scaffold, which not only enhance the mechanical integrity of the device, but also increase its electrical conductivity. After culturing and maturation of cardiomyocytes on the biomimetic scaffold, they show excellent myofiber organization and provide self‐actuating motions aligned with the direction of the contractile force of the cells. The Au microelectrodes placed below the cell layer further provide localized electrical stimulation and control of the beating behavior of the bioinspired soft robot.     

Graphene oxide-filled conducting polyaniline composites as methanol-sensing materials

Polyaniline/graphene oxide (PANI/GO) composites were prepared by polymerization of aniline monomer in the presence of GO under acidic conditions. The synthesized samples were characterized by Fourier transform infra red spectroscopy, ultraviolet–visible absorption, Raman spectroscopy, X-ray diffraction, scanning electron microscopy, transmission electron microscopy and thermogravimetric analysis. The direct current electrical conductivity of the composite was calculated by a four-probe technique. It is found that the conductivity dramatically increased to 241 S m^?1 for PANI/GO (5 wt%) composite at 110 °C compared to pure PANI (7.5 S m^?1). The composite material was investigated as a methanol vapour sensor and compared with pure PANI. The methanol-sensing characteristics of the prepared composite was monitored by measuring the change in electrical resistivity on exposure to methanol vapour at different concentrations. The resistivity of PANI increases on exposure to methanol vapour because of strong hydrogen bonding between methanol with the polymer chain. A density functional theory study was carried out to verify the proposed concept of hydrogen bonding between the polymer chains and methanol. The presence of GO in PANI/GO composite increases the sensitivity towards methanol as compared with the pure PANI.     

Thermoelectric properties of conducting polyaniline/graphite composites

HClO₄-doped polyaniline/graphite composites were prepared by mechanical ball milling and cold pressing. The thermoelectric properties for the composites were investigated as a function of graphite concentration. The thermal conductivity (К) increases slightly with increasing graphite content but the electric conductivity(σ) as well as the Seebeck coefficient (S) increases remarkably, which leads to an obvious enhancement in the figure of merit for the composites. The ZT of the composite with graphite concentration of 50 wt % was calculated to be 1.37 × 10^− 3 at 393 K, which was at least 10000 times greater than that of the HClO₄-doped polyaniline without graphite (1.13 × 10^− 7). This work suggests a new method to improve thermoelectric properties of conducting polymers.     

An overview on the cellulose based conducting composites

Biopolymer based composites have been employed in numerous applications with increasing interest not only due to renewable, eco-friendly nature, but also because of the flexibility in processing conditions and competitive cost of their end products. The conductive materials from biopolymers have been found applicable in robots, medical imaging, sensitive membranes, actuators, visual displays, electronic wiring and shielding, and components in batteries. Cellulose is one of the most abundant biopolymers in the nature, which has received special attention for development of conducting materials due to biocompatibility for protein and drug immobilization and ability to form the composites with synthetic polymers. The present review is aimed to provide concisely the current status in this field of conducting composites from cellulose, with brief discussions of associated problems and future applications.     

Highly aligned ultrahigh density arrays of conducting polymer nanorods using block copolymer templates

Ultrahigh density arrays of conducting polypyrrole (PPy) nanorods are fabricated directly on the indium-tin oxide coated glass by an electropolymerization within a porous diblock copolymer template. The nanorods are shown to have conductivity much higher than thin PPy films, due to the high degree of chain orientation, even though the separation distance for two neighboring PPy main chains is as small as 0.37 nm. The ultrahigh density arrays of conducting polymer nanorods have potential applications as sensor materials, nanoactuators, and organic photovoltaic devices.     

Scanning electron microscope investigations of highly conducting organic composites

Scanning electron microscope (SEM) investigation of the highly conducting organic composites of general formulae (BEDT-TTF) _x /(AuI) and (BEDT-TTF) _x /(AuI₃), where BEDT-TTF is bis(ethylenedithio)tetrathiafulvalene, obtained by direct charge-transfer (CT) reaction in the solid state is performed. The granular structure of the composites with domination of three types of the grains: (BEDT-TTF)₂I₃, (BEDT-TTF)₂AuI₂ and Au, is suggested. The changes in the composite structure caused by their composition and/or thermal treatment are connected with the d.c. electrical conductivity of the samples.     

聚吡咯及其复合材料导电稳定性的研究

对聚吡咯/二氧化硅(PPy/SiO2)、聚吡咯/蒙脱土(PPy/MMT)复合材料的电导率及其导电稳定性进行了研究.通过对材料进行定期和不同条件下的测试发现对PPy/SiO2复合材料而言,导电稳定性随时间延长而下降.对PPy与不同百分含量偶联剂处理过的SiO2的复合材料(PPy/APS-SiO2)而言,聚吡咯导电稳定性提高很多.PPy/MMT复合材料在室温具有较好的导电稳定性,而且合成温度越低,所得产物的稳定性越好.在0℃下合成的PPy/MMT复合物在空气中放置50天,电导率仅降低10%.     

Dielectric properties of conducting polymer composites at microwave frequencies

The dielectric properties of conducting polymer composites containing polypyrrole (PPy) crushed films, PPy powder, polyaniline (PAn) base and acid powders as the dispersants and silicone rubber and vinyl ester as matrix materials have been investigated in the frequency range 2-18 GHz. The dielectric parameters such as the real part, , and imaginary part, , of the permittivity and loss tangent, tan, increase with increasing conductivity and concentration of the dispersant. The geometrical shape of the dispersant governs the ability of conductive network formation which is indicated by a large drop in the resistivity of the composite. Also, dispersant/matrix interactions and physical properties of the matrix influence the agglomeration of the dispersant phase which, in turn, affects the dielectric properties of the composites. Flakes of PPy obtained by crushing highly conductive films and large PAn powder aggregates were unable to form a conducting network. The composites without a network of dispersant exhibit low dielectric parameters. On the other hand, high values of tan ranging from 0.7–1.1 were achieved for the PPy powder (15 parts)/silicone rubber composites where a conducting network was observed.