Conductive fibre composites

The upper limit to the conductance between opposite faces of a composite block is the conductance of a uniform wire of the conductor, whose length equals the distance between the faces and whose volume equals the total 

If we assume that the fibre content is high, inter-fibre contacts will be sufficiently numerous for nearly all of the fibres to be active. Then, neglecting the effects of branch points and dead-ends, the equation for the conductivity of a composite containing randomly oriented fibres becomes 

To cite a practical example, 20% by mass of 12/mi diameter glass fibre coated with silver (70 nm thick) mixed in a thermosetting polyester resin gave a composite conductivity of 2.5 x 104fi-lm 1. From Equation (8.37) the calculated value is 5.3 x 104fl-lm-1, which compares reasonably well with the experiment, if one takes into account that some disintegration of the fibre is inevitable in the mixing process. 

Preferential orientation of the fibres in a particular direction will tend, of course, to increase the conductivity in that direction at the expense of the others. 

In all the examples discussed above the conductive fillers have been inorganic materials. As noted in the previous section, it is possible to make conductive composites with organic metal fillers. The organic charge transfer (CT) salts are ionic materials which contain linear arrays of conjugated ions with strong overlap of the -electrons of adjacent ions. If charge transfer is complete and 

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The poor stability on exposure to air and water, particularly at elevated temperatures, which results in a reduction in conductivity, also poses problems. In the case of polypyrrole it has been found that conductivity can, however, be maintained either by the drastic measure of storing under the protective layer of the inert gas argon or embedding polypyrrole film in a matrix of an epoxide resin-glass-fibre composite. 

This is an important relationship. It states that the modulus of a unidirectional fibre composite is proportional to the volume fractions of the materials in the composite. This is known as the Rule of Mixtures. It may also be used to determine the density of a composite as well as other properties such as the Poisson s Ratio, strength, thermal conductivity and electrical conductivity in the fibre direction. 

Particular examples of using polymer composites as screens are given in [14-16, 67-75], The present review does not touch the properties of the composite materials based on fabrics of conducting fibres due to the fact that manufacturing techniques for such materials are specific and differ greatly from the mixing processes considered above. However, these materials also have an application field, say, in contacts for calculator and computer keyboards [9] and even in small-power electric motor commutators as a partial substitute for copper [76, 77]. 

Jana, P.B., Mallick, A.K., and De, S.K., Electrically conductive rubber and plastic composites with carbon particles or conductive fibres, in Short Fiber-Polymer Composites, De, S.K. and White, J.R. (Eds.), Woodhead Publishing, Cambridge, 1996, Chapter 7. 

A rubber-like copolymer/carbon fibre composite material has also been prepared [170]. Carbon fibres were added directly to o/w highly concentrated emulsions of block copolymers, such as styrene/butadiene triblocks (SBS), in toluene, followed by precipitation in methanol, drying and hot-pressing. The surfactant was found to aid adhesion between the polymer and carbon fibres. The materials obtained had fairly even distributions of carbon fibres, good mechanical properties and conductivities which increased with increasing carbon fibre length. 

Dall Acqua et al.45 reported the development of conductive fibres made by cellulose-based fibres embedded with polypyrrole. Several efforts with cotton, viscose, cupro and lyonell have followed. The conductivity is directly related to the amount of polypyrrole, oxidant ratio and fibre structure with significant differences between viscose and lyonell. Polymerisation occurs uniformly inside the fibre bulk, by producing a coherent composite polypyrrole/cellulose. The mechanical and physical properties of cellulose fibres were not significantly modified as they are the best available45. 

P(l) If aluminium honeycomb is used with electrically conducting composites (e.g. carbon fibre composites), construction shall ensure that the honeycomb is electrically insulated from the core by using a glass fibre interlayer to prevent potential internal galvanic corrosion problems occurring in wet environments. 

Zhdan PA, Bors M, Castle JE, In situ scanning force microscopy (SFM) study of the electrochemical activations of carbon fibres. Composite Sci Technol, 58(3-4), 559-570, 1998. Zhdan PA, Nanoscale surface characterization of conducting and non-conducting materials with STM and contact SFM some problems and solutions. Surface Interface Anal, 33, 879-893, 2002. 

Folkes MJ, Short Fibre Reinforced Thermoplastics, Research Studies Press, Letchworth, 1982. Bigg DM, Characteristics of short, conductive fibre reinforced injection mouldable composites, J. Ind Fabrics, 2(3), 4-14, 1983-1984. 

Agarwal et al. [35] analysed the variation of thermal conductivity and thermal diffusivity of banana fibre-reinforced polyester composites caused by the addition of glass fibre. They observed that the thermal conductivity of composites increased when compared with the matrix. However, the thermal conductivity of the composites with increased percentage of glass fibre decreases in comparison to composite of pure banana fibre. 

Annie Paul et al. [36] studied short randomly oriented PP/banana fibre composites. The thermophysical properties of the above composites were studied on the basis of different banana fibre loading and different chemical treatments given to the banana fibres. The incorporation of banana fibres into PP matrix induced a decrease of the effective thermal conductivity of the composite. The use of the theoretical series conduction model allowed to estimate the transverse thermal conductivity of untreated banana fibre composites. As was expected, the series model appears sufficient for the effective thermal conductivity estimation of this kind of composites. All the chemical treatments enhanced both thermal conductivity and diffusivity of the composite considerably in varying degrees. This indicates that the chemical treatment allows a better contact between the fibre and the matrix and reduces considerably the thermal contact resistance. Nevertheless, a significant increase of the thermal conductivity was observed only for benzoylated and 10% NaOH-treated fibre composites. Besides, the variations of density and specific heat upon fibre chemical treatment are small compared to their associated uncertainties. 

Fig. 23.8 Effect of PALF and glass fibre-volume fraction on experimental and theoretical thermal conductivity of PALF/glass hybrid fibre composites [51]...

Composite-skinned honeycomb panels These may use Nomex nylon paper honeycomb core, aluminium honeycomb or sometimes PVC (polyurethane or acrylic foam core). Dents are not acceptable in composite skins as they indicate fibre damage. Nomex core may split under a dent, so any such damage needs to be cut out and repaired. Nomex honeycomb can absorb considerable amounts of moisture. Wet honeycomb should be thoroughly dried or replaced. Moisture meters are available for use with Fibreglass, Kevlar or Nomex, but these meters will not work with carbon fibre because it is electrically conductive. Instruments that can indicate when carbon-fibre composites are dry enough to repair are few in number and very expensive. 

For carbon fibre composite materials a solution for becoming smart is using the carbon fibre s self-sensing properties — conductive carbon fibres within pockets of... 

Polypyrrole/quartz fibre composite samples [344] and mechanically strong and flexible Kevlar/polypyr-role hybrid fibers with room temperature conductivities of 10 S cm [345,346] have also been obtained. 

Polyaniline grafted onto flexible surfaces can find applications as a new material for the control of EMI/ESD. These materials are free from corrosion and may prove to be better than metal fibre/powder-filled composites, which are susceptible to galvanic corrosion or loss of conductivity due to friction, as in graphite-reinforced fibre composites which are brittle. 

The American ThermalWorks company has developed a new carbon fibre composite printed circuit board and substrate material, STABLCOR, which comprises a high temperature resin incorporating carbon fibre sandwiched between two very thin layers of copper. The material has a Tg of 170 "C. However, the company is developing a version with a Tg of 240 C. The material is designed as a plane , or non-signal carrying layer in a ply stack with other dielectric layers to make a composite PCB or device substrate. The carbon composite material has a dielectric constant of approximately 13.4 which makes it electrically conductive. This material allows the user to tailor the coefficient of thermal expansion of a PCB down to 3 ppm/°C to match that of silicon. 

Conductive polymer composites can be defined as insulating polymer matrices which have been blended with filler particles such as carbon black, metal flakes or powders, or other conductive materials to render them conductive. Although the majority of applications of polymers in the electrical and electronic areas are based on their ability to act as electrical insulators, many cases have arisen more recently when electrical conductivity is required. These applications include the dissipation of electrical charge from rubber and plastic parts and the shielding of plastic boxes from the effects of electromagnetic waves. Consequently, materials scientists have sought to combine the versatility of polymers with the electrical properties of metals. The method currently used to increase the electrical conductivity of plastics is to fill them with conductive additives such as metallic powders, metallic fibres, carbon black and intrinsically conducting polymers such as polypyrrole. 

For some purposes, good electrical conductivity or resistance to development of electrostatic change is needed in a composite structure. A range of conductive fibres is available for incorporation to satisfy these requirements. These include fine metal fibres, already discussed, but also glass, carbon and other fibres coated or impregnated with conductive material, usually a metal such as aluminium. 

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