Carbon-based conductive fillers

Partici Shape and Size. The most common morphology of conductive fillers used for ICAs is flake because flakes tend to have a large surface area, and more contact spots and thus more electrical paths than spherical fillers. The particle size of ICA fillers generally ranges from 1 to 20 /rm. Larger particles tend to provide the material with a higher electrical conductivity and lower viscosity (45). A new class of silver particles, porous nano-sized silver particles, has been introduced in ICA formulations (46,47). ICAs made with this type of particles exhibited improved mechanical properties, but the electrical conductivity is less than ICAs filled with silver flakes. In addition, short carbon fibers have been used as conductive fillers in conductive adhesive formulations (36,48). However, carbon-based conductive adhesives show much lower electrical conductivity than silver-filled ones. 

As far as conducting fillers are concerned, we have rather a wide range of choice. In addition to the traditional and long used fillers, such as carbon black and metal powders [13] fiber and flaky fillers on organic or metal bases, conducting textures, etc recently appeared and came into use. The shape of the filler particles varies widely, but only the particle aspect ratio, the main parameter which determines the probability... 

Although a majority of these composite thermistors are based upon carbon black as the conductive filler, it is difficult to control in terms of particle size, distribution, and morphology. One alternative is to use transition metal oxides such as TiO, VO2, and V2O3 as the filler. An advantage of using a ceramic material is that it is possible to easily control critical parameters such as particle size and shape. Typical polymer matrix materials include poly(methyl methacrylate) PMMA, epoxy, silicone elastomer, polyurethane, polycarbonate, and polystyrene. 

The blend morphology containing conductive filler (e.g., carbon black) was simulated by the model based on Cahn s approach. Figure 16.10 shows the two-dimensional cut explaining the localization of carbon black between two incompatible phases, and Figure 16.11 shows the effect of carbon black concentration on the prediction of conductivity. This simple model of interfacial film partitioning... 

Carbon black is the most widely used conducting filler in composite industry. Carbon black filled immiscible blends based on polar/polar (65), polar/nonpolar (63,66), nonpolar/nonpolar thermoplastics (67,68), plastic/rubber and rubber/mbber blends (69,70) have already been reported in the literature. The properties of carbon black filled immiscible PP/epoxy were reported recently by Li et al. (60). The blend system was interesting because one of the components is semicrystalline and the other is an amorphous polar material with different percolation thresholds. The volume resistivity of carbon black filled individual polymers is shown in Fig. 21.23. 

Semiconductive materials having conductivity between 10 and 10 S/cm have been studied more extensively. They are obtained by dispersion of conductive fillers—metallic and carbon based—in an insulating matrix. High filler contents are required, from 10 to 15% for fibres and 30% for spheres, to reach a percolation level leading to microwave losses. 

Graphene-polymer nanocomposites share with other nanocomposites the characteristic of remarkable improvements in properties and percolation thresholds at very low filler contents. Although the majority of research has focused on polymer nanocomposites based on layered materials of natural origin, such as an MMT type of layered silicate compounds or synthetic clay (layered double hydroxide), the electrical and thermal conductivity of clay minerals are quite poor [177]. To overcome these shortcomings, carbon-based nanofillers, such as CB, carbon nanotubes, carbon nanofibers, and graphite have been introduced to the preparation of polymer nanocomposites. Among these, carbon nanotubes have proven to be very effective as conductive fillers. An important drawback of them as nanofillers is their high production costs, which... 

In carbon, the conductivity varies from 10 (ohm-cm) for amorphous carbon to approximately 300 (ohm-cm) in the longitudinal direction for PAN-based high modulus carbon fibers. Apart from relatively low conductivity, carbon has the same magnetic permeahUity as aluminum, i.e., approximately 1. In order to obtain a given damping, carbon-based fillers have to he added in higher concentrations in comparison with metallic fillers such as steel. However, special carhon hlack grades with microporous structure and increased conductivity can now be found that allow the construction of a conductive network at relatively low concentrations. 

There are a range of conductive polymers on the market that are based on metal fillers such as aluminum flake, brass fibers, stainless steel fibers, graphite-coated fibers, and metal-coated graphite fibers. However, the most cost effective conductive filler is carbon black. Mention should also be made of... 

Recently, nanostructured carbon-based fillers such as Ceo [313,314], single-wall carbon nanotubes, carbon nanohorns (CNHs), carbon nanoballoons (CNBs), ketjenblack (KB), conductive grade and graphitized carbon black (CB) [184], graphene [348], and nanodiamonds [349] have been used to prepare PLA-based composites. These fillers enhance the crystalUza-tion ofPLLA [184,313,314].Nanocomposites incorporating fibrous MWCNTsandSWCNTs are discussed in the section on fibre-reinforced plastics (section 8.12.3). 

Besides metal NPs, carbon micro and nanoparticles as well as carbon nanotubes are usually used as fillers in composites for electrochemical transducers. The following section focuses on the properties and main issues of carbon-polymer conducting composites based on nonconducting binders and polymers. 

Carbon-Based Materials as Conductive Fillers in Composites... 

In case of other fillers, the nanofillers can introduce new functionality into the polymer, e.g. electrical conductivity in case of carbon based nanoparticles, barrier properties in case of platelet like nanofillers (nanoclay, expanded graphite), enhancement of mechanical properties, enhanced flame retardancy, and many others. 

Kazeminejad [20] has described the construction of an apparatus for the measurement of thermal conductivity accordingly to ASTM C177 [21] and DIN 52612-2 [22]. He describes a method of determining thermal conductivity of insulating materials, based on a copper-coated printed circuit board. Thermal conductivity values are reported for pure PE, pure PC, and PE and PC mixed with conductive fillers such as aluminium powder and carbon black. 

There are two kinds of carbon-based fillers used to produce antistatic and conducting PP composites carbon black and carbon fiber. 

Research has shown that the surface functionalization of carbon-based fillers, which can both maximize interfacial adhesion between carbon-based fillers and the polymer matrix and increase the dispersion of CB, CNTs, and graphene in polymer matrix, is one of the best approaches to achieve good dispersion of conducting particles in polymer matrix. At present there are several approaches for functionalization of carbon-based materials including defect functionalization, covalent functionalization, and non-covalent functionalization (Gong et al. 2000 Hirsch 2002). Some functional groups, which can improve the interaction between carbon-based fillers and polymer matrix, are covalently bonded directly to the surface of carbon. These functionalization methods will be discussed in Chap. 25 (Vol. 2). 

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