Composites conductive fillers

In many composites, conducting fillers (carbon black, carbon nanotubes, or metal nanoparticles) are added to make material conductive. The relationship between composite morphology and electrical conductivity has been studied extensively, especially in the context of carbon black filled polymers [156-162]. It is well known that the dependence of conductivity on the loading of conductive filler, percolation theory there is some threshold filler loading below which there is no conductive pathway through the system and conductivity is zero above the threshold, conductivity grows very rapidly as ... 

Conducting polymer composite materials (CPCM) — artificial media based on polymers and conductive fillers, have been known since the early 1940s and widely used in various branches of science and technology. Their properties are described in a considerable number of monographs and articles [1-12]. However, the publications available do not clearly distinguish such materials from other composites and do not provide for specific features of their formation. 

Conducting polymer composite materials are typical disordered structures consisting of randomly (or according to a certain law) arranged particles of a conducting filler that are submerged into a polymer medium. In this case the filler particles have macro-... 

Fig. 2. Relationship of conductivity of polypropylene-based polymer composites and filler concentration (natural graphite) 1 — polymerization filling 2 — mechanical mixture [24]...

Experimental dependences of conductivity cr of the CPCM on conducting filler concentration have, as a rule, the form predicted by the percolation theory (Fig. 2, [24]). With small values of C, a of the composite is close to the conductivity of a pure polymer. In the threshold concentration region when a macroscopic conducting chain appears for the first time, the conductivity of a composite material (CM) drastically rises (resistivity Qv drops sharply) and then slowly increases practically according to the linear law due to an increase in the number of conducting chains. 

Calculation of dependence of o on the conducting filler concentration is a very complicated multifactor problem, as the result depends primarily on the shape of the filler particles and their distribution in a polymer matrix. According to the nature of distribution of the constituents, the composites can be divided into matrix, statistical and structurized systems [25], In matrix systems, one of the phases is continuous for any filler concentration. In statistical systems, constituents are spread at random and do not form regular structures. In structurized systems, constituents form chainlike, flat or three-dimensional structures. 

As already noted, the main merit of fibers used as a filler for conducting composite materials is that only low threshold concentrations are necessary to reach the desired level of composite conductivity. However, introduction of fiber fillers into a polymer with the help of ordinary plastic materials processing equipment presents certain difficulties which are bound up mainly with significant shearing deformations entailing fiber destruction and, thereby, a decrease of parameter 1/d which determines the value of the percolation threshold. 

Quite naturally, novel techniques for manufacturing composite materials are in principal rare. The polymerization filling worked out at the Chemical Physics Institute of the USSR Academy of Sciences is an example of such techniques [49-51], The essence of the technique lies in that monomer polymerization takes place directly on the filler surface, i.e. a composite material is formed in the polymer forming stage which excludes the necessity of mixing constituents of a composite material. Practically, any material may be used as a filler the use of conducting fillers makes it possible to obtain a composite material having electrical conductance. The material thus obtained in the form of a powder can be processed by traditional methods, with polymers of many types (polyolefins, polyvinyl chloride, elastomers, etc.) used as a matrix. 

Any review devoted to conducting composites would be incomplete if the application fields of such composites were not described even if briefly. One of the first, if not the foremost, examples of the utilization of the CPCM is antistatic materials [1], For the materials of this kind resistivity q of less than 106 to 108 Ohm cm is not required, and this is achieved by introducing small amounts (several per cent) of a conducting filler, say, carbon black [4],... 

The main reasons for this lie in feasibility. Conducting fillers are rather expensive and their use increases the cost of an article. Besides, filled polymers have worse physical-mechanical properties, especially impact strength and flexural modulus. The use of fillers is also detrimental to the articles appearance and calls for additional treatment. The continuous development of electronics has also contributed to a loss of interest to conducting composites as screening materials the improvement of components and circuits of devices made it possible to reduce currents consumed and, thereby, noise level a so called can method is practised on a wide scale in order to cover the most sensitive or noisy sections of a circuit with metal housings [14]. 

Hu et al. showed a decrease in electrical resistivity of PVA by four orders of magnitude with a percolation threshold of 6 wt% [68], while biodegradable polylactide-graphene nanocomposites were prepared with a percolation threshold as low as 3 5wt% [46]. For polystyrene-graphene composites, percolation occurred at only 0.1 °/o of graphene filler, a value three times lower than those for other 2D-filler [69]. Figure 6.7(b) shows the variation of conductivity of the polystyrene-graphene composite with filler content. A sharp increase in conductivity occurs at 0.1 % (the percolation threshold) followed by a saturation. The inset shows the four probe set up for in-plane and trans-... 

Where a is the composite conductivity, a0 a proportionally coefficient, Vfc the percolation threshold and t an exponent that depends on the dimensionality of the system. For high aspect ratio nanofillers the percolation threshold is several orders of magnitude lower than for traditional fillers such as carbon black, and is in fact often lower than predictions using statistical percolation theory, this anomaly being usually attributed to flocculation [24] (Fig. 8.3). 

Although this technique is not normally used for thin polymer films for the reasons described before, it can be used for analyzing the surface of polymer composites containing conductive fillers, e.g. carbon fibers. In addition, because of the surface specificity, the sampled area can be maintained almost identically to the beam cross-section so that the scanning Auger microscope (SAM) can have a spatial resolution that is much better than that of microprobe analysis. 

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. 

Lin Q, Harb JN. Implementation of a thick-film composite Li-ion microcathode using carbon nanotubes as the conductive filler. J Electro chem Soc 2004 151 A1115-A1119. 

An explanation of the observed relaxation transition of the permittivity in carbon black filled composites above the percolation threshold is again provided by percolation theory. Two different polarization mechanisms can be considered (i) polarization of the filler clusters that are assumed to be located in a non polar medium, and (ii) polarization of the polymer matrix between conducting filler clusters. Both concepts predict a critical behavior of the characteristic frequency R similar to Eq. (18). In case (i) it holds that R= , since both transitions are related to the diffusion behavior of the charge carriers on fractal clusters and are controlled by the correlation length of the clusters. Hence, R corresponds to the anomalous diffusion transition, i.e., the cross-over frequency of the conductivity as observed in Fig. 30a. In case (ii), also referred to as random resistor-capacitor model, the polarization transition is affected by the polarization behavior of the polymer matrix and it holds that [128, 136,137]... 

Due to unprecedented mechanical, electrical and chemical properties, CNTs have been considered as an ideal material for various applications as well as for new fundamental investigations (1,2). In this review chapter, we will only discuss mechanical and electrical properties. In most composite structures, nanotubes are used as mechanical reinforcing agents or conductive fillers. This is also the case of PVA/nanotubes nanocomposites. 

This paper represents an overview of investigations carried out in carbon nanotube / elastomeric composites with an emphasis on the factors that control their properties. Carbon nanotubes have clearly demonstrated their capability as electrical conductive fillers in nanocomposites and this property has already been commercially exploited in the fabrication of electronic devices. The filler network provides electrical conduction pathways above the percolation threshold. The percolation threshold is reduced when a good dispersion is achieved. Significant increases in stiffness are observed. The enhancement of mechanical properties is much more significant than that imparted by spherical carbon black or silica particles present in the same matrix at a same filler loading, thus highlighting the effect of the high aspect ratio of the nanotubes. 

Composites with filler concentrations close to the percolation threshold exhibit conductivity which is sensitive to compressive deformation, since this brings the metal particles into contact, thereby forming percolation pathways. This sensitivity has been exploited especially in anisotropic composites. These are made by prealigning the metal particles with either electric or magnetic fields. This alignment is identical with that produced by external fields in electro- and magneto-rheological fluids where at a critical field continuous threads of... 

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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