Conductive nanofillers

Carbon materials provide electrical conduction through the pi bonding system that exists between adjacent carbon atoms in the graphite structure [182]. Electrical properties of nanocomposites based on conducting nanofillers such as EG [183-187], CNTs [188-190], and CNFs [191], dispersed in insulating polymer matrix have found widespread applications in industrial sectors. 

In this multiphase system, the term double percolation is defined to describe the conductive mechanism of polymer nanocomposites with a percolated network of nanofiller in one phase, which enables the formation of the conductive network through the whole polymer matrix. It has been proved that addition of conductive nanofillers into an immiscible polymer blend allows for the formation of cocon-tinuous structure and efficiently decreases the percolation threshold of nanofiUers due to the selective localization of the conductive networks. For example, Petra Poitschke et al. [89] introduced CNTs into Polycarbonate/Poly(styrene-acrylonitrile) (PC/SAN) to prepare CPCs. The percolation threshold of CNTs was less than 1 wt %, which is lower than those of CNTs in single PC matrix (1.2 wt%) and in single SAN phase (2.0 wt%).The localization of the conductive fiUer in polymer matrix depends on the interfacial energies of components and can be predicted by following Eq. (2) [86]. 

D-TEM gave 3D images of nano-filler dispersion in NR, which clearly indicated aggregates and agglomerates of carbon black leading to a kind of network structure in NR vulcanizates. That is, filled rubbers may have double networks, one of rubber by covalent bonding and the other of nanofiller by physical interaction. The revealed 3D network structure was in conformity with many physical properties, e.g., percolation behavior of electron conductivity. 

Interfacial behavior of different silicones was extensively studied, as indicated in Section 3.12.4.6. To add a few more examples, solution behavior of water-soluble polysiloxanes carrying different pendant hydrophilic groups, thus differing in hydrophobicity, was reported.584 A study of the aggregation phenomena of POSS in the presence of amphiphilic PDMS at the air/water interface was conducted in an attempt to elucidate nanofiller-aggregation mechanisms.585 An interesting phenomenon of the spontaneous formation of stable microtopographical surface domains, composed primarily of PDMS surrounded by polyurethane matrix, was observed in the synthesis of a cross-linked PDMS-polyurethane films.586... 

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). 

The effects of carbon-based nanofillers of EG, MWCNTs, and CNFs on the AC conductivity and dielectric constant of elastomeric grade EVA (50% vinyl acetate content) at a particular frequency of 12 Hz, are shown in Fig. 29a, b [194]. EVA-EG, EVA-T, and EVA-F represent EVA-based nanocomposites reinforced with EG, MWCNT, and CNF respectively. 

To examine the optimized loading for a PA6/MMT nanocomposite, a series of tests were conducted in the cone calorimeter [24] with various (2%, 5%, and 10%) loadings of nanofillers. The experimental HRR/MLR are reproduced in Figure 19.37, where it was found that the HRR/MLR decrease as the concentration of nanofillers increase up to 10%. The main objective of this section... 

The recognition of the unique properties of carbon nanotubes (CNTs) has stimulated a huge interest in their use as advanced filler in composite materials. In particular, their superior mechanical, thermal and electrical properties are expected to provide much higher property improvement than other nanofillers (18-22). For example, as conductive inclusions in polymeric matrices, CNTs shift the percolation threshold to much lower loading values than traditional carbon black particles. 

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... 

The pol5mier nanocomposite field has been studied heavily in the past decade. However, polymier nanocomposite technology has been around for quite some time in the form of latex paints, carbon-black filled tires, and other pol5mier systems filled with nanoscale particles. However, the nanoscale interface nature of these materials was not truly understood and elucidated until recently [2 7]. Today, there are excellent works that cover the entire field of polymer nanocomposite research, including applications, with a wide range of nanofillers such as layered silicates (clays), carbon nanotubes/nanofibers, colloidal oxides, double-layered hydroxides, quantum dots, nanocrystalline metals, and so on. The majority of the research conducted to date has been with organically treated, layered silicates or organoclays. 

Properties of nanofillers recently developed nano materials are reported to display greater mechanical strength, greater thermal conductivity and improved electrical performance when compared to materials of normal particle sizes. Nano dimensional materials are being studied as fillers in polymer matrices in a variety of formulations for electrically conductive adhesives, thermally conductive adhesives, encapsulants, printed circuit boards, coatings, catalysts, underfills for flip-chip-attached devices and wafer-level connections. ... 

Of particular interest to adhesives formulators are nanofillers such as carbon nanotubes (CNT), silica, alumina, magnesium oxide, titanium dioxide, zirconium oxide (Zn02), silver, copper, and nickel). Of these, carbon nanotubes are the most widely studied for electrically conductive adhesives to attach microdevices, to interconnect microcircuits and to increase I/O densities at the device level. ... 

In addition to improving strength, increasing electrical conductivity and reducing interconnect dimensions, formulations containing nanofillers have been reported to improve flame retardant and humidity resistance properties of polymer resins. ... 

Nanofillers, carbon nanotubes (CNTs), especially single-walled carbon nanotubes (SWNTs), have attracted a great deal of interest due to their low density, large aspect ratio, superior mechanical properties, and unique electrical and thermal conductivities [1-4], They can find potential applications in many fields, such as chemical sensing, gas storage, field emission, scanning microscopy, catalysis, and composite materials. 

Nanofillers have superb thermal and electrical properties. All nanotubes are expected to be very good thermal conductors along the tube axis, exhibiting a property known as ballistic conduction, but good insulators laterally to the tube axis. It has been reported that single-wall carbon nanotubes exhibit thermal conductivity (TC) values as high as 2000-6000 W mK [4] under ideal circumstances. The temperature stability of carbon nanotubes is estimated to be up to 2800 °C in a vacuum, and about 750 °C in air. By comparison, metals have TC values of several hundred W mK , and water and oil have TC values of only 0.6 W mK and 0.2 W mK respectively. Table 19.1 lists the thermal conductivities of various materials, including nanofillers (nanotubes), metals, and oils. 

Nanocomposites of Polymers Made Conductive by Nanofillers 739 Table 19.1 Thermal conductivity of various materials... 

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