Nanocomposites percolation threshold

Alignment of CNTs markedly affects the electrical properties of polymer/CNT composites. For example, the nanocomposites of epoxy/MWCNTs with MWCNTs aligned under a 25 T magnetic field leads to a 35% increase in electric conductivity compared to those similar composites without magnetic aligned CNTs (Kilbride et al., 2002). Improvements on the dispersion and alignment of CNTs in a polymer matrix could markedly decrease the percolation threshold value. 

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-... 

Logakis E, Pissis P, Pospiech D, Korwitz A, Krause B, Reuter U, et al. Low electrical percolation threshold in polyethylene terephthalate)/multi-walled carbon nanotube nanocomposites. European Polymer Journal. 2010 May 46(5) 928-36. 

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. 

The electric properties of soft ferromagnetic nanoparticles in an insulating matrix strongly depend on the concentration of a metallic filler x and are varied between properties of the matrix and those of the filler. In binary nanocomposites a critical concentration (percolation threshold Xq) is reached when a continuous current-conducting cluster of the filler particles is formed through out the sample. 

The results of the systematic SPM study performed with the samples (Coo,45Feo,45Zro.io)x(Al203)i.i nanocomposite films in the vicinity of percolation threshold xc are demonstrated below in (Figs. 1,2). 

As for the linear properties, numerous approaches have been proposed to predict and explain the nonlinear optical response of nanocomposite materials beyond the hypothesis leading to the simple model presented above ( 3.2.2). Especially, Eq. (27) does not hold as soon as metal concentration is large and, a fortiori, reaches the percolation threshold. Several EMT or topological methods have then been developed to account for such regimes and for different types of material morphology, using different calculation methods [38, 81, 83, 88, 96-116]. Let us mention works devoted to ellipsoidal [99, 100, 109] or cylindrical [97] inclusions, effect of a shape distribution [110, 115], core-shell particles [114, 116], layered composites [103], nonlinear inclusions in a nonlinear host medium [88], linear inclusions in a nonlinear host medium [108], percolated media and fractals [101, 104-106, 108]. Attempts to simulate in a nonlinear EMT the influence of temperature have also been reported [107, 113]. 

Abstract Hybrid metal/metal oxide-poly-/>ara-xylylene (PPX) nanocomposites have attracted great interest, because of a broad spectrum of applications. A simple, low-cost preparation technique has been developed and comprises a cold-wall vacuum co-deposition technique. This co-deposition technique has been applied to synthesize nanocomposites, containing PPX and nanoparticles of Al, Sn, Zn, Ti and their oxides. Important is the oxidation kinetics of the metal clusters to their oxides in relation to the percolation threshold. 

TEM analysis of the nanocomposite with an A1 content beyond the percolation threshold reveals spherical pure metal nanoparticles with a mean diameter of about 10 nm (Fig. 6a), while below the percolation threshold the composite contains agglomerates of rhombohedral AI2O3 (corundum) with a mean size of 55 nm (Fig. 6c). A sample with a metal content just at the percolation threshold contains metal nanoparticles of 10nm and alumina aggregates of 28 nm in diameter (Fig. 6b). The inorganic phase is homogeneously dispersed within the polymeric matrix in all of the investigated samples. It has been shown that the nanocomposite structure determines the oxidation behaviour of A1 nanoparticles within the polymeric matrix under air exposure. 

XRD studies show that synthesized composites do not contain any crystal phase, just an amorphous phase. Optical absorption measurements prove that synthesized nanocomposites are containing Ti02 and Ti phases. For comparative analysis the pure Ti containing thin film was deposited onto the cold substrate (77 K) and onto the substrate at room temperature. The same result was obtained XRD analysis shows that the synthesized films only contain the amorphous phase. Kinetics of the electrical resistance increase with the air exposure of Ti/PPX nanocomposites (after synthesis under vacuum) is similar to that of the Al/PPX ones. For a metal content below the percolation threshold the metal particles became insulator within several seconds, whereas for the samples beyond the threshold the observed resistance increase is per cents within several hours. DTA analysis revealed that the heating of amorphous Ti02 nanoparticles up to a temperature of 480°C leads to a phase transformation to anatase, whereas heating up to 580°C results in the anatase transformation to the mtile structure. 

It has been shown that the nanocomposite sttucture determines the oxidation be-havionr of Al, Sn, and Ti nanoparticles within the polymeric matrix under air expo-snre. For the metal content below the percolation threshold the metal particles became insnlators within several seconds, whereas for the samples above the threshold the observed resistance growth is several per cent and full oxidation takes several hours. 

S, Zavyalov, A, Timofeev, A. Pivkina, and J. Schoonman, Metal-polymer nanocomposites formation and properties near the percolation threshold in Nanostructured Materials Selected... 

It was revealed that at deposition of the nanocomposite films in the Ar + O2 gas mixture the region of the superparamagnetic state was expanded far beyond percolation threshold %c determined from the electrical properties of the films deposited in pure Ar. An inductive contribution in equivalent circuits of the studied films shows that the currentconducting routes within the film bulk look like a system of nanocoils embedded in the alumina matrix. 

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... 

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