Dispersion Electrical conductivity

In general, it can be said that, often of necessity, the detector cell may be relative large with a low aspect ratio and thus, would theoretically produce serious band dispersion. In practice the predicted dispersion is reduced by deign of the inlet and outlet tubes, as discussed above, to ensure maximum secondary flow in the cell and thus, minimize dispersion. The success of the procedure to reduce detector cell dispersion depends on the type of detector and the principle of detection. For example.it is far easier to design a low dispersion electrical conductivity cell than a low dispersion UV absorption cell. 

There are two types of conductive adhesives conventional materials that conduct electricity equally in all directions (isotropic conductors) and those materials that conduct in only one direction (anisotropic conductors). Isotropically conductive materials are typically formulated by adding silver particles to an adhesive matrix such that the percolation threshold is exceeded. Electrical currents are conducted throughout the composite via an extensive network of particle-particle contacts. Anisotropically conductive adhesives are prepared by randomly dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. A schematic illustration of an anisotropically conductive adhesive interconnection is shown in Fig. 1. The concentration of particles is controlled such that enough particles are present to assure reliable electrical contacts between the substrate and the device (Z direction), while too few particles are present to achieve conduction in the X-Y plane. The materials become conductive in one direction only after they have been processed under pressure they do not inherently conduct in a preferred direction. Applications, electrical conduction mechanisms, and formulation of both isotropic and anisotropic conductive adhesives are discussed in detail in this chapter. 

From the viewpoint of their conduction and mechanical joining, ACAs are similar to ICAs, except that they have lower concentrations of conductive particles. This lower concentration provides unidirectional conductivity in the vertical or z-direction (perpendicular to the plane of the substrate), which is why they are called anisotropic conductive adhesives. In the same way, ACA materials are prepared by dispersing electrically conductive particles in an adhesive matrix at a concentration far below the percolation threshold. The concentration of particles is controlled, so that sufficient particles are present to ensure reliable electrical conductivity between the assembled parts in the z-direction, while insufficient particles are present to achieve percolation conduction in the x-y plane (Kim et al. 2008b). O Figure 50.6 shows a schematic description of an ACA interconnect, showing the electrical conductivity in the... 

The conductive polymer is usually composed of dispersed electrically conductive particles such as carbon black or metal in a durable polymer matrix such as silicone rubber. As it is possible to obtain conductivity and elasticity in simple materials, this is useful for high density micro-sized electrical contacts as connector or as key-board switch. However, in spite of the excellent properties, it is difficult to obtain low electrical resistivity. In order to approach this problem, performances of the contacts between the polymer surface and metal were studied from the view point of surface contamination. The electrical conduction mechanism and a method to get low resistivity were also studied. The results show that the surface of the polymers containing carbon black shows contamination resistance properties because of their chemical stability. The conduction mechanism changes with particle content. For large content, electric current flows through particles contacting each other and for small content, the current passes through the gap between particles by Schottky conduction. Thus, as the resistance consists of particle resistance, constriction resistance and gap resistance, the resistance can be controlled by particle material, their content and size. 

Fricke, H, A Mathematical Treatment of the Electric Conductivity and Capacity of Disperse Systems, Physical Review 24, 575, 1924. 

For the support material of electro-catalysts in PEMFC, Vulcan XC72(Cabot) has been widely used. This carbon black has been successfully employed for the fuel cell applications for its good electric conductivity and high chemical/physical stability. But higher amount of active metals in the electro-catalysts, compared to the general purpose catalysts, make it difficult to control the metal size and the degree of distribution. This is mainly because of the restricted surface area of Vulcan XC72 carbon black. Thus complex and careM processes are necessary to get well dispersed fine active metal particles[4,5]. 

Electrical conductivity is an easily measured transport property, and percolation in electrical conductivity appears a sensitive probe for characterizing microstructural transformations. A variety of field (intensive) variables have been found to drive percolation in reverse microemulsions. Disperse phase volume fraction has been often reported as a driver of percolation in electrical conductivity in such microemulsions [17-20]. 

Lagues et al. [17] found that the percolation theory for hard spheres could be used to describe dramatic increases in electrical conductivity in reverse microemulsions as the volume fraction of water was increased. They also showed how certain scaling theoretical tools were applicable to the analysis of such percolation phenomena. Cazabat et al. [18] also examined percolation in reverse microemulsions with increasing disperse phase volume fraction. They reasoned the percolation came about as a result of formation of clusters of reverse microemulsion droplets. They envisioned increased transport as arising from a transformation of linear droplet clusters to tubular microstructures, to form wormlike reverse microemulsion tubules. 

Electrical Conductivity of Carbon Black Dispersions in Podecane(16)... 

Pratihar SK, Baus RN, Mazumder S, and Maiti HS. Electrical conductivity and microstructure of Ni-YSZ anode prepared by liquid dispersion method. In Singhal SC, Dokiya M, editors. Proceedings of the Sixth International Symposium on Solid Oxide Fuel cells (SOFC-VI), Pennington, NJ The Electrochemical Society, 1999 99(19) 513-521. 

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