Conduction mechanisms Particles

A polymer electrolyte with acceptable conductivity, mechanical properties and electrochemical stability has yet to be developed and commercialized on a large scale. The main issues which are still to be resolved for a completely successful operation of these materials are the reactivity of their interface with the lithium metal electrode and the decay of their conductivity at temperatures below 70 °C. Croce et al. found an effective approach for reaching both of these goals by dispersing low particle size ceramic powders in the polymer electrolyte bulk. They claimed that this new nanocomposite polymer electrolytes had a very stable lithium electrode interface and an enhanced ionic conductivity at low temperature. combined with good mechanical properties. Fan et al. has also developed a new type of composite electrolyte by dispersing fumed silica into low to moderate molecular weight PEO. 

Ionically conducting polymers and their relevance to lithium batteries were mentioned in a previous section. However, there are several developments which contain both ionically conducting materials and other supporting agents which improve both the bulk conductivity of these materials and the properties of the anode (Li)/electrolyte interface in terms of resistivity, passivity, reversibility, and corrosion protection. A typical example is a composite electrolyte system comprised of polyethylene oxide, lithium salt, and A1203 particles dispersed in the polymeric matrices, as demonstrated by Peled et al. [182], By adding alumina particles, a new conduction mechanism is available, which involved surface conductivity of ions on and among the particles. This enhances considerably the overall conductivity of the composite electrolyte system. There are also a number of other reports that demonstrate the potential of these solid electrolyte systems [183],... 

Figure 13.3 Mechanism of charge alignment of conducting aerosol particles as th y we brought together by an external force.

Mahajan and coworkers [85] studied the impact of abrasive size at different particle concentrations on the oxide removal rate. It was found that the removal rate was a direct function of the particle concentration for monosize abrasives of size 0.2 pm, thereby supporting the contact-area mechanism. The mechanism shifted to indentation for a monodispersed system at 1.5 pm, resulting in reduced removal rates. At 0.5 pm, the removal rate initially increased and then decreased with particle concentration, suggesting a shift in the removal rate mechanism. Particle-size distribution [86] has an equally important effect as the particle size. A larger number of oversized particles in the distribution also cause a shift in the mechanism of material removal. Mahajan and coworkers conducted studies to evaluate the impact of size distribution on oxide removal rates. Baseline commercial slurry was spiked with different concentrations of impurities in the range of 0.5-1.5 pm. The size at different concentrations resulted in removal rates lower than that obtained with the original slurry. Slurry spiked with 1.1 % of 1.5pm particles resulted in a removal rate equal to the baseline slurry, suggesting the predominance of indentation mechanism. Slurries spiked with other concentrations and sizes resulted in a decrease in the removal rate explained by the reduction in the contact area of the abrasives with the oxide substrate. 

Ionic systems, such as water solutions of NaCl, CuS04, K2Cr04, and Ca(NOs)2 and solutions of sulfuric acid in ethyl alcohol, were among the objects of Johnson s experiments (I) that led him to conclude that there exists equilibrium electrical noise of a universal nature that manifests thermal motion of charged particles in conductors on a macroscopic level. Independently of a particular conductivity mechanism, the voltage spectral density, Sv(/), of this noise can be calculated from the real part of the system... 

The measurement of bulk resistivity of a powder includes volume and surface conduction mechanisms. It is generally not possible to separate out the two effects so that the effective powder resistivity, either the volume or surface resistivity, for dielectric and insulating particles such as glass depends on such factors as the presence of surface impurities and the relative humidity. For clean metal powders, the volume resistivity will dominate conduction in a bed of particles whereas, the presence of a surface oxide film can dominate conduction via the contact resistance for only lightly compacted powders. 

Particles remain noncontacting, i.e. restricted to volume conduction mechanism only. 

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. 

Unlike other acids, in concentrated phosphoric acid solutions (where the water concentration is low) hydrogen ions exist not as hydrated ions H+ H20, but as ions solvated by phosphoric acid molecules H+ - H3P04. For this reason, the solution s conductivity is a complex, nonmonotonous function of concentration. As for the conductivity mechanism, these ions do not move like a spherical particle in a viscous medium when an electric field is applied to the solution (the Stokes mechanism), but rather the protons alone jump in the field direction from one acid molecule to another acid molecule (the Grotthuss mechanism, suggested in 1806 as an explanation for the conductivity behavior in aqueous solutions). 

These results forced us to conclude that the conductive polymers are in fact organic metals or they are nanometals, and two different transport mechanisms contribute to the conduction mechanism (a) a purely metallic part within each particle and (b) a thermally activated part from one particle to another (Figure 1.12b). 

There are two different types of fractals in solid state chemistry (a) mass fractals, sets of solid particles that form aggregates and have as measure their mass that scales as I with 0 < D 3 and (b) surface fractals that consist of interfaces between solids and the vacuum and that have as measure the surface, which also scales as IP with 0 < D < 3. The fractal dimension of an object, composite, or aggregate affects the values of the heat capacity, heat conductivity, electric conductivity, mechanical resistance against deformation, specific mass, and light scattering. 

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