Conductivity in-plane

Nitta et al. [216] designed a method to measure the in-plane conductivity of a DL as a function of the compressed thickness. The sample material was placed on a plate and compressed on both ends by graphite current collectors. Steel gages were located between the graphite blocks and the plate in order to maintain constant thickness of the DL while tests were conducted. A specific current range was applied to the apparatus and the voltage drop was measured in order to calculate the total resistance of the system. 

Through assumptions and the use of values for known resistances of the materials used in the apparatus, the actual bulk resistance of the DL material could be calculated. This resistance was then used so that the electrical conductivity could be solved. Nitta and colleagues noted that the in-plane conductivities of the DL materials were a linear fxmction of the compressed thickness (i.e., the conductivity increased when the thickness decreased with increased compression pressure). This resulted from a decrease in thickness that led to a loss of porosity in the DL materials and higher contact between fibers. 

Similarly to in-plane conductivity, through-plane conductivity seems to be a linear function of the compressed thickness of the DL that is, conductivity increases linearly with thickness decrease. Nitta et al. [216] also observed that in-plane conductivity was larger than through-plane conductivity however, the difference was not as large as that found in previous studies [9]. 

LijN has a layer structure. Fig. 2.14, in which sheets of stoichiometry LijN are separated by additional Li" ions, which act to bridge the N atoms in adjacent LijN sheets. In H-doped LijN, 1-2% of the Li" ions in the LijN sheets are vacant, giving rise to a high in-plane conductivity of 1 X 10 S cm at 25 °C, but with a much lower conductivity perpendicular to the sheets, 1 x 10 Scm ... 

Recently Japanese workers have succeeded in forming well ordered LB films from single compounds which show a high in-plane conductivity and whose layer structure has been characterised by low angle X-ray diffraction [351, 352]. 

FIGURE 6.19 Room temperature in-plane resistivity of F GIC C Fy for different fluorine concentrations. O0 is the in-plane conductivity of host graphite pristine graphite (HOPG) and graphite fiber (VGCF). Note that y is defined as y = (1 + x) 1 for C F. (From Vaknin, D., et al., Synth. Metals, 16, 349, 1986. With permission.)... 

Intercalation of electroactive polymers such as polyaniline and polypyrrole in mica-type layered silicates leads to metal-insulator nanocomposites. The conductivity of these nanocomposites in the form of films is highly anisotropic, with the in-plane conductivity 10 to 10 times higher than the conductivity in the direction perpendicular to the film. Conductive polymer/oxide bronze nanocomposites have been prepared by intercalating polythiophene in V2O5 layered phase, which is analogous to clays. °° Studies of these composites are expected not only to provide a fundamental understanding of the conduction mechanism in the polymers, but also to lead to diverse electrical and optical properties. 

As can be seen in Figure 11.16(b), the electric resistance of the film depends on the applied gate voltage and, therefore, its in-plane conductivity becomes a function of the applied electric field. On that basis we can conclude that the sp film clearly demonstrates the field effect. 

It has been predicted theoretically [21] and demonstrated experimentally [22] that one or a few layers of atomically flat graphene exhibits two-dimensional semimetal properties with a small overlap (ca. 0.04 eV) between the valence and conductance bands at six symmetric points in the corner of the Brillouin zone, as shown in Figure 14.4. Thus, a nonzero density of states is found at the Fermi level, although the Fermi surface consists of only isolated points. This is attributed to the delocalization of electrons in the graphene plane and results in high in-plane conductivity. Across the plane, there is little interaction, and thus it shows very small conductivity. 

The in-plane conductivity is the sum of the conductivities of the individual layers, which depend exponentially on the band-bending potential (f>. Thus,... 

Built-up multilayers can be obtained only for transfer achieved at a surface far beyond the plateau regime, generally around 35 mN/m. Hence the resulting films are just a disordered superposition of oken monocrystals, with a random orientation in the plane of the support, as shown in Fig. 2b. As a consequence, the normal to the support behaves as an axis of full rotation. Such a macroscopic configuration is easy to investigate. This was done by the use of two different resonance methods, namely, linear dichroism and electron spin resonance, with the following result TCNQ radical anions lie completely flat on the substrate in the dimeric form [9]. Obviously, such a structure is not compatible with an in-plane conductivity, for which TCNQ molecular planes should be perpendicular to the substrate. In addition, the complete charge transfer between pyridinium and TCNQ prevents any kind of electron conductivity in the film and, as expected, the electrical properties of the pristine film are those of an insulator. 

One-component versus multicomponent description of the in-plane conductivity... 

In-plane conductivity of NR-211 membranes at 30°C (diamonds), (squares) and 80°C (triangles) (Peron etal., 2010). 

Illustration of in-plane conductivity measurement using the four-point probe technique. 

The four-point probe method can be used to measure the in-plane electronic conductivity of the GDM, as shown in Figure 4.10. Two copper strips are placed near the ends of the GDM, and two copper wires separated a distance of L are placed befween the two strips. The strips and the wires are pressed against the GDM to form good contacts. A current passes from one sfrip fhrough the GDM along the in-plane direction to the other strip, and the voltage drop between the two wires is measured. Then the in-plane conductivity is... 

For a GDM made of carbon fiber (e.g., carbon paper), the in-plane conductivity is normally much higher than the through-plane conductivity because most of the carbon fibers stay in the planar direction. Fortunately, since the thickness of the GDM is around 0.2 mm, the voltage loss caused by the through-plane resistance is not significant. For example, if the through-plane conductivity is 20 S cm then the voltage drop caused by one membrane electrode assembly (MEA) (with two pieces of GDM) will be 2 mV at a current density of 1 A cm. ... 

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