Electrical Conduction in Materials

An electric field applied to a material that has free charged particles causes these particles to flow through the material and into the external circuit. How do we define and measure the conductivity of a material On what macroscopic properties does it depend How does it relate to the detailed chemical strnctnre of the material  

The electrical conductivity of a material is measured by placing a cylindrical sample of cross-sectional area. A, and length, , in a simple electrical circuit as a resistor in series with a power supply and ammeter a voltmeter measures the actual voltage across the sample (Fig. 22.18). If the voltage, V, is varied and the resnlting current, I, is measured at each voltage, a plot of / versus V is a straight line (Fig. 22.19)  

The resulting slope is called the conductance and is denoted by G. If V is measnred in volts and I in amperes (A), then G has units of siemens (1 siemens = 1 A/V). If the value of G is constant, the material follows Ohm s law 

FIGURE 22.19 Plot of current against voltage Is a straight line whose slope Is the conductance G. 

Therefore, we define the material property resistivity, denoted by p, as the proportionality constant that summarizes these two effects  

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The most apparently true model of electric conductivity in materials with the system of double conjugated bonds seems to be the change transfer in the ranges of poly conjugation possessing metal conductivity and jump conductivity between poly conjugation spheres [6]. 

Electrical conduction in materials characterizes a charge transport phenomenon of conduction electrons (or holes) which occurs in the presence of an electrical field E. The current density j is described by Ohm s law... 

Humidification. Adding moisture to air has long been used to control static and to help dissipate static electricity in textile mills (see A IR conditioning). Moisture does not improve the electrical conductivity of the air, but it increases the electrical conductivity of materials absorbing moisture. 

Electromagnetic (EM) Conductivity Measures the electrical conductivity of materials in microohms over a range of depths determined by the spacing and orientation of the transmitter and receiver coils, and the nature of the earth materials. Delineates areas of soil and groundwater contamination and the depth to bedrock or buried objects. Surveys to depths of SO to 100 ft are possible. Power lines, underground cables, transformers and other electrical sources severely distort the measurements. Low resistivities of surficial materials makes interpretation difficult. The top layers act as a shunt to the introduction of energy info lower layers. Capabilities for defining the variation of resistivity with depth are limited. In cases where the desired result is to map a contaminated plume in a sand layer beneath a surficial clayey soil in an area of cultural interference, or where chemicals have been spilled on the surface, or where clay soils are present it is probably not worth the effort to conduct the survey. 

This is an important relationship. It states that the modulus of a unidirectional fibre composite is proportional to the volume fractions of the materials in the composite. This is known as the Rule of Mixtures. It may also be used to determine the density of a composite as well as other properties such as the Poisson s Ratio, strength, thermal conductivity and electrical conductivity in the fibre direction. 

The electrical conductivity in the solid state is determined by the product of the carrier concentration and the carrier mobility. In conjugated polymers both entities are material dependent and, i.e., are different for electrons and holes. Electrons or holes placed on a conjugated polymer lead to a relaxation of the surrounding lattice, forming so-called polarons which can be positive or negative. Therefore, the conductivity, o, is the sum of both the conductivity of positive (P+) and negative polarons (P ) ... 

Quite naturally, novel techniques for manufacturing composite materials are in principal rare. The polymerization filling worked out at the Chemical Physics Institute of the USSR Academy of Sciences is an example of such techniques [49-51], The essence of the technique lies in that monomer polymerization takes place directly on the filler surface, i.e. a composite material is formed in the polymer forming stage which excludes the necessity of mixing constituents of a composite material. Practically, any material may be used as a filler the use of conducting fillers makes it possible to obtain a composite material having electrical conductance. The material thus obtained in the form of a powder can be processed by traditional methods, with polymers of many types (polyolefins, polyvinyl chloride, elastomers, etc.) used as a matrix. 

Having considered the material outlined in preceding sections it is easy to conclude that depending on technology of preparation of polycrystalline adsorbent one can encounter various mechanisms of electric conductivity and, naturally, various mechanisms and peculiarities of adsorption induced change of the value of electric conductivity in adsorbents. 

Electrical Conductivity. Measurements of the electrical conductivity of materials under Irradiation, particularly time-resolved conductivity following a pulse of electrons, can provide Information on mobilities and trapping mechanisms, free electron yield, and thermal1sat Ion times and path lengths. Some measurements have been done on polymers (45-47) particularly because of their technological Importance in the electronics Industry (37). More fundamental work Is needed. 

An important point is that the electrochemically driven charge transport in these polymeric materials is not dependent on the presence of mixed valence interactions which are well known to give rise to electronic conductivity — in a number of cation radical crystalline salts. This is clearly seen from the absorption spectrum of the electrochemically oxidized pyrazoline films (Figure 8) which show no evidence for the mixed valence states that are the structural electronic prerequisites for electrical conductivity in the crystalline salts. A more definitive confirmation of this point is provided by the absorption spectrum (Figure 10) of electrochemically oxidized TTF polymer films which shows... 

Electrical and thermal conductivity are important diffusion layer properties that affect the fuel cell s overall performance. The maferial chosen to be the DL in a fuel cell must have a good electrical conductivity in order for the electron flow from the FF plates to the CLs (and vice versa) to have the least possible resistance. Similarly, the DL material must have good thermal properties so that heat generated in the active zones can be removed efficiently. Therefore, in order to choose an optimal material it is critical to be able to measure the electrical and thermal conductivity. In this section, a number of procedures used fo measure fhese paramefers will be discussed. 

In practice, a porous electrically insulating material containing the electrolyte is often placed between the anode and cathode to prevent the anode from directly contacting the cathode. Should the anode and cathode physically touch, the battery will be shorted and its full energy released as heat inside the battery. Electrical conduction in electrolytic solutions follows Ohm s law E = IR. 

Electrical conduction in such disordered materials proceeds by a process of phonon-assisted tunnelling in which the probability of transfer between two sites is given by... 

The earliest considerations of how electrical conductivity might be associated with molecular materials emphasized the polarizability of the constituent molecules as the most important factor (65JCP(42)4307). The efforts of synthetic chemists to devise an organic conductor based solely on these ideas were unsuccessful. Discovery of the electrical conductivity in the material prepared from A-2,2 -bi-l,3-dithiolylidene (tetrathiafulvalene, TTF ... 

In this chapter we take a careful look at the phenomenon of electrical conductivity of materials, particularly electrolytic solutions. In the first section, the nature of electrical conductivity and its relation to the electrolyte composition and temperature is developed. The first section and the second (which deals with the direct-current contact methods for measuring conductance) introduce the basic considerations and techniques of conductance measurement. This introduction to conductance measurements is useful to the scientist, not only for electrolytic conductance, but also for understanding the applications of common resistive indicator devices such as thermistors for temperature, photoconductors for light, and strain gauges for mechanical distortion. The third section of this chapter describes the special techniques that are used to minimize the effects of electrode phenomena on the measurement of electrolytic conductance. In that section you will encounter the most recent solutions to the problems of conductometric measurements, the solutions that have sparked the resurgent interest in analytical conductometry. 

One potential application of the work on oriented nematic phases of rodlike molecules is to solutions containing cylindrical micelles. Orientation could be achieved by a shear field or perhaps by an electric field. Gotz and Heckman (9) confirmed the existence of anisotropic electrical conductivity for a concentrated surfactant solution in a shear field. They used their results to show that the solution contained cylindrical rather than platelike micelles. Of course, the magnitude of the electrical conductivity in an aqueous micellar solution should be quite different from that in the nematic phase of an organic material. So the conditions for and types of electrohydrodynamic instabilities could be different as well. 

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