Electrical resistances

Electrical resistance measurements have been reported on PANI/cerium oxide composites [48], polyester fibres and carbon containing epoxy composites [74]. 

Electrical resistivity has been measured on reinforced ABS terpolymers [75], jute filled phenol-formaldehyde resins [37] and carbon black PP composites [72]. 

Polyaniline is a relatively recent development. Electronically conducting polymers have applications in the field of electronics, micro-electric devices, computers, photographic equipment and, as discussed next, in developments such as electrically conducting electrochemical and fuel cells. 

Charge transport in electrically conducting polymers (ECP) is related to the role of easily polarisable delocalised p-electrons, which determines the electrical properties of conducting polymers [75]. Changes in molecular structure due to the localisation of p-electrons and electronic repulsion between the polycations influence the operation of a conducting polymer micro-acutator [76]. 

The electrical conductivity properties of PANI make it probably the most frequently discussed ECP. Developments of this polymer are discussed next in date order [77-80]. 

The electrical resistance of coatings is composed of the active (ohmic) and reactive (polarization) resistances. The latter is a resistance to alternating current through the capacity and inductance of the coating-substrate system. Ohmic resistance makes up an insignificant portion of the total electrical resistance of pol3Tner coatings and characterizes electrolyte diffusion in the 

Anodic current density (i, 1-4) in electrodes with pentaplast coatings (0.5N solution of H2SO4, 20°C) and delamination area (S, 5 - 7) of the coatings round a puncture (pregnant solution of NaCl, 20°C) versus coating formation temperature (T). Curves (1) 4 correspond to polarization potentials 4.0 3.0 2.0 and 0 V. Curves 7, 6 and (5) after 75, 125 and 150 hours of exposition 

Alternating currents of variable frequency are commonly used to measure the total electrical resistance i t of polymer coatings. On increasing the frequency to 2-25 kHz the ohmic constituent Rq of the total resistance can be estimated accurately enough. At low frequencies (400-1000 Hz) the recorded resistance value characterizes the polarizing resistance Rp of the coating. 

The electrical resistance exerted by a separator on the ionic current is defined as the total resistance of the separator filled with electrolyte minus the resistance of a layer of electrolyte of equal thickness, but without the. separator. The separator resistance has to be considered as an increment over the electrolyte resistance. 

The path taken by an ion from one electrode to the other will not be a straight one, as it has to evade the solid structures by making detours. The ratio of the mean 

For sulfuric acid (H2SO4) of specific density 1.28 gcm at 25 °C, the specific resistance (1/cr) is 1.26 Qcm using this value in the Eq. (6) and selecting values typical for polyethylene starter battery separators ai d = 0.25 mm, P = 0.6 and T =1.3, the electrical resistance for 1 cm of separators area results in 

This formula shows the factorial effect of the separator on the electrical resistance the measured resistance of the electrolyte-filled separator is the () - fold multiple of the electrolyte resistance without the separator by definition, 7 /P 1. 

With increasing tortuosity factor T and lower porosity P, R increases sharply. The electrical resistance of a separator is proportional to the thickness d of the membrane and is subject to the same dependence on temperature or concentration as 

Usually the electrical resistance of a separator is quoted in relation to area in the above case it is 57 mf2 cm. In order to quote it for other areas, because of the parallel connection of individual separator areas, KirchhofFs law has to be taken into account  

According to the modem quantum electronic theory [1,88,89], electrical resistivity of a metal results from the scattering of electrons by the lattice. In a perfect lattice, electrons experience no scattering, and they can carry current without any attenuation. A real metal lattice departs from perfect long-range order, and electrons are 

If we assume that all the current is carried by carriers with the same effective mass and velocity, then the resistivity p is given by 

The resistance of most plastics to the flow of direct current is very high. Both surface and volume electrical resistivities are important properties for applications of plastics insulating materials. The volume resistivity is the electrical resistance of the material measured in ohms as though the material was a conductor. Insulators will not sustain an indefinitely high voltage as the applied voltage is increased, a point is reached where a drastic decrease in resistance takes place accompanied by a physical breakdown of the insulator. This is known as the dielectric strength, which is the electric potential in volts, which would be necessary to cause the failure of a 1/8-in. thick insulator (Chapter 4, ELEC-TRICAL/ELECTR ONICS PRODUCT). 

Specimens for these tests may be any practical form, such as flat plates, sheets, or tubes. These tests describe methods for determining the several properties defined below. T vo electrodes are placed on or embedded in the surface of a test specimen. Different properties are obtained. 

Insulation resistance is the ratio of direct voltage applied to the electrodes to the total current between them dependent upon both volume and surface resistance of the specimen. In materials used to insulate and support components of an electrical network, it is generally desirable to have insulation resistance as high as possible. 

Volume resistivity is the ratio of the potential gradient parallel to the current density. 

Surface resistivity is the ratio of the potential gradient parallel to the current along its surface to the current per unit width of the surface. Knowing the volume and surface resistivity of an insulating material makes it possible to design an insulator for a specific application. 

We can define the principal electrical properties of polymers in terms of four characteristics electrical resistance, capacitive properties, dielectric strength, and arc resistance. We can change the surface characteristics of a polymer by subjecting it to a corona discharge generated by a strong electrical field. Lastly, we must also consider the influence of other physical properties on the application of polymers in electrical applications. 

AB = energy gap bettveen filled and unfilled electronic orbitals 

We measure volume resistance by placing electrodes with known dimensions on opposite sides of a specimen of known thickness. Volume resistivity is calculated from Eq. 8.12. 

Ty = volume resistivity Ry volume resistance A = area of measuring electrode t = sample thickness 

We obtain surface resistance in a similar fashion to volume resistance, except thatthe electrodes are placed on the same side of the sample. We calculate surface resistivity from Eq. 8.13 

Both theoretical premises and some experimental data allow us to try to find an explanation of the observed membrane polarization changes in artificial membranes. The first contributing factor which is involved is the membrane resistance. When the concentrations of electrolytes are the same across the membrane, the resistance is linear. Nonlinear resistance and rectifying properties of the membrane above this restriction are detected. If the lipid-like substances are characterized by high resistance, proteins display lower resistance, and their presence in membranes diminishes it con- 

FIGURE 4. Chromatograms showing ATP produced in a suspension of copoly (asp,glu,cys,leu,tyr) microspheres in an aqueous solution of ADP and inorganic phosphate and with isobutyric acid NH40H EDTA solvent (upper), and rechromatogram of ATP fraction with LiCl2 acetic solvent (lower). Ordinate CPM X 10  

The entropy production rate is equal to a sum of products of generalized forces by generalized fluxes. The laws of thermodynamics of irreversible processes enable us to express these fluxes as functions of these forces. When we do not stray too far from the state of equilibrium, where the fluxes and forces are null, linear relations appear between these terms. The coefficients of these linear laws are the Onsager phenomenological coefficients they are combinations of the coefficients of diffusion, viscosity, heat conduction, etc. In conductive media, the electrical resistance also appears as an Onsager coefficient. 

In order to demonstrate this fact, we envisage an isothermal conductor made up solely of ions and electrons. This is illustrated well by a metal conductor, with the ensemble of ions (fixed) plus electrons (mobile) behaving like a fluid. No chemical reaction takes place, there are no forces work, no viscosity, etc. Only an electrical current is liable to circulate. 

However, we have v, = 0,v =/ v, which, for the diffusion fluxes, gives us  

Because neutrality is assured, the total charge z is zero, and we have  

Additionally, in the absence of a temperature and pressure gradient, the Gibbs-Duhem equation gives us  

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Figure C2.16.1. A nomogram comparing electrical resistivity of pure (intrinsic) and doped Si witli metals and insulators.

Selenium exhibits both photovoltaic action, where light is converted directly into electricity, and photoconductive action, where the electrical resistance decreases with increased illumination. These properties make selenium useful in the production of photocells and exposure meters for photographic use, as well as solar cells. Selenium is also able to convert a.c. electricity to d.c., and is extensively used in rectifiers. Below its melting point selenium is a p-type semiconductor and is finding many uses in electronic and solid-state applications. 

It is a white crystalline, brittle metal with a pinkish tinge. It occurs native. Bismuth is the most diamagnetic of all metals, and the thermal conductivity is lower than any metal, except mercury. It has a high electrical resistance, and has the highest Hall effect of any metal (i.e., greatest increase in electrical resistance when placed in a magnetic field). 

The electronic configuration for an element s ground state (Table 4.1) is a shorthand representation giving the number of electrons (superscript) found in each of the allowed sublevels (s, p, d, f) above a noble gas core (indicated by brackets). In addition, values for the thermal conductivity, the electrical resistance, and the coefficient of linear thermal expansion are included. 

Thermal Conductivity Detector One of the earliest gas chromatography detectors, which is still widely used, is based on the mobile phase s thermal conductivity (Figure 12.21). As the mobile phase exits the column, it passes over a tungsten-rhenium wire filament. The filament s electrical resistance depends on its temperature, which, in turn, depends on the thermal conductivity of the mobile phase. Because of its high thermal conductivity, helium is the mobile phase of choice when using a thermal conductivity detector (TCD). 

Surface conduction is monitored in most humidity sensors through the use of porous ceramics of MgCr204—Ti02 that adsorb water molecules which then dissociate and lower the electrical resistivity. 

Some nonhygroscopic materials such as metals, glass, and plastics, have the abiUty to capture water molecules within microscopic surface crevices, thus forming an invisible, noncontinuous surface film. The density of the film increases as the relative humidity increases. Thus, relative humidity must be held below the critical point at which metals may etch or at which the electrical resistance of insulating materials is significantly decreased. 

Spinel ferrites, isostmctural with the mineral spinel [1302-67-6] MgAl204, combine interesting soft magnetic properties with a relatively high electrical resistivity. The latter permits low eddy current losses in a-c appHcations, and based on this feature spinel ferrites have largely replaced the iron-based core materials in the r-f range. The main representatives are MnZn-ferrites (frequencies up to about 1 MH2) and NiZn-ferrites (frequencies 1 MHz). 

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