Specific resistance value

The bulk (or volume)-specific resistance is one of the most useful general electrical properties. Specific resistance is a physical quantity that may vary more than 10 in readily available materials. This unusually wide range of conductivity allows wide variety of electrical applications. Conductive materials, such as copper, have specific resistance values of about 10 fl-cm, whereas good insulators such as polytetrafluoroethylene and LDPE have values of about 10 fl-cm. Specific resistance is calculated from the following equation where R is the resistance in ohms, a is the pellet area in square centimeters, t is the pellet thickness in centimeter, and P is the specific resistance in ohm-centimeter ... 

A number of quaternary ammonium perchlorates and tetrafluoroborates have been prepared, and their solubilities and specific resistance values have been measured in several solvents (see Table 7.6).35 The resistance values are about 10-fold higher in ethers like tetrahydrofuran and 1,2-dimethyoxyethane relative to solutions in acetonitrile and dimethylformamide. This is due to the greater ion association in the ethers because of their smaller dielectric constants. 

The specific resistance value (a) in the case of incompressible cakes assumes a constant value. For compressible cakes, the resistance varies with the pressure according to equation 2.5 ... 

The electro-catalyst layers, which are porous, conduct both ions and electrons to facilitate oxidation and reduction reactions. For example, the electrocatalyst layers in most PEMFCs are 5-30 pm thick. The ion conductivity of these layers varies from 1 to 5 S/m and hence electro-catalyst layer area specific resistance values vary from 0.01 Q cm to 0.03 Q cm (or 10-300 mQ cm ). The Nafion electrolyte in PEMFC has a conductivity of 10 S/m when hydrated. Hence, for electrolyte thickness of 50-200 pm, the area specific resistance varies from 50 mil cm to 200 mil cm. Thus, it can be seen that the electro-catalyst layer contribution to ohmic resistance is significant. In Table 5.3, typical thickness and area-specific resistance values for selected fuel components are listed. 

The resistance due to a circular junction is given by / = /2ak, where a is the radius of the junction and k is specific conductivity of the metal. For the case of two steel plates, the measured resistance is 5 x 10" Q for a load of 50 kg the yield pressure of steel is 60 kg/mm, and the specific resistance is 5x 10 Q/cm. Calculate the number of junctions, assuming that it is their combined resistance that is giving the measured value. 

For each specific appHcation of a mbber compound as an iasulating material, there is a minimum value of resistivity below which it does not function satisfactorily. In addition, iasulating compounds are required to withstand the effect of water, moist atmosphere, or heat without their resistivity values falling below a satisfactory level. Insulation resistance measurements frequently serve as useful control tests to detect impurities and manufactuting defects ia mbber products. 

The ability of a material to retard the flow of heat is expressed by its thermal conductivity (for unit thickness) or conductance (for a specific thickness). Low values for thermal conduc tivity or conductance (or high thermal resistivity or resistance value) are characteristics of thermal insulation. 

The specific resistance coefficient for the dust layer Ko was originally denned by Williams et al. [Heat. Piping Air Cond., 12, 259 (1940)], who proposed estimating values of the coefficient by use of the Kozeny-Carman equation [Carman, Trans. Inst. Chem. Fng. (London), 15, 150 (1937)]. In practice, K and Ko are measured directly in filtration experiments. The K and Ko values can be corrected for temperature by multiplying by the ratio of the gas viscosity at the desired condition to the gas viscosity at the original experimental conditions. Values of Ko determined for certain dfists by Williams et al. (op. cit.) are presented in Table 17-5. 

The following terms apply to the specific coating resistance which is related to the surface, S r is the value calculated from the specific resistance of the coating material using Eq. (5-1) ... 

The dependence on the temperature of the specific resistance (Q/cm) of the pure MEPBr and MEMBr complexes, and a 1 1 mixture there of, as obtained in Ref. [73], is listed in Table 4. It is remarkable that within the complex phases consisting of Br2 and either pure MEP or MEM the change of specific resistance at the liquid —> solid phase transition amounts to about one order of magnitude, where as the value is only doubled in the 1 1 mixture. The table also indicates that MEMBr complexes possess higher melting temperatures. 

Experiments to determine specific resistance, based on Equation 7, have usually been carried out by some form of vacuum filtration. These methods are time-consuming and subject to error. More rapid techniques such as the measurement of capillary suction time (CST) can be used (8), although these do not give absolute values of specific resistance. Nevertheless, the CST method is very useful for rapidly obtaining comparative data on the flocculation of fairly concentrated suspensions by polymers (9). In the present work, specific resistance has been determined by an automated technique, which will be described below. 

The specific resistances obtained are independent of applied load, suspension concentration and membrane type, as expected for non-compressible filter cakes. Tests with uniform latex particles have given permeabilities in very good agreement with Equation 2, using a value of 5 for the Carman-Kozeny constant. 

The results in Figure 5 compare these procedures and the previous one (rapid mix and tube flow) for a kaolin concentration of 140 mg/1. In this and subsequent Figures, specific resistances are plotted as percentages of the original value (i.e. for the unflocculated kaolin). Evidently, the procedure makes little difference to the results and essentially the same optimum polymer concentration would be chosen in each case. 

Thus, the polarisation data, cyclic voltammetric results and the a.c. impedance measurements all suggest that, when an Ru02/TiC>2 anode exhibits a high overpotential, this is a direct consequence of the surface depletion of Ru. This is also consistent with the estimated Re values of approximately 20 Q for the failed electrodes, in contrast to the known, much higher specific resistivity of Ti02 of... 

Typical values of the specific resistance r of filter cakes, taken from the work of Carman(1°), are given in Table 7.1. In the absence of details of the physical properties of the particles and of the conditions under which they had been formed, these values are approximate although they do provide an indication of the orders of magnitude. 

The blocking of the pores of the filter medium by particles is a complex phenomenon, partly because of the complicated nature of the surface structure of the usual types of filter media, and partly because the lines of movement of the particles are not well defined. At the start of filtration, the manner in which the cake forms will lie between two extremes — the penetration of the pores by particles and the shielding of the entry to the pores by the particles forming bridges. Heertjes(11) considered a number of idealised cases in which suspensions of specified pore size distributions were filtered on a cloth with a regular pore distribution. First, it was assumed that an individual particle was capable on its own of blocking a single pore, then, as filtration proceeded, successive pores would be blocked, so that the apparent value of the specific resistance of the filter cake would depend on the amount of solids deposited. 

Transistors Mg and Mg are the main sources of noise of this ampHfier. Their area is hence optimized in order to meet the noise specifications. Table 5.3 summarizes the transistor dimensions, bias currents and resistance values. 

The significant variation of the barrier height observed for immersed junctions reflects the experimental difficulties associated with determining the tunneling constant, k. Two key issues are contamination of the junction and uncertainty as to the structural and electronic character of the tip [104], Recent data clearly reveal a dependence of the apparent barrier height on tip-substrate separation [7,92-94,104]. Specifically, the effective barrier is observed to diminish for resistance values below <10 Q as shown in Fig. 

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