Electrical resistance dependence

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). 

Besides the transference numbers Schlogl and Schodel were able to calculate the concentration profiles in the membrane. If the solution concentrations are different on both sides of the membrane, these profiles change if the direction of the current is reversed. By consequence, the electrical resistance depends on the direction of the current. This is called the rectifier effect. 

SiC-BN composites show good thermal conductivity. The electrical resistivity depends on the BN content and can be adjusted from a few Q/cm to more than 1010 Q/cm. When the SiC content of the composites is high, the composites can be used as cutting tools [135]. BN additions reduce the friction coefficient. Thus these composites are used for sliding parts [136],... 

Seebeck coeffcient and electrical resistivity depending on temperature. 

Here, A is the thermal conductivity and hi the specific enthalpy of species i either in the gas phase or at the surface. In the radiation term, a is the Stefan-Boltzmann constant, e is the temperature-dependent surface emissivity, Tref is the reference temperature to which the surface radiates. The term PR represents an energy source corresponding to resistive heating of the catalyst, where I is the current and R the electrical resistance depending on temperature. Ns is the number of surface species, Cp the specific heat capacity of the gas at the wall, and Ccat denotes the specific heat capacity of the catalyst while pcat is the density of the catalyst material. 

The oxidation of ethylene in air on a Pt wire is a good example by which to demonstrate the ignition behavior of exothermic catalytic reactions. The experiment was conducted as follows (Table 4.5.4). A coil consisting of a thin Pt-wire is placed in a tubular reactor. Then an ethylene-air mixture of constant temperature and pressure (303 K, 1 bar) is fed into the tubular reactor. The wire is now electrically heated until ignition (jump in temperature) occurs. The current and the voltage is measured and, thus, also the temperature of the wire as the electrical resistance depends on temperature. 

Electrical resistivity— dependence on resistance, specimen cross-sectional area, and distance between measuring points... 

Plasma-deposited siUcon nitride contains large amounts of hydrogen, typically in the range of 20—25 atomic % H, and has polymer-like properties. The electrical resistivity of the film depends on the deposition temperature, the film stoichiometry, and the amounts of hydrogen and oxygen in the film. 

Electrical conduction ia glasses is mainly attributed to the migration of mobile ions such as LE, Na", K", and OH under the influence of an appHed field. At higher temperatures, >250° C, divalent ions, eg, Ca " and Mg ", contribute to conduction, although their mobiUty is much less (14). Conduction ia glass is an activated process and thus the number of conducting ions iacreases with both temperature and field. The temperature—resistivity dependence is given... 

Nonferrous alloys account for only about 2 wt % of the total chromium used ia the United States. Nonetheless, some of these appHcations are unique and constitute a vital role for chromium. Eor example, ia high temperature materials, chromium ia amounts of 15—30 wt % confers corrosion and oxidation resistance on the nickel-base and cobalt-base superaHoys used ia jet engines the familiar electrical resistance heating elements are made of Ni-Cr alloy and a variety of Ee-Ni and Ni-based alloys used ia a diverse array of appHcations, especially for nuclear reactors, depend on chromium for oxidation and corrosion resistance. Evaporated, amorphous, thin-film resistors based on Ni-Cr with A1 additions have the advantageous property of a near-2ero temperature coefficient of resistance (58). 

Resistivity can be used as a guide to the role a material performs ia a specific device. Materials having high values, such as Teflon, serve an iasulation function (see Insulation, electrical). Metals such as silver and copper are excellent conductors. Organic compounds and polymers can cover a wide range of values, and the actual resistivity depends on exact composition. 

Temperature The level of the temperature measurement (4 K, 20 K, 77 K, or higher) is the first issue to be considered. The second issue is the range needed (e.g., a few degrees around 90 K or 1 to 400 K). If the temperature level is that of air separation or liquefact-ing of natural gas (LNG), then the favorite choice is the platinum resistance thermometer (PRT). Platinum, as with all pure metals, has an electrical resistance that goes to zero as the absolute temperature decreases to zero. Accordingly, the lower useful limit of platinum is about 20 K, or liquid hydrogen temperatures. Below 20 K, semiconductor thermometers (germanium-, carbon-, or silicon-based) are preferred. Semiconductors have just the opposite resistance-temperature dependence of metals—their resistance increases as the temperature is lowered, as fewer valence electrons can be promoted into the conduction band at lower temperatures. Thus, semiconductors are usually chosen for temperatures from about 1 to 20 K. 

The electrical-resistance measurement has nothing to do with the electrochemistry of the corrosion reaction. It merely measures a bulk property that is dependent upon the specimens cross-section area. Commercial instruments are available (Fig. 28-5). 

The specific electrical resistances usually depend on the material and the temperature [31]. For the most important pipe materials these are (in 10 Q cm) ... 

The grounding or penetration depth of the electrical resistance in conductors is, according to Eq. (3-42), dependent on the specific resistance and the frequency. The penetration depth, t, is the distance at which the field strength has fallen by 1/e,- is the relative permeability [35] ... 

The specific electrical resistance of concrete can be measured by the method described in Section 3.5. Its value depends on the water/cement value, the type of cement (blast furnace, portland cement), the cement content, additives (flue ash), additional materials (polymers), the moisture content, salt content (chloride), the temperature and the age of the concrete. Comparisons are only meaningful for the... 

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