Tungsten electrical resistivity

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). 

The electrical resistance of rhenium is about 3.5 times higher than tungsten at 20°C. However, the difference is reduced at higher temperatures because of rhenium s lower temperature coefficient, so that at 2500°C the resistivity of rhenium is only about 20% more than tungsten. The higher resistance at low temperature, combined with a lower temperature coefficient, contributes to rapid heating of a filament. 

Heating and Cooling. Heat must be appHed to form the molten zones, and this heat much be removed from the adjacent sohd material (4,70). In principle, any heat source can be used, including direct flames. However, the most common method is to place electrical resistance heaters around the container. In air, nichrome wine is useflil to ca 1000°C, Kanthal to ca 1300°C, and platinum-rhodium alloys to ca 1700°C. In an inert atmosphere or vacuum, molybdenum, tungsten, and graphite can be used to well over 2000°C. 

The above measurements all rely on force and displacement data to evaluate adhesion and mechanical properties. As mentioned in the introduction, a very useful piece of information to have about a nanoscale contact would be its area (or radius). Since the scale of the contacts is below the optical limit, the techniques available are somewhat limited. Electrical resistance has been used in early contact studies on clean metal surfaces [62], but is limited to conducting interfaces. Recently, Enachescu et al. [63] used conductance measurements to examine adhesion in an ideally hard contact (diamond vs. tungsten carbide). In the limit of contact size below the electronic mean free path, but above that of quantized conductance, the contact area scales linearly with contact conductance. They used these measurements to demonstrate that friction was proportional to contact area, and the area vs. load data were best-fit to a DMT model. 

Tip assemblies used for work at helium temperatures may be modified as shown in Fig. 12. The inclusion of the nichrome sections is necessary if temperature control is desired. Since both the heat capacity and the electrical resistivity of tungsten at helium temperatures are extremely low and its heat conductivity is very high, a rise in temperature from I K to about 1000°K corresponds to changes of a few milliamperes in the heating current if all-tungsten assemblies are used. The nichrome sections act as thermal barriers, since alloys do not lose their high-temperature thermal properties at 4°K, and permit fine control of temperature. For... 

Tungsten. Tungsten increases electrical resistance and incorrodibility. Irmann, Metall und Erz, 1915, 12,358 1917,14,21. 

The electrical resistivity of Na W03, Li WOs, and K WOg has been measured at 300° K. The range of x values was 0.25 < x < 0.9. All resistivities were characteristic of a metal and lie on a single curve. Extrapolation of the conductivity curve to zero conductivity indicated that the tungsten bronzes should be semiconductors for x < 0.25. The resistivities measured for tungsten bronzes with x < 0.25 showed semiconducting behavior. The resistivity of Li WOg exhibited an anomalous peak in the p vs. T curve. The Hall coefficient of Li0 37WO3 indicated one free electron per alkali atom, as previously found for Na WOg. The Seebeck coefficient of Na WOg depended linearly on x 2/3, as expected from free electron theory. The implications of these and other data are discussed. 

The more frequently used forms of electrical heating, such as resistance and induction, have limited use above 3000°K. (Ml, Dl, C2, FI, F4, B3). Tungsten, carbon, and some of the carbides are the only solid materials capable of use as resistors or susceptors above that temperature. Conducting liquids, although potentially capable of reaching higher temperatures than these solids as electrical resistance elements, suffer from problems of containment and have not been used above 3000°K. except in levitation melting (01, W6). 

The thermal conductivity detector (TCD), which was one of the earliest detectors for gas chromatography, still finds wide application. This device consists of an electrically heated source whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas. The heated element may be a fine platinum, gold, or tungsten wire (Figure 31 -9a) or, alternatively, a small thermistor. The electrical resistance of this element depends on the thermal conductivity of the gas. Twin detectors are ordinarily used, one located ahead of the sample injection chamber and the other immediately beyond the column alternatively, the gas stream can be split. The detectors are incorporated into two arms of a simple bridge circuit (see Figure 31 -9) such that the thennal conductivity of the carrier gas is... 

TABLE 1.16. Electrical Resistivity of Tungsten at Near-Room and at Elevated Temperature [1.35]... 

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