Resistivity, electrical pure metals

White metal with brdhant metaUic luster face-centered cubic crystals density 10.43 g/cm at 20°C, and 9.18 g/cm at 1,100°C melts at 961.8°C vaporizes at 2,162°C vapor pressure 5 torr at 1,500° C pure metal has the highest electrical and thermal conductive of aU metals, electrical resistivity of pure metal at 25°C 1.617x10 ohm-cm elastic modulus 71GPa (10.3x10 psi) Poisson s ratio 0.39 (hard drawn), 0.37 (annealed) viscosity of hquid silver 3.97 centipoise at 1,043°C thermal neutron absorption cross section 63 1 barns insoluble in water inert to most acids attacked by dilute HNO3 and concentrated H2SO4 soluble in fused caustic soda or caustic potash in the presence of air. 

The resistance per unit length of wires of various metals is tabulated here. Values were calculated from resistivity values in the tables Electrical Resistivity of Pure Metals and Electrical Resistivity of Selected Alloys , which appear in Section 12. In practice, resistance may vary because of differing heat treatments and metal composition. The values in the table refer to 20°C, but values at other temperatures may be calculated from the following resistivity data ... 

The electrical resistivity of pure metals usually falls by orders of magnitude on their being cooled from 300 K to 1K. That of alloys, on the other hand, tends to be relatively temperature independent. In both cases, there is a possible exception. At a range of critical temperatures below about... 

J Bass, KH Fisher. Electrical resistivity of pure metals and dilute alloys. In Landolt-Bomstein New Series. Vol 15. Berlin Springer-Verlag, 1982. 

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

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. 

Resistance thermometers are made of a pure metal, such as platinum, nickel, or copper. The electrical resistance of such a material is almost linearly dependent on temperature. Resistance thermometers are stable, having a small drift. A widely used and the best-known resistance probe is the IW-100 probe, which is platinum, having a resistance of 100 ohms at the temperature of 0 °C. Other resistance values for PT probes are available. The resistance versus temperature values as well as tolerances for platinum probes are standardized. The shape and size of a resistance probe can vary considerably, resulting in changes in probe dynamics. 

The electrical resistance of most conductors, metals in particular, decreases as the temperature of the conductor decreases. For some pure metals and compounds of the metals, the resistance decreases with temperature as usual, but at some critical temperature the resistance drops identically to zero. The resistance remains zero as long as the material is maintained at a temperature below the critical temperature. Such a material is termed a supercon-... 

Vacuum-deposited and electroplated coatings are pure metal with no chemical bond to the underlying surface. The properties will be those of pure aluminium. The presence of lacquer, in the case of vacuum-deposited coatings will, however, afford resistance to the passage of electricity and limit the maximum temperature of use. 

Nickel was first isolated in 1751, and a relatively pure metal was prepared in 1804. In nature, nickel is found primarily as oxide and sulfide ores (USPHS 1977). It has high electrical and thermal conductivities and is resistant to corrosion at environmental temperatures between -20°C and +30°C (Chau and Kulikovsky-Cordeiro 1995). Nickel, also known as carbonyl nickel powder or C.I. No. 77775, has a CAS number of 7440-02-0. Metallic nickel is a hard, lustrous, silvery white metal with a specific gravity of 8.9, a melting point of about 1455°C, and a boiling point at about 2732°C. It is insoluble in water and ammonium hydroxide, soluble in dilute nitric acid or aqua regia, and slightly soluble in hydrochloric and sulfuric acid. Nickel has an atomic weight of 58.71. Nickel is... 

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