Alloys electrical resistivity values

Electrical—Thermal Conductivities. Electrical conductivities of alloys (Table 5) are often expressed as a percentage relative to an International Annealed Copper Standard (lACS), ie, units of % lACS, where the value of 100 % lACS is assigned to pure copper having a measured resistivity value of 0.017241 Q mm /m. The measurement of resistivity and its conversion to % lACS is covered under ASTM B193 (8). 

Electrical Properties at Low Temperatures The eleciiical resistivity of most pure metalhc elements at ambient and moderately low temperatures is approximately proportional to the absolute temperature. At very low temperatures, however, the resistivity (with the exception of superconductors) approaches a residual value almost independent of temperature. Alloys, on the other hand, have resistivities much higher than those of their constituent elements and resistance-temperature coefficients that are quite low. The electrical resistivity of alloys as a consequence is largely independent of temperature and may often be of the same magnitude as the room temperature value. 

More importantly, such alloys also possess a very low temperature coefficient of electrical resistance (of the order of 220 idQ.IQ.rC, typical), which causes only a marginal change in its resistance value with variation in temperature. They can therefore ensure a near-consistent predefined performance of the motor for which the resistance grid is designed, even after frequent starts and stops. They are also capable of absorbing shocks and vibrations during stringent service conditions and are therefore suitable for heavy-duty drives, such as steel mill applications. 

Rhodium melts at 1907° C.4 and boils at about 2500° C. It is less volatile than platinum,5 and when alloyed with that metal not only stiffens it, but, unlike iridium, reduces its volatility at all temperatures above 900° C. It has been suggested,6 therefore, that a useful alloy for best quality crucibles would consist of platinum 95 to 97 per cent., and rhodium 3 to 5 per cent., and containing no other detectable impurities. Below 900° C. the presence of rhodium appears to exert a negligible effect. When cooled to — 80° C. rhodium appears to undergo a molecular transformation of some kind, analogous to that evidenced by copper. At this temperature the electrical resistance is considerably below the calculated value.7 The most intense lines in the spectrum of rhodium are as follow 8 ... 

Since the fact is well established that the formation of solid solutions between two metals brings about a great increase in the electrical resistance beyond the value which would result from simple mixture, it is natural to attribute the permanent increase of resistance in hydrogen alloys such as palladium-hydrogen to this combination and if the supplementary conduction is attributed to the atomic hydrogen, it is evident that, upon the conception just outlined, this conduction must vary with the cathodic current density during electrolysis, as has been found to be the case, and must persist after the interruption of electrolysis until the equilibria of equation (1) have become established. ... 

In general, metals and alloys become better conductors when the temperature is lowered. With several of the softer metals the property of superconduction appears suddenly at very low temperatures (around — 268° C. or 5° absolute).8 The electrical resistance, which is already very small at these low temperatures, practically disappears when the temperature is lowered below a critical value. Most electrolytic conductors, on the other hand, increase in conductivity as tire temperature is raised, that is to say, dectrolytic conductors have positive temperature coeffidents of conductivity. This difference of temperature coeffident is occasionally of use in dedding whether a conductor is metallic or electrolytic in nature. Metallic conductors are, with few exceptions, far better conductors of electridty than are electrolytic conductors. 

Several workers have attempted to rationalise the chemical isomer shifts observed at Au impurity nuclei in various metals with limited success [102, 103]. The most comprehensive set of data comes from Barrett et al. [104], who doped Pt into 20 metals. The shift correlates very approximately with the electronegativity of the host, and a crude interpretation is that electrons are transferred in v ing degrees to the 6j-shell of the gold. An estimate of bR/R — 1-9(6) X 10 was obtained. Additional evidence in favour of an increased 6. -population has come from a comparison of the shift and residual electrical resistivity of Au alloys with Cu, Ag, Pd, and Pt [105]. The pressure dependence up to 70-6 kbar of the chemical isomer shift in a gold foil at 4-2 K has been obtained and with detailed analysis leads to a value for bR/R of +l-5 X 10- [106]. 

Metallic materials for be used as interconnects in SOFCs should fulfil a number of specific requirements [1, 2], Crucial properties of the materials are high oxidation resistance in both air and anode environment, low electrical resistance of the oxide scales formed on the alloy surface as well as good compatibility with the contact materials. Additionally, the value of the coefficient of thermal expansion (CTE) should match with those of the other cell components [3], These requirements can potentially be achieved with high chromium ferritic steels [4], however, previous studies [5] have shown that none of the commercially available ferritic steels seems to possess the suitable combination of properties required for long term reliable cell performance. 

The highest values of approximately 20 Q cm were obtained for the commercial alloy 1.4742 which has in some cases been considered as a potential candidate to be used as interconnect in intermediate temperature SOFCs. These high values can be explained by the fact that this alloy, depending on the exact alloy composition and surface treatment, in some cases tends to form a veiy protective alumina scale [5], which, however, possesses a very poor electrical conductivity. In contrast, the new JS-3 alloys (batches JDA, JEW, and JEX) show very low contact resistance values of approximately 10 mf2cm, i.e., values which are two to three orders of magnitude smaller than those of most commercial alloys. 

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 ... 

Fig. 11.2. Cerium solute contribution to the electrical resistivity of YCe alloys, normalized to its value at 0 K, vs temperature. The Ce contribution at 0 K is equal to 12.0 1.5 ftH cm/at.% Ce [after Sugawara and Yoshida (1968)].

---