Thermal Conductivity of Some Alloys at High Temperature

TABLE 2-329 Thermal Conductivity of Some Alloys at High Temperature  

---

Various high conductivity materials have been alloyed with metal hydrides to form enhanced heat transport composite materials. Eaton et al. [31] experimented with various alloyed metal additives including copper, aluminum, lead, and lead-tin. The samples were alloyed at elevated temperature (200-600 °C) and cycled. In many samples, cycling resulted in the separation and fracture of the alloy and thus a reduction in composite thermal conductivity. Sintered aluminum structures of 20% solid fraction have been integrated with LaNis hydride materials with success, resulting in effective thermal conductivities of 10-33 W/mK [32-34]. Temperatures required for this process and added mass and volume may exclude application to some complex hydrides. 

Adequate prediction of the thermal conductivity for pure metals can be made by means of the Wiedeman-Franz law which states that the ratio of the thermal conductivity to the product of the electrical conductivity and the absolute temperature is a constant. This ratio for high-conductivity metals extrapolates essentially to the Sommerfeld value of 2.449 x 10" W f2/K at 0 K, but falls considerably below it at higher temperatures. High-purity aluminum and copper exhibit peaks in thermal conductivity between 20 to 50 K, but these peaks are rapidly suppressed with increased impurity levels and cold work of the metal. Some metals including Monel, Inconel, stainless steel, and structural and aluminum alloys show a steady decrease in thermal conductivity with a decrease in temperature. 

Nickel is silver-white, with high electrical and thermal conductivities (both 15% of those of silver) and m.p. 1452°, and it can be drawn, rolled, forged and polished. It is quite resistant to attack by air or water at ordinary temperatures when compact and is therefore often electroplated as a protective coating. Because nickel reacts but slowly with fluorine, the metal and certain alloys (Monel) are used to handle F2 and other corrosive fluorides. It is also ferromagnetic, but not so much as iron. The finely divided metal is reactive to air, and it may be pyrophoric under some conditions. 

Some of the most important properties of sodium and lithium for high-temperature nuclear-reactor applications are listed in Table I. Several other popular and potential heat-transfer fluids are shown for comparison purposes. The advantages and disadvantages of various coolants are considered in relation to their application at temperatures in excess of 1200 °F. The undesirable properties of a particular coolant are underlined. Water is not particularly suitable because of its very low boiling point and its poor thermal conductivity. Sodium and the sodium-potassium alloy have properties to which there are no major objections. (Any statement made in this paper concerning the corrosiveness of sodium may be considered as applicable to the sodium-potassium alloys, as differences found... 

---