NaK
NaK, or sodium-potassium alloy (commonly pronounced /næk/), is an alloy of potassium (K) and sodium (Na) which is usually liquid at room temperature. Various commercial grades are available. NaK is highly reactive with water and may catch fire when exposed to air, so must be handled with special precautions.
NaK containing 40% to 90% potassium by weight is liquid at room temperature. The eutectic mixture consists of 77% potassium and 23% sodium, is liquid from −12.6 to 785 °C (9.3 to 1,445.0 °F), and has a density of 866 kg/m3 at 21 °C (70 °F) and 855 kg/m3 at 100 °C (212 °F), making it less dense than water. It is highly reactive with water and is stored usually under hexane or other hydrocarbons, or under an inert gas (usually dry nitrogen or argon) if high purity and low levels of oxidation are required.
When stored in air, it forms a yellow potassium superoxide coating and may ignite. This superoxide reacts explosively with organics. NaK is not dense enough to sink in most hydrocarbons, but will sink in lighter mineral oil. It is unsafe to store in this manner if the superoxide has formed. A large explosion took place at the Oak Ridge Y-12 facility on December 8, 1999, when NaK cleaned up after an accidental spill and inappropriately treated with mineral oil was scratched with a metal tool. The liquid alloy also attacks PTFE ("Teflon").
NaK has a very high surface tension, which makes large amounts of it pull into a bun-like shape. Its specific heat capacity is 982 J/kg⋅K, which is roughly one quarter of that for water, but heat transfer is higher over a temperature gradient due to higher thermal conductivity.
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