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, and has a density of 866 kg/m3 at and 855 kg/m3 at, 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 if high purity and low levels of oxidation are required. NaK has a very high surface tension, which makes large amounts of it pull into a bun-like shape. Its specific heat capacity is, which is roughly one quarter of that for water, but heat transfer is higher over a temperature gradient due to higher thermal conductivity.
Chemical properties
When stored in air, it forms a yellow potassium superoxide coating and may ignite. This superoxide reacts explosively with water and 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 RidgeY-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.
Further alloys with low melting points are Cs77K23 at −37,5 °C, Cs95Na5 at −30 °C and Na8Rb92 mit −5 °C. The alloy consisting of 40.8 % cesium, 11.8 % sodium and 47.4 % potassium has a melting point of −79.4 °C.
Usage
Coolant
NaK has been used as the coolant in experimental fast neutronnuclear reactors. Unlike commercial plants, these are frequently shut down and defuelled. Use of lead or pure sodium, the other materials used in practical reactors, would require continual heating to maintain the coolant as a liquid. Use of NaK overcomes this. The Dounreay Fast Reactor is an example. The SovietRORSAT radar satellites were powered by a BES-5 reactor, which was cooled with NaK. In addition to the wide liquid temperature range, NaK has a very low vapor pressure, which is important in the vacuum of space. An unintended consequence of the usage as a coolant on orbiting satellites has been the creation of additional space debris. NaK coolant has leaked from a number of satellites, including Kosmos 1818 and Kosmos 1867. The coolant self-forms into frozen droplets of solid sodium-potassium of up to several centimeters in size. These solid objects are a source of space debris. The Danamics LMX Superleggera CPU cooler uses NaK to transport heat from the CPU to its cooling fins.
Desiccant
In contact with water, hydrogen is created. Hence, sodium-potassium alloys are used as desiccants in drying solvents prior to distillation.
Hydraulic fluid
NaK-77, a eutectic alloy of sodium-potassium, can be used as a hydraulic fluid in high-temperature and high-radiation environments, for temperature ranges of. Its bulk modulus at is 310,000 psi, higher than of a hydraulic oil at room temperature. Its lubricity is poor, so positive-displacement pumps are unsuitable and centrifugal pumps have to be used. Addition of caesium shifts the useful temperature range to. NaK-77 alloy was tested in hydraulic and fluidic systems for the Supersonic Low Altitude Missile.
Chemical methods
NaK can be used as catalyst in some reactions, such as the production of ibuprofen.
Synthesis and production
Industrially, NaK is produced in a reactive distillation. In this continuous process, a distillation column is fed with potassium chloride and sodium. In the reaction zone, potassium chloride reacts with sodium to form sodium chloride and potassium. The lighter-boiling potassium is enriched in an upper fractionating zone and drawn at the column head while molten sodium chloride is withdrawn from the bottom.
