Sodium-potassium alloy, colloquially called NaK (commonly pronounced /næk/),^[1] is an alloy of two alkali metals sodium (Na, atomic number 11) and potassium (K, atomic number 19) and which is usually liquid at room temperature.^[2] Various commercial grades are available. NaK is highly reactive with water (like its constituent elements) and may catch fire when exposed to air, so must be handled with special precautions.

Physical properties

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.^[2] 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^[3]) 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.^[4] The liquid alloy also attacks PTFE ("Teflon").^[5]

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.^[6]

• Solid-liquid phase diagram of sodium and potassium^[7]

• NaK alloy is liquid at room temperature.

Usage

Coolant

NaK has been used as the coolant in experimental fast neutron nuclear 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 Soviet RORSAT radar satellites were powered by a NaK-cooled reactor.^[8][9] As well as the wide liquid temperature range, NaK has a very low vapor pressure, which is important in the vacuum of space.

An apparently unintended consequence of this usage as a coolant on orbiting satellites has been the creation of additional space debris. A number of these old satellites are punctured by orbiting space debris—calculated to be 8 percent over any 50-year period—and release their NaK coolant into space. The coolant self-forms into frozen droplets of solid sodium-potassium of up to around several centimeters in size^[10] and these solid objects then become a significant source of space debris themselves.^[11]

The Danamics LMX Superleggera CPU cooler uses NaK to transport heat from the CPU to its cooling fins.^[12]

Desiccant

Sodium-potassium alloys are used as desiccants in drying solvents prior to distillation, although they are inferior to molecular sieves.^[13]

Hydraulic fluid

NaK-77, an eutectic alloy of sodium-potassium, can be used as a hydraulic fluid in high-temperature and high-radiation environments, for temperature ranges of 10 to 1,400 °F (−12 to 760 °C). Its bulk modulus at 1,000 °F (538 °C) is 310,000 psi (2.14 GPa), 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 −95 to 1,300 °F (−71 to 704 °C). The NaK-77 alloy was tested in hydraulic and fluidic systems for the Supersonic Low Altitude Missile.^[14]

Synthesis and production

Industrially, NaK is produced in a reactive distillation.^[15] 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.

See also

• Liquid metal