Sodium–potassium alloy, colloquially called NaK (commonly pronounced ), is an

alloy An alloy is a mixture of chemical elements of which in most cases at least one is a metal, metallic element, although it is also sometimes used for mixtures of elements; herein only metallic alloys are described. Metallic alloys often have prop ...

of the

alkali metals The alkali metals consist of the chemical elements lithium (Li), sodium (Na), potassium (K),The symbols Na and K for sodium and potassium are derived from their Latin names, ''natrium'' and ''kalium''; these are still the origins of the names ...

sodium Sodium is a chemical element; it has Symbol (chemistry), symbol Na (from Neo-Latin ) and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 element, group 1 of the peri ...

(Na,

atomic number The atomic number or nuclear charge number (symbol ''Z'') of a chemical element is the charge number of its atomic nucleus. For ordinary nuclei composed of protons and neutrons, this is equal to the proton number (''n''p) or the number of pro ...

11) and

potassium Potassium is a chemical element; it has Symbol (chemistry), symbol K (from Neo-Latin ) and atomic number19. It is a silvery white metal that is soft enough to easily cut with a knife. Potassium metal reacts rapidly with atmospheric oxygen to ...

(K, atomic number 19) that is normally liquid at room temperature. 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.

Properties

Physical properties

NaK containing 40% to 90% potassium by mass is liquid at

room temperature Room temperature, colloquially, denotes the range of air temperatures most people find comfortable indoors while dressed in typical clothing. Comfortable temperatures can be extended beyond this range depending on humidity, air circulation, and ...

. The eutectic mixture consists of 77% potassium and 23% sodium by mass (NaK-77), and it is a liquid from , and has a density of 0.866 g/cm³ at and 0.855 g/cm³ at , making it less dense than water. It is highly reactive with water and is stored usually under

hexane Hexane () or ''n''-hexane is an organic compound, a straight-chain alkane with six carbon atoms and the molecular formula C6H14. Hexane is a colorless liquid, odorless when pure, and with a boiling point of approximately . It is widely used as ...

or other hydrocarbons, or under an inert gas (usually dry

nitrogen Nitrogen is a chemical element; it has Symbol (chemistry), symbol N and atomic number 7. Nitrogen is a Nonmetal (chemistry), nonmetal and the lightest member of pnictogen, group 15 of the periodic table, often called the Pnictogen, pnictogens. ...

argon Argon is a chemical element; it has symbol Ar and atomic number 18. It is in group 18 of the periodic table and is a noble gas. Argon is the third most abundant gas in Earth's atmosphere, at 0.934% (9340 ppmv). It is more than twice as abu ...

) if high purity and low levels of oxidation are required. A solid compound, , exists at low temperatures, containing 46 percent potassium by mass. NaK has a very high

surface tension Surface tension is the tendency of liquid surfaces at rest to shrink into the minimum surface area possible. Surface tension (physics), tension is what allows objects with a higher density than water such as razor blades and insects (e.g. Ge ...

, which makes large amounts of it pull into a

bun A bun is a type of bread that is round and small enough that it can generally be eaten hand-held. Whether a bun is considered sweetened or unsweetened differs between countries: it is considered sweetened in the United Kingdom, a savory bread in ...

-like shape. Its

specific heat capacity In thermodynamics, the specific heat capacity (symbol ) of a substance is the amount of heat that must be added to one unit of mass of the substance in order to cause an increase of one unit in temperature. It is also referred to as massic heat ...

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.

Chemical properties

When stored in

air An atmosphere () is a layer of gases that envelop an astronomical object, held in place by the gravity of the object. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. A stellar atmosph ...

, it forms a yellow

potassium superoxide Potassium superoxide is an inorganic compound with the formula . It is a yellow paramagnetic solid that decomposes in moist air. It is a rare example of a stable salt of the superoxide anion. It is used as a scrubber, dehumidifier, and gene ...

coating and may ignite. This superoxide reacts explosively with water and organics. NaK is not dense enough to sink in most

hydrocarbon In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons are generally colourless and Hydrophobe, hydrophobic; their odor is usually fain ...

s, but will sink in lighter

mineral oil Mineral oil is any of various colorless, odorless, light mixtures of higher alkanes from a mineral source, particularly a distillate of petroleum, as distinct from usually edible vegetable oils. The name 'mineral oil' by itself is imprecise, ...

. 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 Polytetrafluoroethylene (PTFE) is a synthetic fluoropolymer of tetrafluoroethylene, and has numerous applications because it is chemically inert. The commonly known brand name of PTFE-based composition is Teflon by Chemours, a spin-off fro ...

("Teflon"). Sodium–potassium alloy polymerizes

dimethyldichlorosilane Dimethyldichlorosilane is a tetrahedral organosilicon compound with the formula . At room temperature it is a colorless liquid that readily reacts with water to form both linear and cyclic Si-O chains. Dimethyldichlorosilane is made on an indust ...

into

polysilane Polysilanes are organosilicon compounds with the formula (R2Si)n. They are relatives of traditional organic polymers but their backbones are composed of silicon atoms. They exhibit distinctive optical and electrical properties. They are mainly use ...

s with a Si-Si backbone and methyl radicals, primarily

dodecamethylcyclohexasilane Dodecamethylcyclohexasilane is the organosilicon compound with the formula . It is one of the more readily prepared and easily handled polysilanes. Dodecamethylcyclohexasilane is produced by reduction of dimethyldichlorosilane with sodium-potass ...

\ce \ \ce

Further alloys with low melting points

Further alloys with low melting points are at , at and at . The alloy consisting of 40.8 % caesium, 11.8 % sodium and 47.4 % potassium has a melting point of .

Usage

Coolant

NaK has been used as the

coolant A coolant is a substance, typically liquid, that is used to reduce or regulate the temperature of a system. An ideal coolant has high thermal capacity, low viscosity, is low-cost, non-toxic, chemically inert and neither causes nor promotes corr ...

in experimental fast neutron nuclear reactors. Unlike commercial plants, these are frequently shut down and defuelled. Use of

lead Lead () is a chemical element; it has Chemical symbol, symbol Pb (from Latin ) and atomic number 82. It is a Heavy metal (elements), heavy metal that is density, denser than most common materials. Lead is Mohs scale, soft and Ductility, malleabl ...

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 Dounreay (; ) is a small settlement and the site of two large nuclear establishments on the north coast of Caithness in the Highland area of Scotland. It is on the A836 road west of Thurso. The nuclear establishments were created in the 1950s ...

is an example. The first nuclear reactor in space,History of US Astronuclear Reactors part 1: SNAP-2 and 10A
, ''Beyond NERVA'', April 3, 2019. Retrieved 25 April 2024. the United States' experimental

SNAP-10A SNAP-10A (Systems for Nuclear Auxiliary Power, aka Snapshot for Space Nuclear Auxiliary Power Shot, also known as OPS 4682) was a US experimental nuclear powered satellite launched into space in 1965 as part of the SNAPSHOT program.liquid metal direct current conduction-type pump. The satellite was launched in 1965, and as of 2022 is the only

fission reactor A nuclear reactor is a device used to initiate and control a fission nuclear chain reaction. They are used for commercial electricity, marine propulsion, weapons production and research. Fissile nuclei (primarily uranium-235 or plutonium-23 ...

power system launched into space by the United States. The Soviet

RORSAT Upravlyaemy Sputnik Aktivnyy ( for Controlled Active Satellite), or US-A, also known in the Western world as Radar Ocean Reconnaissance Satellite or RORSAT (GRAU index 17F16K), was a series of 33 Soviet reconnaissance satellites. Launched between ...

radar satellites were powered by a

BES-5 BES-5, also known as Bouk or Buk (), was a Soviet thermoelectric generator that was used to power 31 satellites in the US-A (RORSAT) project. The heat source was a uranium 235 fast fission nuclear reactor (FNR). Background Spacecraft nuclea ...

reactor, which was cooled with NaK. In addition to the wide liquid temperature range, NaK has a very low

vapor pressure Vapor pressure or equilibrium vapor pressure is the pressure exerted by a vapor in thermodynamic equilibrium with its condensed phases (solid or liquid) at a given temperature in a closed system. The equilibrium vapor pressure is an indicat ...

, which is important in the vacuum of

space Space is a three-dimensional continuum containing positions and directions. In classical physics, physical space is often conceived in three linear dimensions. Modern physicists usually consider it, with time, to be part of a boundless ...

. An unintended consequence of the usage as a coolant on orbiting satellites has been the creation of additional

space debris Space debris (also known as space junk, space pollution, space waste, space trash, space garbage, or cosmic debris) are defunct human-made objects in spaceprincipally in Earth orbitwhich no longer serve a useful function. These include dere ...

. NaK coolant has leaked from a number of satellites, including

Kosmos 1818 Kosmos 1818 was a nuclear powered Soviet surveillance satellite in the RORSAT program, which monitored NATO vessels using radar. Kosmos 1818 was the first satellite to use the TOPAZ-1 fission reactor. In July 2008, the satellite was damaged, and ...

and

Kosmos 1867 Kosmos 1867 () was a nuclear powered radar ocean reconnaissance satellite (RORSAT) that was launched by the Soviet Union on July 10, 1987. It was put into an orbit of about . Its mission was to monitor the oceans for naval and merchant vessels, a ...

. The coolant self-forms into droplets of sodium–potassium of up to several centimeters in size. These objects are 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 Hydrogen is a chemical element; it has chemical symbol, symbol H and atomic number 1. It is the lightest and abundance of the chemical elements, most abundant chemical element in the universe, constituting about 75% of all baryon, normal matter ...

is created. Hence, sodium–potassium alloys are used as

desiccant A desiccant is a hygroscopic substance that is used to induce or sustain a state of dryness (desiccation) in its vicinity; it is the opposite of a humectant. Commonly encountered pre-packaged desiccants are solids that absorb water. Desiccant ...

s in drying

solvent A solvent (from the Latin language, Latin ''wikt:solvo#Latin, solvō'', "loosen, untie, solve") is a substance that dissolves a solute, resulting in a Solution (chemistry), solution. A solvent is usually a liquid but can also be a solid, a gas ...

s prior to

distillation Distillation, also classical distillation, is the process of separating the component substances of a liquid mixture of two or more chemically discrete substances; the separation process is realized by way of the selective boiling of the mixt ...

Hydraulic fluid

Eutectic NaK (NaK-77, an alloy of 77% potassium and 23% sodium by mass) can be used as a

hydraulic fluid A hydraulic fluid or hydraulic liquid is the medium by which power is transferred in hydraulic machinery. Common hydraulic fluids are based on mineral oil or water. Examples of equipment that might use hydraulic fluids are excavators and backho ...

in high-temperature and high-radiation environments, for temperature ranges of . Its

bulk modulus The bulk modulus (K or B or k) of a substance is a measure of the resistance of a substance to bulk compression. It is defined as the ratio of the infinitesimal pressure increase to the resulting ''relative'' decrease of the volume. Other mo ...

at is 2.14 GPa, higher than of a hydraulic oil at room temperature. Its

lubricity Lubricity is the measure of the reduction in friction and/or wear by a lubricant. The study of lubrication and wear mechanisms is called tribology. Measurement of lubricity The lubricity of a substance is not a material property, and cannot be m ...

is poor, so positive-displacement

pump A pump is a device that moves fluids (liquids or gases), or sometimes Slurry, slurries, by mechanical action, typically converted from electrical energy into hydraulic or pneumatic energy. Mechanical pumps serve in a wide range of application ...

s are unsuitable and centrifugal pumps have to be used. Addition of

caesium Caesium (IUPAC spelling; also spelled cesium in American English) is a chemical element; it has Symbol (chemistry), symbol Cs and atomic number 55. It is a soft, silvery-golden alkali metal with a melting point of , which makes it one of only f ...

shifts the useful temperature range to . NaK-77 was tested in hydraulic and fluidic systems for the

Supersonic Low Altitude Missile The Supersonic Low Altitude Missile or SLAM was a U.S. Air Force nuclear weapons project conceived around 1955, and cancelled in 1964. SLAMs were conceived of as unmanned nuclear-powered ramjets capable of delivering thermonuclear warheads deep ...

. NaK may also be used to transmit forces inside high temperature pressure transducers as an alternative to mercury.

Chemical methods

NaK can be used as catalyst in some reactions, such as isobutylbenzene, a precursor to

ibuprofen Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) that is used to relieve pain, fever, and inflammation. This includes dysmenorrhea, painful menstrual periods, migraines, and rheumatoid arthritis. It can be taken oral administration, ...

Synthesis and production

Industrially, NaK is produced in a

reactive distillation Reactive distillation is a process where the chemical reactor is also the still. Separation of the product from the reaction mixture does not need a separate distillation step which saves energy (for heating) and materials. This technique can be us ...

References

{{Reflist Fusible alloys Potassium alloys Sodium alloys Desiccants Coolants Nuclear reactor coolants Reducing agents Pyrophoric materials