NaK

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For other meanings, see Nak (disambiguation).

NaK, or sodium-potassium alloy, an alloy of potassium (K) and sodium (Na), is usually liquid at room temperature.[1] 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.

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, and has a density of 866 kg/m3 at 21 °C and 855 kg/m3 at 100 °C, making it less dense than water.[1] It is highly reactive with water and is stored under dry nitrogen (or, even better, dry argon) for safety reasons.[2]

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.[3] The liquid alloy also attacks Teflon.[citation needed]

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-fourth of that for water but heat transfer is higher over a temperature gradient due to higher thermal conductivity.[4]

  • 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.[6][7] As well as the wide liquid temperature range, NaK has a very low vapor pressure, which is important in the vacuum of space.

Desiccant

Both sodium and potassium are used as desiccants in drying solvents prior to distillation.

Synthesis and production

Industrially, NaK is produced in a reactive distillation.[8] 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.

References

  1. ↑ 1.0 1.1 "Sodium-Potassium Alloy (NaK)". BASF. Retrieved 2009-03-05. 
  2. ↑ Strem Chemical. "MSDS". Retrieved 4 April 2012. 
  3. ↑ Oak Ridge Explosion U.S. Department of Energy
  4. ↑ "Danamics LM10 - Liquid metal put to the test". NordicHardware.  2010-01-10 nordichardware.com
  5. ↑ G.L.C.M. van Rossen, H. van Bleiswijk: Über das Zustandsdiagramm der Kalium-Natriumlegierungen, in: Z. Anorg. Chem., 1912, 74, S. 152–156.
  6. ↑ Space.com article about RORSATs leaking NaK after jettisoning their reactor core. Retrieved 03/10/2009.
  7. ↑ Klinkrad, Heiner (2006-02-23). Space debris: models and risk analysis. p. 83. ISBN 978-3-540-25448-5. 
  8. ↑ Jackson, C. B.; Werner, R. C. (1957-01-01). 18. "The Manufacture of Potassium and NaK". Advances in Chemistry 19: 169–173. doi:10.1021/ba-1957-0019.ch018. ISBN 9780841221666. 
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