Liquid metal

Liquid metal consists of alloys with very low melting points which form a eutectic that is liquid at room temperature.^[1] The standard metal used to be mercury, but gallium-based alloys, which are lower both in their vapor pressure at room temperature and toxicity, are being used as a replacement in various applications.^[2]

Thermal and electrical conductivity

Alloy systems that are liquid at room temperature have thermal conductivity far superior to ordinary non-metallic liquids,^[3] allowing liquid metal to efficiently transfer energy from the heat source to the liquid. They also have a higher electrical conductivity that allows the liquid to be pumped by more efficient, electromagnetic pumps.^[4] This results in the use of these materials for specific heat conducting and/or dissipation applications.

Another advantage of liquid alloy systems are their inherent high densities.

Wetting to metallic and non-metallic surfaces

Once oxides have been removed from the substrate surface, most liquid metals will wet to most metallic surfaces. Specifically though, room-temperature liquid metal can be very reactive with certain metals. Liquid metal can dissolve most metals; however, at moderate temperatures, only some are slightly soluble, such as sodium, potassium, gold, magnesium, lead, nickel and interestingly mercury.^[5] Gallium is corrosive to all metals except tungsten and tantalum, which have a high resistance to corrosion, more so than niobium, titanium and molybdenum.^[6]

Similar to indium, gallium and gallium-containing alloys have the ability to wet to many non-metallic surfaces such as glass and quartz. Gently rubbing the alloy into the surface may help induce wetting. However, this observation of "wetting by rubbing into glass surface" has created a widely spread misconception that the gallium-based liquid metals wet glass surfaces, as if the liquid breaks free of the oxide skin and wets the surface. The reality is the opposite: the oxide makes the liquid wet the glass. In more details: as the liquid is rubbed into and spread onto the glass surface, the liquid oxidizes and coats the glass with a thin layer of oxide (solid) residues, on which the liquid metal wets. In other words, what is seen is a gallium-based liquid metal wetting its solid oxide, not glass. Apparently, the above misconception was caused by the super-fast oxidation of the liquid gallium in even a trace amount of oxygen, i.e., nobody observed the true behavior of a liquid gallium on glass, until research at the UCLA debunked the above myth by testing Galinstan, a gallium-based alloy that is liquid at room temperature, in an oxygen-free environment.^[7] Note: These alloys form a thin dull looking oxide skin that is easily dispersed with mild agitation. The oxide-free surfaces are bright and lustrous.

Applications

Typical uses of liquid metals include thermostats, switches, barometers, heat transfer systems, and thermal cooling and heating designs.^[8] Uniquely, they can be used to conduct heat and/or electricity between non-metallic and metallic surfaces.

Storage/shelf life

Liquid metal is usually packed in polyethylene bottles. Unopened bottles generally have a one-year shelf life. It is recommended that, as the liquid metal is removed from the bottle, the volume be replaced with dry argon gas. This will minimize the possibility of oxidation at the surface of the alloy. If the liquid metal has been stored below its melting point and has solidified, it should be re-melted and thoroughly shaken or mixed before use. Care should be taken in reheating the liquid metal in the original packaging provided. Temperatures should not exceed 65 °C (149 °F).

General handling guidelines

• Liquid metal may be frozen before shipping and shipped in a solid state, to avoid "sloshing" and uncontrolled movement.
• Liquid metal should be shipped in accordance with the applicable international regulations and reported as a corrosive liquid. Liquid metals are prohibited on most airlines. Due to their corrosive nature, they should not be put in contact with most other metals.
• Liquid metal may be stored at room temperatures, in a cool, dry area away from incompatible materials, including hydrogen peroxide, hydrochloric acid, and halogenated chemicals.
• Before use, liquid metal should be allowed to reach room temperature and liquefy. Shake or mix before use.
• Allow up to four hours for solidified liquid metal to reach room temperature. Remove from storage one day before use.
• Rapid warming of liquid metal on top of an oven or by any other method is not recommended, but a temperature-controlled water bath may be used. Gallium-containing alloys are very corrosive when hot; their temperature should not exceed 65 °C (149 °F).
• Gallium-contained alloys have a specific shelf life and should be managed as a first-in, first-out (FIFO) product. Packaging should be labeled with date and time of opening.
• Gallium may be absorbed through the skin. Rubber or vinyl gloves should be worn at all times when handling gallium-containing alloys.
• It is not recommended to repackage gallium-containing alloys from their original packaging.
• As the liquid metal is removed from the packaging, it is recommended that the volume be replaced with dry argon gas to minimize the possibility of oxidation on the surface of the alloy.

In popular culture

A fictional "mimetic polyalloy" is a form of liquid metal consisting entirely of microscopic nanobots, which are used for production of a Series 1000 Terminator in Terminator 2: Judgment Day.

See also

• Electromagnetic pump
• NaK, Quicksilver - other metals which are liquid at room temperature

References

1. Indalloy Alloys Liquid at Room Temperature
2. Thermal Interface Materials
3. Kunquan, Ma; Jing, Liu (October 2007). Liquid metal cooling in thermal management of computer chips. Frontiers of Energy and Power Engineering in China (Review Article) 1 (Higher Education Press, co-published with Springer-Verlag GmbH). pp. 384–402. doi:10.1007/s11708-007-0057-3. ISSN 1673-7504.
4. Miner, A.; Ghoshal, U. (2004-07-19). "Cooling of high-power-density microdevices using liquid metal coolants". Applied Physics Letters 85 (3): 506–508. doi:10.1063/1.1772862. ISSN 0003-6951.
5. Wade, K.; Banister, A. J. (1975). The Chemistry of ALUMINUM, GALLIUM, INDIUM, and THALLIUM, Pergamon Texts in Inorganic Chemistry, ASIN B0007AXLOA 12. External link in |title= (help)
6. Lyon, Richard N., ed. (1952). Liquid Metals Handbook (2 ed.). Washington, D.C.
7. Liu, T.; S., Prosenjit; Kim, C.-J. (April 2012). Characterization of Nontoxic Liquid-Metal Alloy Galinstan for Applications in Microdevices. Journal of Microelectromechanical Systems (Journal Article) 21 (IEEE). pp. 443–450. doi:10.1109/JMEMS.2011.2174421.
8. Liquid Metal Thermal Interface Materials