Galinstan is a commercial liquid metal alloy whose composition is taken from a family of eutectic alloys mainly consisting of gallium, indium, and tin. Such eutectic alloys are liquids at room temperature, typically melting at −19 °C (−2 °F). Due to the low toxicity and low reactivity of its component metals, galinstan finds use as a replacement for many applications that previously employed the toxic liquid mercury or the reactive NaK (sodium-potassium alloy). An example of a typical eutectic composition is 68 wt% Ga, 22 wt% In and 10 wt% Sn, though it varies between 62 wt% to 95 wt% Ga, 5 wt% to 22 wt% In, 0 wt% to 16 wt% Sn while keeping eutectic ability. The marketing name is a portmanteau of gallium, indium, and stannum (Latin for "tin"). Galinstan is a registered trademark of the German company Geratherm Medical AG. The exact composition of Galinstan is not publicly known.
- Boiling point: >1300 °C
- Melting point: −19 °C (according to the Geratherm MSDS; see Melting point controversy)
- Vapour pressure: <10−8 Torr (at 500 °C)
- Density: 6.44 g/cm3 (at 20 °C)
- Solubility: Insoluble in water or organic solvents
- Viscosity: 0.0024 Pa·s (at 20 °C)
- Thermal conductivity: 16.5 W·m−1·K−1
- Electrical conductivity: 3.46x106 S/m (at 20 °C)
- Surface tension: s = 0.718 N/m (at 20 °C)
Galinstan tends to wet and adhere to many materials, including glass, which limits its use compared to mercury. Galinstan is commercially used as a mercury replacement in thermometers due to its nontoxic properties, but the inner tube must be coated with gallium oxide to prevent the alloy from wetting the glass surface.
Galinstan may be used as thermal interface for computer hardware cooling solutions, though its cost and aggressivity (it corrodes many other metals such as aluminium by dissolving them) are major obstacles for widespread use. It is also electrically conductive, thus needs to be applied more carefully than insulating regular compounds.
Galinstan has higher reflectivity and lower density than mercury; it is investigated as a replacement for mercury in liquid mirror telescopes for astronomy. It is difficult to use for cooling fission-based nuclear reactors, because indium has a high absorption cross section for thermal neutrons, efficiently absorbing them and inhibiting the fission reaction. Conversely, it is being investigated as a possible coolant for fusion reactors. Unlike other liquid metals used in this application, such as lithium and mercury, the nonreactivity makes Galinstan a safer material to use.
Melting point controversy
The melting point of Galinstan has been a source of much debate. By definition, eutectic alloys exhibit the lowest melting point of any composition of the same components involved. It is also known that many available commercially gallium, indium and tin eutectic alloys are advertised with a melting point of about 11 °C which is higher significantly than –19 °C featured by Galinstan. The official MSDS (Material safety data sheet) mentions only that Galinstan is an "eutectic mixture of the metal components gallium, indium and tin" with no further explanation provided. A US Patent to Geraberger Thermometerwerk GmbH describes various related eutectic alloys and mentions that they may contain additionally up to 2 wt% of antimony (Sb) to increase oxidation resistance and up to 2 wt% of bismuth (Bi) to improve fluidity. The resulting eutectic alloy is said to contain 68 wt% to 69 wt% Ga, 21 wt% to 22 wt% In, and 9.5 wt% to 10.5 wt% Sn, with small addition of Sb and Bi and an impurity level less than 0.001 wt%. The resulting material is noted to have a melting point of –19.5 °C and vaporisation point is above 1800 °C.
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- "Experimental Investigations of Electromagnetic Instabilities of Free Surfaces in a Liquid Metal Drop". International Scientific Colloquium Modelling for Electromagnetic Processing, Hannover. March 24–26, 2003. Retrieved 2009-08-08.
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- US patent 6019509, Geraberger Thermometerwerk GmbH, "Low Melting Gallium, Indium and Tin Eutectic Alloys, and Thermometers Employing Same", issued 2000-02-01
- Scharmann, F.; Cherkashinin, G.; Breternitz, V.; Knedlik, Ch.; Hartung, G.; Weber, Th.; Schaefer, J. A. (2004). "Viscosity effect on GaInSn studied by XPS". Surface and Interface Analysis 36 (8): 981. doi:10.1002/sia.1817.
- Dickey, Michael D.; Chiechi, Ryan C.; Larsen, Ryan J.; Weiss, Emily A.; Weitz, David A.; Whitesides, George M. (2008). "Eutectic Gallium-Indium (EGaIn): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature". Advanced Functional Materials 18 (7): 1097. doi:10.1002/adfm.200701216.