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Galinstan is a brand-name and a common name for a liquid metal alloy whose composition is part of a family of eutectic alloys mainly consisting of gallium, indium, and tin. Such eutectic alloys are liquids at room temperature, typically melting at +11 °C (52 °F), while commercial Galinstan melts at −19 °C (−2 °F).
An example of a typical eutectic mixture is 68% Ga, 22% In, and 10% Sn (by weight) though proportions vary between 62–95% Ga, 5–22% In, 0–16% Sn (by weight) while remaining eutectic; the exact composition of the commercial product “Galinstan” is not publicly known.
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).
- 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: 3.46×106 S/m (at 20°C)
- Surface tension: s = 0.535 - 0.718 N/m (at 20°C, dependent on producer)
- Specific heat capacity: 296 J·kg−1·K−1
Galinstan tends to be “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 surface must be coated with gallium oxide to prevent the alloy from wetting the glass surface.
Galinstan may be used as a thermal interface for computer hardware cooling solutions, though major obstacles for widespread use are its cost and aggressive corrosive properties (it corrodes many other metals such as aluminium by dissolving them). It is also electrically conductive, and so needs to be applied more carefully than regular non-conductive compounds.
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.
The reported melting point of commercial Galinstan is inconsistent with the ternary eutectic alloy. Many commercially available gallium, indium, and tin eutectic alloys are advertised with a melting point of about +11°C, which is significantly higher than the −19°C reported for Galinstan.
Differential Scanning Calorimetry (DSC) tests demonstrate the apparent source of discrepancy. On heating, solid Galinstan will melt at +11°C which is the eutectic point. On cooling, the alloy will remain liquid well below this point (depending on specimen geometry, containment surface, etc.). Several US patents  have been allowed for gallium eutectic alloys with additions of bismuth, antimony, and silver. The claims in these patents include melting temperatures below 0°C, however the test methodology described the liquid alloy remaining liquid when stored in a cold box overnight. Reproducing these results in a commercial batch have not been reported.
The official material safety data sheet (MSDS) mentions only that Galinstan is a “eutectic mixture of the metal components gallium, indium, and tin” with no further description provided. Additionally, a US patent to Geraberger Thermometerwerk GmbH describes various related eutectic alloys, and mentions that they may contain up to 2% Bi (by weight) to increase fluidity, and up to 2% Sb to improve oxidation resistance. The resulting eutectic alloy would contain (by weight) 68–69% Ga, 21–22% In, and 9.5–10.5% Sn, with small amounts of Bi and Sb (0–2%, each), and an impurity level less than 0.001%.
The resulting material is reported by its manufacturer to have a melting point of −19.5°C and vaporisation point above 1800°C.
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