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Just to propose something specific in an answer: I would first guess Gallinstan (by mass: 68.5% Ga, 21.5% In, 10.0% Sn), at least if we're talking about near-room-temperature applications: Supposedly, it melts at -19°C, and has about a μPa or less vapor pressure at 500°C. (This is much lower than Hg, but I don't know if it's lower than pure gallium because ...


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