I had the "brilliant" idea of using liquid gallium as a heat transfer medium between a molybdenum and alumina surface, at up to 1700 deg C. and between a stainless steel and alumina surface at up to 800 deg C - now apparently wiser heads are saying this is a bad idea. Is it really ?
The application is temperature measurement in an instrument. There must be no forces between the two surfaces, but they should be in good thermal contact, hence the liquid medium.
Any comments on the merits of the idea would be much appreciated.
Look it up. There is probably a low melting compound that can be formed.
I remember someone wanting to combine the mass and strength of Tungsten with the thermal shock resistance of carbon for an erosion resistant reentry vehicle nosetip. Both of these have very high melting points.
The specimens were made and had good room temperature hypervelocity particle impact behavior.
Then the test temperature was raised to about 1500C , well below the melting points of either of the constituents.
The specimens melted before any impact testing could be done.
Then the phase diagram was FINALLY looked at, and there are some low melting compounds in the Tungsten-Carbon phase diagram that explained the melting.
Just the melting points of the pure substances does not explain what happens when they are heated in contact with each other.
You probably had good advice. Maybe ask the source about the Ga Mo phase diagram. He might take heart, thinking you have learned something.
Remember that Ice melts at about 0C and salt (NaCl) melts at maybe 400C or more. Yet a salt-ice mixture melts well below 0C. Another example, perhaps timely for winter driving.
Only part of my mind seemed to work. Focused on the wrong thing.
The other part of the problem is will the liquid Ga dissolve in the Mo ..... and thus go away eliminating your liquid thermal contact. Or will there be some dissolving of the Mo in the hot liquid Ga?
High temperatures generally make reactions kinetically happen, if they are thermodynamically favorable.
The most direct answer is that I don't know the answer. The liquid metal is great for conductivity, but maybe it promotes the surface reactivity.
Just got no real answer for you. I've no experience with use of liquid metals as a substitute for thermal grease, and maybe someone here has more of that experience, or relevent foundry crucible experience for molten compatibility.
You've gotten some good advice so far. Another issue may be failure due to a phenomenon called Liquid Metal Embrittlement. I know gallium can do this in many solid metal systems, including aluminum alloys. SS may be a good candidate, as well, but I don't have my reference books handy.
Essentially, the liquid gallium separates individual grains of the solid metal, or causes cracking under any stress.
Anyway, comprehensive books on corrosion should list the active systems involving gallium. I know the ASM Metals Hanbook volume does.
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