Cryostage solid state active insulation

Question

I am designing an ultra-high vacuum cryostage for a fluorescence microscope. The cold finger is cooled by a closed loop liquid nitrogen system. The base of the stage is maintained at room temp with ceramic heaters to prevent thermal drift of the piezoelectric positioning stages. The current design is to use polyethyl ether ketone (PEEK) plastic standoffs to insulate the cold finger and base as PEEK has the lowest thermal conductivity I could find (0.25W/m/K).

I am wondering if a peltier/TEC could be used instead as a sort of active insulator where rather than pumping heat across it is preventing heat flow (reversed polarity). My concern is that the inefficiency of TECs is such that it would actually introduce more heat to the cold finger.

Lets assume the PEEK standoffs and TEC are the same thickness for simplicity. Does anyone with thermodynamic expertise have an idea on this?

thanks!! Selene

Answer 1

The cooling power of Peltier effect stages drops to virtually nothing once you get to a 70K temperature difference between the hot and the cold sides of the device.

This means that a single-stage Peltier "Active Thermal Isolation" device won't work, since your entire cold finger will eventually thermalise to 77K and you will have a large heat leak from the 230K "thermal isolation".

You are better just to use teflon. Lots of layers of teflon tape will help too, since thermal boundary resistance is helpful for thermal isolation.