You're looking for phase change materials (PCM), which soften at the desired operating temperature. Prevents pumping out because it's still mostly-solid. E.g.

You're looking for phase change materials (PCM), which soften at the desired operating temperature. Prevents pumping out because it's still mostly-solid. E.g. https://thermalmanagement.honeywell.com/us/en/products/thermal-interface-materials/phase-change-materials https://thermalmanagement.honeywell.com/us/en/products/therm...