Thermal compound

[twodogs]

Is there a material that will remain insulative (?) at low temps (<40'C) and a heat conductor at higher temps? We need a way to allow a preheat strip to warmup fluorescent bulbs when cold, but hten to act as a conductor to heatsinks when the bulbs are too warm. I'm thinking of something like the sodium filled engine valves, but less costly and hazardous...