Melting, Melting Point and Expansion

Polyethylene should be right in that range. Of course there is never enough detail in a request like this to know if a suggestion might work well....

You could try salt. It expands more than 10% but you may need to mix in graphite to make it easier to handle and control its eutectic point.

I've heard that certain waxes are used in the space program to control cooling under high temp applications. The wax melts and triggers cooling flow. I have no idea what level of expansion occurs, however.