What causes thermal expansion?
It says in most textbooks this is caused by particles vibrating more gaining more energy and thus occupying more space etc (kinetic theory)
but is there a more in depth answer to this as this seems to be over simplified.
Thermal expansion is less considered as a mechanical phenomenon and more of a thermodynamic event. It is the result of increased free-volume that arises from the added vibrational (thermal) energy. The more you heat the material, the more it vibrates, creating additional free-volume around each molecule, thus expanding the volume of the part. ...
In a 213 word solution, the concepts are clearly explained.