Thermal expansion of materials has a simple physical explanation:
The thermal energy received by the material is assimilated primarily in the form of vibrational energy of the atoms and molecules that make up the structure of the material.
The oscillatory motion of atoms or molecules is described by the model of an oscillator with an asymmetric potential:
So, we can see that the particles repel each other more as they approach than they attract as they move away from each other.
The oscillator energy is minimal (Ep0) and the equilibrium distance between the oscillating particles (r0) is also minimal at temperatures close to absolute zero. As the oscillator heats up, the energy of the oscillator increases (E1 - >E2 - >E3 ->...). the equilibrium distance between the oscillating particles also increases (r1->r2->r3 ->...).
This increase in the equilibrium distance corresponds to the thermal expansion.