If you look into matter without accounting for phase transformations it expands due to increased vibrational frequency of atoms as they have higher amplitude and simply occupying more space.
In crystalline mater after phase transformation it depends on base lattice size. For example in Fe crystal structure changes from BCC to FCC and causing increase of lattice parameter from 0.287 nm to 0.352 nm as it need to accommodate additional 2 atoms per unit cell.
Yes, this is true. However, because of structure changes, you'll also find materials which shrink when heated above some given temperatures as more compact phases can form. But for one given phase, heating indeed increases the vibration amplitude and therefore the matter expands.
Alain
One example of contraction during heating is the transformation of ferrite(bcc) to austenite(fcc) in steels, and expansion during cooling from Austenite(fcc) to Martensite(Tetragnonal) transformation
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