Atoms in metal crystals are connected by metal bonds. After the metal is heated, the vibration of atoms intensifies, the cell size becomes larger, and the delocalization degree of free electrons is greater. Even if the dislocation is caused by expansion, the metal bond can almost maintain its original strength. This shows that metal has a large thermal expansion coefficient.

Chemical bonds in ceramics include ionic bonds and covalent bonds. For ionic bonds, once the atomic spacing increases due to thermal expansion, the coulomb force will be significantly reduced and the potential energy will be significantly increased, which means that only a little expansion is needed to absorb external energy. In other words, to expand more, more energy must be provided (when the external energy is too large, the ionic bond will break, and the ceramic will break rather than expand). For covalent bonds, due to their obvious directionality, when ceramics thermally expand, the atoms will be dislocated to a certain extent, which will lead to the obvious weakening of covalent bonds and the obvious increase of potential energy, and at the same time, it will absorb a lot of external energy, so that the expansion cannot continue to increase. Therefore, the thermal expansion coefficient of ceramics is very small.