Expanded ceramics are generally divided into two categories: open-cell (skeletal) ceramics and closed-cell ceramics, depending on whether the pores have solid walls. If the solid forming the foam is only contained within the edges of the pores, it is called open-cell ceramic material, with interconnected pores; if there are solid walls, the foam is termed closed-cell ceramic material, with pores separated by continuous ceramic matrix. However, most expanded ceramics have both open-cell pores and a small amount of closed-cell pores. Generally, materials with pore diameters less than 2nm are micro-porous materials; materials with pores between 2-50nm are mesoporous materials; and materials with pores larger than 50nm are macroporous materials.