Zirconium is a Block D, Group 4, Period 5 element. The electronic configuration is [Kr] 4d² 5s². In its elemental form zirconium's CAS number is 7440-67-7. The zirconium atom has a radius of 159.pm and it's Van der Waals radius is 200.pm. Zirconium is primarily used in it's oxide or zirconia form. Zirconium dioxide has a high melting point (2,700° C) and a low thermal conductivity. Its polymorphism, however, restricts its widespread use in ceramic industry. During a heating process, zirconia will undergo a phase transformation process. The change in volume associated with this transformation makes the usage of pure zirconia in many applications impossible. Addition of some oxides, such as CaO, MgO, and Y2O3, into the zirconia structure in a certain degree results in a solid solution, which is a cubic form and has no phase transformation during heating and cooling. This solid solution material is termed as stabilized zirconia, a valuable refractory. Stabilized zirconia is used as a grinding media and engineering ceramics due to its increased hardness and high thermal shock resistivity. Stabilized zirconia is also used in applications such as oxygen sensors and solid oxide fuel cells due to its high oxygen ion conductivity.