Zirconium is a strong, corrosion-resistant transition metal used in nuclear reactors (low neutron absorption) and in ceramics as zirconia (ZrO₂).
Ground-state configuration: \([Kr]4d^2\,5s^2\). The most common and stable oxidation state in compounds is +4 (e.g., ZrO2, ZrCl4). Lower states such as +3 and +2 are rare and usually require strongly reducing conditions.
Zr spontaneously forms a thin, adherent ZrO2 passive film that protects the underlying metal in many environments (water, steam, organic acids). This passivation is analogous to Al2O3 on aluminum and Cr2O3 on stainless steels.
Zirconia is a high-melting ceramic that exists in monoclinic (room T), tetragonal, and cubic phases. Doping with yttria (Y2O3) stabilizes the high-symmetry phases and creates oxygen vacancies, giving tough, thermal-shock-resistant ceramics and an oxide-ion conductor:
\(\mathrm{Zr_{1-x}Y_xO_{2-\frac{x}{2}}}\)
The vacancies \(\mathrm{V_{O}^{\bullet\bullet}}\) enable fast O2− transport used in solid oxide fuel cells and oxygen sensors.
The principal mineral is zircon, \(\mathrm{ZrSiO_4}\). A typical route chlorinates zircon to ZrCl4, then reduces it (Kroll process):
\(\mathrm{ZrSiO_4 + 4\,Cl_2 + 4\,C \rightarrow ZrCl_4 + SiCl_4 + 4\,CO}\)
\(\mathrm{ZrCl_4 + 2\,Mg \xrightarrow{\sim 800^\circ C} Zr + 2\,MgCl_2}\)
The sponge metal is then purified and melted into ingots.
Hafnium (Hf), which accompanies Zr in ores, has a very high neutron-capture cross section. In reactor cores, cladding must absorb as few neutrons as possible, so Zr for nuclear service is purified to extremely low Hf levels (e.g., Zircaloy). Conversely, Hf is valuable in control rods precisely because it absorbs neutrons well.
Zircaloy refers to Zr-based alloys (with Sn, Fe, Cr, Ni, Nb in small amounts) that offer a balance of low neutron absorption, good strength at temperature, and water/steam corrosion resistance. These properties make them ideal for enclosing nuclear fuel pellets in light-water reactors.
ZrCl4 is a strong Lewis acid and hydrolyzes readily to oxo-species and HCl; a simplified net equation is:
\(\mathrm{ZrCl_4 + 2\,H_2O \rightarrow ZrO_2\downarrow + 4\,HCl}\)
In practice, polymeric hydroxy-chloride gels (e.g., ZrOCl2·8H2O) form—useful precursors for sol-gel synthesis of zirconia.
Yes—Zr and especially zirconia ceramics are considered biocompatible and are used for dental implants/crowns and some orthopedic components. The toughened Y-TZP (yttria-tetragonal zirconia polycrystal) grade provides high strength and aesthetic translucency.
Bulk Zr metal is generally safe to handle, but fine Zr powders and chips can be pyrophoric and burn very hot. Avoid ignition sources, use inert atmospheres for powder processing, and employ Class D extinguishing media (do not use water) for metal fires. Zirconia ceramics are stable and inert.