Zirconium dioxide

Zirconium dioxide, often called zirconia, is a white crystalline oxide of zirconium. It is a highly significant ceramic material used in many different industries. In nature, the most common form of this material is a mineral known as baddeleyite. Because of its unique properties, scientists and engineers study it extensively. It is used for everything from medical tools to space exploration technology.

The physical structure of zirconia changes depending on the temperature. This is known as a phase change. Below 1170 °C, the material has a monoclinic crystalline structure. Between 1170 °C and 2370 °C, it enters a tetragonal phase. Above 2370 °C, it becomes a cubic phase. As temperatures rise, the symmetry of the crystal structure typically increases. This shifting structure can cause the material to crack during cooling. This happens because the changes in volume create large internal stresses.

To prevent cracking, scientists use a process called stabilization. They add small amounts of other oxides, known as dopants, to the material. Common dopants include magnesium oxide, yttria, calcium oxide, and cerium(III) oxide. Adding yttria, or yttrium oxide, is a very common method. This prevents the disruptive phase changes from occurring. Stabilized zirconia possesses much better thermal, mechanical, and electrical properties. It is often more useful to engineers in this stabilized state.

One fascinating process is called transformation toughening. This occurs when zirconia contains a metastable tetragonal phase. A metastable phase is one that stays in a state that is not its most stable form. When a crack begins to form, the stress at the crack tip causes the tetragonal phase to convert to the monoclinic phase. This conversion causes a volume expansion. This expansion puts the crack into compression, which stops the crack from growing. This mechanism significantly increases the fracture toughness and lifetime of the product.

Different amounts of dopants create different results. For example, adding 3 mol% of yttrium oxide creates 3Y-ZrO2. At high temperatures of about 1500 °C, this improves hardness and flexural strength. This is because it retains the metastable tetragonal phase at room temperature. In contrast, adding 5 mol% of yttrium oxide creates 5Y-ZrO2. This version is mostly in the cubic phase. While 5Y-ZrO2 has better optical translucency, it does not benefit from transformation toughening.

Zirconia has many important uses in modern life. In dentistry, it is used to make durable crowns, bridges, and dental prostheses. Because it is very hard, it is also used to make ceramic knives that stay sharp longer than steel. In engineering, it serves as a thermal barrier coating in jet and diesel engines. These coatings allow engines to operate at higher temperatures, which increases efficiency. It is also used in oxygen sensors because oxygen ions can move freely through its crystal structure at high temperatures.

Beyond Earth, zirconia might help humans explore Mars. Scientists have proposed using zirconia electrolysis to produce fuel. This process could extract carbon monoxide and oxygen from the Martian atmosphere. This would provide both fuel and oxidizer for surface transportation. On Earth, the material is even used in luxury items. In 2015, the company Omega released a watch called "The Dark Side of The Moon." It featured a ceramic case that was four times harder than stainless steel.