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Basic properties and high-temperature performance of zirconia

wallpapers Tech 2020-07-14
Most of the ZrO2 used in the world is obtained from the extraction of zircon. There are two main methods for refining ZrO2 from zircon: chemical method (alkali metal oxide decomposition method) and electric melting method (reduction melting desalinization method).

Refractory raw materials: basic properties and high zirconia

The former process is involved, the ZrO2 produced is of high purity, but the price is higher, and it is generally used in unique ceramics; the latter is more comfortable to provide, low cost, suitable for large-scale production, and the ZrO2 content can reach more than 95%, which can meet the requirements of the refractory industry demand.

1. Basic properties of zirconia:

Zirconium dioxide is a high-melting metal oxide with molecular formula ZrO2, the relative molecular mass of 123.22, melting point of 2715℃, softening point between 2390~2500℃, boiling point of about 4300℃, Mohs hardness of 7, the density of 5.65 ~6.27g/cm3, the average linear expansion coefficient at 20~1000℃ is 10×10-6/℃, and the thermal conductivity at 1000℃ is 2.30W/(m·K).

Pure zirconia is a white powder with yellow or grey when containing impurities, and contains hafnium dioxide impurities. Because zirconium dioxide has the characteristics of wear resistance, high-temperature resistance, corrosion resistance, non-conduction, and non-magnetic conductivity, it also has a linear expansion coefficient close to that of metal and is the only oxide that has a phase transformation with steel and another martensite The similarity of alloy materials makes zirconia have many essential uses.

Zirconia has three different crystal structures: low-temperature phase, medium-temperature phase and high-temperature phase. Zirconia at high temperature belongs to the cubic fluorite structure; the medium-temperature stage is the tetragonal zirconia crystal structure and low-temperature phase monoclinic zirconia crystal structure. The linear expansion coefficients of the three crystal types are different, the smallest is monoclinic zirconia, the second is tetragonal zirconia, and the largest is cubic zirconia. This is because the thermal properties of the material such as heat capacity, thermal conductivity, and thermal expansion are all related to the thermal shock of the atoms, which directly depends on the vibration of the crystal lattice.

On the premise of considering only the material phase composition, for zirconia, since the cubic phase lattice structure is the simplest, the thermal vibration of atoms is relatively easy, while the monoclinic phase structure is the most complicated, and the atomic thermal wave is relatively tricky.

2. The crystal form transformation of zirconia:

The three crystal forms of zirconia have a reversible phase transition process with the change of temperature. The phase of the tetragonal phase to the monoclinic phase changes to the martensite phase transition. It was first pointed out by GMWolten that this phase transition is in the study of zirconia materials. Of particular importance. The conversion from monoclinic to tetragonal crystal form is reversible, and the volume shrinks by 7%. That is, it shrinks when it heats up and expands when it cools down.

However, the forward conversion of monoclinic to tetragonal starts at 1170℃, while the reverse transformation is between 1000~850℃. Due to the difficulty of monoclinic crystal nucleation, the temperature lag phenomenon will occur in the crystal phase transition. ZrO2 is monoclinic when not higher than 1000℃, and tetragonal when it is higher than 1000℃. Observing the experimental illustrations of the thermal expansion properties of different zirconias, although the linear expansion coefficient of pure monoclinic zirconia is small, its linear expansion has significant anisotropy, and there is also the problem of phase change.

Although the axial expansion of tetragonal zirconia is different, the difference is not significant, and it is close to a linear relationship, so the expansion uniformity is excellent, and mutation and embrittlement will not occur; the thermal expansion of cubic ZrO2 is along a single axis, and It increases with increasing temperature; the thermal expansion of partially stabilized ZrO2 is between monoclinic and tetragonal phases.

3. Stabilization of zirconia:

ZrO2 stabilization is the reconstruction of a tetragonal lattice into a cubic lattice that is stable at any temperature. At the same time as the mesh is reconstructed, a reliable solution composed of a stabilizer and ZrO2 is produced. These robust solutions are Displacement reliable solution with limited solubility is formed by certain oxides with similar cation radius and Z4+ ion radius (0.087nm).

Conventional stabilizers are rare earth or alkaline earth oxides, and only oxides whose ionic radius differs from the Zr radius by no more than 40% can be used as ZrO2 stabilizers. Among them, Y2O3, MgO, CeO2, CaO are more commonly used. At present, the stabilization mechanism is not very clear. It is generally interpreted as the cations of stabilizers such as Y3+, Mg2+, Ce4+, Ca2+ have a specific solubility in ZrO2, which can replace Zr and form a robust replacement solution.

The stability of zirconia stabilized by different stabilizers is substantially the same. Still, the performance after they are supported is not the same, and the performance of the same stabilizer with varying amounts of addition is also much different. Therefore, choosing the right stabilizer and adding value is crucial to the production of zirconia material.

Trunnano is one of the world's largest manufacturers of zirconium dioxide. There are various sizes of zirconium dioxide products. If necessary, please contact Dr Leo, email: brad@ihpa.net.

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Tag: Trunnano   ZrO2   Zirconium dioxide