Zirconia is an inorganic non-metallic material with high temperature resistance, corrosion resistance, abrasion resistance and excellent electrical conductivity, especially its excellent normal temperature mechanical properties, high temperature resistance and corrosion resistance, and it is favored by scientific researchers. In the 1920s It was first used in the field of refractories. Since the mid-1970s, outstanding countries in Europe, America and Japan have invested in research and development of zirconia production technology and production of zirconia series products, and further expanded the application of zirconia to structural materials and functions. Zirconium oxide is also one of the key incentives for the development of high-performance new materials in the national industrial policy.
1. ZrO2 crystal structure
At high temperatures, ZrO2 belongs to a cubic fluorite structure. As shown in Figure 1, because the diameter of Zr4+ is larger than the diameter of O2- ions, it can be considered that Zr4+ forms a face-centered cubic lattice, occupying 1/2 of the octahedral gap, and O2- ions Occupies all four tetrahedral voids in the face-centered cubic lattice.
There are three crystal types of pure zirconia under normal pressure, monoclinic system at low temperature, density 5.68g/cm3, tetragonal system at high temperature, density 6.10g/cm3, and cubic system at higher temperature, density 6.27g/cm3 .
At room temperature, ZrO2 can only be a monoclinic phase. When it is calcined with zirconium salt to reach 650℃, a stable tetragonal phase appears. When the temperature continues to rise, the tetragonal phase gradually transforms into a monoclinic phase. When the temperature continues to rise to 830℃, ZrO2 again It begins to transform into tetragonal phase, and when it reaches 1170℃, it completely transforms into tetragonal phase. When the temperature rises to 2370℃, it transforms into cubic phase; when the temperature decreases, it gradually transforms into tetragonal phase, and when it reaches room temperature, it becomes stable monoclinic phase. The transformation of monoclinic zirconia at 830-1200°C is more complicated and will cause hysteresis. It is this hysteresis that provides a key performance for the application of zirconium dioxide in ceramics and refractories. During the transformation process, the corresponding volume will change. When the temperature rises, the volume will shrink by 5% when the temperature changes from the monoclinic phase to the tetragonal phase, and the volume will expand when the temperature decreases from the tetragonal phase to the monoclinic phase. %, there are three phase structures, and their thermal expansion is different.
2. The properties of zirconia and its role in
refractory materials
In the oxide product system, ZrO2 contains many excellent features, such as high melting point (2700℃), high structural strength at high temperature (200Kpa load at 2000℃, can hold for 0.5~1 hour to produce deformation), good chemical stability, regardless of acid and Alkali or glass are all highly chemically inert and not easy to be wetted by liquid metal, so they also contain high metal stability (for many molten metals and even highly active metals of group IV, V, and VI contain good corrosion resistance), high temperature The vapor pressure and decomposition pressure are both low, and it contains lower volatility than Al2O3 and MgO. ZrO2 has higher stability to vacuum than Al2O3 and MgO, which can be explained by the high bonding strength of zirconium and oxygen. The breaking energy of the Zr-O bond in ZrO2 is 757.8kJ/mol, but the Mg-O bond is 481.5kL/mol, and the Al-O bond is 418.7kJ/mol. The affinity of zirconium for oxygen and the strong Zr-O bond explain its higher metal stability than magnesia and alumina and lower interaction with carbon steel and decarburized steel. Therefore, it can be considered that ZrO2 can meet the technical requirements for smelting many pure metals and alloys at high temperature and high vacuum, and is a key refractory material for metallurgy in the future.
The role of zirconia in refractory materials:
(1) Good chemical stability, prolonging the corrosion of metal ions such as Fe on refractory materials;
(2) Improve material performance and enhance the thermal stability of refractory products;
(3) According to the different performance of the compound item, the production process can be optimized to improve the performance of the refractory product and reduce the production cost;
(4) The compound has a high melting temperature and a higher production temperature of a low melting mixture.
3. ZrO2 material stabilization
Because the reversible transformation between monoclinic and tetragonal zirconium dioxide is accompanied by a volume effect. It causes the refractory to easily crack when it is fired, so it is extremely difficult to produce a sintered product without cracking using pure zirconia alone. If an appropriate amount of CaO, MgO, Y2O3, Nb2O3, CeO2, ScO3 and other cation radius and Zr4+ ion radius are within 12% of zirconia are added to zirconia, after high temperature treatment, all stable cubic types can be obtained from room temperature to 2000℃ Zirconia solid solution, thereby eliminating the volume effect caused by phase change during heating or cooling, and avoiding cracking of zirconia-containing products. The above-mentioned addition of oxides becomes a stabilizer. After this stabilization treatment, zirconia is called stabilized zirconia, and the process of preparing stable zirconia is called zirconia stabilization.
The most widely used stabilizers are CaO, MgO and their mixtures, of which CaO is more effective, followed by MgO. The amount of CaO added is usually 3-8% or more (by mass). ZrO2-MgO cubic solid solution will decompose after a long time heat treatment (1000~1400℃), causing product damage. Although ZrO2-CaO cubic solid solution is relatively stable, it will partially decompose if heated for a long time, and ZrO2 will lose its stabilization effect. Compared with other ZrO2 solid solutions, the key advantage of ZrO2-Y2O3 solid solution is that it will not decompose when heated at 1100~1400℃ for a long time. However, this kind of oxide is scarce and expensive, so it can only be used in places with special requirements. Recently, a number of composite agents have been studied, such as ZrO2-MgO and ZrO2-CaO solid solution by adding 1~2% Y2O3 to significantly improve its thermal shock stability. Adding 3~5% Y2O3 can make the solid solution not decompose at all, and it has high mechanical strength and low thermal expansion coefficient.
The biggest disadvantage of fully stable ZrO2 is its high thermal expansion coefficient and poor thermal shock resistance. Partially stabilized zirconium dioxide can effectively improve its thermal shock resistance. The principle is that when the stabilizer is added in a small amount, only a part of ZrO2 and the stabilizer form a solid solution. When cooling from high temperature to normal temperature, there is still a part of ZrO2 undergoing phase change, from cubic phase or tetragonal phase to monoclinic phase, and Accompanied by a certain volume change. Because this volume change is small and is controlled by the amount of stabilizer added, it will not cause damage to the sintered product. On the contrary, this volume change can produce a certain amount of microcracks in the sintered body of the product. Such microcracks can absorb the crack propagation energy when the material is subjected to thermal stress, inhibit crack propagation, and improve the heat resistance of the material Shock capacity. Therefore, partially stabilized zirconia has more extensive uses than fully stabilized zirconia.
