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Understanding the Preparation and Application of Magnesium Oxide Ceramics

Understanding the Preparation and Application of Magnesium Oxide Ceramics

Oct 20, 2023

Magnesium oxide ceramics is a typical new type of ceramics and also belongs to traditional refractory materials. Magnesium oxide itself has strong anti-erosion ability to alkaline metal solution, the prepared magnesium oxide ceramic crucible has excellent chemical properties and stability to resist metal erosion, and magnesium, nickel, uranium, aluminum, molybdenum and so on do not work. Under the protection of oxidizing atmosphere or nitrogen, magnesium oxide ceramics can work stably up to 2400℃, so magnesium oxide is the key material in the advanced process of modern metallurgical industry.

Performance characteristics of magnesium oxide ceramics

MgO ceramics are cubic crystal system, sodium chloride structure. The main crystalline phase of magnesium oxide ceramics is MgO, the theoretical density is 3.85g/cm³, the melting point reaches 2800°C, and the Moh's hardness is 5-6. MgO belongs to the weakly alkaline substance, and has strong anti-erosion ability to alkaline metal solution. The ion stacking density in the lattice of magnesium oxide is close, the symmetry of ion arrangement is high, the lattice defects are few, and it is difficult to sinter. Therefore, in the preparation process of magnesium oxide ceramics, in order to reduce the sintering temperature, it is necessary to add a certain amount of sintering additives, generally mixed rare earth metal oxides (RxOY), TiO2, ZrO2, SiO2 as a sintering additive. The high temperature stability and corrosion resistance of magnesium oxide ceramics are better than that of alumina ceramics, which can be used in the corrosive environment of strong alkali metals.

Preparation of magnesium oxide ceramics

Preparation of magnesium oxide raw materials

Nature is rich in magnesium-containing compounds, it exists in the earth's crust and the ocean in the form of a variety of minerals, such as magnesite (MgCO3), dolomite (CaMg(CO3)2), hydromagnesite (Mg(OH)2), talc (Mg[Si4O10](OH)2) and so on. Industrially, MgO is mainly extracted from the above minerals, but recently it is developed to be extracted from seawater.

Extracting MgO from minerals or seawater, most of them are first made into magnesium hydroxide or magnesium carbonate, and then calcined and decomposed into MgO, which can be obtained as high-purity MgO through further chemical treatment or heat treatment.

Preparation of MgO ceramics

Preparation of MgO raw materials after treatment, according to the composition of the ingredients. In order to promote sintering as well as can make the grain grow a little bit, at the same time in order to reduce the preparation of the hydration tendency, can add some additives, such as TiO2, Al2O3, V2O3, etc.. If the requirements of high purity MgO ceramics, can not be used to add additives to promote sintering and grain growth, but the method of activation sintering, that is, Mg (OH) 2 calcined at the appropriate temperature, to get a lot of lattice defects of the activated MgO, used to manufacture sintered magnesium oxide ceramics. Generally calcined at 1200 ℃ decomposition of Mg (OH) 2 is appropriate.

MgO chemical activity, soluble in acid, hydration capacity, so MgO ceramics manufacturing process must take into account this characteristic of the raw material. In order to reduce the activity of MgO, should be pre-fired to 1100 ~ 1300 ℃. Grinding liner and grinding balls used in grinding raw materials should be made of ceramic materials, can not use steel balls. Dry grinding is used to prevent hydration, and it is better to use vibrating mill for fine grinding, which can get more fine particles. Molding methods can be used semi-dry pressing method, slurry method, hot die casting method and hot pressing method. Semi-dry pressing method using the binder for glycerol, polyvinyl alcohol, beeswax carbon tetrachloride solution, etc., choose fine-grained ingredients, molding under 50 ~ 70MPa pressure. Preparation of slurry molding slurry, need anhydrous ethanol as a medium to avoid MgO hydration and expansion. Preparation of hot die casting with the best raw materials in the fine grinding to add 2% ~ 3% of oleic acid (general dosage of about 14%), the fine grinding time should not be too long to prevent MgO particles agglomeration. The bulk density of the product obtained by hot pressing method can be close to the theoretical density value. General pressure of 20~30MPa, temperature 1300~1400 ℃, pressurization time 20~40min.

MgO ceramics are mostly produced by slurry injection method. However, because MgO is easily hydrated, its process is more complicated. In the preparation of slurry, in order not to make MgO contact with water, need to use anhydrous ethanol and other organic liquids as a suspension medium. Widely used process is as follows: the MgO raw materials with a sufficient amount of distilled water mixed to form a paste and fully hydrated into Mg(OH)2. stored for a certain period of time, drying below 100 ℃, in 1450 ~ 1600 ℃ in the sealed conditions of calcination, insulation 8h, so that the decomposition of magnesium hydroxide into MgO. then ball milling for 45 ~ 90h, and then add water (the amount of water to add) Then ball mill for 45~90h, then add water (the amount of water added is 50%~60%) and continue to mill for 70~90min. to form a suspension slurry, used for casting molding. In order to improve the performance of the slurry, it can be adjusted by the method of pH value, so that the pH value of 7~8.

After demolding the billet is dried at a temperature of 70°C. In order to remove the moisture from the billet quickly, the humid air should be removed as soon as possible.

Ceramic sintering, is the first in 1250 ℃ temperature for vegetal firing, and then loaded into the corundum porcelain sagger in 1750 ~ 1800 ℃ temperature insulation 2h sintering.

Application of magnesium oxide ceramics

Magnesium oxide ceramics, its theoretical operating temperature up to 2200 ℃, can be used for a long time at 1600 ℃ ~ 1800 ℃. Its high temperature stability and corrosion resistance are better than alumina ceramics, and with Fe, Ni, U, Th, Zn, Al, Mo, Mg, Cu, Pt and other incompatible, so its scope of application can include: iron and steel, glass, and other smelting industry in corrosive conditions in the crucible or other refractory materials.

Magnesium oxide ceramics can be used as crucibles for smelting metals, and is also suitable for smelting high purity uranium and thorium in the atomic energy industry; it can also be used as thermocouple protective cover. The use of it can make electromagnetic waves through the radiation shield and infrared radiation projection window material properties, smelting metals, alloys, such as nickel alloys, radioactive metals, uranium, thorium alloys, iron and its alloys and other crucibles. Piezoelectric, superconducting materials and other raw materials, and no pollution, lead corrosion resistance, etc.; can also be used as a ceramic sintering carrier, especially β-Al2O3 and other high-temperature corrosive, volatile substances such as ceramic products sintering protection.