Magnesium oxide ceramics are a typical new type of ceramics and also belong to traditional refractory materials. Magnesium oxide itself has strong corrosion resistance to alkaline metal solutions. The prepared magnesium oxide ceramic crucible has excellent chemical properties and stability against metal corrosion, and does not work with magnesium, nickel, uranium, aluminum, and molybdenum. Magnesium oxide ceramics can work stably to 2400°C under the protection of oxidizing atmosphere or nitrogen, so magnesium oxide is a key material in the advanced technology of modern metallurgical industry.
01 Performance characteristics of magnesium oxide ceramics
MgO ceramics belong to the cubic crystal system and have a sodium chloride structure. The main crystal phase of magnesium oxide ceramics is MgO, the theoretical density is 3.85g/cm3, the melting point is 2800℃, and the Mohs hardness is 5-6. MgO is a weakly alkaline substance and has strong corrosion resistance to alkaline metal solutions. The ion packing density in the magnesium oxide crystal lattice is close, the ion arrangement symmetry is high, the crystal 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, a certain amount of sintering aids need to be added. Generally, mixed rare earth metal oxides (RxOY), TiO2, ZrO2, and SiO2 are used as sintering aids. Magnesium oxide ceramics have better high temperature stability and corrosion resistance than alumina ceramics, and can be used in strong alkali metal corrosive environments.
02 Preparation of Magnesium Oxide Ceramics
(1) Preparation of magnesium oxide raw materials
Magnesium-containing compounds are abundant in nature. They exist in the crust and ocean in the form of various minerals, such as magnesite (MgCO3), dolomite (CaMg(CO3)2), brucite (Mg(OH)2). ), talc (Mg[Si4O10](OH)2), etc. Industry mainly extracts MgO from the above-mentioned minerals, and recently it has been developed to extract MgO from seawater.
MgO is extracted from minerals or seawater, and most of them are first made into magnesium hydroxide or magnesium carbonate, and then decomposed into MgO by calcination. The high purity MgO can be obtained by further chemical treatment or heat treatment of this MgO.
(2) Preparation of MgO ceramics
After the MgO raw material is processed during the preparation, the ingredients are mixed according to the composition. In order to promote sintering and make the grains grow slightly, and to reduce the hydration tendency of the preparation, some additives can be added, such as TiO2, Al2O3, V2O3, etc. Table 1 lists the effects of some additives on the MgO grain size and sinterability. If high-purity MgO ceramics are required, the method of adding additives cannot be used to promote sintering and grain growth. Instead, the method of activation sintering is used, that is, Mg(OH)2 is calcined at an appropriate temperature to obtain many crystal lattices. Defective active MgO, used to make sintered magnesium oxide ceramics. Generally, it is better to decompose Mg(OH)2 by calcination at 1200℃.
MgO has strong chemical activity, is easily soluble in acid, and has high hydration capacity. Therefore, the manufacturing process of MgO ceramics must consider this characteristic of the raw material. In order to reduce the activity of MgO, it should be pre-fired to 1100~1300℃. The grinding lining and balls used for grinding raw materials must be made of ceramic materials, and steel balls cannot be used. Dry grinding is used to make it water repellent, and it is best to use vibration grinding to finely grind to obtain more fine particles. The forming method can adopt semi-dry pressing method, grouting method, hot die casting method and hot pressing method. The binder used in the semi-dry pressing method is glycerin, polyvinyl alcohol, carbon tetrachloride solution of beeswax, etc., and fine-grained ingredients are selected and molded under a pressure of 50 to 70 MPa. The preparation of grouting slurry requires absolute ethanol as a medium to prevent MgO from hydration and expansion. It is better to add 2% to 3% oleic acid (usually about 14%) when preparing the raw materials for hot die casting. The fine grinding time should not be too long to prevent the agglomeration of MgO particles. The volume density of the product obtained by the hot pressing method can be close to the theoretical density value. The general pressure is 20~30MPa, the temperature is 1300~1400℃, and the pressure time is 20~40min.
MgO ceramics are mostly produced by grouting. However, since MgO is easy to hydrate, its process is more complicated. When preparing the slurry, in order to prevent the MgO from contacting water, an organic liquid such as absolute ethanol should be used as the suspension medium. The widely used process is as follows: MgO raw materials are mixed with a sufficient amount of distilled water to form a paste and fully hydrated into Mg(OH)2. After being stored for a certain period of time, it should be dried under 100℃, calcined at 1450~1600℃ in a sealed condition, and kept for 8h to decompose magnesium hydroxide into MgO again. Then ball mill for 45~90h, then add water (50%~60% of water added) and continue grinding for 70~90min. Form a suspension slurry for casting. In order to improve the performance of the slurry, the pH value can be adjusted to 7-8.
After demolding, the blank is dried at 70°C. In order to quickly remove the moisture in the blank, the humid air should be removed as soon as possible.
The firing of ceramics is carried out at first at 1250 ℃ for biscuit firing, and then placed in a corundum porcelain sagger and sintered at a temperature of 1750 to 1800 ℃ for 2 hours.
03 Application of Magnesium Oxide Ceramics
Magnesium oxide ceramics have a theoretical use temperature of up to 2200℃, and can be used for a long time at 1600℃~1800℃. Its high temperature stability and corrosion resistance are better than alumina ceramics, and it does not interact with MgO such as Fe, Ni, U, Th, Zn, Al, Mo, Mg, Cu, Pt, etc., so its application range can include : Crucibles or other refractory materials under corrosive conditions in steel, glass and other smelting industries.
MgO ceramics can be used as a crucible for metal smelting, and it is also suitable for smelting high-purity uranium and thorium in the atomic energy industry; it can also be used as a thermocouple protection sleeve. It can be used as a radome and infrared radiation projection window material, smelting metals, alloys, such as nickel alloy, radioactive metal uranium, thorium alloy, iron and its alloys, etc. Raw materials for piezoelectric, superconducting materials, etc., without pollution, lead corrosion resistance, etc.; it can also be used as ceramic sintered carrier, especially for sintering protection of ceramic products with corrosive and volatile substances at high temperatures such as β-Al2O3.